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Cystine/glutamate antiporter solute carrier family 7 member 11 (SLC7A11; also known as xCT) plays a key role in antioxidant defense by mediating cystine uptake, promoting glutathione synthesis, and maintaining cell survival under oxidative stress conditions. Recent studies showed that, to prevent toxic buildup of highly insoluble cystine inside cells, cancer cells with high expression of SLC7A11 (SLC7A11 high) are forced to quickly reduce cystine to more soluble cysteine, which requires substantial NADPH supply from the glucose-pentose phosphate pathway (PPP) route, thereby inducing glucose- and PPP-dependency in SLC7A11 high cancer cells. Limiting glucose supply to SLC7A11 high cancer cells results in significant NADPH “debt”, redox “bankruptcy”, and subsequent cell death. This review summarizes our current understanding of NADPH-generating and -consuming pathways, discusses the opposing role of SLC7A11 in protecting cells from oxidative stress–induced cell death such as ferroptosis but promoting glucose starvation–induced cell death, and proposes the concept that SLC7A11-mediated cystine uptake acts as a double-edged sword in cellular redox regulation. A detailed understanding of SLC7A11 in redox biology may identify metabolic vulnerabilities in SLC7A11 high cancer for therapeutic targeting.

Metabolic reprogramming in cancer cells can create metabolic liabilities which could be exploited therapeutically. Tumor suppressor KEAP1 is frequently mutated in lung cancer, and KEAP1-mutant lung cancers are refractory to most current therapies. Here we show that KEAP1 deficiency promotes glucose dependency in lung cancer cells, and KEAP1-mutant or -deficient lung cancer cells are more vulnerable to glucose deprivation than KEAP1-proficient lung cancer cells. Mechanistically, KEAP1 inactivation in lung cancer cells leads to constitutive activation of NRF2 transcription factor and aberrant expression of NRF2 target cystine transporter SLC7A11; under glucose limiting conditions, high cystine uptake in KEAP1-inactivated lung cancer cells results in toxic buildup of intracellular disulfide molecules, NADPH depletion, and subsequent cell death, which can be rescued by either genetic ablation of the NRF2-SLC7A11 signaling axis or treatments that prevent disulfide accumulation. Finally, we show that KEAP1-inactivated lung cancer cells or xenograft tumors are sensitive to glucose transporter (GLUT) inhibitor KL-11743. Together, our results reveal that KEAP1 deficiency induces glucose dependency in lung cancercells and uncover a metabolic liability that can be therapeutically targeted in KEAP1-mutant lung cancer.

RIG-I-like receptor (RLR)-mediated interferon production is critical for antiviral responses. A recent study (Zhang et al., Cell, 2019) uncovered a reciprocal inhibition between RLR signaling and glycolysis: lactate produced by glycolysis inhibits RLR signaling by binding to RLR signaling component mitochondrial antiviral-signaling (MAVS), whereas RLR activation suppresses glycolysis through inhibiting glycolysis enzyme hexokinase.

Cancer cells with specific genetic alterations may be highly dependent on certain nutrients for survival, which can inform therapeutic strategies to target these cancer-specific metabolic vulnerabilities. The glutamate/cystine antiporter solute carrier family 7 member 11 (SLC7A11, also called xCT) is overexpressed in several cancers. Contrasting the established pro-survival roles of SLC7A11 under other stress conditions, here we report the unexpected finding that SLC7A11 overexpression enhances cancer cell dependence on glucose and renders cancer cells more sensitive to glucose starvation-induced cell death and, conversely, that SLC7A11 deficiency by either knockdown or pharmacological inhibition promotes cancer cell survival upon glucose starvation. We further show that glucose starvation induces SLC7A11 expression through ATF4 and NRF2 transcription factors and, correspondingly, that ATF4 or NRF2 deficiency also renders cancer cells more resistant to glucose starvation. Finally, we show that SLC7A11 overexpression decreases whereas SLC7A11 deficiency increases intracellular glutamate levels because of SLC7A11-mediated glutamate export and that supplementation of α-ketoglutarate, a key downstream metabolite of glutamate, fully restores survival in SLC7A11-overexpressing cells under glucose starvation. Together, our results support the notion that both glucose and glutamate have important roles in maintaining cancer cell survival and uncover a previously unappreciated role of SLC7A11 to promote cancer cell dependence on glucose. Our study therefore informs therapeutic strategies to target the metabolic vulnerability in tumors with high SLC7A11 expression.

