Purification of the major substrate for palmitoylation in rat adipocytes: N-terminal homology with CD36 and evidence for cell surface acylation

Inhibition of fatty acid uptake by TGR5 prevents diabetic cardiomyopathy

Diabetic cardiomyopathy is characterized by myocardial lipid accumulation and cardiac dysfunction. Bile acid metabolism is known to play a crucial role in cardiovascular and metabolic diseases. Takeda G-protein-coupled receptor 5 (TGR5), a major bile acid receptor, has been implicated in metabolic regulation and myocardial protection. However, the precise involvement of the bile acid-TGR5 pathway in maintaining cardiometabolic homeostasis remains unclear. Here we show decreased plasma bile acid levels in both male and female participants with diabetic myocardial injury. Additionally, we observe increased myocardial lipid accumulation and cardiac dysfunction in cardiomyocyte-specific TGR5-deleted mice (both male and female) subjected to a high-fat diet and streptozotocin treatment or bred on the diabetic db/db genetic background. Further investigation reveals that TGR5 deletion enhances cardiac fatty acid uptake, resulting in lipid accumulation. Mechanistically, TGR5 deletion promotes localization of CD36 on the plasma membrane through the upregulation of CD36 palmitoylation mediated by the palmitoyl acyltransferase DHHC4. Our findings indicate that the TGR5-DHHC4 pathway regulates cardiac fatty acid uptake, which highlights the therapeutic potential of targeting TGR5 in the management of diabetic cardiomyopathy.

CD36 as a gatekeeper of myocardial lipid metabolism and therapeutic target for metabolic disease

The multifunctional membrane glycoprotein CD36 is expressed in different types of cells and plays a key regulatory role in cellular lipid metabolism, especially in cardiac muscle. CD36 facilitates the cellular uptake of long-chain fatty acids, mediates lipid signaling, and regulates storage and oxidation of lipids in various tissues with active lipid metabolism. CD36 deficiency leads to marked impairments in peripheral lipid metabolism, which consequently impact on the cellular utilization of multiple different fuels because of the integrated nature of metabolism. The functional presence of CD36 at the plasma membrane is regulated by its reversible subcellular recycling from and to endosomes and is under the control of mechanical, hormonal, and nutritional factors. Aberrations in this dynamic role of CD36 are causally associated with various metabolic diseases, in particular insulin resistance, diabetic cardiomyopathy, and cardiac hypertrophy. Recent research in cardiac muscle has disclosed the endosomal proton pump vacuolar-type H+-ATPase (v-ATPase) as a key enzyme regulating subcellular CD36 recycling and being the site of interaction between various substrates to determine cellular substrate preference. In addition, evidence is accumulating that interventions targeting CD36 directly or modulating its subcellular recycling are effective for the treatment of metabolic diseases. In conclusion, subcellular CD36 localization is the major adaptive regulator of cellular uptake and metabolism of long-chain fatty acids and appears a suitable target for metabolic modulation therapy to mend failing hearts.

Post-translational palmitoylation of metabolic proteins

Numerous cellular proteins are post-translationally modified by addition of a lipid group to their structure, which dynamically influences the proteome by increasing hydrophobicity of proteins often impacting protein conformation, localization, stability, and binding affinity. These lipid modifications include myristoylation and palmitoylation. Palmitoylation involves a 16-carbon saturated fatty acyl chain being covalently linked to a cysteine thiol through a thioester bond. Palmitoylation is unique within this group of modifications, as the addition of the palmitoyl group is reversible and enzyme driven, rapidly affecting protein targeting, stability and subcellular trafficking. The palmitoylation reaction is catalyzed by a large family of Asp-His-His-Cys (DHHCs) motif-containing palmitoyl acyltransferases, while the reverse reaction is catalyzed by acyl-protein thioesterases (APTs), that remove the acyl chain. Palmitoyl-CoA serves an important dual purpose as it is not only a key metabolite fueling energy metabolism, but is also a substrate for this PTM. In this review, we discuss protein palmitoylation in regulating substrate metabolism, focusing on membrane transport proteins and kinases that participate in substrate uptake into the cell. We then explore the palmitoylation of mitochondrial proteins and the palmitoylation regulatory enzymes, a less explored field for potential lipid metabolic regulation.

