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Castration-resistant prostate cancer (CRPC) is fatal and therapeutically under-served. We describe a novel CRPC-restraining role for the vasodilatory soluble guanylyl cyclase (sGC) pathway. We discovered that sGC subunits are dysregulated during CRPC progression and its catalytic product, cyclic GMP (cGMP), is lowered in CRPC patients. Abrogating sGC heterodimer formation in castration-sensitive prostate cancer (CSPC) cells inhibited androgen deprivation (AD)-induced senescence, and promoted castration-resistant tumor growth. We found sGC is oxidatively inactivated in CRPC. Paradoxically, AD restored sGC activity in CRPC cells through redox-protective responses evoked to protect against AD-induced oxidative stress. sGC stimulation via its FDA-approved agonist, riociguat, inhibited castration-resistant growth, and the anti-tumor response correlated with elevated cGMP, indicating on-target sGC activity. Consistent with known sGC function, riociguat improved tumor oxygenation, decreasing the PC stem cell marker, CD44, and enhancing radiation-induced tumor suppression. Our studies thus provide the first evidence for therapeutically targeting sGC via riociguat to treat CRPC.Statement of significanceProstate cancer is the second highest cancer-related cause of death for American men. Once patients progress to castration-resistant prostate cancer, the incurable and fatal stage, there are few viable treatment options available. Here we identify and characterize a new and clinically actionable target, the soluble guanylyl cyclase complex, in castration-resistant prostate cancer. Notably we find that repurposing the FDA-approved and safely tolerated sGC agonist, riociguat, decreases castration-resistant tumor growth and re-sensitizes these tumors to radiation therapy. Thus our study provides both new biology regarding the origins of castration resistance as well as a new and viable treatment option.

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Prostate cancer is the second most common cause of cancer-related deaths in American men. About 20-40% of patients with localized castration-sensitive prostate cancer (CSPC) eventually progress, requiring androgen deprivation therapy (ADT). However ADT inevitably fails, giving rise to castration-resistant prostate cancer (CRPC), which is incurable and fatal. A significant fraction of patients undergoing ADT die of treatment-accelerated cardiovascular disease. Pre-existing metabolic dysfunction has been implicated in PC disease incidence, higher rates of prostatectomy failure, and accelerated progression to CRPC. Thus, there is an unmet need to counter the adverse sequelae of ADT through alternative CSPC treatments that do not exacerbate metabolic syndrome but prevent PC progression. Few studies have evaluated whether targeting the metabolic dysfunction associated with early-stage PC can prevent progression and thus the need for ADT. In a recent clinical trial spearheaded by Dr. Bilusic, metformin, an affordable and well-tolerated insulin-sensitizing diabetes drug, has recently shown promise as monotherapy in biologically-recurrent CSPC, preventing PSA rise in about 40% of high-risk patients. To identify the molecular mechanisms and biomarkers of positive response to metformin in PC patients, we performed preliminary cytokine profiling of responder vs non-responder patient sera. We found that mitigation of inflammatory factors implicated in cardiovascular disease as well as markers of pro-apoptotic response are associated with positive metformin response. We also found that metformin responders exhibited pre-treatment cytokine signatures associated with inflammatory metabolic diseases, including diabetes and hyperinsulinemia, which are conditions known to benefit from metformin treatment. We have been able to model the variability of metformin treatment response in two well-characterized CSPC cell lines, LNCaP (highly responsive) and LAPC4 (minimally responsive). Metformin induced cell death in LNCaP but not in LAPC4, which possesses mutant p53 and high Bcl-2 levels. LAPC4 instead showed markers of senescence, which is associated with an inflammatory phenotype. Metformin also elevated levels of damaging high-potency (short-lived) ROS in LNCaP to a greater extent than in LAPC4. LAPC4 instead exhibited elevated levels of hydrogen peroxide upon metformin treatment, a ROS associated with inflammation. Thus the cell models show congruence with factors of positive response seen in patients. Our findings shed light on the potential mechanism of positive response to metformin in CSPC patients, and lay the groundwork for further investigation, while insight into biomarkers predicting positive response could be immediately useful in the clinic. Citation Format: Philip Gregory, Beatriz Mateo-Victoriano, Ling Zhang, Cassie Due, Marijo Bilusic, Priyamvada Rai. Repurposing metformin in early-stage high risk prostate cancer to avoid ADT-induced cardiovascular toxicity. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 4335.

Investigations into the human 8-oxodGTPase, MutT Homolog 1 (MTH1), have risen sharply since the first-in-class MTH1 inhibitors were reported to be highly tumoricidal. However, MTH1 as a cancer therapeutic target is currently controversial because subsequently developed inhibitors did not exhibit similar cytotoxic effects. Here, we provide the first direct evidence for MTH1-independent 8-oxodGTPase function in human cancer cells and human tumors, using a novel ATP-releasing guanine-oxidized (ARGO) chemical probe. Our studies show that this functionally redundant 8-oxodGTPase activity is not decreased by five different published MTH1-targeting small molecules or by MTH1 depletion. Significantly, while only the two first-in-class inhibitors, TH588 and TH287, reduced cancer cell viability, all five inhibitors evaluated in our studies decreased 8-oxodGTPase activity to a similar extent. Thus, the reported efficacy of the first-in-class MTH1 inhibitors does not arise from their inhibition of MTH1-specific 8-oxodGTPase activity. Comparison of DNA strand breaks, genomic 8-oxoguanine incorporation, or alterations in cellular oxidative state by TH287 versus the noncytotoxic inhibitor, IACS-4759, contradict that the cytotoxicity of the former results solely from increased levels of oxidatively damaged genomic DNA. Thus, our findings indicate that mechanisms unrelated to oxidative stress or DNA damage likely underlie the reported efficacy of the first-in-class inhibitors. Our study suggests that MTH1 functional redundancy, existing to different extents in all cancer lines and human tumors evaluated in our study, is a thus far undefined factor which is likely to be critical in understanding the importance of MTH1 and its clinical targeting in cancer.

377 Background: Metformin impacts immune response in several ways. It protects CD8 TILs from apoptotic death, downregulates CD39 and CD73, reduces MDSC activity and suppresses the transcription of PD1 thus enhancing CD8+ T cell antitumor activity (Ma et al, 2000, Nature). There is limited data on the immunologic impact of metformin treatment in hormone sensitive prostate cancer. Methods: We conducted an open label, randomized, phase II trial of bicalutamide with or without metformin of patients with high risk, biochemically recurrent prostate cancer (NCT02614859). Patients were randomized 1:2 to observation for an initial 8 weeks or metformin 1000 mg twice daily. Bicalutamide 50 mg/day was added after 8 weeks to both arms. The study discontinued accrual after interim analysis indicated no difference in PSA at 32 weeks between the two arms; however, metformin monotherapy for 8 weeks induced modest PSA declines observed in 40% of patients. Here we report immune responses in a subgroup of patients (N=12), including systemic cytokine levels and 158 circulating immune cell subsets after 8 weeks of metformin monotherapy. Results: Twelve patients treated at the NCI were analyzed. After 8 weeks, the metformin arm showed increased pDCs and lower Tregs compared to baseline. Significant differences in the percent change of immune parameters after 8 weeks of metformin monotherapy compared to 8 weeks of observation are summarized. Conclusions: Metformin has been shown to reduce markers of immune exhaustion in hormone sensitive prostate cancer which is supported by a greater reduction of PD-1+ and Tim3+ NK cells. Additional immune changes included increases in activated subpopulations of natural killer (NK) cells, which have been reported as a potential predictive biomarker of prolonged treatment response to hormone therapy in prostate cancer (Pasero et al, Oncotarget, 2015). Clinical trial information: NCT02614859 . [Table: see text]

