Impairment of insulin signaling in human skeletal muscle cells by co-culture with human adipocytes.

Adipocyte factors play a major role in the induction of insulin resistance in skeletal muscle. To analyze this cross-talk, we established a system of co-culture of human fat and skeletal muscle cells. Cells of three muscle donors were kept in co-culture with cells of various fat cell donors, and insulin signaling was subsequently analyzed in myocytes. Insulin-induced tyrosine phosphorylation of insulin receptor substrate (IRS)-1 was completely blocked, with unaltered expression of IRS-1. Troglitazone increased insulin action on IRS-1 phosphorylation, in both the absence and presence of co-culture. Insulin-regulated activation of Akt kinase in the myocytes was significantly reduced after co-culture, with troglitazone restoring insulin action. Addition of tumor necrosis factor (TNF)-alpha (2.5 nmol/l) to myocytes for 48 h reduced IRS-1 expression and inhibited IRS-1 and Akt phosphorylation comparable to the effect of co-culture. Lower doses of TNF-alpha were ineffective. After co-culture, TNF-alpha in the culture medium was below the detection limit of 0.3 pmol/l. A very low level of resistin was detected in the supernatant of myocytes, but not of adipocytes. In conclusion, the release of fat cell factors induces insulin resistance in human skeletal muscle cells; however, TNF-alpha and resistin appear not to be involved in this process.

Thiazolidinedione derivatives as novel therapeutic agents to prevent the development of chronic pancreatitis.

INTRODUCTION: Thiazolidinedione derivatives are known to be novel insulin-sensitizing agents and ligands of a nuclear hormone receptor peroxisome proliferator-activated receptor gamma (PPARgamma). Recently, ligands of PPARgamma have been shown to modulate proinflammatory cytokine production and NF-kappaB activation. AIM: To show that thiazolidinedione derivatives interfere with the development of chronic pancreatitis. METHODOLOGY: Rat chow containing 0.2% troglitazone was administered from 1 month to 7 months of age in WBN/Kob rats with spontaneous chronic pancreatitis. Morphologic evaluation of the pancreas was performed at 4 months and 7 months of age. Pancreas weight, protein, amylase, and insulin contents also were determined. Changes of cytokine levels were detected by enzyme-linked immunosorbent assay or semiquantitative reverse transcription-polymerase chain reaction. Localization and expression of PPARgamma in the pancreas and isolated peritoneal macrophages were examined by immunohistochemical study. RESULTS: Administration of troglitazone reduced the severity of morphologic pancreatic damage including inflammatory cell infiltration, and fibrosis markedly improved by the administration of troglitazone. Further, troglitazone was able to prevent the decrease in amylase content and pancreas atrophy that were observed in WBN/Kob rats. Serum IL-8 levels and TNF-alpha mRNA levels in the pancreas were significantly elevated in WBN/Kob rats, and these were dramatically attenuated by troglitazone. Peritoneal macrophages isolated from normal rats expressed PPARgamma at low levels, whereas those from WBN/Kob rat abundantly expressed PPARgamma. CONCLUSION: Troglitazone prevented the progression of pancreatic inflammatory process in an animal model of chronic pancreatitis. Macrophages may be one of the targets of the PPARgamma ligand to attenuate the severity of chronic pancreatitis, partially mediated by the inhibition of proinflammatory cytokine gene expression.

Angptl3 regulates lipid metabolism in mice.

The KK obese mouse is moderately obese and has abnormally high levels of plasma insulin (hyperinsulinemia), glucose (hyperglycemia) and lipids (hyperlipidemia). In one strain (KK/San), we observed abnormally low plasma lipid levels (hypolipidemia). This mutant phenotype is inherited recessively as a mendelian trait. Here we report the mapping of the hypolipidemia (hypl) locus to the middle of chromosome 4 and positional cloning of the autosomal recessive mutation responsible for the hypolipidemia. The hypl locus encodes a unique angiopoietin-like lipoprotein modulator, which we named Allm1. It is identical to angiopoietin-like protein 3, encoded by Angptl3, and has a highly conserved counterpart in humans. Overexpression of Angptl3 or intravenous injection of the purified protein in KK/San mice elicited an increase in circulating plasma lipid levels. This increase was also observed in C57BL/6J normal mice. Taken together, these data suggest that Angptl3 regulates lipid metabolism in animals.