β-Adrenergic receptors and nitric oxide generation in the cardiovascular system

Nitric oxide plays a crucial role in cardiovascular homeostasis, with important vasodilatory, anti-thrombotic and anti-atherogenic properties. β-Adrenergic receptors (βARs), present on a wide variety of cardiovascular cells, including vascular endothelial cells, platelets, cardiac myocytes and leukocytes, have long been established as key players in maintaining cardiovascular homeostatic control. During the last few years a wealth of evidence has emerged which directly links stimulation of these cardiovascular βARs to nitric oxide (NO) generation, suggesting a new and important mechanism of adrenergic control of cardiovascular function. This review explores the cardiovascular cell systems in which this coupling of βARs and NO occurs, the intracellular signalling and regulatory mechanisms involved and the abnormalities in βAR-NO oxide coupling found in cardiovascular disease states. Received 30 September 2005; received after revision 24 November 2005; accepted 24 January 2006     

G proteins as drug targets

The structure and function of heterotrimeric G protein subunits is known in considerable detail. Upon stimulation of a heptahelical receptor by the appropriate agonists, the cognate G proteins undergo a cycle of activation and deactivation; the α-subunits and the βγ-dimers interact sequentially with several reaction partners (receptor, guanine nucleotides and effectors as well as regulatory proteins) by exposing appropriate binding sites. For most of these domains, low molecular weight ligands have been identified that either activate or inhibit signal transduction. These ligands include short peptides derived from receptors, G protein subunits and effectors, mastoparan and related insect venoms, modified guanine nucleotides, suramin analogues and amphiphilic cations. Because compounds that act on G proteins may be endowed with new forms of selectivity, we propose that G protein subunits may therefore be considered as potential drug targets. Received 18 September 1998; received after revision 6 November 1998; accepted 11 November 1998     

Integrins and cardiovascular disease

Cardiovascular diseases involve abnormal cell-cell interactions leading to the development of atherosclerotic plaque, which when ruptured causes massive platelet activation and thrombus formation. Parts of a loose thrombus may detach to form an embolus, blocking circulation at a more distant point. The integrins are a family of adhesive cell receptors interacting with adhesive proteins or with counterreceptors on other cells. There is now solid evidence that the major integrin on platelets, the fibrinogen receptor α _IIb  β ₃ , has an important role in several aspects of cardiovascular diseases and that its regulated inhibition leads to a reduction in incidence and mortality due to these disorders. The development of α _IIb  β ₃ inhibitors is an important strategy of many pharmaceutical companies which foresee a large market for the treatment of acute conditions in surgery, the symptoms of chronic conditions and, it is hoped, maybe even the successful prophylaxis of these conditions. Although all the associated problems have not been solved, the undoubted improvements in patient care resulting from the first of these treatments in the clinic have stimulated further research on the role of integrins on other vascular cells in these processes and in the search for new inhibitors. Both the development of specific inhibitors and of mice with specific integrin subunit genes ablated have contributed to a better understanding of the function of integrins in development of the cardiovascular system.     

Subtypes of functional α1-adrenoceptor

In this review, subtypes of functional α1-adrenoceptor are discussed. These are cell membrane receptors, belonging to the seven-transmembrane-spanning G-protein-linked family of receptors, which respond to the physiological agonist noradrenaline. α1-Adrenoceptors can be divided into α1A-, α1B- and α1D-adrenoceptors, all of which mediate contractile responses involving Gq/11 and inositol phosphate turnover. A fourth α1-adrenoceptor, the α1L-, represents a functional phenotype of the α1A-adrenoceptor. α1-Adrenoceptor subtype knock-out mice have refined our knowledge of the functions of α-adrenoceptor subtypes, particuarly as subtype-selective agonists and antagonists are not available for all subtypes. α1-Adrenoceptors function as stimulatory receptors involved particularly in smooth muscle contraction, especially contraction of vascular smooth muscle, both in local vasoconstriction and in the control of blood pressure and temperature, and contraction of the prostate and bladder neck. Central actions are now being elucidated.     

