Regulated interaction of endothelin B receptor with caveolin-1

Caveolin-3 negatively regulates endocytic recycling of cardiac KATP channels

Sarcolemmal ATP-sensitive potassium (KATP) channels play a vital role in cardioprotection. Cardiac KATP channels are enriched in caveolae and physically interact with the caveolae structural protein caveolin-3 (Cav3). Disrupting caveolae impairs the regulation of KATP channels through several signaling pathways. However, the direct functional effect of Cav3 on KATP channels is still poorly understood. Here, we used the cardiac KATP channel subtype, Kir6.2/SUR2A, and showed that Cav3 greatly reduced KATP channel surface density and current amplitude in a caveolae-independent manner. A screen of Cav3 functional domains revealed that a 25 amino acid region in the membrane attachment domain of Cav3 is the minimal effective segment (MAD1). The peptide corresponding to the MAD1 segment decreased KATP channel current in a concentration-dependent manner with an IC50 of ∼5 μM. The MAD1 segment prevented KATP channel recycling, thus decreasing KATP channel surface density and abolishing the cardioprotective effect of ischemic preconditioning. Our research identified the Cav3 MAD1 segment as a novel negative regulator of KATP channel recycling, providing pharmacological potential in the treatment of diseases with KATP channel trafficking defects.NEW & NOTEWORTHY Cardiac KATP channels physically interact with caveolin-3 in caveolae. In this study, we investigated the functional effect of caveolin-3 on KATP channel activity and identified a novel segment (MAD1) in the C-terminus domain of Caveolin-3 that negatively regulates KATP channel surface density and current amplitude by impairing KATP channel recycling. The peptide corresponding to the MAD1 segment abolished the cardioprotective effect of ischemic preconditioning.

Caveolin-1 in vascular health and glaucoma: A critical vascular regulator and potential therapeutic target

Caveolin-1 (Cav-1) is an integral scaffolding membrane protein found in most cell types. Cav-1 has been found to contribute significantly to ocular function, with mutations of Cav-1 being associated with a genetic risk of glaucoma development. Raised intraocular pressure (IOP) is a major modifiable risk factor for glaucoma. Cav-1 may be involved in both IOP-dependent and independent mechanisms involving vascular dysregulation. Systemic vascular diseases including hypertension, diabetes and hyperlipidaemia, have been shown to be associated with glaucoma development. Cav-1 is closely interlinked with endothelial nitric oxide synthase pathways that mediate vascular function and prevent cardiovascular diseases. Endothelial nitric oxide synthase and endothelin-1 are key vasoactive molecules expressed in retinal blood vessels that function to autoregulate ocular blood flow (OBF). Disruptions in the homeostasis of OBF have led to a growing concept of impaired neurovascular coupling in glaucoma. The imbalance between perfusion and neuronal stimulation arising from Cav-1 depletion may result in relative ischemia of the optic nerve head and glaucomatous injury. OBF is also governed by circadian variation in IOP and systemic blood pressure (BP). Cav-1 has been shown to influence central BP variability and other circadian rhythms such as the diurnal phagolysosomal digestion of photoreceptor fragments and toxic substrates to maintain ocular health. Overall, the vast implications of Cav-1 on various ocular mechanisms leading to glaucoma suggest a potential for new therapeutics to enhance Cav-1 expression, which has seen success in other neurodegenerative diseases.

Recruitment of TNF ligands to lipid rafts is mediated by their physical association with caveolin-1

Activities of the tumour necrosis factor (TNF) family members are associated with their targeting to lipid rafts, specialised regions of the plasma membrane. Herein, we investigated the physical association of TNF and its family members cluster of differentiation 40 ligand (CD40L) and tumour necrosis factor-related apoptosis-inducing ligand with caveolin-1, a lipid raft resident protein. We discovered that the intracellular domains of TNF and CD40L interact with caveolin-1, and the membrane proximal region of TNF is required for the binding of caveolin-1 domains. Full-length TNF can form a complex with caveolin-1 in membrane rafts of HeLa cells, and caveolin-1 knockdown leads to impaired TNF transport to rafts. These findings provide the first evidence of a direct interaction between TNF, CD40L and caveolin-1 and suggest that caveolin-1 may be responsible for recruiting TNF to lipid rafts.

Orthogonal Peptide-Templated Labeling Elucidates Lateral ETA R/ETB R Proximity and Reveals Altered Downstream Signaling

Fine‐tuning of G protein‐coupled receptor (GPCR) signaling is important to maintain cellular homeostasis. Recent studies demonstrated that lateral GPCR interactions in the cell membrane can impact signaling profiles. Here, we report on a one‐step labeling method of multiple membrane‐embedded GPCRs. Based on short peptide tags, complementary probes transfer the cargo (e. g. a fluorescent dye) by an acyl transfer reaction with high spatial and temporal resolution within 5 min. We applied this approach to four receptors of the cardiovascular system: the endothelin receptor A and B (ET_AR and ET_BR), angiotensin II receptor type 1, and apelin. Wild type‐like G protein activation after N‐terminal modification was demonstrated for all receptor species. Using FRET‐competent dyes, a constitutive proximity between hetero‐receptors was limited to ET_AR/ET_BR. Further, we demonstrate, that ET_AR expression regulates the signaling of co‐expressed ET_BR. Our orthogonal peptide‐templated labeling of different GPCRs provides novel insight into the regulation of GPCR signaling.

The ring size of monocyclic ET-1 controls selectivity and signaling efficiency at both endothelin receptor subtypes

Cardiovascular diseases (CVDs) like hypertension are a major cause for death worldwide. In the cardiovascular tissue, the endothelin system-consisting of the receptor subtypes A (ET_A R) and B (ET_B R) and the mixed agonist endothelin 1 (ET-1)-is a major key player in the regulation of vascular tone and blood pressure. Tight control of this system is required to maintain homeostasis; otherwise, the endothelin system can cause severe CVDs like pulmonary artery hypertension. The high sequence homology between both receptor subtypes limits the development of novel and selective ligands. Identification of small differences in receptor-ligand interactions and determination of selectivity constraints are crucial to fine-tune ligand properties and subsequent signaling events. Here, we report on novel ET-1 analogs and their detailed pharmacological characterization. We generated simplified ET-1-derived monocyclic peptides to provide an accessible synthesis route. By detailed in vitro characterization, we demonstrated that both G protein signaling and the subsequent arrestin recruitment of activated ET_B R remain intact, whereas activation of the ET_A R depends on the intramolecular ring size. Increasing of the intramolecular ring structure reduces activity at the ET_A R and shifts the peptide toward ET_B R selectivity. All ET-1 analogs displayed efficient ET_B R-mediated signaling by G protein activation and arrestin 3 recruitment. Our study provides in-depth characterization of the ET-1/ET_A R and ET-1/ET_B R interactions, which has the potential for future development of endothelin-based drugs for CVD treatment. By identification of Lys⁹ for selective labeling, novel analogs for peptide-mediated shuttling by ET-1 are proposed.

