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PDF file - 426K, Histological analysis of lung in primary mice.

Figure S1. Creation of Inducible-BRAF-Inducible Senescent Cells. Figure S2. Summary of lentiviral based shRNA screen for OIS targets. Figure S3. Knockdown of IAPP expression by shRNA. Figure S4. Perturbation of IAPP and ERK2. Figure S5. Loss of IAPP can bypass RAS induced senescence. Figure S6. RAMP3 inhibition affects proliferation arrest. Figure S7. IAPP knockdown cells have a unique profile of glucose metabolites.

PDF file - 697K, S5. In vitro liquid culture.

Mutational activation of BRAF leading to expression of the BRAFV600E oncoprotein was recently identified in a high percentage of specific hematopoietic neoplasms in monocyte/histiocyte and mature B-cell lineages. Although BRAFV600E is a driver oncoprotein and pharmacologic target in solid tumors such as melanoma, lung, and thyroid cancer, it remains unknown whether BRAFV600E is an appropriate therapeutic target in hematopoietic neoplasms. To address this critical question, we generated a mouse model expressing inducible BRAFV600E in the hematopoietic system, and evaluated the efficacy of pathway-targeted therapeutics against primary hematopoietic cells. In this model, BRAFV600E expression conferred cytokine-independent growth to monocyte/macrophage-lineage progenitors leading to aberrant in vivo and in vitro monocyte/macrophage expansion. Furthermore, transplantation of BRAFV600E-expressing bone marrow cells promoted an in vivo pathology most notable for monocytosis in hematopoietic tissues and visceral organs. In vitro analysis revealed that MAP–ERK kinase inhibition, but not RAF inhibition, effectively suppressed cytokine-independent clonal growth of monocyte/macrophage-lineage progenitors. However, combined RAF and phosphoinositide 3-kinase (PI3K) inhibition effectively inhibited cytokine-independent colony formation, suggesting autocrine PI3K pathway activation. Taken together, these results provide evidence that constitutively activated BRAFV600E drives aberrant proliferation of monocyte-lineage cells.Implications: This study supports the development of pathway-targeted therapeutics in the treatment of BRAFV600E-expressing hematopoietic neoplasms in the monocyte/histiocyte lineage. Mol Cancer Res; 11(12); 1530–41. ©2013 AACR.

PDF file - 110K, S6. Imunohistochemistry of tissue sections from BrafV600E bone marrow recipients.

PDF file - 379K, Dose response curves of human PDA cell lines treated with either MEK inhibitor GSK1120212, AKT inhibitor GSK690693, or both together

PDF file - 247K, Histological analysis of spleen and liver in primary mice.

Oncogenic activation of the RTK–RAS–RAF–MEK–ERK pathway occurs in approximately 25% of all human cancers, yet activated RAS, BRAF, or MEK expression in primary cells leads to a prolonged and predominantly irreversible cell-cycle arrest termed oncogene-induced senescence (OIS). OIS acts as an intrinsic tumor suppressor mechanism, serving as a barrier to tumor progression. Screening a library of activated kinases and kinase-regulatory proteins we identified MOB3A, a Mps-one binder coactivator (MOB) protein family member, whose constitutive expression permits proliferation and suppresses senescence in response to oncogenic RAS and BRAF signals. MOB3A is one of seven human MOB genes, which are highly conserved from yeast to human and that function to activate the Hippo pathway kinases (MST/LATS) or NDR kinases through direct association. Here we show that within the MOB family of genes MOB3A and C are unique in their ability to allow primary cell proliferation in the face of sustained oncogene signaling. Unlike the canonical MOB1A/B proteins, MOB3A inhibits Hippo/MST/LATS signaling and constitutive MOB3A membrane localization phenocopies OIS bypass seen with elevated YAP expression. Moreover, inhibition of MOB3 family member expression results in decreased proliferation and tumor growth of cancer cell lines. Together these data identify MOB3A's role in bypass of oncogene induced senescence and its role as a Hippo pathway inhibitor.Implications:These results suggest that MOB3 targeting to re-engage the Hippo pathway, or direct targeting of YAP/TAZ, may be viable therapeutic strategies potential for RAS-pathway driven tumours.

