Mechanical stretch induces hypertrophic responses in cardiac myocytes of angiotensin II type 1a receptor knockout mice

Cell-stretching devices: advances and challenges in biomedical research and live-cell imaging

Basic human functions such as breathing and digestion require mechanical stretching of cells and tissues. However, when it comes to laboratory experiments, the mechanical stretching that cells experience in the body is not often replicated, limiting the biomimetic nature of the studies and the relevance of results. Herein, we establish the importance of mechanical stretching during in vitro investigations by reviewing seminal works performed using cell-stretching platforms, highlighting important outcomes of these works as well as the engineering characteristics of the platforms used. Emphasis is placed on the compatibility of cell-stretching devices (CSDs) with live-cell imaging as well as their limitations and on the research advancements that could arise from live-cell imaging performed during cell stretching.

Dasatinib targets c-Src kinase in cardiotoxicity

Dasatinib is a multitargeted kinase inhibitor used for treatment of chronic myeloid leukemia and acute lymphoblastic leukemia. Unfortunately, treatment of cancer patients with some kinase inhibitors has been associated with cardiotoxicity. Cancer treatment with dasatinib has been reported to be associated with cardiotoxic side effects such as left ventricular dysfunction, heart failure, pericardial effusion and pulmonary hypertension. Here we aimed to investigate the molecular mechanisms underlying the cardiotoxicity of dasatinib. We found that among the resident cardiac cell types, cardiomyocytes were most sensitive to dasatinib-induced cell death. Exposure of cardiomyocytes to dasatinib attenuated the activity of extracellular signal-regulated kinase (ERK), which is a downstream target of dasatinib target kinase c-Src. Similar to dasatinib, c-Src depletion in cardiomyocytes compromised cardiomyocyte viability. Overexpression of dasatinib-resistant mutant of c-Src rescued the toxicity of dasatinib on cardiomyocytes, whereas forced expression of wild type c-Src did not have protective effect. Collectively, our results show that c-Src is a key target of dasatinib mediating the toxicity of dasatinib to cardiomyocytes. These findings may influence future drug design and suggest closer monitoring of patients treated with agents targeting c-Src for possible adverse cardiac effects.

Cyclic stretch increases mitochondrial biogenesis in a cardiac cell line

Unlike stable and immobile cell line conditions, animal hearts contract and relax to pump blood throughout the body. Mitochondria play an essential role by producing biological energy molecules to maintain heart function. In this study, we assessed the effect of heart mimetic cyclic stretch on mitochondria in a cardiac cell line. To mimic the geometric and biomechanical conditions surrounding cells in vivo, cyclic stretching was performed on HL-1 murine cardiomyocytes seeded onto an elastic micropatterned substrate (10% elongation, 0.5 Hz, 4 h/day). Cell viability, semi-quantitative Q-PCR, and western blot analyses were performed in non-stimulated control and cyclic stretch stimulated HL-1 cell lines. Cyclic stretch significantly increased the expression of mitochondria biogenesis-related genes (TUFM, TFAM, ERRα, and PGC1-α) and mitochondria oxidative phosphorylation-related genes (PHB1 and CYTB). Western blot analysis confirmed that cyclic stretch increased protein levels of mitochondria biogenesis-related proteins (TFAM, and ERRα) and oxidative phosphorylation-related proteins (NDUFS1, UQCRC, and PHB1). Consequently, cyclic stretch increased mitochondrial mass and ATP production in treated cells. Our results suggest that cyclic stretch transcriptionally enhanced mitochondria biogenesis and oxidative phosphorylation without detrimental effects in a cultured cardiac cell line.

Size uniformity of animal cells is actively maintained by a p38 MAPK-dependent regulation of G1-length

Animal cells within a tissue typically display a striking regularity in their size. To date, the molecular mechanisms that control this uniformity are still unknown. We have previously shown that size uniformity in animal cells is promoted, in part, by size-dependent regulation of G1 length. To identify the molecular mechanisms underlying this process, we performed a large-scale small molecule screen and found that the p38 MAPK pathway is involved in coordinating cell size and cell cycle progression. Small cells display higher p38 activity and spend more time in G1 than larger cells. Inhibition of p38 MAPK leads to loss of the compensatory G1 length extension in small cells, resulting in faster proliferation, smaller cell size and increased size heterogeneity. We propose a model wherein the p38 pathway responds to changes in cell size and regulates G1 exit accordingly, to increase cell size uniformity.

Dynamic culture yields engineered myocardium with near-adult functional output

Engineered cardiac tissues hold promise for cell therapy and drug development, but exhibit inadequate function and maturity. In this study, we sought to significantly improve the function and maturation of rat and human engineered cardiac tissues. We developed dynamic, free-floating culture conditions for engineering "cardiobundles", 3-dimensional cylindrical tissues made from neonatal rat cardiomyocytes or human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs) embedded in fibrin-based hydrogel. Compared to static culture, 2-week dynamic culture of neonatal rat cardiobundles significantly increased expression of sarcomeric proteins, cardiomyocyte size (∼2.1-fold), contractile force (∼3.5-fold), and conduction velocity of action potentials (∼1.4-fold). The average contractile force per cross-sectional area (59.7 mN/mm²) and conduction velocity (52.5 cm/s) matched or approached those of adult rat myocardium, respectively. The inferior function of statically cultured cardiobundles was rescued by transfer to dynamic conditions, which was accompanied by an increase in mTORC1 activity and decline in AMPK phosphorylation and was blocked by rapamycin. Furthermore, dynamic culture effects did not stimulate ERK1/2 pathway and were insensitive to blockers of mechanosensitive channels, suggesting increased nutrient availability rather than mechanical stimulation as the upstream activator of mTORC1. Direct comparison with phenylephrine treatment confirmed that dynamic culture promoted physiological cardiomyocyte growth rather than pathological hypertrophy. Optimized dynamic culture conditions also augmented function of human cardiobundles made reproducibly from cardiomyocytes derived from multiple hPSC lines, resulting in significantly increased contraction force (∼2.5-fold) and conduction velocity (∼1.4-fold). The average specific force of 23.2 mN/mm² and conduction velocity of 25.8 cm/s approached the functional metrics of adult human myocardium. In conclusion, we have developed a versatile methodology for engineering cardiac tissues with a near-adult functional output without the need for exogenous electrical or mechanical stimulation, and have identified mTOR signaling as an important mechanism for advancing tissue maturation and function in vitro.

Collagen regulates transforming growth factor-β receptors of HL-1 cardiomyocytes through activation of stretch and integrin signaling

