Calcium influx, arachidonic acid,and control of endothelial cell proliferation

Arachidonic acid regulates pluripotency by modulating cellular energetics via fatty acid synthesis and mitochondrial fission

Arachidonic acid (AA) is an important omega-6 fatty acid that can be metabolised into an impressive spectrum of biologically active mediators participating in various cellular functions. Studies have shown that fatty acid synthesis is enhanced in embryonic stem cells (ESCs), and it is crucial for the cellular reprogramming of somatic cells into induced pluripotent stem cells (iPSCs). Fatty acid synthesis increases the cellular lipid contents and, in turn, promotes mitochondrial fission and cellular reprogramming. AA was found to induce acetyl-CoA carboxylase 1 (ACC1) expression, a major enzyme in fatty acid synthesis. In this study, we have investigated the regulation of pluripotency, fatty acid synthesis and mitochondrial activities of the human induced pluripotent stem cells (hiPSCs) and the human embryonal carcinoma (hEC) NTERA-2 cells upon treatment with varying concentrations of AA. Our results indicate that a lower concentration of AA can increase pluripotency, as evidenced by an increased expression of pluripotency markers, increased fatty acid synthesis as evidenced by lipid estimation and modulated mitochondrial fission, as evidenced by mitotracker staining for fissioned mitochondria. Moreover, higher concentrations of AA-induced the opposite effect, leading to pluripotent stem cell differentiation. Molecular docking simulations predicted the possible interactions between AA and its metabolites with fatty acid synthesis regulators ACC1 and CREB1 (Cyclic adenosine monophosphate Response Element Binding Protein 1) as a mechanism for AA regulating pluripotency.

Nitric oxide synthesis stimulated by arachidonic acid accumulation via PPARα in acetylcholine-stimulated gastric mucous cells

NEW FINDINGS

What is the central question of this study? Arachidonic acid (AA) stimulates NO production in antral mucous cells without any increase in [Ca²⁺ ]_i . Given that the intracellular AA concentration is too low to measure, the relationship between AA accumulation and NO production remains uncertain. Is AA accumulation a key step for NO production? What is the main finding and its importance? We demonstrated that AA accumulation is a key step for NO production. The amount of AA released could be measured using fluorescence-HPLC. The intracellular AA concentration was maintained at < 1 μM. Nitric oxide is produced by AA accumulation in antral mucous cells, not as a direct effect of [Ca²⁺ ]_i .

ABSTRACT

In the present study, we demonstrate that NO production is stimulated by an accumulation of arachidonic acid (AA) mediated via peroxisome proliferation-activated receptor α (PPARα) and that the NO produced enhances Ca²⁺ -regulated exocytosis in ACh-stimulated antral mucous cells. The amount of AA released from the antral mucosa, measured by fluorescence high-performance liquid chromatography (F-HPLC), was increased by addition of ionomycin (10 μM) or ACh, suggesting that AA accumulation is stimulated by an increase in [Ca²⁺ ]_i . The AA production was inhibited by an inhibitor of cytosolic phospholipase A2 (cPLA2-inhα). GW6471 (a PPARα inhibitor) and cPLA2-inhα inhibited NO synthesis stimulated by ACh. Moreover, indomethacin, an inhibitor of cyclooxygenase, stimulated AA accumulation and NO production. However, acetylsalicylic acid did not stimulate AA production and NO synthesis. An analogue of AA (AACOCF3) alone stimulated NO synthesis, which was inhibited by GW6471. In antral mucous cells, indomethacin enhanced Ca²⁺ -regulated exocytosis by increasing NO via PPARα, and the enhancement was abolished by GW6471 and cPLA2-inhα. Thus, AA produced via PLA2 activation is the key step for NO synthesis in ACh-stimulated antral mucous cells and plays important roles in maintaining antral mucous secretion, especially in Ca²⁺ -regulated exocytosis.

Arachidonic Acid Evokes an Increase in Intracellular Ca2+ Concentration and Nitric Oxide Production in Endothelial Cells from Human Brain Microcirculation

It has long been known that the conditionally essential polyunsaturated arachidonic acid (AA) regulates cerebral blood flow (CBF) through its metabolites prostaglandin E2 and epoxyeicosatrienoic acid, which act on vascular smooth muscle cells and pericytes to vasorelax cerebral microvessels. However, AA may also elicit endothelial nitric oxide (NO) release through an increase in intracellular Ca²⁺ concentration ([Ca²⁺]_i). Herein, we adopted Ca²⁺ and NO imaging, combined with immunoblotting, to assess whether AA induces intracellular Ca²⁺ signals and NO release in the human brain microvascular endothelial cell line hCMEC/D3. AA caused a dose-dependent increase in [Ca²⁺]_i that was mimicked by the not-metabolizable analogue, eicosatetraynoic acid. The Ca²⁺ response to AA was patterned by endoplasmic reticulum Ca²⁺ release through type 3 inositol-1,4,5-trisphosphate receptors, lysosomal Ca²⁺ mobilization through two-pore channels 1 and 2 (TPC1-2), and extracellular Ca²⁺ influx through transient receptor potential vanilloid 4 (TRPV4). In addition, AA-evoked Ca²⁺ signals resulted in robust NO release, but this signal was considerably delayed as compared to the accompanying Ca²⁺ wave and was essentially mediated by TPC1-2 and TRPV4. Overall, these data provide the first evidence that AA elicits Ca²⁺-dependent NO release from a human cerebrovascular endothelial cell line, but they seemingly rule out the possibility that this NO signal could acutely modulate neurovascular coupling.

Store-independent Orai1-mediated Ca2+ entry and cancer

Ca²⁺ channels play an important role in the development of different types of cancer, and considerable progress has been made to understand the pathophysiological mechanisms underlying the role of Ca²⁺ influx in the development of different cancer hallmarks. Orai1 is among the most ubiquitous and multifunctional Ca²⁺ channels. Orai1 mediates the highly Ca²⁺-selective Ca²⁺ release-activated current (I_CRAC) and participates in the less Ca²⁺-selective store-operated current (I_SOC), along with STIM1 or STIM1 and TRPC1, respectively. Furthermore, Orai1 contributes to a variety of store-independent Ca²⁺ influx mechanisms, including the arachidonate-regulated Ca²⁺ current, together with Orai3 and the plasma membrane resident pool of STIM1, as well as the constitutive Ca²⁺ influx processes activated by the secretory pathway Ca²⁺-ATPase-2 (SPCA2) or supported by physical and functional interaction with the small conductance Ca²⁺-activated K⁺ channel 3 (SK3) or the voltage-dependent K_v10.1 channel. This review summarizes the current knowledge concerning the store-independent mechanisms of Ca²⁺ influx activation through Orai1 channels and their role in the development of different cancer features.

