PDGFRα up-regulation mediated by sonic hedgehog pathway activation leads to BRAF inhibitor resistance in melanoma cells with BRAF mutation

Molecular Susceptibility and Treatment Challenges in Melanoma

Melanoma is the most aggressive subtype of cancer, with a higher propensity to spread compared to most solid tumors. The application of OMICS approaches has revolutionized the field of melanoma research by providing comprehensive insights into the molecular alterations and biological processes underlying melanoma development and progression. This review aims to offer an overview of melanoma biology, covering its transition from primary to malignant melanoma, as well as the key genes and pathways involved in the initiation and progression of this disease. Utilizing online databases, we extensively explored the general expression profile of genes, identified the most frequently altered genes and gene mutations, and examined genetic alterations responsible for drug resistance. Additionally, we studied the mechanisms responsible for immune checkpoint inhibitor resistance in melanoma.

Antibody dependent cellular cytotoxicity-inducing anti-EGFR antibodies as effective therapeutic option for cutaneous melanoma resistant to BRAF inhibitors

Introduction

About 50% of cutaneous melanoma (CM) patients present activating BRAF mutations that can be effectively targeted by BRAF inhibitors (BRAFi). However, 20% of CM patients exhibit intrinsic drug resistance to BRAFi, while most of the others develop adaptive resistance over time. The mechanisms involved in BRAFi resistance are disparate and globally seem to rewire the cellular signaling profile by up-regulating different receptor tyrosine kinases (RTKs), such as the epidermal growth factor receptor (EGFR). RTKs inhibitors have not clearly demonstrated anti-tumor activity in BRAFi resistant models. To overcome this issue, we wondered whether the shared up-regulated RTK phenotype associated with BRAFi resistance could be exploited by using immune weapons as the antibody-dependent cell cytotoxicity (ADCC)-mediated effect of anti-RTKs antibodies, and kill tumor cells independently from the mechanistic roots.

Methods and results

By using an in vitro model of BRAFi resistance, we detected increased membrane expression of EGFR, both at mRNA and protein level in 4 out of 9 BRAFi-resistant (VR) CM cultures as compared to their parental sensitive cells. Increased EGFR phosphorylation and AKT activation were observed in the VR CM cultures. EGFR signaling appeared dispensable for maintaining resistance, since small molecule-, antibody- and CRISPR-targeting of EGFR did not restore sensitivity of VR cells to BRAFi. Importantly, immune-targeting of EGFR by the anti-EGFR antibody cetuximab efficiently and specifically killed EGFR-expressing VR CM cells, both in vitro and in humanized mouse models in vivo, triggering ADCC by healthy donors' and patients' peripheral blood cells.

Conclusion

Our data demonstrate the efficacy of immune targeting of RTKs expressed by CM relapsing on BRAFi, providing the proof-of-concept supporting the assessment of anti-RTK antibodies in combination therapies in this setting. This strategy might be expected to concomitantly trigger the crosstalk of adaptive immune response leading to a complementing T cell immune rejection of tumors.

Kinetics of RTK activation determine ERK reactivation and resistance to dual BRAF/MEK inhibition in melanoma

The combination of BRAF and MEK inhibitors (BRAFi/MEKi) has shown promising response rates in treating BRAF-mutant melanoma by inhibiting ERK activation. However, treatment efficacy is limited by the emergence of drug-tolerant persister cells (persisters). Here, we show that the magnitude and duration of receptor tyrosine kinase (RTK) activation determine ERK reactivation and persister development. Our single-cell analysis reveals that only a small subset of melanoma cells exhibits effective RTK and ERK activation and develops persisters, despite uniform external stimuli. The kinetics of RTK activation directly influence ERK signaling dynamics and persister development. These initially rare persisters form major resistant clones through effective RTK-mediated ERK activation. Consequently, limiting RTK signaling suppresses ERK activation and cell proliferation in drug-resistant cells. Our findings provide non-genetic mechanistic insights into the role of heterogeneity in RTK activation kinetics in ERK reactivation and BRAFi/MEKi resistance, suggesting potential strategies for overcoming drug resistance in BRAF-mutant melanoma.

Non-genetic adaptive resistance to KRASG12C inhibition: EMT is not the only culprit

Adaptions to therapeutic pressures exerted on cancer cells enable malignant progression of the tumor, culminating in escape from programmed cell death and development of resistant diseases. A common form of cancer adaptation is non-genetic alterations that exploit mechanisms already present in cancer cells and do not require genetic modifications that can also lead to resistance mechanisms. Epithelial-to-mesenchymal transition (EMT) is one of the most prevalent mechanisms of adaptive drug resistance and resulting cancer treatment failure, driven by epigenetic reprogramming and EMT-specific transcription factors. A recent breakthrough in cancer treatment is the development of KRASG12C inhibitors, which herald a new era of therapy by knocking out a unique substitution of an oncogenic driver. However, these highly selective agents targeting KRASG12C, such as FDA-approved sotorasib (AMG510) and adagrasib (MRTX849), inevitably encounter multiple mechanisms of drug resistance. In addition to EMT, cancer cells can hijack or rewire the sophisticated signaling networks that physiologically control cell proliferation, growth, and differentiation to promote malignant cancer cell phenotypes, suggesting that inhibition of multiple interconnected signaling pathways may be required to block tumor progression on KRASG12C inhibitor therapy. Furthermore, the tumor microenvironment (TME) of cancer cells, such as tumor-infiltrating lymphocytes (TILs), contribute significantly to immune escape and tumor progression, suggesting a therapeutic approach that targets not only cancer cells but also the TME. Deciphering and targeting cancer adaptions promises mechanistic insights into tumor pathobiology and improved clinical management of KRASG12C-mutant cancer. This review presents recent advances in non-genetic adaptations leading to resistance to KRASG12C inhibitors, with a focus on oncogenic pathway rewiring, TME, and EMT.

The future of targeted kinase inhibitors in melanoma

Melanoma is a cancer of the pigment-producing cells of the body and its incidence is rising. Targeted inhibitors that act against kinases in the MAPK pathway are approved for BRAF-mutant metastatic cutaneous melanoma and increase patients' survival. Response to these therapies is limited by drug resistance and is less durable than with immune checkpoint inhibition. Conversely, rare melanoma subtypes have few therapeutic options for advanced disease and MAPK pathway targeting agents show minimal anti-tumor effects. Nevertheless, there is a future for targeted kinase inhibitors in melanoma: in new applications such as adjuvant or neoadjuvant therapy and in novel combinations with immunotherapies or other targeted therapies. Pre-clinical studies continue to identify tumor dependencies and their corresponding actionable drug targets, paving the way for rational targeted kinase inhibitor combinations as a personalized medicine approach for melanoma.

Ornidazole suppresses CD133+ melanoma stem cells via inhibiting hedgehog signaling pathway and inducing multiple death pathways in a mouse model

AIM

To evaluate the inhibitory effects of ornidazole on the proliferation and migration of metastatic melanoma cell line (B16F10) in vitro and its anti-cancer effects in vivo using a melanoma mouse model.

METHODS

We investigated the effects of ornidazole on cell viability (Crystal Violet and MTT assay) and migration ability (wound-healing assay) of B16F10 melanoma cells, and its ability to trigger DNA damage (Comet assay) in vitro. We also sorted CD133+ and CD133- cells from B16F10 melanoma cell line and injected them subcutaneously into Swiss albino mice to induce tumor formation. Tumor-bearing mice were divided into control and treatment groups. Treatment group received intraperitoneal ornidazole injections. Tumors were resected. Real-time polymerase chain reaction was used to determine the expression of genes involved into Sonic hedgehog (Shh) signaling pathway, stemness, apoptosis, endoplasmic reticulum (ER) stress, ER stress-mediated apoptosis, and autophagy. Shh signaling pathway-related proteins and CD133 protein were analyzed by ELISA.

