Managing leptomeningeal melanoma metastases in the era of immune and targeted therapy

Melanoma frequently metastasizes to the brain, with CNS involvement being clinically evident in ∼30% of patients (as high as 75% at autopsy). In ∼5% cases melanoma cells also metastasize to the leptomeninges, the sub-arachnoid space and cerebrospinal fluid (CSF). Patients with leptomeningeal melanoma metastases (LMM) have the worst prognosis and are characterized by rapid disease progression (mean survival 8-10 weeks) and a death from neurological causes. The recent years have seen tremendous progress in the development of targeted and immune therapies for melanoma that has translated into an increased survival benefit. Despite these gains, the majority of patients fail therapy and there is a suspicion that the brain and the leptomeninges are a "sanctuary" sites for melanoma cells that escape both targeted therapy and immunologic therapies. Emerging evidence suggests that (1) Cancer cells migrating to the CNS may have unique molecular properties and (2) the CNS/leptomeningeal microenvironment represents a pro-survival niche that influences therapeutic response. In this Mini-Review, we will outline the clinical course of LMM development and will describe how the intracranial immune and cellular microenvironments offer both opportunities and challenges for the successful management of this disease. We will further discuss the latest data demonstrating the potential use of BRAF inhibitors and immune therapy in the management of LMM, and will review future potential therapeutic strategies for the management of this most devastating complication of advanced melanoma.

Fibronectin induction abrogates the BRAF inhibitor response of BRAF V600E/PTEN-null melanoma cells

The mechanisms by which some melanoma cells adapt to Serine/threonine-protein kinase B-Raf (BRAF) inhibitor therapy are incompletely understood. In the present study, we used mass spectrometry-based phosphoproteomics to determine how BRAF inhibition remodeled the signaling network of melanoma cell lines that were BRAF mutant and PTEN null. Short-term BRAF inhibition was associated with marked changes in fibronectin-based adhesion signaling that were PTEN dependent. These effects were recapitulated through BRAF siRNA knockdown and following treatment with chemotherapeutic drugs. Increased fibronectin expression was also observed in mouse xenograft models as well as specimens from melanoma patients undergoing BRAF inhibitor treatment. Analysis of a melanoma tissue microarray showed loss of PTEN expression to predict for a lower overall survival, with a trend for even lower survival being seen when loss of fibronectin was included in the analysis. Mechanistically, the induction of fibronectin limited the responses of these PTEN-null melanoma cell lines to vemurafenib, with enhanced cytotoxicity observed following the knockdown of either fibronectin or its receptor α5β1 integrin. This in turn abrogated the cytotoxic response to BRAF inhibition via increased AKT signaling, which prevented the induction of cell death by maintaining the expression of the pro-survival protein Mcl-1. The protection conveyed by the induction of FN expression could be overcome through combined treatment with a BRAF and PI3K inhibitor.

Inhibition of BRAF and BRAF+MEK drives a metastatic switch in melanoma

Recent analyses by our group and others showed that the majority of melanoma patients who fail BRAF inhibitor therapy do so at new disease sites. Using phosphoproteomics we showed that BRAF inhibition mediates a switch to an aggressive/metastatic melanoma phenotype that is driven by ligand-independent erythropoietin-producing hepatocellular receptor A2 (EphA2) signaling.

BRAF Inhibition Generates a Host-Tumor Niche that Mediates Therapeutic Escape

The current study defines a fibroblast-derived niche that facilitates the therapeutic escape of melanoma cells from BRAF inhibition. Vemurafenib treatment led to the release of TGF-β from the melanoma cells that increased the differentiation state of the fibroblasts; an affect associated with fibronectin deposition, increase in α-smooth muscle actin (α–SMA) expression and the release of neuregulin (NRG). At the same time, vemurafenib directly activated the fibroblasts through paradoxical stimulation of the MAPK pathway, causing them to secrete hepatocyte growth factor (HGF). Treatment with the BRAF/MEK inhibitor combination reversed the release of HGF. Adhesion of melanoma cells to fibronectin was critical in amplifying the fibroblast-derived NRG and HGF-mediated PI3K/AKT survival signaling in the melanoma cells following BRAF inhibition. In co-culture studies, combination treatment with inhibitors of BRAF/MET/HER kinase was ineffective at reversing the fibroblast-mediated therapeutic escape from BRAF inhibition. Instead, it was noted that combined BRAF/PI3K inhibition overcame fibroblast-mediated drug resistance in vitro and was associated with enhanced anti-tumor effects in an in vivo xenograft model. Thus, we show melanoma cells and fibroblasts remodel their microenvironment in response to BRAF inhibition and that these adaptations allow tumor cells to evade therapy through increased PI3K/AKT survival signaling.