Live cell PNA labelling enables erasable fluorescence imaging of membrane proteins

DNA nanotechnology is an emerging field that promises fascinating opportunities for the manipulation and imaging of proteins on a cell surface. The key to progress is the ability to create a nucleic acid-protein junction in the context of living cells. Here we report a covalent labelling reaction that installs a biostable peptide nucleic acid (PNA) tag. The reaction proceeds within minutes and is specific for proteins carrying a 2 kDa coiled-coil peptide tag. Once installed, the PNA label serves as a generic landing platform that enables the recruitment of fluorescent dyes via nucleic acid hybridization. We demonstrate the versatility of this approach by recruiting different fluorophores, assembling multiple fluorophores for increased brightness and achieving reversible labelling by way of toehold-mediated strand displacement. Additionally, we show that labelling can be carried out using two different coiled-coil systems, with epidermal growth factor receptor and endothelin receptor type B, on both HEK293 and CHO cells. Finally, we apply the method to monitor internalization of epidermal growth factor receptor on CHO cells.

Endothelin type B receptor interacts with the 78-kDa glucose-regulated protein

Endothelin (ET)-1 is involved in the vascular system, cell proliferation and apoptosis. ET receptors consist of ET type A receptor (ET_A R) and ET type B receptor (ET_B R). ET_A R and ET_B R generally exhibit opposite responses, although many exceptions exist. In the present study, we attempted to identify ET_A R- or ET_B R-specific binding proteins to understand the differences in ET_A R- and ET_B R-mediated responses after ET-1 stimulation. The 78-kDa glucose-regulated protein (GRP78) showed a stronger binding affinity towards ET_B R than towards ET_A R. Moreover, GRP78 overexpression promoted ET_B R-mediated ERK activation and GRP78 silencing suppressed ET_B R-mediated ERK activation. Furthermore, ET_B R can localize GRP78 to the cell periphery. These results suggest that the interaction of ET_B R with GRP78 affects ERK activation and GRP78 localization.

Endothelin-1 differentially directs lineage specification of adipose- and bone marrow-derived mesenchymal stem cells

Blood vessels composed of endothelial cells (ECs) contact with mesenchymal stem cells (MSCs) in different tissues, suggesting possible interaction between these 2 types of cells. We hypothesized that endothelin-1 (ET1), a secreted paracrine factor of ECs, can differentially direct the lineages of adipose-derived stem cells (ASCs) and bone marrow-derived MSCs (BMSCs). Predifferentiated ASCs and BMSCs were treated with ET1 for 2 cell passages and then induced for multilineage differentiation. Our results showed that adipogenesis of ET1-pretreated ASCs and osteogenesis of ET1-pretreated BMSCs were increased compared to those of control cells. The effect of ET1 on enhancing adipogenesis of ASCs and osteogenesis of BMSCs was attenuated by blocking endothelin receptor type A (ETAR) and/or endothelin receptor type B (ETBR). Western blot analysis indicated that regulation by ET1 was mediated through activation of the protein kinase B and ERK1/2 signaling pathways. We analyzed subpopulations of ASCs and BMSCs with or without ETAR and/or ETBR, and we found that ETAR⁺/ETBR^- and ETAR^-/ETBR⁺ subpopulations of ASCs and those of BMSCs pretreated with ET1 were prone to turning into adipocytes and osteoblasts, respectively, after differentiation induction. Our findings provide insight into the differential regulation of MSC specification by ET1, which may help develop viable approaches for tissue regeneration.-Lee, M.-S., Wang, J., Yuan, H., Jiao, H., Tsai, T.-L., Squire, M. W., Li, W.-J. Endothelin-1 differentially directs lineage specification of adipose- and bone marrow-derived mesenchymal stem cells.

Loss of Caveolin-1 in Metastasis-Associated Macrophages Drives Lung Metastatic Growth through Increased Angiogenesis

Although it is well established that tumor-associated macrophages take part in each step of cancer progression, less is known about the distinct role of the so-called metastasis-associated macrophages (MAMs) at the metastatic site. Previous studies reported that Caveolin-1 (Cav1) has both tumor-promoting and tumor-suppressive functions. However, the role of Cav1 in bone-marrow-derived cells is unknown. Here, we describe Cav1 as an anti-metastatic regulator in mouse models of lung and breast cancer pulmonary metastasis. Among all the recruited inflammatory cell populations, we show that MAMs uniquely express abundant levels of Cav1. Using clodronate depletion of macrophages, we demonstrate that macrophage Cav1 signaling is critical for metastasis and not for primary tumor growth. In particular, Cav1 inhibition does not affect MAM recruitment to the metastatic site but, in turn, favors angiogenesis. We describe a mechanism by which Cav1 in MAMs specifically restrains vascular endothelial growth factor A/vascular endothelial growth factor receptor 1 (VEGF-A/VEGFR1) signaling and its downstream effectors, matrix metallopeptidase 9 (MMP9) and colony-stimulating factor 1 (CSF1).

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Macrophage Cav1 signaling is critical for restraining lung metastatic growth

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Cav1 deletion in macrophages favors angiogenesis at the lung metastatic site

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Cav1 suppresses VEGF-A/VEGFR1 activity and its downstream effectors, MMP9 and CSF1

Caveolins as Regulators of Stress Adaptation

Caveolins have been recognized over the past few decades as key regulators of cell physiology. They are ubiquitously expressed and regulate a number of processes that ultimately impact efficiency of cellular processes. Though not critical to life, they are central to stress adaptation in a number of organs. The following review will focus specifically on the role of caveolin in stress adaptation in the heart, brain, and eye, three organs that are susceptible to acute and chronic stress and that show as well declining function with age. In addition, we consider some novel molecular mechanisms that may account for this stress adaptation and also offer potential to drive the future of caveolin research.

Caveolin 1 and G-Protein-Coupled Receptor Kinase-2 Coregulate Endothelial Nitric Oxide Synthase Activity in Sinusoidal Endothelial Cells

Liver injury leads to a vasculopathy in which post-translational modifications of endothelial nitric oxide synthase (eNOS) lead to impaired nitric oxide synthesis. We hypothesized that caveolin 1 (CAV1), a well-known eNOS interactor, regulates eNOS activity in sinusoidal endothelial cells (SECs) via its interaction with G-protein-coupled receptor kinase-2 (GRK2) that also post-translationally modifies eNOS. Liver injury with portal hypertension was established using bile duct ligation in rats. CAV1 function was modified using a CAV1 scaffolding domain construct and cDNAs encoding wild-type CAV1, and CAV1 phosphorylation was increased in injured SECs, resulting in increased GRK2-CAV1 interaction and decreased eNOS activity. In injured SECs, endothelin-1 blocked CAV1 phosphorylation induced by CAV1 scaffolding domain, indicating that CAV1 interaction with GRK2 is inversely regulated by endothelin-1 and CAV1 scaffolding domain after liver injury. In addition, after transduction with DNA encoding wild-type CAV1 into SECs isolated from Cav1-deficient mice, GRK2 association with CAV1 was evident, whereas transduction with a dominant negative CAV1 mutated at tyrosine 14 reduced the interaction. Finally, isoproterenol-induced GRK2 phosphorylation enhanced CAV1-GRK2 interaction and reduced eNOS activity. Our data suggest a novel mechanism and model in which CAV1 phosphorylation facilitates CAV1 scaffolding and GRK2-CAV1 interaction, thus clustering eNOS within a complex that inhibits eNOS activity. This process takes place in injured, but not in normal, SECs.