Nicotinamide adenine dinucleotide (NAD+) is an essential biomolecule involved in many critical processes. Its role as both a driver of energy production and a signaling molecule underscores its importance in health and disease. NAD+ signaling impacts multiple processes that are dysregulated in cancer, including DNA repair, cell proliferation, differentiation, redox regulation, and oxidative stress. Distribution of NAD+ is highly compartmentalized, with each subcellular NAD+ pool differentially regulated and preferentially involved in distinct NAD+-dependent signaling or metabolic events. Emerging evidence suggests that targeting NAD+ metabolism is likely to repress many specific mechanisms underlying tumor development and progression, including proliferation, survival, metabolic adaptations, invasive capabilities, heterotypic interactions with the tumor microenvironment, and stress response including notably DNA maintenance and repair. Here we provide a comprehensive overview of how compartmentalized NAD+ metabolism in mitochondria, nucleus, cytosol, and extracellular space impacts cancer formation and progression, along with a discussion of the therapeutic potential of NAD+-targeting drugs in cancer.

Unlike normal tissues, tumor cells possess a propensity for genomic instability, resulting from elevated oxidant levels produced by oncogenic signaling and aberrant cellular metabolism. Thus, targeting mechanisms that protect cancer cells from the tumor-inhibitory consequences of their redox imbalance and spontaneous DNA-damaging events is expected to have broad-spectrum efficacy and a high therapeutic index. One critical mechanism for tumor cell protection from oxidant stress is the hydrolysis of oxidized nucleotides. Human MutT homolog 1 (MTH1), the mammalian nudix (nucleoside diphosphate X) pyrophosphatase (NUDT1), protects tumor cells from oxidative stress-induced genomic DNA damage by cleansing the nucleotide pool of oxidized purine nucleotides. Depletion or pharmacologic inhibition of MTH1 results in genomic DNA strand breaks in many cancer cells. However, the mechanisms underlying how oxidized nucleotides, thought mainly to be mutagenic rather than genotoxic, induce DNA strand breaks are largely unknown. Given the recent therapeutic interest in targeting MTH1, a better understanding of such mechanisms is crucial to its successful translation into the clinic and in identifying the molecular contexts under which its inhibition is likely to be beneficial. Here we provide a comprehensive perspective on MTH1 function and its importance in protecting genome integrity, in the context of tumor-associated oxidative stress and the mechanisms that likely lead to irreparable DNA strand breaks as a result of MTH1 inhibition.

Major contributors to therapeutic resistance in pancreatic ductal adenocarcinoma (PDAC) include Kras mutations, a dense desmoplastic stroma that prevents drug delivery to the tumor, and activation of redundant signaling pathways. We have previously identified a mechanistic rationale for targeting STAT3 signaling to overcome therapeutic resistance in PDAC. In this study, we investigate the molecular mechanisms underlying the heterogeneous response to STAT3 and RAS pathway inhibition in PDAC. Effects of JAK/STAT3 inhibition (STAT3i) or MEK inhibition (MEKi) were established in Ptf1acre/+; LSL-KrasG12D/+; and Tgfbr2flox/flox (PKT) mice and patient-derived xenografts (PDX). Amphiregulin (AREG) levels were determined in serum from human patients with PDAC, LSL-KrasG12D/+;Trp53R172H/+;Pdx1Cre/+ (KPC), and PKT mice. MEKi/STAT3i–treated tumors were analyzed for integrity of the pancreas and the presence of cancer stem cells (CSC). We observed an inverse correlation between ERK and STAT3 phosphorylation. MEKi resulted in an immediate activation of STAT3, whereas STAT3i resulted in TACE-induced, AREG-dependent activation of EGFR and ERK. Combined MEKi/STAT3i sustained blockade of ERK, EGFR, and STAT3 signaling, overcoming resistance to individual MEKi or STAT3i. This combined inhibition attenuated tumor growth in PDX and increased survival of PKT mice while reducing serum AREG levels. Furthermore, MEKi/STAT3i altered the PDAC tumor microenvironment by depleting tumor fibrosis, maintaining pancreatic integrity, and downregulating CD44+ and CD133+ CSCs. These results demonstrate that resistance to MEKi is mediated through activation of STAT3, whereas TACE-AREG-EGFR–dependent activation of RAS pathway signaling confers resistance to STAT3 inhibition. Combined MEKi/STAT3i overcomes these resistances and provides a novel therapeutic strategy to target the RAS and STAT3 pathway in PDAC. Significance: This report describes an inverse correlation between MEK and STAT3 signaling as key mechanisms of resistance in PDAC and shows that combined inhibition of MEK and STAT3 overcomes this resistance and provides an improved therapeutic strategy to target the RAS pathway in PDAC. Graphical Abstract: http://cancerres.aacrjournals.org/content/canres/78/21/6235/F1.large.jpg. Cancer Res; 78(21); 6235–46. ©2018 AACR.

The development of Barrett's esophagus (BE) and its progression to esophageal adenocarcinoma (EAC) is highly linked to exposure to acidic bile salts due to chronic gastroesophageal reflux disease (GERD). In this study, we investigated the role of Apurinic/apyrimidinic endonuclease 1/redox effector factor-1 (APE1/REF-1) in STAT3 activation in response to acidic bile salts. Our results indicate that APE1 is constitutively overexpressed in EAC, whereas its expression is transiently induced in response to acidic bile salts in non-neoplastic BE. Using overexpression or shRNA knockdown of APE1, we found that APE1 is required for phosphorylation, nuclear localization, and transcriptional activation of STAT3. By using an APE1 redox-specific mutant (C65A) and APE1 redox inhibitor (E3330), we demonstrate that APE1 activates STAT3 in a redox-dependent manner. By using pharmacologic inhibitors and genetic knockdown systems, we found that EGFR is a required link between APE1 and STAT3. EGFR phosphorylation (Y1068) was directly associated with APE1 levels and redox function. Co-immunoprecipitation and proximity ligation assays indicated that APE1 coexists and interacts with the EGFR-STAT3 protein complex. Consistent with these findings, we demonstrated a significant induction in mRNA expression levels of STAT3 target genes (IL-6, IL-17A, BCL-xL, Survivin, and c-MYC) in BE and EAC cells, following acidic bile salts treatment. ChIP assays indicated that acidic bile salts treatment enhances binding of STAT3 to the promoter of its target genes, Survivin and BCL-xL. Inhibition of APE1/REF-1 redox activity using E3330 abrogated STAT3 DNA binding and transcriptional activity. The induction of APE1-STAT3 axis in acidic bile salts conditions provided a survival advantage and promoted cellular proliferation. In summary, our study provides multiple pieces of evidence supporting a critical role for APE1 induction in activating the EGFR-STAT3 signaling axis in response to acidic bile salts, the main risk factor for Barrett's carcinogenesis.

