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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. [ABSTRACT FROM AUTHOR]

PPARγplays a major role in cell differentiation, lipid metabolism and apoptosis and is the target of thiazolidinediones which are widely used to improve insulin sensitivity and for treatment of diabetes mellitus. Recently a SUMOylation (small ubiquitin-like modifier) site was identified on PPARγsuggesting that it might be a target for modification by SUMO-1. SUMOylation, a process similar to ubiquitination, regulates many cellular processes including nuclear transport, transcription, apuptosis and protein stability. SUMOylation of PPARγhas been proposed to play a role in the transrepression of inflammatory genes. We hypothesized that pioglitazone (pin) would increase the amount of PPARγmodified by SUMO-1, and that this could be part of the anti-inflammatory effect of TZDs. To evaluate SUMOylation we examined human adipose tissue mRNA levels for SUMO-1 in 18 impaired glucose tolerant subjects in response to two insulin-sensitizing drugs (pin and metfromin). We found that pin treatment increased expression of SUMO-1 by 23% (p<0.002). When added to adipocytes in vitro, pin also increased SUMO-1 suggesting a direct effect of TZDs on adipocytes. Next, we examined the expression of SUMO-1 and NCOR (nuclear receptor corepressor protein) in adipose tissue from 86 subjects (54 NGT and 32 IGT) covering a broad range of BMI and S[sub 1]. SUMO-1 and NCOR correlated strongly with each other (r=0.8, p<0.001) suggesting that they are tightly associated in a large complex as described previously in cultured macrophages where SUMOylation of PPARγ promotes interaction with the NCOR-histone deacetylase3 complex and represses target genes. Although SUMO-1 and NCOR were not significantly associated with BMI or S[sub 1] in this group, they were highly associated with inflammatory markers. Adipose CD68 and MCP1 mRNA levels correlated strongly with SUMO-1 and NCOR (r=0.5 and 0.6, p<0.001 respectively). Overall these results show that SUMOylation of PPARγcan be regulated by pin treatment and suggest that the anti-inflammatory effects of pio may be controlled by the degree of SUMO-1 modification of PPARγ. [ABSTRACT FROM AUTHOR]

Retinol binding protein 4 (RBP4) was overexpressed in adipose tissue of Glut4 KO mice, and elevated RBP4 has been associated with insulin resistance in mouse and human studies. To determine the relationship between RBP4 and obesity, insulin resistance, and other markers of insulin resistance in humans, RBP4 gene expression measured by real-time RT-PCR and blood levels measured by ELISA and Western blotting were studied in adipose tissue and muscle of 85 non-diabetic humans over a wide range of BMI (19-55 Kg/m²) and S[sub I] (1.02-26.77 x 10[sup -5] x min[sup -1]/pM), and RBP4 expression was also measured from visceral (VAT) and subcutaneous adipose tissue (SAT) from 16 surgical patients. RBP4 expression was 3-fold higher in SAT vs VAT (p<0.005), and RBP4 was 7-fold higher in the adipocyte (vs stromal) fraction, and undetectable in preadipocytes. Neither SAT RBP4 expression nor blood RBP4 level showed a significant relationship with BMI or S[sub I]. However, there was a strong positive correlation between RBP4 mRNA and the inflammation markers MCP1 (r=0.42, p<0.0001) and CD68 (r=0.46, p<0.0001)). Adipose GLUT4 mRNA was also highly associated with RBP4 (r=0.53, p<0.0001). These relationships remained significant after adjustment for BMI and St. Impaired glucose tolerant subjects were treated with pioglitazone, resulting in an increase in S[sub 1] along with a 60% increase in RBP4 expression, but no change in plasma RBP4. The in vitro treatment of cultured adipocytes with pioglitazone yielded a similar increase in RBP4 mRNA. Hence, RBP4 expression in humans was not significantly associated with obesity or insulin resistance, and actually increased with TZD treatment. However, RBP4 was associated with inflammatory markers and GLUT 4 expression, suggesting a complex role in metabolic syndrome. [ABSTRACT FROM AUTHOR]

Adipose triglyceride lipase (ATGL) has been identified as an important mediator of adipocyte lipolysis and energy homeostasis. ATGL deficient mice had increased adipose mass and triglyceride accumulation in cardiac muscle. We studied the expression of ATGL in human fat and muscle, and examined correlations with muscle lipid oxidation. ATGL was expressed in mature adipocytes at a level 10-fold higher than that in the adipose tissue stromal fraction or muscle. We measured ATGL mRNA levels in adipose tissue and muscle of 86 nondiabetic subjects covering a wide range of BMI (19-55 Kg/m²) and S[sub 1] (1.0226.77 x 10[sup -5] x min[sup -1]/pM), with either normal glucose tolerance (NGT) or impaired glucose tolerance (IGT). ATGL expression in adipose tissue or muscle showed no correlation with either BMI or St. In muscle, ATGL mRNA associated strongly with genes involved with lipid oxidation, including CPT1 mRNA (r=0.82, p<0.0001), AdipoR1 mRNA (r=0.71, p<0.0001), and AdipoR2 mRNA (r=0.74, p<0.0001), however there was no association with intramyocellular lipid. Thirty two IGT subjects were randomized to receive either pioglitazone (PIO) or metformin (MET) for 10 weeks, and we examined the effects of these drugs on ATGL expression. PIO, but not MET, resulted in a 31% increase of adipose ATGL expression (p<0.03), with no change in muscle. ATGL in muscle is associated with lipid oxidation. These data tend to support rodent studies which suggest that muscle ATGL may be involved with the release of FFA from lipid storage for oxidation. However, adipose ATGL expression was not associated with obesity or insulin resistance, and was increased following PIO treatment, suggesting an important role in the balance of adipose lipid metabolism. [ABSTRACT FROM AUTHOR]