Abstract IA04: Using comprehensive proteomic approaches to map signaling adaptations to BRAF and BRAF/MEK inhibition

Although small molecule inhibitors of BRAF have shown great promise in advanced melanoma, resistance is commonplace. Many patients with acquired BRAF and BRAF/MEK inhibitor resistance develop disease at new sites, suggesting that drug-induced selection pressure drives metastasis. Here we used mass spectrometry-based phosphoproteomic screening and network modeling to uncover two novel adaptations to BRAF inhibitor therapy. The first involved ligand-independent EphA2 signaling that led to the adoption of an invasive/metastatic phenotype. The EphA2-mediated invasion was epigenetically mediated, AKT-dependent and was readily reversible upon removal of drug as well as through PI3K or HDAC inhibition. In a human melanoma mouse xenograft model, chronic BRAF inhibition led to the development of metastases, which stained positively for EphA2. A second phosphoproteomic screen showed BRAF inhibition to enhance the survival of both melanoma cells and fibroblasts through the induction of fibronectin (FN)/integrin α5β1 signaling. Adhesion to FN amplified the adaptive EGFR, HER2 and c-MET signals required for PI3K/AKT-mediated survival when BRAF was inhibited. At the same time, BRAF inhibition led, directly and indirectly, to the paracrine release of HGF and neuregulin from fibroblasts, with TGF-β release from the melanoma cells increasing both fibroblast differentiation and survival. These effects were partly mediated through paradoxical MAPK activation in the fibroblasts and could be reversed through BRAF/MEK inhibition. Thus, we have used comprehensive phosphoproteomics to identify two key non-genetic adaptations to BRAF inhibitor therapy that offer new insights into the process of therapeutic escape.

Citation Format: Inna V. Fedorenko, Kim H.T. Paraiso, Bin Fang, John M. Koomen, Keiran S.M. Smalley. Using comprehensive proteomic approaches to map signaling adaptations to BRAF and BRAF/MEK inhibition. [abstract]. In: Proceedings of the AACR Precision Medicine Series: Drug Sensitivity and Resistance: Improving Cancer Therapy; Jun 18-21, 2014; Orlando, FL. Philadelphia (PA): AACR; Clin Cancer Res 2015;21(4 Suppl): Abstract nr IA04.

Ligand-independent EPHA2 signaling drives the adoption of a targeted therapy-mediated metastatic melanoma phenotype

Many patients with BRAF inhibitor resistance can develop disease at new sites, suggesting that drug-induced selection pressure drives metastasis. Here, we used mass spectrometry-based phosphoproteomic screening to uncover ligand-independent EPHA2 signaling as an adaptation to BRAF inhibitor therapy that led to the adoption of a metastatic phenotype. The EPHA2-mediated invasion was AKT-dependent and readily reversible upon removal of the drug as well as through PI3K and AKT inhibition. In xenograft models, BRAF inhibition led to the development of EPHA2-positive metastases. A retrospective analysis of patients with melanoma on BRAF inhibitor therapy showed that 68% of those failing therapy develop metastases at new disease sites, compared with 35% of patients on dacarbazine. Further IHC staining of melanoma specimens taken from patients on BRAF inhibitor therapy as well as metastatic samples taken from patients failing therapy showed increased EPHA2 staining. We suggest that inhibition of ligand-independent EPHA2 signaling may limit metastases associated with BRAF inhibitor therapy.

SIGNIFICANCE

This study provides evidence that BRAF inhibition promotes the adoption of a reversible, therapy-driven metastatic phenotype in melanoma. The cotargeting of ligand-independent EPHA2 signaling and BRAF may be one strategy to prevent the development of therapy-mediated disease at new sites.

