LAUSR

BACKGROUND & AIMS We reported that cholangiocyte senescence, regulated by the transcription factor, ETS1, is a pathogenic feature of Primary Sclerosing Cholangitis (PSC). Furthermore, Histone 3 Lysine 27 is acetylated at senescence-associated loci. The epigenetic readers, bromodomain and extra-terminal domain (BET) proteins, bind acetylated histones, recruit transcription factors, and drive gene expression. Thus, we tested the hypothesis that BET proteins interact with ETS1 to drive gene expression and cholangiocyte senescence. METHODS We performed immunofluorescence for BET proteins (BRD2 and 4) in PSC patient and mouse model liver tissue. Using Normal Human Cholangiocytes (NHC), NHC experimentally induced to senescence (NHCsen), and PSC patient-derived cholangiocytes (PSCDCs), we assessed senescence, fibroinflammatory secretome, and apoptosis following BET inhibition or RNAi depletion. We assessed BET interaction with ETS1 in NHCsen and PSC patient tissues, and the effects of BET inhibitors on liver fibrosis, senescence, and inflammatory gene expression in mouse models. RESULTS PSC patient and mouse model tissue exhibited increased cholangiocyte BRD2 and 4 protein (∼5x) compared to non-disease controls. NHCsen exhibited increased BRD2 and 4 (∼2x), while PSCDCs exhibited increased BRD2 protein (∼2x) relative to NHC. BET inhibition in NHCsen and PSCDCs reduced senescence markers and inhibited the fibroinflammatory secretome. ETS1 interacted with BRD2 in NHCsen and BRD2 depletion diminished NHCsen p21 expression. BET inhibitors reduced senescence, fibroinflammatory gene expression, and fibrosis in the DDC-fed and Mdr2-/- mouse models. CONCLUSION Our data suggest that BRD2 is an essential mediator of the senescent cholangiocyte phenotype and is a potential therapeutic target for patients with PSC.