D-amino acids trigger an inflammatory environment in vitro: potential role in modulating age-associated inflammation

Studies in vivo have demonstrated that the accumulation of D-amino acids (D-AAs) is associated with age-related diseases as well as increased immune activation. However, the underlying mechanisms are not well defined. The metabolism of D-AAs produces hydrogen peroxide (H 2 O 2 ), a reactive oxygen species that involves several physiological processes including immune response, cell differentiation and proliferation. Excessive levels of H 2 O 2 contributes to oxidative stress and eventually causes cell death, a characteristic of age-related pathology. We explored the molecular mechanisms of D-serine (D-Ser) and D-alanine (D-Ala) in human liver cancer cells, HepG2 cells, with attention to the production of H 2 O 2 , due to the oxidation of D-AAs by D-amino acid oxidase (DAO), and secretion of pro-inflammatory cytokine and chemokine, which subsequently leads to inflammation and cell death. In HepG2 cells, we demonstrated that D-Ser decreased H 2 O 2 production and induced concentration-dependent depolarization of MMP, with upregulation of activated NF-кB and pro-inflammatory cytokine, TNF-α, and chemokine, IL-8, which subsequently led to apoptosis. Conversely, D-Ala-treated cells induced H 2 O 2 production, accompanied by the upregulation of activated NF-кB, TNF-α and IL-8, but did not cause significant apoptosis. The present study confirms to the role of both D-Ser and D-Ala in inducing inflammatory responses but each via unique activation pathways. This response was associated with apoptotic cell death only with D-Ser. Further research is required to gain a better understanding of the mechanisms underlying inflammation induced by D-AAs and downstream consequences especially in the context of aging given the wide detection of these entities in systemic circulation.