Attilio Rigotti

Identification of Scavenger Receptor SR-BI as a High Density Lipoprotein Receptor

High density lipoprotein (HDL) and low density lipoprotein (LDL) are cholesterol transport particles whose plasma concentrations are directly (LDL) and inversely (HDL) correlated with risk for atherosclerosis. LDL catabolism involves cellular uptake and degradation of the entire particle by a well-characterized receptor. HDL, in contrast, selectively delivers its cholesterol, but not protein, to cells by unknown receptors. Here it is shown that the class B scavenger receptor SR-BI is an HDL receptor. SR-BI binds HDL with high affinity, is expressed primarily in liver and nonplacental steroidogenic tissues, and mediates selective cholesterol uptake by a mechanism distinct from the classic LDL receptor pathway.

Regulation of scavenger receptor, class B, type I, a high density lipoprotein receptor, in liver and steroidogenic tissues of the rat.

The scavenger receptor, class B, type I (SR-BI) binds HDL and mediates the selective transfer of cholesteryl esters from HDL to cultured cells. The tissue distribution of SR-BI in mice suggests that this receptor may deliver HDL-cholesterol to the liver and to nonplacental steroidogenic tissues. To examine the role of SR-BI in vivo, we determined its tissue and cell type-specific expression pattern and regulation in rats. High levels of immunodetectable SR-BI were present in the adrenal gland, ovary, and liver. In pregnant animals, the mammary gland also expressed high levels of the protein. SR-BI was localized by immunofluorescence to the surfaces of steroidogenic cells in the zona fasciculata and zona reticularis of the adrenal gland and to the corpus luteal cells of the ovary. High-dose estrogen treatment dramatically reduced SR-BI in the liver and increased SR-BI in the adrenal gland and corpus luteal cells of the ovary. These estrogen-induced increases in SR-BI in the adrenal gland and ovary were accompanied by enhanced in vivo uptake of fluorescent lipid from HDL. The administration of human chorionic gonadotropin induced a dramatic increase in SR-BI in the steroidogenic Leydig cells of the testes. These findings suggest that SR-BI mediates physiologically relevant uptake of cholesterol from HDL to nonplacental steroidogenic tissues in vivo.

Murine SR-BI, a High Density Lipoprotein Receptor That Mediates Selective Lipid Uptake, Is N-Glycosylated and Fatty Acylated and Colocalizes with Plasma Membrane Caveolae

The class B, type I scavenger receptor, SR-BI, was the first molecularly well defined cell surface high density lipoprotein (HDL) receptor to be described. It mediates transfer of lipid from HDL to cells via selective lipid uptake, a mechanism distinct from receptor-mediated endocytosis via clathrin-coated pits and vesicles. SR-BI is expressed most abundantly in steroidogenic tissues (adrenal gland, ovary), where trophic hormones coordinately regulate its expression with steroidogenesis, and in the liver, where it may participate in reverse cholesterol transport. Here we have used immunochemical methods to study the structure and subcellular localization of murine SR-BI (mSR-BI) expressed either in transfected Chinese hamster ovary cells or in murine adrenocortical Y1-BS1 cells. mSR-BI, an ∼82-kDa glycoprotein, was initially synthesized with multiple high mannose N-linked oligosaccharide chains, and some, but not all, of these were processed to complex forms during maturation of the protein in the Golgi apparatus. Metabolic labeling with [3H]palmitate and [3H]myristate demonstrated that mSR-BI was fatty acylated, a property shared with CD36, another class B scavenger receptor, and other proteins that concentrate in specialized, cholesterol- and glycolipid-rich plasma membrane microdomains called caveolae. OptiPrep density gradient fractionation of plasma membranes established that mSR-BI copurified with caveolin-1, a constituent of caveolae; and immunofluorescence microscopy demonstrated that mSR-BI colocalized with caveolin-1 in punctate microdomains across the surface of cells and on the edges of cells. Thus, mSR-BI colocalizes with caveolae, and this raises the possibility that the unique properties of these specialized cell surface domains may play a critical role in SR-BI-mediated transfer of lipids between lipoproteins and cells.

Regulation by Adrenocorticotropic Hormone of the in Vivo Expression of Scavenger Receptor Class B Type I (SR-BI), a High Density Lipoprotein Receptor, in Steroidogenic Cells of the Murine Adrenal Gland

The class B, type I scavenger receptor, SR-BI, binds high density lipoprotein (HDL) and can mediate selective uptake of HDL cholesteryl esters by cultured cells. The high levels of expression of SR-BI in steroidogenic tissues and the importance of selective uptake from HDL as a source of cholesterol for steroidogenesis raised the possibility that SR-BI may participate in cholesterol delivery to steroidogenic tissues in vivo. We have used immunoblotting and immunohistochemical methods to show that SR-BI is specifically expressed in a distinctive pattern on the surfaces of steroid-producing cells in the murine adrenal glands cortex and that its expression in vivo is induced by adrenocorticotropic hormone and suppressed by glucocorticoids. Thus, expression of SR-BI protein is coordinately regulated with adrenal steroidogenesis. These data provide strong support for the hypothesis that SR-BI is a physiologically relevant HDL receptor that provides substrate cholesterol for steroid hormone synthesis.

