Genome-wide analysis of mitochondrial DNA copy number reveals loci implicated in nucleotide metabolism, platelet activation, and megakaryocyte proliferation.

Mitochondrial DNA copy number (mtDNA-CN) measured from blood specimens is a minimally invasive marker of mitochondrial function that exhibits both inter-individual and intercellular variation. To identify genes involved in regulating mitochondrial function, we performed a genome-wide association study (GWAS) in 465,809 White individuals from the Cohorts for Heart and Aging Research in Genomic Epidemiology (CHARGE) consortium and the UK Biobank (UKB). We identified 133 SNPs with statistically significant, independent effects associated with mtDNA-CN across 100 loci. A combination of fine-mapping, variant annotation, and co-localization analyses was used to prioritize genes within each of the 133 independent sites. Putative causal genes were enriched for known mitochondrial DNA depletion syndromes (p = 3.09 × 10-15) and the gene ontology (GO) terms for mtDNA metabolism (p = 1.43 × 10-8) and mtDNA replication (p = 1.2 × 10-7). A clustering approach leveraged pleiotropy between mtDNA-CN associated SNPs and 41 mtDNA-CN associated phenotypes to identify functional domains, revealing three distinct groups, including platelet activation, megakaryocyte proliferation, and mtDNA metabolism. Finally, using mitochondrial SNPs, we establish causal relationships between mitochondrial function and a variety of blood cell-related traits, kidney function, liver function and overall (p = 0.044) and non-cancer mortality (p = 6.56 × 10-4).

A genotype of exceptional longevity is associated with preservation of cognitive function.

OBJECTIVE: To test whether cholesterol ester transfer protein (CETP) genotype (VV homozygosity for I405V) is associated with preservation of cognitive function in addition to its association with exceptional longevity. METHODS: We studied Ashkenazi Jews with exceptional longevity (n = 158; age 99.2 +/- 0.3 years) for the associations of CETP VV genotype and lipoprotein phenotype, using the Mini-Mental State Examination (MMSE). To confirm the role of CETP in a younger cohort, we studied subjects from the Einstein Aging Study (EAS) for associations between CETP VV and cognitive impairment. RESULTS: Subjects with MMSE > 25 were twice as likely to have the CETP VV genotype (29% vs 14%, p = 0.02), and those with the VV genotype were more likely (61% vs 30%, p = 0.02) to have MMSE > 25. Subjects with the VV genotype had lower levels of CETP (1.73 +/- 0.11 vs 2.12 +/- 0.10 mug/mL, p = 0.01), higher high-density lipoprotein (HDL) levels (p = 0.02), and larger lipoprotein particles (p = 0.03). In the EAS cohort, an approximately fivefold increase in the VV genotype (21% vs 4%, p = 0.02), higher HDL levels, and larger lipoprotein particle sizes were associated with less dementia and improved memory. CONCLUSIONS: Using two independent cohorts, we implicate the longevity CETP gene as a modulator of age-related cognitive function. A specific CETP genotype is associated with lower CETP levels and a favorable lipoprotein profile. It has not been determined whether modulation of this gene prevents age-related decline or AD.

Caloric restriction, body fat and ageing in experimental models.

Caloric restriction in animal models delays many age-related pathological conditions. Ageing rats have characteristically increased body weight, fat mass and a specific body fat distribution. This report will focus on the potential cause-effect relationship between increased fat mass and accelerated ageing. In humans, increased fat mass (obesity), and in particular increases in abdominal obesity as a result of deposition of visceral fat, are associated with the metabolic syndrome of ageing. This syndrome is associated with hyperinsulinaemia, dyslipidaemia, type 2 diabetes mellitus, atherosclerosis, hypercoagulability and hypertension. Fat tissue, however, plays a major role by secreting multiple metabolically active factors, which are potentially responsible for the development of insulin resistance. This article will review various experimental models (in animals) used to prevent insulin resistance of ageing by decreasing fat mass, and in particular, decreasing visceral fat. We suggest that this decrease in fat mass and its beneficial repercussions observed in ageing animal models may apply also to human ageing and its related pathology.