GPR120: a critical role in adipogenesis,inflammation, and energy metabolism in adipose tissue

It is well known that adipose tissue has a critical role in the development of obesity and metabolic diseases and that adipose tissue acts as an endocrine organ to regulate lipid and glucose metabolism. Accumulating in the adipose tissue, fatty acids serve as a primary source of essential nutrients and act on intracellular and cell surface receptors to regulate biological events. G protein-coupled receptor 120 (GPR120) represents a promising target for the treatment of obesity-related metabolic disorders for its involvement in the regulation of adipogenesis, inflammation, glucose uptake, and insulin resistance. In this review, we summarize recent studies and advances regarding the systemic role of GPR120 in adipose tissue, including both white and brown adipocytes. We offer a new perspective by comparing the different roles in a variety of homeostatic processes from adipogenic development to adipocyte metabolism, and we also discuss the effects of natural and synthetic agonists that may be potential agents for the treatment of metabolic diseases.     

Searching for “signal 2”: costimulation requirements of γδ T cells

T cell activation requires the integration of signals that arise from various types of receptors. Although TCR triggering is a necessary condition, it is often not sufficient to induce full T-cell activation, as reflected in cell proliferation and cytokine secretion. This has been firmly demonstrated for conventional αβ T cells, for which a large panel of costimulatory receptors has been identified. By contrast, the area remains more obscure for unconventional, innate-like γδ T cells, as the literature has been scarce and at times contradictory. Here we review the current state of the art on the costimulatory requirements of γδ T cell activation. We highlight the roles of members of the immunoglobulin (like CD28 or JAML) or tumour necrosis factor receptor (like CD27) superfamilies of coreceptors, but also of more atypical costimulatory molecules, such as NKG2D or CD46. Finally, we identify various areas where our knowledge is still markedly insufficient, hoping to provoke future research on γδ T cell costimulation.     

Chaperone-like features of bovine serum albumin: a comparison with α-crystallin

The chaperone behaviour of bovine serum albumin was compared with that of α-crystallin. The chaperone activity was assessed by measuring: (i) the ability to antagonize protein aggregation induced by heat; (ii) the capability to protect the activity of thermally stressed enzymes and (iii) the effectiveness in assisting the functional recovery of chemically denatured sorbitol dehydrogenase. Despite the lack of structural analogies, both proteins show several functional similarities in preventing inactivation of thermally stressed enzymes and in reactivating chemically denatured sorbitol dehydrogenase. As with α-crystallin, the chaperone action of bovine serum albumin appears to be ATP independent. Bovine serum albumin appears significantly less effective than α-crystallin only in preventing thermally induced protein aggregation. A possible relationship between chaperone function and structural organization is proposed. Together, our results indicate that bovine serum albumin acts as a molecular chaperone and that, for its particular distribution, can be included in the extracellular chaperone family. Received 29 August 2005; received after revision 23 September 2005; accepted 12 October 2005     

Phytanic acid: production from phytol, its breakdown and role in human disease

Phytanic acid is a branched-chain fatty acid that accumulates in a variety of metabolic disorders. High levels of phytanic acid found in patients can exceed the millimolar range and lead to severe symptoms. Degradation of phytanic acid takes place by α-oxidation inside the peroxisome. A deficiency of its breakdown, leading to elevated levels, can result from either a general peroxisomal dysfunction or from a defect in one of the enzymes involved in α-oxidation. Research on Refsum disease, belonging to the latter group of disorders and characterized by a deficiency of the first enzyme of α-oxidation, has extended our knowledge of phytanic acid metabolism and pathology of the disease greatly over the past few decades. This review will centre on this research on phytanic acid: its origin, the mechanism by which its α-oxidation takes place, its role in human disease and the way it is produced from phytol. Received 4 October 2005; received after revision 24 February 2006; accepted 26 April 2006     

