Long-Term ERK Inhibition in KRAS-Mutant Pancreatic Cancer Is Associated with MYC Degradation and Senescence-like Growth Suppression

Synthetic Lethal Vulnerabilities in KRAS-Mutant Cancers

KRAS is the most commonly mutated oncogene in human cancer. Most KRAS-mutant cancers depend on sustained expression and signaling of KRAS, thus making it a high-priority therapeutic target. Unfortunately, development of direct small molecule inhibitors of KRAS function has been challenging. An alternative therapeutic strategy for KRAS-mutant malignancies involves targeting codependent vulnerabilities or synthetic lethal partners that are preferentially essential in the setting of oncogenic KRAS. KRAS activates numerous effector pathways that mediate proliferation and survival signals. Moreover, cancer cells must cope with substantial oncogenic stress conferred by mutant KRAS. These oncogenic signaling pathways and compensatory coping mechanisms of KRAS-mutant cancer cells form the basis for synthetic lethal interactions. Here, we review the compendium of previously identified codependencies in KRAS-mutant cancers, including the results of numerous functional genetic screens aimed at identifying KRAS synthetic lethal targets. Importantly, many of these vulnerabilities may represent tractable therapeutic opportunities.

Concurrent HER or PI3K Inhibition Potentiates the Antitumor Effect of the ERK Inhibitor Ulixertinib in Preclinical Pancreatic Cancer Models

Effective treatment for pancreatic ductal adenocarcinoma (PDAC) is an urgent, unmet medical need. Targeting KRAS, the oncogene that is present in >95% of PDAC, is a heavily pursued strategy, but remains unsuccessful in the clinic. Therefore, targeting key effector cascades of KRAS oncoprotein, particularly the mitogenic RAF-MEK-ERK pathway, represents the next best strategy. However, RAF or MEK inhibitors have failed to show clinical efficacy in PDAC. Several studies have shown that cancer cells treated with RAF or MEK inhibitors adopt multiple mechanisms to reactivate ERK signaling. Therefore, development of ERK-specific inhibitors carries the promise to effectively abrogate this pathway. Ulixertinib (or BVD-523) is a first-in-class ERK-specific inhibitor that has demonstrated promising antitumor activity in a phase I clinical trial for advanced solid tumors with NRAS and BRAF mutations, providing a strong rationale to test this inhibitor in PDAC. In this study, we show that ulixertinib effectively inhibits in vitro growth of multiple PDAC lines and potentiates the cytotoxic effect of gemcitabine. Moreover, we found that PDAC cells treated with ulixertinib upregulates the parallel PI3K-AKT pathway through activating the HER/ErbB family proteins. Concurrent inhibition of PI3K or HER proteins synergizes with ulixertinib in suppressing PDAC cell growth in vitro and in vivo Overall, our study provides the preclinical rationale for testing combinations of ulixertinib with chemotherapy or PI3K and HER inhibitors in PDAC patients. Mol Cancer Ther; 17(10); 2144-55. ©2018 AACR.

Advances in the Genetics and Biology of Pancreatic Cancer

Pancreatic ductal adenocarcinoma (PDA) remains one of the most devastating diagnoses in modern medicine. While the clinical management of the disease has improved, the complex biologic underpinnings of PDA enable both its aggressive nature and slow clinical translational progress. In this review, we provide an overview of the key features of PDA genetics and biology, highlighting translational challenges and providing a framework for improved diagnostic and therapeutic approaches.

MYC-induced metabolic stress and tumorigenesis

The MYC oncogene is commonly altered across human cancers. Distinct from the normal MYC proto-oncogene, which is under tight transcriptional, translational, and post-translational control, deregulated oncogenic MYC drives imbalanced, non-linear amplification of transcription that results in oncogenic 'stress.' The term 'stress' had been a euphemism for our lack of mechanistic understanding, but synthesis of many studies over the past decade provides a more coherent picture of oncogenic MYC driving metastable cellular states, particularly altered metabolism, that activate and depend on cellular stress response pathways to allow for continued growth and survival. Both deregulated metabolism and these stress response pathways represent vulnerabilities that can be exploited therapeutically.

E47 Governs the MYC-CDKN1B/p27KIP1-RB Network to Growth Arrest PDA Cells Independent of CDKN2A/p16INK4A and Wild-Type p53

BACKGROUND & AIMS

Oncogenic mutations in KRAS, coupled with inactivation of p53, CDKN2A/p16INK4A, and SMAD4, drive progression of pancreatic ductal adenocarcinoma (PDA). Overexpression of MYC and deregulation of retinoblastoma (RB) further promote cell proliferation and make identifying a means to therapeutically alter cell-cycle control pathways in PDA a significant challenge. We previously showed that the basic helix-loop-helix transcription factor E47 induced stable growth arrest in PDA cells in vitro and in vivo. Here, we identified molecular mechanisms that underlie E47-induced growth arrest in low-passage, patient-derived primary and established PDA cell lines.

METHODS

RNA sequencing was used to profile E47-dependent transcriptomes in 5 PDA cell lines. Gene Ontology analysis identified cell-cycle control as the most altered pathway. Small interfering RNA/short hairpin RNA knockdown, small-molecule inhibitors, and viral expression were used to examine the function of E47-dependent genes in cell-cycle arrest. Cell morphology, expression of molecular markers, and senescence-associated β-galactosidase activity assays identified cellular senescence.

RESULTS

E47 uniformly inhibited PDA cell-cycle progression by decreasing expression of MYC, increasing the level of CDKN1B/p27KIP1, and restoring RB tumor-suppressor function. The molecular mechanisms by which E47 elicited these changes included altering both RNA transcript levels and protein stability of MYC and CDKN1B/p27KIP1. At the cellular level, E47 elicited a senescence-like phenotype characterized by increased senescence-associated β-galactosidase activity and altered expression of senescence markers.

CONCLUSIONS

E47 governs a highly conserved network of cell-cycle control genes, including MYC, CDKN1B/p27KIP1, and RB, which can induce a senescence-like program in PDA cells that lack CDKN2A/p16INK4A and wild-type p53. RNA sequencing data are available at the National Center for Biotechnology Information GEO at https://www.ncbi.nlm.nih.gov/geo/; accession number: GSE100327.

The plasticity of pancreatic cancer metabolism in tumor progression and therapeutic resistance

Pancreatic ductal adenocarcinoma (PDA) is an aggressive cancer that is highly refractory to the current standards of care. The difficulty in treating this disease is due to a number of different factors, including altered metabolism. In PDA, the metabolic rewiring favors anabolic reactions which supply the cancer cell with necessary cellular building blocks for unconstrained growth. Furthermore, PDA cells display high levels of basal autophagy and macropinocytosis. KRAS is the driving oncogene in PDA and many of the metabolic changes are downstream of its activation. Together, these unique pathways for nutrient utilization and acquisition result in metabolic plasticity enabling cells to rapidly adapt to nutrient and oxygen fluctuations. This remarkable adaptability has been implicated as a cause of the intense therapeutic resistance. In this review, we discuss metabolic pathways in PDA tumors and highlight how they contribute to the pathogenesis and treatment of the disease.