Differential gene expression between visceral and subcutaneous fat depots.

Abdominal obesity has been linked to the development of insulin resistance and Type 2 diabetes mellitus (DM2). By surgical removal of visceral fat (VF) in a variety of rodent models, we prevented insulin resistance and glucose intolerance, establishing a cause-effect relationship between VF and the metabolic syndrome. To characterize the biological differences between visceral and peripheral fat depots, we obtained perirenal visceral (VF) and subcutaneous (SC) fat from 5 young rats. We extracted mRNA from the fat tissue and performed gene array hybridization using Affymetrix technology with a platform containing 9 000 genes. Out of the 1 660 genes that were expressed in fat tissue, 297 (17.9 %) genes show a two-fold or higher difference in their expression between the two tissues. We present the 20 genes whose expression is higher in VF fat (by 3 - 7 fold) and the 20 genes whose expression is higher in SC fat (by 3 - 150 fold), many of which are predominantly involved in glucose homeostasis, insulin action, and lipid metabolism. We confirmed the findings of gene array expression and quantified the changes in expression in VF of genes involved in insulin resistance (PPARgamma leptin) and its syndrome (angiotensinogen and plasminogen activating inhibitor-1, PAI-1) by real-time PCR (qRT-PCR) technology. Finally, we demonstrated increased expression of resistin in VF by around 12-fold and adiponectin by around 4-fold, peptides that were not part of the gene expression platform. These results indicate that visceral fat and subcutaneous fat are biologically distinct.

Overfeeding rapidly induces leptin and insulin resistance.

In common forms of obesity, hyperphagia, hyperinsulinemia, and hyperleptinemia coexist. Here, we demonstrate rapid induction of insulin and leptin resistance by short-term overfeeding. After 3 and 7 days on the assigned diet regimen, rats were tested for their biological responses to acute elevations in plasma insulin and leptin concentrations. Severe resistance to the metabolic effects of both leptin and insulin ensued after just 3 days of overfeeding. During the insulin clamp studies, glucose production was decreased by approximately 70% in control rats and 28-53% in overfed rats. Similarly, leptin infusion doubled the contribution of gluconeogenesis to glucose output in control rats but failed to modify gluconeogenesis in overfed animals. These findings demonstrate a paradoxical and rapid collapse of the leptin system in response to nutrient excess. This partial failure is tightly coupled with the onset of insulin resistance.

Superior sagittal sinus thrombosis: a rare complication in a child with nephrotic syndrome.

A 2-year-old boy with new-onset nephrotic syndrome developed recurrent vomiting, apathy and papilloedema. Superior sagittal sinus thrombosis was diagnosed on cranial CT and MRI. He gradually recovered after treatment with heparin, fresh frozen plasma and warfarin with complete resolution of the thrombosis after 1 month. Superior sagittal sinus thrombosis is an extremely rare complication of nephrotic syndrome in children. Early diagnosis is essential for institution of anticoagulation therapy and a successful outcome.

Hyperglycemia-induced production of acute phase reactants in adipose tissue.

Chronic elevation of systemic levels of acute phase reactants and inflammatory cytokines found in patients with diabetes and the often-associated metabolic syndrome X (hypertriglyceridemia, low serum high density lipoprotein cholesterol, hypertension, and accelerated atherosclerosis) may be responsible for the increased incidence of cardiovascular problems in this population. Here we examine the contribution of adipose tissue to the systemic elevation of acute phase reactants associated with chronic hyperglycemia. We demonstrate that adipose tissue expresses a number of acute phase reactants at high levels, including serum amyloid A3 (SAA3), alphal-acid glycoprotein, the lipocalin 24p3 as well as plasminogen activator inhibitor-1 (PAI-1). Additionally, we show SAA3 is expressed at low levels under normal conditions but in the diabetic state is dramatically up-regulated in adipose tissue while down-regulated in liver. Furthermore, pro-inflammatory stimuli and high glucose can lead to the induction of SAA3 in adipose tissue in vivo as well as in the 3T3-L1 adipocyte cell line. Adipose tissue may therefore play a major role in the pathogenic sequelae of Type II diabetes, in particular the cardiovascular problems associated with prolonged hyperglycemia.

