Retinoic acid and host-pathogen interactions: effects on inducible nitric oxide synthase in vivo

Vitamin A and its metabolite retinoic acid modulate the host response to pa thogens through poorly characterized mechanisms. In vitro studies have sugg ested that retinoic acid decreases inducible NO synthase (NOS2, or iNOS) ex pression, a component of innate immunity, in several cell types stimulated with lipopolysaccharide (LPS) or cytokines. This study investigated the eff ect of retinoic acid on LPS-stimulated NOS2 expression in vivo. Wistar-Kyot o rats received all-trans retinoic acid (RA, 10 mg/kg) or vehicle intraperi toneally daily for 5 days followed by LPS (4 mg/kg) or saline intraperitone ally and were killed 6 h later. NOS2 activation was estimated by mRNA (RT-P CR) and protein (Western-blot) expression and plasma nitrate/nitrite accumu lation. In sharp contrast to previous in vitro study reports, RA significan tly enhanced NOS2 mRNA, protein expression, and plasma nitrate/nitrite conc entration in LPS-injected rats but not in saline-injected rats. This was as sociated with increased expression of interleukin-2, interferon (IFN)-gamma and IFN regulatory factor-1 mRNAs in several organs and increased IFN-gamm a plasma concentration. RA significantly increased mortality in LPS-injecte d rats. The NOS inhibitor aminoguanidine (50 mg/kg before LPS injection) si gnificantly attenuated the RA-mediated increase in mortality. These results demonstrate for the first time that RA supplementation in vivo enhances ac tivation of the LPS-triggered NOS2 pathway.