Molecular Mechanisms of PARP-1 in Regulating NLRP3 Inflammasome Activation

Poly(ADP-ribose) polymerase-1 (PARP-1) is a nuclear chromatin-associated multifunctional enzyme found in most eukaryotes. It can catalyze an important post-translational modification named PARylation by polymerizing the ADP-ribose units and conjugating the poly(ADP-ribose) (PAR) complex to the target proteins. PARylation plays diverse roles in many molecular and cellular processes, including DNA damage detection and repair, chromatin modification, transcription and cell death pathways. For inducing cell death, overactivation of PARP-1 leads to depletion of cellular NAD+ and its precursor ATP, and subsequently cell necrosis. In our previous work, we note a novel activating pathway of PARP-1 via ROS, and a transient energy recovery response through mitochondrial biogenesis upon PARP-1 hyperactivation. Recently, the emerging role of PARP-1 in regulating inflammation has been revealed, and several studies have shown that PARP-1 can be simultaneously activated under inflammatory responses and contributes to the pathogenesis in various disease models, including sepsis, diabetes, myocardial dysfunction, stroke, trauma, arthritis, and obesity. Most studies have focused on the transcriptional regulation by PARP-1 in inflammatory genes. However, whether PARP-1 plays roles in NLRP3 inflammasome regulation has not been investigated. Moreover, although the deleterious action of PARP-1 in inflammatory responses is suggested, its involvement in innate immune response and host defense against pathogen infection has not yet been well understood. Inflammsome activation is critical for IL-1 production during microbial infection via generating active caspase-1 for cleavage of pro-IL-1. In our preliminary study using PARP-1-/- primary murine macrophages under lipopolysaccharide (LPS) priming, we found that PARP-1 deficiency impaired IL-1production, caspase-1 maturation and NLRP3 complex assembly caused by various NLRP3 activators, and these actions depend on PARP-1 enzymatic activity, as PARP-1 inhibitor can mimic the responses of PARP-1 deficiency, while PARP-1 activator can enhance the IL-1production via NLRP3 inflammasome activation. Notably PARP-1 did not get involved in pro-IL-1nor NLRP3 induction caused by LPS. Overall, our preliminary data suggest the predominantly nuclear enzyme PARP-1 as a positive regulator of NLRP3 inflammasome, but the detailed molecular events are not yet clarified. Moreover, our preliminary data also suggest PARP-1 might regulate TLRs-induced iNOS and COX-2 expression through different and multiple action mechanisms. Thus in this project we attempt to use genetic, pharmacological and biochemical approaches to explore the roles played by PARP-1 in innate immunity, especially focusing on NLRP3 inflammasome activation and pathogen infection. Three specific aims of this study include Aim 1: Determine the roles of PARP-1 in regulating NLRP3 inflammasome activation; Aim 2: Determine the roles of PARP-1 in TLR signaling pathways and inflammatory responses; Aim 3: Determine the roles of PARP-1 in host defense against pathogens.