Expression of CD68 and macrophage chemoattractant protein-1 genes in human adipose and muscle tissues: association with cytokine expression, insulin resistance, and reduction by pioglitazone.

To examine the role of adipose-resident macrophages in insulin resistance, we examined the gene expression of CD68, a macrophage marker, along with macrophage chemoattractant protein-1 (MCP-1) in human subcutaneous adipose tissue using real-time RT-PCR. Both CD68 and MCP-1 mRNAs were expressed in human adipose tissue, primarily in the stromal vascular fraction. When measured in the adipose tissue from subjects with normal glucose tolerance, covering a wide range of BMI (21-51 kg/m2) and insulin sensitivity (S(I)) (0.6-8.0 x 10(-4)min(-1).microU(-1).ml(-1)), CD68 mRNA abundance, which correlated with the number of CD68-positive cells by immunohistochemistry, tended to increase with BMI but was not statistically significant. However, there was a significant inverse relation between CD68 mRNA and S(I) (r=-0.55, P=0.02). In addition, there was a strong positive relationship among adipose tissue CD68 mRNA, tumor necrosis factor-alpha (TNF-alpha) secretion in vitro (r=0.79, P<0.005), and plasma interleukin-6 (r=0.67, P < 0.005). To determine whether improving S(I) in subjects with impaired glucose tolerance (IGT) was associated with decreased CD68 expression, IGT subjects were treated for 10 weeks with pioglitazone or metformin. Pioglitazone increased S(I) by 60% and in the same subjects reduced both CD68 and MCP-1 mRNAs by >50%. Furthermore, pioglitazone resulted in a reduction in the number of CD68-positive cells in adipose tissue and reduced plasma TNF-alpha. Metformin had no effect on any of these measures. Thus, treatment with pioglitazone reduces expression of CD68 and MCP-1 in adipose tissue, apparently by reducing macrophage numbers, resulting in reduced inflammatory cytokine production and improvement in S(I).

Pioglitazone improves insulin sensitivity through reduction in muscle lipid and redistribution of lipid into adipose tissue.

Patients with insulin resistance often manifest increased intramyocellular lipid (IMCL) along with increased visceral adipose tissue. This study was designed to determine whether the insulin sensitizer drugs pioglitazone and metformin would improve glucose intolerance and insulin sensitivity by decreasing IMCL. In this study, 23 generally healthy subjects with impaired glucose tolerance were randomized to receive either pioglitazone 45 mg/day or metformin 2,000 mg/day for 10 wk. Before and after treatment, we measured insulin sensitivity and abdominal subcutaneous and visceral adipose tissue with CT scanning. In addition, muscle biopsies were performed for measurement of IMCL and muscle oxidative enzymes. After treatment with pioglitazone, 2-h glucose fell from 9.6 mmol/l (172 mg/dl) to 6.1 mmol/l (119 mg/dl), whereas there was no change in 2-h glucose with metformin. With pioglitazone treatment, there was a 65% increase in insulin sensitivity along with a 34% decrease in IMCL (both P < or = 0.002). This decrease in IMCL was not due to increased muscle lipid oxidation, as there were no changes in muscle lipid oxidative enzymes. However, pioglitazone resulted in a 2.6-kg weight gain along with a significant decrease in the visceral-to-subcutaneous adipose tissue ratio. In contrast, metformin treatment resulted in no change in insulin sensitivity, IMCL, oxidative enzymes, or adipose tissue volumes. Pioglitazone improved glucose tolerance and insulin sensitivity by reducing IMCL. This reduction in IMCL was not due to an increase in muscle lipid oxidation but to a diversion of lipid from ectopic sites into subcutaneous adipose tissue.

Expression of FOXC2 in adipose and muscle and its association with whole body insulin sensitivity.

FOXC2 is a winged helix/forkhead transcription factor involved in PKA signaling. Overexpression of FOXC2 in the adipose tissue of transgenic mice protected against diet-induced obesity and insulin resistance. We examined the expression of FOXC2 in fat and muscle of nondiabetic humans with varying obesity and insulin sensitivity. There was no relation between body mass index (BMI) and FOXC2 mRNA in either adipose or muscle. There was a strong inverse relation between adipose FOXC2 mRNA and insulin sensitivity, using the frequently sampled intravenous glucose tolerance test (r = -0.78, P < 0.001). However, there was no relationship between muscle FOXC2 and any measure of insulin sensitivity. To separate insulin resistance from obesity, we examined FOXC2 expression in pairs of subjects who were matched for BMI but who were discordant for insulin sensitivity. Compared with insulin-sensitive subjects, insulin-resistant subjects had threefold higher levels of adipose FOXC2 mRNA (P = 0.03). In contrast, muscle FOXC2 mRNA expression was no different between insulin-resistant and insulin-sensitive subjects. There was no association of adipose or muscle FOXC2 mRNA with either circulating or adipose-secreted TNF-alpha, IL-6, leptin, adiponectin, or non-esterified fatty acids. Thus adipose FOXC2 is more highly expressed in insulin-resistant subjects, and this effect is independent of obesity. This association between FOXC2 and insulin resistance may be related to the role of FOXC2 in PKA signaling.

Lipid and carbohydrate metabolism in mice with a targeted mutation in the IL-6 gene: absence of development of age-related obesity.

