TCDD-inducible poly-ADP-ribose polymerase (TIPARP/PARP7) mono-ADP-ribosylates and co-activates liver X receptors.

Members of the poly-ADP-ribose polymerase family catalyse the ADP-ribosylation of target proteins and are known to play important roles in many cellular processes, including DNA repair, differentiation and transcription. The majority of exhibit mono-ADP-ribosyltransferase activity rather than activity; however, little is known about their biological activity. In the present study, we report that 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-inducible poly-ADP-ribose polymerase mono-ADP-ribosylates and positively regulates liver X receptor α (LXRα) and LXRβ activity. Overexpression of enhanced LXR-reporter gene activity. TIPduanyu37 knockdown or deletion reduced LXR regulated target gene expression levels in HepG2 cells and inTiparp(-/-)mouse embryonic fibroblasts (MEFs) respectively. Deletion and mutagenesis studies showed that zinc-finger and catalytic domains were required to enhance LXR activity. Protein interaction studies using TIPduanyu37 and LXRα/β peptide arrays revealed that LXRs interacted with an N-terminal sequence (a.a. 209-236) of which also overlapped with a putative co-activator domain of TIPduanyu37 (a.a. 200-225). Immunofluorescence studies showed that TIPduanyu37 and LXRα or LXRβ co-localized in the nucleus.In vitroribosylation assays provided evidence that TIPduanyu37 mono-ADP-ribosylated both LXRα and LXRβ. Co-immunoprecipitation (co-IP) studies revealed that ADP-ribosylase macrodomain 1 (MACROD1), but not MACROD2, interacted with LXRs in a manner. This was complemented by reporter gene studies showing that MACROD1, but not MACROD2, prevented the TIPduanyu37-dependent increase in LXR activity. GW3965-dependent increases in hepatic Srebp1 mRNA and protein expression levels were reduced inTiparp(-/-)mice compared withTiparp(+/+)mice. Taken together, these data identify a new mechanism of LXR regulation that involves TIPduanyu37, ADP-ribosylation and MACROD1.

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