Targeted inhibition of BRAF kinase: opportunities and challenges for therapeutics in melanoma

BRAF Inhibition and UVB Light Synergistically Promote Mus musculus Papillomavirus 1-Induced Skin Tumorigenesis

The development of keratinocytic skin tumors, presumably attributable to paradoxical activation of the MAPK pathway, represents a relevant side effect of targeted therapies with BRAF inhibitors (BRAFis). The role of cutaneous papillomavirus infection in BRAFi-associated skin carcinogenesis, however, is still inconclusive. Employing the Mus musculus papillomavirus 1 (MmuPV1) skin infection model, the impact of BRAFis and UVB exposure on papillomavirus induced skin tumorigenesis was investigated in immunocompetent FVB/NCrl mice. Systemic BRAF inhibition in combination with UVB light induced skin tumors in 62% of the MmuPV1-infected animals. In contrast, significantly fewer tumors were observed in the absence of either BRAF inhibition, UVB irradiation or virus infection, as demonstrated by lesional outgrowth in 20%, 5% and 0% of the mice, respectively. Combinatory exposure to BRAFis and UVB favored productive viral infection, which was shown by high numbers of MmuPV1 genome copies and E1^E4 spliced transcripts and an abundance of E6/E7 oncogene mRNA and viral capsid proteins. BRAF inhibition, but not viral infection or UVB light, activated ERK1/2, whereas γH2AX expression, inducible by UVB light, remained unaltered by BRAFis. These results provide experimental evidence that BRAF inhibition and UVB irradiation synergistically promote MmuPV1-induced skin tumor development in vivo. This indicates an alternative pathway by which papillomavirus skin infection may contribute to BRAFi-associated skin tumorigenesis.

Defining melanoma combination therapies that provide senolytic sensitivity in human melanoma cells

Malignant Melanoma that resists immunotherapy remains the deadliest form of skin cancer owing to poor clinically lasting responses. Alternative like genotoxic or targeted chemotherapy trigger various cancer cell fates after treatment including cell death and senescence. Senescent cells can be eliminated using senolytic drugs and we hypothesize that the targeted elimination of therapy-induced senescent melanoma cells could complement both conventional and immunotherapies. We utilized a panel of cells representing diverse mutational background relevant to melanoma and found that they developed distinct senescent phenotypes in response to treatment. A genotoxic combination therapy of carboplatin-paclitaxel or irradiation triggered a mixed response of cell death and senescence, irrespective of BRAF mutation profiles. DNA damage-induced senescent melanoma cells exhibited morphological changes, residual DNA damage, and increased senescence-associated secretory phenotype (SASP). In contrast, dual targeted inhibition of Braf and Mek triggered a different mixed cell fate response including senescent-like and persister cells. While persister cells could reproliferate, senescent-like cells were stably arrested, but without detectable DNA damage and senescence-associated secretory phenotype. To assess the sensitivity to senolytics we employed a novel real-time imaging-based death assay and observed that Bcl2/Bcl-XL inhibitors and piperlongumine were effective in promoting death of carboplatin-paclitaxel and irradiation-induced senescent melanoma cells, while the mixed persister cells and senescent-like cells resulting from Braf-Mek inhibition remained unresponsive. Interestingly, a direct synergy between Bcl2/Bcl-XL inhibitors and Braf-Mek inhibitors was observed when used out of the context of senescence. Overall, we highlight diverse hallmarks of melanoma senescent states and provide evidence of context-dependent senotherapeutics that could reduce treatment resistance while also discussing the limitations of this strategy in human melanoma cells.

