Hyperglycemia down-regulates apolipoprotein M expression in vivo and in vitro

Diabetes is associated with low concentrations of apoM in plasma. In db/db mice, ob/ob mice as well as in the alloxan-induced diabetic mouse, the low apoM levels are paralleled by decreased expression of the apoM gene. In the latter model, insulin substitution tended to reverse the low apoM expression. It is not known whether the impairment in apoM expression can be ascribed to hyperglycemia, insulin deficiency or insulin resistance. In the present study, we investigated apoM levels and expression in rats rendered hyperglycemic by short-term glucose infusion. As expected, serum insulin concentrations rose moderately during the infusions. Serum apoM concentrations and hepatic apoM mRNA levels were significantly reduced in the hyperglycemic rats, indicating that the low expression of apoM in these diabetic models can be ascribed to hyperglycemia rather than to insulin deficiency or insulin resistance. However, in HepG2 cells both glucose and insulin markedly inhibited apoM expression these effects were additive. Thus, the possible effects of insulin in vivo seem to be mediated indirectly.     

Suppression of apolipoprotein M expression and secretion in alloxan-diabetic mouse: Partial reversal by insulin

We previously reported that apolipoprotein M (apoM) expression is reduced in ob/ob mice. Whether such a reduction is specific for this model or is a more general phenomenon in diabetes models is not known. In the present study, we therefore investigated apoM expression and secretion in NMRI mice rendered diabetes through administration of alloxan (120 mg/kg). Plasma glucose levels were markedly increased and plasma insulin levels markedly reduced at 3 days after alloxan. At the same time, that plasma apoM concentrations were decreased by 70%, apoM mRNA levels in liver was decreased by 40%, and apoM mRNA in kidney was decreased by 20% in alloxan-treated mice compared to saline-injected controls. Furthermore we found also that daily sc administration of insulin (5 IU/kg per day) increased plasma apoM levels, and apoM mRNA levels in liver and kidney. We therefore conclude that apoM is reduced in this diabetes model and that exogenous insulin administrations partially reverses the abnormal apoM expression. Based on these results, we suggest that insulin regulates apoM synthesis in vivo and, therefore, that the reduction of apoM expression is a general phenomenon in diabetes models.     

Obese and diabetic db/db mice develop marked liver fibrosis in a model of nonalcoholic steatohepatitis: role of short-form leptin receptors and osteopontin

Obesity and type 2 diabetes are associated with nonalcoholic steatohepatitis (NASH), but an obese/diabetic animal model that mimics human NASH remains undefined. We examined the induction of steatohepatitis and liver fibrosis in obese and type 2 diabetic db/db mice in a nutritional model of NASH and determined the relationship of the expressions of osteopontin (OPN) and leptin receptors to the pathogenesis of NASH. db/db mice and the corresponding lean and nondiabetic db/m mice were fed a diet deficient in methionine and choline (MCD diet) or control diet for 4 wk. Leptin-deficient obese and diabetic ob/ob mice fed similar diets were used for comparison. MCD diet-fed db/db mice exhibited significantly greater histological inflammation and higher serum alanine aminotransferase levels than db/m and ob/ob mice. Trichrome staining showed marked pericellular fibrosis in MCD diet-fed db/db mice but no significant fibrosis in db/m or ob/ob mice. Collagen I mRNA expression was increased 10-fold in db/db mice, 4-fold in db/m mice, and was unchanged in ob/ob mice. mRNA expressions of OPN, TNF-alpha, TGF-beta, and short-form leptin receptors (Ob-Ra) were significantly increased in db/db mice compared with db/m or ob/ob mice. Parallel increases in OPN and Ob-Ra protein levels were observed in db/db mice. Cultured hepatocytes expressed only Ob-Ra, and leptin stimulated OPN mRNA and protein expression in these cells. In conclusion, our results demonstrate the development of an obese/diabetic experimental model for NASH in db/db mice and suggest an important role for Ob-Ra and OPN in the pathogenesis of NASH.     

