Sterol carrier protein-2 directly interacts with caveolin-1 in vitro and in vivo

Sterol carrier protein 2 in lipid metabolism and non-alcoholic fatty liver disease: Pathophysiology, molecular biology, and potential clinical implications

Non-alcoholic fatty liver disease (NAFLD) is considered as the most common chronic liver disease and has become a rapidly global public health problem. Sterol carrier protein 2 (SCP-2), also called non-specific lipid-transfer protein, is predominantly expressed by the liver. SCP-2 plays a key role in intracellular lipid transport and metabolism. SCP-2 has been closely implicated in the development of NAFLD-related metabolic disorders, such as obesity, atherosclerosis, Type 2 diabetes mellitus (T2DM), and gallstones. Recent studies indicate that SCP-2 plays a beneficial role in NAFLD by regulating cholesterol-, endocannabinoid-, and fatty acid-related aspects of lipid metabolism. Hence, in this paper, we summarize the latest findings about the roles of SCP-2 in hepatic steatosis and further describe its molecular function in the pathogenesis of NAFLD.

Deciphering the roles of caveolin in neurodegenerative diseases: The good, the bad and the importance of context

Neurodegenerative diseases (NDDs), which contribute to progressive and irreversible impairments of both the structure and function of the nervous system, pose a substantial socioeconomic burden on society. Mitochondrial dysfunction, oxidative stress, membrane damage, DNA damage, and abnormal protein degradation pathways play pivotal roles in the etiology of NDDs. Recently, growing evidence has demonstrated that caveolins are important in the pathology of NDDs due to their cellular functions in signal transduction, endocytosis, transcytosis, cholesterol transport, and lipid homeostasis. Given the significance of caveolins, here we review the literature to clarify their molecular mechanisms and roles in NDDs. We first briefly introduce the general background on caveolins. Next, we focus on the various important functions of caveolins in the brain. Finally, we emphasize recent progress regarding caveolins, especially Cav-1, which exert both benefit and unfavorable effects in NDDs such as AD and PD. Collectively, the data presented here should advance the investigation of caveolins for the future development of innovative strategies for the treatment of NDDs.

Cholesterol Regulation of Pulmonary Endothelial Calcium Homeostasis

Cholesterol is a key structural component and regulator of lipid raft signaling platforms critical for cell function. Such regulation may involve changes in the biophysical properties of lipid microdomains or direct protein-sterol interactions that alter the function of ion channels, receptors, enzymes, and membrane structural proteins. Recent studies have implicated abnormal membrane cholesterol levels in mediating endothelial dysfunction that is characteristic of pulmonary hypertensive disorders, including that resulting from long-term exposure to hypoxia. Endothelial dysfunction in this setting is characterized by impaired pulmonary endothelial calcium entry and an associated imbalance that favors production vasoconstrictor and mitogenic factors that contribute to pulmonary hypertension. Here we review current knowledge of cholesterol regulation of pulmonary endothelial Ca²⁺ homeostasis, focusing on the role of membrane cholesterol in mediating agonist-induced Ca²⁺ entry and its components in the normal and hypertensive pulmonary circulation.

Intracellular and Plasma Membrane Events in Cholesterol Transport and Homeostasis

Cholesterol transport between intracellular compartments proceeds by both energy- and non-energy-dependent processes. Energy-dependent vesicular traffic partly contributes to cholesterol flux between endoplasmic reticulum, plasma membrane, and endocytic vesicles. Membrane contact sites and lipid transfer proteins are involved in nonvesicular lipid traffic. Only “active" cholesterol molecules outside of cholesterol-rich regions and partially exposed in water phase are able to fast transfer. The dissociation of partially exposed cholesterol molecules in water determines the rate of passive aqueous diffusion of cholesterol out of plasma membrane. ATP hydrolysis with concomitant conformational transition is required to cholesterol efflux by ABCA1 and ABCG1 transporters. Besides, scavenger receptor SR-B1 is involved also in cholesterol efflux by facilitated diffusion via hydrophobic tunnel within the molecule. Direct interaction of ABCA1 with apolipoprotein A-I (apoA-I) or apoA-I binding to high capacity binding sites in plasma membrane is important in cholesterol escape to free apoA-I. ABCG1-mediated efflux to fully lipidated apoA-I within high density lipoprotein particle proceeds more likely through the increase of “active” cholesterol level. Putative cholesterol-binding linear motifs within the structure of all three proteins ABCA1, ABCG1, and SR-B1 are suggested to contribute to the binding and transfer of cholesterol molecules from cytoplasmic to outer leaflets of lipid bilayer. Together, plasma membrane events and intracellular cholesterol metabolism and traffic determine the capacity of the cell for cholesterol efflux.

The Role of Caveolin 1 in HIV Infection and Pathogenesis

Caveolin 1 (Cav-1) is a major component of the caveolae structure and is expressed in a variety of cell types including macrophages, which are susceptible to human immunodeficiency virus (HIV) infection. Caveolae structures are present in abundance in mechanically stressed cells such as endothelial cells and adipocytes. HIV infection induces dysfunction of these cells and promotes pathogenesis. Cav-1 and the caveolae structure are believed to be involved in multiple cellular processes that include signal transduction, lipid regulation, endocytosis, transcytosis, and mechanoprotection. Such a broad biological role of Cav-1/caveolae is bound to have functional cross relationships with several molecular pathways including HIV replication and viral-induced pathogenesis. The current review covers the relationship of Cav-1 and HIV in respect to viral replication, persistence, and the potential role in pathogenesis.

