MULTIPLE ROLES OF PHOSPHATIDYLINOSITOL 3-KINASE IN REGULATION OF GLUCOSE-TRANSPORT, AMINO-ACID-TRANSPORT, AND GLUCOSE TRANSPORTERS IN L6 SKELETAL-MUSCLE CELLS

Phosphatidylinositol 3-kinase (PI3k) activity is required for the insu lin stimulation of glucose transport in adipocytes and Chinese hamster ovary cells. Wortmannin (WM), an inhibitor of PI3k, inhibits the stim ulation of glucose transport by insulin and the gain of glucose transp orters at the cell surface. However, the effect of inhibition of PI3k on the maintenance of the basal and the insulin-stimulated glucose tra nsport and on the intracellular donor pool of glucose transporters has not been clarified. Here we show that in L6 skeletal muscle cells in culture WM significantly inhibits the basal PI3k activity (by 40%), de creases the levels of phosphatidylinositol 3,4-phosphate and 3,4,5-pho sphate (by about 50%) and abolishes the activation of the enzyme by in sulin. WM inhibited the basal rate of transport of glucose (by 45%) an d of amino acids through system A (by 25%) and abolished their stimula tion by insulin. Insulin caused a transient increase in PI3k activity and PI3k products that returned to basal levels within 40 min, whereas glucose and amino acid transport remained elevated. Under these condi tions, WM reduced the rate of glucose and amino acid transport back to basal levels. In unstimulated cells, WM decreased significantly the G LUT4 glucose transporter content at the plasma membrane and prevented the ability of insulin to recruit transporters to this membrane. Inter estingly, the intracellular pools of the GLUT3 and GLUT4 glucose trans porters were significantly reduced in response to WM treatment alone. We conclude that in muscle cells PI3k activity is required to maintain basal and insulin-stimulated glucose and amino acid transport, as wel l as to develop the stimulation of the two transport processes in resp onse to the hormone. We hypothesize that PI3k, likely through producti on of phosphatidylinositol 3,4-phosphate and 3,4,5-phosphate, regulate s the basal plasma membrane glucose transporter recycling and the orga nization of the transporter intracellular pool, in addition to being a n insulin signal.