From the conceptual basis to the discovery of leptin

Two years ago, the Lasker Award was shared by Douglas Coleman and Jeffrey Friedman for their discovery of leptin, a hormone that exerts a key role in the central regulation of appetite and body weight. Douglas Coleman is recognized as the researcher who raised the hypothesis and predicted that a circulating satiety factor was lacking in the ob/ob mouse, and predicted that this factor acted at the hypothalamic level to modulate food intake. After three decades, in an attempt to identify the genes that were mutated in the ob/ob mouse, Jeffrey Friedman found that the ob gene encodes a protein hormone that reverses obesity and other abnormalities of this genetic rodent model of obesity. This discovery was a landmark event in physiology, and revolutionized our understanding of energy homeostasis. This short review aims to summarize the main steps that lead to the identification of leptin, the product of the ob gene.     

Genistein and daidzein modulate hepatic glucose and lipid regulating enzyme activities in C57BL/KsJ-db/db mice

This study examines whether anti-diabetic effects of genistein and daidzein are mediated by hepatic glucose and lipid regulating enzyme activities in type 2 diabetic animals. Male C57BL/KsJ-lepr(db)/lepr(db) (db/db) mice and age-matched non-diabetic littermates (db/+) were used in this study. The db/db mice were divided into control, genistein (0.02%, w/w) and daidzein (0.02%, w/w) groups. The blood glucose and HbA(1c) levels were significantly lower in the genistein and daidzein groups than in the control group, while glucose tolerance only was significantly improved in the genistein-supplemented group. The plasma insulin and C-peptide levels did not differ significantly between groups, yet the glucagon level was lower in the genistein and daidzein groups compared to that in the control db/db or db/+ group. The genistein and daidzein supplements increased the insulin/glucagon ratio in the type 2 diabetic animals. While the hepatic glucokinase activity was significantly lower in the db/db control group, the glucose-6-phosphatase and phosphoenolpyruvate carboxykinase activities were significantly higher in the control group compared to the db/+ group. Interestingly, these hepatic glucose metabolizing enzyme activities were reversed by the genistein and daidzein supplementation in db/db mice compared to the control group. The hepatic fatty acid synthase, beta-oxidation and carnitine palmitoyltransferase activities were all significantly lower in the genistein and daidzein groups than in the control group. The genistein and daidzein supplements also improved the plasma total cholesterol, triglyceride, HDL-cholesterol/total cholesterol, free fatty acid and hepatic triglyceride concentrations in the db/db mice. These results suggest that genistein and daidzein exert anti-diabetic effect in type 2 diabetic conditions by enhancing the glucose and lipid metabolism.     

Hepatic knockdown of mitochondrial GPAT1 in ob/ob mice improves metabolic profile

Glycerol-3-phosphate acyltransferase (GPAT) controls the first step of triglyceride (TAG) synthesis. Three distinct GPAT activities have been identified, two localized in mitochondria and one in microsomes. Mitochondrial GPAT1 (mtGPAT1) is abundantly expressed in the liver and constitutes approximately 50% of total GPAT activities in this organ. Hepatic mtGPAT1 activity is elevated in obese rodents. Mice deficient in mtGPAT1 have an improved lipid profile. To investigate if beneficial effects can result from reduced hepatic expression of mtGPAT1 in adult obese mice, adenoviral vector-based short hairpin RNA interference (shRNA) technology was used to knockdown mtGPAT1 expression in livers of ob/ob mice. Reduced expression of mtGPAT1 mRNA in liver of ob/ob mice resulted in dramatic and dose dependent reduction in mtGPAT1 activity. Reduced hepatic TAG, diacylglycerol, and free fatty acid, as well as reduced plasma cholesterol and glucose, were also observed. Fatty acid composition analysis revealed decrease of C16:0 in major lipid species. Our results demonstrate that acute reduction of mtGPAT1 in liver of ob/ob mice reduces TAG synthesis, which points to a role for mtGPAT1 in the correction of obesity and related disorders.     

Leptin attenuates gene expression for renal 25-hydroxyvitamin D3-1alpha-hydroxylase in mice via the long form of the leptin receptor

