An afferent vagal nerve pathway links hepatic PPARalpha activation to glucocorticoid-induced insulin resistance and hypertension

Glucocorticoid excess causes insulin resistance and hypertension. Hepatic expression of PPARalpha (Ppara) is required for glucocorticoid-induced insulin resistance. Here we demonstrate that afferent fibers of the vagus nerve interface with hepatic Ppara expression to disrupt blood pressure and glucose homeostasis in response to glucocorticoids. Selective hepatic vagotomy decreased hyperglycemia, hyperinsulinemia, hepatic insulin resistance, Ppara expression, and phosphoenolpyruvate carboxykinase (PEPCK) enzyme activity in dexamethasone-treated Ppara(+/+) mice. Selective vagotomy also decreased blood pressure, adrenergic tone, renin activity, and urinary sodium retention in these mice. Hepatic reconstitution of Ppara in nondiabetic, normotensive dexamethasone-treated PPARalpha null mice increased glucose, insulin, hepatic PEPCK enzyme activity, blood pressure, and renin activity in sham-operated animals but not hepatic-vagotomized animals. Disruption of vagal afferent fibers by chemical or surgical means prevented glucocorticoid-induced metabolic derangements. We conclude that a dynamic interaction between hepatic Ppara expression and a vagal afferent pathway is essential for glucocorticoid induction of diabetes and hypertension.     

Effects of Dexamethasone In Vivo and In Vitro on Hexose Transport in Brain Microvasculature

Glucocorticoids induce hyperinsulinemia, hyperglycemia, and depress glucose transport by aortic endothelium. High glucocorticoid doses are used for many diseases, but with unknown effects on brain glucose transport or metabolism. This study tested the hypothesis that glucocorticoids affect glucose transport or metabolism by brain microvascular endothelium. Male rats received dexamethasone (DEX) sc with sucrose feeding for up to seven days. Cerebral microvessels from rats treated with DEX/sucrose demonstrated increased GLUT1 and brain glucose extraction compared to controls. Glucose transport in vivo correlated with hyperinsulinemia. Pre-treatment with low doses of strep-tozotocin blunted hyperinsulinemia and prevented increased glucose extraction induced by DEX. In contrast, isolated brain microvessels exposed to DEX in vitro demonstrated suppression of 2-deox-yglucose uptake and glucose oxidation. We conclude that DEX/sucrose treatment in vivo increases blood-brain glucose transport in a manner that requires the effects of chronic hyperinsulinemia. These effects override any direct inhibitory effects of either hyperglycemia or DEX.     

Effects of hyperglycemia on hepatic gluconeogenic flux during glycogen phosphorylase inhibition in the conscious dog

The aim of these studies was to investigate the effect of hyperglycemia with or without hyperinsulinemia on hepatic gluconeogenic flux, with the hypothesis that inhibition would be greatest with combined hyperglycemia/hyperinsulinemia. A glycogen phosphorylase inhibitor (BAY R3401) was used to inhibit glycogen breakdown in the conscious overnight-fasted dog, and the effects of a twofold rise in plasma glucose level (HI group) accompanied by 1) euinsulinemia (HG group) or 2) a fourfold rise in plasma insulin were assessed over a 5-h experimental period. Hormone levels were controlled using somatostatin with portal insulin and glucagon infusion. In the HG group, net hepatic glucose uptake and net hepatic lactate output substantially increased. There was little or no effect on the net hepatic uptake of gluconeogenic precursors other than lactate (amino acids and glycerol) or on the net hepatic uptake of free fatty acids compared with the control group. Consequently, whereas hyperglycemia had little effect on gluconeogenic flux to glucose 6-phosphate (G-6-P), net hepatic gluconeogenic flux was reduced because of increased hepatic glycolytic flux during hyperglycemia. Net hepatic glycogen synthesis was increased by hyperglycemia. The effect of hyperglycemia on gluconeogenic flux to G-6-P and net hepatic gluconeogenic flux was similar. We conclude that, in the absence of appreciable glycogen breakdown, the increase in glycolytic flux that accompanies hyperglycemia results in decreased net carbon flux to G-6-P but no effect on gluconeogenic flux to G-6-P.     

