Author
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HANEY, PETER - BAYLOR COLL OF MEDICINE |
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Submitted to: Cell Biology International
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 9/3/2000 Publication Date: N/A Citation: N/A Interpretive Summary: Lactose is the main carbohydrate present in milk. Lactose synthesis takes place in the Golgi complex, a network of vesicles inside cells that functions in making proteins and carbohydrates. In order for lactose to be made in mammary epithelial (lining) cells, glucose, a sugar, must be transported into the Golgi. The amount of lactose that is synthesized determines the amount of milk produced by the lactating human mammary gland. The purpose of this study was to test the hypothesis that two hormones, called prolactin and hydrocortisone, cause a glucose transporter called GLUT1 to be targeted to the Golgi complex in cultured mouse mammary epithelial cells. Our results showed that this glucose transporter is, in fact, targeted to the Golgi under the influence of these two hormones which induce lactation. The results suggest that GLUT1 has an important role in the delivery of glucose to the Golgi, in the regulation of lactose synthesis, and in the control of milk production. These scientific findings provide important new information to be contributed to the field of human nutrition. Technical Abstract: The synthesis of lactose, the major carbohydrate and osmotic constituent of human milk, takes place in the Golgi. Therefore, free glucose must be transported into Golgi. The GLUT1 glucose transporter is the only isoform of the facilitated diffusion family of glucose transporters expressed in mammary gland. The hypothesis that lactogenic hormones induce GLUT1 and cause its localization to the Golgi of mammary epithelial cells was tested. CIT**3** mouse mammary epithelial cells treated with prolactin and hydrocortisone showed a 15-fold induction of GLUT1 by Western blotting. Subcellular fractionation demonstrated enrichment of Golgi fractions with GLUT1. Lactogenic hormones enhanced GLUT1 glycosylation, but did not determine whether GLUT1 was targeted to plasma membrane or to Golgi. Confocal immunofluorescent microscopy revealed that lactogenic hormones alter GLUT1 targeting from a plasma membrane pattern to a perinuclear distribution with a punctate pattern scattered through the cytoplasm, characteristic of Golgi. Brefeldin A caused disruption of the trans-Golgi but did not grossly affect targeting of GLUT1, alpha-mannosidase II, or beta-COP. Therefore, in mammary epithelial cells, prolactin and hydrocortisone induce GLUT1, enhance GLUT1 glycosylation and cause glycosylation-independent targeting of GLUT1 to a a subcompartment of cis- and/or medial-Golgi, but not to trans-Golgi. These results suggest a tissue- and developmental stage-specific mechanism for Golgi targeting of GLUT1, and indicate an important role for GLUT1 in the provision of substrate for lactose synthesis. |
