Estrogen regulation of the apolipoprotein AI gene promoter through transcription cofactor sharing.
Estrogen replacement therapy increases plasma concentrations of high density lipoprotein and its major protein constituent, apolipoprotein AI (apoAI). Studies with animal model systems, however, suggest opposite effects. In HepG2 cells stably expressing estrogen receptor alpha (ERalpha), 17beta-estradiol (E2) potently inhibited apoAI mRNA steady state levels. ApoAI promoter deletion mapping experiments indicated that ERalpha plus E2 inhibited apoAI activity through the liver-specific enhancer. Although the ERalpha DNA binding domain was essential but not sufficient for apoAI enhancer inhibition, ERalpha binding to the apoAI enhancer could not be detected by electrophoretic mobility shift assays. Western blotting and cotransfection assays showed that ERalpha plus E2 did not influence the abundance or the activity of the hepatocyte-enriched factors HNF-3beta and HNF-4, two transcription factors essential for apoAI enhancer function. Expression of the ERalpha coactivator RIP140 dramatically repressed apoAI enhancer function in cotransfection experiments, suggesting that RIP140 may also function as a coactivator on the apoAI enhancer. Moreover, estrogen regulation of apoAI enhancer activity was dependent upon the balance between ERalpha and RIP140 levels. At low ratios of RIP140 to ERalpha, E2 repressed apoAI enhancer activity, whereas at high ratios this repression was reversed. Regulation of the apoAI gene by estrogen may thus vary in direction and magnitude depending not only on the presence of ERalpha and E2 but also upon the intracellular balance of ERalpha and coactivators utilized by ERalpha and the apoAI enhancer.
Harnish
D C
DC
Department of Nuclear Receptors, Wyeth-Ayerst Research, Radnor, Pennsylvania 19087, USA.
Evans
M J
MJ
Scicchitano
M S
MS
Bhat
R A
RA
Karathanasis
S K
SK
D048868:Adaptor Proteins, Signal Transducing; D016632:Apolipoprotein A-I / Q000096:biosynthesis* / Q000235:genetics*; D051778:Basic Helix-Loop-Helix Leucine Zipper Transcription Factors; D001665:Binding Sites; D006528:Carcinoma, Hepatocellular; D004268:DNA-Binding Proteins / Q000378:metabolism*; D004742:Enhancer Elements, Genetic; D004958:Estradiol / Q000494:pharmacology*; D015972:Gene Expression Regulation, Neoplastic / Q000187:drug effects*; D051542:Hepatocyte Nuclear Factor 3-beta; D051557:Hepatocyte Nuclear Factor 4; D006801:Humans; D007700:Kinetics; D008113:Liver Neoplasms; D008156:Luciferases / Q000096:biosynthesis; D009687:Nuclear Proteins / Q000378:metabolism*; D000078723:Nuclear Receptor Interacting Protein 1; D010750:Phosphoproteins / Q000378:metabolism; D011401:Promoter Regions, Genetic / Q000187:drug effects; D012333:RNA, Messenger / Q000096:biosynthesis; D011960:Receptors, Estrogen / Q000096:biosynthesis / Q000378:metabolism*; D011993:Recombinant Fusion Proteins / Q000096:biosynthesis / Q000378:metabolism; D014157:Transcription Factors / Q000378:metabolism; D014158:Transcription, Genetic / Q000187:drug effects*; D014162:Transfection; D014407:Tumor Cells, Cultured
D048868:Adaptor Proteins, Signal Transducing; D016632:Apolipoprotein A-I; D051778:Basic Helix-Loop-Helix Leucine Zipper Transcription Factors; D004268:DNA-Binding Proteins; C496502:FOXA2 protein, human; D051557:Hepatocyte Nuclear Factor 4; C403128:MLX protein, human; C000627127:NRIP1 protein, human; D009687:Nuclear Proteins; D000078723:Nuclear Receptor Interacting Protein 1; D010750:Phosphoproteins; D012333:RNA, Messenger; D011960:Receptors, Estrogen; D011993:Recombinant Fusion Proteins; D014157:Transcription Factors; D051542:Hepatocyte Nuclear Factor 3-beta; D004958:Estradiol; D008156:Luciferases