Mutant B-RAF mediates resistance to anoikis via Bad and Bim

Kinase signalling adaptation supports dysfunctional mitochondria in disease

Mitochondria form a critical control nexus which are essential for maintaining correct tissue homeostasis. An increasing number of studies have identified dysregulation of mitochondria as a driver in cancer. However, which pathways support and promote this adapted mitochondrial function? A key hallmark of cancer is perturbation of kinase signalling pathways. These pathways include mitogen activated protein kinases (MAPK), lipid secondary messenger networks, cyclic-AMP-activated (cAMP)/AMP-activated kinases (AMPK), and Ca2+/calmodulin-dependent protein kinase (CaMK) networks. These signalling pathways have multiple substrates which support initiation and persistence of cancer. Many of these are involved in the regulation of mitochondrial morphology, mitochondrial apoptosis, mitochondrial calcium homeostasis, mitochondrial associated membranes (MAMs), and retrograde ROS signalling. This review will aim to both explore how kinase signalling integrates with these critical mitochondrial pathways and highlight how these systems can be usurped to support the development of disease. In addition, we will identify areas which require further investigation to fully understand the complexities of these regulatory interactions. Overall, this review will emphasize how studying the interaction between kinase signalling and mitochondria improves our understanding of mitochondrial homeostasis and can yield novel therapeutic targets to treat disease.

Vertical pathway inhibition of receptor tyrosine kinases and BAD with synergistic efficacy in triple negative breast cancer

Aberrant activation of the PI3K/AKT signaling axis along with the sustained phosphorylation of downstream BAD is associated with a poor outcome of TNBC. Herein, the phosphorylated to non-phosphorylated ratio of BAD, an effector of PI3K/AKT promoting cell survival, was observed to be correlated with worse clinicopathologic indicators of outcome, including higher grade, higher proliferative index and lymph node metastasis. The structural optimization of a previously reported inhibitor of BAD-Ser99 phosphorylation was therefore achieved to generate a small molecule inhibiting the phosphorylation of BAD at Ser99 with enhanced potency and improved oral bioavailability. The molecule 2-((4-(2,3-dichlorophenyl)piperazin-1-yl)(pyridin-3-yl)methyl) phenol (NCK) displayed no toxicity at supra-therapeutic doses and was therefore assessed for utility in TNBC. NCK promoted apoptosis and G0/G1 cell cycle arrest of TNBC cell lines in vitro, concordant with gene expression analyses, and reduced in vivo xenograft growth and metastatic burden, demonstrating efficacy as a single agent. Additionally, combinatorial oncology compound library screening demonstrated that NCK synergized with tyrosine kinase inhibitors (TKIs), specifically OSI-930 or Crizotinib in reducing cell viability and promoting apoptosis of TNBC cells. The synergistic effects of NCK and TKIs were also observed in vivo with complete regression of a percentage of TNBC cell line derived xenografts and prevention of metastatic spread. In patient-derived TNBC xenograft models, NCK prolonged survival times of host animals, and in combination with TKIs generated superior survival outcomes to single agent treatment. Hence, this study provides proof of concept to further develop rational and mechanistic based therapeutic strategies to ameliorate the outcome of TNBC.

Blebs and former blebs: From surface protrusions to extracellular vesicles in cancer signalling, anoikis resistance and beyond

Associations between plasma membrane blebbing and metastatic progression have been widely reported. There are also reports of increased extracellular vesicle release from cancer cells. Yet the ties between these closely related phenomena are incompletely understood. In this commentary, we remark on a recent finding on cellular membrane blebs in melanoma signaling. We discuss possible implications for cancer biology and draw parallels to knowns and unknowns in the relationships of extracellular vesicles and cancer progression.

Therapeutic targeting of anoikis resistance in cutaneous melanoma metastasis

The acquisition of resistance to anoikis, the cell death induced by loss of adhesion to the extracellular matrix, is an absolute requirement for the survival of disseminating and circulating tumour cells (CTCs), and for the seeding of metastatic lesions. In melanoma, a range of intracellular signalling cascades have been identified as potential drivers of anoikis resistance, however a full understanding of the process is yet to be attained. Mechanisms of anoikis resistance pose an attractive target for the therapeutic treatment of disseminating and circulating melanoma cells. This review explores the range of small molecule, peptide and antibody inhibitors targeting molecules involved in anoikis resistance in melanoma, and may be repurposed to prevent metastatic melanoma prior to its initiation, potentially improving the prognosis for patients.

Modulating Tumor Cell Functions by Tunable Nanopatterned Ligand Presentation

Cancer comprises a large group of complex diseases which arise from the misrouted interplay of mutated cells with other cells and the extracellular matrix. The extracellular matrix is a highly dynamic structure providing biochemical and biophysical cues that regulate tumor cell behavior. While the relevance of biochemical signals has been appreciated, the complex input of biophysical properties like the variation of ligand density and distribution is a relatively new field in cancer research. Nanotechnology has become a very promising tool to mimic the physiological dimension of biophysical signals and their positive (i.e., growth-promoting) and negative (i.e., anti-tumoral or cytotoxic) effects on cellular functions. Here, we review tumor-associated cellular functions such as proliferation, epithelial-mesenchymal transition (EMT), invasion, and phenotype switch that are regulated by biophysical parameters such as ligand density or substrate elasticity. We also address the question of how such factors exert inhibitory or even toxic effects upon tumor cells. We describe three principles of nanostructured model systems based on block copolymer nanolithography, electron beam lithography, and DNA origami that have contributed to our understanding of how biophysical signals direct cancer cell fate.

KRAS and BRAF mutations induce anoikis resistance and characteristic 3D phenotypes in Caco‑2 cells

In a number of types of cancer, anoikis, a form of apoptosis induced by loss of extracellular matrix (ECM) attachment, is disturbed. Anoikis resistance is essential in the formation of metastases. A recent study identified carcinoma cell subpopulations surviving without ECM contact in pathological specimens of colorectal cancer. The occurrence of these subpopulations indicated anoikis resistance. In the present study, it is demonstrated that KRAS and BRAF mutations induce anoikis resistance in colon cancer (Caco-2) cells. In 3D cultures, Caco-2 cells transfected with mutated KRAS or BRAF formed multicellular structures analogous to anoikis-resistant subpopulations in actual carcinomas, and serve as an in vitro model for anoikis resistance. Caco-2 cell lines were constructed, with KRAS or BRAF mutations, using retroviral delivery. The current study investigated anoikis resistance using an Annexin V apoptosis test from suspension cultures. 3D in vitro cultures, which were generated in collagen-matrigel mixtures, were assessed using confocal microscopy. 3D cultures embedded in paraffin were analyzed using conventional histopathology. In suspension cultures, Caco-2 cells with KRAS or BRAF mutations indicated a significantly lower proportion of Annexin positivity than the native Caco-2 cells, indicating that these mutations induce anoikis resistance in Caco-2 cells. 3D cultures displayed native Caco-2 cells forming polarized cysts with a single layer thick epithelium, whereas Caco-2 cells with KRAS or BRAF mutations formed partially filled cystic structures or solid round structures where only the outermost layer was in contact with the ECM. Additionally, KRAS mutations induced reversed polarity to Caco-2 cells along with the emergence of solid growth. The present study demonstrated that KRAS and BRAF mutations induce anoikis resistance in Caco-2 colorectal cancer cells. The growth patterns generated from the KRAS and BRAF mutated cells in 3D cultures revealed a resemblance to the putative anoikis-resistant subpopulations in actual carcinomas, including micropapillary structures and solid tumor cell islands. Additionally, KRAS mutation induced the emergence of inverted polarity. In conclusion, 3D cultures with modified Caco-2 cells serve as a valid in vitro model for anoikis resistance and inverted polarity.

