Hepatic scavenger receptor BI protects against polymicrobial-induced sepsis through promoting LPS clearance in mice

Recent studies revealed that scavenger receptor BI (SR-BI or Scarb1) plays a critical protective role in sepsis. However, the mechanisms underlying this protection remain largely unknown. In this study, using Scarb1(I179N) mice, a mouse model specifically deficient in hepatic SR-BI, we report that hepatic SR-BI protects against cecal ligation and puncture (CLP)-induced sepsis as shown by 75% fatality in Scarb1(I179N) mice, but only 21% fatality in C57BL/6J control mice. The increase in fatality in Scarb1(I179N) mice was associated with an exacerbated inflammatory cytokine production. Further study demonstrated that hepatic SR-BI exerts its protection against sepsis through its role in promoting LPS clearance without affecting the inflammatory response in macrophages, the glucocorticoid production in adrenal glands, the leukocyte recruitment to peritoneum or the bacterial clearance in liver. Our findings reveal hepatic SR-BI as a critical protective factor in sepsis and point out that promoting hepatic SR-BI-mediated LPS clearance may provide a therapeutic approach for sepsis.

Recent studies revealed an important function of SR-BI, namely, protection against sepsis, as shown by a marked increase in fatality in mice deficient in SR-BI upon lipopolysaccharides (LPS) or cecal ligation and puncture (CLP) challenge (48)(49)(50)(51). Available evidence suggests that SR-BI may exert its protection via multiple mechanisms. Macrophage SR-BI suppresses inflammatory response by modulating LPS-TLR4 signaling in macrophages, which contributes to protection against septic animal death (50,52); Adrenal SR-BI is a key determinant of inducible glucocorticoid (GC) generation in response to stress (49,50,53), and SR-BI null mice are completely deficient in inducible GC generation in CLP-induced sepsis (50); A number of studies suggest that SR-BI may provide protection against sepsis through detoxifying LPS. SR-BI binds to LPS and mediates the uptake of LPS in vitro (54), and SR-BI null mice display impaired LPS clearance in circulation in LPS-induced endotoxemia or CLP-induced sepsis (49,50). Liver is the most important organ for LPS detoxification. In sepsis, majority of LPS target to Kupffer cells and hepatocytes (55)(56)(57). However, little is known about how the liver detoxifies LPS. It has been speculated that Kupffer cells and hepatocytes participate in uptake and clearance of LPS via a receptor-mediated mechanism. Given the abundant expression of SR-BI in hepatocytes (9) and the role of SR-BI in LPS uptake and clearance, we hypothesized that hepatic SR-BI provides protection against sepsis by promoting LPS clearance.
Huby et al. developed loxp-floxed SR-BI mice but the mice exhibited hypo-phenotypes as shown by a marked reduction in SR-BI expression in all tissues (58). Overexpression of SR-BI by adenoviral vector is a useful and widely used animal model to elucidate the function of hepatic SR-BI in regulating HDL metabolism (20, 59,60). Unfortunately, adenovirus induces host immune response, which may alter the outcomes of sepsis (61). This presents a barrier to use these animal models for septic study. Stylianou et al. recently reported an interesting hepatic specific SR-BI deficient mouse model -Scarb1 I179N mice (62). The Scarb1 I179N mutant mice had a 90% decrease in hepatic SR-BI protein expression, resulting in a 170% increase in plasma cholesterol concentrations compared to C57BL/6J control mice. However, the Scarb1 I179N mice had normal SR-BI expression in non-hepatic tissues such as adrenal gland and ovary (62). In this study, we utilized this unique animal model to assess the role of hepatic SR-BI in sepsis. We demonstrate that hepatic SR-BI protects against CLP-induced septic death by promoting LPS clearance. Our findings reveal hepatic SR-BI as a critical protective factor in sepsis and point out that promoting hepatic SR-BI-mediated LPS clearance may provide a therapeutic approach for sepsis.