Importance of extracellular matrix and growth state for the EA.hy926 endothelial cell response to polyunsaturated fatty acids

Consumption of different PUFAs (polyunsaturated fatty acids) can induce functional changes in blood vessels via endothelial cells, which interact with dietary factors in the circulation. The basement membrane that separates the endothelium from the smooth muscle cells of the medial layer can also influence the functional state of endothelial cells. However, the effect of basement membrane on the endothelial response to dietary PUFAs in relation to growth state (e.g. proliferation versus quiescence) has never been investigated. We therefore compared the viability (CCK kit) and proliferation (bromodeoxyuridine incorporation) of EA.hy926 endothelial cells grown on Matrigel or collagen versus non-coated plates. EA.hy926 viability and proliferation were also assessed after treatment with 0–150 μM of PUFAs [linoleic acid (LA), arachidonic acid (AA), α-linolenic acid (ALA), eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA)]. Our study showed that only cells grown on Matrigel-coated plates reached quiescence after becoming confluent with a decreased level of MCM2 and p-cyclin D1 (T286), increased levels of p27kip1 and a low level of apoptosis and senescence. AA, EPA and DHA decreased the viability and proliferation of subconfluent cells grown on plastic dishes in a dose-dependent manner, while the presence of Matrigel made the cells resistant to these adverse effects. Confluent cell viability was less sensitive to higher concentrations of AA, EPA and DHA than subconfluent cells, and a significant increase in caspase-3 cleavage was only observed in confluent cells treated with DHA. Higher concentrations of AA, EPA and DHA suppressed DNA synthesis by both subconfluent and confluent cells, while precursor C18 PUFAs (LA and ALA) had no negative effects on viability and proliferation. Our study is the first to show that extracellular matrix and growth state are important factors in the EA.hy926 cell response to PUFAs, and that the mechanisms by which individual PUFAs operate may be growth state-dependent.

Actions and Mechanisms of Polyunsaturated Fatty Acids on Voltage-Gated Ion Channels

Polyunsaturated fatty acids (PUFAs) act on most ion channels, thereby having significant physiological and pharmacological effects. In this review we summarize data from numerous PUFAs on voltage-gated ion channels containing one or several voltage-sensor domains, such as voltage-gated sodium (Na_V), potassium (K_V), calcium (Ca_V), and proton (H_V) channels, as well as calcium-activated potassium (K_Ca), and transient receptor potential (TRP) channels. Some effects of fatty acids appear to be channel specific, whereas others seem to be more general. Common features for the fatty acids to act on the ion channels are at least two double bonds in cis geometry and a charged carboxyl group. In total we identify and label five different sites for the PUFAs. PUFA site 1: The intracellular cavity. Binding of PUFA reduces the current, sometimes as a time-dependent block, inducing an apparent inactivation. PUFA site 2: The extracellular entrance to the pore. Binding leads to a block of the channel. PUFA site 3: The intracellular gate. Binding to this site can bend the gate open and increase the current. PUFA site 4: The interface between the extracellular leaflet of the lipid bilayer and the voltage-sensor domain. Binding to this site leads to an opening of the channel via an electrostatic attraction between the negatively charged PUFA and the positively charged voltage sensor. PUFA site 5: The interface between the extracellular leaflet of the lipid bilayer and the pore domain. Binding to this site affects slow inactivation. This mapping of functional PUFA sites can form the basis for physiological and pharmacological modifications of voltage-gated ion channels.

Arachidonic acid-evoked Ca2+ signals promote nitric oxide release and proliferation in human endothelial colony forming cells

Arachidonic acid (AA) stimulates endothelial cell (EC) proliferation through an increase in intracellular Ca²⁺ concentration ([Ca²⁺]_i), that, in turn, promotes nitric oxide (NO) release. AA-evoked Ca²⁺ signals are mainly mediated by Transient Receptor Potential Vanilloid 4 (TRPV4) channels. Circulating endothelial colony forming cells (ECFCs) represent the only established precursors of ECs. In the present study, we, therefore, sought to elucidate whether AA promotes human ECFC (hECFC) proliferation through an increase in [Ca²⁺]_i and the following activation of the endothelial NO synthase (eNOS). AA induced a dose-dependent [Ca²⁺]_i raise that was mimicked by its non-metabolizable analogue eicosatetraynoic acid. AA-evoked Ca²⁺ signals required both intracellular Ca²⁺ release and external Ca²⁺ inflow. AA-induced Ca²⁺ release was mediated by inositol-1,4,5-trisphosphate receptors from the endoplasmic reticulum and by two pore channel 1 from the acidic stores of the endolysosomal system. AA-evoked Ca²⁺ entry was, in turn, mediated by TRPV4, while it did not involve store-operated Ca²⁺ entry. Moreover, AA caused an increase in NO levels which was blocked by preventing the concomitant increase in [Ca²⁺]_i and by inhibiting eNOS activity with NG-nitro-l-arginine methyl ester (l-NAME). Finally, AA per se did not stimulate hECFC growth, but potentiated growth factors-induced hECFC proliferation in a Ca²⁺- and NO-dependent manner. Therefore, AA-evoked Ca²⁺ signals emerge as an additional target to prevent cancer vascularisation, which may be sustained by ECFC recruitment.

Arachidonic acid-induced dilation in human coronary arterioles: convergence of signaling mechanisms on endothelial TRPV4-mediated Ca2+ entry

BACKGROUND

Arachidonic acid (AA) and/or its enzymatic metabolites are important lipid mediators contributing to endothelium-derived hyperpolarizing factor (EDHF)-mediated dilation in multiple vascular beds, including human coronary arterioles (HCAs). However, the mechanisms of action of these lipid mediators in endothelial cells (ECs) remain incompletely defined. In this study, we investigated the role of the transient receptor potential vanilloid 4 (TRPV4) channel in AA-induced endothelial Ca(2+) response and dilation of HCAs.

METHODS AND RESULTS

AA induced concentration-dependent dilation in isolated HCAs. The dilation was largely abolished by the TRPV4 antagonist RN-1734 and by inhibition of endothelial Ca(2+)-activated K(+) channels. In native and TRPV4-overexpressing human coronary artery ECs (HCAECs), AA increased intracellular Ca(2+) concentration ([Ca(2+)]i), which was mediated by TRPV4-dependent Ca(2+) entry. The AA-induced [Ca(2+)]i increase was inhibited by cytochrome P450 (CYP) inhibitors. Surprisingly, the CYP metabolites of AA, epoxyeicosatrienoic acids (EETs), were much less potent activators of TRPV4, and CYP inhibitors did not affect EET production in HCAECs. Apart from its effect on [Ca(2+)]i, AA induced endothelial hyperpolarization, and this effect was required for Ca(2+) entry through TRPV4. AA-induced and TRPV4-mediated Ca(2+) entry was also inhibited by the protein kinase A inhibitor PKI. TRPV4 exhibited a basal level of phosphorylation, which was inhibited by PKI. Patch-clamp studies indicated that AA activated TRPV4 single-channel currents in cell-attached and inside-out patches of HCAECs.

