The human scavenger receptor class B type I is a novel candidate receptor for the hepatitis C virus

We discovered that the hepatitis C virus (HCV) envelope glycoprotein E2 binds to human hepatoma cell lines independently of the previously proposed HCV receptor CD81. Comparative binding studies using recombinant E2 from the most prevalent 1a and 1b genotypes revealed that E2 recognition by hepatoma cells is independent from the viral isolate, while E2-CD81 interaction is isolate specific. Binding of soluble E2 to human hepatoma cells was impaired by deletion of the hypervariable region 1 (HVR1), but the wild-type phenotype was recovered by introducing a compensatory mutation reported previously to rescue infectivity of an HVR1-deleted HCV infectious clone. We have identified the receptor responsible for E2 binding to human hepatic cells as the human scavenger receptor class B type I (SR-BI). E2-SR-BI interaction is very selective since neither mouse SR-BI nor the closely related human scavenger receptor CD36, were able to bind E2. Finally, E2 recognition by SR-BI was competed out in an isolate-specific manner both on the hepatoma cell line and on the human SR-BI-transfected cell line by an anti-HVR1 monoclonal antibody.     

High density lipoproteins facilitate hepatitis C virus entry through the scavenger receptor class B type I

The scavenger receptor class B type I (SR-BI) has recently been shown to interact with hepatitis C virus (HCV) envelope glycoprotein E2, suggesting that it might be involved at some step of HCV entry into host cells. However, due to the absence of a cell culture system to efficiently amplify HCV, it is not clear how SR-BI contributes to HCV entry. Here, we sought to determine how high density lipoproteins (HDLs), the natural ligand of SR-BI, affect HCV entry. By using the recently described infectious HCV pseudotyped particles (HCVpps) that display functional E1E2 glycoprotein complexes, we showed that HDLs are able to markedly enhance HCVpp entry. We did not find any evidence of HDL association with HCVpps, suggesting that HCVpps do not enter into target cells using HDL as a carrier to bind to its receptor. Interestingly, lipid-free apoA-I and apoA-II, the major HDL apolipoproteins, were unable to enhance HCVpp infectivity. In addition, drugs inhibiting HDL cholesteryl transfer (block lipid transport (BLT)-2 and BLT-4) reduced HDL enhancement of HCVpp entry, suggesting a role for lipid transfer in facilitating HCVpp entry. Importantly, silencing of SR-BI expression in target cells by RNA interference markedly reduced HDL-mediated enhancement of HCVpp entry. Finally, enhancement of HCVpp entry was also suppressed when the SR-BI binding region on HCV glycoprotein E2 was deleted. Altogether, these data indicate that HDL-mediated enhancement of HCVpp entry involves a complex interplay between SR-BI, HDL, and HCV envelope glycoproteins, and they highlight the active role of HDLs in HCV entry.     

Scavenger receptor class B type I and hepatitis C virus infection of primary tupaia hepatocytes

Hepatitis C virus (HCV) is a major cause of chronic hepatitis worldwide. The study of early steps during HCV infection has been hampered by the lack of suitable in vitro or in vivo models. Primary Tupaia hepatocytes (PTH) have been shown to be susceptible to HCV infection in vitro and in vivo. Human scavenger receptor class B type I (SR-BI) represents an HCV receptor candidate mediating the cellular binding of E2 glycoprotein to HepG2 hepatoma cells. However, the function of SR-BI for viral infection of hepatocytes is unknown. In this study, we used PTH to assess the functional role of SR-BI as a putative HCV receptor. Sequence analysis of cloned tupaia SR-BI revealed a high homology between tupaia and human SR-BI. Transfection of CHO cells with human or tupaia SR-BI but not mouse SR-BI cDNA resulted in cellular E2 binding, suggesting that E2-binding domains between human and tupaia SR-BI are highly conserved. Preincubation of PTH with anti-SR-BI antibodies resulted in marked inhibition of E2 or HCV-like particle binding. However, anti-SR-BI antibodies were not able to block HCV infection of PTH. In conclusion, our results demonstrate that SR-BI represents an important cell surface molecule for the binding of the HCV envelope to hepatocytes and suggest that other or additional cell surface molecules are required for the initiation of HCV infection. Furthermore, the structural and functional similarities between human and tupaia SR-BI indicate that PTH represent a useful model system to characterize the molecular interaction of the HCV envelope and SR-BI on primary hepatocytes.     