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

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

Abstract 1610: MAPK inhibitor resistance leads to ligand-independent Ephrin A2 receptor signaling and the formation of new melanoma metastases

Melanoma is the deadliest form of skin cancer with the worst prognosis in patients with metastasis to distant sites such as brain, liver and bone. Studies have shown that MAPK reactivation is a key signaling event leading to BRAF inhibitor resistance. As such, clinical investigations are now underway to evaluate the efficacy of combining frontline BRAF plus MEK inhibitors. Though this approach appears to have meaningful clinical benefit, there are a number of patients who do not respond to therapy, or who through unknown mechanisms, succumb to refractory disease. In order to identify the dynamic changes that drive MAPK inhibitor resistance, we have developed a systems level approach combining mass spectrometry based phosphoproteomic and bioinformatics methodologies. Based on significant changes in tyrosine, threonine and serine phosphorylation events between naive and vemurafenib resistant melanoma cell lines, we have mapped a resistance interactome of ∼550 nodes. This resistance network was significantly enriched for pathways associated with metastatic disease where changes in network connectivity resulted in the appearance of new signaling hubs such as EGFR, EphA2, EphB4, STAT3, FAK1 and HDAC1. The clinical relevance of these findings was demonstrated in a retrospective study showing that 65% of BRAFV600E patients on vemurafenib therapy developed metastases at new sites, with 25% of these new metastases involving the brain. Consistent with our bioinformatics prediction, resistant lines had greater metastatic potential as seen by increased migration and invasion across matrigel and endothelial cell barriers. Extending our findings to a panel of BRAF and BRAF plus MEK inhibitor resistant cell lines, we found compelling evidence that EphA2 is essential for maintaining a resistance phenotype. This phenotype was associated with AKT activation and uncoupled Eph-ephrin signaling resulting in S897 phosphorylation and overexpression of EphA2. In vivo analysis of matched primary and metastatic tumors from vemurafenib resistant xenografts showed that EphA2 expression is increased in metastatic but not primary lesions. Importantly, analysis of clinical specimens from melanoma patients undergoing or failing vemurafenib therapy confirmed that EphA2 receptor expression is significantly upregulated in metastatic but not primary tumors. Functionally, EphA2 S897 phosphorylation was increased at the tumor leading edge of metastatic lesions while absent in primary lesions. Our results show that MAPK resistant melanomas can adaptively rewire their signaling leading to a more malignant phenotype and that through proteomics based approaches we can reliably identify resistance pathways that will translate into rational therapeutic strategies for disseminated disease.

Citation Format: Kim H. T. Paraiso, Meghna Das Thakur, Jobin K. John, Bin Fang, John M. Koomen, Inna V. Fedorenko, Hensin Tsao, Keith T. Flaherty, Jane L. Messina, Elena M. Pasquale, Alejandro Villagra, John M. Kirkwood, Friedegund Meier, Sarah Sloot, Geoffrey T. Gibney, Darrin Stuart, Hussein Tawbi, Keiran S.M. Smalley. MAPK inhibitor resistance leads to ligand-independent Ephrin A2 receptor signaling and the formation of new melanoma metastases. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 1610. doi:10.1158/1538-7445.AM2014-1610

Evaluating melanoma drug response and therapeutic escape with quantitative proteomics

