Adipocyte membrane phospholipids and PPAR-γ expression in obese women:: relationship to hyperinsulinemia

We have shown that membrane sphingomyelin (SM) is an independent predictor of the variance of fasting plasma insulin (FPI) concentrations and the homeostasis model assessment (HOMA) estimate of insulin resistance in obese women. The peroxisome proliferator-activated receptor-gamma (PPAR-gamma) is a key component in adipocyte differentiation that may also contribute to the sensitivity of cells to insulin. PPAR-gamma is activated by fatty acids, and the membrane composition may have an impact on the activity of PPAR-gamma and thus on the sensitivity of adipocytes to insulin. We investigated these possible links by determining the phospholipid contents of adipocyte membranes, the mRNA expression of PPAR-gamma, and the FPI and HOMA estimate of insulin resistance in obese women. The mRNA levels of tumor necrosis factor-alpha (TNF-alpha), which is suspected to play a role in insulin resistance and which downregulates PPAR-gamma expression, were also quantified. FPI and HOMA were strongly positively correlated with membrane SM (P < 0.005) and cholesterol (P < 0.005). PPAR-gamma mRNA levels were negatively correlated with FPI (P < 0.05) and HOMA (P < 0.05) and positively correlated with high-density lipoprotein (HDL) cholesterol (P < 0.05), membrane SM (P < 0.05), and cholesterol contents (P < 0.05). TNF-alpha mRNA levels were not correlated with membrane parameters. In stepwise multiple regression analysis, the variations in PPAR-gamma mRNA levels were mainly explained by HDL cholesterol (31.9%) and membrane SM (17.7%). Our study shows that the expression of PPAR-gamma, a major factor controlling adipocyte functions, the lipid composition of the membrane, and insulin sensitivity are probably closely associated in the adipose tissue of obese women.