EGFR kinase regulates volume-sensitive chloride current elicited by integrin stretch via PI-3K and NADPH oxidase in ventricular myocytes

Acute and chronic adrenergic stimulation of submandibular salivary glands. Effects on the endocrine function of epidermal growth factor in mice

Submandibular salivary glands are the major source of epidermal growth factor (EGF) in mice. Acute secretion of EGF from these glands protects the heart against catecholamine-induced injury. Little is known about chronic adrenergic stimulation of salivary glands and the contribution of accumulated EGF to the adaptive hypertrophic response of the heart to such chronic adrenergic stimulation. Here we show that the EGF content of submandibular glands did not recover to normal values 24 h after a single phenylephrine injection or an aggressive encounter. Repeated (twice a day for 2 days) adrenergic stimulation resulted in an almost 90% decrease in EGF content in the submandibular glands. In these conditions, new adrenergic stimulation did not result in an increase in plasma EGF concentration, or in the activation of liver ErbB1 (the EGF receptor). Chronic isoproterenol or phenylephrine administration (7 days) induced atrial natriuretic factor expression in the heart and an increase in both ventricular weight and protein. The surgical removal of submandibular glands (sialoadenectomy) did not affect these adaptive responses of the heart. We conclude that EGF from submandibular glands does not contribute to heart hypertrophy, one of the adaptive responses induced by chronic adrenergic stimulation.

ErbB receptors protect the perfused heart against injury induced by epinephrine combined with low-flow ischemia

ErbB receptor tyrosine kinases are important in maintaining the long-term structural integrity of the heart and in the induction of hypertrophy. In addition, in vivo activation of ErbB1 by epidermal growth factor (EGF) protects the heart against acute stress-induced damage. We examined here whether the ErbB sytem acutely protects the isolated heart in which stress was induced in vitro by ischemia combined with epinephrine infusion (EPI). In perfused mouse hearts, EGF induced Tyr-phosphorylation of ErbB1 but not ErbB2. Neuregulin-1beta (NRG-1beta) induced Tyr-phosphorylation of both ErbB4 and ErbB2. We also found differences in the signaling cascades activated by each growth factor. To stress the perfused mouse heart, we combined EPI with low-flow ischemia. This resulted in (i) loss of left ventricle contraction force ( + dP/dt(max)) and developed pressure (LVDP) after a short period of hypercontractility, (ii) enhanced anaerobic metabolism (lactate production), and (iii) myocyte injury (lactate dehydrogenase (LDH) release). EGF and NRG-1beta had different effects on stressed-heart contractility. EGF reduced to a half the loss of both + dP/dt(max) and LVDP. In contrast, NRG-1beta exacerbated the hypercontractility soon after reperfusion. This is coincident with a transient increase in coronary flow after reperfusion. In spite of these differences in contraction, both EGF and NRG-1beta induced similar early protection as shown by the reduction of LDH release. Our results show that the ErbB system protects the perfused heart against damage induced by acute stress. They reinforce the relevance of ErbB receptors and ligands in cardiac physiology.

Reduced volume-regulated outwardly rectifying anion channel activity in ventricular myocyte of type 1 diabetic mice

The currents through the volume-regulated outwardly rectifying anion channel (VRAC) were measured in single ventricular myocytes obtained from streptozotocin (STZ)-induced diabetic mice, using whole-cell voltage-clamp method. In myocytes from STZ-diabetic mice, the density of VRAC current induced by hypotonic perfusion was markedly reduced, compared with that in the cells form normal control mice. Video-image analysis showed that the regulatory volume decrease (RVD), which was seen in normal cells after osmotic swelling, was almost lost in myocytes from STZ-diabetic mice. Some mice were pretreated with 3-O-methylglucose before STZ injection, to prevent the STZ's beta cell toxicity. In the myocytes obtained from such mice, the magnitude of VRAC current and the degree of RVD seen during hypotonic challenge were almost normal. Incubation of the myocytes from STZ-diabetic mice with insulin reversed the attenuation of VRAC current. These findings suggested that the STZ-induced chronic insulin-deficiency was an important causal factor for the attenuation of VRAC current. Intracellular loading of the STZ-diabetic myocytes with phosphatidylinositol 3,4,5-trisphosphate (PIP3), but not phosphatidylinositol 4,5-bisphosphate (PIP2), also reversed the attenuation of VRAC current. Furthermore, treatment of the normal cells with wortmannin, a phosphatidylinositol 3-kinase (PI3K) inhibitor, suppressed the development of VRAC current. We postulate that an impairment PI3K-PIP3 pathway, which may be insulin-dependent, is responsible for the attenuation of VRAC currents in STZ-diabetic myocytes.

