Hormonal Regulation of Rapidly Labeled Ribonucleic Acid in Mammary Cells in Vitro

Insulin and prolactin stimulate the rate of incorporation of 3H-uridine into rapidly labeled nuclear RNA in mammary epithelial cells in organ culture. Since these hormones do not significantly alter the pool size and specific activity of uridine triphosphate, these effects appear to represent increased rates of RNA synthesis. These RNAs consist of 45 S and 32 S preribosomal RNA and heterogeneous, short lived RNA whose base composition is DNA-like. The hormonal stimulations result in an effective increase in the preribosomal RNA processed into 28 S and 18 S ribosomal RNA. The magnitude of prolactin's stimulation of RNA labeling is greater upon the ribosomal RNA than upon the preribosomal RNA, which suggests that prolactin may stimulate the ribosomal RNA-processing reactions. RNA synthesis was stimulated by prolactin only in daughter epithelial cells formed in vitro and previously treated with hydrocortisone, and this action of prolactin required the synergistic action of insulin. The stimulatory effect of prolactin on the synthesis of nuclear RNA was observed within 30 min, preceding the previously demonstrated increase in cytoplasmic ribosome concentration and the induction of specific milk proteins, and appears to represent an early effect of this hormone.