BRAFi-resistant Melanoma Research Articles

Abstract 4897: An acquired vulnerability of drug resistant melanoma with therapeutic potential

Abstract BRAF(V600E) mutant melanomas treated with inhibitors of the BRAF and MEK kinases almost invariably develop resistance, which is frequently caused by reactivation of the Mitogen Activated Protein Kinase (MAPK) pathway. To identify novel treatment options for such patients, we searched for acquired vulnerabilities of MAPK inhibitor-resistant melanomas. We find that resistance to BRAF+MEK inhibitors is associated with increased levels of reactive oxygen species (ROS). Subsequent treatment with the histone deacetylase inhibitor (HDACi) vorinostat represses SLC7A11 that leads to a lethal increase in the already elevated levels of ROS in drug-resistant cells, thereby causing selective apoptotic death of only the drug resistant tumor cells. Consistently, treatment of BRAF inhibitor-resistant melanoma with HDACi in mice results in a dramatic tumor regression. In a study in patients with advanced BRAF+MEK inhibitor resistant melanoma, we find that HDACi can selectively ablate drug-resistant tumor cells, providing clinical proof of concept for the novel therapy identified here. Citation Format: Rodrigo Leite de Oliveira. An acquired vulnerability of drug resistant melanoma with therapeutic potential [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4897.

Abstract 1543: Upregulation of mannose 6-phosphate receptor (MPR) has a potential to increase the efficacy of combined therapy for melanoma patients

Abstract BRAF -targeting therapy demonstrates impressive clinical response in patients with metastatic melanoma. However, patients quickly develop resistance associated with rapid tumor progression. Therefore, additional therapeutic approaches are necessary. Our group has previously shown that transient induction of cation-independent mannose 6-phosphate receptor (MPR) is important for the antitumor effect of chemo-immunotherapy. In this study, we aimed to decipher the role of MPR during possible combination therapy of BRAF inhibitor with adoptive T-cell therapy of BRAF inhibitor-resistant melanoma. For this aim, we have used PLX4720 as a BRAFV600E inhibitor and observed that in vitro PLX4720 treatment of human V600E-positive melanoma cells, including WM-35, SK-Mel-624 and WM983B, resulted in a dose-dependent upregulation of MPR on the cell surface. Importantly, increased levels of MPR on the cell surface induced the uptake of recombinant Granzyme B, which is a major mechanism of cell killing by cytotoxic T cells (CTL). We found that PLX4720 treatment rendered WM35 cells more prone to lysis by HLA-matching tumor-infiltrating lymphocytes (TILs) and increased cell killing is not due to PLX4720 treatment itself but increased levels of MPR on the cell surface. In order to prove this, we generated MPR-overexpressing WM35 cell line and showed that upregulation of MPR, independently of PLX4720 treatment, sensitizes melanoma cells for T cell killing in vitro. We further showed that PLX4720 treatment leads to upregulation of MPR in vivo by a mouse model bearing xenograft WM35-derived subcutaneous tumors as well. According to our data, 3 days of PLX4720 treatment is enough to induce MPR in tumors and this effect continues up to 6 days after 5 days treatment ends. Unexpectedly, we found that PLX4720 caused upregulation of MPR in resistant cell lines and that those cell lines became more sensitive to cytotoxic effect of TILs. Thus, our data suggest that adoptive T-cell therapy can be potentially effective in melanoma resistant to BRAF inhibitor. (This study is supported by NIH grant P50 CA168536.) Citation Format: Cigdem Atay, Sergio Lavilla-Alonso, Dmitry Gabrilovich. Upregulation of mannose 6-phosphate receptor (MPR) has a potential to increase the efficacy of combined therapy for melanoma patients [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1543.

Effect of ABT-888 on the apoptosis, motility and invasiveness of BRAFi-resistant melanoma cells.