Impact of Fabp1/Scp-2/Scp-x gene ablation (TKO) on hepatic phytol metabolism in mice

Studies in vitro have suggested that both sterol carrier protein-2/sterol carrier protein-x (Scp-2/Scp-x) and liver fatty acid binding protein [Fabp1 (L-FABP)] gene products facilitate hepatic uptake and metabolism of lipotoxic dietary phytol. However, interpretation of physiological function in mice singly gene ablated in the Scp-2/Scp-x has been complicated by concomitant upregulation of FABP1. The work presented herein provides several novel insights: i) An 8-anilino-1-naphthalenesulfonic acid displacement assay showed that neither SCP-2 nor L-FABP bound phytol, but both had high affinity for its metabolite, phytanic acid; ii) GC-MS studies with phytol-fed WT and Fabp1/Scp-2/SCP-x gene ablated [triple KO (TKO)] mice showed that TKO exacerbated hepatic accumulation of phytol metabolites in vivo in females and less so in males. Concomitantly, dietary phytol increased hepatic levels of total long-chain fatty acids (LCFAs) in both male and female WT and TKO mice. Moreover, in both WT and TKO female mice, dietary phytol increased hepatic ratios of saturated/unsaturated and polyunsaturated/monounsaturated LCFAs, while decreasing the peroxidizability index. However, in male mice, dietary phytol selectively increased the saturated/unsaturated ratio only in TKO mice, while decreasing the peroxidizability index in both WT and TKO mice. These findings suggested that: 1) SCP-2 and FABP1 both facilitated phytol metabolism after its conversion to phytanic acid; and 2) SCP-2/SCP-x had a greater impact on hepatic phytol metabolism than FABP1.

Caveolae and Caveolin-1 Integrate Reverse Cholesterol Transport and Inflammation in Atherosclerosis

Lipid disorder and inflammation play critical roles in the development of atherosclerosis. Reverse cholesterol transport is a key event in lipid metabolism. Caveolae and caveolin-1 are in the center stage of cholesterol transportation and inflammation in macrophages. Here, we propose that reverse cholesterol transport and inflammation in atherosclerosis can be integrated by caveolae and caveolin-1.

Impact of SCP-2/SCP-x gene ablation and dietary cholesterol on hepatic lipid accumulation

While a high-cholesterol diet induces hepatic steatosis, the role of intracellular sterol carrier protein-2/sterol carrier protein-x (SCP-2/SCP-x) proteins is unknown. We hypothesized that ablating SCP-2/SCP-x [double knockout (DKO)] would impact hepatic lipids (cholesterol and cholesteryl ester), especially in high-cholesterol-fed mice. DKO did not alter food consumption, and body weight (BW) gain decreased especially in females, concomitant with hepatic steatosis in females and less so in males. DKO-induced steatosis in control-fed wild-type (WT) mice was associated with 1) loss of SCP-2; 2) upregulation of liver fatty acid binding protein (L-FABP); 3) increased mRNA and/or protein levels of sterol regulatory element binding proteins (SREBP1 and SREBP2) as well as increased expression of target genes of cholesterol synthesis (Hmgcs1 and Hmgcr) and fatty acid synthesis (Acc1 and Fas); and 4) cholesteryl ester accumulation was also associated with increased acyl-CoA cholesterol acyltransferase-2 (ACAT2) in males. DKO exacerbated the high-cholesterol diet-induced hepatic cholesterol and glyceride accumulation, without further increasing SREBP1, SREBP2, or target genes. This exacerbation was associated both with loss of SCP-2 and concomitant downregulation of Ceh/Hsl, apolipoprotein B (ApoB), MTP, and/or L-FABP protein expression. DKO diminished the ability to secrete excess cholesterol into bile and oxidize cholesterol to bile acid for biliary excretion, especially in females. This suggested that SCP-2/SCP-x affects cholesterol transport to particular intracellular compartments, with ablation resulting in less to the endoplasmic reticulum for SREBP regulation, making more available for cholesteryl ester synthesis, for cholesteryl-ester storage in lipid droplets, and for bile salt synthesis and/or secretion. These alterations are significant findings, since they affect key processes in regulation of sterol metabolism.

A structural and functional investigation of a novel protein from Mycobacterium smegmatis implicated in mycobacterial macrophage survivability

The success of pathogenic mycobacterial species is owing in part to their ability to parasitize the generally inhospitable phagosomal environment of host macrophages, utilizing a variety of strategies to avoid their antimycobacterial capabilities and thereby enabling their survival. A recently identified gene target in Mycobacterium smegmatis, highly conserved within Mycobacterium spp. and denoted MSMEG_5817, has been found to be important for bacterial survival within host macrophages. To gain insight into its function, the crystal structure of MSMEG_5817 has been solved to 2.40 Å resolution. The structure reveals a high level of structural homology to the sterol carrier protein (SCP) family, suggesting a potential role of MSMEG_5817 in the binding and transportation of biologically relevant lipids required for bacterial survival. The lipid-binding capacity of MSMEG_5817 was confirmed by ELISA, revealing binding to a number of phospholipids with varying binding specificities compared with Homo sapiens SCP. A potential lipid-binding site was probed by alanine-scanning mutagenesis, revealing structurally relevant residues and a binding mechanism potentially differing from that of the SCPs.

