Leptin, troglitazone, and the expression of sterol regulatory element binding proteins in liver and pancreatic islets

The hypotriglyceridemic effect of dietary n-3 FA is associated with increased beta-oxidation and reduced leptin expression

To study the mechanisms responsible for the hypotriglyceridemic effect of marine oils, we monitored the effects of high dietary intake of n-3 PUFA on hepatic and muscular beta-oxidation, plasma leptin concentration, leptin receptor gene expression, and in vivo insulin action. Two groups of male Wistar rats were fed either a high-fat diet [28% (w/w) of saturated fat] or a high-fat diet containing 10% n-3 PUFA and 18% saturated fat for 3 wk. The hypotriglyceridemic effect of n-3 PUFA was accompanied by increased hepatic oxidation of palmitoyl-CoA (125%, P < 0.005) and palmitoyl-L-carnitine (480%, P < 0.005). These findings were corroborated by raised carnitine palmitoyltransferase-2 activity (154%, P < 0.001) and mRNA levels (91%, P < 0.01) as well as by simultaneous elevation of hepatic peroxisomal acyl-CoA oxidase activity (144%, P < 0.01) and mRNA content (82%, P < 0.05). In contrast, hepatic carnitine palmitoyltransferase-1 activity remained unchanged despite a twofold increased mRNA level after n-3 PUFA feeding. Skeletal muscle FA oxidation was less affected by dietary n-3 PUFA, and the stimulatory effect was found only in peroxisomes. Dietary intake of n-3 PUFA was followed by increased acyl-CoA oxidase activity (48%, P < 0.05) and mRNA level (83%, P < 0.05) in skeletal muscle. The increased FA oxidation after n-3 PUFA supplementation of the high-fat diet was accompanied by lower plasma leptin concentration (-38%, P < 0.05) and leptin mRNA expression (-66%, P < 0.05) in retroperitoneal adipose tissue, and elevated hepatic mRNA level for the leptin receptor Ob-Ra (140%, P < 0.05). Supplementation of the high-fat diet with n-3 PUFA enhanced in vivo insulin sensitivity, as shown by normalization of the glucose infusion rate during euglycemic hyperinsulinemic clamp. Our results indicate that the hypotriglyceridemic effect of dietary n-3 PUFA is associated with stimulation of FA oxidation in the liver and to a smaller extent in skeletal muscle. This may ameliorate dyslipidemia, tissue lipid accumulation, and insulin action, in spite of decreased plasma leptin level and leptin mRNA in adipose tissue.

Metformin in the treatment of non-alcoholic steatohepatitis: a pilot open label trial

BACKGROUND

Insulin sensitizing agents may be useful in treatment of non-alcoholic fatty liver disease.

AIM

A pilot study to evaluate the efficacy and safety of metformin in non-alcoholic fatty liver disease.

METHODS

In an open labelled study, patients with histologically confirmed non-alcoholic fatty liver disease were given metformin (20 mg/kg) for 1 year. Insulin resistance (by log homeostasis assessment model analysis for insulin resistance and Quantitative Insulin Sensitivity Check Index) and post-treatment hepatic histology were compared with pre-treatment histology.

RESULTS

Fifteen patients completed 1 year of treatment. During the initial 3 months, there was improvement in alanine aminotransferase and aspartate aminotransferase (P-value 0.01 and 0.02, respectively) along with improvement in insulin sensitivity. However, after 3 months, there was no further improvement in insulin sensitivity and there was gradual rise in aspartate aminotransferase and alanine aminotransferase back to pre-treatment levels. Among the 10 patients with post-treatment biopsy, three (33%), showed improvement in steatosis, two (20%) showed improvement in inflammation score and one (10%) showed improvement in fibrosis.

CONCLUSION

Metformin treatment was associated with only a transient improvement in liver chemistries. A progressive, sustainable reduction in insulin sensitivity was not noted during treatment.

Altered triglyceride-rich lipoprotein production in Zucker diabetic fatty rats

Triglyceride-rich lipoprotein (TRL) production was studied in Zucker diabetic fatty (ZDF) rats, a model of insulin-resistant type 2 diabetes progression. TRL production was measured in vivo by blocking catabolism with Triton WR-1339. Ten-week ZDF rats are hyperinsulinemic with increased TRL production [both triglyceride and apolipoprotein B (apoB)]. Twenty-week ZDF rats are insulinopenic, and TRL production is similar to lean controls. Insulin infusion suppresses glucose and free fatty acids in 10- and 20-wk ZDF rats. Increased TRL production is not reduced by insulin in 10-wk rats; however, at 20 wk, TRL production is suppressed by insulin. In vitro studies with hepatocytes derived from 10-wk ZDF rats showed minimal insulin dose effects on apoB secretion compared with the response and sensitivity of hepatocytes derived from 20-wk ZDF and control lean rats. Hepatic sterol regulatory-binding protein (SREBP)-1c mRNA levels are increased at 10 wk but return to control levels at 20 wk. ApoB mRNA levels are similar to lean controls at 10 and 20 wk. The following two mechanisms for hypertriglyceridemia associated with hyperinsulinemia are suggested: increased TRL synthesis and loss of TRL suppression. Increased triglyceride production in hyperinsulinemic rats likely relates to increased expression of SREBP-1c, whereas increased apoB production involves posttranscriptional processes.

Over-expression of sterol-regulatory-element-binding protein-1c (SREBP1c) in rat pancreatic islets induces lipogenesis and decreases glucose-stimulated insulin release: modulation by 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR)

Accumulation of intracellular lipid by pancreatic islet beta-cells has been proposed to inhibit normal glucose-regulated insulin secretion ('glucolipotoxicity'). In the present study, we determine whether over-expression in rat islets of the lipogenic transcription factor SREBP1c (sterol-regulatory-element-binding protein-1c) affects insulin release, and whether changes in islet lipid content may be reversed by activation of AMPK (AMP-activated protein kinase). Infection with an adenovirus encoding the constitutively active nuclear fragment of SREBP1c resulted in expression of the protein in approx. 20% of islet cell nuclei, with a preference for beta-cells at the islet periphery. Real-time PCR (TaqMan) analysis showed that SREBP1c up-regulated the expression of FAS (fatty acid synthase; 6-fold), acetyl-CoA carboxylase-1 (2-fold), as well as peroxisomal-proliferator-activated receptor-gamma (7-fold), uncoupling protein-2 (1.4-fold) and Bcl2 (B-cell lymphocytic-leukaemia proto-oncogene 2; 1.3-fold). By contrast, levels of pre-proinsulin, pancreatic duodenal homeobox-1, glucokinase and GLUT2 (glucose transporter isoform-2) mRNAs were unaltered. SREBP1c-transduced islets displayed a 3-fold increase in triacylglycerol content, decreased glucose oxidation and ATP levels, and a profound inhibition of glucose-, but not depolarisation-, induced insulin secretion. Culture of islets with the AMPK activator 5-amino-4-imidazolecarboxamide riboside decreased the expression of the endogenous SREBP1c and FAS genes, and reversed the effect of over-expressing active SREBP1c on FAS mRNA levels and cellular triacylglycerol content. We conclude that SREBP1c over-expression, even when confined to a subset of beta-cells, leads to defective insulin secretion from islets and may contribute to some forms of Type II diabetes.