Analysis of leukocyte recruitment to peritoneum: leukocyte recruitment to peritoneal cavity was analyzed by flow cytometry as described (63)(64)(65). Briefly, 8-to 10-week old Scarb1 I179N and C57BL/6J mice were treated with/without CLP for 6 h and the peritoneal fluids were collected. Peritoneal neutrophils (PMN, CD11b hi Ly6C hi ) and inflammatory monocytes (IM, CD11b int Ly6C hi ) were gated by CD11b, and Ly6C expression on CD45 + cells. Ly6G expression was confirmed to be high in gated PMNs and low in gated IMs.
Biochemical assays: 10-to 12-week-old mice were euthanized by CO 2 inhalation 6 and 20 h following CLP. The blood was obtained by cardiac puncture and stored at -80 o C. Serum ALT levels were quantified with the ALT kit, and were used as an indicator for liver damage; the serum TNF-α, IL-6, corticosterone and LPS levels were quantified with corresponding ELISA kits.
Quantification of cytokine generation in LPS-stimulated macrophages: Bone marrowderived macrophages were cultured as described previously (50). Briefly, bone marrow cells were cultured in a 12-well plate at 1.5 x 10 6 cells/well in RPMI1640 medium containing 20% FBS and 15% supernatant of L929 cell culture for 5 days. For measurement of cytokines, the cells were incubated in PBS for 4 h and treated with LPS at 0.5 ng/ml for 20 h in RPMI1640 medium containing 20% FBS. Cytokines in the culture supernatant were quantified with corresponding ELISA kit. Of note, LPS from E coli k12 strain, is much more potent than LPS from other commonly used strains with respect to the activity of stimulating inflammatory cytokine production (Supplemental Figure 1).
Assay for bacterial load: Bacterial load in liver and spleen was analyzed as previously described (50).
Assay for SR-BI expression: Tissue or cells were homogenized in lysis buffer containing 1% proteinase inhibitor cocktail (Sigma), and subjected to Western Blot analysis against SR-BI as described previously (50). The expression of SR-BI was quantified with Fuji LAS-4000 and normalized to actin expression.
Statistical Analysis: The survival assay was analyzed by Log-Rank x 2 test using SAS software. Significance in experiments comparing two groups was determined by 2-tailed Student's t-test. Significance in experiments comparing more than two groups was evaluated by One Way ANOVA, followed by post hoc analysis using Tukey's test. Means were considered different at p < 0.05.

RESULTS
Hepatic SR-BI protects against CLPinduced septic death. The Scarb1 I179N mutation is caused by a T to A transversion in exon 4 which eliminates a BstYI site (R/GATCY). Based on this character, we designed a PCR method to genotype the mutant mice. As shown in Figure 1A, PCR generated a 294bp product. BstYI digestion of the PCR product yielded a single 294 bp band for Scarb1 I179N mice, but 96 bp and 198bp bands for C57BL/6J controls. The Scarb1 I179N mice had a 90% decrease in SR-BI expression in the liver ( Figure 1B) and a 150% increase in serum cholesterol concentrations ( Figure 1C), which was consistent with the previous report (62). Thus, the Scarb1 I179N mice presented a unique animal model to elucidate the role of hepatic SR-BI in sepsis. As shown in Figure 1D, CLP induced 21.4% fatality in C57BL/6J control mice, but 75% fatality in Scarb1 I179N mice, indicating that hepatic SR-BI provides significant protection against CLPinduced septic death. CLP-induced liver injury was assessed by measuring serum ALT levels. Compared to wild type controls, Scarb1 I179N mice had a significant increase in serum ALT levels at 6 and 20 h following CLP ( Figure 1E). These findings indicate that hepatic SR-BI protects against polymicrobial sepsis.
Exacerbated innate immune response in Scarb1 I179N mice during sepsis. To understand why Scarb1 I179N mice were susceptible to septic death, we assessed inflammatory cytokine production at 6 and 20 h following CLP. Upon CLP, C57BL/6J control mice exhibited a typical acute phase response as shown by a rapid and strong induction of TNF-α and IL-6 in the early stage of sepsis (6 h) and significant decreases in TNF-α and IL-6 levels by 20 h (Figure 2, A and  B). Scarb1 I179N mice also had a rapid and strong induction of TNF-α and IL-6 in the early stage of sepsis (6 h), however, Scarb1 I179N mice displayed uncontrolled inflammatory cytokine generation as shown by high serum concentrations of TNF-α and IL-6 20 h after CLP ( Figure 2, A and B).
