No abstract

Extracellular purines, including adenosine and ATP, are potent endogenous immunomodulatory molecules. Inosine, a degradation product of these purines, can reach high concentrations in the extracellular space under conditions associated with cellular metabolic stress such as inflammation or ischemia. In the present study, we investigated whether extracellular inosine can affect inflammatory/immune processes. In immunostimulated macrophages and spleen cells, inosine potently inhibited the production of the proinflammatory cytokines TNF-α, IL-1, IL-12, macrophage-inflammatory protein-1α, and IFN-γ, but failed to alter the production of the anti-inflammatory cytokine IL-10. The effect of inosine did not require cellular uptake by nucleoside transporters and was partially reversed by blockade of adenosine A1 and A2 receptors. Inosine inhibited cytokine production by a posttranscriptional mechanism. The activity of inosine was independent of activation of the p38 and p42/p44 mitogen-activated protein kinases, the phosphorylation of the c-Jun terminal kinase, the degradation of inhibitory factor κB, and elevation of intracellular cAMP. Inosine suppressed proinflammatory cytokine production and mortality in a mouse endotoxemic model. Taken together, inosine has multiple anti-inflammatory effects. These findings, coupled with the fact that inosine has very low toxicity, suggest that this agent may be useful in the treatment of inflammatory/ischemic diseases.

Enhanced formation of nitric oxide (NO) by both the constitutive and the inducible isoforms of NO synthase (NOS) has been implicated in the pathophysiology of a variety of diseases, including circulatory shock. Non-isoform-selective inhibition of NO formation, however, may lead to side effects by inhibiting the constitutive isoform of NOS and, thus, the various physiological actions of NO. S-Methylisothiourea sulfate (SMT) is at least 10-to 30-fold more potent as an inhibitor of inducible NOS (iNOS) in immunostimulated cultured macrophages (EC50, 6 ,AM) and vascular smooth muscle cells (EC50, 2 ,uM) than NG-methyl-L-arginine (MeArg) Moreover, therapeutic administration of SMT (5 mg/kg, i.p., given 2 hr after LPS, 10 mg/kg, i.p.) attenuates the rises in plasma alanine and aspartate aminotransferases, bilirubin, and creatinine and also prevents hypocalcaemia when measured 6 hr after administration of LPS. SMT (1 mg/kg, i.p.) improves 24-hr survival of mice treated with a high dose of LPS (60 mg/kg, i.p.). Thus, SMT is a potent and selective inhibitor of iNOS and exerts beneficial effects in rodent models of septic shock. SMT, therefore, may have considerable value in the therapy of circulatory shock of various etiologies and other pathophysiological conditions associated with induction of iNOS.Nitric oxide (NO) is produced by a group of isoenzymes collectively termed NO synthases (NOSs) (1,2). NO derived from the constitutive isoform is a neurotransmitter in the central and peripheral nervous system (3). NO generated by the constitutive isoform of NOS in the vascular endothelium (eNOS) is involved in the regulation of blood pressure and organ blood flow distribution and inhibits the adhesion of platelets and polymorphonuclear granulocytes to the endothelial surface (1, 2, 4). Dysfunction of NO formation by the vascular endothelium is implicated in the pathogenesis of hypertension, hypercholesterolemia, diabetes mellitus, ischemia-reperfusion injury, angina pectoris, subarachnoid hemorrhage, and various forms of circulatory shock (5-11). NO derived from eNOS also plays a protective role in endotoxininduced gastointestinal damage (12, 13).Another isoform of NOS (inducible NOS, iNOS) can be induced in various cells, including macrophages, by a variety of agents such as endotoxin (bacterial lipopolysaccharide, LPS), interleukin 1, tumor necrosis factor, and y-interferon (IFN-y). The cytotoxicity of NO from activated macrophages plays a key role in their antimicrobial activity (1, 2). Enhanced formation of NO following the induction of iNOS contributes importantly to the circulatory failure (hypotension and vascular hyporeactivity to vasoconstrictor agents) in circulatory shock of various etiologies (14-19). Moreover, iNOS plays a role in the pathophysiology of a variety of other diseases, including diabetes and transplant rejection (20-28). However, non-isoform-selective inhibition of NO formation may lead to side effects by inhibiting the constitutive isoform of NOS and, thus, the various physiological acti...

1 The induction of a calcium-independent isoform of nitric oxide (NO) synthase (iNOS) and a subsequent enhanced formation of NO has been implicated in the pathophysiology of a variety of diseases including inflammation and circulatory shock. Here we demonstrate that the S-substituted isothioureas, S-methylisothiourea (SMT), S-(2-aminoethyl)isothiourea (aminoethyl-TU), Sethylisothiourea (ethyl-TU) and S-isopropylisothiourea (isopropyl-TU) potently inhibit iNOS activity in J774.2 macrophages activated with bacterial endotoxin with EC50 values 8-24 times lower than that of N0-methyl-L-arginine (MeArg) and 200-times lower than that of NG-nitro-L-arginine (L-NO2Arg). 2 The inhibition of iNOS activity by these S-substituted isothioureas is dose-dependently prevented by excess of L-arginine suggesting that these isothioureas are competitive inhibitors of iNOS at the L-arginine binding site. 3 Ethyl-TU and isopropyl-TU are 4-6 times more potent than MeArg in inhibiting the constitutive NOS activity in homogenates of bovine aortic endothelial cells (eNOS) and are more potent pressor agents than MeArg in the anaesthetized rat. SMT is equipotent with MeArg, whereas aminoethyl-TU is 6-times less potent in inhibiting eNOS activity in vitro. Both SMT and aminoethyl-TU, however, elicit only weak pressor responses (approximately 15 mmHg at 10mg kg-', i.v.) in vivo. 4 A comparison of the potencies of ethyl-, iso-propyl-, n-propyl-, t-butyl-and n-butyl-isothioureas on iNOS activity shows that the inhibitory activity of S-substituted isothioureas declines sharply if the side chain exceeds 2 carbon atoms in length. Similarly, substitution of the ethylene side chain of ethyl-TU also results in a diminished potency. Substitution of either one or both nitrogens of SMT with either amino or alkyl groups also substantially reduces its NOS inhibitory potency. 5 In conclusion, isothioureas represent a new class of NOS inhibitors which includes the most potent inhibitors of iNOS activity reported to date. Some members of this class (ethyl-TU and isopropyl-TU) are potent inhibitors of eNOS and iNOS with little selectivity towards either isoform, while others (SMT and aminoethyl-TU) are relatively selective inhibitors of iNOS activity. These latter agents may become useful tools for studying the role of iNOS in various disease models and may be useful in the therapy of diseases that are associated with an enhanced formation of NO due to iNOS induction, such as inflammation, circulatory shock or cancer.