Protective roles and mechanisms of latent TGF-β1 and Buyang Huanwu decoction in diabetic kidney disease

Abstract

Diabetic kidney disease (DKD), the common complication in diabetes patients, is characterized by chronic renal inflammation and fibrosis. As no regimens can completely stop the pathological progress of DKD, most patients eventually develop into end-stage renal disease in decades. Latent transforming growth factor (TGF)-β1 has been reported to protect against obstructive kidney disease and crescentic glomerulonephritis. However, whether latent TGF-β1 protects against DKD remains unknown. This study discovered that latent TGF-β1 alleviates streptozotocin (STZ)-induced diabetic renal injury via several mechanisms. We further elucidated Buyang Huanwu Decoction (BHD) and the compound from BHD, calycosin-7-glucoside (CG), protects against DKD via regulation of TGF-β1/Smad3 signaling.

Firstly, diabetes was induced in latent TGF-β1 transgenic (Tg) and wild-type (WT) mice by STZ. Renal function, inflammation and fibrosis were measured 16 weeks after induction of diabetes mellitus (DM). Compared to diabetic WT mice, the Tg mice overexpressing latent TGF-β1 were protected from the development of DKD as demonstrated by the low microalbuminuria and the decrease of renal fibrosis and inflammation, and no alteration of blood glucose levels in mice. Mechanistically, latent TGF-β1 attenuates TGF-β1/Smad3-mediated renal fibrosis and NF-κB-driven renal inflammation in the diabetic kidney through inhibiting Arkadia-mediated Smad7 ubiquitin degradation.

Recent studies indicate latent TGF-β1/GARP complex involves in the activation of Tregs. In the present study, we determined whether latent TGF-β1 could protect against DKD through the latent TGF-β1/GARP complex-mediated Treg activation. Interestingly, we found that T cells infiltration and the proportion of inflammatory Th1/Th17 cells were largely reduced in diabetic Tg mice, accompanied by the increased proportion of Tregs and GARP expression on Tregs. Mechanistically, overexpressing latent TGF-β1 induces Tregs activation through latent TGF-β1/GARP pathway, which restored the impaired immunosuppressive function of Tregs in diabetic mice by releasing immunosuppressive mediators, IL-10 and CTLA-4. Finally, the recombinant human latent TGF-β1 protein treatment protected against diabetic kidney injury in mice, indicating the therapeutic potential for DKD patients.

Finally, BHD is commonly used for DKD, but the molecular mechanism remains largely unclear. The findings showed that BHD attenuated renal inflammation and fibrosis in STZ-induced diabetic kidney disease via inhibiting the TGF-β1/Smad3 and NF-κB signaling pathway while suppressing the Arkadia and restoring renal Smad7. Notably, CG, one of the compounds in BHD, significantly inhibited the high glucose-induced cell inflammation and fibrosis by regulating TGF-β1/Smad3 signaling, suggesting that CG could be a promising therapeutic drug for DKD.

In conclusion, latent TGF-β1 protects against DKD via multiple mechanisms locally and systemically. In the kidney, latent TGF-β1 inhibits Arkadia-mediated Smad7 ubiquitin degradation; in turn, Smad7 suppresses the activation of TGF-β1/Smad3 and NF-κB pathways. Systemically, latent TGF-β1/GARP mediated activation of Tregs restores the impaired immunosuppressive function of Tregs, inhibiting the renal inflammation. CG could be a potential therapeutic agent for DKD.