Melanoma is a molecularly heterogeneous disease with many genetic mutations and altered signaling pathways. Activating mutations in the BRAF oncogene are observed in approximately 50% of cutaneous melanomas and the use of BRAF inhibitor (BRAFi) compounds has been reported to improve the outcome of patients with BRAF-mutated metastatic melanoma. However, the majority of these patients develop resistance within 6–8 months following the initiation of BRAFi treatment. In this study, we examined the possible use of the poly(ADP-ribose) polymerase 1 (PARP1) inhibitor, ABT-888 (veliparib), as a novel molecule that may be successfully employed in the treatment of BRAFi-resistant melanoma cells. Sensitive and resistant to BRAFi dabrafenib A375 cells were exposed to increasing concentrations of ABT-888. Cell viability and apoptosis were assessed by MTT assay and Annexin V-FITC analysis, respectively. The cell migratory and invasive ability was investigated using the xCELLigence technology and Boyden chamber assays, respectively. ABT-888 was found to reduce cell viability and exhibited pro-apoptotic activity in melanoma cell lines, independently from the BRAF/NRAS mutation status, in a dose-dependent manner, with the maximal effect being reached in the 25–50 µM concentration range. Moreover, ABT-888 promoted apoptosis in both the sensitive and resistant A375 cells, suggesting that ABT-888 may be useful in the treatment of BRAFi-resistant subsets of melanoma cells. Finally, in accordance with the involvement of PARP1 in actin cytoskeletal machinery, we found that the cytoskeletal organization, motility and invasive capability of both the A375 and A375R cells decreased upon exposure to 5 µM ABT-888 for 24 h. On the whole, the findings of this study highlight the pivotal role of PARP1 in the migration and invasion of melanoma cells, suggesting that ABT-888 may indeed be effective, not only as a pro-apoptotic drug for use in the treatment of BRAFi-resistant melanoma cells, but also in suppressing their migratory and invasive activities.

Differential expression of ABCB5 in BRAF inhibitor-resistant melanoma cell lines

BackgroundMore than 50% of metastatic melanoma patients have a specific mutation in the serine/threonine kinase BRAF. This results in constitutive activation of the RAS-RAF-MEK-ERK-MAP kinase pathway, which causes uncontrolled cell growth. Vemurafenib (PLX4032) is an oral chemotherapeutic agent that targets the specific mutation V600E in the BRAF protein. Initial response rates are high in patients with BRAF mutant melanoma treated with a BRAF inhibitor such as vemurafenib, but resistance nearly always develops and disease progression ensues. There are several different mechanisms by which melanoma develops BRAF inhibitor resistance. One potential component of resistance is increased drug efflux. Overexpressed ABCB5 (ATP-binding cassette transporter, subfamily B, member 5) has been shown to efflux anti-cancer drugs from cancer cells. The purpose of this study is to determine whether ABCB5 is highly expressed in BRAF inhibitor-resistant melanoma cells and to evaluate whether ABCB5 is involved in the development of resistance to BRAF inhibitors in cutaneous melanoma.MethodsWe established three BRAF inhibitor-resistant melanoma cell lines with BRAF mutation. The expression level of ABCB5 in PLX-resistant cell lines was checked by real-time PCR and Western blot analysis. SK-MEL-2 melanoma cells with wild-type BRAF were used for comparison. The association of different levels of ABCB5 with the changes of ERK, p-ERK, Akt and p-Akt was also assessed by Western blotting. Re-sensitization of melanoma cells to PLX was tested by p-ERK inhibitor PD58059 and ABCB5 knockdown by ABCB5 siRNA, respectively.ResultsWe showed that ABCB5 was overexpressed in SK-MEL-28PLXr and A2058PLXr cells but not in A375PLXr cells. ABCB5 overexpression is associated with activation of p-ERK status but not Akt. Inhibition of p-ERK re-sensitized SK-MEL-28PLXr and A2058PLXr cells to PLX treatment, but knockdown of ABCB5 did not re-sensitize A2058 PLXr and SK-MEL-28 PLXr cells to PLX treatment.ConclusionThese results confirm that, even though ABCB5 was overexpressed in SK-MEL-28 and A2058 melanoma cells that develop resistance to BRAF inhibitors, ABCB5 may not be a major targetable contributor to BRAF resistance. p-ERK inhibition may play important roles in BRAF resistance in these two melanoma cell lines.