Drug repurposing to overcome resistance to various therapies for colorectal cancer

Emergence of novel treatment modalities provides effective therapeutic options, apart from conventional cytotoxic chemotherapy, to fight against colorectal cancer. Unfortunately, drug resistance remains a huge challenge in clinics, leading to invariable occurrence of disease progression after treatment initiation. While novel drug development is unfavorable in terms of time frame and costs, drug repurposing is one of the promising strategies to combat resistance. This approach refers to the application of clinically available drugs to treat a different disease. With the well-established safety profile and optimal dosing of these approved drugs, their combination with current cancer therapy is suggested to provide an economical, safe and efficacious approach to overcome drug resistance and prolong patient survival. Here, we review both preclinical and clinical efficacy, as well as cellular mechanisms, of some extensively studied repurposed drugs, including non-steroidal anti-inflammatory drugs, statins, metformin, chloroquine, disulfiram, niclosamide, zoledronic acid and angiotensin receptor blockers. The three major treatment modalities in the management of colorectal cancer, namely classical cytotoxic chemotherapy, molecular targeted therapy and immunotherapy, are covered in this review.

Prognostic Value of NRAS Gene for Survival of Colorectal Cancer Patients: A Systematic Review and Meta-Analysis

Introduction: NRAS gene is associated with malignant proliferation and metastasis of colorectal cancer (CRC). But its prognostic value on CRC is still unknown. The objective of this study is to perform a meta-analysis to obtain its prognostic value on survival of CRC patients. Methods: The systematic review and meta-analysis was designed, undertaken and reported using items from the PRISMA statement. Relevant articles were identified through PubMed (containing Medline), Embase, Web of Science databases and Google scholar search engines from their inception up to October 3, 2016. The articles about NRAS on prognosis of CRC patients were enrolled. The association between NRAS and CRC survival time (including overall survival [OS], progression-free survival [PFS], and disease-free survival [DFS]) was evaluated using hazard ratio (HR) with its corresponding 95% confidence interval (CI). Results: A total of fifteen articles were included. High-expression of NRAS was significantly associated with poor OS (HR: 1.36, 95% CI: 1.15–1.61), and poor PFS (HR: 1.75, 95% CI: 1.04–2.94). The combined HR of NRAS on DFS was 0.87 (95% CI: 0.37–2.03). Subgroup analysis showed that NRAS was significantly associated with poor OS for patients from Western countries (HR: 1.38, 95% CI: 1.09–1.73), but not for those from Asian countries. Conclusions: This meta-analysis demonstrate that NRAS gene could predict the poor prognosis for the CRC patients. More large-sample cohort studies are needed to further confirm this conclusion.

Discovery of a small-molecule inhibitor of specific serine residue BAD phosphorylation

Human BCL-2-associated death promoter (hBAD) is an apoptosis-regulatory protein mediating survival signals to carcinoma cells upon phosphorylation of Ser99, among other residues. Herein, we screened multiple small-molecule databases queried in a Laplacian-modified naive Bayesian-based cheminformatics platform and identified a Petasis reaction product as a site-specific inhibitor for hBAD phosphorylation. Based on apoptotic efficacy against mammary carcinoma cells, N-cyclopentyl-3-((4-(2,3-dichlorophenyl) piperazin-1-yl) (2-hydroxyphenyl) methyl) benzamide (NPB) was identified as a potential lead compound. In vitro biochemical analyses demonstrated that NPB inhibited the phosphorylation of hBAD specifically on Ser99. NPB was observed to exert this effect independently of AKT and other kinase activities despite the demonstration of AKT-mediated BAD-Ser99 phosphorylation. Using a structure-based bioinformatics platform, we observed that NPB exhibited predicted interactions with hBAD in silico and verified the same by direct binding kinetics. NPB reduced phosphorylation of BAD-Ser99 and enhanced caspase 3/7 activity with associated loss of cell viability in various human cancer cell lines derived from mammary, endometrial, ovarian, hepatocellular, colon, prostatic, and pancreatic carcinoma. Furthermore, by use of a xenograft model, it was observed that NPB, as a single agent, markedly diminished BAD phosphorylation in tumor tissue and significantly inhibited tumor growth. Similar doses of NPB utilized in acute toxicity studies in mice did not exhibit significant effects. Hence, we report a site-specific inhibitor of BAD phosphorylation with efficacy in tumor models.

Reversal of Resistance in Targeted Therapy of Metastatic Melanoma: Lessons Learned from Vemurafenib (BRAFV600E-Specific Inhibitor)

Malignant melanoma is the most aggressive form of skin cancer and has a very low survival rate. Over 50% of melanomas harbor various BRAF mutations with the most common being the V600E. BRAF^V600E mutation that causes constitutive activation of the MAPK pathway leading to drug-, immune-resistance, apoptosis evasion, proliferation, survival, and metastasis of melanomas. The ATP competitive BRAF^V600E selective inhibitor, vemurafenib, has shown dramatic success in clinical trials; promoting tumor regression and an increase in overall survival of patients with metastatic melanoma. Regrettably, vemurafenib-resistance develops over an average of six months, which renders melanomas resistant to other therapeutic strategies. Elucidation of the underlying mechanism(s) of acquisition of vemurafenib-resistance and design of novel approaches to override resistance is the subject of intense clinical and basic research. In this review, we summarize recent developments in therapeutic approaches and clinical investigations on melanomas with BRAF^V600E mutation to establish a new platform for the treatment of melanoma.

A low microRNA-630 expression confers resistance to tyrosine kinase inhibitors in EGFR-mutated lung adenocarcinomas via miR-630/YAP1/ERK feedback loop

Purpose: MicroRNA-630 plays dual roles in apoptosis and drug resistance in human cancers. However, the role of miR-630 in resistance to tyrosine kinase inhibitors (TKIs) in lung adenocarcinoma remains to be elucidated.

Methods: Manipulation of miR-630 and its targeted gene YAP1 and/or combination of inhibitor treatments was performed to explore whether low miR-630 could confer TKI resistance due to de-targeting YAP1, and this could decrease proapoptotic protein Bad expression through the miR-630/YAP1/ERK feedback loop. A retrospective study was conducted to examine whether the expression of miR-630 and YAP1 could be associated with TKI therapeutic response in patients with lung adenocarcinoma.

Results: Low miR-630 expression may confer TKI resistance via increased SP1 binding to the miR-630 promoter due to ERK activation by YAP1 de-targeting. Persistent activation of ERK signaling via the miR-630/YAP1/ERK feedback loop may be responsible for TKI resistance in EGFR-mutated cells. Moreover, a decrease in Bad expression by its phosphorylation at Serine 75 through ERK activation conferred low miR-630-mediated TKI resistance by modulating the apoptotic pathway. Xenographic tumors induced by miR-630-knockdown PC9 and PC9GR cells in nude mice were nearly suppressed by the combination of gefitinib with the YAP1 inhibitor verteporfin or an MEK/ERK inhibitor AZD6244. Patients with low miR-630 and high YAP1 expressing tumors had a higher prevalence of unfavorable responses to TKI therapy and poorer outcomes when compared with their counterparts.

Conclusion: MiR-630 may be a potential biomarker for the prediction of TKI therapeutic response and outcome in patients with lung adenocarcinoma.

Bad phosphorylation as a target of inhibition in oncology

Bcl-2 agonist of cell death (BAD) is a BH3-only member of the Bcl-2 family which possesses important regulatory function in apoptosis. BAD has also been shown to possess many non-apoptotic functions closely linked to cancer including regulation of glycolysis, autophagy, cell cycle progression and immune system development. Interestingly, BAD can be either pro-apoptotic or pro-survival depending on the phosphorylation state of three specific serine residues (human S75, S99 and S118). Expression of BAD and BAD phosphorylation patterns have been shown to influence tumor initiation and progression and play a predictive role in disease prognosis, drug response and chemosensitivity in various cancers. This review aims to summarize the current evidence on the functional role of BAD phosphorylation in human cancer and evaluate the potential utility of modulating BAD phosphorylation in cancer.