CONCLUSIONS

AA dilates HCAs through a novel mechanism involving endothelial TRPV4 channel-dependent Ca(2+) entry that requires endothelial hyperpolarization, PKA-mediated basal phosphorylation of TRPV4, and direct activation of TRPV4 channels by AA.

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.

Multilevel complexity of calcium signaling: Modeling angiogenesis

Intracellular calcium signaling is a universal, evolutionary conserved and versatile regulator of cell biochemistry. The complexity of calcium signaling and related cell machinery can be investigated by the use of experimental strategies, as well as by computational approaches. Vascular endothelium is a fascinating model to study the specific properties and roles of calcium signals at multiple biological levels. During the past 20 years, live cell imaging, patch clamp and other techniques have allowed us to detect and interfere with calcium signaling in endothelial cells (ECs), providing a huge amount of information on the regulation of vascularization (angiogenesis) in normal and tumoral tissues. These data range from the spatiotemporal dynamics of calcium within different cell microcompartments to those in entire multicellular and organized EC networks. Beside experimental strategies, in silico endothelial models, specifically designed for simulating calcium signaling, are contributing to our knowledge of vascular physiology and pathology. They help to investigate and predict the quantitative features of proangiogenic events moving through subcellular, cellular and supracellular levels. This review focuses on some recent developments of computational approaches for proangiogenic endothelial calcium signaling. In particular, we discuss the creation of hybrid simulation environments, which combine and integrate discrete Cellular Potts Models. They are able to capture the phenomenological mechanisms of cell morphological reorganization, migration, and intercellular adhesion, with single-cell spatiotemporal models, based on reaction-diffusion equations that describe the agonist-induced intracellular calcium events.

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.

A multiscale hybrid approach for vasculogenesis and related potential blocking therapies

Solid tumors must recruit and form new blood vessels for maintenance, growth and detachments of metastases. Discovering drugs that block malignant angiogenesis is thus an important approach in cancer treatment and has given rise to multiple in vitro and in silico models. The present hybrid individual cell-based model incorporates some underlying biochemical events relating more closely the classical Cellular Potts Model (CPM) parameters to subcellular mechanisms and to the activation of specific signaling pathways. The model spans the three fundamental biological levels: at the extracellular level a continuous model describes secretion, diffusion, uptake and decay of the autocrine VEGF; at the cellular level, an extended lattice CPM, based on a system energy reduction, reproduces cell dynamics such as migration, adhesion and chemotaxis; at the subcellular level, a set of reaction-diffusion equations describes a simplified VEGF-induced calcium-dependent intracellular pathway. The results agree with the known interplay between calcium signals and VEGF dynamics and with their role in malignant vasculogenesis. Moreover, the analysis of the link between the microscopic subcellular dynamics and the macroscopic cell behaviors confirms the efficiency of some pharmacological interventions that are currently in use and, more interestingly, proposes some new therapeutic approaches, that are counter-intuitive but potentially effective.

Multiple roles of protein kinase a in arachidonic acid-mediated Ca2+ entry and tumor-derived human endothelial cell migration

We recently showed that arachidonic acid (AA) triggers calcium signals in endothelial cells derived from human breast carcinoma (B-TEC). In particular, AA-dependent Ca(2+) entry is involved in the early steps of tumor angiogenesis in vitro. Here, we investigated the multiple roles of the nitric oxide (NO) and cyclic AMP/protein kinase A (PKA) pathways in AA-mediated Ca(2+) signaling in the same cells. B-TEC stimulation with 5 μmol/L AA resulted in endothelial NO synthase (NOS) phosphorylation at Ser(1177), and NO release was measured with the fluorescent NO-sensitive probe DAR4M-AM. PKA inhibition by the use of the membrane-permeable PKA inhibitory peptide myristoylated PKI(14-22) completely prevented both AA- and NO-induced calcium entry and abolished B-TEC migration promoted by AA. AA-dependent calcium entry and cell migration were significantly affected by both the NOS inhibitor N(G)-nitro-l-arginine methyl ester and the NO scavenger 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide, suggesting that NO release is functionally involved in the signaling dependent on AA. Moreover, pretreatment with carboxyamidotriazole, an antiangiogenic compound that interferes with agonist-activated calcium entry, prevented AA-dependent B-TEC motility. Interestingly, even in the absence of AA, enhancement of the cyclic AMP/PKA pathway with the adenylyl cyclase activator forskolin evoked a calcium entry dependent on NOS recruitment and NO release. The functional relevance of AA-induced calcium entry could be restricted to tumor-derived endothelial cells (EC) because AA evoked a smaller calcium entry in normal human microvascular ECs compared with B-TECs, and even more importantly, it was unable to promote cell motility in wound healing assay. This evidence opens an intriguing opportunity for differential pharmacologic treatment between normal and tumor-derived human ECs.

Modulation of Ca(2+) signalling in human vascular endothelial cells by hydrogen sulfide

Hydrogen sulfide (H(2)S) is now recognised as an important endogenous antihypertensive molecule and is synthesised in the vasculature primarily by endothelial cystathionine gamma lyase. Activity of this enzyme, and the production of other vasoactive substances by the endothelium, are subject to modulation by changes of [Ca(2+)](i). Here, we have used microfluorimetry to investigate whether H(2)S can regulate human endothelial [Ca(2+)](i). H(2)S (applied via the donor NaHS, 5-500 microM) caused concentration-dependent rises of [Ca(2+)](i) which were attributable to release from an ATP- and 4-CEP sensitive intracellular pool. Rises of [Ca(2+)](i) evoked by H(2)S were essentially abolished by prior pool depletion. In the absence of external Ca(2+), H(2)S slowed the decay phase of responses to cyclopiazonic acid, but this could not be attributed to the inhibition of Ca(2+) extrusion since the effects of H(2)S were at least additive with the Na(+)/Ca(2+) exchange inhibitors bepridil and SEA 0400 and the Ca(2+) ATPase inhibitor, carboxyeosin. In some but not all the cells, re-exposure to extracellular Ca(2+) following the addition and removal of H(2)S activated capacitative Ca(2+) entry (CCE), and H(2)S increased ATP-evoked (but not thapsigargin-evoked) CCE. Effects of H(2)S were not mediated by energy depletion or production of cyclic ADP ribose. Our data indicate that H(2)S can modulate endothelial [Ca(2+)](i) via multiple mechanisms, and such effects are likely to contribute to this gasotransmitter's beneficial actions.