An Acquired Vulnerability of Drug-Resistant Melanoma with Therapeutic Potential

BRAF(V600E) mutant melanomas treated with inhibitors of the BRAF and MEK kinases almost invariably develop resistance that is frequently caused by reactivation of the mitogen activated protein kinase (MAPK) pathway. To identify novel treatment options for such patients, we searched for acquired vulnerabilities of MAPK inhibitor-resistant melanomas. We find that resistance to BRAF+MEK inhibitors is associated with increased levels of reactive oxygen species (ROS). Subsequent treatment with the histone deacetylase inhibitor vorinostat suppresses SLC7A11, leading to a lethal increase in the already-elevated levels of ROS in drug-resistant cells. This causes selective apoptotic death of only the drug-resistant tumor cells. Consistently, treatment of BRAF inhibitor-resistant melanoma with vorinostat in mice results in dramatic tumor regression. In a study in patients with advanced BRAF+MEK inhibitor-resistant melanoma, we find that vorinostat can selectively ablate drug-resistant tumor cells, providing clinical proof of concept for the novel therapy identified here.

1214 Palladium based nanoparticles for the treatment of advanced melanoma

Braf and MEK inhibitors are commonly used for the treatment of melanoma; however, resistance to this treatment is encountered frequently. IGFR -1 and CD44 are usually overexpressed in most advanced Braf inhibitor-resistant melanoma. Non-Braf mutated melanomas are also resistant to Braf inhibitors. Thus, a significant need exists for the treatment of these melanomas. Tris DBA Palladium (Tris DBA pd) is a novel inhibitor of N-myristoyltransferase 1 and has proven in vivo activity against melanoma. However, its usefulness is impaired by its poor solubility. To improve its therapeutic efficacy and overcome drug resistance in genetically diverse melanomas, we synthesized Tris DBA pd hyaluronic acid nanoparticles (HANP) and tagged them with IGFR -1 antibody and evaluated them against in vivo xenografts of LM36R, an aggressive human melanoma that is resistant to Braf inhibitors. We induced LM36R tumor growth in four groups of nude mice and then treated with Tris DBA pd HANP with IGFR-1 antibody, Tris DBA pd HANP, free Tris DBA pd, and empty nanoparticle through tail vein injections. We observed that the tumors in the group treated with Tris DBA pd HANP were the most responsive to treatment. In melanoma cells treated with Tris DBA pd HANP, we noted depletion of CD44 positive cells on IHC with CD44 being the receptor for hyaluronic acid, as well as a melanoma stem cell marker. Surprisingly, the IGFR1 containing HANP nanoparticle was less effective, possibly due to steric hindrance of IGFR1 and CD44 binding. This may be an important factor in the design of nanoparticles. Microarray analysis of the treated tumors compared with controls indicates a B lymphocyte response that may be mediating tumor regression. HANP Tris DBA pd nanoparticles are an effective therapy for CD44 positive tumors like melanoma, and may stimulate novel immune methods of anti-tumor immunity.

AXL/AKT axis mediated-resistance to BRAF inhibitor depends on PTEN status in melanoma.

Multiple genetic mutations within melanoma not only cause lesion-specific responses to targeted therapy but also alter the molecular route of resistance to that therapy. Inactivation of PTEN occurs in up to 30% of melanomas, frequently with a concurrent activating BRAF mutation. PTEN loss regulates both acquired and intrinsic drug resistance. Here we show that AXL/AKT axis mediated-resistance to BRAF inhibitor (BRAFi) depends upon PTEN status in melanoma. Hyperactivation of both ERK and AKT pathways was associated with BRAFi resistance in melanoma with wildtype PTEN. The PTEN-impaired melanoma cells required only the ERK resistance mechanism. Moreover, we identified AXL as a key upstream effector of AKT pathway-associated resistance to BRAFi in melanoma with wildtype PTEN, but not in melanoma with impaired PTEN. Notably, we confirmed that blocking AXL by shRNA and a small molecular inhibitor could rescue the sensitivity of resistant melanoma cells with wildtype PTEN to BRAFi and inhibit their growth in vitro and in vivo. Our study has uncovered a mechanism by which PTEN status contributes to acquired resistance to BRAFi and offers a rational strategy to guide clinical testing in pre-identified subsets of patients who relapse during treatment with BRAFi. The identified protein AXL represents a promising therapeutic target for BRAF mutant melanoma patients with wildtype PTEN.