PDF file - 97K, S7. Sequence data from genomic DNA extracted from the SigM5 cell line.

XLS file - 41K, Cell lines used in this study and their genotype

PDF file - 95K, S3. Purity and morphology of the sorted bone marrow populations. S4. Morphology of the colonies developed in the presence of lowdose IL-3.

Supplementary Figure from MOB3A Bypasses BRAF and RAS Oncogene-Induced Senescence by Engaging the Hippo Pathway

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PDF file - 194K, RAF and MEK co inhibition

XLS file - 65K, Interaction indicies of MEK inhibitor GSK1120212, AKT inhibitor GSK690693 in PDA cell lines

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PDF file - 331K, MEK inhibitor PD325901 inhibits MEK in vivo

PDF file - 578K, Characterization of a new, syngeneic, orthotopic mouse model of PDA

Questions persist about the nature and number of cells with tumor-propagating capability in different types of cancer, including melanoma. In part, this is because identification and characterization of purified tumorigenic subsets of cancer cells has not been achieved to date. Here, we report tumor formation after injection of single purified melanoma cells derived from three novel mouse models. Tumor formation occurred after every injection of individual CD34+p75− melanoma cells, with intermediate rates using CD34−p75− cells, and rarely with CD34−p75+ cells. These findings suggest that tumorigenic melanoma cells may be more common than previously thought and establish that multiple distinct populations of melanoma-propagating cells (MPC) can exist within a single tumor. Interestingly, individual CD34−p75− MPCs could regenerate cellular heterogeneity after tumor formation in mice or multiple passages in vitro, whereas CD34+p75− MPCs underwent self-renewal only, showing that reestablishment of tumor heterogeneity is not always a characteristic of individual cells capable of forming tumors. Functionally, single purified MPCs were more resistant to chemotherapy than non-MPCs. We anticipate that purification of these MPCs may allow a more comprehensive evaluation of the molecular features that define tumor-forming capability and chemotherapeutic resistance in melanoma. Cancer Res; 70(1); 388–97

XLS file - 58K, List of differentially expressed genes between Nkx3.1CE2/+; Ptenf/f; BrafCA/+ and Nkx3.1CE2/+; Ptenf/f mouse prostate tumors (p-value

XLS file - 188K, List of differentially expressed genes between RAP + PD treated and Vehicle treated Nkx3.1CE2/+; Ptenf/f; Braf CA/+ mouse prostate tumors (p-value

Supplementary Methods from Characterization of Melanoma Cells Capable of Propagating Tumors from a Single Cell

PDF file - 31K, Myc pathway enrichment analysis for differentially expressed genes between RAP + PD treated and Vehicle treated Nkx3.1CE2/+; Ptenf/f; BrafCA/+ mouse prostate tumors

Supplemental Figure Legend

Supplementary Figures 1-5 from Characterization of Melanoma Cells Capable of Propagating Tumors from a Single Cell

PDF file - 57K, Pathway enrichment analysis for differentially expressed genes between Nkx3.1CE2/+; Ptenf/f; BrafCA/+ and Nkx3.1CE2/+; Ptenf/f mouse prostate tumors

Supplementary Figure Legends 1-5, Table Legends1-2 from Characterization of Melanoma Cells Capable of Propagating Tumors from a Single Cell

