Stimulation of high-affinity GTPase by trypsin and trypsin-like proteinases in membranes of human platelets

An antibody directed against residues 100-119 within the alpha-helical domain of Galpha(s) defines a novel contact site for beta-adrenergic receptors

A polyclonal antiserum that recognizes residues 100-119 within the alpha-helical domain of Galpha(s) (K-20) caused a dissociation of G(s) into its component subunits and activated a cholera toxin-sensitive high affinity GTPase. Consistently, the antibody mimicked the stimulatory effects of the beta-adrenergic agonist, isoproterenol, on adenylyl cyclase, which is mediated by Galpha(s), and its inhibitory action on NADPH-dependent H(2)O(2) generation, a Gbetagamma-mediated response. A peptide corresponding to the target sequence of K-20 not only neutralized the receptor-mimetic effects of the antibody but inhibited the whole spectrum of isoproterenol action as well, including its antagonistic effects on adenylyl cyclase and NADPH-dependent H(2)O(2) generation. By contrast, COOH-terminal anti-Galpha(s) selectively inhibited the stimulatory effect of isoproterenol on cAMP formation without affecting its inhibitory effect on NADPH-dependent H(2)O(2) generation. The data are consistent with the concept that beta-adrenergic receptors interact with multiple sites on Galpha(s) each playing a distinct role, and strongly suggest that antibody K-20 defines a novel contact site for beta-adrenergic receptors that localizes to the alpha-helical domain and is essential for eliciting the complete spectrum of beta-adrenergic responses.

Trypsin inhibits voltage-activated chloride conductance of toad skin

The effect of trypsin on the voltage-activated chloride conductance (GCl) of toad skin was investigated. Serosal application of > 0.1 mg ml-1 trypsin decreased the voltage-activated GCl without notable delay. The maximal inhibition to 38% of the control values, exerted within 15 min, was in some experiments partly or completely reversible. Chymotrypsin had much lower effect than trypsin. Mucosal application of trypsin did not have any effect. Trypsin did neither interfere with the conductive pathway opened by supramaximal concentrations of cAMP nor with the inhibitory effect of epinephrine on the voltage-activated GCl. The effect of trypsin required influx of Ca2+ from the extracellular space. It is concluded that protease-activated receptors or trypsin-sensitive proteins in the basolateral membrane of toad skin epithelial cells interfere with regulative steps involved in the voltage-activation of GCl. This may be harmful for the segregation of epithelial cells using this enzyme.

Kinetic characteristics of thrombin receptor-mediated responses in rat megakaryocytes

Kinetic characteristics of thrombin receptor-mediated responses on rat megakaryocytes were examined by the use of the perforated patch clamp combined with the rapid drug exchange system termed the 'Y-tube method'. The application of thrombin evoked repetitive Ca(2+)-activated K+ current (IKCa) in a concentration-dependent manner. The characteristic features for thrombin-induced response compared with purinoceptor-induced response were the long latency, long washout time and fast desensitization. The similar IKCa as thrombin was induced by trypsin. Thrombin- and trypsin-induced IKCa were both inhibited by the protease inhibitor, SBTI, and the washout time for thrombin was markedly shortened (7.4 +/- 2.2 s) when thrombin was washed out by a solution containing soybean trypsin inhibitor. A synthetic thrombin receptor agonist peptide induced IKCa oscillation with shorter latency than thrombin.

Separation by heparin-affinity chromatography of catalytically active and inactive forms of trypsin which retain the (Na-K)ATPase stimulating property

