Evidence of a role for TRPC channels in VEGF-mediated increased vascular permeability in vivo

High magnesium mitigates the vasoconstriction mediated by different types of calcium influx from monocrotaline-induced pulmonary hypertensive rats

NEW FINDINGS

What is the central question of this study? The aim was to examine and explore the involvement of Mg²⁺ in mitigating the vasoconstriction in PAs and sPAs in the MCT-PAH rat model. What are the main finding and its importance? 1.Both SOCE- and ROCE-mediated vasoconstriction enhanced in the MCT-PAH model. 2.High magnesium inhibited vasoconstriction due to directly antagonizing Ca²⁺ and increasing NO release. 3.The inhibition effect of high magnesium was more notable in sPA.

ABSTRACT

Increased extracellular magnesium concentration ([Mg²⁺ ]_e ) has been evidenced to attenuate the endothelin-1 (ET-1)-induced contractile response via the release of nitric oxide (NO) from the endothelium in proximal pulmonary arteries (PAs) of chronic hypoxic (CH) mice. Here we further examined the involvement of Mg²⁺ in the inhibition of vasoconstriction in PAs and distal smaller pulmonary arteries (sPAs) in a monocrotaline-induced pulmonary arterial hypertension (MCT-PAH) rat model. The data showed that in control rats, vasoconstriction in sPAs is more intense than that in PAs. In MCT-PAH rats, the store-operated Ca²⁺ entry (SOCE)-, and receptor-operated Ca²⁺ entry (ROCE)-mediated contraction was significantly strengthened. However, there was no upregulation of the vasoconstriction mediated by voltage-dependent calcium entry (VDCE). Furthermore, high magnesium greatly inhibited the VDCE-mediated contraction in PAs instead of sPAs, which was opposite to the ROCE-mediated contraction. Moreover, MCT pretreatment partly eliminated the endothelium-dependent vasodilation in PAs, which in sPAs, however, was still promoted by magnesium due to the increased NO release in pulmonary microvascular endothelial cells (PMVECs). In conclusion, the findings suggest that both SOCE- and ROCE-mediated vasoconstriction in the MCT-PAH model are enhanced, especially in sPAs. The inhibition effect of high magnesium on vasoconstriction can be achieved partly by its direct role as a Ca²⁺ antagonist and partly by increasing the NO release in PMVECs. This article is protected by copyright. All rights reserved.

Transient receptor potential channel regulation by growth factors

Calcium (Ca²⁺) is one of the most universal secondary messengers, owing its success to the immense concentration gradient across the plasma membrane. Dysregulation of Ca²⁺ homeostasis can result in severe cell dysfunction, thereby initiating several pathologies like tumorigenesis and fibrosis. Transient receptor potential (TRP) channels represent a superfamily of Ca²⁺-permeable ion channels that convey diverse physical and chemical stimuli into a physiological signal. Their broad expression pattern and gating promiscuity support their potential involvement in the cellular response to an altering environment. Growth factors (GF) are essential biochemical messengers that contribute to these environmental changes. Since Ca²⁺ is essential in GF signaling, altering TRP channel expression or function could be a valid strategy for GF to exert their effect onto their target. In this review, a comprehensive understanding of the current knowledge regarding the activation and/or modulation of TRP channels by GF is presented.

TRPM Channels in Human Diseases

The transient receptor potential melastatin (TRPM) subfamily belongs to the TRP cation channels family. Since the first cloning of TRPM1 in 1989, tremendous progress has been made in identifying novel members of the TRPM subfamily and their functions. The TRPM subfamily is composed of eight members consisting of four six-transmembrane domain subunits, resulting in homomeric or heteromeric channels. From a structural point of view, based on the homology sequence of the coiled-coil in the C-terminus, the eight TRPM members are clustered into four groups: TRPM1/M3, M2/M8, M4/M5 and M6/M7. TRPM subfamily members have been involved in several physiological functions. However, they are also linked to diverse pathophysiological human processes. Alterations in the expression and function of TRPM subfamily ion channels might generate several human diseases including cardiovascular and neurodegenerative alterations, organ dysfunction, cancer and many other channelopathies. These effects position them as remarkable putative targets for novel diagnostic strategies, drug design and therapeutic approaches. Here, we review the current knowledge about the main characteristics of all members of the TRPM family, focusing on their actions in human diseases.

Regulation of Vessel Permeability by TRP Channels

The vascular endothelium constitutes a semi-permeable barrier between blood and interstitial fluids. Since an augmented endothelial permeability is often associated to pathological states, understanding the molecular basis for its regulation is a crucial biomedical and clinical challenge. This review focuses on the processes controlling paracellular permeability that is the permeation of fluids between adjacent endothelial cells (ECs). Cytosolic calcium changes are often detected as early events preceding the alteration of the endothelial barrier (EB) function. For this reason, great interest has been devoted in the last decades to unveil the molecular mechanisms underlying calcium fluxes and their functional relationship with vessel permeability. Beyond the dicotomic classification between store-dependent and independent calcium entry at the plasma membrane level, the search for the molecular components of the related calcium-permeable channels revealed a difficult task for intrinsic and technical limitations. The contribution of redundant channel-forming proteins including members of TRP superfamily and Orai1, together with the very complex intracellular modulatory pathways, displays a huge variability among tissues and along the vascular tree. Moreover, calcium-independent events could significantly concur to the regulation of vascular permeability in an intricate and fascinating multifactorial framework.

Introduction to ion channels and calcium signaling in the microcirculation

The microcirculation is the network of feed arteries, arterioles, capillaries and venules that supply and drain blood from every tissue and organ in the body. It is here that exchange of heat, oxygen, carbon dioxide, nutrients, hormones, water, cytokines, and immune cells takes place; essential functions necessary to maintenance of homeostasis throughout the life span. This chapter will outline the structure and function of each microvascular segment highlighting the critical roles played by ion channels in the microcirculation. Feed arteries upstream from the true microcirculation and arterioles within the microcirculation contribute to systemic vascular resistance and blood pressure control. They also control total blood flow to the downstream microcirculation with arterioles being responsible for distribution of blood flow within a tissue or organ dependent on the metabolic needs of the tissue. Terminal arterioles control blood flow and blood pressure to capillary units, the primary site of diffusional exchange between blood and tissues due to their large surface area. Venules collect blood from capillaries and are important sites for fluid exchange and immune cell trafficking. Ion channels in microvascular smooth muscle cells, endothelial cells and pericytes importantly contribute to all of these functions through generation of intracellular Ca²⁺ and membrane potential signals in these cells.

