Poly (ADP-ribose) polymerases (PARPs) constitute a family of 18 cell signaling enzymes, which are involved in the regulation of DNA-binding proteins and DNA repair. These enzymes catalyze the transfer of ADP-ribose units from NAD+ to a number of acceptor proteins (by a process called poly (ADP) ribosylation) and can also interact directly with their target proteins. Among the PARPs, PARP1 is the most abundant isoform and has been described to regulate the expression of diverse proinflammatory mediators, through its interaction with numerous transcription factors such as NF-kB and AP-1. Recent published observations (including our group) suggest that PARP-1 is a cofactor for NF-kB-dependent gene expression. Several studies in Parp1 KO mice have shown a decreased expression of host inflammatory mediators, cytokines, chemokines, adhesion molecules as well as reduced immune tissue infiltration in various model of inflammation including, streptomycin induced diabetes, LPS induced septic shock, ischemia and reperfusion injury. Interestingly, in vivo studies have demonstrated that genetic ablation or pharmacological inhibition of PARP1 ameliorates the pathophysiological changes of experimental colitis. However, the studies published to date have failed to characterize the exact mechanism by which PARP1 inhibition is protective in colitis and in which cell type PARP1 inhibition would have the most significant impact in IBD. Such studies are critical in guiding future development of therapeutics targeting PARP1, with proper pharmacokinetics and pharmacodynamics. Our preliminary data obtained with adoptive T-cell transfer model of colitis indicate that Parp1 is dispensable in T-cells, while Parp1-/- mice are protected from colitis resulting from epithelial damage induced by DSS. These data collectively suggests that the pathogenic role of Parp1 in colitis is restricted to either colonic epithelial cells or other cells of the innate immune system. A novel mouse model for conditional cell-type specific Parp1 targeting, Parp1/Rag1 double knockout mice, and bone marrow chimeras developed as part of this proposal will be used in the following specific aims: (1) we are planning to differentiate the pathogenic role of PARP1 in hematopoietic or parenchymal cells in the development of colitis, (2) to investigate the role of Parp1 deletion in intestinal epithelial cells and (3) to characterize the role of PARP1 on neutrophil and macrophage functions.
Those specific aims will help address the following hypothesis: PARP1 contributes to aggravated mucosal immune response via modulation of inflammatory signaling pathways within the cells of the innate immune system.
Inflammatory Bowel Disease (IBD) is characterized by an uncontrolled inflammation resulting from an inappropriate and persistent activation of the mucosal immune system against normal intestinal bacterial flora. The regulation of the mucosal immune response is a very complex physiological process with many cell types involved and many overlapping signaling mechanisms identified as main regulators. Among those, poly (ADP-ribose) polymerases (PARPs) are cell signaling enzymes which interact directly with their target proteins including major transcription factors, and modulate the expression of pro-inflammatory mediators, thereby determining the extent and outcome of inflammation. In well-established animal models of IBD, studies have demonstrated that genetic ablation or pharmacological inhibition of PARP1, a well characterized member of the PARP family, ameliorates the biological and physical manifestations of experimental colitis. However, all published studies have failed to identify the mechanism of the detrimental effects of PARP1 activity during the course of disease, and did not identify the cell type(s) in which PARP1 inhibition would have the most significant protective impact in IBD. Our preliminary data demonstrated that while PARP1 is dispensable in T-cells, the key role of PARP1 in colitis may be restricted to either colonic epithelial cells or other cells of the innate immune system, such a neutrophil and macrophages. Approach &Significance considering the essential role of PARP1 in the regulation of inflammation in IBD, the remarkable effects of its inhibition, and the relativly superficial descriptions in the available literature, we aim to shed more light at PARP1 biology, which would lay foundation for more translational studies. The objective of the proposed research project is to determine how PARP1 contributes to aggravated mucosal immune response via modulation of inflammatory signaling pathways within the cells of the innate immune system, primarily colonic epithelial cells, neutrophils, and macrophages. We plan to employ several state-of- the-art animal models which will identify the type of cells in which PARP1 plays a detrimental role in colitis. We will evaluate the role of PARP1 in the activation of the master regulator of inflammation, NF-kB, and address the role of PARP1 in the function of epithelial cells, neutrophil, and macrophages in a functional and mechanistic fashion. We believe that the proposed research plan and the resulting findings will guide the development of novel tailored therapeutic strategies in IBD targeting PARP1 activity, complementing or improving upon the currently available treatments aimed at successful long lasting remission in IBD patients. Relevance of the project to IBD: Preclinical studies strongly suggest that PARP1 is not only a potent modulator of the immune response in IBD, but also a potential target for novel pharmaceutical approaches. Moreover, PARP1 has been implicated in multiple steps leading to epithelial cell malignant transformation, including cell cycle control, proliferation, differentiaton, and epithelial cell death. Therefore, findings from the proposed study can provide essential and novel insights not only into the role of PARP1 in the inflammatory process in the intestine, but also be of high relevance for IBD-related colorectal cancer.