Increased metabolic activity of cancer cells due to uncontrolled proliferation, often leads to the accumulation of reactive oxygen species (ROS). High ROS can produce tumor-suppressive oxidative DNA damage either directly in the genome or through oxidation of deoxynucleotides that can then be incorporated into DNA. The human 8-oxoguanine glycosylase 1 (OGG1), a base excision repair (BER) enzyme, protects cells from the former by excising genomic 8-oxodG; MutT Homolog 1 (MTH1), the mammalian 8-oxodGTPase, protects cells from the latter by degrading 8-oxo-dGTP. There has been much interest in increasing oxidative DNA damage in tumors as a therapeutic option. Accordingly, inhibitors are being developed that can target both MTH1 and 8-oxodG glycosylase 1 (OGG1) with the speculation that this co-targeting will significantly increase tumor suppression efficiency. Our lab has shown that MTH1 inhibition leads to DNA strand breaks and tumor-suppressive effects in cancers characterized by high MTH1enzymatic activity. Here we assessed whether co-inhibition of OGG1 would provide benefits in tumor-suppressive effects over MTH1 inhibition alone. We first compared OGG1 and MTH1 expression in paired normal versus tumor tissue from lung adenocarcinoma patients and found that, unlike MTH1, OGG1 tends to remain within normal range in cancerous tissue. Similarly, TCGA lung adenocarcinoma patients show minimal OGG1 elevations and a positive correlation between OGG1 and MTH1 mRNA levels even when both are low. This data suggests that tumors with low OGG1 and MTH1 are robustly represented in the tumor continuum. We then co-depleted OGG1/MTH1 in the A549 lung cancer cell line and found this co-depletion led to lower DNA strand breaks and decreased cell senescence over those caused by MTH1 depletion alone. The shMTH1-transduced cells were less sensitive to SU0268, a potent OGG1 inhibitor compared to shGFP-transduced counterparts. We also treated A549 cells with SU0383, a dual OGG1/MTH1 inhibitor. The dual-inhibitor did not consistently increase cytotoxicity either with duration or dose of treatment beyond single inhibitors or combined single MTH1 and OGG1 inhibitors. In conclusion, our findings suggest that the co-inhibition of OGG1 with MTH1 does not produce enhanced therapeutic benefit in tumor cells, and could instead diminish the tumor-inhibitory effects of MTH1 inhibition that arise from induction of DNA strand breaks and p53-induced senescence. Citation Format: Laura Misiara Lincheta, Ling Zhang, Govindi Samaranayake, Nisha Sharma, Dao Nguyen, Yu-Ki Tahara, Eric Kool, Priyamvada Rai. Low OGG1 protects against the DNA damage induced by MTH1 inhibition [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 2076.

Androgen deprivation (AD) therapy failure leads to terminal and incurable castration-resistant prostate cancer (CRPC). We show that the redox-protective protein thioredoxin-1 (TRX1) increases with prostate cancer progression and in androgen-deprived CRPC cells, suggesting that CRPC possesses an enhanced dependency on TRX1. TRX1 inhibition via shRNA or a phase I-approved inhibitor, PX-12 (untested in prostate cancer), impedes the growth of CRPC cells to a greater extent than their androgen-dependent counterparts. TRX1 inhibition elevates reactive oxygen species (ROS), p53 levels and cell death in androgen-deprived CRPC cells. Unexpectedly, TRX1 inhibition also elevates androgen receptor (AR) levels under AD, and AR depletion mitigates both TRX1 inhibition-mediated ROS production and cell death, suggesting that AD-resistant AR expression in CRPC induces redox vulnerability. In vivo TRX1 inhibition via shRNA or PX-12 reverses the castration-resistant phenotype of CRPC cells, significantly inhibiting tumor formation under systemic AD. Thus, TRX1 is an actionable CRPC therapeutic target through its protection against AR-induced redox stress., Identifying actionable components in castration–resistant prostate cancer (CRPC) is critical for the development of effective treatments. Here, the authors show that the inhibition of the redox-protective protein TRX1 decreases the growth of CRPC cells through the regulation of ROS levels, p53 and androgen receptor expression.

Nicotinamide adenine dinucleotide (NAD(+)) is an essential biomolecule involved in many critical processes. Its role as both a driver of energy production and a signaling molecule underscores its importance in health and disease. NAD(+) signaling impacts multiple processes that are dysregulated in cancer, including DNA repair, cell proliferation, differentiation, redoxregulation, and oxidative stress. Distribution of NAD(+) is highly compartmentalized, with each subcellular NAD(+) pool differentially regulated and preferentially involved in distinct NAD(+)-dependent signaling or metabolic events. Emerging evidence suggests that targeting NAD(+) metabolism is likely to repress many specific mechanisms underlying tumor development and progression, including proliferation, survival, metabolic adaptations, invasive capabilities, heterotypic interactions with the tumor microenvironment, and stress response including notably DNA maintenance and repair. Here we provide a comprehensive overview of how compartmentalized NAD(+) metabolism in mitochondria, nucleus, cytosol, and extracellular space impacts cancer formation and progression, along with a discussion of the therapeutic potential of NAD(+)-targeting drugs in cancer.

Cancer cells develop protective adaptations against oxidative DNA damage, providing a strong rationale for targeting DNA repair proteins. There has been a high degree of recent interest in inhibiting the mammalian Nudix pyrophosphatase MutT Homolog 1 (MTH1). MTH1 degrades 8-oxo-dGTP, thus limiting its incorporation into genomic DNA. MTH1 inhibition has variously been shown to induce genomic 8-oxo-dG elevation, genotoxic strand breaks in p53-functional cells, and tumor-inhibitory outcomes. Genomically incorporated 8-oxo-dG is excised by the base excision repair enzyme, 8-oxo-dG glycosylase 1 (OGG1). Thus, OGG1 inhibitors have been developed with the idea that their combination with MTH1 inhibitors will have anti-tumor effects by increasing genomic oxidative DNA damage. However, contradictory to this idea, we found that human lung adenocarcinoma with low OGG1 and MTH1 were robustly represented in patient datasets. Furthermore, OGG1 co-depletion mitigated the extent of DNA strand breaks and cellular senescence in MTH1-depleted p53-wildtype lung adenocarcinoma cells. Similarly, shMTH1-transduced cells were less sensitive to the OGG1 inhibitor, SU0268, than shGFP-transduced counterparts. Although the dual OGG1/MTH1 inhibitor, SU0383, induced greater cytotoxicity than equivalent combined or single doses of its parent scaffold MTH1 and OGG1 inhibitors, IACS-4759 and SU0268, this effect was only observed at the highest concentration assessed. Collectively, using both genetic depletion as well as small molecule inhibitors, our findings suggest that OGG1/MTH1 co-inhibition is unlikely to yield significant tumor-suppressive benefit. Instead such co-inhibition may exert tumor-protective effects by preventing base excision repair-induced DNA nicks and p53 induction, thus potentially conferring a survival advantage to the treated tumors., Graphical abstract Image 1, Highlights • Low MTH1/low OGG1 tumors are robustly represented in patient lung adenocarcinoma datasets but low MTH1/high OGG1 are not. • Co-depletion of OGG1 in lung adenocarcinoma cells mitigates shMTH1-induced DNA strand breaks and p53-induced senescence. • p53-null tumor cells have lower OGG1 vs. wt p53 counterparts and are more resistant to MTH1 loss-induced anti-tumor effects. • Pharmacologic co-inhibition of OGG1 and MTH1 does not enhance cytotoxicity over the respective single inhibitors.