BH3-only protein BIM: An emerging target in chemotherapy

BH3-only proteins constitute major proportion of pro-apoptotic members of B-cell lymphoma 2 (Bcl-2) family of apoptotic regulatory proteins and participate in embryonic development, tissue homeostasis and immunity. Absence of BH3-only proteins contributes to autoimmune disorders and tumorigenesis. Bim (Bcl-2 Interacting Mediator of cell death), most important member of BH3-only proteins, shares a BH3-only domain (9-16 aa) among 4 domains (BH1-BH4) of Bcl-2 family proteins and highly pro-apoptotic in nature. Bim initiates the intrinsic apoptotic pathway under both physiological and patho-physiological conditions. Reduction in Bim expression was found to be associated with tumor promotion and autoimmunity, while overexpression inhibited tumor growth and drug resistance as cancer cells suppress Bim expression and stability. Apart from its role in normal homeostasis, Bim has emerged as a central player in regulation of tumorigenesis, therefore gaining attention as a plausible target for chemotherapy. Regulation of Bim expression and stability is complicated and regulated at multiple levels viz. transcriptional, post-transcriptional, post-translational (preferably by phosphorylation and ubiquitination), epigenetic (by promoter acetylation or methylation) including miRNAs. Furthermore, control over Bim expression and stability may be exploited to enhance chemotherapeutic efficacy, overcome drug resistance and select anticancer drug regimen as various chemotherapeutic agents exploit Bim as an executioner of cell death. Owing to its potent anti-tumorigenic activity many BH3 mimetics e.g. ABT-737, ABT-263, obatoclax, AT-101and A-1210477 have been developed and entered in clinical trials. It is more likely that in near future strategies commanding Bim expression and stability ultimately lead to Bim based therapeutic regimen for cancer treatment.

The RhoJ-BAD signaling network: An Achilles' heel for BRAF mutant melanomas

Genes and pathways that allow cells to cope with oncogene-induced stress represent selective cancer therapeutic targets that remain largely undiscovered. In this study, we identify a RhoJ signaling pathway that is a selective therapeutic target for BRAF mutant cells. RhoJ deletion in BRAF mutant melanocytes modulates the expression of the pro-apoptotic protein BAD as well as genes involved in cellular metabolism, impairing nevus formation, cellular transformation, and metastasis. Short-term treatment of nascent melanoma tumors with PAK inhibitors that block RhoJ signaling halts the growth of BRAF mutant melanoma tumors in vivo and induces apoptosis in melanoma cells in vitro via a BAD-dependent mechanism. As up to 50% of BRAF mutant human melanomas express high levels of RhoJ, these studies nominate the RhoJ-BAD signaling network as a therapeutic vulnerability for fledgling BRAF mutant human tumors.

BRAF^V600E is the most common mutation in human melanomas, and kinase inhibitors that block BRAF^V600E signaling can rapidly induce tumor regression but only modestly improve melanoma long-term survival. In this study, we identify a novel therapeutic vulnerability for BRAF mutant melanoma tumors. Targeting RhoJ signaling with existing PAK inhibitors was more efficient at blocking the progression of BRAF mutant tumors in vivo when compared to kinase inhibitors currently used to treat melanoma. Approximately half of all human BRAF mutant melanoma tumors express high levels of RhoJ, identifying the RhoJ-BAD pathway as a novel target for melanoma.

hvTRA, a novel TRAIL receptor agonist, induces apoptosis and sustained growth retardation in melanoma

In recent years, new treatment options for malignant melanoma patients have enhanced the overall survival for selected patients. Despite new hope, most melanoma patients still relapse with drug-resistant tumors or experience intrinsic resistance to the therapy. Therefore, novel treatment modalities beneficial for subgroups of patients are needed. TRAIL receptor agonists have been suggested as promising candidates for use in cancer treatment as they preferentially induce apoptosis in cancer cells. Unfortunately, the first generation of TRAIL receptor agonists showed poor clinical efficacy. hvTRA is a second-generation TRAIL receptor agonist with improved composition giving increased potency, and in the present study, we showed hvTRA-induced activation of apoptosis leading to an efficient and sustained reduction in melanoma cell growth in cell lines and xenograft models. Furthermore, the potential of hvTRA in a clinical setting was demonstrated by showing efficacy on tumor cells harvested from melanoma patients with lymph node metastasis in an ex vivo drug sensitivity assay. Inhibition of mutated BRAF has been shown to regulate proteins in the intrinsic apoptotic pathway, making the cells more susceptible for apoptosis induction. In an attempt to increase the efficacy of hvTRA, combination treatment with the mutated BRAF inhibitor vemurafenib was investigated. A synergistic effect by the combination was observed for several cell lines in vitro, and an initial cytotoxic effect was observed in vivo. Unfortunately, the initial increased reduction in tumor growth compared with hvTRA mono treatment was not sustained, and this was related to downregulation of the DR5 level by vemurafenib. Altogether, the presented data imply that hvTRA efficiently induce apoptosis and growth delay in melanoma models and patient material, and the potential of this TRAIL receptor agonist should be further evaluated for treatment of subgroups of melanoma patients.

BH3-only protein Bim predicts advanced stage of cutaneous melanoma

BACKGROUND

Bim having strong pro-apoptotic effects belongs to the BH3-only proteins of the Bcl-2 protein family and contributes to survival pathways in cancer cells.

OBJECTIVES

We aimed to investigate Bim protein expression in cutaneous melanoma (CM).

METHODS

Bim protein expression was assessed by immunohistochemistry in primary and metastatic melanomas and correlated with clinical and histopathological features.

RESULTS

The Bim immunoreactivity score of the primary melanomas investigated (4.6 ± 1.5) was significantly (P < 0.0001) higher than that observed in metastases (2.8 ± 1.1). Low Bim expression was significantly associated with primary nodular melanoma type (P = 0.005). Moreover, Bim expression was significantly inversely correlated with tumour thickness (r = -0.36; P = 0.0035), advanced stage of disease (stage III and IV; r = -0.60; P < 0.0001), disease relapse (r = -0.18; P = 0.034) and disease-related death (r = -0.19; P = 0.026). Advanced stage of disease was independently predicted by low Bim expression (P = 0.0010, odds ratio: 0.22, 95% CI: 0.10-0.56) on multivariate analysis; however, Bim was not shown to be an independent predictor for disease relapse (P = 0.40) and disease-related death (P = 0.77).

CONCLUSIONS

Our data demonstrate that Bim protein expression is significantly inversely correlated with melanoma features that are associated with worse prognosis. We have shown that Bim protein expression in CM is an independent predictor for advanced disease confirming that this pro-apoptotic BH3-only protein might be a potent biomarker and promising therapeutic target.