RESULTS

Ornidazole effectively induced DNA damage in CD133+ melanoma cells and reduced their viability and migration ability in vitro. Moreover, it significantly suppressed tumor growth in melanoma mouse model seemingly by inhibiting the Shh signaling pathway and ER-stress mediated autophagy, as well as by activating multiple apoptosis pathways.

CONCLUSIONS

Our preclinical findings suggest the therapeutic potential of ornidazole in the treatment of metastatic melanoma. However, larger and more comprehensive studies are required to validate our results and to further explore the safety and clinical effectiveness of ornidazole.

Potential Biomarkers of Skin Melanoma Resistance to Targeted Therapy—Present State and Perspectives

Simple Summary

Around 5–10% of advanced melanoma patients progress early on anti-BRAF targeted therapy and 20–30% respond only with the stabilization of the disease. Presumably, these patients could benefit more from first-line immunotherapy. Resistance to BRAF/MEK inhibitors is generated by genetic and non-genetic factors inherent to a tumor or acquired during therapy. Some of them are well documented as a cause of treatment failure. They are potential predictive markers that could improve patients’ selection for both standard and also alternative therapy as some of them have therapeutic potential. Here, a summary of the most promising predictive and therapeutic targets is presented. This up-to-date knowledge may be useful for further study on implementing more accurate genetic/molecular tests in melanoma treatment.

Abstract

Melanoma is the most aggressive skin cancer, the number of which is increasing worldwide every year. It is completely curable in its early stage and fatal when spread to distant organs. In addition to new therapeutic strategies, biomarkers are an important element in the successful fight against this cancer. At present, biomarkers are mainly used in diagnostics. Some biological indicators also allow the estimation of the patient’s prognosis. Still, predictive markers are underrepresented in clinics. Currently, the only such indicator is the presence of the V600E mutation in the BRAF gene in cancer cells, which qualifies the patient for therapy with inhibitors of the MAPK pathway. The identification of response markers is particularly important given primary and acquired resistance to targeted therapies. Reliable predictive tests would enable the selection of patients who would have the best chance of benefiting from treatment. Here, up-to-date knowledge about the most promising genetic and non-genetic resistance-related factors is described. These are alterations in MAPK, PI3K/AKT, and RB signaling pathways, e.g., due to mutations in NRAS, RAC1, MAP2K1, MAP2K2, and NF1, but also other changes activating these pathways, such as the overexpression of HGF or EGFR. Most of them are also potential therapeutic targets and this issue is also addressed here.

Hedgehog-signalling in papillary fibroblasts is essential for hair follicle regeneration during wound healing

Patients suffering from large scars such as burn victims not only encounter aesthetical challenges but also ongoing itching or pain that substantially deteriorates their quality of life. Skin appendages such as hair follicles (HFs) rarely regenerate within the healing wound. As they are crucial for skin homeostasis and the lack thereof constitutes one of the main limitations to scarless wound healing, their regeneration represents a major objective for regenerative medicine. Fibroblasts, the main resident cell type of the skin dermis, mediate embryonic HF morphogenesis and are particularly involved in wound healing as they orchestrate extracellular matrix (ECM) remodeling and collagen deposition in the wound bed. Importantly, dermal fibroblasts originate from two distinct developmental lineages with unique functions that differently mediate the response to epidermal signals such as Hedgehog (Hh)-signalling. In the present study, we show that Hh-signalling in the reticular fibroblast lineage promotes the initial phase of wound repair, possibly by modulating angiogenesis and fibroblast proliferation, while Hh-signalling in papillary fibroblasts is essential to induce de novo HF formation within the healing wound.

Cholesterol Regulates the Tumor Adaptive Resistance to MAPK Pathway Inhibition

Although targeted MAPK pathway inhibition has achieved remarkable patient responses in many cancers, the development of resistance has remained a critical challenge. Adaptive tumor response underlies the drug resistance. Furthermore, such bypass mechanisms often lead to the activation of many pro-survival kinases, which complicates the rational design of combination therapies. Here, we performed global tyrosine phosphoproteomic (pTyr) analyses and demonstrated that targeted MAPK signaling inhibition in melanoma leads to a profound remodeling of the pTyr proteome. Intriguingly, altered cholesterol metabolism might drive, in a coordinated fashion, the activation of these kinases. Indeed, we found an accumulation of intracellular cholesterol in melanoma cells (with BRAF^V600E mutations) and non-small cell lung cancer cells (with KRAS^G12C mutations) treated with MAPK and KRAS^G12C inhibitors, respectively. Importantly, depletion of cholesterol not only prevents the feedback activation of pTyr signaling but also enhances the cytotoxic effects of MAPK pathway inhibitors, both in vitro and in vivo. Together, our findings suggest that cholesterol contributes to the tumor adaptive response upon targeted MAPK pathway inhibitors. These results also suggest that MAPK pathway inhibitors could be combined with cholesterol-lowering agents to achieve a more complete and durable response in tumors with hyperactive MAPK signaling.

Mitochondrial Metabolism in Melanoma

Melanoma and its associated alterations in cellular pathways have been growing areas of interest in research, especially as specific biological pathways are being elucidated. Some of these alterations include changes in the mitochondrial metabolism in melanoma. Many mitochondrial metabolic changes lead to differences in the survivability of cancer cells and confer resistance to targeted therapies. While extensive work has gone into characterizing mechanisms of resistance, the role of mitochondrial adaptation as a mode of resistance is not completely understood. In this review, we wish to explore mitochondrial metabolism in melanoma and how it impacts modes of resistance. There are several genes that play a major role in melanoma mitochondrial metabolism which require a full understanding to optimally target melanoma. These include BRAF, CRAF, SOX2, MCL1, TRAP1, RHOA, SRF, SIRT3, PTEN, and AKT1. We will be discussing the role of these genes in melanoma in greater detail. An enhanced understanding of mitochondrial metabolism and these modes of resistance may result in novel combinatorial and sequential therapies that may lead to greater therapeutic benefit.

RICTOR Affects Melanoma Tumorigenesis and Its Resistance to Targeted Therapy

The network defined by phosphatidylinositol-3-kinase (PI3K), AKT, and mammalian target of rapamycin (mTOR) plays a major role in melanoma oncogenesis and has been implicated in BRAF inhibitor resistance. The central role of RICTOR (rapamycin-insensitive companion of mTOR) in this pathway has only recently begun to be unraveled. In the present study, we assessed the role of mTORC2/RICTOR in BRAF-mutated melanomas and their resistance to BRAF inhibition. We showed that RICTOR was significantly overexpressed in melanoma and associated with bad prognoses. RICTOR overexpression stimulated melanoma-initiating cells (MICs) with ‘stemness’ properties. We also showed that RICTOR contributed to melanoma resistance to BRAF inhibitors and rendered the cells very sensitive to mTORC2 inhibition. We highlighted a connection between mTORC2/RICTOR and STAT3 in resistant cells and revealed an interaction between RAS and RICTOR in resistant melanoma, which, when disrupted, impeded the proliferation of resistant cells. Therefore, as a key signaling node, RICTOR contributes to BRAF-dependent melanoma development and resistance to therapy and, as such, is a valuable therapeutic target in melanoma.

Metalloproteinases Suppression Driven by the Curcumin Analog DM-1 Modulates Invasion in BRAF-Resistant Melanomas

BACKGROUND

Melanoma is the most aggressive skin cancer, and BRAF (V600E) is the most frequent mutation that led to the development of BRAF inhibitors (BRAFi). However, patients treated with BRAFi usually present recidivism after 6-9 months. Curcumin is a turmeric substance, and it has been deeply investigated due to its anti-inflammatory and antitumoral effects. Still, the low bioavailability and biodisponibility encouraged the investigation of different analogs. DM-1 is a curcumin analog and has shown an antitumoral impact in previous studies.

