The Functional Role of MicroRNA-21 in Renal Fibrosis

Objectives: TGF-β/Smad signaling plays a critical role in renal fibrosis in chronic kidney disease (CKD). It is well known that Smad3 is a key mediator of downstream TGF-β/Smad signaling in renal fibrosis, however, the exact mode of TGF-β/Smad3 in renal fibrosis remains unclear. In this thesis, we tested a novel hypothesis that TGF-β may act by regulating the Smad3-dependent microRNA-21 (miR-21) to mediate renal fibrosis and that specific targeting miR-21 may represent an effective and novel therapy for chronic kidney disease.;Methods: The regulatory mechanism of TGF-β1-induced miR-21 expression via the Smad3-dependent pathway was studied in a rat NRK52E tubular epithelial cell (TEC) line and mesangial cell (MC) line. The functional role of miR-21 in renal fibrosis was investigated by overexpressing or down-regulating of miR-21 both in TGF-β1 and high glucose (HG) conditions in TEC and MC. The therapeutic potential role of miR-21 in kidney diseases were examined in unilateral ureteral obstructive (UUO) mouse model and in db/db mice by applying an ultrasound-microbubble-mediated anti-miR-21 gene transfer technique. The target gene of miR-21 was identified by luciferase reporter assays.;Results: By microarray and realtime PCR, upregulation of miR-21 was observed in tubular epithelial cells (TECs) and mesangial cells (MCs) in response to TGF-β1 and high glucose (HG). Both in vitro and in vivo studies demonstrated that the upregulation of miR-21 expression during renal fibrosis and diabetic conditions was dependent on the activation of TGF-β/Smad3 signaling. The findings that overexpression of miR-21 promoted but knockdown of miR-21 suppressed TGF-β1-induced renal fibrosis and HG-induced diabetic kidney injury demonstrated the functional importance for miR-21 in fibrosis and inflammation in vitro. More importantly, ultrasound-microbubble-mediated gene transfer of a miR-21 knockdown plasmid into the mouse kidney before and after established unilateral ureteral obstructive (UUO) nephropathy was able to prevent and halt the progression of renal fibrosis. Furthermore, we also found that blockade of miR-21 was capable of attenuating diabetic kidney injury including progressive renal fibrosis and inflammation, as well as renal functional injury in a mouse model of type 2 diabetes in db/db mice. The functional role of miR-21 on renal fibrosis and inflammation was through Smad7, which was identified as a direct target gene of miR-21. All these results revealed a therapeutic potential for targeting miR-21 in chronic kidney disease.;Conclusions: In conclusion, miR-21 is a downstream mediator of TGF-β/Smad3 signaling and plays a critical role in the development of renal fibrosis. Targeting miR-21 may represent a novel and effective therapy to combat renal fibrosis in chronic kidney disease.