The extracellular matrix (ECM) and transforming growth factor-β (TGF)-β are important in cardiac fibrosis, however, the effects of the ECM on TGF‑β signaling remain to be fully elucidated. The aims of the present study were to evaluate the role of collagen in TGF‑β signaling and examine the underlying mechanisms. In the present study, western blot analysis was used to examine TGF‑β signaling in HL‑1 cells treated with and without (control) type I collagen (10 µg/ml), which was co‑administered with either an anti‑β1 integrin antibody (10 µg/ml) or a stretch‑activated channel inhibitor (gadolinium; 50 µM). Cell proliferation and adhesion assays were used to investigate the roles of integrin, mechanical stretch and mitogen‑activated protein kinases (MAPKs) on cell proliferation and adhesion. The type I collagen (10 µg/ml)‑treated HL‑1 cells were incubated with or without anti‑β1 integrin antibody (10 µg/ml), gadolinium (50 µM) or inhibitors of p38 (SB203580; 3 µM), extracellular signal‑regulated kinase (ERK; PD98059; 50 µM) and c‑Jun N‑terminal kinase (JNK; SP600125; 50 µM). Compared with the control cells, the collagen‑treated HL‑1 cells had lower expression levels of type I and type II TGF‑β receptors (TGFβRI and TGFβRII), with an increase in phosphorylated focal adhesion kinase (FAK), p38 and ERK1/2, and a decrease in JNK. Incubation with the anti‑β1 integrin antibody reversed the collagen‑induced downregulation of the expression of TGFβRII and phosphorylated FAK. Gadolinium downregulated the expression levels of TGFβRI and small mothers against decapentaplegic (Smad)2/3, and decreased the levels of phosphorylated p38, ERK1/2 and JNK. In addition, gadolinium reversed the collagen‑induced activation of p38 and ERK1/2. In the presence of gadolinium and anti‑β1 integrin antibody, collagen regulated the expression levels of TGFβRI, TGFβRII and Smad2/3, but did not alter the phosphorylation of p38, ERK1/2 or JNK. In addition, collagen increased cell proliferation and adhesion, and this collagen‑induced cell proliferation was inhibited by the anti‑β1 integrin antibody and ERK inhibitor. Taken together, the data obtained suggested that collagen differentially regulated the expression levels of TGFβRI and TGFβRII, and modulated the phosphorylation of MAPKs through integrin‑ or stretch‑dependent and ‑independent signaling pathways.

Stretch-activation of angiotensin II type 1a receptors contributes to the myogenic response of mouse mesenteric and renal arteries

RATIONALE

Vascular wall stretch is the major stimulus for the myogenic response of small arteries to pressure. The molecular mechanisms are elusive, but recent findings suggest that G protein-coupled receptors can elicit a stretch response.

OBJECTIVE

To determine whether angiotensin II type 1 receptors (AT1R) in vascular smooth muscle cells exert mechanosensitivity and identify the downstream ion channel mediators of myogenic vasoconstriction.

METHODS AND RESULTS

We used mice deficient in AT1R signaling molecules and putative ion channel targets, namely AT1R, angiotensinogen, transient receptor potential channel 6 (TRPC6) channels, or several subtypes of the voltage-gated K+ (Kv7) gene family (KCNQ3, 4, or 5). We identified a mechanosensing mechanism in isolated mesenteric arteries and in the renal circulation that relies on coupling of the AT1R subtype a to a Gq/11 protein as a critical event to accomplish the myogenic response. Arterial mechanoactivation occurs after pharmacological block of AT1R and in the absence of angiotensinogen or TRPC6 channels. Activation of AT1R subtype a by osmotically induced membrane stretch suppresses an XE991-sensitive Kv channel current in patch-clamped vascular smooth muscle cells, and similar concentrations of XE991 enhance mesenteric and renal myogenic tone. Although XE991-sensitive KCNQ3, 4, and 5 channels are expressed in vascular smooth muscle cells, XE991-sensitive K+ current and myogenic contractions persist in arteries deficient in these channels.

CONCLUSIONS

Our results provide definitive evidence that myogenic responses of mouse mesenteric and renal arteries rely on ligand-independent, mechanoactivation of AT1R subtype a. The AT1R subtype a signal relies on an ion channel distinct from TRPC6 or KCNQ3, 4, or 5 to enact vascular smooth muscle cell activation and elevated vascular resistance.

Promyelocytic leukemia zinc finger protein activates GATA4 transcription and mediates cardiac hypertrophic signaling from angiotensin II receptor 2

BACKGROUND

Pressure overload and prolonged angiotensin II (Ang II) infusion elicit cardiac hypertrophy in Ang II receptor 1 (AT(1)) null mouse, whereas Ang II receptor 2 (AT(2)) gene deletion abolishes the hypertrophic response. The roles and signals of the cardiac AT(2) receptor still remain unsettled. Promyelocytic leukemia zinc finger protein (PLZF) was shown to bind to the AT(2) receptor and transmit the hypertrophic signal. Using PLZF knockout mice we directed our studies on the function of PLZF concerning the cardiac specific transcription factor GATA4, and GATA4 targets.

METHODOLOGY AND PRINCIPAL FINDINGS

PLZF knockout and age-matched wild-type (WT) mice were treated with Ang II, infused at a rate of 4.2 ng·kg(-1)·min(-1) for 3 weeks. Ang II elevated systolic blood pressure to comparable levels in PLZF knockout and WT mice (140 mmHg). WT mice developed prominent cardiac hypertrophy and fibrosis after Ang II infusion. In contrast, there was no obvious cardiac hypertrophy or fibrosis in PLZF knockout mice. An AT(2) receptor blocker given to Ang II-infused wild type mice prevented hypertrophy, verifying the role of AT(2) receptor for cardiac hypertrophy. Chromatin immunoprecipitation and electrophoretic mobility shift assay showed that PLZF bound to the GATA4 gene regulatory region. A Luciferase assay verified that PLZF up-regulated GATA4 gene expression and the absence of PLZF expression in vivo produced a corresponding repression of GATA4 protein.

CONCLUSIONS

PLZF is an important AT(2) receptor binding protein in mediating Ang II induced cardiac hypertrophy through an AT(2) receptor-dependent signal pathway. The angiotensin II-AT(2)-PLZF-GATA4 signal may further augment Ang II induced pathological effects on cardiomyocytes.

Stretch-induced hypertrophy activates NFkB-mediated VEGF secretion in adult cardiomyocytes

Hypertension and myocardial infarction are associated with the onset of hypertrophy. Hypertrophy is a compensatory response mechanism to increases in mechanical load due to pressure or volume overload. It is characterized by extracellular matrix remodeling and hypertrophic growth of adult cardiomyocytes. Production of Vascular Endothelial Growth Factor (VEGF), which acts as an angiogenic factor and a modulator of cardiomyocyte function, is regulated by mechanical stretch. Mechanical stretch promotes VEGF secretion in neonatal cardiomyocytes. Whether this effect is retained in adult cells and the molecular mechanism mediating stretch-induced VEGF secretion has not been elucidated. Our objective was to investigate whether cyclic mechanical stretch induces VEGF secretion in adult cardiomyocytes and to identify the molecular mechanism mediating VEGF secretion in these cells. Isolated primary adult rat cardiomyocytes (ARCMs) were subjected to cyclic mechanical stretch at an extension level of 10% at 30 cycles/min that induces hypertrophic responses. Cyclic mechanical stretch induced a 3-fold increase in VEGF secretion in ARCMs compared to non-stretch controls. This increase in stretch-induced VEGF secretion correlated with NFkB activation. Cyclic mechanical stretch-mediated VEGF secretion was blocked by an NFkB peptide inhibitor and expression of a dominant negative mutant IkBα, but not by inhibitors of the MAPK/ERK1/2 or PI3K pathways. Chromatin immunoprecipitation assays demonstrated an interaction of NFkB with the VEGF promoter in stretched primary cardiomyocytes. Moreover, VEGF secretion is increased in the stretched myocardium during pressure overload-induced hypertrophy. These findings are the first to demonstrate that NFkB activation plays a role in mediating VEGF secretion upon cyclic mechanical stretch in adult cardiomyocytes. Signaling by NFkB initiated in response to cyclic mechanical stretch may therefore coordinate the hypertrophic response in adult cardiomyocytes. Elucidation of this novel mechanism may provide a target for developing future pharmacotherapy to treat hypertension and heart disease.

