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Accumulating evidence suggests that both the nature of oncogenic lesions and the cell-of-origin can strongly influence cancer histopathology, tumor aggressiveness and response to therapy. Although oncogenic Kras expression and loss of Trp53 tumor suppressor gene function have been demonstrated to initiate murine lung adenocarcinomas (LUADs) in alveolar type II (AT2) cells, clear evidence that Club cells, representing the second major subset of lung epithelial cells, can also act as cells-of-origin for LUAD is lacking. Equally, the exact anatomic location of Club cells that are susceptible to Kras transformation and the resulting tumor histotype remains to be established. Here, we provide definitive evidence for Club cells as progenitors for LUAD. Using in vivo lineage tracing, we find that a subset of Kras12V -expressing and Trp53-deficient Club cells act as precursors for LUAD and we define the stepwise trajectory of Club cell-initiated tumors leading to lineage marker conversion and aggressive LUAD. Our results establish Club cells as cells-of-origin for LUAD and demonstrate that Club cell-initiated tumors have the potential to develop aggressive LUAD.

In mammals, the KRAS locus encodes two protein isoforms, KRAS4A and KRAS4B, which differ only in their C terminus via alternative splicing of distinct fourth exons. Previous studies have shown that whereas KRAS expression is essential for mouse development, the KRAS4A isoform is expendable. Here, we have generated a mouse strain that carries a terminator codon in exon 4B that leads to the expression of an unstable KRAS4B154 truncated polypeptide, hence resulting in a bona fide Kras4B-null allele. In contrast, this terminator codon leaves expression of the KRAS4A isoform unaffected. Mice selectively lacking KRAS4B expression developed to term but died perinatally because of hypertrabeculation of the ventricular wall, a defect reminiscent of that observed in embryos lacking the Kras locus. Mouse embryonic fibroblasts (MEFs) obtained from Kras4B-/- embryos proliferated less than did wild-type MEFs, because of limited expression of KRAS4A, a defect that can be compensated for by ectopic expression of this isoform. Introduction of the same terminator codon into a Kras FSFG12V allele allowed expression of an endogenous KRAS4AG12V oncogenic isoform in the absence of KRAS4B. Exposure of Kras +/FSF4AG12V4B- mice to Adeno-FLPo particles induced lung tumors with complete penetrance, albeit with increased latencies as compared with control Kras +/FSFG12V animals. Moreover, a significant percentage of these mice developed proximal metastasis, a feature seldom observed in mice expressing both mutant isoforms. These results illustrate that expression of the KRAS4AG12V mutant isoform is sufficient to induce lung tumors, thus suggesting that selective targeting of the KRAS4BG12V oncoprotein may not have significant therapeutic consequences. We thank Marta San Roman, Raquel Villar, and Nuria Cabrera for excellent technical assistance; Mayte Lamparero and Isabel Blanco (Animal Facility) for mouse work; the Histopathology Unit for processing of mouse tissues; Lola Martinez (Flow Cytometry Unit) for her help with flow cytometry analyses; Diego Megias and Manuel Perez (Confocal Microscopy Unit) for assistance with confocal microscopy; and the Mouse Genome Editing Unit for support with the generation of the mouse strains described here. We also thank Ignacio Perez de Castro (Instituto de Salud Carlos III, Madrid, Spain) for sharing the EGFP-KRAS4B plasmid and Orlando Dominguez (Genomics Unit) and Pedro P. Lopez-Casas (Clinical Research Program) for their advice on exome sequencing. This work was supported by grants from the European Research Council (ERC-2015-AdG/695566, THERACAN), the Spanish Ministry of Science, Innovation and Universities (RTC-2017-6576-1), and the Autonomous Community of Madrid (B2017/BMD-3884 iLUNG-CM); a grant from the CRIS Cancer Foundation (to M.B.); and a grant from the Spanish Ministry of Science, Innovation and Universities (RTI2018-094664-B-I00, to M.B. and M.M.). M.B. is a recipient of an Endowed Chair from the AXA Research Fund. M.S. was supported by predoctoral contract "Severo Ochoa" (BES-2016-079096) from the SpanishMinistry of Science, Innovation and Universities. G.P. was a recipient of a "Young Ph.D." grant from the Government of the Community of Madrid. F.F.-G. was supported by a formacion de profesorado universitario (FPU) fellowship from the Spanish Ministry of Science, Innovation and Universities. Sí

Pancreatic acinar cells are a cell type of origin for pancreatic cancer that become progressively less sensitive to tumorigenesis induced by oncogenic Kras mutations after birth. This sensitivity is increased when Kras mutations are combined with pancreatitis. Molecular mechanisms underlying these observations are still largely unknown. To identify these mechanisms, we generated the first CRISPR-edited mouse models that enable detection of wild-type and mutant KRAS proteins in vivo. Analysis of these mouse models revealed that more than 75% of adult acinar cells are devoid of detectable KRAS protein. In the 25% of acinar cells expressing KRAS protein, transcriptomic analysis highlighted a slight upregulation of the RAS and MAPK pathways. However, at the protein level, only marginal pancreatic expression of essential KRAS effectors, including C-RAF, was observed. The expression of KRAS and its effectors gradually decreased after birth. The low sensitivity of adult acinar cells to Kras mutations resulted from low expression of KRAS and its effectors and the subsequent lack of activation of RAS/MAPK pathways. Pancreatitis triggered expression of KRAS and its effectors as well as subsequent activation of downstream signaling; this induction required the activity of EGFR. Finally, expression of C-RAF in adult pancreas was required for pancreatic tumorigenesis. In conclusion, our study reveals that control of the expression of KRAS and its effectors regulates the sensitivity of acinar cells to transformation by oncogenic Kras mutations. Significance: This study generates new mouse models to study regulation of KRAS during pancreatic tumorigenesis and highlights a novel mechanism through which pancreatitis sensitizes acinar cells to Kras mutations.

International audience; Platinum-based chemotherapy in combination with immune-checkpoint inhibitors is the current standard of care for patients with advanced lung adenocarcinoma (LUAD). However, tumor progression evolves in most cases. Therefore, predictive biomarkers are needed for better patient stratification and for the identification of new therapeutic strategies, including enhancing the efficacy of chemotoxic agents. Here, we hypothesized that discoidin domain receptor 1 (DDR1) may be both a predictive factor for chemoresistance in patients with LUAD and a potential target positively selected in resistant cells. By using biopsies from patients with LUAD, KRAS-mutant LUAD cell lines, and in vivo genetically engineered KRAS-driven mouse models, we evaluated the role of DDR1 in the context of chemotherapy treatment. We found that DDR1 is upregulated during chemotherapy both in vitro and in vivo. Moreover, analysis of a cohort of patients with LUAD suggested that high DDR1 levels in pretreatment biopsies correlated with poor response to chemotherapy. Additionally, we showed that combining DDR1 inhibition with chemotherapy prompted a synergistic therapeutic effect and enhanced cell death of KRAS-mutant tumors in vivo. Collectively, this study suggests a potential role for DDR1 as both a predictive and prognostic biomarker, potentially improving the chemotherapy response of patients with LUAD.