Mutant BRAF Upregulates MCL-1 to Confer Apoptosis Resistance that Is Reversed by MCL-1 Antagonism and Cobimetinib in Colorectal Cancer

Oncogenic BRAF^V600E mutations activate MAPK signaling and are associated with treatment resistance and poor prognosis in patients with colorectal cancer. In BRAF^V600E-mutant colorectal cancers, treatment failure may be related to BRAF^V600E-mediated apoptosis resistance that occurs by an as yet undefined mechanism. We found that BRAF^V600E can upregulate anti-apoptotic MCL-1 in a gene dose-dependent manner using colorectal cancer cell lines isogenic for BRAF BRAF^V600E-induced MCL-1 upregulation was confirmed by ectopic BRAF^V600E expression that activated MEK/ERK signaling to phosphorylate (MCL-1^Thr163) and stabilize MCL-1. Upregulation of MCL-1 was mediated by MEK/ERK shown by the ability of ERK siRNA to suppress MCL-1. Stabilization of MCL-1 by phosphorylation was shown by a phosphorylation-mimicking mutant and an unphosphorylated MCL-1 mutant that decreased or increased MCL-1 protein turnover, respectively. MEK/ERK inhibition by cobimetinib suppressed MCL-1 expression/phosphorylation and induced proapoptotic BIM to a greater extent than did vemurafenib in BRAF^V600E cell lines. MCL-1 knockdown versus control shRNA significantly enhanced cobimetinib-induced apoptosis in vitro and in HT29 colon cancer xenografts. The small-molecule MCL-1 inhibitor, A-1210477, also enhanced cobimetinib-induced apoptosis in vitro that was due to disruption of the interaction of MCL-1 with proapoptotic BAK and BIM. Knockdown of BIM attenuated BAX, but not BAK, activation by cobimetinib plus A-1210477. In summary, BRAF^V600E-mediated MEK/ERK activation can upregulate MCL-1 by phosphorylation/stabilization to confer apoptosis resistance that can be reversed by MCL-1 antagonism combined with cobimetinib, suggesting a novel therapeutic strategy against BRAF^V600E-mutant CRCs. Mol Cancer Ther; 15(12); 3015-27. ©2016 AACR.

Receptor tyrosine kinase Met promotes cell survival via kinase-independent maintenance of integrin α3β1

This study identifies a new mechanism by which the receptor tyrosine kinase Met promotes cell survival. The ectodomain and transmembrane domain of Met, independently of kinase activity, are required to maintain integrin α3β1 on the cell surface to prevent activation of intrinsic and extrinsic cell death pathways and maintain autophagic flux.

Matrix adhesion via integrins is required for cell survival. Adhesion of epithelial cells to laminin via integrin α3β1 was previously shown to activate at least two independent survival pathways. First, integrin α3β1 is required for autophagy-induced cell survival after growth factor deprivation. Second, integrin α3β1 independently activates two receptor tyrosine kinases, EGFR and Met, in the absence of ligands. EGFR signaling to Erk promotes survival independently of autophagy. To determine how Met promotes cell survival, we inhibited Met kinase activity or blocked its expression with RNA interference. Loss of Met expression, but not inhibition of Met kinase activity, induced apoptosis by reducing integrin α3β1 levels, activating anoikis, and blocking autophagy. Met was specifically required for the assembly of autophagosomes downstream of LC3II processing. Reexpression of wild-type Met, kinase-dead Met, or integrin α3 was sufficient to rescue death upon removal of endogenous Met. Integrin α3β1 coprecipitated and colocalized with Met in cells. The extracellular and transmembrane domain of Met was required to fully rescue cell death and restore integrin α3 expression. Thus Met promotes survival of laminin-adherent cells by maintaining integrin α3β1 via a kinase-independent mechanism.

Encorafenib (LGX818), a potent BRAF inhibitor, induces senescence accompanied by autophagy in BRAFV600E melanoma cells

Encorafenib (LGX818) is a new-generation BRAF inhibitor that is under evaluation in clinical trials. However, the underlying mechanism remains to be elucidated. Here we show that LGX818 potently decreased ERK phosphorylation and inhibited proliferation in BRAFV600E melanoma cell lines. Moreover, LGX818 downregulated CyclinD1 in a glycogen synthase kinase 3β-independent manner and induced cell cycle arrest in the G1 phase, Surprisingly, LGX818 triggered cellular senescence in BRAFV600E melanoma cells, as evidenced by increased β-galactosidase staining, while no appreciable induction of apoptosis was detected, as determined by Annexin V and propidium iodide staining and immunoblot analysis of caspase-3 processing and poly (ADP-ribose) polymerase cleavage. Increased p27KIP1 expression and retinoblastoma protein activation were detected during LGX818-induced senescence. Additionally, inhibition of dual-specificity tyrosine phosphorylation-regulated kinase 1B by AZ191 reversed LGX818-induced CyclinD1 turnover and senescence. Interestingly, autophagy is triggered through inhibition of the mTOR/70S6K pathway during LGX818-induced senescence. Moreover, autophagy inhibition by pharmacological and genetic regulation attenuates LGX818-induced senescence. Notably, combining LGX818 with autophagy modulators has anti-proliferative effect in LGX818-resistant BRAF mutant melanoma cells. Altogether, we uncovered a mechanism by which LGX818 exerts its anti-tumor activity in BRAFV600E melanoma cells.

The Novel ATP-Competitive MEK/Aurora Kinase Inhibitor BI-847325 Overcomes Acquired BRAF Inhibitor Resistance through Suppression of Mcl-1 and MEK Expression

Resistance to BRAF inhibitors is a major clinical problem. Here, we evaluate BI-847325, an ATP-competitive inhibitor of MEK and Aurora kinases, in treatment-naïve and drug-resistant BRAF-mutant melanoma models. BI-847325 potently inhibited growth and survival of melanoma cell lines that were both BRAF inhibitor naïve and resistant in 2D culture, 3D cell culture conditions, and in colony formation assays. Western blot studies showed BI-847325 to reduce expression of phospho-ERK and phospho-histone 3 in multiple models of vemurafenib resistance. Mechanistically, BI-847325 decreased the expression of MEK and Mcl-1 while increasing the expression of the proapoptotic protein BIM. Strong suppression of MEK expression was observed after 48 hours of treatment, with no recovery following >72 hours of washout. siRNA-mediated knockdown of Mcl-1 enhanced the effects of BI-847325, whereas Mcl-1 overexpression reversed this in both 2D cell culture and 3D spheroid melanoma models. In vivo, once weekly BI-847325 (70 mg/kg) led to durable regression of BRAF-inhibitor naïve xenografts with no regrowth seen (>65 days of treatment). In contrast, treatment with the vemurafenib analog PLX4720 was associated with tumor relapse at >30 days. BI-847325 also suppressed the long-term growth of xenografts with acquired PLX4720 resistance. Analysis of tumor samples revealed BI-847325 to induce apoptosis associated with suppression of phospho-ERK, total MEK, phospho-Histone3, and Mcl-1 expression. Our studies indicate that BI-847325 is effective in overcoming BRAF inhibitor resistance and has long-term inhibitory effects upon BRAF-mutant melanoma in vivo, through a mechanism associated with the decreased expression of both MEK and Mcl-1.

Melanoma never says die

Drug resistance in melanoma is commonly attributed to ineffective apoptotic pathways. Targeting apoptosis regulators is thus considered a promising approach to sensitizing melanoma to therapy. In the previous issue of Experimental Dermatology, Plötz and Eberle discuss the role that apoptosis plays in melanoma progression and drug resistance and the utility of apoptosis-inducing BH3-mimetics as targeted therapy. There are a number of compounds in clinical development and the field seems close to translating recent findings into benefits for patients with melanoma. Thus, this viewpoint is timely and achieves a valuable summary of the current state of apoptosis-inducing therapy of melanoma.

BRAF inhibitor resistance mediated by the AKT pathway in an oncogenic BRAF mouse melanoma model

BRAF (v-raf murine sarcoma viral oncogene homolog B) inhibitors elicit a transient anti-tumor response in ∼ 80% of BRAF(V600)-mutant melanoma patients that almost uniformly precedes the emergence of resistance. Here we used a mouse model of melanoma in which melanocyte-specific expression of Braf(V618E) (analogous to the human BRAF(V600E) mutation) led to the development of skin hyperpigmentation and nevi, as well as melanoma formation with incomplete penetrance. Sleeping Beauty insertional mutagenesis in this model led to accelerated and fully penetrant melanomagenesis and synchronous tumor formation. Treatment of Braf(V618E) transposon mice with the BRAF inhibitor PLX4720 resulted in tumor regression followed by relapse. Analysis of transposon insertions identified eight genes including Braf, Mitf, and ERas (ES-cell expressed Ras) as candidate resistance genes. Expression of ERAS in human melanoma cell lines conferred resistance to PLX4720 and induced hyperphosphorylation of AKT (v-akt murine thymoma viral oncogene homolog 1), a phenotype reverted by combinatorial treatment with PLX4720 and the AKT inhibitor MK2206. We show that ERAS expression elicits a prosurvival signal associated with phosphorylation/inactivation of BAD, and that the resistance of hepatocyte growth factor-treated human melanoma cells to PLX4720 can be reverted by treatment with the BAD-like BH3 mimetic ABT-737. Thus, we define a role for the AKT/BAD pathway in resistance to BRAF inhibition and illustrate an in vivo approach for finding drug resistance genes.

