Growth factors rescue cutaneous melanoma cells from apoptosis induced by knockdown of mutated (V600E) B-RAF

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Abstract
Constitutive activation of the RAS-RAF-MEK-ERK signaling cascade is a hallmark of cutaneous malignant melanoma. A single activating mutation (c.1799T>A; p.V600E) in the gene encoding the serine/threonine kinase B-RAF occurs in >60% of the tumors. Previous work has shown that knockdown of V600EB-RAF by RNA interference induces a variety of phenotypic changes in cultured melanoma cells, including lower proliferation rates, reduced anchorage-independent growth and apoptosis. Here, we show that the majority of melanomas harboring the V600EB-RAF mutation have retained the wild-type (WT) B-RAF allele, and that these cells can be rescued from the effects of V600EB-RAF knockdown by stimulation with growth factors. Ectopic expression of short hairpin RNAs specifically suppressing V600EB-RAF in melanoma cell lines reduced colony formation by 80%. This response could be rescued by basic fibroblast growth factor, hepatocyte growth factor or, to a lesser extent, endothelin-1. Rescue with growth factors was not possible in cell lines lacking WTB-RAF. Single-cell clones with efficient knockdown of V600EB-RAF could be propagated in the presence of basic fibroblast growth factor but underwent apoptosis or senescence-like growth arrest upon withdrawal of this growth factor. The ability of growth factors to modulate the response of V600EB-RAF knockdown in melanoma cells may have both experimental and therapeutic implications.