Arachidonic acid-induced Ca2+ entry is involved in early steps of tumor angiogenesis

Growth factor-induced intracellular calcium signals in endothelial cells regulate cytosolic and nuclear events involved in the angiogenic process. Among the intracellular messengers released after proangiogenic stimulation, arachidonic acid (AA) plays a key role and its effects are strictly related to calcium homeostasis and cell proliferation. Here, we studied AA-induced intracellular calcium signals in endothelial cells derived from human breast carcinomas (B-TEC). AA promotes B-TEC proliferation and organization of vessel-like structures in vitro. The effect is directly mediated by the fatty acid without a significant contribution of its metabolites. AA induces Ca(2+)(i) signals in the entire capillary-like structure during the early phases of tubulogenesis in vitro. No such responses are detectable in B-TECs organized in more structured tubules. In B-TECs growing in monolayer, AA induces two different signals: a Ca(2+)(i) increase due to Ca(2+) entry and an inhibition of store-dependent Ca(2+) entry induced by thapsigargin or ATP. An inhibitor of Ca(2+) entry and angiogenesis, carboxyamidotriazole, significantly and specifically decreases AA-induced B-TEC tubulogenesis, as well as AA-induced Ca(2+) signals in B-TECs. We conclude that (a) AA-activated Ca(2+) entry is associated with the progression through the early phases of angiogenesis, mainly involving proliferation and tubulogenesis, and it is down-regulated during the reorganization of tumor-derived endothelial cells in capillary-like structures; and (b) inhibition of AA-induced Ca(2+) entry may contribute to the antiangiogenic action of carboxyamidotriazole.

Novel role for polycystin-1 in modulating cell proliferation through calcium oscillations in kidney cells

OBJECTIVES

Polycystin-1 (PC1), a signalling receptor regulating Ca(2+)-permeable cation channels, is mutated in autosomal dominant polycystic kidney disease, which is typically characterized by increased cell proliferation. However, the precise mechanisms by which PC1 functions on Ca(2+) homeostasis, signalling and cell proliferation remain unclear. Here, we investigated the possible role of PC1 as a modulator of non-capacitative Ca(2+) entry (NCCE) and Ca(2+) oscillations, with downstream effects on cell proliferation.

RESULTS AND DISCUSSION

By employing RNA interference, we show that depletion of endogenous PC1 in HEK293 cells leads to an increase in serum-induced Ca(2+) oscillations, triggering nuclear factor of activated T cell activation and leading to cell cycle progression. Consistently, Ca(2+) oscillations and cell proliferation are increased in PC1-mutated kidney cystic cell lines, but both abnormal features are reduced in cells that exogenously express PC1. Notably, blockers of the NCCE pathway, but not of the CCE, blunt abnormal oscillation and cell proliferation. Our study therefore provides the first demonstration that PC1 modulates Ca(2+) oscillations and a molecular mechanism to explain the association between abnormal Ca(2+) homeostasis and cell proliferation in autosomal dominant polycystic kidney disease.

Novel role for polycystin-1 in modulating cell proliferation through calcium oscillations in kidney cells

OBJECTIVES

Polycystin-1 (PC1), a signalling receptor regulating Ca(2+)-permeable cation channels, is mutated in autosomal dominant polycystic kidney disease, which is typically characterized by increased cell proliferation. However, the precise mechanisms by which PC1 functions on Ca(2+) homeostasis, signalling and cell proliferation remain unclear. Here, we investigated the possible role of PC1 as a modulator of non-capacitative Ca(2+) entry (NCCE) and Ca(2+) oscillations, with downstream effects on cell proliferation.

RESULTS AND DISCUSSION

By employing RNA interference, we show that depletion of endogenous PC1 in HEK293 cells leads to an increase in serum-induced Ca(2+) oscillations, triggering nuclear factor of activated T cell activation and leading to cell cycle progression. Consistently, Ca(2+) oscillations and cell proliferation are increased in PC1-mutated kidney cystic cell lines, but both abnormal features are reduced in cells that exogenously express PC1. Notably, blockers of the NCCE pathway, but not of the CCE, blunt abnormal oscillation and cell proliferation. Our study therefore provides the first demonstration that PC1 modulates Ca(2+) oscillations and a molecular mechanism to explain the association between abnormal Ca(2+) homeostasis and cell proliferation in autosomal dominant polycystic kidney disease.

Impaired capsaicin-induced relaxation of coronary arteries in a porcine model of the metabolic syndrome

Recent studies implicate channels of the transient receptor potential vanilloid family (e.g., TRPV1) in regulating vascular tone; however, little is known about these channels in the coronary circulation. Furthermore, it is unclear whether metabolic syndrome alters the function and/or expression of TRPV1. We tested the hypothesis that TRPV1 mediates coronary vasodilation through endothelium-dependent mechanisms that are impaired by the metabolic syndrome. Studies were conducted on coronary arteries from lean and obese male Ossabaw miniature swine. In lean pigs, capsaicin, a TRPV1 agonist, relaxed arteries in a dose-dependent manner (EC50 = 116 +/- 41 nM). Capsaicin-induced relaxation was blocked by the TRPV1 antagonist capsazepine, endothelial denudation, inhibition of nitric oxide synthase, and K+ channel antagonists. Capsaicin-induced relaxation was impaired in rings from pigs with metabolic syndrome (91 +/- 4% vs. 51 +/- 10% relaxation at 100 microM). TRPV1 immunoreactivity was prominent in coronary endothelial cells. TRPV1 protein expression was decreased 40 +/- 11% in obese pigs. Capsaicin (100 microM) elicited divalent cation influx that was abolished in endothelial cells from obese pigs. These data indicate that TRPV1 channels are functionally expressed in the coronary circulation and mediate endothelium-dependent vasodilation through a mechanism involving nitric oxide and K+ channels. Impaired capsaicin-induced vasodilation in the metabolic syndrome is associated with decreased expression of TRPV1 and cation influx.

Anandamide and NADA bi-directionally modulate presynaptic Ca2+ levels and transmitter release in the hippocampus

BACKGROUND AND PURPOSE

Inhibitory CB(1) cannabinoid receptors and excitatory TRPV(1) vanilloid receptors are abundant in the hippocampus. We tested if two known hybrid endocannabinoid/endovanilloid substances, N-arachidonoyl-dopamine (NADA) and anandamide (AEA), presynapticaly increased or decreased intracellular calcium level ([Ca(2+)](i)) and GABA and glutamate release in the hippocampus.