Putative Role of Protein Palmitoylation in Cardiac Lipid-Induced Insulin Resistance

In the heart, inhibition of the insulin cascade following lipid overload is strongly associated with contractile dysfunction. The translocation of fatty acid transporter CD36 (SR-B2) from intracellular stores to the cell surface is a hallmark event in the lipid-overloaded heart, feeding forward to intracellular lipid accumulation. Yet, the molecular mechanisms by which intracellularly arrived lipids induce insulin resistance is ill-understood. Bioactive lipid metabolites (diacyl-glycerols, ceramides) are contributing factors but fail to correlate with the degree of cardiac insulin resistance in diabetic humans. This leaves room for other lipid-induced mechanisms involved in lipid-induced insulin resistance, including protein palmitoylation. Protein palmitoylation encompasses the reversible covalent attachment of palmitate moieties to cysteine residues and is governed by protein acyl-transferases and thioesterases. The function of palmitoylation is to provide proteins with proper spatiotemporal localization, thereby securing the correct unwinding of signaling pathways. In this review, we provide examples of palmitoylations of individual signaling proteins to discuss the emerging role of protein palmitoylation as a modulator of the insulin signaling cascade. Second, we speculate how protein hyper-palmitoylations (including that of CD36), as they occur during lipid oversupply, may lead to insulin resistance. Finally, we conclude that the protein palmitoylation machinery may offer novel targets to fight lipid-induced cardiomyopathy.

CD36 palmitoylation disrupts free fatty acid metabolism and promotes tissue inflammation in non-alcoholic steatohepatitis

BACKGROUND AND AIMS

Fatty acid translocase CD36 (CD36) is a membrane protein with multiple immuno-metabolic functions. Palmitoylation has been suggested to regulate the distribution and functions of CD36, but little is known about its significance in non-alcoholic steatohepatitis (NASH).

METHODS

Human liver tissue samples were obtained from patients undergoing liver biopsy for diagnostic purposes. CD36 knockout mice were injected with lentiviral vectors expressing wild-type CD36 or CD36 with mutated palmitoylation sites. Liver histology, immunofluorescence, mRNA expression profile, subcellular distributions and functions of CD36 protein were assessed.

RESULTS

The localization of CD36 on the plasma membrane of hepatocytes was markedly increased in patients with NASH compared to patients with normal liver and those with simple steatosis. Increased CD36 palmitoylation and increased localization of CD36 on the plasma membrane of hepatocytes were also observed in livers of mice with NASH. Furthermore, inhibition of CD36 palmitoylation protected mice from developing NASH. The absence of palmitoylation decreased CD36 protein hydrophobicity reducing its localization on the plasma membrane as well as in lipid raft of hepatocytes. Consequently, a lack of palmitoylation decreased fatty acid uptake and CD36/Fyn/Lyn complex in HepG2 cells. Inhibition of CD36 palmitoylation not only ameliorated intracellular lipid accumulation via activation of the AMPK pathway, but also inhibited the inflammatory response through the inhibition of the JNK signaling pathway.

CONCLUSIONS

Our findings demonstrate the key role of palmitoylation in regulating CD36 distributions and its functions in NASH. Inhibition of CD36 palmitoylation may represent an effective therapeutic strategy in patients with NASH.

LAY SUMMARY

Fatty acid translocase CD36 (CD36) is a multifunctional membrane protein which contributes to the development of liver steatosis. In the present study, we demonstrated that the localization of CD36 on the plasma membrane of hepatocytes is increased in patients with non-alcoholic steatohepatitis. Blocking the palmitoylation of CD36 reduces CD36 distribution in hepatocyte plasma membranes and protects mice from non-alcoholic steatohepatitis. The inhibition of CD36 palmitoylation not only improved fatty acid metabolic disorders but also reduced the inflammatory response in vitro and in vivo. The present study suggests that CD36 palmitoylation is important for non-alcoholic steatohepatitis development and inhibition of CD36 palmitoylation could be used to cure non-alcoholic steatohepatitis.

