Martin, Sally; (1993) Glucose transport and its' regulation in rodent mammary gland. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Specific, anti-peptide antibodies against the five known mammalian facilitative D-glucose transporter isoforms (GLUT1-5) were used to investigate the transporter content of lactating rat mammary gland. Western blots of gland homogenate showed the apparent presence of GLUT1 and GLUT4. However, in isolated epithelial cells only GLUT1 was present, a result which indicated the mammary GLUT4 is present within adipocytes. The putative GLUT1 glucose transporter was recognised by antibodies against several hydrophilic regions of human erythrocyte GLUT1 and endoglycosidase F digestion decreased its apparent Mr from 50,000 to 42,000, a value essentially identical to that of the deglycosylated human protein. Its identity as a glucose transporter was confirmed by the ability of anti-GLUT1 antibodies to immunoprecipitate a mammary protein of apparent 50,000, which could be photolabelled in a D-glucose-sensitive fashion by the transport inhibitor [4-3H]cytochalasin B. Immunocytochemical staining of sections confirmed that the primary location of GLUT1 in the mammary gland is the epithelial cell. Sub-cellular fractionation experiments showed that the transporter is not only located at the cell surface, but also within the Golgi. However, quantitative Western blotting indicated that GLUT1 could only account for about half of the D-glucose-sensitive cytochalasin B binding sites in Golgi vesicles, suggesting the presence of a second, so far unidentified, glucose transporter isoform. Developmental changes in transporter expression in the gland were investigated both at the level of protein and mRNA. GLUT1 protein levels became detectable in the epithelial cells during the final 24-48hr of pregnancy, rose to a peak immediately following parturition, then fell slightly at the time of peak lactation. No major changes in GLUT1 mRNA were observed, implying a post-transcriptional control of GLUT1 expression. Removal of the litter for 24hr at peak lactation resulted in a total loss of GLUT1, suggesting feed-back inhibition influencing GLUT1 expression. The hormonal basis of these changes was investigated using explant culture of alveoli from mid-pregnant mammary gland. These experiments indicated that GLUT1 expression was dependent upon a lactogenic hormone, prolactin. Insulin and cortisol were also required for explant viability, but neither alone could stimulate synthesis of GLUT1.

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