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Insulin and Prolactin Synergistically Stimulate ß-Casein Messenger Ribonucleic Acid Translation by Cytoplasmic Polyadenylation

Department of Biochemistry and Molecular Biology (K.M.C., R.E.R.), Louisiana State University Health Sciences Center, Shreveport, Louisiana, 71130-3932; and Institute of Animal Science (I.B.), The Volcani Center, 50250 Bet-Degan, Israel

Address all correspondence and requests for reprints to: Robert E. Rhoads, Department of Biochemistry and Molecular Biology, Louisiana State University Health Sciences Center, Shreveport, Louisiana, 71130-3932. E-mail: rrhoad{at}lsuhsc.edu.

Previous studies have shown that the synthesis and stability of milk protein mRNAs are regulated by lactogenic hormones. We demonstrate here in cultured mouse mammary epithelial cells (CID 9) that insulin plus prolactin also synergistically increases the rate of milk protein mRNA translation. Insulin alone stimulates synthesis of both milk and nonmilk proteins, whereas prolactin alone has no effect, but insulin plus prolactin selectively stimulate synthesis of milk proteins more than insulin alone. The increase in ß-casein mRNA translation is also reflected in a shift to larger polysomes, indicating an effect on translational initiation. Inhibitors of the phosphatidylinositol 3-kinase, mammalian target of rapamycin, and MAPK pathways block insulin-stimulated total protein and ß-casein synthesis but not the synergistic stimulation. Conversely, cordycepin abolishes synergistic stimulation of protein synthesis without affecting insulin-stimulated translation. The poly(A) tract of ß-casein mRNA progressively increases from approximately 20 to about 200 A residues over 30 min of treatment with insulin plus prolactin. The 3'-untranslated region of ß-casein mRNA containing an unaltered cytoplasmic polyadenylation element is sufficient for the translational enhancement and mRNA-specific polyadenylation, based on transient transfection of cells with a reporter construct. Insulin and prolactin stimulate cytoplasmic polyadenylation element binding protein phosphorylation with no increase of cytoplasmic poly(A) polymerase activity.

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