Multifaceted roles of TNF‐α in myoblast destruction: A multitude of signal transduction pathways

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Abstract
In catabolic conditions, such as cancer cachexia, a balance favouring a cytokine environment culminates in muscle destruction. Utilising an in vitro model to mimic muscle wasting, we elucidate here the multifaceted roles that one such cytokine, TNF‐α, invokes in the degeneration process. Treatment of C2 skeletal myoblasts with TNF‐α not only suppresses morphological and biochemical differentiation, but following an initial wave of proliferation, and of survival (24 h), induces apoptosis. Investigating the mechanisms underlying these diverse actions of TNF‐α, we demonstrate that cell replication is dependent on rapid and sustained activation of MAP kinase. Map kinase is not, however, central to the death process, which is associated with a progressive rise in caspase‐8 activity, and is accompanied by sustained activation of JNK1 and transient activation of JNK2. Caspase inhibition caused a dose responsive reduction in cell death, while inhibition of the JNKs caused a significant increase in apoptosis. We further report that PI3 kinase is not involved in conferring early protection against TNF‐α‐induced death. By contrast, inhibition of NF‐κB in the presence of TNF‐α culminates in increased cell cycle progression, decreased gadd45β expression and significant and precociously increased cell death, when compared with TNF‐α alone. Our results begin to characterise the mechanisms underlying the acute mitogenic and anti‐apoptotic roles of TNF‐α, which appear to be defined by a balance between MAP kinase, Jun kinase (JNK), NF‐κB and gadd45β. They establish that inhibition of any one of these molecules, as may occur following caspase activation, could eliminate vital stem cells required for skeletal muscle regeneration during chronic catabolic conditions. J. Cell. Physiol. 198: 237–247, 2004