Selective activation of heparin cofactor II by a sulfated polysaccharide isolated from the leaves of Artemisia princeps

Effects of leaf extracts from Croton zambesicus Müell. Arg. on hemostasis

ETHNOPHARMACOLOGICAL RELEVANCE

The leaf decoction of Croton zambesicus Müell. Arg. (Euphorbiaceae; syn. Croton amabilis Müell. Arg., Croton gratissimus Burch) is traditionally used in Benin to treat hypertension.

AIM OF THE STUDY

As hypertension and thromboembolism are often associated in several cardiovascular diseases, we studied the potential effects of leaf extracts from Croton zambesicus on hemostasis.

MATERIALS AND METHODS

We prepared the dichloromethane and aqueous extracts from the air-dried leaves of Croton zambesicus and separated the aqueous extract in its aqueous and dichloromethane fractions. The potential effects of these four extracts/fractions were investigated on red blood cells integrity using spectrophotometric lysis assays, on primary hemostasis using platelet aggregation studies and on secondary hemostasis using calibrated automated thrombin generation assays and coagulation factors inhibition tests.

RESULTS

In the in vitro testing, we found that none of the tested extracts/fractions exhibit hemolytic or antiplatelet activity. However, they display a moderate but significant anticoagulant activity which would be mediated through the direct inhibition of thrombin, FXa and TF/FVIIa. The active anticoagulant compound(s) seem to be mainly in the aqueous extract and especially in its aqueous fraction.

CONCLUSIONS

This experimental work reported for the first time the anticoagulant effect of leaf extracts from Croton zambesicus. These findings are of particular interest as the leaves from Croton zambesicus are commonly used in infusion by local population and may provide a new natural source for the development of original anticoagulant agents. Furthermore, this activity, associated with the vasorelaxant properties of some of its diterpenes may prove to be interesting for the prevention of cardiovascular diseases in traditional medicine.

Adenovirus-mediated expression of heparin cofactor II inhibits thrombin-induced cellular responses in fibroblasts and vascular smooth muscle cells

Heparin cofactor II functions as a physiological inhibitor of thrombin activity. The rate of inactivation of thrombin by heparin cofactor II is increased in the presence of dermatan sulfate, which is produced by fibroblasts or smooth muscle cells. To elucidate the role of heparin cofactor II in the extravascular cells, we induced expression of heparin cofactor II in cultured human fibroblasts or vascular smooth muscle cells using adenovirus-mediated gene transfer. After infection of adenovirus vector, these cells secreted heparin cofactor II protein into culture medium. The expressed heparin cofactor II formed the complex with exogenous thrombin and inhibited the proteolytic activity of thrombin. Expression of heparin cofactor II by infection of adenovirus vector inhibited thrombin-induced tissue-type plasminogen activator and interleukin-6 releases from fibroblasts and thrombin-induced interleukin-6 release from vascular smooth muscle cells. These findings show that fibroblasts and vascular smooth muscle cells expressing heparin cofactor II are resistant to thrombin-induced cellular responses.

The Preferred Pathway of Glycosaminoglycan-accelerated Inactivation of Thrombin by Heparin Cofactor II

