Presence of functional endothelin-1 receptors in nuclear membranes of human aortic vascular smooth muscle cells

Difference in the response to angiotensin II between left and right ventricular endocardial endothelial cells

Previous studies focused on the right ventricular endocardial endothelial cells (EECRs) and showed that angiotensin II (Ang II) induced increase in cytosolic and nuclear calcium via AT₁ receptor activation. In the present study, we verified whether the response of left EECs (EECLs) to Ang II is different than that of EECRs. Our results showed that the EC₅₀ of the Ang II-induced increase of cytosolic and nuclear calcium in EECLs was 10× higher (around 2 × 10^-13 mol/L) than in EECRs (around 8 × 10^-12 mol/L). The densities of both AT₁ and AT₂ receptors were also higher in EECLs than those previously reported in EECRs. The effect of Ang II was mediated in both cell types via the activation of AT₁ receptors. Treatment with Ang II induced a significant increase of cytosolic and nuclear AT₁ receptors in EECRs, whereas the opposite was found in EECLs. In both cell types, there was a transient increase of cytosolic and nuclear AT₂ receptors following the Ang II treatment. In conclusion, our results showed that both AT₁ and AT₂ receptors densities are higher in both EECLs compared to what was reported in EECRs. The higher density of AT₁ receptors in EECLs compared to REECs may explain, in part, the higher sensitivity of EECLs to Ang II.

New insights into mechanisms of nuclear translocation of G-protein coupled receptors

The G-protein coupled receptor (GPCR) signaling was long believed to involve activation of receptor exclusively at the cell surface, followed by its binding to heterotrimeric G-proteins and arrestins to trigger various intracellular signaling cascades, and termination of signaling by internalization of the receptor. It is now accepted that many GPCRs continue to signal after internalization in the endosomes. Since the breakthrough discoveries of nuclear binding sites for their ligands in 1980s, several GPCRs have been detected at cell nuclei. But mechanisms of nuclear localization of GPCRs, many of whom contain putative nuclear localization signals, remain poorly understood to date. Nevertheless, it is known that subcellular trafficking of GPCRs is regulated by members of Ras superfamily of small GTPases, most notably by Rab and Arf GTPases. In this commentary, we highlight several recent studies which suggest novel roles of small GTPases, importins and sorting nexin proteins in the nuclear translocation of GPCRs via vesicular transport pathways. Taken together with increasing evidence for in vivo functionality of the nuclear GPCRs, better understanding of their trafficking will provide valuable clues in cell biology.

Developmental and adult expression patterns of the G-protein-coupled receptor GPR88 in the rat: Establishment of a dual nuclear-cytoplasmic localization

GPR88 is a neuronal cerebral orphan G-protein-coupled receptor (GPCR) that has been linked to various psychiatric disorders. However, no extensive description of its localization has been provided so far. Here, we investigate the spatiotemporal expression of the GPR88 in prenatal and postnatal rat tissues by using in situ hybridization and immunohistochemistry. GPR88 protein was initially detected at embryonic day 16 (E16) in the striatal primordium. From E16-E20 to adulthood, the highest expression levels of both protein and mRNA were observed in striatum, olfactory tubercle, nucleus accumbens, amygdala, and neocortex, whereas in spinal cord, pons, and medulla GPR88 expression remains discrete. We observed an intracellular redistribution of GPR88 during cortical lamination. In the cortical plate of the developing cortex, GPR88 presents a classical GPCR plasma membrane/cytoplasmic localization that shifts, on the day of birth, to nuclei of neurons progressively settling in layers V to II. This intranuclear localization remains throughout adulthood and was also detected in monkey and human cortex as well as in the amygdala and hypothalamus of rats. Apart from the central nervous system, GPR88 was transiently expressed at high levels in peripheral tissues, including adrenal cortex (E16-E21) and cochlear ganglia (E19-P3), and also at moderate levels in retina (E18-E19) and spleen (E21-P7). The description of the GPR88 anatomical expression pattern may provide precious functional insights into this novel receptor. Furthermore, the GRP88 nuclear localization suggests nonclassical GPCR modes of action of the protein that could be relevant for cortical development and psychiatric disorders. J. Comp. Neurol. 524:2776-2802, 2016. © 2016 Wiley Periodicals, Inc.

Mechanisms of estrogen effects on the endothelium: an overview

In this review, we aim to provide an overview of the recent advances in understanding estrogen effects on the vascular endothelium. Epidemiological studies suggest the female sex hormone estrogen mediates the relative protection of premenopausal women against cardiovascular disease, compared with age-matched men. However, results from clinical trials of exogenous estrogen supplementation in postmenopausal women have been disappointing, generating much controversy about the role of estrogen and demonstrating the need for further research in this field. Here we have discussed the roles of different estrogen receptors (ERs) such as ERα, ERβ, and G-protein coupled receptor 30; the complex genomic and nongenomic signalling pathways downstream to ER activation and the factors such as age, menopause, pregnancy, and diabetes that might alter estrogen responses. The common themes of this discussion are the complexity and diversity of endothelial estrogen responses and their modulation by 1 or more coexisting factors. Finally, we summarize the emerging therapeutic options including improved targeting of individual ERs and signalling pathways that might maximize the therapeutic potential of estrogenic compounds while minimizing their harmful side effects.

Intracrine endothelin signaling evokes IP3-dependent increases in nucleoplasmic Ca²⁺ in adult cardiac myocytes

Endothelin receptors are present on the nuclear membranes in adult cardiac ventricular myocytes. The objectives of the present study were to determine 1) which endothelin receptor subtype is in cardiac nuclear membranes, 2) if the receptor and ligand traffic from the cell surface to the nucleus, and 3) the effect of increased intracellular ET-1 on nuclear Ca(2+) signaling. Confocal microscopy using fluorescently-labeled endothelin analogs confirmed the presence of ETB at the nuclear membrane of rat cardiomyocytes in skinned-cells and isolated nuclei. Furthermore, in both cardiac myocytes and aortic endothelial cells, endocytosed ET:ETB complexes translocated to lysosomes and not the nuclear envelope. Although ETA and ETB can form heterodimers, the presence or absence of ETA did not alter ETB trafficking. Treatment of isolated nuclei with peptide: N-glycosidase F did not alter the electrophoretic mobility of ETB. The absence of N-glycosylation further indicates that these receptors did not originate at the cell surface. Intracellular photolysis of a caged ET-1 analog ([Trp-ODMNB(21)]ET-1) evoked an increase in nucleoplasmic Ca(2+) ([Ca(2+)]n) that was attenuated by inositol 1,4,5-trisphosphate receptor inhibitor 2-aminoethoxydiphenyl borate and prevented by pre-treatment with ryanodine. A caged cell-permeable analog of the ETB-selective antagonist IRL-2500 blocked the ability of intracellular cET-1 to increase [Ca(2+)]n whereas extracellular application of ETA and ETB receptor antagonists did not. These data suggest that 1) the endothelin receptor in the cardiac nuclear membranes is ETB, 2) ETB traffics directly to the nuclear membrane after biosynthesis, 3) exogenous endothelins are not ligands for ETB on nuclear membranes, and 4) ETB associated with the nuclear membranes regulates nuclear Ca(2+) signaling.

