Hypoxia inhibits the induction of argininosuccinate synthetase by endotoxin in lung endothelial cells

Pulmonary artery endothelial cells (PAEC) possess a two-step pathway for synthesizing L-arginine from L-citrulline. The first and rate-limiting step is catalyzed by argininosuccinate synthetase (AS). We have previously shown that hypoxia inhibits synthesis of L-arginine from L-citrulline in PAEC. In this study, we examined the effect of hypoxia on the induction of AS in PAEC. Porcine PAEC were incubated with or without endotoxin under normoxia (air-5% CO₂) or hypoxia (0% O₂-95% N₂-5% CO₂) for 24 h, and then AS activity and AS mRNA content were determined. Incubation with endotoxin resulted in increases in AS activity and mRNA, and the latter was blocked by actinomycin D. Exposure to hypoxia for 24 h decreased AS activity and mRNA content and stability, and it also abolished the increases in AS activity and mRNA induced by endotoxin. These results indicate that hypoxia inhibits endotoxin- mediated induction of AS. This inhibition might reduce the availability of intracellular L-arginine and thereby limit immunostimulant-induced nitric oxide production by lung endothelial cells.