The Effect and Mechanism of TRPC1, 3, and 6 on the Proliferation, Migration, and Lumen Formation of Retinal Vascular Endothelial Cells Induced by High Glucose

OBJECTIVE

Transient receptor potential canonical (TRPC) channels are involved in neovascularization repairing after vascular injury in many tissues. However, whether TRPCs play a regulatory role in the development of diabetic retinopathy (DR) has rarely been reported. In the present study, we selected TRPC1, 3, and 6 to determine their roles and mechanism in human retina vascular endothelial cells (HREC) under high glucose (HG) conditions.

METHODS

HRECs were cultured in vitro under HG, hyper osmosis, and normal conditions. The expression of TRPC1, 3, and 6 in the cells at 24 and 48 h were detected by RT-polymerase chain reaction (PCR), Western blot and cell immunohistochemistry (IHC); In various concentrations, SKF96365 acted on HG cultured HRECs, the expression of vascular endothelial growth factor (VEGF) were detected by the same methods above; and the CCK-8, Transwell, cell scratch assay, and Matrigel assay were used to assess cell proliferation, migration, and lumen formation.

RESULTS

The RT-PCR, Western blot, and IHC results showed that TRPC1 expression was increased, and TRPC6 mRNA expression was increased under high-glucose conditions. SKF96365 acted on HG cultured HRECs that VEGF expression was significantly decreased. The CCK-8 assay, Transwell assay, cell scratch assay, and Matrigel assay showed that cell proliferation, migration, and lumen formation were downregulated by SKF96365.

CONCLUSION

HG can induce increased expression of TRPC1 and 6 in HRECs. Inhibition of the TRPC pathway not only can decrease VEGF expression but also can prevent proliferation, migration, and lumen formation of HRECs induced by HG. Inhibition of TRPC channels is expected to become a drug target for DR.

Alternative Strategies to Inhibit Tumor Vascularization

Endothelial cells present in tumors show different origin, phenotype, and genotype with respect to the normal counterpart. Various mechanisms of intra-tumor vasculogenesis sustain the complexity of tumor vasculature, which can be further modified by signals deriving from the tumor microenvironment. As a result, resistance to anti-VEGF therapy and activation of compensatory pathways remain a challenge in the treatment of cancer patients, revealing the need to explore alternative strategies to the classical anti-angiogenic drugs. In this review, we will describe some alternative strategies to inhibit tumor vascularization, including targeting of antigens and signaling pathways overexpressed by tumor endothelial cells, the development of endothelial vaccinations, and the use of extracellular vesicles. In addition, anti-angiogenic drugs with normalizing effects on tumor vessels will be discussed. Finally, we will present the concept of endothelial demesenchymalization as an alternative approach to restore normal endothelial cell phenotype.

Microvascular ion transport through endothelial glycocalyx layer: new mechanism and improved Starling principle

Ion transport through the endothelial glycocalyx layer is closely associated with many vascular diseases. Clarification of ion behaviors around the endothelial glycocalyx layer under varying circumstances will benefit pathologies related to cardiovascular and renal diseases. In this research, a series of large-scale molecular dynamics simulations are conducted to study the response of ion transport to the changing blood flow velocity and the shedding of endothelial glycocalyx sugar chains. Results indicate that blood flow promotes the outward Na+ transport from the near-membrane region to the lumen via the endothelial glycocalyx layer. Scrutiny of sugar-chain dynamics and their interactions with Na+ suggests that corner conformation of endothelial glycocalyx sugar chains confines the movement of the Na+, whereas stretching conformation facilitates the motion of Na+ ions. The flow impact on ion transport of Na+ is nonlinear. Based on the findings, the Starling principle and its revised version, which are prevailingly used to predict the ion transport of the endothelial glycocalyx layer, are further improved. An estimation based on the further revised Starling principle indicates that physiological flow changes the osmotic part of transendothelial water flux by 8% compared with the stationary situation.

NEW & NOTEWORTHY The biophysical roles of negatively charged oligosaccharides of the endothelial glycocalyx have gained increasing attention due to their importance in regulating microvascular fluid exchange. The Starling principle and its revisions are at the heart of the understanding of fluid homeostasis in the periphery. Here, the blood flow changes the conformations of glycocalyx sugar chains, thereby influencing availability of Na+ for transport. Based on the findings, the Starling principle and its revision are further improved.

TRP Channels in Angiogenesis and Other Endothelial Functions

Angiogenesis is the growth of blood vessels mediated by proliferation, migration, and spatial organization of endothelial cells. This mechanism is regulated by a balance between stimulatory and inhibitory factors. Proangiogenic factors include a variety of VEGF family members, while thrombospondin and endostatin, among others, have been reported as suppressors of angiogenesis. Transient receptor potential (TRP) channels belong to a superfamily of cation-permeable channels that play a relevant role in a number of cellular functions mostly derived from their influence in intracellular Ca²⁺ homeostasis. Endothelial cells express a variety of TRP channels, including members of the TRPC, TRPV, TRPP, TRPA, and TRPM families, which play a relevant role in a number of functions, including endothelium-induced vasodilation, vascular permeability as well as sensing hemodynamic and chemical changes. Furthermore, TRP channels have been reported to play an important role in angiogenesis. This review summarizes the current knowledge and limitations concerning the involvement of particular TRP channels in growth factor-induced angiogenesis.

Cholesterol Regulation of Pulmonary Endothelial Calcium Homeostasis

Cholesterol is a key structural component and regulator of lipid raft signaling platforms critical for cell function. Such regulation may involve changes in the biophysical properties of lipid microdomains or direct protein-sterol interactions that alter the function of ion channels, receptors, enzymes, and membrane structural proteins. Recent studies have implicated abnormal membrane cholesterol levels in mediating endothelial dysfunction that is characteristic of pulmonary hypertensive disorders, including that resulting from long-term exposure to hypoxia. Endothelial dysfunction in this setting is characterized by impaired pulmonary endothelial calcium entry and an associated imbalance that favors production vasoconstrictor and mitogenic factors that contribute to pulmonary hypertension. Here we review current knowledge of cholesterol regulation of pulmonary endothelial Ca²⁺ homeostasis, focusing on the role of membrane cholesterol in mediating agonist-induced Ca²⁺ entry and its components in the normal and hypertensive pulmonary circulation.