Acetylation of a conserved lysine residue in the ATP binding pocket of p38 augments its kinase activity during hypertrophy of cardiomyocytes

Like phosphorylation, acetylation of lysine residues within a protein is considered a biologically relevant modification that controls the activity of target proteins. During stress of cells, massive protein acetylation takes place. Here, we show that p38 mitogen-activated protein kinase (MAPK), which controls many biological functions during stress, is reversibly acetylated by PCAF/p300 and HDAC3. We identified two acetylated lysine residues, K152 and K53, located in the substrate binding domain and in the ATP-binding pocket of p38, respectively. Acetylation of lysine 53 enhanced the activity of p38 by increasing its affinity for ATP binding. The enhanced acetylation and activation of p38 were found to be in parallel with reduced intracellular ATP levels in cardiomyocytes under stress, as well as in vivo models of cardiac hypertrophy. Thus, our data show, for the first time, that p38 activity is critically regulated by, in addition to phosphorylation, reversible acetylation of a lysine residue, which is conserved in other kinases, implying the possibility of a similar mechanism regulating their activity.

Involvement of Src family kinase activation in angiotensin II-induced hyperresponsiveness of rat bronchial smooth muscle

Angiotensin II (Ang II) might be an important mediator in pathogenesis of airway hyperresponsiveness (AHR) that is the asthmatic characteristic feature of asthma, although the mechanisms of AHR caused by Ang II are not yet clear. Presently, the RT-PCR analyses revealed that all the Src family kinases (SFKs), such as Fyn, Lck, Lyn, Hck, Src, Yes, Blk, Fgr and Frk, were expressed in the lungs and main bronchi of rats. The phosphorylation (activation) of SFK (Tyr416) was increased in bronchial smooth muscle (BSM) by Ang II. The Ang II-induced SFK phosphorylation was inhibited by pretreatment with SU6656, an SFK inhibitor. The concentration-contraction curves to carbachol (CCh) were shifted to the left in the presence of Ang II. The maximal contraction of CCh was also significantly increased by pretreatment with Ang II. These results indicate that Ang II causes BSM hyperresponsiveness. The Ang II-induced BSM hyperresponsiveness was significantly inhibited by SU6656, although the carbachol (CCh)-induced contraction itself was not changed by SU6656. In conclusion, Ang II induced a BSM hyperresponsiveness through activation of SFK, and might play an important role in pathophysiology of bronchial asthma.

Mechanical strain-induced RhoA activation requires NADPH oxidase-mediated ROS generation in caveolae

Increased intraglomerular pressure leads to kidney fibrosis, and can be modeled by exposing glomerular mesangial cells (MC) to mechanical strain. We previously showed that RhoA mediates strain-induced matrix production. Here we investigate whether reactive oxygen species (ROS) are required for RhoA activation. Maximal RhoA activation (1 min) was inhibited by ROS scavenge or NADPH oxidase inhibition. Strain activated NADPH oxidase, with Rac1, p47(phox), and p67(phox) membrane translocation, and Rac1 activation, observed within 30 sec. Epidermal growth factor receptor (EGFR) inhibition blocked RhoA and Rac1 activation, p67(phox) membrane translocation, and ROS generation. However, EGFR activation was unaffected by ROS inhibitors, placing it upstream of ROS generation. We previously showed, using chemical disruption, that caveolae mediate strain-induced EGFR and RhoA activation. In MC from caveolin-1 knockout mice, which lack caveolae, RhoA and Rac1 activation, p67(phox) membrane translocation, and ROS generation were absent. These were rescued by caveolin-1 re-expression. ROS generation, Rac1 activation, and p67(phox) membrane translocation were also prevented by Src inhibition. They were absent in MC stably infected with caveolin-1 Y14A, a mutant resistant to Src phosphorylation. In MC, caveolae are thus important mediators of strain-induced ROS generation through NADPH oxidase, mediating a signaling cascade which results in RhoA activation.

Angiotensin II signaling through the AT1a and AT1b receptors does not have a role in the development of cerulein-induced chronic pancreatitis in the mouse

The intraorgan renin-angiotensin system (RAS) plays an important role in the pathophysiology of a variety of diseases and has been implicated in fibrogenesis. The role of RAS in the development of chronic pancreatitis is not well established. The blockade of RAS in rat models with angiotensin-converting enzyme inhibitors (ACEi) or angiotensin receptor 1 (AT1) blockers (ARBs) mostly have reduced pancreatic inflammation and fibrosis with a few exceptions. At the same time, the use of ACEi and ARBs in humans is associated with a modest risk of acute pancreatitis. The aim of this study was to elucidate the effect of the AT1 signaling pathway in the development of pancreatitis using AT1a- and AT1b-deficient mice as well as the ARB losartan. Chronic pancreatitis was induced by repetitive cerulein administration in C57BL/6J wild-type (WT) and AT1a- and AT1b-deficient mice (AT1a-/- and AT1b-/-), and pancreatic injury was assessed at day 10. Pancreatic weight of cerulein treated groups was significantly reduced. There was severe parenchymal atrophy and fibrosis assessed by histological examination. Fibrosis was accompanied by activation of pancreatic stellate cells (PSC) evaluated by Western blot analysis for alpha-smooth muscle actin. No differences were seen between cerulein-treated WT, AT1a-/- , AT1b-/- mice, or losartan treated-WT mice with regards to morphological or molecular alterations induced by cerulein. Our results demonstrate that AT1a and AT1b receptor pathways do not seem to be essential for the development of pancreatitis in the mouse model of pancreatitis induced by repetitive cerulein injury.

Effect of pressure overload-induced hypertrophy on the expression and localization of p38 MAP kinase isoforms in the mouse heart

p38 mitogen-activated protein kinases (MAPKs) are serine/threonine specific protein kinases that respond to cellular stress and regulate a broad range of cellular activities. There are four major isoforms of p38 MAPK: alpha, beta, gamma, and delta. To date, the prominent isoform in heart has been thought to be p38alpha. We examined the expression of each p38 isoform at both the mRNA and protein level in murine heart. mRNA for all four p38 isoforms was detected. p38gamma and p38delta were expressed at protein levels comparable to p38alpha and 38beta, respectively. In the early phase of pressure-overload hypertrophy (1-7 days after constriction of the transverse aorta), the abundance of p38beta, p38gamma and p38delta mRNA increased; however, no corresponding changes were detected at the protein level. Confocal immunofluorescence studies revealed p38alpha and p38gamma in both the cytoplasm and nucleus. In the established phase of hypertrophy induced by chronic pressure overload (7-28 days after constriction of the transverse aorta), p38gamma immunoreactivity accumulated in the nucleus whereas the distribution of p38alpha remained unaffected. Hence, both p38alpha and p38gamma are prominent p38 isoforms in heart and p38gamma may play a role in mediating the changes in gene expression associated with cardiac remodeling during pressure-overload hypertrophy.

Cardiac-Specific Activation of Angiotensin II Type 1 Receptor–Associated Protein Completely Suppresses Cardiac Hypertrophy in Chronic Angiotensin II–Infused Mice

We cloned a novel molecule interacting with angiotensin II type 1 receptor, which we named ATRAP (for angiotensin II type 1 receptor–associated protein). Previous in vitro studies showed that ATRAP significantly promotes constitutive internalization of the angiotensin II type 1 receptor and further attenuates angiotensin II–mediated hypertrophic responses in cardiomyocytes. The present study was designed to investigate the putative functional role of ATRAP in cardiac hypertrophy by angiotensin II infusion in vivo. We first examined the effect of angiotensin II infusion on endogenous ATRAP expression in the heart of C57BL/6J wild-type mice. The angiotensin II treatment promoted cardiac hypertrophy, concomitant with a significant decrease in cardiac ATRAP expression, but without significant change in cardiac angiotensin II type 1 receptor expression. We hypothesized that a downregulation of the cardiac ATRAP to angiotensin II type 1 receptor ratio is involved in the pathogenesis of cardiac hypertrophy. To examine this hypothesis, we next generated transgenic mice expressing ATRAP specifically in cardiomyocytes under control of the α-myosin heavy chain promoter. In cardiac-specific ATRAP transgenic mice, the development of cardiac hypertrophy, activation of p38 mitogen-activated protein kinase, and expression of hypertrophy-related genes in the context of angiotensin II treatment were completely suppressed, in spite of there being no significant difference in blood pressure on radiotelemetry between the transgenic mice and littermate control mice. These results demonstrate that cardiomyocyte-specific overexpression of ATRAP in vivo abolishes the cardiac hypertrophy provoked by chronic angiotensin II infusion, thereby suggesting ATRAP to be a novel therapeutic target in cardiac hypertrophy.