Dehydroepiandrosterone decreases elevated hepatic glucose production in C57BL/KsJ-db/db mice

Dehydroepiandrosterone (DHEA) is known to improve hyperglycemia in diabetic db/db mice that are obese and insulin resistant. In a previous study, we reported that DHEA suppresses the elevated hepatic gluconeogenic glucose-6-phosphatase (G6Pase) activity and gene expression in C57BL/KsJ-db/db mice. In the present study, we evaluated the total amount of gluconeogenesis using NaH[(14)C]CO(3) and hepatic glucose production using fructose as a substrate in primary cultured hepatocytes. Despite hyperinsulinemia, the glucose production of db/db mice in the total body and hepatocytes was elevated as compared to their heterozygote littermate C57BL/KsJ-db/+m mice. Administration of DHEA significantly decreased the blood glucose level and increased the plasma insulin level in db/db mice. Administration of DHEA decreased the elevated total body and hepatic glucose production in db/db mice. In addition, the glucose production in the primary cultured hepatocytes of db/db mice was decreased significantly by the direct addition of DHEA or DHEA-S to the medium. These results suggest that administration of DHEA suppresses the elevated total body and hepatic glucose production in db/db mice, and this effect on the liver is considered to result from increased plasma insulin and DHEA or DHEA-S itself.     

Intergenerational consequences of fetal programming by in utero exposure to glucocorticoids in rats

Epidemiological studies linking low birth weight and subsequent cardiometabolic disease have given rise to the hypothesis that events in fetal life permanently program subsequent cardiovascular risk. The effects of fetal programming may not be limited to the first-generation offspring. We have explored intergenerational effects in the dexamethasone-programmed rat, a model in which fetal exposure to excess glucocorticoid results in low birth weight with subsequent adult hyperinsulinemia and hyperglycemia underpinned by increased activity of the key hepatic gluconeogenic enzyme, phosphoenolpyruvate carboxykinase (PEPCK). We found that the male offspring of female rats that had been exposed prenatally to dexamethasone, but were not manipulated in their own pregnancy, also had reduced birth weight (5.66 +/- 0.06 vs. 6.12 +/- 0.06 g, P < 0.001), glucose intolerance, and elevated hepatic PEPCK activity (5.7 +/- 0.6 vs. 3.3 +/- 0.2 nmol.min(-1).mg protein(-1), P < 0.001). These effects resolved in a third generation. Similar intergenerational programming was observed in offspring of male rats exposed prenatally to dexamethasone mated with control females. The persistence of such programming effects through several generations, transmitted by either maternal or paternal lines, indicates the potential importance of epigenetic factors in the intergenerational inheritance of the "programming phenotype" and provides a basis for the inherited association between low birth weight and cardiovascular risk factors.     

Antipsychotic induced metabolic abnormalities: an interaction study with various PPAR modulators in mice

Abnormalities in glucose and lipid regulation have been reported in schizophrenia during antipsychotic medications. The objectives of the present study were to evaluate the effect of various peroxisome proliferator-activated receptor modulators viz. glimepiride, rosiglitazone and fenofibrate on chlorpromazine, clozapine and ziprasidone induced hyperglycemia and hyperlipidemia in mice. Male Swiss albino mice were orally treated with chlorpromazine, clozapine and ziprasidone concurrently with the antidiabetic medications for 7 days. Plasma glucose, insulin and triglyceride levels were determined at the end of the study. Chlorpromazine and clozapine elevated the glucose and triglyceride levels in normal mice, with no effect on insulin but ziprasidone increased the basal triglyceride and insulin levels and did not have any effect on glucose. Glimepiride and rosiglitazone showed beneficial glucose and triglyceride lowering effects in chlorpromazine and clozapine animals and no effect on insulin levels. Fenofibrate significantly reduced the glucose levels only in animals treated with clozapine, and exhibited significant reduction of triglyceride levels in chlorpromazine, clozapine and ziprasidone treated animals. All three antidiabetic/hypolipidemic agents lowered triglyceride and insulin levels in ziprasidone treated animals. The results of the present studies suggest that hyperglycemia, hyperinsulinemia and hypertriglyceridemia induced by various antipsychotics may involve diverse mechanisms.     

