Alternative (M2) macrophage activation driven through interleukin 4 receptor α (IL-4Rα) is important for immunity to parasites, wound healing, the prevention of atherosclerosis and metabolic homeostasis. M2 polarization is dependent on fatty acid oxidation (FAO), but the source of fatty acids to support this metabolic program has not been clear. We show that the uptake of triacylglycerol substrates via CD36 and their subsequent lipolysis by lysosomal acid lipase (LAL) was important for the engagement of elevated oxidative phosphorylation (OXPHOS), enhanced spare respiratory capacity (SRC), prolonged survival and expression of genes that together define M2 activation. Inhibition of lipolysis suppressed M2 activation during infection with a parasitic helminth, and blocked protective responses against this pathogen. Our findings delineate a critical role for cell-intrinsic lysosomal lipolysis in M2 activation.

SummaryThe prevalence of obesity and type 2-diabetes is increasing worldwide and threatens to shorten lifespan. Impaired insulin action in peripheral tissues is a major pathogenic factor. Insulin stimulates glucose uptake in adipose tissue through the Glut4-glucose transporter and alterations in adipose-Glut4 expression or function regulate systemic insulin sensitivity. Downregulation of adipose tissue-Glut4 occurs early in diabetes development. Here we report that adipose tissue-Glut4 regulates the expression of carbohydrate responsive-element binding protein (ChREBP), a transcriptional regulator of lipogenic and glycolytic genes. Furthermore, adipose-ChREBP is a major determinant of adipose tissue fatty acid synthesis and systemic insulin sensitivity. We discovered a new mechanism for glucose-regulation of ChREBP: Glucose-mediated activation of the canonical ChREBP isoform (ChREBPα) induces expression of a novel, potent isoform (ChREBPβ) that is transcribed from an alternative promoter. ChREBPβ expression in human adipose tissue predicts insulin sensitivity indicating that it may be an effective target for treating diabetes.

Visceral adipose tissue (VAT) is an important risk factor for obesityrelated metabolic disorders. Therefore, a reduction in VAT has become a key goal in obesity management. However, VAT is correlated with intrahepatic triglyceride (IHTG) content, so it is possible that IHTG, not VAT, is a better marker of metabolic disease. We determined the independent association of IHTG and VAT to metabolic function, by evaluating groups of obese subjects, who differed in IHTG content (high or normal) but matched on VAT volume or differed in VAT volume (high or low) but matched on IHTG content. Stable isotope tracer techniques and the euglycemic-hyperinsulinemic clamp procedure were used to assess insulin sensitivity and very-lowdensity lipoprotein-triglyceride (VLDL-TG) secretion rate. Tissue biopsies were obtained to evaluate cellular factors involved in ectopic triglyceride accumulation. Hepatic, adipose tissue and muscle insulin sensitivity were 41, 13, and 36% lower (P < 0.01), whereas VLDLtriglyceride secretion rate was almost double (P < 0.001), in subjects with higher than normal IHTG content, matched on VAT. No differences in insulin sensitivity or VLDL-TG secretion were observed between subjects with different VAT volumes, matched on IHTG content. Adipose tissue CD36 expression was lower (P < 0.05), whereas skeletal muscle CD36 expression was higher (P < 0.05), in subjects with higher than normal IHTG. These data demonstrate that IHTG, not VAT, is a better marker of the metabolic derangements associated with obesity. Furthermore, alterations in tissue fatty acid transport could be involved in the pathogenesis of ectopic triglyceride accumulation by redirecting plasma fatty acid uptake from adipose tissue toward other tissues.abdominal fat ͉ insulin resistance ͉ NAFLD ͉ steatosis ͉ VLDL V isceral adipose tissue (VAT) is an important and independent predictor of metabolic risk factors for coronary heart disease, particularly diabetes and dyslipidemia (1, 2). Moreover, data from metabolic studies conducted on human subjects (3, 4) indicate that an increase in VAT is associated with impaired glucose tolerance, insulin resistance, and increased very-low-density lipoproteintriglyceride (VLDL-TG) secretion. These observations and the unique anatomical location of visceral fat, which releases free fatty acids (FFA) and adipokines into the portal vein for direct transport to the liver, have led to the concept that VAT is responsible for many of the metabolic abnormalities associated with abdominal obesity (5, 6). Therefore, a reduction in visceral fat has become a key therapeutic goal in the management of obesity (6, 7).Although VAT is associated with metabolic disease, a causal link between VAT and metabolic dysfunction has not been demonstrated in humans. Recently, it has become clear that VAT correlates directly with intrahepatic triglyceride (IHTG) content (8-10), and an increase in IHTG is associated with the same metabolic abnormalities linked to an increase in VAT (9-12). Therefore, it is possible that VAT itself is n...

