Inhibition of myoblast differentiation by tumor necrosis factor α is mediated by c-Jun N-terminal kinase 1 and leukemia inhibitory factor

The proinflammatory cytokine, TNF alpha plays a major role in muscle wasting occurring in chronic diseases and muscular dystrophies. Among its other functions, TNF alpha perturbs muscle regeneration by preventing satellite cell differentiation. In the present study, the role of c-Jun N-terminal kinase (JNK), a mediator of TNF alpha, was investigated in differentiating myoblast cell lines. Addition of TNF alpha to C2 myoblasts induced immediate and delayed phases of JNK activity. The delayed phase is associated with myoblast proliferation. Inhibition of JNK activity prevented proliferation and restored differentiation to TNF alpha-treated myoblasts. Studies with cell lines expressing MyoD: ER chimera and lacking JNK1 or JNK2 genes indicate that JNK1 activity mediates the effects of TNF alpha on myoblast proliferation and differentiation. TNF alpha does not induce proliferation or inhibit differentiation of JNK1-null myoblasts. However, differentiation of JNK1-null myoblasts is inhibited when they are grown in conditioned medium derived from cell lines affected by TNF alpha. We investigated the induced synthesis of several candidate growth factors and cytokines following treatment with TNF alpha. Expression of IL-6 and leukemia inhibitory factor (LIF) was induced by TNF alpha in wildtype and JNK2-null myoblasts. However, LIF expression was not induced by TNF alpha in JNK1-null myoblasts. Addition of LIF to the growth medium of JNK1-null myoblasts prevented their differentiation. Moreover, LIF-neutralizing antibodies added to the medium of C2 myoblasts prevented inhibition of differentiation mediated by TNF alpha. Hence, TNF alpha promotes myoblast proliferation through JNK1 and prevents myoblast differentiation through JNK1-mediated secretion of LIF.