Regulation of adiponectin secretion and expression by endothelin-1

Adiponectin is a 30 kDa hormone produced exclusively by adipocytes that has become well-known for its insulin-sensitizing and antiatherogenic properties. Circulating concentrations of adiponectin are normally high; however circulating concentrations of this hormone are significantly reduced in obesity, insulin resistance and diabetes. In order to further elucidate the regulation of adiponectin expression and secretion from adipose tissue, we investigated the effects of endothelin-1 (ET-1) on its expression and secretion from 3T3-L1 adipocytes. ET-1 is a small, 21 amino acid vasoactive peptide produced by the endothelial cells of the vasculature. Levels of ET-1 are elevated in many disease states including obesity and diabetes. Mature 3T3-L1 adipocytes were incubated with vehicle, insulin (0.01--100 nM) or ET-1 (0.001--100 nM) for 1--24 hours and media and RNA were collected for analysis of protein secretion and gene expression, respectively. Insulin and ET-1 significantly stimulated adiponectin secretion 1 hour following treatment compared to vehicle-treated cells. The stimulatory effects of insulin declined within 4 hours of treatment, while the stimulatory effects of ET-1 lasted over 8 hours following treatment. Insulin significantly downregulated adiponectin gene expression 12 hours following treatment in a concentration-dependent manner. ET-1 did not significantly affect adiponectin gene expression in either a time or concentration dependent manner. Chronic (24 hr) exposure to ET-1 (100 nM) significantly inhibited insulin and ET-1 stimulated adiponectin secretion. Previous studies demonstrated that ET-1 has profound effects on adipocyte metabolism including glucose uptake and utilization. In order to investigate whether glucose metabolism was one mechanism responsible for the stimulatory effect of ET-1 on adiponectin secretion, 3T3-L1 adipocytes were incubated in the presence or absence of phloretin (an inhibitor of glucose uptake, 0.1 mM) and stimulated with vehicle, insulin or ET-1 for 0--12 hrs. Phloretin significantly inhibited basal, insulin-stimulated and ET-1 stimulated glucose uptake for 0--12 hours. Phloretin also significantly inhibited insulin and ET-1 stimulated adiponectin secretion one hour following treatment. Further investigations utilizing the competitive glucose metabolism inhibitor 2-Deoxy-D-glucose (2DG, 25 mM) revealed that competitive inhibition of glucose uptake inhibited insulin and ET-1 stimulated adiponectin secretion. Thus, it appears that ET-1 acutely stimulates and chronically inhibits adiponectin secretion and factors such as glucose uptake and utilization play a role in the acute stimulatory and chronic inhibitory effects of ET-1 on adiponectin secretion.