Targeting ERK enhances the cytotoxic effect of the novel PI3K and mTOR dual inhibitor VS-5584 in preclinical models of pancreatic cancer

Pancreatic ductal adenocarcinoma (PDAC) is a deadly disease in urgent need of newer therapeutic modalities. Majority of patients with PDAC have mutations in KRAS, which unfortunately remains an ineffectual target. Our strategy here is to target KRAS downstream effectors PI3K and mTOR. In this study, we investigated the antitumor efficacy of the novel PI3K and mTOR dual inhibitor VS-5584 in PDAC. Our data shows that PI3K/mTOR dual inhibition causes ERK activation in all tested PDAC cell lines. Although the MEK inhibitor GSK1120212 could abrogate VS-5584-induced ERK activation, it did not substantially enhance cell death in all the cell lines tested. However, combination with ERK inhibitor SCH772984 not only mitigated VS-5584-induced ERK activation but also enhanced VS-5584-induced cell death. In a xenograft model of PDAC, we observed 28% and 44% tumor inhibition for individual treatment with VS-5584 and SCH772984, respectively, while the combined treatment showed superior tumor inhibition (80%) compared to vehicle control treatment. Our findings support the clinical development of VS-5584 and ERK inhibitor combination for PDAC treatment.

Phosphoinositide 3-Kinase Signaling Pathway in Pancreatic Ductal Adenocarcinoma Progression, Pathogenesis, and Therapeutics

Pancreatic ductal adenocarcinoma (PDAC) is a highly aggressive malignancy characterized by its sudden manifestation, rapid progression, poor prognosis, and limited therapeutic options. Genetic alterations in key signaling pathways found in early pancreatic lesions are pivotal for the development and progression of pancreatic intraepithelial neoplastic lesions into invasive carcinomas. More than 90% of PDAC tumors harbor driver mutations in K-Ras that activate various downstream effector-signaling pathways, including the phosphoinositide-3-kinase (PI3K) pathway. The PI3K pathway also responds to stimuli from various growth factor receptors present on the cancer cell surface that, in turn, modulate downstream signaling cascades. Thus, the inositide signaling acts as a central node in the complex cellular signaling networks to impact cancer cell growth, motility, metabolism, and survival. Also, recent publications highlight the importance of PI3K signaling in stromal cells, whereby PI3K signaling modifies the tumor microenvironment to dictate disease outcome. The high incidence of mutations in the PI3K signaling cascade, accompanied by activation of parallel signaling pathways, makes PI3K a promising candidate for drug therapy. In this review, we describe the role of PI3K signaling in pancreatic cancer development and progression. We also discuss the crosstalk between PI3K and other major cellular signaling cascades, and potential therapeutic opportunities for targeting pancreatic ductal adenocarcinoma.

Inhibition of mTORC2 component RICTOR impairs tumor growth in pancreatic cancer models

Mammalian Target of Rapamycin complex 2 (mTORC2) and its regulatory component Rapamycin-insensitive companion of mTOR (RICTOR) are increasingly recognized as important players in human cancer development and progression. However, the role of RICTOR in human pancreatic ductal adenocarcinoma (PDAC) is unclear so far. Here, we sought to analyze the effects of RICTOR inhibition in human pancreatic cancer cell lines in vitro and in vivo. Furthermore, RICTOR expression was determined in human PDAC samples. Results demonstrate that depletion of RICTOR with siRNA (transient knock-down) or shRNA (stable knock-down) has an inhibitory effect on tumor growth in vitro. Moreover, RICTOR inhibition led to impaired phosphorylation/activity of AGC kinases (AKT, SGK1). Interestingly, hypoxia-induced expression of hypoxia-induced factor-1α (HIF-1α) was diminished and secretion of vascular-endothelial growth factor-A (VEGF-A) was impaired upon targeting RICTOR. Stable RICTOR knock-down led to significant inhibition of tumor growth in subcutaneous and orthotopic tumor models which was accompanied by significant reduction of tumor cell proliferation. Finally, immunohistochemical analyses of 85 human PDAC samples revealed significantly poorer survival in patients with higher RICTOR expression. In conclusion, these findings provide first evidence for mTORC2/RICTOR as an attractive novel target for treatment of human PDAC.

Deletion of sphingosine kinase 1 inhibits liver tumorigenesis in diethylnitrosamine-treated mice

Primary liver cancer is the 3rd leading cause of cancer deaths worldwide with very few effective treatments. Sphingosine kinase 1 (SphK1), a key regulator of sphingolipid metabolites, is over-expressed in human hepatocellular carcinoma (HCC) and our previous studies have shown that SphK1 is important in liver injury. We aimed to explore the role of SphK1 specifically in liver tumorigenesis using the SphK1 knockout (SphK1^-/-) mouse. SphK1 deletion significantly reduced the number and the size of DEN-induced liver cancers in mice. Mechanistically, fewer proliferating but more apoptotic and senescent cells were detected in SphK1 deficient tumors compared to WT tumors. There was an increase in sphingosine rather than a decrease in sphingosine 1-phosphate (S1P) in SphK1 deficient tumors. Furthermore, the STAT3-S1PR pathway that has been reported previously to mediate the effect of SphK1 on colorectal cancers was not altered by SphK1 deletion in liver cancer. Instead, c-Myc protein expression was down-regulated by SphK1 deletion. In conclusion, this is the first in vivo evidence that SphK1 contributes to hepatocarcinogenesis. However, the downstream signaling pathways impacting on the development of HCC via SphK1 are organ specific providing further evidence that simply transferring known oncogenic molecular pathway targeting into HCC is not always valid.

Development of MK-8353, an orally administered ERK1/2 inhibitor, in patients with advanced solid tumors

BACKGROUND

Constitutive activation of ERK1/2 occurs in various cancers, and its reactivation is a well-described resistance mechanism to MAPK inhibitors. ERK inhibitors may overcome the limitations of MAPK inhibitor blockade. The dual mechanism inhibitor SCH772984 has shown promising preclinical activity across various BRAFV600/RAS-mutant cancer cell lines and human cancer xenografts.

METHODS

We have developed an orally bioavailable ERK inhibitor, MK-8353; conducted preclinical studies to demonstrate activity, pharmacodynamic endpoints, dosing, and schedule; completed a study in healthy volunteers (P07652); and subsequently performed a phase I clinical trial in patients with advanced solid tumors (MK-8353-001). In the P07652 study, MK-8353 was administered as a single dose in 10- to 400-mg dose cohorts, whereas in the MK-8353-001 study, MK-8353 was administered in 100- to 800-mg dose cohorts orally twice daily. Safety, tolerability, pharmacokinetics, pharmacodynamics, and antitumor activity were analyzed.

RESULTS

MK-8353 exhibited comparable potency with SCH772984 across various preclinical cancer models. Forty-eight patients were enrolled in the P07652 study, and twenty-six patients were enrolled in the MK-8353-001 study. Adverse events included diarrhea (44%), fatigue (40%), nausea (32%), and rash (28%). Dose-limiting toxicity was observed in the 400-mg and 800-mg dose cohorts. Sufficient exposure to MK-8353 was noted that correlated with biological activity in preclinical data. Three of fifteen patients evaluable for treatment response in the MK-8353-001 study had partial response, all with BRAFV600-mutant melanomas.

CONCLUSION

MK-8353 was well tolerated up to 400 mg twice daily and exhibited antitumor activity in patients with BRAFV600-mutant melanoma. However, antitumor activity was not particularly correlated with pharmacodynamic parameters.

TRIAL REGISTRATION

ClinicalTrials.gov NCT01358331.

FUNDING

Merck Sharp & Dohme Corp., a subsidiary of Merck & Co. Inc., and NIH (P01 CA168585 and R35 CA197633).

