Estrogen induction of progestophilins in rat estrogen-sensitive cells grown in media supplemented with sera from castrated rats and from rats bearing an α-fetoprotein-secreting hepatoma

The purpose of this work was to study the effect of α-fetoprotein (AFP) over cell multiplication and the induction of an estradiol-17β (E₂)-dependent marker, i.e., progestophilins in E-sensitive cells C₂9RAP derived from a W/Fu rat pituitary tumor. These cells proliferate in isogeneic hosts under the influence of E₂, while they proliferate in culture regardless of the presence of E₂. C₂9RAP cells were grown in medium supplemented with 10% horse serum. Progestophilin levels were measured 48 h after adding serum (20% horse, or castrated rat, or AFP-secreting tumor-bearing rat) and estrogen to the 10% horse serum-supplemented medium in which the cells were growing. Maximal induction of progestophilins was obtained at 3 × 10⁻¹⁰ M E₂ in cells grown in medium containing horse or castrated rat serum. In contrast, maximal induction of progestophilins required 3 × 10⁻⁸ M E₂ in cells grown in medium supplemented with the serum of Morris hepatoma 7777-bearing rats. This serum contained AFP levels comparable to those present at birth in the rat. 11-Methoxy-17β ethynylestradiol (R₂₈₅₈), a synthetic estrogen with little affinity for AFP, was also tested for its ability to induce progestophilins. The degree of maximal induction of progestophilins expressed as percentage of the respective control, was similar for all experimental groups, both with E₂ and with R₂₈₅₈.In addition, we compared the free E₂ levels in the culture medium with the progestophilin levels and the cell proliferation rate. We found that the progestophilin levels were maximal at free E₂ concentrations above 11 pg E₂/ml, whereas there was no correlation between the free E₂ levels and the proliferation rate. Moreover, the proliferation rate of cells in medium supplemented with horse or castrated rat serum was maximal at concentrations of free E₂ below 0.4 pg/ml, whereas cell proliferation was inhibited with hepatoma serum even at concentrations of free E₂ of 44 pg/ml. We conclude that the effect of hepatoma serum on the E₂ induction of progestophilins seems to be mediated by the effect of AFP on the availability of free estrogen, since it is abolished by the addition of both natural and synthetic estrogens. The inhibitory effect of hepatoma serum upon cell proliferation is not reversed by estrogens and thus seems to be mediated by mechanisms other than E₂ trapping by AFP.