Regulation of human apolipoprotein A-I gene expression by equine estrogens.
Estrogen replacement therapies, such as conjugated equine estrogen (CEE, Premarin), reduce the risk of coronary heart disease among postmenopausal women. In the present study, a HepG2 stable cell line (HepG2/S) that harbors a luciferase reporter gene cassette with the human apolipoprotein A-I (apoA-I) promoter region was used to examine the activity of CEE components in modulating human apoA-I promoter activity. A number of estrogens modulated apoA-I promoter activity, with equilenin (Eqn) being the most potent. Eqn produced a 3-fold increase in apoA-I promoter activity and a similar increase in apoA-I mRNA without affecting its degradation rate. Nuclear runoff assays indicated that the transcription rate of the apoA-I gene was increased 2.5-fold in Eqn-treated cells. When HepG2/S cells were exposed to Eqn, apoA-I protein secretion increased by 80%, whereas the level of secreted apoA-II remained unchanged. Transient transfection studies with human apoA-I promoter constructs derived from pGL3-luciferase reporter plasmid were used to identify the cis-acting element involved in Eqn-mediated induction. The results demonstrated that the apoA-I electrophile/antioxidant response element (EpRE/ARE) might be responsible for the increase in apoA-I promoter activity by Eqn. Cotransfection experiments using estrogen receptor (ERalpha and/or ERbeta) expression vectors have indicated that neither receptor can potentiate the Eqn-mediated induction of apoA-I promoter activity. In addition, mobility shift analysis using antibody against either ERalpha or ERbeta cannot detect the presence of these receptors in the DNA-protein complex. The data indicate that Eqn can induce the promoter activity of the human apoA-I gene, leading to an increase in apoA-I mRNA levels and apoA-I protein secretion through an ER-independent pathway involving apoA-I EpRE/ARE.
Zhang
X
X
Department of Biochemistry, Queen's University, Kingston, Ontario, Canada.
Jiao
J J
JJ
Bhavnani
B R
BR
Tam
S P
SP
D000818:Animals; D016632:Apolipoprotein A-I / Q000235:genetics*; D001053:Apolipoproteins / Q000096:biosynthesis / Q000378:metabolism; D006528:Carcinoma, Hepatocellular; D004855:Equilenin / Q000494:pharmacology; D004965:Estrogen Antagonists / Q000494:pharmacology; D004967:Estrogens / Q000494:pharmacology*; D005786:Gene Expression Regulation / Q000187:drug effects*; D006736:Horses; D006801:Humans; D007700:Kinetics; D008113:Liver Neoplasms; D008156:Luciferases / Q000235:genetics / Q000378:metabolism; D011401:Promoter Regions, Genetic; D012333:RNA, Messenger / Q000032:analysis; D011960:Receptors, Estrogen / Q000235:genetics / Q000502:physiology; D020218:Response Elements; D014162:Transfection; D014407:Tumor Cells, Cultured
D016632:Apolipoprotein A-I; D001053:Apolipoproteins; D004965:Estrogen Antagonists; D004967:Estrogens; D012333:RNA, Messenger; D011960:Receptors, Estrogen; D008156:Luciferases; D004855:Equilenin