GLUCOCORTICOIDS UP-REGULATE HIGH-AFFINITY, HIGH-DENSITY-LIPOPROTEIN BINDING-SITES IN RAT HEPATOCYTES

Glucocorticoid hormones (GL) regulate high-density lipoprotein (HDL) p lasma concentrations by increasing synthesis and secretion of HDL by t he liver. However, little is known about the effect of GL on the uptak e and processing of HDL by hepatocytes (HEP). To investigate this ques tion, we studied the effects of dexamethasone (DEX) on the expression of high-affinity HDL-binding sites via the specific binding and intern alization of iodine-labeled apolipoprotein E (ape E)-free HDL(3) in a culture of rat HEP. Specific binding and internalization of HDL(3) dec reased by 60% in cells cultured in the absence of DEX for 48 hours. At concentrations of 10(-7) and 10(-5) mol/L, DEX prevented the decrease , maintaining specific binding and internalization versus the control level (at 24 hours). HDL-binding sites with a K-d Of 20 mu g/mL were r evealed on the surface of cultured HEP. HEP demonstrated a greater bin ding capacity in the presence of DEX at concentrations of 10(-7) and 1 0(-5) mol/L (125 v 45 ng/mg cell protein). The effect of the hormone h as demonstrated to be dose-dependent at concentrations between 10(-9) and 10(-7) mol/L, leveling off at 10(-7). Higher concentrations did no t induce a further increase in specific binding and internalization. W ithdrawal of the hormone from culture medium was associated with a dec rease in specific binding of the ligand by 60% in the following 24 hou rs. To investigate the effect of glucocorticoid deficiency on liver up take of HDL in vivo, specific binding and internalization were studied in a culture of HEP isolated from adrenalectomized rats (AER) at 2 ho urs after seeding. The HEP demonstrated a 2.5-fold decrease in specifi c binding versus those from normal rats (NR). Correction of the glucoc orticoid deficiency in AER by intramuscular injection of DEX at a dose of 5 mu g per rat restored the specific binding of HDL(3) at 2 hours after seeding. Replacement of the hormone by incubation of HEP from AE R with DEX increased specific binding and internalization of the ligan d in a dose-dependent fashion until NR levels were attained. In both c ases, the upregulating effect of DEX was stopped by 0.5 mmol/L cyclohe ximide. Lovastatin (100 nmol/L) blocked the DEX-dependent increase in cholesterol synthesis, but failed to prevent the DEX-induced increase in specific binding and internalization of I-125.HDL(3). It is conclud ed that the specific binding and internalization of I-125.HDL(3) by li ver parenchymal cells are reversibly regulated by physiologic and phar macologic concentrations of DEX. Its effect does not depend on availab ility of the pool of newly synthesized cholesterol, but it seems to be a result of the increase in the synthesis of HDL-binding protein. Cop yright (C) 1995 by W.B. Saunders Company