Impaired DNA crosslink repair leads to Fanconi anemia (FA), characterized by a unique manifestation of bone marrow failure and pancytopenia among diseases caused by DNA damage response defects. As a germline disorder, why the hematopoietic hierarchy is specifically affected is not fully understood. We find that reprogramming transcription during hematopoietic differentiation results in an overload of genotoxic stress, which causes aborted differentiation and depletion of FA mutant progenitor cells. DNA damage onset most likely arises from formaldehyde, an obligate by-product of oxidative protein demethylation during transcription regulation. Our results demonstrate that rapid and extensive transcription reprogramming associated with hematopoietic differentiation poses a major threat to genome stability and cell viability in the absence of the FA pathway. The connection between differentiation and DNA damage accumulation reveals a novel mechanism of genome scarring and is critical to exploring therapies to counteract the aplastic anemia for the treatment of FA patients.

SummaryTo assess telomerase as a cancer therapeutic target and determine adaptive mechanisms to telomerase inhibition, we modeled telomerase reactivation and subsequent extinction in T cell lymphomas arising in Atm−/− mice engineered with an inducible telomerase reverse transcriptase allele. Telomerase reactivation in the setting of telomere dysfunction enabled full malignant progression with alleviation of telomere dysfunction-induced checkpoints. These cancers possessed copy number alterations targeting key loci in human T cell lymphomagenesis. Upon telomerase extinction, tumor growth eventually slowed with reinstatement of telomere dysfunction-induced checkpoints, yet growth subsequently resumed as tumors acquired alternative lengthening of telomeres (ALT) and aberrant transcriptional networks centering on mitochondrial biology and oxidative defense. ALT+ tumors acquired amplification/overexpression of PGC-1β, a master regulator of mitochondrial biogenesis and function, and they showed marked sensitivity to PGC-1β or SOD2 knockdown. Genetic modeling of telomerase extinction reveals vulnerabilities that motivate coincidental inhibition of mitochondrial maintenance and oxidative defense mechanisms to enhance antitelomerase cancer therapy.

SummaryA hallmark feature of glioblastoma is its strong self-renewal potential and immature differentiation state, which contributes to its plasticity and therapeutic resistance. Here, integrated genomic and biological analyses identified PLAGL2 as a potent protooncogene targeted for amplification/gain in malignant gliomas. Enhanced PLAGL2 expression strongly suppresses neural stem cell (NSC) and glioma-initiating cell differentiation while promoting their self-renewal capacity upon differentiation induction. Transcriptome analysis revealed that these differentiation-suppressive activities are attributable in part to PLAGL2 modulation of Wnt/β-catenin signaling. Inhibition of Wnt signaling partially restores PLAGL2-expressing NSC differentiation capacity. The identification of PLAGL2 as a glioma oncogene highlights the importance of a growing class of cancer genes functioning to impart stem cell-like characteristics in malignant cells.

FIP200 (FAK family-interacting protein of 200 kDa) is a conserved protein recently identified as a potential mammalian counterpart of yeast autophagy protein Atg17. However, it remains unknown whether mammalian FIP200 regulates autophagy in vivo. Here we show that neural-specific deletion of FIP200 resulted in cerebellar degeneration accompanied by progressive neuronal loss, spongiosis, and neurite degeneration in the cerebellum. Furthermore, deletion of FIP200 led to increased apoptosis in cerebellum as well as accumulation of ubiquitinated protein aggregates without any deficiency in proteasome catalytic functions. We also observed an increased p62/SQSTM1 accumulation in the cerebellum and reduced autophagosome formation as well as accumulation of damaged mitochondria in the mutant mice. Lastly, analysis of cerebellar neurons in vitro showed reduced JNK activation and increased susceptibility to serum deprivation-induced apoptosis in cerebellar neurons from the mutant mice. Taken together, these results provide strong genetic evidence for a role of FIP200 in the regulation of neuronal homeostasis through its function in autophagy in vivo.