Endothelial Protrusions in Junctional Integrity and Barrier Function

Endothelial cells of the microcirculation form a semi-permeable diffusion barrier between the blood and tissues. This permeability of the endothelium, particularly in the capillaries and postcapillary venules, is a normal physiological function needed for blood-tissue exchange in the microcirculation. During inflammation, microvascular permeability increases dramatically and can lead to tissue edema, which in turn can lead to dysfunction of tissues and organs. The molecular mechanisms that control the barrier function of endothelial cells have been under investigation for several decades and remain an important topic due to the potential for discovery of novel therapeutic strategies to reduce edema. This review highlights current knowledge of the cellular and molecular mechanisms that lead to endothelial hyperpermeability during inflammatory conditions associated with injury and disease. This includes a discussion of recent findings demonstrating temporal protrusions by endothelial cells that may contribute to intercellular junction integrity between endothelial cells and affect the diffusion distance for solutes via the paracellular pathway.

TRP Channels as Sensors of Bacterial Endotoxins

The cellular and systemic effects induced by bacterial lipopolysaccharides (LPS) have been solely attributed to the activation of the Toll-like receptor 4 (TLR4) signalling cascade. However, recent studies have shown that LPS activates several members of the Transient Receptor Potential (TRP) family of cation channels. Indeed, LPS induces activation of the broadly-tuned chemosensor TRPA1 in sensory neurons in a TLR4-independent manner, and genetic ablation of this channel reduced mouse pain and inflammatory responses triggered by LPS and the gustatory-mediated avoidance to LPS in fruit flies. LPS was also shown to activate TRPV4 channels in airway epithelial cells, an effect leading to an immediate production of bactericidal nitric oxide and to an increase in ciliary beat frequency. In this review, we discuss the role of TRP channels as sensors of bacterial endotoxins, and therefore, as crucial players in the timely detection of invading gram-negative bacteria.

Novel noncanonical regulation of soluble VEGF/VEGFR2 signaling by mechanosensitive ion channel TRPV4

VEGF signaling via VEGF receptor-2 (VEGFR2) is a major regulator of endothelial cell (EC) functions, including angiogenesis. Although most studies of angiogenesis focus on soluble VEGF signaling, mechanical signaling also plays a critical role. Here, we examined the consequence of disruption of mechanical signaling on soluble signaling pathways. Specifically, we observed that small interfering RNA (siRNA) knockdown of a mechanosensitive ion channel, transient receptor potential vanilloid 4 (TRPV4), significantly reduced perinuclear (Golgi) VEGFR2 in human ECs with a concomitant increase in phosphorylation at Y1175 and membrane translocation. TRPV4 knockout (KO) ECs exhibited increased plasma membrane localization of phospho-VEGFR2 compared with normal ECs. The knockdown also increased phospho-VEGFR2 in whole cell lysates and membrane fractions compared with control siRNA-treated cells. siRNA knockdown of TRPV4 enhanced nuclear localization of mechanosensitive transcription factors, yes-associated protein/transcriptional coactivator with PDZ-binding motif via rho kinase, which were shown to increase VEGFR2 trafficking to the plasma membrane. Furthermore, TRPV4 deletion/knockdown enhanced VEGF-mediated migration in vitro and increased expression of VEGFR2 in vivo in the vasculature of TRPV4 KO tumors compared with wild-type tumors. Our results thus show that TRPV4 channels regulate VEGFR2 trafficking and activation to identify novel cross-talk between mechanical (TRPV4) and soluble (VEGF) signaling that controls EC migration and angiogenesis.-Kanugula, A. K., Adapala, R. K., Midha, P., Cappelli, H. C., Meszaros, J. G., Paruchuri, S., Chilian, W. M., Thodeti, C. K., Novel noncanonical regulation of soluble VEGF/VEGFR2 signaling by mechanosensitive ion channel TRPV4.

Endothelial dysfunction in pulmonary arterial hypertension: an evolving landscape (2017 Grover Conference Series)

Endothelial dysfunction is a major player in the development and progression of vascular pathology in pulmonary arterial hypertension (PAH), a disease associated with small vessel loss and obstructive vasculopathy that leads to increased pulmonary vascular resistance, subsequent right heart failure, and premature death. Over the past ten years, there has been tremendous progress in our understanding of pulmonary endothelial biology as it pertains to the genetic and molecular mechanisms that orchestrate the endothelial response to direct or indirect injury, and how their dysregulation can contribute to the pathogenesis of PAH. As one of the major topics included in the 2017 Grover Conference Series, discussion centered on recent developments in four areas of pulmonary endothelial biology: (1) angiogenesis; (2) endothelial-mesenchymal transition (EndMT); (3) epigenetics; and (4) biology of voltage-gated ion channels. The present review will summarize the content of these discussions and provide a perspective on the most promising aspects of endothelial dysfunction that may be amenable for therapeutic development.

Molecular Regulation of Sprouting Angiogenesis

The term angiogenesis arose in the 18th century. Several studies over the next 100 years laid the groundwork for initial studies performed by the Folkman laboratory, which were at first met with some opposition. Once overcome, the angiogenesis field has flourished due to studies on tumor angiogenesis and various developmental models that can be genetically manipulated, including mice and zebrafish. In addition, new discoveries have been aided by the ability to isolate primary endothelial cells, which has allowed dissection of various steps within angiogenesis. This review will summarize the molecular events that control angiogenesis downstream of biochemical factors such as growth factors, cytokines, chemokines, hypoxia-inducible factors (HIFs), and lipids. These and other stimuli have been linked to regulation of junctional molecules and cell surface receptors. In addition, the contribution of cytoskeletal elements and regulatory proteins has revealed an intricate role for mobilization of actin, microtubules, and intermediate filaments in response to cues that activate the endothelium. Activating stimuli also affect various focal adhesion proteins, scaffold proteins, intracellular kinases, and second messengers. Finally, metalloproteinases, which facilitate matrix degradation and the formation of new blood vessels, are discussed, along with our knowledge of crosstalk between the various subclasses of these molecules throughout the text. Compr Physiol 8:153-235, 2018.

