Inhibition of insulin signaling and glycogen synthesis by phorbol dibutyrate in rat skeletal muscle

Chronic central leptin decreases food intake and improves glucose tolerance in diet-induced obese mice independent of hypothalamic malonyl CoA levels and skeletal muscle insulin sensitivity

Although acute leptin administration in the hypothalamus decreases food intake and increases peripheral energy metabolism, the peripheral actions of central chronic leptin administration are less understood. In this study, we investigated what effects chronic (7 d) intracerebroventricular (ICV) administration of leptin has on energy metabolism and insulin sensitivity in diet-induced obese mice. C57/BL mice were fed a low-fat diet (LFD; 10% total calories) or high-fat diet (HFD; 60% total calories) for 8 wk after which leptin was administered ICV for 7 consecutive days. Mice fed a HFD showed signs of insulin resistance, as evidenced by an impaired glucose tolerance test. Chronic leptin treatment resulted in a decrease in food intake and body weight and normalization of glucose clearance but no improvement in insulin sensitivity. Chronic ICV leptin increased hypothalamic signal transducer and activator of transcription-3 and AMP-activated protein kinase phosphorylation but did not change hypothalamic malonyl CoA levels in HFD fed and LFD-fed mice. In the gastrocnemius muscles, the levels of malonyl CoA in both leptin-treated groups were lower than their respective control groups, suggesting an increase in fatty acid oxidation. However, only in the muscles of ICV leptin-treated LFD mice was there a decrease in lipid metabolites including diacylglycerol, triacylglycerol, and ceramide. Our results suggest that chronic ICV leptin decreases food consumption and body weight via a mechanism different from acute ICV leptin administration. Although chronic ICV leptin treatment in HFD mice improves glucose tolerance, this occurs independent of changes in insulin sensitivity in the muscles of HFD mice.

Intracerebroventricular leptin administration differentially alters cardiac energy metabolism in mice fed a low-fat and high-fat diet

Leptin directly acts on peripheral tissues and alters energy metabolism in obese mice. It also has acute beneficial effects on these tissues via its hypothalamic action. However, it is not clear what effect chronic intracerebroventrical (ICV) leptin administration has on cardiac energy metabolism. We examined the effects of chronic ICV leptin on glucose and fatty acid metabolism in isolated working hearts from high-fat-fed and low-fat-fed mice. Mice were fed a high-fat (60% calories from fat) or low-fat (10% calories from fat) diet for 8 weeks before ICV leptin (5 [mu]g/d) for 7 days. In low-fat-fed mice, leptin increased glucose oxidation rates in isolated working hearts when compared with control [203 +/- 21 vs. 793 +/- 93 nmol[middle dot](g dry weight)-1[middle dot]min-1]. In high-fat-fed mice leptin inhibited fatty acid oxidation [476 +/- 73 vs. 251 +/- 38 nmol[middle dot](g[middle dot]dry[middle dot]wt)-1[middle dot]min-1]. The increase in glucose oxidation in low-fat-fed mice was accompanied by increased pyruvate dehydrogenase activity. In high-fat-fed mice, leptin increased cardiac malonyl coenzyme A levels, secondary to a decrease in malonyl coenzyme A decarboxylase expression. These results suggest that ICV leptin alters cardiac energy metabolism opposite to its peripheral effects and that these effects differ depending on energy substrate supply to the mice.

Focal adhesion kinase regulates insulin resistance in skeletal muscle

AIMS/HYPOTHESIS

On the basis of our previous studies, we investigated the possible role of focal adhesion kinase (FAK) in the development of insulin resistance in skeletal muscle, a major organ responsible for insulin-stimulated glucose uptake.

MATERIALS AND METHODS

Insulin-resistant C2C12 skeletal muscle cells were transfected with FAK wild-type or FAK mutant plasmids, knocked down using small interfering RNA (siRNA), and their effects on the levels and activities of insulin-signalling molecules and on glucose uptake were determined.