Hepatic SR-BI is required for LPS clearance in sepsis. Macrophages are one of the major types of cells that generate inflammatory cytokines during sepsis. SR-BI is moderately expressed in macrophages and recent studies demonstrated that macrophage SR-BI suppresses inflammatory response of macrophages to LPS (50,52). To test whether a deficiency of hepatic SR-BI affects inflammatory response in macrophages, we utilized bone marrow-derived macrophages from Scarb1 I179N and control mice. Upon stimulation by 0.5 ng/ml of LPS for 20 h, macrophages from Scarb1 I179N mice produced similar levels of TNF-α and IL-6 compared to macrophages from the control mice ( Figure 3, A and B).
We then looked for an alternative explanation for hepatic SR-BI's protective role in sepsis. Endotoxemia is a hallmark of sepsis. LPS released by Gram-negative bacteria is a major stimulator for systemic inflammatory cytokine production. We speculated that the exacerbated inflammatory cytokine production observed in Scarb1 I179N mice is caused by elevated LPS in the circulation. To test this, we measured serum LPS levels in CLP-challenged mice. Scarb1 I179N mice had a 3-fold increase in serum LPS levels compared with wild type controls 6 and 20 h following CLP ( Figure 4A), suggesting that hepatic SR-BI is responsible for LPS clearance in sepsis.
SR-BI is abundantly expressed in adrenal glands. Adrenal SR-BI mediates the intracellular uptake of cholesterol ester from HDL, which provides a substrate for glucocorticoid synthesis (9,12,49,66,67). Given the strong effects of glucocorticoids in regulating inflammatory response, we tested whether the exacerbated inflammatory cytokine generation is caused by a disruption in adrenal glucocorticoid production. As shown in Figure 4B, CLP induced a significant increase in corticosterone generation in both Scarb1 I179N mice and wild type mice 6 and 20 h following CLP. Unexpectedly, CLP-Scarb1 I179N mice actually had a moderate increase in corticosterone concentrations compared with CLPwild type mice. It is unlikely that the exacerbated inflammatory cytokine generation in Scarb1 I179N mice is caused by altered adrenal glucocorticoid production.
SR-BI is capable of binding Gramnegative bacteria and facilitating the uptake of Gram-negative bacteria in vitro (54,68). This raises a possibility that SR-BI might play a role in bacterial clearance in sepsis. Kupffer cells in the liver constitute a major pool of macrophages in the body. To determine whether hepatic SR-BI affects bacterial uptake by macrophages during sepsis, we isolated total DNA from the liver and quantified bacteria with qPCR using bacteria specific primers. No significant difference in the number of bacteria was observed between Scarb1 I179N mice and C57BL/6J control mice ( Figure 4C). We also quantified living bacteria in spleen, no significant difference in the number of living bacteria was observed between Scarb1 I179N mice and C57BL/6J mice (1.5x10 5 ±2.0x10 5 /g tissue in Scarb1 I179N mice versus 1.2x10 5 ±2.5x10 5 /g tissue in C57BL/6J mice).
SR-BI is a well-established HDL receptor. Hepatic SR-BI mediates the uptake of cholesterol ester from HDL, which plays a critical role in regulating HDL metabolism. Stylianou et al. reported that Scarb1 I179N mice display moderately larger HDL particles. We had similar observations (data not shown). HDL regulates the expression of adhesion molecules (69), and a deficiency of HDL has been shown to impair the recruitment of leukocytes to peritoneal cavity following CLP (65). To elucidate if hepatic SR-BI affects leukocyte recruitment in sepsis, we quantified peritoneal neutrophils and inflammatory monocytes with flow cytometry in Scarb1 I179N mice and C57BL/6J mice. Almost no peritoneal neutrophils and inflammatory monocytes were observed in non-CLP animals ( Figure 5A). Six hours following CLP, a significant number of neutrophils and inflammatory monocytes were recruited to the peritoneum ( Figure 5B) in both Scarb1 I179N and C57BL/6J mice, but no difference in leukocyte recruitment was observed between these two strains ( Figure 5C).