Deregulation of microRNAs (miRs) contributes to progression and metastasis of prostate and other cancers. miR-23b and -27b, encoded in the same miR cluster (miR-23b/-27b), are downregulated in human metastatic prostate cancer compared with primary tumors and benign tissue. Expression of miR-23b/-27b decreases prostate cancer cell migration, invasion and results in anoikis resistance. Conversely, antagomiR-mediated miR-23b and -27b silencing produces the opposite result in a more indolent prostate cancer cell line. However, neither miR-23b/-27b expression or inhibition impacts prostate cancer cell proliferation suggesting that miR-23b/-27b selectively suppresses metastasis. To examine the effects of miR-23b/-27b on prostate cancer metastasis in vivo, orthotopic prostate xenografts were established using aggressive prostate cancer cells transduced with miR-23b/-27b or non-targeting control miRNA. Although primary tumor formation was similar between miR-23b/-27b-transduced cells and controls, miR-23b/-27b expression in prostate cancer cells decreased seminal vesicle invasion and distant metastases. Gene-expression profiling identified the endocytic adaptor, Huntingtin-interacting protein 1-related (HIP1R) as being downregulated by miR-23b/-27b. Increased HIP1R expression in prostate cancer cells inversely phenocopied the effects of miR-23b/-27b overexpression on migration, invasion and anchorage-independent growth. HIP1R rescued miR-23b/-27b-mediated repression of migration in prostate cancer cells. HIP1R mRNA levels were decreased in seminal vesicle tissue from mice bearing miR-23b/-27b-transduced prostate cancer cell xenografts compared with scrambled controls, suggesting HIP1R is a key functional target of miR-23b/-27b. In addition, depletion of HIP1R led to a more rounded, less mesenchymal-like cell morphology, consistent with decreased metastatic properties. Together, these data demonstrate that the miR-23b/-27b cluster functions as a metastasis-suppressor by decreasing HIP1R levels in pre-clinical models of prostate cancer.

The classification of specific cancers into distinct subgroups, based on emerging genomic technologies, has ushered in a new era of targeted therapeutics and companion diagnostics. This promise, however, is inconsistent with a model in which the growth of solid tumors, and their resistance to various chemotherapeutics, is driven by a small number of tumor-propagating cells whose transcriptome is unlikely to be captured by these technologies. We isolated such tumor propagating cells from a mouse model for SONIC HEDGEHOG (SHH)-subgroup medulloblastoma. Medulloblastoma sphere cultures were enriched for the medulloblastoma progenitor marker SOX2, resistant to SHH inhibitors, and formed tumors in vivo. Surprisingly, these cultures, and their ability to self-renew were WNT-dependent. We show that Trp53 loss in these sphere cultures activated canonical WNT signaling and a small molecule WNT inhibitor was able to reduce the propagation and growth of SHH-subgroup medulloblastoma in vivo, in an on-target manner, leading to increased survival. Our results suggest that the tumor propagating cells driving the growth of bulk SHH-dependent medulloblastoma are themselves WNT dependent. Further, these results suggest WNT pathway inhibition as a novel therapeutic strategy to treat TRP53 deficient SHH-subgroup medulloblastoma patients, for whom few therapeutic options exist.

The metalloid arsenic is a worldwide environmental toxicant, exposure to which is associated with many adverse outcomes. Arsenic is also an effective therapeutic agent in certain disease settings. Arsenic was recently shown to regulate the activity of the Hedgehog (HH) signal transduction pathway, and this regulation of HH signaling was proposed to be responsible for a subset of arsenic’s biologic effects. Surprisingly, these separate reports proposed contradictory activities for arsenic, as either an agonist or antagonist of HH signaling. Here we provide in vitro and in vivo evidence that arsenic acts as a modulator of the activity of the HH effector protein glioma-associated oncogene family zinc finger (GLI), activating or inhibiting GLI activity in a context-dependent manner. This arsenic-induced modulation of HH signaling is observed in cultured cells, patients with colorectal cancer who have received arsenic-based therapy, and a mouse colorectal cancer xenograft model. Our results show that arsenic activates GLI signaling when the intrinsic GLI activity is low but inhibits signaling in the presence of high-level GLI activity. Furthermore, we show that this modulation occurs downstream of primary cilia, evidenced by experiments in suppressor of fused homolog (SUFU) deficient cells. Combining our findings with previous reports, we present an inclusive model in which arsenic plays dual roles in GLI signaling modulation: when GLIs are primarily in their repressor form, arsenic antagonizes their repression capacity, leading to low-level GLI activation, but when GLIs are primarily in their activator form, arsenic attenuates their activity.

The anti-apoptotic protein Mcl-1 is highly expressed in castration-resistant prostate cancer (CRPC), resulting in resistance to apoptosis and association with poor prognosis. Although predominantly localized in the cytoplasm, there is evidence that Mcl-1 exhibits nuclear localization where it is thought to protect against DNA damage-induced cell death. The role of Mcl-1 in mediating resistance to chemotherapy-induced DNA damage in prostate cancer (PCa) is not known. We show in human PCa cell lines and in TRAMP, a transgenic mouse model of PCa, that the combination of the antimitotic agent ENMD-1198 (analog of 2-methoxyestradiol) with betulinic acid (BA, increases proteotoxic stress) targets Mcl-1 by increasing its proteasomal degradation, resulting in increased γH2AX (DNA damage) and apoptotic/necrotic cell death. Knockdown of Mcl-1 in CRPC cells leads to elevated γH2AX, DNA strand breaks, and cell death after treatment with 1198 + BA- or doxorubicin. Additional knockdowns in PC3 cells suggests that cytoplasmic Mcl-1 protects against DNA damage by blocking the mitochondrial release of apoptosis-inducing factor and thereby preventing its nuclear translocation and subsequent interaction with the cyclophilin A endonuclease. Overall, our results suggest that chemotherapeutic agents that target Mcl-1 will promote cell death in response to DNA damage, particularly in CRPC.