RESULTS

A significant decrease in tyrosine phosphorylation of FAK in insulin-resistant C2C12 cells was observed. A similar decrease was observed in skeletal muscle obtained from insulin-resistant Sprague-Dawley rats fed a high-fat diet. Increased levels of FAK in insulin-resistant C2C12 skeletal muscle cells increased insulin sensitivity and glucose uptake. These effects were reversed by an increase in the level of kinase activity mutant FAK or suppression of endogenous FAK by siRNA. FAK was also found to interact downstream with insulin receptor substrate-1, phosphatidylinositol 3-kinase and protein kinase C and glycogen synthase kinase 3beta, leading to translocation of glucose transporter 4 and resulting in the regulation of glucose uptake.

CONCLUSIONS/INTERPRETATION

The present study provides strong evidence that the modulation of FAK level regulates the insulin sensitivity of skeletal muscle cells. The results demonstrate a direct role of FAK in insulin-resistant skeletal muscle cells for the first time.

Sphingosine kinase 1 participates in insulin signalling and regulates glucose metabolism and homeostasis in KK/Ay diabetic mice

AIMS/HYPOTHESIS

The aim of this study was to determine the potential role of sphingosine kinase 1 (SPHK1), a key sphingolipid metabolic enzyme, in glucose metabolism and homeostasis.

METHODS

SMMC-7721 hepatoma cells and C2C12 myotube cells were used to explore the role of SPHK1 in glucose uptake in vitro. KK/Ay type 2 diabetic mice, which were transfected with adenovirus harbouring the human SPHK1 gene by i.v. injection, were used to investigate the glucose-lowering effects of SPHK1 in vivo.

RESULTS

The basal glucose uptake and the insulin-stimulated glucose uptake in both 7721 cells and C2C12 cells were markedly enhanced when SPHK1 was overexpressed by adenovirus-mediated gene transfer, whereas they were substantially reduced when the expression of SPHK1 was inhibited or the activity of SPHK1 was blocked. Insulin could activate SPHK1 of both cell lines in a dose-dependent manner. SPHK1 gene delivery significantly reduced the blood glucose level of KK/Ay diabetic mice, but had no effect on that of normal animals. It also attenuated elevated levels of plasma insulin, NEFA, triacylglycerol, cholesterol and LDL, significantly ameliorated hyperglycaemia-induced injury of liver, heart and kidney, and enhanced phosphorylation of insulin-signalling kinases such as Akt and glycogen synthase kinase 3beta in livers of the diabetic animals.

CONCLUSIONS/INTERPRETATION

SPHK1 is involved in insulin signalling and plays an important role in the regulation of glucose and fat metabolism; adenovirus-mediated SPHK1 gene transfer might provide a novel strategy in the treatment of type 2 diabetes mellitus.

Potential utility of natural polyphenols for reversing fat-induced insulin resistance

There is intriguing recent evidence that the beta subunit of the signalsome--IKKbeta, a crucial catalyst of NF-kappaB activation--is an obligate mediator of the disruption of insulin signaling induced by excessive exposure of tissues to free fatty acids and by hypertrophy of adipocytes. Thus, agents which safely inhibit or suppress the activation of IKKbeta may have utility for reversing insulin resistance syndrome and aiding control of type 2 diabetes. Two natural agents which can achieve this effect in vitro--and which may have clinical potential in this regard--are the polyphenols resveratrol and silibinin. To date, limited absorbability and/or rapid glucuronidation have prevented these agents from achieving full therapeutic utility, but, by administering these agents in optimally absorbable forms, and co-administering inhibitors of glucuronidation such as probenecid, it may prove feasible to make these agents more clinically viable. Oral silibinin, in the guise of the milk thistle extract silymarin, already has documented clinical utility in a range of hepatic disorders, and recent evidence that dietary silibinin can inhibit the growth of certain cancers in rodents suggests that this agent may indeed have clinical potential as an IKKbeta inhibitor. A report that silymarin has a favorable impact on glycemic and lipidemic control in type 2 diabetics with cirrhosis, may or may not be indicative of IKKbeta inhibition in skeletal muscle and adipocytes. In light of the fact that IKKbeta plays a crucial role, not only in the induction of insulin resistance, but also atherogenesis, a host of inflammatory disorders, and the survival and spread of cancer, the development of pharmaceutical agents that could safely and feasibly achieve a down-regulation of IKKbeta activity would have broad therapeutic and preventive implications.