The evolution of cancer therapy into complex regimens with multiple drugs requires novel approaches for the development and evaluation of companion biomarkers. Liquid chromatography-multiple reaction monitoring mass spectrometry (LC-MRM) is a versatile platform for biomarker measurement. In this study, we describe the development and use of the LC-MRM platform to study the adaptive signaling responses of melanoma cells to inhibitors of HSP90 (XL888) and MEK (AZD6244). XL888 had good anti-tumor activity against NRAS mutant melanoma cell lines as well as BRAF mutant cells with acquired resistance to BRAF inhibitors both in vitro and in vivo. LC-MRM analysis showed HSP90 inhibition to be associated with decreased expression of multiple receptor tyrosine kinases, modules in the PI3K/AKT/mammalian target of rapamycin pathway, and the MAPK/CDK4 signaling axis in NRAS mutant melanoma cell lines and the inhibition of PI3K/AKT signaling in BRAF mutant melanoma xenografts with acquired vemurafenib resistance. The LC-MRM approach targeting more than 80 cancer signaling proteins was highly sensitive and could be applied to fine needle aspirates from xenografts and clinical melanoma specimens (using 50 μg of total protein). We further showed MEK inhibition to be associated with signaling through the NFκB and WNT signaling pathways, as well as increased receptor tyrosine kinase expression and activation. Validation studies identified PDGF receptor β signaling as a potential escape mechanism from MEK inhibition, which could be overcome through combined use of AZD6244 and the PDGF receptor inhibitor, crenolanib. Together, our studies show LC-MRM to have unique value as a platform for the systems level understanding of the molecular mechanisms of drug response and therapeutic escape. This work provides the proof-of-principle for the future development of LC-MRM assays for monitoring drug responses in the clinic.

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

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

Senescent fibroblasts in melanoma initiation and progression: an integrated theoretical, experimental, and clinical approach

We present an integrated study to understand the key role of senescent fibroblasts in driving melanoma progression. Based on the hybrid cellular automata paradigm, we developed an in silico model of normal skin. The model focuses on key cellular and microenvironmental variables that regulate interactions among keratinocytes, melanocytes, and fibroblasts, key components of the skin. The model recapitulates normal skin structure and is robust enough to withstand physical as well as biochemical perturbations. Furthermore, the model predicted the important role of the skin microenvironment in melanoma initiation and progression. Our in vitro experiments showed that dermal fibroblasts, which are an important source of growth factors in the skin, adopt a secretory phenotype that facilitates cancer cell growth and invasion when they become senescent. Our coculture experiments showed that the senescent fibroblasts promoted the growth of nontumorigenic melanoma cells and enhanced the invasion of advanced melanoma cells. Motivated by these experimental results, we incorporated senescent fibroblasts into our model and showed that senescent fibroblasts transform the skin microenvironment and subsequently change the skin architecture by enhancing the growth and invasion of normal melanocytes. The interaction between senescent fibroblasts and the early-stage melanoma cells leads to melanoma initiation and progression. Of microenvironmental factors that senescent fibroblasts produce, proteases are shown to be one of the key contributing factors that promoted melanoma development from our simulations. Although not a direct validation, we also observed increased proteolytic activity in stromal fields adjacent to melanoma lesions in human histology. This leads us to the conclusion that senescent fibroblasts may create a prooncogenic skin microenvironment that cooperates with mutant melanocytes to drive melanoma initiation and progression and should therefore be considered as a potential future therapeutic target. Interestingly, our simulations to test the effects of a stroma-targeting therapy that negates the influence of proteolytic activity showed that the treatment could be effective in delaying melanoma initiation and progression.

Abstract 5222: A novel mechanism of microenvironment-mediated drug resistance in BRAF V600E melanoma with loss of PTEN