Post-translational modifications of CD36 (SR-B2): Implications for regulation of myocellular fatty acid uptake

The membrane-associated protein CD36, now officially designated as SR-B2, is present in various tissues and fulfills multiple cellular functions. In heart and muscle, CD36 is the main (long-chain) fatty acid transporter, regulating myocellular fatty acid uptake via its vesicle-mediated reversible trafficking (recycling) between intracellular membrane compartments and the cell surface. CD36 is subject to various types of post-translational modification. This review focusses on the role of these modifications in further regulation of myocellular fatty acid uptake. Glycosylation, ubiquitination and palmitoylation are involved in regulating CD36 stability, while phosphorylation at extracellular sites affect the rate of fatty acid uptake. In addition, CD36 modification by O-linked N-acetylglucosamine may regulate the translocation of CD36 from endosomes to the cell surface. Acetylation of CD36 has also been reported, but possible effects on CD36 expression and/or functioning have not yet been addressed. Taken together, CD36 is subject to a multitude of post-translational modifications of which their functional implications are beginning to be understood. Moreover, further investigations are needed to disclose whether these post-translational modifications play a role in altered fatty acid uptake rates seen in several pathologies of heart and muscle. This article is part of a special issue entitled: The role of post-translational protein modifications on heart and vascular metabolism edited by Jason R.B. Dyck and Jan F.C. Glatz.

FGF21 Lowers Plasma Triglycerides by Accelerating Lipoprotein Catabolism in White and Brown Adipose Tissues

FGF21 decreases plasma triglycerides (TGs) in rodents and humans; however, the underlying mechanism or mechanisms are unclear. In the present study, we examined the role of FGF21 in production and disposal of TG-rich lipoproteins (TRLs) in mice. Treatment with pharmacological doses of FGF21 acutely reduced plasma non-esterified fatty acids (NEFAs), liver TG content, and VLDL-TG secretion. In addition, metabolic turnover studies revealed that FGF21 facilitated the catabolism of TRL in white adipose tissue (WAT) and brown adipose tissue (BAT). FGF21-dependent TRL processing was strongly attenuated in CD36-deficient mice and transgenic mice lacking lipoprotein lipase in adipose tissues. Insulin resistance in diet-induced obese and ob/ob mice shifted FGF21 responses from WAT toward energy-combusting BAT. In conclusion, FGF21 lowers plasma TGs through a dual mechanism: first, by reducing NEFA plasma levels and consequently hepatic VLDL lipidation and, second, by increasing CD36 and LPL-dependent TRL disposal in WAT and BAT.

Proteomic analysis of protein palmitoylation in adipocytes

Protein palmitoylation, by modulating the dynamic interaction between protein and cellular membrane, is involved in a wide range of biological processes, including protein trafficking, sorting, sub-membrane partitioning, protein-protein interaction and cell signaling. To explore the role of protein palmitoylation in adipocytes, we have performed proteomic analysis of palmitoylated proteins in adipose tissue and 3T3-L1 adipocytes and identified more than 800 putative palmitoylated proteins. These include various transporters, enzymes required for lipid and glucose metabolism, regulators of protein trafficking and signaling molecules. Of note, key proteins involved in membrane translocation of the glucose-transporter Glut4 including IRAP, Munc18c, AS160 and Glut4, and signaling proteins in the JAK-STAT pathway including JAK1 and 2, STAT1, 3 and 5A and SHP2 in JAK-STAT, were palmitoylated in cultured adipocytes and primary adipose tissue. Further characterization showed that palmitoylation of Glut4 and IRAP was altered in obesity, and palmitoylation of JAK1 played a regulatory role in JAK1 intracellular localization. Overall, our studies provide evidence to suggest a novel and potentially regulatory role for protein palmitoylation in adipocyte function.

Each of the four intracellular cysteines of CD36 is essential for insulin- or AMP-activated protein kinase-induced CD36 translocation

Stimulation of cellular fatty acid uptake by induction of insulin signalling or AMP-kinase (AMPK) activation is due to translocation of the fatty acid-transporter CD36 from intracellular stores to the plasma membrane (PM). For investigating the role of the four Cys-residues within CD36's cytoplasmic tails in CD36 translocation, we constructed CHO-cells expressing CD36 mutants in which all four, two, or one of the intracellular Cys were replaced by Ser. Intracellular and PM localization of all mutants was similar to wild-type CD36 (CD36wt). Hence, the four Cys do not regulate sub-cellular CD36 localization. However, in contrast to CD36wt, insulin or AMPK activation failed to induce translocation of any of the mutants, indicating that all four intracellular Cys residues are essential for CD36 translocation. The mechanism of defective translocation of mutant CD36 is unknown, but appears not due to loss of S-palmitoylation of the cytoplasmic tails or to aberrant oligomerization of the mutants.