Tagging strategies strongly affect the fate of overexpressed caveolin-1

Caveolin-1 (Cav1) is the primary scaffolding protein of caveolae, flask-shaped invaginations of the plasma membrane thought to function in endocytosis, mechanotransduction, signaling and lipid homeostasis. A significant amount of our current knowledge about caveolins and caveolae is derived from studies of transiently overexpressed, C-terminally tagged caveolin proteins. However, how different tags affect the behavior of ectopically expressed Cav1 is still largely unknown. To address this question, we performed a comparative analysis of the subcellular distribution, oligomerization state and detergent resistance of transiently overexpressed Cav1 labeled with three different C-terminal tags (EGFP, mCherry and myc). We show that addition of fluorescent protein tags enhances the aggregation and/or degradation of both wild-type Cav1 and an oligomerization defective P132L mutant. Strikingly, complexes formed by overexpressed Cav1 fusion proteins excluded endogenous Cav1 and Cav2, and the properties of native caveolins were largely preserved even when abnormal aggregates were present in cells. These findings suggest that differences in tagging strategies may be a source of variation in previously published studies of Cav1 and that overexpressed Cav1 may exert functional effects outside of caveolae. They also highlight the need for a critical re-evaluation of current knowledge based on transient overexpression of tagged Cav1.

Systems Chemo-Biology and Transcriptomic Meta-Analysis Reveal the Molecular Roles of Bioactive Lipids in Cardiomyocyte Differentiation

Lipids, which are essential constituents of biological membranes, play structural and functional roles in the cell. In recent years, certain lipids have been identified as regulatory signaling molecules and have been termed "bioactive lipids". Subsequently, the importance of bioactive lipids in stem cell differentiation and cardiogenesis has gained increasing recognition. Therefore, the aim of this study was to identify the biological processes underlying murine cardiac differentiation and the mechanisms by which bioactive lipids affect these processes. For this purpose, a transcriptomic meta-analysis of microarray and RNA-seq data from murine stem cells undergoing cardiogenic differentiation was performed. The differentially expressed genes identified via this meta-analysis, as well as bioactive lipids, were evaluated using systems chemo-biology tools. These data indicated that bioactive lipids are associated with the regulation of cell motility, cell adhesion, cytoskeletal rearrangement, and gene expression. Moreover, bioactive lipids integrate the signaling pathways involved in cell migration, the secretion and remodeling of extracellular matrix components, and the establishment of the cardiac phenotype. In conclusion, this study provides new insights into the contribution of bioactive lipids to the induction of cellular responses to various stimuli, which may originate from the extracellular environment and morphogens, and the manner in which this contribution directly affects murine heart morphogenesis.

A role for G-proteins in directing G-protein-coupled receptor-caveolae localization

Caveolae are membrane domains that may influence cell signaling by sequestering specific proteins such as G-protein-coupled receptors (GPCRs). While previous reports largely show that Gα_q subunits, but not other G-proteins, interact strongly with the caveolae protein, Caveolin-1 (Cav1), the inclusion of GPCRs in caveolae is controversial. Here, we have used fluorescence methods to determine the effect of caveolae on the physical and functional properties of two GPCRs that have been reported to reside in caveolae, bradykinin receptor type 2 (B₂R), which is coupled to Gα_q, and the μ-opioid receptor (μOR), which is coupled to Gα_i. While caveolae do not affect cAMP signals mediated by μOR, they prolong Ca²⁺ signals mediated by B₂R. In A10 cells that endogenously express B₂R and Cav1, downregulation of Cav1 ablates the prolonged recovery seen upon bradykinin stimulation in accord with the idea that the presence of caveolae prolongs Gα_q activation. Immunofluorescence and Förster resonance energy transfer (FRET) studies show that a significant fraction of B₂R resides at or close to caveolae domains while none or very little μOR resides in caveolae domains. The level of FRET between B₂R and caveolae is reduced by downregulation of Gα_q or by addition of a peptide that interferes with Gα_q–Caveolin-1 interactions, suggesting that Gα_q promotes localization of B₂R to caveolae domains. Our results lead to the suggestion that Gα_q can localize its associated receptors to caveolae domains to enhance their signals.

Evaluating caveolin interactions: do proteins interact with the caveolin scaffolding domain through a widespread aromatic residue-rich motif?

Caveolins are coat proteins of caveolae, small flask-shaped pits of the plasma membranes of most cells. Aside from roles in caveolae formation, caveolins recruit, retain and regulate many caveolae-associated signalling molecules. Caveolin-protein interactions are commonly considered to occur between a ∼20 amino acid region within caveolin, the caveolin scaffolding domain (CSD), and an aromatic-rich caveolin binding motif (CBM) on the binding partner (фXфXXXXф, фXXXXфXXф or фXфXXXXфXXф, where ф is an aromatic and X an unspecified amino acid). The CBM resembles a typical linear motif - a short, simple sequence independently evolved many times in different proteins for a specific function. Here we exploit recent improvements in bioinformatics tools and in our understanding of linear motifs to critically examine the role of CBMs in caveolin interactions. We find that sequences conforming to the CBM occur in 30% of human proteins, but find no evidence for their statistical enrichment in the caveolin interactome. Furthermore, sequence- and structure-based considerations suggest that CBMs do not have characteristics commonly associated with true interaction motifs. Analysis of the relative solvent accessible area of putative CBMs shows that the majority of their aromatic residues are buried within the protein and are thus unlikely to interact directly with caveolin, but may instead be important for protein structural stability. Together, these findings suggest that the canonical CBM may not be a common characteristic of caveolin-target interactions and that interfaces between caveolin and targets may be more structurally diverse than presently appreciated.

Evidence for the nitric oxide pathway as a potential mode of action in fenoldopam-induced vascular injury

Fenoldopam, a dopaminergic DA1 agonist, induces vasodilatation via nitric oxide (NO), and this may be associated with mesenteric arterial injury. NO is produced from the enzymatic action of nitric oxide synthase (NOS), which is regulated by the shear-stress mediating protein caveolin-1. Profound vasodilatation and accompanied decreased shear are early events that could initiate vascular injury. Therefore, it is of interest to determine the role of caveolin-1 and the NO pathway in fenoldopam-induced vascular injury. At sites of fenoldopam-induced mesenteric arterial injury, decreased caveolin-1 expression and apoptosis were prominent immunohistochemical findings. An additional finding at these sites of injury were loss and/or reduced expression of caveolin-1 regulated structural proteins, connexin-43, (gap junction) ZO-1, and claudin (tight junctions). Because functional loss of caveolin-1 is associated with increased NOS activity and vasodilatation via NO, studies were conducted to show a NO donor produced vascular lesions in the mesenteric arteries morphologically similar to those induced by fenoldopam. Moreover, the incidence and severity of fenoldopam-induced vascular injury were reduced when an NOS inhibitor or a scavenger of NO-generated free radicals were coadministered with fenoldopam. Collectively, these data suggest that caveolin-1 and its regulated NO pathway may play an important role in vasodilatory drug-induced vascular injury.