Phorbol esters affect skeletal muscle glucose transport in a fiber type-specific manner

Recent evidence has shown that activation of lipid-sensitive protein kinase C (PKC) isoforms leads to skeletal muscle insulin resistance. However, earlier studies demonstrated that phorbol esters increase glucose transport in skeletal muscle. The purpose of the present study was to try to resolve this discrepancy. Treatment with the phorbol ester 12-deoxyphorbol-13-phenylacetate 20-acetate (dPPA) led to an approximately 3.5-fold increase in glucose transport in isolated fast-twitch epitrochlearis and flexor digitorum brevis muscles. Phorbol ester treatment was additive to a maximally effective concentration of insulin in fast-twitch skeletal muscles. Treatment with dPPA did not affect insulin signaling in the epitrochlearis. In contrast, phorbol esters had no effect on basal glucose transport and inhibited maximally insulin-stimulated glucose transport approximately 50% in isolated slow-twitch soleus muscle. Furthermore, dPPA treatment inhibited the insulin-stimulated tyrosine phosphorylation of insulin receptor substrate (IRS)-1 and the threonine and serine phosphorylation of PKB by approximately 50% in the soleus. dPPA treatment also caused serine phosphorylation of IRS-1 in the slow-twitch soleus muscle. In conclusion, our results show that phorbol esters stimulate glucose transport in fast-twitch skeletal muscles and inhibit insulin signaling in slow-twitch soleus muscle of rats. These findings suggest that mechanisms other than PKC activation mediate lipotoxicity-induced whole body insulin resistance.

Characterizing the effects of saturated fatty acids on insulin signaling and ceramide and diacylglycerol accumulation in 3T3-L1 adipocytes and C2C12 myotubes

A strong correlation between intramyocellular lipid concentrations and the severity of insulin resistance has fueled speculation that lipid oversupply to skeletal muscle, fat, or liver may desensitize these tissues to the anabolic effects of insulin. To identify free fatty acids (FFAs) capable of inhibiting insulin action, we treated 3T3-L1 adipocytes or C2C12 myotubes with either the saturated FFA palmitate (C16:0) or the monounsaturated FFA oleate (C18:1), which were shown previously to be the most prevalent FFAs in rat soleus and gastrocnemius muscles. In C2C12 myotubes, palmitate, but not oleate, inhibited insulin-stimulation of glycogen synthesis, as well as its activation of Akt/Protein Kinase B (PKB), an obligate intermediate in the regulation of anabolic metabolism. Palmitate also induced the accrual of ceramide and diacylglycerol (DAG), two lipid metabolites that have been shown to inhibit insulin signaling in cultured cells and to accumulate in insulin resistant tissues. Interestingly, in 3T3-L1 adipocytes, neither palmitate nor oleate inhibited glycogen synthesis or Akt/PKB activation, nor did they induce ceramide or DAG synthesis. Using myotubes, we also tested whether other saturated fatty acids blocked insulin signaling while promoting ceramide and DAG accumulation. The long-chain fatty acids stearate (18:0), arachidate (20:0), and lignocerate (24:0) reproduced palmitate's effects on these events, while saturated fatty acids with shorter hydrocarbon chains [i.e., laurate (12:0) and myristate (14:0)] failed to induce ceramide accumulation or inhibit Akt/PKB activation. Collectively these findings implicate excess delivery of long-chain fatty acids in the development of insulin resistance resulting from lipid oversupply to skeletal muscle.