Hyperglycemia modulates angiotensinogen gene expression.

Elevated plasma angiotensinogen (AGT) levels have been demonstrated in insulin-resistant states such as obesity and type 2 diabetes mellitus (DM2), conditions that are directly correlated to hypertension. We examined whether hyperinsulinemia or hyperglycemia may modulate fat and liver AGT gene expression and whether obesity and insulin resistance are associated with abnormal AGT regulation. In addition, because the hexosamine biosynthetic pathway is considered to function as a biochemical sensor of intracellular nutrient availability, we hypothesized that activation of this pathway would acutely mediate in vivo the induction of AGT gene expression in fat and liver. We studied chronically catheterized lean (approximately 300 g) and obese (approximately 450 g) Sprague-Dawley rats in four clamp studies (n = 3/group), creating physiological hyperinsulinemia (approximately 60 microU/ml, by an insulin clamp), hyperglycemia (approximately 18 mM, by a pancreatic clamp using somatostatin to prevent endogenous insulin secretion), or euglycemia with glucosamine infusion (GlcN; 30 micromol. kg(-1). min(-1)) and equivalent saline infusions (as a control). Although insulin infusion suppressed AGT gene expression in fat and liver of lean rats, the obese rats demonstrated resistance to this effect of insulin. In contrast, hyperglycemia at basal insulin levels activated AGT gene expression in fat and liver by approximately threefold in both lean and obese rats (P < 0.001). Finally, GlcN infusion simulated the effects of hyperglycemia on fat and liver AGT gene expression (2-fold increase, P < 0.001). Our results support the hypothesis that physiological nutrient "pulses" may acutely induce AGT gene expression in both adipose tissue and liver through the activation of the hexosamine biosynthetic pathway. Resistance to the suppressive effect of insulin on AGT expression in obese rats may potentiate the effect of nutrients on AGT gene expression. We propose that increased AGT gene expression and possibly its production may provide another link between obesity/insulin resistance and hypertension.

The role of fat cell derived peptides in age-related metabolic alterations.

Aging in humans is associated with alterations in body fat distribution and a parallel gradual increase in the prevalence of atherosclerotic cardiovascular disease, as well as mortality of all causes. Because of nutrient cost, availability, and the sedentary life-style, half of the western world population has fat mass in excess of 30% of the body weight that weighs 3-4 times more than the fat mass of lean subjects. Recent discoveries of various hormones, cytokines and complement factors secreted by adipose cells opened a new avenue of research, looking at the role of these fat derived peptides in different conditions. We will focus here on the potential role of fat tissue in different physiological and physiopathological conditions associated with age-related metabolism and risk factors for diseases. We will also exemplify how body fat capacity, distribution and function can be directly linked, and may play a central role in energy metabolism and homeostasis, atherosclerosis, and possibly in the defense against cancer. We hypothesize that biological pathways involved in nutrient regulation in fat tissue may be important in inducing longevity in calorie restricted animals.

Food deprivation limits insulin secretory capacity in postpubertal rats.

ABSTRACT Because prenatal and perinatal undernutrition are associated with type 2 diabetes later in life, we posed the question whether nutrient deprivation during puberty would also result in a decreased ability to secrete insulin. Chronically catheterized, unstressed Sprague Dawley rats, fed ad libitum, were studied before puberty (Pre, n = 14) and after puberty (Post, n = 8). Moderately caloric-restricted rats (fed 70% of the control diet, n = 9), were studied after puberty. Insulin secretion was assessed using a hyperglycemic clamp at a glucose concentration of 300 mg/dL, or with a primed continuous infusion of intralipid (plasma FFA levels approximately 1.5 mM) at a plasma glucose concentration of 200 mg/dL. Stimulated insulin levels increased in Post rats by 3- to 4-fold compared with Pre rats (from 4.6 +/- 0.4 ng/mL Pre to 12.8 +/- 0.7 ng/mL Post, and from 4.5 +/- 0.4 ng/mL Pre to 15.8 +/- 0.7 ng/mL Post, respectively, p < 0.001, at a glucose concentration of 300 mg/dL, and 200 mg/dL with intralipid). Caloric restriction prevented any rise in insulin secretion (3.8 +/- 0.5 and 4.6 +/- 0.5 ng/mL in the caloric-restricted rats at glucose concentrations of 300 mg/dL and 200 mg/dL with intralipid, respectively). A semiquantitative reverse-transcriptase PCR procedure was used to assess basal and stimulated insulin mRNA levels. Caloric restriction did not compensate by enhancing insulin mRNA levels in response to glucose stimulation. Moderate food deprivation during puberty reduced the capacity of the pancreas to secrete insulin in response to different nutrient stimuli. We hypothesize that puberty has an important role in beta-cell maturation and any major nutrient modification may have deleterious consequences later in life.