Sterol carrier protein 2 regulates proximal tubule size in the Xenopus pronephric kidney by modulating lipid rafts

The kidney is a homeostatic organ required for waste excretion and reabsorption of water, salts and other macromolecules. To this end, a complex series of developmental steps ensures the formation of a correctly patterned and properly proportioned organ. While previous studies have mainly focused on the individual signaling pathways, the formation of higher order receptor complexes in lipid rafts is an equally important aspect. These membrane platforms are characterized by differences in local lipid and protein compositions. Indeed, the cells in the Xenopus pronephric kidney were positive for the lipid raft markers ganglioside GM1 and Caveolin-1. To specifically interfere with lipid raft function in vivo, we focused on the Sterol Carrier Protein 2 (scp2), a multifunctional protein that is an important player in remodeling lipid raft composition. In Xenopus, scp2 mRNA was strongly expressed in differentiated epithelial structures of the pronephric kidney. Knockdown of scp2 did not interfere with the patterning of the kidney along its proximo-distal axis, but dramatically decreased the size of the kidney, in particular the proximal tubules. This phenotype was accompanied by a reduction of lipid rafts, but was independent of the peroxisomal or transcriptional activities of scp2. Finally, disrupting lipid micro-domains by inhibiting cholesterol synthesis using Mevinolin phenocopied the defects seen in scp2 morphants. Together these data underscore the importance for localized signaling platforms in the proper formation of the Xenopus kidney.

Mutational analysis of the rotavirus NSP4 enterotoxic domain that binds to caveolin-1

Background

Rotavirus (RV) nonstructural protein 4 (NSP4) is the first described viral enterotoxin, which induces early secretory diarrhea in neonatal rodents. Our previous data show a direct interaction between RV NSP4 and the structural protein of caveolae, caveolin-1 (cav-1), in yeast and mammalian cells. The binding site of cav-1 mapped to the NSP4 amphipathic helix, and led us to examine which helical face was responsible for the interaction.

Methods

A panel of NSP4 mutants were prepared and tested for binding to cav-1 by yeast two hybrid and direct binding assays. The charged residues of the NSP4 amphipathic helix were changed to alanine (NSP4_46-175-ala6); and three residues in the hydrophobic face were altered to charged amino acids (NSP4_46-175-HydroMut). In total, twelve mutants of NSP4 were generated to define the cav-1 binding site. Synthetic peptides corresponding to the hydrophobic and charged faces of NSP4 were examined for structural changes by circular dichroism (CD) and diarrhea induction by a neonatal mouse study.

Results

Mutations of the hydrophilic face (NSP4_46-175-Ala6) bound cav-1 akin to wild type NSP4. In contrast, disruption of the hydrophobic face (NSP4_46-175-HydroMut) failed to bind cav-1. These data suggest NSP4 and cav-1 associate via a hydrophobic interaction. Analyses of mutant synthetic peptides in which the hydrophobic residues in the enterotoxic domain of NSP4 were altered suggested a critical hydrophobic residue. Both NSP4_{HydroMut112-140,} that contains three charged amino acids (aa113, 124, 131) changed from the original hydrophobic residues and NSP4_{AlaAcidic112-140} that contained three alanine residues substituted for negatively charged (aa114, 125, 132) amino acids failed to induce diarrhea. Whereas peptides NSP4wild type _112−140 and NSP4_{AlaBasic112-140} that contained three alanine substituted for positively charged (aa115, 119, 133) amino acids, induced diarrhea.

Conclusions

These data show that the cav-1 binding domain is within the hydrophobic face of the NSP4 amphipathic helix. The integrity of the helical structure is important for both cav-1 binding and diarrhea induction implying a connection between NSP4 functional and binding activities.

Mandarin fish caveolin 1 interaction with major capsid protein of infectious spleen and kidney necrosis virus and its role in early stages of infection

Infectious spleen and kidney necrosis virus (ISKNV) is the type species of the genus Megalocytivirus from the family Iridoviridae. ISKNV is one of the major agents that cause mortality and economic losses to the freshwater fish culture industry in Asian countries, particularly for mandarin fish (Siniperca chuatsi). In the present study, we report that the interaction of mandarin fish caveolin 1 (mCav-1) with the ISKNV major capsid protein (MCP) was detected by using a virus overlay assay and confirmed by pulldown assay and coimmunoprecipitation. This interaction was independent of the classic caveolin 1 scaffolding domain (CSD), which is responsible for interacting with several signaling proteins and receptors. Confocal immunofluorescence microscopy showed that ISKNV MCP colocalized with mCav-1 in the perinuclear region of virus-infected mandarin fish fry (MFF-1) cells, which appeared as soon as 4 h postinfection. Subcellular fractionation analysis showed that ISKNV MCP was associated with caveolae in the early stages of viral infection. RNA interference silencing of mCav-1 did not change virus-cell binding but efficiently inhibited the entry of virions into the cell. Taken together, these results suggested that mCav-1 plays an important role in the early stages of ISKNV infection.

Loss of liver FA binding protein significantly alters hepatocyte plasma membrane microdomains

Although lipid-rich microdomains of hepatocyte plasma membranes serve as the major scaffolding regions for cholesterol transport proteins important in cholesterol disposition, little is known regarding intracellular factors regulating cholesterol distribution therein. On the basis of its ability to bind cholesterol and alter hepatic cholesterol accumulation, the cytosolic liver type FA binding protein (L-FABP) was hypothesized to be a candidate protein regulating these microdomains. Compared with wild-type hepatocyte plasma membranes, L-FABP gene ablation significantly increased the proportion of cholesterol-rich microdomains. Lack of L-FABP selectively increased cholesterol, phospholipid (especially phosphatidylcholine), and branched-chain FA accumulation in the cholesterol-rich microdomains. These cholesterol-rich microdomains are important, owing to enrichment therein of significant amounts of key transport proteins involved in uptake of cholesterol [SR-B1, ABCA-1, P-glycoprotein (P-gp), sterol carrier binding protein (SCP-2)], FA transport protein (FATP), and glucose transporters 1 and 2 (GLUT1, GLUT2) insulin receptor. L-FABP gene ablation enhanced the concentration of SCP-2, SR-B1, FATP4, and GLUT1 in the cholesterol-poor microdomains, with functional implications in HDL-mediated uptake and efflux of cholesterol. Thus L-FABP gene ablation significantly impacted the proportion of cholesterol-rich versus -poor microdomains in the hepatocyte plasma membrane and altered the distribution of lipids and proteins involved in cholesterol uptake therein.