Hepatitis B virus X protein regulates hepatic glucose homeostasis via activation of inducible nitric oxide synthase

Dysregulation of liver functions leads to insulin resistance causing type 2 diabetes mellitus and is often found in chronic liver diseases. However, the mechanisms of hepatic dysfunction leading to hepatic metabolic disorder are still poorly understood in chronic liver diseases. The current work investigated the role of hepatitis B virus X protein (HBx) in regulating glucose metabolism. We studied HBx-overexpressing (HBxTg) mice and HBxTg mice lacking inducible nitric oxide synthase (iNOS). Here we show that gene expressions of the key gluconeogenic enzymes were significantly increased in HepG2 cells expressing HBx (HepG2-HBx) and in non-tumor liver tissues of hepatitis B virus patients with high levels of HBx expression. In the liver of HBxTg mice, the expressions of gluconeogenic genes were also elevated, leading to hyperglycemia by increasing hepatic glucose production. However, this effect was insufficient to cause systemic insulin resistance. Importantly, the actions of HBx on hepatic glucose metabolism are thought to be mediated via iNOS signaling, as evidenced by the fact that deficiency of iNOS restored HBx-induced hyperglycemia by suppressing the gene expression of gluconeogenic enzymes. Treatment of HepG2-HBx cells with nitric oxide (NO) caused a significant increase in the expression of gluconeogenic genes, but JNK1 inhibition was completely normalized. Furthermore, hyperactivation of JNK1 in the liver of HBxTg mice was also suppressed in the absence of iNOS, indicating the critical role for JNK in the mutual regulation of HBx- and iNOS-mediated glucose metabolism. These findings establish a novel mechanism of HBx-driven hepatic metabolic disorder that is modulated by iNOS-mediated activation of JNK.     

Hypoglycemic effects of brassinosteroid in diet-induced obese mice

The prevalence of obesity is increasing globally, and obesity is a major risk factor for metabolic diseases such as type 2 diabetes. Previously, we reported that oral administration of homobrassinolide (HB) to healthy rats triggered a selective anabolic response that was associated with lower blood glucose. Therefore, the aim of this study was to evaluate the effects of HB administration on glucose metabolism, insulin sensitivity, body composition, and gluconeogenic gene expression profiles in liver of C57BL/6J high-fat diet-induced obese mice. Acute oral administration of 50-300 mg/kg HB to obese mice resulted in a dose-dependent decrease in fasting blood glucose within 3 h of treatment. Daily chronic administration of HB (50 mg/kg for 8 wk) ameliorated hyperglycemia and improved oral glucose tolerance associated with obesity without significantly affecting body weight or body composition. These changes were accompanied by lower expression of two key gluconeogenic enzymes, phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G-6-Pase), and increased phosphorylation of AMP-activated protein kinase in the liver and muscle tissue. In vitro, HB treatment (1-15 μM) inhibited cyclic AMP-stimulated but not dexamethasone-stimulated upregulation of PEPCK and G-6-Pase mRNA levels in H4IIE rat hepatoma cells. Among a series of brassinosteroid analogs related to HB, only homocastasterone decreased glucose production in cell culture significantly. These results indicate the antidiabetic effects of brassinosteroids and begin to elucidate their putative cellular targets both in vitro and in vivo.     

Prevention of diabetes,hepatic injury,and colon cancer with dehydroepiandrosterone

The levels of dehydroepiandrosterone (DHEA) and its sulfate (DHEA-S) peak in human in their twenties, then decrease gradually with age. The physiological importance of DHEA was not clear until recent research reports showing that DHEA has beneficial effects on preventing diabetes, malignancy, inflammation, osteoporosis, and collagen disease. We summarize our results concerning diabetes, hepatitis, and colon cancer.

In 1982, Coleman et al. [Diabetes 31 (1982) 830] reported that DHEA decreased hyperglycemia in diabetic db/db mice, which become insulin resistant. We measured hepatic gluconeogenic enzymes in an attempt to elucidate the mechanical mechanism of DHEA action. The activity and gene expression of hepatic gluconeogenic enzyme such as glucose-6-phosphatase (G6Pase) was increased in db/db mice despite hyperinsulinemia compared to control db/+m mice. DHEA, like troglitazone, decreased these levels in db/db mice. We also showed that DHEA improved the insulin resistance caused by aging or obesity using the glucose clamp technique in another animal model. In humans, the serum DHEA concentration was shown to be associated with hyperinsulinemia in diabetes. It also became clear that DHEA increased insulin secretion in old-aged db/db mice. DHEA increases not only insulin sensitivity due to the effects in the liver and muscle, but also insulin secretion.