Endothelin induces rapid, dynamin-mediated budding of endothelial caveolae rich in ET-B

Clathrin-independent trafficking pathways for internalizing G protein-coupled receptors (GPCRs) remain undefined. Clathrin-mediated endocytosis of receptors including ligand-engaged GPCRs can be very rapid and comprehensive (<10 min). Caveolae-mediated endocytosis of ligands and antibodies has been reported to be much slower in cell culture (≫10 min). Little is known about the role of physiological ligands and specific GPCRs in regulating caveolae trafficking. Here, we find that one receptor for endothelin, ET-B but not ET-A, resides on endothelial cell surfaces in both tissue and cell culture primarily concentrated within caveolae. Reconstituted cell-free budding assays show that endothelins (ETs) induce the fission of caveolae from endothelial plasma membranes purified from rat lungs. Electron microcopy of lung tissue sections and tissue subcellular fractionation both show that endothelin administered intravascularly in rats also induces a significant loss of caveolae at the luminal surface of lung vascular endothelium. Endothelial cells in culture show that ET stimulates very rapid internalization of caveolae and cargo including caveolin, caveolae-targeting antibody, and itself. The ET-B inhibitor BQ788, but not the ET-A inhibitor BQ123, blocks the ET-induced budding of caveolae. Both the pharmacological inhibitor Dynasore and the genetic dominant negative K44A mutant of dynamin prevent this induced budding and internalization of caveolae. Also shRNA lentivirus knockdown of caveolin-1 expression prevents rapid internalization of ET and ET-B. It appears that endothelin can engage ET-B already highly concentrated in caveolae of endothelial cells to induce very rapid caveolae fission and endocytosis. This transport requires active dynamin function. Caveolae trafficking may occur more rapidly than previously documented when it is stimulated by a specific ligand to signaling receptors already located in caveolae before ligand engagement.

Increased interaction of connexin43 with zonula occludens-1 during inhibition of gap junctions by G protein-coupled receptor agonists

Astrocytes are extensively coupled through gap junctions (GJs) that are composed of channels mostly constituted by connexin43 (Cx43). This astroglial gap junctional intercellular communication (GJIC) allows propagation of ions and signaling molecules critical for neuronal activity and survival. It is drastically inhibited by a short-term exposure to endothelin-1 (ET-1) or to sphingosine-1-phosphate (S1P), both compounds being inflammatory mediators acting through activation of GTP-binding protein-coupled receptors (GPCRs). Previously, we have identified the GTPases G(i/o) and Rho as key actors in the process of S1P-induced inhibition. Here, we asked whether similar mechanisms underlied the effects of ET-1 and S1P by investigating changes in the phosphorylation status of Cx43 and in the molecular associations of Cx43 with zonula occludens (ZO) proteins and occludin. We showed that the inhibitory effect of ET-1 on GJIC was entirely dependent on the activation of G(i/o) but not on Rho and Rho-associated kinase. Both ET-1 and S1P induced dephosphorylation of Cx43 located at GJs through a process mediated by G(i/o) and calcineurin. Thanks to co-immunoprecipitation approaches, we found that a population of Cx43 (likely junctional Cx43) was associated to ZO-1-ZO-2-occludin multiprotein complexes and that acute treatments of astrocytes with ET-1 or S1P induced a G(i/o)-dependent increase in the amount of Cx43 linked to these complexes. As a whole, this study identifies a new mechanism of GJIC regulation in which two GPCR agonists dynamically alter interactions of Cx43 with its molecular partners.

Sf9 cells: a versatile model system to investigate the pharmacological properties of G protein-coupled receptors

The Sf9 cell/baculovirus expression system is widely used for high-level protein expression, often with the purpose of purification. However, proteins may also be functionally expressed in the defined Sf9 cell environment. According to the literature, the pharmacology of G-protein-coupled receptors (GPCRs) functionally reconstituted in Sf9 cells is similar to the receptor properties in mammalian cells. Sf9 cells express both recombinant GPCRs and G-proteins at much higher levels than mammalian cells. Sf9 cells can be grown in suspension culture, providing an inexpensive way of obtaining large protein amounts. Co-infection with various baculoviruses allows free combination of GPCRs with different G-proteins. The absence of constitutively active receptors in Sf9 cells provides an excellent signal-to background ratio in functional assays, allowing the detection of agonist-independent receptor activity and of small ligand-induced signals including partial agonistic and inverse agonistic effects. Insect cell Gα(i)-like proteins mostly do not couple productively to mammalian GPCRs. Thus, unlike in mammalian cells, Sf9 cells do not require pertussis toxin treatment to obtain a Gα(i)-free environment. Co-expression of GPCRs with Gα(i1), Gα(i2), Gα(i3) or Gα(o) in Sf9 cells allows the generation of a selectivity profile for these Gα(i/o)-isoforms. Additionally, GPCR-G-protein combinations can be compared with defined 1:1 stoichiometry by expressing GPCR-Gα fusion proteins. Sf9 cells can also be employed for ligand screening in medicinal chemistry programs, using radioligand binding assays or functional assays, like the steady-state GTPase- or [(35)S]GTPγS binding assay. This review shows that Sf9 cells are a versatile model system to investigate the pharmacological properties of GPCRs.

Localisation of endothelin B receptor variants to plasma membrane microdomains and its effects on downstream signalling

The endothelin B (ET(B)) receptor can undergo a proteolytic cleavage resulting in an unglycosylated N-terminally truncated receptor. We investigated whether ET(B) receptor processing affects caveolar localisation and mitogenic signalling. Distinct subcellular localisations of ET(B) receptor constructs and epidermal growth factor (EGF) receptor ligands were analysed performing detergent-free caveolae preparations and total internal reflection fluorescence microscopy. ET(B) receptor-induced transactivation of the EGF receptor and its downstream signalling was investigated performing shedding assays and ERK1/2 phosphorylation analyses. In COS7 cells, the N-terminally truncated but not the full-length or glycosylation-deficient ET(B) receptor localised to caveolae. In caveolae-free HEK293 cells, only ET(B) receptor constructs fused to caveolin-2 localised to membrane microdomains. A caveolar accumulation of the ET(B) receptor disfavoured EGF receptor ligand shedding. Nonetheless, the activation of ERK1/2 was efficient and long-lasting. In HEK293 cells, the shedding activity was also impaired by N-terminal truncation. The subsequent ERK1/2 phosphorylation was long-lasting only for the full-length ET(B) receptor. We conclude that the ET(B) receptor localisation might depend on the presence of caveolae within the cell investigated. The data further suggest that caveolar enrichment of ET(B) receptors does not facilitate the release of EGF receptor ligands. However, independent of their localisation, ET(B) receptors are able to induce an ERK1/2 phosphorylation.

