Preclinical and clinical pharmacology of platelet-activating factor receptor antagonists

Genome mining of Streptomyces sp. CB00271 as a natural high-producer of β-rubromycin and the resulting discovery of β-rubromycin acid

β-rubromycin (β-RUB) (1) is an efficient inhibitor of human telomerase possessing a unique spiroketal moiety as a potential pharmacophore and regarded as a promising anticancer drug lead. But the development of (β-RUB) (1) has long been hampered by its low titer and very poor water solubility. By adopting a genome mining strategy, an FAD-dependent monooxygenase RubN involving with the formation of the spiro system was applied as the probe and Streptomyces sp. CB00271 was screened out from our strain collection as an alternative natural high producer of β-RUB (1). After a series of fermentation optimizations, CB00271 could produce 124.8 ± 3.4 mg/L β-RUB (1), which was the highest titer up to now. Moreover, the enhanced production of β-RUB (1) in fermentation broth also led to the discovery of a new congener β-RUB acid (7), which was structurally elucidated as the acid form of β-RUB (1). Comparing to β-RUB (1), the substituted carboxyl group endowed β-RUB acid (7) much better solubility in serum and resulted in its higher activity towards tumor cells. Our work set up a solid base for the pilot-scale production of β-RUB (1) and its congeners to facilitate their future development as promising anticancer drug leads, and also provide an alternative and practical strategy for the exploitation of other important microbial natural products.

UVB-generated Microvesicle Particles: A Novel Pathway by Which a Skin-specific Stimulus Could Exert Systemic Effects

Ultraviolet B radiation (UVB) exerts profound effects on human skin. Much is known regarding the ability of UVB to generate a plethora of bioactive agents ranging from cytokines and other bioactive proteins, lipid mediators and microRNAs. It is presumed that these agents are in large part responsible for the effects of UVB, which is only absorbed appreciably in the epidermis. However, the exact mechanism by which these bioactive agents can leave the epidermis are as yet unclear. This review addresses the potential role of microvesicle particles (MVP) as UVB signaling agents through transmitting biologic mediators. New data are provided that UVB treatment of human skin explants also generates MVP production. We hypothesize that UVB production of MVPs (UVB-MVP) could serve this important function of transmitting keratinocyte-derived bioactive agents. Moreover, we propose that UVB-MVP formation involves the lipid mediator platelet-activating factor. This novel pathway has the potential to be exploited pharmacologically to modulate UVB effects.

2-Phenyl-3-(quinolizidin-1-yl)-5-substituted indoles as platelet antiaggregating agents

A set of ten 2-phenyl-3-(quinolizidin-1-yl)-5-substituted indoles was prepared through the Fischer cyclization of lupinyl- and epi-lupinylphenylketone 4-substituted phenylhydrazones. Compounds were tested for antiaggregating activity on human platelets activated by adenosine diphosphate (ADP), collagen and adrenaline. At 2.5 x 10(-4) M concentration most compounds strongly inhibited the aggregation induced by all the agonists considered and many of them still displayed good activity at 0.625 x 10(-4) M concentration. The least active (1c) and one of the most active (1d) compounds were also tested for antiaggregating activity on rabbit platelets activated by ADP, PAF and sodium arachidonate. Both the compounds were active against ADP and PAF, but only 1d inhibited the arachidonate-induced aggregation (100% at 8 x 10(-6) M concentration) and increased the bleeding time in mice. The same compounds were subjected to a general pharmacological screening and found to display several activities; of particular interest was the dose dependent reduction of serum cholesterol and heparin precipitating betalipoproteins in hypercholesterolemic mice exerted by 1c, which was still significant at the oral dose of 10 mg/kg.

Role of platelet-activating factor in cardiovascular pathophysiology

Platelet-activating factor (PAF) is a phospholipid mediator that belongs to a family of biologically active, structurally related alkyl phosphoglycerides. PAF acts via a specific receptor that is coupled with a G protein, which activates a phosphatidylinositol-specific phospholipase C. In this review we focus on the aspects that are more relevant for the cell biology of the cardiovascular system. The in vitro studies provided evidence for a role of PAF both as intercellular and intracellular messenger involved in cell-to-cell communication. In the cardiovascular system, PAF may have a role in embryogenesis because it stimulates endothelial cell migration and angiogenesis and may affect cardiac function because it exhibits mechanical and electrophysiological actions on cardiomyocytes. Moreover, PAF may contribute to modulation of blood pressure mainly by affecting the renal vascular circulation. In pathological conditions, PAF has been involved in the hypotension and cardiac dysfunctions occurring in various cardiovascular stress situations such as cardiac anaphylaxis and hemorrhagic, traumatic, and septic shock syndromes. In addition, experimental studies indicate that PAF has a critical role in the development of myocardial ischemia-reperfusion injury. Indeed, PAF cooperates in the recruitment of leukocytes in inflamed tissue by promoting adhesion to the endothelium and extravascular transmigration of leukocytes. The finding that human heart can produce PAF, expresses PAF receptor, and is sensitive to the negative inotropic action of PAF suggests that this mediator may have a role also in human cardiovascular pathophysiology.

Platelet-activating factor in the CNS

Platelet-activating factor (PAF) is a phospholipid synthesized in a variety of cells throughout the body. Platelet-activating factor has been identified in the CNS and has a number of diverse physiological and pathological functions. It has been shown to be a modulator of many CNS processes, ranging from long-term potentiation (LTP) to neuronal differentiation. Excessive levels of PAF appear to play an important role in neuronal cell injury, such as that resulting from ischaemia, inflammation, human immunodeficiency syndrome (HIV) and meningitis. The beneficial effects of PAF receptor antagonists are many and give rise to possible therapeutic strategies for neurotrauma.