Oncogenic BRAF, endoplasmic reticulum stress, and autophagy: Crosstalk and therapeutic targets in cutaneous melanoma

BRAF is a member of the RAF family of serine/threonine-specific protein kinases. Oncogenic BRAF, in particular, BRAF V600E, can disturb the normal protein folding machinery in the endoplasmic reticulum (ER) leading to accumulation of unfolded/misfolded proteins in the ER lumen, a condition known as endoplasmic reticulum (ER) stress. To alleviate such conditions, ER-stressed cells have developed a highly robust and adaptable signaling network known as unfolded protein response (UPR). UPR is ordinarily a cytoprotective response and usually operates through the induction of autophagy, an intracellular lysosomal degradation pathway that directs damaged proteins, protein aggregates, and damaged organelles for bulk degradation and recycling. Both ER stress and autophagy are involved in the progression and chemoresistance of melanoma. Melanoma, which arises as a result of malignant transformation of melanocytes, exhibits exceptionally high therapeutic resistance. Many mechanisms of therapeutic resistance have been identified in individual melanoma patients and in preclinical BRAF-driven melanoma models. Recently, it has been recognized that oncogenic BRAF interacts with GRP78 and removes its inhibitory influence on the three fundamental ER stress sensors of UPR, PERK, IRE1α, and ATF6. Dissociation of GRP78 from these ER stress sensors prompts UPR that subsequently activates cytoprotective autophagy. Thus, pharmacological inhibition of BRAF-induced ER stress-mediated autophagy can potentially resensitize BRAF mutant melanoma tumors to apoptosis. However, the underlying molecular mechanism of how oncogenic BRAF elevates the basal level of ER stress-mediated autophagy in melanoma tumors is not well characterized. A better understanding of the crosstalk between oncogenic BRAF, ER stress and autophagy may provide a rationale for improving existing cancer therapies and identify novel targets for therapeutic intervention of melanoma.

Inducible Mouse Models for Cancer Drug Target Validation

Genetically-engineered mouse (GEM) models have provided significant contributions to our understanding of cancer biology and developing anticancer therapeutic strategies. The development of GEM models that faithfully recapitulate histopathological and clinical features of human cancers is one of the most pressing needs to successfully conquer cancer. In particular, doxycycline-inducible transgenic mouse models allow us to regulate (induce or suppress) the expression of a specific gene of interest within a specific tissue in a temporal manner. Leveraging this mouse model system, we can determine whether the transgene expression is required for tumor maintenance, thereby validating the transgene product as a target for anticancer drug development (target validation study). In addition, there is always a risk of tumor recurrence with cancer therapy. By analyzing recurrent tumors derived from fully regressed tumors after turning off transgene expression in tumor-bearing mice, we can gain an insight into the molecular basis of how tumor cells escape from their dependence on the transgene (tumor recurrence study). Results from such studies will ultimately allow us to predict therapeutic responses in clinical settings and develop new therapeutic strategies against recurrent tumors. The aim of this review is to highlight the significance of doxycycline-inducible transgenic mouse models in studying target validation and tumor recurrence.

Overexpression of Mcl-1 confers resistance to BRAFV600E inhibitors alone and in combination with MEK1/2 inhibitors in melanoma

Melanoma harboring BRAF mutations frequently develop resistance to BRAF inhibitors, limiting the impact of treatment. Here, we establish a mechanism of resistance and subsequently identified a suitable drug combination to overcome the resistance. Single treatment of BRAF mutant melanoma cell lines with vemurafenib or dabrafenib (BRAF inhibitors) alone or in combination with trametinib (MEK1/2 inhibitor) resulted in overexpression of Mcl-1. Overexpression of Mcl-1 in A375 and SK-MEL-28 by transfection completely blocked BRAF and MEK1/2 inhibitor-mediated inhibition of cell survival and apoptosis. Melanoma cells resistant to BRAF inhibitors showed massive expression of Mcl-1 as compared to respective sensitive cell lines. Silencing of Mcl-1 using siRNA completely sensitized resistant melanoma cells to growth suppression and induction of apoptosis by BRAF inhibitors. In vivo, vemurafenib resistant A375 xenografts implanted in athymic nude mice showed substantial tumor growth inhibition when treated with a combination of vemurafenib and Mcl-1 inhibitor or siRNA. Immunohistochemistry and western blot analyses demonstrated enhanced expression of Mcl-1 and activation of ERK1/2 in vemurafenib-resistant tumors whereas level of Mcl-1 or p-ERK1/2 was diminished in the tumors of mice treated with either of the combination. Biopsied tumors from the patients treated with or resistant to BRAF inhibitors revealed overexpression of Mcl-1. These results suggest that the combination of BRAF inhibitors with Mcl-1 inhibitor may have therapeutic advantage to melanoma patients with acquired resistance to BRAF inhibitors alone or in combination with MEK1/2 inhibitors.

