Study of the transcriptional responses to the inhibition of the expression of the mitochondrial genome: analysis of the activation of the mitochondrial unfolded protein response and the integrated stress response.

Mitochondria are multifunctional organelles and their continued maintenance is required for cell homeostasis and function. Upon organelle dysfunction, a mitochondrial-to-nucleus communication, known as retrograde signalling, triggers an orchestrated expression of nuclear genes to relieve the stress and/or compensate the defect. The importance of the good working of these stress response pathways can be highlighted by the various pathophysiological developments associated with their impairment. The activation of two stress responsive pathways, known as the mitochondrial unfolded protein response (mtUPR) and the integrated stress response (ISR), was assessed upon various stresses such as the inhibition of the mitochondrial DNA (mtDNA) expression. While there is no induction of mtUPR-related gene markers HSPD1 (heat shock 60kDa protein 1) and ClpP (caseinolytic mitochondrial matrix peptidase proteolytic subunit), the transcription factor CHOP-10 (C/EBP homologous protein-10) was systematically overexpressed when mtDNA expression is inhibited. Furthermore, increased expression of this transcription factor correlates with induction of three genes associated with ISR activation: HERP (homocysteine-inducible, endoplasmic reticulum stress-inducible, ubiquitin-like domain member 1), ATF3 (activating transcription factor 3) and Trib3 (tribbles pseudokinase 3). These results were confirmed in three cancer cell lines (HeLa, Hep3B, MCF-7) as well as in primary human hepatocytes. In addition, the requirement of the ISR key transcription factor ATF4 (activating transcription factor 4) for CHOP-10 induction was demonstrated as well as the involvement of the GCN2 (General control non-derepressible kinase 2) kinase in the phosphorylation of eIF2α. Altogether, these data demonstrate that a GCN2-eIF2α-ATF4-CHOP-10 axis is activated in response to mitochondrial dysfunction. The decrease in cytosolic translation that occurs consequently to eIF2α phosphorylation could help to reduce the mitochondrial stress.