Mechanism-based targeting of NMDA receptor functions

NMDA receptors (NRs) are key signaling proteins in the central nervous system and represent important targets for drug development in several neurologic disorders. They are critically involved with fundamental brain processes, and thus indiscriminate pharmacological suppression of NR currents has seen only modest therapeutic success so far. Targeting harmful NR receptor activities while sparing the receptor’s vital functions requires a better understanding of the complexity of NR activation reaction; of the range of mechanisms that modulate discrete receptor activities; and of the consequences of this modulation on specific receptor functions. A quantitative account of the NR activation pathway was recently proposed and validated. It describes the gating reaction as a sequential, multi-step process rather than a binary, on-off switch. Alongside isoform-specific modulators, state-specific modulators may represent sophisticated interventions with high potential for narrow, functional specifi city. Here I review physiologic mechanisms that control NR responses; the salient features of the NR activation reaction; and discuss the model’s validity and its implications for drug development and characterization.Submitted 25 May 2005; accepted 29 June 2005     

Dietary vitamin E does not protect from endotoxin-induced hepatic microvascular dysfunction

Starting from the concept that lipopolysaccharide (LPS)-associated hepatotoxicity involves the action of reactive oxygen species, the present study was conducted to test whether vitamin E, a lipophilic antioxidant, prevents LPS-induced hepatic microvascular dysfunction and liver injury. Fifty-two rats were divided into three groups and fed diets containing 0 (n=16), 75 (n=18) or 8000 mg (n=18) α-tocopherol acetate/kg food for four weeks. At 1 h and 6 h after intravenous LPS-exposure (10 mg/kg E. coli LPS) hepatic microvascular response and liver injury were assessed by the analysis of Kupffer cell phagocytic activity, leukocyte-endothelial cell interaction and nutritive sinusoidal perfusion (intravital fluorescence epi- illumination technique) as well as bile flow, serum liver enzyme activities and tissue histomorphology. In animals fed with 75 mg vitamin E/kg (standard diet), LPS caused hepatic Kupffer cell activation (increased phagocytic activity) and hepatic microvascular leukocyte activation, with stasis in sinusoids and adherence in postsinusoidal venules (1 h) followed by leukocytic infiltration into tissue (6 h) and progredient sinusoidal perfusion failure (6 h). Hepatic microvascular injury was accompanied by reduced bile flow and enhanced liver enzyme release. Vitamin E-enriched diet (8000 mg/kg) and even vitamin E-deficient diet did not significantly affect LPS-induced hepatic microvascular cell activation and perfusion failure. Thus, we conclude, that vitamin E is not effective to protect from endotoxin-induced hepatic microvascular dysfunction. Received 7 November 1996; received after revision 30 December 1996; accepted 20 January 1997     

Mammalian olfactory receptors: pharmacology, G protein coupling and desensitization

The vertebrate olfactory system recognizes and discriminates between thousands of structurally diverse odorants. Detection of odorants in mammals is mediated by olfactory receptors (ORs), which comprise the largest superfamily of G protein-coupled receptors (GPCRs). Upon odorant binding, ORs couple to G proteins, resulting in an increase in intracellular cAMP levels and subsequent receptor signaling. In this review, we will discuss recently published studies outlining the molecular basis of odor discrimination, focusing on pharmacology, G protein activation, and desensitization of ORs. A greater understanding of the molecular mechanisms underlying OR activity may help in the discovery of agonists and antagonists of ORs, and of GPCRs with potential therapeutic applications.     