Physiology of cell volume regulation in vertebrates

The ability to control cell volume is pivotal for cell function. Cell volume perturbation elicits a wide array of signaling events, leading to protective (e.g., cytoskeletal rearrangement) and adaptive (e.g., altered expression of osmolyte transporters and heat shock proteins) measures and, in most cases, activation of volume regulatory osmolyte transport. After acute swelling, cell volume is regulated by the process of regulatory volume decrease (RVD), which involves the activation of KCl cotransport and of channels mediating K(+), Cl(-), and taurine efflux. Conversely, after acute shrinkage, cell volume is regulated by the process of regulatory volume increase (RVI), which is mediated primarily by Na(+)/H(+) exchange, Na(+)-K(+)-2Cl(-) cotransport, and Na(+) channels. Here, we review in detail the current knowledge regarding the molecular identity of these transport pathways and their regulation by, e.g., membrane deformation, ionic strength, Ca(2+), protein kinases and phosphatases, cytoskeletal elements, GTP binding proteins, lipid mediators, and reactive oxygen species, upon changes in cell volume. We also discuss the nature of the upstream elements in volume sensing in vertebrate organisms. Importantly, cell volume impacts on a wide array of physiological processes, including transepithelial transport; cell migration, proliferation, and death; and changes in cell volume function as specific signals regulating these processes. A discussion of this issue concludes the review.

Thyroid hormone-induced angiogenesis

A series of reports in the past decade have ascribed pro-angiogenic activity to several thyroid hormone analogues, including L-thyroxine (T(4)), 3,5,3-triiodo-L-thyronine (T(3)) and diiodothyropropionic acid (DITPA). Model systems of angiogenesis have demonstrated that thyroid hormone-induced neovascularization is initiated at a cell surface receptor for the hormone on an integrin. The hormone signal is transduced within the cell by extracellular regulated kinase 1/2 (ERK1/2) into secretion of basic fibroblast growth factor (bFGF) and other vascular growth factors and consequent angiogenesis. Intact animal studies have shown that endogenous thyroid hormone supports blood vessel density in heart and brain and that thyroid hormone administration can induce angiogenesis in ischemic limbs.

Cell volume regulation: physiology and pathophysiology

Cell volume perturbation initiates a wide array of intracellular signalling cascades, leading to protective and adaptive events and, in most cases, activation of volume-regulatory osmolyte transport, water loss, and hence restoration of cell volume and cellular function. Cell volume is challenged not only under physiological conditions, e.g. following accumulation of nutrients, during epithelial absorption/secretion processes, following hormonal/autocrine stimulation, and during induction of apoptosis, but also under pathophysiological conditions, e.g. hypoxia, ischaemia and hyponatremia/hypernatremia. On the other hand, it has recently become clear that an increase or reduction in cell volume can also serve as a specific signal in the regulation of physiological processes such as transepithelial transport, cell migration, proliferation and death. Although the mechanisms by which cell volume perturbations are sensed are still far from clear, significant progress has been made with respect to the nature of the sensors, transducers and effectors that convert a change in cell volume into a physiological response. In the present review, we summarize recent major developments in the field, and emphasize the relationship between cell volume regulation and organism physiology/pathophysiology.

Plasmodium falciparum regulatory subunit of cAMP-dependent PKA and anion channel conductance

Malaria symptoms occur during Plasmodium falciparum development into red blood cells. During this process, the parasites make substantial modifications to the host cell in order to facilitate nutrient uptake and aid in parasite metabolism. One significant alteration that is required for parasite development is the establishment of an anion channel, as part of the establishment of New Permeation Pathways (NPPs) in the red blood cell plasma membrane, and we have shown previously that one channel can be activated in uninfected cells by exogenous protein kinase A. Here, we present evidence that in P. falciparum-infected red blood cells, a cAMP pathway modulates anion conductance of the erythrocyte membrane. In patch-clamp experiments on infected erythrocytes, addition of recombinant PfPKA-R to the pipette in vitro, or overexpression of PfPKA-R in transgenic parasites lead to down-regulation of anion conductance. Moreover, this overexpressing PfPKA-R strain has a growth defect that can be restored by increasing the levels of intracellular cAMP. Our data demonstrate that the anion channel is indeed regulated by a cAMP-dependent pathway in P. falciparum-infected red blood cells. The discovery of a parasite regulatory pathway responsible for modulating anion channel activity in the membranes of P. falciparum-infected red blood cells represents an important insight into how parasites modify host cell permeation pathways. These findings may also provide an avenue for the development of new intervention strategies targeting this important anion channel and its regulation.