β-Catenin mRNA Silencing and MEK Inhibition Display Synergistic Efficacy in Preclinical Tumor Models

Colorectal carcinomas harbor well-defined genetic abnormalities, including aberrant activation of Wnt/β-catenin and MAPK pathways, often simultaneously. Although the MAPK pathway can be targeted using potent small-molecule drugs, including BRAF and MEK inhibitors, β-catenin inhibition has been historically challenging. RNAi approaches have advanced to the stage of clinical viability and are especially well suited for transcriptional modulators, such as β-catenin. In this study, we report therapeutic effects of combined targeting of these pathways with pharmacologic agents. Using a recently described tumor-selective nanoparticle containing a β-catenin-targeting RNAi trigger, in combination with the FDA-approved MEK inhibitor (MEKi) trametinib, we demonstrate synergistic tumor growth inhibition in in vivo models of colorectal cancer, melanoma, and hepatocellular carcinoma. At dose levels that were insufficient to significantly impact tumor growth as monotherapies, combination regimens resulted in synergistic efficacy and complete tumor growth inhibition. Importantly, dual MEKi/RNAi therapy dramatically improved survival of mice bearing colorectal cancer liver metastases. In addition, pharmacologic silencing of β-catenin mRNA was effective against tumors that are inherently resistant or that acquire drug-induced resistance to trametinib. These results provide a strong rationale for clinical evaluation of this dual-targeting approach for cancers harboring Wnt/β-catenin and MAPK pathway mutations. Mol Cancer Ther; 17(2); 544-53. ©2017 AACR.

KRAS promotes tumor metastasis and chemoresistance by repressing RKIP via the MAPK-ERK pathway in pancreatic cancer

Oncogenic KRAS plays a crucial role in pancreatic ductal adenocarcinoma (PDAC) development and progression. However, the mechanism has not been clearly elucidated. RKIP is a tumor repressor, and loss of RKIP has been shown in PDAC. Here, we found that KRAS expression was inversely correlated with RKIP expression in PDAC fresh tissue regardless of the KRAS mutant status. The negative correlation between KRAS and RKIP was further confirmed in our PDAC tissue microarray. KRAS overexpression and RKIP downregulation were associated with poor clinical outcomes. Knockdown or overexpression of KRAS in PDAC cell lines robustly increased or decreased, respectively, RKIP protein and mRNA levels. Furthermore, the MAPK-ERK pathway was involved in the regulation of RKIP. KRAS-regulated RKIP expression, which in turn affected the expression of pivotal epithelial-mesenchymal transition (EMT) and apoptosis factors. The biological function of the KRAS-RKIP axis was demonstrated in human pancreatic cancer cells in vitro and in vivo. KRAS knockdown increased RKIP expression and inhibited metastasis and chemoresistance. Moreover, the feature of metastasis and chemoresistance was rescued in the KRAS-knockdown cells through the inhibition of RKIP by RNA interference. In conclusion, our studies demonstrate how KRAS inhibits the tumor suppressor RKIP, thus offering novel justification for targeting RKIP as a strategy to overcome KRAS-induced tumor metastasis and chemoresistance in PDAC.

Periostin promotes tumor angiogenesis in pancreatic cancer via Erk/VEGF signaling

Pancreatic cancer (PaC) consists of a bulk of stroma cells which contribute to tumor progression by releasing angiogenic factors. Recent studies have found that periostin (POSTN) is closely associate with the metastatic potential and prognosis of PaC. The purpose of this study is to determine the role of POSTN in tumor angiogenesis and explore the precise mechanisms. In this study, we used lentiviral shRNA and human recombinant POSTN protein (rPOSTN) to negatively and positively regulate POSTN expression in vitro. We found that increased POSTN expression promoted the tubule formation dependent on human umbilical vein endothelial cells (HUVECs). Moreover, knockdown of POSTN in PaC cells reduced tumor growth and VEGF expression in vivo. In accordance with these observations, we found that Erk phosphorylation and its downstream VEGF expression were upregulated achieved in rPOSTN-treated groups, opposing results were obversed in POSTN-slienced group. Meanwhile, Erk inhibitor SCH772984 significantly decreased VEGF expression as well as tubule formation of HUVECs in rPOSTN-treated PaC cells. Taken together, these findings suggest that POSTN promotes tumor angiogenesis via Erk/VEGF signaling in PaC and POSTN may be a new target for cancer anti-vascular treatment.

Oncogenic RAS-induced downregulation of ATG12 is required for survival of malignant intestinal epithelial cells

Activating mutations of RAS GTPase contribute to the progression of many cancers, including colorectal carcinoma. So far, attempts to develop treatments of mutant RAS-carrying cancers have been unsuccessful due to insufficient understanding of the salient mechanisms of RAS signaling. We found that RAS downregulates the protein ATG12 in colon cancer cells. ATG12 is a mediator of autophagy, a process of degradation and reutilization of cellular components. In addition, ATG12 can kill cells via autophagy-independent mechanisms. We established that RAS reduces ATG12 levels in cancer cells by accelerating its proteasomal degradation. We further observed that RAS-dependent ATG12 loss in these cells is mediated by protein kinases MAP2K/MEK and MAPK1/ERK2-MAPK3/ERK1, known effectors of RAS. We also demonstrated that the reversal of the effect of RAS on ATG12 achieved by the expression of exogenous ATG12 in cancer cells triggers both apoptotic and nonapoptotic signals and efficiently kills the cells. ATG12 is known to promote autophagy by forming covalent complexes with other autophagy mediators, such as ATG5. We found that the ability of ATG12 to kill oncogenic RAS-carrying malignant cells does not require covalent binding of ATG12 to other proteins. In summary, we have identified a novel mechanism by which oncogenic RAS promotes survival of malignant intestinal epithelial cells. This mechanism is driven by RAS-dependent loss of ATG12 in these cells.

Nrf2 promotes mutant K-ras/p53-driven pancreatic carcinogenesis

The Keap1-Nrf2 system contributes to the maintenance of homeostasis by regulating oxidative stress responses in normal tissues and organs, and is exploited in various cancers for proliferation, survival and acquisition of therapy resistance. Pancreatic cancer remains one of the intractable cancers, despite the improved clinical outcomes of other types of cancer, due to its invasive and refractory nature to therapeutic intervention. The current study aimed to clarify the contribution of Nrf2 to pancreatic carcinogenesis using a pancreas-specific mutant K-ras and p53 (KPC) mouse model. Deletion of Nrf2 in KPC mice (KPCN) decreased the formation of precancerous lesions as well as the development of invasive pancreatic cancer. The pancreatic tumor-derived cancer cell lines from KPCN mouse showed decreased expression of glutathione S-transferases (GST), UDP glucuronosyltransferases (UGT) and ABC transporters. Along with these biochemical changes, cell lines from KPCN mice revealed increased sensitivity to oxidative stress and chemotherapeutic agent. The current study revealed that Nrf2 contributes to pancreatic carcinogenesis in a way distinct from the chemoresistance of lung and esophagus, and that Nrf2 could be a novel therapeutic target of pancreatic cancer.

Non-canonical PI3K-Cdc42-Pak-Mek-Erk Signaling Promotes Immune-Complex-Induced Apoptosis in Human Neutrophils

Neutrophils are peripheral blood leukocytes that represent the first line of immune cell defense against bacterial and fungal infections but are also crucial players in the generation of the inflammatory response. Many neutrophil cell surface receptors regulate important cellular processes via activation of agonist-activated PI3Ks. We show here that activation of human neutrophils with insoluble immune complexes drives a previously uncharacterized, PI3K-dependent, non-canonical, pro-apoptotic signaling pathway, FcγR-PI3Kβ/δ-Cdc42-Pak-Mek-Erk. This is a rare demonstration of Ras/Raf-independent activation of Erk and of PI3K-mediated activation of Cdc42. In addition, comparative analysis of immune-complex- and fMLF-induced signaling uncovers key differences in pathways used by human and murine neutrophils. The non-canonical pathway we identify in this study may be important for the resolution of inflammation in chronic inflammatory diseases that rely on immune-complex-driven neutrophil activation.