The role of fat depletion in the biological benefits of caloric restriction.

One of the most robust observations in the biology of aging is that caloric restriction (CR) extends life in a variety of species. Although CR results in substantial decrease in fat mass, the role of fat in life extension was considered minimal. Indeed, in the fields of obesity and diabetes, the amount of fat has been directly implicated in the metabolic consequences. Since it became apparent that fat is a massive endocrine tissue, some of its roles have been recently revised. Many of the systemic effects of CR can now be explained by the chronic effects related to decreased plasma levels of peptides, cytokines, complement factors and substrates that are produced in fat. Most of the benefits of CR on the neuroendocrine system and those related to the improvement in glucose homeostasis can be attributed to a decrease in adipose cells and their products. If all or most of the life-extending benefits of CR can be attributed to decreased fat stores, the expression of specific candidate substrates and proteins may be explored and manipulated in searching for the most powerful adipose-dependent signals that modulate life expectancy.

Physiological increase in plasma leptin markedly inhibits insulin secretion in vivo.

The demonstration of leptin receptors on the pancreatic beta-cells suggests the possibility of direct actions of leptin on insulin secretion. In vitro studies on islets or perfused pancreas and beta-cell lines produced inconsistent results. We performed an in vivo study to distinctly examine whether leptin has an effect on glucose-stimulated insulin secretion. Young chronically catheterized Sprague-Dawley rats (n = 28) were subjected to a 4-h hyperglycemic clamp study (approximately 11 mmol/l). At minute 120 to 240, rats were assigned to receive either saline or leptin (0.1, 0.5, and 5 microg x kg(-1) x min) infusion. Leptin decreased plasma insulin levels abruptly, and an approximately twofold decrease in plasma insulin levels compared with saline control was sustained over the 2 h of the study (14.8 +/- 5.8 vs. 34.8 +/- 2.6 ng/ml with leptin and saline infusion, respectively, P < 0.001). Moreover, a dose-dependent decrease in plasma insulin levels was noted (r = -0.731, P < 0.01). Since milrinone, an inhibitor of cAMP phosphodiesterase (PDE) 3, did not reverse the effect of leptin on glucose-induced insulin secretion, its action may be independent of PDE3. These findings suggest that acute physiological increase in plasma leptin levels acutely and significantly inhibits glucose-stimulated insulin secretion in vivo. The site of leptin effects on insulin secretion remains to be determined.

Searching for human longevity genes: the future history of gerontology in the post-genomic era.

Over the last 30 years, a number of genetic and environmental factors that lead to decreased length of life have been identified. Unfortunately, much less progress has been achieved in identifying genes associated with longevity that protect from common diseases or slow the aging process. Recent compelling evidence supports a role for important genetic and environmental interactions on longevity in lower organisms. Although less is known in humans, commonality in molecular and biological processes, evolutionary arguments, and epidemiological data would strongly suggest that similar mechanisms also apply. The completion of the Human Genome Project and the rapid innovations in technology will make possible the identification of human longevity-assurance genes. This article reviews such evidence, its implications for the identification of human longevity-assurance genes, and the significance of finding longevity genes to human health and disease.

Offspring of centenarians have a favorable lipid profile.