METHODS

Evaluated DM-1 stability and cytotoxic effects for BRAFi-sensitive and resistant melanomas, as well as the role in the metalloproteinases modulation.

RESULTS

DM-1 showed growth inhibitory potential for melanoma cells, demonstrated by reduction of colony formation, migration and endothelial tube formation, and cell cycle arrest. Subtoxic doses were able to downregulate important Metalloproteinases (MMPs) related to invasiveness, such as MMP-1, -2 and -9. Negative modulations of TIMP-2 and MMP-14 reduced MMP-2 and -9 activity; however, the reverse effect is seen when increased TIMP-2 and MMP-14 resulted in raised MMP-2.

CONCLUSION

These findings provide essential details into the functional role of DM-1 in melanomas, encouraging further studies in the development of combinatorial treatments for melanomas.

Resistance to Targeted Therapy and RASSF1A Loss in Melanoma: What Are We Missing?

Melanoma is one of the most aggressive forms of skin cancer and is therapeutically challenging, considering its high mutation rate. Following the development of therapies to target BRAF, the most frequently found mutation in melanoma, promising therapeutic responses were observed. While mono- and combination therapies to target the MAPK cascade did induce a therapeutic response in BRAF-mutated melanomas, the development of resistance to MAPK-targeted therapies remains a challenge for a high proportion of patients. Resistance mechanisms are varied and can be categorised as intrinsic, acquired, and adaptive. RASSF1A is a tumour suppressor that plays an integral role in the maintenance of cellular homeostasis as a central signalling hub. RASSF1A tumour suppressor activity is commonly lost in melanoma, mainly by aberrant promoter hypermethylation. RASSF1A loss could be associated with several mechanisms of resistance to MAPK inhibition considering that most of the signalling pathways that RASSF1A controls are found to be altered targeted therapy resistant melanomas. Herein, we discuss resistance mechanisms in detail and the potential role for RASSF1A reactivation to re-sensitise BRAF mutant melanomas to therapy.

The Role of Notch, Hedgehog, and Wnt Signaling Pathways in the Resistance of Tumors to Anticancer Therapies

Resistance to therapy is the major hurdle in the current cancer management. Cancer cells often rewire their cellular process to alternate mechanisms to resist the deleterious effect mounted by different therapeutic approaches. The major signaling pathways involved in the developmental process, such as Notch, Hedgehog, and Wnt, play a vital role in development, tumorigenesis, and also in the resistance to the various anticancer therapies. Understanding how cancer utilizes these developmental pathways in acquiring the resistance to the multi-therapeutic approach cancer can give rise to a new insight of the anti-therapy resistance mechanisms, which can be explored for the development of a novel therapeutic approach. We present a brief overview of Notch, Hedgehog, and Wnt signaling pathways in cancer and its role in providing resistance to various cancer treatment modalities such as chemotherapy, radiotherapy, molecular targeted therapy, and immunotherapy. Understanding the importance of these molecular networks will provide a rational basis for novel and safer combined anticancer therapeutic approaches for the improvement of cancer treatment by overcoming drug resistance.

Crenolanib Regulates ERK and AKT/mTOR Signaling Pathways in RAS/BRAF-Mutated Colorectal Cancer Cells and Organoids

Recently developed molecularly targeted therapies such as EGFR inhibitors have notably improved the prognosis of patients with cancer. However, patients with KRAS and BRAF mutations do not currently benefit from these therapies. Here, we aimed to examine potential effects of crenolanib as a new molecularly targeted therapy in colorectal cancer. We used multiple colorectal cancer cell lines to investigate the growth-inhibitory effect of crenolanib and its effect in combination with other cytotoxic agents. Primary cultures of patient-derived organoids (PDO), a model that reflects the heterogeneity of clinical colorectal cancer, were used to further validate the effects of crenolanib. Unlike cetuximab, crenolanib remarkably suppressed ERK and AKT/mTOR pathways in HT29 cells with BRAF mutation and in HCT116 cells with KRAS mutation with corresponding growth-suppressing effects. Additive or synergistic effects were observed in treatments with combination of crenolanib and other cytotoxic drugs. Moreover, crenolanib suppressed the expression of stem cell markers, such as OCT4, NANOG, and SOX2. These observations were substantiated in seven PDOs with KRAS mutation and two PDOs without KRAS/BRAF mutations, with crenolanib suppressing the growth of all PDOs regardless of their KRAS mutation status. Furthermore, crenolanib abrogated PDGF- and TGFβ-induced increase of OCT4-positive cells in PDOs. Together, these findings suggest that crenolanib may have clinical utility for patients with colorectal cancer, especially patients with KRAS/BRAF mutations. IMPLICATIONS: These findings indicate that crenolanib can be a useful target agent for patients with colorectal cancer, especially patients with KRAS/BRAF mutations.

The Carbonic Anhydrase IX inhibitor SLC-0111 as emerging agent against the mesenchymal stem cell-derived pro-survival effects on melanoma cells

Mesenchymal stem cells (MSC) take part to solid tumour-associated stroma and critically influence progression of malignancy. Our study represents a striking example of melanoma progression to a more malignant and resistant phenotype promoted by MSC and the possibility to contrast this diabolic liaison using CAIX inhibitors. In particular, we demonstrated that melanoma cells exposed to a MSC-conditioned medium switch to a more malignant phenotype, characterised by resistance to programmed cell death and endowed with an epithelial-to-mesenchymal transition and stem cell characteristics. These effects were reversed abrogating MSC CAIX activity using SLC-0111, a CAIX inhibitor. Moreover, the acquisition by melanoma cells of a Vemurafenib-resistant phenotype upon MSC-conditioned medium exposure was removed when MSC were treated with SLC-0111. Therefore, MSC may profoundly reprogramme melanoma cells towards a wide resistant phenotype through CAIX involvement, as the use of SLC-0111 is able to contrast the development of this highly risky adaptation for disease progression.

BRAF paradox breakers PLX8394, PLX7904 are more effective against BRAFV600Ε CRC cells compared with the BRAF inhibitor PLX4720 and shown by detailed pathway analysis

PLX7904 and PLX8394 are novel BRAFV600E inhibitors-BRAFi that are designed to evade the paradoxical MAPK activation, a trait for the name "paradox breakers"-PB. Current FDA approved inhibitors (Vemurafenib, Dabrafenib, Encorafenib) although improved progression-free survival of mtBRAF melanoma patients suffer from this treatment related side effect. mtBRAF Colorectal Cancer (CRC) is resistant to the approved BRAF inhibitors, although combinatorial treatment co-targeting BRAF and EGFR/MEK is offering a promising prospect. In an effort to explore the potential of the novel BRAF inhibitors-PB to impede CRC cell proliferation, they were tested on RKO, HT29 and Colo-205 cells, bearing the BRAFV600E mutation. This study shows that the BRAF paradox breakers PLX7904 and PLX8394 cause a more prolonged MAPK pathway inhibition and achieve a stronger blockage of proliferation and reduced viability than PLX4720, the sister compound of Vemurafenib. In some treatment conditions, cells can undergo apoptosis. Genomic analysis on the more resistant RKO cells treated with PLX7904, PLX8394 and PLX4720 showed similar gene expression pattern, but the alterations imposed by the PB were more intense. Bioinformatic analysis resulted in a short list of genes representing potential master regulators of the cellular response to BRAF inhibitors' treatments. From our results, it is clear that the BRAF paradox breakers present a notable differential regulation of major pathways, like MAPK signalling, apoptosis, cell cycle, or developmental signalling pathways. Combinatorial treatments of BRAFi with Mcl-1 and Notch modulators show a better effect than mono-treatments. Additional pathways could be further exploited in novel efficient combinatorial treatment protocols with BRAFi.