High insulin levels are required for FAT/CD36 plasma membrane translocation and enhanced fatty acid uptake in obese Zucker rat hepatocytes

In myocytes and adipocytes, insulin increases fatty acid translocase (FAT)/CD36 translocation to the plasma membrane (PM), enhancing fatty acid (FA) uptake. Evidence links increased hepatic FAT/CD36 protein amount and gene expression with hyperinsulinemia in animal models and patients with fatty liver, but whether insulin regulates FAT/CD36 expression, amount, distribution, and function in hepatocytes is currently unknown. To investigate this, FAT/CD36 protein content in isolated hepatocytes, subfractions of organelles, and density-gradient isolated membrane subfractions was analyzed in obese and lean Zucker rats by Western blotting in liver sections by immunohistochemistry and in hepatocytes by immunocytochemistry. The uptake of oleate and oleate incorporation into lipids were assessed in hepatocytes at short time points (30-600 s). We found that FAT/CD36 protein amount at the PM was higher in hepatocytes from obese rats than from lean controls. In obese rat hepatocytes, decreased cytoplasmatic content of FAT/CD36 and redistribution from low- to middle- to middle- to high-density subfractions of microsomes were found. Hallmarks of obese Zucker rat hepatocytes were increased amount of FAT/CD36 protein at the PM and enhanced FA uptake and incorporation into triglycerides, which were maintained only when exposed to hyperinsulinemic conditions (80 mU/l). In conclusion, high insulin levels are required for FAT/CD36 translocation to the PM in obese rat hepatocytes to enhance FA uptake and triglyceride synthesis. These results suggest that the hyperinsulinemia found in animal models and patients with insulin resistance and fatty liver might contribute to liver fat accumulation by inducing FAT/CD36 functional presence at the PM of hepatocytes.

Palmitoylation of CD36/FAT regulates the rate of its post-transcriptional processing in the endoplasmic reticulum

CD36/FAT is a transmembrane glycoprotein that functions in the cellular uptake of long-chain fatty acids and also as a scavenger receptor. As such it plays an important role in lipid homeostasis and, pathophysiologically, in the progression of type 2 diabetes and atherosclerosis. CD36 expression is tightly regulated at the levels of both transcription and translation. Here we show that its expression and location are also regulated post-translationally, by palmitoylation. Although palmitoylation of CD36 was not required for receptor maturation and cell surface expression, inhibition of palmitoylation either pharmacologically with cerulenin or by mutation of the relevant cysteines delayed processing at the ER and trafficking through the secretory pathway. The absence of palmitoylation also reduced the half life of the CD36 protein. Additionally, the CD36 palmitoylation mutant did not incorporate efficiently into lipid rafts, a site known to be required for its function of fatty acid uptake, and this reduced the efficiency of uptake of oxidized low density lipoprotein. These findings provide an added level of sophistication where translocation of CD36 to the plasma membrane may be physiologically regulated by palmitoylation.

Membrane Fatty Acid Transporters as Regulators of Lipid Metabolism: Implications for Metabolic Disease

Long-chain fatty acids and lipids serve a wide variety of functions in mammalian homeostasis, particularly in the formation and dynamic properties of biological membranes and as fuels for energy production in tissues such as heart and skeletal muscle. On the other hand, long-chain fatty acid metabolites may exert toxic effects on cellular functions and cause cell injury. Therefore, fatty acid uptake into the cell and intracellular handling need to be carefully controlled. In the last few years, our knowledge of the regulation of cellular fatty acid uptake has dramatically increased. Notably, fatty acid uptake was found to occur by a mechanism that resembles that of cellular glucose uptake. Thus, following an acute stimulus, particularly insulin or muscle contraction, specific fatty acid transporters translocate from intracellular stores to the plasma membrane to facilitate fatty acid uptake, just as these same stimuli recruit glucose transporters to increase glucose uptake. This regulatory mechanism is important to clear lipids from the circulation postprandially and to rapidly facilitate substrate provision when the metabolic demands of heart and muscle are increased by contractile activity. Studies in both humans and animal models have implicated fatty acid transporters in the pathogenesis of diseases such as the progression of obesity to insulin resistance and type 2 diabetes. As a result, membrane fatty acid transporters are now being regarded as a promising therapeutic target to redirect lipid fluxes in the body in an organ-specific fashion.