Thrombin (T) inactivation by the serpin, heparin cofactor II (HCII), is accelerated by the glycosaminoglycans (GAGs) dermatan sulfate (DS) and heparin (H). Equilibrium binding and thrombin inactivation kinetics at pH 7.8 and ionic strength (I) 0.125 m demonstrated that DS and heparin bound much tighter to thrombin (K(T(DS)) 1-5.8 microm; K(T(H)) 0.02-0.2 microm) than to HCII (K(HCII(DS)) 236-291 microm; K(HCII(H)) 25-35 microm), favoring formation of T.GAG over HCII.GAG complexes as intermediates for T.GAG.HCII complex assembly. At [GAG] << K(HCII(GAG)) the GAG and HCII concentration dependences of the first-order inactivation rate constants (k(app)) were hyperbolic, reflecting saturation of T.GAG complex and formation of the T.GAG.HCII complex from T.GAG and free HCII, respectively. At [GAG] >> K(HCII(GAG)), HCII.GAG complex formation caused a decrease in k(app). The bell-shaped logarithmic GAG dependences fit an obligatory template mechanism in which free HCII binds GAG in the T.GAG complex. DS and heparin bound fluorescently labeled meizothrombin(des-fragment 1) (MzT(-F1)) with K(MzT(-F1)(GAG)) 10 and 20 microm, respectively, demonstrating a binding site outside of exosite II. Exosite II ligands did not attenuate the DS-accelerated thrombin inactivation markedly, but DS displaced thrombin from heparin-Sepharose, suggesting that DS and heparin share a restricted binding site in or nearby exosite II, in addition to binding outside exosite II. Both T.DS and MzT(-F1).DS interactions were saturable at DS concentrations substantially below K(HCII(DS)), consistent with DS bridging T.DS and free HCII. The results suggest that GAG template action facilitates ternary complex formation and accommodates HCII binding to GAG and thrombin exosite I in the ternary complex.

Activation of heparin cofactor II by calcium spirulan

Heparin cofactor II (HCII) is a plasma serine protease inhibitor whose ability to inhibit alpha-thrombin is accelerated by a variety of sulfated polysaccharides in addition to heparin and dermatan sulfate. Previous investigations have indicated that calcium spirulan (Ca-SP), a novel sulfated polysaccharide, enhanced the rate of inhibition of alpha-thrombin by HCII. In this study, we investigated the mechanism of the activation of HCII by Ca-SP. Interestingly, in the presence of Ca-SP, an N-terminal deletion mutant of HCII (rHCII-Delta74) inhibited alpha-thrombin, as native recombinant HCII (native rHCII) did. The second-order rate constant for the inhibition of alpha-thrombin by rHCII-Delta74 was 2.0 x 10(8) M(-1) min(-1) in the presence of 50 microgram/ml Ca-SP and 10, 000-fold higher than in the absence of Ca-SP. The rates of native rHCII and rHCII-Delta74 for the inhibition of gamma-thrombin were increased only 80- and 120-fold, respectively. Our results suggested that the anion-binding exosite I of alpha-thrombin was essential for the rapid inhibition reaction by HCII in the presence of Ca-SP and that the N-terminal acidic domain of HCII was not required. Therefore, we proposed a mechanism by which HCII was activated allosterically by Ca-SP and could interact with the anion-binding exosite I of thrombin not through the N-terminal acidic domain of HCII. The Arg(103) --> Leu mutant bound to Ca-SP-Toyopearl with normal affinity and inhibited alpha-thrombin in a manner similar to native rHCII. These results indicate that Arg(103) in HCII molecule is not critical for the interaction with Ca-SP.

Sulfated polysaccharide from the leaves of Artemisia Princeps activates heparin cofactor II independently of the Lys173 and Arg189 residues of heparin cofactor II

A sulfated polysaccharide (AFE-HCD) purified from the leaves of Artemisia princeps Pamp selectively accelerated the rate of thrombin inhibition by heparin cofactor II (HCII). By using plasma derived HCII and bacterial expressed recombinant HCII molecules, the interaction between each HCII molecule and AFE-HCD was analyzed. AFE-HCD accelerated thrombin inhibition by plasma derived HCII or bacterial expressed wild type HCII to the same extent (IC50: 0.056 micrograms/ml for plasma derived HCII and 0.066 micrograms/ml for recombinant HCII under the experimental condition). The recombinant HCII (rHCII) molecule with Lys173-->Leu or Arg189-->His substitution, which is defective in interactions with heparin and dermatan sulfate, respectively, is activated by AFE-HCD to inhibit thrombin in a manner similar to wild type rHCII. These results suggested that activation of HCII was independent of its Lys173 or Arg189 residue. Although AFE-HCD is a selective activator of HCII like dermatan sulfate, the amino acid residue required for the activation of HCII was distinct form that of dermatan sulfate as well as heparin.