Mechanisms of Acquired BRAF Inhibitor Resistance in Melanoma: A Systematic Review

This systematic review investigated the literature on acquired v-raf murine sarcoma viral oncogene homolog B1 (BRAF) inhibitor resistance in patients with melanoma. We searched MEDLINE for articles on BRAF inhibitor resistance in patients with melanoma published since January 2010 in the following areas: (1) genetic basis of resistance; (2) epigenetic and transcriptomic mechanisms; (3) influence of the immune system on resistance development; and (4) combination therapy to overcome resistance. Common resistance mutations in melanoma are BRAF splice variants, BRAF amplification, neuroblastoma RAS viral oncogene homolog (NRAS) mutations and mitogen-activated protein kinase kinase 1/2 (MEK1/2) mutations. Genetic and epigenetic changes reactivate previously blocked mitogen-activated protein kinase (MAPK) pathways, activate alternative signaling pathways, and cause epithelial-to-mesenchymal transition. Once BRAF inhibitor resistance develops, the tumor microenvironment reverts to a low immunogenic state secondary to the induction of programmed cell death ligand-1. Combining a BRAF inhibitor with a MEK inhibitor delays resistance development and increases duration of response. Multiple other combinations based on known mechanisms of resistance are being investigated. BRAF inhibitor-resistant cells develop a range of 'escape routes', so multiple different treatment targets will probably be required to overcome resistance. In the future, it may be possible to personalize combination therapy towards the specific resistance pathway in individual patients.

Drug resistance of BRAF-mutant melanoma: Review of up-to-date mechanisms of action and promising targeted agents

Melanoma onset and progression are associated with a high variety of activating mutations in the MAPK-pathway, most frequently involving BRAF (35-45%) and NRAS (15-25%) genes, but also c-KIT and PTEN. Targeted therapies with BRAF and MEK inhibitors showed promising results over the past years, but it is known that most responses are temporary, and almost all of patients develop a tumor relapse within one year. Different drug-resistance mechanisms underlie the progression of disease and activation of both MAPK and PI3K/AKT/mTOR pathways. Therefore, in this article we reviewed the main studies about clinical effects of several target inhibitors, describing properly the most prominent mechanisms of both intrinsic and acquired resistance. Furthermore, suggestive strategies for overcoming drug resistance and the most recent alternative combination therapies to optimize the use of MAPK pathway inhibitors were also discussed.

Safety and Tolerability of Sonic Hedgehog Pathway Inhibitors in Cancer

The hedgehog pathway, for which sonic hedgehog (Shh) is the most prominent ligand, is highly conserved and is tightly associated with embryonic development in a number of species. This pathway is also tightly associated with the development of several types of cancer, including basal cell carcinoma (BCC) and acute promyelocytic leukemia, among many others. Inactivating mutations in Patched-1 (PTCH1), leading to ligand-independent pathway activation, are frequent in several cancer types, but most prominent in BCC. This has led to the development of several compounds targeting this pathway as a cancer therapeutic. These compounds target the inducers of this pathway in Smoothened (SMO) and the GLI transcription factors, although targeting SMO has had the most success. Despite the many attempts at targeting this pathway, only three US FDA-approved drugs for cancers affect the Shh pathway. Two of these compounds, vismodegib and sonidegib, target SMO to suppress signaling from either PTCH1 or SMO mutations that lead to upregulation of the pathway. The other approved compound is arsenic trioxide, which can suppress this pathway at the level of the GLI proteins, although current evidence suggests it also has other targets. This review focuses on the safety and tolerability of these clinically approved drugs targeting the Shh pathway, along with a discussion on other Shh pathway inhibitors being developed.

EGFR/uPAR interaction as druggable target to overcome vemurafenib acquired resistance in melanoma cells

Background

BRAF inhibitor (BRAF-I) therapy for melanoma patients harboring the V600E mutation is initially highly effective, but almost all patients relapse within a few months. Understanding the molecular mechanisms behind BRAF-I responsiveness and acquired resistance is therefore an important issue. Here we assessed the role of urokinase type plasminogen activator receptor (uPAR) as a potentially valuable biomarker in the acquisition of BRAF-I resistance in V600E mutant melanoma cells.

Methods

We examined uPAR and EGFR levels by real time PCR and western blot analysis. uPAR loss of function was realized by knocking down uPAR by RNAi or using M25, a peptide that uncouples uPAR-integrin interaction. We investigated uPAR-β1integrin-EGFR association by co-immunoprecipitation and confocal immuno-fluorescence analysis. Acquired resistance to BRAF-I was generated by chronic exposure of cells to vemurafenib.

Findings

We proved that uPAR knockdown in combination with vemurafenib inhibits melanoma cell proliferation to greater extent than either treatment alone causing a decrease in AKT and ERK1/2 phosphorylation. Conversely, we demonstrated that uPAR enforced over-expression results in reduced sensitivity to BRAF inhibition. Moreover, by targeting uPAR and EGFR interaction with an integrin antagonist peptide we restored vemurafenib responsiveness in melanoma resistant cells. Furthermore, we found significant detectable uPAR and EGFR levels in tumor biopsies of 4 relapsed patients.

Interpretation

We disclosed an unpredicted mechanism of reduced sensitiveness to BRAF inhibition, driven by elevated levels of uPAR and identified a potential therapeutic strategy to overcome acquired resistance.

Funds

Associazione Italiana Ricerca sul Cancro (AIRC); Ente Cassa di Risparmio di Firenze.

BRAF inhibition causes resilience of melanoma cell lines by inducing the secretion of FGF1

Approximately half of all melanoma patients harbour activating mutations in the serine/threonine kinase BRAF. This is the basis for one of the main treatment strategies for this tumor type, the targeted therapy with BRAF and MEK inhibitors. While the initial responsiveness to these drugs is high, resistance develops after several months, frequently at sites of the previously responding tumor. This indicates that tumor response is incomplete and that a certain tumor fraction survives even in drug-sensitive patients, e.g., in a therapy-induced senescence-like state. Here, we show in several melanoma cell lines that BRAF inhibition induces a secretome with stimulating effect on fibroblasts and naive melanoma cells. Several senescence-associated factors were found to be transcribed and secreted in response to BRAF or MEK inhibition, among them members of the fibroblast growth factor family. We identified the growth factor FGF1 as mediator of resilience towards BRAF inhibition, which limits the pro-apoptotic effects of the drug and activates fibroblasts to secrete HGF. FGF1 regulation was mediated by the PI3K pathway and by FRA1, a direct target gene of the MAPK pathway. When FGFR inhibitors were applied in parallel to BRAF inhibitors, resilience was broken, thus providing a rationale for combined therapeutical application.

Fibroblasts potentiate melanoma cells in vitro invasiveness induced by UV-irradiated keratinocytes

Melanoma represents a malignant disease with steadily increasing incidence. UV-irradiation is a recognized key factor in melanoma initiation. Therefore, the efficient prevention of UV tissue damage bears a critical potential for melanoma prevention. In this study, we tested the effect of UV irradiation of normal keratinocytes and their consequent interaction with normal and cancer-associated fibroblasts isolated from melanoma, respectively. Using this model of UV influenced microenvironment, we measured melanoma cell migration in 3-D collagen gels. These interactions were studied using DNA microarray technology, immunofluorescence staining, single cell electrophoresis assay, viability (dead/life) cell detection methods, and migration analysis. We observed that three 10 mJ/cm² fractions at equal intervals over 72 h applied on keratinocytes lead to a 50% increase (p < 0.05) in in vitro invasion of melanoma cells. The introduction cancer-associated fibroblasts to such model further significantly stimulated melanoma cells in vitro invasiveness to a higher extent than normal fibroblasts. A panel of candidate gene products responsible for facilitation of melanoma cells invasion was defined with emphasis on IL-6, IL-8, and CXCL-1. In conclusion, this study demonstrates a synergistic effect between cancer microenvironment and UV irradiation in melanoma invasiveness under in vitro condition.

