A biphasic effect of TNF-α in regulation of the Keap1/Nrf2 pathway in cardiomyocytes

Antagonizing TNF-alpha signaling attenuates chronic inflammatory disease, but is associated with adverse effects on the cardiovascular system. Therefore the impact of TNF-alpha on basal control of redox signaling events needs to be understand in more depth. This is particularly important for the Keap1/Nrf2 pathway in the heart and in the present study we hypothesized that inhibition of a low level of TNF-alpha signaling attenuates the TNF-alpha dependent activation of this cytoprotective pathway. HL-1 cardiomyocytes and TNF receptor1/2 (TNER1/2) double knockout mice (DKO) were used as experimental models. TNF-alpha (2-5 ng/ ml, for 2 h) evoked significant nuclear translocation of Nrf2 with increased DNA/promoter binding and transactivation of Nrf2 targets. Additionally, this was associated with a 1.5 fold increase in intracellular glutathione (GSH). Higher concentrations of TNF-alpha (> 10-50 ng/ml) were markedly suppressive of the Keapl/Nrf2 response and associated with cardiomyocyte death marked by an increase in cleavage of caspase-3 and PARP. In vivo experiments with TNER1/2-DKO demonstrates that the expression of Nrf2 regulated proteins (NQO1, HO-1, G6PD) were significantly downregulated in hearts of the DKO when compared to WT mice indicating a weakened antioxidant system under basal conditions. Overall, these results indicate that TNF-alpha exposure has a bimodal effect on the Keap1/Nrf2 system and while an intense inflammatory activation suppresses expression of antioxidant proteins a low level appears to be protective. (C) 2016 The Authors. Published by Elsevier B.V.