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Mitochondrial mutations cause bioenergetic defects associated with failures to use the electron transfer chain and oxidize substrates. These defects are exacerbated under energetic stress conditions and ultimately cause cell deterioration and death. However, little is known about cellular strategies that rescue mitochondrial stress failures and maintain cell survival under these conditions. Here, we have designed and performed a high-throughput chemical screen to identify small molecules that rescue human mitochondrial complex I mutations from energetic stress-induced cell death. The top positive hits were a series of sulfonylureas that efficiently maintain prolonged cell survival and growth under energetic stress conditions. The addition of galactose instead of glucose, to experimentally force mitochondrial respiration, triggered an initial ER stress response that was associated with IRE1α-dependent inflammatory signals including JNK and p38 MAP kinases in mutant cells. Sulfonylureas, similar to inhibition of IRE1α and p38 MAP kinase, potently blocked this ER stress inflammatory and cell death pathway and maintained viability and cell growth under severe energetic stress conditions. These studies reveal that sulfonylureas and specific inhibition of the IRE1α inflammatory pathway protect against cell death and can be used to rescue bioenergetic failures in mitochondrial complex I-mutated cells under stress conditions.

Mitochondrial diseases comprise a heterogeneous group of genetically inherited disorders that cause failures in energetic and metabolic function. Boosting residual oxidative phosphorylation (OXPHOS) activity can partially correct these failures. Herein, using a high-throughput chemical screen, we identified the bromodomain inhibitor I-BET 525762A as one of the top hits that increases COX5a protein levels in complex I (CI) mutant cybrid cells. In parallel, bromodomain-containing protein 4 (BRD4), a target of I-BET 525762A, was identified using a genome-wide CRISPR screen to search for genes whose loss of function rescues death of CI-impaired cybrids grown under conditions requiring OXPHOS activity for survival. We show that I-BET525762A or loss of BRD4 remodeled the mitochondrial proteome to increase the levels and activity of OXPHOS protein complexes, leading to rescue of the bioenergetic defects and cell death caused by mutations or chemical inhibition of CI. These studies show that BRD4 inhibition may have therapeutic implications for the treatment of mitochondrial diseases., (Copyright © 2016 Elsevier Inc. All rights reserved.)