Phosphoproteomic analysis of basal and therapy-induced adaptive signaling networks in BRAF and NRAS mutant melanoma

Basal and kinase inhibitor driven adaptive signaling has been examined in a panel of melanoma cell lines using phosphoproteomics in conjunction with pathway analysis. A considerable divergence in the spectrum of tyrosine-phosphorylated peptides was noted at the cell line level. The unification of genotype-specific cell line data revealed the enrichment for the tyrosine-phosphorylated cytoskeletal proteins to be associated with the presence of a BRAF mutation and oncogenic NRAS to be associated with increased receptor tyrosine kinase phosphorylation. A number of proteins including cell cycle regulators (cyclin dependent kinase 1, cyclin dependent kinase 2, and cyclin dependent kinase 3), MAPK pathway components (Extracellular signal regulated kinase 1 and Extracellular signal regulated kinase 2), interferon regulators (tyrosine kinase-2), GTPase regulators (Ras-Rasb interactor 1), and controllers of protein tyrosine phosphorylation (dual specificity tyrosine (Y) phosphorylation regulated kinase 1A and protein tyrosine phosphatase receptor type A) were common to all genotypes. Treatment of a BRAF-mutant/phosphatase and tensin homologue (PTEN) null melanoma cell line with vemurafenib led to decreased phosphorylation of ERK, phospholipase C1, and β-catenin with increases in receptor tyrosine kinase phosphorylation, signal transduction and activator of signaling 3, and glycogen synthase kinase 3α noted. In NRAS-mutant melanoma, MEK inhibition led to increased phosphorylation of epidermal growth factor receptor signaling pathway components, Src family kinases, and protein kinase Cδ with decreased phosphorylation seen in STAT3 and ERK1/2. Together these data present the first systems level view of adaptive and basal phosphotyrosine signaling in BRAF- and NRAS-mutant melanoma.

Comparing BRAF mutation status in matched primary and metastatic cutaneous melanomas: implications on optimized targeted therapy

BACKGROUND

Selective BRAF inhibitors have shown dramatic results with regard to improving outcome in patients with melanoma. Testing the BRAF status in matched primary and metastatic melanomas to optimize individual targeted therapy is not well investigated.

METHODS

Extended BRAF testing using PCR for 9 mutations and VE1 immunohistochemistry for BRAF V600E detection on 95 lesions including 40 primary melanomas with their matched metastases (n = 42), recurrences (n = 9) and second primaries (n = 4) was performed. Nine patients had multiple metastases.

RESULTS

V600E was the only identified mutation type; 35.4% of primary vs. 18.9% of metastatic melanomas. The overall primary-metastatic BRAF status discordance rate was 32.3% using PCR and 27.5% with immunohistochemistry, and was significantly more frequent in primary lesions with mutant BRAF (67%). Males and patients with metastasis to lymph nodes were less likely to be discordant compared to females and those with metastasis to other sites (p = 0.023). Discordant BRAF mutation status was predicted by multivariate binary logistic regression: the presence of a mutant BRAF in the primary melanoma [OR (95% C.I.) = 23.4 (2.4-229.7)] and female gender [OR = 10.6 (1.08-95)]. Inter-metastases BRAF concordance was 100% (6 comparisons).

CONCLUSION

A high discordant rate implies the need for clinical trials addressing the response to targeted therapy in patients with discordant BRAF statuses between their primary and metastatic lesions.

Cancer cell survival during detachment from the ECM: multiple barriers to tumour progression

Epithelial cells require attachment to the extracellular matrix (ECM) for survival. However, during tumour progression and metastasis, cancerous epithelial cells must adapt to and survive in the absence of ECM. During the past 20 years, several cellular changes, including anoikis, have been shown to regulate cell viability when cells become detached from the ECM. In this Opinion article, we review in detail how cancer cells can overcome or take advantage of these specific processes. Gaining a better understanding of how cancer cells survive during detachment from the ECM will be instrumental in designing chemotherapeutic strategies that aim to eliminate ECM-detached metastatic cells.

Critical role of STAT3 in melanoma metastasis through anoikis resistance

Anoikis is an anchorage-independent cell death. Resistance to anoikis is one of the key features of metastatic cells. Here, we analyzed the role of STAT3 in anoikis resistance in melanoma cells leading to metastasis. When grown under anchorage-independent conditions, significant proportion of cells resisted anoikis and these resistant cells had higher rate of migration and invasion as compared to the cells grown under anchorage-dependent conditions. The anoikis resistant cells also had significantly higher expression and phosphorylation of STAT3 at Y705 than the cells that were attached to the basement membrane. STAT3 inhibitors, AG 490 and piplartine (PL) induced anoikis in a concentration-dependent manner in anoikis resistant cells. Over-expression of STAT3 or treatment with IL-6 not only increased anoikis resistance, but also protected the cancer cells from PL-induced anoikis. On the other hand, silencing STAT3 decreased the potential of cancer cells to resist anoikis and to migrate. STAT3 knock-down cells and PL treated cells did not form tumors as well as failed to metastasize in SCID-NSG mice as compared to untreated anchorage-independent cells, which formed big tumors and extensively metastasized. In summary, our results for the first time establish STAT3 as a critical player that renders anoikis resistance to melanoma cells and enhance their metastatic potential.

Xanthoceraside induces apoptosis in melanoma cells through the activation of caspases and the suppression of the IGF-1R/Raf/MEK/ERK signaling pathway

Xanthoceraside, a saponin extracted from the husks of Xanthoceras sorbifolia Bunge, suppresses inflammation and oxidative stress. However, the antitumor properties of xanthoceraside as well as its mechanism of action remain unclear. Therefore, we proposed to investigate its potential anticancer property. In this study, the viability of cells was measured by the MTT assay. Cell cycle and mitochondrial membrane potential were measured by flow cytometry, and the expressions of procaspase-9, procaspase-3, Cyto.c, Apaf-1, Bcl-2, Bcl-xL, Bad, p53, and IGF-1R/Raf/MEK/ERK were tested by Western blotting. Xanthoceraside significantly inhibited the proliferation of human melanoma A375.S2 cells in a concentration- and time-dependent manner but did not impair the viability of normal cells (peripheral blood mononuclear cells). Further analysis revealed that xanthoceraside induced apoptosis by activating caspase-3 and caspase-9 in a time-dependent manner through the mitochondrial pathway but did not activate caspase-8 in the cells. In addition, xanthoceraside inhibited the expression of the insulin-like growth factor-1 receptor (IGF-1R), which is an important prosurvival, antiapoptotic signaling growth factor receptor that is frequently overexpressed in cancer cells and used as a therapeutic target for multiple cancers. Interestingly, xanthoceraside also decreased the expression of Raf, p-MEK, and p-ERK, the downstream effectors of IGF-1R. Taken together, these findings indicate that xanthoceraside induces apoptosis through a mitochondria-mediated apoptotic pathway, which is induced by the downregulation of IGF-1R/Raf/MEK/ERK cascades in A375.S2 cells.