TRPC3- and ETB receptor-mediated PI3K/AKT activation induces vasogenic edema formation following status epilepticus

Status epilepticus (SE, a prolonged seizure activity) is a high risk factor of developing vasogenic edema, which leads to secondary complications following SE. In the present study, we investigated whether transient receptor potential canonical channel-3 (TRPC3) may link vascular endothelial growth factor (VEGF) pathway to NFκB/ET_B receptor axis in the rat piriform cortex during vasogenic edema formation. Following SE, TRPC3 and ET_B receptor independently activated phosphatidylinositol 3 kinase (PI3K)/AKT/eNOS signaling pathway. SN50 (a NFκB inhibitor) attenuated the up-regulations of eNOS, TRPC3 and ET_B receptor expressions following SE, accompanied by reductions in PI3K/AKT phosphorylations. Inhibition of SE-induced VEGF over-expression by leptomycin B also abrogated PI3K and AKT phosphorylations, but not TRPC3 expression. Wortmannin (a PI3K inhibitor) and 3CAI (an AKT inhibitor) effectively inhibited up-regulation of eNOS expressions and vasogenic edema lesion following SE. These findings indicate that PI3K/AKT may be common down-stream molecules for TRPC3- and ET_B receptor signaling pathways during vasogenic edema formation. In addition, the present data demonstrate for the first time that TRPC3 may integrate VEGF- and NFκB-mediated vasogenic edema formation following SE. Thus, we suggest that PI3K/AKT signaling pathway may be one of considerable therapeutic targets for vasogenic edema.

The Role of Transient Receptor Potential Channel 6 Channels in the Pulmonary Vasculature

Canonical or classical transient receptor potential channel 6 (TRPC6) is a Ca²⁺-permeable non-selective cation channel that is widely expressed in the heart, lung, and vascular tissues. The use of TRPC6-deficient (“knockout”) mice has provided important insights into the role of TRPC6 in normal physiology and disease states of the pulmonary vasculature. Evidence indicates that TRPC6 is a key regulator of acute hypoxic pulmonary vasoconstriction. Moreover, several studies implicated TRPC6 in the pathogenesis of pulmonary hypertension. Furthermore, a unique genetic variation in the TRPC6 gene promoter has been identified, which might link the inflammatory response to the upregulation of TRPC6 expression and ultimate development of pulmonary vascular abnormalities in idiopathic pulmonary arterial hypertension. Additionally, TRPC6 is critically involved in the regulation of pulmonary vascular permeability and lung edema formation during endotoxin or ischemia/reperfusion-induced acute lung injury. In this review, we will summarize latest findings on the role of TRPC6 in the pulmonary vasculature.

Role of Transient Receptor Potential Channels in Heart Transplantation: A Potential Novel Therapeutic Target for Cardiac Allograft Vasculopathy

Heart transplantation has evolved as the criterion standard therapy for end-stage heart failure, but its efficacy is limited by the development of cardiac allograft vasculopathy (CAV), a unique and rapidly progressive form of atherosclerosis in heart transplant recipients. Here, we briefly review the key processes in the development of CAV during heart transplantation and highlight the roles of transient receptor potential (TRP) channels in these processes during heart transplantation. Understanding the roles of TRP channels in contributing to the key procedures for the development of CAV during heart transplantation could provide basic scientific knowledge for the development of new preventive and therapeutic approaches to manage patients with CAV after heart transplantation.

Protein Interactions at Endothelial Junctions and Signaling Mechanisms Regulating Endothelial Permeability

The monolayer of endothelial cells lining the vessel wall forms a semipermeable barrier (in all tissue except the relatively impermeable blood-brain and inner retinal barriers) that regulates tissue-fluid homeostasis, transport of nutrients, and migration of blood cells across the barrier. Permeability of the endothelial barrier is primarily regulated by a protein complex called adherens junctions. Adherens junctions are not static structures; they are continuously remodeled in response to mechanical and chemical cues in both physiological and pathological settings. Here, we discuss recent insights into the post-translational modifications of junctional proteins and signaling pathways regulating plasticity of adherens junctions and endothelial permeability. We also discuss in the context of what is already known and newly defined signaling pathways that mediate endothelial barrier leakiness (hyperpermeability) that are important in the pathogenesis of cardiovascular and lung diseases and vascular inflammation.

ROS-activated calcium signaling mechanisms regulating endothelial barrier function

Increased vascular permeability is a common pathogenic feature in many inflammatory diseases. For example in acute lung injury (ALI) and its most severe form, the acute respiratory distress syndrome (ARDS), lung microvessel endothelia lose their junctional integrity resulting in leakiness of the endothelial barrier and accumulation of protein rich edema. Increased reactive oxygen species (ROS) generated by neutrophils (PMNs) and other inflammatory cells play an important role in increasing endothelial permeability. In essence, multiple inflammatory syndromes are caused by dysfunction and compromise of the barrier properties of the endothelium as a consequence of unregulated acute inflammatory response. This review focuses on the role of ROS signaling in controlling endothelial permeability with particular focus on ALI. We summarize below recent progress in defining signaling events leading to increased endothelial permeability and ALI.

Involvement of ion channels and transporters in carcinoma angiogenesis and metastasis

Angiogenesis is a finely tuned process, which is the result of the equilibrium between pro- and antiangiogenic factors. In solid tumor angiogenesis, the balance is highly in favor of the production of new, but poorly functional blood vessels, initially intended to provide growing tumors with nutrients and oxygen. Among the numerous proteins involved in tumor development, several types of ion channels are overexpressed in tumor cells, as well as in stromal and endothelial cells. Ion channels thus actively participate in the different hallmarks of cancer, especially in tumor angiogenesis and metastasis. Indeed, from their strategic localization in the plasma membrane, ion channels are key operators of cell signaling, as they sense and respond to environmental changes. This review aims to decipher how ion channels of different families are intricately involved in the fundamental angiogenesis and metastasis hallmarks, which lead from a nascent tumor to systemic dissemination. An overview of the possible use of ion channels as therapeutic targets will also be given, showing that ion channel inhibitors or specific antibodies may provide effective tools, in the near future, in the treatment of carcinomas.