A null mutation in the scavenger receptor gene CD36 was created in mice by targeted homologous recombination. These mice produced no detectable CD36 protein, were viable, and bred normally. A significant decrease in binding and uptake of oxidized low density lipoprotein was observed in peritoneal macrophages of null mice as compared with those from control mice. CD36 null animals had a significant increase in fasting levels of cholesterol, nonesterified free fatty acids, and triacylglycerol. The increase in cholesterol was mainly within the high density lipoprotein fraction, while the increase in triacylglycerol was within the very low density lipoprotein fraction. Null animals had lower fasting serum glucose levels when compared with wild type controls. Uptake of 3 H-labeled oleate was significantly reduced in adipocytes from null mice. However, the decrease was limited to the low ratios of fatty acid:bovine serum albumin, suggesting that CD36 was necessary for the high affinity component of the uptake process. The data provide evidence for a functional role for CD36 in lipoprotein/fatty acid metabolism that was previously underappreciated.Scavenger receptors are integral membrane glycoproteins, distinct from the classic low density lipoprotein (LDL) 1 receptor, that mediate binding and uptake of native and modified lipoproteins by macrophages (1-8). There are at least two major classes of mammalian monocyte/macrophage scavenger receptors, SR-A and SR-B, based on molecular sequence and protein structural homology (1, 2, 9 -11). Scavenger receptors have broad ligand specificity and may have evolved from the primitive immune system as pattern recognition molecules, which are able to recognize common structural motifs on microbial surfaces (1,6,(12)(13)(14)(15)(16)(17). They also function in the recognition and clearance of damaged, senescent, or apoptotic cells before lysis, tissue damage, and inflammation can ensue (11, 18 -21) and in the modulation of cytokine release and host immune responses (14,15,22). Scavenger receptors may be important in the pathogenesis of atherosclerosis, since there is significant evidence in support of the hypothesis that uptake of oxidatively modified LDL by monocytes/macrophages is one of the key early events in lesion development (23-26).The class A receptors, which are expressed on liver sinusoidal endothelial and Kupffer cells (27)(28)(29), and monocytes/ macrophages (9, 10, 30) result from an alternative splice from a single gene (31, 32). SR-AI/II are trimeric, integral membrane glycoprotein receptors for oxidized LDL, acetylated LDL, and other anionic ligands including polyinosinic acid and maleylated albumin (5, 9, 10, 33-35). Monocytes/macrophages isolated from a null mouse carrying a mutation in the class A receptors showed partial loss in the ability to bind and internalize oxidized LDL (ϳ30%) (36), and a lack of murine SR-AI/II receptors in the context of an atherogenic environment was partially protective against the formation of atherogenic lesions, decreasing lesion...

The transmembrane protein CD36 has been identified in isolated cell studies as a putative transporter of long chain fatty acids. In humans, an association between CD36 deficiency and defective myocardial uptake of the fatty acid analog 15-(p-iodophenyl)-3-(R,S)-methyl pentadecanoic acid (BMIPP) has been reported. To determine whether this association represents a causal link and to assess the physiological role of CD36, we compared tissue uptake and metabolism of two iodinated fatty acid analogs BMIPP and 15-(p-iodophenyl) pentadecanoic acid (IPPA) in CD36 null and wild type mice. We also investigated the uptake and lipid incorporation of palmitate by adipocytes isolated from both groups. Compared with wild type, uptake of BMIPP and IPPA was reduced in heart (50 -80%), skeletal muscle (40 -75%), and adipose tissues (60 -70%) of null mice. The reduction was associated with a 50 -68% decrease in label incorporation into triglycerides and in 2-3-fold accumulation of label in diglycerides. Identical results were obtained from studies of [ 3 H]palmitate uptake in isolated adipocytes. The block in diglyceride to triglyceride conversion could not be explained by changes in specific activities of the key enzymes long chain acylCoA synthetase and diacylglycerol acyltransferase, which were similar in tissues from wild type and null mice. It is concluded that CD36 facilitates a large fraction of fatty acid uptake by heart, skeletal muscle, and adipose tissues and that CD36 deficiency in humans is the cause of the reported defect in myocardial BMIPP uptake. In CD36-expressing tissues, uptake regulates fatty acid esterification at the level of diacylglycerol acyltransferase by determining fatty acyl-CoA supply. The membrane transport step may represent an important control site for fatty acid metabolism in vivo.Studies with isolated and cultured cells have provided evidence for the existence of a protein-facilitated component in the membrane transport of long chain fatty acids (FA) 1 in adipose(1, 2), liver (3, 4), and muscle tissues (3, 5). Among the proteins proposed to enhance FA uptake is the transmembrane protein CD36. Expression of this protein in fibroblasts, which do not endogenously express CD36, was associated with an increase in FA uptake and incorporation into phospholipids (6). This increase reflected the appearance of a saturable, phloretinsensitive component exhibiting a transport K m within the physiologic range of unbound FA concentrations (7-10 nM) (7). The distribution of CD36 favors tissues with a high metabolic capacity for FA such as adipose, heart, and skeletal muscle (8). In muscle tissues, CD36 expression is most abundant in red oxidative fibers and is up-regulated with muscle stimulation concomitant with an increase in the V max of FA transport (9). Expression is also high in tissues experiencing large fluxes of FA such as capillary endothelia, mammary epithelia (10), or epithelia of the small intestine (11).Mice null for CD36 were recently developed and found to exhibit increased serum FA, triglyce...