Role of hepatic SR-BI in Gram-positive bacteria-induced sepsis. The above findings suggest that hepatic SR-BI provides protection primarily through its role in promoting LPS clearance. To determine whether hepatic SR-BI provides protection other than LPS clearance, we infected Scarb1 I179N mice and C57BL/6J mice with Gram-positive bacteria S aureus, and monitored survival for 7 days. A moderate decrease in survival was observed in Scarb1 I179N mice compare to C57BL/6J mice (p = 0.68) (Figure 6).

DISCUSSION
Using Scarb1 I179N mice, a unique hepatic specific SR-BI deficient mouse model, we demonstrate that hepatic SR-BI is a critical protective factor in sepsis. Scarb1 I179N mice had a 3-fold increase in CLP-induced septic fatality associated with exacerbated inflammatory cytokine production compared with wild type control mice.
To understand the mechanisms by which hepatic SR-BI protects against sepsis, we tested five possible effects of hepatic SR-BI: 1) on LPS clearance. SR-BI has been shown to bind LPS and mediate the uptake of LPS in vitro (54), and SR-BI null mice display impaired LPS clearance in circulation in LPS-induced endotoxemia or CLPinduced sepsis (49,50). These findings suggest that SR-BI plays a critical role in LPS clearance. Given that majority of LPS target to the liver in sepsis (55)(56)(57) and SR-BI is abundantly expressed in the liver, it is plausible that hepatic SR-BI functions to clear LPS. In this study, by measuring serum LPS levels, we found that Scarb1 I179N mice had a 3-fold increase in serum LPS concentrations compared with wild type control mice; 2) on macrophage inflammatory response. We found that macrophages from Scarb1 I179N and wild type control mice exhibit similar inflammatory cytokine production in response to LPS, suggesting that a deficiency of hepatic SR-BI does not alter the macrophage inflammatory response; 3) on adrenal steroidogenesis. We found that Scarb1 I179N mice have a moderate increase in corticosterone production compared with wild type control mice in sepsis. Given the potent suppressive effects of glucocorticoid on inflammation, it is unlikely that the increase in corticosterone production contributes to the elevated inflammatory cytokine production; 4) on bacterial clearance. We found no difference in the number of liver bacteria between Scarb1 I179N mice and wild type mice; 5) on leukocyte recruitment. Given that HDL regulates leukocyte recruitment, we assessed neutrophils and inflammatory monocytes in the peritoneum in response to CLP. We found no difference in the number of peritoneal neutrophils or inflammatory monocytes between Scarb1 I179N mice and wild type mice.
While LPS-induced inflammatory cell damage has been considered as a major cause of septic death, the failure of anti-LPS monoclonal antibodies in clinical trials questions the contribution of LPS to septic death. An issue associated with LPS neutralization is that the binding of antibody to LPS sequesters LPS, which delays LPS clearance (70), suggesting that simple neutralization of LPS may have limited effect on detoxifying LPS. It is worth noting that, different from neutralizing LPS, SR-BI detoxifies LPS through mediating the intracellular uptake of LPS and promoting LPS clearance (49,50,54). Combined with the observations that SR-BI null mice are susceptible to LPS-induced endotoxic animal death (48,49), SR-BI is abundantly expressed in liver (9), liver is a major organ for LPS clearance (55-57) and a deficiency of hepatic SR-BI leads to impaired LPS clearance, we get our conclusion that hepatic SR-BI-mediated LPS clearance contributes to protection against sepsis. To further clarify this issue, we administered polymyxin B (PMB, a potent LPS neutralizer (71), 0.6mg/kg body weight, i.p.) to Scarb 1I179N and C57BL/6J mice 2h post CLP challenge. PMB profoundly inhibited IL-6 production in Scarb 1I179N mice but not in B6 mice 18 h post CLP, compared to mice without PMB treatment (0.47±0.08 ng/ml in CLP-Scarb1 I179N mice with PMB treatment versus 25.08±8.32 ng/ml in CLP-Scarb1 I179N mice without PMB treatment; 5.22±4.66 ng/ml in C57BL/6J mice with PMB treatment versus 2.66±1.21 ng/ml in C57BL/6J mice without PMB treatment). We only observed a slight increase in survival in PMB-treated septic Scarb 1I179N (33.3% survival in CLP-Scarb1 I179N mice with PMB treatment versus 25% survival in CLP-Scarb1 I179N mice without PMB treatment). The PMB treatment actually decreased the survival in septic B6 mice (33.3% survival in CLP-C57BL/6J mice with PMB treatment versus 78.6% survival in CLP-C57BL/6J mice without PMB treatment). These suggest that simple neutralization of LPS by LPS neutralizer cannot provide efficient protection against septic death and promoting LPS clearance is a more efficient way for LPS detoxification.