Malignant transformation entails important changes in the control of cell proliferation through the rewiring of selected signaling pathways. Cancer cells then become very dependent on the proper function of those pathways, and their inhibition offers therapeutic opportunities. Here we identify the stress kinase p38α as a nononcogenic signaling molecule that enables the progression of KrasG12V-driven lung cancer. We demonstrate in vivo that, despite acting as a tumor suppressor in healthy alveolar progenitor cells, p38α contributes to the proliferation and malignization of lung cancer epithelial cells. We show that high expression levels of p38α correlate with poor survival in lung adenocarcinoma patients, and that genetic or chemical inhibition of p38α halts tumor growth in lung cancer mouse models. Moreover, we reveal a lung cancer epithelial cell-autonomous function for p38α promoting the expression of TIMP-1, which in turn stimulates cell proliferation in an autocrine manner. Altogether, our results suggest that epithelial p38α promotes KrasG12V-driven lung cancer progression via maintenance of cellular self-growth stimulatory signals. Fil: Vitos Faleato, Jessica. Instituto de Investigación Biomédica de Barcelona.; España Fil: Real Varela, Sebastián Martín. Instituto de Investigación Biomédica de Barcelona.; España Fil: Gutiérrez Prat, Nuria. Instituto de Investigación Biomédica de Barcelona.; España Fil: Villanueva, Alberto. Instituto Catalán de Oncología; España Fil: Llonch llongueras, Elisabet. Instituto de Investigación Biomédica de Barcelona.; España Fil: Drosten, Matthias. Centro Nacional de Investigaciones Oncologicas; España Fil: Barbacid, Mariano. Centro Nacional de Investigaciones Oncologicas; España Fil: Nebreda, Ángel R.. Instituto de Investigación Biomédica de Barcelona.; España

KRAS mutations occur in a quarter of all of human cancers, yet no selective drug has been approved to treat these tumors. Despite the recent development of drugs that block KRASG12C, the majority of KRAS oncoproteins remain undruggable. Here, we review recent efforts to validate individual components of the mitogen-activated protein kinase (MAPK) pathway as targets to treat KRAS-mutant cancers by comparing genetic information derived from experimental mouse models of KRAS-driven lung and pancreatic tumors with the outcome of selective MAPK inhibitors in clinical trials. We also review the potential of RAF1 as a key target to block KRAS-mutant cancers. This work was supported by grants from the European Research Council (ERC-AG/695566, THERACAN), the Spanish Ministry of Science, Innovation and Universities (RTI2018-094664-B-I00 and RTC2017-6576-1), the Autonomous Community of Madrid (B2017/BMD-3884 iLUNG-CM), and the Asociacion Espanola contra el Cancer (GC166173694BARB). M.B. is a recipient of an Endowed Chair from the AXA Research Fund. Sí

Myocardial uptake can hamper visualization of lung tumors, atherosclerotic plaques, and inflammatory diseases in 2-deoxy-2-[18F]fluoro-D-glucose ([18F]FDG) studies because it leads to spillover in adjacent structures. Several preparatory pre-imaging protocols (including dietary restrictions and drugs) have been proposed to decrease physiological [18F]FDG uptake by the heart, although their effect on tumor glucose metabolism remains largely unknown. The objective of this study was to assess the effects of a ketogenic diet (as an alternative protocol to fasting) on tumor glucose metabolism assessed by [18F]FDG positron emission tomography (PET) in a mouse model of lung cancer. PET scans were performed 60 min after injection of 18.5 MBq of [18F]FDG. PET data were collected for 45 min, and an x-ray computed tomograph (CT) image was acquired after the PET scan. A PET/CT study was obtained for each mouse after fasting and after the ketogenic diet. Quantitative data were obtained from regions of interest in the left ventricular myocardium and lung tumor. Three days on a ketogenic diet decreased mean standard uptake value (SUVmean) in the myocardium (SUVmean 0.95 ± 0.36) more than one night of fasting (SUVmean 1.64 ± 0.93). Tumor uptake did not change under either dietary condition. These results show that 3 days on high-fat diets prior to [18F]FDG-PET imaging does not change tumor glucose metabolism compared with one night of fasting, although high-fat diets suppress myocardial [18F]FDG uptake better than fasting.

Mutations at multiple sites in MEK1 occur in cancer, suggesting that their mechanisms of activation might be different. We analyzed 17 tumor-associated MEK1 mutants and found that they drove ERK signaling autonomously or in a RAS/RAF-dependent manner. The latter are sensitive to feedback inhibition of RAF, which limits their functional output, and often cooccur with RAS or RAF mutations. They act as amplifiers of RAF signaling. In contrast, another class of mutants deletes a hitherto unrecognized negative regulatory segment of MEK1, is RAF- and phosphorylation-independent, is unaffected by feedback inhibition of upstream signaling, and drives high ERK output and transformation in the absence of RAF activity. Moreover, these RAF-independent mutants are insensitive to allosteric MEK inhibitors, which preferentially bind to the inactivated form of MEK1. All the mutants are sensitive to an ATP-competitive MEK inhibitor. Thus, our study comprises a novel therapeutic strategy for tumors driven by RAF-independent MEK1 mutants. Significance: Mutants with which MEK1 mutants coexist and their sensitivity to inhibitors are determined by allele-specific properties. This study shows the importance of functional characterization of mutant alleles in single oncogenes and identifies a new class of MEK1 mutants, insensitive to current MEK1 inhibitors but treatable with a new ATP-competitive inhibitor. Cancer Discov; 8(5); 648–61. ©2018 AACR. See related commentary by Maust et al., p. 534. This article is highlighted in the In This Issue feature, p. 517

The initiating oncogenic event in almost half of human lung adenocarcinomas is still unknown, a fact that complicates the development of selective targeted therapies. Yet these tumours harbour a number of alterations without obvious oncogenic function including BRAF-inactivating mutations. Inactivating BRAF mutants in lung predominate over the activating V600E mutant that is frequently observed in other tumour types. Here we demonstrate that the expression of an endogenous Braf(D631A) kinase-inactive isoform in mice (corresponding to the human BRAF(D594A) mutation) triggers lung adenocarcinoma in vivo, indicating that BRAF-inactivating mutations are initiating events in lung oncogenesis. Moreover, inactivating BRAF mutations have also been identified in a subset of KRAS-driven human lung tumours. Co-expression of Kras(G12V) and Braf(D631A) in mouse lung cells markedly enhances tumour initiation, a phenomenon mediated by Craf kinase activity, and effectively accelerates tumour progression when activated in advanced lung adenocarcinomas. We also report a key role for the wild-type Braf kinase in sustaining Kras(G12V)/Braf(D631A)-driven tumours. Ablation of the wild-type Braf allele prevents the development of lung adenocarcinoma by inducing a further increase in MAPK signalling that results in oncogenic toxicity; this effect can be abolished by pharmacological inhibition of Mek to restore tumour growth. However, the loss of wild-type Braf also induces transdifferentiation of club cells, which leads to the rapid development of lethal intrabronchiolar lesions. These observations indicate that the signal intensity of the MAPK pathway is a critical determinant not only in tumour development, but also in dictating the nature of the cancer-initiating cell and ultimately the resulting tumour phenotype.