Receptors and ionic transporters in nuclear membranes: new targets for therapeutical pharmacological interventions

Work from our group and other laboratories showed that the nucleus could be considered as a cell within a cell. This is based on growing evidence of the presence and role of nuclear membrane G-protein coupled receptors and ionic transporters in the nuclear membranes of many cell types, including vascular endothelial cells, endocardial endothelial cells, vascular smooth muscle cells, cardiomyocytes, and hepatocytes. The nuclear membrane receptors were found to modulate the functioning of ionic transporters at the nuclear level, and thus contribute to regulation of nuclear ionic homeostasis. Nuclear membranes of the mentioned types of cells possess the same ionic transporters; however, the type of receptors is cell-type dependent. Regulation of cytosolic and nuclear ionic homeostasis was found to be dependent upon a tight crosstalk between receptors and ionic transporters of the plasma membranes and those of the nuclear membrane. This crosstalk seems to be the basis for excitation–contraction coupling, excitation–secretion coupling, and excitation – gene expression coupling. Further advancement in this field will certainly shed light on the role of nuclear membrane receptors and transporters in health and disease. This will in turn enable the successful design of a new class of drugs that specifically target such highly vital nuclear receptors and ionic transporters.

Cysteinyl leukotriene signaling through perinuclear CysLT(1) receptors on vascular smooth muscle cells transduces nuclear calcium signaling and alterations of gene expression

Leukotrienes are pro-inflammatory mediators that are locally produced in coronary atherosclerotic plaques. The response induced by cysteinyl leukotrienes (CysLT) in human coronary arteries may be altered under pathological conditions, such as atherosclerosis. The aim of the present study was to elucidate cysteinyl leukotriene signaling in vascular smooth muscle cells (SMCs) and the effects of inflammation on this process. Immunohistochemical analysis of human carotid endarterectomy samples revealed that the CysLT₁ leukotriene receptor was expressed in areas that also stained positive for α-smooth muscle actin. In human coronary artery smooth muscle cells, lipopolysaccharide significantly upregulated the CysLT₁ receptor and significantly enhanced the changes in intracellular calcium induced by leukotriene C₄ (LTC₄). In these cells, the CysLT₁ receptor exhibited a perinuclear expression, and LTC₄ stimulation predominantly enhanced nuclear calcium increase, which was significantly inhibited by the CysLT₁ receptor antagonist MK-571. Microarray analysis revealed, among a number of significantly upregulated genes after 24 h stimulation of human coronary artery smooth muscle cells with LTC₄, a 5-fold increase in mRNA levels for plasminogen activator inhibitor (PAI)-2. The LTC₄-induced increase in PAI-2 expression was confirmed by real-time quantitative PCR and ELISA and was inhibited by the CysLT₁ receptor antagonist MK-571 and by calcium chelators. In summary, pro-inflammatory stimulation of vascular SMCs upregulated a perinuclear CysLT₁ receptor expression coupled to nuclear calcium signaling and changes in gene expression, such as upregulation of PAI-2. Taken together, these findings suggest a role of nuclear CysLT₁ receptor signaling in vascular SMCs inducing gene expression patterns associated with atherosclerosis.

Nuclear membrane receptors for ET-1 in cardiovascular function

Plasma membrane endothelin type A (ETA) receptors are internalized and recycled to the plasma membrane, whereas endothelin type B (ETB) receptors undergo degradation and subsequent nuclear translocation. Recent studies show that G protein-coupled receptors (GPCRs) and ion transporters are also present and functional at the nuclear membranes of many cell types. Similarly to other GPCRs, ETA and ETB are present at both the plasma and nuclear membranes of several cardiovascular cell types, including human cardiac, vascular smooth muscle, endocardial endothelial, and vascular endothelial cells. The distribution and density of ETARs in the cytosol (including the cell membrane) and the nucleus (including the nuclear membranes) differ between these cell types. However, the localization and density of ET-1 and ETB receptors are similar in these cell types. The extracellular ET-1-induced increase in cytosolic ([Ca]c) and nuclear ([Ca]n) free Ca2+ is associated with an increase of cytosolic and nuclear reactive oxygen species. The extracellular ET-1-induced increase of [Ca]c and [Ca]n as well as intracellular ET-1-induced increase of [Ca]n are cell-type dependent. The type of ET-1 receptor mediating the extracellular ET-1-induced increase of [Ca]c and [Ca]n depends on the cell type. However, the cytosolic ET-1-induced increase of [Ca]n does not depend on cell type. In conclusion, nuclear membranes' ET-1 receptors may play an important role in overall ET-1 action. These nuclear membrane ET-1 receptors could be targets for a new generation of antagonists.

Nuclear localization of vasoactive intestinal peptide (VIP) receptors in human breast cancer

Vasoactive intestinal peptide (VIP) and its receptors (VPACs) are involved in proliferation, survival, and differentiation in human breast cancer cells. Its mechanism of action is traditionally thought to be through specific plasma membrane receptors. There is compelling evidence for a novel intracrine mode of genomic regulation by G-protein-coupled receptors (GPCRs) that implies both endocytosis and nuclear translocation of peripheral GPCR and/or the activation of nuclear-located GPCRs by endogenously-produced, non-secreted ligands. Regarding to VPAC receptors, which are GPCRs, there is only a report suggesting them as a dynamic system for signaling from plasma membrane and nuclear membrane complex. In this study, we show that VPAC(1) receptor is localized in cell nuclear fraction whereas VPAC(2) receptor presents an extranuclear localization and its protein expression is lower than that of VPAC(1) receptor in human breast tissue samples. Both receptors as well as VIP are overexpressed in breast cancer as compared to non-tumor tissue. Moreover, we report the markedly nuclear localization of VPAC(1) receptors in estrogen-dependent (T47D) and independent (MDA-MB-468) human breast cancer cell lines. VPAC(1) receptors are functional in plasma membrane and nucleus as shown by VIP stimulation of cAMP production in both cell lines. In addition, VIP increases its own intracellular and extracellular levels, and could be involved in the regulation of VPAC(1)-receptor traffic from the plasma membrane to the nucleus. These results support new concepts on function and regulation of nuclear GPCRs which could have an impact on development of new therapeutic drugs.

Presence of tubular and reticular structures in the nucleus of human vascular smooth muscle cells

In recent decades, studies addressing nuclear calcium (Ca(2+)) homeostasis and signaling contributed to redefining the role of the nucleus. Yet many aspects of nuclear Ca(2+) signaling and homeostasis are only modestly understood. The present study aimed at investigating the presence of nuclear structures which could contribute to the regulation of nuclear Ca(2+) homeostasis. Using real 3D confocal microscopy, coupled to utilization of appropriate organelle probes and specific antibodies, we identified two entities in the nuclei of intact human vascular smooth muscle cells (hVSMCs) as well as in isolated hVSMCs nuclei. Our results demonstrate the presence of an ER-like nuclear reticular structure in nuclei of intact hVSMCs and in isolated nuclei. Similar to the ER/SR, this structure possesses thapsigargin binding sites, IP(3)Rs and RyRs, thus it was named nucleoplasmic reticulum (NR). Furthermore, nuclear tubular structures were also detected. The latter, similar to the nuclear envelope membranes, possess nuclear pores, thapsigargin binding sites, Angiotensin II receptor AT(2), and are associated with Lamin A/C. However, unlike the NR and the nuclear envelope membranes, these tubular structures disappeared when the nuclei were isolated from the cells. The nuclear tubular structures were called Nuclear T-Tubules (NTTs). Our calcium studies in isolated nuclei utilizing IP(3) and Ryanodine suggest that the NR may participate in nuclear Ca(2+) signaling. On the other hand, presence of nuclear pores on the NTTs suggests that these structures can play a role in cytosol-nucleus exchange. In conclusion, two distinct structures are present in the nucleus of hVSMCs and might play an important role in nuclear Ca(2+) homeostasis.

