Sepsis is a leading cause of death that is characterized by uncontrolled inflammatory response. In this study, we report that scavenger receptor BI (SR-BI), a high density lipoprotein receptor, is a critical survival factor of sepsis. We induced sepsis using an established septic animal model, cecal ligation and puncture (CLP). CLP induced 100% fatality in SR-BI-null mice but only 21% fatality in wild type littermates. SR-BI-null mice exhibited aberrant inflammatory responses with delayed inflammatory cytokine generation at the early stage of sepsis and highly elevated inflammatory cytokine production 20 h after CLP treatment. To understand the mechanisms underlying SR-BI protection, we elucidated the effect of macrophage SR-BI on inflammatory cytokine generation. Macrophages from SR-BI-null mice produced significantly higher levels of inflammatory cytokines than those of wild type controls in response to LPS. Importantly, transgenic mice overexpressing SR-BI were more resistant to CLP-induced septic death. Using an HEKBlue TM cell system, we demonstrated that expression of SR-BI suppressed TLR4-mediated NF-B activation. To understand why SR-BI-null mice had a delayed inflammatory response, we elucidated the effect of SR-BI on LPS clearance during sepsis. Compared with wild type controls, SR-BI-null mice had lower plasma LPS levels in the early stage of sepsis and elevated plasma LPS levels 20 h following CLP treatment. In conclusion, our findings demonstrate that SR-BI is a critical protective modulator of sepsis in mice. SR-BI exerts its protective function through its role in modulating inflammatory response in macrophages and facilitating LPS recruitment and clearance.Sepsis is one of the major causes of death that claims over 215,000 lives and costs $16.7 billion per year in America alone (1-4). The death rate from sepsis is high, exceeding 50%, due to poor understanding of the disease (5). Identifying molecules involved in sepsis, especially endogenous protective modulators, is of great importance not only in understanding the mechanisms but also in providing new insights for efficient therapies.Scavenger receptor BI (SR-BI 2 or Scarb1) is a 75-kDa membrane protein expressed in the liver, endothelial cells, macrophages, and steroidogenic tissues (6, 7). It is a well established high density lipoprotein (HDL) receptor. It mediates intracellular uptake of cholesterol ester from HDL, which plays a key role in regulating plasma cholesterol levels and steroidogenesis (8 -11). Mice deficient in SR-BI have a 2-fold increase in plasma cholesterol levels and develop cardiovascular diseases (10,(12)(13)(14)(15)(16). Recent studies reveal that SR-BI is a multifunctional protein. It activates endothelial nitric-oxide synthase in endothelial cells in the presence of HDL (17-20), induces apoptosis in the absence of HDL/endothelial nitric-oxide synthase (21), and protects against nitric oxide (NO)-induced oxidative damage (22). Emerging evidence indicates that specific expression of SR-BI in macrophages provides protecti...

Sepsis is a leading cause of death, which is characterized by uncontrolled inflammatory response. In this study, we report that caveolin-1, a major component of caveolae, is a critical survival factor of sepsis. We induced sepsis using a well established sepsis animal model, cecal ligation and puncture (CLP). CLP induced 67% fatality in caveolin-1 null mice, but only 27% fatality in wild type littermates (p ‫؍‬ 0.015). Further studies revealed that mice deficient in caveolin-1 exhibited marked increase in tumor necrosis factor-␣ and interleukin-6 production 20 h following CLP treatment, indicating uncontrolled inflammatory responses in the absence of caveolin-1. Caveolin-1 null mice also had a significant increase in bacteria number recovered from liver and spleen, indicating elevated bacterial burdens. In addition, caveolin-1 null mice had a 2-fold increase in thymocyte apoptosis compared with wild type littermates, indicating caveolin-1 as a critical modulator of thymocyte apoptosis during sepsis. In conclusion, our findings demonstrate that caveolin-1 is a critical protective modulator of sepsis in mice. Caveolin-1 exerts its protective function likely through its roles in modulating inflammatory response, alleviating bacterial burdens, and suppressing thymocyte apoptosis.Sepsis is one of the major causes of death, which claims over 215,000 lives and costs $16.7 billion per year in America alone (1-3). The death rate from sepsis is high, exceeding 50%, due to poor understanding of the disease (4). Identifying