Both the PI3K → Akt → mTOR and mitogen-activated protein kinase (MAPK) signaling pathways are often deregulated in prostate tumors with poor prognosis. Here we describe a new genetically engineered mouse model of prostate cancer in which PI3K-Akt-mTOR signaling is activated by inducible disruption of PTEN, and extracellular signal-regulated kinase 1/2 (ERK1/2) MAPK signaling is activated by inducible expression of a BRAFV600E oncogene. These tissue-specific compound mutant mice develop lethal prostate tumors that are inherently resistant to castration. These tumors bypass cellular senescence and disseminate to lymph nodes, bone marrow, and lungs where they form overt metastases in approximately 30% of the cases. Activation of PI3K → Akt → mTOR and MAPK signaling pathways in these prostate tumors cooperate to upregulate c-Myc. Accordingly, therapeutic treatments with rapamycin and PD0325901 to target these pathways, respectively, attenuate c-Myc levels and reduce tumor and metastatic burden. Together, our findings suggest a generalized therapeutic approach to target c-Myc activation in prostate cancer by combinatorial targeting of the PI3K → Akt → mTOR and ERK1/2 MAPK signaling pathways. Cancer Res; 72(18); 4765–76. ©2012 AACR.

Temporal activation of the p53R270H cooperates with BRAFV600E to form lung adenocarcinomas.

Diminished frequency of adenocarcinoma formation with increased time between recombination events.

Supplementary Table 2 from Characterization of Melanoma Cells Capable of Propagating Tumors from a Single Cell

PDF file - 404K, Supplementary Figure 1: Strategy for inducible recombination in the prostate. Supplementary Figure 2: Analyses of tumor regression following castration. Supplementary Figure 3: Gene Set Enrichment Analysis (GSEA). Supplementary Table 1: List of the primers used in this study. Supplementary Table 2: List of antibodies for used in this study. Supplementary Table 3: Summary of phenotypic analyses. Supplementary Table 7: Summary of preclinical data

Supplementary Table 1 from Characterization of Melanoma Cells Capable of Propagating Tumors from a Single Cell

PDF file - 36K, Pathway enrichment analysis for differentially expressed genes between Nkx3.1CE2/+; Ptenf/f; BrafCA/+ and Nkx3.1CE2/+; Ptenf/f mouse prostate tumors

Lung carcinogenesis is a multistep process in which normal lung epithelial cells are converted to cancer cells through the sequential acquisition of multiple genetic or epigenetic events. Despite the utility of current genetically engineered mouse (GEM) models of lung cancer, most do not allow temporal dissociation of the cardinal events involved in lung tumor initiation and cancer progression. Here we describe a novel two-switch GEM model for BRAFV600E-induced lung carcinogenesis allowing temporal dissociation of these processes. In mice carrying a Flp recombinase-activated allele of Braf (BrafFA) in conjunction with Cre-regulated alleles of Trp53, Cdkn2a, or c-MYC, we demonstrate that secondary genetic events can promote bypass of the senescence-like proliferative arrest displayed by BRAFV600E-induced lung adenomas, leading to malignant progression. Moreover, restoring or activating TP53 in cultured BRAFV600E/TP53Null or BRAFV600E/INK4A-ARFNull lung cancer cells triggered a G1 cell-cycle arrest regardless of p19ARF status. Perhaps surprisingly, neither senescence nor apoptosis was observed upon TP53 restoration. Our results establish a central function for the TP53 pathway in restricting lung cancer development, highlighting the mechanisms that limit malignant progression of BRAFV600E-initiated tumors. Cancer Res; 75(15); 3167–80. ©2015 AACR.

