Stability of prostacyclin in human and rabbit whole blood and plasma

De Novo Synthesis of Dihydrobenzofurans and Indolines and Its Application to a Modular, Asymmetric Synthesis of Beraprost

Dihydrobenzofurans and indolines are important constituents of pharmaceuticals. Herein, we describe a novel strategy for their construction in which the aromatic ring is created de novo through an inverse-electron demand Diels-Alder reaction and cheletropic extrusion sequence of a 2-halothiophene-1,1-dioxide with an enol ether/enamide, followed by aromatization. Unusually, the aromatization process proved to be highly challenging, but it was discovered that treatment of the halocyclohexadienes with a base effected an α-elimination-aromatization reaction. Mechanistic investigation of this step using deuterium-labeling studies indicated the intermediacy of a carbene which undergoes a 1,2-hydrogen shift and subsequent aromatization. The methodology was applied to a modular and stereoselective total synthesis of the antiplatelet drug beraprost in only 8 steps from a key enal-lactone. This lactone provided the core of beraprost to which both its sidechains could be appended through a 1,4-conjugate addition process (lower ω-sidechain), followed by de novo construction of beraprost's dihydrobenzofuran (upper α-sidechain) using our newly developed methodology. Additionally, we have demonstrated the breadth of our newly established protocol in the synthesis of functionalized indolines, which occurred with high levels of regiocontrol. According to density-functional theory (DFT) calculations, the high selectivity originates from attractive London dispersion interactions in the TS of the Diels-Alder reaction.

Prostacyclin Released by Cancer-Associated Fibroblasts Promotes Immunosuppressive and Pro-Metastatic Macrophage Polarization in the Ovarian Cancer Microenvironment

Metastasis of high-grade ovarian carcinoma (HGSC) is orchestrated by soluble mediators of the tumor microenvironment. Here, we have used transcriptomic profiling to identify lipid-mediated signaling pathways encompassing 41 ligand-synthesizing enzymes and 23 cognate receptors in tumor, immune and stroma cells from HGSC metastases and ascites. Due to its strong association with a poor clinical outcome, prostacyclin (PGI₂) synthase (PTGIS) is of particular interest in this signaling network. PTGIS is highly expressed by cancer-associated fibroblasts (CAF), concomitant with elevated PGI₂ synthesis, whereas tumor-associated macrophages (TAM) exhibit the highest expression of its surface receptor (PTGIR). PTGIR activation by PGI₂ agonists triggered cAMP accumulation and induced a mixed-polarization macrophage phenotype with altered inflammatory gene expression, including CXCL10 and IL12A repression, as well as reduced phagocytic capability. Co-culture experiments provided further evidence for the interaction of CAF with macrophages via PGI₂, as the effect of PGI₂ agonists on phagocytosis was mitigated by cyclooxygenase inhibitors. Furthermore, conditioned medium from PGI₂-agonist-treated TAM promoted tumor adhesion to mesothelial cells and migration in a PTGIR-dependent manner, and PTGIR activation induced the expression of metastasis-associated and pro-angiogenic genes. Taken together, our study identifies a PGI₂/PTGIR-driven crosstalk between CAF, TAM and tumor cells, promoting immune suppression and a pro-metastatic environment.

Prostanoid Metabolites as Biomarkers in Human Disease

Prostaglandins (PGD₂, PGE₂, PGF_2α), prostacyclin (PGI₂), and thromboxane A₂ (TXA₂) together form the prostanoid family of lipid mediators. As autacoids, these five primary prostanoids propagate intercellular signals and are involved in many physiological processes. Furthermore, alterations in their biosynthesis accompany a wide range of pathological conditions, which leads to substantially increased local levels during disease. Primary prostanoids are chemically instable and rapidly metabolized. Their metabolites are more stable, integrate the local production on a systemic level, and their analysis in various biological matrices yields valuable information under different pathological settings. Therefore, prostanoid metabolites may be used as diagnostic, predictive, or prognostic biomarkers in human disease. Although their potential as biomarkers is great and extensive research has identified major prostanoid metabolites that serve as target analytes in different biofluids, the number of studies that correlate prostanoid metabolite levels to disease outcome is still limited. We review the metabolism of primary prostanoids in humans, summarize the levels of prostanoid metabolites in healthy subjects, and highlight existing biomarker studies. Since analysis of prostanoid metabolites is challenging because of ongoing metabolism and limited half-lives, an emphasis of this review lies on the reliable measurement and interpretation of obtained levels.