By replicating within red blood cells malaria parasites are largely hidden from immune recognition, but within mature erythrocytes nutrients are limiting and accumulation of potentially hazardous metabolic end products can rapidly become critical. In order to survive within red blood cells malaria parasites, therefore, alter the permeability of the erythrocyte plasma membrane either by up-regulating existing carriers, or by creating new permeation pathways. Recent electrophysiological studies of Plasmodium-infected erythrocytes have demonstrated that these changes reflect trans-membrane transport through ion channels in the infected erythrocyte plasma membrane. Protein phosphorylation has been documented in protozoan parasites for a number of years and is implicated in key processes of both parasites and parasitized host cells. It has been established that cAMP-dependent regulated pathways are able to activate ion channels in the red cell membrane and a better understanding of how the parasite manipulates cAMP-dependent signaling to activate anion channels could be important in developing novel strategies for future anti-malarial chemotherapies.

Autocrine signaling involved in cell volume regulation: the role of released transmitters and plasma membrane receptors

Cell volume regulation is a basic homeostatic mechanism transcendental for the normal physiology and function of cells. It is mediated principally by the activation of osmolyte transport pathways that result in net changes in solute concentration that counteract cell volume challenges in its constancy. This process has been described to be regulated by a complex assortment of intracellular signal transduction cascades. Recently, several studies have demonstrated that alterations in cell volume induce the release of a wide variety of transmitters including hormones, ATP and neurotransmitters, which have been proposed to act as extracellular signals that regulate the activation of cell volume regulatory mechanisms. In addition, changes in cell volume have also been reported to activate plasma membrane receptors (including tyrosine kinase receptors, G-protein coupled receptors and integrins) that have been demonstrated to participate in the regulatory process of cell volume. In this review, we summarize recent studies about the role of changes in cell volume in the regulation of transmitter release as well as in the activation of plasma membrane receptors and their further implications in the regulation of the signaling machinery that regulates the activation of osmolyte flux pathways. We propose that the autocrine regulation of Ca2+-dependent and tyrosine phosphorylation-dependent signaling pathways by the activation of plasma membrane receptors and swelling-induced transmitter release is necessary for the activation/regulation of osmolyte efflux pathways and cell volume recovery. Furthermore, we emphasize the importance of studying these extrinsic signals because of their significance in the understanding of the physiology of cell volume regulation and its role in cell biology in vivo, where the constraint of the extracellular space might enhance the autocrine or even paracrine signaling induced by these released transmitters.

Stretch-activated currents in cardiomyocytes isolated from rabbit pulmonary veins

Evidence is growing of a relationship between atrial dilation and atrial fibrillation (AF), the most prevalent type of arrhythmia. Pulmonary veins, which are important ectopic foci for provoking AF, are of increasing interest in relation to the early development of AF. Here, using single cardiomyocytes isolated from rabbit pulmonary veins, we characterised the stretch-activated currents induced by swelling and axial mechanical stretching. Swelling induced both a stretch-activated nonselective cationic current (NSC) and a Cl(-) current. The swelling-induced Cl(-) current (I Cl,swell) was inhibited by DIDS, whereas the swelling-induced NSC (I NSC,swell) was inhibited by Gd3+. The cationic selectivity of the I NSC,swell was K+ >Cs+ >Na+ >Li+, whilst the PK/PNa, PCs/PNa, and PLi/PNa permeability ratios were 2.84, 1.86, and 0.85, respectively. Activation of the I NSC,swell was faster than that of the I Cl,swell. Given a high K+ concentration in the bath solution, the I NSC,swell showed limited amplitude (<-70 mV). Mechanical stretching induced an immediate Gd3+- and streptomycin-sensitive NSC (I NSC,stretch) that was permeable to Na+, K+, Cs+ and NMDG. Persistent stretching activated a DIDS-sensitive current (I Cl,stretch). The I NSC,stretch, but not the I NSC,swell, was completely blocked by 400 microM streptomycin; therefore, the two currents may not be associated with the same channel. In addition, the type of current induced may depend on the type of stretching. Thus, stretch-induced anionic and cationic currents are functionally present in the cardiomyocytes of the main pulmonary veins of rabbits, and they may have pathophysiological roles in the development of AF under stretched conditions.

Bending of z-lines by mechanical stimuli: an input signal for integrin dependent modulation of ion channels?