The possibility that different structural determinants on trypsin, other than catalytic sites, are involved in the cell membrane (Na-K)ATPase stimulating property was investigated by submitting bovine trypsin to two purification procedures: gel filtration on Sephadex G-50 and heparin-Sepharose chromatography. The latter procedure was also chosen in consideration of the known affinity for heparin displayed by serine proteinases. Trypsin peaks eluted from both columns were analysed by measuring esterolytic and proteolytic activities, the beef heart (Na-K)ATPase stimulating property and amino acid content. Fluorescence emission spectra and both non-denaturing and SDS-gel electrophoresis were also performed to test structural modifications on trypsin peaks. Four peaks eluted from Sephadex G-50 with variable estero-proteolytic and (Na-K)ATPase stimulating activities; the latter was also present in two peaks which displayed the lowest estero-proteolytic activities. All peaks proved to be trypsin in amino acid composition. Two peaks eluted from the heparin-Sepharose column with distinct biological activities: a first minor peak, eluted with the void volume, was catalytically inactive but it retained the (Na-K)ATPase stimulating activity. The second, major peak eluted mostly with 0.5 mol/l NaCl, displayed only esteroproteolytic activities, but no (Na-K)ATPase-stimulating activity. It overlapped control trypsin in both electrophoretic patterns, fluorescence emission spectrum and amino acid composition. The first peak showed differences with the parent compound, as revealed by the amino acid composition and tryptophan fluorescence emission spectrum. Marked differences were also observed in the electrophoretic pattern which only showed bands of low molecular mass mostly confined to the anode. NH2-terminus analysis confirmed that the first peak contained trypsin fragments originated from the parent compound after passage through the heparin column. It is hypothesized that trypsin binding to heparin causes structural alteration of the proteinase and primes the catalytic cleavage of fragments which lose heparin affinity and elute in the void volume. The results also confirm that the proteolytic mechanism is not involved in trypsin-mediated (Na-K)ATPase stimulation and indicate that heparin-Sepharose chromatography is a useful tool to separate catalytically active and inactive forms of trypsin.

Protease inhibitors and carcinogenesis: a review

This brief review article deals with the subject of anticarcinogenic activity of protease inhibitors (PI). Three basic premises are made: (1) Although PI are prevalent constituents of dietary staples such as soy products, which have been epidemiologically associated with reduced cancer incidences at multiple target sites, they are unlikely to be the active anticarcinogenic entities. Cooked soy products, which are devoid of PI activity, are equally as effective at reducing cancer development as raw soy products. Isoflavones are likely to represent major chemopreventive agents in soy, although other constituents may well contribute. (2) Although supplementation of diets with PI (natural or synthetic), or direct topical administration, results in lower cancer incidences in many experimental models in vivo, this effect appears to be indirect. Dietary PI are, in general, poorly absorbed from the GI tract, and never reach target organs in any measurable quantity. The most attractive hypothesis is that dietary PI could induce synthesis and distribution of endogenous PI (acute-phase reactants), which have widespread effects on cell growth and behavior. Effects of topical administration of PI also encompass prominent anti-inflammatory effects. (3) A spectrum of PI inhibit in vitro transformation induced by a variety of carcinogenic agents. Their effects can be grouped into three basic categories, affecting: (a) signal transduction pathways; (b) DNA repair processes; and (c) nuclear proteases. I suggest that the nuclear multicatalytic protease activity, in particular the chymotrypsin-like activity, represents an important cellular target for which considerable anecdotal support can be garnered.

Pharmacological characterization of epinephrine-stimulated GTPase activity in human platelet membranes

The alpha 2-adrenergic receptor-mediated stimulation of GTPase activity was investigated in human platelet membranes. The stimulatory effect of (-)-epinephrine was strictly dependent on Mg2+ and derived from a high-affinity GTPase activation. (-)-Epinephrine and (-)-norepinephrine stimulated GTPase activity in a concentration-dependent manner with EC50 values of 200 and 600 nM, respectively. These effects were stereospecific, since (+/-)-epinephrine, (+/-)-norepinephrine, and (+)-epinephrine were less potent in stimulating the enzyme activity with EC50 values of 4, 1 and 3 microM, respectively. Thrombin also stimulated GTPase activity concentration dependently with an EC50 value of 0.02 U/mL. The maximal effects of (-)-epinephrine, (-)-norepinephrine, and thrombin were not additive in any combination. Clonidine did not stimulate GTPase activity, whereas another synthetic alpha 2-adrenergic agonist, p-aminoclonidine, had the characteristics of a partial agonist. The rank order of potency for antagonists to inhibit the activation of GTPase by 1 microM (-)-epinephrine was yohimbine = rauwolscine > idazoxan = oxymetazoline = phentolamine = WB4101 = (+)-mianserin > (-)-mianserin > prazosin > (-)-propranolol. Negative logarithms of the IC50 values of these antagonists corresponded well with the negative logarithmic values of Ki(pKi) for the alpha 2A-adrenergic receptors determined by a receptor-binding technique in human platelets. These results indicate that epinephrine stimulates high-affinity GTPase activity of G proteins (putatively Gi2), which are also coupled with thrombin receptors, in a Mg(2+)-dependent and stereospecific manner, via alpha 2A-adrenergic receptor activation in human platelet membrane preparations.