COX-2 Deficiency Promotes White Adipogenesis via PGE2-Mediated Paracrine Mechanism and Exacerbates Diet-Induced Obesity

Cyclooxygenase-2 (COX-2) plays a critical role in regulating innate immunity and metabolism by producing prostaglandins (PGs) and other lipid mediators. However, the implication of adipose COX-2 in obesity remains largely unknown. Using adipocyte-specific COX-2 knockout (KO) mice, we showed that depleting COX-2 in adipocytes promoted white adipose tissue development accompanied with increased size and number of adipocytes and predisposed diet-induced adiposity, obesity, and insulin resistance. The increased size and number of adipocytes by COX-2 KO were reversed by the treatment of prostaglandin E2 (PGE2) but not PGI2 and PGD2 during adipocyte differentiation. PGE2 suppresses PPARγ expression through the PKA pathway at the early phase of adipogenesis, and treatment of PGE2 or PKA activator isoproterenol diminished the increased lipid droplets in size and number in COX-2 KO primary adipocytes. Administration of PGE2 attenuated increased fat mass and fat percentage in COX-2 deficient mice. Taken together, our study demonstrated the suppressing effect of adipocyte COX-2 on adipogenesis and reveals that COX-2 restrains adipose tissue expansion via the PGE2-mediated paracrine mechanism and prevents the development of obesity and related metabolic disorders.

Epoprostenol Therapy for a Pediatric Patient With Subacute Heparin-Induced Thrombocytopenia and a Ventricular Assist Device Undergoing Heart Transplant: A Case Report

Concerns remain regarding the use of direct thrombin inhibitors for cardiopulmonary bypass anticoagulation in pediatric patients with heparin-induced thrombocytopenia undergoing complex cardiac surgery. We describe the safe and effective use of epoprostenol sodium as an alternative therapy before heparin exposure for a pediatric patient with subacute heparin-induced thrombocytopenia and a ventricular assist device undergoing heart transplant.

Protein kinase A determines platelet life span and survival by regulating apoptosis

Apoptosis delimits platelet life span in the circulation and leads to storage lesion, which severely limits the shelf life of stored platelets. Moreover, accumulating evidence indicates that platelet apoptosis provoked by various pathological stimuli results in thrombocytopenia in many common diseases. However, little is known about how platelet apoptosis is initiated or regulated. Here, we show that PKA activity is markedly reduced in platelets aged in vitro, stored platelets, and platelets from patients with immune thrombocytopenia (ITP), diabetes, and bacterial infections. Inhibition or genetic ablation of PKA provoked intrinsic programmed platelet apoptosis in vitro and rapid platelet clearance in vivo. PKA inhibition resulted in dephosphorylation of the proapoptotic protein BAD at Ser155, resulting in sequestration of prosurvival protein BCL-XL in mitochondria and subsequent apoptosis. Notably, PKA activation protected platelets from apoptosis induced by storage or pathological stimuli and elevated peripheral platelet levels in normal mice and in a murine model of ITP. Therefore, these findings identify PKA as a homeostatic regulator of platelet apoptosis that determines platelet life span and survival. Furthermore, these results suggest that regulation of PKA activity represents a promising strategy for extending platelet shelf life and has profound implications for the treatment of platelet number-related diseases and disorders.

Effects of prostaglandin lipid mediators on agonist-induced lung endothelial permeability and inflammation

