PARP1 Suppresses the Transcription of PD-L1 by Poly(ADP-Ribosyl)ating STAT3.

Studies have pointed to a role of PARP1 in regulating gene expression through poly(ADP-ribosyl)ating， sequence-specific， DNA-binding transcription factors. However， few examples exist that link this role of PARP1 to the immunogenicity of cancer cells. Here， we report that PARP1 poly(ADP-ribosyl)ates STAT3 and subsequently promotes STAT3 dephosphorylation， resulting in reduced transcriptional activity of STAT3 and expression of PD-L1. In this study， we showed that PARP1 silencing or pharmacologic inhibition enhanced the transcription of PD-L1 in cancer cells， which was accompanied by the upregulation of PD-L1 protein expression， both in the cytoplasm and on the cell surface. This induction of PD-L1 was attenuated in the absence of the transcription factor STAT3. Cell-based studies indicated that PARP1 interacted directly with STAT3 and caused STAT3 poly(ADP-ribosyl)ation. STAT3's activation of PD-L1 transcription was abolished by the overexpression of wild-type PARP1 but not mutant PARP1， which lacks catalytic activity. PARP1 downregulation or catalytic inhibition enhanced the phosphorylation of STAT3， which was reversed by the ectopic expression of wild-type PARP1 but not by mutated PARP1. An inverse correlation between PARP1 and PD-L1 was also observed in clinical ovarian cancer samples. Overall， our study revealed PARP1-mediated poly(ADP-ribosyl)ation of STAT3 as a key step in inhibiting the transcription of PD-L1， and this mechanism exists in a variety of cancer cells.