Differential involvement of membrane (Na-K)ATPase in thrombin- and trypsin-mediated platelet activation

The possibility that platelet activation may also involve membrane (Na-K)ATPase was investigated by testing the effects of both proteinases on platelet shape change and aggregation in the absence and presence of the specific (Na-K)ATPase inhibitor ouabain. Ouabain (8 to 80 microM) completely antagonized trypsin-induced platelet shape change and aggregation when it was preincubated with platelet suspension before the addition of trypsin. Unlike trypsin, thrombin-induced platelet activation was significantly enhanced by ouabain. It was also observed that on partially purified beef heart (Na-K)ATPase preparation, thrombin significantly enhanced the enzyme inhibition caused by submaximal inhibitory concentrations of ouabain. Soybean trypsin inhibitor (4 micrograms/ml) employed as the agent capable to counteract proteinase effects on the (Na-K)ATPase, was shown both to prevent and antagonize the platelet activation induced by trypsin (0.3 to 1.5 micrograms/ml), but it failed to modify the responses evoked by thrombin. It is concluded that membrane (Na-K)ATPase is involved differently in platelet activation by trypsin and thrombin probably because receptor sites to which either proteinase on the platelet surface binds, are distinct. Direct enzyme involvement is indeed apparent only in trypsin-induced platelet activation.

Purification of proteinase-like and Na+/K(+)-ATPase stimulating substance from plasma of insulin-dependent diabetics and its identification as alpha 1-antitrypsin

The purpose of this study was to purify and identify the proteinase-like substance previously recognized as responsible for the Na+/K(+)-ATPase stimulating property of plasma from insulin-dependent diabetic subjects. Anion-exchange chromatography followed by two-step heparin affinity chromatography resulted in a fraction highly enriched in both potent Na+/K(+)-ATPase stimulating activity and potent proteolytic activity. Approx. 400 micrograms of purified protein was isolated from 62 mg of starting plasma proteins. When analyzed on sodium dodecyl sulfate gels the active fraction consisted mainly of one polypeptide band with an apparent molecular mass of 66 kDa under either reducing or nonreducing conditions. The proteinase-like properties of the purified fraction were further revealed by its ability to clot plasma, split fibrinogen with production of fibrinopeptide A and induce shape change in human platelets and irreversible platelet aggregation in the presence of the stable analogue of endoperoxides U46619. Its additional capacity to affect platelet phosphoinositol metabolism was shown by the stimulation of protein kinase C-dependent phosphorylation of 47 kDa platelet membrane protein. In designing an identification protocol for the purified fraction, it was postulated that plasma proteinases are probably bound to their inhibitors, to form a stable covalently linked complex. The possibility that a proteinase-proteinase inhibitor complex was purified instead of single proteinase(s) was investigated. Neither trypsin nor neutrophil elastase were present in the active fraction whereas, among the possible plasma proteinase inhibitors tested, immunoreactivity was observed only in the presence of alpha 1-antitrypsin (alpha 1 AT) antiserum. Double immunodiffusion showed that control human alpha 1 AT and the plasma-purified fraction shared common antigens. Furthermore, both isoelectric focusing and amino acid composition analysis showed that the two substances were similar. The results obtained indicate that alpha 1 AT is apparently the only active component of the purified fraction from the plasma of insulin-dependent diabetics, thus suggesting that an altered form of the inhibitor is responsible for the broad range of proteinase-like effects elicited by the plasma-purified fraction.