Leptin, the ob gene product secreted by adipocytes, controls overall energy balance. We previously showed that leptin administration to leptin-deficient obese (ob/ob) mice suppressed mRNA expression and activity of renal 25-hydroxyvitamin D(3)-1alpha-hydroxylase (CYP27B1). In leptin receptor-deficient (db/db) mice, we presently examined whether leptin affects 1alpha-hydroxylase expression in renal tubules through the active form of the leptin receptor (ObRb). Elevated serum concentrations of calcium and 1,25-dihydroxyvitamin D(3) [1,25-(OH)(2)D(3)] in untreated ob/ob mice showed sharp reduction with leptin administration (4 mg/kg, i.p. every 12h for 2 days); no such reduction of elevation occurred in db/db mice. ObRb mRNA was expressed in kidney, brain, fat, lung, and bone in wild-type and ob/ob mice, but not db/db mice. The ob/ob and db/db mice showed large increases in renal 1alpha-hydroxylase mRNA expression and activity. Leptin administration (4 mg/kg) completely abrogated these increases in ob/ob but not db/db mice. Renal 25-hydroxyvitamin D(3)-24-hydroxylase (CYP24) mRNA synthesis also was greatly elevated in ob/ob and db/db mice; excesses decreased significantly with leptin administration in ob/ob mice, but increased in db/db mice. Renal tubular cells in primary culture expressed mRNAs including proximal tubules markers (1alpha-hydroxylase and megalin), parathyroid hormone receptor, and vitamin D receptor. Calcitonin receptor mRNA, synthesized mainly in distal tubules, was scant, indicating that most cultured cells were from proximal tubules. Cells did not express ObRb mRNA. Forskolin exposure at 10(-6)M for 3 or 6h significantly increased 1alpha-hydroxylase mRNA. Leptin at 10(-6)M did not change mRNA expression in either presence or absence of forskolin. Accordingly, leptin attenuates renal 1alpha-hydroxylase gene expression through ObRb. Furthermore, leptin appears to act indirectly on renal proximal tubules to regulate 1alpha-hydroxylase gene expression.     

Oleoyl-estrone lowers the body weight of both ob/ob and db/db mice.

Homozygous obese db/db (BKS-Lepr(db) and ob/ob (B6-Lep(ob)) mice were treated for 14 days with a continuous infusion of a fat emulsion (controls) or loaded with oleoyl-estrone at doses of 12.5 and 50 nmol/g x d using surgically inserted osmotic minipumps. Treatment with oleoyl-estrone resulted in a marked decrease in body weight in both strains, compared with the unchecked growth of controls. In db/db mice, plasma urea and insulin, as well as liver lipid decreased with treatment. In ob/ob mice, the effect on insulin was more marked, in parallel with higher plasma lipids pointing to increased fat mobilisation. The results suggest that oleoyl-estrone effects on body fat reserves and insulin resistance are not mediated by leptin, since ob/ob mice lack this hormone and in the db/db it is present but cannot induce effects because of defective leptin receptors; in both cases oleoyl-estrone treatment lowers body weight.     

Obesity modulates the expression of haptoglobin in the white adipose tissue via TNFalpha.

Increase in adipose mass results in obesity and modulation of several factors in white adipose tissue (WAT). Two important examples are tumor necrosis factor alpha (TNFalpha) and leptin, both of which are upregulated in adipose tissue in obesity. In order to isolate genes differentially expressed in the WAT of genetically obese db/db mice compared to their lean littermates, we performed RNA fingerprinting and identified haptoglobin (Hp), which is significantly upregulated in the obese animals. Hp is a glycoprotein induced by a number of cytokines, LPS (Lipopolysaccharide), and more generally by inflammation. A significant upregulation of WAT Hp expression was also evident in several experimental obese models including the yellow agouti (/) A(y), ob/ob and goldthioglucose-treated mice (10-, 8-, and 7-fold, respectively). To identify the potential signals for an increase in Hp expression in obesity, we examined leptin and TNFalpha in vivo. Wild type animals treated with recombinant leptin did not show any alteration in WAT Hp expression compared to controls that were food restricted to the level of intake of the treated animals. On the other hand, Hp expression was induced in mice transgenically expressing TNFalpha in adipose tissue. Finally, a significant downregulation of WAT Hp mRNA was observed in ob/ob mice deficient in TNFalpha function, when compared to the ob/ob controls. These results demonstrate that haptoglobin expression in WAT is increased in obesity in rodents and TNFalpha is an important signal for this regulation.     

