Mitochondrial metabolism regulates cellular proteostasis

Protein homeostasis involving protein synthesis and degradation by the proteasome system is intricately balanced in the cell and fine-tuned according to cellular needs and upon stress. Metabolic regulation of protein synthesis and proteasome activity by respiratory chain dysfunction has not been investigated so far. In our study, we used mouse embryonic fibroblasts (MEFs) isolated from the mitochondrial (mt)DNA mutator mouse model that is characterized by genetic respiratory chain dysfunction. Cellular proteostasis including protein synthesis and proteasome activity was significantly decreased in mutator MEFs. In particular, mTOR signaling was downregulated and the formation of 26S proteasome complexes was impaired. Electron microscopy and proteomic analysis of isolated mitochondria revealed massive respiratory chain complex I deficiency in mutator mitochondria resulting in the accumulation of NADH and altered Krebs cycle metabolism. Targeted metabolomics of mutator MEFs identified impaired biosynthesis of aspartate as a consequence of disturbed Krebs cycle function. Notably, supplementation with aspartate restored both mTOR mediated protein translation and proteasome activity in mutator cells. Furthermore, we delineated impairment of respiratory chain complex I activity as the driving factor of this metabolic regulation in primary human lung fibroblasts treated with the complex I inhibitor Metformin. Metformin downregulated both protein synthesis and proteasome activity, a phenotype that was rescued by aspartate supplementation. Our data thus unravel a previously unrecognized adaptation of proteostasis to metabolic alterations of the cell, which might be of therapeutic relevance for the treatment of lung diseases.