Adaptive upregulation of FOXD3 and resistance to PLX4032/4720-induced cell death in mutant B-RAF melanoma cells

Melanoma cells driven by mutant v-raf murine sarcoma viral oncogene homolog B1 (B-RAF) are highly resistant to chemotherapeutic treatments. Recent phase 1 results with PLX4032/RG7204/vemurafenib, which selectively inhibits B-RAF/mitogen-activated protein kinase kinase (MEK)/extracellular signal-regulated kinase (ERK)1/2 signaling in mutant B-RAF cells, has given encouragement to this struggling field. Nearly all patients in the phase 1-3 studies saw at least some response and the overall response rates ranged from 48 and 81%. However, despite initial tumor shrinkage, most responders in the trial experienced tumor relapse over time. These findings indicate that both intrinsic and acquired resistance may affect the clinical efficacy of PLX4032. It is critical to optimize PLX4032 activity to improve response rates and understand why some patients with the B-RAF mutation do not respond. We have previously shown that the stemness factor, Forkhead box D3 (FOXD3), is upregulated following inhibition of B-RAF-MEK signaling in mutant B-RAF melanoma cells. Here, we show that upregulation of FOXD3 following treatment with PLX4032 and PLX4720 (the non-clinical tool compound for PLX4032) confers resistance to cell death. Small interfering RNA-mediated knockdown of FOXD3 significantly enhanced the cell death response after PLX4032/4720 treatment in mutant B-RAF melanoma cell lines. Additionally, upregulation of FOXD3 after PLX4720 treatment was attenuated in non-adherent conditions and correlated with enhanced cell death. Ectopic expression of FOXD3 in non-adherent cells significantly reduced cell death in response to PLX4720 treatment. Together, these data indicate that upregulation of FOXD3 is an adaptive response to RAF inhibitors that promotes a state of drug resistance.