Clinical profiling of BCL-2 family members in the setting of BRAF inhibition offers a rationale for targeting de novo resistance using BH3 mimetics

While response rates to BRAF inhibitiors (BRAFi) are high, disease progression emerges quickly. One strategy to delay the onset of resistance is to target anti-apoptotic proteins such as BCL-2, known to be associated with a poor prognosis. We analyzed BCL-2 family member expression levels of 34 samples from 17 patients collected before and 10 to 14 days after treatment initiation with either vemurafenib or dabrafenib/trametinib combination. The observed changes in mRNA and protein levels with BRAFi treatment led us to hypothesize that combining BRAFi with a BCL-2 inhibitor (the BH3-mimetic navitoclax) would improve outcome. We tested this hypothesis in cell lines and in mice. Pretreatment mRNA levels of BCL-2 negatively correlated with maximal tumor regression. Early increases in mRNA levels were seen in BIM, BCL-XL, BID and BCL2-W, as were decreases in MCL-1 and BCL2A. No significant changes were observed with BCL-2. Using reverse phase protein array (RPPA), significant increases in protein levels were found in BIM and BID. No changes in mRNA or protein correlated with response. Concurrent BRAF (PLX4720) and BCL2 (navitoclax) inhibition synergistically reduced viability in BRAF mutant cell lines and correlated with down-modulation of MCL-1 and BIM induction after PLX4720 treatment. In xenograft models, navitoclax enhanced the efficacy of PLX4720. The combination of a selective BRAF inhibitor with a BH3-mimetic promises to be an important therapeutic strategy capable of enhancing the clinical efficacy of BRAF inhibition in many patients that might otherwise succumb quickly to de novo resistance. Trial registrations: ClinicalTrials.gov NCT01006980; ClinicalTrials.gov NCT01107418; ClinicalTrials.gov NCT01264380; ClinicalTrials.gov NCT01248936; ClinicalTrials.gov NCT00949702; ClinicalTrials.gov NCT01072175.

Molecular pathways: targeting NRAS in melanoma and acute myelogenous leukemia

Successful targeting of specific oncogenic "driver" mutations with small-molecule inhibitors has represented a major advance in cancer therapeutics over the past 10 to 15 years. The most common activating oncogene in human malignancy, RAS (rat sarcoma), has proved to be an elusive target. Activating mutations in RAS induce mitogen-activated protein kinase (MAPK) and phosphoinositide 3-kinase-AKT pathway signaling and drive malignant progression in up to 30% of cancers. Oncogenic NRAS mutations occur in several cancer types, notably melanoma, acute myelogenous leukemia (AML), and less commonly, colon adenocarcinoma, thyroid carcinoma, and other hematologic malignancies. Although NRAS-mutant tumors have been recalcitrant to targeted therapeutic strategies historically, newer agents targeting MAP/ERK kinase 1 (MEK1)/2 have recently shown signs of clinical efficacy as monotherapy. Combination strategies of MEK inhibitors with other targeted agents have strong preclinical support and are being evaluated in clinical trials. This review discusses the recent preclinical and clinical studies about the role of NRAS in cancer, with a focus on melanoma and AML.

Targeting the intrinsic apoptosis pathway as a strategy for melanoma therapy

Melanoma drug resistance is often attributed to abrogation of the intrinsic apoptosis pathway. Targeting regulators of apoptosis is thus considered a promising approach to sensitizing melanomas to treatment. The development of small-molecule inhibitors that mimic natural antagonists of either antiapoptotic members of the BCL-2 family or the inhibitor of apoptosis proteins (IAPs), known as BH3- or SMAC-mimetics, respectively, are helping us to understand the mechanisms behind apoptotic resistance. Studies using BH3-mimetics indicate that the antiapoptotic BCL-2 protein MCL-1 and its antagonist NOXA are particularly important regulators of BCL-2 family signaling, while SMAC-mimetic studies show that both XIAP and the cIAPs must be targeted to effectively induce apoptosis of cancer cells. Although most solid tumors, including melanoma, are insensitive to these mimetic drugs as single agents, combinations with other therapeutics have yielded promising results, and tests combining them with BRAF-inhibitors, which have already revolutionized melanoma treatment, are a clear priority.

Inhibition of autophagy enhances the effects of the AKT inhibitor MK-2206 when combined with paclitaxel and carboplatin in BRAF wild-type melanoma

This study investigates the mechanism of action behind the long-term responses (12-16 months) of two BRAF WT melanoma patients to the AKT inhibitor MK-2206 in combination with paclitaxel and carboplatin. Although single agent MK-2206 inhibited phospho-AKT signaling, it did not impact in vitro melanoma growth or survival. The combination of MK-2206 with paclitaxel and carboplatin was cytotoxic in long-term colony formation and 3D spheroid assays, and induced autophagy. Autophagy was initially protective with autophagy inhibitors and deletion of ATG5 found to enhance cytotoxicity. Although prolonged autophagy induction (>6 days) led to caspase-dependent apoptosis, drug resistant clones still emerged. Autophagy inhibition enhanced the cell death response through reactive oxygen species and could be reversed by anti-oxidants. We demonstrate for the first time that AKT inhibition in combination with chemotherapy may have clinical activity in BRAF WT melanoma and show that an autophagy inhibitor may prevent resistance to these drugs.

Marker utility for combination therapy

Melanoma is a heterogeneous disease for which monotherapies are likely to fail in the majority of patients due to genomic variations between individuals. Novel treatments, such as vemurafenib and ipilimumab, offer clinical promise in metastatic melanoma and the increased potential for combined therapeutic strategies, necessary given the differences in response between patients. Together with these new approaches, the development of clinically relevant biomarkers that predict treatment outcomes are required to ensure these new therapies are targeted at those patients most likely to benefit. Here we review the utility of some potential biomarkers of treatment response in patients with metastatic melanoma.

Inhibition of Bim enhances replication of varicella-zoster virus and delays plaque formation in virus-infected cells

Programmed cell death (apoptosis) is an important host defense mechanism against intracellular pathogens, such as viruses. Accordingly, viruses have evolved multiple mechanisms to modulate apoptosis to enhance replication. Varicella-zoster virus (VZV) induces apoptosis in human fibroblasts and melanoma cells. We found that VZV triggered the phosphorylation of the proapoptotic proteins Bim and BAD but had little or no effect on other Bcl-2 family members. Since phosphorylation of Bim and BAD reduces their proapoptotic activity, this may prevent or delay apoptosis in VZV-infected cells. Phosphorylation of Bim but not BAD in VZV-infected cells was dependent on activation of the MEK/extracellular signal-regulated kinase (ERK) pathway. Cells knocked down for Bim showed delayed VZV plaque formation, resulting in longer survival of VZV-infected cells and increased replication of virus, compared with wild-type cells infected with virus. Conversely, overexpression of Bim resulted in earlier plaque formation, smaller plaques, reduced virus replication, and increased caspase 3 activity. Inhibition of caspase activity in VZV-infected cells overexpressing Bim restored levels of virus production similar to those seen with virus-infected wild-type cells. Previously we showed that VZV ORF12 activates ERK and inhibits apoptosis in virus-infected cells. Here we found that VZV ORF12 contributes to Bim and BAD phosphorylation. In summary, VZV triggers Bim phosphorylation; reduction of Bim levels results in longer survival of VZV-infected cells and increased VZV replication.

Future perspectives in melanoma research. Meeting report from the "Melanoma Bridge. Napoli, December 2nd-4th 2012"

Recent insights into the genetic and somatic aberrations have initiated a new era of rapidly evolving targeted and immune-based treatments for melanoma. After decades of unsuccessful attempts to finding a more effective cure in the treatment of melanoma now we have several drugs active in melanoma. The possibility to use these drugs in combination to improve responses to overcome the resistance, to potentiate the action of immune system with the new immunomodulating antibodies, and identification of biomarkers that can predict the response to a particular therapy represent new concepts and approaches in the clinical management of melanoma. The third "Melanoma Research: "A bridge from Naples to the World" meeting, shortened as "Bridge Melanoma Meeting" took place in Naples, December 2 to 4th, 2012. The four topics of discussion at this meeting were: advances in molecular profiling and novel biomarkers, combination therapies, novel concepts toward integrating biomarkers and therapies into contemporary clinical management of patients with melanoma across the entire spectrum of disease stage, and the knowledge gained from the biology of tumor microenvironment across different tumors as a bridge to impact on prognosis and response to therapy in melanoma. This international congress gathered more than 30 international faculty members who in an interactive atmosphere which stimulated discussion and exchange of their experience regarding the most recent advances in research and clinical management of melanoma patients.