Mechanisms regulating endothelial permeability

The endothelial monolayer partitioning underlying tissue from blood components in the vessel wall maintains tissue fluid balance and host defense through dynamically opening intercellular junctions. Edemagenic agonists disrupt endothelial barrier function by signaling the opening of the intercellular junctions leading to the formation of protein-rich edema in the interstitial tissue, a hallmark of tissue inflammation that, if left untreated, causes fatal diseases, such as acute respiratory distress syndrome. In this review, we discuss how intercellular junctions are maintained under normal conditions and after stimulation of endothelium with edemagenic agonists. We have focused on reviewing the new concepts dealing with the alteration of adherens junctions after inflammatory stimulus.

Functional properties of ion channels and transporters in tumour vascularization

Vascularization is crucial for solid tumour growth and invasion, providing metabolic support and sustaining metastatic dissemination. It is now accepted that ion channels and transporters play a significant role in driving the cancer growth at all stages. They may represent novel therapeutic, diagnostic and prognostic targets for anti-cancer therapies. On the other hand, although the expression and role of ion channels and transporters in the vascular endothelium is well recognized and subject of recent reviews, only recently has their involvement in tumour vascularization been recognized. Here, we review the current literature on ion channels and transporters directly involved in the angiogenic process. Particular interest will be focused on tumour angiogenesis in vivo as well as in the different steps that drive this process in vitro, such as endothelial cell proliferation, migration, adhesion and tubulogenesis. Moreover, we compare the 'transportome' system of tumour vascular network with the physiological one.

Cytoskeletal dynamics and lung fluid balance

This article examines the role of the endothelial cytoskeleton in the lung's ability to restrict fluid and protein to vascular space at normal vascular pressures and thereby to protect lung alveoli from lethal flooding. The barrier properties of microvascular endothelium are dependent on endothelial cell contact with other vessel-wall lining cells and with the underlying extracellular matrix (ECM). Focal adhesion complexes are essential for attachment of endothelium to ECM. In quiescent endothelial cells, the thick cortical actin rim helps determine cell shape and stabilize endothelial adherens junctions and focal adhesions through protein bridges to actin cytoskeleton. Permeability-increasing agonists signal activation of "small GTPases" of the Rho family to reorganize the actin cytoskeleton, leading to endothelial cell shape change, disassembly of cortical actin rim, and redistribution of actin into cytoplasmic stress fibers. In association with calcium- and Src-regulated myosin light chain kinase (MLCK), stress fibers become actinomyosin-mediated contractile units. Permeability-increasing agonists stimulate calcium entry and induce tyrosine phosphorylation of VE-cadherin (vascular endothelial cadherin) and β-catenins to weaken or pull apart endothelial adherens junctions. Some permeability agonists cause latent activation of the small GTPases, Cdc42 and Rac1, which facilitate endothelial barrier recovery and eliminate interendothelial gaps. Under the influence of Cdc42 and Rac1, filopodia and lamellipodia are generated by rearrangements of actin cytoskeleton. These motile evaginations extend endothelial cell borders across interendothelial gaps, and may initiate reannealing of endothelial junctions. Endogenous barrier protective substances, such as sphingosine-1-phosphate, play an important role in maintaining a restrictive endothelial barrier and counteracting the effects of permeability-increasing agonists.

TLR4 activation of TRPC6-dependent calcium signaling mediates endotoxin-induced lung vascular permeability and inflammation

Lung vascular endothelial barrier disruption and the accompanying inflammation are primary pathogenic features of acute lung injury (ALI); however, the basis for the development of both remains unclear. Studies have shown that activation of transient receptor potential canonical (TRPC) channels induces Ca(2+) entry, which is essential for increased endothelial permeability. Here, we addressed the role of Toll-like receptor 4 (TLR4) intersection with TRPC6-dependent Ca(2+) signaling in endothelial cells (ECs) in mediating lung vascular leakage and inflammation. We find that the endotoxin (lipopolysaccharide; LPS) induces Ca(2+) entry in ECs in a TLR4-dependent manner. Moreover, deletion of TRPC6 renders mice resistant to endotoxin-induced barrier dysfunction and inflammation, and protects against sepsis-induced lethality. TRPC6 induces Ca(2+) entry in ECs, which is secondary to the generation of diacylglycerol (DAG) induced by LPS. Ca(2+) entry mediated by TRPC6, in turn, activates the nonmuscle myosin light chain kinase (MYLK), which not only increases lung vascular permeability but also serves as a scaffold to promote the interaction of myeloid differentiation factor 88 and IL-1R-associated kinase 4, which are required for NF-κB activation and lung inflammation. Our findings suggest that TRPC6-dependent Ca(2+) entry into ECs, secondary to TLR4-induced DAG generation, participates in mediating both lung vascular barrier disruption and inflammation induced by endotoxin.

Effect of normobaric hypoxia on the testis in a murine model

High-altitude hypoxia generates spermiogram impairment due to germinal epithelium, Leydig cells, sperm and seminal plasma alterations, but precise mechanisms involved are unknown. The objective of this work was to analyse the effect of normobaric hypoxia on the morphology of testicular interstitium and some associated molecular and hormonal factors. Twenty-four mice were exposed to normobaric hypoxia (8.1% inspired oxygen fraction) during 20 days. The effects on body weight, testicular weight, vascularisation, testosterone, HIF1-α and VEGF were analysed at different periods of exposure and compared to controls. Hypoxic mice had lower body weight than mice kept in normoxia. Testicular weight raised significantly the 1st day, but remained normal during the rest of experiment. Number of blood vessels per field and mean diameter of vessels were higher in hypoxic mice. Plasmatic and testicular testosterone raised during first 24 h of hypoxia, but decreased on the 5th day. Vascular/interstitial ratio (proportion of interstice occupied by blood vessels) duplicated at the end of the experiment. Most substantial early effects of hypoxia were testicular oedema, increase in number and diameter of blood vessels and elevation of plasmatic and testicular testosterone. Normobaric hypoxia generates similar effects to those induced by hypobaric hypoxia.

