Abstract
The proinflammatory cytokine Tumour Necrosis Factor (TNF)-α is implicated in a variety of skeletal muscle pathologies. Here, we have investigated how in vitro cotreatment of skeletal muscle C2C12 cells with β-agonists modulates the TNF-α-induced inflammatory program. We observed that C2C12 myotubes express functional TNF receptor 1 (TNF-R1) and β2-adrenoreceptors (β2-ARs). TNF-α activated the canonical Nuclear Factor-κB (NF-κB) pathway and Mitogen-Activated Protein Kinases (MAPKs), culminating in potent induction of NF-κB-dependent proinflammatory genes. Cotreatment with the β-agonist isoproterenol potentiated the expression of inflammatory mediators, including Interleukin-6 (IL-6) and several chemokines. The enhanced production of chemotactic factors upon TNF-α/isoproterenol cotreatment was also suggested by the results from migrational analysis. Whereas we could not explain our observations by cytoplasmic crosstalk, we found that TNF-R1-and β2-AR-induced signalling cascades cooperate in the nucleus. Using the IL-6 promoter as a model, we demonstrated that TNF-α/isoproterenol cotreatment provoked phosphorylation of histone H3 at serine 10, concomitant with enhanced promoter accessibility and recruitment of the NF-κB p65 subunit, cAMP-response element-binding protein (CREB), CREB-binding protein (CBP) and RNA polymerase II. In summary, we show that β-agonists potentiate TNF-α action, via nuclear crosstalk, that promotes chromatin relaxation at selected gene promoters. Our data warrant further study into the mode of action of β-agonists and urge for caution in their use as therapeutic agents for muscular disorders.
Highlights
Skeletal muscle atrophy is a devastating consequence of a large number of diseases, including cancer and myopathies, but is apparent in physiological processes, such as aging or disuse
We did observe that multiple high-molecular weight immunoreactive bands appear upon myogenic differentiation of C2C12 cells (Figure 1B)
Whereas we observed increased Tumour Necrosis Factor (TNF)-R1 expression at the mRNA level, this was not reflected at the protein level
Summary
Skeletal muscle atrophy is a devastating consequence of a large number of diseases, including cancer and myopathies, but is apparent in physiological processes, such as aging or disuse. NF-kB is the generic term for members of a family of ubiquitously expressed transcription factors, that act as homo- or heterodimers to regulate genes involved in immunity and inflammation [4]. Whereas NF-kB function has been mainly studied in immune cells, recent reports have demonstrated a role for NF-kB in a variety of other cell types, including skeletal muscle. It was shown that interference with NF-kB activity, via overexpression of IkB supersuppressor or p65 knock-out, reduces inflammation and improves the regeneration process in different skeletal muscle disease models [1,5]
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