Metabolic syndrome as a risk factor for neurological disorders

The metabolic syndrome is a cluster of common pathologies: abdominal obesity linked to an excess of visceral fat, insulin resistance, dyslipidemia and hypertension. At the molecular level, metabolic syndrome is accompanied not only by dysregulation in the expression of adipokines (cytokines and chemokines), but also by alterations in levels of leptin, a peptide hormone released by white adipose tissue. These changes modulate immune response and inflammation that lead to alterations in the hypothalamic ‘bodyweight/appetite/satiety set point,’ resulting in the initiation and development of metabolic syndrome. Metabolic syndrome is a risk factor for neurological disorders such as stroke, depression and Alzheimer’s disease. The molecular mechanism underlying the mirror relationship between metabolic syndrome and neurological disorders is not fully understood. However, it is becoming increasingly evident that all cellular and biochemical alterations observed in metabolic syndrome like impairment of endothelial cell function, abnormality in essential fatty acid metabolism and alterations in lipid mediators along with abnormal insulin/leptin signaling may represent a pathological bridge between metabolic syndrome and neurological disorders such as stroke, Alzheimer’s disease and depression. The purpose of this review is not only to describe the involvement of brain in the pathogenesis of metabolic syndrome, but also to link the pathogenesis of metabolic syndrome with neurochemical changes in stroke, Alzheimer’s disease and depression to a wider audience of neuroscientists with the hope that this discussion will initiate more studies on the relationship between metabolic syndrome and neurological disorders.     

Anti-adipogenesis by 6-thioinosine is mediated by downregulation of PPAR γ through JNK-dependent upregulation of iNOS

Adipocyte dysfunction is associated with the development of obesity. This study shows that 6-thioinosine inhibits adipocyte differentiation. The mRNA levels of PPAR γ and C/EBPα, but not C/EBPβ and δ, were reduced by 6-thioinosine. Moreover, the mRNA levels of PPAR γ target genes (LPL, CD36, aP2, and LXRα) were down-regulated by 6-thioinosine. We also demonstrated that 6-thioinosine inhibits the transactivation activity and the mRNA level of PPAR γ. Additionally, attempts to elucidate a possible mechanism underlying the 6-thioinosine-mediated effects revealed that 6-thioinosine induced iNOS gene expression without impacting eNOS expression, and that this was mediated through activation of AP-1, especially, JNK. In addition, 6-thioinosine was found to operate upstream of MEKK-1 in JNK activation signaling. Taken together, these findings suggest that the inhibition of adipocyte differentiation by 6-thioinosine occurs primarily through the reduced expression of PPAR γ, which is mediated by upregulation of iNOS via the activation of JNK.     

Intra-vascular glucocorticoid metabolism as a modulator of vascular structure and function

The ability of glucocorticoids to directly alter arterial function, structure and the inflammatory response to vascular injury may contribute to their well-established link with the development of cardiovascular disease. Recent studies have emphasised the importance of tissue-specific regulation of glucocorticoid availability by the 11 β-hydroxysteroid dehydrogenase (11HSD) isozymes, which inter-convert active glucocorticoids and their inactive metabolites. The expression of both type 1 and type 2 11HSDs in the arterial wall suggests that prereceptor metabolism of glucocorticoids may have a direct impact on vascular physiology. Indeed there is evidence that 11HSDs influence glucocorticoid-mediated changes in vascular contractility, vascular structure, the inflammatory response to injury and the growth of new blood vessels. Hence, inhibition of 11HSD isozymes may provide a novel therapeutic target in vascular disease. Received 19 September 2005; received after revision 1 November 2005; accepted 25 November 2005     

Functional expression of mammalian opioid receptors in insect cells and high-throughput screening platforms for receptor ligand mimetics

Lepidopteran cell lines have been engineered to constitutively express high levels of mouse opioid receptors either alone or in combination with human G16 protein. Biochemical and pharmacological studies demonstrate that these lines contain all the mediator G proteins and downstream effectors required for opioid receptor function, including phospholipase C, and that expression of exogenous G16 does not contribute significantly to increased receptor responses upon activation. The activation of the phospholipase C pathway in the transformed cells upon stimulation with known receptor ligands results in easily and quantitatively measurable increases in free intracellular calcium, which can be monitored by automated fluorescent methods, while the addition of specific antagonists blocks the agonist-induced responses. Therefore, the transformed lepidopteran cell lines can be used as sensitive high-throughput screening platforms for fast detection of opioid receptor ligand mimetics (agonists and antagonists) in collections of natural products and synthetic compounds.Received 3 December 2004; received after revision 3 February 2005; accepted 10 February 2005L. Swevers and E. Morou contributed equally to this work.     