Left ventricular mass in chronic kidney disease and ESRD

Chronic kidney disease (CKD) and ESRD, treated with conventional hemo- or peritoneal dialysis are both associated with a high prevalence of an increase in left ventricular mass (left ventricular hypertrophy [LVH]), intermyocardial cell fibrosis, and capillary loss. Cardiac magnetic resonance imaging is the best way to detect and quantify these abnormalities, but M-Mode and 2-D echocardiography can also be used if one recognizes their pitfalls. The mechanisms underlying these abnormalities in CKD and ESRD are diverse but involve afterload (arterial pressure and compliance), preload (intravascular volume and anemia), and a wide variety of afterload/preload independent factors. The hemodynamic, metabolic, cellular, and molecular mediators of myocardial hypertrophy, fibrosis, apoptosis, and capillary degeneration are increasingly well understood. These abnormalities predispose to sudden cardiac death, most likely by promotion of electrical instability and re-entry arrhythmias and congestive heart failure. Current treatment modalities for CKD and ESRD, including thrice weekly conventional hemodialysis and peritoneal dialysis and metabolic and anemia management regimens, do not adequately prevent or correct these abnormalities. A new paradigm of therapy for CKD and ESRD that places prevention and reversal of LVH and cardiac fibrosis as a high priority is needed. This will require novel approaches to management and controlled interventional trials to provide evidence to fuel the transition from old to new treatment strategies. In the meantime, key management principles designed to ameliorate LVH and its complications should become a routine part of the care of the patients with CKD and ESRD.

Mechanical stretch-induced RhoA activation is mediated by the RhoGEF Vav2 in mesangial cells

Increased intraglomerular pressure is an important hemodynamic determinant of glomerulosclerosis, and can be modelled in vitro by exposing mesangial cells (MC) to cyclic mechanical stretch. We have previously shown that the GTPase RhoA mediates stretch-induced fibronectin production. Here we investigate the role of the RhoGEF Vav2 in the activation of RhoA by stretch. Primary rat MC were exposed to 1 Hz cyclic stretch, previously shown to induce maximal RhoA activation at 1 min. Total Vav2 tyrosine phosphorylation and specific phosphorylation on Y172, required for activation, were increased by 1 min of stretch. Overexpression of dominant-negative Vav2 Y172/159F in COS-1 cells or downregulation of Vav2 by siRNA in MC prevented stretch-induced RhoA activation. Vav2 is known to be activated in response to growth factors, and we have previously shown the epidermal growth factor receptor (EGFR) to be transactivated by stretch in MC. Both Vav2 Y172 phosphorylation and RhoA activation were blocked by the EGFR inhibitor AG1478 and prevented in MC overexpressing kinase inactive EGFR. Stretch led to physical association between the EGFR and Vav2, and this was dependent on EGFR activation. EGFR Y992 phosphorylation, required for growth factor-induced Vav2 phosphorylation, was also induced by stretch. Activation of both Src and PI3K were necessary upstream mediators of stretch-induced Vav2 Y172 phosphorylation and RhoA activation. In summary, stretch-induced RhoA activation is dependent on transactivation of the EGFR and activation of the RhoGEF Vav2. Src and PI3K are both required upstream of Vav2 and RhoA activation.

Sartan-AT1 receptor interactions: in vitro evidence for insurmountable antagonism and inverse agonism

Sartans are non-peptide AT(1) receptor antagonists used to treat hypertension and related pathologies. Their effects on the G protein-dependent responses of angiotensin II (Ang II) were the same in vascular tissues and in isolated cell systems. All are competitive but, when pre-incubated, they act surmountably (only rightward shift of the Ang II concentration-response curve) or insurmountably (also decreasing the maximal response). Insurmountable behaviour reflects the formation of tight sartan-receptor complexes; it is often partial due to the co-existence of tight and loose complexes. Their ratio positively correlates with the dissociation half-life of the tight complexes and depends on the sartan: i.e. candesartan>olmesartan>telmisartan approximately equal EXP3174>valsartan>irbesartan>>losartan. When AT(1) receptors display sufficient basal activity (in case of receptor over-expression, mutation and, especially, tissue stretching) sartans may also act as inverse agonists. This rather affects long-term, G protein-independent hypertrophic responses leading to cardiovascular remodelling.

Protective role of angiotensin II type 2 receptor signaling in a mouse model of pancreatic fibrosis

The renin-angiotensin system contributes to pathological processes in a variety of organs. In the pancreas, blocking the angiotensin II (AII) type 1 receptor (AT1) attenuates pancreatic fibrogenesis in animal models of pancreatitis. Because the role of the AII type 2 receptor (AT2) in modulating pancreatic injury is unknown we investigated the role of AT2 in pancreatic injury and fibrosis. Pancreatic fibrosis was induced by repetitive cerulein administration in C57BL/6 wild-type (WT) or AT2-deficient (AT2-/-) mice and assessed by morphology and gene expression at 10 days. There was no difference between WT and AT2-/- mice in the degree of acute pancreatic injury as assessed by amylase release at 9 and 12 h and by histological examination of the pancreas at 12 h. In contrast, parenchymal atrophy and fibrosis were more pronounced in AT2-/- mice compared with WT mice at 10 days. Fibrosis was accompanied by activation of pancreatic stellate cells (PSC) evaluated by Western blot analysis for alpha-smooth muscle actin and by immunocytochemistry; PSC activation was further increased in AT2-/- mice compared with WT mice. The level of pancreatic transforming growth factor-beta1 mRNA and protein after repetitive cerulein treatment was higher in AT2-/- mice than in WT mice. Our results demonstrate that, in contrast to AT1 receptor signaling, AT2 receptor signaling modulates protective antifibrogenic effects in a mouse model of cerulein-induced pancreatic fibrogenesis. We propose that the effects of AII on injury-induced pancreatic fibrosis may be determined by the balance between AT1 and AT2 receptor signaling.

Angiotensin II: a hormone involved in and contributing to pro-hypertrophic cardiac networks and target of anti-hypertrophic cross-talks

Angiotensin II (Ang II) plays a major role in the progression of myocardial hypertrophy to heart failure. Inhibiting the angiotensin converting enzyme (ACE) or blockade of the corresponding Ang II receptors is used extensively in clinical practice, but there is scope for refinement of this mode of therapy. This review summarizes the current understanding of the direct effects of Ang II on cardiomyocytes and then focus particularly on interaction of components of the renin-angiotensin system with other hormones and cytokines. New findings described in approximately 400 papers identified in the PubMed database and published during the 2.5 years are discussed in the context of previous relevant literature. The cardiac action of Ang II is influenced by the activity of different isoforms of ACE leading to different amounts of Ang II by comparison with other angiotensinogen-derived peptides. The effect of Ang II is mediated by at least two different AT receptors that are differentially expressed in cardiomyocytes from neonatal, adult and failing hearts. The intracellular effects of Ang II are influenced by nitric oxide (NO)/cGMP-dependent cross talk and are mediated by the release of autocrine factors, such as transforming growth factor (TGF)-beta1 and interleukin (IL)-6. Besides interactions with cytokines, Ang II is involved in systemic networks including aldosterone, parathyroid hormone and adrenomedullin, which have their own effects on cardiomyocytes that modify, amplify or antagonize the primary effect of Ang II. Finally, hyperinsulemia and hyperglycaemia influence Ang II-dependent processes in diabetes and its cardiac sequelae.

