TRANSCRIPTIONAL ACTIVATION OF THE C-MYC ONCOGENE IN THE T-47D MAMMARY-CANCER CELLS BY CONDITIONED MEDIA FROM EMBRYONIC MOUSE FIBROBLASTS (BALB C-3T3) - EFFECT OF THE ANTIESTROGEN ICI-164,384/

The effects of the conditioned medium (CM) of embryonic mouse fibrobla st (BALB/c-3T3) cells (clone A31) on the mRNA of the c-myc oncogene of the hormone-dependent T-47D mammary cancel cells were explored. After 6h incubation a significant stimulation of c-myc expression was obser ved with a 10% v/v concentration of CM, and the effect was even higher using a CM concentration of 30% v/v. It is suggested that this action of CM could be related to the stimulatory effect on proliferation and DNA synthesis induced by the CM, as was recently demonstrated for the T-47D cells. It was also observed that the antiestrogen ICI 164,384 ( 5 x 10(-6)M) can inhibit the stimulatory action of CM on c-myc express ion. It is concluded that the fibroblast mouse embryonic cells contain and secrete factor(s) involved in the mechanism of proliferation of h uman hormone-dependent mammary cancer cells.