Abstract

Although many types of ancient bacteria and archea rely on hydrogen sulfide (H₂S) for their energy production, eukaryotes generate ATP in an oxygen-dependent fashion. We hypothesize that endogenous H₂S remains a regulator of energy production in mammalian cells under stress conditions, which enables the body to cope with energy demand when oxygen supply is insufficient. Cystathionine γ-lyase (CSE) is a major H₂S-producing enzyme in the cardiovascular system that uses cysteine as the main substrate. Here we show that CSE is localized only in the cytosol, not in mitochondria, of vascular smooth-muscle cells (SMCs) under resting conditions, revealed by Western blot analysis and confocal microscopy of SMCs transfected with GFP-tagged CSE plasmid. After SMCs were exposed to A23187, thapsigargin, or tunicamycin, intracellular calcium level was increased, and CSE translocated from the cytosol to mitochondria. CSE was coimmunoprecipitated with translocase of the outer membrane 20 (Tom20) in mitochondrial membrane. Tom20 siRNA significantly inhibited mitochondrial translocation of CSE and mitochondrial H₂S production. The cysteine level inside mitochondria is approximately three times that in the cytosol. Translocation of CSE to mitochondria metabolized cysteine, produced H₂S inside mitochondria, and increased ATP production. Inhibition of CSE activity reversed A23187-stimulated mitochondrial ATP production. H₂S improved mitochondrial ATP production in SMCs with hypoxia, which alone decreased ATP production. These results suggest that translocation of CSE to mitochondria on specific stress stimulations is a unique mechanism to promote H₂S production inside mitochondria, which subsequently sustains mitochondrial ATP production under hypoxic conditions.