Initiation of hepatitis C virus infection is dependent on cholesterol and cooperativity between CD81 and scavenger receptor B type I

In the past several years, a number of cellular proteins have been identified as candidate entry receptors for hepatitis C virus (HCV) by using surrogate models of HCV infection. Among these, the tetraspanin CD81 and scavenger receptor B type I (SR-BI), both of which localize to specialized plasma membrane domains enriched in cholesterol, have been suggested to be key players in HCV entry. In the current study, we used a recently developed in vitro HCV infection system to demonstrate that both CD81 and SR-BI are required for authentic HCV infection in vitro, that they function cooperatively to initiate HCV infection, and that CD81-mediated HCV entry is, in part, dependent on membrane cholesterol.     

An interplay between hypervariable region 1 of the hepatitis C virus E2 glycoprotein, the scavenger receptor BI, and high-density lipoprotein promotes both enhancement of infection and protection against neutralizing antibodies

Hepatitis C virus (HCV) circulates in the bloodstream in different forms, including complexes with immunoglobulins and/or lipoproteins. To address the significance of such associations, we produced or treated HCV pseudoparticles (HCVpp), a valid model of HCV cell entry and its inhibition, with na?ve or patient-derived sera. We demonstrate that infection of hepatocarcinoma cells by HCVpp is increased more than 10-fold by human serum factors, of which high-density lipoprotein (HDL) is a major component. Infection enhancement requires scavenger receptor BI, a molecule known to mediate HDL uptake into cells as well as HCVpp entry, and involves conserved amino acid positions in hypervariable region 1 (HVR1) of the E2 glycoprotein. Additionally, we show that the interaction with human serum or HDL, but not with low-density lipoprotein, leads to the protection of HCVpp from neutralizing antibodies, including monoclonal antibodies and antibodies present in patient sera. Finally, the deletion or mutation of HVR1 in HCVpp abolishes infection enhancement and leads to increased sensitivity to neutralizing antibodies/sera compared to that of parental HCVpp. Altogether, these results assign to HVR1 new roles which are complementary in helping HCV to survive within its host. Besides immune escape by mutation, HRV1 can mediate the enhancement of cell entry and the protection of virions from neutralizing antibodies. By preserving a balance between these functions, HVR1 may be essential for the viral persistence of HCV.     

The cellular biology of scavenger receptor class B type I

The HDL receptor scavenger receptor class B type I plays an important role in meditating the uptake of HDL-derived cholesterol and cholesteryl ester in the liver and steroidogenic tissues. However, the mechanism by which scavenger receptor class B type I mediates selective cholesterol uptake is unclear. In hepatocytes scavenger receptor class B type I mediates the transcytosis of cholesterol into bile, appears to be expressed on both basolateral and apical membranes, and directly interacts with a PDZ domain containing protein that may modulate the activity of scavenger receptor class B type I. This suggests the involvement of scavenger receptor class B type I in higher order complexes in polarized cells. Scavenger receptor class B type I expression has been shown to alter plasma membrane cholesterol distribution and induce the formation of novel membrane structures, suggesting multiple roles for scavenger receptor class B type I in the cell. A close examination of scavenger receptor class B type I function in polarized cells may yield new insights into the mechanism of scavenger receptor class B type I-mediated HDL selective uptake and the effects of scavenger receptor class B type I on cellular cholesterol homeostasis.     