Oncogenic activation of the RTK–RAS–RAF–MEK–ERK pathway occurs in approximately 25% of all human cancers, yet activated RAS, BRAF, or MEK expression in primary cells leads to a prolonged and predominantly irreversible cell-cycle arrest termed oncogene-induced senescence (OIS). OIS acts as an intrinsic tumor suppressor mechanism, serving as a barrier to tumor progression. Screening a library of activated kinases and kinase-regulatory proteins we identified MOB3A, a Mps-one binder coactivator (MOB) protein family member, whose constitutive expression permits proliferation and suppresses senescence in response to oncogenic RAS and BRAF signals. MOB3A is one of seven human MOB genes, which are highly conserved from yeast to human and that function to activate the Hippo pathway kinases (MST/LATS) or NDR kinases through direct association. Here we show that within the MOB family of genes MOB3A and C are unique in their ability to allow primary cell proliferation in the face of sustained oncogene signaling. Unlike the canonical MOB1A/B proteins, MOB3A inhibits Hippo/MST/LATS signaling and constitutive MOB3A membrane localization phenocopies OIS bypass seen with elevated YAP expression. Moreover, inhibition of MOB3 family member expression results in decreased proliferation and tumor growth of cancer cell lines. Together these data identify MOB3A's role in bypass of oncogene induced senescence and its role as a Hippo pathway inhibitor. Implications: These results suggest that MOB3 targeting to re-engage the Hippo pathway, or direct targeting of YAP/TAZ, may be viable therapeutic strategies potential for RAS-pathway driven tumours.

Cellular senescence is characterized by a prolonged and predominantly irreversible cell-cycle arrest state, which is linked to loss of tissue function and aging in mammals. Moreover, in response to aberrant oncogenic signals such as those from oncogenic RAS or BRAF, senescence functions as an intrinsic tumor suppressor mechanism restraining tumor progression. In addition to this durable proliferative block, senescent cells adopt altered morphologies, transcriptional profiles, and metabolism, while often possessing unusual heterochromatin formation termed senescence-associated heterochromatic foci. To uncover genes that are required to permit proliferation in the face of sustained oncogene signaling, we conducted an shRNA-based genetic screen in primary cells expressing inducible BRAF. Here we show that depletion of a known glycolysis regulator, islet amylin polypeptide (IAPP also known as amylin), prevents RAS and BRAF oncogene-induced senescence (OIS) in human cells. Importantly, depletion of IAPP resulted in changes of the cells' metabolome and this metabolic reprogramming was associated with widespread alterations in chromatin modifications compared with senescent cells. Conversely, exogenous treatment of IAPP-depleted cells with amylin restored OIS. Together, our results demonstrate that the metabolic regulator IAPP is important regulator of OIS. Moreover, they suggest that IAPP analog treatment or activation of IAPP signaling in RAS/BRAF mutant tumors may have therapeutic potential through senescence induction. Implications: These findings demonstrate that IAPP is a novel metabolic regulator of oncogene-induced senescence and use of IAPP analogs may be therapeutically effective to restore growth arrest to BRAF and/or RAS mutant cancers.

Melanomas commonly undergo a phenotype switch, from a proliferative to an invasive state. Such tumor cell plasticity contributes to immunotherapy resistance; however, the mechanisms are not completely understood and thus are therapeutically unexploited. Using melanoma mouse models, we demonstrated that blocking the MNK1/2-eIF4E axis inhibited melanoma phenotype switching and sensitized melanoma to anti–PD-1 immunotherapy. We showed that phospho-eIF4E–deficient murine melanomas expressed high levels of melanocytic antigens, with similar results verified in patient melanomas. Mechanistically, we identified phospho-eIF4E–mediated translational control of NGFR, a critical effector of phenotype switching. Genetic ablation of phospho-eIF4E reprogrammed the immunosuppressive microenvironment, exemplified by lowered production of inflammatory factors, decreased PD-L1 expression on dendritic cells and myeloid-derived suppressor cells, and increased CD8(+) T cell infiltrates. Finally, dual blockade of the MNK1/2-eIF4E axis and the PD-1/PD-L1 immune checkpoint demonstrated efficacy in multiple melanoma models regardless of their genomic classification. An increase in the presence of intratumoral stem-like TCF1(+)PD-1(+)CD8(+) T cells, a characteristic essential for durable antitumor immunity, was detected in mice given a MNK1/2 inhibitor and anti–PD-1 therapy. Using MNK1/2 inhibitors to repress phospho-eIF4E thus offers a strategy to inhibit melanoma plasticity and improve response to anti–PD-1 immunotherapy.