Update on vascular endothelial Ca2+ signalling: A tale of ion channels, pumps and transporters

A monolayer of endothelial cells (ECs) lines the lumen of blood vessels and forms a multifunctional transducing organ that mediates a plethora of cardiovascular processes. The activation of ECs from as state of quiescence is, therefore, regarded among the early events leading to the onset and progression of potentially lethal diseases, such as hypertension, myocardial infarction, brain stroke, and tumor. Intracellular Ca²⁺ signals have long been know to play a central role in the complex network of signaling pathways regulating the endothelial functions. Notably, recent work has outlined how any change in the pattern of expression of endothelial channels, transporters and pumps involved in the modulation of intracellular Ca²⁺ levels may dramatically affect whole body homeostasis. Vascular ECs may react to both mechanical and chemical stimuli by generating a variety of intracellular Ca²⁺ signals, ranging from brief, localized Ca²⁺ pulses to prolonged Ca²⁺ oscillations engulfing the whole cytoplasm. The well-defined spatiotemporal profile of the subcellular Ca²⁺ signals elicited in ECs by specific extracellular inputs depends on the interaction between Ca²⁺ releasing channels, which are located both on the plasma membrane and in a number of intracellular organelles, and Ca²⁺ removing systems. The present article aims to summarize both the past and recent literature in the field to provide a clear-cut picture of our current knowledge on the molecular nature and the role played by the components of the Ca²⁺ machinery in vascular ECs under both physiological and pathological conditions.

Lung endothelial Ca2+ and permeability response to platelet-activating factor is mediated by acid sphingomyelinase and transient receptor potential classical 6

RATIONALE

Platelet-activating factor (PAF) increases lung vascular permeability within minutes by activation of acid sphingomyelinase (ASM) and a subsequent nitric oxide (NO)-inhibitable and Ca(2+)-dependent loss in barrier function.

OBJECTIVES

To elucidate the molecular mechanisms underlying this response.

METHODS

In isolated perfused rat and mouse lungs, endothelial Ca(2+) concentration ([Ca(2+)](i)) was quantified by real-time fluorescence imaging, and caveolae of endothelial cells were isolated and probed for Ca(2+) entry channels. Regulation of transient receptor potential classical (TRPC) 6-mediated currents in lung endothelial cells was assessed by patch clamp technique.

MEASUREMENTS AND MAIN RESULTS

PAF increased lung weight gain and endothelial [Ca(2+)](i). This response was abrogated by inhibitors of ASM or in ASM-deficient mice, and replicated by lung perfusion with exogenous ASM or C2-ceramide. PAF increased the caveolar abundance of TRPC6 channels, which was similarly blocked by ASM inhibition. PAF-induced increases in lung endothelial [Ca(2+)](i), vascular filtration coefficient, and edema formation were attenuated by the TRPC inhibitor SKF96365 and in TRPC6-deficient mice, whereas direct activation of TRPC6 replicated the [Ca(2+)](i) and edema response to PAF. The exogenous NO donor PapaNONOate or the cyclic guanosine 3',5'-monophosphate analog 8Br-cGMP blocked the endothelial [Ca(2+)](i) and permeability response to PAF, in that they directly blocked TRPC6 channels without interfering with their PAF-induced recruitment to caveolae.

CONCLUSIONS

The present findings outline a new signaling cascade in the induction of PAF-induced lung edema, in that stimulation of ASM causes recruitment of TRPC6 channels to caveolae, thus allowing for Ca(2+) influx and subsequent increases in endothelial permeability that are amplified in the absence of endothelial NO synthesis.

Regulation of cardiovascular TRP channel functions along the NO-cGMP-PKG axis

There is growing body of evidence that nitric oxide (NO)-cGMP-PKG signaling plays a central role in negative regulation of cardiovascular (CV) responses and its disorders through suppressed Ca(2+) dynamics. Other lines of evidence also reveal the stimulatory effects of this signaling on some CV functions. Recently, transient receptor potential (TRP) channels have received much attention as non-voltage-gated Ca(2+) channels involved in CV physiology and pathophysiology. Available information suggests that these channels undergo both inhibition and activation by NO via PKG-mediated phosphorylation and S-nitrosylation, respectively, and also act as upstream regulators to promote endothelial NO production. This review summarizes the roles of NO-cGMP-PKG signaling pathway, particularly in regulating TRP channel functions with their associated physiology and pathophysiology.

Hydrogen sulfide promotes calcium signals and migration in tumor-derived endothelial cells

Hydrogen sulfide (H(2)S) is a gasotransmitter that plays several roles in various tissues, including the cardiovascular system. Because it has been recently proposed to act as a mediator of angiogenesis progression, here we investigate the effects of H(2)S in a well-established model of tumor angiogenesis: endothelial cells obtained from human breast carcinoma (B-TECs). Ca(2+) imaging and patch-clamp experiments reveal that acute perfusion with NaHS, a widely employed H(2)S donor, activates cytosolic calcium (Ca(c)) increase, as well as potassium and nonselective cationic currents, in B-TECs. Stimulation with NaHS in the same concentration range (1 nM-200 μM) evoked Ca(c) signals also in "normal" human microvascular endothelial cells (HMVECs), but the amplitude was significantly lower. Moreover, although NaHS failed to promote either migration or proliferation on HMVECs, B-TEC migration was enhanced at low-micromolar NaHS concentrations (1-10 μM). Remarkably H(2)S mediates tumor proangiogenic signaling triggered by vascular endothelial growth factor (VEGF). B-TECs pretreated with dl-propargylglycine (5mM, 30 min), an inhibitor of the H(2)S-producing enzyme cystathionine γ-lyase, showed drastically reduced migration and Ca(c) signals induced by VEGF (20 ng/ml). We conclude that H(2)S plays a role in proangiogenic signaling of tumor-derived but not normal human ECs. Furthermore the ability of this gasotransmitter to interfere with B-TEC responsiveness to VEGF suggests that it could be an interesting target for antiangiogenic strategies in tumor treatment.

