Growth hormone can induce expression of four major milk protein genes in transfected MAC-T cells.
Growth hormone (GH) can increase milk production in cattle, and this effect was thought to be mediated by an indirect mechanism because traditional ligand binding assays failed to detect GH binding sites in the mammary gland. However, recent findings that GH receptor (GHR) mRNA and protein are expressed in the epithelial cells of the bovine mammary gland suggest that GH may directly act on these cells to affect milk production. Therefore, the objective of this study was to determine whether GH could affect milk protein gene expression, nutrient uptake, and cell proliferation in bovine mammary epithelial cells using the bovine mammary epithelial cell-derived MAC-T cells as a model. Native MAC-T cells had low expression of GHR. Thus, we transfected them with expression plasmids for GHR and signal transducer and activator of transcription 5 (STAT5), 2 key components of GHR signaling, to maximize their GH response. Growth hormone increased the expression of alphaS1-casein, alphaS2-casein, beta-casein, and alpha-lactalbumin mRNA 16- to 117-fold in the transfected MAC-T cells, whereas it had no effect on the expression of kappa-casein, beta-lactoglobulin, or insulin-like growth factor I mRNA. Cotransfection analyses showed that GH also strongly induced reporter gene expression from alphaS1-casein, alphaS2-casein, beta-casein, and alpha-lactalbumin gene promoters. Growth hormone had no effect on the uptake of 2-deoxyglucose, an unmetabolizable glucose analog, amino acids, or oleic acid; neither did it affect cell proliferation or death. These observations together with the fact that GH receptor mRNA and protein are expressed in the epithelial cells of the bovine mammary gland raise the possibility that GH might act directly on the mammary epithelial cells in cows to stimulate transcription of major milk protein genes, as part of the mechanism by which GH stimulates milk production.
Zhou
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Department of Animal and Poultry Sciences, Virginia Polytechnic Institute and State University, Blacksburg, 24061, USA.
Akers
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Jiang
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D000596:Amino Acids / Q000378:metabolism; D000818:Animals; D002417:Cattle / Q000502:physiology*; D049109:Cell Proliferation / Q000187:drug effects; D004247:DNA / Q000737:chemistry / Q000235:genetics; D003847:Deoxyglucose / Q000378:metabolism; D004847:Epithelial Cells; D005260:Female; D005786:Gene Expression Regulation / Q000502:physiology*; D013006:Growth Hormone / Q000096:biosynthesis / Q000235:genetics / Q000502:physiology*; D008321:Mammary Glands, Animal / Q000502:physiology*; D008894:Milk Proteins / Q000096:biosynthesis* / Q000235:genetics; D019301:Oleic Acid / Q000378:metabolism; D016133:Polymerase Chain Reaction; D011986:Receptors, Somatotropin / Q000502:physiology; D050799:STAT5 Transcription Factor / Q000502:physiology; D014162:Transfection / Q000662:veterinary
D000596:Amino Acids; D008894:Milk Proteins; D011986:Receptors, Somatotropin; D050799:STAT5 Transcription Factor; D019301:Oleic Acid; D013006:Growth Hormone; D004247:DNA; D003847:Deoxyglucose