The SMOOTHENED inhibitor vismodegib is FDA approved for advanced basal cell carcinoma (BCC), and shows promise in clinical trials for SONIC HEDGEHOG (SHH)-subgroup medulloblastoma (MB) patients. Clinical experience with BCC patients shows that continuous exposure to vismodegib is necessary to prevent tumor recurrence, suggesting the existence of a vismodegib-resistant reservoir of tumor-propagating cells. We isolated such tumor-propagating cells from a mouse model of SHH-subgroup MB and grew them as sphere cultures. These cultures were enriched for the MB progenitor marker SOX2 and formed tumors in vivo. Moreover, while their ability to self-renew was resistant to SHH inhibitors, as has been previously suggested, this self-renewal was instead WNT-dependent. We show here that loss of Trp53 activates canonical WNT signaling in these SOX2-enriched cultures. Importantly, a small molecule WNT inhibitor was able to reduce the propagation and growth of SHH-subgroup MB in vivo, in an on-target manner, leading to increased survival. Our results imply that the tumor- propagating cells driving the growth of bulk SHH-dependent MB are themselves WNT dependent. Further, our data suggest combination therapy with WNT and SHH inhibitors as a therapeutic strategy in patients with SHH-subgroup MB, in order to decrease the tumor recurrence commonly observed in patients treated with vismodegib

As a clinical researcher whose area of expertise is Geriatric Pelvic Medicine, the physical changes that go along with increasing age is incredibly interesting to me. However, from a more practical perspective, the biology of aging, although fascinating, is not something that I would be pushed to address in a routine clinical or administrative situation. All that notwithstanding, questions, presumptions, and theories surrounding the definition of aging are inescapable. In fact, more often than I like to admit, I will find myself getting side tracked and drifting off into deep thought about what “aging really means,” or what patients do I consider “old”? Are they really “old”? Which ones might I consider “young” and compare my assessments to their actual chronological age in years? Perhaps, the most frustrating piece of all, is that, almost invariably, after I come out of my intensely contemplative trance on” “aging,” that I end up with more questions than answers.

Cellular homeostasis is dependent on a balance between DNA damage and DNA repair mechanisms. Cells are constantly assaulted by both exogenous and endogenous stimuli leading to high levels of reactive oxygen species (ROS) that cause oxidation of the nucleotide dGTP to 8-oxodGTP. If this base is incorporated into DNA and goes unrepaired, it can result in G > T transversions, leading to genomic DNA damage. MutT Homolog 1 (MTH1) is a nucleoside diphosphate X (Nudix) pyrophosphatase that can remove 8-oxodGTP from the nucleotide pool before it is incorporated into DNA by hydrolyzing it into 8-oxodGMP. MTH1 expression has been shown to be elevated in many cancer cells and is thought to be a survival mechanism by which a cancer cell can stave off the effects of high ROS that can result in cell senescence or death. It has recently become a target of interest in cancer because it is thought that inhibiting MTH1 can increase genotoxic damage and cytotoxicity. Determining the role of MTH1 in normal and cancer cells is confounded by an inability to reliably and directly measure its native enzymatic activity. We have used the chimeric A TP- r eleasing g uanine- o xidized (ARGO) probe that combines 8-oxodGTP and ATP to measure MTH1 enzymatic activity in colorectal cancer (CRC), non-small cell lung cancer (NSCLC) and pancreatic ductal adenocarcinoma (PDAC) along with patient-matched normal tissue. MTH1 8-oxodGTPase activity is significantly increased in tumors across all three tissue types, indicating that MTH1 is a marker of cancer. MTH1 activity measured by ARGO assay was compared to mRNA and protein expression measured by RT-qPCR and Western blot in the CRC tissue pairs, revealing a positive correlation between ARGO assay and Western blot, but little correlation with RT-qPCR in these samples. The adoption of the ARGO assay will help in establishing the level of MTH1 activity in model systems and in assessing the effects of MTH1 modulation in the treatment of cancer.

MutT Homolog 1 (MTH1) is a NUDIX pyrophosphorylase that hydrolyzes oxidized purine nucleoside triphosphates in the nucleotide pool, thus preventing their incorporation into DNA. Our prior work has shown that MTH1 is critical for the maintenance of multiple pro-tumorigenic phenotypes in oncogenic RAS-driven cancer cells, with its depletion leading to decreased tumor formation in vivo. Our subsequent analyses of TCGA patient datasets showed elevated MTH1 expression to be significantly associated with poorer disease-free survival in RAS-mutated cancers, such as that of the lung and pancreas. We found that MTH1 mRNA levels were positively correlated with KRAS levels even in early-stage non-small cell lung cancer patient tissues, and that the introduction of oncogenic KRAS was sufficient to upregulate MTH1 mRNA and protein levels. The aim of this study is to identify RAS-effector signaling intermediates affecting MTH1 expression and activity. Chemical inhibitors of the MAPK/ERK, PI3K/AKT and NOX pathways, plus oncogenic RASV12-effector domain mutants (RASV12- S35/ E38/ G37/ C40), were used to identify key signaling molecular mediators of MTH1 expression in the distinct RAS isoforms (H- and K-RAS). The dependencies of the different KRASG12-mutant polymorphisms (KRASG12- C/ D/ V) on MTH1 expression and activity, as well as candidate transcription factors regulating MTH1 expression, were evaluated. Our work shows MTH1 at the nexus of crosstalk between different effector pathways activated downstream of RAS. Dissecting these signaling intermediates are important in identifying alternate pathways of MTH1 regulation, which may manifest as resistance mechanisms to standard-of-care cancer treatments. Our work will also help understand how to best leverage MTH1 as a therapeutic target in oncogenic RAS-driven cancers driven by the different isoforms, and their respective mutant polymorphisms. Citation Format: Govindi J. Samaranayake, Clara I. Troccoli, Christina Jayaraj, Brittany C. Durden, Nagaraj Nagathihalli, Nipun Merchant, Priyamvada Rai. Exploring the signaling and activity interactome between oncogenic RAS and the nucleotide pool-detoxifying enzyme MTH1 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 888.

Castration-resistant prostate cancer (CRPC) arises from a failure of standard-of-care androgen deprivation (AD) therapy to suppress emergence of castration-resistant variants from the original AD-responsive prostate cancer (ADPC). Identification of molecular drivers of AD-resistance and CRPC emergence is critical for developing curative therapies. Comparative transcriptomic analysis of parental ADPC cells and their acutely-emergent CRPC counterparts, developed in our laboratory, identified two distinct and novel redox-regulated pathways in mediating castration resistance. The redox-protective protein thioredoxin-1 (TRX1) was elevated under AD in CRPC cells, but not ADPC cells. TRX1 inhibition, via shRNA or the Phase-I approved TRX1 inhibitor PX-12 (not tested in prostate cancer), elevated ROS levels and induced cell death in androgen-deprived CRPC cells and reduced xenograft tumor formation in castrate in vivo models. Unexpectedly, TRX1 inhibition also elevated androgen receptor (AR) levels under AD, and AR depletion mitigated both TRX1 inhibition-mediated ROS production and loss of viability. Our results suggest maintaining AR expression in CRPC induces oxidative stress that requires redox-protective adaptations (such as elevated TRX1) to avoid AD-induced cell death. In contrast to TRX1, we found GC1a, the main subunit of the nitric oxide receptor, soluble guanylate cyclase (sGC), was decreased in CRPC relative to ADPC. Treatment with sGC agonists, including an FDA-approved anti-hypertensive agent, led to reduction in castrate-resistant tumor burden in xenograft models. Tumor suppression was significantly correlated to the extent by which sGC signaling was stimulated in the treated animals. Our preliminary findings indicate sGC signaling is inhibitory to CRPC and that oxidation of the sGC complex is likely to be an important mechanism of signal dampening in CRPC. Our research points to a ROS duality in CRPC, comprising oxidation-mediated inhibition of anti-tumor signaling alongside a concomitant vulnerability to oxidative stress, requiring enhanced redox-protective mechanisms that could serve as novel avenues for therapeutic intervention in CRPC.