The role of beta(1)Pix/caveolin-1 interaction in endothelin signaling through Galpha subunits

Endothelin-1 (ET-1) is a potent mitogen that transmits signals through its cognate G protein-coupled receptors to stimulate extracellular signal-regulated kinase Erk1/2. Endothelin-1 receptors (ET-Rs) are known to interact with caveolin-1 and co-localize in caveolae which integrate different receptor and signaling proteins. We have recently shown that beta(1)Pix binds specifically to ET-Rs. Here, we show that beta(1)Pix binding to caveolin-1 is dependent on heterotrimeric G proteins activation state. beta(1)Pix interaction with different G proteins is increased in the presence of the G protein activator AMF. Moreover, extraction of cholesterol with methyl-beta-cyclodextrin disrupts the binding of beta(1)Pix to Galpha(q), Galpha(12) and phospho-Erk1/2 but not the binding of beta(1)Pix to G(beta1). The disruption of beta(1)Pix dimerization strongly reduced the binding of caveolin-1, Galpha(q) and Galpha(12). Constitutively active mutants of Galpha(q) and Galpha(12) increased Cdc42 activation when co-expressed with beta(1)Pix but not in the presence of beta(1)Pix dimerization deficient mutant beta(1)PixDelta (602-611). ET-1 stimulation increased the binding of phosphorylated Erk1/2 to beta(1)Pix but not to beta(1)PixDelta (602-611). RGS3 decreased ET-1-induced Cdc42 activation. These results strongly suggest that the activation of ET-Rs leads to the compartmentalization and the binding of Galpha(q) to beta(1)Pix in caveolae, where dimeric beta(1)Pix acts as platform to facilitate the binding and the activation of Erk1/2.

Endothelin-1 induces p66Shc activation through EGF receptor transactivation: Role of beta(1)Pix/Galpha(i3) interaction

Endothelin-1 (ET-1) is a vasoconstrictor peptide known to be a potent mitogen for glomerular mesangial cells. We have shown that ET-1 stimulates the adaptor protein p66Shc through Rac/Cdc42 guanine nucleotide exchange factor beta(1)Pix. In this study, we demonstrate that ET-1-induced serine phosphorylation of p66Shc is mediated through Galpha(i3). Pertussis toxin treatment of cells induced a significant decrease in the interaction between beta(1)Pix and ET(A)-R, and an increase in the binding of Galpha(i3) and G(beta1) to beta(1)Pix. Activation of heterotrimeric G proteins by AlF(4)(-) resulted in an increase of Galpha(i3) binding to beta(1)Pix, which was significantly disrupted in cells expressing beta(1)Pix dimerization deficient mutant, beta(1)PixDelta (602-611). In cells expressing beta(1)PixDelta (602-611), ET-1-induced p66Shc activation was also significantly decreased. Specific inhibition of EGF receptor by AG1478 blocked ET-1-induced p66Shc activation and the binding of p66Shc and Galpha(i3) to beta(1)Pix. Inhibition of Erk1/2 blocked p66Shc activation induced by ET-1. Altogether, our results indicate that ET-1 activates p66Shc through EGF receptor transactivation, leading to the activation of Galpha(i3), beta(1)Pix and Erk1/2.

Caveolin-1 Expression and Hemodynamics in COPD Patients

Caveolin-1 is a regulator of both intracellular calcium homeostasis and endothelial nitric oxide synthase and may play a pathogenetic role in pulmonary hypertension. In the present study, we aimed to investigate the correlations between pulmonary hemodynamics and vessel morphology including the expression of Caveolin-1 in pulmonary arterioles from patients with chronic obstructive pulmonary disease (COPD) who underwent lung-volume reduction surgery. Staining and subsequent analysis was performed on paraffin-embedded lung tissue from COPD patients (n = 12). Pulmonary arteries with an external diameter of 100-500µm were analysed. Immunhistochemistry with antibodies against caveolin-1 was performed and intensity was assessed. Morphometric data were obtained by using computer-assisted imaging software. The findings were quantified and correlated to hemodynamic data obtained by right-heart catheterization. In COPD patients with pulmonary hypertension (n = 5), the expression of caveolin-1 within the medial smooth muscle cell layer was found to be increased, whereas the intimal caveolin-1 was more prominently expressed in COPD patients with normal pulmonary pressures (n = 7). The ratio between these expression patterns was positively correlated to the mean pulmonary artery pressure. Similar findings were observed for the ratio between intimal and medial thickness as well as for the expression of smooth muscle actin (SMA).

Taken together, the expression of caveolin-1 within medial smooth muscle cells of pulmonary arteries in patients with COPD is associated with pulmonary hypertension. Our results thus emphasize a potential novel player in the pathogenesis of COPD-associated pulmonary hypertension.

NAADP-induced Ca(2+ signaling in response to endothelin is via the receptor subtype B and requires the integrity of lipid rafts/caveolae

We have investigated the role of NAADP-mediated Ca(2+) mobilization in endothelin (ET) signaling via endothelin receptor subtype A (ETA) and endothelin receptor subtype B (ETB) in rat peritubular smooth muscle cells. Microinjection and extracellular application of NAADP were both able to elicit Ca(2+) release which was blocked by inhibitory concentrations of NAADP, by impairing Ca(2+) uptake in acidic stores with bafilomycin, and by thapsigargin. Ca(2+) release in response to selective ETB stimulation was abolished by inhibition of NAADP signaling through the same strategies, while these treatments only partially impaired ETA-dependent Ca(2+) signaling, showing that transduction of the ETB signal is dependent on NAADP. In addition, we show that lipid rafts/caveolae contain ETA, ETB, and NAADP/cADPR generating enzyme CD38 and that stimulation of ETB receptors results in increased CD38 activity; interestingly, ETB- (but not ETA-) mediated Ca(2+) responses were antagonized by disruption of lipid rafts/caveolae with methyl-beta-cyclodextrin. These data demonstrate a primary role of NAADP in ETB-mediated Ca(2+) signaling and strongly suggest a novel role of lipid rafts/caveolae in triggering ET-induced NAADP signaling.

Catestatin (chromogranin A344-364) is a novel cardiosuppressive agent: inhibition of isoproterenol and endothelin signaling in the frog heart

The catecholamine release-inhibitory catestatin [Cts; human chromogranin (Cg) A(352-372), bovine CgA(344-364)] is a vasoreactive and anti-hypertensive peptide derived from CgA. Using the isolated avascular frog heart as a bioassay, in which the interactions between the endocardial endothelium and the subjacent myocardium can be studied without the confounding effects of the vascular endothelium, we tested the direct cardiotropic effects of bovine Cts and its interaction with beta-adrenergic (isoproterenol, ISO) and endothelin-1 (ET-1) signaling. Cts dose-dependently decreased stroke volume and stroke work, with a threshold concentration of 11 nM, approaching the in vivo level of the peptide. Cts reduced contractility by inhibiting phosphorylation of phospholamban (PLN). Furthermore, the Cts effect was abolished by pretreatment with either nitric oxide synthase (N(G)-monomethyl-l-arginine) or guanylate cyclase (ODQ) inhibitors, or an ET(B) receptor (ET(BR)) antagonist (BQ-788). Cts also noncompetitively inhibited the positive inotropic action of ISO. In addition, Cts inhibited the positive inotropic effect of ET-1, mediated by ET(A) receptors, and did not alter the negative inotropic ET-1 influence mediated by ET(BR). Cts action through ET(BR) was further suggested when, in the presence of BQ-788, Cts failed to inhibit the positive inotropism of both ISO and ET-1 stimulation and PLN phosphorylation. We concluded that the cardiotropic actions of Cts, including the beta-adrenergic and ET-1 antagonistic effects, support a novel role of this peptide as an autocrine-paracrine modulator of cardiac function, particularly when the stressed heart becomes a preferential target of both adrenergic and ET-1 stimuli.