CD36: implications in cardiovascular disease

CD36 is a broadly expressed membrane glycoprotein that acts as a facilitator of fatty acid uptake, a signaling molecule, and a receptor for a wide range of ligands, including apoptotic cells, modified forms of low density lipoprotein, thrombospondins, fibrillar beta-amyloid, components of Gram positive bacterial walls and malaria infected erythrocytes. CD36 expression on macrophages, dendritic and endothelial cells, and in tissues including muscle, heart, and fat, suggest diverse roles, and indeed, this is truly a multi-functional receptor involved in both homeostatic and pathological conditions. Despite an impressive increase in our knowledge of CD36 functions, in depth understanding of the mechanistic aspects of this protein remains elusive. This review focuses on CD36 in cardiovascular disease-what we know, and what we have yet to learn.

Do lipid rafts mediate virus assembly and pseudotyping?

Co-infection of a host cell by two unrelated enveloped viruses can lead to the production of pseudotypes: virions containing the genome of one virus but the envelope proteins of both viruses. The selection of components during virus assembly must therefore be flexible enough to allow the incorporation of unrelated viral membrane proteins, yet specific enough to exclude the bulk of host proteins. This apparent contradiction has been termed the pseudotypic paradox. There is mounting evidence that lipid rafts play a role in the assembly pathway of non-icosahedral, enveloped viruses. Viral components are concentrated initially in localized regions of the plasma membrane via their interaction with lipid raft domains. Lateral interactions of viral structural proteins amplify the changes in local lipid composition which in turn enhance the concentration of viral proteins in the rafts. An affinity for lipid rafts may be the common feature of enveloped virus proteins that leads to the formation of pseudotypes.

CD36 and atherosclerosis

CD36 has been associated with diverse normal and pathologic processes. These include scavenger receptor functions (uptake of apoptotic cells and modified lipid), lipid metabolism and fatty acid transport, adhesion, angiogenesis, modulation of inflammation, transforming growth factor-beta activation, atherosclerosis, diabetes and cardiomyopathy. Although CD36 was identified more than 25 years ago, it is only with the advent of recent genetic technology that in-vivo evidence has emerged for its physiologic and pathologic relevance. As these in-vivo studies are expanded, we will gain further insight into the mechanism(s) by which CD36 transmits a cellular signal, and this will allow the design of specific therapeutics that impact on a particular function of CD36.

CD36 is a ditopic glycoprotein with the N-terminal domain implicated in intracellular transport

The CD36 receptor sequence predicts two hydrophobic domains located at the N- and C-termini of the protein, but there are conflicting reports as to whether the N-terminal uncleaved leader sequence functions as a transmembrane domain. To investigate the topology of CD36, we generated a panel of mutants lacking either one or both hydrophobic regions and analyzed their folding and transport in COS-7 cells. The N- and the C-terminal hydrophobic regions were both sufficient to anchor CD36 in the membrane, and a FLAG epitope inserted at the N-terminus was located intracellularly. These results indicate that CD36 adopts a ditopic configuration. Accordingly, neither N- nor C-terminal truncation mutants were secreted. Analysis with conformation-specific monoclonal antibodies showed that the N-terminal transmembrane domain truncated molecule was slowly transported through the exocytic pathway and largely accumulated intracellularly. Thus, dual membrane insertion dictates the correct topogenesis and seems to be necessary for efficient folding and intracellular transport.

Cellular fatty acid transport in heart and skeletal muscle as facilitated by proteins

Despite the importance of long-chain fatty acids (FA) as fuels for heart and skeletal muscles, the mechanism of their cellular uptake has not yet been clarified. There is dispute as to whether FA are taken up by the muscle cells via passive diffusion and/or carrier-mediated transport. Kinetic studies of FA uptake by cardiac myocytes and the use of membrane protein-modifying agents have suggested the bulk of FA uptake is due to a protein component. Three membrane-associated FA-binding proteins were proposed to play a role in FA uptake, a 40-kDa plasma membrane FA-binding protein (FABPpm), an 88-kDa FA translocase (FAT/CD36), and a 60-kDa FA transport protein (FATP). In cardiac and skeletal myocytes the intracellular carrier for FA is cytoplasmic heart-type FA-binding protein (H-FABP), which likely transports FA from the sarcolemma to their intracellular sites of metabolism. A scenario is discussed in which FABPpm, FAT/CD36, and H-FABP, probably assisted by an albumin-binding protein, cooperate in the translocation of FA across the sarcolemma.