molecules involved in sepsis, especially endogenous protective modulators, is of great importance, not only in understanding the mechanisms but also in providing new insights for efficient therapies.Caveolae, a subset of lipid rafts, are microdomains of the plasma membrane that are enriched in cholesterol and sphingolipids (5). In addition to its specific lipid compositions, caveolae also contain abundant signaling molecules such as nitricoxide synthase (NOS) 3 and TLR4 and Src family tyrosine kinases, which provide a platform for signal transduction. Caveolin-1, a 24-kDa protein, is a major component of caveolae and has been used as a marker protein of caveolae (6). Disruption of the caveolin-1 gene leads to loss of caveolae (7), indicating an essential role of caveolin-1 in caveolae formation. Given the importance of caveolae in signal transduction, it is not surprising that caveolin-1 has been implicated in a variety of cellular processes such as endocytosis, phagocytosis, and cholesterol trafficking (5-6, 8).Evidence from the caveolin-1 null mouse model has established an inhibitory role of caveolin-1 in cell proliferation and tissue homeostasis. For example, mice deficient in caveolin-1 display hypercellularity in lungs and heart (7, 9 -10). A number of studies also suggest a role of caveolin-1 in apoptosis. However, the role of caveolin-1 in regulating apoptotic cell death is controversial and seems to be cell type-specific and depend on stimuli. For example, knockdown of caveolin-1 by short hairpi...

This article is available online at http://www.jlr.org from HDL into hepatocytes and subsequently secretes cholesterol from bile. SR-BI-mediated cholesteryl ester uptake is also required for steroidogenesis, in which SR-BI uptakes cholesteryl ester from HDL into endocrine tissues for steroid synthesis ( 3-7 ). The importance of SR-BI-mediated cholesterol transport has been established in mouse models. Mice defi cient in SR-BI have a 2-fold increase in total cholesterol concentration and a 3-fold increase in free cholesterol concentration in circulation, and they develop cardiovascular diseases ( 8-13 ). Mice overexpressing SR-BI have lower plasma cholesterol levels ( 14-17 ) and are protected against the development of atherosclerosis ( 11,18 ). A recent report showed that a family with functional mutation in SR-BI has elevated plasma HDL and changes in cholesterol metabolism in macrophages and platelets, indicating a role of SR-BI in humans ( 19 ). In addition to modulating HDL metabolism, recent studies revealed that SR-BI is a multifunctional protein. It activates eNOS in endothelial cells in the presence of HDL ( 20-23 ), induces apoptosis in the absence of HDL/eNOS ( 24 ), protects against nitric oxide (NO)-induced oxidative damage ( 25 ), and prevents endotoxic and septic animal death ( 25-27 ).SR-BI facilitates intracellular uptake of HDL cholesteryl esters in a two-step process involving binding of HDL through its extracellular domain and transferring cholesteryl ester from HDL into cells. The importance of extracellular domain of SR-BI in cholesteryl ester uptake has been explored by the use of chimeric receptors ( 28 ), by insertion of epitope tags into various regions of the domain of SR-BI ( 29 ), by blocking antibody against the extracellular domain, and by mutations ( 30, 31 ). However, SR-BI has a large extracellular domain that contains 403 amino acid residues, and the HDL binding site remains to be determined. Identifying the HDL binding site and understanding the regulation of SR-BI-mediated cholesterol Abstract Scavenger receptor BI (SR-BI) is an HDL receptor. It binds HDL and mediates the uptake of cholesteryl ester from HDL. Early studies have pointed out that the extracellular domain of SR-BI is critical for SR-BI-mediated cholesteryl ester uptake. However, the extracellular loop of SR-BI is large: it contains 403 amino acids. The HDL binding site and the modulation of SR-BI-mediated cholesteryl ester uptake remain to be identifi ed. In this study, using C323G mutant SR-BI, we showed that C323G mutant SR-BI lost its HDL binding and cholesteryl ester uptake activity, indicating that the highly conserved C323 is required for SR-BI-mediated HDL binding and cholesteryl ester uptake. Using a blocking antibody against C323 region, we demonstrated that C323 is directly involved in HDL binding and likely an HDL binding site. Using C323G mutant transgenic mouse model, we further demonstrated that C323 of SR-BI is required for regulating plasma cholesterol levels in vivo.Using redox reagents, we show...