Pseudohypoxia-stabilized HIF2⍺ transcriptionally inhibits MNRR1, a druggable target in MELAS

The observation that amounts of the mitochondrial regulator MNRR1 (CHCHD2, AAG10, PARK22) are reduced in several pathologies, and that restoration of its level normalizes the pathological phenotype, prompted a search for compounds that could increase MNRR1 levels. High throughput screening of a 2400-compound drug and natural products library uncovered the antifungal drug nitazoxanide and its metabolite tizoxanide as effective enhancers of MNRR1 transcription. Using the mitochondrial disease MELAS (in which various mixtures, called heteroplasmy, of wild-type and mutant mitochondrial DNA (mtDNA) coexist) as a test, we showed that treating a cybrid MELAS model with tizoxanide could restore cellular respiration, enhance mitophagy, and, importantly, shift heteroplasmy toward more wild-type mtDNA. Furthermore, in MELAS patient fibroblasts, the compound could improve mitochondrial biogenesis, enhance autophagy, and protect the fibroblasts from LPS-induced inflammation. Chemical activation of MNRR1 is thus a potential strategy to improve mitochondrial deficits seen in MELAS. Investigation of the mechanism by which MNRR1 is reduced identified that two factors compete to regulate transcription at the MNRR1 promoter – RBPJκ, which stimulates it, and HIF2α, which inhibits it. In MELAS cells there is a pseudohypoxic state that stabilizes HIF2α, leading to transcriptional inhibition of MNRR1. Nitazoxanide reduces the levels of HIF2α by increasing the levels of PHD3, the prolyl hydroxylase that degrades HIF2α.