Activation of the ERK1/2 cascade via pulsatile interstitial fluid flow promotes cardiac tissue assembly

Deciphering the cellular signals leading to cardiac muscle assembly is a major challenge in ex vivo tissue regeneration. For the first time, we demonstrate that pulsatile interstitial fluid flow in three-dimensional neonatal cardiac cell constructs can activate ERK1/2 sixfold, as compared to static-cultivated constructs. Activation of ERK1/2 was attained under physiological shear stress conditions, without activating the p38 cell death signal above its basic level. Activation of the ERK1/2 signaling cascade induced synthesis of high levels of contractile and cell-cell contact proteins by the cardiomyocytes, while its inhibition diminished the inducing effects of pulsatile flow. The pulsed medium-induced cardiac cell constructs showed improved cellularity and viability, while the regenerated cardiac tissue demonstrated some ultra-structural features of the adult myocardium. The cardiomyocytes were elongated and aligned into myofibers with defined Z-lines and multiple high-ordered sarcomeres. Numerous intercalated disks were positioned between adjacent cardiomyocytes, and deposits of collagen fibers surrounded the myofibrils. The regenerated cardiac tissue exhibited high density of connexin 43, a major protein involved in electrical cellular connections. Our research thus demonstrates that by judiciously applying fluid shear stress, cell signaling cascades can be augmented with subsequent profound effects on cardiac tissue regeneration.

Stretch-induced MAP kinase activation in cardiac myocytes: differential regulation through beta1-integrin and focal adhesion kinase

Mitogen-activated protein (MAP) kinases have been implicated in hemodynamic load induced heart failure. Both angiotensin II (Ang II) and mechanical stretch activate MAP kinases in cardiac myocytes. In this study, we used a neonatal rat ventricular myocyte (NRVM) model to determine the role of focal-adhesion kinase (FAK) in beta1 integrin mediated MAP kinase activation in response to mechanical stretch in presence and absence of Ang II receptor blockade (ATB). NRVM plated on deformable membranes coated with collagen IV were exposed to 20% equiaxial static-stretch. beta1 integrin signaling was blocked by adenovirus-mediated expression of a dominant-negative form of beta1D integrin (tac-beta1D). FAK signaling was disrupted by infecting NRVM with adenovirus expressing FAK-related non-kinase (FRNK). Western blot analysis was used to assess the phosphorylation of MAP kinases. In the presence and absence of ATB, mechanical stretch caused maximal phosphorylation of ERK, p38 and JNK at 5 min, which was significantly attenuated in NRVM expressing tac-beta1D. In the presence of ATB, FRNK overexpression significantly increased basal phosphorylation of ERK (40.2+/-8.6% P<0.05), p38 (39.5+/-11.7%, P<0.05), JNK (86+/-29.4%, P<0.05) and stretch-induced p38 (48.1+/-8.7%, P<0.05) and JNK (85.0+/-19.4%, P<0.05) phosphorylation. However, in the absence of ATB, FRNK overexpression significantly reduced basal and stretch-induced phosphorylation of only ERK. Examination of FAK activation revealed that beta1 integrin was required for stretch-induced phosphorylation of FAK at Y397 and Y925, but not Y861. In summary, mechanical stretch-activated ERK1/2, p38 and JNK through FAK independent and dependent mechanisms. Beta1 integrin was required for FAK independent activation of all three MAP kinases, whereas cross-talk between beta1 integrin and Ang II receptors mediated FAK dependent regulation of ERK1/2.

Apical epidermal growth factor receptor signaling: regulation of stretch-dependent exocytosis in bladder umbrella cells

The apical surface of polarized epithelial cells receives input from mediators, growth factors, and mechanical stimuli. How these stimuli are coordinated to regulate complex cellular functions such as polarized membrane traffic is not understood. We analyzed the requirement for growth factor signaling and mechanical stimuli in umbrella cells, which line the mucosal surface of the bladder and dynamically insert and remove apical membrane in response to stretch. We observed that stretch-stimulated exocytosis required apical epidermal growth factor (EGF) receptor activation and that activation occurred in an autocrine manner downstream of heparin-binding EGF-like growth factor precursor cleavage. Long-term changes in apical exocytosis depended on protein synthesis, which occurred upon EGF receptor-dependent activation of mitogen-activated protein kinase signaling. Our results indicate a novel physiological role for the EGF receptor that couples upstream mechanical stimuli to downstream apical EGF receptor activation that may regulate apical surface area changes during bladder filling.

Inhibition of EGFR signaling abrogates smooth muscle proliferation resulting from sustained distension of the urinary bladder

Urinary bladder outlet obstruction results in sustained stretch of the detrusor muscle and can lead to pathological smooth muscle hyperplasia and hypertrophy. The epidermal growth factor receptor (EGFR) is a cognate receptor for mitogens implicated in bladder hyperplasia/hypertrophy. Here, we investigated the potential for modulation of this pathway by pharmacologic targeting with a clinically available EGFR antagonist using an organ culture model of bladder stretch injury as a test system. Urinary bladders from adult female rats were distended in vivo with medium containing the EGFR inhibitor ZD1839 (gefitinib, Iressa). The bladders were excised and incubated in ex vivo organ culture for 4-24 h. EGFR phosphorylation, DNA proliferation, and the extent of apoptosis in the cultured tissues were assessed. To verify that the smooth muscle cells (SMC) are a target of the EGFR inhibitor, primary culture human and rat bladder SMC were subjected to cyclic mechanical stretch in vitro in the presence of ZD1839. Levels of phosphorylated EGFR were significantly increased in the detrusor muscle with 12 h of stretch in the organ cultures. This activation coincided with a subsequent 23-fold increase in DNA synthesis and a 30-fold decrease in apoptosis in the muscle compartment at 24 h. In the presence of ZD1839, DNA synthesis was reduced to basal levels without an increase in the rate of apoptosis under ex vivo conditions. Mechanical stretch of bladder SMC in vitro resulted in a significant increase in DNA synthesis, which was completely abrogated by treatment with ZD1839 but not by AG825, an inhibitor of the related receptor, ErbB2. Our results indicate that the EGFR pathway is a physiologically relevant signaling mechanism in hypertrophic bladder disease resulting from mechanical distension and may be amenable to pharmacologic intervention.

Effects of olmesartan, an AT1 receptor antagonist, on hypoxia-induced activation of ERK1/2 and pro-inflammatory signals in the mouse lung

The present study aimed to investigate the effects of olmesartan, an antagonist for angiotensin II receptor type 1(AT1), on the activation of extracellular signal-regulated kinases (ERK)1/2, tissue remodeling, and pro-inflammatory signals in the right ventricle and lung of mice during the early phase of hypobaric hypoxia. Phosphorylation of ERK1/2 in both tissue types in response to hypoxia peaked at 1-3 days, and declined rapidly in the right ventricle, whereas in the lung it was sustained for at least 8 days. Upregulation of angiotensinogen mRNA was observed in the hypoxic lung at 4-9 days, but not in the hypoxic right ventricle and pulmonary artery. Olmesartan inhibited the hypoxia-induced phosphorylation of ERK1/2 in the lung, but not in the right ventricle. Neither right ventricular hypertrophy nor the thickening of the intrapulmonary arterial wall was ameliorated by olmesartan. However, this drug inhibited the expression of the mRNA for angiotensinogen and several pro-inflammatory factors, including interleukin-6 and inducible nitric oxide synthase in the hypoxic lung. These results suggest that olmesartan blocks a potential positive feedback loop of the angiotensin II-AT1 receptor system, which may lead to attenuate pro-inflammatory signals in the mouse lung, that are associated with hypoxic pulmonary hypertension, without inducing any appreciable effects on the compensatory cardiopulmonary hypertrophy at an early phase of exposure to a hypobaric hypoxic environment.

