4-Hydroxylation of estradiol by human uterine myometrium and myoma microsomes: implications for the mechanism of uterine tumorigenesis.
Estradiol is converted to catechol estrogens via 2- and 4-hydroxylation by cytochrome P450 enzymes. 4-Hydroxyestradiol elicits biological activities distinct from estradiol, most notably an oxidant stress response induced by free radicals generated by metabolic redox cycling reactions. In this study, we have examined 2- and 4-hydroxylation of estradiol by microsomes of human uterine myometrium and of associated myomata. In all eight cases studied, estradiol 4-hydroxylation by myoma has been substantially elevated relative to surrounding myometrial tissue (minimum, 2-fold; mean, 5-fold). Estradiol 2-hydroxylation in myomata occurs at much lower rates than 4-hydroxylation (ratio of 4-hydroxyestradiol/2-hydroxyestradiol, 7.9 +/- 1.4) and does not significantly differ from rates in surrounding myometrial tissue. Rates of myometrial 2-hydroxylation of estradiol were also not significantly different from values in patients without myomata. We have used various inhibitors to establish that 4-hydroxylation is catalyzed by a completely different cytochrome P450 than 2-hydroxylation. In myoma, alpha-naphthoflavone and a set of ethynyl polycyclic hydrocarbon inhibitors (5 microM) each inhibited 4-hydroxylation more efficiently (up to 90%) than 2-hydroxylation (up to 40%), indicating > 10-fold differences in Ki (<0.5 microM vs. > 5 microM). These activities were clearly distinguished from the selective 2-hydroxylation of estradiol in placenta by aromatase reported previously (low Km, inhibition by Fadrozole hydrochloride or ICI D1033). 4-Hydroxylation was also selectively inhibited relative to 2-hydroxylation by antibodies raised against cytochrome P450 IB1 (rat) (53 vs. 17%). These data indicate that specific 4-hydroxylation of estradiol in human uterine tissues is catalyzed by a form(s) of cytochrome P450 related to P450 IB1, which contribute(s) little to 2-hydroxylation. This enzyme(s) is therefore a marker for uterine myomata and may play a role in the etiology of the tumor.
Liehr
J G
JG
Department of Pharmacology and Toxicology, University of Texas Medical Branch, Galveston 77555-1031, USA.
Ricci
M J
MJ
Jefcoate
C R
CR
Hannigan
E V
EV
Hokanson
J A
JA
Zhu
B T
BT
D001571:Benzoflavones / Q000494:pharmacology; D002471:Cell Transformation, Neoplastic; D065607:Cytochrome P-450 Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D004791:Enzyme Inhibitors / Q000494:pharmacology*; D004958:Estradiol / Q000031:analogs & derivatives* / Q000032:analysis / Q000378:metabolism*; D002393:Estrogens, Catechol / Q000378:metabolism*; D005260:Female; D006801:Humans; D006900:Hydroxylation; D007889:Leiomyoma / Q000378:metabolism*; D008861:Microsomes / Q000187:drug effects / Q000378:metabolism*; D009215:Myometrium / Q000378:metabolism*; D009281:Naphthalenes / Q000494:pharmacology; D010616:Phenanthrenes / Q000494:pharmacology; D010920:Placenta / Q000378:metabolism*; D011247:Pregnancy; D014594:Uterine Neoplasms / Q000378:metabolism*
D001571:Benzoflavones; D065607:Cytochrome P-450 Enzyme Inhibitors; D004791:Enzyme Inhibitors; D002393:Estrogens, Catechol; D009281:Naphthalenes; D010616:Phenanthrenes; D004958:Estradiol; C011512:alpha-naphthoflavone; D003577:Cytochrome P-450 Enzyme System; C001390:2-hydroxyestradiol; C014036:4-hydroxyestradiol