BRAF(V600E)-induced, tumor intrinsic PD-L1 can regulate chemotherapy-induced apoptosis in human colon cancer cells and in tumor xenografts

Programmed death ligand 1 (PD-L1) is an immune checkpoint protein, however, emerging data suggest that tumor cell PD-L1 may regulate immune-independent and intrinsic cellular functions. We demonstrate regulation of PD-L1 by oncogenic BRAF(V600E) and investigated its ability to influence apoptotic susceptibility in colorectal cancer (CRC) cells. Endogenous or exogenous mutant vs wild-type BRAF were shown to increase PD-L1 mRNA and protein expression that was attenuated by MEK inhibition or c-JUN and YAP knockdown. Deletion of PD-L1 reduced tumor cell growth in vitro and in vivo. Loss of PD-L1 was also shown to attenuate DNA damage and apoptosis induced by diverse anti-cancer drugs that could be reversed by restoration of wild-type PD-L1, but not mutants with deletion of its extra- or intra-cellular domain. The effect of PD-L1 on chemosensitivity was confirmed in MC38 murine tumor xenografts generated from PD-L1 knockout vs parental cells. Deletion of PD-L1 suppressed BH3-only BIM and BIK proteins that could be restored by re-expression of PD-L1; re-introduction of BIM enhanced apoptosis. PD-L1 expression was significantly increased in BRAF(V600E) human colon cancers, and patients whose tumors had high vs low PD-L1 had significantly better survival. In summary, BRAF(V600E) can transcriptionally up-regulate PD-L1 expression that was shown to induce BIM and BIK to enhance chemotherapy-induced apoptosis. These data indicate an intrinsic, non-immune function of PD-L1, and suggest the potential for PD-L1 as a predictive biomarker.