Involvement of the transforming growth factor-β system in the pathogenesis of diabetic nephropathy

Bardoxolone ameliorates TGF-β1-associated renal fibrosis through Nrf2/Smad7 elevation

Transforming growth factor-β (TGF-β) is a potent pathogenic factor of renal injury through the upregulation of extracellular matrix (ECM) expression and facilitation of renal fibrosis. Nuclear factor erythroid 2-like 2 (Nfe2l2; Nrf2), a master regulator of antioxidant and detoxifying systems, is mainly controlled by the binding with cytosolic protein Kelch-like ECH-associated protein 1 (Keap1) and subsequent proteasomal degradation. The protective effect of Nrf2 on renal injury has been attributed to its antioxidant role, where it aids in coping with oxidative stress-associated progression of renal disease. In this study, we investigated the effect of Nrf2 activation on ECM production and TGF-β/Smad signaling using Keap1-silenced MES-13 cells (a genetic glomerular mesangial cell model with Nrf2 overexpression). The TGF-β1-inducible expression of fibronectin and α-smooth muscle actin (α-Sma) was suppressed and Smad2/3 phosphorylation was blocked in Nrf2-high mesangial cells as compared with that in control cells. Notably, in these Nrf2-high mesangial cells, levels of TGF-β1 receptor 1 (TβR1) were substantially diminished, and the protein levels of Smad7, an inhibitor TGF-β1/Smad signaling, were increased. Nrf2-mediated Smad7 elevation and its anti-fibrotic role in Keap1-silenced cells were confirmed by studies with Nrf2-or Smad7-silencing. As a molecular link for Smad7 elevation in Nrf2-high cells, the reduction of Smad-ubiquitination-regulatory factor 1 (Smurf1), an E3 ubiquitin ligase for Smad7, was notable. Silencing of Smurf1 increased Smad7 in the control mesangial cells; however, forced expression of Smurf1 repressed Smad7 levels in Keap1-silenced cells. Additionally, we demonstrate that bardoxolone (BARD; CDDO-methyl), a pharmacological activator of Nrf2, increased Smad7 levels and attenuated TGF-β/Smad/ECM expression in MES-13. Moreover, in an aristolochic acid (AA)-mediated nephropathy mouse model, the renal expression of Nrf2 and Smad7 was elevated by BARD treatment, and AA-induced tubular necrosis and interstitial fibrosis were substantially ameliorated by BARD. Collectively, these results indicate that the Nrf2-Smad7 axis plays a key role in the protection of TGF-β-induced renal fibrosis, and further suggest a novel molecular mechanism of beneficial effect of BARD on renal disease.

A case of "hidden" diabetic nephropathy diagnosed by a combination of renal biopsy findings and continuous glucose monitoring system

Although diabetic nephropathy is a microvascular complication of diabetes mellitus, some reports suggest that renal biopsy often shows this pathological change without a diagnosis of diabetes mellitus. Here, we report a case of a 65-year-old man who presented with proteinuria, hypoalbuminemia and hypertension without a diagnosis of diabetes mellitus. He drank alcohol regularly and was a heavy smoker. Renal biopsy revealed a diffuse increase in the mesangial area, mesangial nodules or well-developed hyalinosis, interstitial fibrosis, and arteriosclerosis consistent with the changes of diabetic nephropathy. Although we had initially diagnosed him with idiopathic nodular glomerulosclerosis, use of a continuous glucose monitoring system (CGMS) revealed that the changes in his daily blood glucose concentrations met with the diagnostic criteria of diabetes mellitus. Accordingly, we diagnosed him with diabetic nephropathy and initiated treatment for diabetes mellitus. This case suggests that some cases of diabetic nephropathy may be hidden among patients with impaired glucose tolerance, who are not diagnosed with diabetes mellitus. Use of a CGMS may be helpful in diagnosing this type of "hidden" diabetes mellitus. In addition to diet therapy, smoking control, treatment for hypertension, and strict control of hyperglycemia may be important for these patients.