Sterol regulation of scavenger receptor class B type I in macrophages

Scavenger receptor class B type I (SR-BI) is expressed in macrophages, but its role in sterol trafficking in these cells remains controversial. We examined the effect of sterol loading on SR-BI expression in human monocytes/macrophages, mouse peritoneal macrophages, and a cultured mouse macrophage cell line (J774 cells). Sterol loading using either acetylated LDL or 25-hydroxycholesterol resulted in a time- and concentration-dependent decrease in SR-BI protein and mRNA levels. Treatment of lipid-loaded J774 cells with cyclodextrin or HDL to promote cellular sterol efflux was associated with an increase in SR-BI expression. Studies were performed to determine if the sterol-associated downregulation of SR-BI in macrophages was mediated by either sterol regulatory element binding proteins (SREBPs) or the liver X receptor (LXR). Expression of constitutively active SREBPs failed to alter the expression of a luciferase reporter placed downstream of a 2556 bp 5' flanking sequence from the mouse SR-BI gene. Reduction in SR-BI expression was also seen in sterol-loaded peritoneal macrophages from mice expressing no LXRalpha and LXRbeta. We conclude that SR-BI levels in macrophages are responsive to changes in intracellular sterol content and that these sterol-associated changes are not mediated by LXR and are unlikely to be mediated by an SREBP pathway.     

Liver receptor homolog 1 controls the expression of the scavenger receptor class B type I

The scavenger receptor class B type I (SR-BI), which mediates selective cellular cholesterol uptake from high-density lipoproteins (HDLs), plays a key role in reverse cholesterol transport. The orphan nuclear receptor liver receptor homolog 1 (LRH-1) and SR-BI are co-expressed in liver and ovary, suggesting that LRH-1 might control the expression of SR-BI in these tissues. LRH-1 induces human and mouse SR-BI promoter activity by binding to an LRH-1 response element in the promoter. Retroviral expression of LRH-1 robustly induces SR-BI, an effect associated with histone H3 acetylation on the SR-BI promoter. The decrease in SR-BI mRNA levels in livers of LRH-1(+/-) animals provides in vivo evidence that LRH-1 regulates SR-BI expression. Our data demonstrate that SR-BI is an LRH-1 target gene and underscore the pivotal role of LRH-1 in reverse cholesterol transport.     

Monoclonal antibodies against 27.8 kDa protein receptor efficiently block lymphocystis disease virus infection in flounder Paralichthys olivaceus gill cells

In previous research using co-immunoprecipitation, a 27.8 kDa protein in flounder Paralichthys olivaceus gill (FG) cells was found to bind lymphocystis disease virus (LCDV). In this paper, 13 hybridomas secreting monoclonal antibodies (MAbs) against the 27.8 kDa protein were obtained, and 2 MAbs designated as 2G11 and 3D9 were cloned by limiting dilution. Analyzed by indirect enzyme-linked immunosorbent assay (ELISA) and western blotting, the MAbs specifically reacted with the 27.8 kDa protein of FG cells. Confocal fluorescence microscopy and immunogold electron microscopy (IEM) provided evidence that the epitopes recognized by these MAbs were located primarily on the cell membrane and occasionally in the cytoplasm near the cell membrane of FG cells. The MAbs could block LCDV binding after MAbs were pre-incubated with isolated membrane proteins of FG cells in a blocking ELISA, and MAbs also could inhibit LCDV infection of FG cells in culture. Moreover, several target tissues of LCDV in flounder, including gill, stomach, intestine and liver, displayed the presence of the LCDV receptor-27.8 kDa. These results strongly supported the possibility that the 27.8 kDa protein is the putative receptor specific for LCDV infection of FG cells in flounder.     

Properties of monoclonal antibodies directed against hepatitis B virus polymerase protein

Hepadnavirus polymerases are multifunctional enzymes that play critical roles during the viral life cycle but have been difficult to study due to a lack of a well-defined panel of monoclonal antibodies (MAbs). We have used recombinant human hepatitis B virus (HBV) polymerase (Pol) expressed in and purified from baculovirus-infected insect cells to generate a panel of six MAbs directed against HBV Pol protein. Such MAbs were subsequently characterized with respect to their isotypes and functions in analytical and preparative assays. Using these MAbs as probes together with various deletion mutants of Pol expressed in insect cells, we mapped the B-cell epitopes of Pol recognized by these MAbs to amino acids (aa) 8 to 20 and 20 to 30 in the terminal protein (TP) region of Pol, to aa 225 to 250 in the spacer region, and to aa 800 to 832 in the RNase H domain. Confocal microscopy and immunocytochemical studies using various Pol-specific MAbs revealed that the protein itself appears to be exclusively localized to the cytoplasm. Finally, MAbs specific for the TP domain, but not MAbs specific for the spacer or RNase H regions of Pol, appeared to inhibit Pol function in the in vitro priming assay, suggesting that antibody-mediated interference with TP may now be assessed in the context of HBV replication.     

