NFκB-regulation of c-FLIP promotes TNFα-mediated RAF inhibitor resistance in melanoma

Targeted inhibitors elicit heterogeneous clinical responses in genetically stratified groups of patients. While most studies focus on tumor intrinsic properties, factors in the tumor microenvironment were recently found to modulate the response to inhibitors. Here, we show that in cutaneous BRAF V600E melanoma, the cytokine TNFα blocks RAF-inhibitor-induced apoptosis via activation of nuclear factor κB (NFκB). Several NFκB-dependent factors are up-regulated following TNFα and RAF inhibitor treatment. Of these factors, we show that death receptor inhibitor cellular caspase 8 (FLICE)-like inhibitory protein (c-FLIP) is required for TNFα-induced protection against RAF inhibitor. Overexpression of c-FLIP_S or c-FLIP_L isoform decreased RAF inhibitor-induced apoptosis in the absence of TNFα. Importantly, targeting NFκB enhances response to RAF inhibitor in vitro and in vivo. Together, our results show mechanistic evidence for cytokine-mediated resistance to RAF inhibitor and provide a preclinical rationale for the strategy of co-targeting the RAF-MEK-ERK1/2 pathway and the TNFα/NFκB axis to treat mutant BRAF melanomas.