YAP-Induced PD-L1 Expression Drives Immune Evasion in BRAFi-Resistant Melanoma.

Activation of YAP, a Hippo pathway effector, is an important resistance mechanism to BRAF inhibitor (BRAFi) in melanoma. Emerging evidence also suggests that YAP is involved in suppression of the antitumor immune response. However, the potential direct impact of YAP activity on cytotoxic T-cell immune responses has not been explored yet. Here, we show that BRAFi-resistant melanoma cells evade CD8+ T-cell immune responses in a PD-L1-dependent manner by activating YAP, which synchronously supports melanoma cell survival upon BRAF inhibition. PD-L1 expression is elevated in BRAFi-resistant melanoma cells, in which YAP is robustly activated, and YAP knockdown decreases PD-L1 expression. In addition, constitutively active YAP (YAP-5SA) increases PD-L1 expression by binding to an upstream enhancer of the PD-L1 gene and potentiating its transcription. Both BRAFi-resistant and YAP-5SA-expressing melanoma cells suppress the cytotoxic function and cytokine production of Melan-A-specific CD8+ T cells, whereas anti-PD-1 antibody reverses the YAP-mediated T-cell suppression. Moreover, nuclear enrichment of YAP in clinical melanoma samples correlates with increased PD-L1 expression. Our findings show that YAP directly mediates evasion of cytotoxic T-cell immune responses in BRAFi-resistant melanoma cells by upregulating PD-L1, and targeting of YAP-mediated immune evasion may improve prognosis of melanoma patients. Cancer Immunol Res; 6(3); 255-66. ©2018 AACR.

Glucose metabolism and NRF2 coordinate the antioxidant response in melanoma resistant to MAPK inhibitors

Targeted therapies as BRAF and MEK inhibitor combination have been approved as first-line treatment for BRAF-mutant melanoma. However, disease progression occurs in most of the patients within few months of therapy. Metabolic adaptations have been described in the context of acquired resistance to BRAF inhibitors (BRAFi). BRAFi-resistant melanomas are characterized by an increase of mitochondrial oxidative phosphorylation and are more prone to cell death induced by mitochondrial-targeting drugs. BRAFi-resistant melanomas also exhibit an enhancement of oxidative stress due to mitochondrial oxygen consumption increase. To understand the mechanisms responsible for survival of BRAFi-resistant melanoma cells in the context of oxidative stress, we have established a preclinical murine model that accurately recapitulates in vivo the acquisition of resistance to MAPK inhibitors including several BRAF or MEK inhibitors alone and in combination. Using mice model and melanoma cell lines generated from mice tumors, we have confirmed that the acquisition of resistance is associated with an increase in mitochondrial oxidative phosphorylation as well as the importance of glutamine metabolism. Moreover, we have demonstrated that BRAFi-resistant melanoma can adapt mitochondrial metabolism to support glucose-derived glutamate synthesis leading to increase in glutathione content. Besides, BRAFi-resistant melanoma exhibits a strong activation of NRF-2 pathway leading to increase in the pentose phosphate pathway, which is involved in the regeneration of reduced glutathione, and to increase in xCT expression, a component of the xc—amino acid transporter essential for the uptake of cystine required for intracellular glutathione synthesis. All these metabolic modifications sustain glutathione level and contribute to the intracellular redox balance to allow survival of BRAFi-resistant melanoma cells.

MiR-204-5p and miR-211-5p Contribute to BRAF Inhibitor Resistance in Melanoma.

Melanoma treatment with the BRAF V600E inhibitor vemurafenib provides therapeutic benefits but the common emergence of drug resistance remains a challenge. We generated A375 melanoma cells resistant to vemurafenib with the goal of investigating changes in miRNA expression patterns that might contribute to resistance. Increased expression of miR-204-5p and miR-211-5p occurring in vemurafenib-resistant cells was determined to impact vemurafenib response. Their expression was rapidly affected by vemurafenib treatment through RNA stabilization. Similar effects were elicited by MEK and ERK inhibitors but not AKT or Rac inhibitors. Ectopic expression of both miRNA in drug-naïve human melanoma cells was sufficient to confer vemurafenib resistance and more robust tumor growth in vivo Conversely, silencing their expression in resistant cells inhibited cell growth. Joint overexpression of miR-204-5p and miR-211-5p durably stimulated Ras and MAPK upregulation after vemurafenib exposure. Overall, our findings show how upregulation of miR-204-5p and miR-211-5p following vemurafenib treatment enables the emergence of resistance, with potential implications for mechanism-based strategies to improve vemurafenib responses.Significance: Identification of miRNAs that enable resistance to BRAF inhibitors in melanoma suggests a mechanism-based strategy to limit resistance and improve clinical outcomes. Cancer Res; 78(4); 1017-30. ©2017 AACR.

