BACKGROUND: 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.
METHODS: 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.
RESULTS: 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.
CONCLUSIONS: 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.