Objective
Reduction of oxidized methionines is emerging as a major protein repair pathway. The lack of methionine sulfoxide reductase A (MsrA) exacerbates cardiovascular disease phenotypes driven by increased oxidative stress. However, the role of MsrA on maintaining cellular homeostasis in the absence of excessive oxidative stress is less well understood.
Results
Constitutive genetic deletion of MsrA increased formation of p62-containing protein aggregates, activated autophagy, and decreased a marker of apoptosis in vascular smooth muscle cells (VSMC). The association of Keap1 with p62 was augmented in MsrA-/- VSMC. Keap1 targets the transcription factor Nrf2, which regulates antioxidant genes, for proteasomal degradation. However, in MsrA-/- VSMC, the association of Nrf2 with Keap1 was diminished. Whereas Nrf2 mRNA levels were not decreased in MsrA-/- VSMC, we detected decreased ubiquitination of Nrf2 and a corresponding increase in total Nrf2 protein in the absence of biochemical markers of oxidative stress. Moreover, nuclear-localized Nrf2 was increased under MsrA deficiency, resulting in upregulation of Nrf2-dependent transcriptional activity. Consequently, transcription, protein levels and enzymatic activity of glutamate-cysteine ligase and glutathione reductase were greatly augmented in MsrA-/- VSMC. Summary: Our findings demonstrate that reversal of methionine oxidation is required for maintenance of cellular homeostasis in the absence of increased oxidative stress. These data provide the first link between autophagy and activation of Nrf2 in the setting of MsrA deletion.