Multiple roles of phosphatidylinositol 3-kinase in regulation of glucose transport, amino acid transport, and glucose transporters in L6 skeletal muscle cells
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Phosphatidylinositol 3-kinase (PI3k) activity is required for the insulin stimulation of glucose transport in adipocytes and Chinese hamster ovary cells. Wortmannin (WM), an inhibitor of PI3k, inhibits the stimulation of glucose transport by insulin and the gain of glucose transporters at the cell surface. However, the effect of inhibition of PI3k on the maintenance of the basal and the insulin-stimulated glucose transport 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%), decreases the levels of phosphatidylinositol 3,4-phosphate and 3,4,5-phosphate (by about 50%) and abolishes the activation of the enzyme by insulin. WM inhibited the basal rate of transport of glucose (by 45%) and of amino acids through system A (by 25%) and abolished their stimulation 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 conditions, WM reduced the rate of glucose and amino acid transport back to basal levels. In unstimulated cells, WM decreased significantly the GLUT4 glucose transporter content at the plasma membrane and prevented the ability of insulin to recruit transporters to this membrane. Interestingly, the intracellular pools of the GLUT3 and GLUT4 glucose transporters 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 well as to develop the stimulation of the two transport processes in response to the hormone. We hypothesize that PI3k, likely through production of phosphatidylinositol 3,4-phosphate and 3,4,5-phosphate, regulates the basal plasma membrane glucose transporter recycling and the organization of the transporter intracellular pool, in addition to being an insulin signal.