Oxidative stress as a consequence of oncogenic signaling-induced ROS is a targetable vulnerability in many cancers. Our previous work implicated the mammalian 8-oxodGTPase human MutT Homolog 1 (MTH1) as an oncogenic adaptation in RAS-driven lung cancers that facilitates tumorigenesis and inhibits oncogenic ROS-associated DNA breaks and tumor-suppressive responses. Although the first-in-class MTH1 small molecule inhibitors initially highlighted their therapeutic potential, more recent reports have questioned their efficacy against cancer cells. In this study, we have attempted to provide some clarity to the current controversy in the field. Here we utilize a newly-developed assay for accurately assessing MTH1 8-oxodGTPase activity in a panel of cancer cell lines as well as human tumor tissue to investigate the effects of first-in-class MTH1 inhibitors, TH588 and TH287, alongside IACS-4759, an independently developed second-generation MTH1 inhibitor that does not recapitulate the anti-tumor effects of the first-in-class inhibitors despite comparably inhibiting MTH1 8-oxodGTPase activity. Our findings show that while MTH1 expression is almost universally elevated in lung tumors relative to normal tissue, the association between MTH1-specific 8-oxodGTPase activity and MTH1 expression is not strong. Our results further indicate that efficacy of the first-in-class inhibitors correlate broadly with MTH1-specific but not total cellular 8-oxodGTPase activity in lung cancer lines, pointing to the existence of as-yet-unidentified redundant 8-oxodGTPases that function either alongside or in the absence of MTH1. Additionally when we compare the inhibitors against shRNA-mediated MTH1 depletion, our collective findings show the first-in-class inhibitors can recapitulate the effects of our previous shMTH1 studies but not IACS-4759, despite all three inhibiting MTH1-specific 8-oxodGTPase activity as effectively as the shRNA. An obvious implication of our findings is that the efficacy of the first-in-class inhibitors is due to some as-yet unidentified mechanism(s) of MTH1 action that appears to be independent from its 8-oxodGTPase activity. Thus a better understanding of noncanonical MTH1 function in cancer as well as of alternate 8-oxodGTPase mechanisms is needed for the development and use of more universally efficacious inhibitors.

Androgen deprivation therapy (ADT) initially suppresses prostate cancer (PC) progression. However, castration-resistant PC (CRPC) cells inevitably emerge, resulting in incurable disease. We recently demonstrated that AD produces a form of cellular senescence leading to outgrowth of CRPC subpopulations from the AD-sensitive parental cells. Gene expression profiling studies comparing the parental LNCaP line to the senescence-resistant, early CRPC variant, LNCaP SB5, revealed an enrichment of redox-protective proteins in the latter under AD conditions. This finding suggests that redox stress due to hyperactivated mitogenic/survival signaling or metabolic stresses may be an important and understudied Achilles heel in the progression to CRPC. Here we present the effects of inhibiting thioredoxin-1 (TRX1), a redox-protective protein that was identified as being elevated in our early CRPC LNCaP SB5 model as well as in human CRPC vs. AD-responsive PC datasets. Suppression of TRX1 expression via shRNA led to profound growth suppression of CRPC cells, relative to their AD-sensitive counterparts, and significantly reduced CRPC xenograft tumor growth. Furthermore, under AD, TRX1 suppression promoted p53-induced cell death, which was accompanied by increased reactive oxygen species (ROS). Curiously, TRX1 inhibition led to increased AR and Sp1 expression under AD, and the oxidative stress effects associated with TRX1 inhibition could be mitigated by depleting AR expression. These in vitro and in vivo results were recapitulated using PX-12 a phase I-approved pharmacologic TRX1 inhibitor. Thus our results point to TRX1 as a critical requirement for protecting against redox stress evoked by the inappropriate reactivation of AR in a low-androgen environment in CRPC. Our work provides a rationale for using TRX1 inhibitors in conjunction with ADT to limit CRPC. Citation Format: Govindi J. Samaranayake, Clara I. Troccoli, Mai Q. Huynh, Rolando D.Z. Lyles, Deukwoo Kwon, Yuguang Ban, Steven Xi Chen, Merce Jorda, Kerry Burnstein, Priyamvada Rai. Inappropriate AR reactivation-induced redox stress as a novel therapeutic target in castration-resistant prostate cancer [abstract]. In: Proceedings of the AACR Special Conference: Prostate Cancer: Advances in Basic, Translational, and Clinical Research; 2017 Dec 2-5; Orlando, Florida. Philadelphia (PA): AACR; Cancer Res 2018;78(16 Suppl):Abstract nr B032.

The classification of specific cancers into distinct subgroups, based on emerging genomic technologies, has ushered in a new era of targeted therapeutics and companion diagnostics. This promise, however, is inconsistent with a model in which the growth of solid tumors, and their resistance to various chemotherapeutics, is driven by a small number of tumor-propagating cells whose transcriptome is unlikely to be captured by these technologies. We isolated such tumor propagating cells from a mouse model for SONIC HEDGEHOG (SHH)-subgroup medulloblastoma. Medulloblastoma sphere cultures were enriched for the medulloblastoma progenitor marker SOX2, resistant to SHH inhibitors, and formed tumors in vivo. Surprisingly, these cultures, and their ability to self-renew were WNT-dependent. We show that Trp53 loss in these sphere cultures activated canonical WNT signaling and a small molecule WNT inhibitor was able to reduce the propagation and growth of SHH-subgroup medulloblastoma in vivo, in an on-target manner, leading to increased survival. Our results suggest that the tumor propagating cells driving the growth of bulk SHH-dependent medulloblastoma are themselves WNT dependent. Further, these results suggest WNT pathway inhibition as a novel therapeutic strategy to treat TRP53 deficient SHH-subgroup medulloblastoma patients, for whom few therapeutic options exist.

Androgen deprivation therapy (ADT) initially suppresses prostate cancer (PC) progression. However, castration resistant PC (CRPC) cells inevitably emerge, resulting in incurable disease. We recently demonstrated that AD produces a form of cellular senescence leading to outgrowth of CRPC subpopulations from the AD-sensitive parental cells. Gene expression profiling studies comparing the parental LNCaP line to the senescence-resistant, early CRPC variant, LNCaP SB5, revealed an enrichment of thiol-based redox-protective proteins in the latter under AD conditions. This finding suggests that redox stress due to hyperactivated mitogenic/survival signaling or metabolic stresses may be an important and understudied Achilles heel in the progression to CRPC. Here we present the effects of inhibiting thioredoxin-1 (TRX1), a redox-protective protein that was identified as being elevated in our early CRPC LNCaP SB5 model. Suppression of TRX1 expression via shRNA led to profound growth suppression of CRPC cells, relative to their AD-sensitive counterparts, and significantly reduced CRPC xenograft tumor growth. Furthermore, under AD, TRX1 suppression promoted p53-induced cell death, which was accompanied by increased reactive oxygen species (ROS). These in vitro and in vivo results were recapitulated using a Phase II clinical trial-tested chemical TRX1 inhibitor. Thus our results point to TRX1 as a critical requirement for CRPC progression and provide a rationale for using TRX1 inhibitors in conjunction with ADT to limit CRPC. Citation Format: Clara I. Troccoli, Govindi J. Samaranayake, Mai Q. Huynh, Karen Kage, Deukwoo Kwon, Yuguang Ban, Xi S. Chen, Enrique R. Zarco, Merce Jorda, Kerry L. Burnstein, Priyamvada Rai. Redox stress as a therapeutic Achilles heel in castration-resistant prostate cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5479. doi:10.1158/1538-7445.AM2017-5479