Effect of ATP depletion on the palmitoylation of myelin proteolipid protein in young and adult rats

The present study was designed to determine whether the palmitoylation of the hydrophobic myelin proteolipid protein (PLP) is dependent on cellular energy. To this end, brain slices from 20- and 60-day-old rats were incubated with [3H]palmitate for 1 h in the presence or absence of various metabolic poisons. In adult rats, the inhibition of mitochondrial ATP production with KCN (5 mM), oligomycin (10 microM), or rotenone (10 microM) reduced the incorporation of [3H]palmitate into fatty acyl-CoA and glycerolipids by 50-60%, whereas the labeling of PLP was unaltered. Incubation in the presence of rotenone (10 microM) plus NaF (5 mM) abolished the synthesis of acyl-CoA and lipid palmitoylation, but the incorporation of [3H]palmitate into PLP was still not different from that in controls. In rapidly myelinating animals, the inhibition of both mitochondrial electron transport and glycolysis obliterated the palmitoylation of lipids but reduced that of PLP by only 40%. PLP acylation was reduced to a similar extent when slices were incubated for up to 3 h, indicating that exogenously added palmitate is incorporated into PLP by ATP-dependent and ATP-independent mechanisms. Determination of the number of PLP molecules modified by each of these reactions during development suggests that the ATP-dependent process is important during the formation and/or compaction of the myelin sheath, whereas the ATP-independent mechanism is likely to play a role in myelin maintenance, perhaps by participating in the periodic repair of thioester linkages between the fatty acids and the protein.

Regulation of fatty acid transport protein and fatty acid translocase mRNA levels by endotoxin and cytokines

The cloning of two novel fatty acid (FA) transport proteins, FA transport protein (FATP) and FA translocase (FAT), has recently been reported; however, little is known about their in vivo regulation. Endotoxin [lipopolysaccharide (LPS)], tumor necrosis factor (TNF), and interleukin-1 (IL-1) stimulate adipose tissue lipolysis and enhance hepatic lipogenesis and reesterification while suppressing FA oxidation in multiple tissues. Hence, in this study we examined their effects on FATP and FAT mRNA levels in Syrian hamsters. Our results demonstrate that LPS decreased FATP and FAT mRNA expression in adipose tissue, heart, skeletal muscle, brain, spleen, and kidney, tissues in which FA uptake and/or oxidation is decreased during sepsis. In the liver, where FA oxidation is decreased during sepsis but the uptake of peripherally derived FA is increased to support reesterification, LPS decreased FATP mRNA expression by 70-80% but increased FAT mRNA levels by four- to fivefold. The effects of LPS on FATP and FAT mRNA levels in liver were observed as early as 4 h after administration and were maximal by 16 h. TNF and IL-1 mimicked the effect of LPS on FATP and FAT mRNA levels in both liver and adipose tissue. These results indicate that the mRNAs for both transport proteins are downregulated by LPS in tissues in which FA uptake and/or oxidation are decreased during sepsis. On the other hand, differential regulation of FATP and FAT mRNA in liver raises the possibility that these proteins may be involved in transporting FA to different locations inside the cell. FATP may transport FA toward mitochondria for oxidation, which is decreased in sepsis, whereas FAT may transport FA to cytosol for reesterification, which is enhanced in sepsis.

CD36 forms covalently associated dimers and multimers in platelets and transfected COS-7 cells

CD36 is a transmembrane glycoprotein expressed on the surface of a number of cell types. The analysis of CD36 from platelets using immunoblotting, gel filtration, and native PAGE indicated the presence of high molecular complexes exceeding the Mr of monomeric CD36. Experiments using transfected COS-7 cells revealed these complexes were homodimers and -multimers of CD36. The multimers could be dissociated by treatment with a reducing agent, indicating they were formed by intermolecular cysteine-bridging. Mutagenesis of the cDNA for CD36 implicated the cysteines in the extracellular domain of the molecule. The potential physiological roles of CD36 multimerisation are discussed.

