Treatment of advanced melanoma has been improved with the advent of the BRAF inhibitors.,However, a limitation to such treatment is the occurrence of resistance.,Several mechanisms have been identified to be responsible for the development of resistance, either MEK-dependent or MEK-independent.,In order to overcome resistance due to reactivation of MEK signaling, MEK inhibitors are being clinically developed with promising results.,However, also in this case resistance inevitably occurs.,It has been recently reported that ErbB3, a member of the EGFR receptor family, may be involved in the establishment of drug resistance.,Three melanoma cell lines were tested: LOX IMVI (BRAF V600E), MST-L (BRAF V600R) and WM266 (BRAF V600D).,Phosphorylation of Receptor Tyrosine Kinases (RTKs) was assessed by an RTK array.,Western blot analysis was performed on total protein extracts using anti-ErbB3, anti-AKT and anti-ERK 1/2 antibodies.,The expression of neuregulin after vemurafenib treatment was assessed by Real Time PCR and Western blotting.,The growth inhibitory effects of vemurafenib, GSK1120212b and/or anti-ErbB3 mAbs were evaluated by in vitro colony formation assays.,In the present study we demonstrate that ErbB3 is the main RTK undergoing rapidly hyperphosphorylation upon either treatment with a BRAF inhibitor or with a MEK inhibitor in a panel of melanoma cell lines harboring a variety of V600BRAF mutations and that this results in a strong activation of phospho-AKT.,Importantly, ErbB3 activation is fully abrogated by the simultaneous use of anti-ErbB3 monoclonal antibodies, which are also shown to potently synergize with BRAF inhibitors in the inactivation of both AKT and ERK pathways and in the inhibition of melanoma cell growth.,We show that upregulation of phospho-ErbB3 is due to an autocrine loop involving increased transcription and production of neuregulin by melanoma cells.,On the basis of these results, we propose that initial co-treatment with BRAF and/or MEK inhibitors and anti-ErbB3 antibodies should be pursued as a strategy to reduce the ErbB3-dependent feedback survival mechanism and enhance duration of clinical response.
Drug resistance remains a vexing problem in the treatment of cancer patients.,While many studies have focused on cell autonomous mechanisms of drug resistance, we hypothesized that the tumor microenvironment may confer innate resistance to therapy.,Here we developed a co-culture system to systematically assay the ability of 23 stromal cell types to influence the innate resistance of 45 cancer cell lines to 35 anti-cancer drugs.,We found that stroma-mediated resistance is surprisingly common - particularly to targeted agents.,We further characterized the stroma-mediated resistance of BRAF-mutant melanoma to RAF inhibition because most of these patients exhibit some degree of innate resistance1-4.,Proteomic analysis showed that stromal secretion of the growth factor hepatocyte growth factor (HGF) resulted in activation of the HGF receptor MET, reactivation of the MAPK and PI3K/AKT pathways, and immediate resistance to RAF inhibition.,Immunohistochemistry confirmed stromal HGF expression in patients with BRAF-mutant melanoma and a statistically significant correlation between stromal HGF expression and innate resistance to treatment.,Dual inhibition of RAF and MET resulted in reversal of drug resistance, suggesting RAF/MET combination therapy as a potential therapeutic strategy for BRAF-mutant melanoma.,A similar resistance mechanism was uncovered in a subset of BRAF-mutant colorectal and glioblastoma cell lines.,More generally, these studies indicate that the systematic dissection of tumor-microenvironment interactions may reveal important mechanisms underlying drug resistance.