CIC (also known as Capicua) is a transcriptional repressor negatively regulated by RAS/MAPK signaling. Whereas the functions of Cic have been well characterized in Drosophila, little is known about its role in mammals. CIC is inactivated in a variety of human tumors and has been implicated recently in the promotion of lung metastases. Here, we describe a mouse model in which we inactivated Cic by selectively disabling its DNA-binding activity, a mutation that causes derepression of its target genes. Germline Cic inactivation causes perinatal lethality due to lung differentiation defects. However, its systemic inactivation in adult mice induces T-cell acute lymphoblastic lymphoma (T-ALL), a tumor type known to carry CIC mutations, albeit with low incidence. Cic inactivation in mice induces T-ALL by a mechanism involving derepression of its well-known target, Etv4. Importantly, human T-ALL also relies on ETV4 expression for maintaining its oncogenic phenotype. Moreover, Cic inactivation renders T-ALL insensitive to MEK inhibitors in both mouse and human cell lines. Finally, we show that Ras-induced mouse T-ALL as well as human T-ALL carrying mutations in the RAS/MAPK pathway display a genetic signature indicative of Cic inactivation. These observations illustrate that CIC inactivation plays a key role in this human malignancy, This work was supported by grants from the Fundació La Marató de TV3 (20131730/1) to G.J. and M.B., and the European Research Council (ERC-AG/ 250297-RAS AHEAD), the EU-Framework Programme (HEALTH-F2-2010-259770/LUNGTARGET and HEALTH-2010- 260791/EUROCANPLATFORM), the Spanish Ministry of Economy and Competitiveness (SAF2014-59864-R), the Autonomous Community of Madrid (S2011/BDM-2470/ONCOCYCLE), and the Asociación Española contra el Cáncer (AECC) (GC16173694BARB) to M.B. M.B. is the recipient of an Endowed Chair from the AXA Research Fund. L.S.-C. was supported by a fellowship from the Programa de Formación de Personal Investigator (FPI) of the Spanish Ministry of Economy and Competitiveness.

Activating mutations in KRAS and EGFR, the two most frequent oncogenes in human lung adenocarcinoma, are mutually exclusive, a phenotype attributed to functional redundancy implying lack of positive selection. Employing a mouse model expressing EGFRL858R in advanced KrasG12V-driven tumors we show that their mutual exclusivity can be explained by detrimental effects of their co-expression in lung adenocarcinoma. In vivo, expression of EGFRL858R in KrasG12V-driven tumors triggers replicative stress and apoptosis, while the surviving cells enter a transient cytostatic state incompatible with tumor development that is fully reversible upon discontinued EGFRL858R expression. Eventually, sustained expression of both mutants induces attenuation of oncogenic signaling to levels compatible with proliferation and tumor growth resulting in high sensitivity to Mek inhibition. Our results indicate that the mutual exclusivity of KRAS and EGFR mutations occurs as a combination of cellular toxicity and signal adjustment resulting in lack of selective advantage for cells expressing both oncogenes.

KRAS oncogenes are responsible for one fourth of all human tumors, including lung and pancreatic adenocarcinomas, two tumor types with some of the worse prognoses. Unfortunately, development of suitable therapies to treat these tumors has remained elusive for the last 35 years. Although KRAS remains an undruggable target (with the possible exception of the KRASG12C isoform), most of its downstream effectors are druggable kinases. Indeed, pharma and biotech companies have already developed dozens of selective inhibitors against these kinases. Yet none of them has been approved by the FDA, mostly due to unacceptable toxicities at antitumor effective doses. Hence, patients are still treated with old chemotherapy drugs. To address this important health issue, we have used a new generation of genetically engineered mouse (GEM) tumor models of KRas/Tp53 driven lung and pancreatic tumors to deconstruct oncogenic KRas signaling to identify those effectors whose systemic ablation/inactivation in tumor-bearing mice will result in therapeutic responses without inducing unacceptable toxicities. We have used this experimental strategy to interrogate the effect of ablating/inactivating each of the members of the MAPK cascade, including the Raf, Mek, and Erk kinases, as well as key effectors of the PI3K pathway, including PI3K p110alpha and mTOR. We have also evaluated additional downstream signaling elements such as the cell cycle Cdks. This systematic approach has revealed that most of the KRas signaling effectors are not suitable therapeutic targets due to either lack of therapeutic activity, such as Cdk2, Cdk6 A-Raf, or B-Raf, or to the induction of unacceptable toxicities such as the Mek1/2 and Erk1/2 kinases, PI3K p110alpha, mTOR, and Cdk1. Only Raf1 and Cdk4 turned out to be suitable therapeutic targets, based not only on their antitumor properties but also on the well-tolerated toxicities. Surprisingly, EGFR, an upstream effect of KRas, also has suitable therapeutic activity in pancreatic but not in lung tumors. We are now combining these targets to define more efficacious therapeutic strategies that could be eventually translated to the clinic. Moreover, we are using these experimental GEM tumor models to identify potential resistance mechanisms to these therapies. Citation Format: Mariano Barbacid. The use of GEM tumor models to identify effective therapies against KRAS mutant tumors [abstract]. In: Proceedings of the AACR Special Conference on the Evolving Landscape of Cancer Modeling; 2020 Mar 2-5; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2020;80(11 Suppl):Abstract nr IA11.

The presence of K-RAS oncogenes in human tumors has been known for over 35 years. Indeed, extensive analysis of cancer genomes has revealed that they are present in at least a quarter of all solid tumors. Moreover, there is overwhelming evidence that they represent the initiating event in lung and pancreatic adenocarcinomas, two cancer types with some of the worse prognoses. Recent studies in genetically engineered mouse (GEM) tumor models have shown that inhibition of K-RAS oncogene expression results in rapid tumor regression. Yet in spite of recent progress, K-RAS oncogenes remain for the most part undruggable. Ironically, the main K-RAS downstream effectors are druggable kinases. Selective inhibitors against members of the MAPK and PI3K signaling pathways have been developed over the last two decades. Yet none of them has yielded suitable results in clinical trials to merit FDA approval against K-RAS mutant malignancies. In an effort to identify K-RAS vulnerabilities that may have been missed using biochemical or pharmacologic approaches, we decided a decade ago to deconstruct K-Ras oncogenic signaling in GEM tumor models by interrogating the effect of ablating or inactivating its downstream kinases during tumor initiation. In addition, we examined the consequences of systemically eliminating or inhibiting these targets during adult homeostasis, in order to evaluate the potential toxic effects that may result from tampering with the MAPK and PI3K signaling pathways. These studies revealed three classes of downstream targets: those whose ablation had no consequences on tumor development, those that prevented tumor development but were unacceptably toxic when ablated systemically, and a limited number (c-Raf and Cdk4) that prevented or thwarted tumor development without causing significant toxicities upon systemic elimination. In addition to these downstream targets, we were also able to identify the EGFR as a potential target in pancreatic but not in lung tumors. More recently, we have established a new generation of “therapeutic” GEM tumor models in which we can separate, both spatially and temporally, tumor development from target ablation/inactivation by using two independent recombinase systems. In these models, whereas K-Ras or K-Ras/TP53-driven tumor development is mediated by the Flp(o)/frt system, we use the inducible CreERT2/loxP system for systemic target ablation/inactivation. These “therapeutic” models have allowed us to explore the therapeutic potential of these previously validated targets in tumor-bearing mice. Our results have revealed that c-Raf ablation induces significant levels of tumor regression in K-Ras/TP53-driven lung tumors, a therapeutic effect that is further potentiated by the concomitant expression of a kinase dead isoform of Cdk4. In the corresponding GEM pancreatic tumor model, ablation or inactivation of c-Raf, Cdk4, or EGFR failed to prevent tumor regression, although tampering with Cdk4 or EGFR resulted in significant tumor delays. Importantly, concomitant ablation of c-Raf and EGFR in K-Ras/TP53-driven advanced pancreatic tumors resulted in complete tumor regression in half of the mice. Preliminary comparison of these “responder” tumors with those that did not respond to c-Raf/EGFR ablation is revealing some vulnerabilities that may provide additional targets against these advanced tumors. We hope that these results may serve to pave the way to define more efficacious therapeutic strategies against K-RAS mutant tumors that might eventually be translated to the clinic. Citation Format: Mariano Barbacid. Deconstructing K-RAS oncogenic signaling in lung and pancreatic tumors [abstract]. In: Proceedings of the AACR Special Conference on Targeting RAS-Driven Cancers; 2018 Dec 9-12; San Diego, CA. Philadelphia (PA): AACR; Mol Cancer Res 2020;18(5_Suppl):Abstract nr IA01.