Caveolin-1 is enriched in the peroxisomal membrane of rat hepatocytes

Caveolae are a subtype of cholesterol-enriched lipid microdomains/rafts that are routinely detected as vesicles pinching off from the plasma membrane. Caveolin-1 is an essential component of caveolae. Hepatic caveolin-1 plays an important role in liver regeneration and lipid metabolism. Expression of caveolin-1 in hepatocytes is relatively low, and it has been suggested to also reside at other subcellular locations than the plasma membrane. Recently, we found that the peroxisomal membrane contains lipid microdomains. Like caveolin-1, hepatic peroxisomes are involved in lipid metabolism. Here, we analyzed the subcellular location of caveolin-1 in rat hepatocytes. The subcellular location of rat hepatocyte caveolin-1 was analyzed by cell fractionation procedures, immunofluorescence, and immuno-electron microscopy. Green fluorescent protein (GFP)-tagged caveolin-1 was expressed in rat hepatocytes. Lipid rafts were characterized after Triton X-100 or Lubrol WX extraction of purified peroxisomes. Fenofibric acid-dependent regulation of caveolin-1 was analyzed. Peroxisome biogenesis was studied in rat hepatocytes after RNA interference-mediated silencing of caveolin-1 and caveolin-1 knockout mice. Cell fractionation and microscopic analyses reveal that caveolin-1 colocalizes with peroxisomal marker proteins (catalase, the 70 kDa peroxisomal membrane protein PMP70, the adrenoleukodystrophy protein ALDP, Pex14p, and the bile acid-coenzyme A:amino acid N-acyltransferase BAAT) in rat hepatocytes. Artificially expressed GFP-caveolin-1 accumulated in catalase-positive organelles. Peroxisomal caveolin-1 is associated with detergent-resistant microdomains. Caveolin-1 expression is strongly repressed by the peroxisome proliferator-activated receptor-alpha agonist fenofibric acid. Targeting of peroxisomal matrix proteins and peroxisome number and shape were not altered in rat hepatocytes with 70%-80% reduced caveolin-1 levels and in livers of caveolin-1 knockout mice.

CONCLUSION

Caveolin-1 is enriched in peroxisomes of hepatocytes. Caveolin-1 is not required for peroxisome biogenesis, but this unique subcellular location may determine its important role in hepatocyte proliferation and lipid metabolism.

Use of dansyl-cholestanol as a probe of cholesterol behavior in membranes of living cells

While plasma membrane cholesterol-rich microdomains play a role in cholesterol trafficking, little is known about the appearance and dynamics of cholesterol through these domains in living cells. The fluorescent cholesterol analog 6-dansyl-cholestanol (DChol), its biochemical fractionation, and confocal imaging of L-cell fibroblasts contributed the following new insights: i) fluorescence properties of DChol were sensitive to microenvironment polarity and mobility; (ii) DChol taken up by L-cell fibroblasts was distributed similarly as cholesterol and preferentially into cholesterol-rich vs. -poor microdomains resolved by affinity chromatography of purified plasma membranes; iii) DChol reported similar polarity (dielectric constant near 18) but higher mobility near phospholipid polar head group region for cholesterol in purified cholesterol-rich versus -poor microdomains; and iv) real-time confocal imaging, quantitative colocalization analysis, and fluorescence resonance energy transfer with cholesterol-rich and -poor microdomain markers confirmed that DChol preferentially localized in plasma membrane cholesterol-rich microdomains of living cells. Thus, DChol sensed a unique, relatively more mobile microenvironment for cholesterol in plasma membrane cholesterol-rich microdomains, consistent with the known, more rapid exchange dynamics of cholesterol from cholesterol-rich than -poor microdomains.

Developmental iodine deficiency and hypothyroidism impair neural development, up-regulate caveolin-1 and down-regulate synaptophysin in rat hippocampus

Developmental iodine deficiency leads to inadequate thyroid hormone, which damages the hippocampus. In the present study, we implicate hippocampal caveolin-1 and synaptophysin in developmental iodine deficiency and hypothyroidism. Two developmental rat models were established: pregnant rats were administered either an iodine-deficient diet or propylthiouracil (PTU)-adulterated (5 p.p.m. or 15 p.p.m.) drinking water from gestational day 6 until postnatal day (PN) 28. Nissl staining and the levels of caveolin-1 and synaptophysin in several hippocampal subregions were assessed on PN14, PN21, PN28 and PN42. The results obtained show that surviving cells in the iodine-deficient and PTU-treated rats were lower than in controls. Up-regulation of caveolin-1 and down-regulation of synaptophysin were observed in the iodine-deficient and PTU-treated rats. Our findings implicate decreases in the number of surviving cells and alterations in the levels of caveolin-1 and synaptophysin in the impairments in neural development induced by developmental iodine deficiency and hypothyroidism.

Caveolin, sterol carrier protein-2, membrane cholesterol-rich microdomains and intracellular cholesterol trafficking

While the existence of membrane lateral microdomains has been known for over 30 years, interest in these structures accelerated in the past decade due to the discovery that cholesterol-rich microdomains serve important biological functions. It is increasingly appreciated that cholesterol-rich microdomains in the plasma membranes of eukaryotic cells represent an organizing nexus for multiple cellular proteins involved in transmembrane nutrient uptake (cholesterol, fatty acid, glucose, etc.), cell-signaling, immune recognition, pathogen entry, and many other roles. Despite these advances, however, relatively little is known regarding the organization of cholesterol itself in these plasma membrane microdomains. Although a variety of non-sterol markers indicate the presence of microdomains in the plasma membranes of living cells, none of these studies have demonstrated that cholesterol is enriched in these microdomains in living cells. Further, the role of cholesterol-rich membrane microdomains as targets for intracellular cholesterol trafficking proteins such as sterol carrier protein-2 (SCP-2) that facilitate cholesterol uptake and transcellular transport for targeting storage (cholesterol esters) or efflux is only beginning to be understood. Herein, we summarize the background as well as recent progress in this field that has advanced our understanding of these issues.

