Cytokine-activated endothelial cells express an isotype of nitric oxide synthase which is tetrahydrobiopterin-dependent, calmodulin-independent and inhibited by arginine analogs with a rank-order of potency characteristic of activated macrophages
We have previously reported that cultured murine brain endothelial cells (MBE) produce large quantities of nitric oxide (NO) after activation with interferon-gamma in combination with any of several immunoactivators including: bacterial endotoxin, tumor necrosis factor and interleukin-1. Since endothelial cells are the first example of a cell-type which may possess both a constitutive and an inducible type of NO synthase, it was of interest to compare the requirements of these two enzyme activities. Induction of NO synthesis in MBE by cytokines was abolished by the protein synthesis inhibitor, cycloheximide, and by 2,4-diamino-6-hydroxypyridine (DAHP), a selective inhibitor of GTP cyclohydrolase I, the rate-limiting enzyme for de novo synthesis of tetrahydrobiopterin (THB). In the presence of DAHP, NO synthesis was restored by sepiapterin (SEP), a substrate for the alternative pathway of THB synthesis occurring via pterin salvage. Moreover, SEP increased NO synthesis to greater than 150% of control values, suggesting that THB availability is rate-limiting for NO synthesis by cytokine-induced MBE. Methotrexate, an inhibitor of the pterin salvage pathway of THB synthesis, completely reversed the stimulation of NO synthesis by sepiapterin. Thus, cytokine-induced MBE NO synthase appears to have an absolute requirement for THB as cofactor. In additional studies we found that NO synthesis by cytokine-activated MBE was inhibited by NG-monosubstituted arginine analogs with a rank-order of potency NH2 greater than CH3 greater than NO2, in contrast with the rank-order of NO2 greater than NH2 greater than CH3 previously described for inhibition of the constitutive endothelial cell enzyme. Using a kinetic assay for NO synthase activity, based on oxidation of myoglobin heme-iron, we have found that these rank orders of potency also apply to cytosol preparations of cytokine-induced and untreated endothelial cells, respectively. Further differences between constitutive and cytokine-induced NO synthase were observed with regard to calmodulin requirements. Whereas constitutive NO synthase was potently inhibited by the calmodulin antagonists mellitin and trifluoperazine, cytokine-induced NO synthase was unaffected. In summary, NO synthesis by cytokine-activated MBE is THB-dependent, calmodulin-independent and inhibited by NG-substituted arginine analogs with a rank-order profile distinct from that for untreated endothelial cells but identical to that for cytokine-activated macrophages.