Epigallocatechin gallate upregulates NRF2 to prevent diabetic nephropathy via disabling KEAP1

Epigallocatechin gallate (EGCG) is the most abundant and effective green tea catechin and has been reported to attenuate diabetic nephropathy (DN). However, the mechanism by which EGCG ameliorates DN, till now, has remained unclear. EGCG is known as a potent activator of nuclear factor erythroid 2-related factor 2 (NRF2), which plays a key role in cellular defense against diabetes-induced oxidative stress and in the prevention of DN. In the present study, we tested whether NRF2 is required for EGCG protection against DN. Therefore, C57BL/6 wild type (WT) and Nrf2 knockout mice were induced to diabetes by streptozotocin, in the presence or absence of a 24-week treatment with EGCG. In the WT mice, EGCG activated Nrf2 expression and function without altering the expression of Kelch-like ECH-associated protein 1 (Keap1). Diabetes-induced renal oxidative damage, inflammation, fibrosis and albuminuria were significantly prevented by EGCG. Notably, deletion of the Nrf2 gene completely abrogated these actions of EGCG. To further determine the effect of EGCG on KEAP1/NRF2 signaling, mouse mesangial cells were treated with high glucose, in the presence of both Keap1 siRNA and EGCG. Interestingly, EGCG failed to enhance NRF2 signaling and alleviate oxidative, inflammatory and fibrotic indicators, in the presence of Keap1 siRNA. The present study demonstrated, for the first time, that NRF2 plays a critical role in EGCG protection against DN. Other findings indicated that inactivation of KEAP1 protein by EGCG may mediate EGCG function in activating NRF2.