Despite recent success of the BRAF kinase inhibitor vemurafenib in the clinic, nearly all of those treated follow a course in which initial tumor regression was followed by eventual relapse, which constitutes a major barrier to long-term therapeutic management of melanoma. We have identified a novel mechanism of microenvironment-mediated drug resistance that arises from the release of TGFβ from melanomas that lack PTEN expression leading to the recruitment of host fibroblasts and the generation of a protective microenvironment through autocrine and paracrine fibronectin secretion facilitating therapeutic escape. Preliminary studies show that BRAF V600E mutant melanoma cell lines that lacked PTEN expression dramatically increased their secretion of TGF-β following vemurafenib treatment, as detected by western blot, immunofluorescence and ELISA assays. Furthermore, autocrine TGFβ secretion initiated with vemurafinib treatment stimulated the upregulation and secretion of fibronectin in PTEN- cells. The vemurafenib-mediated ECM remodeling in this case was dependent upon autocrine TGFβ signaling and could be attenuated following pre-treatment with the TGFβ receptor kinase inhibitor SB505124 or by siRNA knockdown of TGFβ receptor I. A mass spectrometry based phospho-proteomic screen in conjunction with innovative network modeling approaches suggested that increased integrin α5β1 signaling was a key adaptive response of PTEN- melanoma cells to BRAF inhibitor therapy. The importance of increased fibronectin expression and integrin α5β1 signaling in the therapeutic escape of PTEN- melanoma cells was confirmed by the significant increases in vemurafenib-induced apoptosis observed following siRNA knockdown of either integrin α5, β1, or fibronectin expression. A mass spectrometry screen (LC-MRM) of 18 BH3 proteins demonstrated Mcl-1 and Bak to be critical apoptosis regulators in the microenvironment-mediated drug resistance response. The importance of PTEN loss and Mcl-1 downregulation in therapeutic escape was validated in PTEN- melanoma cell lines expressing doxycycline inducible forms of PTEN, lipid phosphatase mutant PTEN and Mcl-1. Novel organotypic co-culture models were developed to demonstrate that TGF-β released from PTEN- melanoma cell lines activated host fibroblasts leading to the attenuation of drug response in the tumor cells. Our overall hypothesis is that BRAF inhibition remodels both the melanoma and host microenvironments to provide a protective “sanctuary” for the minor populations of melanoma cells that escape therapy. We postulate that tumor eradication can only be achieved by a dual therapeutic strategy that targets both the bulk tumor and the microenvironment.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 5222. doi:1538-7445.AM2012-5222

Acquired and intrinsic BRAF inhibitor resistance in BRAF V600E mutant melanoma

The discovery of activating BRAF V600E mutations in 50% of all cutaneous melanomas has revolutionized the understanding of melanoma biology and provided new strategies for the therapeutic management of this deadly disease. Highly potent small molecule inhibitors of BRAF are now showing great promise as a novel therapeutic strategy for melanomas harboring activating BRAF V600E mutations and are associated with high levels of response. This commentary article discusses the latest data on the role of mutated BRAF in the development and progression of melanoma as the basis for understanding the mechanism of action of BRAF inhibitors in the preclinical and clinical settings. We further address the issue of BRAF inhibitor resistance and outline the latest insights into the mechanisms of therapeutic escape as well as describing approaches to prevent and abrogate the onset of both intrinsic and acquired drug resistance. It is likely that our evolving understanding of melanoma genetics and signaling will allow for the further personalization of melanoma therapy with the goal of improving clinical responses.

Abstract 5111: Integrative phospho-proteomic and genomic analyses identify AXL as a potential biomarker and therapeutic target for NRAS-mutated melanoma

Currently, few therapeutic options exist for the 15-20% of patients whose melanomas harbor activating mutations in NRAS. Here, we have used comprehensive phospho-proteomic and genomic analysis to characterize the patterns of intracellular signaling in a panel of NRAS and BRAF-mutated melanoma cell lines, with the goal of identifying new biomarkers and therapeutic targets for NRAS mutated melanomas. Our preliminary analyses showed BRAF mutated melanomas to have less diverse intracellular signaling than the NRAS group, which tended to have constitutive phosphorylation in a wide-range of receptor tyrosine kinases (RTKs), such as c-MET, EGFR, HER2 and c-Abl. Although these patterns of RTK activity were heterogeneous across the NRAS mutated melanoma cell line panel, all of the NRAS mutated cell lines examined showed constitutive activity in the receptor tyrosine kinase Axl. A role for Axl in the oncogenic behavior of NRAS mutated melanomas was suggested by array CGH studies showing genomic amplification of Axl in a significant fraction of the NRAS mutated melanoma cell lines and by Western Blot experiments demonstrating increased expression of Axl and its ligand Gas6. Mass spectrometry analysis and immunoprecipitation revealed Axl to be constitutively phosphorylated at the Tyr702 autophosphorylation site in NRAS mutated melanoma cell lines only. Functionally, siRNA knockdown and pharmacological inhibition of Axl impaired the survival and invasion of NRAS mutated melanoma cell lines grown under 3D organotypic cell culture conditions. Studies are ongoing to confirm the role of Axl in the initiation and progression of melanomas harboring NRAS mutations and determine whether Axl constitutes a therapeutic target for this melanoma sub-group.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 5111. doi:10.1158/1538-7445.AM2011-5111