Ion channels and transporters in cancer. 6. Vascularizing the tumor: TRP channels as molecular targets

Tumor vascularization is a critical process that determines tumor growth and metastasis. In the last decade new experimental evidence obtained from in vitro and in vivo studies have challenged the classical angiogenesis model forcing us to consider new scenarios for tumor neovascularization. In particular, the genetic stability of tumor-derived endothelial cells (TECs) has been recently questioned in several studies, which show that TECs, as well as pericytes, differ significantly from their normal counterparts at genetic and functional levels. In addition to such an epigenetic action of tumor microenvironment on endothelial cells (ECs) commitment, the distinct characteristics of TECs could be due to differences in their origin compared with preexisting differentiated ECs. Intracellular Ca(2+) signals are involved at different critical phases in the regulation of the complex process of angiogenesis and tumor progression. These signals are generated by a wide variety of intrinsic and extrinsic factors. Several key components of Ca(2+) signaling including Ca(2+) channels in the plasma membrane, endoplasmic reticulum, calcium pumps, and mitochondria contribute to the generation, amplitude, and frequency of these Ca(2+) change. In particular, several members of the transient receptor potential (TRP) family of calcium-permeable channels have profound effects on the function of ECs. Because of its multifaceted role in the control of cell function, proliferation, and motility, TRP channels have been suggested as a potential molecular target for control of tumor neovascularization. Since plasma membrane Ca(2+) channels are easily and directly accessible via the bloodstream, they are potential targets for a number of pharmacological and antibody-targeted therapeutic strategies, with specificity being the main limitation. In this review we discuss recent advances in understanding the role of Ca(2+) channels, with specific reference to TRP channels, in tumor vascularization process.

TRPV4 mediates tumor-derived endothelial cell migration via arachidonic acid-activated actin remodeling

Changes in intracellular calcium [Ca(2+)](i) levels control critical cytosolic and nuclear events that are involved in the initiation and progression of tumor angiogenesis in endothelial cells (ECs). Therefore, the mechanism(s) involved in agonist-induced Ca(2+)(i) signaling is a potentially important molecular target for controlling angiogenesis and tumor growth. Several studies have shown that blood vessels in tumors differ from normal vessels in their morphology, blood flow and permeability. We had previously reported a key role for arachidonic acid (AA)-mediated Ca(2+) entry in the initial stages of tumor angiogenesis in vitro. In this study we assessed the mechanism involved in AA-induced EC migration. We report that TRPV4, an AA-activated channel, is differentially expressed in EC derived from human breast carcinomas (BTEC) as compared with 'normal' EC (HMVEC). BTEC display a significant increase in TRPV4 expression, which was correlated with greater Ca(2+) entry, induced by AA or 4αPDD (a selective TRPV4 agonist) in the tumor-derived ECs. Wound-healing assays revealed a key role of TRPV4 in regulating cell migration of BTEC but not HMVEC. Knockdown of TRPV4 expression completely abolished AA-induced BTEC migration, suggesting that TRPV4 mediates the pro-angiogenic effects promoted by AA. Furthermore, pre-incubation of BTEC with AA induced actin remodeling and a subsequent increase in the surface expression of TRPV4. This was consistent with the increased plasma membrane localization of TRPV4 and higher AA-stimulated Ca(2+) entry in the migrating cells. Together, the data presented herein demonstrate that: (1) TRPV4 is differentially expressed in tumor-derived versus 'normal' EC; (2) TRPV4 has a critical role in the migration of tumor-derived but not 'normal' EC migration; and (3) AA induces actin remodeling in BTEC, resulting in a corresponding increase of TRPV4 expression in the plasma membrane. We suggest that the latter is critical for migration of EC and thus in promoting angiogenesis and tumor growth.

TRP channels in vascular endothelial cells

Endothelial cells regulate multiple vascular functions, such as vascular tone, permeability, remodeling, and angiogenesis. It is known for long that cytosolic Ca(2+) level ([Ca(2+)](i)) and membrane potential of endothelial cells are crucial factors to initiate the signal transduction cascades, leading to diverse vascular functions. Among the various kinds of endothelial ion channels that regulate ion homeostasis, transient receptor potential (TRP) channels emerge as the prime mediators for a diverse range of vascular signaling. The characteristics of TRP channels, including subunit heteromultimerization, diverse ion selectivity, and multiple modes of activation, permit their versatile functional roles in vasculatures. Substantial amount of evidence demonstrates that many TRP channels in endothelial cells participate in physiological and pathophysiological processes of vascular system. In this article, we summarize the recent findings of TRP research in endothelial cells, aiming at providing up-to-date information to the researchers in this rapidly growing field.

Moderation of calpain activity promotes neovascular integration and lumen formation during VEGF-induced pathological angiogenesis

Background

Successful neovascularization requires that sprouting endothelial cells (ECs) integrate to form new vascular networks. However, architecturally defective, poorly integrated vessels with blind ends are typical of pathological angiogenesis induced by vascular endothelial growth factor-A (VEGF), thereby limiting the utility of VEGF for therapeutic angiogenesis and aggravating ischemia-related pathologies. Here we investigated the possibility that over-exuberant calpain activity is responsible for aberrant VEGF neovessel architecture and integration. Calpains are a family of intracellular calcium-dependent, non-lysosomal cysteine proteases that regulate cellular functions through proteolysis of numerous substrates.

Methodology/Principal Findings

In a mouse skin model of VEGF-driven angiogenesis, retroviral transduction with dominant-negative (DN) calpain-I promoted neovessel integration and lumen formation, reduced blind ends, and improved vascular perfusion. Moderate doses of calpain inhibitor-I improved VEGF-driven angiogenesis similarly to DN calpain-I. Conversely, retroviral transduction with wild-type (WT) calpain-I abolished neovessel integration and lumen formation. In vitro, moderate suppression of calpain activity with DN calpain-I or calpain inhibitor-I increased the microtubule-stabilizing protein tau in endothelial cells (ECs), increased the average length of microtubules, increased actin cable length, and increased the interconnectivity of vascular cords. Conversely, WT calpain-I diminished tau, collapsed microtubules, disrupted actin cables, and inhibited integration of cord networks. Consistent with the critical importance of microtubules for vascular network integration, the microtubule-stabilizing agent taxol supported vascular cord integration whereas microtubule dissolution with nocodazole collapsed cord networks.