Inhibition of Wee1, AKT, and CDK4 underlies the efficacy of the HSP90 inhibitor XL888 in an in vivo model of NRAS-mutant melanoma

The HSP90 inhibitor XL888 is effective at reversing BRAF inhibitor resistance in melanoma, including that mediated through acquired NRAS mutations. The present study has investigated the mechanism of action of XL888 in NRAS-mutant melanoma. Treatment of NRAS-mutant melanoma cell lines with XL888 led to an inhibition of growth, G2-M phase cell-cycle arrest, and the inhibition of cell survival in three-dimensional spheroid and colony formation assays. In vitro, HSP90 inhibition led to the degradation of ARAF, CRAF, Wee1, Chk1, and cdc2 and was associated with decreased mitogen-activated protein kinase (MAPK), AKT, mTOR, and c-jun NH2 kinase (JNK) signaling. Apoptosis induction was associated with increased BIM expression and a decrease in the expression of the prosurvival protein Mcl-1. The critical role of increased BIM and decreased Mcl-1 expression in the survival of NRAS-mutant melanoma cell lines was shown through siRNA knockdown and overexpression studies. In an animal xenograft model of NRAS-mutant melanoma, XL888 treatment led to reduced tumor growth and apoptosis induction. Important differences in the pattern of client degradation were noted between the in vivo and in vitro studies. In vivo, XL888 treatment led to degradation of CDK4 and Wee1 and the inhibition of AKT/S6 signaling with little or no effect observed upon ARAF, CRAF, or MAPK. Blockade of Wee1, using either siRNA knockdown or the inhibitor MK1775, was associated with significant levels of growth inhibition and apoptosis induction. Together, these studies have identified Wee1 as a key target of XL888, suggesting novel therapeutic strategies for NRAS-mutant melanoma.

Anti-apoptotic proteins on guard of melanoma cell survival

Apoptosis plays a pivotal role in sustaining proper tissue development and homeostasis. Evading apoptosis by cancer cells is a part of their adaption to microenvironment and therapies. Cellular integrity is predominantly maintained by pro-survival members of Bcl-2 family and IAPs. Melanoma cells are characterized by a labile and stage-dependent phenotype. Pro-survival molecules can protect melanoma cells from apoptosis and mediate other processes, thus enhancing aggressive phenotype. The essential role of Bcl-2, Mcl-1, Bcl-X(L), livin, survivin and XIAP was implicated for melanoma, often in a tumor stage-dependent fashion. In this review, the current knowledge of pro-survival machinery in melanoma is discussed.

NRAS mutant melanoma: biological behavior and future strategies for therapeutic management

The recent years have seen a significant shift in the expectations for the therapeutic management of disseminated melanoma. The clinical success of BRAF targeted therapy suggests that long-term disease control may one day be a reality for genetically defined subgroups of melanoma patients. Despite this progress, few advances have been made in developing targeted therapeutic strategies for the 50% of patients whose melanomas are BRAF wild-type. The most well-characterized subgroup of BRAF wild-type tumors is the 15-20% of all melanomas that harbor activating NRAS (Neuroblastoma Rat Sarcoma Virus) mutations. Emerging preclinical and clinical evidence suggests that NRAS mutant melanomas have patterns of signal transduction and biological behavior that is distinct from BRAF mutant melanomas. This overview will discuss the unique clinical and prognostic behavior of NRAS mutant melanoma and will summarize the emerging data on how NRAS-driven signaling networks can be translated into novel therapeutic strategies.

MAP kinase signaling and inhibition in melanoma

The mitogen-activated protein kinase (MAPK) pathway is critical to oncogenic signaling in the majority of patients with malignant melanoma. Driver mutations in both NRAS and BRAF have important implications for prognosis and treatment. The development of inhibitors to mediators of the MAPK pathway, including those to CRAF, BRAF, and MEK, has led to major advances in the treatment of patients with melanoma. In particular, the selective BRAF inhibitor vemurafenib has been shown to improve overall survival in patients with tumors harboring BRAF mutations. However, the duration of benefit is limited in many patients and highlights the need for understanding the limitations of therapy in order to devise more effective strategies. MEK inhibitors have proven to particularly active in BRAF mutant melanomas also. Emerging knowledge about mechanisms of resistance as well as a more complete understanding of the biology of MAPK pathway signaling provides insight into rational combination regimens and sequences of molecularly targeted therapies.

BH3-only protein silencing contributes to acquired resistance to PLX4720 in human melanoma

B-RAF is mutated to a constitutively active form in 8% of human cancers including 50% of melanomas. In clinical trials, the RAF inhibitor, PLX4032 (vemurafenib), caused partial or complete responses in 48-81% of mutant B-RAF harboring melanoma patients. However, the average duration of response was 6-7 months before tumor regrowth, indicating the acquisition of resistance to PLX4032. To understand the mechanisms of resistance, we developed mutant B-RAF melanoma cells that displayed resistance to RAF inhibition through continuous culture with PLX4720 (the tool compound for PLX4032). Resistance was associated with a partial reactivation of extracellular signal-regulated kinase 1/2 (ERK1/2) signaling, recovery of G1/S cell-cycle events, and suppression of the pro-apoptotic B-cell leukemia/lymphoma 2 (Bcl-2) homology domain 3 (BH3)-only proteins, Bcl-2-interacting mediator of cell death-extra large (Bim-EL) and Bcl-2 modifying factor (Bmf). Preventing ERK1/2 reactivation with MEK (mitogen-activated protein/extracellular signal-regulated kinase kinase) inhibitors blocked G1-S cell-cycle progression but failed to induce apoptosis or upregulate Bim-EL and Bmf. Treatment with the histone deacetylase (HDAC) inhibitor, suberoylanilide hydroxamic acid, led to de-repression of Bim-EL and enhanced cell death in the presence of PLX4720 or AZD6244 in resistant cells. These data indicate that acquired resistance to PLX4032/4720 likely involves ERK1/2 pathway reactivation as well as ERK1/2-independent silencing of BH3-only proteins. Furthermore, combined treatment of HDAC inhibitors and MEK inhibitors may contribute to overcoming PLX4032 resistance.

The current state of targeted therapy in melanoma: this time it's personal

Treatment of metastatic melanoma has long been a challenge. Over the past 8 years significant advances have been made in understanding the genetic changes that drive melanoma development and progression. These studies have shown melanoma to be a heterogeneous group of tumors, driven by a diverse array of oncogenic mutations. There is now good evidence that activating mutations in the serine/threonine kinase BRAF and the receptor tyrosine kinase KIT constitute good therapeutic targets for restricted subgroups of melanoma. In this article, we discuss the genetics and etiology of cutaneous and noncutaneous melanoma and review some of the latest preclinical and clinical data on the new targeted therapy agents that are beginning to make an impact on the lives of melanoma patients.

