9-CIS-RETINOIC ACID INCREASES APOLIPOPROTEIN AI SECRETION AND MESSENGER-RNA EXPRESSION IN HEPG2 CELLS

HepG2 cells were studied as a model for regulation of hepatic apolipoprotein AI (ape AI) secretion and gene expression by 9-cis-retinoic acid. HepG2 cells cultured on plastic dishes were exposed to 9-cis-retinoic acid (9-cis-RA) for 48 h with a complete media change at 24 h. Apo AI mass in culture media was determined by ELISA, by quantitative immunoblotting and by steady-state S-35-methionine labeling. Messenger RNA levels were determined by RNase protection using probes for apo AI and the housekeeping gene, glyceraldehyde 3-phosphate dehydrogenase (G3PDH). 9-cis-RA increased secretion of apo Al by 52% at doses of 10 and 1 mu M (6.3 +/- 0.6 vs. 4.2 +/- 0.3; P < 0.005; 6.1 +/- 0.3 vs. 4.0 +/- 0.7 ng of apo AI/mg cell protein, P < 0.05) and by 35% at 0.1 mu M (5.5 +/- 0.6 vs. 4.1 +/- 0.4 ng apo AI/mg protein, P < 0.05, n = 4). Immunoblotting results were consistent with results from ELISA (70% increase at 10 mu M 9-cis-RA, P < 0.001; 34% increase at 1 mu M, P < 0.005, n = 3). Metabolically labeled apoAI in the medium was increased by 39%, following steady-state labeling in the presence of 10 mu M 9-cis-RA(597 +/- 7 vs. 430 +/- 13 DPM/mu l media; P < 0.001; rz = 4). 9-cis-RA (10 mu M) also increased HepG2 cell apo AI mRNA expression by 76% (68 700 +/- 400 vs, 38 900 +/- 2700 DPM, P < 0.01, n = 4), whereas expression of G3PDH mRNA was slightly decreased (14%, P < 0.05). Thus, 9-cis-RA stimulates apo AI expression in HepG2 cells, suggesting a role for retinoids in activating endogenous apo AI gene expression.