Prostaglandins (PG), the products of cyclooxygenase-mediated conversion of arachidonic acid, become upregulated in many situations including allergic response, inflammation, and injury, and exhibit a variety of biological activities. Previous studies described barrier-enhancing and anti-inflammatory effects of PGE₂ and PGI₂ on vascular endothelial cells (EC). Yet, the effects of other PG members on EC barrier and inflammatory activation have not been systematically analyzed. This study compared effects of PGE₂, PGI₂, PGF_2α, PGA₂, PGJ₂, and PGD₂ on human pulmonary EC. EC permeability was assessed by measurements of transendothelial electrical resistance and cell monolayer permeability for FITC-labeled tracer. Anti-inflammatory effects of PGs were evaluated by analysis of expression of adhesion molecule ICAM1 and secretion of soluble ICAM1 and cytokines by EC. PGE₂, PGI₂, and PGA₂ exhibited the most potent barrier-enhancing effects and most efficient attenuation of thrombin-induced EC permeability and contractile response, whereas PGI₂ effectively suppressed thrombin-induced permeability but was less efficient in the attenuation of prolonged EC hyperpermeability caused by interleukin-6 or bacterial wall lipopolysaccharide, LPS. PGD₂ showed a modest protective effect on the EC inflammatory response, whereas PGF_2α and PGJ₂ were without effect on agonist-induced EC barrier dysfunction. In vivo, PGE₂, PGI₂, and PGA₂ attenuated LPS-induced lung inflammation, whereas PGF_2α and PGJ₂ were without effect. Interestingly, PGD₂ exhibited a protective effect in the in vivo model of LPS-induced lung injury. This study provides a comprehensive analysis of barrier-protective and anti-inflammatory effects of different prostaglandins on lung EC in vitro and in vivo and identifies PGE₂, PGI₂, and PGA₂ as prostaglandins with the most potent protective properties.

COX-2 Inhibition by Use of Rofecoxib or High Dose Aspirin Enhances ADP-Induced Platelet Aggregation in Fresh Blood

Aim:

Increased cardiovascular risk after use of selective or nonselective cyclooxygenase-2 (COX-2)-inhibitors might partly be caused by enhanced platelet aggregability. However, an effect of COX-2 inhibition on platelets has so far not been observed in humans.

Methods:

We tested in healthy volunteers the effect of COX-2-inhibition nearly in-vivo, i.e. immediately after and even during blood sampling.

Results:

Measurement within 2 minutes after venipuncture, but not 60 minutes later, showed that 50 mg of rofecoxib (n=12) or 500 (n=8) or 1000 (n=8) mg of aspirin increased ADP-induced platelet aggregation in a whole-blood aggregometer to, respectively, 152, 176 and 204 % of basal level (p<0.01). No significant differences in aggregability were observed after ingestion of 80 mg of aspirin (n=16), or placebo (n=8). Plasma 6-keto-PGF1α was decreased to 74 % after rofecoxib and to 76 and 70 % after 500 and 1000 mg of aspirin but did not change after low dose aspirin. Continuous photometrical measurement of aggregation in blood flowing from a cannulated vein revealed that high dose aspirin did not elicit aggregation by itself, but increased ADP-induced aggregation in proportion to the decrease in prostacyclin formation (r=0.68, p = 0.004). Since in these experiments thromboxane production was virtually absent, the enhanced aggregation after partial COX-2 inhibition was not caused by unopposed thromboxane formation.

Conclusions:

We conclude that both selective and nonselective COX-2 inhibition enhances ADP-induced platelet aggregation in humans. This effect can only be detected during or immediately after venipuncture, possibly because of the short half-life of prostacyclin.

Physiology of Kidney Renin

The protease renin is the key enzyme of the renin-angiotensin-aldosterone cascade, which is relevant under both physiological and pathophysiological settings. The kidney is the only organ capable of releasing enzymatically active renin. Although the characteristic juxtaglomerular position is the best known site of renin generation, renin-producing cells in the kidney can vary in number and localization. (Pro)renin gene transcription in these cells is controlled by a number of transcription factors, among which CREB is the best characterized. Pro-renin is stored in vesicles, activated to renin, and then released upon demand. The release of renin is under the control of the cAMP (stimulatory) and Ca2+(inhibitory) signaling pathways. Meanwhile, a great number of intrarenally generated or systemically acting factors have been identified that control the renin secretion directly at the level of renin-producing cells, by activating either of the signaling pathways mentioned above. The broad spectrum of biological actions of (pro)renin is mediated by receptors for (pro)renin, angiotensin II and angiotensin-( 1 – 7 ).

