Inhibition of mitochondrial complex III or dihydroorotate dehydrogenase (DHODH) triggers formation of poly(A)⁺ RNA foci adjacent to nuclear speckles following activation of ATM (ataxia telangiectasia mutated)

Intracellular and intercellular signalling networks play an essential role in optimizing cellular homoeostasis and are thought to be partly reflected in nuclear mRNA dynamics. However, the regulation of nuclear mRNA dynamics by intracellular and intercellular signals remains largely unexplored, and research tools are lacking. Through an original screening based on the mRNA metabolic mechanism, we discovered that eight well-known inhibitors cause significant nuclear poly(A)⁺ RNA accumulation. Among these inhibitors, we discovered a new mRNA metabolic response in which the addition of antimycin A, an inhibitor of mitochondrial respiratory-chain complex III (complex III), resulted in a marked accumulation of poly(A)⁺ RNA near the nuclear speckles. Furthermore, dihydroorotate dehydrogenase (DHODH) inhibitors, a rate-limiting enzyme in the intracellular de novo pyrimidine synthesis reaction that specifically exchanges electrons with complex III, also caused a remarkable accumulation of nuclear poly(A)⁺ RNA adjacent to the nuclear speckles, which was abolished by extracellular uridine supply, indicating that the depletion of intracellular pyrimidine affects poly(A)⁺ RNA metabolism. Further analysis revealed that ataxia telangiectasia mutated (ATM), a serine and threonine kinase and a master regulator of DNA double-strand break (DSB) and nucleolar stress, is required for this poly(A)⁺ RNA nuclear accumulation phenomenon. This study reports new insights into novel aspects of nuclear poly(A)⁺ RNA metabolism, especially the relationship between mitochondrial respiratory-chain functions, pyrimidine metabolism, and nuclear RNA metabolism.