Targeting Colon Cancers with Mutated BRAF and Microsatellite Instability

The subgroup of colon cancer (CRC) characterized by mutation in the BRAF gene and high mutation rate in the genomic DNA sequence, known as the microsatellite instability (MSI) phenotype, accounts for roughly 10% of the patients and derives from polyps with a serrated morphology. In this review, both features are discussed with regard to therapeutic opportunities. The most prevalent cancer-associated BRAF mutation is BRAF V600E that causes constitutive activation of the pro-proliferative MAPK pathway. Unfortunately, the available BRAF-specific inhibitors had little clinical benefit for metastatic CRC patients due to adaptive MAPK reactivation. Recent contributions for the development of new combination therapy approaches to pathway inhibition will be highlighted. In addition, we review the promising role of the recently developed immune checkpoint therapy for the treatment of this CRC subtype. The MSI phenotype of this subgroup results from an inactivated DNA mismatch repair system and leads to frameshift mutations with translation of new amino acid stretches and the generation of neo-antigens. This most likely explains the observed high degree of infiltration by tumour-associated lymphocytes. As cytotoxic lymphocytes are already part of the tumour environment, their activation by immune checkpoint therapy approaches is highly promising.

Response and Resistance to Paradox-Breaking BRAF Inhibitor in Melanomas In Vivo and Ex Vivo

FDA-approved BRAF inhibitors produce high response rates and improve overall survival in patients with BRAF V600E/K-mutant melanoma, but are linked to pathologies associated with paradoxical ERK1/2 activation in wild-type BRAF cells. To overcome this limitation, a next-generation paradox-breaking RAF inhibitor (PLX8394) has been designed. Here, we show that by using a quantitative reporter assay, PLX8394 rapidly suppressed ERK1/2 reporter activity and growth of mutant BRAF melanoma xenografts. Ex vivo treatment of xenografts and use of a patient-derived explant system (PDeX) revealed that PLX8394 suppressed ERK1/2 signaling and elicited apoptosis more effectively than the FDA-approved BRAF inhibitor, vemurafenib. Furthermore, PLX8394 was efficacious against vemurafenib-resistant BRAF splice variant-expressing tumors and reduced splice variant homodimerization. Importantly, PLX8394 did not induce paradoxical activation of ERK1/2 in wild-type BRAF cell lines or PDeX. Continued in vivo dosing of xenografts with PLX8394 led to the development of acquired resistance via ERK1/2 reactivation through heterogeneous mechanisms; however, resistant cells were found to have differential sensitivity to ERK1/2 inhibitor. These findings highlight the efficacy of a paradox-breaking selective BRAF inhibitor and the use of PDeX system to test the efficacy of therapeutic agents. Mol Cancer Ther; 17(1); 84-95. ©2017 AACR.

Crosstalk signaling in targeted melanoma therapy

Inhibition of the BRAF/MAPK pathway belongs to the standard therapies for patients with activating BRAF^V600E/K mutations. However, even in well-responding tumors, anti-tumorigenic effect and clinical benefit are only transient, and the original tumors often relapse. This demonstrates that there are remaining residual tumors, which have withstood therapy-induced apoptosis and which have the potential to resume growth. Although BRAF mutant melanoma cells seem to depend on BRAF/MAPK signaling, the inhibition of this pathway triggers several events, which modulate the tumor as well as the tumor niche. After a certain adaptation period, this can turn out to be beneficial for tumor growth and metastasis-even in cases of good initial tumor response. This review sheds light on the biology of BRAF/MEK inhibitor-sensitive melanoma cells, which survive targeted therapy and will address the crosstalk signaling events occurring in BRAF mutant melanomas when the BRAF/MAPK pathway is fully blocked. The knowledge of these events is important for potential future drug combinations, which enhance the inhibitory effect of BRAF/MEK inhibition, particularly in patients not eligible for immune therapy.

Unusual Skin Carcinomas Induced by BRAF Inhibitor for Metastatic Melanoma: A Case Report

The most frequently reported skin tumours during treatment with targeted therapies for BRAF (B type Rapidly Accelerated Fibrosarcoma kinase) mutated metastatic melanoma are squamous cell carcinomas (SCCs). Basal cell carcinomas (BCCs) have been described in such setting, but no cases of multiple and recurring tumours have been reported so far. A patient with a history of chronic sun exposure and more than 10 BCCs removed since 1998 started treatment with vemurafenib for BRAF mutated metastatic melanoma. Therapy was complicated by sporadic episodes of atrial fibrillation and by the development of recurrent, multiple and diffuse BCCs. So, vemurafenib was discontinued and dabrafenib and trametinib were started. Since then, only four BCCs occurred in the patient. Histopathological re-examination showed that most BCCs occurred under vemurafenib presented with squamous features. Such characteristic was significantly less evident before therapy start and in lesions removed under treatment with dabrafenib and trametinib. BRAF inhibition (BRAFi) without MEK inhibition induces mitogen activated kinases overactivation, with consequent skin toxicity and acquired drug resistance. The BCCs removed from our patient showed squamous features, more evident during vemurafenib monotherapy. Both the switch from vemurafenib to dabrafenib and the addition of MEK inhibitor (MEKi) might have reduced the incidence of BCCs and their squamous differentiation.

Overcoming resistance to targeted therapy with immunotherapy and combination therapy for metastatic melanoma

Resistance to targeted therapy is an ongoing problem for the successful treatment of Stage IV metastatic melanoma. For many patients, the use of targeted therapies, such as BRAF kinase inhibitors, were initially promising yet resistance inevitably occurred. Even after combining BRAF kinase inhibitors with MEK pathway inhibitors to offset re-activation of the MAP kinase pathway, resistance is still documented. Similarly, outcomes with immune checkpoint inhibitors as monotherapy were optimistic for some patients without relapse or progression, yet the majority of patients undergoing monotherapy have progressive disease. Will immunotherapy and combination therapy trials overcome resistance in metastatic melanoma? In an effort to treat resistant disease, new clinical trials evaluating the combination of immunotherapy with other therapies, such as kinase inhibitors, adoptive cell therapy, chimeric CD40 ligand to boost costimulation, or a tumor-specific oncolytic virus enhancing granulocyte macrophage colony-stimulating factor (GM-CSF) expression, are currently underway. Updated studies on the mechanisms of resistance, immune escape and options to reinvigorate immune cells support the continued discovery of new and improved forms of therapy.

Sonic Hedgehog Signaling in Thyroid Cancer

Thyroid cancer is the most common malignancy of the endocrine system. The initiation of thyroid cancer is often triggered by a genetic mutation in the phosphortidylinositol-3 kinase (PI3K) or mitogen-activated protein kinase (MAPK) pathway, such as RAS and BRAF, or by the rearrangement of growth factor receptor tyrosine kinase genes such as RET/PTC. The sonic hedgehog (Shh) pathway is evolutionarily conserved and plays an important role in the embryonic development of normal tissues and organs. Gene mutations in the Shh pathway are involved in basal cell carcinomas (BCC). Activation of the Shh pathway due to overexpression of the genes encoding the components of this pathway stimulates the growth and spread of a wide range of cancer types. The Shh pathway also plays an important role in cancer stem cell (CSC) self-renewal. GDC-0449 and LDE-225, two inhibitors of this pathway, have been approved for treating BCC and are being tested as a single agent or in combination with other drugs for treating various other cancers. Here, we review the recent findings on activation of the Shh pathway in thyroid cancer and its role in maintaining thyroid CSC self-renewal. We also summarize the recent developments on crosstalk of the Shh pathway with the MAPK and PI3K oncogenic pathways, and its implications for combination therapy.

