Background
Hydrogen sulfide (H2S) is an endogenous gasotransmitter produced by mammalian cells. The current study investigated the potential role of H2S in the regulation of heme biosynthesis using mice deficient in cystathionine gamma-lyase (CSE), one of the three major mammalian H2S-producing enzymes.
Conclusions
The CSE system regulates the expression of CPOX and consequent heme synthesis. These effects in turn, do not influence global oxygen transport parameters, but may regulate mitochondrial electron transport.
Methods
Wild-type and global CSE-/- mice, as well as mitochondria prepared from their liver were used. In vivo, arterial and venous blood gases were measured, and survival of the mice to severe global hypoxia was monitored. Ex vivo, expression of various heme biosynthetic enzymes including coproporphyrinogen oxidase (CPOX) was measured, and mitochondrial function was evaluated using Extracellular Flux Analysis. Urine samples were collected to measure the oxidized porphyrinogen intermediates. The in vivo/ex vivo studies were complemented with mitochondrial bioenergetic studies in hepatocytes in vitro. Moreover, the potential effect of H2S on the CPOX promoter was studied in cells expressing a CPOX promoter construct system.
Results
The main findings are as follows: (1) CSE-/- mice exhibit elevated red blood cell counts and red blood cell mean corpuscular volumes compared to wild-type mice; (2) these changes are associated with elevated plasma and liver heme levels and (3) these alterations are likely due to an induction of CPOX (the sixth enzyme involved in heme biosynthesis) in CSE-/- mice. (4) Based on in vitro promoter data the promoter activation of CPOX is directly influenced by H2S, the product of CSE. With respect to the potential functional relevance of these findings, (5) the increased circulating red blood cell numbers do not correspond to any detectable alterations in blood gas parameters under resting conditions, (6) nor do they affect the hypoxia tolerance of the animals in an acute severe hypoxia model. However, there may be a functional interaction between the CSE system and the CPOX system in terms of mitochondrial bioenergetics: (7) CSE-/- hepatocytes and mitochondria isolated from them exhibit increased oxidative phosphorylation parameters, and (8) this increase is partially blunted after CPOX silencing. Although heme is essential for the biosynthesis of mitochondrial electron chain complexes, and CPOX is required for heme biosynthesis, (9) the observed functional mitochondrial alterations are not associated with detectable changes in mitochondrial electron transport chain protein expression. Conclusions: The CSE system regulates the expression of CPOX and consequent heme synthesis. These effects in turn, do not influence global oxygen transport parameters, but may regulate mitochondrial electron transport.