EXPERIMENTAL APPROACH

Resting and K(+)-evoked levels of [Ca(2+)](i) and the release of [(3)H]GABA and [(3)H]glutamate were measured in rat hippocampal nerve terminals.

KEY RESULTS

NADA and AEA per se triggered a rise of [Ca(2+)](i) and the release of both transmitters in a concentration- and external Ca(2+)-dependent fashion, but independently of TRPV(1), CB(1), CB(2), or dopamine receptors, arachidonate-regulated Ca(2+)-currents, intracellular Ca(2+) stores, and fatty acid metabolism. AEA was recently reported to block TASK-3 potassium channels thereby depolarizing membranes. Common inhibitors of TASK-3, Zn(2+), Ruthenium Red, and low pH mimicked the excitatory effects of AEA and NADA, suggesting that their effects on [Ca(2+)](i) and transmitter levels may be attributable to membrane depolarization upon TASK-3 blockade. The K(+)-evoked Ca(2+) entry and Ca(2+)-dependent transmitter release were inhibited by nanomolar concentrations of the CB(1) receptor agonist WIN55212-2; this action was sensitive to the selective CB(1) receptor antagonist AM251. However, in the low micromolar range, WIN55212-2, NADA and AEA inhibited the K(+)-evoked Ca(2+) entry and transmitter release independently of CB(1) receptors, possibly through direct Ca(2+) channel blockade.

CONCLUSIONS AND IMPLICATIONS

We report here for hybrid endocannabinoid/endovanilloid ligands novel dual functions which were qualitatively similar to activation of CB(1) or TRPV(1) receptors, but were mediated through interactions with different targets.

Role of intracellular calcium and phospholipase A2 in arachidonic acid-induced toxicity in liver cells overexpressing CYP2E1

Liver cells (HepG2 and primary hepatocytes) overexpressing CYP2E1 and exposed to arachidonic acid (AA) were previously shown to lose viability together with enhanced lipid peroxidation. These events were blocked in cells pre-incubated with antioxidants (alpha-tocopherol, glutathione ethyl ester), or in HepG2 cells not expressing CYP2E1. The goal of the current study was to evaluate the role of calcium and calcium-activated hydrolases in these CYP2E1-AA interactions. CYP2E1-expressing HepG2 cells treated with AA showed an early increase in cytosolic calcium and partial depletion of ionomycin-sensitive calcium stores. These changes in calcium were blocked by alpha-tocopherol. AA activated phospholipase A2 (PLA2) in CYP2E1-expressing liver cells, and this was inhibited by PLA2 inhibitors or alpha-tocopherol. PLA2 inhibitors prevented the cell death caused by AA, without affecting CYP2E1 activity or lipid peroxidation. AA toxicity and PLA2 activation were inhibited in calcium-depleted cells, but not by removal of extracellular calcium alone. Removal of extracellular calcium inhibited the early increase in cytosolic calcium caused by AA. CYP2E1 overexpressing HepG2 cells exposed to AA showed a decrease in mitochondrial membrane potential, which was prevented by the PLA2 inhibitors. These results suggest that AA-induced toxicity to CYPE1-expressing cells: (i) is associated with release of Ca2+ from intracellular stores that depends mainly on oxidative membrane damage; (ii) is associated with activation of PLA2 that depends on intracellular calcium and lipid peroxidation; (iii) does not depend on increased influx of extracellular calcium, and (iv) depends on the effect of converging events (lipid peroxidation, intracellular calcium, activation of PLA2) on mitochondria to induce bioenergetic failure and necrosis. These interactions may play a role in alcohol liver toxicity, which requires polyunsaturated fatty acids, and involves induction of CYP2E1.

Calcium signalling in the endothelium

Elevations in cytosolic Ca2+ concentration are the usual initial response of endothelial cells to hormonal and chemical transmitters and to changes in physical parameters, and many endothelial functions are dependent upon changes in Ca2+ signals produced. Endothelial cell Ca2+ signalling shares similar features with other electrically non-excitable cell types, but has features unique to endothelial cells. This chapter discusses the major components of endothelial cell Ca2+ signalling.

PLA2 and TRPV4 channels regulate endothelial calcium in cerebral arteries

We previously demonstrated that endothelium-derived hyperpolarizing factor (EDHF)-mediated dilations in cerebral arteries are significantly reduced by inhibitors of PLA(2). In this study we examined possible mechanisms by which PLA(2) regulates endothelium-dependent dilation, specifically whether PLA(2) is involved in endothelial Ca(2+) regulation through stimulation of TRPV4 channels. Studies were carried out with middle cerebral arteries (MCA) or freshly isolated MCA endothelial cells (EC) of male Long-Evans rats. Nitro-l-arginine methyl ester (l-NAME) and indomethacin were present throughout. In pressurized MCA, luminally delivered UTP produced increased EC intracellular Ca(2+) concentration ([Ca(2+)](i)) and MCA dilation. Incubation with PACOCF(3), a PLA(2) inhibitor, significantly reduced both EC [Ca(2+)](i) and dilation responses to UTP. EC [Ca(2+)](i) was also partially reduced by a transient receptor potential vanilloid (TRPV) channel blocker, ruthenium red. Manganese quenching experiments demonstrated Ca(2+) influx across the luminal and abluminal face of the endothelium in response to UTP. Interestingly, PLA(2)-sensitive Ca(2+) influx occurred primarily across the abluminal face. Luminal application of arachidonic acid, the primary product of PLA(2) and a demonstrated activator of certain TRPV channels, increased both EC [Ca(2+)](i) and MCA diameter. TRPV4 mRNA and protein was demonstrated in the endothelium by RT-PCR and immunofluorescence, respectively. Finally, application of 4alpha-phorbol 12,13-didecanoate (4alphaPDD), a TRPV4 channel activator, produced an increase in EC [Ca(2+)](i) that was significantly reduced in the presence of ruthenium red. We conclude that PLA(2) is involved in EC Ca(2+) regulation through its regulation of TRPV4 channels. Furthermore, the PLA(2)-sensitive component of Ca(2+) influx may be polarized to the abluminal face of the endothelium.