Simultaneous blocking of IL-6 and IL-8 is sufficient to fully inhibit CAF-induced human melanoma cell invasiveness

Tumour microenvironment plays a critical role in cell invasion and metastasis. To investigate the role of cancer-associated fibroblasts (CAFs) in melanoma cell invasiveness, we used 3D spheroid invasion assay. The effect of conditioned media from normal fibroblasts and CAFs cultivated alone or co-cultivated with melanoma cells on BLM or A2058 melanoma spheroid invasion was analysed. We found that conditioned media from CAFs and CAFs co-cultured with melanoma cells, especially, promote invasion and migration, without significant effect on melanoma cell proliferation. We further analysed the expression of pro-invasive cytokines IL-8 and IL-6 in media and found that melanoma cells are dominant producers of IL-8 and fibroblasts are dominant producers of IL-6 in 2D monocultures, while co-cultivation of CAFs with melanoma cells induces production/secretion of IL-6 and IL-8 into the media. The analyses of IL-6 levels in 3D cultures and human melanoma samples, however, revealed that at least in some cases IL-6 is also produced directly by melanoma cells. Analysis of the role of IL-6 and IL-8 in CAF-induced melanoma invasion, using neutralising antibodies, revealed that simultaneous blocking of IL-6 and IL-8 is sufficient to fully inhibit CAF-induced human melanoma cell invasiveness. In summary, these experiments indicate the important role of CAFs and IL-8 and IL-6 cytokines in melanoma cell invasiveness.

Structure of the BRAF-MEK complex reveals a kinase activity independent role for BRAF in MAPK signaling

Numerous oncogenic mutations occur within the BRAF kinase domain (BRAF(KD)). Here we show that stable BRAF-MEK1 complexes are enriched in BRAF(WT) and KRAS mutant (MT) cells but not in BRAF(MT) cells. The crystal structure of the BRAF(KD) in a complex with MEK1 reveals a face-to-face dimer sensitive to MEK1 phosphorylation but insensitive to BRAF dimerization. Structure-guided studies reveal that oncogenic BRAF mutations function by bypassing the requirement for BRAF dimerization for activity or weakening the interaction with MEK1. Finally, we show that conformation-specific BRAF inhibitors can sequester a dormant BRAF-MEK1 complex resulting in pathway inhibition. Taken together, these findings reveal a regulatory role for BRAF in the MAPK pathway independent of its kinase activity but dependent on interaction with MEK.

Phenformin enhances the therapeutic benefit of BRAF(V600E) inhibition in melanoma

Biguanides, such as the diabetes therapeutics metformin and phenformin, have demonstrated antitumor activity both in vitro and in vivo. The energy-sensing AMP-activated protein kinase (AMPK) is known to be a major cellular target of biguanides. Based on our discovery of cross-talk between the AMPK and v-Raf murine sarcoma viral oncogene homolog B1 (BRAF) signaling pathways, we investigated the antitumor effects of combining phenformin with a BRAF inhibitor PLX4720 on the proliferation of BRAF-mutated melanoma cells in vitro and on BRAF-driven tumor growth in vivo. Cotreatment of BRAF-mutated melanoma cell lines with phenformin and PLX4720 resulted in synergistic inhibition of cell viability, compared with the effects of the single agent alone. Moreover, treatment with phenformin significantly delayed the development of resistance to PLX4720 in cultured melanoma cells. Biochemical analyses showed that phenformin and PLX4720 exerted cooperative effects on inhibiting mTOR signaling and inducing apoptosis. Noticeably, phenformin selectively targeted subpopulations of cells expressing JARID1B, a marker for slow cycling melanoma cells, whereas PLX4720 selectively targeted JARID1B-negative cells. Finally, in contrast to their use as single agents, the combination of phenformin and PLX4720 induced tumor regression in both nude mice bearing melanoma xenografts and in a genetically engineered BRAF(V600E)/PTEN(null)-driven mouse model of melanoma. These results strongly suggest that significant therapeutic advantage may be achieved by combining AMPK activators such as phenformin with BRAF inhbitors for the treatment of melanoma.

