Radiation-induced fibrosis (RIF) is thought to involve the excessive accumulation of collagen and other extracellular matrix components; previously, we reported that ionizing radiation increased the type I collagen expression and that transforming growth factor (TGF)-β was involved in this increase through activating its downstream mediator, Smad3. A recent study found that microRNAs (miRNAs)-small, noncoding sequences approximately 20 nucleotides long-negatively regulate the gene expression posttranscriptionally, and it has been suggested that miRNAs play essential roles in cellular processes, including fibrosis. However, their role in the development of RIF remains unexplored. In the present study, we examined the effects of miRNA on the expression of type I collagen induced by ionizing radiation and the mechanisms underlying the miRNA expression observed following ionizing radiation. We analyzed the regulation of miRNA following ionizing radiation by an miRNA real-time PCR, and found that miR-29 family members were downregulated in irradiated mouse fibroblasts and directly targeted type I collagen genes by specifically binding to the 3' untranslated region. We also found that the overexpression of miR-29 inhibited the ionizing radiation-induced expression of type I collagen, whereas the knockdown of miR-29 enhanced it. In addition, TGF-β/Smad-signaling significantly decreased the transcription of miR-29, whereas the inhibition of this signaling pathway cancelled this decrease. In conclusion, miR-29 was involved in the regulation of type I collagen expression through the TGF-β/Smad-signaling pathway in irradiated cells, suggesting that miR-29 may be an important regulator of RIF.
Keywords: Fibrosis; Ionizing radiation; MicroRNA; Type I collagen.