PTEN loss confers BRAF inhibitor resistance to melanoma cells through the suppression of BIM expression

This study addresses the role of PTEN loss in intrinsic resistance to the BRAF inhibitor PLX4720. Immunohistochemical staining of a tissue array covering all stages of melanocytic neoplasia (n = 192) revealed PTEN expression to be lost in >10% of all melanoma cases. Although PTEN expression status did not predict for sensitivity to the growth inhibitory effects of PLX4720, it was predictive for apoptosis, with only limited cell death observed in melanomas lacking PTEN expression (PTEN-). Mechanistically, PLX4720 was found to stimulate AKT signaling in the PTEN- but not the PTEN+ cell lines. Liquid chromatography multiple reaction monitoring mass spectrometry (LC-MRM) was performed to identify differences in apoptosis signaling between the two cell line groups. PLX4720 treatment significantly increased BIM expression in the PTEN+ (>14-fold) compared with the PTEN- cell lines (four-fold). A role for PTEN in the regulation of PLX4720-mediated BIM expression was confirmed by siRNA knockdown of PTEN and through reintroduction of PTEN into cells that were PTEN-. Further studies showed that siRNA knockdown of BIM significantly blunted the apoptotic response in PTEN+ melanoma cells. Dual treatment of PTEN- cells with PLX4720 and a PI3K inhibitor enhanced BIM expression at both the mRNA and protein level and increased the level of apoptosis through a mechanism involving AKT3 and the activation of FOXO3a. In conclusion, we have shown for the first time that loss of PTEN contributes to intrinsic BRAF inhibitor resistance via the suppression of BIM-mediated apoptosis.

PTEN loss confers BRAF inhibitor resistance to melanoma cells through the suppression of BIM expression

This study addresses the role of PTEN loss in intrinsic resistance to the BRAF inhibitor PLX4720. Immunohistochemical staining of a tissue array covering all stages of melanocytic neoplasia (n = 192) revealed PTEN expression to be lost in >10% of all melanoma cases. Although PTEN expression status did not predict for sensitivity to the growth inhibitory effects of PLX4720, it was predictive for apoptosis, with only limited cell death observed in melanomas lacking PTEN expression (PTEN-). Mechanistically, PLX4720 was found to stimulate AKT signaling in the PTEN- but not the PTEN+ cell lines. Liquid chromatography multiple reaction monitoring mass spectrometry (LC-MRM) was performed to identify differences in apoptosis signaling between the two cell line groups. PLX4720 treatment significantly increased BIM expression in the PTEN+ (>14-fold) compared with the PTEN- cell lines (four-fold). A role for PTEN in the regulation of PLX4720-mediated BIM expression was confirmed by siRNA knockdown of PTEN and through reintroduction of PTEN into cells that were PTEN-. Further studies showed that siRNA knockdown of BIM significantly blunted the apoptotic response in PTEN+ melanoma cells. Dual treatment of PTEN- cells with PLX4720 and a PI3K inhibitor enhanced BIM expression at both the mRNA and protein level and increased the level of apoptosis through a mechanism involving AKT3 and the activation of FOXO3a. In conclusion, we have shown for the first time that loss of PTEN contributes to intrinsic BRAF inhibitor resistance via the suppression of BIM-mediated apoptosis.

Abstract A66: Integrative phosphoproteomic analysis of signaling pathways in MMS-mutated melanoma

Introduction: Recent years have seen great progress in the development of targeted therapy strategies for melanoma, with impressive therapeutic responses now being observed in patients whose melanomas harbor either activating mutations in BRAFor c-KIT. In contrast, few therapeutic options exist for the 15-20% of melanomas with mutations in NRAS. Previous attempts to define the biology of NRAS mutant melanomas have focused upon microarray analysis-an approach that failed to distinguish NRAS from BRAF-mutated melanomas. Despite apparently overlapping RNA expression profiles between the two sub-groups, BRAFand NRAS mutated melanomas were found to respond differently to small molecule signal transduction inhibitors, suggesting important differences in cell signaling. The aim of the current study was to perform comparative phosphoproteomic analysis on a panel of NRAS and BRAF mutated melanoma cell lines and to build intracellular signaling networks of the two groups.