Melanomas commonly undergo a phenotype switch, from a proliferative to an invasive state. Melanoma plasticity exhibited as phenotype switching contributes to immunotherapy resistance, however the mechanisms are not completely understood and thus therapeutically unexploited. Here, using a transgenic melanoma mouse model, we demonstrated a critical role of the MNK1/2-eIF4E axis in melanoma plasticity and resistance to immunotherapy. We showed that phospho-eIF4E deficient murine melanomas express high levels of melanocytic antigens, with similar results verified in patient melanomas. Mechanistically, we identified that phospho-eIF4E controls the translation of NGFR, a critical effector of phenotype switching. In patients with melanoma, the expression of MKNK1, the kinase for eIF4E, positively correlated with markers of immune exhaustion. Genetic ablation of phospho-eIF4E reprogrammed the immunosuppressive microenvironment, exemplified by lowered production of inflammatory factors and increased CD8+ T cell infiltrates. Blocking phospho-eIF4E, using MNK1/2 inhibitors, offers a new strategy to inhibit melanoma plasticity and improve the survival response to anti-PD-1 immunotherapy.

Lung cancer arises through the acquisition of a number of genetic lesions, with a preponderance of activating mutations in the canonical mitogen-activated protein kinase (MAPK) cascade (RTK-RAS-RAF-MEK). BrafV600E expression induces benign lung adenomas that fail to progress to adenocarcinoma because of oncogene-induced senescence (OIS). BrafV600E expression, coupled with simultaneous p53 ablation, permits bypass of senescence and progression to lung adenocarcinoma. However, spontaneous human tumors sustain mutations in a temporally separated manner. Here, we use a mouse lung cancer model where oncogene activation (BrafV600E expression) and tumor suppressor loss (p53 ablation) are independently controlled through the actions of Flp and Cre recombinase, respectively. We show that p53 loss before OIS is permissive for the transition from lung adenoma to adenocarcinoma. In contrast, p53 loss after senescence is established fails to enable escape from senescence and disease progression. This study demonstrates that BrafV600E induced senescence is irreversible in vivo and suggests that therapy-induced senescence would halt further tumor progression.