Nitric oxide and reactive oxygen species in the nucleus revisitedThis review is one of a selection of papers published in a Special Issue on Oxidative Stress in Health and Disease

Recent work from our group showed that the nuclear envelope membranes contain several G protein-coupled receptors, including prostaglandin E2 (EP3R) and endothelin-1 (ET-1) receptors. Activation of EP3R increased endothelial nitric oxide synthase (eNOS) RNA expression in nuclei. eNOS and inducible NOS (iNOS) are reported to also be present at the nuclear level. Furthermore, reactive oxygen species (ROS) were also localized at the nuclear level. In this review, we show that stimulation with NO donor sodium nitroprusside results in an increase of intranuclear calcium that was dependent on guanylate cyclase activation, but independent of MAPK. This increase in nuclear calcium correlated with an increase in nuclear transcription of iNOS. H2O2 and ET-1 increase both cytosolic and nuclear ROS in human endocardial endothelial cells and in human aortic vascular smooth muscle cells. This increase in ROS levels by H2O2 and ET-1 was reversed by the antioxidant glutathione. In addition, our results strongly suggest that cytosolic signalization is not only transmitted to the nucleus but is also generated by the nucleus. Furthermore, we demonstrate that oxidative stress can be sensed by the nucleus. These results highly suggest that ROS formation is also generated directly by the nucleus and that free radicals may contribute to ET-1 regulation of nuclear Ca2+ homeostasis.

The cardiovascular physiology and pharmacology of endothelin-1

One year after the discovery in 1980 that the endothelium was obligatory for acetylcholine to relax isolated arteries, it was clearly shown that the endothelium could also promote contraction. In 1988, Dr Yanagisawa's group identified endothelin-1 (ET-1) as the first endothelium-derived contracting factor. The circulating levels of this short (21 amino acids) peptide were quickly determined in humans and it was reported that in most cardiovascular diseases, circulating levels of ET-1 were increased and ET-1 was then recognized as a likely mediator of pathological vasoconstriction in human. The discovery of two receptor subtypes in 1990, ET(A) and ET(B), permitted optimization of bosentan, which entered clinical development in 1993, and was offered to patients with pulmonary arterial hypertension in 2001. In this report, we discuss the physiological and pathophysiological role of endothelium-derived ET-1, the pharmacology of its two receptors, focusing on the regulation of the vascular tone and as much as possible in humans. The coronary bed will be used as a running example, but references to the pulmonary, cerebral, and renal circulation will also be made. Many of the cardiovascular complications associated with aging and cardiovascular risk factors are initially attributable, at least in part, to endothelial dysfunction, particularly dysregulation of the vascular function associated with an imbalance in the close interdependence of NO and ET-1, in which the implication of the ET(B) receptor may be central.

Nuclear membrane receptors and channels as targets for drug development in cardiovascular diseases

The use of confocal microscopy has shown that the nucleus plays an important role in excitation-contraction and excitation-secretion coupling of several excitable and nonexcitable cardiovascular cells. It has shown that the nuclear membranes, like the sarcolemmal membrane, possess ionic transporters as well as G protein-coupled receptors (GPCRs), which play a major role in modulating both cytosolic and nuclear ionic homeostasis and nuclear signalling. During spontaneous contraction of heart cells, the increase in cytosolic Ca2+ was immediately followed by a transient increase in nuclear Ca2+. The nuclear Ca2+ rise during excitation-contraction and excitation-secretion coupling was both dependent and independent of changes in cytosolic Ca2+. Nuclear membrane GPCRs, such as those of angiotensin II, neuropeptide Y, and ET-1, were functional and contributed to modulation of nuclear ionic homeostasis via direct and (or) indirect modulation of nuclear membrane ionic transporters such as channels, pumps, and exchangers. The signalling of nuclear membrane GPCRs may also contribute to modulation of gene expression, which may regulate proliferation and remodelling of cells and, indeed, life and death. Direct or indirect targeting of nuclear membrane ionic transporters and GPCRs may constitute a new target for drug action.

The neurokinin-3 (NK3) and the neurokinin-1 (NK1) receptors are differentially targeted to mesocortical and mesolimbic projection neurons and to neuronal nuclei in the rat ventral tegmental area

Tonic activation of neurokinin-3 (NK(3)) receptors in dopamine neurons of the ventral tegmental area (VTA) has been implicated in the pathophysiology of schizophrenia. This psychiatric disorder is associated with a dysfunctional activity in VTA projection neurons that can affect cognitive function at the level of the medial prefrontal cortex (mPFC) as well as motor and motivational states controlled in part by mesolimbic output to the nucleus accumbens (Acb). To determine the relevant sites for NK(3) receptor activation within this neuronal network, we used confocal and electron microscopy to examine NK(3) receptors (Cy5; immunogold) and retrograde labeling of fluorogold (FG, FITC; immunoperoxidase) in the VTA of rats receiving either Acb or mPFC injections of FG. Comparison was made with neurokinin-1 (NK(1)) receptors, which are also present, but less abundant then NK(3) receptors, in dopaminergic and GABAergic VTA neurons. There were no observable differences between NK(3) and NK(1) receptors in their primary locations in the cytoplasm and on the plasma membrane of VTA somata and dendrites with or without FG. Dendrites labeled with FG retrogradely transported from mPFC, however, contained more NK(3) or less NK(1) immunogold particles (plasmalemmal + cytoplasmic) then those retrogradely labeled following FG injection in the Acb. Moreover, only the NK(3) receptors were detected in neuronal nuclei in the VTA and in the nuclei of human HEK-293T NK(3)-transfected cells. The enrichment of NK(3) receptors in mesocortical projection neurons and nuclear distribution of these receptors may provide insight for understanding the selective antipsychotic effectiveness of NK(3) antagonists.

Nonpeptidic antagonists of ETA and ETB receptors reverse the ET-1-induced sustained increase of cytosolic and nuclear calcium in human aortic vascular smooth muscle cells

Our previous work showed that ET-1 induced a concentration-dependent increase of cytosolic Ca2+ ([Ca]c) and nuclear Ca2+ ([Ca]n) in human aortic vascular smooth muscle cells (hVSMCs). In the present study, using hVSMCs and 3-dimensional confocal microscopy coupled to the Ca2+ fluorescent probe Fluo-3, we showed that peptidic antagonists of ETA and ETB receptors (BQ-123 (10(-6) mol/L) and BQ-788 (10(-7) mol/L), respectively) prevented, but did not reverse, ET-1-induced sustained increase of [Ca]c and [Ca]n. In contrast, nonpeptidic antagonists of ETA and ETB (respectively, BMS-182874 (10(-8)-10(-6) mol/L) and A-192621 (10(-7) mol/L)) both prevented and reversed ET-1-induced sustained increase of [Ca]c and [Ca]n. Furthermore, activation of the ETB receptor alone using the specific agonist IRL-1620 (10(-9) mol/L) induced sustained increases of [Ca]c and [Ca]n, and subsequent administration of ET-1 (10(-7) mol/L) further increased nuclear Ca2+. ET-1-induced increase of [Ca]c and [Ca]n was completely blocked by extracellular application of the Ca2+ chelator EGTA. Pretreatment with the G protein inhibitors pertussis toxin (PTX) and cholera toxin (CTX) also prevented the ET-1 response; however, strong membrane depolarization with KCl (30 mmol/L) subsequently induced sustained increase of [Ca]c and [Ca]n. Pretreatment of hVSMCs with either the PKC activator phorbol-12,13-dibutyrate or the PKC inhibitor bisindolylmaleimide did not affect ET-1-induced sustained increase of intracellular Ca2+. These results suggest that both ETA- and ETB-receptor activation contribute to ET-1-induced sustained increase of [Ca]c and [Ca]n in hVSMCs. Moreover, in contrast to the peptidic antagonists of ET-1 receptors, the nonpeptidic ETA-receptor antagonist BMS-182874 and the nonpeptidic ETB-receptor antagonist A-192621 were able to reverse the effect of ET-1. Nonpeptidic ETA- and ETB-receptor antagonists may therefore be better pharmacological tools for blocking ET-1-induced sustained increase of intracellular Ca2+ in hVSMCs. Our results also suggest that the ET-1-induced sustained increase of [Ca]c and [Ca]n is not mediated via activation of PKC, but via a PTX- and CTX-sensitive G protein calcium influx through the R-type Ca2+ channel.

