ISOPROTERENOL DECREASES LDL RECEPTOR EXPRESSION IN RAT ADIPOSE-CELLS - ACTIVATION OF CYCLIC-AMP-DEPENDENT PROTEOLYSIS

The low density lipoprotein (LDL) receptor is part of a family of prot eins that mediate the uptake of lipoproteins into cells. In this paper we have demonstrated the over-expression in E. coli of a rat LDL rece ptor fusion protein that contains the region of the receptor sharing h omology with the EGF precursor. The fusion protein was utilized to imm u nize rabbits and successfully generate antibodies that recognize the intact LDL receptor. These anti-LDL receptor/fusion protein antibodie s were used to examine the effects of cyclic AMP on the expression of LDL receptors in isolated rat adipocytes. Incubation of adipocytes wit h isoproterenol caused a dose-dependent diminution in intact LDL recep tors in the plasma membrane with the concomitant appearance of smaller immunoreactive proteins. Pulse-chase experiments demonstrated that is oproterenol rapidly shortened the initial half-life of intact, immunop recipitable LDL receptors in the plasma membrane. The effects of isopr oterenol on LDL receptor expression were mimicked by forskolin, by an analog of cyclic AMP, and by ACTH. In contrast, incubation with propra nolol blocked the effects of isoproterenol on LDL receptor expression. While antioxidants and several different protease inhibitors had no e ffects, N-acetyl-leucine-lcucine methionine (ALLM) was able to prevent the isoproterenol-induced effects on LDL receptors. Thus, it appears that agents acting via cyclic AMP cause a rapid decrease in LDL recept ors in the plasma membranes of isolated adipose cells due to the appar ent stimulation of an ALLM-sensitive protease that degrades the LDL re ceptor. These results suggest a novel mechanism for the posttranscript ional regulation of LDL receptor expression in adipocytes.