Estrogen and related compounds: biphasic dose responses.

This article documents and quantitatively assesses the capacity of estrogen, phytoestrogens, and antiestrogens to affect biphasic dose-response relationships in animal/human models and across a broad range of cell types, affecting multiple endpoints. The range of endpoints displaying such biphasic dose responses includes plasminogen activation, oxytocin secretion, angiogenesis, cell proliferation, bone growth, monocyte chemotaxis, secretion of various cytokines, and other effects. The quantitative features of the dose response relationships revealed that the magnitude of the stimulatory responses was typically less than twofold, whereas the stimulatory responses were markedly variable ranging from about 5- to 10(6)-fold. Mechanistic explanations of the biphasic responses are addressed.

Impaired fibrinolysis in cyclosporine-treated renal transplant patients. Analysis of the defect and beneficial effect of fish-oil.

Cyclosporine treatment has been associated with thrombotic vascular complications. We investigated the activity of the fibrinolytic system and its capacity to respond upon DDAVP stimulation in a group of 20 cyclosporine-treated patients as compared with a group of 9 azathioprine-treated patients. Furthermore, the effect of the administration of fish-oil to these patients on the endogenous fibrinolytic activity was studied in a double-blind randomized, placebo-controlled cross-over study. The cyclosporine-treated patients showed a significantly reduced plasminogen activator activity and plasmin generation response upon the infusion of DDAVP as compared with the azathioprine group. In the cyclosporine group 60% of the patients had an impaired fibrinolytic response, whereas this was found in only 11% of the azathioprine-treated patients (P < 0.05). The impairment of the endogenous fibrinolysis activity could be attributed either to a defective release of plasminogen activator from the vessel wall (67% of patients) or to high plasma levels of plasminogen activator inhibitor 1 (33% of patients). Administration of fish-oil resulted in a significant improvement of the impaired fibrinolysis in the cyclosporine group. Particularly, in patients with a defective release of plasminogen activator from the vessel wall, fish-oil treatment resulted in a normalization of the fibrinolytic activity. These results indicate that cyclosporine treatment induces an impaired fibrinolysis that may contribute to the frequent occurrence of thromboembolic complications and eventually the impairment of renal function in cyclosporine-treated patients. The beneficial effect of the administration of fish-oil on the endogenous fibrinolysis may result in a reduction of the adverse events associated with cyclosporine treatment.

A comparative study of the efficacy and specificity of tissue plasminogen activator and pro-urokinase: demonstration of synergism and of different thresholds of non-selectivity.

Clot lysis and non-specific plasminogen activation in human plasma by tissue tissue plasminogen activator (t-PA) and/or pro-urokinase (pro-UK) were studied. The fibrinolytic activity of pro-UK was expressed as latent units, i.e. measured after activation with plasmin on a fibrin plate against the reference standard. The t-PA unitage was assigned on a weight basis of a similar equivalence of 100,000 IU/mg. To simplify comparison, both activators were expressed in IU (1 IU = approximately 10 ng). At low concentration (1-50 IU/ml), t-PA induced more effective and more linear clot lysis, whereas pro-UK induced lysis was preceded by a lag phase. The two activators were equivalently effective at higher concentrations and saturated at the same lysis rate. Clots made from platelet rich plasma or whole blood were more responsive to lysis by pro-UK but not t-PA than corresponding platelet poor clots. At very low concentrations (2.5-5 IU/ml) of t-PA combined with moderate concentrations (25-50 IU/ml) of pro-UK, a synergistic effect on clot lysis, which was fibrin-specific, was observed. Plasminogen and fibrinogen and the appearance of plasmin-inhibitor complexes in plasma were measured after incubation with either activator with and without a clot present. Non-specific plasminogen activation occurred above a certain concentration of either activator but was found at lower concentrations of t-PA than pro-UK. In the absence of a clot, plasmin generation occurred with t-PA at about 30% of the concentration at which pro-UK induced a corresponding effect. It is concluded that there are important differences in the fibrinolytic and clot selective properties of t-PA and pro-UK, and that some of these properties may be complementary resulting in a fibrin specific, synergistic fibrinolytic effect.

Protein C.

The protein C anticoagulant pathway provides many new insights into control mechanisms for regulating coagulation. The observation that protein C deficiency is associated with thrombotic tendencies in the heterozygote (106-109) and early, lethal thrombosis in the homozygote (110, 111) points to the importance of the system as a major regulatory pathway. The complexity of the system has only recently begun to emerge. Thrombin activation of protein C at the endothelial cell surface requires not only the synthesis of thrombomodulin but the coupling of the receptor to a protein C binding site. It is reasonable to assume that an inherited or acquired deficiency in thrombomodulin might lead to thrombotic tendencies. This aspect of the system may explain, in part, the association between vascular disease and thrombosis. Once activated, protein C has an almost total dependence on protein S to express anticoagulant activity. (98) This suggests that deficiencies of protein S may also be associated with thrombotic tendencies. Protein S offers an additional intriguing property. Protein S, a regulatory protein of the coagulation system, is found both free and associated with C4BP, a regulatory protein of the complement system. The high affinity, very stable interaction between these components (85) suggests that the interaction is likely to be involved in regulation. (89) The importance of the interaction remains to be demonstrated, but clearly this is a potential direct link between major control proteins of the coagulation and complement system. Clinical studies suggest that protein C and/or thrombomodulin might be effective therapeutically. Certainly, protein C supplementation during the onset of oral anticoagulant therapy would be expected to circumvent the transient rapid decrease in protein C levels that may influence the early effectiveness of oral anticoagulants. (119) In addition to the systems clinical importance, protein C, its activation, and its function offer a variety of intriguing biochemical problems. For instance, how does thrombomodulin alter the specificity of thrombin? What is the protein C binding site on the cell surface, and what role does Factor Va or its degradation products play in the formation and regulation of this site? How does protein S facilitate activated protein C anticoagulant activity and what roles do membrane surfaces play in this system? What role does beta-hydroxyaspartic acid play in protein C activation and function? How does activated protein C influence fibrinolytic activity? The answers to these questions will undoubtedly add to our understanding of the fundamental mechanisms involved in regulating blood coagulation.(ABSTRACT TRUNCATED AT 400 WORDS)