Hepatic insig-1 or -2 overexpression reduces lipogenesis in obese Zucker diabetic fatty rats and in fasted/refed normal rats

To determine whether the antilipogenic actions of insulin-induced gene 1 (insig-1) demonstrated in cultured preadipocytes also occur in vivo, we infected Zucker diabetic fatty (ZDF) (fa/fa) rats, with recombinant adenovirus containing insig-1 or -2 cDNA. An increase of both proteins appeared in their livers. In control ZDF (fa/fa) rats infected with adenovirus containing the beta-galactosidase (beta-gal) cDNA, triacylglycerols in the liver and plasma rose steeply whereas the insig-infected rats exhibited substantial attenuation of the increase in hepatic steatosis and hyperlipidemia. Insig overexpression was associated with a striking reduction in the elevated level of nuclear sterol regulatory element-binding protein (SREBP)-1c, the activated form of the transcription factor. The mRNA of SREBP-1c lipogenic target enzymes also fell. The mRNA of endogenous insig-1, but not -2a and -2b, was higher in the fatty livers of untreated obese ZDF (fa/fa) rats compared with controls, but the elevation was not sufficient to block the approximately 3-fold increase in SREBP-1c expression and activity. In normal animals, adenovirus-induced overexpression of the insigs reduced the increase in SREBP-1c mRNA and its target enzymes caused by refeeding. The findings demonstrated that both insigs have antilipogenic action when transgenically overexpressed in livers with increased SREBP-1c-mediated lipogenesis. However, the increase in endogenous insig-1 expression associated with augmented lipogenesis may limit it, but is insufficient to prevent it.

Glucose induces de novo lipogenesis in rat muscle satellite cells through a sterol-regulatory-element-binding-protein-1c-dependent pathway

We previously reported that sterol-regulatory-element-binding-protein-1c (SREBP-1c) mediates insulin upregulation of genes encoding glycolytic and lipogenic enzymes in rat skeletal muscle. Here, we assessed whether glucose could regulate gene expression in contracting myotubes deriving from cultured muscle satellite cells. Glucose uptake increased twofold after a 30 minute treatment with a high glucose concentration, suggesting an acute glucose-stimulated glucose uptake. Time-course experiments showed that, within 3 hours, glucose stimulated the expression of hexokinase II, fatty acid synthase and acetyl-CoA-carboxylase-2 proteins, leading to an increased lipogenic flux and intracellular lipid accumulation in contracting myotubes. Furthermore, kinetic experiments indicated that glucose upregulated SREBP-1c precursor and nuclear proteins within 30 minutes, SREBP-1c nuclear translocation being confirmed using immunocytochemistry. In addition, the knockdown of SREBP-1 mRNA using a RNA-interference technique totally abrogated the glucose-induced upregulation of lipogenic enzymes, indicating that SREBP-1c mediates the action of glucose on these genes in rat skeletal muscle. Finally, we found that glucose rapidly stimulated SREBP-1c maturation through a Jak/STAT dependent pathway. We propose that increased intramuscular lipid accumulation associated with muscle insulin resistance in obesity or type-2 diabetes could arise partly from de novo fatty acid synthesis in skeletal muscle.

In vivo regulation of SREBP-1c in skeletal muscle: effects of nutritional status, glucose, insulin, and leptin

Sterol regulatory element binding protein-1c (SREBP-1c), a transcription factor that is important for mediating insulin effects on metabolic gene expression in liver during the fasted-to-fed transition, is also expressed in skeletal muscle. However, the regulation and role of SREBP-1c in skeletal muscle are poorly understood. The present study compared the effects of nutritional status, physiological hyperinsulinemic clamps, and adenovirus-mediated hyperleptinemia (HLEP) in rats on expression of SREBP-1c and other metabolic genes in skeletal muscle. Three- and 6-h refeeding of 18-h-fasted animals increased levels of SREBP-1c mRNA and the SREBP-1 protein (full length and mature) in gastrocnemius muscle (P < 0.05). Fatty acid synthase (FAS) and hexokinase II (HKII) mRNA levels were also increased by refeeding, and uncoupling protein 3 (UCP3) mRNA level was decreased (all P < 0.05). Surprisingly, 3-h hyperinsulinemic clamps did not increase gastrocnemius muscle SREBP-1c and FAS mRNA levels or SREBP-1 protein levels but did increase HKII mRNA levels and decrease UCP3 mRNA levels (P < 0.05). HLEP reduced refeeding-induced increases of SREBP-1c and FAS mRNA levels but did not reduce the level of SREBP-1 protein. We conclude that 1) skeletal muscle SREBP-1c gene expression is regulated by nutritional status in a fashion similar to that observed in liver and adipose tissue, 2) physiological hyperinsulinemia is not sufficient to imitate the effects of refeeding on SREBP-1c gene expression, and 3) leptin suppresses refeeding effects on SREBP-1c mRNA levels.

Pioglitazone prevents alcohol-induced fatty liver in rats through up-regulation of c-Met

BACKGROUND & AIMS

Treatment of steatosis is important in preventing development of fibrosis in alcoholic liver diseases. This study aimed to examine if pioglitazone, an antidiabetic reagent serving as a ligand of peroxisome proliferator-activated receptor gamma (PPAR gamma), could prevent alcoholic fatty liver.

METHODS

Rats fed with an ethanol-containing liquid diet were given the reagent at 10 mg/kg per day intragastrically for 6 weeks. Hepatic genes involved in actions of the reagent were mined by transcriptome analyses, and their changes were confirmed by real-time polymerase chain reaction and Western blotting analyses. The direct effects of pioglitazone on primary-cultured hepatocytes were also assessed in vitro.

RESULTS

Pioglitazone significantly attenuated steatosis and lipid peroxidation elicited by chronic ethanol exposure without altering insulin resistance. Mechanisms for improving effects of the reagent appeared to involve restoration of the ethanol-induced down-regulation of c-Met and up-regulation of stearoyl-CoA desaturase (SCD). Such effects of pioglitazone on the c-Met signaling pathway resulted from its tyrosine phosphorylation and resultant up-regulation of the apolipoprotein B (apoB)-mediated lipid mobilization from hepatocytes through very low-density lipoprotein (VLDL) as well as down-regulation of sterol regulatory element binding protein (SREBP) -1c and SCD levels and a decrease in triglyceride synthesis in the liver.

CONCLUSIONS

Pioglitazone activates c-Met and VLDL-dependent lipid retrieval and suppresses triglyceride synthesis and thereby serves as a potentially useful stratagem to attenuate ethanol-induced hepatic steatosis.

Dietary n-3 polyunsaturated fatty acids decrease hepatic triglycerides in Fischer 344 rats

Dietary fatty acid composition modifies hepatic lipid metabolism. To determine the effects of fatty acids on hepatic triglyceride storage, rats were fed diets enriched in carbohydrates (control), fish oil, or lard. After 4 weeks, the animals were fasted overnight. In the morning, the animals were either sacrificed or fed 8 g of their respective diets before sacrifice. Animals ingested more food calories with diets containing fish oil than with other diets. However, fish oil-fed animals weighed less and had less body fat. In fish oil-fed animals, liver triglyceride was lower by 27% (P <.05) and 73% (P <.01) than in control- and lard-fed animals, respectively. Fish oil altered the postprandial gene expression of hepatic regulators of fatty acid degradation and synthesis. Fish oil feeding blunted the normal postprandial decline in fatty acid degradation genes (PPARalpha, CPT1, and ACO) and blunted the normal postprandial rise in triglyceride synthesis genes (SREBP1-c, FAS, SCD-1). Therefore, the direct postprandial effect of fish oil ingestion decreases the propensity for hepatic triglyceride storage. In conclusion, n-3 polyunsaturated fatty acids decrease total body weight, total body fat, and hepatic steatosis.

Genetic variants in human sterol regulatory element binding protein-1c in syndromes of severe insulin resistance and type 2 diabetes

The transcription factor sterol regulatory element binding protein (SREBP)-1c is intimately involved in the regulation of lipid and glucose metabolism. To investigate whether mutations in this gene might contribute to insulin resistance, we screened the exons encoding the aminoterminal transcriptional activation domain in a cohort of 85 unrelated human subjects with severe insulin resistance. Two missense mutations (P87L and P416A) were found in single affected patients but not in 47 control subjects. However, these variants were indistinguishable from the wild-type in their ability to bind DNA or to transactivate an SREBP-1 responsive promoter construct. We also identified a common intronic single nucleotide polymorphism (C/T) located between exon 18c and 19c. In a case-control study of 517 U.K. Caucasian case subjects and 517 age- and sex-matched control subjects, the T-allele at this locus was significantly associated with type 2 diabetes in men (odds ratio = 1.42 [1.11-1.82], P = 0.005) but not women. In a separate population-based study of 1,100 Caucasians, carriers of the T-allele showed significantly higher levels of total and LDL cholesterol (P < 0.05) compared with wild-type individuals. In summary, we have conducted the first study of the SREBP-1c gene as a candidate for human insulin resistance. Although the rare mutations identified were functionally silent in the assays used, we obtained some evidence, which requires conformation in other populations, that a common variant in the SREBP-1c gene might influence diabetes risk and plasma cholesterol level.

