LOCALIZATION OF THE GLUT1 GLUCOSE TRANSPORTER TO BREFELDIN A-SENSITIVE VESICLES OF DIFFERENTIATED CIT3MOUSE MAMMARY EPITHELIAL CELLS

Authors

  • Peter M. Haney M.D., Ph.D.

    Corresponding author
    1. Section of Neonatology and USDA/ARS Children's Nutrition Research Center, Department of Pediatrics, Baylor College of Medicine, One Baylor Plaza, Houston, TX, 77030, U.S.A.
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To whom correspondence should be addressed: Peter M. Haney, M.D., Ph.D., Baylor College of Medicine, Children's Nutrition Research Center, Department of Pediatrics, 1100 Bates St, Houston, TX 77030. Tel.: (713)-798-7067; Fax: (713)-798-7057. email: phaney@neo.bcm.tmc.edu

Abstract

Glucose is a precursor of lactose, the major carbohydrate and osmotic constituent of human milk, which is synthesized in the Golgi. The GLUT1 glucose transporter is the only glucose transporter isoform expressed in the mammary gland. The hypothesis that lactogenic hormones induce GLUT1 and cause its localization to the Golgi of mammary epithelial cells was tested in CIT3mouse mammary epithelial cells. Treatment with prolactin and hydrocortisone caused a 15-fold induction of GLUT1 by Western blotting, but 2-deoxyglucose uptake decreased. Subcellular fractionation and density gradient centrifugation 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 microscopy revealed that lactogenic hormones alter GLUT1 targeting from a plasma membrane pattern to a predominant perinuclear distribution with punctate scattering through the cytoplasm. GLUT1 is targeted to a compartment which is more sensitive to Brefeldin A than the compartments in which GM130 and β-COP reside. Targeting of GLUT1 to endosomes was specifically excluded. We conclude that prolactin and hydrocortisone induce GLUT1, enhance GLUT1 glycosylation, and cause glycosylation-independent targeting of GLUT1 to Brefeldin A-sensitive vesicles which may represent a subcompartment of cis-Golgi. These results demonstrate a hormonally-regulated targeting mechanism for GLUT1 and are consistent with an important role for GLUT1 in the provision of substrate for lactose synthesis.

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