G protein-coupled receptors in and on the cell nucleus: a new signaling paradigm?

Signaling from internalizing and endosomal receptors has almost become a classic GPCR paradigm in the last several years. However, it has become clear in recent years that GPCRs also elicit signals when resident at other subcellular sites including the endoplasmic reticulum, Golgi apparatus, and the nucleus. In this review we discuss the nature, function, and trafficking of nuclear GPCR signaling complexes, as well as potential sources of endogenous and exogenous ligands. Finally, we pose a series of questions that will need to be answered in the coming years to confirm and extend this as a new paradigm for GPCR signaling.

Interactions of multiple signaling pathways in neuropeptide Y-mediated bimodal vascular smooth muscle cell growth

Neuropeptide Y (NPY), a sympathetic cotransmitter, acts via G protein-coupled receptors to stimulate constriction and vascular smooth muscle cell (VSMC) proliferation through interactions with its Y1 receptors. However, VSMC proliferation appears bimodal, with high- and low-affinity peaks differentially blocked by antagonists of both Y1 and Y5 receptors. Here, we sought to determine the signaling mechanisms of NPY-mediated bimodal mitogenesis. In rat aortic VSMCs, NPY's mitogenic effect at all concentrations was blocked by pertussis toxin and was associated with decreased forskolin-stimulated cAMP levels. NPY also increased intracellular calcium levels; in contrast to mitogenesis, this effect was dose dependent. The rise in intracellular Ca2+ depended on extracellular Ca2+ and was mediated via activation of Y1 receptors, but not Y5 receptors. Despite differences in calcium, the signaling pathways activated at low and high NPY concentrations were similar. The mitogenic effect of the peptide at all doses was completely blocked by inhibitors of calcium/calmodulin-dependent kinase II (CaMKII), protein kinase C (PKC), and mitogen-activated protein kinase kinase, MEK1/2. Thus, in VSMCs, NPY-mediated mitogenesis signals primarily via Y1 receptors activating 2 Ca2+-dependent, growth-promoting pathways -- PKC and CaMKII. At the high-affinity peak, these 2 pathways are amplified by Y5 receptor-mediated, calcium-independent inhibition of the adenylyl cyclase - protein kinase A (PKA) pathway. All 3 mechanisms converge to the extracellular signal-regulated kinases (ERK1/2) signaling cascade and lead to VSMC proliferation.

G-protein-coupled receptors signalling at the cell nucleus: an emerging paradigm

G-protein-coupled receptors (GPCRs) comprise a wide family of monomeric heptahelical glycoproteins that recognize a broad array of extracellular mediators including cationic amines, lipids, peptides, proteins, and sensory agents. Thus far, much attention has been given towards the comprehension of intracellular signaling mechanisms activated by cell membrane GPCRs, which convert extracellular hormonal stimuli into acute, non-genomic (e.g., hormone secretion, muscle contraction, and cell metabolism) and delayed, genomic biological responses (e.g., cell division, proliferation, and apoptosis). However, with respect to the latter response, there is compelling evidence for a novel intracrine mode of genomic regulation by GPCRs that implies either the endocytosis and nuclear translocation of peripheral-liganded GPCR and (or) the activation of nuclearly located GPCR by endogenously produced, nonsecreted ligands. A noteworthy example of the last scenario is given by heptahelical receptors that are activated by bioactive lipoids (e.g., PGE(2) and PAF), many of which may be formed from bilayer membranes including those of the nucleus. The experimental evidence for the nuclear localization and signalling of GPCRs will be reviewed. We will also discuss possible molecular mechanisms responsible for the atypical compartmentalization of GPCRs at the cell nucleus, along with their role in gene expression.

Intracrine signaling through lipid mediators and their cognate nuclear G-protein-coupled receptors: a paradigm based on PGE2, PAF, and LPA1 receptors

Prostaglandins (PGs), platelet-activating factor (PAF), and lysophosphatidic acid (LPA) are ubiquitous lipid mediators that play important roles in inflammation, cardiovascular homeostasis, and immunity and are also known to modulate gene expression of specific pro-inflammatory genes. The mechanism of action of these lipids is thought to be primarily dependent on their specific plasma membrane receptors belonging to the superfamily of G-protein-coupled receptors (GPCR). Increasing evidence suggests the existence of a functional intracellular GPCR population. It has been proposed that immediate effects are mediated via cell surface receptors whereas long-term responses are dependent upon intracellular receptor effects. Indeed, receptors for PAF, LPA, and PGE(2) (specifically EP(1), EP(3), and EP(4)) localize at the cell nucleus of cerebral microvascular endothelial cells of newborn pigs, rat hepatocytes, and cells overexpressing each receptor. Stimulation of isolated nuclei with these lipids reveals biological functions including transcriptional regulation of major genes, namely c-fos, cylooxygenase-2, and endothelial as well as inducible nitric oxide synthase. In the present review, we shall focus on the nuclear localization and signaling of GPCRs recognizing PGE(2), PAF, and LPA phospholipids as ligands. Mechanisms on how nuclear PGE2, PAF, and LPA receptors activate gene transcription and nuclear localization pathways are presented. Intracrine signaling for lipid mediators uncover novel pathways to elicit their effects; accordingly, intracellular GPCRs constitute a distinctive mode of action for gene regulation.

Roles of nuclear NPY and NPY receptors in the regulation of the endocardial endothelium and heart functionThis paper is one of a selection of papers published in this Special issue, entitled Second Messengers and Phosphoproteins—12th International Conference

It is now well accepted that the heart is a multifunctional organ in which endothelial cells, and more particularly endocardial endothelial cells (EECs), seem to play an important role in regulating and maintaining cardiac excitation–contraction coupling. Even if major differences exist between vascular endothelial cells (VECs) and EECs, all endothelial cells including EECs release a variety of auto- and paracrine factors such as nitric oxide, endothelin-1, angiotensin II, and neuropeptide Y. All these factors were reported to affect cardiomyocyte contractile performance and rhythmicity. In this review, findings on the morphology of EECs, differences between EECs and other types of endothelial cells, interactions between EECs and the adjacent cardiomyocytes, and effects of NPY on the heart will be presented. We will also show evidence on the presence and localization of NPY and the Y1receptor in the endocardial endothelium and discuss their role in the regulation of cytosolic and nuclear free calcium.