Blood plasma influences anti-aggregatory potency of prostaglandins: effect of albumin

The anti-aggregatory effects of prostaglandins (PGs) were compared in platelet-rich plasma (PRP) with that in washed platelets (WP). PGE(1), 13,14-dihydro-PGE(1), 5,6-dihydro-PGE(3) and ethyl ester of PGE(1) had the same potency in both platelet preparations. Iloprost was significantly more potent in WP compared to PRP, while 5,6-trans-PGE(2) and PGE(3) had higher potency in PRP than in WP. Albumin (35 mg/ml) in WP decreased the potency of iloprost but did not change the IC(50) value for PGE(3). In PRP, PGs were 1.5-2.8 times more potent when incubated 3 min as compared to 30 s, while in WP, the incubation time did not affect the IC(50) values for PGs. In WP with albumin, the potency of iloprost was significantly lower when it was incubated 30 s as compared to 3 min. Thus, plasma albumin is responsible for time-dependent effect of PGs and for the lower potency of iloprost but not for the higher potency of PGs in PRP.

Schwann cells express IP prostanoid receptors coupled to an elevation in intracellular cyclic AMP

We have shown previously that prostaglandin E(2) (PGE(2)) and prostaglandin I(2) (PGI(2)) are each produced in an explant model of peripheral nerve injury. We report that IP prostanoid receptor mRNA and protein are present in primary rat Schwann cells. IP prostanoid receptor stimulation using prostacyclin produced an elevation in intracellular cyclic AMP concentration ([cAMP](i)) in primary Schwann cells. Peak [cAMP](i) was observed between 5-15 min of stimulation followed by a gradual recovery toward basal level. Phosphorylation of cyclic AMP-response element binding protein (CREB) on Ser(133) was also detected after IP prostanoid receptor stimulation and CREB phosphorylation was inhibited completely by the protein kinase A inhibitor, H-89. Intracellular calcium levels were not affected by IP prostanoid receptor stimulation. Unlike forskolin, IP prostanoid receptor stimulation did not significantly augment Schwann cell proliferation in response to growth factor treatment. However, IP prostanoid receptor stimulation increased the number of Schwann cells that were able to generate a calcium transient in response to P2 purinergic receptor activation. These findings suggest that signaling via the IP prostanoid receptor may by relevant to Schwann cell biology in vivo.

Heparin-induced thrombocytopenia type II in an infant with a congenital heart defect--anticoagulation during cardiopulmonary bypass with epoprostenol sodium and heparin

BACKGROUND

Heparin-induced thrombocytopenia type II (HIT II) is a rare but potentially life-threatening complication of heparin therapy. Hitherto, only few reports on HIT II in infants and children have been published. In particular, infants and children who have to be operated under cardiopulmonary bypass are at risk as an alternative anticoagulation is required.

CASE PRESENTATION

We report on an infant with a congenital heart defect who was scheduled for cardiac surgery (Damus Kaye-Stansel procedure) with cardiopulmonary bypass. In the intensive care unit, an HIT II was diagnosed. Before surgery, the infant was pretreated with epoprostenol sodium (incrementally increasing up to a maximum dose of 30 ng/kg/min) before heparin was administered shortly after sternotomy. Mean arterial pressure was kept stable with an infusion of norepinephrine and the course of the cardiopulmonary bypass showed no signs of thrombosis. Drainage loss in the postoperative period was moderate.

CONCLUSION

In HIT II infants, pretreatment with epoprostenol sodium before reexposure to heparin may offer a safe and effective anticoagulation for cardiopulmonary bypass.

Epoprostenol sodium, a prostaglandin I2, lacks tumor promoting effects in a medium-term liver carcinogenesis bioassay in rats

Potential modifying effects of epoprostenol sodium administration on liver carcinogenesis were investigated in male F344/DuCrj rats initially treated with N-nitrosodiethylamine (DEN). Two weeks after a single dose of DEN (200 mg/kg, intraperitoneally), rats daily received subcutaneously epoprostenol sodium at doses of 0, 1, 10 and 100 microg/kg, or were fed phenobarbital sodium (PB) at a dietary level of 500 parts per million (ppm) as positive control for 6 weeks. All animals were subjected to partial hepatectomy at week 3, and were killed at week 8. Prominent flushing of extremis and signs of behavioural depression occurred after injection and lasted for 1 h in rats given 100 microg/kg epoprostenol sodium. Such clinical signs were slight in rats treated with 10 microg/kg, but not observed with 1 microg/kg. Marked decrease in body weight gain was noted in rats given 100 microg/kg. Statistically significant changes in relative liver weights were not found in any group given the test chemical. Epoprostenol sodium did not significantly increase the quantitative values for glutathione S-transferase placental form (GST-P) positive liver cell foci observed after DEN initiation, in clear contrast to the positive control. The results thus demonstrate that epoprostenol sodium lacks modifying potential for liver carcinogenesis in our medium-term bioassay system.

