PDGFRa up-regulation mediated by sonic hedgehog pathway activation leads to BRAF inhibitor resistance in melanoma cells with BRAF mutation

Control of BRAF(V600E) metastatic melanoma by BRAF inhibitor (BRAF-I) is limited by intrinsic and acquired resistance. Growth factor receptor up-regulation is among the mechanisms underlying BRAF-I resistance of melanoma cells. Here we demonstrate for the first time that PDGFRa up-regulation causes BRAF-I resistance. PDGFRa inhibition by PDGFRa-specific short hairpin (sh)RNA and by PDGFRa inhibitors restores and increases melanoma cells' sensitivity to BRAF-I in vitro and in vivo. This effect reflects the inhibition of ERK and AKT activation which is associated with BRAF-I resistance of melanoma cells. PDGFRa up-regulation is mediated by Sonic Hedgehog Homolog (Shh) pathway activation which is induced by BRAF-I treatment. Similarly to PDGFRa inhibition, Shh inhibition by LDE225 restores and increases melanoma cells' sensitivity to BRAF-I. These effects are mediated by PDGFRa down-regulation and by ERK and AKT inhibition. The clinical relevance of these data is indicated by the association of PDGFRa up-regulation in melanoma matched biopsies of BRAF-I +/- MEK inhibitor treated patients with shorter time to disease progression and less tumor regression. These findings suggest that monitoring patients for early PDGFRa up-regulation will facilitate the identification of those who may benefit from the treatment with BRAF-I in combination with clinically approved PDGFRa or Shh inhibitors.