Hypothalamic neuronal histamine in genetically obese animals: its implication of leptin action in the brain

Leptin regulates feeding behavior and energy metabolism by affecting hypothalamic neuromodulators. The present study was designed to examine hypothalamic neuronal histamine, a recently identified mediator of leptin signaling in the brain, in genetic obese animals. Concentrations of hypothalamic histamine and tele-methylhistamine (t-MH), a major histamine metabolite, were significantly lower in obese (ob/ob) and diabetic (db/db) mice, and Zucker fatty (fa/fa) rats, leptin-deficient and leptin-receptor defective animals, respectively, relative to lean littermates (P < 0.05 for each). A bolus infusion of leptin (1.0 microg) into the lateral ventricle (ilvt) significantly elevated the turnover rate of hypothalamic neuronal histamine, as assessed by pargyline-induced accumulation of t-MH, in ob/ob mice compared with phosphate-buffered saline (PBS) infusions (P < 0.05). However, this same treatment did not affect hypothalamic histamine turnover in db/db mice. In agouti yellow (A(y)/a) mice, animals defective in pro-opiomelanocortin (POMC) signaling, normal levels of histamine, and t-MH were seen in the hypothalamus at 4 weeks of age when obesity had not yet developed. These amine levels in A(y)/a mice showed no change until 16 weeks of age, although the mice were remarkably obese by this time. Infusions of corticotropin releasing hormone (CRH), one of neuropeptide related to leptin signaling, into the third ventricle (i3vt) increased histamine turnover in the hypothalamus of Wistar King A rats (P < 0.05 versus PBS infusion). Infusion of neuropeptide Y (NPY) or alpha-melanocyte stimulating hormone (MSH), a POMC-derived peptide failed to increase histamine turnover. These results indicate that lowered activity of hypothalamic neuronal histamine in ob/ob and db/db mice, and fa/fa rats may be due to insufficiency of leptin action in the brains of these animals. These results also suggest that disruption of POMC signaling in A(y)/a mice may not impact on neuronal histamine. Moreover, CRH but neither POMC-derived peptide nor NPY may act as a signal to neuronal histamine downstream of the leptin signaling pathway.     

Timing of Ca2+ response in pancreatic beta-cells is related to mitochondrial mass

The timing and magnitude of calcium response are cell-specific in individual beta-cells. This may indicate that the cells have different roles in the intact islet. It is unknown what mechanisms determine these characteristics. We previously found that the mechanisms setting cell-specific response timing are disturbed in beta-cells from hyperglycemic mice and one of the causes is likely to be an altered mitochondrial metabolism. Mitochondria play a key role in the control of nutrient-induced insulin secretion. Here, we used confocal microscopy with the fluorescent probe MitoTracker Red CMXRos and Fluo-3 to study how the amount of active mitochondria is related to the lag-time and the magnitude of calcium response to 20mM glucose in isolated beta-cells and in cells within intact lean and ob/ob mouse islets. Results show that the mitochondrial mass is inversely correlated with the lag-times for calcium response both in lean and ob/ob mouse beta-cells (r=-0.73 and r=-0.43, respectively, P<0.05). Thus, the state of mitochondria may determine the timing of calcium response.     

Decreased p110&alpha; catalytic activity accompanies increased myocyte apoptosis and cardiac hypertrophy in leptin deficient ob/ob mice

Disruption of leptin signaling has been associated with both obesity and heart failure. We recently demonstrated that leptin deficiency in ob/ob mice and leptin insensitivity in db/db mice leads to increased myocyte apoptosis and left ventricular (LV) hypertrophy. We showed that LV mass, while similar among young ob/ob, db/db, and WT (WT) mice, is significantly higher in old ob/ob and db/db versus WT. Ob/ob and db/db mice developed markedly increased rates of myocyte apoptosis by TUNEL and activated caspase 3 levels. An intriguing candidate for the study of obesity-associated cardiac hypertrophy and apoptosis is PI3K, which functions to regulate not only cell size but also maintains cell integrity through protection from apoptosis. Here we further show that ob/ob mice have decreased catalytic activity of PI3K (p110α) which is reversed with leptin treatment. Leptin repletion in ob/ob mice reduced both myocyte apoptosis and LV hypertrophy to WT levels. We have therefore concluded that normal leptin signaling is necessary to prevent age-related myocyte apoptosis and LV hypertrophy and that PI3K is a critical component of the leptin signaling axis. The decrease in p110α catalytic activity could explain the development of increased myocyte apoptosis and cardiac hypertrophy in these obese mouse models.     

Leptin signaling deficiency impairs humoral and cellular immune responses and attenuates experimental arthritis.

