Argininosuccinate synthetase overexpression in vascular smooth muscle cells potentiates immunostimulant-induced NO production.

Immunostimulants trigger vascular smooth muscle cells (VSMC) to express both the inducible isoform of NO synthase (iNOS) and argininosuccinate synthetase (AS). With constitutively expressed argininosuccinate lyase (AL), AS confers cells with an Arg/Cit cycle that can sustain NO production via continuous regeneration of the NOS substrate, L-arginine (Arg), from the NOS coproduct, L-citrulline (Cit). To assess whether NO synthesis can be rate-limited by Arg recycling, we tested whether AS-overexpressing cells have an enhanced capacity for immununostimulant-induced NO synthesis. Rat VSMC were stably transfected with human AS cDNA in a eukaryotic cell expression vector, driven by a strong viral promoter. AS activity in transfected VSMC exceeded that induced in untransfected cells treated for 24 h with a combination of bacterial lipopolysaccharide and interferon-gamma (LPS/IFN). AS activity was predominantly associated with membranes but was also found in cytosol. Recombinant AS was purified from cytosol and possessed a specific activity exceeding that reported for native AS. Western blotting verified the basal expression of AS antigen in membranes from untreated AS-transfected VSMC and from untransfected VSMC after 24 h exposure to LPS/IFN. Epifluorescence histochemistry revealed a punctate distribution of AS antigen in transfected cells, consistent with a predominant membrane localization. Remarkably, on a per cell basis, LPS/IFN-induced NO production was 3-4-fold greater in AS-transfected cells than untransfected VSMC. In untransfected VSMC, maximal NO production during 48 h required millimolar Arg; notably, Cit was needed at approximately 3-fold higher concentrations than Arg for a comparable NO synthesis rate. In contrast, AS-transfected VSMC utilized Arg and Cit equi-effectively and at much lower concentrations; 100 microM of either precursor supported a maximal rate of NO synthesis for 48 h. The enhanced ability of AS-transfected cells to produce NO, compared with untransfected cells, could not be ascribed to differences in iNOS protein content or LPS/IFN potency for immunoactivation. We conclude that transfection with AS provides a continuous flux of Arg which drives NO synthesis in immunoactivated VSMC. Arg regeneration by AS is rate-limiting to NO synthesis and apparently provides iNOS with a preferred cellular source of Arg. In accord with the reported "channeling" of substrates by urea cycle enzymes, we hypothesize that the Arg/Cit cycle sequesters a discrete pool of recyclable substrate that sustains high-output NO synthesis.