Nuclear factor-erythroid 2-related factor-2 (Nrf2) is a master regulator of the anti-oxidant response. However, studies in models of Friedreich's ataxia (FA), a neuro- and cardio-degenerative disease associated with oxidative stress, reported decreased Nrf2 expression due to unknown mechanisms. Using a mouse conditional frataxin knockout (KO) model in the heart and skeletal muscle, we examined the Nrf2 pathway in these tissues. Frataxin KO results in fatal cardiomyopathy, while skeletal muscle was asymptomatic. In the KO heart, protein oxidation and a decreased GSH:GSSG ratio was observed, but the opposite was found in skeletal muscle. Decreased total and nuclear Nrf2 and increased levels of its inhibitor, Keap1, were evident in the KO heart, but not skeletal muscle. Moreover, a mechanism involving activation of the nuclear Nrf2 export/degradation machinery via Gsk3β-signaling was demonstrated in the KO heart. This process involved: (i) increased Gsk3β activation; (ii) β-TrCP nuclear accumulation; and (iii) Fyn phosphorylation. A corresponding decrease in Nrf2-DNA-binding activity and a general decrease in Nrf2-target mRNA was observed in KO hearts. Paradoxically, protein levels of some Nrf2 anti-oxidant targets were significantly increased in KO mice. Collectively, cardiac frataxin-deficiency reduces Nrf2 levels via two potential mechanisms: increased levels of cytosolic Keap1, and activation of Gsk3β-signaling that decreases nuclear Nrf2. These findings are in contrast to the frataxin-deficient skeletal muscle, where Nrf2 was not decreased.
Read the entire article HERE