Galectin-7

Galectins are a family of animal lectins with an affinity for β-galactosides. They are differentially expressed by various tissues and appear to be functionally multivalent, exerting a wide range of biological activities both during development and in adult tissue. Galectin-7, a member of this family, contributes to different events associated with the differentiation and development of pluristratified epithelia. It is also associated with epithelial cell migration, which plays a crucial role in the re-epithelialization process of corneal or epidermal wounds. In addition, recent evidence indicates that galectin-7, designated as the product of the p53-induced gene 1 (PIG1), is a regulator of apoptosis through JNK activation and mitochondrial cytochrome c release. Defects in apoptosis constitute one of the major hallmarks of human cancers, and galectin-7 can act as either a positive or a negative regulatory factor in tumour development, depending on the histological type of the tumour. Received 30 October 2005; received after revision 15 November 2005; accepted 25 November 2005     

δ-Protocadherins: unique structures and functions

δ-Protocadherins constitute a group of cadherins characterized by several conserved motifs in their cytoplasmic domains. We present a phylogenetic analysis that further divides this group into δ1-protocadherins (comprising protocadherin-1, −7, −9 and −11 or -X/Y) and δ2-protocadherins (comprising protocadherin-8, −10, −17, −18 and −19). The δ-protocadherin genes, which are located on different chromosomes in man and mouse, have a similar gene structure. They are expressed as multiple splice forms, differing mostly in their cytoplasmic domains. Some δ-protocadherins were reported to mediate weak cell-cell adhesion in vitro and cell sorting in vivo. In addition, individual δ-protocadherins might play important roles in signaling pathways, as they bind to proteins such as TAF1/Set, protein phosphatase-1α and the Frizzled 7 receptor. The spatiotemporally restricted expression of δ-protocadherins in different tissues and species and the results of their functional analysis, mainly in Xenopus, suggest that they play multiple, tightly regulated roles in vertebrate development. Received 18 July 2005; received after revision 26 August 2005; accepted 2 September 2005     

Hormone-induced subcellular redistribution of trimeric G proteins

Trimeric guanine nucleotide-binding proteins (G proteins) function as the key regulatory elements in a number of transmembrane signaling cascades where they convey information from agonist-activated receptors to effector molecules. The subcellular localization of G proteins is directly related to their functional role, i.e., the dominant portion of the cellular pool of G proteins resides in the plasma membrane. An intimate association of G protein subunits with the plasma membrane has been well known for a long time. However, results of a number of independent studies published in the past decade have indicated clearly that exposure of intact target cells to agonists results in subcellular redistribution of the cognate G proteins from plasma membranes to the light-vesicular membrane fractions, in internalization from the cell surface into the cell interior and in transfer from the membrane to the soluble cell fraction (high-speed supernatant), i.e., solubilization. Solubilization of G protein α subunits as a consequence of stimulation of G protein-coupled receptors (GPCRs) with agonists has also been observed in isolated membrane preparations. The membrane-cytosol shift of G proteins was detected even after direct activation of these proteins by non-hydrolyzable analogues of GTP or by cholera toxin-induced ADP-ribosylation. In addition, prolonged stimulation of GPCRs with agonists has been shown to lead to down-regulation of the relevant G proteins. Together, these data suggest that G proteins might potentially participate in a highly complex set of events, which are generally termed desensitization of the hormone response. Internalization, subcellular redistribution, solubilization, and down-regulation of trimeric G proteins may thus provide an additional means (i.e., beside receptor-based mechanisms) to dampen the hormone or neurotransmitter response after sustained (long-term) exposure. Received 31 August 2001; received after revision 31 October 2001; accepted 7 November 2001