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Immune-complex-activated human neutrophils use PI3Kβ/δ-Cdc42-Pak-Mek-Erk signaling

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Immune-complex-induced non-canonical neutrophil signaling is pro-apoptotic

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Other immune-complex-induced neutrophil functions depend on alternative PI3K effectors

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Immune-complex-induced PI3K signaling is not conserved between humans and mice

Uncoupling the Oncogenic Engine

Inhibition of oncogenic signaling and correction of aberrant metabolic processes may be key paradigms to eliminate cancer cells. The high incidence of activating RAS mutations and hyperactivated ERK1/2 signaling observed in many human tumors and the lack of effective targeted therapies to elicit long-term inhibition of the RAS-ERK1/2 signaling pathway add to the importance of discovering novel strategies to treat malignancies characterized by elevated RAS-ERK1/2 signaling. In this review, we describe connections between oncogenic signaling and cancer cell metabolism and how these links may be exploited for novel modern molecular medicine approaches. Cancer Res; 77(22); 6060-4. ©2017 AACR.

Cucurbitacin B and SCH772984 exhibit synergistic anti-pancreatic cancer activities by suppressing EGFR, PI3K/Akt/mTOR, STAT3 and ERK signaling

Cucurbitacin B (CuB) is a natural tetracyclic triterpene product and displays antitumor activity across a wide array of cancers. In this study, we explored the anti-pancreatic cancer activity of CuB alone and in combination with SCH772984, an ERK inhibitor, in vitro and in vivo. CuB inhibited proliferation of pancreatic cancer cells by arresting them in the G2/M cell cycle phase. This was associated with inhibition of EGFR expression and activity and downstream signaling, including PI3K/Akt/mTOR and STAT3. Interestingly, ERK activity was markedly enhanced by activating AMPK signaling after 12 h of CuB treatment. SCH772984 potentiates the cytotoxic effect of CuB on pancreatic cancer cells through complementary inhibition of EGFR, PI3K/Akt/mTOR, STAT3 and ERK signaling, followed by an increase in the pro-apoptotic protein Bim and a decrease in the anti-apoptotic proteins Mcl-1, Bcl-2, Bcl-xl and survivin. Furthermore, combined therapy with CuB and SCH772984 resulted in highly significant growth inhibition of pancreatic cancer xenografts. These results may provide a basis for further development of combining CuB and ERK inhibitors to treat pancreatic cancer.

Evaluation of the selectivity and sensitivity of isoform- and mutation-specific RAS antibodies

There is intense interest in developing therapeutic strategies for RAS proteins, the most frequently mutated oncoprotein family in cancer. Development of effective anti-RAS therapies will be aided by the greater appreciation of RAS isoform-specific differences in signaling events that support neoplastic cell growth. However, critical issues that require resolution to facilitate the success of these efforts remain. In particular, the use of well-validated anti-RAS antibodies is essential for accurate interpretation of experimental data. We evaluated 22 commercially available anti-RAS antibodies with a set of distinct reagents and cell lines for their specificity and selectivity in recognizing the intended RAS isoforms and mutants. Reliability varied substantially. For example, we found that some pan- or isoform-selective anti-RAS antibodies did not adequately recognize their intended target or showed greater selectivity for another; some were valid for detecting G12D and G12V mutant RAS proteins in Western blotting, but none were valid for immunofluorescence or immunohistochemical analyses; and some antibodies recognized nonspecific bands in lysates from "Rasless" cells expressing the oncoprotein BRAFV600E Using our validated antibodies, we identified RAS isoform-specific siRNAs and shRNAs. Our results may help to ensure the accurate interpretation of future RAS studies.

Targeting the MAPK Signaling Pathway in Cancer: Promising Preclinical Activity with the Novel Selective ERK1/2 Inhibitor BVD-523 (Ulixertinib)

Aberrant activation of signaling through the RAS-RAF-MEK-ERK (MAPK) pathway is implicated in numerous cancers, making it an attractive therapeutic target. Although BRAF and MEK-targeted combination therapy has demonstrated significant benefit beyond single-agent options, the majority of patients develop resistance and disease progression after approximately 12 months. Reactivation of ERK signaling is a common driver of resistance in this setting. Here we report the discovery of BVD-523 (ulixertinib), a novel, reversible, ATP-competitive ERK1/2 inhibitor with high potency and ERK1/2 selectivity. In vitro BVD-523 treatment resulted in reduced proliferation and enhanced caspase activity in sensitive cells. Interestingly, BVD-523 inhibited phosphorylation of target substrates despite increased phosphorylation of ERK1/2. In in vivo xenograft studies, BVD-523 showed dose-dependent growth inhibition and tumor regression. BVD-523 yielded synergistic antiproliferative effects in a BRAF^V600E-mutant melanoma cell line xenograft model when used in combination with BRAF inhibition. Antitumor activity was also demonstrated in in vitro and in vivo models of acquired resistance to single-agent and combination BRAF/MEK-targeted therapy. On the basis of these promising results, these studies demonstrate BVD-523 holds promise as a treatment for ERK-dependent cancers, including those whose tumors have acquired resistance to other treatments targeting upstream nodes of the MAPK pathway. Assessment of BVD-523 in clinical trials is underway (NCT01781429, NCT02296242, and NCT02608229). Mol Cancer Ther; 16(11); 2351-63. ©2017 AACR.

KRAS-dependent suppression of MYC enhances the sensitivity of cancer cells to cytotoxic agents

KRAS is the most commonly mutated oncogene, frequently associated with some of the deadliest forms of cancer. However, the need for potent and specific KRAS inhibitors remains unmet. Here, we evaluated the effects of selected cytotoxic agents on oncogenic KRAS signaling and drug response. The data provided new insights into the functional interaction between the KRAS and MYC pathways and revealed key differences between WT and mutant KRAS expressing cells. Systematic investigation of non-small cell lung cancer cell lines revealed that KRAS mutation can paradoxically increase the sensitivity of cells to cytotoxic agents. We identify MYC as a key regulator of the cellular stress responses and tumor cell viability as MYC expression was suppressed in drug-sensitive but not resistant cells. Furthermore, this suppression was driven by hyperactive KRAS/MAPK signaling. Our findings support a direct link between MYC and cancer cell viability, and raise the possibility that inactivation of MYC may be an effective therapeutic strategy for KRAS mutant tumors across various cancer types.

Targeting macrophages for cancer therapy disrupts bone homeostasis and impairs bone marrow erythropoiesis in mice bearing Lewis lung carcinoma tumors

Macrophages are represented in all tissues by phenotypically distinct resident populations that show great functional diversity. Macrophages generally play a protumoral role, and they are attractive targets for cancer therapy. In this study, we found that CD169⁺ macrophages depletion inhibited the growth of established Lewis lung carcinoma tumors in mice. Benefits must be weighed against potential adverse effects in cancer therapy. Here, we investigated the adverse effects of CD169⁺ macrophages depletion on bone and bone marrow in mice bearing Lewis lung carcinoma tumors. Our studies showed that depletion of CD169⁺ macrophages in LLC tumor-bearing mice disrupted bone homeostasis, including bone weight loss and bone mineral density decrease. Further studies revealed that bone marrow erythropoiesis was severely impaired after depletion of CD169⁺ macrophages in LLC tumor-bearing mice. Our findings suggest that depletion of macrophages for cancer therapy may be associated with potential adverse effects that need to be recognized, prevented, and optimally managed.