Hepatic phenotype of liver fatty acid binding protein gene-ablated mice

Although the function of liver fatty acid binding protein in hepatic fatty acid metabolism has been extensively studied, its potential role in hepatic cholesterol homeostasis is less clear. Although hepatic cholesterol accumulation was initially reported in L-FABP-null female mice, that study was performed with early N2 backcross generation mice. To resolve whether the hepatic cholesterol phenotype in these L-FABP(-/-) mice was attributable to genetic inhomogeneity, these L-FABP(-/-) mice were further backcrossed to C57Bl/6 mice up to the N10 (99.9% homogeneity) generation. Hepatic total cholesterol accumulation was observed in female, but not male, L-FABP(-/-) mice at all (N2, N4, N6, N10) backcross generations examined. The greater total cholesterol was due to increased hepatic levels of both unesterified (free) cholesterol and esterified cholesterol. Altered hepatic cholesterol accumulation correlated directly with L-FABP's ability to bind cholesterol with high affinity as shown by direct L-FABP binding of fluorescent cholesterol analogs (NBD-cholesterol, dansyl-cholesterol), a photoactivatable cholesterol analog [free cholesterol benzophenone (FCBP)], and free cholesterol (circular dichroism, isothermal titration microcalorimetry). One mole of fluorescent sterol was bound per mole of L-FABP. This was confirmed by photo-cross-linking studies with the photoactivatable cholesterol analog FCBP and by isothermal titration calorimetry with free cholesterol, which showed that L-FABP bound only one sterol molecule per L-FABP molecule. In contrast, the hepatic phenotype of male, but not female, L-FABP(-/-) mice was characterized by decreased hepatic triacylglycerol levels at all backcross generations examined. Taken together, these data support the hypothesis that L-FABP plays a role in physiological regulation of not only hepatic fatty acid metabolism, but also that of hepatic cholesterol.

Caveolin-1 restoration by cholesterol enhances the inhibitory effect of simvastatin on arginine vasopressin-induced cardiac fibroblasts proliferation

Caveolin-1 (cav1) has been implicated in the regulation of cell growth, and its expression can be regulated by cellular cholesterol content. In this study, we examined the effect of manipulating cellular cholesterol content on cav1 expression and the proliferation of adult rat cardiac fibroblasts (CFs) in the presence of arginine vasopressin (AVP). We found that AVP concentration-dependently down-regulated the expression of cav1 protein. Cav1 antisense treatment enhanced the proliferatory effect of AVP. Simvastatin, a HMG-CoA reductase inhibitor, further down-regulated cav1 protein, whereas repleting cells with cholesterol increased cav1 protein and enhanced the anti-growth effect of simvastatin. Our results provide a novel finding that cholesterol restoration may confer an additional inhibitory effect over simvastatin on AVP-induced CFs proliferation through cholesterol-cav1 interaction.

Overexpression of sterol carrier protein-2 differentially alters hepatic cholesterol accumulation in cholesterol-fed mice

Although in vitro studies suggest a role for sterol carrier protein-2 (SCP-2) in cholesterol trafficking and metabolism, the physiological significance of these observations remains unclear. This issue was addressed by examining the response of mice overexpressing physiologically relevant levels of SCP-2 to a cholesterol-rich diet. While neither SCP-2 overexpression nor cholesterol-rich diet altered food consumption, increased weight gain, hepatic lipid, and bile acid accumulation were observed in wild-type mice fed the cholesterol-rich diet. SCP-2 overexpression further exacerbated hepatic lipid accumulation in cholesterol-fed females (cholesterol/cholesteryl esters) and males (cholesterol/cholesteryl esters and triacyglycerol). Primarily in female mice, hepatic cholesterol accumulation induced by SCP-2 overexpression was associated with increased levels of LDL-receptor, HDL-receptor scavenger receptor-B1 (SR-B1) (as well as PDZK1 and/or membrane-associated protein 17 kDa), SCP-2, liver fatty acid binding protein (L-FABP), and 3alpha-hydroxysteroid dehydrogenase, without alteration of other proteins involved in cholesterol uptake (caveolin), esterification (ACAT2), efflux (ATP binding cassette A-1 receptor, ABCG5/8, and apolipoprotein A1), or oxidation/transport of bile salts (cholesterol 7alpha-hydroxylase, sterol 27alpha-hydroxylase, Na(+)/taurocholate cotransporter, Oatp1a1, and Oatp1a4). The effects of SCP-2 overexpression and cholesterol-rich diet was downregulation of proteins involved in cholesterol transport (L-FABP and SR-B1), cholesterol synthesis (related to sterol regulatory element binding protein 2 and HMG-CoA reductase), and bile acid oxidation/transport (via Oapt1a1, Oatp1a4, and SCP-x). Levels of serum and hepatic bile acids were decreased in cholesterol-fed SCP-2 overexpression mice, especially in females, while the total bile acid pool was minimally affected. Taken together, these findings support an important role for SCP-2 in hepatic cholesterol homeostasis.