As an effect of DHEA on T-cell mediated hepatitis induced by concanavalin A (ConA), DHEA reduced hepatic injury by inhibiting several inflammatory mediators and apoptosis. As an effect of DHEA on carcinogenesis, DHEA would be a potential chemopreventative agent against colon cancer because it decreases the number of azoxymethane (AOM) induced aberrant crypt foci, which is a possible precursor to adenoma and cancer in a murine model.

Thus, since DHEA has many beneficial effects experimentally, we should consider administration of DHEA in the future, and common mechanisms among these actions of DHEA should be elucidated in further studies.     

Impact of hyperinsulinemia and hyperglycemia on valvular interstitial cells – A link between aortic heart valve degeneration and type 2 diabetes

Type 2 diabetes is a known risk factor for cardiovascular diseases and is associated with an increased risk to develop aortic heart valve degeneration. Nevertheless, molecular mechanisms leading to the pathogenesis of valve degeneration in the context of diabetes are still not clear. Hence, we hypothesized that classical key factors of type 2 diabetes, hyperinsulinemia and hyperglycemia, may affect signaling, metabolism and degenerative processes of valvular interstitial cells (VIC), the main cell type of heart valves. Therefore, VIC were derived from sheep and were treated with hyperinsulinemia, hyperglycemia and the combination of both. The presence of insulin receptors was shown and insulin led to increased proliferation of the cells, whereas hyperglycemia alone showed no effect. Disturbed insulin response was shown by impaired insulin signaling, i.e. by decreased phosphorylation of Akt/GSK-3α/β pathway. Analysis of glucose transporter expression revealed absence of glucose transporter 4 with glucose transporter 1 being the predominantly expressed transporter. Glucose uptake was not impaired by disturbed insulin response, but was increased by hyperinsulinemia and was decreased by hyperglycemia. Analyses of glycolysis and mitochondrial respiration revealed that VIC react with increased activity to hyperinsulinemia or hyperglycemia, but not to the combination of both. VIC do not show morphological changes and do not acquire an osteogenic phenotype by hyperinsulinemia or hyperglycemia. However, the treatment leads to increased collagen type 1 and decreased α-smooth muscle actin expression. This work implicates a possible role of diabetes in early phases of the degeneration of aortic heart valves.     

Both insulin signaling defects in the liver and obesity contribute to insulin resistance and cause diabetes in Irs2(-/-) mice

We previously reported that insulin receptor substrate-2 (IRS-2)-deficient mice develop diabetes as a result of insulin resistance in the liver and failure of beta-cell hyperplasia. In this study we introduced the IRS-2 gene specifically into the liver of Irs2(-/-) mice with adenovirus vectors. Glucose tolerance tests revealed that the IRS-2 restoration in the liver ameliorated the hyperglycemia, but the improvement in hyperinsulinemia was only partial. Endogenous glucose production (EGP) and the rate of glucose disappearance (Rd) were measured during hyperinsulinemic-euglycemic clamp studies: EGP was increased 2-fold in the Irs2(-/-) mice, while Rd decreased by 50%. Restoration of IRS-2 in the liver suppressed EGP to a level similar to that in wild-type mice, but Rd remained decreased in the Adeno-IRS-2-infected Irs2(-/-) mice. Irs2(-/-) mice also exhibit obesity and hyperleptinemia associated with impairment of hypothalamic phosphatidylinositol 3-kinase activation. Continuous intracerebroventricular leptin infusion or caloric restriction yielded Irs2(-/-) mice whose adiposity was comparable to that of Irs2(+/+) mice, and both the hyperglycemia and the hyperinsulinemia of these mice improved with increased Rd albeit partially. Finally combination treatment consisting of adenovirus-mediated gene transfer of IRS-2 and continuous intracerebroventricular leptin infusion completely reversed the hyperglycemia and hyperinsulinemia in Irs2(-/-) mice. EGP and Rd also became normal in these mice as well as in mice treated by caloric restriction plus adenoviral gene transfer. We therefore concluded that a combination of increased EGP due to insulin signaling defects in the liver and reduced Rd due to obesity accounts for the systemic insulin resistance in Irs2(-/-) mice.     

