Inhibition of Mammary Gland Differentiation in Vitro by 5-Bromo-2'-deoxyuridine

Mammary epithelial cells in organ culture incorporate 5-bromo-2'-deoxyuridine (BUdR) into DNA during a single cycle of DNA replication. The specific milk proteins, casein and α-lactalbumin, are induced by prolactin in control mammary cells, but the induction of these proteins is markedly inhibited in cells which contain BUdR-DNA. This is a selective effect, since the induction of the galactosyltransferase of lactose synthetase is not inhibited, and the rate of total protein synthesis is only slightly decreased in such cells. Incubation of mammary cells with BUdR after completion of DNA synthesis did not inhibit the induction of casein or α-lactalbumin. Thus BUdR appears to selectively inhibit the expression of genes which are uniquely expressed in differentiated mammary secretory cells. Formation of transfer RNA, total rapidly labeled nuclear RNA, and ribosomal RNA was unaltered in cells containing BUdR-DNA. However, a marked reduction of prolactin-stimulated polysome formation was observed. Initiation of casein synthesis in these cells has been previously shown to be dependent upon the formation of prolactin-induced polysomes. BUdR inhibited milk protein induction by more than 90%. This result suggests that only mammary daughter cells containing the newly synthesized sense strand of DNA may express differentiated function.