Leptin is produced almost exclusively by adipocytes and regulates body weight at the hypothalamic level. In addition, recent studies showed that leptin plays an important role in T lymphocyte responses. To examine the role of leptin in Ag-induced arthritis, the development of joint inflammation was assessed in immunized leptin-deficient mice (ob/ob), +/?, and wild-type mice (+/+) following the administration of methylated BSA into the knees. The results showed that ob/ob mice developed less severe arthritis compared with control mice. The levels of IL-1beta and TNF-alpha mRNA in the synovium of arthritic knees were lower in ob/ob than in +/? mice. In vitro Ag-specific T cell proliferative responses were significantly decreased in ob/ob mice with lower IFN-gamma and higher IL-10 production, suggesting a shift toward a Th2-type response in ob/ob mice. The serum levels of anti-methylated BSA Abs of any isotype were significantly decreased in arthritic ob/ob mice compared with controls. Essentially identical results were obtained in db/db mice, which lack the expression of the long isoform of leptin receptor. By RT-PCR, we observed that B lymphocytes express leptin receptor mRNA, indicating that in addition to its effect on the cellular response, leptin may exert a direct effect on B cell function. In conclusion, leptin contributes to the mechanisms of joint inflammation in Ag-induced arthritis by regulating both humoral and cell-mediated immune responses.     

Partial leptin deficiency favors diet-induced obesity and related metabolic disorders in mice

Partial leptin deficiency is not uncommon in the general population. We hypothesized that leptin insufficiency could favor obesity, nonalcoholic steatohepatitis (NASH), and other metabolic abnormalities, particularly under high calorie intake. Thus, mice partially deficient in leptin (ob/+) and their wild-type (+/+) littermates were fed for 4 mo with a standard-calorie (SC) or a high-calorie (HC) diet. Some ob/+ mice fed the HC diet were also treated weekly with leptin. Our results showed that, when fed the SC diet, ob/+ mice did not present significant metabolic abnormalities except for elevated levels of plasma adiponectin. Under high-fat feeding, increased body fat mass, hepatic steatosis, higher plasma total cholesterol, and glucose intolerance were observed in +/+ mice, and these abnormalities were further enhanced in ob/+ mice. Furthermore, some metabolic disturbances, such as blunted plasma levels of leptin and adiponectin, reduced UCP1 expression in brown adipose tissue, increased plasma liver enzymes, beta-hydroxybutyrate and triglycerides, and slight insulin resistance, were observed only in ob/+ mice fed the HC diet. Whereas de novo fatty acid synthesis in liver was decreased in +/+ mice fed the HC diet, it was disinhibited in ob/+ mice along with the restoration of the expression of several lipogenic genes. Enhanced expression of several genes involved in fatty acid oxidation was also observed only in ob/+ animals. Leptin supplementation alleviated most of the metabolic abnormalities observed in ob/+ fed the HC diet. Hence, leptin insufficiency could increase the risk of obesity, NASH, glucose intolerance, and hyperlipidemia in a context of calorie overconsumption.     

β‐Aminoisobutyric Acid Prevents Diet‐induced Obesity in Mice With Partial Leptin Deficiency

β‐Aminoisobutyric acid (BAIBA), a thymine catabolite, increases fatty acid oxidation (FAO) in liver and reduces the gain of body fat mass in Swiss (lean) mice fed a standard chow. We determined whether BAIBA could prevent obesity and related metabolic disorders in different murine models. To this end, BAIBA (100 or 500 mg/kg/day) was administered for 4 months in mice totally deficient in leptin (ob/ob). BAIBA (100 mg/kg/day) was also given for 4 months in wild‐type (+/+) mice and mice partially deficient in leptin (ob/+) fed a high‐calorie (HC) diet. BAIBA did not limit obesity and hepatic steatosis in ob/ob mice, but reduced liver cytolysis and inflammation. In ob/+ mice fed the HC diet, BAIBA fully prevented, or limited, the gain of body fat, steatosis and necroinflammation, glucose intolerance, and hypertriglyceridemia. Plasma β‐hydroxybutyrate was increased, whereas expression of carnitine palmitoyltransferase‐1 was augmented in liver and white adipose tissue. Acetyl‐CoA carboxylase was more phosphorylated, and de novo lipogenesis was less induced in liver. These favorable effects of BAIBA in ob/+ mice were associated with a restoration of plasma leptin levels. The reduction of body adiposity afforded by BAIBA was less marked in +/+ mice. Finally, BAIBA significantly stimulated the secretion of leptin in isolated ob/+ adipose cells, but not in +/+ cells. Thus, BAIBA could limit triglyceride accretion in tissues through a leptin‐dependent stimulation of FAO. As partial leptin deficiency is not uncommon in the general population, supplementation with BAIBA may help to prevent diet‐induced obesity and related metabolic disorders in low leptin secretors.     

