Effects of tumor necrosis factor-alpha on insulin action in cultured humanmuscle cells

Reported discrepancies in the effects of tumor necrosis factor (TNF)-alpha in modulating insulin sensitivity of cultured cells may relate both to cell types studied and to the time course of exposure to the cytokine, Addition ally, the relationship of effects on glucose metabolism to changes in the i nsulin signaling pathway cannot be assumed, For in vitro study, the cell ty pe most relevant to insulin resistance in humans is the cultured human musc le cell, In the present study, TNF brought about no change in the rate of g lycogen synthesis in cultured human muscle cells unless present during diff erentiation. The presence of TNF (5 ng/ml) during the process of differenti ation of myoblasts into mature myotubes diminished the response of glycogen synthesis to acute insulin stimulation. This finding was associated with a n impairment of differentiation-dependent increases in total cellular glyco gen synthase (GS) activity. Under the same conditions of TNF exposure, ther e was no effect on the response to acute insulin stimulation of the fractio nal activity of GS, Similarly, there was no effect on the insulin stimulati on of protein kinase B (PI(B) and inhibition of glycogen synthase kinase 3 (GSK-3), Acute insulin stimulation brought about a 4.08 +/- 0.44-fold stimu lation of activity of PKB in the absence of TNF, with 4.81 +/- 0.70-fold st imulation in cells exposed to TNF. GSK-3 activity decreased to 74.0 +/- 5.8 % of basal after insulin stimulation without TNF and 78.3 +/- 5.0% after TN F exposure. However, differentiation of myocytes, as defined by an increase in the acetylcholine receptor, myogenin, and mature creatine kinase isofor m expression, was impaired in TNF-treated cells, These studies demonstrate that TNF, if present during differentiation, decreases insulin-stimulated r ates of storage of glucose as glycogen and total GS activity but does not d ownregulated the insulin-signaling system to GS, More generally, TNF also i nhibits differentiation of human muscle cells in culture.