Phosphorylation of the C-Raf N Region Promotes Raf Dimerization

The activation of Raf kinases by the small GTPase Ras requires two major sets of phosphorylations. One set lies within the activation loop, and the other lies within the N-terminal acidic region (N region). In the most abundant isoform of Raf, C-Raf, N-region phosphorylations occur on serine 338 (S338) and tyrosine 341 (Y341) and are thought to provide allosteric activation of the Raf dimer. We show that the phosphorylations of these N-region sites does not require C-Raf dimerization, but rather, they precede dimerization. One of these phosphorylations (phospho-Y341) is required for C-Raf dimerization, and this action can be replicated by phosphomimetic mutants both in vivo and in vitro The role of the phosphorylation of Y341 in promoting Raf dimerization is distinct from its well-known function in facilitating S338 phosphorylation. In Ras mutant pancreatic cancer cell lines, the phosphorylation and dimerization of C-Raf are basally elevated. Dimerization is thought to contribute to their elevated growth rate through their activation of the mitogen-activated protein (MAP) kinase (extracellular signal-regulated kinase [ERK]) signaling cascade. Blocking the tyrosine phosphorylation of C-Raf with Src family inhibitors blocks growth, basal dimerization, and ERK activation in these cells. We suggest that the kinases mediating C-Raf Y341 phosphorylation are potential candidate drug targets in selected Ras-dependent cancers.

STK38L kinase ablation promotes loss of cell viability in a subset of KRAS-dependent pancreatic cancer cell lines

Pancreatic ductal adenocarcinomas (PDACs) are highly aggressive malignancies, associated with poor clinical prognosis and limited therapeutic options. Oncogenic KRAS mutations are found in over 90% of PDACs, playing a central role in tumor progression. Global gene expression profiling of PDAC reveals 3-4 major molecular subtypes with distinct phenotypic traits and pharmacological vulnerabilities, including variations in oncogenic KRAS pathway dependencies. PDAC cell lines of the aberrantly differentiated endocrine exocrine (ADEX) subtype are robustly KRAS-dependent for survival. The KRAS gene is located on chromosome 12p11-12p12, a region amplified in 5-10% of primary PDACs. Within this amplicon, we identified co-amplification of KRAS with the STK38L gene in a subset of primary human PDACs and PDAC cell lines. Therefore, we determined whether PDAC cell lines are dependent on STK38L expression for proliferation and viability. STK38L encodes a serine/threonine kinase, which shares homology with Hippo pathway kinases LATS1/2. We show that STK38L expression is elevated in a subset of primary PDACs and PDAC cell lines displaying ADEX subtype characteristics, including overexpression of mutant KRAS. RNAi-mediated depletion of STK38L in a subset of ADEX subtype cell lines inhibits cellular proliferation and induces apoptosis. Concomitant with these effects, STK38L depletion causes increased expression of the LATS2 kinase and the cell cycle regulator p21. LATS2 depletion partially rescues the cytostatic and cytotoxic effects of STK38L depletion. Lastly, high STK38L mRNA expression is associated with decreased overall patient survival in PDACs. Collectively, our findings implicate STK38L as a candidate targetable vulnerability in a subset of molecularly-defined PDACs.

A screen for inducers of bHLH activity identifies pitavastatin as a regulator of p21, Rb phosphorylation and E2F target gene expression in pancreatic cancer

The average survival for patients with Pancreatic Ductal Adenocarcinoma (PDA) is merely 6 months, underscoring the need for new therapeutic approaches. During PDA progression, pancreatic acinar cells lose activity of the ClassI/II bHLH factors that regulate quiescence. We previously found that promoting transcriptional activity of the Class I bHLH factor E47 in highly aggressive PDA cells induced stable growth arrest in vitro and in vivo. To translate these findings for clinical utility, we developed a high throughput screening platform to identify small molecule inducers of Class I/II bHLH activity. A screen of 4,375 known drugs identified 70 bHLH activators. Prominent among the hits were members of the statin class of HMG-CoA reductase inhibitors, cholesterol lowering drugs that are also being evaluated in cancer. Studies with pitavastatin in primary patient derived tumor cells and established PDA lines, revealed dose dependent growth inhibition. At the molecular level, pitavastatin induced expression of the cyclin dependent kinase (CDK) inhibitor p21 in a cholesterol independent manner, blocked repressive phosphorylation of the Retinoblastoma tumor suppressor protein at CDK targeted sites, and reduced expression of E2F target genes required for progression through the G1/S boundary. Together, the data provide new insight into mechanisms by which statins constrain proliferation in cancer and establish the effectiveness of a novel screening platform to identify small molecules of clinical relevance in pancreatic cancer.

Fructose-1,6-bisphosphatase Inhibits ERK Activation and Bypasses Gemcitabine Resistance in Pancreatic Cancer by Blocking IQGAP1-MAPK Interaction

Dysregulation of the MAPK pathway correlates with progression of pancreatic ductal adenocarcinoma (PDAC) progression. IQ motif containing GTPase-activating protein 1 (IQGAP1) is a MAPK scaffold that directly regulates the activation of RAF, MEK, and ERK. Fructose-1,6-bisphosphatase (FBP1), a key enzyme in gluconeogenesis, is transcriptionally downregulated in various cancers, including PDAC. Here, we demonstrate that FBP1 acts as a negative modulator of the IQGAP1-MAPK signaling axis in PDAC cells. FBP1 binding to the WW domain of IQGAP1 impeded IQGAP1-dependent ERK1/2 phosphorylation (pERK1/2) in a manner independent of FBP1 enzymatic activity. Conversely, decreased FBP1 expression induced pERK1/2 levels in PDAC cell lines and correlated with increased pERK1/2 levels in patient specimens. Treatment with gemcitabine caused undesirable activation of ERK1/2 in PDAC cells, but cotreatment with the FBP1-derived small peptide inhibitor FBP1 E4 overcame gemcitabine-induced ERK activation, thereby increasing the anticancer efficacy of gemcitabine in PDAC. These findings identify a primary mechanism of resistance of PDAC to standard therapy and suggest that the FBP1-IQGAP1-ERK1/2 signaling axis can be targeted for effective treatment of PDAC. Cancer Res; 77(16); 4328-41. ©2017 AACR.

Signaling pathway profiling using reverse-phase protein array and its clinical applications

INTRODUCTION

Increased accessibility to next-generation sequencing within the last decade has led to a paradigm shift in cancer treatment from one-size-fits-all medicine to precision medicine providing therapeutic strategies tailored to the requirements of individual patients. However, the effect of even the most successful agent yet tested is only transient, and durable efficacy has yet to be achieved. Genome- and transcriptome-based approaches cannot fully predict the diversity of protein expression patterns or post-translational modifications that directly contribute to cancer pathogenesis and physiology. This underscores the need for concordant proteomic analysis in the next stage of precision medicine. Areas covered: This review begins with an overview of the recent advances and trends in precision medicine that currently rely on genomics, and highlights the utility of antibody-based reverse-phase protein array (RPPA) technology as a proteomic tool in this context. Expert commentary: RPPA is well suited for pharmacodynamics analysis in view of its ability to precisely map signaling status using limited amounts of clinical samples. In addition, the cost-effectiveness and rapid turn-around time of the RPPA platform offer a substantial advantage over existing molecular profiling technologies in clinical settings.

The G protein-coupled receptor GPR31 promotes membrane association of KRAS

Mutant KRAS drives oncogenesis when associated with the plasma membrane. Fehrenbacher et al. identify GPR31, a G protein–coupled receptor, as a secretory pathway chaperone that guides the KRAS protein to the plasma membrane.