OBJECTIVES: It is well recognized that a favorable lipid profile provides protection from atherosclerotic cardiovascular disease. Because the major cause of nontraumatic death in the western world is considered to be due to cardiovascular disease, centenarians (defined here as subjects over 95 years of age) are believed to possess "atherosclerotic protective" factors. However, it is impossible to study comparatively the lipid profile in centenarians because of lack of controls. Assuming that certain genes responsible for encoding the lipid phenotype may be inherited, we studied the lipid profile characteristics of offspring of centenarians and compared them with control groups. DESIGN: Prospective cohort study. SETTING: The study was part of the Longevity Genes Project at Albert Einstein College of Medicine. PARTICIPANTS: Ashkenazi Jewish centenarians (n = 27, 98.4 +/- 10.4 years) and their offspring (n = 33, 67.4 +/- 1.4 years). The Ashkenazi Jewish offsprings' spouses, who were not related by blood to the centenarians or their offspring, were used as a control group (n = 26, 68.4 +/- 1.2 years). MEASUREMENTS: The lipoprotein profile of the offspring was compared with the above control group and to a larger control group (age and gender matched) from the National Health and Nutrition Examination Survey (NHANES) III study (without the sample weights, n = 394, 60 to 69 years). RESULTS: Female offspring of centenarians had significantly higher plasma levels of high density lipoprotein-cholesterol (HDL-C) levels compared with controls (70.2 +/- 3.1 vs 59.0 +/- 4.1 mg/dl, P = .029). Male offspring of centenarians had higher plasma levels of HDL-C levels (56.2 +/- 7.1 vs 44.3 +/- 3.4 mg/dl, P = 0.130) and significantly lower LDL-cholesterol (LDL-C) levels (95.0 +/- 6.0 vs 127.0 +/- 8.0 mg/dl, P = .009) compared with controls. CONCLUSION: Offspring of centenarians have a favorable lipid profile compared with controls. These data support the notion that a certain phenotypic lipid profile may be transmitted in families and suggest that a favorable lipid profile may play a role in longevity.

The regulation of body fat distribution and the modulation of insulin action.

Body fat distribution may determine insulin resistance and its metabolic syndrome in humans, independent of obesity. Surgical removal of visceral fat (VF) in obese rats was associated with decreased leptin plasma levels and its gene expression in subcutaneous fat (SC). Chronic leptin treatment to rats decreased VF specifically supporting the role of leptin in determining fat distribution. Surgical removal of selected VF provided direct evidence of improved in vivo insulin action on hepatic glucose production (HGP) by over 2-fold vs sham-operated control. The impact of decreased VF on improved in vivo insulin action was further supported by obtaining similar decreases in VF by treating rats with leptin (Lep), beta3-aderenoreceptor agonist, or by severe caloric restriction (CR). All these three interventions improved insulin action on the modulation of HGP and were mostly attributed to preservation of hepatic glycogen stores. Because free fatty acids (FFA) plasma levels were unchanged, this effect may not be mediated portally by substrates. Improved peripheral insulin sensitivity and glycogen synthesis was demonstrated only in Lep. These data suggest that VF is a major determinant of hepatic insulin action. In obese rats, the ability of leptin to prevent visceral adiposity and its own expression is attenuated. Thus, the failure of leptin to regulate fat distribution and its own secretion suggest that 'leptin resistance' may be a pathologic feature in obesity.

Ability of insulin to modulate hepatic glucose production in aging rats is impaired by fat accumulation.

