Kinetics of estrogen-dependent modulation of apolipoprotein A-I synthesis in human hepatoma cells.
Apolipoprotein (apo) A-I is the principal protein component of high density lipoproteins and the major in vivo activator of lecithin-cholesterol acyltransferase. We have used the human hepatoma cell line, HepG2, as a model to examine the ability of estrogen to modulate hepatic synthesis of apo-A-I. Primer extension studies have demonstrated that the major transcriptional initiation site of the apo-A-I gene utilized in the hepatoma cells is the same as that used in human liver. The kinetics of induction of high- and low-affinity estrogen-binding sites, rates of secretion of apolipoproteins, and apo-A-I mRNA levels were examined following treatment of the cells with estrogen. Initial concentrations of 20 nM 17 beta-estradiol resulted in a 14-15-fold increase in the levels of high-affinity nuclear estrogen-binding sites within 8 h, while the level of low-affinity sites increased by only 10%. During the same period, the levels of apo-A-I mRNA and the rate of accumulation of the secreted protein increased by 55 and 50%, respectively. New steady state levels of apo-A-I mRNA and rates of accumulation of protein, approximately twice those in control cultures, were established within 24-48 h of exposure to hormone. Experiments with a 50-fold higher concentration of estrogen resulted in only an additional 10% increase in mRNA levels. The increase in mRNA levels following estrogen treatment was adequate to account for 85-90% of the elevation observed in the rate of accumulation of secreted apo-A-I. Comparison of the apo-A-I mRNA levels in HepG2 cells with those present in human liver revealed that the concentration of the mRNA was approximately 3-fold lower than that found in vivo.
D016632:Apolipoprotein A-I; D001054:Apolipoproteins A / Q000096:biosynthesis* / Q000235:genetics; D001665:Binding Sites / Q000187:drug effects; D006528:Carcinoma, Hepatocellular / Q000378:metabolism*; D002460:Cell Line; D002467:Cell Nucleus / Q000187:drug effects / Q000378:metabolism; D004958:Estradiol / Q000494:pharmacology*; D004967:Estrogens / Q000378:metabolism; D006801:Humans; D007700:Kinetics; D008099:Liver / Q000378:metabolism; D008113:Liver Neoplasms / Q000378:metabolism*; D008214:Lymphocytes / Q000378:metabolism; D009693:Nucleic Acid Hybridization; D012333:RNA, Messenger / Q000378:metabolism; D014158:Transcription, Genetic
D016632:Apolipoprotein A-I; D001054:Apolipoproteins A; D004967:Estrogens; D012333:RNA, Messenger; D004958:Estradiol