Sustained peripheral expression of transgene adiponectin offsets the development of diet-induced obesity in rats

Adiponectin (Acrp30) is a physiologically active polypeptide hormone secreted by adipose tissue that shows insulin-sensitizing, antiinflammatory, and antiatherogenic properties. In humans, Acrp30 levels are inversely related to the degree of adiposity. In the current study, we tested the long-term weight-reducing and insulin-enhancing effects of Acrp30 cDNA delivered peripherally by a viral vector. To this end, we have generated a series of recombinant adeno-associated virus vectors of serotypes 1 and 5 encoding mouse Acrp30 cDNAs. The long-term expression of recombinant adeno-associated virus-Acrp30 vectors was tested after intramuscular or intraportal injection in female Sprague-Dawley rats with diet-induced obesity. We show that a single peripheral injection of 10(12) physical particles of Acrp30-encoding vectors resulted in sustained (up to 280 days) significant reduction in body weight, concomitant with the reduction in daily food intake. Acrp30 treatment resulted in higher peripheral insulin sensitivity measured by the i.p. glucose tolerance test in fasted animals. Ectopic expression of the Acrp30 transgene resulted in modulation of hepatic gluconeogenesis and lipogenesis, as demonstrated by the reduction of the expression of two key genes: PEPCK (phosphoenolpyruvate carboxykinase) and SREBP-1c (sterol regulatory element-binding protein 1c) in the liver. These data show successful peripheral therapy in a clinically relevant model for human obesity and insulin resistance.

PKClambda in liver mediates insulin-induced SREBP-1c expression and determines both hepatic lipid content and overall insulin sensitivity

PKClambda is implicated as a downstream effector of PI3K in insulin action. We show here that mice that lack PKClambda specifically in the liver (L-lambdaKO mice), produced with the use of the Cre-loxP system, exhibit increased insulin sensitivity as well as a decreased triglyceride content and reduced expression of the sterol regulatory element-binding protein-1c (SREBP-1c) gene in the liver. Induction of the hepatic expression of Srebp1c and of its target genes involved in fatty acid/triglyceride synthesis by fasting and refeeding or by hepatic expression of an active form of PI3K was inhibited in L-lambdaKO mice compared with that in control animals. Expression of Srebp1c induced by insulin or by active PI3K in primary cultured rat hepatocytes was inhibited by a dominant-negative form of PKClambda and was mimicked by overexpression of WT PKClambda. Restoration of PKClambda expression in the liver of L-lambdaKO mice with the use of adenovirus-mediated gene transfer corrected the metabolic abnormalities of these animals. Hepatic PKClambda is thus a determinant of hepatic lipid content and whole-body insulin sensitivity.

PKClambda in liver mediates insulin-induced SREBP-1c expression and determines both hepatic lipid content and overall insulin sensitivity

PKClambda is implicated as a downstream effector of PI3K in insulin action. We show here that mice that lack PKClambda specifically in the liver (L-lambdaKO mice), produced with the use of the Cre-loxP system, exhibit increased insulin sensitivity as well as a decreased triglyceride content and reduced expression of the sterol regulatory element-binding protein-1c (SREBP-1c) gene in the liver. Induction of the hepatic expression of Srebp1c and of its target genes involved in fatty acid/triglyceride synthesis by fasting and refeeding or by hepatic expression of an active form of PI3K was inhibited in L-lambdaKO mice compared with that in control animals. Expression of Srebp1c induced by insulin or by active PI3K in primary cultured rat hepatocytes was inhibited by a dominant-negative form of PKClambda and was mimicked by overexpression of WT PKClambda. Restoration of PKClambda expression in the liver of L-lambdaKO mice with the use of adenovirus-mediated gene transfer corrected the metabolic abnormalities of these animals. Hepatic PKClambda is thus a determinant of hepatic lipid content and whole-body insulin sensitivity.

The physiology of cellular liporegulation

Here we explore the physiologic role of leptin as a liporegulatory hormone responsible for maintaining intracellular homeostasis in the face of wide variations in caloric intake. Normally, rats can tolerate a 60% fat diet because 96% of the surplus fat is deposited in adipocytes. In contrast, when leptin is congenitally absent or inactive, even on a normal diet, unutilized dietary fat is deposited in nonadipose tissues, causing dysfunction (lipotoxicity) and possible cell death (lipoapoptosis). We theorize that in diet-induced obesity, acquired leptin resistance may also develop as the result of increase in certain leptin resistance factors. Acquired leptin resistance occurs in aging, obesity, Cushing's syndrome, and acquired lipodystrophy, and preliminary evidence suggests that ectopic lipid deposition is increased. We speculate that the metabolic syndrome may be the human equivalent of the lipotoxic syndrome of rodents.

The transcription factor SREBP-1c is instrumental in the development of beta-cell dysfunction

Accumulation of lipids in non-adipose tissues is often associated with Type 2 diabetes and its complications. Elevated expression of the lipogenic transcription factor, sterol regulatory element binding protein-1c (SREBP-1c), has been demonstrated in islets and liver of diabetic animals. To elucidate the molecular mechanisms underlying SREBP-1c-induced beta-cell dysfunction, we employed the Tet-On inducible system to achieve tightly controlled and conditional expression of the nuclear active form of SREBP-1c (naSREBP-1c) in INS-1 cells. Controlled expression of naSREBP-1c induced massive accumulation of lipid droplets and blunted nutrient-stimulated insulin secretion in INS-1 cells. K(+)-evoked insulin exocytosis was unaltered. Quantification of the gene expression profile in this INS-1 stable clone revealed that naSREBP-1c induced beta-cell dysfunction by targeting multiple genes dedicated to carbohydrate metabolism, lipid biosynthesis, cell growth, and apoptosis. naSREBP-1c elicits cell growth-arrest and eventually apoptosis. We also found that the SREBP-1c processing in beta-cells was irresponsive to acute stimulation of glucose and insulin, which was distinct from that in lipogenic tissues. However, 2-day exposure to these agents promoted SREBP-1c processing. Therefore, the SREBP-1c maturation could be implicated in the pathogenesis of beta-cell glucolipotoxicity.

Hepatic VLDL production in ob/ob mice is not stimulated by massive de novo lipogenesis but is less sensitive to the suppressive effects of insulin

Type 2 diabetes in humans is associated with increased de novo lipogenesis (DNL), increased fatty acid (FA) fluxes, decreased FA oxidation, and hepatic steatosis. In this condition, VLDL production is increased and resistant to suppressive effects of insulin. The relationships between hepatic FA metabolism, steatosis, and VLDL production are incompletely understood. We investigated VLDL-triglyceride and -apolipoprotein (apo)-B production in relation to DNL and insulin sensitivity in female ob/ob mice. Hepatic triglyceride (5-fold) and cholesteryl ester (15-fold) contents were increased in ob/ob mice compared with lean controls. Hepatic DNL was increased approximately 10-fold in ob/ob mice, whereas hepatic cholesterol synthesis was not affected. Basal rates of hepatic VLDL-triglyceride and -apoB100 production were similar between the groups. Hyperinsulinemic clamping reduced VLDL-triglyceride and -apoB100 production rates by approximately 60% and approximately 75%, respectively, in lean mice but only by approximately 20% and approximately 20%, respectively, in ob/ob mice. No differences in hepatic expression of genes encoding apoB and microsomal triglyceride transfer protein were found. Hepatic expression and protein phosphorylation of insulin receptor and insulin receptor substrate isoforms were reduced in ob/ob mice. Thus, strongly induced hepatic DNL is not associated with increased VLDL production in ob/ob mice, possibly related to differential hepatic zonation of apoB synthesis (periportal) and lipid accumulation (perivenous) and/or relatively low rates of cholesterogenesis. Insulin is unable to effectively suppress VLDL-triglyceride production in ob/ob mice, presumably because of impaired insulin signaling.