The product of the KRAS oncogene, KRAS4B, promotes tumor growth when associated with the plasma membrane (PM). PM association is mediated, in part, by farnesylation of KRAS4B, but trafficking of nascent KRAS4B to the PM is incompletely understood. We performed a genome-wide screen to identify genes required for KRAS4B membrane association and identified a G protein–coupled receptor, GPR31. GPR31 associated with KRAS4B on cellular membranes in a farnesylation-dependent fashion, and retention of GPR31 on the endoplasmic reticulum inhibited delivery of KRAS4B to the PM. Silencing of GPR31 expression partially mislocalized KRAS4B, slowed the growth of KRAS-dependent tumor cells, and blocked KRAS-stimulated macropinocytosis. Our data suggest that GPR31 acts as a secretory pathway chaperone for KRAS4B.

The Atypical Kinase RIOK1 Promotes Tumor Growth and Invasive Behavior

Despite being overexpressed in different tumor entities, RIO kinases are hardly characterized in mammalian cells. We investigated the role of these atypical kinases in different cancer cells. Using isogenic colon-, breast- and lung cancer cell lines, we demonstrate that knockdown of RIOK1, but not of RIOK2 or RIOK3, strongly impairs proliferation and invasiveness in conventional and 3D culture systems. Interestingly, these effects were mainly observed in RAS mutant cancer cells. In contrast, growth of RAS wildtype Caco-2 and Bcr-Abl-driven K562 cells is not affected by RIOK1 knockdown, suggesting a specific requirement for RIOK1 in the context of oncogenic RAS signaling. Furthermore, we show that RIOK1 activates NF-κB signaling and promotes cell cycle progression. Using proteomics, we identified the pro-invasive proteins Metadherin and Stathmin1 to be regulated by RIOK1. Additionally, we demonstrate that RIOK1 promotes lung colonization in vivo and that RIOK1 is overexpressed in different subtypes of human lung- and breast cancer. Altogether, our data suggest RIOK1 as a potential therapeutic target, especially in RAS-driven cancers.

MYC Deregulation in Primary Human Cancers

MYC regulates a complex biological program by transcriptionally activating and repressing its numerous target genes. As such, MYC is a master regulator of many processes, including cell cycle entry, ribosome biogenesis, and metabolism. In cancer, the activity of the MYC transcriptional network is frequently deregulated, contributing to the initiation and maintenance of disease. Deregulation often leads to constitutive overexpression of MYC, which can be achieved through gross genetic abnormalities, including copy number alterations, chromosomal translocations, increased enhancer activity, or through aberrant signal transduction leading to increased MYC transcription or increased MYC mRNA and protein stability. Herein, we summarize the frequency and modes of MYC deregulation and describe both well-established and more recent findings in a variety of cancer types. Notably, these studies have highlighted that with an increased appreciation for the basic mechanisms deregulating MYC in cancer, new therapeutic vulnerabilities can be discovered and potentially exploited for the inhibition of this potent oncogene in cancer.

Strategies for monitoring and combating resistance to combination kinase inhibitors for cancer therapy

Targeted therapies such as kinase inhibitors and monoclonal antibodies have dramatically altered cancer care in recent decades. Although these targeted therapies have improved patient outcomes in several cancer types, resistance ultimately develops to these agents. One potential strategy proposed to overcome acquired resistance involves taking repeat tumor biopsies at the time of disease progression, to identify the specific molecular mechanism driving resistance in an individual patient and to select a new agent or combination of agents capable of surmounting that specific resistance mechanism. However, recent studies sampling multiple metastatic lesions upon acquired resistance, or employing “liquid biopsy” analyses of circulating tumor DNA, have revealed that multiple, heterogeneous resistance mechanisms can emerge in distinct tumor subclones in the same patient. This heterogeneity represents a major clinical challenge for devising therapeutic strategies to overcome resistance. In many cancers, multiple drug resistance mechanisms often converge to reactivate the original pathway targeted by the drug. This convergent evolution creates an opportunity to target a common signaling node to overcome resistance. Furthermore, integration of liquid biopsy approaches into clinical practice may allow real-time monitoring of emerging resistance alterations, allowing intervention prior to standard detection of radiographic progression. In this review, we discuss recent advances in understanding tumor heterogeneity and resistance to targeted therapies, focusing on combination kinase inhibitors, and we discuss approaches to address these issues in the clinic.

Enhanced dependency of KRAS-mutant colorectal cancer cells on RAD51-dependent homologous recombination repair identified from genetic interactions in Saccharomyces cerevisiae

Activating KRAS mutations drive colorectal cancer tumorigenesis and influence response to anti‐EGFR‐targeted therapy. Despite recent advances in understanding Ras signaling biology and the revolution in therapies for melanoma using BRAF inhibitors, no targeted agents have been effective in KRAS‐mutant cancers, mainly due to activation of compensatory pathways. Here, by leveraging the largest synthetic lethal genetic interactome in yeast, we identify that KRAS‐mutated colorectal cancer cells have augmented homologous recombination repair (HRR) signaling. We found that KRAS mutation resulted in slowing and stalling of the replication fork and accumulation of DNA damage. Moreover, we found that KRAS‐mutant HCT116 cells have an increase in MYC‐mediated RAD51 expression with a corresponding increase in RAD51 recruitment to irradiation‐induced DNA double‐strand breaks (DSBs) compared to genetically complemented isogenic cells. MYC depletion using RNA interference significantly reduced IR‐induced RAD51 foci formation and HRR. On the contrary, overexpression of either HA‐tagged wild‐type (WT) MYC or phospho‐mutant S62A increased RAD51 protein levels and hence IR‐induced RAD51 foci. Likewise, depletion of RAD51 selectively induced apoptosis in HCT116‐mutant cells by increasing DSBs. Pharmacological inhibition targeting HRR signaling combined with PARP inhibition selectivity killed KRAS‐mutant cells. Interestingly, these differences were not seen in a second isogenic pair of KRAS WT and mutant cells (DLD‐1), likely due to their nondependency on the KRAS mutation for survival. Our data thus highlight a possible mechanism by which KRAS‐mutant‐dependent cells drive HRR in vitro by upregulating MYC‐RAD51 expression. These data may offer a promising therapeutic vulnerability in colorectal cancer cells harboring otherwise nondruggable KRAS mutations, which warrants further investigation in vivo.

Oncogenic KRAS Targets MUC16/CA125 in Pancreatic Ductal Adenocarcinoma

Pancreatic ductal adenocarcinoma (PDAC) is a devastating disease with the 5-year survival rate less than 6%. Previous results indicated that serum levels of CA125 (encoded by MUC16) could be used to predict which groups of pancreatic cancer patients may benefit from surgery. However, the underlying mechanism remains elusive. Herein, using the Cancer Genome Atlas and clinicopathologic data obtained from our center, we demonstrate that high CA125 serum levels and expression levels of MUC16 are predictive of poor prognosis. MUC16 is also validated as a downstream target of KRAS, and their expression strongly correlated with each other in vitro and in vivo Mechanistically, the KRAS/ERK axis induced upregulation of MUC16 and shedding of CA125 via its effector c-Myc in SW1990 and PANC-1 pancreatic cancer cells. Notably, proto-oncogene c-Myc could bind to the promoter of MUC16 and transcriptionally activate its expression. Taken together, these data establish CA125 as a prognostic marker for pancreatic cancer, and mechanistic studies uncovered the KRAS/c-Myc axis as a driving factor for upregulation of MUC16.

