Estrogen increases apolipoprotein (apo) A-I secretion in hep G2 cells by modulating transcription of the apo A-I gene promoter.
Estrogen administration to postmenopausal women has been shown to increase plasma levels of apolipoprotein (apo) A-I. A human hepatoma cell line, Hep G2, was used to test the hypothesis that estrogen increases the hepatic production of apo A-I by modulating gene expression. When Hep G2 cells were treated for 24 hours with E(2), the apo A-I content in the medium increased 4.3+/-1.0-fold at 10 micromol/L E(2) and 1.8+/-0.4-fold at 1 micromol/L E(2) compared with untreated cells. A time-course experiment indicated that there was no E(2)-dependent (10 micromol/L) increase in apo A-I medium content at 1 hour and 2 hours and that apo A-I was 165% of controls at 6 hours and 440% at 24 hours. Hep G2 cells were transfected, by the cationic lipid method, with constructs containing serial deletions of the 5' region of the apo A-I gene (-41/+397, -256/+397, and -2500/+397) cloned in front of the luciferase gene and with or without a 7-kb region spanning the apo C-III/A-IV intergenic region, which has been shown to contain regulatory elements for the expression of the apo A-I gene. With the exception of the construct containing only the basal promoter (-41/+397), the expression of all constructs was 2- to 3-fold greater in the presence of E(2). The smallest construct that maintained E(2) responsiveness, the -256/+397 construct, does not contain a typical estrogen-responsive element. In the same transfection experiments, the 4-fold increase in apo A-I in the culture medium was preserved. However, when the same set of transfections was performed by the calcium phosphate precipitation method, the E(2) effect on the apo A-I content in the culture medium and on transcription activation was nearly abolished. This effect was probably mediated by Ca(2+), because incubation of cells with 20 mmol/L CaCl(2) abolished the E(2) response. In conclusion, E(2) increases apo A-I production in hepatic cells by increasing the transcription of the apo A-I gene.
Lamon-Fava
S
S
Lipid Metabolism Laboratory, Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University, Boston, MA 02111, USA. sfava@hnrc.tufts.edu
Ordovas
J M
JM
Schaefer
E J
EJ
D020413:3' Untranslated Regions / Q000502:physiology; D020121:5' Untranslated Regions / Q000502:physiology; D016632:Apolipoprotein A-I / Q000235:genetics* / Q000378:metabolism*; D002118:Calcium / Q000494:pharmacology; D002478:Cells, Cultured; D004958:Estradiol / Q000494:pharmacology*; D005786:Gene Expression Regulation / Q000187:drug effects / Q000502:physiology; D006801:Humans; D008099:Liver / Q000737:chemistry / Q000166:cytology* / Q000378:metabolism; D010957:Plasmids; D011401:Promoter Regions, Genetic / Q000502:physiology; D012333:RNA, Messenger / Q000032:analysis; D011960:Receptors, Estrogen / Q000502:physiology; D015533:Transcriptional Activation / Q000187:drug effects*; D014162:Transfection
D020413:3' Untranslated Regions; D020121:5' Untranslated Regions; D016632:Apolipoprotein A-I; D012333:RNA, Messenger; D011960:Receptors, Estrogen; D004958:Estradiol; D002118:Calcium