Current Vascular Pharmacology - Volume 7, Issue 4, 2009

Cardiovascular disease is characterized by abnormal function of the endothelium and smooth muscle of blood vessels. Several mechanisms had been proposed to explain, at least partially, the bases of these pathologies, and a significant contribution has come from cellular and molecular studies. Altered vascular reactivity has been detected in human subjects suffering cardiovascular disease as well as in isolated blood vessels, and altered biochemical mechanisms have been characterized in vascular cells in culture or in freshly isolated circulating cells. Complementary explanation for an altered vascular dysfunction is the phenomenon of epigenetics where the environment plays a critical role. Angiogenesis and vasculogenesis are adaptive mechanisms triggered at the vascular tree playing specific roles to secure healthiness of blood vessels, and disturbances of these phenomena are crucial in, for example, tumor growth or fetal development. Several pathologies are associated with altered activity and expression of plasma membrane proteins that are essential to take up metabolic substrates from the extracellular space or to allow the secretion of metabolites that are toxic or unnecessary for the cell. These proteins are grouped in several families and act as membrane transporters, or could also allow the semi-selective transfer of substrates through a group of proteins known as hemi-channels. This issue focuses into the insights of selected membrane transport processes, which could be determinant in pathologies associated with altered vascular cell function. Information regarding diabetes mellitus, pre-eclampsia, intrauterine growth restriction, cell programming, epigenetics and cancer is included.

Concentrative nucleoside transporters (CNT; SLC28) and equilibrative nucleoside transporters (ENT; SLC29) mediate the uptake of natural nucleosides and a variety of nucleoside-derived drugs, mostly used in anticancer therapy. SLC28 and SLC29 families consist in three and four members, respectively, which differ in their substrate selectivity and their energy requirements. Tissue distribution of these transporters is not homogeneous among tissues, and their expression can be regulated. In epithelia, CNT and ENT proteins are mostly localized in the apical and basolateral membranes, respectively, which results in nucleoside and nucleoside-derived drugs vectorial flux. Nucleoside transporters can play physiological roles other than salvages, such as the modulation of extracellular and intracellular adenosine concentrations. Moreover, these transporters also have clinical significance. ENT proteins are target of dipyridamole and dilazep, used as vasodilatory drugs in the treatment of heart and vascular diseases. On the other hand, nucleoside transporters are responsible for the cellular uptake of currently used anticancer nucleoside-derived drugs, thus these membrane proteins might play a significant role in nucleoside-based chemotherapy. Finally, several polymorphisms have been described in CNT and ENT proteins that could affect nucleoside homeostasis, adenosine signalling events or nucleoside-derived drug cytotoxicity or pharmacokinetics.

Diabetes mellitus types 1 and 2, and gestational diabetes are characterized by abnormal D-glucose metabolism and hyperglycaemia, and induce foetal endothelial dysfunction with implications in adult life increasing the risk of vascular diseases. Synthesis of nitric oxide (NO) and uptake of L-arginine (i.e. the L-arginine/NO signalling pathway) and adenosine (a vasoactive endogenous nucleoside) by the umbilical vein endothelium is altered in pathological pregnancies, including pregnancies with pre-established diabetes mellitus or in gestational diabetes. The mechanisms underlying these alterations include differential expression of equilibrative nucleoside transporters (ENTs), amino acid transporters and NO synthases (NOS). Modulation of ENTs and NOS expression and activity in endothelium involves several signalling molecules, including protein kinase C, mitogen-activated protein kinases p42 and p44, calcium and phosphatidyl inositol 3 kinase. Elevated extracellular D-glucose and diabetes alters human endothelial function. However, information regarding modulation the transport capacity as well as expression of ENTs is limited. This review focuses on the effect of diabetes mellitus and gestational diabetes, and hyperglycaemia on the reported mechanisms described for transcriptional and posttranscriptional regulation of ENTs, and the potential consequences for foetal endothelial function in these pathologies. Recent available information regarding functional consequences of an abnormal environment on the functionality of the endothelium from microvasculature of the human placenta is mentioned. The available information is scarce, but it could contribute to a better understanding of the cell and molecular basis of the altered vascular endothelial function in this pathological conditions, emphasizing the key role of this type of epithelium in fetal-placental function and the normal foetal development and growth.

