9-cis-retinoic acid increases apolipoprotein AI secretion and mRNA expression in HepG2 cells Original Research Article

HepG2 cells were studied as a model for regulation of hepatic apolipoprotein AI (apo 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 35S-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 AI by 52% at doses of 10 and 1 μM (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 μM (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 μM 9-cis-RA, P < 0.001; 34% increase at 1 μM, P < 0.005, N = 3). Metabolically labeled apoAI in the medium was increased by 39% following steady-state labeling in the presence of 10 μM 9-cis-RA (597 ± 7 vs. 430 ± 13 DPM/μl media; P < 0.001; N = 4). 9-cis-RA (10 μM) 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.