Biphasic effects of leptin in porcine granulosa cells

The direct effects of recombinant porcine leptin on porcine granulosa cells were studied to test the hypothesis that leptin, acting through the nuclear transcription factor signal transducer and activator of transcription 3 (STAT-3), modulates sterol regulatory element-binding protein 1 (SREBP1) thereby increasing steroidogenesis. In porcine granulosa cells in culture over 48 h, leptin at 10 ng/ml increased progesterone accumulation 3-fold while it was reduced by leptin at 1000 ng/ml. Leptin had no effect on progression of granulosa cells through the cell cycle nor on the frequency of cell death. Leptin treatment at 24 or 48 h of culture resulted in dose-dependent 2- to 4-fold increases in tyrosine phosphorylation of STAT-3. Leptin had a biphasic effect on the abundance of membrane-bound and transcriptionally active forms of SREBP1. In transient transfection of primary porcine granulosa cells, the plasmid expressing the transcriptionally active form of SREPB-1 induced transcription of the key regulator of steroidogenesis, the steroidogenic acute regulatory protein (StAR). StAR transcription was also increased by the low dose of leptin and was further upregulated in the presence of the SREBP plasmid. Leptin at 1000 ng/ml inhibited SREBP1-induced StAR expression. Thus, leptin, acting through STAT-3, modulates steroidogenesis in a biphasic and dose-dependent manner, and SREBP1 induction of StAR expression may be in the cascade of regulatory events.

Lipoapoptosis: its mechanism and its diseases

The balance between cell division and cell death determines the cell population of an organ. When cell death exceeds cell replacement in an organ, a functional deficit is created. A metabolic cause of programmed cell death, lipoapoptosis, has recently been identified to occur in obesity and aging. If nonadipose tissues are exposed to an excess of long-chain fatty acids, unless leptin action increases their oxidation sufficiently, unoxidized fatty acids enter nonoxidative pathways. While initially they are sequestered as harmless neutral fat, ultimately some will enter more toxic pathways. One of these, the de novo ceramide pathway, has been implicated in the lipoapoptosis of beta-cells and myocardiocytes of congenitally obese rats in which leptin action is defective. Here we review the mechanisms of lipoapoptosis and the diseases that result from this cause of a diminishing cell population of these organs. We suggest that some of the components of the metabolic syndrome of obese humans and the sarcopenia of aging may be result of failure of leptin liporegulation to prevent lipid overload of lean body mass and lipoapoptosis in certain organ systems.

Malonyl-CoA signaling, lipid partitioning, and glucolipotoxicity: role in beta-cell adaptation and failure in the etiology of diabetes

Beta-cells possess inherent mechanisms to adapt to overnutrition and the prevailing concentrations of glucose, fatty acids, and other fuels to maintain glucose homeostasis. However, this is balanced by potentially harmful actions of the same nutrients. Both glucose and fatty acids may cause good/adaptive or evil/toxic actions on the beta-cell, depending on their concentrations and the time during which they are elevated. Chronic high glucose dramatically influences beta-cell lipid metabolism via substrate availability, changes in the activity and expression of enzymes of glucose and lipid metabolism, and modifications in the expression level of key transcription factors. We discuss here the emerging view that beta-cell "glucotoxicity" is in part indirectly caused by "lipotoxicity," and that beta-cell abnormalities will become particularly apparent when both glucose and circulating fatty acids are high. We support the concept that elevated glucose and fatty acids synergize in causing toxicity in islets and other organs, a process that may be instrumental in the pleiotropic defects associated with the metabolic syndrome and type 1 and type 2 diabetes. The mechanisms by which hyperglycemia and hyperlipidemia alter insulin secretion are discussed and a model of beta-cell "glucolipotoxicity" that implicates alterations in beta-cell malonyl-CoA concentrations; peroxisome proliferator-activated receptor-alpha and -gamma and sterol regulatory element binding protein-1c expression; and lipid partitioning is proposed.

Genetic manipulations of fatty acid metabolism in beta-cells are associated with dysregulated insulin secretion

Triacylglyceride (TG) accumulation in pancreatic beta-cells is associated with impaired insulin secretion, which is called lipotoxicity. To gain a better understanding of the pathophysiology of lipotoxicity, we generated three models of dysregulated fatty acid metabolism in beta-cells. The overexpression of sterol regulatory element binding protein-1c induced lipogenic genes and TG accumulation. Under these conditions, we observed a decrease in glucose oxidation and upregulation of uncoupling protein-2, which might be causally related to the decreased glucose-stimulated insulin secretion. The overexpression of AMP-activated protein kinase was accompanied by decreased lipogenesis, increased fatty acid oxidation, and decreased glucose oxidation; insulin secretions to glucose and depolarization stimuli were decreased, probably because of the decrease in glucose oxidation and cellular insulin content. It was notable that the secretory response to palmitate was blunted, which would suggest a role of the fatty acid synthesis pathway, but not its oxidative pathway in palmitate-stimulated insulin secretion. Finally, we studied islets of PPAR-gamma(+/-) mice that had increased insulin sensitivity and low TG content in white adipose tissue, skeletal muscle, and liver. On a high-fat diet, glucose-stimulated insulin secretion was decreased in association with increased TG content in the islets, which might be mediated through the elevated serum free fatty acid levels and their passive transport into beta-cells. These results revealed some aspects about the mechanisms by which alterations of fatty acid metabolism affect beta-cell functions.

Muscle triglyceride and insulin resistance

Skeletal muscle contains the majority of the body's glycogen stores and a similar amount of readily accessible energy as intramyocellular triglyceride (imTG). While a number of factors have been considered to contribute to the pathogenesis of insulin resistance (IR) in obesity and type 2 diabetes mellitus (DM), this review will focus on the potential role of skeletal muscle triglyceride content. In obesity and type 2 DM, there is an increased content of lipid within and around muscle fibers. Changes in muscle in fuel partitioning of lipid, between oxidation and storage of fat calories, almost certainly contribute to accumulation of imTG and to the pathogenesis of both obesity and type 2 DM. In metabolic health, skeletal muscle physiology is characterized by the capacity to utilize either lipid or carbohydrate fuels, and to effectively transition between these fuels. We will review recent findings that indicate that in type 2 DM and obesity, skeletal muscle manifests inflexibility in the transition between lipid and carbohydrate fuels. This inflexibility in fuel selection by skeletal muscle appears to be related to the accumulation of imTG and is an important aspect of IR of skeletal muscle in obesity and type 2 DM.

Regulation of the uncoupling protein-2 gene in INS-1 beta-cells by oleic acid

Current evidence suggests that uncoupling protein-2 (UCP2) is a regulator of insulin secretion. It is also known that chronic exposure of pancreatic islets to free fatty acids (FFAs) blunts glucose-stimulated insulin secretion and is accompanied by elevated levels of UCP2. However, the mechanisms regulating expression of UCP2 in beta-cells are unknown. Here, we show that UCP2 mRNA and protein levels were increased after a 48-h exposure of INS-1(832/13) beta-cells to oleic acid (0.5 mm) by activation of the UCP2 promoter. Furthermore, progressive deletions of the mouse UCP2 promoter (from -7.3 kb to +12 bp) indicated that an enhancer region (-86/-44) was responsible for both basal and FFA-stimulated UCP2 gene transcription. This enhancer contains tightly clustered Sp1, sterol regulatory element (SRE), and double E-Box elements. While all three sequence motifs were required for basal activity of the UCP2 promoter, the mutations in either the SRE or the E-Box elements eliminated the response to FFAs. The SRE and sterol regulatory element binding protein-1 (SREBP1) appear to be crucial for the response of the UCP2 gene to FFAs, since overexpression of the nuclear forms of the SREBPs increased UCP2 promoter activity by 7-10-fold and restored the ability of E-Box mutants to respond to oleic acid. These data support a model in which SREBP is the major modulator of UCP2 gene transcription by FFA, while E-Box binding factors play a supportive role.