Fluorescence techniques using dehydroergosterol to study cholesterol trafficking

Cholesterol itself has very few structural/chemical features suitable for real-time imaging in living cells. Thus, the advent of dehydroergosterol [ergosta-5,7,9(11),22-tetraen-3beta-ol, DHE] the fluorescent sterol most structurally and functionally similar to cholesterol to date, has proven to be a major asset for real-time probing/elucidating the sterol environment and intracellular sterol trafficking in living organisms. DHE is a naturally occurring, fluorescent sterol analog that faithfully mimics many of the properties of cholesterol. Because these properties are very sensitive to sterol structure and degradation, such studies require the use of extremely pure (>98%) quantities of fluorescent sterol. DHE is readily bound by cholesterol-binding proteins, is incorporated into lipoproteins (from the diet of animals or by exchange in vitro), and for real-time imaging studies is easily incorporated into cultured cells where it co-distributes with endogenous sterol. Incorporation from an ethanolic stock solution to cell culture media is effective, but this process forms an aqueous dispersion of DHE crystals which can result in endocytic cellular uptake and distribution into lysosomes which is problematic in imaging DHE at the plasma membrane of living cells. In contrast, monomeric DHE can be incorporated from unilamellar vesicles by exchange/fusion with the plasma membrane or from DHE-methyl-beta-cyclodextrin (DHE-MbetaCD) complexes by exchange with the plasma membrane. Both of the latter techniques can deliver large quantities of monomeric DHE with significant distribution into the plasma membrane. The properties and behavior of DHE in protein-binding, lipoproteins, model membranes, biological membranes, lipid rafts/caveolae, and real-time imaging in living cells indicate that this naturally occurring fluorescent sterol is a useful mimic for probing the properties of cholesterol in these systems.

Role of an ancestral d-bifunctional protein containing two sterol-carrier protein-2 domains in lipid uptake and trafficking in Toxoplasma

The inability to synthesize cholesterol is universal among protozoa. The intracellular pathogen Toxoplasma depends on host lipoprotein-derived cholesterol to replicate in mammalian cells. Mechanisms of cholesterol trafficking in this parasite must be important for delivery to proper organelles. We characterized a unique d-bifunctional protein variant expressed by Toxoplasma consisting of one N-terminal d-3-hydroxyacyl-CoA dehydrogenase domain fused to two tandem sterol carrier protein-2 (SCP-2) domains. This multidomain protein undergoes multiple cleavage steps to release free SCP-2. The most C-terminal SCP-2 carries a PTS1 that directs the protein to vesicles before processing. Abrogation of this signal results in SCP-2 accumulation in the cytoplasm. Cholesterol specifically binds to parasite SCP-2 but with 10-fold lower affinity than phosphatidylcholine. In mammalian cells and Toxoplasma, the two parasite SCP-2 domains promote the circulation of various lipids between organelles and to the surface. Compared with wild-type parasites, TgHAD-2SCP-2-transfected parasites replicate faster and show enhanced uptake of cholesterol and oleate, which are incorporated into neutral lipids that accumulate at the basal end of Toxoplasma. This work provides the first evidence that the lipid transfer capability of an ancestral eukaryotic SCP-2 domain can influence the lipid metabolism of an intracellular pathogen to promote its multiplication in mammalian cells.

Structure and function of the sterol carrier protein-2 N-terminal presequence

Although sterol carrier protein-2 (SCP-2) is encoded as a precursor protein (proSCP-2), little is known regarding the structure and function of the 20-amino acid N-terminal presequence. As shown herein, the presequence contains significant secondary structure and alters SCP-2: (i) secondary structure (CD), (ii) tertiary structure (aqueous exposure of Trp shown by UV absorbance, fluorescence, and fluorescence quenching), (iii) ligand binding site [Trp response to ligands, peptide cross-linked by photoactivatable free cholesterol (FCBP)], (iv) selectivity for interaction with anionic phospholipid-rich membranes, (v) interaction with a peroxisomal import protein [FRET studies of Pex5p(C) binding], the N-terminal presequence increased SCP-2's affinity for Pex5p(C) by 10-fold, and (vi) intracellular targeting in living and fixed cells (confocal microscopy). Nearly 5-fold more SCP-2 than proSCP-2 colocalized with plasma membrane lipid rafts and caveolae (AF488-CTB); 2.8-fold more SCP-2 than proSCP-2 colocalized with a mitochondrial marker (Mitotracker), but nearly 2-fold less SCP-2 than proSCP-2 colocalized with peroxisomes (AF488 antibody to PMP70). These data indicate the importance of the N-terminal presequence in regulating SCP-2 structure, cholesterol localization within the ligand binding site, membrane association, and, potentially, intracellular targeting.

Effect of sterol carrier protein-2 expression on sphingolipid distribution in plasma membrane lipid rafts/caveolae

Although sphingolipids are highly important signaling molecules enriched in lipid rafts/caveolae, relatively little is known regarding factors such as sphingolipid binding proteins that may regulate the distribution of sphingolipids to lipid rafts/caveolae of living cells. Since early work demonstrated that sterol carrier protein-2 (SCP-2) enhanced glycosphingolipid transfer from membranes in vitro, the effect of SCP-2 expression on sphingolipid distribution to lipid rafts/caveolae in living cells was examined. Using a non-detergent affinity chromatography method to isolate lipid rafts/caveolae and non-rafts from purified L-cell plasma membranes, it was shown that lipid rafts/caveolae were highly enriched in multiple sphingolipid species including ceramides, acidic glycosphingolipids (ganglioside GM1); neutral glycosphingolipids (monohexosides, dihexosides, globosides), and sphingomyelin as compared to non-raft domains. SCP-2 overexpression further enriched the content of total sphingolipids and select sphingolipid species in the lipid rafts/caveolae domains. Analysis of fluorescence binding and displacement data revealed that purified human recombinant SCP-2 exhibited high binding affinity (nanomolar range) for all sphingolipid classes tested. The binding affinity decreased in the following order: ceramides > acidic glycosphingolipid (ganglioside GM1) > neutral glycosphingolipid (monohexosides, hexosides, globosides) > sphingomyelin. Enrichment of individual sphingolipid classes to lipid rafts/caveolae versus non-rafts in SCP-2 expressing plasma membranes followed closely with those classes most strongly bound to SCP-2 (ceramides, GM1 > the neutral glycosphingolipids (monohexosides, dihexosides, and globosides) > sphingomyelin). Taken together these data suggested that SCP-2 acts to selectively regulate sphingolipid distribution to lipid rafts/caveolae in living cells.