Contraction activates glucose uptake and glycogen synthase normally in muscles from dexamethasone-treated rats

Glucocorticoids cause insulin resistance in skeletal muscle. The aims of the present study were to investigate the effects of contraction on glucose uptake, insulin signaling, and regulation of glycogen synthesis in skeletal muscles from rats treated with the glucocorticoid analog dexamethasone (1 mg x kg(-1) x day(-1) ip for 12 days). Insulin resistance in dexamethasone-treated rats was confirmed by reduced insulin-stimulated glucose uptake (approximately 35%), glycogen synthesis (approximately 70%), glycogen synthase activation (approximately 80%), and PKB Ser(473) phosphorylation (approximately 40%). Chronic dexamethasone treatment did not impair glucose uptake during contraction in soleus or epitrochlearis muscles. In epitrochlearis (but not in soleus), the presence of insulin during contraction enhanced glucose uptake to similar levels in control and dexamethasone-treated rats. Contraction also increased glycogen synthase fractional activity and dephosphorylated glycogen synthase at Ser(645), Ser(649), Ser(653), and Ser(657) normally in muscles from dexamethasone-treated rats. After contraction, insulin-stimulated glycogen synthesis was completely restored in epitrochlearis and improved in soleus from dexamethasone-treated rats. Contraction did not increase insulin-stimulated PKB Ser(473) or glycogen synthase kinase-3 (GSK-3) phosphorylation. Instead, contraction increased GSK-3beta Ser(9) phosphorylation in epitrochlearis (but not in soleus) in muscles from control and dexamethasone-treated rats. In conclusion, contraction stimulates glucose uptake normally in dexamethasone-induced insulin resistant muscles. After contraction, insulin's ability to stimulate glycogen synthesis was completely restored in epitrochlearis and improved in soleus from dexamethasone-treated rats.     

Hepatic insulin resistance in ob/ob mice involves increases in ceramide,aPKC activity,and selective impairment of Akt-dependent FoxO1 phosphorylation

Pathogenesis of insulin resistance in leptin-deficient ob/ob mice is obscure. In another form of diet-dependent obesity, high-fat-fed mice, hepatic insulin resistance involves ceramide-induced activation of atypical protein kinase C (aPKC), which selectively impairs protein kinase B (Akt)-dependent forkhead box O1 protein (FoxO1) phosphorylation on scaffolding protein, 40 kDa WD(tryp-x-x-asp)-repeat propeller/FYVE protein (WD40/ProF), thereby increasing gluconeogenesis. Resultant hyperinsulinemia activates hepatic Akt and mammalian target of rapamycin C1, and further activates aPKC; consequently, lipogenic enzyme expression increases, and insulin signaling in muscle is secondarily impaired. Here, in obese minimally-diabetic ob/ob mice, hepatic ceramide and aPKC activity and its association with WD40/ProF were increased. Hepatic Akt activity was also increased, but Akt associated with WD40/ProF was diminished and accounted for reduced FoxO1 phosphorylation and increased gluconeogenic enzyme expression. Most importantly, liver-selective inhibition of aPKC decreased aPKC and increased Akt association with WD40/ProF, thereby restoring FoxO1 phosphorylation and reducing gluconeogenic enzyme expression. Additionally, lipogenic enzyme expression diminished, and insulin signaling in muscle, glucose tolerance, obesity, hepatosteatosis, and hyperlipidemia improved. In conclusion, hepatic ceramide accumulates in response to CNS-dependent dietary excess irrespective of fat content; hepatic insulin resistance is prominent in ob/ob mice and involves aPKC-dependent displacement of Akt fromWD40/ProF and subsequent impairment of FoxO1 phosphorylation and increased expression of hepatic gluconeogenic and lipogenic enzymes; and hepatic alterations diminish insulin signaling in muscle.