Identification of the BRAF V600E mutation in gastroenteropancreatic neuroendocrine tumors

Genomic profiles of gastroenteropancreatic neuroendocrine tumors (GEP-NETs) are still insufficiently understood, and the genetic alterations associated with drug responses have not been studied. Here, we performed whole exome sequencing of 12 GEP-NETs from patients enrolled in a nonrandomized, open-labeled, single-center phase II study for pazopanib, and integrated our results with previously published results on pancreas (n = 12) and small intestine NETs (n = 50). The mean numbers of somatic mutations in each case varied widely from 20 to 4682. Among 12 GEP-NETs, eight showed mutations of more than one cancer-related gene, including TP53, CNBD1, RB1, APC, BCOR, BRAF, CTNNB1, EGFR, EP300, ERBB3, KDM6A, KRAS, MGA, MLL3, PTEN, RASA1, SMARCB1, SPEN, TBC1D12, and VHL. TP53 was recurrently mutated in three cases, whereas CNBD1 and RB1 mutations were identified in two cases. Three GEP-NET patients with TP53 mutations demonstrated a durable response and one small intestinal grade (G) 1 NET patient with BRAF V600E mutation showed progression after pazopanib treatment. We found BRAF V600E (G1 NET from rectum and two G3 NETs from colon) and BRAF G593S (G2 NET from pancreas) missense mutations (9.1%) in an independent cohort of 44 GEP-NETs from the rectum (n = 26), colon (n = 7), pancreas (n = 4), small intestine (n = 3), stomach (n = 3) and appendix (n = 1) by Sanger sequencing. All tumor specimens were obtained before chemotherapy. In conclusion, BRAF V600E mutation is likely to result in resistance to pazopanib but may be a potentianally actionable mutation in metastatic GEP-NETs patients.

Targeting the hedgehog transcription factors GLI1 and GLI2 restores sensitivity to vemurafenib-resistant human melanoma cells

BRAF inhibitor (BRAFi) therapy for melanoma patients harboring the V600E mutation is initially highly effective, but almost all patients relapse within a few months. Understanding the molecular mechanisms underpinning BRAFi-based therapy is therefore an important issue. Here we identified a previously unsuspected mechanism of BRAFi resistance driven by elevated Hedgehog (Hh) pathway activation that is observed in a cohort of melanoma patients after vemurafenib treatment. Specifically, we demonstrate that melanoma cell lines, with acquired in vitro-induced vemurafenib resistance, show increased levels of glioma-associated oncogene homolog 1 and 2 (GLI1/GLI2) compared with naïve cells. We also observed these findings in clinical melanoma specimens. Moreover, the increased expression of the transcription factors GLI1/GLI2 was independent of canonical Hh signaling and was instead correlated with the noncanonical Hh pathway, involving TGFβ/SMAD (transforming growth factor-β/Sma- and Mad-related family) signaling. Knockdown of GLI1 and GLI2 restored sensitivity to vemurafenib-resistant cells, an effect associated with both growth arrest and senescence. Treatment of vemurafenib-resistant cells with the GLI1/GLI2 inhibitor Gant61 led to decreased invasion of the melanoma cells in a three-dimensional skin reconstruct model and was associated with a decrease in metalloproteinase (MMP2/MMP9) expression and microphthalmia transcription factor upregulation. Gant61 monotherapy did not alter the drug sensitivity of naïve cells, but could reverse the resistance of melanoma cells chronically treated with vemurafenib. We further noted that alternating dosing schedules of Gant61 and vemurafenib prevented the onset of BRAFi resistance, suggesting that this could be a potential therapeutic strategy for the prevention of therapeutic escape. Our results suggest that targeting the Hh pathway in BRAFi-resistant melanoma may represent a viable therapeutic strategy to restore vemurafenib sensitivity, reducing or even inhibiting the acquired chemoresistance in melanoma patients.

Antitumor Activity of BRAF Inhibitor and IFNα Combination in BRAF-Mutant Melanoma

BACKGROUND

BRAF(V600E)-mediated MAPK pathway activation is associated in melanoma cells with IFNAR1 downregulation. IFNAR1 regulates melanoma cell sensitivity to IFNα, a cytokine used for the adjuvant treatment of melanoma. These findings and the limited therapeutic efficacy of BRAF-I prompted us to examine whether the efficacy of IFNα therapy of BRAF(V600E) melanoma can be increased by its combination with BRAF-I.

METHODS

BRAF/NRAS genotype, ERK activation, IFNAR1, and HLA class I expression were tested in 60 primary melanoma tumors from treatment-naive patients. The effect of BRAF-I on IFNAR1 expression was assessed in three melanoma cell lines and in four biopsies of BRAF(V600E) metastases. The antiproliferative, pro-apoptotic and immunomodulatory activity of BRAF-I and IFNα combination was tested in vitro and in vivo utilizing three melanoma cell lines, HLA class I-MA peptide complex-specific T-cells and immunodeficient mice (5 per group for survival and 10 per group for tumor growth inhibition). All statistical tests were two-sided. Differences were considered statistically significant when the P value was less than .05.

RESULTS

The IFNAR1 level was statistically significantly (P < .001) lower in BRAF(V600E) primary melanoma tumors than in BRAF wild-type tumors. IFNAR1 downregulation was reversed by BRAF-I treatment in the three melanoma cell lines (P ≤ .02) and in three out of four metastases. The IFNAR1 level in the melanoma tumors analyzed was increased as early as 10 to 14 days following the beginning of the treatment. These changes were associated with: 1) an increased susceptibility in vitro of melanoma cells to the antiproliferative (P ≤ .04), pro-apoptotic (P ≤ .009) and immunomodulatory activity, including upregulation of HLA class I antigen APM component (P ≤ .04) and MA expression as well as recognition by cognate T-cells (P < .001), of BRAF-I and IFNα combination and 2) an increased survival (P < .001) and inhibition of tumor growth of melanoma cells (P < .001) in vivo by BRAF-I and IFNα combination.

CONCLUSIONS

The described results provide a strong rationale for the clinical trials implemented in BRAF(V600E) melanoma patients with BRAF-I and IFNα combination.

Targeting the PI3K/AKT/mTOR pathway overcomes the stimulating effect of dabrafenib on the invasive behavior of melanoma cells with acquired resistance to the BRAF inhibitor

BRAF inhibitors (BRAFi) have proven clinical benefits in patients with BRAF-mutant melanoma. However, acquired resistance eventually arises. The effects of BRAFi on melanoma cell proliferation and survival have been extensively studied, and several mechanisms involved in acquired resistance to the growth suppressive activity of these drugs have been identified. Much less is known about the impact of BRAFi, and in particular of dabrafenib, on the invasive potential of melanoma cells. In the present study, the BRAF-mutant human melanoma cell line A375 and its dabrafenib-resistant subline A375R were analyzed for invasive capacity, expression of vascular endothelial growth factor receptor (VEGFR)-2, and secretion of VEGF-A and matrix metalloproteinase (MMP)-9, under basal conditions or in response to dabrafenib. The consequences of inhibiting the PI3K/AKT/mTOR pathway on A375R cell responses to dabrafenib were also evaluated. We found that A375R cells were more invasive and secreted higher levels of VEGF-A and MMP-9 as compared with A375 cells. Dabrafenib reduced invasiveness, VEGFR-2 expression and VEGF-A secretion in A375 cells, whereas it increased invasiveness, VEGF-A and MMP-9 release in A375R cells. In these latter cells, the stimulating effects of dabrafenib on the invasive capacity were markedly impaired by the anti-VEGF‑A antibody bevacizumab, or by AKT1 silencing. A375R cells were not cross-resistant to the PI3K/mTOR inhibitor GSK2126458A. Moreover, this inhibitor given in combination with dabrafenib efficiently counteracted the stimulating effects of the BRAFi on invasiveness and VEGF-A and MMP-9 secretion. Our data demonstrate that melanoma cells with acquired resistance to dabrafenib possess a more invasive phenotype which is further stimulated by exposure to the drug. Substantial evidence indicates that continuing BRAFi therapy beyond progression produces a clinical benefit. Our results suggest that after the development of resistance, a regimen combining BRAFi with bevacizumab or with inhibitors of the PI3K/AKT/mTOR pathway might be more effective than BRAFi monotherapy.

