Lipopolysaccharide‐induced transcriptional upregulation of interleukin‐6 and tumor necrosis factor‐α precedes muscle specific expression of the ubiquitin ligases, MAFbx and MuRF1 in the rat extensor digitorum longus muscle

Abstract

The cytokines, tumor necrosis factor-α (TNF-α) and interleukin-6 (Il-6), are capable of inducing skeletal muscle catabolism. Here, we characterized the time course of Lipopolysaccharide (LPS)-induced expression of TNF-α and Il-6 mRNA in the rat fast-twitch skeletal muscle, the extensor digitorum longus (EDL), and compared it to the time course of mRNA expression of the muscle specific ubiquitin(Ub)-ligases, MuRF1 and MAFbx, of the Ub-proteasome system. Conscious, male, Sprague-Dawley rats were infused i.v. with either saline (0.4 ml h-1) or LPS (150 μg kg-1 h-1) for 2h, 6h or 24h (n = 6-8 per group), after which the EDL was removed. Realtime PCR was used to quantify the TNF-α, Il-6, MAFbx and MuRF1 mRNA transcripts. Levels of TNF-α and IL-6 mRNA expression were elevated as soon as 2h after onset of LPS-infusion (10.9 ± 2.7 and 175 ± 51 fold increase over controls, respectively; P<0.01), and thereafter levels declined (4.4 ± 1.4 and 97 ± 56 fold increase at 24h). In contrast, significant elevation in the Ub-ligases did not occur until after 6h of LPS infusion, with maximal levels of MAFbx and MuRF1 expression seen after 24h LPS infusion (3.6 ± 0.7, and 19.5 ± 1.9 fold increase; respectively, P<0.05). These data are consistent with muscle-derived TNF-α and Il-6 contributing to the increase in the atrophy-inducing Ub-ligases, MAFbx and MuRF1 in sepsis. This work was supported by the Medical Research Council UK.