Insulin and prolactin synergistically stimulate beta-casein messenger ribonucleic acid translation by cytoplasmic polyadenylation.
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 beta-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 beta-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 beta-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 beta-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.
Choi
Kyoung Moo
KM
Department of Biochemistry and Molecular Biology, Louisiana State University Health Sciences Center, Shreveport, Louisiana, 71130-3932, USA.
Barash
Itamar
I
Rhoads
Robert E
RE
D020413:3' Untranslated Regions; D048868:Adaptor Proteins, Signal Transducing; D000818:Animals; D002352:Carrier Proteins / Q000187:drug effects / Q000378:metabolism; D002364:Caseins / Q000187:drug effects / Q000235:genetics* / Q000378:metabolism; D018797:Cell Cycle Proteins; D002478:Cells, Cultured; D003593:Cytoplasm / Q000187:drug effects / Q000235:genetics* / Q000378:metabolism; D004259:DNA-Directed DNA Polymerase / Q000187:drug effects / Q000378:metabolism; D004357:Drug Synergism; D004847:Epithelial Cells / Q000187:drug effects / Q000378:metabolism; D039642:Eukaryotic Initiation Factors; D005260:Female; D024601:Glyceraldehyde 3-Phosphate Dehydrogenase (NADP+) / Q000187:drug effects / Q000235:genetics; D007328:Insulin / Q000378:metabolism / Q000494:pharmacology*; D008321:Mammary Glands, Animal / Q000166:cytology / Q000187:drug effects; D051379:Mice; D008894:Milk Proteins / Q000096:biosynthesis / Q000187:drug effects; D010750:Phosphoproteins / Q000187:drug effects / Q000378:metabolism; D026723:Polyadenylation; D011132:Polyribosomes / Q000378:metabolism; D011388:Prolactin / Q000378:metabolism / Q000494:pharmacology*; D014176:Protein Biosynthesis / Q000187:drug effects*; D012333:RNA, Messenger / Q000187:drug effects / Q000378:metabolism; D016601:RNA-Binding Proteins / Q000187:drug effects / Q000378:metabolism; D015398:Signal Transduction; D014157:Transcription Factors; D039221:mRNA Cleavage and Polyadenylation Factors
D020413:3' Untranslated Regions; D048868:Adaptor Proteins, Signal Transducing; D002352:Carrier Proteins; D002364:Caseins; D018797:Cell Cycle Proteins; D004259:DNA-Directed DNA Polymerase; D039642:Eukaryotic Initiation Factors; D024601:Glyceraldehyde 3-Phosphate Dehydrogenase (NADP+); D007328:Insulin; D008894:Milk Proteins; D010750:Phosphoproteins; D011388:Prolactin; D012333:RNA, Messenger; D016601:RNA-Binding Proteins; C499527:Eif4ebp1 protein, mouse; C490387:Cpeb1 protein, mouse; D014157:Transcription Factors; D039221:mRNA Cleavage and Polyadenylation Factors