Phorbol 12-myristate 13-acetate-dependent protein kinase C delta-Tyr311 phosphorylation in cardiomyocyte caveolae

Protein kinase Cdelta (PKCdelta) activation is generally attributed to lipid cofactor-dependent allosteric activation mechanisms at membranes. However, recent studies indicate that PKCdelta also is dynamically regulated through tyrosine phosphorylation in H(2)O(2)- and phorbol 12-myristate 13-acetate (PMA)-treated cardiomyocytes. H(2)O(2) activates Src and related Src-family kinases (SFKs), which function as dual PKCdelta-Tyr(311) and -Tyr(332) kinases in vitro and contribute to H(2)O(2)-dependent PKCdelta-Tyr(311)/Tyr(332) phosphorylation in cardiomyocytes and in mouse embryo fibroblasts. H(2)O(2)-dependent PKCdelta-Tyr(311)/Tyr(332) phosphorylation is defective in SYF cells (deficient in SFKs) and restored by Src re-expression. PMA also promotes PKCdelta-Tyr(311) phosphorylation, but this is not associated with SFK activation or PKCdelta-Tyr(332) phosphorylation. Rather, PMA increases PKCdelta-Tyr(311) phosphorylation by delivering PKCdelta to SFK-enriched caveolae. Cyclodextrin treatment disrupts caveolae and blocks PMA-dependent PKCdelta-Tyr(311) phosphorylation, without blocking H(2)O(2)-dependent PKCdelta-Tyr(311) phosphorylation. The enzyme that acts as a PKCdelta-Tyr(311) kinase without increasing PKCdelta phosphorylation at Tyr(332) in PMA-treated cardiomyocytes is uncertain. Although in vitro kinase assays implicate c-Abl as a selective PKCdelta-Tyr(311) kinase, PMA-dependent PKCdelta-Tyr(311) phosphorylation persists in cardiomyocytes treated with the c-Abl inhibitor ST1571 and c-Abl is not detected in caveolae; these results effectively exclude a c-Abl-dependent process. Finally, we show that 1,2-dioleoyl-sn-glycerol mimics the effect of PMA to drive PKCdelta to caveolae and increase PKCdelta-Tyr(311) phosphorylation, whereas G protein-coupled receptor agonists such as norepinephrine and endothelin-1 do not. These results suggest that norepinephrine and endothelin-1 increase 1,2-dioleoyl-sn-glycerol accumulation and activate PKCdelta exclusively in non-caveolae membranes. Collectively, these results identify stimulus-specific PKCdelta localization and tyrosine phosphorylation mechanisms that could be targeted for therapeutic advantage.

Mechanisms of ET-1-induced endothelial dysfunction

There is now increasing evidence that endothelial dysfunction is an early event in the pathophysiology of cardiovascular diseases and can be corrected with certain therapies such as angiotensin converting enzyme inhibitors angiotensin type I receptor antagonists and stains independently of blood pressure lowering effects. Restoring endothelial function appears to be a crucial target since endothelial dysfunction predicts cardiovascular events in various situations such as coronary artery disease peripheral artery disease, or hypertension and in patients undergoing vascular surgery. Preclinical and clinical data strongly support that endothelin receptor antagonists belong to this restricted class of pharmacological agents able to act on the endothelium, and offer a potential therapeutic approach for numerous diseases associated with endothelial dysfunction. The purpose of this review will be therefore, 1) to propose mechanisms by which ET-1 can cause endothelial dysfunction; 2) to provide an overview of pathological situations associated with endothelial dysfunction related to ET-1; and 3) to assemble evidence on efficacy of endothelin receptor antagonists for improvement of endothelial function.

Elevated plasma endothelin-1 and pulmonary arterial pressure in children exposed to air pollution

BACKGROUND

Controlled exposures of animals and humans to particulate matter (PM) or ozone air pollution cause an increase in plasma levels of endothelin-1, a potent vasoconstrictor that regulates pulmonary arterial pressure.

OBJECTIVES

The primary objective of this field study was to determine whether Mexico City children, who are chronically exposed to levels of PM and O(3) that exceed the United States air quality standards, have elevated plasma endothelin-1 levels and pulmonary arterial pressures.

METHODS

We conducted a study of 81 children, 7.9 +/- 1.3 years of age, lifelong residents of either northeast (n = 19) or southwest (n = 40) Mexico City or Polotitlán (n = 22), a control city with PM and O(3) levels below the U.S. air quality standards. Clinical histories, physical examinations, and complete blood counts were done. Plasma endothelin-1 concentrations were determined by immunoassay, and pulmonary arterial pressures were measured by Doppler echocardiography.

RESULTS

Mexico City children had higher plasma endothelin-1 concentrations compared with controls (p < 0.001). Mean pulmonary arterial pressure was elevated in children from both northeast (p < 0.001) and southwest (p < 0.05) Mexico City compared with controls. Endothelin-1 levels in Mexico City children were positively correlated with daily outdoor hours (p = 0.012), and 7-day cumulative levels of PM air pollution < 2.5 mum in aerodynamic diameter (PM(2.5)) before endothelin-1 measurement (p = 0.03).

CONCLUSIONS

Chronic exposure of children to PM(2.5) is associated with increased levels of circulating endothelin-1 and elevated mean pulmonary arterial pressure.

Caveolin-1 regulates cellular trafficking and function of the glucagon-like Peptide 1 receptor

The glucagon-like peptide 1 receptor (GLP-1R) mediates important effects on beta-cell function and glucose homeostasis and is one of the most promising therapeutic targets for type 2, and possibly type 1, diabetes. Yet, little is known regarding the molecular and cellular mechanisms that regulate its function. Therefore, we examined the cellular trafficking of the GLP-1R and the relation between receptor localization and signaling activity. In resting human embryonic kidney 293 and insulinoma MIN6 cells, a fully functional green fluorescent protein-tagged GLP-1R was localized both at the cell membrane and in highly mobile intracellular compartments. Real-time confocal fluorescence microscopy allowed direct visualization of constitutive cycling of the receptor. Overexpression of K44A-dynamin increased the number of functional receptors at the cell membrane. Immunoprecipitation, sucrose sedimentation, and microscopy observations demonstrated that the GLP-1R localizes in lipid rafts and interacts with caveolin-1. This interaction is necessary for membrane localization of the GLP-1R, because overexpression of a dominant-negative form of caveolin-1 (P132L-cav1) or specific mutations within the putative GLP-1R's caveolin-1 binding domain completely inhibited GLP-1 binding and activity. Upon agonist stimulation, the GLP-1R underwent rapid and extensive endocytosis independently from arrestins but in association with caveolin-1. Finally, GLP-1R-stimulated activation of ERK1/2, which involves transactivation of epidermal growth factor receptors, required lipid raft integrity. In summary, the interaction of the GLP-1R with caveolin-1 regulates subcellular localization, trafficking, and signaling activity. This study provides further evidence of the key role of accessory proteins in specifying the cellular behavior of G protein-coupled receptors.