Phosphorylation of BRAF by AMPK impairs BRAF-KSR1 association and cell proliferation

BRAF is an oncogenic protein kinase that drives cell growth and proliferation through the MEK-ERK signaling pathway. BRAF inhibitors have demonstrated antitumor efficacy in melanoma therapy but have also been found to be associated with the development of cutaneous squamous cell carcinomas (cSCCs) in certain patients. Here, we report that BRAF is phosphorylated at Ser729 by AMP-activated protein kinase (AMPK), a critical energy sensor. This phosphorylation promotes the association of BRAF with 14-3-3 proteins and disrupts its interaction with the KSR1 scaffolding protein, leading to attenuation of the MEK-ERK signaling. We also show that phosphorylation of BRAF by AMPK impairs keratinocyte cell proliferation and cell-cycle progression. Furthermore, AMPK activation attenuates BRAF inhibitor-induced ERK hyperactivation in keratinocytes and epidermal hyperplasia in mouse skin. Our findings reveal a mechanism for regulating BRAF signaling in response to energy stress and suggest a strategy for preventing the development of cSCCs associated with BRAF-targeted therapy.

SOX10 ablation arrests cell cycle, induces senescence, and suppresses melanomagenesis

The transcription factor SOX10 is essential for survival and proper differentiation of neural crest cell lineages, where it plays an important role in the generation and maintenance of melanocytes. SOX10 is also highly expressed in melanoma tumors, but a role in disease progression has not been established. Here, we report that melanoma tumor cell lines require wild-type SOX10 expression for proliferation and SOX10 haploinsufficiency reduces melanoma initiation in the metabotropic glutamate receptor 1 (Grm1(Tg)) transgenic mouse model. Stable SOX10 knockdown in human melanoma cells arrested cell growth, altered cellular morphology, and induced senescence. Melanoma cells with stable loss of SOX10 were arrested in the G1 phase of the cell cycle, with reduced expression of the melanocyte determining factor microphthalmia-associated transcription factor, elevated expression of p21WAF1 and p27KIP2, hypophosphorylated RB, and reduced levels of its binding partner E2F1. As cell-cycle dysregulation is a core event in neoplastic transformation, the role for SOX10 in maintaining cell-cycle control in melanocytes suggests a rational new direction for targeted treatment or prevention of melanoma.

Is combination therapy the next step to overcome resistance and reduce toxicities in melanoma?

In the last few years, several drugs targeting signalling proteins critical for melanoma entered clinical evaluation. In 2011 vemurafenib (Zelboraf®, F. Hoffman-La Roche Ltd.) was approved for BRAF V600-positive melanoma and showed high overall response rates (48-53%). However recent results from a phase II clinical trial also showed that the median duration of response was 6.7months and median progression free survival was 6.8months with tumour relapse. Resistance to targeted agents is quite common and understanding of the underlying molecular mechanisms might predict response or failure. The knowledge of the mechanisms involved in intrinsic and acquired resistance to mutated BRAF is increasing swiftly. Subsequently the elucidation of these mechanisms resulted in the development of rational combination therapies to overcome toxicity and resistance. These combination therapies will be discussed.

Rapid, Microwave-Assisted Organic Synthesis of Selective (V600E)BRAF Inhibitors for Preclinical Cancer Research

We report a dramatically improved total synthesis of two highly selective (V600E)BRAF inhibitors, PLX4720 and PLX4032, that leverages microwave-assisted organic synthesis (MAOS). Compared with previously reported approaches, our novel MAOS method significantly reduces overall reaction time without compromising yield. In addition to providing a gram-scale route to these compounds for preclinical oncology research, we anticipate this approach could accelerate the synthesis of azaindoles in high-throughput, library-based formats.