Connexin32 Promotes the Activation of Foxo3a to Ameliorate Diabetic Nephropathy Inhibiting the Polyubiquitination and Degradation of Sirt1.

Renal oxidative stress (OSS) is the leading cause of diabetic nephropathy (DN). The silent information regulator 1/forkhead boxo3a (Sirt1/Foxo3a) pathway plays an essential role in regulating the antioxidant enzyme system. In this study, we aimed to investigate the mechanism of connexin32 (Cx32) on the antioxidant enzyme system in DN. In this study, Cx32 overexpression significantly reduced reactive oxygen species generation and effectively inhibited the excessive production of extracellular matrix such as fibronectin (FN) and intercellular adhesion molecule-1 (ICAM-1) in high-glucose (HG)-induced glomerular mesangial cells. In addition, Cx32 overexpression reversed the downregulation of Sirt1, and promoted the nuclear transcription of Foxo3a, subsequently activating the antioxidant enzymes including catalase and manganese superoxide dismutase (MnSOD), however, Cx32 knockdown showed the opposite effects. A further mechanism study showed that Cx32 promoted the autoubiquitination and degradation of Smad ubiquitylation regulatory factor-1 (Smurf1), thereby reducing the ubiquitination of Sirt1 at Lys and the degradation of Sirt1. Moreover, the results showed that adenovirus-mediated Cx32 overexpression activated the Sirt1/Foxo3a pathway, and inhibited OSS in the kidney tissues, eventually improving the renal function and glomerulosclerosis in diabetic mice. This study highlighted the antioxidant role of Cx32-Sirt1-Foxo3a axis to alleviate DN, which is a new mechanism of Cx32 alleviating DN. Cx32 alleviated DN activating the Sirt1/Foxo3a antioxidant pathway. The specific mechanism was that Cx32 upregulated the Sirt1 expression through reducing the ubiquitination of Lys of Sirt1 by inhibiting Smurf1. 39, 241-261.