E-type cyclins E1 (CcnE1) and E2 (CcnE2) are regulatory subunits of cyclin-dependent kinase 2 (Cdk2) and thought to control the transition of quiescent cells into the cell cycle. Initial findings indicated that CcnE1 and CcnE2 have largely overlapping functions for cancer development in several tumor entities including hepatocellular carcinoma (HCC). In the present study, we dissected the differential contributions of CcnE1, CcnE2, and Cdk2 for initiation and progression of HCC in mice and patients. To this end, we tested the HCC susceptibility in mice with constitutive deficiency for CcnE1 or CcnE2 as well as in mice lacking Cdk2 in hepatocytes. Genetic inactivation of CcnE1 largely prevented development of liver cancer in mice in two established HCC models, while ablation of CcnE2 had no effect on hepatocarcinogenesis. Importantly, CcnE1-driven HCC initiation was dependent on Cdk2. However, isolated primary hepatoma cells typically acquired independence on CcnE1 and Cdk2 with increasing progression in vitro, which was associated with a gene signature involving secondary induction of CcnE2 and up-regulation of cell cycle and DNA repair pathways. Importantly, a similar expression profile was also found in HCC patients with elevated CcnE2 expression and poor survival. In general, overall survival in HCC patients was synergistically affected by expression of CcnE1 and CcnE2, but not through Cdk2. Our study suggests that HCC initiation specifically depends on CcnE1 and Cdk2, while HCC progression requires expression of any E-cyclin, but no Cdk2.

K-RAS signaling has been intensely studied for over 40 years. Yet, as of today, no drugs have been approved to treat K-RAS mutant cancers. Since the turn of the century, scientists have used genetically engineered mouse (GEM) models to reproduce K-RAS mutant cancers in a laboratory setting to elucidate those molecular events responsible for the onset and progression of these tumors and to identify suitable therapies. In this review, we outline a brief description of available GEM models for two tumor types known to be driven by K-RAS mutations: lung adenocarcinoma and pancreatic ductal adenocarcinoma. In addition, we summarize a series of studies that have used these GEM tumor models to validate, either by genetic or pharmacological approaches, the therapeutic potential of a variety of targets, with the ultimate goal of translating these results to the clinical setting. This work was supported by grants from the European Research Council (ERC-ADG/695566-THERACAN), the Spanish Ministry of Economy and Competitiveness (SAF2014-59864-R) and the Spanish Association against Cancer (AECC, GC16173694BARB). Sí

MEK inhibition in combination with a glycogen synthase kinase-3β (GSK3β) inhibitor, referred as the 2i condition, favors pluripotency in embryonic stem cells (ESCs). However, the mechanisms by which the 2i condition limits ESC differentiation and whether RAS proteins are involved in this phenomenon remain poorly understood. Here we show that RAS nullyzygosity reduces the growth of mouse ESCs (mESCs) and prohibits their differentiation. Upon RAS deficiency or MEK inhibition, ERF (E twenty-six 2 [Ets2]-repressive factor), a transcriptional repressor from the ETS domain family, translocates to the nucleus, where it binds to the enhancers of pluripotency factors and key RAS targets. Remarkably, deletion of Erf rescues the proliferative defects of RAS-devoid mESCs and restores their capacity to differentiate. Furthermore, we show that Erf loss enables the development of RAS nullyzygous teratomas. In summary, this work reveals an essential role for RAS proteins in pluripotency and identifies ERF as a key mediator of the response to RAS/MEK/ERK inhibition in mESCs. We thank Cian Lynch, Jorge Monsech, and Diego Megias for their help with microarray, ChIP-seq, and high-throughput microscopy analyses. We also thank Dr. Manuel Serrano and Dr. André Nussenzweig for their input on the manuscript, and Dr. Diego Sanz for his support throughout the project. C.M.-R. was funded by a PhD fellowship from La Caixa Foundation, T.O. was funded by a PhD fellowship from the Boehringer Ingelheim Fonds, and S.R. was funded by a Ramon y Cajal contract (RYC-2011-09242). Research was funded by Fundación Botín and Banco Santander through its Santander Universities Global Division; grants from the Spanish Ministry of Economy and Competitiveness (SAF2011-23753 and SAF2014- 57791-REDC; these projects were cofinanced with European Fonds Européen de Développement Économique et Régional [FEDER] funds), Fundació La Marato de TV3, Howard Hughes Medical Institute, and the European Research Council (ERC- 617840) to O.F.-C.; and grants from the Spanish Ministryof Economy and Competitiveness (SAF2013-49147-P and SAF2016-80874-P; these projects were cofinanced with European FEDER funds) to S.R. Author contributions: C.M.-R. and S.R. conducted most of the experiments; T.O. helped with the characterization of RASlox/lox mESCs and with ERF localization studies; E.L. helped with ChIP-seq experiments; M.D., S.O., and M.B. contributed to the work on RAS-deficient cells; M.V.-S. provided technical help; O.D. helped with genomics experiments and bioinformatics analysis; and S.R. and O.F.-C. coordinated the study and wrote the manuscript. Sí