Our earlier study indicated that mice with whole body SR-BI deficiency are susceptible to CLP-induced septic death (50). While both whole body SR-BI null mice and the hepatic SR-BI deficient mice are susceptible to CLP-induced sepsis, we have noticed an interesting difference in inflammatory cytokine generation between these two animal models. Compared to strain-marched wild type control mice, Scarb1 I179N mice had a rapid induction of TNF-α and IL-6 in the early stage of sepsis (6 h), and uncontrolled inflammatory cytokine generation in the late stage of sepsis (20 h). Meanwhile, whole body SR-BI null mice displayed delayed inflammatory cytokine generation in the early stage of sepsis and uncontrolled inflammatory cytokine generation in the late stage of sepsis (50); interestingly, the difference in inflammatory cytokine generation patterns is in line with the LPS levels in the circulation. Scarb1 I179N mice had elevated serum LPS in both the early and late stage of sepsis; but, the whole body SR-BI null mice had lower serum LPS in the early stage of sepsis and elevated serum LPS in the late stage of sepsis (50). These findings suggest that hepatic SR-BI is responsible for LPS removal and a defect in hepatic SR-BImediated LPS removal leads to endotoxemia which likely accounts for exacerbated inflammatory cytokine production observed in Scarb1 I179N mice; These findings also imply that non-hepatic SR-BI may be required for the recruitment of LPS to circulation, and a deficiency of non-hepatic SR-BI leads to a delayed inflammatory response in the early stage of sepsis. Endothelial cells have been shown to uptake LPS in a receptor dependent manner (72,73). Interestingly, earlier studies showed that endothelial cells express high levels of SR-BI (23,24,74). It is likely that endothelial SR-BI is responsible for recruitment of LPS to circulation. Further study is warranted to test these speculations.
An earlier study pointed out that hepatic SR-BI expression is suppressed by LPS and by inflammatory cytokines such as TNF-α (75). Sepsis induces profound increases in circulating LPS and inflammatory cytokines, which likely causes the down-regulation of hepatic SR-BI. Indeed, we found that the hepatic SR-BI expression is down-regulated in C57BL/6J mice 20 h post CLP (data not shown). Combined with our finding that hepatic SR-BI is required for LPS clearance, these observations suggest that promoting hepatic SR-BI expression may provide a therapeutic approach for sepsis. Further study is needed to test this speculation.
In summary, using the unique hepatic specific SR-BI deficient animal model, we demonstrate that hepatic SR-BI provides protection against sepsis through promoting LPS clearance. Given that the expression of SR-BI is down regulated in sepsis, we propose that promoting hepatic SR-BI-mediated LPS removal may provide a novel therapeutic strategy for sepsis. The PCR products were digested with/without BstYI and subjected to 2.5% agarose gel electrophoresis; B, Western blot analysis of SR-BI expression in the liver; C, serum cholesterol concentrations. The Scarb1 I179N and C57BL/6J mice were fasted for 4 h. n = 6 each group with duplicate measurements, mean ± SEM.; D, survival analysis. Scarb1 I179N (n = 12) and C57BL/6J mice (n = 14) were treated with CLP and survival was observed for 7 days. The data were expressed as the percentage of mice surviving at indicated times, and survival was analyzed by Log-Rank x 2 test. E, CLP induced liver injury. Scarb1 I179N and C57BL/6J mice were treated with CLP for the indicated times, and liver injury was assessed by measuring serum alanine aminotransferase (ALT) levels. n = 6 -11 each group with duplicate measurements, mean ± SEM.