RAS mutations occur in more than 30% of all human
cancers but efforts to directly target mutant RAS signaling as a
cancer therapy have yet to succeed. As alternative strategies, RAF and
MEK inhibitors have been developed to block oncogenic signaling downstream of RAS. As might be expected, studies of these inhibitors have indicated that
tumors with RAS or BRAF mutations display resistance RAF or
MEK inhibitors. In order to better understand the mechanistic basis for this resistance, we conducted a RNAi-based screen to identify genes that mediated chemoresistance to the
RAF kinase inhibitor
RAF265 in a BRAF (V600E) mutant
melanoma cell line that is resistant to this
drug. In this way, we found that knockdown of
protein kinase D3 (PRKD3) could enhance cell killing of RAF and
MEK inhibitors across multiple
melanoma cell lines of various genotypes and sensitivities to
RAF265. PRKD3 blockade cooperated with
RAF265 to prevent reactivation of the MAPK signaling pathway, interrupt cell cycle progression, trigger apoptosis, and inhibit colony formation growth. Our findings offer initial proof-of-concept that PRKD3 is a valid target to overcome drug resistance being encountered widely in the clinic with RAF or
MEK inhibitors.