Leptin and insulin down-regulate angiopoietin-like protein 3, a plasma triglyceride-increasing factor

We reported previously that angiopoietin-like protein3 (ANGPTL3), a liver-specific secretory factor, increased plasma triglyceride (TG) via inhibition of lipoprotein lipase and free fatty acid (FFA) by activating adipose-lipolysis. The current study examined the regulation of Angptl3 by leptin and insulin, both of which are key players in the metabolic syndrome. Angptl3 expression and plasma ANGPTL3 levels were increased in leptin-resistant C57BL/6J(db/db) and -deficient C57BL/6J(ob/ob) mice, relative to the control. Leptin supplements decreased Angptl3 gene expression and plasma ANGPTL3 in C57BL/6J(ob/ob) mice. The changes of Angptl3 were associated with alterations of plasma TG and FFA levels. Leptin treatment directly suppressed Angptl3 gene expression in hepatocytes. Angptl3 gene expression and plasma protein levels were also increased in insulin-deficient streptozotocin-treated mice. Insulin treatment of hepatocytes decreased Angptl3 gene expression and protein secretion. Our results suggest that elevated ANGPTL3 by leptin- or insulin-resistance is attributed to increased plasma TG and FFA concentrations in obesity.     

Defects in the leptin axis reduce abundance of the ABCG5-ABCG8 sterol transporter in liver

ABGG5 (G5) and ABCG8 (G8) are ABC half-transporters that dimerize within the endoplasmic reticulum, traffic to the cell surface, and mediate cholesterol excretion into bile. Mice harboring defects in the leptin axis (db/db and ob/ob) have reduced biliary cholesterol concentrations. Rapid weight loss brought about by administration of leptin or dietary restriction increases biliary cholesterol excretion. We hypothesized that the reduction in biliary cholesterol in mice harboring defects in the leptin axis is associated with a reduction in G5G8 transporters and that levels of the transporter would increase with leptin administration and dietary restriction. We examined mRNA and protein levels for G5 and G8 in db/db and ob/ob mice. In both models G5 and G8 protein levels were reduced. In ob/ob mice, both leptin administration and dietary restriction increased G5 and G8 protein and biliary cholesterol concentrations. Finally, we examined the effects of tauroursodeoxycholate, which has been shown to increase biliary cholesterol excretion and function as a molecular chaperone. Tauroursodeoxycholate increased G5 and G8 protein and biliary cholesterol concentrations in both wild-type and db/db mice. Our results indicate that the mechanism for reduced biliary cholesterol excretion in db/db and ob/ob mice involves reductions in G5 and G8 protein levels and that this may occur at the level of G5G8 heterodimer assembly within the endoplasmic reticulum.     

Sphingolipid therapy in myocardial ischemia–reperfusion injury

Sphingolipids are known to play a significant physiological role in cell growth, cell differentiation, and critical signal transduction pathways. Recent studies have demonstrated a significant role of sphingolipids and their metabolites in the pathogenesis of myocardial ischemia–reperfusion injury. Our laboratory has investigated the cytoprotective effects of N,N,N-trimethylsphingosine chloride (TMS), a stable N-methylated synthetic sphingolipid analogue on myocardial and hepatic ischemia–reperfusion injury in clinically relevant in vivo murine models of ischemia–reperfusion injury. TMS administered intravenously at the onset of ischemia reduced myocardial infarct size in the wild-type and obese (ob/ob) mice. Following myocardial I/R, there was an improvement in cardiac function in the wild-type mice. Additionally, TMS also decreased serum liver enzymes following hepatic I/R in wild-type mice. The cytoprotective effects did not extend to the ob/ob mice following hepatic I/R or to the db/db mice following both myocardial and hepatic I/R. Our data suggest that although TMS is cytoprotective following I/R in normal animals, the cytoprotective actions of TMS are largely attenuated in obese and diabetic animals which may be due to altered signaling mechanisms in these animal models. Here we review the therapeutic role of TMS and other sphingolipids in the pathogenesis of myocardial ischemia–reperfusion injury and their possible mechanisms of cardioprotection.