IMPLICATIONS

The current study uncovers the contribution of oncogenic KRAS to serum marker CA125 production through a mechanism that involves the ERK/c-Myc axis. Mol Cancer Res; 15(2); 201-12. ©2016 AACR.

Genetic alterations affecting GTPases and T-cell receptor signaling in peripheral T-cell lymphomas

Peripheral T-cell lymphomas (PTCLs) are rare, heterogeneous tumors with poor response to standard therapy and few targeted treatments available. The identification of mutations in the T-cell receptor (TCR) signaling pathway that either directly or indirectly affect Ras- and Rho-family GTPases is an emerging theme across PTCL subtypes. This review summarizes the role of GTPases in TCR signaling and highlights the constellation of mutations in this pathway among PTCLs. In particular, focus is given to the functional impact of the mutations and opportunities for targeted therapy. These mutations include activating mutations and gene fusions involving the guanine nucleotide exchange factor, VAV1, as well as activating and dominant negative mutations in the GTPases KRAS and RHOA, respectively. In addition to mutations directly affecting the GTPase pathway, TCR signaling mutations indirectly affecting Ras- and Rho-family GTPases involving genes such as CD28, FYN, LCK, and PLCG1 are also reviewed.

mTOR Signaling Confers Resistance to Targeted Cancer Drugs

Cancer is a complex disease and a leading cause of death worldwide. Extensive research over decades has led to the development of therapies that target cancer-specific signaling pathways. However, the clinical benefits of such drugs are at best transient due to tumors displaying intrinsic or adaptive resistance. The underlying compensatory pathways that allow cancer cells to circumvent a drug blockade are poorly understood. We review here recent studies suggesting that mammalian TOR (mTOR) signaling is a major compensatory pathway conferring resistance to many cancer drugs. mTOR-mediated resistance can be cell-autonomous or non-cell-autonomous. These findings suggest that mTOR signaling should be monitored routinely in tumors and that an mTOR inhibitor should be considered as a co-therapy.

Constitutive IRAK4 Activation Underlies Poor Prognosis and Chemoresistance in Pancreatic Ductal Adenocarcinoma

Purpose: Aberrant activation of the NF-κB transcription factors underlies the aggressive behavior and poor outcome of pancreatic ductal adenocarcinoma (PDAC). However, clinically effective and safe NF-κB inhibitors are not yet available. Because NF-κB transcription factors can be activated by the interleukin-1 receptor-associated kinases (IRAKs) downstream of the Toll-like receptors (TLRs), but has not been explored in PDAC, we sought to investigate the role of IRAKs in the pathobiology of PDAC.Experimental Design: We examined the phosphorylation status of IRAK4 (p-IRAK4), the master regulator of TLR signaling, in PDAC cell lines, in surgical samples and commercial tissue microarray. We then performed functional studies using small-molecule IRAK1/4 inhibitor, RNA-interference, and CRISPR/Cas9n techniques to delineate the role of IRAK4 in NF-κB activity, chemoresistance, cytokine production, and growth of PDAC cells in vitro and in vivoResults: p-IRAK4 staining was detectable in the majority of PDAC lines and about 60% of human PDAC samples. The presence of p-IRAK4 strongly correlated with phospho-NF-κB/p65 staining in PDAC samples and is predictive of postoperative relapse and poor overall survival. Inhibition of IRAK4 potently reduced NF-κB activity, anchorage-independent growth, chemoresistance, and secretion of proinflammatory cytokines from PDAC cells. Both pharmacologic suppression and genetic ablation of IRAK4 greatly abolished PDAC growth in mice and augmented the therapeutic effect of gemcitabine by promoting apoptosis, reducing tumor cell proliferation and tumor fibrosis.Conclusions: Our data established IRAK4 as a novel therapeutic target for PDAC treatment. Development of potent IRAK4 inhibitors is needed for clinical testing. Clin Cancer Res; 23(7); 1748-59. ©2016 AACR.

KRAS-related proteins in pancreatic cancer

Pancreatic ductal adenocarcinoma (PDAC) is a highly metastatic disease with a high mortality rate. Genetic and biochemical studies have shown that RAS signaling mediated by KRAS plays a pivotal role in disease initiation, progression and drug resistance. RAS signaling affects several cellular processes in PDAC, including cellular proliferation, migration, cellular metabolism and autophagy. 90% of pancreatic cancer patients harbor somatic oncogenic point mutations in KRAS, which lead to constitutive activation of the molecule. Pancreatic cancers lacking KRAS mutations show activation of RAS via upstream signaling through receptor mediated tyrosine kinases, like EGFR, and in a small fraction of patients, oncogenic activation of the downstream B-RAF molecule is detected. RAS-stimulated signaling of RAF/MEK/ERK, PI3K/AKT/mTOR and RalA/B is active in human pancreatic cancers, cancer cell lines and mouse models of PDAC, although activation levels of each signaling arm appear to be variable across different tumors and perhaps within different subclones of single tumors. Recently, several targeted therapies directed towards MEK, ERK, PI3K and mTOR have been assayed in pancreatic cancer cell lines and in mouse models of the disease with promising results for their ability to impede cellular growth or delay tumor formation, and several inhibitors are currently in clinical trials. However, therapy-induced cross activation of RAS effector molecules has elucidated the complexities of targeting RAS signaling. Combinatorial therapies are now being explored as an approach to overcome RAS-induced therapeutic resistance in pancreatic cancer.

ERK/MAPK Signaling Drives Overexpression of the Rac-GEF, PREX1, in BRAF- and NRAS-Mutant Melanoma

Recently, we identified that PREX1 overexpression is critical for metastatic but not tumorigenic growth in a mouse model of NRAS-driven melanoma. In addition, a PREX1 gene signature correlated with and was dependent on ERK MAPK activation in human melanoma cell lines. In the current study, the underlying mechanism of PREX1 overexpression in human melanoma was assessed. PREX1 protein levels were increased in melanoma tumor tissues and cell lines compared with benign nevi and normal melanocytes, respectively. Suppression of PREX1 by siRNA impaired invasion but not proliferation in vitro PREX1-dependent invasion was attributable to PREX1-mediated activation of the small GTPase RAC1 but not the related small GTPase CDC42. Pharmacologic inhibition of ERK signaling reduced PREX1 gene transcription and additionally regulated PREX1 protein stability. This ERK-dependent upregulation of PREX1 in melanoma, due to both increased gene transcription and protein stability, contrasts with the mechanisms identified in breast and prostate cancers, in which PREX1 overexpression was driven by gene amplification and HDAC-mediated gene transcription, respectively. Thus, although PREX1 expression is aberrantly upregulated and regulates RAC1 activity and invasion in these three different tumor types, the mechanisms of its upregulation are distinct and context dependent.

IMPLICATIONS

This study identifies an ERK-dependent mechanism that drives PREX1 upregulation and subsequent RAC1-dependent invasion in BRAF- and NRAS-mutant melanoma. Mol Cancer Res; 14(10); 1009-18. ©2016 AACR.

Concepts to Target MYC in Pancreatic Cancer

Current data suggest that MYC is an important signaling hub and driver in pancreatic ductal adenocarcinoma (PDAC), a tumor entity with a strikingly poor prognosis. No targeted therapies with a meaningful clinical impact were successfully developed against PDAC so far. This points to the need to establish novel concepts targeting the relevant drivers of PDAC, like KRAS or MYC. Here, we discuss recent developments of direct or indirect MYC inhibitors and their potential mode of action in PDAC. Mol Cancer Ther; 15(8); 1792-8. ©2016 AACR.

