2-hydroxyestradiol and Carcinoma--Hepatocellular

The effect of endogenous estrogens on sex-hormone-binding globulin (SHBG) production was studied in HepG2 cells. 17 beta-estradiol, estrone, and estrogens from both 2- and 16 alpha-hydroxylative pathways stimulated SHBG production, but not in parallel with their binding affinities for the estrogen receptor. Thus, the underlying mechanism may be other than a pure interaction with the estrogen receptor.

Topics: 2-Methoxyestradiol; Carcinoma, Hepatocellular; Estradiol; Estriol; Estrogens; Estrone; Humans; Hydroxyestrones; Liver Neoplasms; Receptors, Estrogen; RNA, Messenger; Sex Hormone-Binding Globulin; Tumor Cells, Cultured

Twelve forms of human cytochrome P450 were synthesized in human hepatoma Hep G2 cells by means of cDNA-directed expression using vaccinia virus. The cDNA-expressed enzymes were tested for their ability to oxidize estradiol. Incubation of [14C]estradiol with cell lysates containing P450 IA2 resulted in the production of 2-hydroxy and 4-hydroxy metabolites with substrate turnovers of 2.74 and 0.27 min-1, respectively. P450s IIIA3 and IIIA4 yielded the same metabolites at about one third the rate of P450 IA2. Low levels of estradiol hydroxylation were also catalyzed by P450s IIC9, IIIA5, and IVB1. Six other P450 forms yielded no detectable metabolism. The roles of P450s IA2, IIA3, and IIIA4 were further established by immunoinhibition using antirat P450 antibodies. Antibody that specifically binds to P450 IIIA3 and IIIA4 inhibited 60-70% of estradiol hydroxylation, and antibody against P450 IA2 inhibited 20-40% of the estradiol hydroxylase activity in microsomes from two human liver specimens, suggesting that these enzymes constitute the major forms catalyzing estradiol oxidation in human liver. Immunoinhibition results also suggest that 2-hydroxy- and/or 4-hydroxycatechol estrogens are further metabolized to other yet uncharacterized metabolites by P450s IIIA3 and IIIA4.

Topics: Antibodies; Carcinoma, Hepatocellular; Cytochrome P-450 Enzyme System; DNA; Estradiol; Estrogens, Catechol; Gene Expression; Humans; Hydroxylation; Liver Neoplasms; Microsomes, Liver; Recombinant Proteins; Transfection; Tumor Cells, Cultured; Vaccinia virus