Effects of tumour necrosis factor-alpha and inhibition of protein kinase Con glucose uptake in L6 myoblasts

Clinical and experimental stud ies have implicated high circulating levels of the cytokine tumour necrosis factor-alpha (TNF-alpha) in the pathogenesi s of insulin resistance, not only in obesity and diabetes, but also in clin ical conditions associated with cachexia and sepsis. TNF-alpha impairs insu lin-mediated glucose uptake in adipocytes, but because of lipolytic effects the interpretation of clinical stud ies and the extent to which TNF-alpha affects muscle insulin sensitivity are unclear. In addition, protein kinase C (PKC) has recently been implicated in the mechanism of TNF-alpha -induce d insulin resistance. The present study investigated the effects of TNF-alp ha and a PKC inhibitor (RO-318220) on basal and insulin-stimulated 2-[H-3]d eoxyglucose uptake in cultured L6 myoblasts. Reverse transcriptase-PCR anal ysis confirmed that L6 myoblasts express TNF-alpha receptors I and II (p60 and p80). Dose-response curves for glucose uptake were fitted to a quadrati c function to derive C1-150 values (concentration of insulin required to in crease glucose uptake by 50%). Incubation with TNF-alpha at 1 or 10 ng/ml f or 24 h had no significant effect on basal glucose uptake, insulin sensitiv ity or maximal insulin responsiveness. C1-150 values (means +/- S.E.M.) wer e as follows: basal, 91.2 +/- 13 nM; 1 ng/ml TNF-alpha, 102 +/- 12 nM; and basal, 70.8 +/- 13 nM; 10 ng/ml TNF-alpha, 43.7 +/- 40 nM. PKC inhibition m arkedly attenuated glucose uptake, but there was no difference in insulin s ensitivity with RO-318220 alone compared with RO-318229 + TNF-alpha. In con clusion, although increased TNF-alpha. expression and plasma concentrations have been implicated in the pathogenesis of insulin resistance in various clinical states, there is no evidence that TNF-alpha impairs insulin-stimul ated glucose uptake in a skeletal-muscle-derived cell line.