Endothelial nitric oxide production is tightly coupled to the citrulline-NO cycle.

Nitric oxide (NO) is an important vasorelaxant produced along with L-citrulline from L-arginine in a reaction catalyzed by endothelial nitric oxide synthase (eNOS). Previous studies suggested that the recycling of L-citrulline to L-arginine is essential for NO production in endothelial cells. However, there is no direct evidence demonstrating the degree to which the recycling of L-citrulline to L-arginine is coupled to NO production. We hypothesized that the amount of NO formed would be significantly higher than the amount of L-citrulline formed due to the efficiency of L-citrulline recycling via the citrulline-NO cycle. To test this hypothesis, endothelial cells were incubated with [14C]-L-arginine and stimulated by various agents to produce NO. The extent of NO and [14C]-L-citrulline formation were simultaneously determined. NO production exceeded apparent L-citrulline formation of the order of 8 to 1, under both basal and stimulated conditions. As further support, alpha-methyl-DL-aspartate, an inhibitor of argininosuccinate synthase (AS), a component of the citrulline-NO cycle, inhibited NO production in a dose-dependent manner. The results of this study provide evidence for the essential and efficient coupling of L-citrulline recycling, via the citrulline-NO cycle, to endothelial NO production.

The caveolar nitric oxide synthase/arginine regeneration system for NO production in endothelial cells.

The enzyme endothelial nitric oxide synthase (eNOS) catalyzes the conversion of arginine, oxygen and NADPH to NO and citrulline. Previous results suggest an efficient, compartmentalized system for recycling of citrulline to arginine utilized for NO production. In support of this hypothesis, the recycling enzymes, argininosuccinate synthase (AS) and argininosuccinate lyase (AL), have been shown to colocalize with eNOS in caveolae, a subcompartment of the plasma membrane. Under unstimulated conditions, the degree of recycling is minimal. Upon stimulation of NO production by bradykinin, however, recycling is co-stimulated to the extent that more than 80% of the citrulline produced is recycled to arginine. These results suggest an efficient caveolar recycling complex that supports the receptor-mediated stimulation of endothelial NO production. To investigate the molecular basis for the unique location and function of endothelial AS and AL, endothelial AS mRNA was compared with liver AS mRNA. No differences were found in the coding region of the mRNA species, but significant differences were found in the 5'-untranslated region (5'-UTR). The results of these studies suggest that sequence in the endothelial AS-encoding gene, represented by position -92 nt to -43 nt from the translation start site in the extended AS mRNA 5'-UTRs, plays an important role in differential and tissue-specific expression. Overall, a strong evidential case has been developed supporting the proposal that arginine availability, governed by a caveolar-localized arginine regeneration system, plays a key role in receptor-mediated endothelial NO production.

Endothelial argininosuccinate synthase mRNA 5'-untranslated region diversity. Infrastructure for tissue-specific expression.

Based on the integral role that argininosuccinate synthase (AS) plays in the production of nitric oxide in vascular endothelial cells and urea in liver, an analysis was carried out to determine whether signals reside in the AS mRNA to account for tissue differences in AS function and location. Reverse transcriptase-PCR and sequence analysis showed that the AS mRNA coding region was the same for both endothelial cells and liver; however, 5'-RACE analysis (rapid amplification of cDNA ends) identified AS mRNA species in endothelial cells in addition to a major 43-nucleotide (nt) 5'-untranslated region (UTR) AS mRNA with overlapping extended 5'-UTRs of 66 and 92 nt. Comparison to the genomic sequence immediately upstream of the reported transcription start site for the human and mouse AS gene suggested that expression of all three species of bovine endothelial AS mRNA are driven by a common promoter and that 5'-UTR diversity in endothelial cells results from three transcriptional initiation sites within exon 1. RNase protection analysis and real-time reverse transcriptase-PCR verified and quantitated the differential expression of the extended 5'-UTR species relative to the major 43-nt 5'-UTR AS mRNA. In vitro translation studies showed a less pronounced but similar discordant expression. Sequential deletions starting from the 5' terminus of the 92-nt 5'-UTR construct resulted in a corresponding increase in translational efficiency, but the most pronounced effect resulted from mutation of an upstream open reading frame, which restored translational efficiency of the 92-nt 5'-UTR AS mRNA. When the different AS mRNA 5'-UTRs, cloned in front of a luciferase reporter gene, were transfected into endothelial cells, the pattern of luciferase expression was nearly identical to that observed for the different 5'-UTR AS mRNAs in endothelial cells. Given the different roles ascribed for argininosuccinate synthase, urea versus NO production, these results suggest that sequence in the AS gene represented by position -92 to -43 nt from the translation start site in the extended AS mRNA 5'-UTRs plays an important role in differential and tissue-specific expression.