Insulin signal transduction and glucose transport in human adipocytes: effects of obesity and low calorie diet

Role of Distinct Fat Depots in Metabolic Regulation and Pathological Implications

People suffering from obesity and associated metabolic disorders including diabetes are increasing exponentially around the world. Adipose tissue (AT) distribution and alteration in their biochemical properties play a major role in the pathogenesis of these diseases. Emerging evidence suggests that AT heterogeneity and depot-specific physiological changes are vital in the development of insulin resistance in peripheral tissues like muscle and liver. Classically, AT depots are classified into white adipose tissue (WAT) and brown adipose tissue (BAT); WAT is the site of fatty acid storage, while BAT is a dedicated organ of metabolic heat production. The discovery of beige adipocyte clusters in WAT depots indicates AT heterogeneity has a more central role than hither to ascribed. Therefore, we have discussed in detail the current state of understanding on cellular and molecular origin of different AT depots and their relevance toward physiological metabolic homeostasis. A major focus is to highlight the correlation between altered WAT distribution in the body and metabolic pathogenesis in animal models and humans. We have also underscored the disparity in the molecular (including signaling) changes in various WAT tissues during diabetic pathogenesis. Exercise-mediated beneficial alteration in WAT physiology/distribution that protects against metabolic disorders is evolving. Here we have discussed the depot-specific biochemical adjustments induced by different forms of exercise. A detailed understanding of the molecular details of inter-organ crosstalk via substrate utilization/storage and signaling through chemokines provide strategies to target selected WAT depots to pharmacologically mimic the benefits of exercise countering metabolic diseases including diabetes.

Impact of Sleep and Circadian Disruption on Energy Balance and Diabetes: A Summary of Workshop Discussions

A workshop was held at the National Institute for Diabetes and Digestive and Kidney Diseases with a focus on the impact of sleep and circadian disruption on energy balance and diabetes. The workshop identified a number of key principles for research in this area and a number of specific opportunities. Studies in this area would be facilitated by active collaboration between investigators in sleep/circadian research and investigators in metabolism/diabetes. There is a need to translate the elegant findings from basic research into improving the metabolic health of the American public. There is also a need for investigators studying the impact of sleep/circadian disruption in humans to move beyond measurements of insulin and glucose and conduct more in-depth phenotyping. There is also a need for the assessments of sleep and circadian rhythms as well as assessments for sleep-disordered breathing to be incorporated into all ongoing cohort studies related to diabetes risk. Studies in humans need to complement the elegant short-term laboratory-based human studies of simulated short sleep and shift work etc. with studies in subjects in the general population with these disorders. It is conceivable that chronic adaptations occur, and if so, the mechanisms by which they occur needs to be identified and understood. Particular areas of opportunity that are ready for translation are studies to address whether CPAP treatment of patients with pre-diabetes and obstructive sleep apnea (OSA) prevents or delays the onset of diabetes and whether temporal restricted feeding has the same impact on obesity rates in humans as it does in mice.

Reduced Slc2a4/GLUT4 expression in subcutaneous adipose tissue of monosodium glutamate obese mice is recovered after atorvastatin treatment

BACKGROUND

Decreased expression of glucose transporter protein GLUT4, encoded by the solute carrier 2A4 (Slc2a4) gene, is involved in obesity-induced insulin resistance. Local tissue inflammation, by nuclear factor-κB (NFκB)-mediated pathway, has been related to Slc2a4 repression; a mechanism that could be modulated by statins. Using a model of obesity with insulin resistance, this study investigated whether (1) inflammatory markers and Slc2a4 expression are altered; (2) atorvastatin has beneficial effects on inflammation and Slc2a4 expression; and (3) inhibitor of NFκB (IKK)/NFκB pathway is involved in subcutaneous adipose tissue (SAT).

FINDINGS

Obese mice showed insulin resistance, decreased expression of Slc2a4 mRNA (66%, P < 0.01) and GLUT4 protein (30%, P < 0.05), and increased expression of interleukin 6 (Il6) mRNA (44%, P < 0.05) in SAT. Obese mice treated with atorvastatin had enhanced in vivo insulin sensitivity, besides increased Slc2a4/GLUT4 expression and reduced Il6 expression in SAT. No alterations of tumor necrosis factor-α, interleukin 1β and adiponectin expression or IKKα/β activity in SAT of obese mice or obese mice treated with atorvastatin were observed.