Introduction: Melanoma is the deadliest form of skin cancer. Melanoma phenotype switching is characterized by reduced expression of melanocyte lineage transcription factor MITF and its downstream targets, leading to increased invasiveness of melanoma cells and resistance to both targeted therapy and immunotherapy, and worse prognosis. In melanoma, MAPK and PI3K pathways ultimately converge upon eukaryotic translation initiation factor 4E (eIF4E) to induce its phosphorylation (p-eIF4E), which is critical for the oncogenicity of eIF4E. Here, we investigate the role of p-eIF4E in melanoma progression and tumor immunity. Methods: We crossed p-eIF4E deficient (eIF4EKI) mice with an inducible melanoma mouse model. We monitored the primary tumor outgrowth, metastasis, and survival of the eIF4EKI mice versus eIF4EWT mice. Melanoma samples were isolated for further investigation. Results: Compared to the eIF4EWT mice, eIF4EKI mice exhibit significantly delayed tumor growth, reduced metastasis, and increased survival. Increased expression of MITF and downstream melanoma antigens were observed in eIF4EKI tumors, suggesting a p-eIF4E-mediated phenotype switching. Cytokine array analysis revealed a novel proinvasive cytokine signature in eIF4EWT melanoma primary culture, further supporting a role of phospho-eIF4E in phenotype switching. The cytokine profiling also revealed a pro-myeloid-derived suppressor cell (MDSC) cytokine signature in the eIF4EWT tumor, indicating a p-eIF4E-linked immunosuppression. In support of the immune suppressive cytokine signature associated with phospho-eIF4E expressing melanomas, immune phenotyping of eIF4EWT melanomas showed a significant increase in MDSCs and less cytotoxic T cells, compared to eIF4EKI melanomas. Finally, pharmacologic inhibition of p-eIF4E in combination with anti-PD-1 immunotherapy results in a synergistic delay in primary tumor outgrowth and reduced metastasis. Conclusions: Here we showed that phosphorylation of eIF4E promotes melanoma phenotype switching, leading to increased invasiveness and reduced expression of tumor-associated antigens. Further, by increasing the secretion of pro-MDSC cytokines, p-eIF4E permits an immunosuppressive microenvironment. Pharmacologic inhibition of p-eIF4E sensitizes melanoma to anti-PD-1 immunotherapy, potentially by increasing melanoma antigen expression and compromising MDSC-mediated immunosuppression. This study provides a novel therapeutic approach for the treatment of melanoma. Citation Format: Fan Huang, Christophe Gonçalves, Qianyu Guo, Joelle Rémy-Sarrazin, Audrey Emond, William Yang, Dany Plourde, Margarita Bartish, Jie Su, Yao Zhan, Marina G. Gimeno, Elie Khoury, Alexandre Benoit, David Dankort, Wilson H. Miller, Sonia V. del Rincón. Phosphorylation of eIF4E promotes phenotype switching and MDSC-mediated immunosuppression in melanoma [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology and Immunotherapy; 2019 Nov 17-20; Boston, MA. Philadelphia (PA): AACR; Cancer Immunol Res 2020;8(3 Suppl):Abstract nr A53.

Elucidating gene function is heavily reliant on the ability to modulate gene expression in biological model systems. Although transient expression systems can provide useful information about the biological outcome resulting from short-term gene overexpression or silencing, methods providing stable integration of desired expression constructs (cDNA or RNA interference) are often preferred for functional studies. To this end, lentiviral vectors offer the ability to deliver long-term and regulated gene expression to mammalian cells, including the expression of gene targeting small hairpin RNAs (shRNAmirs). Unfortunately, constructing vectors containing the desired combination of cDNAs, markers, and shRNAmirs can be cumbersome and time-consuming if using traditional sequence based restriction enzyme and ligation-dependent methods. Here we describe the use of a recombination based Gateway cloning strategy to rapidly and efficiently produce recombinant lentiviral vectors for the expression of one or more cDNAs with or without simultaneous shRNAmir expression. Additionally, we describe a luciferase-based approach to rapidly triage shRNAs for knockdown efficacy and specificity without the need to create stable shRNAmir expressing cells.

Elucidating gene function is heavily reliant on the ability to modulate gene expression in biological model systems. Although transient expression systems can provide useful information about the biological outcome resulting from short-term gene overexpression or silencing, methods providing stable integration of desired expression constructs (cDNA or RNA interference) are often preferred for functional studies. To this end, lentiviral vectors offer the ability to deliver long-term and regulated gene expression to mammalian cells, including the expression of gene targeting small hairpin RNAs (shRNAmirs). Unfortunately, constructing vectors containing the desired combination of cDNAs, markers, and shRNAmirs can be cumbersome and time-consuming if using traditional sequence based restriction enzyme and ligation-dependent methods. Here we describe the use of a recombination based Gateway cloning strategy to rapidly and efficiently produce recombinant lentiviral vectors for the expression of one or more cDNAs with or without simultaneous shRNAmir expression. Additionally, we describe a luciferase-based approach to rapidly triage shRNAs for knockdown efficacy and specificity without the need to create stable shRNAmir expressing cells.

