Alteration in gene expression at the onset of hormone-dependent mammary tumor regression.

Two hormone-dependent mammary carcinomas regressing after hormone removal, [one primary 7,12-dimethylbenz(alpha)-anthracene induced, and the other transplanted MTW9] were used as models to test the hypothesis that the regression of hormone-dependent mammary tumors is triggered by a modification of the genetic expression. In support of the hypothesis, we observed that: (1) the in vitro translation products of poly(A)+ RNAs differ between growing and regressing tumors; (b) the difference is localized in four protein bands (mr 53,000, 35,000, 22,000 and 20,000) of the electrophoretic pattern that decrease in their concentrations and one protein band (Mr 20,500) that increases during regression; (c) the changes in the translation products occur within 6 hr post-hormone removal, and the changes are reversed when resumption of tumor growth was induced by replenishment of the hormone to the host rats: and (d) three autonomously growing tumors exhibit a unique translation pattern which differs from that of hormone-dependent tumors, and the pattern does not change upon hormone removal. The results suggest that hormonal regulation of mammary tumor growth is related to specific genetic transcripts, and loss of hormone dependence of growth may correlate with the changes in this genetic control.