Targeting the Senescence-Overriding Cooperative Activity of Structurally Unrelated H3K9 Demethylases in Melanoma

Oncogene-induced senescence, e.g., in melanocytic nevi, terminates the expansion of pre-malignant cells via transcriptional silencing of proliferation-related genes due to decoration of their promoters with repressive trimethylated histone H3 lysine 9 (H3K9) marks. We show here that structurally distinct H3K9-active demethylases-the lysine-specific demethylase-1 (LSD1) and several Jumonji C domain-containing moieties (such as JMJD2C)-disable senescence and permit Ras/Braf-evoked transformation. In mouse and zebrafish models, enforced LSD1 or JMJD2C expression promoted Braf-V600E-driven melanomagenesis. A large subset of established melanoma cell lines and primary human melanoma samples presented with a collective upregulation of related and unrelated H3K9 demethylase activities, whose targeted inhibition restored senescence, even in Braf inhibitor-resistant melanomas, evoked secondary immune effects and controlled tumor growth invivo.

Inhibition of YAP function overcomes BRAF inhibitor resistance in melanoma cancer stem cells.

Treating BRAF inhibitor-resistant melanoma is an important therapeutic goal. Thus, it is important to identify and target mechanisms of resistance to improve therapy. The YAP1 and TAZ proteins of the Hippo signaling pathway are important drivers of cancer cell survival, and are BRAF inhibitor resistant factors in melanoma. We examine the role of YAP1/TAZ in melanoma cancer stem cells (MCS cells). We demonstrate that YAP1, TAZ and TEAD (TEA domain transcription factor) levels are elevated in BRAF inhibitor resistant MCS cells and enhance cell survival, spheroid formation, matrigel invasion and tumor formation. Moreover, increased YAP1, TAZ and TEAD are associated with sustained ERK1/2 activity that is not suppressed by BRAF inhibitor. Xenograft studies show that treating BRAF inhibitor-resistant tumors with verteporfin, an agent that interferes with YAP1 function, reduces YAP1/TAZ level, restores BRAF inhibitor suppression of ERK1/2 signaling and reduces tumor growth. Verteporfin is highly effective as concentrations of verteporfin that do not impact tumor formation restore BRAF inhibitor suppression of tumor formation, suggesting that co-treatment with agents that inhibit YAP1 and BRAF(V600E) may be a viable therapy for cancer stem cell-derived BRAF inhibitor-resistant melanoma.

Degradation of AMPK-α1 sensitizes BRAF inhibitor-resistant melanoma cells to arginine deprivation.

Melanomas harboring BRAF mutation (V600E) are known to recur frequently following treatment with BRAF inhibitors (BRAFi) despite a high initial response rate. Our previous study has uncovered that BRAFi‐resistant melanoma (BR) cells are vulnerable to arginine deprivation. It has been reported that naïve melanoma cells undergo autophagy and re‐express argininosuccinate synthetase 1 (ASS1) to enable them to synthesize arginine for survival when encountering arginine deprivation. Abolishing these two factors in BR cells confers sensitivity to arginine deprivation. In this report, we further demonstrated that downregulation of AMPK‐α1 in BR cells is a major factor contributing to impairment of autophagy as evidenced by decreased autophagosome formation. These BR cells also showed a metabolic shift from glucose to arginine dependence, which was supported by decreased expressions of GLUT1 (glucose transporter) and hexokinase II (HKII) coupled with less glucose uptake but high levels of arginine transporter CAT‐2 expression. Furthermore, silencing CAT‐2 expression also distinctly attenuated BR cell proliferation. Notably, when naïve melanoma cells became BR cells by long‐term exposure to BRAFi, a stepwise degradation of AMPK‐α1 was initiated via ubiquitin‐proteasome system (UPS). We discovered that a novel E3 ligase, RING finger 44 (RNF44), is responsible for promoting AMPK‐α1 degradation in BR cells. RNF44 expression in BR cells was upregulated by transcription factor CREB triggered by hyperactivation of ERK/AKT. High levels of RNF44 corresponding to low levels of AMPK‐α1 appeared in BR xenografts and melanoma tumor samples from BR and BRAFi/MEK inhibitor (MEKi)‐resistant (BMR) melanoma patients. Similar to BR cells, BMR cells were also sensitive to arginine deprivation. Our study provides a novel insight into the mechanism whereby BRAFi or BRAFi/MEKi resistance drives proteasomal degradation of AMPK‐α1 and consequently regulates autophagy and metabolic reprogramming in melanoma cells.