Effects of leptin, troglitazone, and dietary fat on stearoyl CoA desaturase

Leptin, troglitazone, and high fat feeding profoundly influence the lipid content of various tissues. To determine if they affect the expression of stearoyl CoA desaturase (SCD)-1 and -2, their mRNA was measured in livers of normal, hyperleptinemic, troglitazone-treated, and fat-fed rats. Hyperleptinemia, which reduces tissue TG by downregulating lipogenic enzymes and upregulating fatty acid oxidation, lowered SCD-1 96% below controls and reduced SCD-2 slightly. By contrast, hepatic SCD-1 mRNA of leptin-resistant fa/fa rats was five times wild-type controls, but SCD-2 mRNA was 66% lower. High fat feeding lowered SCD-1 by 80%, possibly by inducing hyperleptinemia. Troglitazone treatment, which reduces nonadipose tissue TG of fa/fa rats without downregulating lipogenic enzymes, raised SCD-2 13-fold but lowered SCD-1 by 25%. The findings suggest that leptin controls SCD-1 expression and that troglitazone's antilipotoxic action may involve SCD-2 upregulation.

New perspectives in the regulation of hepatic glycolytic and lipogenic genes by insulin and glucose: a role for the transcription factor sterol regulatory element binding protein-1c

The regulation of hepatic glucose metabolism has a key role in whole-body energy metabolism, since the liver is able to store (glycogen synthesis, lipogenesis) and to produce (glycogenolysis, gluconeogenesis) glucose. These pathways are regulated at several levels, including a transcriptional level, since many of the metabolism-related genes are expressed according to the quantity and quality of nutrients. Recent advances have been made in the understanding of the regulation of hepatic glycolytic, lipogenic and gluconeogenic gene expression by pancreatic hormones, insulin and glucagon and glucose. Here we review the role of the transcription factors forkhead and sterol regulatory element binding protein-1c in the inductive and repressive effects of insulin on hepatic gene expression, and the pathway that leads from glucose to gene regulation with the recently discovered carbohydrate response element binding protein. We discuss how these transcription factors are integrated in a regulatory network that allows a fine tuning of hepatic glucose storage or production, and their potential importance in metabolic diseases.

Stimulation of acetyl-CoA carboxylase gene expression by glucose requires insulin release and sterol regulatory element binding protein 1c in pancreatic MIN6 beta-cells

Acetyl-CoA carboxylase I (ACCI) is a key lipogenic enzyme whose induction in islet beta-cells may contribute to glucolipotoxicity. Here, we provide evidence that enhanced insulin release plays an important role in the activation of this gene by glucose. Glucose (30 vs. 3 mmol/l) increased ACCI mRNA levels approximately 4-fold and stimulated ACCI (pII) promoter activity >30-fold in MIN6 cells. The latter effect was completely suppressed by blockade of insulin release or of insulin receptor signaling. However, added insulin substantially, but not completely, mimicked the effects of glucose, suggesting that intracellular metabolites of glucose may also contribute to transcriptional stimulation. Mutational analysis of the ACCI promoter, and antibody microinjection, revealed that the effect of glucose required sterol response element binding protein (SREBP)-1c. Moreover, adenoviral transduction with dominant-negative-acting SREBP1c blocked ACCI gene induction, whereas constitutively active SREBP1c increased ACCI mRNA levels. Finally, glucose also stimulated SREBP1c transcription, although this effect was independent of insulin release. These data suggest that glucose regulates ACCI gene expression in the beta-cell by complex mechanisms that may involve the covalent modification of SREBP1c. However, overexpression of SREBP1c also decreased glucose-stimulated insulin release, implicating SREBP1c induction in beta-cell lipotoxicity in some forms of type 2 diabetes.

Serum leptin in NASH correlates with hepatic steatosis but not fibrosis: a manifestation of lipotoxicity?

Nonalcoholic steatohepatitis (NASH) is a disorder characterized by hepatic steatosis, inflammation, and fibrosis. Leptin is an adipocyte-derived antiobesity hormone that in rodents prevents "lipotoxicity" by limiting triglyceride accumulation and also regulates matrix deposition (fibrosis) during wound healing. We therefore determined serum leptin levels in patients with NASH to determine whether relationships existed between leptin levels and severity of hepatic steatosis or fibrosis. We used a radioimmunoassay to determine serum [total] leptin concentrations in 27 men and 20 women with NASH and 47 controls matched for gender and body mass index (BMI; and partly for age). Serum leptin values were correlated with hepatic steatosis, fibrosis, and inflammation (each categorized semiquantitatively on liver histology), and with anthropometric indices, serum lipids, glucose, insulin, c-peptide, and alanine aminotransferase (ALT) levels. Compared with the controls, mean serum leptin levels were raised in both men and women with NASH (men 14 +/- 11 ng/mL vs. 7.2 +/- 4.1 ng/mL, P =.003; women 35 +/- 16 ng/mL vs. 15 +/- 8.2 ng/mL, P <.001). Leptin values correlated with serum c-peptide levels but not with BMI. In a multivariate analysis, serum leptin (P =.027), serum c-peptide (P =.001), and age (P =.027) were selected as independent predictors of the severity of hepatic steatosis. However, serum leptin was not an independent predictor of hepatic inflammation or fibrotic severity. In conclusion, hyperleptinemia occurs in NASH and is not explained simply by gender, obesity, or the presence of type 2 diabetes. Furthermore, leptin levels correlate directly with the severity of hepatic steatosis but not with inflammation or fibrosis. We propose that the relationship between leptin and steatosis reflects a pathogenic role of leptin in hepatic insulin resistance and/or a failure of the antisteatotic actions of leptin ("peripheral leptin resistance").

Brain-derived neurotrophic factor ameliorates lipid metabolism in diabetic mice

AIM

It has been reported previously that brain-derived neurotrophic factor (BDNF) regulates blood glucose metabolism in rodent obese diabetic models such as C57BL/KsJ-leprdb/leprdb (db/db) mice. BDNF further regulates energy expenditure, possibly through the central and autonomous nervous systems. In this study, we evaluated the effect of BDNF on both lipid and glucose metabolisms to clarify its action mechanism.

METHODS

To control the energy intake, we used a pellet pair-feeding apparatus to synchronize food intake precisely between BDNF-treated and vehicle-treated db/db mice. BDNF (50 mg/kg/week) was subcutaneously injected to male db/db mice twice weekly for 3 weeks, and blood glucose, serum biochemical lipid parameters and tissue weights were measured. Liver triglyceride contents were measured and liver sections were histologically analysed.

RESULTS

Twice weekly BDNF treatment for 3 weeks significantly lowered blood glucose compared with pellet pair-fed, vehicle-treated db/db mice (294 +/- 109 vs. 529 +/- 91 mg/dL). Serum non-esterified free fatty acid (726 +/- 72 vs. 999 +/- 220 microEq/l), total cholesterol (125 +/- 8 vs. 151 +/- 23 mg/dL) and phospholipid levels (215 +/- 13 vs. 257 +/- 36 mg/dL) of the BDNF-treated db/db mice decreased significantly. Liver weights (1.51 +/- 0.11 vs. 2.05 +/- 0.11 g), liver triglyceride contents (17.5 +/- 1.4 vs. 26.1 +/- 2.1 mg/g) and fatty liver in histological appearance were reduced with BDNF treatment. There were no significant differences in body weights and white adipose tissue weights between the two groups.

CONCLUSIONS

Taken together with the accelerating effect of BDNF on energy metabolism, these findings indicate that BDNF improves glucose and lipid metabolism in obese diabetic animals without enlarging liver or adipose tissues.

