CYTOKINES MODULATE GLUCOSE-TRANSPORT IN SKELETAL-MUSCLE BY INDUCING THE EXPRESSION OF INDUCIBLE NITRIC-OXIDE SYNTHASE

The principal goal of the present study was to test the hypothesis tha t cytokines modulate glucose transport in skeletal muscle by increasin g nitric oxide production. Cultured L6 skeletal muscle cells were incu bated in the presence of tumour necrosis factor-alpha, interferon-gamm a or lipopolysaccharide (LPS) alone or in combination for 24 h. Neithe r cytokines nor LPS alone induced NO production, as measured by nitrit e concentrations in the medium. However, when used in combination, the two cytokines significantly stimulated NO production, and this effect was synergistically enhanced by the presence of LPS. Reverse transcri ptase-PCR (RT-PCR) analysis revealed that NO release was associated wi th the induction of inducible (macrophage-type) NO synthase (iNOS). Th e increase in iNOS expression was confirmed at the protein level by We stern-blot analysis and NADPH/diaphorase histochemical staining. Cytok ines and LPS markedly increased basal glucose transport in L6 myocytes . Insulin also-stimulated basal glucose transport, but significantly l ess in cells chronically exposed to cytokines/LPS. The sensitivity of L6 muscle cells to insulin-stimulated glucose transport was also signi ficantly decreased by cytokines/LPS treatment, The NOS inhibitor N-G-n itro-L-arginine methyl ester (L-NAME) inhibited nitrite production in cytokine/LPS-treated cells, and this prevented the increase in basal g lucose transport and restored muscle cell responsiveness to insulin, C ytokines/LPS exposure significantly increased GLUT1 transporter protei n levels but decreased GLUT4 expression in L6 cells. L-NAME treatment prevented the increase in GLUT1 protein content but failed to restore GLUT4 transporter levels, These results demonstrate that cytokines and LPS affect glucose transport and insulin action by inducing iNOS expr ession and NO production in skeletal muscle cells. The data further in dicate that cytokines and LPS increase the expression of the GLUT1 tra nsporter protein by an NO-dependent mechanism.