A new N-terminal recognition domain in caveolin-1 interacts with sterol carrier protein-2 (SCP-2)

Although plasma membrane domains, such as caveolae, provide an organizing principle for signaling pathways and cholesterol homeostasis in the cell, relatively little is known regarding specific mechanisms, whereby intracellular lipid-binding proteins are targeted to caveolae. Therefore, the interaction between caveolin-1 and sterol carrier protein-2 (SCP-2), a protein that binds and transfers both cholesterol and signaling lipids (e.g., phosphatidylinositides and sphingolipids), was examined by yeast two-hybrid, in vitro binding and fluorescence resonance energy transfer (FRET) analyses. Results of the in vivo and in vitro assays identified for the first time the N-terminal amino acids (aa) 1-32 amphipathic alpha helix of SCP-2 functionally interacted with caveolin-1. This interaction was independent of the classic caveolin-1 scaffolding domain, in which many signaling proteins interact. Instead, SCP-2 bound caveolin-1 through a new domain identified in the N-terminal domain of caveolin-1 between aa 34-40. Modeling studies suggested that electrostatic interactions between the SCP-2 N-terminal aa 1-32 amphipathic alpha-helical domain (cationic, positively charged face) and the caveolin-1 N-terminal aa 33-59 alpha helix (anionic, negatively charged face) may significantly contribute to this interaction. These findings provide new insights on how SCP-2 enhances cholesterol retention within the cell as well as regulates the distribution of signaling lipids, such as phosphoinositides and sphingolipids, at plasma membrane caveolae.

SCP-2/SCP-x gene ablation alters lipid raft domains in primary cultured mouse hepatocytes

Although reverse cholesterol transport from peripheral cell types is mediated through plasma membrane microdomains termed lipid rafts, almost nothing is known regarding the existence, protein/lipid composition, or structure of these putative domains in liver hepatocytes, cells responsible for the net removal of cholesterol from the body. Lipid rafts purified from hepatocyte plasma membranes by a nondetergent affinity chromatography method were: i) present at 33 +/- 3% of total plasma membrane protein; ii) enriched in key proteins of the reverse cholesterol pathway [scavenger receptor class B type I (SR-B1), ABCA1, P-glycoprotein (P-gp), sterol carrier protein-2 (SCP-2)]; iii) devoid of caveolin-1; iv) enriched in cholesterol, sphingomyelin, GM1, and phospholipids low in polyunsaturated fatty acid and double bond index; and v) exhibited an intermediate liquid-ordered lipid phase with significant transbilayer fluidity gradient. Ablation of the gene encoding SCP-2 significantly altered lipid rafts to: i) increase the proportion of lipid rafts present, thereby increasing raft total content of ABCA1, P-gp, and SR-B1; ii) increase total phospholipids while decreasing GM1 in lipid rafts; iii) decrease the fluidity of lipid rafts, consistent with the increased intermediate liquid-ordered phase; and iv) abolish the lipid raft transbilayer fluidity gradient. Thus, despite the absence of caveolin-1 in liver hepatocytes, lipid rafts represented nearly one-third of the mouse hepatocyte plasma membrane proteins and displayed unique protein, lipid, and biophysical properties that were differentially regulated by SCP-2 expression.

Sterol carrier protein-2: new roles in regulating lipid rafts and signaling

Sterol carrier protein-2 (SCP-2) was independently discovered as a soluble protein that binds and transfers cholesterol as well as phospholipids (nonspecific lipid transfer protein, nsLTP) in vitro. Physiological functions of this protein are only now beginning to be resolved. The gene encoding SCP-2 also encodes sterol carrier protein-x (SCP-x) arising from an alternate transcription site. In vitro and in vivo SCP-x serves as a peroxisomal 3-ketoacyl-CoA thiolase in oxidation of branched-chain lipids (cholesterol to form bile acids; branched-chain fatty acid for detoxification). While peroxisomal SCP-2 facilitates branched-chain lipid oxidation, the role(s) of extraperoxisomal (up to 50% of total) are less clear. Studies using transfected fibroblasts overexpressing SCP-2 and hepatocytes from SCP-2/SCP-x gene-ablated mice reveal that SCP-2 selectively remodels the lipid composition, structure, and function of lipid rafts/caveolae. Studies of purified SCP-2 and in cells show that SCP-2 has high affinity for and selectively transfers many lipid species involved in intracellular signaling: fatty acids, fatty acyl CoAs, lysophosphatidic acid, phosphatidylinositols, and sphingolipids (sphingomyelin, ceramide, mono-di-and multi-hexosylceramides, gangliosides). SCP-2 selectively redistributes these signaling lipids between lipid rafts/caveolae and intracellular sites. These findings suggest SCP-2 serves not only in cholesterol and phospholipid transfer, but also in regulating multiple lipid signaling pathways in lipid raft/caveolae microdomains of the plasma membrane.