Biomarkers and mechanisms of drug-induced vascular injury in non-rodents

In preclinical safety studies, drug-induced vascular injury can negatively impact candidate-drug selection because there are no obvious diagnostic markers for monitoring this pathology preclinically or clinically. Furthermore, our current understanding of the pathogenesis of this lesion is limited. While vasodilatation and increased shear stress appear to play a role, the exact mechanism(s) of injury to the primary target cells, smooth muscle (SMC) and endothelial cell (EC), are unknown. Evaluation of potential novel markers for clinical monitoring with a mechanistic underpinning would add value in risk assessment and risk management. This mini review focuses on the efforts and progress to identify diagnostic markers as well as understanding the mechanism of action in nonrodent drug-induced vascular injury.

Accessory proteins for G proteins: partners in signaling

Accessory proteins involved in signal processing through heterotrimeric G proteins are generally defined as proteins distinct from G protein-coupled receptor (GPCR), G protein, or classical effectors that regulate the strength/efficiency/specificity of signal transfer upon receptor activation or position these entities in the right microenvironment, contributing to the formation of a functional signal transduction complex. A flurry of recent studies have implicated an additional class of accessory proteins for this system that provide signal input to heterotrimeric G proteins in the absence of a cell surface receptor, serve as alternative binding partners for G protein subunits, provide unexpected modes of G protein regulation, and have introduced additional functional roles for G proteins. This group of accessory proteins includes the recently discovered Activators of G protein Signaling (AGS) proteins identified in a functional screen for receptor-independent activators of G protein signaling as well as several proteins identified in protein interaction screens and genetic screens in model organisms. These accessory proteins may influence GDP dissociation and nucleotide exchange at the G(alpha) subunit, alter subunit interactions within heterotrimeric G(alphabetagamma) independent of nucleotide exchange, or form complexes with G(alpha) or G(betagamma) independent of the typical G(alphabetagamma) heterotrimer. AGS and related accessory proteins reveal unexpected diversity in G protein subunits as signal transducers within the cell.

Cardiovascular endothelins: essential regulators of cardiovascular homeostasis

The endothelin (ET) system consists of 3 ET isopeptides, several isoforms of activating peptidases, and 2 G-protein-coupled receptors, ETA and ETB, that are linked to multiple signaling pathways. In the cardiovascular system, the components of the ET family are expressed in several tissues, notably the vascular endothelium, smooth muscle cells, and cardiomyocytes. There is general agreement that ETs play important physiological roles in the regulation of normal cardiovascular function, and excessive generation of ET isopeptides has been linked to major cardiovascular pathologies, including hypertension and heart failure. However, several recent clinical trials with ET receptor antagonists were disappointing. In the present review, the authors take the stance that ETs are mainly and foremost essential regulators of cardiovascular function, hence that antagonizing normal ET actions, even in patients, will potentially do more harm than good. To support this notion, we describe the predominant roles of ETs in blood vessels, which are (indirect) vasodilatation and ET clearance from plasma and interstitial spaces, against the background of the subcellular mechanisms mediating these effects. Furthermore, important roles of ETs in regulating and adapting heart functions to different needs are addressed, including recent progress in understanding the effects of ETs on diastolic function, adaptations to changes in preload, and the interactions between endocardial-derived ET-1 and myocardial pump function. Finally, the potential dangers (and gains) resulting from the suppression of excessive generation or activity of ETs occurring in some cardiovascular pathological states, such as hypertension, myocardial ischemia, and heart failure, are discussed.

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 enhances oxidative stress, cell proliferation and reduces apoptosis in human umbilical vein endothelial cells: role of ETB receptor, NADPH oxidase and caveolin-1

1 Endothelin-1 (ET-1), an endothelium-derived vasoactive peptide, participates in the regulation of endothelial function through mechanisms that are not fully elucidated. This study examined the impact of ET-1 on oxidative stress, apoptosis and cell proliferation in human umbilical vein endothelial cells (HUVEC). HUVECs were challenged for 24 h with ET-1 (10 pM-10 nM) in the absence or presence of the ET(B) receptor antagonist BQ788 (1 microM) or the NADPH oxidase inhibitor apocynin (1 microM). Reactive oxygen species (ROS) were detected using chloromethyl-2',7'-dichlorodihydrofluorescein diacetate. Apoptosis was evaluated with 4',6'-diamidino-2'-phenylindoladihydrochloride staining and by the caspase-3 assay. Cell proliferation was measured by a colorimetric assay. Expression of NADPH oxidase, Akt, pAkt, Bcl-2, Bax, IkappaB, caveolin-1 and eNOS was evaluated by Western blot analysis. 2 ET-1 significantly enhanced ROS generation and cell proliferation following 24-h incubation, both of which were prevented by BQ788 or apocynin, consistent with the ability of ET-1 to directly upregulate NADPH oxidase. ET-1 itself did not affect apoptosis but attenuated homocysteine-induced apoptosis through an ET(B) receptor-mediated mechanism. Western blot analysis indicated that ET-1 alleviated homocysteine (Hcy)-induced apoptosis, likely acting by antagonizing the Hcy-induced decreases in Akt, pAkt, pAkt-to-Akt, Bcl-2-to-Bax ratios and increases in Bax and caveolin-1 expression. Furthermore, ET-1 downregulated expression of caveolin-1 and eNOS, which was attenuated by BQ788 or apocynin. 3 In summary, our results suggest that ET-1 affects oxidative stress, proliferation and apoptosis possibly through ET(B), NADPH oxidase, Akt, Bax and caveolin-1-mediated mechanisms.

Gene Expression Profile Following Stable Expression of the Cellular Prion Protein

1. The cellular prion protein (PrPC) is expressed widely in neural and nonneural tissues at the highest level in neurons in the central nervous system (CNS). 2. Recent studies indicated that transgenic mice with the cytoplasmic accumulation of PrPC exhibited extensive neurodegeneration in the cerebellum, although the underlying mechanism remains unknown. To identify the genes whose expression is controlled by over-expression of PrPC in human cells, we have established a stable PrPC-expressing HEK293 cell line designated P1 by the site-specific recombination technique. 3. Microarray analysis identified 33 genes expressed differentially between P1 and the parent PrPC-non-expressing cell line among 12,814 genes examined. They included 18 genes involved in neuronal and glial functions, 5 related to production of extracellular matrix proteins, and 2 located in the complement cascade. 4. Northern blot analysis verified marked upregulation in P1 of the brain-specific protein phosphatase 2A beta subunit (PPP2R2B), a causative gene of spinocerebellar ataxia 12, and the cerebellar degeneration-related autoantigen (CDR34) gene associated with development of paraneoplastic cerebellar degeneration. 5. These results indicate that accumulation of PrPC in the cell caused aberrant regulation of a battery of the genes important for specific neuronal function. This represents a possible mechanism underlying PrPC-mediated selective neurodegeneration.

