DIFFERENTIAL POST-TRANSLATIONAL MODIFICATION OF POLYMORPHIC MSH3 AND NOVEL BINDING PARTNER NEMO IS ASSOCIATED WITH ITS NUCLEAR-TO-CYTOSOL SHUTTLING

from their immunoregulatory roles.Colorectal cancer (CRC) cells with mutant BRAF(V600E) or KRAS alleles show increased PD-L1 transcripts (TCGA data) and we found increased tumor cell expression of cell surface PD-L1 protein compared to those with wild-type copies.In human colon cancer cells, ectopic expression of BRAF V600E or mutant KRAS induced PD-L1 whose upregulation was attenuated by inhibition of RAS-RAF-MEK-ERK signaling.Combined knockdown of MEK/ERK effectors c-JUN and YAP were shown to markedly reduce PD-L1.We then explored the role of PD-L1 in colon cancer cell sensitivity to chemotherapy given that treatment resistance in this disease is a major obstacle to improving therapeutic outcomes.Knockout of PD-L1 in RKO cells resulted in a reduction in chemotherapy-induced DNA double strand breaks (pH2AX) and caspase-3 cleavage compared to parental cells.Consistent results were seen for diverse cytotoxic drugs (oxaliplatin, irinotecan, gemcitabine).Results: were confirmed in PD-L1 knockout MC38 murine CRC cells where re-expression of PD-L1 but not its deletion mutants, was shown to promote DNA double strand breaks and apoptosis.In cells with knockout of PD-L1, reductions in p-AKT and the BH3-only proteins BIM and BIK were observed that could be restored by re-expression of PD-L1.Treatment of cells with an anti-PD-L1 antibody reduced p-AKT, BIM and BIK and also attenuated chemotherapy-induced apoptosis.Re-expression of BIM in PD-L1 knockout cells restored apoptosis.Using a murine model where tumor xenografts were generated from cells with knockout of PD-L1, tumors displayed resistance to oxaliplatin-induced regression compared to tumors derived from parental cells.To establish clinical relevance, stage III and IV primary human CRCs from TCGA datasets were identified with high vs low PD-L1 mRNA levels that were shown to be significantly associated with better patient survival.In conclusion, we identified a non-immune function of tumor cell-intrinsic PD-L1 that was shown to regulate apoptosis induction by anti-cancer drugs due to upregulation of proapoptotic BH3-only BIM and BIK proteins.Further study of the potential of PD-L1 to serve as predictive biomarker appears warranted.