l-arginine inhibited apoptosis of fish leukocytes via regulation of NF-κB-mediated inflammation, NO synthesis, and anti-oxidant capacity.

The increased apoptosis plays an important role in bacterial invasion. In addition, LPS can induce inflammation and apoptosis of leukocytes via the production of reactive oxygen and nitrogen species. In the present study, we investigated the potential protective role of l-arginine (L-Arg) against the apoptosis of fish leukocytes in vitro. The results of Annexin V-FITC/PI staining and TUNEL assay indicated that L-Arg significantly alleviated the apoptosis of fish leukocytes induced by LPS at 24 h and 72 h post incubation (hpi). High caspase-3 activities induced by LPS at 72 hpi were significantly inhibited by L-Arg. Moreover, L-Arg supplementation also significantly decreased the mRNA expression levels of caspases at most time points, which contributed to the anti-apoptotic roles of L-Arg. Further analysis showed that L-Arg significantly inhibited the expression of several pro-inflammatory cytokines including IL-8 and TNF-α, partially via the down-regulation of the genes involved in NF-κB/MyD88 including NF-κB, IKKα and IKKγ. The down-regulation of these pro-inflammatory cytokines by L-Arg supplementation led to the further decrease in the expression of death receptor FasL, contributing to the anti-apoptotic effect of L-Arg. In addition, L-Arg supplementation increased both iNOS mRNA expression and NO production. The mRNA expressions of several anti-oxidant enzymes including SOD, CAT and GSHPx were also significantly increased after L-Arg supplementation, which accelerated the clearance of reactive oxygen species. In all, L-Arg inhibited apoptosis of fish leukocytes both via the increased NO production and antioxidant capacity and via the inhibition of inflammation mediated by NF-κB/MyD88 pathway.