Beneficial synergistic interactions of TNF-alpha and IL-6 in C2 skeletal myoblasts--potential cross-talk with IGF system.

The interaction effects of tumour necrosis factor-alpha (TNF-alpha) and interlukin-6 (IL-6) on skeletal muscle proliferation and differentiation remains controversial. We therefore investigated the potential interactive effects of TNF-alpha and IL-6 on murine C2 skeletal myoblast survival, differentiation and proliferation. A novel and unexpected positive temporal interaction between TNF-alpha and IL-6 on cell growth was identified (90%), with maximal beneficial effects obtained in myoblasts treated with TNF-alpha (10 ng/ml) for 24 h prior to being dosed with IL-6 (2.5 ng/ml) for a further 24 h. This combined treatment significantly (p < 0.05) increased the level of total cellular protein (330%), extracellular signal-regulated kinase (ERK) phosphorylation (55%), and S-phase of cell cycle (2.5-fold), confirming cell growth. The expression of mRNAs of key regulators of muscle mass: insulin-like growth factor binding protein-5, insulin-like growth factor-II (IGF-II), IGF-I receptor (IGF-IR) and IGF-II receptor (IGF-IIR) were also significantly (p < 0.05) increased by 1600-, 1.6-, 27- and 6-fold, respectively, giving an indication of the regulatory mechanisms of this interaction. Moreover, in response to this treatment, the expression level of signal-transducing glycoprotein 130 (gp130) was induced up to 3.5-fold but not after either treatments alone. This may not only explain the beneficial effects of this treatment on skeletal myoblast numbers but also define a functional role of gp130 in skeletal muscle cells. Our data suggest that in the presence of TNF-alpha/IL-6 functions positively and potentially also cooperatively with the IGF system to achieve the maximal beneficial effect on skeletal myoblast numbers.