Pancreatic ductal adenocarcinoma (PDAC) is characterized by the presence of abundant desmoplastic stroma primarily composed of cancer-associated fibroblasts (CAFs). It is generally accepted that CAFs stimulate tumor progression and might be implicated in drug resistance and immunosuppression. Here, we have compared the transcriptional profile of PDGFRα + CAFs isolated from genetically engineered mouse PDAC tumors with that of normal pancreatic fibroblasts to identify genes potentially implicated in their protumorigenic properties. We report that the most differentially expressed gene, Saa3, a member of the serum amyloid A (SAA) apolipoprotein family, is a key mediator of the protumorigenic activity of PDGFRα + CAFs. Whereas Saa3-competent CAFs stimulate the growth of tumor cells in an orthotopic model, Saa3-null CAFs inhibit tumor growth. Saa3 also plays a role in the cross talk between CAFs and tumor cells. Ablation of Saa3 in pancreatic tumor cells makes them insensitive to the inhibitory effect of Saa3-null CAFs. As a consequence, germline ablation of Saa3 does not prevent PDAC development in mice. The protumorigenic activity of Saa3 in CAFs is mediated by Mpp6, a member of the palmitoylated membrane protein subfamily of the peripheral membrane-associated guanylate kinases (MAGUK). Finally, we interrogated whether these observations could be translated to a human scenario. Indeed, SAA1, the ortholog of murine Saa3, is overexpressed in human CAFs. Moreover, high levels of SAA1 in the stromal component correlate with worse survival. These findings support the concept that selective inhibition of SAA1 in CAFs may provide potential therapeutic benefit to PDAC patients., This work was supported by European Research Council Grants ERC-AG/250297-RAS AHEAD and ERC-AG/695566-THERACAN, Spanish Ministry of Economy and Competitiveness Grant SAF2014-59864-R, and Asociación Española contra el Cáncer Grant GC16173694BARB (to M. Barbacid). M.D. was supported by a fellowship from La Caixa International Fellowship Program. M. Barbacid is the recipient of an Endowed Chair from the AXA Research Fund

Telomeres are considered anti-cancer targets, as telomere maintenance above a minimum length is necessary for cancer growth. Telomerase abrogation in cancer-prone mouse models, however, only decreased tumor growth after several mouse generations when telomeres reach a critically short length, and this effect was lost upon p53 mutation. Here, we address whether induction of telomere uncapping by inhibition of the TRF1 shelterin protein can effectively block cancer growth independently of telomere length. We show that genetic Trf1 ablation impairs the growth of p53-null K-Ras(G12V)-induced lung carcinomas and increases mouse survival independently of telomere length. This is accompanied by induction of telomeric DNA damage, apoptosis, decreased proliferation, and G2 arrest. Long-term whole-body Trf1 deletion in adult mice did not impact on mouse survival and viability, although some mice showed a moderately decreased cellularity in bone marrow and blood. Importantly, inhibition of TRF1 binding to telomeres by small molecules blocks the growth of already established lung carcinomas without affecting mouse survival or tissue function. Thus, induction of acute telomere uncapping emerges as a potential new therapeutic target for lung cancer.

On the basis of clinical trials using first-generation epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors (TKIs), it became a doctrine that V-Ki-ras2 Kirsten rat sarcoma viral oncogene homolog (K-RAS) mutations drive resistance to EGFR inhibition in non-small cell lung cancer (NSCLC). Conversely, we provide evidence that EGFR signaling is engaged in K-RAS-driven lung tumorigenesis in humans and in mice. Specifically, genetic mouse models revealed that deletion of Egfr quenches mutant K-RAS activity and transiently reduces tumor growth. However, EGFR inhibition initiates a rapid resistance mechanism involving non-EGFR ERBB family members. This tumor escape mechanism clarifies the disappointing outcome of first-generation TKIs and suggests high therapeutic potential of pan-ERBB inhibitors. On the basis of various experimental models including genetically engineered mouse models, patient-derived and cell line-derived xenografts, and in vitro experiments, we demonstrate that the U.S. Food and Drug Administration-approved pan-ERBB inhibitor afatinib effectively impairs K-RAS-driven lung tumorigenesis. Our data support reconsidering the use of pan-ERBB inhibition in clinical trials to treat K-RAS-mutated NSCLC.

Cancer evolution is a process that is still poorly understood because of the lack of versatile in vivo longitudinal studies. By generating murine non–small cell lung cancer (NSCLC) orthoallobanks and paired primary cell lines, we provide a detailed description of an in vivo, time-dependent cancer malignization process. We identify the acquisition of metastatic dissemination potential, the selection of co-driver mutations, and the appearance of naturally occurring intratumor heterogeneity, thus recapitulating the stochastic nature of human cancer development. This approach combines the robustness of genetically engineered cancer models with the flexibility of allograft methodology. We have applied this tool for the preclinical evaluation of therapeutic approaches. This system can be implemented to improve the design of future treatments for patients with NSCLC. Cancer Res; 74(21); 5978–88. ©2014 AACR.

Noonan syndrome (NS) is an autosomal dominant genetic disorder characterized by short stature, craniofacial dysmorphism, and congenital heart defects. NS also is associated with a risk for developing myeloproliferative disorders (MPD), including juvenile myelomonocytic leukemia (JMML). Mutations responsible for NS occur in at least 11 different loci including KRAS. Here we describe a mouse model for NS induced by K-Ras(V14I), a recurrent KRAS mutation in NS patients. K-Ras(V14I)-mutant mice displayed multiple NS-associated developmental defects such as growth delay, craniofacial dysmorphia, cardiac defects, and hematologic abnormalities including a severe form of MPD that resembles human JMML. Homozygous animals had perinatal lethality whose penetrance varied with genetic background. Exposure of pregnant mothers to a MEK inhibitor rescued perinatal lethality and prevented craniofacial dysmorphia and cardiac defects. However, Mek inhibition was not sufficient to correct these defects when mice were treated after weaning. Interestingly, Mek inhibition did not correct the neoplastic MPD characteristic of these mutant mice, regardless of the timing at which the mice were treated, thus suggesting that MPD is driven by additional signaling pathways. These genetically engineered K-Ras(V14I)-mutant mice offer an experimental tool for studying the molecular mechanisms underlying the clinical manifestations of NS. Perhaps more importantly, they should be useful as a preclinical model to test new therapies aimed at preventing or ameliorating those deficits associated with this syndrome.

Despite the pressing need for novel cancer treatments, our improved understanding of tumor biology is not being successfully translated into better therapies. Here we present a lentiviral vector that enables in vivo validation of cancer therapeutic targets when combined with existing cancer animal models that faithfully reproduce the natural history of human disease. Unlike the conventional genetic approaches with targeted alleles, the outlined experimental strategy could be used to assess the preclinical efficacy of a growing number of putative therapeutic hits in a rapid and cost-effective manner.