Involvement of the "tethered-ligand" receptor in thrombin inhibition of platelet adenylate cyclase

Thrombin is thought to activate platelets through multiple signaling pathways. Recently a new thrombin receptor was identified (Vu et al., Cell 64:1057-1068, 1991) that recognizes alpha-thrombin's anion-binding exosite. Thrombin cleaves this receptor generating a new N-terminal ("tethered-ligand") that activates the receptor. We report here that this receptor is involved in alpha-thrombin inhibition of platelet adenylate cyclase, a process thought mediated by thrombin's high-affinity pathway. In gel-filtered human platelets, iloprost-stimulated cAMP levels were lowered by alpha- and zeta-thrombin addition and, to a much lesser extent, by gamma-thrombin. The alpha- and zeta-thrombin mediated decreases in cAMP were prevented by the thrombin anion-binding exosite inhibitor, BMS 180742, implying that binding to thrombin's anion-binding exosite was required. The iloprost-stimulated increase in cAMP was also reversed (in a concentration-dependent fashion) by a peptide mimicking the new N-terminal of the "tethered-ligand" thrombin receptor (SFLLRNPNDKYEPF). In broken cell preparations, platelet adenylate cyclase activity was also inhibited by SFLLRNPNDKYEPF (but not by a similar peptide used as a control, FSLLRNPNDKYEPF). These results support the hypothesis that thrombin inhibition of platelet adenylate cyclase activity is mediated, at least in part, via the "tethered-ligand" receptor. Moreover, this data is consistent with the "tethered-ligand" receptor mediating the high affinity actions of alpha-thrombin.

Blood platelet function in canine acute pancreatitis with reference to treatment with Nafamostat mesilate (FUT-175)

The aim of this study was to investigate the effect of Nafamostat mesilate (FUT-175) on some blood platelet properties during the first hours of acute experimental pancreatitis (AEP) in dogs. A significant decrease in platelet count, hyperaggregability of platelets by ADP and PAF as well as an increased level of TXB2, were found in the early stage of AEP. No changes in platelet aggregation induced with AA were demonstrated. FUT-175 prevented a decrease in platelet number and inhibited platelet aggregation induced with ADP, PAF and AA when it was given immediately after induction of AEP. No evident changes in TXB2 levels in dogs treated with FUT-175 were found. Our results indicate that the positive effect of FUT-175 in AEP in part depends on its antiaggregatory action.

Mechanism of Na+/K(+)-ATPase activation by trypsin and kallikrein

The mechanism of the Na+/K(+)-ATPase activation by trypsin (from bovine pancreas) and kallikrein (from human plasma) was investigated on enzyme preparations from different sources (beef heart and dog kidney) and at different degrees of purification (beef heart). Kallikrein was effective on both beef and dog enzymes, whereas trypsin stimulated only the beef-heart Na+/K(+)-ATPase. The extent of activation by the proteinases was inversely related to the degree of purification (maximal enzyme activation about 60 and 20% on the partially purified and the more purified enzymes, respectively). Enzyme activation was observed up to 0.5-0.6 microgram/ml of proteinase. At higher concentrations the activation decreased and was converted into inhibition at proteinase concentrations above 1.0 micrograms/ml. Na+/K(+)-ATPase stimulation was due to an increase in the Vmax of the enzyme reaction. Km for ATP remained unaffected. The activating effect was favoured by sodium and counteracted by potassium. Accordingly, Na(+)-ATPase activity was stimulated to a greater extent (up to 350%), whereas K(+)-dependent p-nitrophenylphosphatase activity proved to be insensitive to the actions of the proteinases. The Na+/K(+)-ATPase stimulation by both proteinases was antagonized by either ouabain or canrenone, two drugs that bind on the extracellular side of the Na+/K(+)-ATPase molecule. On the contrary, the enzyme inactivation observed at high proteinase concentrations was not counteracted by these two drugs. The stimulation of either Na+/K(+)- or Na(+)-ATPase activity was shown to be an irreversible effect without any significant protein degradation detectable by SDS gel electrophoresis. The results obtained suggest that proteinases exert their stimulatory effects by interacting preferentially with the E2 conformation of Na+/K(+)-ATPase at site(s) located on the extracellular moiety of the enzyme.