Prostacyclin receptor/thromboxane receptor interactions and cellular responses in human atherothrombotic disease

Selenium and selenoproteins in prostanoid metabolism and immunity

Selenium (Se) is an essential trace element that functions in the form of the 21st amino acid, selenocysteine (Sec) in a defined set of proteins. Se deficiency is associated with pathological conditions in humans and animals, where incorporation of Sec into selenoproteins is reduced along with their expression and catalytic activity. Supplementation of Se-deficient population with Se has shown health benefits suggesting the importance of Se in physiology. An interesting paradigm to explain, in part, the health benefits of Se stems from the observations that selenoprotein-dependent modulation of inflammation and efficient resolution of inflammation relies on mechanisms involving a group of bioactive lipid mediators, prostanoids, which orchestrate a concerted action toward maintenance and restoration of homeostatic immune responses. Such an effect involves the interaction of various immune cells with these lipid mediators where cellular redox gatekeeper functions of selenoproteins further aid in not only dampening inflammation, but also initiating an effective and active resolution process. Here we have summarized the current literature on the multifaceted roles of Se/selenoproteins in the regulation of these bioactive lipid mediators and their immunomodulatory effects.

The potential role of endothelial dysfunction and platelet activation in the development of thrombotic risk in COPD patients

INTRODUCTION

Despite lack of knowledge in the field, several studies have underlined the role of endothelium dysfunction and platelet activation as significant players in the development and progression of chronic obstructive pulmonary disease (COPD). Indeed, endothelium plays a crucial role in vascular homeostasis and impairment, due to the inflammation process enhanced by smoking. Chronic inflammation and endothelial dysfunction have been proved to drive platelet activity. Consequently, thrombotic risk is enhanced in COPD, and might explain the higher percentage of cardiovascular death in such patients. Areas covered: This review aims to clarify the role of endothelium function and platelet hyper-activity as the pathophysiological mechanisms of the increased thrombotic risk in COPD. Expert commentary: In COPD patients, chronic inflammation does not impact only on lung parenchyma, but potentially involves all systems, including the endothelium of blood vessels. Impaired endothelium has several consequences, such as reduced vasodilatation capacity, enhanced blood coagulation, and increased platelet activation resulting in higher risk of thrombosis in COPD patients. Endothelium dysfunction and platelet activation are potential targets of therapy in patients with COPD aiming to reduce their risk of cardiovascular events.

Molecular mechanisms of maternal vascular dysfunction in preeclampsia

In preeclampsia, as a heterogeneous syndrome, multiple pathways have been proposed for both the causal as well as the perpetuating factors leading to maternal vascular dysfunction. Postulated mechanisms include imbalance in the bioavailability and activity of endothelium-derived contracting and relaxing factors and oxidative stress. Studies have shown that placenta-derived factors [antiangiogenic factors, microparticles (MPs), cell-free nucleic acids] are released into the maternal circulation and act on the vascular wall to modify the secretory capacity of endothelial cells and alter the responsiveness of vascular smooth muscle cells to constricting and relaxing stimuli. These molecules signal their deleterious effects on the maternal vascular wall via pathways that provide the molecular basis for novel and effective therapeutic interventions.

Thromboxane receptor hyper-responsiveness in hypoxic pulmonary hypertension requires serine 324

BACKGROUND AND PURPOSE

Dysregulation of the thromboxane A₂ (TP) receptor, resulting in agonist hypersensitivity and hyper-responsiveness, contributes to exaggerated vasoconstriction in the hypoxic pulmonary artery in neonatal persistent pulmonary hypertension. We previously reported that hypoxia inhibits TP receptor phosphorylation, causing desensitization. Hence, we examined the role of PKA-accessible serine residues in determining TP receptor affinity, using site-directed mutational analysis.

EXPERIMENTAL APPROACH

Vasoconstriction to a thromboxane mimetic and phosphorylation of TP receptor serine was examined in pulmonary arteries from neonatal swine with persistent pulmonary hypertension and controls. Effects of hypoxia were determined in porcine and human TP receptors. Human TPα serines at positions 324, 329 and 331 (C-terminal tail) were mutated to alanine and transiently expressed in HEK293T cells. Saturation binding and displacement kinetics of a TP antagonist and agonist were determined in porcine TP, wild-type human TPα and all TP mutants. Agonist-elicited calcium mobilization was determined for each TP mutant, in the presence of a PKA activator or inhibitor, and in hypoxic and normoxic conditions.