Unlocking Pandora's box: personalising cancer cell death in non-small cell lung cancer

Evasion of apoptosis is a hallmark of tumorigenesis and a recognised cause of multidrug resistance. Over the last decade, insights into how apoptosis might be exploited in non-small cell lung cancer (NSCLC) and how cancer therapeutics might be used to engage apoptotic signalling in a personalised manner have changed markedly. We are now in the wake of a paradigm shift in stratified therapeutic approaches related to NSCLC. At the heart of this shift in thinking is the emerging knowledge that even the most drug-resistant cancers exhibit a functional death pathway and, critically, that this pathway can be efficiently engaged, leading to clinical benefit. This review will summarise current knowledge of mitochondrial apoptotic pathway dysfunction in NSCLC and how the next generation of targeted therapeutics might be used to exploit deficiencies in apoptotic signalling in a personalised manner to improve clinical outcome and predict therapeutic benefit.

Targeting mutant BRAF in melanoma: current status and future development of combination therapy strategies

The discovery of activating BRAF mutations in ∼50% of all melanomas has proved to be a turning point in the therapeutic management of the disseminated disease. In this commentary, we review the latest research delineating the role of mutant BRAF in melanoma initiation and progression and discuss the remarkable 10-year journey leading up to the recent U.S. Food and Drug Administration approval of the small-molecule BRAF inhibitor vemurafenib. We further outline the most recent findings on the mechanisms that underlie intrinsic and acquired BRAF inhibitor resistance and describe ongoing preclinical and clinical studies designed to delay or abrogate the onset of therapeutic escape. It is hoped that our evolving understanding of melanoma genetics and intracellular signaling coupled with a growing armamentarium of signal transduction inhibitors will lead to significant improvements in the level and durability of therapeutic response in metastatic melanoma.

The HSP90 inhibitor XL888 overcomes BRAF inhibitor resistance mediated through diverse mechanisms

PURPOSE

The clinical use of BRAF inhibitors is being hampered by the acquisition of drug resistance. This study shows the potential therapeutic use of the HSP90 inhibitor (XL888) in six different models of vemurafenib resistance.

EXPERIMENTAL DESIGN

The ability of XL888 to inhibit growth and to induce apoptosis and tumor regression of vemurafenib-resistant melanoma cell lines was shown in vitro and in vivo. A novel mass spectrometry-based pharmacodynamic assay was developed to measure intratumoral HSP70 levels following HSP90 inhibition in melanoma cell lines, xenografts, and melanoma biopsies. Mechanistic studies were carried out to determine the mechanism of XL888-induced apoptosis.

RESULTS

XL888 potently inhibited cell growth, induced apoptosis, and prevented the growth of vemurafenib-resistant melanoma cell lines in 3-dimensional cell culture, long-term colony formation assays, and human melanoma mouse xenografts. The reversal of the resistance phenotype was associated with the degradation of PDGFRβ, COT, IGFR1, CRAF, ARAF, S6, cyclin D1, and AKT, which in turn led to the nuclear accumulation of FOXO3a, an increase in BIM (Bcl-2 interacting mediator of cell death) expression, and the downregulation of Mcl-1. In most resistance models, XL888 treatment increased BIM expression, decreased Mcl-1 expression, and induced apoptosis more effectively than dual mitogen-activated protein-extracellular signal-regulated kinase/phosphoinositide 3-kinase (MEK/PI3K) inhibition.

CONCLUSIONS

HSP90 inhibition may be a highly effective strategy at managing the diverse array of resistance mechanisms being reported to BRAF inhibitors and appears to be more effective at restoring BIM expression and downregulating Mcl-1 expression than combined MEK/PI3K inhibitor therapy.

A novel role for BDNF-TrkB in the regulation of chemotherapy resistance in head and neck squamous cell carcinoma

Mechanisms of resistance for HNSCC to cisplatin (CDDP), the foundational chemotherapeutic agent in the treatment of this disease, remain poorly understood. We previously demonstrated that cisplatin resistance (CR) can be overcome by targeting Trk receptor. In the current study, we explored the potential mechanistic role of the BDNF-TrkB signaling system in the development of CDDP resistance in HNSCC. Utilizing an in vitro system of acquired CR, we confirmed a substantial up-regulation of both BDNF and TrkB at the protein and mRNA levels in CR cells, suggesting an autocrine pathway dysregulation in this system. Exogenous BDNF stimulation led to an enhanced expression of the drug-resistance and anti-apoptotic proteins MDR1 and XiAP, respectively, in a dose-dependently manner, demonstrating a key role for BDNF-TrkB signaling in modulating the response to cytotoxic agents. In addition, modulation of TrkB expression induced an enhanced sensitivity of cells to CDDP in HNSCC. Moreover, genetic suppression of TrkB resulted in changes in expression of Bim, XiAP, and MDR1 contributing to HNSCC survival. To elucidate intracellular signaling pathways responsible for mechanisms underlying BDNF/TrkB induced CDDP-resistance, we analyzed expression levels of these molecules following inhibition of Akt. Inhibition of Akt eliminated BDNF effect on MDR1 and Bim expression in OSC-19P cells as well as modulated expressions of MDR1, Bim, and XiAP in OSC-19CR cells. These results suggest BDNF/TrkB system plays critical roles in CDDP-resistance development by utilizing Akt-dependent signaling pathways.

ERK2 phosphorylation of serine 77 regulates Bmf pro-apoptotic activity

B-cell lymphoma 2 (Bcl-2) homology 3 (BH3)-only proteins represent a class of pro-apoptotic factors that neutralize pro-survival Bcl-2 proteins, and, in some cases, directly activate Bax. The mechanisms of control and the role of BH3-only proteins, such as Bcl-2 like protein 11 extra large and Bad are well studied. By contrast, relatively little is known about the regulation and role of Bcl-2 modifying factor (Bmf). The B-RAF oncogene is mutated in ∼8% of human tumors. We have previously shown that Bmf is upregulated at the transcript level and is required for apoptosis induced by targeting B-RAF signaling in tumor cells harboring mutant B-RAF. In this study, we show that Bmf is regulated at the post-translational level by mutant B-RAF-MEK-ERK2 signaling. Extracellular signal-regulated kinase (ERK2) directly phosphorylates Bmf on serine 74 and serine 77 residues with serine 77 being the predominant site. In addition, serine 77 phosphorylation reduces Bmf pro-apoptotic activity likely through a mechanism independent of altering Bmf localization to the mitochondria and/or interactions with dynein light chain 2 and the pro-survival proteins, B-cell lymphoma extra large, Bcl-2 and Mcl-1. These data identify a novel mode of regulation in Bmf that modulates its pro-apoptotic activity in mutant B-RAF tumor cells.

Wnt/β-catenin signaling and AXIN1 regulate apoptosis triggered by inhibition of the mutant kinase BRAFV600E in human melanoma

Because the Wnt/β-catenin signaling pathway is linked to melanoma pathogenesis and to patient survival, we conducted a kinome small interfering RNA (siRNA) screen in melanoma cells to expand our understanding of the kinases that regulate this pathway. We found that BRAF signaling, which is constitutively activated in many melanomas by the BRAF(V600E) mutation, inhibits Wnt/β-catenin signaling in human melanoma cells. Because inhibitors of BRAF(V600E) show promise in ongoing clinical trials, we investigated whether altering Wnt/β-catenin signaling might enhance the efficacy of the BRAF(V600E) inhibitor PLX4720. We found that endogenous β-catenin was required for PLX4720-induced apoptosis of melanoma cells and that activation of Wnt/β-catenin signaling synergized with PLX4720 to decrease tumor growth in vivo and to increase apoptosis in vitro. This synergistic enhancement of apoptosis correlated with reduced abundance of an endogenous negative regulator of β-catenin, AXIN1. In support of the hypothesis that AXIN1 is a mediator rather than a marker of apoptosis, siRNA directed against AXIN1 rendered resistant melanoma cell lines susceptible to apoptosis in response to treatment with a BRAF(V600E) inhibitor. Thus, Wnt/β-catenin signaling and AXIN1 may regulate the efficacy of inhibitors of BRAF(V600E), suggesting that manipulation of the Wnt/β-catenin pathway could be combined with BRAF inhibitors to treat melanoma.