Highlights from Recent Cancer Literature

Highlights from Recent Cancer Literature

Abstract IA10: Finding vulnerabilities of drug resistant cancers

Abstract Intrinsic and acquired drug resistance represent major obstacles to effective cures for cancer. Understanding these mechanisms is instrumental for the design of strategies to counter these resistance mechanisms. In my presentation, I will focus on an example of intrinsic drug resistance and one of acquired drug resistance. As an example of intrinsic drug resistance, I will discuss the lack of response to MEK inhibitors seen in KRAS mutant tumors of various. The consistently poor response of KRAS mutant tumors to inhibition of its downstream kinases is unexpected, as it defies the concept of oncogene addiction. Nevertheless, a subset of tumors responds well to MEK inhibition, but it remains unclear which biomarkers can be used to identify MEK inhibitor sensitive cancer cells. I will demonstrate that inactivating mutations in both MAP3K1 and MAP2K4 will confer strong sensitivity to MEK inhibition in cancers of any tissue origin. Given that most major cancers have a mutation rate in these two genes of around 10%, the use of these mutations may help select patient subgroups that benefit from MEK inhibitor therapy. As an example of acquired drug resistance, I will discuss resistance of BRAF(V600E) mutant melanomas to drugs that target the BRAF and/or MEK kinases. This acquired resistance is almost always caused by reactivation of signaling through this pathway in the presence of drug. Drug withdrawal in such patients leads to a further hyper-activation of MAPK pathway, leading to a cellular state that has hallmarks of oncogene-induced senescence. I will show that drugs can be identified that maintain high levels of MAPK pathway signaling in BRAF inhibitor resistant melanomas upon cessation of BRAF inhibitor treatment. These drugs are only active in melanoma cells that have acquired resistance to BRAF inhibitors and not in parental melanoma cells, because they require MAPK hyper-activation that is induced by the gained resistance to BRAF inhibitors to be effective. In general, second line treatment of tumors that have acquired resistance to first line therapy is less effective. The unique aspect of the second line therapy identified here is that it exploits a vulnerability that is only gained upon acquisition of resistance to BRAFi therapy. The proposed therapy also takes advantage of the fact that acquisition of resistance to BRAFi follows a predictable path in the clinic making their responses to second line also more predictable. Citation Format: René Bernards. Finding vulnerabilities of drug resistant cancers [abstract]. In: Proceedings of the AACR Precision Medicine Series: Opportunities and Challenges of Exploiting Synthetic Lethality in Cancer; Jan 4-7, 2017; San Diego, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2017;16(10 Suppl):Abstract nr IA10.

Intercellular Resistance to BRAF Inhibition Can Be Mediated by Extracellular Vesicle–Associated PDGFRβ

Treatment of BRAF mutant melanoma with kinase inhibitors has been associated with rapid tumor regression; however, this clinical benefit is short-lived, and most patients relapse. A number of studies suggest that the extracellular environment promotes BRAF inhibitor resistance and tumor progression. Extracellular vesicles, such as exosomes, are functional mediators in the extracellular environment. They are small vesicles known to carry a concentrated group of functional cargo and serve as intercellular communicators not only locally but also systemically. Increasingly, it is reported that extracellular vesicles facilitate the development of drug resistance in cancer; however, their role in BRAF inhibitor resistance in melanoma is unclear. Here we investigated if extracellular vesicles from BRAF inhibitor–resistant melanoma could influence drug sensitivity in recipient melanoma cells. We demonstrate that the resistance driver, PDGFRβ, can be transferred to recipient melanoma cells via extracellular vesicles, resulting in a dose-dependent activation of PI3K/AKT signaling and escape from MAPK pathway BRAF inhibition. These data suggest that the BRAF inhibitor–sensitive phenotype of metastatic melanoma can be altered by delivery of PDGFRβ by extracellular vesicles derived from neighboring drug-resistant melanoma cells.

