Growth inhibition of estrogen-sensitive rat mammary tumors. Effect of an alpha-fetoprotein-secreting hepatoma.

The growth and regression patterns of estrogen (E)-sensitive mammary tumors (SNMU) and autonomous mammary tumors (ANMU) were studied in normal, castrated, and alpha-fetoprotein (AFP)-secreting and nonsecreting hepatoma-bearing female BUF rats. Both SNMU and ANMU tumors had comparable amounts of E receptor. SNMU tumors grew faster in female hosts than in male hosts. The latency period of SNMU tumors was lengthened when the tumor was inoculated into newborn hosts. These E-sensitive tumors regressed after castration of the adult hosts. Tumor regression was also observed in SNMU-bearing rats inoculated with AFP-secreting hepatomas. The growth pattern of the ANMU tumor was not affected by the sex or age of the host. Castration and inoculation of hepatomas to ANMU tumor-bearing rats did not result in regression of the ANMU tumors. We suggest that AFP inhibited the growth of E-sensitive cells in newborn and hepatoma 7777-bearing hosts. This inhibitory effect of AFP could not be attributed to a "functional castration" resulting from the trapping of 17 beta-estradiol (E2) by AFP, because the plasma levels of E2 in these animals affect levels of gonadotropins. Moreover, castration of hepatoma 7777-bearing rats resulted in increased plasma follicle-stimulating hormone and luteinizing hormones levels. The results strongly suggested that a) the malignant properties of E2-sensitive cells in adult hosts can be controlled by AFP, and b) the mechanism by which AFP inhibits the growth of E2-sensitive cells is independent of the E2 "trapping" phenomenon. An ontogenetic approach to the mechanism of action of estrogens may account for available results.