Hepatic steatosis and type 2 diabetes mellitus

Type 2 diabetes is strongly associated with nonalcoholic fatty liver disease (NAFLD), a spectrum of liver damage that ranges from relatively benign hepatic steatosis to potentially fatal cirrhosis. The severities of insulin resistance and liver damage parallel each other, with the greatest prevalence of cirrhosis occurring in cirrhotics. However, it is unknown whether one of these conditions causes the other, or if both are consequences of another process. Experimental evidence suggests that both insulin resistance and NAFLD result from a chronic inflammatory state. The mechanisms driving this chronic inflammation are unknown but might include the egress of products from intestinal bacteria into the portal blood, liver, and systemic circulation to trigger a sustained inflammatory cytokine response in genetically susceptible individuals. More research is needed to evaluate this hypothesis and to determine the benefits of treatments that interrupt this pathogenic cascade.

Lipotoxic diseases

I review evidence that leptin is a liporegulatory hormone that controls lipid homeostasis in nonadipose tissues during periods of overnutrition. When adipocytes store excess calories as triacylglycerol (TG), leptin secretion rises so as to prevent accumulation of lipids in nonadipose tissues, which are not adapted for TG storage. Whenever leptin action is lacking, whether through leptin deficiency or leptin resistance, overnutrition causes disease of nonadipose tissues with generalized steatosis, lipotoxicity, and lipoapoptosis. Examples of such disorders of liporegulation include generalized lipodystrophies, mutations of leptin and leptin receptor genes, and diet-induced obesity. Lipotoxicity of pancreatic beta-cells, myocardium, and skeletal muscle leads, respectively, to type 2 diabetes, cardiomyopathy, and insulin resistance. In humans this constellation of abnormalities is referred to as the metabolic syndrome, a major health problem in the United States. When lipids overaccumulate in nonadipose tissues during overnutrition, fatty acids enter deleterious pathways such as ceramide production, which, through increased nitric oxide formation, causes apoptosis of lipid-laden cells, such as beta-cells and cardiomyocytes. Lipoapoptosis can be prevented by caloric restriction, by thiazolidinedione treatment, and by administration of nitric oxide blockers. There is now substantial evidence that complications of human obesity may reflect lipotoxicity similar to that described in rodents.

Loss of regulation of lipogenesis in the Zucker diabetic rat. II. Changes in stearate and oleate synthesis

De novo lipogenesis and dietary fat uptake are two major sources of fatty acid deposits in fat of obese animals. To determine the relative contribution of fatty acids from these two sources in obesity, we have determined the distribution of c16 and c18 fatty acids of triglycerides in plasma, liver, and epididymal fat pad of Zucker diabetic fatty (ZDF) rats and their lean littermates (ZL) under two isocaloric dietary fat conditions. Lipogenesis was also determined using the deuterated water method. Conversion of palmitate to stearate and stearate to oleate was calculated from the deuterium incorporation by use of the tracer dilution principle. In the ZL rat, lipogenesis was suppressed from 70 to 24%, conversion of palmitate to stearate from 86 to 78%, and conversion of stearate to oleate from 56 to 7% in response to an increase in the dietary fat-to-carbohydrate ratio. The results suggest that suppression of fatty acid synthase and stearoyl-CoA desaturase activities is a normal adaptive mechanism to a high-fat diet. In contrast, de novo lipogenesis, chain elongation, and desaturation were not suppressed by dietary fat in the ZDF rat. The lack of ability to adapt to a high-fat diet resulted in a higher plasma triglyceride concentration and excessive fat accumulation from both diet and de novo synthesis in the ZDF rat.

Atrophic change of rat salivary gland during adenovirus-induced hyperleptinemia

Sustained hyperleptinemia in normal rats induced by infusing a recombinant adenovirus containing the rat leptin cDNA (AdCMV-leptin) exhibited a remarkable reduction in food intake (AdCMV-leptin, 9.3 +/- 2.6 vs untreated, 20.6 +/- 1.0 g/day) and ablated body fat without any significant changes in wet weight of liver and left ventricle. In those hyperleptinemic rats, we found a 52% reduction in wet weight of salivary gland compared with that in the pair-fed AdCMV-beta-gal-treated rats, which received a recombinant virus containing the beta-galactosidase gene (AdCMV-beta-gal) and were fed on the same amount of food as had been consumed by the AdCMV-leptin-treated group on the previous day. Microscopic examination with hematoxylin-eosin staining revealed that atrophic change was induced in both serous and mucous gland only in the AdCMV-leptin-treated group, but not in the pair-fed controls. Thus, the atrophic changes in hyperleptinemic rats were due to neither a decrease of food intake nor disuse of the salivary gland related with anorexia. Our data suggested that size of the salivary gland was controlled, at lease in part, by "non-anorexic" effect of leptin.

Role of AMP-activated protein kinase in mechanism of metformin action

Metformin is a widely used drug for treatment of type 2 diabetes with no defined cellular mechanism of action. Its glucose-lowering effect results from decreased hepatic glucose production and increased glucose utilization. Metformin's beneficial effects on circulating lipids have been linked to reduced fatty liver. AMP-activated protein kinase (AMPK) is a major cellular regulator of lipid and glucose metabolism. Here we report that metformin activates AMPK in hepatocytes; as a result, acetyl-CoA carboxylase (ACC) activity is reduced, fatty acid oxidation is induced, and expression of lipogenic enzymes is suppressed. Activation of AMPK by metformin or an adenosine analogue suppresses expression of SREBP-1, a key lipogenic transcription factor. In metformin-treated rats, hepatic expression of SREBP-1 (and other lipogenic) mRNAs and protein is reduced; activity of the AMPK target, ACC, is also reduced. Using a novel AMPK inhibitor, we find that AMPK activation is required for metformin's inhibitory effect on glucose production by hepatocytes. In isolated rat skeletal muscles, metformin stimulates glucose uptake coincident with AMPK activation. Activation of AMPK provides a unified explanation for the pleiotropic beneficial effects of this drug; these results also suggest that alternative means of modulating AMPK should be useful for the treatment of metabolic disorders.

[Leptin-induced regulation of fat metabolism and its accumulation]

Recent findings have shown that supplementation of leptin decreases body weight in leptin-deficient ob/ob mice through its suppressive effect on food intake and accelerating effect on energy expenditure, particularly on peripheral fat lipolysis. When endogenously hyperleptinemic obese rats were further induced to be hyperleptinemic exogeously using adenovirus vector, their body fat mass was reduced but not food intake. These findings implicate a direct lipolytic action of leptin on peripheral adipose tissues in obese rats because leptin transport capacity across the blood-brain barrier is almost saturated by the relative hyperleptinemia. Recovery from excessive body fat accumulation after adenovirus-induced hyperleptinemia is much slower than that after caloric restriction because there may be difference between those treatments in decreased lipogenic enzymes activities and/or increased activities of fatty acid oxidative enzymes and thermogenic uncoupling proteins. The fat melting effects of leptin may show its crucial pharmacologic potencies to design therapeutic strategies against morbid obesity. The studies on leptin provide a better understanding for creative approaches to anti-obesity drug that are efficient for reducing body fat mass without harmful side-effects.

Increased expression of the sterol regulatory element-binding protein-1 gene in insulin receptor substrate-2(-/-) mouse liver

Insulin receptor substrate (IRS)-2(-/-) mice develop diabetes because of insulin resistance in the liver and failure to undergo beta-cell hyperplasia. Here we show by DNA chip microarray analysis that expression of the sterol regulatory element-binding protein (SREBP)-1 gene, a downstream target of insulin, was paradoxically increased in 16-week-old IRS-2(-/-) mouse liver, where insulin-mediated intracellular signaling events were substantially attenuated. The expression of SREBP-1 downstream genes, such as the spot 14, ATP citrate-lyase, and fatty acid synthase genes, was also increased. Increased liver triglyceride content in IRS-2(-/-) mice assures the physiological importance of SREBP-1 gene induction. IRS-2(-/-) mice showed leptin resistance; low dose leptin administration, enough to reduce food intake and body weight in wild-type mice, failed to do so in IRS-2(-/-) mice. Interestingly, high dose leptin administration reduced SREBP-1 expression in IRS-2(-/-) mouse liver. Thus, IRS-2 gene disruption results in leptin resistance, causing an SREBP-1 gene induction, obesity, fatty liver, and diabetes.