Type 1 angiotensin receptor pharmacology: signaling beyond G proteins

Drugs that inhibit the production of angiotensin II (AngII) or its access to the type 1 angiotensin receptor (AT₁R) are prescribed to alleviate high blood pressure and its cardiovascular complications. Accordingly, much research has focused on the molecular pharmacology of AT₁R activation and signaling. An emerging theme is that the AT₁R generates G protein dependent as well as independent signals and that these transduction systems separately contribute to AT₁R biology in health and disease. Regulatory molecules termed arrestins are central to this process as is the capacity of AT₁R to crosstalk with other receptor systems, such as the widely studied transactivation of growth factor receptors. AT₁R function can also be modulated by polymorphisms in the AGTR gene, which may significantly alter receptor expression and function; a capacity of the receptor to dimerize/oligomerize with altered pharmacology; and by the cellular environment in which the receptor resides. Together, these aspects of the AT₁R “flavour” the response to angiotensin; they may also contribute to disease, determine the efficacy of current drugs and offer a unique opportunity to develop new therapeutics that antagonize only selective facets of AT₁R function.

EGFR-activated signaling and actin remodeling regulate cyclic stretch-induced NRF2-ARE activation

Cyclic stretch (CS) associated with mechanical ventilation (MV) can cause excessive alveolar and endothelial distention, resulting in lung injury and inflammation. Antioxidant enzymes (AOEs) play a major role in suppressing these effects. The transcription factor Nrf2, via the antioxidant response element (ARE), alleviates pulmonary toxicant- and oxidant-induced oxidative stress by up-regulating the expression of several AOEs. Although gene expression profiling has revealed the induction of AOEs in the lungs of rodents exposed to MV, the mechanisms by which mechanical forces, such as CS, regulate the activation of Nrf2-dependent ARE-transcriptional responses are poorly understood. To mimic mechanical stress associated with MV, we have cultured pulmonary alveolar epithelial and endothelial cells on collagen I-coated BioFlex plates and subjected them to CS. CS exposure stimulated ARE-driven transcriptional responses and subsequent AOE expression. Ectopic expression of a dominant-negative Nrf2 suppressed the CS-stimulated ARE-driven responses. Our findings suggest that actin remodeling is necessary but not sufficient for high-level CS-induced ARE activation in both epithelial and endothelial cells. We also found that inhibition of EGFR activity by a pharmacologic agent ablated the CS-induced ARE transcriptional response in both cell types. Additional studies revealed that amphiregulin, an EGFR ligand, regulates this process. We further demonstrated that the PI3K-Akt pathway acts as the downstream effector of EGFR and regulates CS-induced ARE-activation in an oxidative stress-dependent manner. Collectively, these novel findings suggest that EGFR-activated signaling and actin remodeling act in concert to regulate the CS-induced Nrf2-ARE transcriptional response and subsequent AOE expression.

Oxidative signaling in renal epithelium: Critical role of cytosolic phospholipase A2 and p38(SAPK)

Previous studies from this laboratory have demonstrated a critical role of cytosolic phospholipase A2 (cPLA2) and arachidonic acid in angiotensin II (Ang II) AT2 receptor-mediated signal transduction in renal epithelium. In primary proximal tubular epithelial cells exposed to hydrogen peroxide (H2O2), both the selective cPLA2 inhibitors and the cPLA2 antisense oligonucleotides significantly attenuated H2O2-induced arachidonic acid liberation and activation of p38(SAPK), ERK1/2, and Akt1. This H2O2-induced kinase activation was significantly attenuated by a Src kinase inhibitor PP2, or by transient transfection of carboxyl-terminal Src kinase (CSK) that maintained Src in the dormant form. Under basal conditions, Src coimmunoprecipitated with epidermal growth factor receptor (EGFR), while H2O2 increased EGFR phosphorylation in the complex. We observed that inhibition of EGFR kinase activity with AG1478 significantly attenuated H2O2-induced p38(SAPK) and ERK1/2 activation, but did not inhibit Akt1 activation. Furthermore, it seems that p38(SAPK) is upstream of ERK1/2 and Akt1, since a p38(SAPK) inhibitor SB203580 significantly blocked H2O2-induced activation of ERK1/2 and Akt1. Interestingly, overexpression of the dominant-negative p38(SAPK) isoform alpha inhibited ERK1/2 but not Akt1 activation. Our observations demonstrate that in these nontransformed cells, activation of cPLA2 is a converging point for oxidative stress and Ang II, which share common downstream signaling mechanisms including Src and EGFR. In addition, p38(SAPK) provides a positive input to both growth and antiapoptotic signaling pathways induced by acute oxidative stress.

p38 MAP-kinase in cultured adult rat ventricular cardiomyocytes: expression and involvement in hypertrophic signalling

Both alpha-adrenoceptor- and beta-adrenoceptor-stimulation lead to hypertrophic growth of the myocardium. But only beta-adrenoceptor-stimulation requires the pre-cultivation of cells with active TGF-beta. In order to define signalling molecules that are specifically involved in beta-adrenoceptor-dependent hypertrophy, changes in expression and hypertrophic responsiveness during pre-cultivation with TGF-beta were investigated. Isolated adult ventricular cardiomyocytes from rats were either cultured in 20% (v/v) foetal calf serum (FCS) to activate autocrine released TGF-beta or used without pre-treatment. Protein synthesis was analysed by (14)C-phenylalanine incorporation. Expression of signalling molecules was determined by immunoblotting. During cultivation of cardiomyocytes with active TGF-beta only the expression of p38 MAP-kinase increased. Subsequent stimulation of beta-adrenoceptors induced protein synthesis in a p38 MAP-kinase-dependent way. However, stimulation of beta-adrenoceptors activated p38 MAP-kinase irrespective of pre-treatment with TGF-beta. In the absence of this cytokine, hyperosmolarity or reconstitution of mechanical activity increased protein synthesis via p38 MAP-kinase activation in freshly isolated cells. In conclusion, activation of p38 MAP-kinase is a newly identified necessary signalling step required for beta-adrenoceptor induced hypertrophic growth. Like activation of adenyl cyclase, activation of p38 MAP-kinase is up-stream of the TGF-beta-induced coupling to the regulation of protein synthesis. Reconstitution of mechanical activity mimics the co-activation required and induced by TGF-beta.

Maternal nutrient restriction and the fetal left ventricle: decreased angiotensin receptor expression

BACKGROUND

Adequate maternal nutrition during gestation is requisite for fetal nutrition and development. While a large group of epidemiological studies indicate poor fetal nutrition increases heart disease risk and mortality in later life, little work has focused on the effects of impaired maternal nutrition on fetal heart development. We have previously shown that 50% global nutrient restriction from 28-78 days of gestation (early to mid-pregnancy; term = 147 days) in sheep at mid-gestation retards fetal growth while protecting growth of heart and results in hypertensive male offspring at nine months of age. In the present study, we evaluate LV gene transcription using RNA protection assay and real-time reverse transcriptase polymerase chain reaction, and protein expression using western blot, of VEGF and AT1 and AT2 receptors for AngII at mid-gestation in fetuses from pregnant ewes fed either 100% (C) or 50% (NR) diet during early to mid-gestation.

