L-Arginine-dependent suppression of apoptosis in Trypanosoma cruzi: Contribution of the nitric oxide and polyamine pathways

Until recently, a capacity for apoptosis and synthesis of nitric oxide ((NO )-N-.) were viewed as exclusive to multicellular organisms. The existence o f these processes in unicellular parasites was recently described, with the ir biological significance remaining to be elucidated. We have evaluated L- arginine metabolism in Trypanosoma cruzi in the context of human serum-indu ced apoptotic death. Apoptosis was evidenced by the induction of DNA fragme ntation and the inhibition of [H-3]thymidine incorporation, which were inhi bited by the caspase inhibitor Ac-Asp-Glu-Val-aspartic acid aldehyde (DEVD- CHO). In T, cruzi exposed to death stimuli, supplementation with L-arginine inhibited DNA fragmentation, restored [H-3]thymidine incorporation, and au gmented parasite (NO)-N-. production. These effects were inhibited by the ( NO)-N-. synthase inhibitor Nw-nitroarginine methyl ester(L-NAME), Exogenous (NO)-N-. limited DNA fragmentation but did not restore proliferation rates . Because L-arginine is also a substrate for arginine decarboxylase (ADC), and its product agmatine is a precursor for polyamine synthesis, we evaluat ed the contribution of polyamines to limiting apoptosis, Addition of agmati ne, putrescine, and the polyamines spermine and spermidine to T, cruzi sust ained parasite proliferation and inhibited DNA fragmentation. Also, the ADC inhibitor difluoromethylarginine inhibited L-arginine-dependent restoratio n of parasite replication rates, while the protection from DNA fragmentatio n persisted. In aggregate, these results indicate that T, cruzi epimastigot es can undergo programmed cell death that can be inhibited by L-arginine by means of (i) a (NO)-N-. synthase-dependent (NO)-N-. production that suppre sses apoptosis and (ii) an ADC-dependent production of polyamines that supp ort parasite proliferation.