Methods: Phosphotyrosine enrichment was performed using the PhosphoScan protocol (Cell Signaling Technology) where phosphotyrosine-containing peptides were immunoprecipitated with immobilized anti-phosphotyrosine antibodies before being analyzed by liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS). These data yielded a catalog of tyrosine phosphorylations across a wide range of signaling proteins and enabled the relative quantification of each site based on exctracted ion chromatograms from high resolution accurate mass measurements of the peptides. Analysis of the phosphorylated proteins and the quantitative data was performed using GeneGO software, so that signaling maps of BRAFand NRAS mutated melanoma cell lines could be constructed.

Results: A preliminary analysis of the data revealed BRAFmutated melanomas to be less diverse than the NRAS group, which tended to have constitutive phosphorylation in a wide-range of receptor tyrosine kinases (RTKs), such as c-MET, EGFR, and HER2, as well as non-receptor tyrosine kinases, such as c-Abl. Studies are ongoing to validate the role of these RTKs in the oncogenic behavior of the NRAS-mutated melanoma cell line panel and address whether any of the identified RTKs would be suitable therapeutic targets for this group of melanomas.

Conclusions: Phosphoproteomics has proven useful in elucidating cancer biology of melanomas that harbor mutations in BRAF and NRAS. These differences in kinase activation and protein phosphorylation may prove to be useful in selection of targeted therapy regimens for melanoma patients and serve as candidate biomarkers for patient assessment.

∗This presenting student is supported by an ARRA supplement to the NCI Cancer Center Support Grant awarded to Moffitt (3P30 CA076292-11S6 PI WS Dalton) to create a training program for underrepresented undergraduate students in clinical proteomics Project LINK (Leaders In New Knowledge-Emerging Technologies) that also emphasizes education in health disparities and community outreach.

Citation Information: Cancer Epidemiol Biomarkers Prev 2010;19(10 Suppl):A66.

Recovery of phospho-ERK activity allows melanoma cells to escape from BRAF inhibitor therapy

Background:

Resistance to BRAF inhibitors is an emerging problem in the melanoma field. Strategies to prevent and overcome resistance are urgently required.

Methods:

The dynamics of cell signalling, BrdU incorporation and cell-cycle entry after BRAF inhibition was measured using flow cytometry and western blot. The ability of combined BRAF/MEK inhibition to prevent the emergence of resistance was demonstrated by apoptosis and colony formation assays and in 3D organotypic cell culture.

Results:

BRAF inhibition led to a rapid recovery of phospho-ERK (pERK) signalling. Although most of the cells remained growth arrested in the presence of drug, a minor population of cells retained their proliferative potential and escaped from BRAF inhibitor therapy. A function for the rebound pERK signalling in therapy escape was demonstrated by the ability of combined BRAF/MEK inhibition to enhance the levels of apoptosis and abrogate the onset of resistance.

Conclusion:

Combined BRAF/MEK inhibition may be one strategy to prevent the emergence of drug resistance in BRAF-V600E-mutated melanomas.

Enhanced imaging and accelerated photothermalysis of A549 human lung cancer cells by gold nanospheres

Background & aims: Gold nanoparticles are excellent photon-thermal energy converters. The purpose of this work was to investigate the influence of gold nanoparticles on the photothermalysis of A549 lung tumor cells. Materials & methods: A549 lung tumor cells were exposed to goat antihuman immunoglobulin (Ig)G-conjugated gold nanospheres (40 nm) and were then imaged under a dark-field microscope. The live cells were then subjected to photoirradiation using a 633-nm laser at different power levels. The viability of tumor cells under laser irradiation was monitored by confocal microscopy using a viability-assay kit. Results & discussion: The death rates of A549 lung tumor cells after gold nanoparticle exposure increased significantly under laser irradiation. The maximum initial cell death rate was observed at a laser power level of 3.75 mW, with the initial deactivation rate accelerated by a factor of 6.6 and a total loss of 92% of cell viability. Conclusion: This work demonstrated potential applications of gold nanospheres as both imaging probes and enhancing agents for photothermal therapy of cancer.