FIP200 (focal adhesion kinase family interacting protein of 200 kDa) has been shown to interact with other proteins to regulate several intracellular signaling pathways. To study a potential role of FIP200 in tumorigenesis and possibly other disease processes in vivo, we created and analyzed murine mammary tumor virus-Cre-mediated FIP200 conditional knock-out (CKO) mice. We found that deletion of FIP200 in mammary epithelial cells did not result in spontaneous development of breast cancer. Moreover, deletion of FIP200 did not further accelerate or inhibit lymphomagenesis induced by inactivation of p53 in mice. Interestingly, however, FIP200 and p53 double conditional knock-out (dCKO) mice exhibited significant hyperplasia of epidermis (acanthosis), thickening of the cornified layer (hyperkeratosis), and increased vascularity in the dermis. FIP200 CKO mice also showed similar, although less severe, skin defects as dCKO mice. Analyses of primary keratinocytes isolated from dCKO mice did not detect increased proliferation of these cells in vitro, suggesting that epidermis hyperproliferation is not epidermal cell-autonomous but may be a consequence of increased inflammation triggered by immune cells in vivo. Consistent with this possibility, we found infiltration of leukocytes including T cells, macrophages, and granulocytes into the dermis and epidermis, associated with activation of NF-κB and increased expression of several proinflammatory cytokines and chemokines in skin of the dCKO mice. We further found that cultured FIP200 KO keratinocytes showed reduced NF-κB phosphorylation in response to tumor necrosis factor α stimulation, suggesting a paracrine regulation of aberrant NF-κB activation in the skin microenviroment of dCKO and FIP200 CKO mice. Together, these results demonstrate that ablation of FIP200, although not promoting tumorigenesis, can lead to skin inflammatory disorders, suggesting a novel function of FIP200 in the maintenance of normal skin homeostasis in vivo.

Integrin-mediated cell adhesion and signaling is required for mammary gland development and functions. As a major mediator of integrin signaling, focal adhesion kinase (FAK) has been implicated to play a role in the survival, proliferation, and differentiation of mammary epithelial cells in previously studies in vitro. To assess the role of FAK in vivo, we created mice in which FAK is selectively deleted in mammary epithelial cells. The mammary gland FAK conditional knock-out (MFCKO) mice are viable, fertile, and macroscopically indistinguishable from the control littermates. In virgin MFCKO mice, mammary ductal elongation is retarded at 5 weeks of age but reaches the full extent by 8 weeks of age compared with the control mice. However, the MFCKO females are unable to nurse their pups due to severe lobulo-alveolar hypoplasia and secretory immaturity during pregnancy and lactation. Analysis of the mammary epithelial cells in MFCKO mice showed reduced Erk phosphorylation, expression of cyclin D1, and a corresponding decrease in proliferative capability compared with the littermate controls. In addition, phosphorylation of STAT5 and expression of whey acidic protein are significantly reduced in the mammary glands of MFCKO mice, suggesting defective secretory maturation in these mice. Therefore, the combination of the severe lobulo-alveolar hypoplasia and defective secretory differentiation is responsible for the inability of the MFCKO females to nurse their pups. Together, these results provide strong support for a role of FAK in the mammary gland development and function in vivo.

SummaryFocal adhesion kinase (FAK) is an important mediator of integrin signaling in the regulation of cell proliferation, survival, migration, and invasion. To understand how FAK contributes to cell invasion, we explored the regulation of matrix metalloproteinases (MMPs) by FAK. We found that v-Src-transformed cells activate a FAK-dependent mechanism that attenuates endocytosis of MT1-MMP. This in turn increases cell-surface expression of MT1-MMP and cellular degradation of extracellular matrix. Further, we identified an interaction between FAK’s second Pro-rich motif and endophilin A2’s SH3 domain. This interaction served as an autophosphorylation-dependent scaffold to allow Src phosphorylation of endophilin A2 at Tyr315. Tyr315 phosphorylation inhibited endophilin/dynamin interactions, and blockade of Tyr315 phosphorylation promoted endocytosis of MT1-MMP. Together, these results suggest a regulatory mechanism of cell invasion whereby FAK promotes cell-surface presentation of MT1-MMP by inhibiting endophilin A2-dependent endocytosis.