Oncogenic RAS signaling-generated reactive oxygen species (ROS) drive tumor progression by the hyperactivation of proliferative, anti-apoptotic, and metastatic pathways. Elevated ROS levels however can also cause oxidative DNA damage, leading to oncogene-induced senescence (OIS) and cell death. To avoid such tumor-suppressive outcomes, RAS-driven tumors often upregulate redox-protective proteins. The collective research from our group over the last few years has shown that oncogenic RAS elevates expression of the mammalian 8-oxo-dGTPase MutT Homolog 1 (MTH1), which in turn enables evasion of OIS, promotes transformation efficiency, and facilitates tumorigenicity. Our prior work has therefore demonstrated that MTH1 is beneficial to the spectrum of RAS-driven transformation, and that its shRNA-mediated targeting in oncogenic RAS-harboring lung cancer cells produces robust tumor-suppressive outcomes. However the first wave of chemical MTH1 inhibitors has led to controversial and conflicting results regarding MTH1 as a chemotherapeutic target. Here we evaluate the benefit of MTH1 inhibitors in wildtype. vs. oncogenic KRAS expressing cells, and directly assess the effects of oncogenic KRAS as well as three recently developed inhibitors on MTH1 8-oxo-dGTPase activity. Our results support that introduction of oncogenic KRAS elevates both MTH1 expression and activity, presumably through its elevation of ROS levels, and thus sensitizes cells to MTH1 inhibitors. The degree of this sensitization has a complex dependence on residual 8-oxodGTPase activity in the different cells following treatment with the MTH1 inhibitors, alternate antioxidant responses, and induction of different tumor suppressor pathways. Our data therefore highlight the importance of evaluating the molecular contexts and outcomes of MTH1 inhibition when determining its utility as a chemotherapeutic target. Citation Format: Govindi J. Samaranayake, Clara I. Troccoli, Mai Q. Huynh, Andrew Win, Debin Ji, Eric T. Kool, Priyamvada Rai. Towards a better understanding of MTH1 as a therapeutic target in RAS-driven cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5473. doi:10.1158/1538-7445.AM2017-5473

Many tumors sustain elevated levels of reactive oxygen species (ROS), which drive oncogenic signaling. However, ROS can also trigger anti-tumor responses, such as cell death or senescence, through induction of oxidative stress and concomitant DNA damage. To circumvent the adverse consequences of elevated ROS levels, many tumors develop adaptive responses, such as enhanced redox-protective or oxidatively-generated damage repair pathways. Targeting these enhanced oxidative stress-protective mechanisms is likely to be both therapeutically effective and highly specific to cancer, as normal cells are less reliant on such mechanisms. In this review, we discuss one such stress-protective protein human MutT Homolog1 (MTH1), an enzyme that eliminates 8-oxo-7,8-dihydro-2’-deoxyguanosine triphosphate (8-oxodGTP) through its pyrophosphatase activity, and is found to be elevated in many cancers. Our studies, and subsequently those of others, identified MTH1 inhibition as an effective tumor-suppressive strategy. However, recent studies with the first wave of MTH1 inhibitors have produced conflicting results regarding their cytotoxicity in cancer cells and have led to questions regarding the validity of MTH1 as a chemotherapeutic target. To address the proverbial "elephant in the room" as to whether MTH1 is a bona fide chemotherapeutic target, we provide an overview of MTH1 function in the context of tumor biology, summarize the current literature on MTH1 inhibitors, and discuss the molecular contexts likely required for its efficacy as a therapeutic target.

Oncogenic RAS mutations confer multiple malignant traits to cancer cells, many of which are mediated by oncogene-induced reactive oxygen species (ROS). However, oncogenic ROS also create vulnerabilities in the cancer cells, sensitizing them to oxidative DNA damage and strand breaks which can trigger cellular senescence or cell death. Thus RAS-driven tumor cells require adaptive redox protective mechanisms to inhibit ROS-associated tumor suppression. We previously reported that MutT Homolog 1 (MTH1), the mammalian 8-oxodGTPase, comprises one such critical adaptation. We have shown MTH1 is important for facilitating the full gamut of RAS-driven malignancy by promoting evasion of the first barrier to transformation, viz. oncogene-induced senescence (OIS), enhancing RAS-mediated transformation and related pro-malignant traits, and maintaining proliferation and tumorigenicity in established RAS-driven tumor cells. Recently chemical inhibitors against MTH1 have been developed, although there is some controversy surrounding their efficacy and mode of action. Our work here discusses RAS regulation of MTH1 expression, and the molecular contexts under which MTH1 inhibition can synergize with oncogenic oxidative stress in RAS-driven tumor cells for optimal tumor suppressive response. Citation Format: Govindi Jayanika Samaranayake, Clara Issabella Troccoli, Tyler Andrew Cunningham, Priyamvada Rai. MTH1 Inhibition as an effective tumor-suppressive strategy in RAS-driven cancer. [abstract]. In: Proceedings of the AACR Precision Medicine Series: Targeting the Vulnerabilities of Cancer; May 16-19, 2016; Miami, FL. Philadelphia (PA): AACR; Clin Cancer Res 2017;23(1_Suppl):Abstract nr A38.