Diabetes mellitus is currently considered to be an epidemic disease. Approximately a third of individuals with type 1 and type 2 diabetes develop persistent albuminuria, lose renal function, and are at increased risk of cardiovascular and other microvascular complications. Diabetic nephropathy (DN) is the primary cause of end stage renal disease throughout the world. Microvascular dysfunction in the glomerulus appears as an early pathogenic event in progression of this renal complication. In recent years, studies with animal knockout (KO) models have revealed that uncoupling of the vascular endothelial growth factor/nitric oxide (VEGF/NO) axis leads to the glomerular alterations that characterize diabetic nephropathy. Therefore, new therapeutic alternatives may usefully target VEGF overproduction or endothelial nitric oxide availability. Recent studies also demonstrate a role for adenosine receptors in glomerular physiology and VEGF production that looks promising for therapeutic intervention of the evolution of diabetic nephropathy. However further progress is required in order to understand the dynamics of local adenosine production, in particular the extracellular metabolism of adenine nucleotides by ectoenzymes and the role of nucleoside transporters on external adenosine accumulation in the glomerulus in the pathological state. So far there is no assay that is sufficiently sensitive and accurate for subclinical diagnoses of this renal disease, which is complicated and costly to patients with often devastating effects. Current studies using proteomics offer promising alternatives for the identification of new renal injury markers. It is hoped these will permit evaluation of new therapeutic tools for more opportune intervention of this disease.

The placental endothelium is unique among the entire human vasculature. The blood enriched in oxygen and nutrients is transported in the veins, whereas the arteries contain deoxygenated blood coming from the fetus. The placental vasculature has to develop rapidly to ensure adequate supply of the fetus. Therefore, factors present in the fetal circulation will stimulate placental angiogenesis. In the third trimester of pregnancy the placental endothelium is richly endowed with insulin receptors. In a pregnancy complicated by maternal diabetes, fetal hyperinsulinemia resulting from maternal and, hence, fetal hyperglycaemia induces changes in the placental vasculature such as increased growth and angiogenesis. This review will discuss general effects of insulin on endothelial cells and further focus on insulin effects on the placental endothelium. Isolation and culture of placental endothelial cells has allowed the identification of insulin effects in vitro. These include metabolic effects of insulin i.e. stimulation of glycogen synthesis, and modulation of angiogenesis on the placental arterial endothelium i.e. regulation of ephrin-B2 expression, an arterial specific signalling molecule implicated in sprouting. The effect of insulin on ephrin-B2 in placental arterial endothelial cells as well as their particularly high expression levels of insulin receptors and receptors for vascular endothelial growth factors indicate that placental angiogenesis is likely to emanate from the arterial compartment and is stimulated by insulin.

Endothelial cells are key in the regulation of vascular tone through the release of vasoactive molecules, including nitric oxide (NO). NO is a gas synthesized from the cationic amino acid L-arginine via the endothelial NO synthase (eNOS). The semi-essential amino acid L-arginine is a taken up by endothelial cells via systems y+ and y+L in primary cultures of human umbilical vein endothelial cells (HUVEC). System y+ is a family of membrane transporters including at least five transport systems for cationic amino acids (CAT) of which HUVEC express human CAT-1 (hCAT-1) and hCAT-2B. Exposure of HUVEC to high extracellular concentrations of D-glucose increases L-arginine transport, hCAT-1 mRNA expression and eNOS activity. These phenomena are also related with increased production of reactive oxygen species (ROS), thus supporting the possibility that changes in L-arginine/NO signalling pathway result from elevated ROS. It has been shown that insulin blocks D-glucose-increased L-arginine transport and cGMP accumulation in HUVEC, whereas in this cell type insulin also modulates high D-glucose effects by activating the transcriptional factors Sp1 and NFκB. These transcription factors have response elements in SLC7A1 (for hCAT-1) gene promoter region, thus representing 2 possible targets for regulation of the expression of this transporter by D-glucose and/or insulin in this cell type. Recent evidences suggest that insulin blocks the stimulatory effect of D-glucose on L-arginine transport by reducing the transcriptional activity of SLC7A1 via Sp1-, NFκB- and ROS-dependent mechanisms. Thus, a role for these transcription factors in response to insulin is proposed in fetal endothelial cells exposed to hyperglycaemia.

