Abstract B8: FOXD3 enhances expression of ERBB3 and promotes resistance to vemurafenib

Abstract Melanoma cells driven by mutant B-RAF are sensitive to vemurafenib (PLX4032), which selectively inhibits BRAF/MEK/ERK1/2 signaling. Despite initial tumor shrinkage, most responders in the trials experienced tumor relapse over time. Furthermore, approximately 5–15% of patients show tumor progression. These findings indicate that resistance mechanisms will hamper the clinical efficacy of vemurafenib. We have previously shown that a stemness factor, FOXD3, is upregulated following inhibition of B-RAF-MEK signaling in mutant B-RAF melanoma cells. Here, we show that siRNA-mediated knockdown of FOXD3 significantly enhanced the cell death response after PLX4032 treatment in mutant B-RAF melanoma cell lines. Additionally, ectopic expression of FOXD3 in nonadherent cells significantly reduced cell death in response to PLX4720 treatment. Genome wide analyses revealed that FOXD3 significantly increased expression of ERBB3 through direct binding to a known enhancer region of the ERBB3 gene. Knockdown of endogenous FOXD3 reduced ERBB3 upregulation after treatment with PLX4032. Furthermore, activation of ERBB3 in the presence of ligand was enhanced by B-RAF inhibition in an ERBB2-dependent manner. Treatment with the EGFR/ERBB2 inhibitor, lapatinib, in combination with PLX4032/4720 significantly reduced viability in both in vitro and in vivo assays. These data indicate that upregulation of FOXD3 is an adaptive response to B-RAF inhibitors that enhances ERBB3 signaling and promotes a state of drug resistance.