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 apolipop rotein AI (ape AI) secretion and gene expression by 9-cis-retinoic aci d. HepG2 cells cultured on plastic dishes were exposed to 9-cis-retino ic 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 immu noblotting and by steady-state S-35-methionine labeling. Messenger RNA levels were determined by RNase protection using probes for apo AI an d the housekeeping gene, glyceraldehyde 3-phosphate dehydrogenase (G3P DH). 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). Im munoblotting results were consistent with results from ELISA (70% incr ease 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 3 9%, following steady-state labeling in the presence of 10 mu M 9-cis-R A(597 +/- 7 vs. 430 +/- 13 DPM/mu l media; P < 0.001; rz = 4). 9-cis-R A (10 mu M) also increased HepG2 cell apo AI mRNA expression by 76% (6 8 700 +/- 400 vs, 38 900 +/- 2700 DPM, P < 0.01, n = 4), whereas expre ssion of G3PDH mRNA was slightly decreased (14%, P < 0.05). Thus, 9-ci s-RA stimulates apo AI expression in HepG2 cells, suggesting a role fo r retinoids in activating endogenous apo AI gene expression.