KEY RESULTS

The Ser324A mutant was insensitive to PKA activation and hypoxia, had a high affinity for agonist and increased agonist-induced calcium mobilization. Ser329A was no different from wild-type TP receptors. Ser331A was insensitive to hypoxia and PKA with a decreased agonist-mediated response.

CONCLUSIONS AND IMPLICATIONS

In hypoxic pulmonary hypertension, loss of site-specific phosphorylation of the TP receptor causes agonist hyper-responsiveness. Ser324 is the primary residue phosphorylated by PKA, which regulates TP receptor-agonist interactions. Ser331 mutation confers loss of TP receptor-agonist interaction, regardless of PKA activity.

Inverse agonism of SQ 29,548 and Ramatroban on Thromboxane A2 receptor

G protein-coupled receptors (GPCRs) show some level of basal activity even in the absence of an agonist, a phenomenon referred to as constitutive activity. Such constitutive activity in GPCRs is known to have important pathophysiological roles in human disease. The thromboxane A2 receptor (TP) is a GPCR that promotes thrombosis in response to binding of the prostanoid, thromboxane A2. TP dysfunction is widely implicated in pathophysiological conditions such as bleeding disorders, hypertension and cardiovascular disease. Recently, we reported the characterization of a few constitutively active mutants (CAMs) in TP, including a genetic variant A160T. Using these CAMs as reporters, we now test the inverse agonist properties of known antagonists of TP, SQ 29,548, Ramatroban, L-670596 and Diclofenac, in HEK293T cells. Interestingly, SQ 29,548 reduced the basal activity of both, WT-TP and the CAMs while Ramatroban was able to reduce the basal activity of only the CAMs. Diclofenac and L-670596 showed no statistically significant reduction in basal activity of WT-TP or CAMs. To investigate the role of these compounds on human platelet function, we tested their effects on human megakaryocyte based system for platelet activation. Both SQ 29,548 and Ramatroban reduced the platelet hyperactivity of the A160T genetic variant. Taken together, our results suggest that SQ 29,548 and Ramatroban are inverse agonists for TP, whereas, L-670596 and Diclofenac are neutral antagonists. Our findings have important therapeutic applications in the treatment of TP mediated pathophysiological conditions.

New insights into structural determinants for prostanoid thromboxane A2 receptor- and prostacyclin receptor-G protein coupling

G protein-coupled receptors (GPCRs) interact with heterotrimeric G proteins and initiate a wide variety of signaling pathways. The molecular nature of GPCR-G protein interactions in the clinically important thromboxane A2 (TxA(2)) receptor (TP) and prostacyclin (PGI(2)) receptor (IP) is poorly understood. The TP activates its cognate G protein (Gαq) in response to the binding of thromboxane, while the IP signals through Gαs in response to the binding of prostacyclin. Here, we utilized a combination of approaches consisting of chimeric receptors, molecular modeling, and site-directed mutagenesis to precisely study the specificity of G protein coupling. Multiple chimeric receptors were constructed by replacing the TP intracellular loops (ICLs) with the ICL regions of the IP. Our results demonstrate that both the sequences and lengths of ICL2 and ICL3 influenced G protein specificity. Importantly, we identified a precise ICL region on the prostanoid receptors TP and IP that can switch G protein specificities. The validities of the chimeric technique and the derived molecular model were confirmed by introducing clinically relevant naturally occurring mutations (R60L in the TP and R212C in the IP). Our findings provide new molecular insights into prostanoid receptor-G protein interactions, which are of general significance for understanding the structural basis of G protein activation by GPCRs in basic health and cardiovascular disease.

Milrinone attenuates thromboxane receptor-mediated hyperresponsiveness in hypoxic pulmonary arterial myocytes

BACKGROUND AND PURPOSE

Neonatal pulmonary hypertension (PPHN) is characterized by pulmonary vasoconstriction, due in part to dysregulation of the thromboxane prostanoid (TP) receptor. Hypoxia induces TP receptor-mediated hyperresponsiveness, whereas serine phosphorylation mediates desensitization of TP receptors. We hypothesized that prostacyclin (IP) receptor activity induces TP receptor phosphorylation and decreases ligand affinity; that TP receptor sensitization in hypoxic myocytes is due to IP receptor inactivation; and that this would be reversible by the cAMP-specific phosphodiesterase inhibitor milrinone.