Increased total fat mass (FM) and visceral fat (VF) may account in part for age-associated decrease in hepatic insulin action. This study determined whether preventing the changes in body fat distribution abolished this defect throughout aging. We studied the F(1) hybrid of Brown Norway-Fischer 344 rats (n = 29), which we assigned to caloric restriction (CR) or fed ad libitum (AL). CR (55% of the calories consumed by AL) was initiated and used at 2 mo to prevent age-dependent increases in FM and VF. AL rats were studied at 2, 8, and 20 mo; CR rats were studied at 8 and 20 mo. VF and FM remained unchanged throughout aging in CR rats. AL-fed rats at 8 and 20 mo had over fourfold higher FM and VF compared with both CR groups. Insulin clamp studies (3 mU. kg(-1). min(-1) with somatostatin) were performed to assess hepatic insulin sensitivity. Prevention of fat accretion resulted in a marked improvement in insulin action in the suppression of hepatic glucose production (HGP) (6.3 +/- 0.3 and 7.2 +/- 1.2 mg. kg(-1). min(-1) in 8- and 20-mo CR rats vs. 8.3 +/- 0.5 and 10.8 +/- 0.9 mg. kg(-1). min(-1) in 8- and 20-mo AL rats, respectively). The rate of gluconeogenesis (by enrichment of hepatic uridine diphosphate glucose and phosphoenolpyruvate pools by [(14)C]lactate) was unchanged in all groups. The improvement in hepatic insulin action in the CR group was mostly due to effective suppression of glycogenolysis (4.4 +/- 0.3 and 4.9 +/- 0.3 mg. kg(-1). min(-1) in 8- and 20-mo CR rats vs. 5.8 +/- 0.6 and 8.2 +/- 1.0 mg. kg(-1). min(-1) in 8- and 20-mo AL rats, respectively). The results demonstrated the preservation of hepatic insulin action in aging CR rats. Therefore, body fat and its distribution are major determinants of age-associated hepatic insulin resistance.

Aging does not contribute to the decline in insulin action on storage of muscle glycogen in rats.

Increase in fat mass (FM) and changes in body composition may account for the age-associated impairment in insulin action on muscle glycogen storage. We wish to examine whether preventing the increase in FM abolishes this defect seen with aging. We studied the novel aging model of F1 hybrids of BN/F344 NIA rats fed ad libitum (AL) at 2 (weighing 259+/-17 g), 8 (459+/-17 g), and 20 (492+/-10 g) mo old. To prevent the age-dependent growth in FM, rats were caloric restricted (CR) at 2 mo by decreasing their daily caloric intake by 45% (weighing 292+/-5 g at 8 mo, 294+/-9 g at 20 mo). As designed, the lean body mass (LBM) and %FM remained unchanged through aging (8 and 20 mo old) in the CR rats and was similar to that of 2-mo-old AL rats. However, 8- and 20-mo-old AL-fed rats had three- to fourfold higher FM than both CR groups. Peripheral insulin action at physiological hyperinsulinemia was determined (by 3 mU x kg(-1). min(-1) insulin clamp). Prevention of fat accretion maintained glucose uptake (R(d); 29+/-2, 29+/-2, and 31+/-4 mg x kg LBM(-1) x min(-1)) and glycogen synthesis rates (GS, 12+/-1, 12 +/-1, and 14+/-2 mg x kg LBM(-1) x min(-1)) at youthful levels (2 mo AL) in 8- and 20-mo-old CR rats, respectively. These levels were significantly increased (P<0.001) compared with AL rats with higher %FM (R(d), 22+/-1 and 22+/-2 and GS, 7+/-1 and 8+/-2 mg x kg LBM(-1). min(-1) in 8- and 20-mo-old rats, respectively). The increase in whole body GS in age-matched CR rats was accompanied by approximately 40% increased accumulation of [(3)H] glucose into glycogen and a similar increase in insulin-induced muscle glycogen content. Furthermore, the activation of glycogen synthase increased, i.e., approximately 50% decrease in the Michaelis constant, in both CR groups (P<0.01). We conclude that chronic CR designed to prevent an increase in storage of energy in fat maintained peripheral insulin action at youthful levels, and aging per se does not result in a defect on the pathway of glycogen storage in skeletal muscle.

Obesity: age-associated weight gain and the development of disease. An interview with Nir Barzilai. Interview by Marc E. Weksler.

Geriatrics is pleased to highlight the clinical implications of research topics supported by the American Federation for Aging Research (AFAR). AFAR is a leading private organization supporting research on the aging process and diseases of older populations. More than 900 physicians, scientists and students have received AFAR grants totaling more than $20 million since AFAR was founded by Irving S. Wright, MD in 1981. The articles in the New Frontiers series are designed to provide primary care physicians with insight into the pathogenesis, diagnosis, prevention, and treatment of the diseases of aging.

