TRAFFICKING OF GLUCOSE TRANSPORTERS - SIGNALS AND MECHANISMS

The uptake of glucose into mammalian cells, catalysed by members of th e GLUT family of glucose transporters, is regulated by a variety of ho rmones, growth factors and other agents. In adipocytes, skeletal muscl e and heart the principal regulator is the hormone insulin, which rapi dly stimulates glucose uptake by bringing about the translocation of t he GLUT4 glucose transporter isoform from an intracellular vesicular c ompartment to the cell surface. Recent studies have implicated the C-t erminal hydrophilic region of this protein as being primarily responsi ble for its insulin-regulated trafficking. In an attempt to identify t he protein machinery involved in this trafficking, we have used glutat hione S-transferase fusion proteins bearing hydrophilic domains of var ious GLUT transporters in affinity purification experiments on deterge nt-solubilized extracts of 3T3-L1 adipocyte intracellular membranes. T he C-terminal region of GLUT4 was found specifically to bind a number of polypeptides in these extracts, which are therefore candidates for components of the trafficking machinery. Although these proteins did n ot bind to the corresponding region of the more widely-distributed GLU T1 glucose transporter isoform, regulation of this transporter also ap pears to be of physiological importance in some cell types. To study s uch regulation we have used as a model system the interleukin-3 (IL-3) -dependent haemopoietic cell line IC.DP. These cells express a tempera ture-sensitive mutane of the upsilon-abl tyrosine kinase, whose activa tion at the permissive temperature permits cell survival in the absenc e of IL-3 by suppression of apoptosis, although the growth factor is s till required for proliferation. Both IL-3 and activation of the kinas e were found to stimulate glucose transport by promoting the transloca tion of GLUT1 to the cell surface. Moreover, inhibition of glucose upt ake by addition of transport inhibitors markedly increased the rate of apoptosis, an effect which could be reversed by the provision of alte rnative energy sources. These observations suggest that the traffickin g of GLUT1, regulated by growth factors or oncogenes, may play an impo rtant role in the suppression of apoptosis in haemopoietic cells.