et al., The Cip/Kip family, namely, p21Cip1, p27Kip1, and p57Kip2, are stoichiometric cyclin-dependent kinase inhibitors (CKIs). Paradoxically, they have been proposed to also act as positive regulators of Cdk4/6-cyclin D by stabilizing these heterodimers. Loss of p21Cip1 and p27Kip1 reduces Cdk4/6-cyclin D complexes, although with limited phenotypic consequences compared to the embryonic lethality of Cdk4/6 or triple cyclin D deficiency. This milder phenotype was attributed to Cdk2 compensatory mechanisms. To address this controversy using a genetic approach, we generated Cdk2-/- p21-/- p27-/- mice. Triple-knockout mouse embryonic fibroblasts (MEFs) displayed minimal levels of D-type cyclins and Cdk4/6-cyclin D complexes. p57Kip2 downregulation in the absence of p21Cip1 and p27Kip1 aggravated this phenotype, yet MEFs lacking all Cip/Kip proteins exhibited increased retinoblastoma phosphorylation, together with enhanced proliferation and transformation capacity. In vivo, Cdk2 ablation induced partial perinatal lethality in p21-/- p27-/- mice, suggesting partial Cdk2-dependent compensation. However, Cdk2-/- p21-/- p27-/- survivors displayed all phenotypes described for p27-/- mice, including organomegalia and pituitary tumors. Thus, Cip/Kip deficiency does not impair interphasic Cdk activity even in the absence of Cdk2, suggesting that their Cdk-cyclin assembly function is dispensable for homeostatic control in most cell types., This work was supported by grants from the European Research Council (ERC-AG/250297-RAS AHEAD), EU-Framework Programme (LSHG-CT-2007-037665/CHEMORES, HEALTH-F2-2010-259770/LUNGTARGET, and HEALTH-2010-260791/EUROCANPLAT-FORM), Spanish Ministry of Economy and Competitiveness (SAF2011-30173), and Autonomous Community of Madrid (S2011/BDM-2470/ONCOCYCLE) to M.B. C.E.S. is supported by a grant from the La Caixa/CNIO International Ph.D. Fellowship Program sponsored by the La Caixa Foundation (Spain).

In this implementation phase of the European Cancer Patient's Bill of Rights (BoR), we confirm the following three patient-centred principles that underpin this initiative: The right of every European citizen to receive the most accurate information and to be proactively involved in his/her care. The right of every European citizen to optimal and timely access to a diagnosis and to appropriate specialised care, underpinned by research and innovation. The right of every European citizen to receive care in health systems that ensure the best possible cancer prevention, the earliest possible diagnosis of their cancer, improved outcomes, patient rehabilitation, best quality of life and affordable health care. The key aspects of working towards implementing the BoR are: Agree our high-level goal. The vision of 70% long-term survival for patients with cancer in 2035, promoting cancer prevention and cancer control and the associated progress in ensuring good patient experience and quality of life. Establish the major mechanisms to underpin its delivery. (1) The systematic and rigorous sharing of best practice between and across European cancer healthcare systems and (2) the active promotion of Research and Innovation focused on improving outcomes; (3) Improving access to new and established cancer care by sharing best practice in the development, approval, procurement and reimbursement of cancer diagnostic tests and treatments. Work with other organisations to bring into being a Europe based centre that will (1) systematically identify, evaluate and validate and disseminate best practice in cancer management for the different countries and regions and (2) promote Research and Innovation and its translation to maximise its impact to improve outcomes., SCOPUS: re.j, info:eu-repo/semantics/published

Background: Nab-paclitaxel and gemcitabine have demonstrated a survival benefit over gemcitabine alone in advanced pancreatic cancer (PDA). This study aimed to investigate the clinical, biological, and imaging effects of the regimen in patients with operable PDA. Methods: Patients with operable PDA received two cycles of nab-paclitaxel and gemcitabine before surgical resection. FDG-PET and CA19.9 tumour marker levels were used to measure clinical activity. Effects on tumour stroma were determined by endoscopic ultrasound (EUS) elastography. The collagen content and architecture as well as density of cancer-associated fibroblasts (CAFs) were determined in the resected surgical specimen and compared with a group of untreated and treated with conventional chemoradiation therapy controls. A co-clinical study in a mouse model of PDA was conducted to differentiate between the effects of nab-paclitaxel and gemcitabine. Results: A total of 16 patients were enrolled. Treatment resulted in significant antitumour effects with 50% of patients achieving a >75% decrease in circulating CA19.9 tumour marker and a response by FDG-PET. There was also a significant decrement in tumour stiffness as measured by EUS elastography. Seven of 12 patients who completed treatment and were operated had major pathological regressions. Analysis of residual tumours showed a marked disorganised collagen with a very low density of CAF, which was not observed in the untreated or conventionally treated control groups. The preclinical co-clinical study showed that these effects were specific of nab-paclitaxel and not gemcitabine. Conclusion: These data suggest that nab-paclitaxel and gemcitabine decreases CAF content inducing a marked alteration in cancer stroma that results in tumour softening. This regimen should be studied in patients with operable PDA.

Proliferation in the epidermis is a tightly controlled process. During skin development, epidermis formation and hair follicle morphogenesis crucially depend on the regulated balance between proliferation and differentiation. Here we deleted all three Ras loci (H-Ras, N-Ras and K-Ras) from keratinocytes in vitro as well as specifically from the epidermis in mice using a K5Cre strain. Upon Ras elimination, keratinocytes ceased proliferation and entered into senescence without any signs of apoptosis induction. Constitutive activation of the mitogen-activated protein kinase pathway was able to partially rescue the proliferative defects. In mice, Ras signaling was essential for proper development of the epidermis and hair follicles. Deletion of the three Ras loci during epidermis formation in mouse embryos caused a dramatic decrease in proliferation, resulting in a substantially thinner epidermis and delayed appearance of differentiation markers. We could not detect apoptotic or senescent cells in these embryos suggesting that loss of Ras protein expression only leads to severe hypoproliferation. These observations provide genetic evidence for an essential role of Ras proteins in the control of keratinocyte and epidermal proliferation.

Understanding the early evolution of cancer heterogeneity during the initial steps of tumorigenesis can uncover vulnerabilities of cancer cells that may be masked at later stages. We describe a comprehensive approach employing gene expression analysis in early lesions to identify novel therapeutic targets and the use of mouse models to test synthetic lethal drug combinations to treat human Kirsten rat sarcoma viral oncogene homologue (KRAS)-driven lung adenocarcinoma.

SummaryClinical evidence indicates that mutation/activation of EGF receptors (EGFRs) is mutually exclusive with the presence of K-RAS oncogenes in lung and colon tumors. We have validated these observations using genetically engineered mouse models. However, development of pancreatic ductal adenocarcinomas driven by K-Ras oncogenes are totally dependent on EGFR signaling. Similar results were obtained using human pancreatic tumor cell lines. EGFRs were also essential even in the context of pancreatic injury and absence of p16Ink4a/p19Arf. Only loss of p53 made pancreatic tumors independent of EGFR signaling. Additional inhibition of PI3K and STAT3 effectively prevented proliferation of explants derived from these p53-defective pancreatic tumors. These findings may provide the bases for more rational approaches to treat pancreatic tumors in the clinic.

Oncogene-induced replicative stress activates an Atr- and Chk1-dependent response, which has been proposed to be widespread in tumors. We explored whether the presence of replicative stress could be exploited for the selective elimination of cancer cells. To this end, we evaluated the impact of targeting the replicative stress-response on cancer development. In mice (Mus musculus), the reduced levels of Atr found on a mouse model of the Atr-Seckel syndrome completely prevented the development of Myc-induced lymphomas or pancreatic tumors, both of which showed abundant levels of replicative stress. Moreover, Chk1 inhibitors were highly effective in killing Myc-driven lymphomas. By contrast, pancreatic adenocarcinomas initiated by K-Ras(G12V) showed no detectable evidence of replicative stress and were nonresponsive to this therapy. Besides its impact on cancer, Myc overexpression aggravated the phenotypes of Atr-Seckel mice, revealing that oncogenes can modulate the severity of replicative stress-associated diseases.

