The regulation of lipoprotein lipase gene expression by dexamethasone in isolated rat adipocytes

Lipoprotein lipase (LPL) is an enzyme found in adipose tissue that is important in the hydrolysis of triglyceride-rich lipoprotéine, and in the uptake of FFA lipid into the adipocyte. To examine the effects of glucocorticoids on adipose tissue LPL, male Sprague-Dawley rats were injected with dexamethasone (1 mg/kg) every other day for 10 days, followed by measurement of LPL in epididymal adipose tissue. Compared to sham-injected controls, heparin-releasable LPL activity and LPL mass in the dexamethasone-treated rats were 44% and 62% of those in control rats, respectively. Adipocytes were prepared from the fat pads and pulse labeled with [³⁵S] methionine, demonstrating a decrease in the LPL synthetic rate in the treated rats to 57% of the rate in control rats. In addition, LPL mRNA was quantitated by Northern blotting, demonstrating a decrease in LPL mRNA in the dexamethasone-treated rats. A simultaneous decrease in the message for γ-actin was also noted. To examine the effects of dexamethasone on LPL in vitro, adipocytes were prepared from normal rats and treated with dexamethasone for 24 h in vitro. Dexamethasone decreased heparin-releasable LPL activity in cultured adipocytes to 40 ± 6% of the control value (P < 0.01). This decrease in LPL activity was accompanied by a decrease in the LPL synthetic rate using [³⁵S]methionine labeling, to 33% of the control value, and no specific change in LPL turnover or secretion. In addition, dexamethasone added to adipocytes decreased LPL mRNA levels. Because the combination of insulin plus dexamethasone has been shown to yield synergistic increases in LPL in adipose tissue pieces, insulin was added to isolated adipocytes in combination with dexamethasone. Whereas insulin and dexamethasone individually had opposite effects on LPL, the combination of insulin plus dexamethasone resulted in no change in any aspect of LPL gene expression. Thus, dexamethasone resulted in a decrease in adipocyte LPL mRNA levels both when added to cultured adipocytes in vitro as well as when injected into rats. This decreased LPL mRNA level yielded corresponding changes in the LPL synthetic rate and LPL activity.