Mitochondrial translation inhibition triggers ATF4 activation, leading to integrated stress response but not to mitochondria unfolded protein response

Mitochondrial-nuclear communication, known as retrograde signaling, is important for regulating nuclear gene expression in response to mitochondrial dysfunction. Previously, we have found that p32/C1qbp-deficient mice, which have a mitochondrial translation defect, show ER stress response and integrated stress response (ISR) gene expression in the heart and brain. However, the mechanism by which mitochondrial translation inhibition elicits these responses is not clear. Among the transcription factors that respond to mitochondrial stress, ATF4 is a key transcription factor in the ISR. Herein, chloramphenicol, which inhibits mitochondrial DNA-encoded protein expression, induced eIF2a phosphorylation and ATF4 induction, leading to ISR gene expression. However, the expression of the mitochondrial unfolded protein response genes, which has been shown in Caenorhabditis Elegans, was not induced. Short hairpin RNA-based knockdown of ATF4 markedly inhibited the chloramphenicol-induced ISR gene expression. We also observed by ChIP analysis that induced ATF4 bound to the promoter region of several ISR genes, suggesting that mitochondrial translation inhibition induces ISR gene expression through ATF4 activation. In this study, we showed that mitochondrial translation inhibition induced the ISR through ATF4 activation rather than the mitochondrial unfolded protein response.