Fetal and neonatal morbidity and mortality is high in severe pre-eclampsia compared with mild pre-eclampsia and normotensive pregnancies. Causes for these fetal disturbances had been associated with iatrogenic prematurity and reduction in placental blood flow. Actual evidences suggest that in severe (early-onset) pre-eclampsia a reduction in placental angiogenesis could be a mechanism responsible for the reduced placental blood flow, while in mild (late-onset) preeclampsia normal placental blood flow could result from either no alteration or increased placental angiogenesis, or reduced vessel resistance. Since adenosine is involved in endothelium proliferation and angiogenesis, and umbilical and maternal blood level of this nucleoside is elevated in pre-eclampsia compared with normal pregnancies, it is feasible that placental angiogenesis in mild and/or severe pre-eclampsia involves adenosine-dependent cell signaling mechanisms. There are not reports regarding adenosine role in placental angiogenesis neither in normal nor in pathological pregnancies. However, it is well established that adenosine stimulates adenosine receptors triggering expression of angiogenic factors such as vascular endothelial growth factor (VEGF). VEGF stimulates VEGF receptors type 1 and 2, activating signaling cascades that involve increased synthesis of endothelial-derived nitric oxide (NO). On the other hand, the soluble VEGF receptor type 1 (sFlt-1), whose plasma concentration is increased in severe compared with mild pre-eclampsia, reduces angiogenesis, spotting sFlt-1 as a factor that could potentially be involved in this phenomenon. This review focuses on the available evidence regarding a potential differential mechanism of placental angiogenesis in mild compared with severe pre-eclampsia, and analyzes the potential role of adenosine/VEGF/VEGF receptors/NO signaling cascade in this phenomenon.

Connexins and pannexins comprise two families of transmembrane proteins ubiquitously distributed in vertebrates. Most cell types express more than 1 connexin or pannexin. Members of the same protein family form homo- or hetero-hexamers termed hemichannels. Hemichannels are pathways for the transmembrane diffusional exchange of ions and small molecules. Several human genetic diseases are associated with connexin mutants that may form hemichannels with increased or reduced activity. Pro-inflammatory conditions of different duration and/or intensity can lead to acute or chronic increase in hemichannel activity. Non-lethal stimuli can lead to transient increases in hemichannel activity (required for normal autocrine and/or paracrine cell signaling that might lead to preconditioning responses), whereas lethal stimuli induce long lasting hemichannel-mediated membrane permeabilization that accelerate cell death. Thus, in addition to transporters that mediate active and facilitated transport, the plasma membrane of most cells contains diffusional transporters (hemichannels) that are essential for normal cell functioning; their malfunctioning can cause or worsen a pathological condition.

Vascular calcification is caused by the deposition of basic calcium phosphate crystals in blood vessels, myocardium, and/or cardiac valves. Calcification decreases artery wall compliance, and arterial calcification is associated to mortality in hyperphosphatemic renal failure and diabetes mellitus. The calcification of the tunica media characterizes the arteriosclerosis observed with age, diabetes and end stage-renal disease, and it can develop independently from intima calcification. As part of the vascular calcification mechanism, vascular smooth muscle cells (VSMC) experience a transition from a contractile to an osteochondrogenic phenotype and a sequence of molecular events that are typical of endochondral ossification. The current evidence indicates a key role of increased phosphate uptake by VSMC for calcification, which supplies the substrate for hydroxyapatite formation and could trigger or potentiate VSMC transdiferentiation. The present review analyzes the sodium-phosphate cotransporter Pit-1, which is implicated in calcification. On the basis of the available data obtained in the study of vascular and osteoblastic experimental models, we discuss potential regulatory mechanisms that could lead to increased sodium-dependent phosphate uptake in vascular calcification.

The hypothesis of ‘Developmental Origins of Health and Disease’ (DOHaD) relies on the presence of mechanisms sensing and signalling a diversity of stimuli during fetal development. The mechanisms that have been broadly suggested to be involved in these processes are the epigenetic modifications that could ‘record’ perinatal stimuli. Since the definition of epigenetic and the associated mechanisms are conflictive, in this review epigenetic was defined as ‘chromosome- based mechanisms that can change the phenotypic plasticity in a cell or organism’. The most understood epigenetic mechanisms (i.e. DNA methylation, histone post-translational modifications (PTM), ATP-dependent chromatin modifications and non-coding RNAs) and reported evidence for their role in fetal programming were briefly reviewed. The development of the vascular system is strongly influenced by epigenetic mechanisms. For that reason vascular cells are good candidates to be explored regarding epigenetic programming since its proved susceptibility to be imprinted. This has been described in pregnancy diseases such as intra-uterine growth restriction, gestational diabetes and pre-eclampsia, where changes in vascular function are preserved in vitro.