Key roles of EMT for adaptive resistance to MEK inhibitor in KRAS mutant lung cancer

KRAS is frequently mutated in a variety of cancers including lung cancer. Whereas the mitogen-activated protein kinase (MAPK) is a well-known effector pathway of KRAS, blocking this pathway with MEK inhibitors is relatively ineffective. One major contributor to limited efficacy is attributed to the reactivation of MAPK signal following MEK inhibition by multiple feedback mechanisms. In a recent study, we have identified that epithelial-to-mesenchymal transition defines feedback activation of receptor tyrosine kinase signaling following MEK inhibition in KRAS mutant lung cancer. In epithelial-like cells, this feedback was mediated by ERBB3. In contrast, in mesenchymal-like cells, the feedback was attributed to the fibroblast growth factor receptor 1 (FGFR1) pathway. FGFR1 was dominantly expressed in mesenchymal-like cells: suppression of SPRY proteins by MEK inhibition relieved negative feedback control of basal FGFR-FRS2 function, resulting in reactivation of MAPK signaling via FGFR1. Therapeutically, the combination of MEK inhibitor trametinib with an FGFR inhibitor induced tumor regressions in tumor xenografts derived from mesenchymal-like KRAS mutant cancer cell lines as well as a patient derived xenograft model with a representative mesenchymal phenotype. Collectively, feedback activation of MAPK by FGFR1 signaling mitigates the effect of MEK inhibitor in mesenchymal-like KRAS mutant lung tumors, and combinations of clinically available FGFR1 inhibitors and MAPK inhibitors constitute a therapeutic approach to treat these cancers effectively.

Interaction of tRNA with MEK2 in pancreatic cancer cells

Although the translational function of tRNA has long been established, extra translational functions of tRNA are still being discovered. We previously developed a computational method to systematically predict new tRNA-protein complexes and experimentally validated six candidate proteins, including the mitogen-activated protein kinase kinase 2 (MEK2), that interact with tRNA in HEK293T cells. However, consequences of the interaction between tRNA and these proteins remain to be elucidated. Here we tested the consequence of the interaction between tRNA and MEK2 in pancreatic cancer cell lines. We also generated disease and drug resistance-derived MEK2 mutants (Q60P, P128Q, S154F, E207K) to evaluate the function of the tRNA-MEK2 interaction. Our results demonstrate that tRNA interacts with the wild-type and mutant MEK2 in pancreatic cancer cells; furthermore, the MEK2 inhibitor U0126 significantly reduces the tRNA-MEK2 interaction. In addition, tRNA affects the catalytic activity of the wild type and mutant MEK2 proteins in different ways. Overall, our findings demonstrate the interaction of tRNA with MEK2 in pancreatic cancer cells and suggest that tRNA may impact MEK2 activity in cancer cells.

Protein Kinase CK2α Maintains Extracellular Signal-regulated Kinase (ERK) Activity in a CK2α Kinase-independent Manner to Promote Resistance to Inhibitors of RAF and MEK but Not ERK in BRAF Mutant Melanoma

The protein kinase casein kinase 2 (CK2) is a pleiotropic and constitutively active kinase that plays crucial roles in cellular proliferation and survival. Overexpression of CK2, particularly the α catalytic subunit (CK2α, CSNK2A1), has been implicated in a wide variety of cancers and is associated with poorer survival and resistance to both conventional and targeted anticancer therapies. Here, we found that CK2α protein is elevated in melanoma cell lines compared with normal human melanocytes. We then tested the involvement of CK2α in drug resistance to Food and Drug Administration-approved single agent targeted therapies for melanoma. In BRAF mutant melanoma cells, ectopic CK2α decreased sensitivity to vemurafenib (BRAF inhibitor), dabrafenib (BRAF inhibitor), and trametinib (MEK inhibitor) by a mechanism distinct from that of mutant NRAS. Conversely, knockdown of CK2α sensitized cells to inhibitor treatment. CK2α-mediated RAF-MEK kinase inhibitor resistance was tightly linked to its maintenance of ERK phosphorylation. We found that CK2α post-translationally regulates the ERK-specific phosphatase dual specificity phosphatase 6 (DUSP6) in a kinase dependent-manner, decreasing its abundance. However, we unexpectedly showed, by using a kinase-inactive mutant of CK2α, that RAF-MEK inhibitor resistance did not rely on CK2α kinase catalytic function, and both wild-type and kinase-inactive CK2α maintained ERK phosphorylation upon inhibition of BRAF or MEK. That both wild-type and kinase-inactive CK2α bound equally well to the RAF-MEK-ERK scaffold kinase suppressor of Ras 1 (KSR1) suggested that CK2α increases KSR facilitation of ERK phosphorylation. Accordingly, CK2α did not cause resistance to direct inhibition of ERK by the ERK1/2-selective inhibitor SCH772984. Our findings support a kinase-independent scaffolding function of CK2α that promotes resistance to RAF- and MEK-targeted therapies.

KRAS Mutant Pancreatic Cancer: No Lone Path to an Effective Treatment

Pancreatic ductal adenocarcinoma (PDAC) is among the deadliest cancers with a dismal 7% 5-year survival rate and is projected to become the second leading cause of cancer-related deaths by 2020. KRAS is mutated in 95% of PDACs and is a well-validated driver of PDAC growth and maintenance. However, despite comprehensive efforts, an effective anti-RAS drug has yet to reach the clinic. Different paths to inhibiting RAS signaling are currently under investigation in the hope of finding a successful treatment. Recently, direct RAS binding molecules have been discovered, challenging the perception that RAS is an “undruggable” protein. Other strategies currently being pursued take an indirect approach, targeting proteins that facilitate RAS membrane association or downstream effector signaling. Unbiased genetic screens have identified synthetic lethal interactors of mutant RAS. Most recently, metabolic targets in pathways related to glycolytic signaling, glutamine utilization, autophagy, and macropinocytosis are also being explored. Harnessing the patient’s immune system to fight their cancer is an additional exciting route that is being considered. The “best” path to inhibiting KRAS has yet to be determined, with each having promise as well as potential pitfalls. We will summarize the state-of-the-art for each direction, focusing on efforts directed toward the development of therapeutics for pancreatic cancer patients with mutated KRAS.

The ever-changing landscape of pancreatic cancer stem cells

Over the past decade, the cancer stem cell (CSC) concept in solid tumors has gained enormous momentum as an attractive model to explain tumor heterogeneity. The model proposes that tumors contain a subpopulation of rare cancer cells with stem-like properties that maintain the hierarchy of the tumor and drive tumor initiation, progression, metastasis, and chemoresistance. The identification and subsequent isolation of CSCs in pancreatic ductal adenocarcinoma (PDAC) in 2007 provided enormous insight into this extremely metastatic and chemoresistant tumor and renewed hope for developing more specific therapies against this disease. Unfortunately, we have made only marginal advances in applying the knowledge learned to the development of new and more effective treatments for pancreatic cancer. The latter has been partly due to the lack of adequate in vitro and in vivo systems compounded by the use of markers that do not reproducibly nor exclusively select for an enriched CSC population. Thus, attempts to define a pancreatic CSC-specific genetic, epigenetic or proteomic signature has been challenging. Fortunately recent advances in the CSC field have overcome many of these challenges and have opened up new opportunities for developing therapies that target the CSC population. In this review, we discuss these current advances, specifically new methods for the identification and isolation of pancreatic CSCs, new insights into the metabolic profile of CSCs at the level of mitochondrial respiration, and the utility of genetically engineered mouse models as surrogate systems to both study CSC biology and evaluate CSC-specific targeted therapies in vivo.