CONCLUSIONS

Atorvastatin has beneficial effect upon glycemic homeostasis, which may be related to its positive impact on Il6 and Slc2a4/GLUT4 expression in SAT.

Impaired insulin signaling in human adipocytes after experimental sleep restriction: a randomized, crossover study

BACKGROUND

Insufficient sleep increases the risk for insulin resistance, type 2 diabetes, and obesity, suggesting that sleep restriction may impair peripheral metabolic pathways. Yet, a direct link between sleep restriction and alterations in molecular metabolic pathways in any peripheral human tissue has not been shown.

OBJECTIVE

To determine whether sleep restriction results in reduced insulin sensitivity in subcutaneous fat, a peripheral tissue that plays a pivotal role in energy metabolism and balance.

DESIGN

Randomized, 2-period, 2-condition, crossover clinical study.

SETTING

University of Chicago Clinical Resource Center.

PARTICIPANTS

Seven healthy adults (1 woman, 6 men) with a mean age of 23.7 years (SD, 3.8) and mean body mass index of 22.8 kg/m(2) (SD, 1.6).

INTERVENTION

Four days of 4.5 hours in bed or 8.5 hours in bed under controlled conditions of caloric intake and physical activity.

MEASUREMENTS

Adipocytes collected from subcutaneous fat biopsy samples after normal and restricted sleep conditions were exposed to incremental insulin concentrations. The ability of insulin to increase levels of phosphorylated Akt (pAkt), a crucial step in the insulin-signaling pathway, was assessed. Total Akt (tAkt) served as a loading control. The insulin concentration for the half-maximal stimulation of the pAkt-tAkt ratio was used as a measure of cellular insulin sensitivity. Total body insulin sensitivity was assessed using a frequently sampled intravenous glucose tolerance test.

RESULTS

The insulin concentration for the half-maximal pAkt-tAkt response was nearly 3-fold higher (mean, 0.71 nM [SD, 0.27] vs. 0.24 nM [SD, 0.24]; P = 0.01; mean difference, 0.47 nM [SD, 0.33]; P = 0.01), and the total area under the receiver-operating characteristic curve of the pAkt-tAkt response was 30% lower (P = 0.01) during sleep restriction than during normal sleep. A reduction in total body insulin sensitivity (P = 0.02) paralleled this impaired cellular insulin sensitivity.

LIMITATION

This was a single-center study with a small sample size.

CONCLUSION

Sleep restriction results in an insulin-resistant state in human adipocytes. Sleep may be an important regulator of energy metabolism in peripheral tissues.

PRIMARY FUNDING SOURCE

National Institutes of Health.

Anti-inflammatory effect of atorvastatin ameliorates insulin resistance in monosodium glutamate-treated obese mice

Considering that inflammation contributes to obesity-induced insulin resistance and that statins have been reported to have other effects beyond cholesterol lowering, the present study aimed to investigate whether atorvastatin treatment has anti-inflammatory action in white adipose tissue of obese mice, consequently improving insulin sensitivity. Insulin sensitivity in vivo (by insulin tolerance test); metabolic-hormonal profile; plasma tumor necrosis factor (TNF)-alpha, interleukin (IL)-6, and adiponectin; adipose tissue immunohistochemistry; glucose transporter (GLUT) 4; adiponectin; TNF-alpha; IL-1 beta; and IL-6 gene expression; and I kappaB kinase (IKK)-alpha/beta activity were assessed in 23-week-old monosodium glutamate-induced obese mice untreated or treated with atorvastatin for 4 weeks. Insulin-resistant obese mice had increased plasma triglyceride, insulin, TNF-alpha, and IL-6 plasma levels. Adipose tissue of obese animals showed increased macrophage infiltration, IKK-alpha (42%, P < .05) and IKK-beta (73%, P < .05) phosphorylation, and TNF-alpha and IL-6 messenger RNA (mRNA) ( approximately 15%, P < .05) levels, and decreased GLUT4 mRNA and protein (30%, P < .05) levels. Atorvastatin treatment lowered cholesterol, triglyceride, insulin, TNF-alpha, and IL-6 plasma levels, and restored whole-body insulin sensitivity. In adipose tissue, atorvastatin decreased macrophage infiltration and normalized IKK-alpha/beta phosphorylation; TNF-alpha, IL-6, and GLUT4 mRNA; and GLUT4 protein to control levels. The present findings demonstrate that atorvastatin has anti-inflammatory effects on adipose tissue of obese mice, which may be important to its local and whole-body insulin-sensitization effects.

