Clearance of gut derived LPS by hepatic Stabilin receptors: Mechanisms and Implications

About This Grant

Project Summary or Abstract: Gut barrier dysfunction leads to endotoxemia, characterized by increased levels of endotoxin/lipopolysaccha- ride (LPS) in the blood circulation, which affects multiple organs and is associated with liver diseases and sev- eral other LPS-associated diseases. LPS, a potent microbial ligand from gram negative bacterial membrane, induces intense systemic inflammation via TLR4 in immune cells. As a proactive host defense mechanism, the liver clears gut derived LPS from portal circulation. However, the cell types in the liver, receptors involved in LPS clearance, and inflammatory response of those cells is unknown. Identifying the innate immune cells, re- ceptor and the molecular mechanism involved in rapid clearance of circulating endotoxin by liver will provide critical insights to develop therapeutic options for endotoxemia. In this proposal, we present six novel findings. First, we found that liver sinusoidal endothelial cells (LSEC) eliminate a major portion of LPS from blood circu- lation very rapidly within a few minutes, and that clearance of LPS is facilitated by high density lipoprotein (HDL). Second, LSEC clear circulating LPS via Stabilin-1 (Stab1) and Stabilin-2 (Stab2) receptor mediated endocytosis and localize to lysosomes for degradation. Third, the lack of both Stab1 and Stab2 (double knock out mice) results in diminished LPS uptake, clearance, and endocytosis by LSEC, but escalated systemic in- flammation and early death. Fourth, Stab1, and to a lesser extent Stab2, participates in LPS clearance and host defense. Fifth, Stabilin and TLR4 are functionally opposite receptors for LPS mediated immune response. Six, Liquid chromatography-tandem mass spectrometry has identified novel serum and intracellular proteins as potential facilitators of Stabilin receptor-mediated LPS clearance. These results lead us to hypothesize that Stabilin receptors clear LPS through a distinct pathway involving serum and intracellular proteins, leading to enzymatic inactivation of LPS in human and mouse LSEC. Upregulating the clearance function of Stabilin re- ceptors will control TLR4-mediated systemic inflammation. This hypothesis will be tested with following aims: Aim 1: Determine the molecular mechanism of Stabilin-mediated LPS clearance. Aim 2: Determine the relative immune function of Stabilin receptors with TLR4. Aim 3: Determine how the enhancement of Stabilin receptor- mediated LPS clearance protects mice from endotoxemia. This project presents a new paradigm in which Sta- bilin receptors expressed by LSEC offer a host defense mechanism and protection against LPS -associated diseases, and points to novel targets to treat endotoxemia both prophylactically and therapeutically.