Obesity-related insulin resistance may be caused by adipokines such as IL-6, which is known to be elevated with the insulin resistance syndrome. A previous study reported that IL-6 knockout mice (IL-6(-/-)) developed maturity onset obesity, with disturbed carbohydrate and lipid metabolism, and increased leptin levels. Because IL-6 is associated with insulin resistance, one might have expected IL-6(-/-) mice to be more insulin sensitive. We examined body weights of growing and older IL-6(-/-) mice and found them to be similar to wild-type (IL-6(+/+)) mice. Dual-energy X-ray absorptiometry analysis at 3 and 14 mo revealed no differences in body composition. There were no differences in fasting blood insulin and glucose or in triglycerides. To further characterize these mice, we fed 11-mo-old IL-6(-/-) and IL-6(+/+) mice a high- (HF)- or low-fat diet for 14 wk, followed by insulin (ITT) and glucose tolerance tests (GTT). An ITT showed insulin resistance in the HF animals but no difference due to genotype. In the GTT, IL-6(-/-) mice demonstrated elevated postinjection glucose levels by 60% compared with IL-6(+/+) but only in the HF group. Although IL-6(-/-) mice gained weight and white adipose tissue (WAT) with the HF diet, they gained less weight than the IL-6(+/+) mice. Total lipoprotein lipase activity in WAT, muscle, and postheparin plasma was unchanged in the IL-6 (-/-) mice compared with IL-6(+/+) mice. There were no differences in plasma leptin or TNF-alpha due to genotype. Plasma adiponectin was approximately 53% higher (71.7 +/- 14.1 microg/ml) in IL-6(-/-) mice than in IL-6(+/+) mice but only in the HF group. Thus these data show that IL-6(-/-) mice do not demonstrate obesity, fasting hyperglycemia, or abnormal lipid metabolism, although HF IL-6(-/-) mice demonstrate elevated glucose after a GTT.

Perilipin expression in human adipose tissue is elevated with obesity.

The perilipins are highly phosphorylated adipocyte proteins that are localized at the surface of the lipid droplet. With activation by protein kinase A, perilipins translocate away from the lipid droplet and allow hormone-sensitive lipase to hydrolyze the adipocyte triglycerides to release nonesterified fatty acids (NEFA). Because of the potential importance of adipocyte lipolysis to obesity and insulin resistance, we measured perilipin protein and mRNA levels in nondiabetic subjects with varying degrees of insulin resistance. By Northern and Western blotting, we could detect perilipin A, but not perilipin B. Perilipin A protein and mRNA levels were quantitated and were highly correlated with each other. There was a significant positive relationship between perilipin expression and obesity (r = 0.55; P < 0.01, perilipin mRNA vs. percent body fat). However, there was no significant relationship between perilipin expression and blood NEFA, nor was there a significant relationship between perilipin expression and insulin resistance, using the insulin sensitivity index derived from the iv glucose tolerance test with minimal modeling. In addition, there was no significant relationship between perilipin and adipocyte or systemic inflammatory markers, such as TNFalpha, IL-6, and adiponectin. Thus, perilipin was elevated in obese subjects, perhaps as a compensatory mechanism to limit basal lipolysis. However, there was no relationship between perilipin and insulin resistance.

Adiponectin expression from human adipose tissue: relation to obesity, insulin resistance, and tumor necrosis factor-alpha expression.

Adiponectin is a 29-kDa adipocyte protein that has been linked to the insulin resistance of obesity and lipodystrophy. To better understand the regulation of adiponectin expression, we measured plasma adiponectin and adipose tissue adiponectin mRNA levels in nondiabetic subjects with varying degrees of obesity and insulin resistance. Plasma adiponectin and adiponectin mRNA levels were highly correlated with each other (r = 0.80, P < 0.001), and obese subjects expressed significantly lower levels of adiponectin. However, a significant sex difference in adiponectin expression was observed, especially in relatively lean subjects. When men and women with a BMI <30 kg/m(2) were compared, women had a twofold higher percent body fat, yet their plasma adiponectin levels were 65% higher (8.6 +/- 1.1 and 14.2 +/- 1.6 micro g/ml in men and women, respectively; P < 0.02). Plasma adiponectin had a strong association with insulin sensitivity index (S(I)) (r = 0.67, P < 0.0001, n = 51) that was not affected by sex, but no relation with insulin secretion. To separate the effects of obesity (BMI) from S(I), subjects who were discordant for S(I) were matched for BMI, age, and sex. Using this approach, insulin-sensitive subjects demonstrated a twofold higher plasma level of adiponectin (5.6 +/- 0.6 and 11.2 +/- 1.1 micro g/ml in insulin-resistant and insulin-sensitive subjects, respectively; P < 0.0005). Adiponectin expression was not related to plasma levels of leptin or interleukin-6. However, there was a significant inverse correlation between plasma adiponectin and tumor necrosis factor (TNF)-alpha mRNA expression (r = -0.47, P < 0.005), and subjects with the highest levels of adiponectin mRNA expression secreted the lowest levels of TNF-alpha from their adipose tissue in vitro. Thus, adiponectin expression from adipose tissue is higher in lean subjects and women, and is associated with higher degrees of insulin sensitivity and lower TNF-alpha expression.