PARP1 facilitates EP300 recruitment to the promoters of the subset of RBL2-dependent genes

Differentiation of human monocytes is associated with proliferation arrest resulting from activation of the inter alia retinoblastoma protein family of gene repressors, which target gene promoters in an E2F-dependent manner. To investigate RBL2 contribution to defining monocyte phenotype and function, we used primer libraries. We identified genes encoding two surface receptors (CXCR1 and IL17RE) and two TLR signaling mediators (CD86 and NFKB2) that are repressed by the RBL2-E2F4-HDAC1-BRM complex. Surprisingly, PARP1 co-regulated 24 out of the 28 identified genes controlled by RBL2. Upon RBL2 silencing, PARP1 was recruited to one subset of RBL2-dependent genes, represented by MAP2K6 and MAPK3. RBL2 silencing also restored PARP1 transcription. Gene promoters enriched in PARP1 were characterized by increased histone acetylation and the replacement of HDAC1 with EP300. While PARP1 was dispensable for HDAC1 dissociation, EP300 was found only at gene promoters enriched in PARP1. EP300 activated transcription of PARP1/RBL2 co-regulated genes, but not genes solely controlled by RBL2. DNA was a prerequisite to the formation of an immunoprecipitated PARP1-EP300 complex, suggesting that PARP1 enabled EP300 binding, which in turn activated gene transcription. Notably, PARP1 overexpression failed to overcome the inhibitory effect of RBL2 on MAP2K6 and MAPK3 transcription. The same interdependence was observed in proliferating cancer cells; the low abundance of RBL2 resulted in PARP1-mediated EP300 recruitment to promoters of the MAP2K6 and MAPK3 genes. We conclude that RBL2 may indirectly regulate transcription of some genes by controlling PARP1-mediated EP300 recruitment.