Hydrogen sulfide metabolism in cancer and homocystinuria: towards the development of new pharmacological strategies

Hydrogen sulfide (H2S), along with nitric oxide (NO) and carbon monoxide (CO), belongs to a small group of gaseous signalling molecules termed ‘gasotransmitters’. H2S is involved in important physiological and pathological processes and exerts a dose-dependent effect on cellular bioenergetics, acting at higher levels as an inhibitor of mitochondrial cytochrome c oxidase or stimulating ATP synthesis at lower concentrations. A growing number of pathologies are reportedly associated with alterations of H2S metabolism. Here, the human H2S metabolism was investigated in classical homocystinuria and in cancer; furthermore, it was explored a new pharmacological strategy against cancer based on human ferritin (Hft) as a drug delivery system. Classical homocystinuria is a rare genetic disease, associated with mutations in the gene encoding the heme-containing enzyme cystathionine β-synthase (CBS), a major source of H2S in humans. CBS is allosterically activated by S-adenosyl-L-methionine, but inhibited by CO or NO. Here, it was found that the pathogenic p.P49L variant of human CBS, as produced in E. coli and purified, displays reduced affinity for the PLP (pyridoxal 5’-phosphate) cofactor and increased affinity of the ferrous heme for CO. This could lead to enzyme inhibition at physiological CO concentrations, thereof representing a pathogenic mechanism in classical homocystinuria. Hydrogen sulfide plays an important role also in colon cancer where it promotes angiogenesis and, interestingly, energy metabolism. Here, the H2S catabolism was explored in colon cancer model cells (SW480) exposed to hypoxia, a common factor of tumoral microenvironment that promotes tumor cell survival and propagation. Cells were grown under either normoxic (20% O2) or hypoxic (1% O2) conditions and their ability to metabolize H2S at the mitochondrial level was assayed by high resolution respirometry. Intriguingly, exposure to hypoxic conditions for 24 hours, while reducing the mitochondrial mass and the maximal efficacy to metabolize sulfide, was found to increase the expression of the H2S-metabolizing mitochondrial enzyme sulfide:quinone oxidoreductase. These effects induced by hypoxia may affect the H2S levels in SW480 cells, pointing to H2S catabolism as a potential drug target in colon cancer. Finally, in order to design new therapeutic strategies against cancer, it was obtained an engineered human ferritin-based nanocarrier (HFt-MP-PASE), able to effectively incorporate and deliver the chemotherapeutic drug mitoxantrone with high killing efficacy against model cells of colon cancer progression (SW480 and SW620).