Remodeling of HDL remnants generated by scavenger receptor class B type I

Scavenger receptor class B type I (SR-BI) mediates the selective transfer of cholesteryl ester from HDL to cells. We previously established that SR-BI overexpressed in livers of apolipoprotein A-I-deficient mice processes exogenous human HDL2 to incrementally smaller HDL particles. When mixed with normal mouse plasma either in vivo or ex vivo, SR-BI-generated HDL "remnants" rapidly remodel to form HDL-sized lipoproteins. In this study, we analyzed HDLs throughout the process of HDL remnant formation and investigated the mechanism of conversion to larger particles. Upon interacting with SR-BI, alpha-migrating HDL2 is initially converted to a prealpha-migrating particle that is ultimately processed to a smaller alpha-migrating HDL remnant. SR-BI does not appear to generate prebeta-1 HDL particles. When incubated with isolated lipoprotein fractions, HDL remnants are converted to lipoprotein particles corresponding in size to the particle incubated with the HDL remnant. HDL remnant conversion is not altered in phospholipid transfer protein (PLTP)-deficient mouse plasma or by the addition of purified PLTP. Although LCAT-deficient plasma promoted only partial conversion, this deficiency was attributable to the nature of HDL particles in LCAT-/- mice rather than to a requirement for LCAT in the remodeling process. We conclude that HDL remnants, generated by SR-BI, are converted to larger particles by rapidly reassociating with existing HDL particles in an enzyme-independent manner.     

Differentiation-dependent expression and localization of the class B type I scavenger receptor in intestine

The current study used the human Caco-2 cell line and mouse intestine to explore the topology of expression of the class B type I scavenger receptor (SR-BI) in intestinal cells. Results showed that intestinal cells expressed only the SR-BI isoform with little or no expression of the SR-BII variant. The expression of SR-BI in Caco-2 cells is differentiation dependent, with little or no expression in preconfluent undifferentiated cells. Analysis of Caco-2 cells cultured in Transwell porous membranes revealed the presence of SR-BI on both the apical and basolateral cell surface. Immunoblot analysis of mouse intestinal cell extracts demonstrated a gradation of SR-BI expression along the gastrocolic axis of the intestine, with the highest level of expression in the proximal intestine and decreasing to minimal expression levels in the distal intestine. Immunofluorescence studies with SR-BI-specific antibodies also confirmed this expression pattern. Importantly, the immunofluorescence studies also revealed that SR-BI immunoreactivity was most intense in the apical membrane of the brush border in the duodenum. The crypt cells did not show any reactivity with SR-BI antibodies. The localization of SR-BI in the jejunum was found to be different from that observed in the duodenum. SR-BI was present on both apical and basolateral surfaces of the jejunum villus. Localization of SR-BI in the ileum was also different, with little SR-BI detectable on either apical or basolateral membranes.Taken together, these results suggest that SR-BI has the potential to serve several functions in the intestine. The localization of SR-BI on the apical surface of the proximal intestine is consistent with the hypothesis of its possible role in dietary cholesterol absorption, whereas SR-BI present on the basolateral surface of the distal intestine suggests its possible involvement in intestinal lipoprotein uptake.     

The inhibition of endocytosis affects HDL-lipid uptake mediated by the human scavenger receptor class B type I

The scavenger receptor SR-BI plays an important role in the hepatic clearance of HDL cholesterol and other lipids, driving reverse cholesterol transport and contributing to protection against atherosclerosis in mouse models. We characterized the role of endocytosis in lipid uptake from HDL, mediated by the human SR-BI, using a variety of approaches to inhibit endocytosis, including hypertonic shock, potassium or energy depletion and disassembly of the actin cytoskeleton. Our studies revealed that unlike mouse SR-BI, human SR-BI-mediated HDL-lipid uptake was reduced by inhibition of endocytosis. This was not dependent on the cytoplasmic C-terminus of SR-BI. Monitoring the uptake of both the protein and lipid components of HDL revealed that although overall lipid uptake was decreased, the degree of selective lipid uptake was increased. These data suggest that that endocytosis is a dynamic regulator of SR-BI's selective lipid uptake activity.