Role of AMP-activated protein kinase in mechanism of metformin action

Metformin is a widely used drug for treatment of type 2 diabetes with no defined cellular mechanism of action. Its glucose-lowering effect results from decreased hepatic glucose production and increased glucose utilization. Metformin's beneficial effects on circulating lipids have been linked to reduced fatty liver. AMP-activated protein kinase (AMPK) is a major cellular regulator of lipid and glucose metabolism. Here we report that metformin activates AMPK in hepatocytes; as a result, acetyl-CoA carboxylase (ACC) activity is reduced, fatty acid oxidation is induced, and expression of lipogenic enzymes is suppressed. Activation of AMPK by metformin or an adenosine analogue suppresses expression of SREBP-1, a key lipogenic transcription factor. In metformin-treated rats, hepatic expression of SREBP-1 (and other lipogenic) mRNAs and protein is reduced; activity of the AMPK target, ACC, is also reduced. Using a novel AMPK inhibitor, we find that AMPK activation is required for metformin's inhibitory effect on glucose production by hepatocytes. In isolated rat skeletal muscles, metformin stimulates glucose uptake coincident with AMPK activation. Activation of AMPK provides a unified explanation for the pleiotropic beneficial effects of this drug; these results also suggest that alternative means of modulating AMPK should be useful for the treatment of metabolic disorders.

Intramyocellular lipid is associated with resistance to in vivo insulin actions on glucose uptake, antilipolysis, and early insulin signaling pathways in human skeletal muscle

To examine whether and how intramyocellular lipid (IMCL) content contributes to interindividual variation in insulin action, we studied 20 healthy men with no family history of type 2 diabetes. IMCL was measured as the resonance of intramyocellular CH(2) protons in lipids/resonance of CH(3) protons of total creatine (IMCL/Cr(T)), using proton magnetic resonance spectroscopy in vastus lateralis muscle. Whole-body insulin sensitivity was measured using a 120-min euglycemic-hyperinsulinemic (insulin infusion rate 40 mU/m(2). min) clamp. Muscle biopsies of the vastus lateralis muscle were taken before and 30 min after initiation of the insulin infusion to assess insulin signaling. The subjects were divided into groups with high IMCL (HiIMCL; 9.5 +/- 0.9 IMCL/Cr(T), n = 10) and low IMCL (LoIMCL; 3.0 +/- 0.5 IMCL/Cr(T), n = 10), the cut point being median IMCL (6.1 IMCL/Cr(T)). The groups were comparable with respect to age (43 +/- 3 vs. 40 +/- 3 years, NS, HiIMCL versus LoIMCL), BMI (26 +/- 1 vs. 26 +/- 1 kg/m(2), NS), and maximal oxygen consumption (33 +/- 2 vs. 36 +/- 3 ml. kg(-1). min(-1), NS). Whole-body insulin-stimulated glucose uptake was lower in the HiIMCL group (3.0 +/- 0.4 mg. kg(-1). min(-1)) than the LoIMCL group (5.1 +/- 0.5 mg. kg(-1). min(-1), P < 0.05). Serum free fatty acid concentrations were comparable basally, but during hyperinsulinemia, they were 35% higher in the HiIMCL group than the LoIMCL group (P < 0.01). Study of insulin signaling indicated that insulin-induced tyrosine phosphorylation of the insulin receptor (IR) was blunted in HiIMCL compared with LoIMCL (57 vs. 142% above basal, P < 0.05), while protein expression of the IR was unaltered. IR substrate-1-associated phosphatidylinositol (PI) 3-kinase activation by insulin was also lower in the HiIMCL group than in the LoIMCL group (49 +/- 23 vs. 84 +/- 27% above basal, P < 0.05 between HiIMCL and LoIMCL). In conclusion, IMCL accumulation is associated with whole-body insulin resistance and with defective insulin signaling in skeletal muscle independent of body weight and physical fitness.

Nonalcoholic steatosis and steatohepatitis. I. Molecular mechanism for polyunsaturated fatty acid regulation of gene transcription

This review addresses the hypothesis that polyunsaturated fatty acids (PUFA), particularly those of the n-3 family, play pivotal roles as "fuel partitioners" in that they direct fatty acids away from triglyceride storage and toward oxidation and they enhance glucose flux to glycogen. In doing this, PUFA may reduce the risk of enhanced cellular apoptosis associated with excessive cellular lipid accumulation. PUFA exert their beneficial effects by upregulating the expression of genes encoding proteins involved in fatty acid oxidation while simultaneously downregulating genes encoding proteins of lipid synthesis. PUFA govern oxidative gene expression by activating the transcription factor peroxisome proliferator-activated receptor-alpha. PUFA suppress lipogenic gene expression by reducing the nuclear abundance and DNA binding affinity of transcription factors responsible for imparting insulin and carbohydrate control to lipogenic and glycolytic genes. In particular, PUFA suppress the nuclear abundance and expression of sterol regulatory element binding protein-1 and reduce the DNA binding activities of nuclear factor Y, stimulatory protein 1, and possibly hepatic nuclear factor-4. Collectively, the studies discussed suggest that the fuel "repartitioning" and gene expression actions of PUFA should be considered among the criteria used in defining the dietary needs of n-6 and n-3 fatty acids and in establishing the dietary ratio of n-6 to n-3 fatty acids needed for optimum health benefit.

Antiretroviral therapy-associated hyperlipidaemia in HIV disease

Widespread utilization of highly active antiretroviral therapy (HAART) for HIV-infection, primarily protease inhibitors in combination with nucleoside analogue reverse transcriptase inhibitors, has recently led to a sustained reduction in the morbidity and mortality of this disease. However, administration of HAART is frequently associated with the development of lipid disorders. The severity and prevalence of dyslipidaemia vary, depending on the type of HAART, nutritional status, HIV disease stage, and concomitant presence of lipodystrophy and insulin resistance (two additional adverse effects of HAART). The mechanism that is responsible for HAART-associated dyslipidaemia remains incompletely understood. Recent data indicate that this effect may be, at least in part, accounted for by protease inhibitor-mediated inhibition of the proteasome activity and accumulation of the active portion of sterol regulatory element-binding protein-1c in liver cells and adipocytes. Whether lipid disorders in HIV-infected patients receiving HAART translate into an increased cardiovascular risk, and the indications for lipid-lowering interventions in this population, remain to be established.

Mechanisms of nutritional and hormonal regulation of lipogenesis

Fat build-up is determined by the balance between lipogenesis and lipolysis/fatty acid oxidation. In the past few years, our understanding of the nutritional, hormonal and particularly transcriptional regulation of lipogenesis has expanded greatly. Lipogenesis is stimulated by a high carbohydrate diet, whereas it is inhibited by polyunsaturated fatty acids and by fasting. These effects are partly mediated by hormones, which inhibit (growth hormone, leptin) or stimulate (insulin) lipogenesis. Recent research has established that sterol regulatory element binding protein-1 is a critical intermediate in the pro- or anti-lipogenic action of several hormones and nutrients. Another transcription factor implicated in lipogenesis is the peroxisome proliferator activated receptor gamma. Both transcription factors are attractive targets for pharmaceutical intervention of disorders such as hypertriglyceridemia and obesity.