EXPERIMENTAL APPROACH

We examined functional regulation of TP receptors by serine phosphorylation and effects of IP receptor stimulation and protein kinase A (PKA) activity on TP receptor sensitivity in myocytes from neonatal porcine resistance pulmonary arteries after 72 h hypoxia in vitro. Ca(2+) response curves to U46619 (TP receptor agonist) were determined in hypoxic and normoxic myocytes incubated with or without iloprost (IP receptor agonist), forskolin (adenylyl cyclase activator), H8 (PKA inhibitor) or milrinone. TP and IP receptor saturation binding kinetics were measured in presence of iloprost or 8-bromo-cAMP.

KEY RESULTS

Ligand affinity for TP receptors was normalized in vitro by IP receptor signalling intermediates. However, IP receptor affinity was compromised in hypoxic myocytes, decreasing cAMP production. Milrinone normalized TP receptor sensitivity in hypoxic myocytes by restoring PKA-mediated regulatory TP receptor phosphorylation.

CONCLUSIONS AND IMPLICATIONS

TP receptor sensitivity and EC(50) for TP receptor agonists was regulated by PKA, as TP receptor serine phosphorylation by PKA down-regulated Ca(2+) mobilization. Hypoxia decreased IP receptor activity and cAMP generation, inducing TP receptor hyperresponsiveness, which was reversed by milrinone.

Dominant negative actions of human prostacyclin receptor variant through dimerization: implications for cardiovascular disease

OBJECTIVE

Prostacyclin and thromboxane mediate opposing cardiovascular effects through their receptors, the prostacyclin receptor (IP) and thromboxane receptor (TP). Individuals heterozygous for an IP variant, IP(R212C), displayed exaggerated loss of platelet IP responsiveness and accelerated cardiovascular disease. We examined association of IP(R212C) into homo- and heterodimeric receptor complexes and the impact on prostacyclin and thromboxane biology.

METHODS AND RESULTS

Dimerization of the IP, IP(R212C), and TPalpha was examined by bioluminesence resonance energy transfer in transfected HEK293 cells. We observed an equal propensity for formation of IPIP homodimers and IPTPalpha heterodimers. Compared with the IP alone, IP(R212C) displayed reduced cAMP generation and increased endoplasmic reticulum localization but underwent normal homo- and heterodimerization. When the IP(R212C) and IP were coexpressed, a dominant negative action of the variant was evident with enhanced wild-type IP localization to the endoplasmic reticulum and reduced agonist-dependent signaling. Further, the TPalpha activation response, which was shifted from inositol phosphate to cAMP generation following IPTPalpha heterodimerization, was normalized when the TPalpha instead dimerized with IP(R212C).

CONCLUSIONS

IP(R212C) exerts a dominant action on the wild-type IP and TPalpha through dimerization. This likely contributes to accelerated cardiovascular disease in individuals carrying 1 copy of the variant allele.

Gender differences in cardiovascular disease: hormonal and biochemical influences

OBJECTIVE

Atherosclerosis is a complex process characterized by an increase in vascular wall thickness owing to the accumulation of cells and extracellular matrix between the endothelium and the smooth muscle cell wall. There is evidence that females are at lower risk of developing cardiovascular disease (CVD) as compared to males. This has led to an interest in examining the contribution of genetic background and sex hormones to the development of CVD. The objective of this review is to provide an overview of factors, including those related to gender, that influence CVD.

METHODS

Evidence analysis from PubMed and individual searches concerning biochemical and endocrine influences and gender differences, which affect the origin and development of CVD.

RESULTS

Although still controversial, evidence suggests that hormones including estradiol and androgens are responsible for subtle cardiovascular changes long before the development of overt atherosclerosis.

CONCLUSION

Exposure to sex hormones throughout an individual's lifespan modulates many endocrine factors involved in atherosclerosis.

Thromboxane and the thromboxane receptor in cardiovascular disease

Thromboxane A(2) (TXA(2)), the primary product of COX-1-dependent metabolism of arachidonic acid, mediates its biological actions through the TXA(2) receptor, termed the TP. Irreversible inhibition of platelet COX-1-derived TXA(2) with low-dose aspirin affords protection against primary and secondary vascular thrombotic events, underscoring the central role of TXA(2) as a platelet agonist in cardiovascular disease. The limitations associated with aspirin use include significant gastrointestinal toxicity, bleeding complications, potential interindividual response variability and poor efficacy in some disease states. This, together with the broad role of TXA(2) in cardiovascular disease beyond the platelet, has refocused interest towards additional TXA(2)-associated drug targets, in particular TXA(2) synthase and the TP. The superiority of these agents over low-dose aspirin, in terms of clinical efficacy, tolerability and commercial viability, remain open questions that are the focus of ongoing research.