Depot-specific release of leptin from subcutaneous and omental adipocytes in suspension culture: effect of tumor necrosis factor-alpha and transforming growth factor-beta1.

Abstract

Leptin, the product of the ob gene, is overexpressed in human obesity and increased serum leptin levels are closely correlated with adipose tissue mass, but the regulation of leptin production is not completely understood. The aim of this study was to characterize the role of tumor necrosis factor (TNF)-alpha and transforming growth factor (TGF)-beta1 in depot-specific secretion of leptin from cultured human adipocytes. We measured the leptin concentrations in the culture medium of omental and subcutaneous abdominal adipocytes taken from severely obese individuals and kept in suspension culture, and studied the effect of TNF-alpha and TGF-beta1 on leptin release. Leptin protein was measured by radioimmunoassay, leptin mRNA was assessed by reverse transcriptase (RT)-PCR relative to a housekeeping gene. Leptin secretion from subcutaneous fat cells was 2- to 3-fold higher than that from omental fat cells after incubation for 2 and 24h respectively. A 2-h exposure of adipocytes to 1nmol/l TNF-alpha and 400pmol/l TGF-beta1 respectively did not significantly affect leptin secretion. Whereas a 24-h incubation with 1nmol/l TNF-alpha also did not influence leptin secretion from fat cells from both depots, exposure of omental fat cells to 400pmol/l TGF-beta1 for 24h resulted in a significant inhibitory effect (by 33%) on leptin secretion (P<0.05). A 24- and 48-h exposure of in vitro differentiated human adipocytes to TNF-alpha led to a significant decrease in leptin mRNA levels to 70 +/- 8% and 49 +/- 13% of controls respectively. Similarly, TGF-beta1 decreased leptin mRNA expression in newly differentiated human adipocytes to 77 +/- 12% after 24h and to 54 +/- 8% after 48h compared with control cultures. These data provide evidence that long-term exposure of human fat cells to TNF-alpha or TGF-beta1 may suppress leptin expression in human adipose tissue. The inhibitory effect of TGF-beta1 appears to be more pronounced in omental as compared with subcutaneous adipocytes.