REMOTE TRAUMA SENSITIZES HEPATIC MICROCIRCULATION TO ENDOTHELIN VIA CAVEOLIN INHIBITION OF eNOS ACTIVITY

This study addresses the microvascular mechanisms by which a remote, mild stress such as blunt trauma sensitizes the liver to injury. Rats received closed femur fracture (FFx), and 24 h later livers were isolated and perfused at a similar starting flow rate for assessment of vascular response to endothelin-1 (ET-1). Sinusoidal volumetric flow (QS), red blood cell velocity (VRBC), and sinusoidal diameter (Ds) were determined by intravital microscopy. Baseline portal resistance in livers from FFx rats was not changed. The FFx group showed a lower baseline VRBC (322.9 +/- 26.4 and 207.3 +/- 17.2 microm/s in sham and FFx,) and QS (28.4 +/- 4.2 and 17.6 +/- 2.1 pL/s in sham and FFx, P < 0.05). ET-1 caused a decrease in the VRBC in sham but no change after FFx. In contrast, Ds was unchanged by ET-1 in sham but decreased in FFx (10.3 +/- 0.4 to 10.7 +/- 0.5 vs. 10.6 +/- 0.4 to 9.0 +/- 0.4 microm at 10 min in sham and FFx groups, P < 0.05). The overall result of these changes was a greater decrease in sinusoidal flow in FFx compared with sham. There was no significant change in mRNA for ET-1, endothelin A (ETA) receptor, or iNOS (inducible nitric oxide synthase) in FFx compared with sham. However, endothelin B (ETB) receptor mRNA and eNOS (endothelial nitric oxide synthase) mRNA were increased in the FFx group (ETB, 54.81 +/- 8.08 in sham vs. 83.28 +/- 8.19 in FFx; eNOS, 56.11 +/- 2.53 in sham vs. 83.31 +/- 5.51 in FFx; P < 0.05) while the levels of these proteins remained unchanged. Caveolin-1 (cav-1) protein levels were elevated in FFx, and coimmunoprecipitation with both ETB and eNOS showed increased associations with these proteins, suggesting a possible inactivation of eNOS. The eNOS activity was also blunted in FFx animals in the presence of increased cav-1 expression. Taken together, these results demonstrate that remote trauma sensitizes the liver to the sinusoidal constrictor effect of ET-1. We propose that this hyperresponsiveness occurs as a result of uncoupling of the ETB receptor from eNOS activity mediated by interaction of eNOS and possibly the ETB receptor with increased caveolin-1. This vascular sensitization that occurs after FFx may contribute to the exacerbation of injury during subsequent stresses.

Air pollutants increase gene expression of the vasoconstrictor endothelin-1 in the lungs

Inhalation of urban pollutants elevates the circulating levels of the vasoactive peptides endothelin (ET)-1 and ET-3 in rats. This effect could explain the association between episodic variations of urban pollutants and acute cardiopulmonary morbidity and mortality documented in epidemiological studies. Because the lungs are the primary source of circulating ET-1 and the main site of clearance from circulation, we investigated the response of endothelin system genes in the lungs of Fischer-344 rats after 4-h nose-only inhalation of 0.8 ppm ozone plus 49 mg/m(3) EHC-93 (Ottawa particles). The mRNA levels for preproET-1, preproET-3, endothelin-converting enzyme (ECE)-1, and ET receptor subtypes A and B were determined at 2 h, and 1, 2, 3, 7, and 14 days after exposure. The pollutants induced preproET-1 and ECE-1 (P<0.05) after 2 h, consistent with the notion of increased synthesis and conversion of the peptide ET-1 in lung endothelial cells. PreproET-3 mRNA was down-regulated at 2 h post-exposure (P<0.05), and returned to control levels by 24 h, indicating that induction of ET-3 in the lungs is not responsible for the sustained elevation of ET-3 in plasma reported after inhalation of pollutants. Our results indicate that lung endothelin system genes respond rapidly and transiently to inhalation of urban pollutants, consistent with the dynamics of urban pollutant health effects in the human population.

Lipid rafts-protein association and the regulation of protein activity

Lipid rafts are membrane microdomains enriched in saturated phospholipids, sphingolipids, and cholesterol. They have a varied but distinct protein composition and have been implicated in diverse cellular processes including polarized traffic, signal transduction, endo- and exo-cytoses, entrance of obligate intracellular pathogens, and generation of pathological forms of proteins associated with Alzheimer's and prion diseases. Raft proteins can be permanently or temporarily associated to lipid rafts. Here, we review recent advances on the biochemical and cell biological characterization of rafts, and on the emerging concept of the temporary residency of proteins in rafts as a regulatory mechanism of their biological activity.

Arteries and veins desensitize differently to endothelin

Hypertension is accompanied by increased arterial endothelin-1 (ET-1) and decreased arterial contraction to ET-1. By contrast, veins remain responsive to ET-1 in hypertension. Isometric contraction was used to test the hypothesis that veins do not desensitize to ET-1 to the extent of arteries, possibly because of the presence of functional ETA and ETB receptors on veins and only functional ETA receptors on arteries. Contraction to ET-1 after exposure to ET-1 (100 nmol/L) was abolished in aortae, while in veins 36.3 +/- 0.2% of maximal contraction to ET-1 remained. Aortae were unresponsive to the ETA receptor agonist ET-1(1-31) (100 nmol/L) after ET-1 exposure, while 21.9 +/- 0.6% of maximum venous contraction to ET-1 (1-31) remained. In a similar manner, the venous ETB receptor did not lose responsiveness to the ETB receptor agonist sarafotoxin 6c (S6c, 100 nmol/L); aortae did not contract to S6c. In ET-1-desensitized veins, the ETB receptor antagonist BQ-788 (100 nmol/L) decreased maximum contraction to ET-1, but did not alter potency (-log EC50 control = 8.14 +/- 0.01 mol/L; BQ-788 = 8.13 +/- 0.04 mol/L). The ETA receptor antagonist atrasentan (100 nmol/L) blocked remaining venous contraction to ET-1 (control = 8.05 +/- 0.05 mol/L; atrasentan = unmeasurable). Maintained responsiveness to ET-1 in veins occurs primarily via the ETA receptor, while in arteries the ETA receptor is responsible for desensitization to ET-1.

Contribution of PKC-dependent and -independent processes in temporal ERK regulation by ET-1, PDGF, and EGF in rat myometrial cells

Endothelin-1 (ET-1), platelet-derived growth factor (PDGF), and epidermal growth factor (EGF) stimulated thymidine incorporation with different efficiency (PDGF ≫ EGF = ET-1) in rat myometrial cells. They also stimulated ERK activation, which culminated at 5 min and then declined to reach a plateau (at 45 min: EGF > 90%, PDGF = 50%, and ET-1 < 10% of maximum). Inhibition and downregulation of PKC demonstrated that ERK activation at 5 min involved PKCδ and -ζ for ET-1 and PKCα plus another PKC isoform for PDGF. By contrast, the EGF response did not involve PKC. Stimulation of Ras was more important with EGF than with PDGF, with ET-1 being the weakest activator. The simultaneous incubation of the cells with EGF and ET-1 potentiated the ERK activation at 5 min and mimicked the plateau phase obtained with PDGF. Under these conditions thymidine incorporation was comparable to that induced by PDGF. Taken together, our results indicated that the kinetic profile of ERK activation and its impact on cell proliferation can be modulated by the differential involvement of PKC isoforms and the amplitude of Ras activation.