Evidence against Calcium as a Mediator of Mitochondrial Dysfunction during Apoptosis Induced by Arachidonic Acid and Other Free Fatty Acids

Apoptosis is often accompanied by activation of phospholipase A(2), causing release of free fatty acids (FFAs), which in turn are thought to contribute to the loss of mitochondrial transmembrane potential (Deltapsi(m)). In these experiments, we asked whether calcium plays a role as an intermediate in this process. A total of 14 FFAs were compared for their ability to cause loss of Deltapsi(m) and for their ability to affect levels of intracellular calcium. Among the FFAs, unsaturated FFAs tended to induce apoptosis while saturated FFAs did not. Arachidonic acid (AA) was most damaging, causing loss of Deltapsi(m) and cell death in 8-10 h while linoleic acid, gamma-linolenic acid, and docosapentaenoic also strongly induced apoptosis. Effects of the FFAs on levels of intracellular calcium were very different. Many caused strong calcium responses; however, the ability to induce a strong calcium response was not predictive of ability to induce apoptosis, and overall, we did not find a correlation between apoptosis and calcium induction. Also, verapamil and TMB-8 were able to block the calcium response, but these inhibitors did not prevent loss of Deltapsi(m), indicating that the calcium response is not necessary for FFA-induced loss of Deltapsi(m). In contrast, we found that cyclosporine A could inhibit the AA-induced loss of Deltapsi(m) with both whole cells and isolated mitochondria, confirming that the antimitochondrial effects of FFA can stem from direct effects on the mitochondrial permeability transition pore. Finally, we show that the strong apoptosis-inducing activity of AA may stem from its ability to selectively induce its own release.

Interaction between TRPC channel subunits in endothelial cells

Transient Receptor Potential Canonical (TRPC) proteins have been identified in mammals as a family of plasma membrane calcium-permeable channels activated by different kinds of stimuli in several cell types. We have studied TRPC subunit expression in bovine aortic endothelial (BAE-1) cells, where stimulation with basic fibroblast growth factor (bFGF), a potent angiogenetic factor, induces calcium entry carried at least partially by TRPC1 channels. By means of a RT-PCR approach, we have found that, in addition to TRPC1, only TRPC4 is expressed, both at the mRNA and protein level, as confirmed by immunoblotting and immunocytochemical analysis. Because functional TRPC channels are formed by assembly of four subunits in either homo- or heterotetrameric structures, we have carried out immunoprecipitation experiments and showed that TRPC1 and TRPC4 interact to form heteromers in these cells, independently from culture conditions (high or low percent of fetal calf serum, stimulation with bFGF). Moreover, the data show that TRPC subunits are not tyrosine-phosphorylated after bFGF stimulation and they do not co-immunoprecipitate with the type 1 FGF receptor. These results suggest that BAE-1 cells are a suitable model to study function and regulation of endogenous TRPC1/TRPC4 heteromers.

Arachidonic acid influences intracellular calcium handling in human osteoblasts

The effect of arachidonic acid (AA) on intracellular Ca(2+) concentration ([Ca(2+)]i) in human osteoblasts MG63 was studied. AA caused a concentration-dependent increase in [Ca(2+)]i, mainly due to inward Ca(2+) transport from extracellular environment. Moreover, AA in Ca(2+) -free medium produced a small, transient increase of [Ca(2+)]i, indicating that AA may also trigger Ca(2+) release from intracellular stores. Because the [Ca(2+)]i response to AA was inhibited by the cyclooxygenase (COX) inhibitor indomethacin, we tested the effect of prostaglandins (PGs), products of COX pathway. PGs E1 and E2 caused an increase in [Ca(2+)]i, which, however, was far lower than that obtained with AA. The [Ca(2+)]i response to AA was not inhibited by nifedipine, suggesting that AA did not activate a voltage-dependent Ca(2+) channel. Our results indicate that AA could modulate [Ca(2+)]i in MG63 human osteoblasts, where it may influence Ca(2+) transport across both plasma and endoplasmic membranes. Furthermore, they suggest that osteoblast activity may be modulated by AA.

Arachidonic acid is a physiological activator of the ryanodine receptor in pancreatic β-cells

Pancreatic beta-cells have ryanodine receptors but little is known about their physiological regulation. Previous studies have shown that arachidonic acid releases Ca(2+) from intracellular stores in beta-cells but the identity of the channels involved in the Ca(2+) release has not been elucidated. We studied the mechanism by which arachidonic acid induces Ca(2+) concentration changes in pancreatic beta-cells. Cytosolic free Ca(2+) concentration was measured in fura-2-loaded INS-1E cells and in primary beta-cells from Wistar rats. The increase of cytosolic Ca(2+) concentration induced by arachidonic acid (150microM) was due to both Ca(2+) release from intracellular stores and influx of Ca(2+) from extracellular medium. 5,8,11,14-Eicosatetraynoic acid, a non-metabolizable analogue of arachidonic acid, mimicked the effect of arachidonic acid, indicating that arachidonic acid itself mediated Ca(2+) increase. The Ca(2+) release induced by arachidonic acid was from the endoplasmic reticulum since it was blocked by thapsigargin. 2-Aminoethyl diphenylborinate (50microM), which is known to inhibit 1,4,5-inositol-triphosphate-receptors, did not block Ca(2+) release by arachidonic acid. However, ryanodine (100microM), a blocker of ryanodine receptors, abolished the effect of arachidonic acid on Ca(2+) release in both types of cells. These observations indicate that arachidonic acid is a physiological activator of ryanodine receptors in beta-cells.

Ca2+ signaling, intracellular pH and cell volume in cell proliferation

Mitogens control progression through the cell cycle in non-transformed cells by complex cascades of intracellular messengers, such as Ca2+ and protons, and by cell volume changes. Intracellular Ca2+ and proton concentrations are critical for linking external stimuli to proliferation, motility, apoptosis and differentiation. This review summarizes the role in cell proliferation of calcium release from intracellular stores and the Ca2+ entry through plasma membrane Ca2+ channels. In addition, the impact of intracellular pH and cell volume on cell proliferation is discussed.

Calcium Signals Activated by Arachidonic Acid in Embryonic Chick Ciliary Ganglion Neurons

Arachidonic acid (AA, 20:4) has been reported to modulate a variety of calcium-permeable ionic channels, both in the plasma membrane and in the endoplasmic reticulum. We have studied the effects of AA on calcium signaling in a well-characterized model of developing peripheral neurons, embryonic chick ciliary ganglion neurons in culture. When given at low non-micellar concentrations (5 microM), in the majority of cells AA directly activated a delayed and long-lasting increase in [Ca2+]i, involving both the cytoplasm and the nucleoplasm, that was completely reversed by abolition of extracellular calcium. Other fatty acids (FAs), either saturated like arachidic acid (20:0), or unsaturated like linoleic (18:2) and docosahexaenoic acid (22:6), shared its ability to activate calcium influx. This entry was not suppressed by voltage-dependent calcium channel inhibitors omega-conotoxin and nifedipine, by the voltage-independent calcium channel antagonist LOE-908, by pre-treatment with blockers of AA metabolic pathways or with pertussis toxin. The arachidonate-activated calcium pathway was permeable to Mn2+ and blocked by La3+, Gd3+ and Ni2+. In a neuronal subpopulation, AA at the same concentration was also able to elicit calcium release from thapsigargin-sensitive intracellular stores; we provide evidence that cytochrome P450 epoxygenase is involved in this process.

