Age-dependent growth inhibition of estrogen-sensitive rat mammary tumors. Probable role of alpha-fetoprotein.

The growth patterns of estrogen (E)-sensitive mammary tumors (SNMU) and autonomous mammary tumors (ANMU) were compared in normal host F344 and BUF rats of different ages. Both SNMU and ANMU tumors have comparable amounts of E receptors of similar physicochemical properties. The E sensitivity was apparent in SNMU tumors regardless of the age of inoculation. The latency period before tumors became palpable was considerably lengthened when the SNMU tumors were inoculated into newborn hosts. The growth pattern of the ANMU tumor was not affected by the age or sex of the host. Comparable results previously obtained with two hypophyseal cell lines suggest that this growth pattern could now be extended to other E-sensitive autonomous systems and become a generalized principle. These results suggest that: 1) the presence of 17 beta-estradiol (E2) receptors is not sufficient to qualify cells to be considered E-sensitive, 2) E-sensitive cells may be delayed in expressing their malignant growth properties if inoculated during the perinatal period, and 3) alpha-fetoprotein may be the substance responsible for the inhibition of the growth of E-sensitive tumor cells during the perinatal stage.