The human insulin-resistance syndromes, type 2 diabetes, obesity, combined hyperlipidaemia and essential hypertension, are complex disorders whose genetic basis is unknown. The spontaneously hypertensive rat (SHR) is insulin resistant and a model of these human syndromes. Quantitative trait loci (QTLs) for SHR defects in glucose and fatty acid metabolism, hypertriglyceridaemia and hypertension map to a single locus on rat chromosome 4. Here we combine use of cDNA microarrays, congenic mapping and radiation hybrid (RH) mapping to identify a defective SHR gene, Cd36 (also known as Fat, as it encodes fatty acid translocase), at the peak of linkage to these QTLs. SHR Cd36 cDNA contains multiple sequence variants, caused by unequal genomic recombination of a duplicated ancestral gene. The encoded protein product is undetectable in SHR adipocyte plasma membrane. Transgenic mice overexpressing Cd36 have reduced blood lipids. We conclude that Cd36 deficiency underlies insulin resistance, defective fatty acid metabolism and hypertriglyceridaemia in SHR and may be important in the pathogenesis of human insulin-resistance syndromes.

CD36 is a scavenger receptor that functions in high affinity tissue uptake of long chain fatty acids (FA) and contributes under excessive fat supply to lipid accumulation and metabolic dysfunction. This review describes recent evidence regarding the CD36 FA binding site and a potential mechanism for FA transfer. It also presents the view that CD36 and FA signaling coordinate fat utilization based on newly identified CD36 actions that involve oral fat perception, intestinal fat absorption, secretion of the peptides cholecystokinin and secretin, regulation of hepatic lipoprotein output, activation of beta oxidation by muscle and regulation of the production of the FA derived bioactive eicosanoids. Thus abnormalities of fat metabolism and the associated pathology might involve dysfunction of CD36-mediated signal transduction in addition to the changes of FA uptake.

On the basis of temperature dependency, saturability, selective inhibition, and substrate specificity, it has been proposed that an anandamide transporter exists. However, all of these studies have examined anandamide accumulation at long time points when downstream effects such as metabolism and intracellular sequestration are operative. In the current study, we have investigated the initial rates (<1 min) of anandamide accumulation in neuroblastoma and astrocytoma cells in culture and have determined that uptake is not saturable with increasing concentrations of anandamide. However, anandamide hydrolysis, after uptake in neuroblastoma cells, was saturable at steady-state time points (5 min), suggesting that fatty acid amide hydrolase (FAAH) may be responsible for observed saturation of uptake at long time points. In general, arvanil, olvanil, and N-(4-hydroxyphenyl)arachidonylamide (AM404) have been characterized as transport inhibitors in studies using long incubations. However, we found these ''transport inhibitors'' did not inhibit anandamide uptake in neuroblastoma and astrocytoma cells at short time points (40 sec or less). Furthermore, we confirmed that these inhibitors in vitro were actually inhibitors of FAAH. Therefore, the likely mechanism by which the transport inhibitors raise anandamide levels to exert pharmacological effects is by inhibiting FAAH, and they should be reevaluated in this context. Immunofluorescence has indicated that FAAH staining resides mainly on intracellular membranes of neuroblastoma cells, and this finding is consistent with our observed kinetics of anandamide hydrolysis. In summary, these data suggest that anandamide uptake is a process of simple diffusion. This process is driven by metabolism and other downstream events, rather than by a specific membrane-associated anandamide carrier. T he endocannabinoids, including anandamide, are a class of neurotransmitters, similar to ⌬ 9 -tetrahydracannabinol, involved in multiple physiological events including nociception, memory, blood pressure, locomotion, and immunity (for review, see ref. 1). These compounds bind the CB1 and CB2 cannabinoid receptors, which are G i -coupled receptors that modulate ion channels and signal transduction pathways (2-4).Anandamide is readily taken up into cells. The first step of this process has been characterized by several laboratories as a process of facilitated diffusion (for reviews, see refs. 5 and 6). Although an anandamide transporter has never been isolated, its existence is based on an anandamide uptake process that is temperature-dependent, selective, and saturable. In addition, several studies identified compounds that inhibit anandamide accumulation, including N-(4-hydroxyphenyl)arachidonylamide (AM404) and the vanilloids arvanil and olvanil (7-16).After uptake, if fatty acid amide hydrolase (FAAH) is present, anandamide is hydrolyzed to arachidonic acid and ethanolamine (for review, see ref. 17). A recent report investigating the role of FAAH in anandamide metabolism showed that FAAH Ϫ...