The c-Jun/RHOB/AKT pathway confers resistance of BRAF-mutant melanoma cells to MAPK inhibitors

The response of BRAF-mutant melanoma patients to BRAF inhibitors is dramatically impaired by secondary resistances and rapid relapse. So far, the molecular mechanisms driving these resistances are not completely understood. Here, we show that, in BRAF-mutant melanoma cells, inhibition of BRAF or its target MEK induces RHOB expression by a mechanism that depends on the transcription factor c-Jun. In those cells, RHOB deficiency causes hypersensitivity to BRAF and MEK inhibitors-induced apoptosis. Supporting these results, loss of RHOB expression in metastatic melanoma tissues is associated with an increased progression-free survival of BRAF-mutant patients treated with vemurafenib. Following BRAF inhibition, RHOB activates AKT whose inhibition causes hypersensitivity of BRAF-mutant melanoma cells to BRAF inhibitors. In mice, AKT inhibition synergizes with vemurafenib to block tumor growth of BRAF-mutant metastatic melanoma. Our findings reveal that BRAF inhibition activates a c-Jun/RHOB/AKT pathway that promotes tumor cell survival and further support a role of this pathway in the resistance of melanoma to vemurafenib. Our data also highlight the importance of using RHOB tumor levels as a biomarker to predict vemurafenib patient's response and to select those that would benefit of the combination with AKT inhibitors.

A melanoma subtype with intrinsic resistance to BRAF inhibition identified by receptor tyrosine kinases gene-driven classification

Dysregulation of receptor tyrosine kinases (RTKs) contributes to several aspects of oncogenesis including drug resistance. In melanoma, distinct RTKs have been involved in BRAF inhibitors (BRAFi) resistance, yet the utility of RTKs expression pattern to identify intrinsically resistant tumors has not been assessed. Transcriptional profiling of RTKs and integration with a previous classification, reveals three robust subtypes in two independent datasets of melanoma cell lines and one cohort of melanoma samples. This classification was validated by Western blot in a panel of patient-derived melanoma cell lines. One of the subtypes identified here for the first time displayed the highest and lowest expression of EGFR and ERBB3, respectively, and included BRAF-mutant tumors all intrinsically resistant to BRAFi PLX4720, as assessed by analysis of the Cancer Cell Line Encyclopedia pharmacogenomic study and by in vitro growth inhibition assays. High levels of EGFR were detected, even before therapy, in tumor cells of one of three melanoma patients unresponsive to BRAFi. Use of different pharmacological inhibitors highlighted the relevance of PI3K/mTOR signaling for growth of this PLX4720-resistant subtype. Our results identify a specific molecular profile of melanomas intrinsically resistant to BRAFi and suggest the PI3K/mTOR pathway as a potential therapeutic target for these tumors.

Combinatorial drug screening and molecular profiling reveal diverse mechanisms of intrinsic and adaptive resistance to BRAF inhibition in V600E BRAF mutant melanomas

Over half of BRAFV600E melanomas display intrinsic resistance to BRAF inhibitors, in part due to adaptive signaling responses. In this communication we ask whether BRAFV600E melanomas share common adaptive responses to BRAF inhibition that can provide clinically relevant targets for drug combinations. We screened a panel of 12 treatment-naïve BRAFV600E melanoma cell lines with MAP Kinase pathway inhibitors in pairwise combination with 58 signaling inhibitors, assaying for synergistic cytotoxicity. We found enormous diversity in the drug combinations that showed synergy, with no two cell lines having an identical profile. Although the 6 lines most resistant to BRAF inhibition showed synergistic benefit from combination with lapatinib, the signaling mechanisms by which this combination generated synergistic cytotoxicity differed between the cell lines. We conclude that adaptive responses to inhibition of the primary oncogenic driver (BRAFV600E) are determined not only by the primary oncogenic driver but also by diverse secondary genetic and epigenetic changes ("back-seat drivers") and hence optimal drug combinations will be variable. Because upregulation of receptor tyrosine kinases is a major source of drug resistance arising from diverse adaptive responses, we propose that inhibitors of these receptors may have substantial clinical utility in combination with inhibitors of the MAP Kinase pathway.

BMI1 induces an invasive signature in melanoma that promotes metastasis and chemoresistance

Melanoma can switch between proliferative and invasive states, which have identifying gene expression signatures that correlate with good and poor prognosis, respectively. However, the mechanisms controlling these signatures are poorly understood. In this study, we identify BMI1 as a key determinant of melanoma metastasis by which its overexpression enhanced and its deletion impaired dissemination. Remarkably, in this tumor type, BMI1 had no effect on proliferation or primary tumor growth but enhanced every step of the metastatic cascade. Consistent with the broad spectrum of effects, BMI1 activated widespread gene expression changes, which are characteristic of melanoma progression and also chemoresistance. Accordingly, we showed that up-regulation or down-regulation of BMI1 induced resistance or sensitivity to BRAF inhibitor treatment and that induction of noncanonical Wnt by BMI1 is required for this resistance. Finally, we showed that our BMI1-induced gene signature encompasses all of the hallmarks of the previously described melanoma invasive signature. Moreover, our signature is predictive of poor prognosis in human melanoma and is able to identify primary tumors that are likely to become metastatic. These data yield key insights into melanoma biology and establish BMI1 as a compelling drug target whose inhibition would suppress both metastasis and chemoresistance of melanoma.

Increased CD271 expression by the NF-kB pathway promotes melanoma cell survival and drives acquired resistance to BRAF inhibitor vemurafenib

Specific BRAFV600E inhibitors (BRAFi) are highly effective in the treatment of melanoma. However, acquired drug resistances invariably develop after the initial response. Therefore, the identification of new mechanisms of acquired resistance gives important clues towards the development of therapies that could elicit long lasting responses. Here we report that CD271 confers resistance to BRAFi in melanoma cells. The expression of CD271 is increased by BRAFi through a stimulation of tumor necrosis factor-alpha (TNFα) secretion that leads to NF-κB signaling pathway activation. CD271 is upregulated in a subset of BRAFi-resistant melanoma cells. The inhibition of TNFα/NF-κB pathway and CD271 silencing restore the BRAFi sensitivity of resistant melanoma cells. Finally, increase of CD271 expression is validated in BRAFi-resistant xenografts tumors and also in tumors from the patients who relapsed under BRAFi. In summary, these results reveal a novel TNFα/NF-κB/CD271 axis whose activation contributes to the acquisition of resistance to BRAFi and therefore may represent a novel therapeutic target to improve the efficacy of therapy in melanoma.

