Hyperglycaemia-induced subcellular redistribution of GLUT1 glucose transporters in cultured human term placental trophoblast cells

We have recently shown that hyperglycaemia down-regulates the GLUT1 glucose transport system of term placental trophoblast. The reduction in GLUT1 protein alone was, however, not sufficient to explain the decrease in net glucose uptake, suggesting additional mechanisms. Therefore, we hypothesised that hyperglycaemia in vitro leads to a GLUT1 translocation from the trophoblast surface to intracellular sites. This was tested in our study by determining the subcellular distribution of GLUT1 in human term placental trophoblast (n = 5 placentas) cultured for 48 h with 5 compared with 25 mmol/l D-glucose in vitro using immunogold labelling. Electron microscopic examination of cell profiles showed that 73% of total GLUT1 molecules reside in the trophoblast plasma membrane under basal conditions. The reduced GLUT1 expression (-20%; p < 0.05) after culture of the cells with 25 mmol/l glucose was accompanied by an internalisation of plasma membrane GLUT1, resulting in a loss of 40% (p < 0.05) in cell surface transporter labelling. Western blotting identified a characteristically broad band between 55-65 kDa, confirming the specificity of the GLUT1 antiserum. We postulate that in addition to down-regulating human GLUT1 protein concentrations, glucose exerts its autoregulatory effect on hexose transport in term placental trophoblast by altering GLUT1 partitioning between the plasma membrane and intracellular sites in favour of the latter.