Reggie-1 and reggie-2 localize in non-caveolar rafts in epithelial cells: cellular localization is not dependent on the expression of caveolin proteins

Reggie-1 and reggie-2 are highly conserved and widely expressed proteins associated with membrane rafts. The molecular function of reggies remains to be clarified, but recent data indicate that they are involved in various cellular processes such as insulin signaling, phagocytosis and actin remodeling. However, there is discrepancy in the literature if reggies are associated with caveolae or non-caveolar rafts. Reggies are expressed and raft associated also in many cells which do not contain caveolae, such as neurons and lymphocytes. However, it is not clear if the function or localization of reggies are dependent on the presence of caveolae and expression of caveolin-1 protein. In this study, we directly addressed this question in epithelial cells. We could show that ectopic expression of caveolin-1 does not result in any change in the cellular localization of reggie-1, which is present at the plasma membrane also in the absence of caveolin-1. On the other hand, caveolin-2, which localizes in caveolae, is dependent on caveolin-1 expression in order to be localized at the plasma membrane. Although reggie-1 and reggie-2 strongly interact with each other, we did not detect a direct interaction between caveolin-1 and reggies by means of a yeast two-hybrid assay, nor could reggies be co-immunoprecipitated with caveolin-1. Furthermore, endogenous reggie-1 and -2 were found not to colocalize with caveolin-1 in epithelial cells. Thus, our data indicate that reggies are localized in microdomains different from caveolae, and the function of reggies is different from and independent of caveolin-1.

Phospholipid transfer proteins in perspective

Since their discovery and subsequent purification from mammalian tissues more than 30 years ago an impressive number of studies have been carried out to characterize and elucidate the biological functions of phosphatidylcholine transfer protein (PC-TP), phosphatidylinositol transfer protein (PI-TP) and non-specific lipid transfer protein, more commonly known as sterol carrier protein 2 (SCP-2). Here I will present information to show that these soluble, low-molecular weight proteins constitute domain structures in StArR-related lipid transfer (START) proteins (i.e. PC-TP), in retinal degeneration protein, type B (RdgB)-related PI-TPs (e.g. Dm RdgB, Nir2, Nir3) and in peroxisomal beta-oxidation enzyme-related SCP-2 (i.e. 3-oxoacyl-CoA thiolase, also denoted as SCP-X and the 80-kDa D-bifunctional protein). Further I will summarize the most recent studies pertaining to the physiological function of these soluble phospholipid transfer proteins in metazoa.

Caveolin-1 and regulation of cellular cholesterol homeostasis

Caveolae are 50- to 100-nm cell surface plasma membrane invaginations present in terminally differentiated cells. They are characterized by the presence of caveolin-1, sphingolipids, and cholesterol. Caveolin-1 is thought to play an important role in the regulation of cellular cholesterol homeostasis, a process that needs to be properly controlled to limit and prevent cholesterol accumulation and eventually atherosclerosis. We have recently generated caveolin-1-deficient [Cav-1(-/-)] mice in which caveolae organelles are completely eliminated from all cell types, except cardiac and skeletal muscle. In the present study, we examined the metabolism of cholesterol in wild-type (WT) and Cav-1(-/-) mouse embryonic fibroblasts (MEFs) and mouse peritoneal macrophages (MPMs). We observed that Cav-1(-/-) MEFs are enriched in esterified cholesterol but depleted of free cholesterol compared with their wild-type counterparts. Similarly, Cav-1(-/-) MPMs also contained less free cholesterol and were enriched in esterified cholesterol on cholesterol loading. In agreement with this finding, caveolin-1 deficiency was associated with reduced free cholesterol synthesis but increased acyl-CoA:cholesterol acyl-transferase (ACAT) activity. In wild-type MPMs, we observed that caveolin-1 was markedly upregulated on cholesterol loading. Despite these differences, cellular cholesterol efflux from MEFs and MPMs to HDL was not affected in the Cav-1-deficient cells. Neither ATP-binding cassette transporter G1 (ABCG1)- nor scavenger receptor class B type I (SR-BI)-mediated cholesterol efflux was affected. Cellular cholesterol efflux to apolipoprotein A-I was not significantly reduced in Cav-1(-/-) MPMs compared with wild-type MPMs. However, ABCA1-mediated cholesterol efflux was clearly more sensitive to the inhibitory effects of glyburide in Cav-1(-/-) MPMs versus WT MPMs. Taken together, these findings suggest that caveolin-1 plays an important role in the regulation of intracellular cholesterol homeostasis and can modulate the activity of other proteins that are involved in the regulation of intracellular cholesterol homeostasis.

Peculiarities of host cholesterol transport to the unique intracellular vacuole containing Toxoplasma

The intracellular protozoan Toxoplasma gondii is auxotrophic for low-density lipoprotein (LDL)-derived cholesterol (C). We previously showed that T. gondii scavenges this essential lipid from host endolysosomal compartments and that C delivery to the parasitophorous vacuole (PV) does not require transit through host Golgi or endoplasmic reticulum. In this study, we explore the itinerary of C from the host endolysosomes to the PV. Labeled C incorporated into LDL is rapidly detected in intravacuolar parasites and partially esterified by the parasites. In contrast to diverse mammalian organelles, the post-endolysosomal transfer of C to the PV does not involve the host plasma membrane as an intermediate. Nevertheless, the PV membrane is accessible to extracellular sterol acceptors, suggesting C trafficking from intracellular parasites to host plasma membrane. C movement to the PV requires temperatures permissive for vesicular transport, metabolic energy and functional microtubules. Host caveolae vesicles and the sterol carrier protein-2 do not participate in this process. Proteolytic treatment of purified PV or free parasites abolishes C acquisition by the parasites. Altogether, these results support a vesicular transport system from host endolysosomes to the PV, and a requirement for PV membrane and parasite plasma membrane proteins in C delivery to T. gondii.