Arachidonic acid mediates non-capacitative calcium entry evoked by CB1-cannabinoid receptor activation in DDT1 MF-2 smooth muscle cells

Cannabinoid CB1-receptor stimulation in DDT1 MF-2 smooth muscle cells induces a rise in [Ca2+]i, which is dependent on extracellular Ca2+ and modulated by thapsigargin-sensitive stores, suggesting capacitative Ca2+ entry (CCE), and by MAP kinase. Non-capacitative Ca2+ entry (NCCE) stimulated by arachidonic acid (AA) partly mediates histamine H1-receptor-evoked increases in [Ca2+]i in DDT1 MF-2 cells. In the current study, both Ca2+ entry mechanisms and a possible link between MAP kinase activation and increasing [Ca2+]i were investigated. In the whole-cell patch clamp configuration, the CB-receptor agonist CP 55, 940 evoked a transient, Ca2+-dependent K+ current, which was not blocked by the inhibitors of CCE, 2-APB, and SKF 96365. AA, but not its metabolites, evoked a transient outward current and inhibited the response to CP 55,940 in a concentration-dependent manner. CP 55,940 induced a concentration-dependent release of AA, which was inhibited by the CB1 antagonist SR 141716. The non-selective Ca2+ channel blockers La3+ and Gd3+ inhibited the CP 55,940-induced current at concentrations that had no effect on thapsigargin-evoked CCE. La3+ also inhibited the AA-induced current. CP 55,940-induced AA release was abolished by Gd3+ and by phospholipase A2 inhibition using quinacrine; this compound also inhibited the outward current. The CP 55,940-induced AA release was strongly reduced by the MAP kinase inhibitor PD 98059. The data suggest that in DDT1 MF-2 cells, AA is an integral component of the CB1 receptor signaling pathway, upstream of NCCE and, via PLA2, downstream of MAP kinase.

Regulation of noncapacitative calcium entry by arachidonic acid and nitric oxide in endothelial cells

Several peptides, including vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF), activate the release of arachidonic acid (AA) and nitric oxide (NO) in endothelial cells (ECs). Both messengers are involved in EC proliferation and vascular permeability and control calcium homeostasis in different ways. Interestingly, it has been recently suggested that NO acts as a downstream mediator of AA-induced calcium entry in smooth muscle cells and isolated mouse parotid cells. In this paper, we have investigated the complex relationships that link intracellular calcium, AA, and NO in cultured endothelial cells. Using different experimental approaches, mainly simultaneous Ca2+ and NO fluorimetric confocal imaging, we provide evidence for a complex pathway leading to noncapacitative calcium entry (NCCE) in bovine aortic endothelial cells (BAECs). In particular, AA is able to induce NCCE through two different pathways: one dependent on eNOS recruitment and NO release, the other NO-independent. Finally, we show that NO increase is involved in the control of BAEC proliferation.

Arachidonic acid inhibits the store-operated Ca2+ current in rat liver cells

Vasopressin and other phospholipase-C-coupled hormones induce oscillations (waves) of [Ca2+]cyt (cytoplasmic Ca2+ concentration) in liver cells. Maintenance of these oscillations requires replenishment of Ca2+ in intracellular stores through Ca2+ inflow across the plasma membrane. While this may be achieved by SOCs (store-operated Ca2+ channels), some studies in other cell types indicate that it is dependent on AA (arachidonic acid)-activated Ca2+ channels. We studied the effects of AA on membrane conductance of rat liver cells using whole-cell patch clamping. We found no evidence that concentrations of AA in the physiological range could activate Ca2+-permeable channels in either H4IIE liver cells or rat hepatocytes. However, AA (1-10 microM) did inhibit (IC50=2.4+/-0.1 microM) Ca2+ inflow through SOCs (ISOC) initiated by intracellular application of Ins(1,4,5)P3 in H4IIE cells. Pre-incubation with AA did not inhibit ISOC development, but decreased maximal amplitude of the current. Iso-tetrandrine, widely used to inhibit receptor-activation of phospholipase A2, and therefore AA release, inhibited ISOC directly in H4IIE cells. It is concluded that (i) in rat liver cells, AA does not activate an AA-regulated Ca2+-permeable channel, but does inhibit SOCs, and (ii) iso-tetrandrine and tetrandrine are effective blockers of CRAC (Ca2+-release-activated Ca2+) channel-like SOCs. These results indicate that AA-activated Ca2+-permeable channels do not contribute to hormone-induced increases or oscillations in [Ca2+]cyt in liver cells. However, AA may be a physiological modulator of Ca2+ inflow in these cells.

Arachidonic acid cascade in endothelial pathobiology

Arachidonic acid (AA) and its metabolites (eicosanoids) represent powerful mediators, used by organisms to induce and suppress inflammation as a part of the innate response to disturbances. Several cell types participate in the synthesis and release of AA metabolites, while many cell types represent the targets for eicosanoid action. Endothelial cells (EC), forming a semi-permeable barrier between the interior space of blood vessels and underlying tissues, are of particular importance for the development of inflammation, since endothelium controls such diverse processes as vascular tone, homeostasis, adhesion of platelets and leukocytes to the vascular wall, and permeability of the vascular wall for cells and fluids. Proliferation and migration of endothelial cells contribute significantly to new vessel development (angiogenesis). This review discusses endothelial-specific synthesis and action of arachidonic acid derivatives with a particular focus on the mechanisms of signal transduction and associated intracellular protein targets.

Intracellular calcium signals and control of cell proliferation: how many mechanisms?

The progression through the cell cycle in non-transformed cells is under the strict control of extracellular signals called mitogens, that act by eliciting complex cascades of intracellular messengers. Among them, increases in cytosolic free calcium concentration have been long realized to play a crucial role; however, the mechanisms coupling membrane receptor activation to calcium signals are still only partially understood, as are the pathways of calcium entry in the cytosol. This article centers on the role of calcium influx from the extracellular medium in the control of proliferative processes, and reviews the current understanding of the pathways responsible for this influx and of the second messengers involved in their activation.

ARC Channels: A Novel Pathway for Receptor-Activated Calcium Entry

In many nonexcitable cells, stimulation with low agonist concentrations specifically activates Ca2+entry via arachidonic acid-regulated, highly Ca2+-selective ARC channels. Only at high agonist concentrations are the more widely studied store-operated channels activated, producing sustained elevated cytosolic Ca2+concentration signals. These signals activate calcineurin, which in turn inhibits the ARC channels, resulting in a “reciprocal regulation” of these two distinct Ca2+-entry pathways that may have important functional implications for the cell.

