Introduction: Kidney fibrosis is the hallmark of chronic kidney disease progression. At present, no pharmacological therapy against kidney fibrosis exists. The circadian clock gene Bmal1 has been implicated in the multiple diseases including cancer and atherosclerosis. However, the role of Bmal1 in kidney fibrosis remain largely unknown. We therefore examined whether Bmal1 modulates TGF-β-induced renal fibrosis.
Methods: We used the unilateral ureteric obstruction (UUO) mouse model to examine the alteration of Bmal1 expression during renal fibrosis. Also, this study used TGF-β-stimulated NRK-49F renal cells as an in-vitro model of renal interstitial fibrosis. Masson‘s trichrome stain was applied to identify the degree of kidney fibrosis. Real-time PCR and western blotting techniques were used for gene expression and protein expression, respectively. 
Results: Kidneys from UUO mice displayed marked renal fibrosis compared with the control mice. Western blotting analyses revealed increased Bmal1 protein expression in the kidney after UUO, with increase in NRK-49F cells following TGF-β treatment, accompanied by increased fibrotic markers including α-SMA and collagen I. In addition, Bmal1 mRNA expression was increased in TGF-β stimulation and the increase of Bmal1 inhibited by the pretreatment with MEK inhibitor, U0126. Furthermore, knockdown of Bmal1 or SR9009, a REV-ERB agonist that inhibits the transcriptional activity of the Bmal1 expression alleviated TGF-β-induced α-SMA and collagen I mRNA expression. Knockdown of Bmal1 or SR9009 also TGF-β-induced NOX4 mRNA and phosphorylation of p38 that is involved in TGF-β-induced fibrotic markers. 
Conclusion: These data suggest that increased Bmal1 expression plays a crucial role in the induction of fibrotic markers, which leads to the development of kidney fibrosis. Thus, Bmal1 may be a potential therapeutic target for the prevention or treatment of kidney fibrosis.