NPY, ET-1, and Ang II nuclear receptors in human endocardial endothelial cellsThis paper is one of a selection of papers published in this Special Issue, entitled The Nucleus: A Cell Within A Cell

Neuropeptide Y (NPY), endothelin-1 (ET-1), and angiotensin II (Ang II) are peptides that are known to play many important roles in cardiovascular homeostasis. The physiological actions of these peptides are thought to be primarily mediated by plasma membrane receptors that belong to the G-protein-coupled receptor superfamily. However, there is increasing evidence that suggests the existence of functional G-protein-coupled receptors at the level of the nucleus and that the nucleus could be a cell within a cell. Here, we review our work showing the presence in the nucleus of the NPY Y1receptor, the ETAand ETBreceptors, as well as the AT1and AT2receptors and their respective ligands. This work was carried out in 20-week-old fetal human endocardial endothelial cells. Our results demonstrate that nuclear Y1, AT1, and ETAreceptors modulate nuclear calcium in these cells.

G-protein-coupled receptors, channels, and Na+–H+exchanger in nuclear membranes of heart, hepatic, vascular endothelial, and smooth muscle cellsThis paper is one of a selection of papers published in this Special Issue, entitled The Nucleus: A Cell Within A Cell

The action of several peptides and drugs is thought to be primarily dependent on their interactions with specific cell surface G-protein-coupled receptors and ionic transporters such as channels and exchangers. Recent development of 3-D confocal microscopy allowed several laboratories, including ours, to identify and study the localization of receptors, channels, and exchangers at the transcellular level of several cell types. Using this technique, we demonstrated in the nuclei of several types of cells the presence of Ca2+channels as well as Na+–H+exchanger and receptors such as endothelin-1 and angiotensin II receptors. Stimulation of these nuclear membrane G-protein-coupled receptors induced an increase of nuclear Ca2+. Our results suggest that, similar to the plasma membrane, nuclear membranes possess channels, exchangers and receptors such as those for endothelin-1 and angiotensin II, and that the nucleus seems to be a cell within a cell. This article will emphasize these findings.

Caveolin-1-deficient aortic smooth muscle cells show cell autonomous abnormalities in proliferation, migration, and endothelin-based signal transduction

We previously showed that ablation of caveolin-1 (Cav-1) gene expression in mice promotes neointimal hyperplasia in vivo, a phenomenon normally characterized by smooth muscle cell (SMC) migration and proliferation. Whether these defects are cell autonomous, i.e., due to loss of Cav-1 within SMCs or loss of Cav-1 expression in other adjacent cell types in vivo, remains unknown. Cav-1 has been shown to associate with receptors for many vasoactive factors on the SMC surface. Therefore, Cav-1 might be an important regulator of SMC proliferation, migration, and signal transduction. To mechanistically dissect the role of Cav-1 in SMC signaling, we isolated SMCs from the aortas (AoSMCs) of Cav-1-deficient (Cav-1(-/-)) mice and characterized these cells with respect to their proliferation, migration, and Ca(2+) response to an important vasoactive factor, endothelin-1 (ET-1). 5-Bromo-2'-deoxyuridine incorporation and a wound-healing assay showed an increase in proliferation and migration rates in Cav-1(-/-) compared with wild-type (Cav-1(+/+)) AoSMCs. Cav-1(-/-) AoSMCs demonstrated upregulation of phosphorylated ERK1/2, cyclin D1, and proliferating cell nuclear antigen and reduced expression of the cyclin-dependent kinase inhibitor p27(Kip1). The Ca(2+) response was examined in the presence of ET-1 and assessed by confocal microscopy with the Ca(2+)-sensitive fluorescent probe fluo 3. When treated with ET-1, Cav-1(-/-) AoSMCs exhibited a faster and larger increase in free intracellular Ca(2+) than Cav-1(+/+) cells. The ET-1-induced response in Cav-1(-/-) cells was mediated by the ET(B) receptor, as shown using the ET(B) receptor antagonist BQ-788 and the ET(A) receptor antagonist BQ-123. In Cav-1(-/-) cells, ET(A) receptor expression was reduced and ET(B) receptor expression was upregulated. Therefore, Cav-1 ablation increased the ET-1-induced Ca(2+) response in SMCs by altering the type and expression level of the ET receptor (i.e., receptor isoform switching). These data suggest a novel regulatory role for Cav-1 in SMCs with respect to their proliferation, migration, and Ca(2+)-mediated signaling.

Endothelin-1 inhibits apoptosis in prostate cancer

Endothelin-1 (ET-1), produced by the prostate epithelia, likely plays an important role in the progression of prostate cancer. ET-1 can bind two receptor subtypes; generally, binding of the endothelin receptor A (ET(A)) induces a survival pathway, whereas binding of the endothelin receptor B (ET(B)) mediates clearance of circulating ET-1 as well as promotes apoptosis. In prostate carcinoma, hypermethylation of the ET(B) promoter results in repression of ET(B) expression, thereby eliminating the negative growth response that ET-1 binding elicits through this receptor. Therefore, activation of ET(A) exclusively provides a pathway for survival advantage. Our current studies examine the mechanisms by which activation of the ET(A) may allow growth/survival. ET-1 treatment of prostate tumor cells significantly decreased paclitaxel-induced apoptosis through activation of the ET(A) subtype. The anti-apoptotic effects of ET-1 are mediated, at least in part, through the Bcl-2 family. Although no significant changes in Bcl-2 expression occurred with ET-1 treatment, the pro-apoptotic family members Bad, Bax, and Bak all decreased significantly. Further analysis of the survival pathway demonstrated that phosphorylation of Akt occurs with ET-1 treatment in a time- and dose-dependent manner through phosphatidyinositol 3-kinase activation. These data support the combination of ET(A) antagonists and apoptosis-inducing therapies for prostate cancer treatment.

Intracellular signaling of lipid mediators via cognate nuclear G protein-coupled receptors

Platelet-activating factor (PAF) and lysophosphatidic acid (LPA) are ubiquitous lipid mediators that play important roles in inflammation, cardiovascular homeostasis, and immunity and are also known to modulate gene expression of specific proinflammatory genes. The mechanism of action of these phospholipids is thought to be primarily dependent on their specific plasma membrane receptors belonging to the superfamily of G protein-coupled receptors (GPCRs). However, increasing evidence suggests the existence of a functional intracellular GPCR population. It has been suggested that immediate effects are mediated by cell surface receptors, whereas long-term responses are mediated by intracellular receptors. PAF and LPA(1) receptors localize at the cell nucleus of cerebral microvascular endothelial cells of newborn pig, rat hepatocytes, and cells overexpressing each receptor, and stimulation of isolated nuclei reveal biological functions, including transcriptional regulation of major genes, namely cylooxygenase-2 and inducible nitric oxide synthase. This mini review focuses on the nuclear localization and signaling of GPCRs, recognizing PAF and LPA phospholipids as ligands. Theories on how nuclear PAF and LPA1 receptors activate gene transcription and nuclear localization pathways are discussed. Intracrine signaling for lipid mediators uncover novel pathways to elicit their effects; moreover, intracellular GPCRs constitute a distinctive mode of action for gene regulation.

