Hypoxia poses a stress to cells and decreases mitochondrial respiration, in part by electron transport chain (ETC) complex reorganization. While metabolism under acute
hypoxia is well characterized, alterations under chronic
hypoxia largely remain unexplored. We followed oxygen consumption rates in THP-1 monocytes during acute (16 h) and chronic (72 h)
hypoxia, compared to normoxia, to analyze the electron flows associated with glycolysis,
glutamine, and
fatty acid oxidation. Oxygen consumption under acute
hypoxia predominantly demanded
pyruvate, while under chronic
hypoxia,
fatty acid- and
glutamine-oxidation dominated. Chronic
hypoxia also elevated
electron-transferring flavoproteins (ETF), and the knockdown of ETF⁻ubiquinone
oxidoreductase lowered mitochondrial respiration under chronic
hypoxia. Metabolomics revealed an increase in
citrate under chronic
hypoxia, which implied
glutamine processing to α-ketoglutarate and
citrate. Expression regulation of
enzymes involved in this metabolic shunting corroborated this assumption. Moreover, the expression of
acetyl-CoA carboxylase 1 increased, thus pointing to
fatty acid synthesis under chronic
hypoxia. Cells lacking complex I, which experienced a markedly impaired respiration under normoxia, also shifted their metabolism to
fatty acid-dependent synthesis and usage. Taken together, we provide evidence that chronic
hypoxia fuels the ETC via ETFs, increasing
fatty acid production and consumption via the
glutamine-
citrate-
fatty acid axis.