Intravenous epoprostenol sodium does not increase hepatic microsomal enzyme activity in rats

Previous studies have indicated that epoprostenol may increase hepatic microsomal enzyme activity both in animals and humans. However, interpretation of the results of these studies may be confounded by the route of epoprostenol administration or small sample sizes. The primary objective of the present investigation was to evaluate the effects of epoprostenol (given as a continuous intravenous infusion) on hepatic microsomal enzyme activity in rats. Male Sprague Dawley rats (220-290 g) received infusions of either vehicle (glycine buffer, 1 mL/hr) or 0.2 microgram/kg/min epoprostenol through a jugular vein cannula for 24 hr or 7 days. At the end of the infusion, a 25 mg/kg i.v. bolus of antipyrine was administered and blood samples were collected over 6 hr. Serum antipyrine concentrations were determined by HPLC. Twenty-four hr post-infusion, hepatic microsomes were prepared, and cytochrome P-450 content was determined by difference spectroscopy. Cytochrome P-450 content and antipyrine clearance values determined from serum antipyrine concentration-time profiles were not significantly different between treatment groups. Antipyrine clearance [mean (SD)] in the 24-hr vehicle-treated group was 3.68 (0.49) mL/min/kg versus 4.35 (1.1)mL/min/kg in the epoprostenol-treated group. In the 7-day vehicle-treated rats, antipyrine clearance was 5.43 (1.0) mL/min/kg compared to 4.68 (0.61) mL/min/kg in epoprostenol-treated rats. A statistically significant effect of infusion duration was observed in the control group, i.e., antipyrine clearance in rats treated with vehicle for 7 days was significantly greater than that observed in rats treated with vehicle for 24 hr. However, the increase was less than 50%. These data suggest that when epoprostenol is administered as an intravenous infusion to rats, no significant alterations in hepatic microsomal enzyme activity occur. Based on these data, long term changes in hepatic metabolism in response to chronic epoprostenol administration are not expected.

Biochemical and pharmacological activity of arene-fused prostacyclin analogues on human platelets

Human platelets have been employed as an assay system to evaluate the pharmacological activity of a group of stable, arene-fused prostacyclin analogs. Prostacyclin (PGI2) is a highly active member of the eicosanoid family and is relatively unstable under physiological conditions. Prostacyclin's best characterized activities are those of inhibition of platelet aggregation and relaxation of vascular smooth muscle. These activities are mediated in large part via elevation of intracellular levels of cyclic AMP subsequent to receptor occupation and activation of adenylate cyclase. We previously described the synthesis of a series of arene-fused prostacyclin analogs with stability in aqueous media at physiological pH. Several of these compounds have prostacyclin-like activities, i.e., competitive binding at the platelet prostacyclin receptor, elevation of intraplatelet cyclic AMP levels and inhibition of human platelet aggregation. One compound in particular (11a) demonstrated these activities with potency similar to PGI2, i.e., Kd at platelet receptor of 3.7 nM and IC50 for inhibition of collagen-induced human platelet aggregation in plasma of 2.9 nM.

Regulation of PGI2 activity by serum proteins: serum albumin but not high density lipoprotein is the PGI2 binding and stabilizing protein in human blood

Although previous studies have shown that serum albumin binds PGI2 and protects it from rapid degradation, it remains debatable whether it is physiologically important due to its low binding affinity for PGI2. We were intrigued by the observations of Yui et al. (J. Clin. Invest. 82 (1988) 803-807) which suggested that apo A-I of the high density lipoprotein (HDL) is the "serum PGI2 stabilizing factor". To clarify this, we carried out experiments to determine the binding kinetics and parameters of HDL and albumin purified from normal pooled human serum. Despite the use of multiple binding assays, we could not detect any binding activity in HDL2, HDL3 or nascent HDL preparations, nor could we demonstrate any PGI2 protecting activity by these molecules. By contrast, purified albumin exhibited essentially identical binding parameters as the native serum from which the albumin was purified. The binding activity of various albumin preparations was not due to the contamination of apo A-I. Computer simulation analysis also failed to provide evidence to support the notion that HDL bound and prolonged PGI2 activity. To determine whether physiological concentrations of albumin influence PGI2 binding to platelet receptors, we measured PGI2 binding to platelet membrane in the absence and presence of albumin. Albumin at 40 mg/ml increased the KD of PGI2 binding to the receptors by 2-3 fold. These findings indicate that albumin plays a major role in protecting PGI2 activity and regulating its availability for platelet PGI2 receptors.