A role for ceramide, but not diacylglycerol, in the antagonism of insulin signal transduction by saturated fatty acids

Multiple studies suggest that lipid oversupply to skeletal muscle contributes to the development of insulin resistance, perhaps by promoting the accumulation of lipid metabolites capable of inhibiting signal transduction. Herein we demonstrate that exposing muscle cells to particular saturated free fatty acids (FFAs), but not mono-unsaturated FFAs, inhibits insulin stimulation of Akt/protein kinase B, a serine/threonine kinase that is a central mediator of insulin-stimulated anabolic metabolism. These saturated FFAs concomitantly induced the accumulation of ceramide and diacylglycerol, two products of fatty acyl-CoA that have been shown to accumulate in insulin-resistant tissues and to inhibit early steps in insulin signaling. Preventing de novo ceramide synthesis negated the antagonistic effect of saturated FFAs toward Akt/protein kinase B. Moreover, inducing ceramide buildup recapitulated and augmented the inhibitory effect of saturated FFAs. By contrast, diacylglycerol proved dispensable for these FFA effects. Collectively these results identify ceramide as a necessary and sufficient intermediate linking saturated fats to the inhibition of insulin signaling.

Alterations of nPKC distribution, but normal Akt/PKB activation in denervated rat soleus muscle

Denervation has been shown to impair the ability of insulin to stimulate glycogen synthesis and, to a lesser extent, glucose transport in rat skeletal muscle. Insulin binding to its receptor, activation of the receptor tyrosine kinase and phosphatidylinositol 3'-kinase do not appear to be involved. On the other hand, it has been shown that denervation causes an increase in the total diacylglycerol (DAG) content and membrane-associated protein kinase C (PKC) activity. In this study, we further characterize these changes in PKC and assess other possible signaling abnormalities that might be related to the decrease of glycogen synthesis. The results reveal that PKC-epsilon and -theta;, but not -alpha or -zeta, are increased in the membrane fraction 24 h after denervation and that the timing of these changes parallels the impaired ability of insulin to stimulate glycogen synthesis. At 24 h, these changes were associated with a 65% decrease in glycogen synthase (GS) activity ratio and decreased electrophoretic mobility, indicative of phosphorylation in GS in muscles incubated in the absence of insulin. Incubation of the denervated soleus with insulin for 30 min minimally increased glucose incorporation into glycogen; however, it increased GS activity threefold, to a value still less than that of control muscle, and it eliminated the gel shift. In addition, insulin increased the apparent abundance of GS kinase (GSK)-3 and protein phosphatase (PP)1 alpha in the supernatant fraction of muscle homogenate to control values, and it caused the same increases in GSK-3 and Akt/protein kinase B (PKB) phosphorylation and Akt/PKB activity that it did in nondenervated muscle. No alterations in hexokinase I or II activity were observed after denervation; however, in agreement with a previous report, glucose 6-phosphate levels were diminished in 24-h-denervated soleus, and they did not increase after insulin stimulation. These results indicate that alterations in the distribution of PKC-epsilon and -theta; accompany the impairment of glycogen synthesis in the 24-h-denervated soleus. They also indicate that the basal rate of glycogen synthesis and its stimulation by insulin in these muscles are diminished despite a normal activation of Akt/PKB and phosphorylation of GSK-3. The significance of the observed alterations to GSK-3 and PP1 alpha distribution remain to be determined.

Inhibition of glycogen synthesis by fatty acid in C(2)C(12) muscle cells is independent of PKC-alpha, -epsilon, and -theta

We have previously shown that glycogen synthesis is reduced in lipid-treated C(2)C(12) skeletal muscle myotubes and that this is independent of changes in glucose uptake. Here, we tested whether mitochondrial metabolism of these lipids is necessary for this inhibition and whether the activation of specific protein kinase C (PKC) isoforms is involved. C(2)C(12) myotubes were pretreated with fatty acids and subsequently stimulated with insulin for the determination of glycogen synthesis. The carnitine palmitoyltransferase-1 inhibitor etomoxir, an inhibitor of beta-oxidation of acyl-CoA, did not protect against the inhibition of glycogen synthesis caused by the unsaturated fatty acid oleate. In addition, although oleate caused translocation, indicating activation, of individual PKC isoforms, inhibition of PKC by pharmacological agents or adenovirus-mediated overexpression of dominant negative PKC-alpha, -epsilon, or -theta mutants was unable to prevent the inhibitory effects of oleate on glycogen synthesis. We conclude that neither mitochondrial lipid metabolism nor activation of PKC-alpha, -epsilon, or -theta plays a role in the direct inhibition of glycogen synthesis by unsaturated fatty acids.