PAF effects on eicosanoid release in neonatal rat cardiomyocytes

The aim of this study was to find out whether, in neonatal rat cardiomyocytes, platelet-activating factor (PAF) can stimulate eicosanoid formation. For this purpose neonatal cardiomyocytes were incubated for 60 min at 37 degrees C in HANKS buffer with PAF (10-1000 nM), and the eicosanoids thromboxane A2 (TXA2) and prostacyclin (PGI2) were assessed in the supernatant as TXB2 and 6-keto-PGF1 alpha, respectively, by an enzyme immunoassay. PAF caused concentration-dependent release of PGI2; TXA2, however, was significantly released only at the highest concentration of PAF (1000 nM). Acetylsalicylic acid (556 microM) and the PAF antagonist WEB 2086 (10 microM) significantly attenuated PAF-induced eicosanoid formation. We conclude that in neonatal rat cardiomyocytes PAF can induce eicosanoid formation and this effect is brought about by activation of a specific PAF receptor.

Platelet-activating factor inhibits proteoglycan synthesis and enhances neutrophil-mediated proteoglycan degradation in cartilage explants

OBJECTIVE

Platelet-activating factor (PAF), which stimulates the release of tissue-destructive enzymes and reactive oxygen metabolites from neutrophils, was investigated for its role in neutrophil-mediated cartilage breakdown.

METHODS

Bovine cartilage explants were incubated with or without human neutrophils, PAF, and other reagents. Cartilage damage was measured as either proteoglycan degradation (percent release of 35S-labeled proteoglycan from 35S-labeled cartilage) or inhibition of proteoglycan synthesis (rate of incorporation of 35S into proteoglycan).

RESULTS

PAF increased neutrophil-mediated proteoglycan degradation in the 2-20 microM range. Three specific PAF-receptor antagonists, WEB2086, CV3988, and CV6209, reversed this effect of PAF. These antagonists also reduced the enhancement of neutrophil-mediated cartilage damage caused by granulocyte-macrophage colony-stimulating factor (GM-CSF) and tumor necrosis factor alpha (TNF alpha). The results suggest that there may be a positive feedback mechanism whereby cytokine-primed neutrophils produce PAF, which amplifies the release of other tissue-damaging substances from neutrophils. In the absence of neutrophils, PAF (2-20 microM) inhibited the synthesis of proteoglycan by bovine cartilage. Neutrophils also inhibit proteoglycan synthesis, but PAF probably is not involved in this effect of neutrophils because the PAF receptor antagonists had no consistent effect.

CONCLUSION

PAF increases neutrophil-mediated cartilage proteoglycan degradation in vitro. GM-CSF and TNF alpha enhancement of neutrophil damage to cartilage is partly due to PAF. PAF alone inhibits cartilage proteoglycan synthesis.

Molecular and cellular actions of platelet-activating factor in rat heart cells

Platelet-activating factor (PAF) is a phospholipid with cardiovascular actions at low concentrations (1-100 nM) but with uncertain direct myocardial actions. We investigated the cellular and molecular effects of PAF on heart cells using isolated adult and neonatal rat myocytes. Addition of PAF, in the superfusion solution, decreased twitch amplitude and contractile velocity in both systems. Concentrations of PAF below 1 nM stimulated reproducible responses with maximal effects seen at 100 nM. These functional actions of PAF could be blocked by the known PAF antagonist, BN 50739, in a dose-dependent manner. Parallel biochemical studies showed that nanomolar PAF rapidly stimulated the phosphoinositide pathway in cultured myocytes, evidenced by the accumulation of [3H]inositol phosphates in prelabeled cultured myocytes. The potency and specificity of PAF, as well as the time course, for the response were nearly identical in the biochemical and functional assays. PAF produced no functional changes in protein kinase C-depleted myocytes, but it did stimulate inositol trisphosphate accumulation in such cells. We conclude that: (a) PAF exerts a direct negative inotropic effect on myocardial tissue; (b) the effects of PAF are mediated by a specific, high affinity cardiac receptor; (c) an underlying biochemical mechanism for the action of PAF includes the activation of the phospholipase C/phosphatidylinositol intracellular signaling pathway, which leads to activation of protein kinase C.

Platelet-activating factor in stroke and brain injury

Platelet-activating factor, an endogenous phospholipid of proinflammatory, hemostatic, and vasoactive properties, is synthesized by neurons and in injured brain. Platelet-activating factor is released together with eicosanoids such as thromboxane A2, prostacyclin, and leukotrienes. Its effects in neurons are mediated through a specific receptor coupled to phospholipase C and phosphoinositol metabolism. The cerebrovascular effects of platelet-activating factor include disruption of the blood-brain barrier, edema formation, and vasospasm. It has also been described to possess direct toxicity to neuronal cells in culture. Discovery and development of several highly potent and selective antagonists to platelet-activating factor receptors facilitated experimental studies underscoring the role of this factor as an endogenous mediator in cerebral disorders, particularly cerebral ischemia and trauma. Significant biochemical, microvascular, functional, and behavioral recovery has been demonstrated using these antagonists in an array of experimental models of focal and global ischemia in the central nervous system (CNS). Clearly, studies of platelet-activating factor in experimental models of CNS ischemia and reperfusion injury open a new perspective on phospholipid metabolism in stroke and offer an exceptionally promising therapeutic prospect. Data supporting this factor as a mediator of specific pathological sequelae in stroke and neuroinjury are surveyed in this review. We discuss the mechanisms and significance of platelet-activating factor-mediated effects and propose directions for future studies.