Decreased prostaglandin-I2 stability in acute myocardial infarction

It has been demonstrated that under certain conditions the in-vitro half-life of biologically active PGI2 in plasma is extremely shortened, which may result in-vivo in a local haemostatic imbalance. In 36 patients suffering from acute myocardial infarction a sequential change in in-vitro half-life of synthetic PGI2 was therefore studied during 3 weeks. 21 patients admitted turning out not to develop myocardial infarction served as follow-up controls. During and shortly after the acute episode the plasmatic half-life of PGI2 in-vitro was shortened by about 40%, improving continuously thereafter. No certain influence of either risk factors, sex or age could be discovered. A possible influence of various drugs administered in the hospital period has been excluded in 43 patients with proven coronary artery disease. No such changes occurred during acute angina pectoris attack in 12 patients. It remains to be established, whether the short-lasting destabilisation of PGI2 may be an acute disease-associated finding, or an important pathogenetic factor.

Characterization of the interaction between prostacyclin and human serum albumin using a fluorescent analogue, 2,6-dichloro-4-aminophenol iloprost

We synthesized a fluorescent probe, 2,6-dichloro-4-aminophenol iloprost or dichlorohydroxyphenylamide of iloprost (DCHPA-iloprost) by reacting the stable prostacyclin analog, iloprost (ZK 35 374), with 2,6-dichloro-4-aminophenol with a yield of 60%. This probe exhibited an optical spectrum which overlapped with the emission spectrum of the sole tryptophan of human serum albumin (HSA). Energy transfer from the tryptophan residue to the phenol moiety of DCHPA-iloprost was observed. We utilized this donor-quenching phenomenon to quantitate the binding stoichiometry and affinity as well as the association rate of DCHPA-iloprost binding to HSA. As DCHPA-iloprost showed similar binding characteristics similar to those of iloprost and prostacyclin and competed with iloprost for HSA binding sites, we used DCHPA-iloprost as a probe to locate the binding domain of prostacyclin (PGI2) in HSA. The distance between the tryptophan indole and the phenol group of DCHPA-iloprost was estimated to be 15-18 A. Because iloprost binding to HSA was competitive with warfarin and not with free fatty acid, we propose that PGI2 binds to the 'domain 2' of HSA was competitive with warfarin and not with free fatty acid, we propose that PGI2 binds to the 'domain 2' of HSA molecules. A possible molecular mechanism by which HSA reduces the chemical degradation of PGI2 and stabilizes its activity could be derived from this model.

Stabilization mechanism of prostacyclin by human serum: an approach by binding kinetics using a stable prostaglandin I2 analogue, iloprost

We used a gel filtration method and a stable prostaglandin I2 (prostacyclin) analogue, iloprost, to study the kinetics of prostaglandin I2 binding by human serum proteins. Binding equilibrium experiments conducted at physiological prostaglandin I2 concentration (nM) yielded a KD of 10(-9) and a capacity of approx. 50 nM for the serum binding protein(s). Kinetic measurements gave a dissociation rate constant of 10(-3) s-1. When binding equilibrium was established at various ligand concentrations ranging from nM to microM, a result indicating an unsaturable binding was obtained utilizing this method. On the other hand, saturation was achieved with a ligand concentration as high as 50-100 microM by another binding method. A KD of 7 X 10(-5) and a capacity of approx. 600 microM was obtained. This apparent discrepancy was resolved by performing parallel experiments using purified human serum albumin samples and serum. It is concluded that the large quantity of serum albumin, approx. 600 microM, in serum may compensate for its low KD (approx. 10(-5] for prostaglandin I2, thus simulating a binding protein with a KD of 10(-9) and a limited capacity. These data offer direct information regarding how prostaglandin I2 is stabilized by serum and is transported to the platelet prostaglandin I2 receptors. There is a strong implication that serum albumin is the major if not the only protein responsible for binding of prostaglandin I2.