Undifferentiated pleomorphic sarcoma (UPS) is one of the most common soft tissue malignancies. Patients with large, high grade sarcomas often develop fatal lung metastases. Understanding the mechanisms underlying sarcoma metastasis are needed to improve treatment of these patients. Micro-RNAs (miRNAs) are a class of small RNAs that post-transcriptionally regulate gene expression. Global alterations in miRNAs are frequently observed in a number of disease states including cancer. The signaling pathways that regulate miRNA biogenesis are beginning to emerge. To test the relevance of specific oncogenic mutations on miRNA biogenesis in sarcoma, we used primary soft tissue sarcomas expressing either BrafV600E or KrasG12D. We find that BrafV600E mutant tumors, which have increased MAPK signaling, have higher levels of mature miRNAs and enhanced miRNA processing. To investigate the relevance of oncogene dependent alterations in miRNA biogenesis, we introduce conditional mutations in Dicer and show that Dicer haploinsufficiency promotes the development of distant metastases in an oncogene dependent manner. These results demonstrate that a specific oncogenic mutation can cooperate with mutation in Dicer to promote tumor progression in vivo.

KRAS mutation is a hallmark of pancreatic ductal adenocarcinoma (PDA) but remains an intractable pharmacologic target. Consequently, defining RAS effector pathway(s) required for PDA initiation and maintenance is critical to improve treatment of this disease. Here, we show that expression of BRAFV600E, but not PIK3CAH1047R, in the mouse pancreas leads to pancreatic intraepithelial neoplasia (PanIN) lesions. Moreover, concomitant expression of BRAFV600E and TP53R270H result in lethal PDA. We tested pharmacologic inhibitors of RAS effectors against multiple human PDA cell lines. Mitogen-activated protein (MAP)/extracellular signal–regulated (ERK) kinase (MEK) inhibition was highly effective both in vivo and in vitro and was synergistic with AKT inhibition in most cell lines tested. We show that RAF→MEK→ERK signaling is central to the initiation and maintenance of PDA and to rational combination strategies in this disease. These results emphasize the value of leveraging multiple complementary experimental systems to prioritize pathways for effective intervention strategies in PDA. Significance: PDA is difficult to treat, in large part, due to recurrent mutations in the KRAS gene. Here, we define rational treatment approaches for the disease achievable today with existing drug combinations by thorough genetic and pharmacologic dissection of the major KRAS effector pathways, RAF→MEK→ERK and phosphoinositide 3′-kinase (PI3′K)→AKT. Cancer Discov; 2(8); 685–93. ©2012 AACR. Read the Commentary on this article by Hanrahan et al., p. 666. This article is highlighted in the In This Issue feature, p. 653.

Human melanocytic nevi (moles) are benign lesions harboring activated oncogenes, including BRAF. Although this oncogene initially acts mitogenically, eventually, oncogene-induced senescence (OIS) ensues. Nevi can infrequently progress to melanomas, but the mechanistic relationship with OIS is unclear. We show here that PTEN depletion abrogates BRAFV600E-induced senescence in human fibroblasts and melanocytes. Correspondingly, in established murine BRAFV600E-driven nevi, acute shRNA-mediated depletion of PTEN prompted tumor progression. Furthermore, genetic analysis of laser-guided microdissected human contiguous nevus–melanoma specimens recurrently revealed identical mutations in BRAF or NRAS in adjacent benign and malignant melanocytes. The PI3K pathway was often activated through either decreased PTEN or increased AKT3 expression in melanomas relative to their adjacent nevi. Pharmacologic PI3K inhibition in melanoma cells suppressed proliferation and induced the senescence-associated tumor suppressor p15INK4B. This treatment also eliminated subpopulations resistant to targeted BRAFV600E inhibition. Our findings suggest that a significant proportion of melanomas arise from nevi. Furthermore, these results demonstrate that PI3K pathway activation serves as a rate-limiting event in this setting, acting at least in part by abrogating OIS. The reactivation of senescence features and elimination of cells refractory to BRAFV600E inhibition by PI3K inhibition warrants further investigation into the therapeutic potential of simultaneously targeting these pathways in melanoma.