Impaired subcutaneous adipocyte lipogenesis is associated with systemic insulin resistance and increased apolipoprotein B/AI ratio in men and women

OBJECTIVES

To study the association between subcutaneous fat cell lipogenesis and components of the metabolic syndrome (systemic insulin resistance, dyslipidaemia and hypertension).

DESIGN AND SETTING

A university hospital-based cross-sectional study of 383 subjects (303 women and 80 men) with a wide range of BMI (18-53 kg m(-2)).

RESULTS

Strong negative correlations between in vitro fat cell lipogenesis (basal and after maximal insulin stimulation) and HOMA-IR were present in both genders (r = -0.42 to -0.56, P < 0.0001 and r = -0.37 to -0.44, P < 0.001, for women and men respectively). Insulin sensitivity measured using the insulin tolerance test (K(ITT)) was also correlated with adipocyte lipogenesis (r = 0.47-0.57, P < 0.0001, women, r = 0.52-0.70, P < 0.001, men). Plasma apolipoprotein B/AI-ratio negatively associated with fat cell lipogenesis in women (r = -0.41 to-0.51, P < 0.0001,) and men (r = -0.49 to -0.55, P < 0.0001). The negative relationship of fat cell lipogenesis with blood pressure was significant in women (r = -0.27 to -0.28, P < 0.0002,) but not in men (P = 0.08-0.09). All correlations remained significant after adjusting for differences in fat cell volume, body mass index, waist- to hip- ratio or age (P < 0.01).

CONCLUSIONS

Subcutaneous adipocyte lipogenesis is negatively associated with systemic insulin resistance, plasma apolipoprotein B/AI-ratio and blood pressure supporting the view that impaired fat cell function per se may contribute to the development of the metabolic syndrome.

Retinol-binding protein 4 in human obesity

Studies in mice suggest that adipocytes serve as glucose sensors and regulate systemic glucose metabolism through release of serum retinol-binding protein 4 (RBP4). This model has not been validated in humans. RBP4 was highly expressed in isolated mature human adipocytes and secreted by differentiating human adipocytes. In contrast to the animal data, RBP4 mRNA was downregulated in subcutaneous adipose tissue of obese women, and circulating RBP4 concentrations were similar in normal weight, overweight, and obese women (n = 74). RBP4 was positively correlated with GLUT4 expression in adipose tissue, independent of any obesity-associated variable. Five percent weight loss slightly decreased adipose RBP4 expression but did not influence circulating RBP4. In another set of experiments, we stratified patients (n = 14) by low or high basal fasting interstitial glucose concentrations, as determined by the microdialysis technique. Venous glucose concentrations were similar throughout oral glucose tolerance testing, and basal RBP4 expression in adipose tissue and serum RBP4 concentrations were similar in the groups with higher and lower interstitial glucose levels. Our findings point to profound differences between rodents and humans in the regulation of adipose or circulating RBP4 and challenge the notion that glucose uptake by adipocytes has a dominant role in the regulation of RBP4.

Insulin signaling in human visceral and subcutaneous adipose tissue in vivo

In this study, we evaluated the activation of various insulin signaling molecules in human fat in vivo and compared signaling reactions in visceral and subcutaneous fat depots. Paired abdominal omental and subcutaneous fat biopsies were obtained from nonobese subjects with normal insulin sensitivity under basal conditions and 6 and 30 min following administration of intravenous insulin. Insulin receptor phosphorylation was more intense and rapid and insulin receptor protein content was greater in omental than in subcutaneous adipose tissue (P < 0.05). Insulin-induced phosphorylation of Akt also occurred to a greater extent and earlier in omental than in subcutaneous fat (P < 0.05) in the absence of significant changes in Akt protein content. Accordingly, phosphorylation of the Akt substrate glycogen synthase kinase-3 was more responsive to insulin stimulation in omental fat. Protein content of extracellular signal-regulated kinase (ERK)-1/2 was threefold higher in omental than in subcutaneous fat (P < 0.05), and ERK phosphorylation showed an early 6-min peak in omental fat, in contrast with a more gradual increase observed in subcutaneous fat. In conclusion, the adipocyte insulin signaling system of omental fat shows greater and earlier responses to insulin than that of subcutaneous fat. These findings may contribute to explain the biological diversity of the two fat depots.