Significance of cross-linking of alpha 2-plasmin inhibitor to fibrin in inhibition of fibrinolysis and in hemostasis.

When blood is clotted, alpha(2)-plasmin inhibitor (alpha(2)PI) is cross-linked to fibrin by activated fibrin-stabilizing factor (activated coagulation Factor XIII, plasma transglutaminase). The amount of cross-linked alpha(2)-PI is proportional to the amount of alpha(2)PI present at the time of clotting. Plasma from a patient with congenital deficiency of alpha(2)PI was supplemented with various amounts of purified alpha(2)PI. Clots were prepared from these plasmas and were suspended in plasma containing a normal concentration of alpha(2)PI, and spontaneous clot lysis was observed. When the clot was formed in the presence of calcium ions and thereby allowing cross-linking to occur, the rate and extent of fibrinolysis were found to be inversely proportional to the concentrations of alpha(2)PI present in the clot at the time of clotting. When the clot was formed in the absence of calcium ions so that no cross-linking occurred, the clot underwent fibrinolysis at similar rates, regardless of the concentrations of alpha(2)PI in the clot. When the clot formed in the presence of calcium ions was squeezed and washed to remove unbound proteins before being suspended in plasma, the extent of fibrinolysis was also inversely proportional to the amount of alpha(2)PI cross-linked to fibrin. Similar results were obtained when the clot was suspended in buffered saline instead of plasma. These observations suggest that spontaneous fibrinolysis is mainly carried out by plasminogen/plasminogen activator bound to fibrin, and this fibrinolysis caused by fibrin-associated activation of plasminogen was mainly inhibited by alpha(2)PI cross-linked to fibrin. To further support this concept, alpha(2)PI treated with activated fibrin-stabilizing factor and that had lost most of its cross-linking capacity was used in similar experiments. This modified alpha(2)PI had the same inhibitory activity on plasmin as the native inhibitor, but gave significantly less inhibition of fibrinolysis in every experiment, particularly when the clot was compacted by platelet-mediated clot retraction or by squeezing. Thus, it was concluded that alpha(2)PI cross-linked to fibrin plays a significant role in inhibition of physiologically occurring fibrinolysis. It is further suggested that the absence of cross-linked alpha(2)PI contributes to accelerated fibrinolysis and hemorrhagic tendency in patients with congenital deficiency of fibrin-stabilizing factor.

Prostaglandins in hypothalamo-pituitary and ovarian function.

PIP: Some aspects of prostaglandin (PG) functions are reviewed including: 1) the role of PGs in the hypothalamic and pituitary control of gonadotropin secretions; and 2) their roles in ovulation, 3) in luteinization, and 4) in corpus luteum regression. PGE1 is known for its role in stimulation of increased cyclic adenosine 3',5' monophosphate (cAMP) and hormone secretion in the anterior pituitary. Direct effects of PGs on the secretion of luteinizing hormone, follicle stimulating hormone, and adrenal cortex hormones are not clearly known, but surmised. Such actions may not be the direct effects of PGs on pituitary action. Instead, more studies on receptor functions for PGs in pituitary cells are needed. Systemic administration of PGs has been shown to increase circulating levels of gonadotropins, adrenal cortex hormones, prolactin, follicle stimulating hormone, and luteinizing hormone; and in general, PGs of the E series are more potent than those of the F series. This response to systemic administration seems to be caused by an hypothalamic site of action, a conclusion based on several observations, including the observation that direct application of PGs to brain tissue causes a mimicking of endogenous PG effects of gonadotropin secretion. PGs also play a role in ovulation. Elevated PG levels in follicular tissues are induced by gonadotropins; cyclic nucleotides may be involved in mediating the action of gonadotropins on follicular PG production; a recognized time lag after exposure of the follicle to gonadotropin or cyclic nucleotides indicates that macromolecular synthesis may be involved in follicular PG production; and plasminogen activator may play a role in the process of follicular rupture that leads ot ovulation. The role of PGs in luteinization has been suggested by experiments which showed that granulosa cells cultured with PGE1 and PGE2 luteinized. PGs, particularly PGF2 alpha, cause luteal regression in many species, except perhaps in humans. And PGF2 alpha may be an antagonist of gonadotropin action in the corpus luteum. A proposed mechanism of PGF2 alpha-induced luteolysis in rats is also presented.^ieng