The distribution and density of ET-1 and its receptors are different in human right and left ventricular endocardial endothelial cells

Evidence suggests that endocardial endothelial cells (EECs) may play a role in the regulation of cardiac function by releasing ET-1. Furthermore, reports in the literature suggested that differences may exist in peptide receptor distribution between the left and right EECs. In this study, we verified if the distribution and density of ET-1 and its receptors could be different in right as compared to left ventricular EECs, and whether this difference may affect ET-1-induced increase of intracellular calcium. Using immunofluorescence and 3D confocal microscopy, our results showed that in both cell types, the ET(A) receptor is present and is homogeneously distributed throughout the two cell types. The relative density of the ET(A) receptor is similar in both right and left ventricular EECs. The ET(B) receptor is also present in right and left ventricular EECs, however, the relative density of the ET(B) receptor is higher in the nucleus as compared to the cytosol. In addition, the ET(B) receptor density was found to be higher in left EECs as compared to right EECs. In addition, our results showed that ET-1 is present in the cytosol and the nucleus of both types of cells and that the relative density of ET-1 is higher in right as compared to left ventricular EECs. Moreover, using the Fura-2 calcium measurement technique, our results showed that in left ventricular EECs, both ET(A) and ET(B) receptor activation mediated the effect of ET-1 on intracellular calcium, whereas in right ventricular EECs, this effect was solely mediated by the ET(A) receptor. In conclusion, our results showed that ET-1 and its receptors are present in both right and left ventricular EECs. However, the distribution and relative density of ET-1 and its receptors seem to be different in right EECs as compared to left EECs.

Functional metabotropic glutamate receptors on nuclei from brain and primary cultured striatal neurons. Role of transporters in delivering ligand

G-protein-coupled receptors are well known for converting an extracellular signal into an intracellular response. Here we showed that the metabotropic glutamate receptor 5 (mGlu5) plays a dynamic intracellular role in signal transduction. Activation of endogenously expressed mGlu5 on striatal nuclear membranes leads to rapid, sustained calcium (Ca2+) responses within the nucleoplasm that can be blocked by receptor-specific antagonists. Extracellular ligands such as glutamate and quisqualate reach nuclear receptors via both sodium-dependent transporters and cystine glutamate exchangers. Inhibition of either transport system blocks radiolabeled agonist uptake as well as agonist-induced nuclear Ca2+ changes. Impermeable antagonists like LY393053 and LY367366 not only blocked [3H]quisqualate binding but also prevented nontransported agonists such as (RS)-3,5-dihydroxyphenylglycine from inducing intracellular Ca2+ changes in heterologous cells. In contrast, neither LY compound prevented quisqualate or glutamate from activating intracellular receptors leading to Ca2+ responses. Inasmuch as Ca2+ can enter the nucleoplasm via the nuclear pore complex or from the nuclear lumen, the presence of nuclear mGlu5 receptors appeared to amplify the latter process generating a faster nuclear response in heterologous cells. In isolated striatal nuclei, nuclear receptor activation results in the de novo appearance of phosphorylated CREB protein. Thus, activation of nuclear mGlu5 receptors initiates a signaling cascade that is known to alter gene transcription and regulate many paradigms of synaptic plasticity. These studies demonstrated that mGlu5 receptors play a dynamic role in signaling both on and off the plasma membrane.

New insights into the retinal circulation: inflammatory lipid mediators in ischemic retinopathy

Ischemic proliferative retinopathy develops in various retinal disorders, including retinal vein occlusion, diabetic retinopathy and retinopathy of prematurity. Ischemic retinopathy remains a common cause of visual impairment and blindness in the industrialized world due to relatively ineffective treatment. Oxygen-induced retinopathy (OIR) is an established model of retinopathy of prematurity associated with vascular cell injury culminating in microvascular degeneration, which precedes an abnormal neovascularization. The retina is a tissue particularly rich in polyunsaturated fatty acids and the ischemic retina becomes highly sensitive to lipid peroxidation initiated by oxygenated free radicals. Consequently, the retina constitutes an excellent model for testing the functional consequences of membrane lipid peroxidation. Retinal tissue responds to physiological and pathophysiological stimuli by the activation of phospholipases and the consequent release from membrane phospholipids of biologically active metabolites. Activation of phospholipase A(2) is the first step in the synthesis of two important classes of lipid second messengers, the eicosanoids and a membrane-derived phospholipid mediator platelet-activating factor (PAF). These lipid mediators accumulate in the retina in response to injury and a physiologic role of these metabolites in retinal vasculature remains for the most part to be determined; albeit proposed roles have been suggested for some. The eicosanoids, in particular the prostanoids, thromboxane (TXA2) and PAF are abundantly generated following an oxidant stress and contribute to neurovascular injury. TXA2 and PAF play an important role in the retinal microvacular degeneration of OIR by directly inducing endothelial cell death and potentially could contribute to the pathogenesis of ischemic retinopathies. Despite these advances there are still a number of important questions that remain to be answered before we can confidently target pathological signals. This review focuses on mechanisms that precede the development of neovascularization, most notably regarding the role of lipid mediators that partake in microvascular degeneration.

Spontaneous Ca2+ oscillations in subcellular compartments of vascular smooth muscle cells rely on different Ca2+ pools

Spontaneous Ca2+ oscillations in vascular smooth muscle cells have been modeled using a single Ca2+ pool. This report describes spontaneous Ca2+ oscillations dependent on two separate Ca2+ sources for the nuclear versus cytoplasmic compartments. Changes in free intracellular Ca2+ were monitored with ratiometric Ca2+- fluorophores using confocal microscopy. On average, spontaneous oscillations developed in 79% of rat aortic smooth muscle cells that were synchronous between the cytoplasm and nucleus. Reduction of extracellular Ca2+ (less than 1 microM)decreased the frequency and amplitude of the cytoplasmic oscillations with 48% of the oscillations asynchronous between the nuclear and cytoplasmic compartments. Similar results were obtained with the Ca2+ channel blockers, nimodipine and diltiazem. Arg-vasopressin (AVP) induced a rapid release of intracellular Ca2+ stores that was greater in the nuclear compartment (4.20 +/- 0.23 ratio units, n = 56) than cytoplasm (2.54 +/- 0.28) in cells that had spontaneously developed prior oscillations. Conversely, cells in the same conditions lacking oscillations had a greater AVP-induced Ca2+ transient in the cytoplasm (4.99 +/- 0.66, n = 17) than in the nucleus (2.67 +/- 0.29). Pre-treatment with Ca2+ channel blockers depressed the AVP responses in both compartments with the cytoplasmic Ca2+ most diminished. Depletion of internal Ca2+ stores prior to AVP exposure blunted the nuclear response, mimicking the response of cells that lacked prior oscillations. Spontaneous oscillating cells had a greater sarcoplasmic reticulum network than cells that did not oscillate. We propose that spontaneous nuclear oscillations rely on perinuclear sarcoplasmic reticulum stores, while the cytoplasmic oscillations rely on Ca2+ influx.

Proliferative response to phenytoin and nifedipine in gingival fibroblasts cultured from humans with gingival fibromatosis

The response of gingival fibroblasts cultured from humans with gingival fibromatosis to phenytoin (PHT) and nifedipine (NIF) was investigated. PHT and NIF induced proliferation, and increased the expression of immunoreactive endothelin-1 (ET-1). ET-1 (0.1 nm-1 microm) itself also induced proliferation in a concentration-dependent manner. The proliferation was inhibited by BQ-123 (ETA receptor antagonist; 1 microm) and TAK044 (ETA/ETB receptor antagonist; 1 microm), but not by BQ-788 (ETB receptor antagonist; 1 microm). The proliferation induced by PHT (0.25 microm) and NIF (0.25 microm) was inhibited by BQ-123 (1 microm). In addition, the results of Western blot analysis indicated the presence of ETA and ETB receptors in/on the fibroblasts. These findings suggest that PHT- and NIF-induced gingival proliferation may be mediated by endogenously generated ET-1, possibly via ETA receptors.