RESULTS

No difference between the NR (n = 6) and C (n = 6) groups was found in gene transcription of the AngII receptors. Immunoreactive AT1 (1918.4 +/- 154.2 vs. 3881.2 +/- 494.9; P < 0.01) and AT2 (1729.9 +/- 293.6 vs. 3043.3 +/- 373.2; P < 0.02) was decreased in the LV of NR fetuses compared to C fetuses. The LV of fetuses exposed to NR had greater transcription of mRNA for VEGF (5.42 +/- 0.85 vs. 3.05 +/- 0.19; P < 0.03) than respective C LV, while no change was observed in immunoreactive VEGF.

CONCLUSION

The present study demonstrates that VEGF, AT1 and AT2 message and protein are not tightly coupled, pointing to post-transcriptional control points in the mid gestation NR fetus. The present data also suggest that the role of VEGF and the renin-angiotensin system receptors during conditions inducing protected cardiac growth is distinct from the role these proteins may play in normal fetal cardiac growth. The present findings may help explain epidemiological studies that indicate fetuses with low birth weight carry an increased risk of mortality from coronary and cardiovascular disease, particularly if these individuals have reduced cardiovascular reserve due to an epigenetic decrease in vascularization.

Protein kinase C isozymes in hypertension and hypertrophy: insight from SHHF rat hearts

Chronic hypertension results in cardiac hypertrophy and may lead to congestive heart failure. The protein kinase C (PKC) family has been identified as a signaling component promoting cardiac hypertrophy. We hypothesized that PKC activation may play a role mediating hypertrophy in the spontaneously hypertensive heart failure (SHHF) rat heart. Six-month-old SHHF and normotensive control Wistar Furth (WF) rats were used. Hypertension and cardiac hypertrophy were confirmed in SHHF rats. PKC expression and activation were analyzed by Western blots using isozyme-specific antibodies. Compared to WF, untreated SHHF rats had increased phospho-active alpha (10-fold), delta (4-fold), and epsilon (3-fold) isozyme expression. Furthermore, we analyzed the effect of an angiotensin II type 1 receptor blocker (ARB) and hydralazine (Hy) on PKC regulation in SHHF rat left ventricle (LV). Both the ARB and Hy normalized LV blood pressure, but only the ARB reduced heart mass. Neither treatment affected PKC expression or activity. Our data show differential activation of PKC in the hypertensive, hypertrophic SHHF rat heart. Regression of hypertrophy elicited by an ARB in this model occurred independently of changes in the expression and activity of the PKC isoforms examined.

The biochemical response of the heart to hypertension and exercise

Mechanical stress on the heart can lead to crucially different outcomes. Exercise is beneficial because it causes heart muscle cells to enlarge (hypertrophy). Chronic hypertension also causes hypertrophy, but in addition it causes an excessive increase in fibroblasts and extracellular matrix (fibrosis), death of cardiomyocytes and ultimately heart failure. Recent research shows that stimulation of physiological (beneficial) hypertrophy involves several signaling pathways, including those mediated by protein kinase B (also known as Akt) and the extracellular-signal-regulated kinases 1 and 2 (ERK1/2). Hypertension, beta-adrenergic stimulation and agonists such as angiotensin II (Ang II) activate not only ERK1/2 but also p38 and the Jun N-terminal kinase (JNK), leading to pathological heart remodeling. Despite this progress, the mechanisms that activate fibroblasts to cause fibrosis and those that differentiate between exercise and hypertension to produce physiological and pathological responses, respectively, remain to be established.

Immunolocalization of type 2 inositol 1,4,5-trisphosphate receptors in cardiac myocytes from newborn mice

The precise localization and role of inositol 1,4,5-trisphosphate (InsP3) receptors (InsP3Rs) in cardiac muscle cells are largely unknown. It is believed that waves and oscillations in cytosolic free calcium triggered by activation of InsP3Rs underlie modifications of cellular responses that lead to changes in gene expression in other cells. However, how changes in cytosolic calcium alter gene expression in cardiac cells is unknown. Moreover, it is unclear how changes in cytosolic calcium that alter gene expression do so independently of effects of calcium on other cellular functions, such as contraction. Here we show that InsP3R type 2 is the only isoform present in cardiac myocytes isolated from neonatal mouse ventricles. We also show that type 2 InsP3Rs are associated with the nucleus and that activation of type 2 InsP3Rs with endothelin-1 or phenylephrine selectively increases transcription of atrial natriuretic factor and skeletal α-actin. Type 2 InsP3Rs are also in striations. Activation of InsP3Rs with adenophostin A in permeabilized cells induced calcium release in the nuclear domain and other regions of the cell away from the nucleus. Agonist-induced increase in gene expression and calcium release were blocked by the InsP3R inhibitors 2-aminoethoxydiphenyl borate and xestospongin C. The spatial separation of type 2 InsP3Rs provides support for the concept that microdomains of calcium discretely alter various cell processes. Our experiments suggest that calcium released by InsP3Rs in the nuclear domain provides a direct mechanism for the control of gene expression, whereas release of calcium in the cytoplasm may modulate other processes, such as contraction.

Atrial natriuretic peptide inhibits cardiomyocyte hypertrophy through mitogen-activated protein kinase phosphatase-1

Cardiac hypertrophy is formed in response to hemodynamic overload. Although a variety of factors such as catecholamines, angiotensin II (AngII), and endothelin-1 (ET-1) have been reported to induce cardiac hypertrophy, little is known regarding the factors that inhibit the development of cardiac hypertrophy. Production of atrial natriuretic peptide (ANP) is increased in the hypertrophied heart and ANP has recently been reported to inhibit the growth of various cell types. We therefore examined whether ANP inhibits the development of cardiac hypertrophy. Pretreatment of cultured cardiomyocytes with ANP inhibited the AngII- or ET-1-induced increase in the cell size and the protein synthesis. ANP also inhibited the AngII- or ET-1-induced hypertrophic responses such as activation of mitogen-activated protein kinase (MAPK) and induction of immediate early response genes and fetal type genes. To determine how ANP inhibits cardiomyocyte hypertrophy, we examined the mechanism of ANP-induced suppression of the MAPK activation. ANP strongly induced expression of MAPK phosphatase-1 (MKP-1) and overexpression of MKP-1 inhibited AngII- or ET-1-induced hypertrophic responses. These growth-inhibitory actions of ANP were mimicked by a cyclic GMP analog 8-bromo-cyclic GMP. Taken together, ANP directly inhibits the growth factor-induced cardiomyocyte hypertrophy at least partly via induction of MKP-1. Our present study suggests that the formation of cardiac hypertrophy is regulated not only by positive but by negative factors in response to hemodynamic load.

A gammaGT-AT1A receptor transgene protects renal cortical structure in AT1 receptor-deficient mice

To understand the physiological role of angiotensin type 1 (AT(1)) receptors in the proximal tubule of the kidney, we generated a transgenic mouse line in which the major murine AT(1) receptor isoform, AT(1A), was expressed under the control of the P1 portion of the gamma-glutamyl transpeptidase (gammaGT) promoter. In transgenic mice, this promoter has been shown to confer cell-specific expression in epithelial cells of the renal proximal tubule. To avoid random integration of multiple copies of the transgene, we used gene targeting to produce mice with a single-copy transgene insertion at the hypoxanthine phosphoribosyl transferase (Hprt) locus on the X chromosome. The physiological effects of the gammaGT-AT(1A) transgene were examined on a wild-type background and in mice with targeted disruption of one or both of the murine AT(1) receptor genes (Agtr1a and Agtr1b). On all three backgrounds, gammaGT-AT(1A) transgenic mice were healthy and viable. On the wild-type background, the presence of the transgene did not affect development, blood pressure, or kidney structure. Despite relatively low levels of expression in the proximal tubule, the transgene blunted the increase in renin expression typically seen in AT(1)-deficient mice and partially rescued the kidney phenotype associated with Agtr1a(-/-)Agtr1b(-/-) mice, significantly reducing cortical cyst formation by more than threefold. However, these low levels of cell-specific expression of AT(1) receptors in the renal proximal tubule did not increase the low blood pressures or abolish sodium sensitivity, which are characteristic of AT(1) receptor-deficient mice. Although our studies do not clearly identify a role for AT(1) receptors in the proximal tubules of the kidney in blood pressure homeostasis, they support a major role for these receptors in modulating renin expression and in maintaining structural integrity of the renal cortex.

