Effects of insulin and dexamethasone on lipoprotein lipase in human adipose tissue

The mechanisms by which insulin and glucocorticoids modulate lipoprotein lipase (LPL) synthesis and degradation were examined in human adipose tissue fragments maintained in organ culture. Tissue fragments were cultured for 7 days in serum-free medium supplemented with or without insulin (7 nM) and with or without dexamethasone (30 nM), a synthetic glucocorticoid. Responses of LPL activity to both insulin and dexamethasone were obtained at doses within the physiological range. At a maximal dose, insulin increased heparin-releasable and total LPL activity (approximately 7-fold) by specifically increasing the rate of LPL synthesis (approximately 5-fold) determined by pulse labeling with [35S]methionine and [35S]cysteine and immunoprecipitation. Dexamethasone added in the presence of insulin increased heparin-releasable and total LPL activity approximately 8-fold but did not alter rates of LPL synthesis compared with insulin alone. Pulse-chase studies showed that the rate of LPL degradation was markedly slowed in the presence of dexamethasone plus insulin compared with insulin alone. These data suggest that, in human adipose tissue, insulin is essential for maintaining rates of LPL synthesis and that cortisol may play a key role in regulating human adipose tissue LPL at the posttranslational level by inhibiting the degradation of newly synthesized LPL.