Focal adhesion kinase (FAK) regulates insulin-stimulated glycogen synthesis in hepatocytes

Experimental data support a role for FAK, an important component of the integrin signaling pathway, in insulin action. To test the hypothesis that FAK plays a regulatory role in hepatic insulin action, we overexpressed wild type (WT), a kinase inactive (KR), or a COOH-terminal focal adhesion targeting (FAT) sequence-truncated mutant of FAK in HepG2 hepatoma cells. In control untransfected (NON) and vector (CMV2)- and WT-transfected cells, insulin stimulated an expected 54 +/- 13, 37 +/- 4, and 47 +/- 12 increase in [U-(14)C]glucose incorporation into glycogen, respectively. This was entirely abolished in the presence of either KR (-1 +/- 7%) or FAT mutants (0 +/- 8%, n = 5, p < 0.05 for KR or FAT versus other groups), and this was associated with a significant attenuation of incremental insulin-stimulated glycogen synthase (GS) activity. Insulin-stimulated serine phosphorylation of Akt/protein kinase B was significantly impaired in mutant-transfected cells. Moreover, the ability of insulin to inactivate GS kinase-3beta (GSK-3beta), the regulatory enzyme immediately upstream of GS, by serine phosphorylation (308 +/- 16, 321 +/- 41, and 458 +/- 34 optical densitometric units (odu) in NON, CMV2, and WT, respectively, p < 0.02 for WT versus CMV2) was attenuated in the presence of either FAT (205 +/- 14, p < 0.01) or KR (189 +/- 4, p < 0.005) mutants. FAK co-immunoprecipitated with GSK-3beta, but only in cells overexpressing the KR (374 +/- 254 odu) and FAT (555 +/- 308) mutants was this association stimulated by insulin compared with NON (-209 +/- 92), CMV2 (-47 +/- 70), and WT (-39 +/- 31 odu). This suggests that FAK and GSK-3beta form both a constitutive association and a transient complex upon insulin stimulation, the dissociation of which requires normal function and localization of FAK. We conclude that FAK regulates the activity of Akt/protein kinase B and GSK-3beta and the association of GSK-3beta with FAK to influence insulin-stimulated glycogen synthesis in hepatocytes. Insulin action may be subject to regulation by the integrin signaling pathway, ensuring that these growth and differentiation-promoting pathways act in a coordinated and/or complementary manner.

Hyperglycemia-induced apoptosis in human umbilical vein endothelial cells: inhibition by the AMP-activated protein kinase activation

Apoptosis has been observed in vascular cells, nerve, and myocardium of diabetic humans and experimental animals, although whether it contributes to or is a marker of complications in these tissues is unclear. Previous studies have shown that incubation of human umbilical vein endothelial cells (HUVECs) with 30 vs. 5 mmol/l glucose for 72 h causes a significant increase in apoptosis, possibly related to an increase in oxidative stress. We report here that this increase in apoptosis (assessed morphologically by TdT-mediated dUTP nick- end labeling staining) is preceded (24 h of incubation) by inhibition of fatty acid oxidation, by increases in diacylglycerol synthesis, the concentration of malonyl CoA, and caspase-3 activity, and by decreases in mitochondrial membrane potential and cellular ATP content. In addition, the phosphorylation of Akt in the presence of 150 microU/ml insulin was impaired. No increases in ceramide content or its de novo synthesis were observed. AMP-activated protein kinase (AMPK) activity was not diminished; however, incubation with the AMPK activator 5-aminoimidazole-4-carboxamide-riboside increased AMPK activity twofold and completely prevented all of these changes. Likewise, expression of a constitutively active AMPK in HUVEC prevented the increase in caspase-3 activity. The results indicate that alterations in fatty-acid metabolism, impaired Akt activation by insulin, and increased caspase-3 activity precede visible evidence of apoptosis in HUVEC incubated in a hyperglycemic medium. They also suggest that AMPK could play an important role in protecting the endothelial cell against the adverse effects of sustained hyperglycemia.