Mechanical stress activates angiotensin II type 1 receptor without the involvement of angiotensin II

The angiotensin II type 1 (AT1) receptor has a crucial role in load-induced cardiac hypertrophy. Here we show that the AT1 receptor can be activated by mechanical stress through an angiotensin-II-independent mechanism. Without the involvement of angiotensin II, mechanical stress not only activates extracellular-signal-regulated kinases and increases phosphoinositide production in vitro, but also induces cardiac hypertrophy in vivo. Mechanical stretch induces association of the AT1 receptor with Janus kinase 2, and translocation of G proteins into the cytosol. All of these events are inhibited by the AT1 receptor blocker candesartan. Thus, mechanical stress activates AT1 receptor independently of angiotensin II, and this activation can be inhibited by an inverse agonist of the AT1 receptor.

Integrins in mechanotransduction

Mechanical forces are crucial to the regulation of cell and tissue morphology and function. At the cellular level, forces influence cytoskeletal organization, gene expression, proliferation, and survival. Integrin-mediated adhesions are intrinsically mechanosensitive and a large body of data implicates integrins in sensing mechanical forces. We review the relationship between integrins and mechanical forces, the role of integrins in cellular responses to stretch and fluid flow, and propose that some of these events are mechanistically related.

Cyclic stretch-induced cPLA2 mediates ERK 1/2 signaling in rabbit proximal tubule cells

BACKGROUND

Recent evidence from this laboratory have demonstrated a critical role of phospholipase A2 (PLA2) and arachidonic acid in angiotensin II type 2 (AT2) receptor-mediated kinase activation in renal epithelium independent of phosphoinositide- specific phospholipase C (PLC) and without the necessity of eicosanoid biosynthesis. In the present study, we investigated whether cyclic stress phosphorylates and activates the mitogen-activated protein kinase (MAPK) pathway and whether PLA2 activation mediates mechanotransduction in renal epithelial cells. The rational for studying kidney epithelial cells relates to their similarity to podocytes, which undergo mechanical stretch related to changes in intraglomerular pressure.

METHODS

To produce strain or stretch, primary cultures of rabbit proximal tubular cell cells are grown in tissue culture wells having a collagen-coated Silastic deformable membrane bottoms and applying vacuum to the well to generate alternating cycles of stretch and relaxation (30 cycles/min).

RESULTS

We found that cyclic stretching of rabbit proximal tubular cells caused a time- and intensity-dependent activation of extracellular signal-regulated kinases 1 and 2 (ERK 1/2) in proximal tubular cells as detected by its phosphorylation. In addition, mechanical stretch induced PLA2 activation and a subsequent rapid release of arachidonic acid. Inhibition of PLA2 by mepacrine and methyl arachidonyl fluorophosphonate ketone (AACOCF3) attenuated both arachidonic acid release and ERK 1/2 activation by cyclic stretch, supporting the importance of PLA2 as a mediator of mechanotransduction in renal proximal tubular cells. A requirement for extracellular Ca2+ and stretch-activated Ca2+ channels was also documented. Complete inhibition of ERK 1/2 by PD98059, a MAPK kinase (MEK) inhibitor, did not suppress stretch- induced PLA2 activation and arachidonic acid release, suggesting the later events were upstream of ERK 1/2. Cyclic stretch also caused rapid phosphorylation of the EGF receptor kinase and c-Src. Furthermore, arachidonic acid itself induced time- and dose-dependent phosphorylation of c-Src. In addition, the c-Src inhibitor PP2 and selective EGF receptor kinase inhibitor AG1478 attenuated both ERK 1/2 and EGF receptor phosphorylation by cyclic stretch.

CONCLUSION

PLA2 dependence for ERK 1/2 activation in response to cyclic stretch in proximal tubular epithelial cells was established in this report. In addition, these findings indicate cyclic stretch increased the tyrosine phosphorylation of the EGF receptor and c-Src and that c-Src acts upstream of the EGF receptor to mediate its phosphorylation, whereby both are critical for stretch- induced ERK 1/2 activation in rabbit proximal tubular cells. These observations documents for the first time a mechanism of mechanical stretch-induced kinase activation mediated by stretch activated Ca2+ channels and PLA2-dependent release of arachidonic acid.

Mechanical stretch induces fetal type II cell differentiation via an epidermal growth factor receptor-extracellular-regulated protein kinase signaling pathway

Mechanical forces are important for fetal alveolar epithelial cell differentiation. However, the signal transduction pathways regulating this process remain largely unknown. Based on the importance of the extracellular-regulated protein kinase (ERK) pathway in cell differentiation, we hypothesized that this cascade mediates stretch-induced fetal type II cell differentiation. We demonstrate that ERK1/2 was maximally activated (> 3-fold) after 15 min of cyclic stretch. Blockage of the ERK pathway with U0126 (a selective MEK1/2 inhibitor) significantly decreased stretch-inducible surfactant protein-C (SP-C) mRNA expression. We examined upstream activators of ERK1/2 and found that stretch induced phosphorylation of Raf-1 and activation of Ras. Moreover, GW5074, a selective c-Raf-1 inhibitor, decreased stretch-inducible SP-C mRNA accumulation. Mechanical stretch also stimulated epidermal growth factor receptor (EGFR) phosphorylation. Finally, blockage of the EGFR, either with tyrphostin AG1478 or neutralizing antibody, decreased stretch-inducible SP-C mRNA expression. We conclude that stretch, at least in part, induces differentiation of fetal epithelial cells via EGFR activation of the ERK pathway. These results suggest that EGFR may be a mechanosensor during fetal lung development. These findings may have significant implications for the design of strategies to accelerate lung maturation.

An essential role for angiotensin II type 1a receptor in pregnancy-associated hypertension with intrauterine growth retardation

Little is known about an in vivo significance of angiotensin II Type-1 receptor (AT1) for pregnancy-associated diseases, including hypertension and intrauterine growth retardation (IUGR). We previously demonstrated that female mice carrying the human angiotensinogen gene (hAG+/+), when mated with human renin transgenic (hRN+/+) male mice, displayed hypertension in late pregnancy due to secretion of human renin from the fetal side into the maternal circulation. In the present study, to investigate a role for AT1 in pregnancy-associated hypertension, we generated a new strain of hAG+/+/mAT1a-/- mice by genetically deleting the AT1a gene from hAG+/+ mice. When mated with hRN+/+ male mice, excessive increases in human renin, angiotensin, and plasma renin activity were detected in the plasma of pregnant hAG+/+/mAT1a-/- mice as found in that of pregnant hAG+/+ mice. Surprisingly, however, blood pressure of hAG+/+/mAT1a-/- mice was not elevated in late pregnancy despite the presence of AT1b, a subtype of AT1. The maternal and fetal defects, such as cardiac and placental abnormalities, and IUGR observed in pregnant hypertensive hAG+/+ mice were not recognized in pregnant hAG+/+/mAT1a-/- mice. The limited term administration of AT1 antagonists to hypertensive hAG+/+ mice in late pregnancy dramatically improved hypertension and IUGR, showing the clinical importance of AT1a.