Interactions between bradykinin (BK) and cell adhesion molecule (CAM) expression in peptidoglycan‐polysaccharide (PG‐PS)‐induced arthritis

Bradykinin (BK), a vasoactive, proinflammatory nonapeptide, promotes cell adhesion molecule (CAM) expression, leukocyte sequestration, inter-endothelial gap formation, and protein extravasation in postcapillary venules. These effects are mediated by bradykinin-1 (B1R) and-2 (B2R) receptors. We delineated some of the mechanisms by which BK could influence chronic inflammation by altering CAM expression on leukocytes, endothelium, and synovium in joint sections of peptidoglycan-polysaccharide-injected Lewis rats. Blocking B1R results in significantly increased joint inflammation. Immunohistochemistry of the B1R antagonist group revealed increased leukocyte and synovial CD11b and CD54 expression and increased CD11b and CD44 endothelial expression. B2R antagonism decreased leukocyte and synovial CD44 and CD54 and endothelial CD11b expression. Although these findings implicate B2R involvement in the acute phase of inflammation by facilitating leukocyte activation (CD11b), homing (CD44), and transmigration (CD54). Treatment with a B2R antagonist did not affect the disease evolution in this model. In contrast, when both BK receptors are blocked, the aggravation of inflammation by B1R blockade is neutralized and there is no difference from the disease-untreated model. Our findings suggest that B1R and B2R signaling show physiologic antagonism. B1R signaling suggests involvement in down-regulation of leukocyte activation, transmigration, and homing. Further studies are needed to evaluate the B1 receptor agonist's role in this model.

High molecular mass kininogen inhibits cathepsin G-induced platelet activation by forming a complex with cathepsin G

INTRODUCTION

Preliminary studies have shown that high molecular mass kininogen (HK) inhibits cathepsin G-induced platelet activation. However, the potential mechanism underlying this inhibitory effect remains to be elucidated.

MATERIALS AND METHODS

Suspensions of washed and gel-filtered platelets were used in radioligand binding and aggregation studies. The amidolytic activity of cathepsin G was measured using specific chromogenic substrate. Western blot technique was utilised to explore the potential complex formation between cathepsin G and HK. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis was used to analyse the cleavage products of HK.

RESULTS

At a concentration of 1 microM, HK completely blocked cathepsin G-induced platelet shape change and secretion of ATP. HK inhibited cathepsin G-induced platelet aggregation in a concentration-dependent manner with an IC(50) of 0.48 microM. Moreover, HK was found to inhibit binding of (125)I-cathepsin G to gel-filtered platelets. (125)I-cathepsin G forms a complex with HK. The complex formation did not affect the amidolytic activity of cathepsin G. HK was proteolysed upon interaction with cathepsin G.

CONCLUSION

Our results show that high molecular mass kininogen down-regulates cathepsin G-induced platelet activation by forming a complex with cathepsin G and thus prevents binding of cathepsin G to platelets. These kininogen-cathepsin G interactions may be potential targets for pharmacological intervention.

Activation of plasma contact and coagulation systems and neutrophils in the active phase of ulcerative colitis

We have shown that the contact (kallikrein-kinin) system is involved in the pathogenesis of experimental enterocolitis. We now investigate activation of the contact and coagulation pathways, platelets, and neutrophils in active and inactive ulcerative colitis patients as compared to normal controls. In active ulcerative colitis patients, a significant decrease of plasma prekallikrein, high molecular weight kininogen, and C1 inhibitor levels was observed as compared with controls, as well as prekallikrein activation on western blots. Significant elevation of prothrombin fragment (F1 + 2), which indicates thrombin generation, and elastase-alpha 1-antitrypsin complexes, reflecting neutrophil activation, were found in patients with active disease. Plasma beta-thromboglobulin, a marker of platelet activation, was elevated in both active and inactive disease and appears to be a feature of ulcerative colitis. Activation of contact and coagulation pathways, as well as neutrophils, may mediate inflammation in the active phase of ulcerative colitis.

Human kininogens regulate thrombin binding to platelets through the glycoprotein Ib-IX-V complex

We and others have shown that both high and low molecular mass kininogens are able to inhibit the thrombin-induced aggregation of gel-filtered platelets, indicating that the locus for inhibition resides in the heavy chain. The inhibitory site is present in domain 3, confined to the C-terminal portion of the region encoded by exon 7 (K270-G292), and the minimal effective sequence is a heptapeptide (L271-A277; Kunapuli et al, J Biol Chem 271:11228, 1996). Kininogens inhibit thrombin binding to platelets and thus inhibit thrombin-induced aggregation. The molecular mechanism by which kininogens inhibit thrombin-induced aggregation of platelets is unknown. Thrombin has previously been shown to bind to two receptors on the platelet surface, glycoprotein (GP) Ib-IX-V complex and the hepta-spanning transmembrane receptor coupled to G protein(s). We now show that, unlike its effect on normal platelets, kininogen (2 micromol/L) did not inhibit the thrombin-induced aggregation of Bernard-Soulier platelets, which lack the GP Ib-IX-V complex, suggesting that kininogen interacts either directly or indirectly with that complex and restricts access by thrombin to this receptor. We further show that both recombinant K270-G292 polypeptide and the synthetic peptide L271-A277 derived from high molecular mass kininogen lower thrombin binding to platelets in a manner similar to monoclonal antibodies to or ligands (von Willebrand factor and echicetin) of GP Ib-IX. The anti-GP Ib-IX-V complex antibodies, TM-60 and SZ 2, can inhibit 125I-high molecular mass kininogen binding to platelets. Conversely, kininogen could block the binding of biotinylated TM-60 or of 125I-SZ 2. Kininogen inhibited the binding of biotinylated thrombin bound to a mouse fibroblast cell line transfected with the GP Ib-IX-V complex. These results indicated that kininogen binds to the GP Ib-IX-V complex modulating thrombin binding to platelets and the consequent platelet aggregation. Kininogen can thus serve as an important regulator of the early stages of platelet stimulation by thrombin.

Inhibition of plasma kallikrein prevents peptidoglycan-induced arthritis in the Lewis rat

We investigate whether the previously shown contact system activation plays a pathogenetic role in a rat model of acute inflammation induced by peptidoglycan-polysaccharide (PG-APS) using a new specific plasma kallikrein inhibitor, Bz-Pro-Phe-boroArg-OH (P8720). Group I (control) received neither PG-APS nor inhibitor. Group II (disease-treated) received PG-APS intraperitoneally (IP) and P8720 orally. Group III (disease-untreated) received PG-APS IP. Anemia was evident at 49 h in group III but was not present (P < 0.01) in groups I and II. Spleen weight was significantly decreased in group II compared to group III. Acute arthritis progressively developed in group III from 27 to 49 h, but P8720 decreased the joint swelling in group II by 61% (P < 0.0005). We observed a significant fall in prekallikrein and factor XI (P < 0.01) in groups II and III but not in group I. The decrease in the functional levels of high molecular weight kininogen (P < 0.05) observed in group III were prevented by P8720 in group II. The changes in T-kininogen and alpha 1-inhibitor 3 acute-phase proteins were partially prevented by P8720. We conclude that the inflammatory reactions leading to arthritis and anemia, as well as the acute-phase reaction, are due in part to contact activation, and that specific kallikrein inhibitors may have therapeutic potential.