Posttranslational modification of proteins with fatty acids in platelets

Direct modification of proteins by fatty acid can occur as cotranslational N-myristoylation of an N-terminal glycine residue or as posttranslational thioesterification of cysteine residue(s). Platelets provide an excellent model system for studying the posttranslational type of modification in the absence of active protein synthesis and in the absence of protein synthesis-related protein modifications with lipids. Using this model system it was shown that thioesterification of proteins with fatty acid is less specific for palmitate than it was thought earlier and that other saturated, mono- and even polyunsaturated long chain fatty acids can also participate. The chain length and the extent of unsaturation of the protein-linked fatty acid moiety can, very likely, modulate hydrophobic protein-membrane lipid and protein-protein interactions. CD9, HLA class I glycoprotein, glycoproteins Ib, IX and IV, P-selectin and alpha subunits of G proteins have been demonstrated unequivocally as S-fatty acid acylated platelet proteins.

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 approximately 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.

CD36 is palmitoylated on both N- and C-terminal cytoplasmic tails

The membrane protein CD36 has been reported to carry out a wide range of potential functions, including serving as a receptor for thrombospondin, collagen, oxidized low density lipoprotein, fatty acids, anionic phospholipids, and Plasmodium falciparum malaria parasitized erythrocytes. This implicates CD36 in cellular adhesion, human atherosclerotic lesion formation, lipid metabolism, and malaria. A presumed rat homolog of CD36 was previously reported to be palmitoylated. We confirmed that human CD36 is palmitoylated and identified cysteines 3, 7, 464, and 466 as the palmitoylation sites using a mutagenesis approach. This result suggests that both the N- and C-terminal tails of CD36 are cytoplasmic. Published models for the topology of CD36 have the C terminus located in the cytoplasm but differ as to whether the N terminus is cytoplasmic or extracellular. To address this question, a C-terminal truncation mutant of CD36 was made by introducing a stop codon just upstream of the C-terminal transmembrane domain. This mutant was found membrane-bound when expressed in human embryonic kidney 293 cells, indicating that the N-terminal hydrophobic domain serves as a transmembrane anchor, and thus supporting a CD36 topology with two transmembrane domains.

CD36 participates in the phagocytosis of rod outer segments by retinal pigment epithelium

Mechanisms of phagocytosis are complex and incompletely understood. The retinal pigment epithelium provides an ideal system to study the specific aspects of phagocytosis since an important function of this cell is the ingestion of packets of membranous discs that are normally discarded at the apical ends of rod and cone cells during outer segment renewal. Here we provide evidence that rod outer segment phagocytosis by retinal pigment epithelium is mediated by CD36, a transmembrane glycoprotein which has been previously characterized on hematopoietic cells as a receptor for apoptotic neutrophils and oxidized low density lipoprotein. Immunocytochemical staining with monoclonal and polyclonal antibodies demonstrated CD36 expression by both human and rat retinal pigment epithelium in transverse cryostat sections of normal retina and in primary cultured cells. By western blot analysis of retinal pigment epithelial cell lysates, polyclonal and monoclonal antibodies to CD36 recognized an 88 kDa protein which comigrated with platelet CD36. Furthermore, the synthesis of CD36 mRNA by retinal pigment epithelium was confirmed by reverse transcriptase-PCR using specific CD36 oligonucleotides. The addition of CD36 antibodies to cultured retinal pigment epithelial cells reduced the binding and internalization of 125I-labeled rod outer segments by 60%. Immunofluorescence confocal microscopy confirmed that outer segment uptake was significantly diminished by an antibody to CD36. Moreover, we found that transfection of a human melanoma cell line with CD36 cDNA enabled these cells to bind and internalize isolated photoreceptor outer segments as seen by double immunofluorescent staining for surface bound and total cell-associated rod outer segments, and by measurement of cell-associated 125I-labeled rod outer segments. We conclude that the multifunctional scavenger receptor CD36 participates in the clearance of photoreceptor outer segments by retinal pigment epithelium and thus, participates in the visual process.

