Non-electron transfer chain mitochondrial defects differently regulate HIF-1α degradation and transcription

Mitochondria are the main consumers of molecular O-2 in a cell as well as an abundant source of reactive oxygen species (ROS). Both, molecular oxygen and ROS are powerful regulators of the hypoxia-inducible factor-1 alpha-subunit (HIF-alpha). While a number of mechanisms in the oxygen-dependent HIF-alpha regulation are quite well known, the view with respect to mitochondria is less clear. Several approaches using pharmacological or genetic tools targeting the mitochondrial electron transport chain (ETC) indicated that ROS, mainly formed at the Rieske cluster of complex III of the ETC, are drivers of HIF-1 alpha activation. However, studies investigating non-ETC located mitochondrial defects and their effects on HIF-1 alpha regulation are scarce, if at all existing. Thus, in the present study we examined three cell lines with non-ETC mitochondrial defects and focused on HIF-1 alpha degradation and transcription, target gene expression, as well as ROS levels. We found that cells lacking the key enzyme 2-enoyl thioester reductase/mitochondrial enoyl-CoA reductase (MECR), and cells lacking manganese superoxide dismutase (MnSOD) showed a reduced induction of HIF-1 alpha under long-term (20 h) hypoxia. By contrast, cells lacking the mitochondrial DNA depletion syndrome channel protein Mpv17 displayed enhanced levels of HIF-1 alpha already under normoxic conditions. Further, we show that ROS do not exert a uniform pattern when mediating their effects on HIF-1 alpha, although all mitochondrial defects in the used cell types increased ROS formation. Moreover, all defects caused a different HIF-1 alpha regulation via promoting HIF-1 alpha degradation as well as via changes in HIF-1 alpha transcription. Thereby, MECR- and MnSOD-deficient cells showed a reduction in HIF-1 alpha mRNA levels whereas the Mpv17 lacking cells displayed enhanced HIF-1 alpha mRNA levels under normoxia and hypoxia. Altogether, our study shows for the first time that mitochondrial defects which are not related to the ETC and Krebs cycle contribute differently to HIF-1 alpha regulation by affecting HIF-1 alpha degradation and HIF-1 alpha transcription where ROS play not a major role.