Estradiol-activated alpha-fetoprotein suppresses the uterotropic response to estrogens.

Abstract

The binding of estrogen to a-fetoprotein (AFP) in the plasma cannot account for the impaired estrogen response seen in immature rodents because estradiol (E2) doses that far exceed the total body burden of AFP will stimulate only modest uterine growth. We investigated this phenomenon in immature female mice by determining their uterine weights 23 hr after intraperitoneal injection of estrogens or AFP or both. Administration of either 0.5 gg of E2 or 10 ng of moxestrol (MOX) approximately doubled the uterine weight. Giving 1 pg of AFP 1 hr before injection of either estrogen did not alter thatresponse. Combining the E2 and AFP just prior to injection resulted in decreased uterine growth (34% inhibition). Preincubating the estrogens with purified AFP (0.1-50 jug) did not affect the growth response to moxestrol but markedly decreased the response to E2. This was not due to sequestering of hormone because maximal reduction of the E2 response (ca. 65% inhibition) required only 1.0 jug of AFP(AFP/ E2 molar ratio, 1:130), and higher AFP doses inhibited less. About 40% of the growth elicited by injection of either 0.5 jig of E2 or 10 ng of MOX was inhibited when these doses were preceded by injection of the preincubated AFP/E2 mixture but not when preceded by either of the components. In each experiment, the mitotic index of luminal epithelium was affected to the same degree as uterine weight. AFP and E2 incubated for 1 hr thus produce a potent inhibitor of estrogen-stimulated mitotic activity and growth. This inhibitor might act upon estrogen-responsive cells at specific sites at which competition by an inactive component of AFP can block the process.The estrogen contained in the amniotic fluid of pregnant rats and mice is largely protein-bound. The binding component that imparts that property is a-fetoprotein (AFP) which constitutes about one-third of the total protein present in rodent amniotic fluid. In fetal and newborn rodents the plasma contains this protein in high concentrations (3-5 mg/ml) which persist for a few days post partum. The concentration then declines rapidly over the next 3 weeks of life and, in most rodent strains, attains the low levels common to adults after the fifth week (1).Rodent AFP molecules bear sites that bind the ovarian estrogens with high affinity and specificity (2, 3). These properties would ensure that, in animals given submicrogram doses of estradiol (E2), even up to 15 days of age, the concentration of unbound hormone in the circulation that would be available for interaction with target cells is small. This argument has been advanced to account for the apparent insensitivity to E2 exhibited by those immature rodent organs which in the adult are quite sensitive to estrogen (4). However, this assertion seems flawed in that the uterotropic response to administered estrogen in immature rodents is impaired even when the estrogen doses are well in excess of the total body burden of AFP.At the developmental state of the immature rodent, high plasma.AFP levels and insensitivity...

Alpha-fetoprotein, the major fetal serum protein, is not essential for embryonic development but is required for female fertility

“…Among the various properties ascribed to AFP, its capacity to bind estrogen (30,34) is obviously relevant to the observed phenotype. Early exposure to estrogen results in defeminization of female animals (such as female rats), characterized by anovulatory sterility associated with altered neuroendocrine production of gonadotropin (13,35,36).…”

“…The number of each sample tested for each genotype is indicated in parentheses. siveness of the immature mouse uterus to estrogens (30) and is present in significant amounts (Ͼ0.1 mg͞ml) in the circulation of young mice up to the age of 2-3 weeks (31). Hyperstimulation of the uterus in the absence of AFP is thus not surprising and likely makes the uterus of mutant mice incompatible for embryo implantation.…”

Alpha-fetoprotein can regulate growth in the uterus of the immature and adult ovariectomized mouse

“…This inhibitory effect of oestradiol-activated AFP cannot be explained simply by the ability of this molecule to bind to and sequester oestrogen (Raynaud, 1973), since a significant inhibitory effect was seen when the molar amount of AFP in the reaction mixture was less than 1% ofthe molar amount of oestradiol employed, and subsequent additional injections of oestradiol in an amount well in excess of that which AFP could sequester could not overcome the inhibition (Mizejewski et al, 1983). The effect is protein specific for AFP being non-reactive with albumin and transferrin and steroid-specific for oestradiol and oestrone but not moxoestrol.…”

“…AFP inhibits the oestrogen-stimulated uterine growth response in immature mice when the AFP is incubated with excessive amounts of oestradiol for 1 h before injection (Mizejewski et al, 1983(Mizejewski et al, , 1986). This inhibitory effect of oestradiol-activated AFP cannot be explained simply by the ability of this molecule to bind to and sequester oestrogen (Raynaud, 1973), since a significant inhibitory effect was seen when the molar amount of AFP in the reaction mixture was less than 1% ofthe molar amount of oestradiol employed, and subsequent additional injections of oestradiol in an amount well in excess of that which AFP could sequester could not overcome the inhibition (Mizejewski et al, 1983).…”

“…with µ of this oestradiol solution and/or protein mixtures and were returned to their original cage. The transient increase in uterine weight that this treatment evokes is maximal 20-24 h after the injection and is the result of cell proliferation as determined by mitotic indices (Mizejewski et al, 1983). For convenience, a standard 23-h interval was adopted.…”