Anandamide initiates Ca(2+) signaling via CB2 receptor linked to phospholipase C in calf pulmonary endothelial cells

The endocannabinoid anandamide has been reported to affect neuronal cells, immune cells and smooth muscle cells via either CB1 or CB2 receptors. In endothelial cells, the receptors involved in activating signal transduction are still unclear, despite the fact that anandamide is produced in this cell type. The present study was designed to explore in detail the effect of this endocannabinoid on Ca2+ signaling in single cells of a calf pulmonary endothelial cell line. Anandamide initiated a transient Ca2+ elevation that was prevented by the CB2 receptor antagonist SR144528, but not by the CB1 antagonist SR141716A. These data were confirmed by molecular identification of the bovine CB2 receptor in these endothelial cells by partial sequencing. The phospholipase C inhibitor 1-[6-[[(17beta)-3-methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl]-1H-pyrrole-2,5dione and the inositol 1,4,5-trisphosphate receptor antagonist 2-aminoethoxydiphenylborate prevented Ca2+ signaling in response to anandamide. Using an improved cameleon probe targeted to the endoplasmic reticulum (ER), fura-2 and ratiometric-pericam, which is targeted to the mitochondria, anandamide was found to induce Ca2+ depletion of the ER accompanied by the activation of capacitative Ca2+ entry (CCE) and a transient elevation of mitochondrial Ca2+. These data demonstrate that anandamide stimulates the endothelial cells used in this study via CB2 receptor-mediated activation of phospholipase C, formation of inositol 1,4,5-trisphosphate, Ca2+ release from the ER and subsequent activation of CCE. Moreover, the cytosolic Ca2+ elevation was accompanied by a transient Ca2+ increase in the mitochondria. Thus, in addition to its actions on smooth muscle cells, anandamide also acts as a powerful stimulus for endothelial cells.

Control of endothelial cell proliferation by calcium influx and arachidonic acid metabolism: A pharmacological approach

In physiological conditions, endothelial cell proliferation is strictly controlled by several growth factors, among which bFGF and VEGF are the most effective. Both bind to specific tyrosine kinase receptors and trigger intracellular signal cascades. In particular, bFGF stimulates the release of arachidonic acid (AA) and its metabolites in many types of endothelial cells in culture. In bovine aortic endothelial cells, it has been suggested that AA is released by the recruitment of cytosolic phospholipase A2 (cPLA2). AA metabolites are involved in the control of both endothelial cell motility (mostly via the cyclooxygenase pathway) and proliferation (via the lipoxygenase (LOX) cascade). On the other hand, evidence has been provided for a proliferative role of AA-induced calcium influx. By using a pharmacological approach, we have tried to elucidate the contribution to bovine aortic endothelial proliferation of the different pathways leading to production of AA and its metabolites. Two main informations were obtained by our experiments: first, AA release is not entirely due to cPLA2 involvement, but also to DAG lipase recruitment; second, cyclooxygenase derivatives play a role in the control of cell proliferation, and not only of motility. Moreover, by combining proliferation assays and single cell calcium measurements, we show that the blocking effect of carboxyamido-triazole (CAI), an inhibitor of tumor growth and angiogenesis acting on calcium influx-dependent pathways, including AA metabolism, is at least in part due to a direct effect on AA-induced calcium influx.

Regulation of TRP channels via lipid second messengers

In Drosophila photoreceptors, the light-sensitive current is mediated downstream of phospholipase C by TRP (transient receptor potential) channels. Recent evidence suggests that Drosophila TRP channels are activated by diacylglycerol (DAG) or its metabolites (polyunsaturated fatty acids), possibly in combination with the reduction in phosphatidyl inositol 4,5 bisphosphate (PIP2). Consistent with this view, diacylglycerol kinase is identified as a key enzyme required for response termination. Signaling is critically dependent upon efficient PIP2 synthesis; mutants of this pathway in combination with genetically targeted PIP2 reporters provide unique insights into the kinetics and regulation of PIP2 turnover. Recent evidence indicates that a growing number of mammalian TRP homologues are also regulated by lipid messengers, including DAG, arachidonic acid, and PIP2.

Arachidonic acid in astrocytes blocks Ca(2+) oscillations by inhibiting store-operated Ca(2+) entry, and causes delayed Ca(2+) influx

ATP-elicited oscillations of the concentration of free intracellular Ca(2+) ([Ca(2+)](i)) in rat brain astrocytes were abolished by simultaneous arachidonic acid (AA) addition, whereas the tetraenoic analogue 5,8,11,14-eicosatetraynoic acid (ETYA) was ineffective. Inhibition of oscillations is due to suppression by AA of intracellular Ca(2+) store refilling. Short-term application of AA, but not ETYA, blocked Ca(2+) influx, which was evoked by depletion of stores with cyclopiazonic acid (CPA) or thapsigargin (Tg). Addition of AA after ATP blocked ongoing [Ca(2+)](i) oscillations. Prolonged AA application without or with agonist could evoke a delayed [Ca(2+)](i) increase. This AA-induced [Ca(2+)](i) rise developed slowly, reached a plateau after 5 min, could be reversed by addition of bovine serum albumin (BSA), that scavenges AA, and was blocked by 1 microM Gd(3+), indicative for the influx of extracellular Ca(2+). Specificity for AA as active agent was demonstrated by ineffectiveness of C16:0, C18:0, C20:0, C18:2, and ETYA. Moreover, the action of AA was not affected by inhibitors of oxidative metabolism of AA (ibuprofen, MK886, SKF525A). Thus, AA exerted a dual effect on astrocytic [Ca(2+)](i), firstly, a rapid reduction of capacitative Ca(2+) entry thereby suppressing [Ca(2+)](i) oscillations, and secondly inducing a delayed activation of Ca(2+) entry, also sensitive to low Gd(3+) concentration.

Expression and functional role of bTRPC1 channels in native endothelial cells

We have analyzed the expression and localization of bovine transient receptor potential-C1 (bTRPC1) in bovine aortic endothelial cells, and its possible involvement in the store-independent calcium influx induced by basic fibroblast growth factor (bFGF). RT-PCR experiments confirmed the existence of two btrpc1 mRNA isoforms; conversely, the btrpc3 gene was not transcribed. Anti-TRPC1 antibody revealed the presence of the protein in the membrane-rich compartment only. Application of anti-TRPC1 during the response to bFGF caused a partial but significant reduction of calcium entry. This is the first evidence of TRP channel involvement in a non-capacitative calcium influx induced by a biologically relevant agonist such as the angiogenic factor bFGF in native endothelial cells.