Adipocyte lipases and defect of lipolysis in human obesity

The mobilization of fat stored in adipose tissue is mediated by hormone-sensitive lipase (HSL) and the recently characterized adipose triglyceride lipase (ATGL), yet their relative importance in lipolysis is unknown. We show that a novel potent inhibitor of HSL does not inhibit other lipases. The compound counteracted catecholamine-stimulated lipolysis in mouse adipocytes and had no effect on residual triglyceride hydrolysis and lipolysis in HSL-null mice. In human adipocytes, catecholamine- and natriuretic peptide-induced lipolysis were completely blunted by the HSL inhibitor. When fat cells were not stimulated, glycerol but not fatty acid release was inhibited. HSL and ATGL mRNA levels increased concomitantly during adipocyte differentiation. Abundance of the two transcripts in human adipose tissue was highly correlated in habitual dietary conditions and during a hypocaloric diet, suggesting common regulatory mechanisms for the two genes. Comparison of obese and nonobese subjects showed that obesity was associated with a decrease in catecholamine-induced lipolysis and HSL expression in mature fat cells and in differentiated preadipocytes. In conclusion, HSL is the major lipase for catecholamine- and natriuretic peptide-stimulated lipolysis, whereas ATGL mediates the hydrolysis of triglycerides during basal lipolysis. Decreased catecholamine-induced lipolysis and low HSL expression constitute a possibly primary defect in obesity.

Mechanisms regulating phosphoinositide 3-kinase signalling in insulin-sensitive tissues

A great deal of evidence has accumulated indicating that the activity of PI 3-kinase is necessary, and in some cases sufficient, for a wide range of insulin's actions in the cell. Most biochemical, genetic and pharmacological studies have focused on identifying potential roles for the class-Ia PI 3-kinases which are rapidly activated following insulin stimulation. However, recent evidence indicates the alpha isoform of class-II PI 3-kinase (PI3K-C2alpha) may also play a role as insulin causes a very rapid activation of this as well. The basic mechanisms by which insulin activates the various members of the PI 3-kinase family are increasingly well understood and these studies reveal multiple mechanisms for modulating the activity and functionality of PI 3-kinase and for down regulating the signals they generate. These include inhibitory phosphorylation events, lipid phosphatases such as PTEN and SHIP2 and inhibitor proteins of the suppressors of cytokine signalling (SOCS) family. The current review will focus on these mechanisms and how defects in these might contribute to the development of insulin resistance.

Luminal flow induces eNOS activation and translocation in the rat thick ascending limb. II. Role of PI3-kinase and Hsp90

Endothelial nitric oxide synthase (eNOS) regulates NaCl absorption by the thick ascending limb of the loop of Henle (THAL). We found that augmenting luminal flow induces eNOS activation and translocation to the apical membrane of THALs (Ortiz PA, Hong NJ, and Garvin JL. Am J Physiol Renal Physiol 287: F274-F280, 2004). In other cells, eNOS activation by shear stress is mediated by phosphatidylinositol 3-OH kinase (PI3)-kinase. We hypothesized that luminal flow induces eNOS activation via PI3-kinase. Pretreatment of THALs with wortmannin, a PI3-kinase inhibitor, significantly reduced flow-induced nitric oxide (NO) release by 75% (from 53.6 +/- 6 to 13.2 +/- 5.7 pA/mm). Increasing luminal flow from 0 to 20 nl/min induced eNOS translocation to the apical membrane, whereas in the presence of wortmannin eNOS translocation was prevented (basolateral = 32 +/- 2%, middle = 38 +/- 1%, apical = 30 +/- 1%, n = 5, not significant vs. no flow). We next studied which PI3-kinase product mediates eNOS translocation. Addition of PI(3,4,5)P(3) (5 microM) in the absence of flow increased NO levels (P < 0.05) and induced eNOS translocation to the apical membrane (from 40 +/- 4 to 60 +/- 2% of total eNOS, n = 6, P < 0.05). Incubation with PI(3,4)P(2) or PI(4,5)P(2) did not change eNOS localization. We next tested whether heat shock protein (Hsp)90 is involved in eNOS translocation. The Hsp90 inhibitor geldanamycin blocked flow-induced eNOS translocation to the apical membrane (n = 6). Flow also induced translocation of Hsp90 to the apical membrane (from 35 +/- 2 to 57 +/- 2%; P < 0.05) in a PI3-kinase-dependent manner. We conclude that luminal flow induces eNOS translocation and activation in the THAL via PI3-kinase and that Hsp90 is involved in eNOS translocation to the apical membrane.