Mitochondrial complex II regulates a distinct oxygen sensing mechanism in monocytes

Abstract

Mutations in mitochondrial complex II (succinate dehydrogenase; SDH) genes predispose to paraganglioma tumors that show constitutive activation of hypoxia responses. We recently showed that SDHB mRNAs in hypoxic monocytes gain a stop codon mutation by APOBEC3A-mediated C-to-U RNA editing. Here, we test the hypothesis that inhibition of complex II facilitates hypoxic gene expression in monocytes using an integrative experimental approach. By RNA sequencing, we show that specific inhibition of complex II by atpenin A5 in normoxic conditions mimics hypoxia and induces hypoxic transcripts as well as APOBEC3A-mediated RNA editing in human monocytes. Myxothiazol, a complex III inhibitor, has similar effects in normoxic monocytes. Atpenin A5 partially inhibits oxygen consumption, and neither hypoxia nor atpenin A5 in normoxia robustly stabilizes hypoxia-inducible factor (HIF)-1α in primary monocytes. Several earlier studies in transformed cell lines suggested that normoxic stabilization of HIF-1α explains the persistent expression of hypoxic genes upon complex II inactivation. On the contrary, we find that atpenin A5 antagonizes the stabilization of HIF-1α and reduces hypoxic gene expression in transformed cell lines. Accordingly, compound germline heterozygosity of mouse Sdhb/Sdhc/Sdhd null alleles blunts chronic hypoxia-induced increases in hemoglobin levels, an adaptive response mainly regulated by HIF-2α. In contrast, atpenin A5 or myxothiazol does not reduce hypoxia-induced gene expression or RNA editing in monocytes. These results reveal a novel role for mitochondrial respiratory inhibition in induction of the hypoxic transcriptome in monocytes and suggest that inhibition of complex II activates a distinct hypoxia signaling pathway in a cell-type specific manner.

Mitochondrial hypoxic stress induces widespread RNA editing by APOBEC3G in natural killer cells

“…Previously, we have shown that A3A-mediated RNA editing is induced by high cell density and hypoxia in hundreds of mRNAs in monocytes [15]. Furthermore, normoxic inhibition of the mitochondrial complex II by atpenin A5 (AtA5) and of the complex III by myxothiazol (MXT) mimics hypoxia and induces RNA editing as well as hypoxic gene expression in monocytes [39]. Since A3G-mediated RNA editing in NK and HuT78 cells is also induced by hypoxia, we tested the effect of these mitochondrial inhibitors on RNA editing in HuT78 cells cultured in normoxia.…”

Increased nuclear DNA damage precedes mitochondrial dysfunction in peripheral blood mononuclear cells from Huntington’s disease patients

“…In particular, SDHB protects against 3-nitropropionic acid-induced striatal degeneration. Interestingly, SDHB is suppressed in monocytes exposed to inflammation/hypoxia in an APOBEC3-mediated manner 50 . GO analyses identified altered hypoxic signalling in the HD although not in the patients with highest CAG size (Supplementary Table S1).…”

Glutaminases as a Novel Target for SDHB-Associated Pheochromocytomas/Paragangliomas

“…Atpenin A5 (atpenin) is an SDH inhibitor that binds in the ubiquinone binding pocket comprised of residues from SDH subunits B, C, and D, blocking the electron transfer between the enzyme and ubiquinone [44,45]. It is important to note that the inhibition of SDH with atpenin could not induce hypoxia mediated gene expression in monocytes [46] and a dose dependent reduction of cell survival after treatment with atpenin analogues has been shown [47].…”