Inhibitory effects of cerulenin on protein palmitoylation and insulin internalization in rat adipocytes

Protein acylation by long-chain fatty acids has been suggested as a necessary step in membrane trafficking. Because several insulin effects are dependent upon membrane trafficking, the cellular effects of the protein acylation inhibitor cerulenin were examined. Cerulenin blocked palmitoylation of selected rat adipocyte proteins including CD36, the dominant marker for palmitoylation in adipocytes. To measure cerulenin's effects on insulin internalization, rat adipocytes were incubated with 125I-insulin at 37 degrees C in the presence or absence of cerulenin. Surface-bound and intracellular insulin were discriminated by the sensitivity of the former to rapid dissociation by a pH 3 buffer at 4 degrees C. Insulin internalization was inhibited 85% by 0.3 mM cerulenin. Inhibition required preincubation with the agent, was irreversible, was not dependent upon protein synthesis, and was not the result of ATP depletion. Cerulenin was also found to inhibit insulin-stimulated glucose uptake and acetyl-CoA carboxylase activity. Cerulenin had no effect on basal glucose uptake and utilization or on the uptake and retention of fatty acids. In summary, protein acylation may be an important step in insulin-regulated cellular functions dependent upon membrane trafficking, such as insulin internalization.

Isolation of myocardial membrane long-chain fatty acid-binding protein: homology with a rat membrane protein implicated in the binding or transport of long-chain fatty acids

Abnormal myocardial long-chain fatty acid uptake is suspected of being involved in certain types of heart disease, but the mechanism by which the heart takes up long-chain fatty acids remains unclear. The sulfo-N-succinimidyl derivatives of long-chain fatty acids have been reported to undergo covalent binding to a membrane protein and to irreversibly inhibit the transport of long-chain fatty acids by rat adipocytes (Harmon et al., 1991). It has been suggested that the membrane protein bound by these derivatives is a candidate transporter for long-chain fatty acids in adipocytes. However, myocardial membrane long-chain fatty acid-binding proteins have not yet been fully investigated. Rat hearts were isolated and perfused with a sulfo-N-succinimidyl derivative of tritium-labeled palmitate ([3H]SSP). Then the [3H]SSP-binding protein was characterized by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) autoradiography and histological autoradiography. Myocardial palmitic acid uptake was examined after pretreatment of isolated perfused rat hearts with SSP. The SSP-binding protein was isolated from bovine hearts by successive chromatography, and the amino acid sequences of lysylendopeptidase-digested peptide fragments were determined. SDS-PAGE autoradiography revealed that [3H]SSP bound to an 85-90 kDa protein derived from the myocardial microsomal fraction, and histological autoradiography demonstrated that [3H]SSP radioactivity was localized to the myocardial cell membrane. Pre-incubation with SSP inhibited palmitic acid uptake by isolated perfused rat hearts. A [3H]SSP-binding protein was also found in canine and bovine hearts, and was isolated from the bovine cardiac membrane fraction. Amino acid sequencing revealed that four peptide fragments showed strong sequence homology with rat adipocyte membrane protein, which is implicated in the binding or transport of long-chain fatty acids (Abumrad et al., 1993). We conclude that the SSP-binding protein is localized to the myocardial cell membrane and might be involved in the uptake or transport of long-chain fatty acids.

The major integral membrane glycoprotein in adipocytes is a novel 200-kDa heterodimer

The major glycoprotein in adipocytes was purified from rat adipocyte membranes by affinity chromatography with wheat germ agglutinin-agarose followed by DEAE-Sepharose ion exchange chromatography. The protein had an apparent molecular weight of 200-kDa when analysed by SDS-PAGE under non-reducing conditions. When electroeluted from the gel, boiled in the presence of beta-mercaptoethanol, and re-analysed by gel electrophoresis, it was found to be composed of 100- and 160-kDa subunits. The N-terminal sequences were determined through 20 amino acids for each subunit, were found to be identical, and were homologous with no previously described protein sequences. The protein was not extractable from the membrane by high salt concentrations, indicating it was an integral membrane protein. Membrane fractionation by differential ultracentrifugation showed it was present predominantly in the plasma membrane fraction. The protein was susceptible to cell surface radiolabelling, further suggesting it was a plasma membrane protein. In summary, the major membrane glycoprotein in adipocytes is a novel 200-kDa heterodimer whose disulfide-linked subunits possess identical N-terminal sequences.