Porphyromonas gingivalis (P. gingivalis) is a Gram-negative anaerobic bacterium and a causative agent of periodontitis. Studies assessing the host inflammatory response to P. gingivalis have demonstrated that Toll-like receptor (TLR)-2 is the predominant TLR involved in the recognition of LPS and various other microbe-associated molecular patterns of P. gingivalis. However, the underlying cell-signaling pathways, especially the intracellular regulatory molecules, regulating the P. gingivalis (TLR2)-mediated inflammatory response are not fully understood. Using siRNA and Cre/loxP system, we have, for the first time, identified that serum- and glucocorticoid-inducible kinase 1 (SGK1) suppresses P. gingivalis-mediated inflammatory responses in innate immune cells. SGK1 is a ubiquitous serine/theronine kinase belonging to AGC kinase family (Protein kinase A, -G and -C family) and can be activated by PI3 kinase. Whereas the PI3K-Akt-GSK3- pathway has been demonstrated to play a critical regulatory role in TLR-mediated inflammatory responses, the functional role of SGK1 in TLR-mediated immune responses is entirely unknown. Therefore, the studies proposed here are intend to identify and characterize how SGK1 regulates P. gingvalis induced inflammatory mediators, as well as to delineate how the TLRs are involved in the phosphorylation and activation of SGK1. We hypothesize SGK1 is a critical kinase that controls inflammation of innate immune cells upon stimulation with P. gingivalis by regulating activation of Forkhead box protein O1 (FoxO1) and its downstream CCAAT/enhancer-binding protein beta (C/EBP-). This hypothesis is further substantiated by (i) the prediction of phosphorylation sites (by the software NetworKIN) identified FoxO1 as the optimum substrate of SGK1; (ii) our preliminary data showing that P. gingivalis stimulation phospho-activates SGK1 and abolishes the binding of FoxO1 to the promoter of C/EBP- in BMDC; and (iii) our recent publication demonstrating that PDK2 inhibition mediates FoxO1 nuclear sequestration and enhances TLR4 mediated inflammatory responses in innate immune cells (15). These observations provide a strong rationale for SGK1 in modulating P. gingivalis mediated inflammatory response via the TLR2-SGK1-FoxO1-C/EBP- axis. The specific aims will establish the role for SGK1 as an endogenous inhibitor of the P. gingivalis-mediated inflammatory response and delineate how SGK1 is activated in response to P. gingivalis stimulation, as well as determine if SGK1 negatively regulates P. gingivalis induced pro-inflammatory cytokines production via modification of the activity of FoxO1 and its downstream C/EBP- . Successful completion of this project will result in the isolation a novel and potentiall critical signaling pathway (TLR2-PI3K-SGK1-FoxO1-C/EBP- ) and greatly aid in the understanding of how the host inflammatory response is being regulated, which will pave the way for the development of immunoregulatory therapeutics that should have relevance well beyond the oral cavity.
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