Lipin expression is attenuated in adipose tissue of insulin-resistant human subjects and increases with peroxisome proliferator-activated receptor gamma activation.

Lipin-alpha and -beta are the alternatively spliced gene products of the Lpin1 gene, whose product lipin is required for adipocyte differentiation. Lipin deficiency causes lipodystrophy, fatty liver, and insulin resistance in mice, whereas adipose tissue lipin overexpression results in increased adiposity but improved insulin sensitivity. To assess lipin expression and its relation to insulin resistance in humans, we examined lipin-alpha and -beta mRNA levels in subjects with normal or impaired glucose tolerance. We found higher expression levels of both lipin isoforms in lean, insulin-sensitive subjects. When compared with normal glucose-tolerant subjects, individuals with impaired glucose tolerance were more insulin resistant, demonstrated higher levels of intramyocellular lipids (IMCLs), and expressed approximately 50% lower levels of lipin-alpha and -beta. In addition, there was a strong inverse correlation between adipose tissue lipin expression and muscle IMCLs but no evidence for an increase in muscle lipid oxidation. After treatment of the impaired glucose-tolerant subjects with insulin sensitizers for 10 weeks, pioglitazone (but not metformin) resulted in a 60% increase in the insulin sensitivity index (Si) and a 32% decrease in IMCLs (both P < 0.01), along with an increase in lipin-beta (but not lipin-alpha) expression by 200% (P < 0.005). Lipin expression in skeletal muscle, however, was not related to obesity or insulin resistance. Hence, high adipose tissue lipin expression is found in insulin-sensitive subjects, and lipin-beta expression increases following treatment with pioglitazone. These results suggest that increased adipogenesis and/or lipogenesis in subcutaneous fat, mediated by the LPIN1 gene, may prevent lipotoxicity in muscle, leading to improved insulin sensitivity.

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).

Increased plasma adiponectin in response to pioglitazone does not result from increased gene expression.

Plasma levels of adiponectin are lower in obese and insulin-resistant subjects compared with lean and insulin-sensitive ones. Thiazolidinediones increase plasma adiponectin levels in diabetic subjects, although the mechanism of this increased plasma adiponectin has not been well studied. In the present study, we compared the plasma levels and adipose tissue expression of adiponectin in subjects with normal (NGT) and impaired glucose tolerance (IGT) and also studied the effects of metformin and pioglitazone on plasma and adipose tissue mRNA level of adiponectin in IGT subjects. IGT subjects had lower plasma adiponectin levels compared with NGT subjects, and similarly IGT subjects had lower adiponectin mRNA levels. In contrast, the increased plasma levels of adiponectin in response to pioglitazone were not associated with increased adiponectin expression in adipose tissue. Metformin did not cause any change in plasma or expression levels of adiponectin. Other adipokines were examined, and both pioglitazone and metformin decreased plasma levels of resistin in IGT subjects, and pioglitazone (but not metformin) decreased plasma levels of leptin. These data suggest that pioglitazone increases plasma adiponectin levels by posttranscriptional regulation in contrast to transcriptional regulation of adiponectin in relation to insulin sensitivity in NGT vs. IGT subjects.

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.