ACYLATION-STIMULATING PROTEIN (ASP) REGULATES GLUCOSE-TRANSPORT IN THE RAT L6 MUSCLE-CELL LINE

Acylation-stimulating protein (ASP), a human plasma protein, is a pote nt stimulator of triglyceride synthesis and glucose transport in both human adipocytes and fibroblasts. The purpose of the present in vitro study was to examine the effect of ASP on glucose transport in muscle cells. ASP stimulated 2-deoxy-glucose transport (2-DG) in differentiat ed rat L6 myotubes in a time (30 min to 24 h) and concentration depend ent manner (97% increase). The magnitude of the ASP effect on glucose transport was comparable to the time- and concentration-dependent effe cts seen with insulin (125% increase), but was additive to insulin, po inting to involvement of differential signalling pathway. ASP stimulat ion was dependent on cell differentiation in that glucose transport in creased by only 12% in myoblasts, comparable to the effect of insulin in myoblasts (15% increase) demonstrating selective responsiveness of the differentiated myotubes to ASP and insulin. The mechanism for the ASP induced increase in glucose transport was also examined, ASP incre ased the V-max for 2-DG transport by 183% (4.02 vs. 2.42 nmol/mg cell protein/30 s; ASP vs. Control, respectively). This could be explained by an increased translocation of glucose transporters (GLUT 1, GLUT 4 and GLUT 3) to the plasma membrane surface as demonstrated by Western analysis (+43% P < 0.05, + 30% P < 0.05, and +49% P < 0.05, respective ly). The effects of ASP were equal to those of insulin (+47%, +26% and +53% for GLUT 1, GLUT 4 and GLUT 3, respectively) and in all cases we re paralleled by comparable glucose transport increases under the same incubation conditions. After long-term stimulation (24 h), Western an alysis indicated that ASP had a permissive effect on insulin stimulate d increases in total GLUT3 and GLUT4 cellular transporter content. The se results suggest that muscle is also responsive to ASP and that ASP may play a role in glucose metabolism in both muscle and adipose tissu e.