Influence of the HDL Receptor SR-BI on Lipoprotein Metabolism and Atherosclerosis

Abstract—The scavenger receptor class B type I (SR-BI) was the first molecularly well-defined cell-surface HDL receptor to be described. SR-BI mediates selective HDL cholesterol uptake by formation of a productive lipoprotein/receptor complex, which requires specific structural domains and conformation states of apolipoprotein A-I present in HDL particles. SR-BI is abundantly expressed in several tissues, including the liver, where its expression is regulated by various mechanisms, including the transcriptional activity of nuclear receptors. The importance of SR-BI in overall HDL cholesterol metabolism and its antiatherogenic activity in vivo has been definitively established by SR-BI gene manipulation in mice. Remarkably, SR-BI/apolipoprotein E double-knockout mice develop complex coronary artery disease, myocardial infarction, and heart failure. Additional studies should help to define the importance of SR-BI in human health and disease.

Scavenger receptor BI-a cell surface receptor for high density lipoprotein

The receptor-mediated transfer of lipids between cells and lipoproteins plays an important role in lipoprotein metabolism and cardiovascular disease. Although there have been many valuable studies of HDL binding to tissues, cells and membranes, and of the potential role of such binding in the transport of lipids between HDL and cells, much less is known about HDL receptors than about receptors for other lipoproteins (e.g. LDL, chylomicrons, vitellogenin). Here we review recent studies of the class B, type I scavenger receptor, which appears to be a physiologically relevant, cell surface HDL receptor that mediates the selective uptake of lipids by cells.

The role of the high-density lipoprotein receptor SR-BI in cholesterol metabolism.

The HDL receptor scavenger receptor class B type I (SR-BI), which mediates selective HDL cholesterol uptake, plays a role in murine HDL metabolism, reverse cholesterol transport and whole-body cholesterol homeostasis. SR-BI is found in the liver, where its expression is regulated by estrogen, dietary cholesterol and fat, and controls murine plasma HDL cholesterol levels and bile cholesterol secretion. SR-BI is also highly expressed in rodent steroidogenic cells, where it facilitates cholesterol uptake for storage or steroid hormone synthesis and where its expression is regulated by trophic hormones. The detailed mechanism(s) underlying SR-BI-mediated selective cholesterol uptake have not yet been elucidated. Further analysis of the molecular and cellular bases of SR-BI regulation and function should provide new insights into the physiology and pathophysiology of cholesterol metabolism.

Targeted disruption of the PDZK1 gene in mice causes tissue-specific depletion of the high density lipoprotein receptor scavenger receptor class B type I and altered lipoprotein metabolism.

PDZK1, a multi-PDZ domain containing adaptor protein, interacts with various membrane proteins, including the high density lipoprotein (HDL) receptor scavenger receptor class B type I (SR-BI). Here we show that PDZK1 controls in a tissue-specific and post-transcriptional fashion the expression of SR-BI in vivo. SR-BI protein expression in PDZK1 knock-out (KO) mice was reduced by 95% in the liver, 50% in the proximal intestine, and not affected in steroidogenic organs (adrenal, ovary, and testis). Thus, PDZK1 joins a growing list of adaptors that control tissue-specific activity of cell surface receptors. Hepatic expression of SR-BII, a minor splice variant with an alternative C-terminal cytoplasmic domain, was not affected in PDZK1 KO mice, suggesting that binding of PDZK1 to SR-BI is required for controlling hepatic SR-BI expression. The loss of hepatic SR-BI was the likely cause of the elevation in plasma total and HDL cholesterol and the increase in HDL particle size in PDZK1 KO mice, phenotypes similar to those observed in SR-BI KO mice. PDZK1 KO mice differed from SR-BI KO mice in that the ratio of unesterified to total plasma cholesterol was normal, females were fertile, and cholesteryl ester stores in steroidogenic organs were essentially unaffected. These differences may be due to nearly normal extrahepatic expression of SR-BI in PDZK1 KO mice. The PDZK1-dependent regulation of hepatic SR-BI and, thus, lipoprotein metabolism supports the proposal that this adaptor may represent a new target for therapeutic intervention in cardiovascular disease.

Non‐alcoholic fatty liver disease and its association with obesity, insulin resistance and increased serum levels of C‐reactive protein in Hispanics

Background: Non‐alcoholic fatty liver disease (NAFLD) is a metabolic disorder of the liver, which may progress to fibrosis or cirrhosis. Recent studies have shown a significant impact of ethnicity on susceptibility to steatosis‐related liver disease.

Increased expression of syndecan-1 protects against cardiac dilatation and dysfunction after myocardial infarction

Background— The cell-associated proteoglycan syndecan-1 (Synd1) closely regulates inflammation and cell-matrix interactions during wound healing and tumorigenesis. The present study investigated whether Synd1 may also regulate cardiac inflammation, matrix remodeling, and function after myocardial infarction (MI). Methods and Results— First, we showed increased protein and mRNA expression of Synd1 from 24 hours on, reaching its maximum at 7 days after MI and declining thereafter. Targeted deletion of Synd1 resulted in increased inflammation and accelerated, yet functionally adverse, infarct healing after MI. In concordance, adenoviral gene expression of Synd1 protected against exaggerated inflammation after MI, mainly by reducing transendothelial adhesion and migration of leukocytes, as shown in vitro. Increased inflammation in the absence of Synd1 resulted in increased monocyte chemoattractant protein-1 expression, increased activity of matrix metalloproteinase-2 and -9, and decreased activity of tissue transglutaminase, associated with increased collagen fragmentation and disorganization. Exaggerated inflammation and adverse matrix remodeling in the absence of Synd1 increased cardiac dilatation and impaired systolic function, whereas gene overexpression of Synd1 reduced inflammation and protected against cardiac dilatation and failure. Conclusions— Increased expression of Synd1 in the infarct protects against exaggerated inflammation and adverse infarct healing, thereby reducing cardiac dilatation and dysfunction after MI in mice.