Identification of NRAS isoform 2 overexpression as a mechanism facilitating BRAF inhibitor resistance in malignant melanoma

Activating mutations in BRAF are found in 50% of melanomas and although treatment with BRAF inhibitors (BRAFi) is effective, resistance often develops. We now show that recently discovered NRAS isoform 2 is up-regulated in the setting of BRAF inhibitor resistance in melanoma, in both cell lines and patient tumor tissues. When isoform 2 was overexpressed in BRAF mutant melanoma cell lines, melanoma cell proliferation and in vivo tumor growth were significantly increased in the presence of BRAFi treatment. shRNA-mediated knockdown of isoform 2 in BRAFi resistant cells restored sensitivity to BRAFi compared with controls. Signaling analysis indicated decreased mitogen-activated protein kinase (MAPK) pathway signaling and increased phosphoinositol-3-kinase (PI3K) pathway signaling in isoform 2 overexpressing cells compared with isoform 1 overexpressing cells. Immunoprecipitation of isoform 2 validated a binding affinity of this isoform to both PI3K and BRAF/RAF1. The addition of an AKT inhibitor to BRAFi treatment resulted in a partial restoration of BRAFi sensitivity in cells expressing high levels of isoform 2. NRAS isoform 2 may contribute to resistance to BRAFi by facilitating PI3K pathway activation.

Role Played by Signalling Pathways in Overcoming BRAF Inhibitor Resistance in Melanoma.

The discovery of the BRAFV600E mutation led to the development of vemurafenib (PLX4032), a selective BRAF inhibitor specific to the kinase, for the treatment of metastatic melanomas. However, initial success of the drug was dampened by the development of acquired resistance. Melanoma was shown to relapse in patients following treatment with vemurafenib which eventually led to patients’ deaths. It has been proposed that mechanisms of resistance can be due to (1) reactivation of the mitogen-activated protein kinase (MAPK) signalling pathway via secondary mutations, amplification or activation of target kinase(s), (2) the bypass of oncogenic pathway via activation of alternative signalling pathways, (3) other uncharacterized mechanisms. Studies showed that receptor tyrosine kinases (RTK) such as PDGFRβ, IGF1R, EGFR and c-Met were overexpressed in melanoma cells. Along with increased secretion of growth factors such as HGF and TGF-α, this will trigger intracellular signalling cascades. This review discusses the role MAPK and Phosphatidylinositol-3-kinase-protein kinase B-mammalian target of rapamycin (PI3K-AKT-mTOR) pathways play in the mechanism of resistance of melanomas.

Abstract 5895: Targeting RhoA-regulated gene transcription in drug-resistant melanoma