Discordant effects of glucosamine on insulin-stimulated glucose metabolism and phosphatidylinositol 3-kinase activity.

The impact of increased GlcN availability on insulin-stimulated p85/p110 phosphatidylinositol 3-kinase (PI3K) activity in skeletal muscle was examined in relation to GlcN-induced defects in peripheral insulin action. Primed continuous GlcN infusion (750 micromol/kg bolus; 30 micromol/kg.min) in conscious rats limited both maximal stimulation of muscle PI3K by acute insulin (I) (1 unit/kg) bolus (I + GlcN = 1.9-fold versus saline = 3.3-fold above fasting levels; p < 0.01) and chronic activation of PI3K following 3-h euglycemic, hyperinsulinemic (18 milliunits/kg.min) clamp studies (I + GlcN = 1.2-fold versus saline = 2.6-fold stimulation; p < 0.01). To determine the time course of GlcN-induced defects in insulin-stimulated PI3K activity and peripheral insulin action, GlcN was administered for 30, 60, 90, or 120 min during 2-h euglycemic, hyperinsulinemic clamp studies. Activation of muscle PI3K by insulin was attenuated following only 30 min of GlcN infusion (GlcN 30 min = 1.5-fold versus saline = 2.5-fold stimulation; p < 0.05). In contrast, the first impairment in insulin-mediated glucose uptake (Rd) developed following 110 min of GlcN infusion (110 min = 39.9 +/- 1.8 versus 30 min = 42.8 +/- 1.4 mg/kg.min, p < 0.05). However, the ability of insulin to stimulate phosphatidylinositol 3,4, 5-trisphosphate production and to activate glycogen synthase in skeletal muscle was preserved following up to 180 min of GlcN infusion. Thus, increased GlcN availability induced (a) profound and early inhibition of proximal insulin signaling at the level of PI3K and (b) delayed effects on insulin-mediated glucose uptake, yet (c) complete sparing of insulin-mediated glycogen synthase activation. The pattern and time sequence of GlcN-induced defects suggest that the etiology of peripheral insulin resistance may be distinct from the rapid and marked impairment in insulin signaling.

Decreased visceral adiposity accounts for leptin effect on hepatic but not peripheral insulin action.

Leptin decreases visceral fat (VF) and increases peripheral and hepatic insulin action. Here, we generated similar decreases in VF using leptin (Lep), beta(3)-adrenoreceptor agonism (beta3), or food restriction (FR) and asked whether insulin action would be equally improved. For 8 days before the in vivo study, Sprague-Dawley rats (n = 24) were either fed ad libitum [control (Con)], treated with Lep or beta3 (CL-316,243) by implanted osmotic mini-pumps, or treated with FR. Total VF was similarly decreased in the latter three groups (Lep, 3.11 +/- 0.96 g; beta3, 2.87 +/- 0.48 g; and FR, 3.54 +/- 0.77 g compared with 6.91 +/- 1.41 g in Con; P < 0.001) independent of total fat mass (by (3)H(2)O) and food intake. Insulin (3 mU. kg(-1). min(-1)) clamp studies were performed to assess hepatic and peripheral insulin sensitivity. Decreased VF resulted in similar and marked improvements in insulin action on glucose production (GP) (Lep, 1.19 +/- 0.51; beta3, 1.46 +/- 0.68; FR, 2.27 +/-0.71 compared with 6.06 +/- 0.70 mg. kg(-1). min(-1) in Con; P < 0.001). By contrast, reduction in VF by beta3 and FR failed to reproduce the stimulation of insulin-mediated glucose uptake ( approximately 60%), glycogen synthesis ( approximately 80%), and glycolysis ( approximately 25%) observed with Lep. We conclude that 1) a moderate decrease in VF uniformly leads to a marked increase in hepatic insulin action, but 2) the effects of leptin on peripheral insulin action are not due to the associated changes in VF or beta3 activation.