Increased Expression of Vasopressin V1aReceptors after Traumatic Brain Injury

Experimental evidence obtained in various animal models of brain injury indicates that vasopressin promotes the formation of cerebral edema. However, the molecular and cellular mechanisms underlying this vasopressin action are not fully understood. In the present study, we analyzed the temporal changes in expression of vasopressin V1a receptors after traumatic brain injury (TBI) in rats. In the intact brain, the V1a receptor was expressed in neurons located in all layers of the frontoparietal cortex. The V1a receptor-immunoreactive product was predominantly localized to neuronal nuclei and had both a diffused and punctate staining pattern. The V1a receptors were also expressed in astrocytes, especially in layer 1 of the frontoparietal cortex. In these cells, two distinctive patterns of immunopositive staining for V1a receptors were observed: a diffused cytosolic staining of cell bodies and processes and a clearly punctate staining pattern that was predominantly localized to the astrocytic cell bodies. The real-time reverse-transcriptase polymerase chain reaction analysis of changes in mRNA for the V1a receptor demonstrated that after TBI, there is an early (4 h post-TBI) increase in the number of transcripts in the ipsilateral frontoparietal cortex, when compared to the contralateral hemisphere or the sham-injured rats. This increase in the message was followed by the up-regulation of expression of the V1a receptors at the protein level. This was most evident in cortical astrocytes in the areas surrounding the lesion. The number of the V1a receptor-immunopositive astrocytes in the traumatized parenchyma gradually increased, starting at 8 h and peaking at 4-6 days after TBI. Furthermore, a redistribution of V1a receptors from the astrocytic cell bodies to the astrocytic processes was observed. In addition to astrocytes, an increased expression of V1a receptors was found in the endothelium of both blood microvessels and the large-diameter blood vessels in the frontoparietal cortex ipsilateral to injury. This increase in the V1a receptor expression was apparent between 2 and 4 days after TBI. As early as 1-2 h following the impact, there was also a striking increase in the number of the V1a receptor-immunopositive beaded axonal processes, with greatly enlarged varicosities, that were localized to various areas of the injured parenchyma. It is suggested that the increased expression of V1a receptors plays an important role in the vasopressin-mediated formation of edema in the injured brain.

Functional endothelin receptors are present on nuclei in cardiac ventricular myocytes

Endothelins are thought to act through two specific, plasmalemmal G protein-coupled receptor subtypes, ETAR and ETBR. However, in subfractionated cardiac membranes, ETAR immunoreactivity was detected only in the plasma membrane whereas ETBR immunoreactivity was detected predominantly in membranes of intracellular origin. Confocal microscopy demonstrated the presence of intracellular ETAR and ETBR in ventricular myocytes. ETAR were primarily on plasma membrane (surface membranes and transverse-tubules) and to a lesser extent on the nucleus while ETBR localized primarily to the nuclei. Western blot analysis of nuclei isolated from the heart indicated the presence of endothelin receptors: both ETAR and ETBR copurified with nucleoporin 62, whereas markers of endoplasmic reticulum and Golgi membranes were depleted. Radioligand binding studies revealed that isolated nuclei contain specific [125I]ET-1 binding sites. Specific [125I]ET-1 binding was reduced by 70-80% using the ETAR-selective antagonist BQ610 and 20-30% using the ETBR-specific antagonist BQ788. IRL-1620, an ETBR-specific agonist, also reduced [125I]ET-1 binding. Furthermore, ET-1 and IRL-1620 altered the incorporation of 32P into nuclear proteins and caused a transient increase in nuclear Ca2+ concentration. Hence, cardiac nuclei possess both ETAR and ETBR subtypes, which are functional with respect to ligand binding and are coupled to signaling mechanisms within the nuclear membrane.

Activation of metabotropic glutamate receptor mGlu5 on nuclear membranes mediates intranuclear Ca2+ changes in heterologous cell types and neurons

Nuclear Ca2+ plays a critical role in many cellular functions although its mode (s) of regulation is unclear. This study shows that the metabotropic glutamate receptor, mGlu5, mobilizes nuclear Ca2+ independent of cytosolic Ca2+ regulation. Immunocytochemical, ultrastructural, and subcellular fractionation techniques revealed that the metabotropic glutamate receptor, mGlu5, can be localized to nuclear membranes in heterologous cells as well as midbrain and cortical neurons. Nuclear mGlu5 receptors derived from HEK cells or cortical cell types bound [3H]quisqualate. When loaded with Oregon Green BAPTA, nuclei isolated from mGlu5-expressing HEK cells responded to the addition of glutamate with rapid, oscillatory [Ca2+] elevations that were blocked by antagonist or EGTA. In contrast, carbachol-activation of endogenous muscarinic receptors led to cytoplasmic but not nuclear Ca2+ responses. Similarly, activation of mGlu5 receptors expressed on neuronal nuclei led to sustained Ca2+ oscillatory responses. These results suggest mGlu5 may mediate intranuclear signaling pathways.

Activation of sarcolemma and nuclear membranes ET-1 receptors regulates transcellular calcium levels in heart and vascular smooth muscle cells

The use of an ET-1 fluorescent probe in human heart and vascular smooth muscle cells showed that ET-1 receptors are present at both the sarcolemma and nuclear envelope membranes. The use of immunofluorescence studies showed that the ETA receptor was mainly present at the sarcolemma and cytosolic levels. However, the ETB receptor was present at the sarcolemma and the cytosol, as well as the nuclear envelope membranes and the nucleoplasm. In addition, ET-1 immunoreactivity was seen in the cytosol and the nucleus. Using Ca2+ fluorescent probes such as Fluo-3, Indo 1, and yellow cameleon, as well as confocal microscopy three-dimensional image measurement technique, stimulation of ET-1 receptors at the sarcolemma membranes induced an increase of cytosolic and nuclear free Ca2+ levels. This effect of extracellular ET-1 was blocked by removal of extracellular calcium. Direct stimulation of ET-1 receptors at the nuclear envelope membranes also induced an increase of intranuclear free Ca2+ level. Our results suggest that the stimulation of sarcolemmal Ca2+ influx by ET-1 seems to be due to the activation of ETA and ETB receptors. However, the increase of nucleoplasmic Ca2+ levels by cytosolic ET-1 seems to be mediated via the activation of ETB receptors. Activation of nuclear membranes ETB receptors seems to prevent nuclear Ca2+ overload and may protect the cell from apoptosis.

