High homocysteine levels prevent via H2S the CoCl2‐induced alteration of lymphocyte viability

High homocysteine (HCy) levels are associated with lymphocyte‐mediated inflammatory responses that are sometimes in turn related to hypoxia. Because adenosine is a potent lymphocyte suppressor produced in hypoxic conditions and shares metabolic pathways with HCy, we addressed the influence of high HCy levels on the hypoxia‐induced, adenosine‐mediated, alteration of lymphocyte viability. We treated mitogen‐stimulated human lymphocytes isolated from healthy individuals and the human lymphoma T‐cell line CEM with cobalt chloride (CoCl₂)to reproduce hypoxia. We found that CoCl₂‐altered cell viability was dose‐dependently reversed using HCy. In turn, the HCy effect was inhibited using DL‐propargylglycine, a specific inhibitor of the hydrogen sulphide (H₂S)‐synthesizing enzyme cystathionine‐γ‐lyase involved in HCy catabolism. We then addressed the intracellular metabolic pathway of adenosine and HCy, and the role of the adenosine A_2A receptor (A_2AR). We observed that: (i) hypoxic conditions lowered the intracellular concentration of HCy by increasing adenosine production, which resulted in high A_2AR expression and 3′, 5′‐cyclic adenosine monophosphate production; (ii) increasing intracellular HCy concentration reversed the hypoxia‐induced adenosinergic signalling despite high adenosine concentration by promoting both S‐adenosylhomocysteine and H₂S production; (iii) DL‐propargylglycine that inhibits H₂S production abolished the HCy effect. Together, these data suggest that high HCy levels prevent, via H₂S production and the resulting down‐regulation of A_2AR expression, the hypoxia‐induced adenosinergic alteration of lymphocyte viability. We point out the relevance of these mechanisms in the pathophysiology of cardiovascular diseases.