Landscape of Targeted Anti-Cancer Drug Synergies in Melanoma Identifies a Novel BRAF-VEGFR/PDGFR Combination Treatment

A newer generation of anti-cancer drugs targeting underlying somatic genetic driver events have resulted in high single-agent or single-pathway response rates in selected patients, but few patients achieve complete responses and a sizeable fraction of patients relapse within a year. Thus, there is a pressing need for identification of combinations of targeted agents which induce more complete responses and prevent disease progression. We describe the results of a combination screen of an unprecedented scale in mammalian cells performed using a collection of targeted, clinically tractable agents across a large panel of melanoma cell lines. We find that even the most synergistic drug pairs are effective only in a discrete number of cell lines, underlying a strong context dependency for synergy, with strong, widespread synergies often corresponding to non-specific or off-target drug effects such as multidrug resistance protein 1 (MDR1) transporter inhibition. We identified drugs sensitizing cell lines that are BRAFV600E mutant but intrinsically resistant to BRAF inhibitor PLX4720, including the vascular endothelial growth factor receptor/kinase insert domain receptor (VEGFR/KDR) and platelet derived growth factor receptor (PDGFR) family inhibitor cediranib. The combination of cediranib and PLX4720 induced apoptosis in vitro and tumor regression in animal models. This synergistic interaction is likely due to engagement of multiple receptor tyrosine kinases (RTKs), demonstrating the potential of drug- rather than gene-specific combination discovery approaches. Patients with elevated biopsy KDR expression showed decreased progression free survival in trials of mitogen-activated protein kinase (MAPK) kinase pathway inhibitors. Thus, high-throughput unbiased screening of targeted drug combinations, with appropriate library selection and mechanistic follow-up, can yield clinically-actionable drug combinations.

Mitogen-activated protein kinases and Hedgehog-GLI signaling in cancer: A crosstalk providing therapeutic opportunities?

The Hedgehog-GLI (HH-GLI) signaling is of critical importance during embryonic development, where it regulates a number of cellular processes, including patterning, proliferation and differentiation. Its aberrant activation has been linked to several types of cancer. HH-GLI signaling is triggered by binding of ligands to the transmembrane receptor patched and is subsequently mediated by transcriptional effectors belonging to the GLI family, whose function is fine tuned by a series of molecular interactions and modifications. Several HH-GLI inhibitors have been developed and are in clinical trials. Similarly, the mitogen-activated protein kinases (MAPK) are involved in a number of biological processes and play an important role in many diseases including cancer. Inhibiting molecules targeting MAPK signaling, especially those elicited by the MEK1/2-ERK1/2 pathway, have been developed and are moving into clinical trials. ERK1/2 may be activated as a consequence of aberrant activation of upstream signaling molecules or during development of drug resistance following treatment with kinase inhibitors such as those for PI3K or BRAF. Evidence of a crosstalk between HH-GLI and other oncogenic signaling pathways has been reported in many tumor types, as shown by recent reviews. Here we will focus on the interaction between HH-GLI and the final MAPK effectors ERK1/2, p38 and JNK in cancer in view of its possible implications for cancer therapy. Several reports highlight the existence of a consistent crosstalk between HH signaling and MAPK, especially with the MEK1/2-ERK1/2 pathway, and this fact should be taken into consideration for designing optimal treatment and prevent tumor relapse.

The broad-spectrum receptor tyrosine kinase inhibitor dovitinib suppresses growth of BRAF-mutant melanoma cells in combination with other signaling pathway inhibitors

BRAF inhibitors have revolutionized treatment of mutant BRAF metastatic melanomas. However, resistance develops rapidly following BRAF inhibitor treatment. We have found that BRAF-mutant melanoma cell lines are more sensitive than wild-type BRAF cells to the small molecule tyrosine kinase inhibitor dovitinib. Sensitivity is associated with inhibition of a series of known dovitinib targets. Dovitinib in combination with several agents inhibits growth more effectively than either agent alone. These combinations inhibit BRAF-mutant melanoma and colorectal carcinoma cell lines, including cell lines with intrinsic or selected BRAF inhibitor resistance. Hence, combinations of dovitinib with second agents are potentially effective therapies for BRAF-mutant melanomas, regardless of their sensitivity to BRAF inhibitors.

Immune Effects of Chemotherapy, Radiation, and Targeted Therapy and Opportunities for Combination With Immunotherapy

There have been significant advances in cancer treatment over the past several years through the use of chemotherapy, radiation therapy, molecularly targeted therapy, and immunotherapy. Despite these advances, treatments such as monotherapy or monomodality have significant limitations. There is increasing interest in using these strategies in combination; however, it is not completely clear how best to incorporate molecularly targeted and immune-targeted therapies into combination regimens. This is particularly pertinent when considering combinations with immunotherapy, as other types of therapy may have significant impact on host immunity, the tumor microenvironment, or both. Thus, the influence of chemotherapy, radiation therapy, and molecularly targeted therapy on the host anti-tumor immune response and the host anti-host response (ie, autoimmune toxicity) must be taken into consideration when designing immunotherapy-based combination regimens. We present data related to many of these combination approaches in the context of investigations in patients with melanoma and discuss their potential relationship to management of patients with other tumor types. Importantly, we also highlight challenges of these approaches and emphasize the need for continued translational research.

EPHA2 is a mediator of vemurafenib resistance and a novel therapeutic target in melanoma

BRAF(V600E) is the most common oncogenic lesion in melanoma and results in constitutive activation of the MAPK pathway and uncontrolled cell growth. Selective BRAF inhibitors such as vemurafenib have been shown to neutralize oncogenic signaling, restrain cellular growth, and improve patient outcome. Although several mechanisms of vemurafenib resistance have been described, directed solutions to overcome these resistance lesions are still lacking. Herein, we found that vemurafenib resistance can be (i) mediated by EPHA2, a member of the largest receptor tyrosine kinases (RTK) subfamily erythropoietin-producing hepatocellular (EPH) receptors, and (ii) associated with a greater phenotypic dependence on EPHA2. Furthermore, we developed a series of first-in-class EPHA2 inhibitors and show that these new compounds potently induce apoptosis, suppress viability, and abrogate tumorigenic growth of melanoma cells, including those that are resistant to vemurafenib. These results provide proof of concept that RTK-guided growth, and therapeutic resistance, can be prospectively defined and selectively targeted.

SIGNIFICANCE

In this study, we show that resistance to selective BRAF inhibitors can be mediated by the RTK EPHA2. Furthermore, direct targeting of EPHA2 can successfully suppress melanoma growth and mitigate therapeutic resistance.

Effect of p53 activity on the sensitivity of human glioblastoma cells to PARP-1 inhibitor in combination with topoisomerase I inhibitor or radiation

Poly (ADP-Ribose) polymerase-1 (PARP-1) is involved in the DNA repairing system by sensing and signaling the presence of DNA damage. Inhibition of PARP-1 is tested in combination with DNA damaging agents such as topoisomerase I inhibitors or ionizing radiations (RT) for the treatment of glioblastoma (GBM). Disruption of p53, widely prevalent in GBMs, plays a major role in DNA repairing system. The current study investigates whether p53 activity has an effect on the sensitivity of human GBM cells to PARP-1 inhibitors in combination with topoisomerase I inhibitor topotecan (TPT) and/or RT. Human GBM cell lines carrying a different functional status of p53 were treated with PARP-1 inhibitor NU1025, in combination with TPT and/or RT. Cytotoxic effects were examined by analyzing the antiproliferative activity, the cell cycle perturbations, and the DNA damage induced by combined treatments. PARP inhibition enhanced the antiproliferative activity, the cell cycle perturbations and the DNA damage induced by both TPT or RT in GBM cells. These effects were influenced by the p53 activity: cells carrying an active p53 were more sensitive to the combination of PARP inhibitor and RT, while cells carrying an inactive p53 displayed a higher sensitivity to the combination of PARP inhibitor and TPT. Our study suggests that p53 activity influences the differential sensitivity of GBM cells to combined treatments of TPT, RT, and PARP inhibitors. © 2014 International Society for Advancement of Cytometry.