Angiotensin II AT1 receptor internalization, translocation and de novo synthesis modulate cytosolic and nuclear calcium in human vascular smooth muscle cells

The present study was designed to verify if human (h) Angiotensin II (Ang II) type-1 receptor (hAT1R) undergoes internalization, nuclear translocation, and de novo synthesis in primary culture of human aortic vascular smooth muscle cells (hVSMCs) and if overexpression of this receptor modulates sustained free cytosolic ([Ca]c) and nuclear ([Ca]n) calcium. 3-dimensional (3-D) confocal microscopy was used to monitor free intracellular Ca2+ and hAT1R-green fluorescence protein (GFP) fusion protein in cultured hVSMCs. Immunofluorescence studies showed the presence of hAT1R and the absence of hAT2R in normal hVSMCs. Using 3-D imaging technique, hAT1 receptors were localized at the sarcolemma and in the cytosolic and nuclear compartments. In native as well as in normal hAT1R or hAT1R-GFP overexpressing hVSMCs, Ang II (10(-9) and 10(-4) M) induced internalization and nuclear translocation of this type of receptor. The internalization of hAT1Rs is mediated via clathrin-coated pits and vesicles pathway. This phenomenon of trancellular trafficking of receptors was associated with an increase of hAT1R. The Ang II induced increase of hAT1R density was prevented by the protein synthesis inhibitor cycloheximide. Overexpression of hAT1R and hAT1R-GFP decreased both basal cytosolic and nuclear Ca2+. In normal hVSMCs and low hAT1R-GFP overexpressing hVSMCs, Ang II (10(-15) to 10(-4) M) induced a dose-dependent sustained increase of [Ca]c and [Ca]n with an EC50 near 5 x 10(-11) and 5 x 10(-9) M, respectively. Our results suggest that hAT1Rs are the predominant type of Ang II receptors in aortic hVSMCs and are present in the sarcolemma, the cytosolic and the nuclear compartments. Ang II rapidly induces hAT1R internalization, nuclear translocation, as well as nuclear de novo synthesis of this receptor. The hAT1R overexpression in hVSMCs modulates sustained [Ca]c and [Ca]n.

Nucleoplasmic calcium regulation in rabbit aortic vascular smooth muscle cells

In this study, we investigated whether nucleoplasmic free Ca2+ in aortic vascular smooth muscle cells (VSMCs) might be independently regulated from cytosolic free Ca2+. Understanding mechanisms and pathways responsible for this regulation is especially relevant given the role of a numerous intranuclear Ca2+-sensitive proteins in transcriptional regulation, apoptosis and cell division. The question of an independent regulatory mechanism remains largely unsettled because the previous use of intensitometric fluorophores (e.g., Fluo-3) has been criticized on technical grounds. To circumvent the potential problem of fluorescence artifact, we utilized confocal laser scanning microscopy to image intracellular Ca2+ movements with the ratiometric fluorophore Indo-1. In cultured rabbit VSMCs, we found sarcoplasmic reticulum (SR) Ca2+ ATPase (SERCA) pumps and ryanodine receptor (RyR) Ca2+ channel proteins to be discretely arranged within a perinuclear locus, as determined by fluorescent staining patterns of BODIPY FL thapsigargin and BODIPY FL-X Ry. When intracellular Ca2+ stores were mobilized by addition of thapsigargin (5 microM) and activatory concentrations of ryanodine (1 microM), Indo-1 ratiometric signals were largely restricted to the nucleoplasm. Cytosolic signals, by comparison, were relatively small and even then its spatial distribution was largely perinuclear rather homogeneous. These observations indicate perinuclear RyR and SERCA proteins are intimately involved in regulating VSMC nucleoplasmic Ca2+ concentrations. We also observed a similar pattern of largely nucleoplasmic Ca2+ mobilization upon exposure of cells to the immunosuppressant drug FK506 (tacrolimus), which binds to the RyR-associated immunophillin-binding proteins FKBP12 and FKBP12.6. However, initial FK506-induced nucleoplasmic Ca2+ mobilization was followed by marked reduction of Indo-1 signal intensity close to pretreatment levels. This suggested FK506 exerts both activatory and inhibitory effects upon RyR channels. The latter was reinforced by observed effects of FK506 to only reduce nucleoplasmic Indo-1 signal intensity when added following pretreatment with both activatory and inhibitory concentrations of ryanodine. These latter observations raise the possibility that VSMC nuclei represent an important sink of intracellular Ca2+ and may help explain vasodilatory actions of FK506 observed by others.

Presence of neuropeptide Y and the Y1 receptor in the plasma membrane and nuclear envelope of human endocardial endothelial cells: modulation of intracellular calcium

The aims of the present study were to investigate the presence and distribution of NPY and the Y1 receptor in endocardial endothelial cells (EECs), to verify if EECs can release NPY, and to determine if the effect of NPY on intracellular calcium is mediated via the Y1 receptor. Immunofluorescence, 3-D confocal microscopy and radioimmunoassay techniques were used on 20-week-old human fetal EECs. Our results showed that NPY and the Y1 receptor are present in human EECs (hEECs) and that their distributions are similar, the fluorescence labelling being higher in the nucleus and more particularly at the level of the nuclear envelope when compared with the cytosol. Using radioimmunoassay, we demonstrated that EECs are a source of NPY and can secrete this peptide upon a sustained increase of intracellular calcium ([Ca]i). Using fluo-3 and 3-D confocal microscopy technique, superfusion of hEECs as well as EECs isolated from rat adult hearts with increasing concentrations of NPY induced a dose-dependent, sustained increase in free cytosolic and nuclear Ca2+ levels. This effect of NPY on EEC [Ca]i was completely reversible upon washout of NPY and was partially blocked by BIBP3226, a selective Y1 receptor antagonist. The results suggest that NPY and Y1 receptors are present in the EECs of 20-week-old human fetal heart and they share the same distribution and localization inside the cell. In addition, EECs are able to secrete NPY in response to an increase in [Ca]i, and the Y1 receptor as well as other NPY receptors seem to participate in mediating the effects of NPY on [Ca]i in these cells. Thus, NPY released by EECs may modulate excitation-secretion coupling of these cells.

Proliferative effects of angiotensin II and endothelin-1 on guinea pig gingival fibroblast cells in culture

We investigated whether phenytoin (PHT) and nifedipine (NIF) induce angiotensin II (Ang II) and endothelin-1 (ET-1) generation by cultured gingival fibroblasts derived from guinea pigs and whether Ang II and ET-1 induce proliferation of these cells. Immunohistochemical experiments showed that PHT (250 nM) and NIF (250 nM) increased the immunostaining intensities of immunoreactive Ang II and ET-1 (IRET-1) in these cells. Captopril (3 microM), an angiotensin-converting enzyme inhibitor, reduced these enhanced intensities to control levels. Ang II (100 nM) enhanced the immunostaining intensity of IRET-1. PHT (250 nM) and NIF (250 nM)-induced cell proliferation. Both PHT- and NIF-induced proliferation was inhibited by captopril (3 microM). Ang II (100 nM) and ET-1 (100 nM) also induced cell proliferation. Ang II-induced proliferation was inhibited by CV11974 (1 microM), an AT(1) receptor antagonist and saralasin (1 microM), an AT(1)/AT(2) receptor antagonist, but not by PD123,319 (1 microM), an AT(2) receptor antagonist. ET-1-induced proliferation was inhibited by BQ123 (10 microM), an ET(A) receptor antagonist, but not by BQ788 (1 microM), an ET(B) receptor antagonist. These findings suggest that PHT- and NIF-induced gingival fibroblast proliferation is mediated indirectly through the induction of Ang II and ET-1 and probably mediated through AT(1) and ET(A) receptors present in or on gingival fibroblasts.

Regulation of apoptosis by vasoactive peptides

Although originally discovered because of their ability to affect hemodynamics, vasoactive peptides have been found to function in a variety of capacities including neurotransmission, endocrine functions, and the regulation of cell proliferation. A growing body of evidence describes the ability of vasoactive peptides to regulate cell death by apoptosis in either a positive or negative fashion depending on the peptide and the type of target cell. The available evidence to date is strongest for the peptides endothelin, angiotensin II, vasoactive intestinal peptide, atrial natriuretic peptide, and adrenomedullin. Each of these peptides is discussed, with specific regard to apoptosis, in terms of regulatory activity, target cell specificity, and potential role in pulmonary physiology.