Depot-related gene expression in human subcutaneous and omental adipocytes.
Human omental adipocytes display a range of biochemical properties that distinguish them from adipocytes of subcutaneous origin. However, information about site-related gene expression in human fat cells is limited. We have previously demonstrated that leptin mRNA is markedly overexpressed in abdominal subcutaneous (SC) compared with omental (Om) adipocytes. To further investigate depot-specific differences in adipocyte gene expression, we have measured, in paired samples of isolated human adipocytes obtained from SC and Om fat depots, the expression of mRNAs encoding a number of proteins involved in the control of adipocyte metabolism. In contrast to the marked site-related expression of leptin, genes encoding lipoprotein lipase (LPL), hormone-sensitive lipase (HSL), peroxisome proliferator-activated receptor-gamma (PPAR-gamma), tumor necrosis factor-alpha (TNF-alpha), and adipsin were not consistently differentially expressed. Of note, a highly significant inverse correlation between adipocyte PPAR-gamma expression and BMI (r = -0.7, P = 0.0005) was found. In parallel experiments, differential display was used in an attempt to identify novel and/or unexpected adipocyte genes that were expressed in a site-related manner. No transcript that was unique to one or another depot was found, but cellular inhibitor of apoptosis protein-2 (cIAP2) mRNA, which has not previously been reported in adipocytes, was expressed at higher levels in Om than SC adipocytes (Om > SC in all eight subjects; mean Om:SC ratio 1.9 +/- 0.2, P < 0.01). Because cIAP2 may be involved in the regulation of TNF-alpha signaling, this raises the possibility that depot-specific differences may exist in the regulation of adipocyte apoptosis. Thus, of the mRNAs examined to date, only leptin and cIAP2 show consistent site-related expression, suggesting that these molecules may have important roles in determining functional properties particular to individual adipose depots. Given the importance of PPAR-gamma in adipocyte development and insulin sensitivity, the inverse correlation between adipocyte PPAR-gamma mRNA levels and adiposity may represent a local regulatory mechanism restraining fat accumulation and/or may be related to the reduction of insulin sensitivity that occurs with increasing fat mass.