Fibrosis is common after skeletal muscle injury, undermining tissue regeneration and function. The mechanism underlying skeletal muscle fibrosis remains unveiled. Transforming growth factor-β/Smad signaling pathway is supposed to play a pivotal role. However, how microRNAs interact with transforming growth factor-β/Smad related muscle fibrosis remains unclear. We showed that miR-24-3p and miR-122-5p declined in skeletal muscle fibrosis, which was a consequence of transforming growth factor-β. Up-regulating Smad4 suppressed two microRNAs, while inhibiting Smad4 elevated microRNAs. Luciferase reporter assay and chromatin immunoprecipitation confirmed that Smad4 directly inhibited two microRNAs. On the other hand, overexpression of these two miRs retarded fibrotic process. We further identified that Smad2 was a direct target of miR-24-3p, while miR-122-5p targeted transforming growth factor-β receptor-II. Both targets were important participants in transforming growth factor-β/Smad signaling. Taken together, a positive feedback loop in transforming growth factor-β/Smad4 signaling pathway in skeletal muscle fibrosis was identified. Transforming growth factor-β/Smad axis could be down-regulated by microRNAs. This effect, however, was suppressed by Smad4, the downstream of transforming growth factor-β.

骨骼肌损伤后纤维化很常见，破坏了组织的再生和功能。骨骼肌纤维化的潜在机制仍未阐明。转化生长因子-β/ Smad信号传导途径应该起关键作用。但是，microRNA如何与转化生长因子-β/ Smad相关的肌肉纤维化相互作用尚不清楚。我们显示骨骼肌纤维化中miR-24-3p和miR-122-5p下降，这是转化生长因子-β的结果。上调Smad4抑制了两个microRNA，同时抑制了Smad4升高的microRNA。荧光素酶报告基因测定和染色质免疫沉淀证实Smad4直接抑制了两个microRNA。另一方面，这两个miRs的过度表达会延迟纤维化过程。我们进一步确定Smad2是miR-24-3p的直接靶标，而miR-122-5p靶向转化生长因子-β受体II。两个靶都是转化生长因子-β/ Smad信号传导的重要参与者。两者合计，确定了骨骼肌纤维化中转化生长因子-β/ Smad4信号通路的正反馈回路。转化生长因子-β/ Smad轴可能被microRNA下调。但是，这种作用被转化生长因子-β的下游Smad4抑制了。