SummaryPancreatic acinar cells of adult mice (≥P60) are resistant to transformation by some of the most robust oncogenic insults including expression of K-Ras oncogenes and loss of p16Ink4a/p19Arf or Trp53 tumor suppressors. Yet, these acinar cells yield pancreatic intraepithelial neoplasias (mPanIN) and ductal adenocarcinomas (mPDAC) if exposed to limited bouts of non-acute pancreatitis, providing they harbor K-Ras oncogenes. Pancreatitis contributes to tumor progression by abrogating the senescence barrier characteristic of low-grade mPanINs. Attenuation of pancreatitis-induced inflammation also accelerates tissue repair and thwarts mPanIN expansion. Patients with chronic pancreatitis display senescent PanINs, providing they have received antiinflammatory drugs. These results support the concept that antiinflammatory treatment of people diagnosed with pancreatitis may reduce their risk of developing PDAC.

The presence of neurogenic precursors in the adult mammalian brain is now widely accepted, but the mechanisms coupling their proliferation with the onset of neuronal differentiation remain unknown. Here, we unravel the major contribution of the G1 regulator cyclin-dependent kinase 6 (Cdk6) to adult neurogenesis. We found that Cdk6 was essential for cell proliferation within the dentate gyrus of the hippocampus and the subventricular zone of the lateral ventricles. Specifically, Cdk6 deficiency prevents the expansion of neuronally committed precursors by lengthening G1 phase duration, reducing concomitantly the production of newborn neurons. Altogether, our data support G1 length as an essential regulator of the switch between proliferation and neuronal differentiation in the adult brain and Cdk6 as one intrinsic key molecular regulator of this process.

Keratinocytes contribute to melanocyte transformation by affecting their microenvironment, in part, through the secretion of paracrine factors. Here we report a loss of expression of nuclear receptor RXRα in epidermal keratinocytes during human melanoma progression. We show that absence of keratinocytic RXRα in combination with mutant Cdk4 generated cutaneous melanoma that metastasized to lymph nodes in a bigenic mouse model. Expression of several keratinocyte-derived mitogenic growth factors (Et-1, Hgf, Scf, α-MSH and Fgf2) were elevated in skin of bigenic mice, while Fas, E-cadherin and Pten, implicated in apoptosis, cellular invasion and melanomagenesis, respectively, were downregulated within the microdissected melanocytic tumors. We demonstrated that RXRα is recruited on the proximal promoter of both Et-1 and Hgf, possibly directly regulating their transcription in keratinocytes. These studies demonstrate the contribution of keratinocytic paracrine signaling during the cellular transformation and malignant conversion of melanocytes. Significance Malignant melanoma continues to evade modern curative efforts as a result of the complex and elusive nature of metastatic tumors. We demonstrated that RXRα expression is lost in epidermal keratinocytes during melanoma progression in humans. We have presented evidence for the association of keratinocytic nuclear receptor RXRα-mediated paracrine/juxtacrine signaling and the malignant transformation of melanocytic tumors. The present work highlights cooperative effects of aberrant keratinocytic signaling and activated Cdk4 in melanoma metastasis. This mouse model will allow greater understanding of the mechanisms responsible for transition towards a malignant phenotype. Manipulation of these receptor-mediated signaling pathways could hold therapeutic value when combating metastatic disease.

Lung cancer is the leading cause of cancer-related deaths worldwide, and even today, the 5-year survival rate is still below 15%. Lung adenocarcinoma is the most frequent subtype, and approximately 25% of the cases harbor activating mutations in the KRAS gene. To date, there is no effective treatment for patients carrying KRAS mutations due, at least in part, to the challenge posed by direct targeting of the KRAS oncoprotein. During the last decade, scientists have developed genetically engineered mouse models that faithfully recapitulate the natural history of the human tumors. These models have been used as a preclinical platform to validate a number of relevant downstream effectors of KRAS signaling. Targets displaying synthetic lethality with the KRAS oncoprotein have also been validated in these models. Here, we review these studies and discuss their potential value in the clinical setting. We also provide an outlook as of how to improve the significance of target validation studies in preclinical platforms.

H-Ras must adhere to the plasma membrane to be functional. This is accomplished by posttranslational modifications, including palmitoylation, a reversible process whereby H-Ras traffics between the plasma membrane and the Golgi complex. At the plasma membrane, H-Ras has been proposed to occupy distinct sublocations, depending on its activation status: lipid rafts/detergent- resistant membrane fractions when bound to GDP, diffusing to disordered membrane/soluble fractions in response to GTP loading. Herein, we demonstrate that H-Ras sublocalization is dictated by its degree of palmitoylation in a cell type-specific manner. Whereas H-Ras localizes to detergent-resistant membrane fractions in cells with low palmitoylation activity, it locates to soluble membrane fractions in lineages where it is highly palmitoylated. Interestingly, in both cases GTP loading results in H-Ras diffusing away from its original sublocalization. Moreover, tilting the equilibrium between palmitoylation and depalmitoylation processes can substantially alter H-Ras segregation and, subsequently, its biochemical and biological functions. Thus, the palmitoylation/depalmitoylation balance not only regulates H-Ras cycling between endomembranes and the plasma membrane but also serves as a key orchestrator of H-Ras lateral diffusion between different types of plasma membrane and thereby of H-Ras signaling., The P. Crespo laboratory is supported by grant BFU2011-23807 from the Spanish Ministry of Economy-Fondos FEDER and by the Red Temática de Investigación Cooperativa en Cáncer (RTICC) (RD/12/0036/0033), Spanish Ministry of Health. Work in the M. Barbacid laboratory was supported by grants from the European Research Council (ERC-AG/250297-RAS AHEAD) and the Spanish Ministry of Science and Innovation (SAF2006-11773 and CSD2007-00017).

Oncogenic mutations in the K- ras gene occur in ≈50% of human colorectal cancers. However, the precise role that K- ras oncogenes play in tumor formation is still unclear. To address this issue, we have conditionally expressed an oncogenic K- ras V12 allele in the small intestine of adult mice either alone or in the context of Apc deficiency. We found that expression of K- ras V12 does not affect normal intestinal homeostasis or the immediate phenotypes associated with Apc deficiency. Mechanistically we failed to find activation of the Raf/MEK/ERK pathway, which may be a consequence of the up-regulation of a number of negative feedback loops. However, K- ras V12 expression accelerates intestinal tumorigenesis and confers invasive properties after Apc loss over the long term. In renal epithelium, expression of the oncogenic K- ras V12 allele in the absence of Apc induces the rapid development of renal carcinoma. These tumors, unlike those of intestinal origin, display activation of the Raf/MEK/ERK and Akt signaling pathways. Taken together, these data indicate that normal intestinal and kidney epithelium are resistant to malignant transformation by an endogenous K- ras oncogene. However, activation of K- ras V12 after Apc loss results in increased tumorigenesis with distinct kinetics. Whereas the effect of K- ras oncogenes in the intestine can been observed only after long latencies, they result in rapid carcinogenesis in the kidney epithelium. These data imply a window of opportunity for anti-K-ras therapies after tumor initiation in preventing tumor growth and invasion.

