Repression of PARP1 transcription is associated with cell cycle exit to G0 and the recruitment of RBL2 protein to PARP1 promoter. Panel (a) shows the graphic representation of PARP1 regulatory region adjacent to TSS, which spans POLR2A and H3K27ac according to the ENCODE. Small black boxes indicate the binding sites of transcription factors, which are considered as possibly involved in the regulation of PARP1 transcription in hematopoietic stem and progenitor cells and monocytes. The association of E2F1 and E2F4 transcription factors as well as RB1 and RBL2 proteins with PARP1 promoter in undifferentiated and differentiated THP-1 cells was determined with ChIP (b). The effect of LPS (100 ng/ml, 24 h) and cell cycle inhibitors (phase specific arrest in G1 and G2 was induced by cell treatment with 200 μM mimosine (Mim) and 0.5 μM nocodasol (Noco) for 48 h) on PARP1 protein level was monitored with western blot (upper c). In order to eliminate the impact of caspases, likely activated by cell cycle inhibitors, on the level of PARP1 protein LPS, mimosine and nocodasol were added in a mixture with caspase-3 and -7 inhibitor Z-DEVD-FMK (10 μM) (lower c). PARP1 mRNA was quantified in cells treated with LPS, Mim and Noco using real-time PCR (d). The recruitment of E2F1, E2F4, RB1 and RBL2 to PARP1 promoter in cells arrested in G1 phase was determined with ChIP (e).