Intrauterine growth restriction (IUGR) and fetal overgrowth occur in 15% of all pregnancies and lead to the delivery of a baby with an abnormally low or high birth weight, respectively. Both these conditions of pathological fetal growth increase the risk for perinatal complications and predispose the baby for the development of cardiovascular disease and diabetes in childhood and later in life. Fetal growth is closely related to the capacity of the placenta to transport nutrients and ions, which is dependent of the expression and activity of specific transporter proteins in the plasma membrane of the syncytiotrophoblast, the transporting epithelium of the human placenta. In human IUGR, some trophoblast nutrient and ion transporters are down regulated, whereas fetal overgrowth is associated with an up-regulation of transporters for amino acids and glucose in the placental barrier. Experimental studies have provided evidence to suggest that these changes in placental transport capacity constitute a direct cause of altered fetal growth. Therefore, regulation of placental nutrient transporters play a critical role in determining fetal growth and development, as well as the future health of the baby. This review is focused on the human and (i) summarizes the evidence that changes in the activity and expression of trophoblast nutrient and ion transporters play a central role in determining fetal growth, (ii) discusses the molecular mechanisms regulating trophoblast transporters, and (iii) highlights the implications of these findings for the development of pregnancy complications and fetal programming of cardiovascular and metabolic disease.

Cancer cells, as with most mammalian cells, depend on a continuous supply of glucose; not only as a precursor of glycoproteins, triglycerides and glycogen, but also as an important source of energy. This review concentrates on GLUT transporter expression in both normal and cancerous classical sex-steroid hormone tissues (i.e. breast, uterus, ovary, testis and prostate, among others). Given the importance of estrogen, progesterone and androgens in carcinogenesis, as well as in survival and propagation of these cancers, this review also highlights the current literature on hormone regulation of glucose transporters and on the role of hypoxia in their expression. Furthermore, the recent explosion of information on the newer GLUT6-12 family members, a brief overview on their function and general expression has been included. Finally, an insight into the use of glucose transporters as markers of cancer progression and clinical outcome is also discussed.

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High-density lipoprotein (HDL) therapy is an emerging area of therapeutic development in the cardiovascular field, aimed at supplementing and improving the vascular benefit exerted by current treatments. Several studies have clearly established that HDL-cholesterol (HDL-C) levels are a potent and independent epidemiologic risk factor for cardiovascular diseases; moreover, studies in animal models have established that HDL-C raising interventions, such as overexpression of apolipoprotein A-I (apoA-I), the major HDL protein component, reduce the extent of atherosclerosis. In vitro and in vivo experiments have provided mechanistic explanations for the atheroprotective effects of HDL. HDL not only mediates reverse cholesterol transport, but also exerts antioxidant, anti-inflammatory, antithrombotic and vasodilatory effects. These multiple antiatherosclerotic properties provide an excellent rationale for designing therapeutic interventions targeted at enhancing HDL/apoA-I levels, but also for considering a direct administration of HDL-apoA-I in a variety of cardiovascular diseases. We provide an overview and an update of all therapeutic applications of synthetic HDL tested in animal models or in clinical trials. HDL therapy has proven to be effective in promoting atherosclerosis regression not only in experimental models, but also in humans, whereas applications to other areas of cardiovascular disease have only, up to now, been tested in animal models.

Cardiopulmonary exercise testing (CPX) is a well-recognized assessment technique in patients with HF. Ventilatory efficiency, aerobic capacity and heart rate recovery are several parameters obtained from CPX that accurately reflect physiologic function and provide robust prognostic information. Pharmacotherapy is a vital component to the management of patients with HF. Numerous pharmacologic interventions, such as ACE inhibition and beta-blockade have demonstrated significant physiologic and prognostic improvement in this population. Furthermore, a number of investigations demonstrating a positive change in the CPX response resulting from a pharmacologic intervention now exist. Because CPX variables reflect pathophysiologic processes differently, their response to a given pharmacologic is unique. For example, beta-blockade has been shown to significantly improve ventilatory efficiency, one of the most powerful prognostic markers obtained from CPX, while not altering aerobic capacity or heart rate recovery. Conversely, ACE and phosphodiesterase- 5 inhibition appears to improve ventilatory efficiency and aerobic capacity. Given the prognostic value of CPX, gauging its improvement from pharmacotherapy may be advantageous in facilitating optimal titration of medications. A comprehensive review describing the physiologic and prognostic importance of CPX in the context of pharmacotherapy does not exist. This mini review will: 1. Identify key CPX variables obtained from CPX including aerobic capacity, ventilatory efficiency and heart rate recovery, 2. Describe the physiologic and prognostic significance of CPX in the heart failure population, and, 3. Summarize the present body of evidence addressing the change in CPX in response to different pharmacologic interventions including beta-blockade, renin-angiotensin-aldosterone axis inhibition and sildenafil.