We studied which components of mechanical cell deformation are involved in "stretch modulated ion currents" (SMIC). Murine ventricular myocytes were attached to glass coverslips and deformed in x, y and z with a 16 microm thin glass stylus (S) of calibrated stiffness. Three-dimensional confocal microscopy characterized cell deformation (T-tubular membranes, mitochondria) and bending of S (indicative of the applied force). Axial (x-) displacement of S sheared the upper cell part versus the attached bottom, close to S, it changed sarcomere length and bent z-lines ("z-line displacement"). Vertical (z-press) or transversal (y-shear) displacement of S bulged cytoplasm and mitochondria transversally without detectable z-line displacement. Axial stiffness increased with the extent of stress ("stress stiffening"). Depolymerization of F-actin or block of integrin receptors reduced stiffness. SMIC served as a proxy readout of deformation-induced signaling. Axial deformation activated a non-selective cation conductance (Gns) and deactivated an inwardly rectifying K+ conductance (GK1), z-press or y-shear did not induce SMIC. Depolymerization of F-actin or block of integrin receptors reduced SMIC. SMIC did not depend on changes in sarcomere length but correlated with the extent of z-line bending. We discuss that both shear stress at the attached cell bottom and z-line bending could activate mechanosensors. Since SMIC was absent during deformations without z-line bending we postulate that z-line bending is a necessary component for SMIC signaling.

Regulation of swelling-activated Cl(-) current by angiotensin II signalling and NADPH oxidase in rabbit ventricle

AIMS

We assessed whether hypoosmotic swelling of cardiac myocytes activates volume-sensitive Cl(-) current (I Cl,swell) via the angiotensin II (AngII)-reactive oxygen species (ROS) signalling cascade. The AngII-ROS pathway previously was shown to elicit I(Cl,swell) upon mechanical stretch of beta(1D) integrin. Integrin stretch and osmotic swelling are, however, distinct stimuli. For example, blocking Src kinases stimulates swelling-induced but inhibits stretch-induced I Cl,swell.

METHODS AND RESULTS

I Cl,swell was measured in rabbit ventricular myocytes by whole-cell voltage clamp. Swelling-induced I Cl,swell was completely blocked by losartan and eprosartan, AngII type I receptor (AT1) antagonists. AT1 stimulation transactivates epidermal growth factor receptor (EGFR) kinase. Blockade of EGFR kinase with AG1478 abolished both I Cl,swell and AngII-induced Cl(-) current, whereas exogenous EGF evoked a Cl(-) current that was suppressed by osmotic shrinkage. Phosphatidylinositol 3-kinase (PI-3K) is downstream of EGFR kinase, and PI-3K inhibitors LY294002 and wortmannin blocked I Cl,swell. Ultimately, AngII signals via NADPH oxidase (NOX) and superoxide anion, O2*. NOX inhibitors, diphenyleneiodonium, apocynin and gp91ds-tat, eliminated I Cl,swell, whereas scramb-tat, an inactive gp91ds-tat analogue, was ineffective. O2* rapidly dismutates to H2O2. Consistent with H2O2 being a downstream effector, catalase inhibited I Cl,swell, and exogenous H2O2 overcame suppression of I Cl,swell by AT1 receptor, EGFR kinase, and PI-3K blockers. H2O2-induced current was not blocked by osmotic shrinkage, however.

CONCLUSION

Activation of I Cl,swell by osmotic swelling is controlled by the AngII-ROS cascade, the same pathway previously implicated in I Cl,swell activation by integrin stretch. This in part explains why I Cl,swell is persistently activated in several models of cardiac disease.

Osmosensing by integrins in rat liver

Changes in hepatocyte hydration are induced not only by ambient hypo- or hyperosmolarity, but also under isosmotic condition by hormones, substrates, and oxidative stress. The perfused rat liver is a well-established intact organ model with preservation of the three-dimensional hepatocyte anchoring to the extracellular matrix and/or adjacent cells, parenchymal cell polarity, liver cell heterogeneity, acinar construction, and gene expression gradients. Originally, data from the perfused rat liver indicated that changes of cell hydration independent of their origin critically contribute to the control of autophagic proteolysis and canalicular bile acid excretion. Meanwhile, the concept that cell hydration changes trigger signal transduction processes that control metabolism, gene expression, transport, and the susceptibility to stress is well accepted. This chapter summarizes evidence obtained from experiments with the perfused rat liver that integrins are osmosensors in the liver and thereby critically contribute to the Src- and MAP-kinase-dependent inhibition of autophagic proteolysis, stimulation of canalicular taurocholate excretion, and regulatory volume decrease as induced by hypoosmotic swelling. Moreover, integrin-dependent sensing of hepatocyte swelling is essential for signaling and proteolysis inhibition by insulin and glutamine. These findings define a novel role of integrins in insulin and glutamine signaling and set an example for mechanotransduction as an integral part of overall growth factor and nutrient signaling.