Insulin‐like growth factors (IGF‐I and IGF‐II) inhibit C2 skeletal myoblast differentiation and enhance TNFα‐induced apoptosis

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Abstract
IGF-I and IGF-II are thought to be unique in their ability to promote muscle cell differentiation. Murine C2 myoblasts differentiate when placed into low serum media (LSM), accompanied by increased IGF-II and IGF binding protein-5 (IGFBP-5) production. Addition of 20 ng/ml TNFα on transfer into LSM blocked differentiation, IGF-II and IGFBP-5 secretion and induced apoptosis. We, therefore, wished to assess whether IGFs could protect against the effects of TNFα. Neither inhibition of differentiation or induction of apoptosis was rescued by co-incubation with IGF-I or IGF-II. A lower dose of TNFα (1 ng/ml) while not inducing apoptosis still inhibited myoblast differentiation by 56% ± 12, (P < 0.001), indicating that induction of apoptosis is not the sole mechanism by which TNFα inhibits myoblast differentiation. Addition of IGF-I or IGF-II alone reduced differentiation by 49% ± 15 and 33% ± 20, respectively, (P < 0.001), although neither induced apoptosis. For muscle cells to differentiate, they must arrest in G0. We established that addition of IGF-I, IGF-II or TNFα to the myoblasts promoted proliferation. The myoblasts could not exit the cell cycle as efficiently as controls and differentiation was thus reduced. Unexpectedly, co-incubation of IGF-I or IGF-II with 1 ng/ml TNFα enhanced the inhibition of differentiation and induced apoptosis. In the absence of apoptosis we show an association between IGF-induced inhibition of differentiation and increased IGFBP-5 secretion. These results indicate that the effects of the IGFs on muscle may depend on the cytokine environment. In the absence of TNFα, the IGFs delay differentiation and promote myoblast proliferation whereas in the presence of TNFα the IGFs induce apoptosis.

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