Abstract Much of the recent focus of melanoma targeted therapy has been on the ERK pathway, which is aberrantly activated in approximately 90% of melanoma tumors. Over half of these express BRAFV600E. Current targeted therapies such as Vemurafenib (BRAFV600E inhibitor), or a combination therapy of BRAF + MEK inhibitors show profound initial effects in a majority of BRAFV600E expressing tumors. However, these responses are often short-lived and resistance typically develops within months. The goal of this work is to identify pharmacologically targetable resistance mechanisms so that effective combination therapies can be developed. Preliminary bioinformatics analysis suggests that the RhoA subfamily of Rho GTPases is activated in BRAFi-resistant cancer cell lines and BRAFi/MEKi-resistant human melanoma tumors. Thus, we hypothesize that RhoA promotes BRAFi resistance, so simultaneously targeting RhoA and BRAF may reverse drug resistance. Using cell line models of acquired BRAFi-resistance we demonstrated that the RhoA pathway is activated in a subset of resistant cell lines. The cell lines with increased RhoA activation have increased sensitivity to multiple ROCK inhibitors. In addition to regulating the cytoskeleton, RhoA can also regulate gene transcription through activation of multiple transcriptional co-activators, including MRTF and YAP. MRTF and YAP regulated gene transcription through their role as transcriptional co-activators and their interaction with chromatin remodeling complexes. MRTF and YAP are both activated in BRAFi-resistant cell lines. Indirectly targeting MRTF with CCG-222740 or YAP with the YES1 inhibitor Dasatinib effectively kills BRAFi-resistant melanoma cells. Sox10 was previously identified as a gene which promotes BRAFi resistance. Expression of Sox10 is downregulated, and Sox9 is upregulated, 100-1000-fold exclusively in the resistant cell lines which have RhoA activation. Further, a Sox10 gene signature is inversely correlated with a RhoA/C signature in the TCGA dataset and melanoma scRNA-seq data. Sox10 loss is a major drug resistance melanoma in melanoma. These data suggest that targeting RhoA-regulated gene transcription may be an effective mechanism to target Sox9High/Sox10Low cells, and ultimately prevent or reverse drug resistance. Taken together these data suggest that MRTF and YAP promote Vemurafenib resistance in Sox9High/Sox10Low melanoma cells. Citation Format: Sean A. Misek, Kathleen A. Gallo, Richard R. Neubig. Targeting RhoA-regulated gene transcription in drug-resistant melanoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 5895.

Abstract 1196: Epigenetic drugs modulate long noncoding RNAs expression in BRAF inhibitor-resistant melanoma

Abstract Emergence of drug resistance is the major cause of failure of BRAF inhibitors (BRAFi) treatment in cutaneous melanoma (CM). Epigenetic modifications are known to physiologically trigger massive modifications in cellular commitment and several studies report a correlation between the drug-resistant phenotype and epigenetic alterations of tumor cells. In this setting, long noncoding RNAs (lncRNAs) represent a class of gene regulators acting at epigenetic, transcriptional and post-transcriptional level. Several studies have implicated lncRNAs in chemoresistance through their ability to impair cell cycle arrest and apoptosis, but also to induce and modulate epithelial-mesenchymal transition and cell adhesion-associated signaling pathways. LncRNAs interact with histone modifying complexes and/or DNA methyltransferases, being also targets of these epigenetic mediators. Furthermore, epigenetic drugs have been recently identified as modulators for lncRNAs function as well as their related targeting signals. Starting from these evidences, we asked the question whether epigenetic drugs could differentially affect the survival of BRAFi-resistant (VR) and -sensitive CM cells, investigating the mechanistic network involved, with a specific focus on the role of lncRNA. A panel of BRAFi-sensitive and VR CM cell lines was treated with the FDA-approved HDAC inhibitor vorinostat (SAHA). FACS analysis of annexin V-FITC/propidium iodide stained cells showed that SAHA cytotoxic activity was more pronounced on VR CM cells than on their parental counterparts. RNA-Seq analysis revealed that a large number of differentially expressed lncRNAs was modulated in VR CM cells treated with SAHA. Intriguingly, the expression of several VR up-regulated lncRNAs was decreased to levels similar to those observed in the matched parental cells. Functional analysis indicated these lncRNAs were statistically enriched in pathways involving cellular growth and proliferation, but also cellular assembly and organization. Though additional studies are required, epigenetic modulation of VR-associated lncRNAs promises to have significant therapeutic potential to restore BRAFi sensivity in CM, being concomitantly effective in killing VR cells as monotherapy. Based on our preliminary data, we could anticipate that the combined use of epigenetic and targeted drugs would increase therapeutic efficacy in CM patients relapsing to BRAFi. Citation Format: Barbara Montico, Giorgio Giurato, Katy Mastorci, Aurora Rizzo, Maria Ravo, Francesca Rizzo, Alessandro Weisz, Riccardo Dolcetti, Francesca Colizzi, Luca Sigalotti, Elisabetta Fratta. Epigenetic drugs modulate long noncoding RNAs expression in BRAF inhibitor-resistant melanoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1196. doi:10.1158/1538-7445.AM2017-1196