Human melanoma susceptibility is often characterized by germ-line inactivating CDKN2A (INK4A/ARF) mutations, or mutations that activate CDK4 by preventing its binding to and inhibition by INK4A. We have previously shown that a single neonatal UV radiation (UVR) dose delivered to mice that carry melanocyte-specific activation of Hras (TPras) increases melanoma penetrance from 0% to 57%. Here, we report that activated Cdk4 cooperates with activated Hras to enhance susceptibility to melanoma in mice. Whereas UVR treatment failed to induce melanomas in Cdk4R24C/R24C mice, it greatly increased the penetrance and decreased the age of onset of melanoma development in Cdk4R24C/R24C/TPras animals compared with TPras alone. This increased penetrance was dependent on the threshold of Cdk4 activation as Cdk4R24C/+/TPras animals did not show an increase in UVR-induced melanoma penetrance compared with TPras alone. In addition, Cdk4R24C/R24C/TPras mice invariably developed multiple lesions, which occurred rarely in TPras mice. These results indicate that germ-line defects abrogating the pRb pathway may enhance UVR-induced melanoma. TPras and Cdk4R24C/R24C/TPras tumors were comparable histopathologically but the latter were larger and more aggressive and cultured cells derived from such melanomas were also larger and had higher levels of nuclear atypia. Moreover, the melanomas in Cdk4R24C/R24C/TPras mice, but not in TPras mice, readily metastasized to regional lymph nodes. Thus, it seems that in the mouse, Hras activation initiates UVR-induced melanoma development whereas the cell cycle defect introduced by mutant Cdk4 contributes to tumor progression, producing more aggressive, metastatic tumors. (Cancer Res 2006; 66(6): 2946-52)

We evaluated the efficacy of a candidate melanoma vaccine approach in mice genetically prone to develop melanoma due to the introduction of an oncogenic mutation (R24C) in the germline sequence of the cyclin-dependent kinase 4 (cdk4), a protein critically involved in cell cycle regulation. Melanomas were induced in cdk4-mutant mice by chemical carcinogenesis and UVB irradiation. A genetic prime-boost strategy targeting the clinically relevant differentiation antigen tyrosinase-related protein 2 (TRP2) was performed which was able to stimulate a melanocyte-specific cellular immune response associated with localized autoimmune vitiligo-like depigmentation. However, significant destruction of carcinogen-induced autochthonous melanocytic neoplasms in the skin was not observed following immunization. We provide evidence that autochthonous melanomas expressed TRP2 but not the MHC molecule H2-Kb and are immunologically tolerated in the skin. Our results highlight the importance of assessing melanoma vaccines in genetic mouse models that more adequately represent the expected clinical situation in order to identify strategies, which eventually may be of benefit for melanoma patients. © 2005 Wiley-Liss, Inc.

SummaryCell cycle regulators such as E2F1 and retinoblastoma (RB) play crucial roles in the control of adipogenesis, mostly by controlling the transition between preadipocyte proliferation and adipocyte differentiation. The serine-threonine kinase cyclin-dependent kinase 4 (cdk4) works in a complex with D-type cyclins to phosphorylate RB, mediating the entry of cells into the cell cycle in response to external stimuli. Because cdk4 is an upstream regulator of the E2F-RB pathway, we tested whether cdk4 was a target for new factors that regulate adipogenesis. Here we find that cdk4 inhibition impairs adipocyte differentiation and function. Disruption of cdk4 or activating mutations in cdk4 in primary mouse embryonic fibroblasts results in reduced and increased adipogenic potential, respectively, of these cells. We show that the effects of cdk4 are not limited to the control of differentiation; cdk4 also participates in adipocyte function through activation of PPARγ.

Lack of Cdk4 expression in mice leads to insulin-deficient diabetes and female infertility owing to a reduced number of pancreatic beta cells and prolactin-producing pituitary lactotrophs, respectively. Cdk4 null mice display also reduced body and organ size. Here, we show that Cdk4 is essential for the postnatal proliferation of pancreatic beta cells but not for embryonic neogenesis from ductal epithelial cells. Re-expression of endogenous Cdk4 in beta cells and in the pituitary gland of Cdk4 null mice restores cell proliferation and results in fertile and normoglycemic animals, thus, demonstrating that the proliferation defects in these cellular populations are cell autonomous because of the lack of Cdk4 expression. However, these mice remain small in size, indicating that this phenotype is not because of pancreatic- or pituitary-mediated endocrine defects. This phenotype is a consequence of reduced cell numbers rather than reduced cell size. Thus, mammalian Cdk4 is not only involved in controlling proliferation of specific cell types but may play a wider role in establishing homeostatic cell numbers.

Activating BRAF mutants drive human tumors by dysregulating ERK signaling despite ERK-dependent feedback suppression of RAS. These RAF mutants become RAS independent, by either of two mechanisms, and they are thus unaffected by feedback inhibition of upstream signaling. Recently, it has become clear that mutation activation of MEK1 or MEK2 occur at appreciable frequency in human tumors, but the mechanism of activation of these mutants and whether they remain dependent on upstream activation of RAS or RAF signaling remains unknown. Here we characterized the mechanism of activation of 18 recurrent MEK1 mutants identified in human cancer. Based on these data, they fall into three classes. The kinase activity of first class remains dependent on RAF mediated phosphorylation of S218 and S222; the second class of MEK mutants has basal RAF-independent activity, but can be further stimulated by RAF and the third class no longer requires phosphorylation of S218 and S222 and signals in a RAF independent manner. These features determined their sensitivity to ERK-dependent feedback regulation and the ability to drive ERK signaling output in cells. The more RAF-independent activity the mutants acquired, the better they could activate downstream ERK pathway, leading to increased transforming activities in MEF cells in the absence of RAF. This is consistent with their genetic association with RAS, RAF and NF1 mutations in human tumors. The Class 3 RAF-independent MEK1 mutants tend to be mutually exclusive with those mutants, while coexistence with RAS/RAF/NF1 mutations is frequently observed in tumors with Class 1 or 2 MEK1 mutants. Moreover, functional class correlated with sensitivities of ERK signaling driven by these mutants to MEK inhibitors that function by different mechanisms. Unlike RAF dependent or regulated MEK1 mutants, the ERK signaling driven by RAF independent MEK1 mutants is insensitive to an allosteric MEK inhibitor that functions by preventing RAF mediated MEK phosphorylation. However, signaling driven by all classes of MEK1 mutants is sensitive to an ATP competitive MEK1 inhibitor which targets MEK1 kinase activity. Citation Format: Yijun Gao, Matthew T. Chang, Daniel McKay, Rona D. Yaeger, Merna Torres, Keven Muniz, Drosten Matthias, Omar I. Abdel-Wahab, Mariano Barbacid, Giordano Caponigro, Darrin Stuart, David Solit, Barry S. Taylor, Zhan Yao, Neal Rosen. Activation mechanisms of cancer associated MEK1 mutants [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 395. doi:10.1158/1538-7445.AM2017-395