Conclusions/Significance

These findings implicate VEGF-induction of calpain activity and impairment of cytoskeletal dynamics in the failure of VEGF-induced neovessels to form and integrate properly. Accordingly, calpain represents an important target for rectifying key vascular defects associated with pathological angiogenesis and for improving therapeutic angiogenesis with VEGF.

A new role for PTEN in regulating transient receptor potential canonical channel 6-mediated Ca2+ entry, endothelial permeability, and angiogenesis

Phosphatase and tensin homologue (PTEN) is a dual lipid-protein phosphatase that catalyzes the conversion of phosphoinositol 3,4,5-triphosphate to phosphoinositol 4,5-bisphosphate and thereby inhibits PI3K-Akt-dependent cell proliferation, migration, and tumor vascularization. We have uncovered a previously unrecognized role for PTEN in regulating Ca(2+) entry through transient receptor potential canonical channel 6 (TRPC6) that does not require PTEN phosphatase activity. We show that PTEN tail-domain residues 394-403 permit PTEN to associate with TRPC6. The inflammatory mediator thrombin promotes this association. Deletion of PTEN residues 394-403 prevents TRPC6 cell surface expression and Ca(2+) entry. However, PTEN mutant, C124S, which lacks phosphatase activity, did not alter TRPC6 activity. Thrombin failed to increase endothelial monolayer permeability in the endothelial cells, transducing the Δ394-403 PTEN mutant. Paradoxically, we also show that thrombin failed to induce endothelial cell migration and tube formation in cells transducing the Δ394-403 PTEN mutant. Our results demonstrate that PTEN, through residues 394-403, serves as a scaffold for TRPC6, enabling cell surface expression of the channel. Ca(2+) entry through TRPC6 induces an increase in endothelial permeability and directly promotes angiogenesis. Thus, PTEN is indicated to play a role beyond suppressing PI3K signaling.

Altered protein kinase C regulation of pulmonary endothelial store- and receptor-operated Ca2+ entry after chronic hypoxia

Chronic hypoxia (CH)-induced pulmonary hypertension is associated with decreased basal pulmonary artery endothelial cell (EC) Ca(2+), which correlates with reduced store-operated Ca(2+) (SOC) entry. Protein kinase C (PKC) attenuates SOC entry in ECs. Therefore, we hypothesized that PKC has a greater inhibitory effect on EC SOC and receptor-operated Ca(2+) entry after CH. To test this hypothesis, we assessed SOC in the presence or absence of the nonselective PKC inhibitor GF109203X [2-[1-(3-dimethylaminopropyl)-1H-indol-3-yl]-3-(1H-indol-3-yl)maleimide] in freshly isolated, Fura-2-loaded ECs obtained from intrapulmonary arteries of control and CH rats (4 weeks at 0.5 atm). We found that SOC entry and 1-oleoyl-2-acetyl-sn-glycerol (OAG)- and ATP-induced Ca(2+) influx were attenuated in ECs from CH rats versus controls, and GF109203X restored SOC and OAG responses to the level of controls. In contrast, nonselective PKC inhibition with GF109203X or the selective PKC(epsilon) inhibitor myristoylated V1-2 attenuated ATP-induced Ca(2+) entry in ECs from control but not CH pulmonary arteries. ATP-induced Ca(2+) entry was also attenuated by the T-type voltage-gated Ca(2+) channel (VGCC) inhibitor mibefradil in control cells. Consistent with the presence of endothelial T-type VGCC, we observed depolarization-induced Ca(2+) influx in control cells that was inhibited by mibefradil. This response was largely absent in ECs from CH arteries. We conclude that CH enhances PKC-dependent inhibition of SOC- and OAG-induced Ca(2+) entry. Furthermore, these data suggest that CH may reduce the ATP-dependent Ca(2+) entry that is mediated, in part, by PKCepsilon and mibefradil-sensitive Ca(2+) channels in control cells.

Vascular endothelial growth factors and vascular permeability

Vascular endothelial growth factors (VEGFs) are key regulators of permeability. The principal evidence behind how they increase vascular permeability in vivo and in vitro and the consequences of that increase are addressed here. Detailed analysis of the published literature has shown that in vivo and in vitro VEGF-mediated permeability differs in its time course, but has common involvement of many specific signalling pathways, in particular VEGF receptor-2 activation, calcium influx through transient receptor potential channels, activation of phospholipase C gamma and downstream activation of nitric oxide synthase. Pathways downstream of endothelial nitric oxide synthase appear to involve the guanylyl cyclase-mediated activation of the Rho–Rac pathway and subsequent involvement of junctional signalling proteins such as vascular endothelial cadherin and the tight junctional proteins zona occludens and occludin linked to the actin cytoskeleton. The signalling appears to be co-ordinated through spatial organization of the cascade into a signalplex, and arguments for why this may be important are considered. Many proteins have been identified to be involved in the regulation of vascular permeability by VEGF, but still the mechanisms through which these are thought to interact to control permeability are dependent on the experimental system, and a synthesis of existing data reveals that in intact vessels the co-ordination of the pathways is still not understood.

Anti-angiogenic properties of calcium trifluoroacetate

Endothelial cell proliferation and the formation of new vessels are strictly regulated by angiogenic factors (e.g., VEGF) that induce the activation of signal transduction pathways controlled by calcium dynamics. Using in vitro and in vivo experiments, we investigated the effect of calcium trifluoroacetate (CaTFAc), a complex, poorly dissociated salt that is characterized by its low toxicity, on angiogenesis. In vitro, CaTFAc inhibited VEGF-induced effects on endothelial cell proliferation. In two in vivo models of angiogenesis, a Matrigel plug in mice and a chick embryo chorioallantoic membrane, CaTFAc inhibited the VEGF-induced formation of new vessels. The exact mechanism of action is still under investigation, but in vitro experiments demonstrate that CaTFAc induced a reversible increase in the levels of intracellular calcium under basal conditions and prevented calcium signaling induced by VEGF. These results are the first to suggest that CaTFAc may be useful for the treatment of diseases caused by enhanced angiogenesis.