The physiological role of Nrf2 in diabetic kidney disease

Diabetic nephropathy (DN) is one of the leading causes of end-stage kidney disease (ESKD). ESKD is a major risk factor for heart failure and stroke. Dysfunction of intrarenal renin angiotensin system (iRAS) is considered as one of the main reasons that caused the DN. All  components of the iRAS are identified in the renal proximal tubule cells (RPTCs), including  angiotensinogen (Agt), the sole precursor of all angiotensins. Our lab has previously reported that  specific overexpression of Agt in RPTCs induces hypertension, proteinuria, kidney fibrosis,  apoptosis and kidney injury.  Nuclear factor erythroid 2-related factor 2 (Nrf2) is a transcription factor that abundantly  expresses in RPTCs and has been considered as a master regulator of redox balance in cellular  cytoprotective responses. The role of Nrf2 activation in DN, however, is not clear. The overall aim  of this study is to understand the physiological role of Nrf2 in DN and investigate the molecular  mechanism(s) of Nrf2 action.  First, we have demonstrated that genetic deletion of Nrf2 or pharmacological blockade of  Nrf2 with trigonelline in type 1 diabetic Akita mice effectively upregulates Ace2/MasR and  suppresses Agt/ACE expression in isolated RPTCs, resulting in attenuation of systemic  hypertension and kidney injury. Consistently, in cultured IRPTCs, Nrf2 siRNA transfection or  trigonelline treatment prevents high glucose-induced upregulation of Agt/ACE with  downregulation of Ace2/MasR gene expression. These data identified a novel mechanism in  which Nrf2 activation stimulates iRAS gene expression and activation, leading to the development  of hypertension and nephropathy in diabetes.  Second, we have generated Nrf2 transgenic mice under the kidney specific androgen regulated protein (KAP) promoter which specifically overexpress Nrf2 in RPTCs (Nrf2RPTC Tg mice).  We further crossbred the Nrf2RPTC Tg mice with Akita Nrf2-/- mice to generate Akita Nrf2-/- /Nrf2RPTC Tg mice. We have found that overexpression of Nrf2 in RPTCs of Akita Nrf2-/- mice significantly  unregulated sodium-glucose transporter-2 (SGLT2) expression, resulting in elevation of blood  glucose, glomerular filtration rate, albumin-creatinine ratio and tubulointerstitial fibrosis. In 8 immortalized human proximal tubular cells (HK2), oltipraz treatment or NRF2 cDNA transfection  stimulated SGLT2 mRNA expression and its promoter activity. Furthermore, NRF2 bound to NRF2- RE of SGLT2 promoter were identified by gel mobility shift assay and chromatin  immunoprecipitation assay. Moreover, human diabetic kidneys exhibited higher expression of  NRF2 and SGLT2 in RPTCs than non-diabetic kidneys. These data established a novel mechanism  of NRF2’s regulation on SGLT2, leading to exacerbation of blood glucose, hyperfiltration and  kidney injury in diabetes. In summary, this study documented that activation of Nrf2 in  hyperglycemia contributed to the progression of DN via regulation of iRAS and SGLT2, suggesting that Nrf2 might be a potential therapeutic target in the treatment of DN.

Keyphrases: Nrf2, SGLT2, Oxidative stress, Diabetic nephropathy, Intrarenal renin-angiotensin system