Oncogenic oxidative stress is an intrinsic part of the transformation process and needs to be mitigated for optimal malignancy in the transformed cancer cells. The source of oncogenic reactive oxygen species (ROS) likely differs depending on the stage of transformation and can be, separately or collectively, the result of hyperactivated oncogenic signaling, inflammation, aberrant mitochondrial metabolism, and instigation of invasion-promoting pathways[1]. In RAS-driven tumor cells, we have found that MTH1, the mammalian Nudix hydrolase that degrades oxidized purine nucleotides, effectively counteracts the resistance to transformation and malignancy from this inherent oxidative stress. Our work was the first to show that MTH1 is required to facilitate the formation, proliferation and tumorigenic capability of RAS-transformed tumor cells [2–4], thereby establishing a critical role for the oxidation state of the nucleotide pool in determining the malignancy of RAS-driven cancer cells. It is plausible that in the context of other tumor-associated oxidative stress, besides RAS activation, MTH1 would perform a similar protective function although this premise has yet to be explicitly tested. In effect, MTH1 loss compromises the overall robustness of the transformation circuit and enables the negative consequences of oncogenic oxidative stress on tumor formation. Elevated oncogenic ROS can impair the transformation process by promoting genomic DNA damage and anti-tumor processes such as oncogene-induced senescence (OIS) or cell death. Yet ROS-dependent signaling is essential for oncogenic transformation [5]. This duality of ROS drives the acquisition of molecular adaptations that uncouple the tumor-promoting aspects of ROS from their tumor-suppressive consequences. Therefore, the importance of MTH1 in RAS transformation likely lies in its prevention of oxidative DNA damage and the resulting anti-proliferative consequences in the absence of any ROS scavenging functionality[1]. This characteristic places MTH1 in a unique class of non-oncogenic adaptation – the ability to mitigate the negative influences of ROS on tumor formation without directly altering the ROS levels required for oncogenic signaling. Other molecules in this class of adaptations, deriving from DNA damage repair or non-antioxidant redox-protective pathways, are anticipated to possess a similar uncoupling effect. Whilst MTH1 may not be able to directly alter ROS levels, its loss can force reductions in oncogenic oxidants by selectively eradicating cells with high levels of RAS oncoprotein and/or ROS-generating downstream pathways that cannot cope with the consequences of MTH1 inhibition due to their elevated oxidant status. Significantly, our recent findings [4] demonstrate that this phenomenon occurs in p53-nonfunctional lung cancer cells that are able to resist MTH1 inhibition-induced genomic DNA damage and continue to proliferate despite MTH1 loss, albeit at a slower rate. It is well known that approximately 50% of all tumors contain p53 mutations or loss that make them refractory to stresses that induce DNA damage. We find that p53 is required for MTH1 inhibition-induced DNA strand breaks and senescence[4]. However even in the absence of functional p53, MTH1 inhibition reduces tumor formation and proliferation rates. This seems to occur through an additional adaptation that involves a gradual decrease in ROS levels, arising ostensibly from a striking reduction in RAS oncoprotein and activated Akt levels in both the MTH1-inhibited cultured cells and xenograft tumors[4]. We observed a similar effect when we introduced a CMV promoter-driven high RASV12-expressing construct into shMTH1-transduced immortalized lung cells[3]. Significantly, the reduction in ROS and in RAS levels was much milder when we instead introduced a low RASV12-expressing construct, which generated lower oncogenic ROS levels relative to the high RASV12 construct[3]. Similarly KRAS-driven cells in a low oxidative stress environment, created through low oxygen culture, no longer responded to MTH1 inhibition by a selective reduction in RAS oncoprotein levels[4]. Conversely, if ROS were maintained at elevated levels under MTH1 inhibition, for instance by the enforced expression of activated Akt, RAS-driven cells suffered a greater proliferative deficit than if they were able to shift to a lower ROS-producing population[4]. Taken collectively, these results suggest three things: 1) the higher the RAS oncoprotein expression (as seen in advanced tumors), the greater the requirement for MTH1 expression to counteract the related high oncogenic ROS-associated negative effects, 2) the loss of MTH1 leads to a selective decrease in cells with the highest RAS expression (and consequently ROS levels) in a tumor population, and 3) in cells resistant to MTH1 inhibition-induced DNA damage and senescence, the very fact of their continued proliferation necessitates the adaptive response of decreasing RAS-mediated ROS production, at the cost of reduced malignancy. It thus appears that MTH1 could function as a molecular rheostat for modulating oncogene expression at optimal levels, in turn controlling cancer survival and growth in RAS-driven cells (Figure ​(Figure11). Figure 1 MTH1 functions as a molecular rheostat in oncogenic transformation As an increase in genomic strand breaks was not detected in the p53-nonfunctional RAS-driven cells following MTH1 depletion in our study[4], it begs the question as to what specific stressor(s) the cells are sensing and responding to by shifting to an overall lower ROS-producing population. An understanding of the molecular processes involved is essential for the development of further therapeutic strategies to indirectly target RAS activity or response via MTH1 inhibitors. One such avenue of investigation could involve the understudied interplay between unrepaired genomic 8-oxoG, ROS and RAS activity. Our unpublished results suggest that p53-nonfunctional cells possess low levels of the base excision repair (BER) protein, OGG1. A recent study postulated that OGG1 can activate RAS signaling by acting as a guanine nucleotide exchange factor (GEF) when bound to the excised 8-oxoguanine moiety[6]. Accordingly impaired OGG1-mediated repair of genomic 8-oxoG resulting from MTH1 inhibition could in turn lower RAS activation and associated ROS production. In addition to producing ROS, RAS activity is itself modulated by oxidants such as the superoxide radical[7], suggesting the existence of a ROS-mediated positive feedback loop that sustains RAS activation. Thus the net effect of low OGG1 in the context of MTH1 inhibition may be to further reduce RAS activity and associated ROS levels, leading to a reduction in associated protective effects and the shift to a less malignant cell population. Regardless of the specific mechanism(s) involved, the ability to indirectly modulate RAS activity/expression by targeting MTH1 is an attractive approach, given the historical difficulties in direct therapeutic targeting of RAS and its downstream effectors. Our approach further suggests that targeting an unavoidable oncogenic vulnerability such as oxidative stress may provide a more efficacious broad-spectrum therapeutic response than targeting oncogenic pathways, which have proven to be pleiotropic and mutable.

Metabolomic profiling is an increasingly important method for identifying potential biomarkers in cancer cells with a view towards improved diagnosis and treatment. Nuclear magnetic resonance (NMR) provides a potentially noninvasive means to accurately characterize differences in the metabolomic profiles of cells. In this work, we use1H NMR to measure the metabolomic profiles of water soluble metabolites extracted from isogenic control and oncogenic HRAS-, KRAS-, and NRAS-transduced BEAS2B lung epithelial cells to determine the robustness of NMR metabolomic profiling in detecting differences between the transformed cells and their untransformed counterparts as well as differences among the RAS-transformed cells. Unique metabolomic signatures between control and RAS-transformed cell lines as well as among the three RAS isoform-transformed lines were found by applying principal component analysis to the NMR data. This study provides a proof of principle demonstration that NMR-based metabolomic profiling can robustly distinguish untransformed and RAS-transformed cells as well as cells transformed with different RAS oncogenic isoforms. Thus, our data may potentially provide new diagnostic signatures for RAS-transformed cells.

Non-small cell lung carcinoma (NSCLC), a chemoresistant form of lung cancer, is characterized by oncogenic KRAS mutations that confer multiple tumor-promoting traits. Several of these traits are mediated by oncogene-induced reactive oxygen species (ROS). However, oncogenic ROS also produce DNA damage, a major effector of tumor suppressor mechanisms. We previously reported overexpression of the mammalian 8-oxo-dGTP triphosphatase, human MutT Homolog 1 (MTH1), inhibits RAS oncogene-induced senescence (OIS) and any associated DNA damage without substantially affecting ROS levels. Here we report that MTH1 expression is elevated in NSCLC tumors relative to adjacent normal tissue, particularly in tumors with high KRAS expression. We show that MTH1 overexpression facilitates transformation of lung epithelial cells by oncogenic KRAS. We further report that MTH1 suppression in NSCLC cell lines reduces proliferation and tumorigenicity either via DNA strand break-induced elevation of the p53/p21 pathway or by inhibition of Akt activation in p53-nonfunctional tumor cells. Collectively our results indicate MTH1 as a novel potential chemotherapeutic target in NSCLC with activating KRAS mutations. Citation Format: Asmita Patel, Katherine Halvorsen, Wayne Balkan, Alexander Cohen, Teresita Reiner, David Robbins, Dao Nguyen, Priyamvada Rai. Human Mut T homolog 1 (MTH1) as a novel facilitator of KRAS-driven lung cancer. [abstract]. In: Proceedings of the AACR Special Conference on RAS Oncogenes: From Biology to Therapy; Feb 24-27, 2014; Lake Buena Vista, FL. Philadelphia (PA): AACR; Mol Cancer Res 2014;12(12 Suppl):Abstract nr B14. doi: 10.1158/1557-3125.RASONC14-B14