Polyunsaturated fatty acid regulation of gene transcription: a molecular mechanism to improve the metabolic syndrome

This review addresses the hypothesis that polyunsaturated fatty acids (PUFA), particularly those of the (n-3) family, play pivotal roles as "fuel partitioners" in that they direct fatty acids away from triglyceride storage and toward oxidation, and that they enhance glucose flux to glycogen. In doing this, PUFA may protect against the adverse symptoms of the metabolic syndrome and reduce the risk of heart disease. PUFA exert their beneficial effects by up-regulating the expression of genes encoding proteins involved in fatty acid oxidation while simultaneously down-regulating genes encoding proteins of lipid synthesis. PUFA govern oxidative gene expression by activating the transcription factor peroxisome proliferator-activated receptor alpha. PUFA suppress lipogenic gene expression by reducing the nuclear abundance and DNA-binding affinity of transcription factors responsible for imparting insulin and carbohydrate control to lipogenic and glycolytic genes. In particular, PUFA suppress the nuclear abundance and expression of sterol regulatory element binding protein-1 and reduce the DNA-binding activities of nuclear factor Y, Sp1 and possibly hepatic nuclear factor-4. Collectively, the studies discussed suggest that the fuel "repartitioning" and gene expression actions of PUFA should be considered among criteria used in defining the dietary needs of (n-6) and (n-3) and in establishing the dietary ratio of (n-6) to (n-3) needed for optimum health benefit.

Liporegulation in diet-induced obesity. The antisteatotic role of hyperleptinemia

To test the hypothesis that the physiologic liporegulatory role of hyperleptinemia is to prevent steatosis during caloric excess, we induced obesity by feeding normal Harlan Sprague-Dawley rats a 60% fat diet. Hyperleptinemia began within 24 h and increased progressively to 26 ng/ml after 10 weeks, correlating with an approximately 150-fold increase in body fat (r = 0.91, p < 0.0001). During this time, the triacylglycerol (TG) content of nonadipose tissues rose only 1-2.7-fold implying antisteatotic activity. In rodents without leptin action (fa/fa rats and ob/ob and db/db mice) receiving a 6% fat diet, nonadipose tissue TG was 4-100 times normal. In normal rats on a 60% fat diet, peroxisome proliferator-activated receptor alpha protein and liver-carnitine palmitoyltransferase-1 (l-CPT-1) mRNA increased in liver. In their pancreatic islets, fatty-acid oxidation increased 30% without detectable increase in the expression of peroxisome proliferator-activated receptor-alpha or oxidative enzymes, whereas lipogenesis from [14C]glucose was slightly below that of the 4% fat-fed rats (p < 0.05). Tissue-specific overexpression of wild-type leptin receptors in the livers of fa/fa rats, in which marked steatosis is uniformly present, reduced TG accumulation in liver but nowhere else. We conclude that a physiologic role of the hyperleptinemia of caloric excess is to protect nonadipocytes from steatosis and lipotoxicity by preventing the up-regulation of lipogenesis and increasing fatty-acid oxidation.

Nutrient sensing, leptin and insulin action

The glucose-fatty acid cycle as proposed four decades ago by Randle suggests that insulin resistance develops in consequence of alterations of the metabolic pressure of lipids. The more recently published 'hexosamine pathway theory' and the 'malonyl-CoA hypothesis' depict insulin resistance as a consequence of an imbalance between utilization of lipids and carbohydrates. The latter is finely tuned by entry of fatty acids into the mitochondria and/or by entry of glucose to the hexosamine pathway. A significant body of evidence has also been accumulated which points to the complex effects of leptin, an adipocyte-derived signal of lipid stores, on the storage and metabolism of fats and carbohydrates. These are mediated either directly, through actions on specific tissues, or indirectly, via CNS, endocrine and neural mechanisms. The available literature also provides good evidence that leptin orchestrates the metabolic changes in a number of organs and tissues, and alters nutrient fluxes to favor energy expenditure over energy storage. In this article, the proposed lipopenic effects of leptin as studied in various animal models of diet-induced insulin resistance, and possible regulations of leptin production and action by marine fish oil feeding are reviewed.

Diseases of liporegulation: new perspective on obesity and related disorders

Obesity-related diseases now threaten to reach epidemic proportions in the United States. Here we review in a rodent model of genetic obesity, the fa/fa Zucker diabetic fatty (ZDF) rat, the mechanisms involved in the most common complications of diet-induced human obesity, i.e., noninsulin-dependent diabetes mellitus, and myocardial dysfunction. In ZDF rats, hyperphagia leads to hyperinsulinemia, which up-regulates transcription factors that stimulate lipogenesis. This causes ectopic deposition of triacylglycerol in nonadipocytes, providing fatty acid (FA) substrate for damaging pathways of nonoxidative metabolism, such as ceramide synthesis. In beta cells and myocardium, the resulting functional impairment and apoptosis cause diabetes and cardiomyopathy. Interventions that lower ectopic lipid accumulation or block nonoxidative metabolism of FA and ceramide formation completely prevent these complications. Given the evidence for a similar etiology for the complications of human obesity, it would be appropriate to develop strategies to avert the predicted epidemic of lipotoxic disorders.

Slow recovery of body fat lost during adenovirus-induced hyperleptinemia

In normal rats, adenovirus-induced hyperleptinemia causes disappearance of visible body fat, downregulation of lipogenic enzymes, and upregulation of oxidative enzymes and thermogenic proteins. In addition, preadipocyte markers replace mature adipocyte markers, suggesting dedifferentiation. In weight loss induced by caloric restriction, by contrast, the lipogenic machinery is essentially intact. To determine if the radical changes induced by leptin would slow the reappearance of body fat, we compared normal lean rats made hyperleptinemic by infusing an adenovirus-leptin construct with diet-matched littermates. Initially, in plasma leptin the hyperleptinemic rats averaged approximately 50x the controls and, although it declined progressively, it was still slightly elevated at 150 days (P < 0.05). In the hyperleptinemics, body fat mass, quantified by magnetic resonance spectroscopy, remained below the pretreatment value for 60 days, while in diet-matched controls it exceeded the pretreatment value. Epididymal fat pad weight in hyperleptinemics was still 28% below paired controls at 150 days posttreatment. Histologic examination revealed adipocytes of hyperleptinemic animals to be smaller 60 days after treatment. At 60 days, adipose tissue UCP-2 gene expression in hyperleptinemics was still above controls, but expression of other lipogenic and oxidative enzymes had returned to baseline expression levels. We conclude that in normal rats recovery of body fat following adenovirus-induced hyperleptinemia is much slower than after caloric restriction, possibly because of persistent upregulation of adipocyte UCP-2.

Human genomics and obesity: finding appropriate drug targets

The increasing prevalence of obesity worldwide has prompted the World Health Organization (WHO) to classify it as a global epidemic. Around the globe, more than a half billion people are overweight, and the chronic disease of obesity represents a major threat to health care systems in developed and developing countries. The major health hazards associated with obesity are the risks of developing diabetes, cardiovascular disease, stroke, osteoarthritis and some forms of cancer. In this paper, we review the prevalence of obesity and its cost to health care systems and present the relative contribution of environmental conditions and genetic makeup to the development of obesity in people. We also discuss the concept of "essential" obesity in an "obesigenic" environment. Though weight gain results from a sustained imbalance between energy intake and energy expenditure, it is only recently that studies have identified important new mechanisms involved in the regulation of body weight. The etiology of the disease is presented as a feedback model in which afferent signals inform the central controllers in the brain as to the state of the external and internal environment and elicit responses related to the regulation of food intake and energy metabolism. Pharmaceutical agents may intervene at different levels of this feedback model, i.e., reinforce the afferent signals from the periphery, target the central pathways involved in the regulation of food intake and energy expenditure, and increase peripheral energy expenditure and fat oxidation directly. Since obesity results from genetic predisposition, combined with the proactive environmental situation, we discuss new potential targets for generation of drugs that may assist people in gaining control over appetite as well as increasing total energy expenditure and fat oxidation.

Diabetes: the importance of the liver

Insulin resistance, the hallmark of non-insulin dependent diabetes mellitus, is characterized by the failure of tissues to take up and store glucose in response to insulin. Two recent studies shed new light on the importance of insulin signalling in the liver and how this may become defective in diabetes.