Role of the epidermal growth factor receptor in signaling strain-dependent activation of the brain natriuretic peptide gene

Cannabinoid receptor agonist attenuates angiotensin II-induced enlargement and mitochondrial dysfunction in rat atrial cardiomyocytes

Pathological remodeling of atrial tissue renders the atria more prone to arrhythmia upon arrival of electrical triggers. Activation of the renin-angiotensin system is an important factor that contributes to atrial remodeling, which may result in atrial hypertrophy and prolongation of P-wave duration. In addition, atrial cardiomyocytes are electrically coupled via gap junctions, and electrical remodeling of connexins may result in dysfunction of coordinated wave propagation within the atria. Currently, there is a lack of effective therapeutic strategies that target atrial remodeling. We previously proposed that cannabinoid receptors (CBR) may have cardioprotective qualities. CB13 is a dual cannabinoid receptor agonist that activates AMPK signaling in ventricular cardiomyocytes. We reported that CB13 attenuates tachypacing-induced shortening of atrial refractoriness and inhibition of AMPK signaling in the rat atria. Here, we evaluated the effects of CB13 on neonatal atrial rat cardiomyocytes (NRAM) stimulated by angiotensin II (AngII) in terms of atrial myocyte enlargement and mitochondrial function. CB13 inhibited AngII-induced enhancement of atrial myocyte surface area in an AMPK-dependent manner. CB13 also inhibited mitochondrial membrane potential deterioration in the same context. However, AngII and CB13 did not affect mitochondrial permeability transition pore opening. We further demonstrate that CB13 increased Cx43 compared to AngII-treated neonatal rat atrial myocytes. Overall, our results support the notion that CBR activation promotes atrial AMPK activation, and prevents myocyte enlargement (an indicator that suggests pathological hypertrophy), mitochondrial depolarization and Cx43 destabilization. Therefore, peripheral CBR activation should be further tested as a novel treatment strategy in the context of atrial remodeling.

Quantification of model and data uncertainty in a network analysis of cardiac myocyte mechanosignalling

Cardiac myocytes transduce changes in mechanical loading into cellular responses via interacting cell signalling pathways. We previously reported a logic-based ordinary differential equation model of the myocyte mechanosignalling network that correctly predicts 78% of independent experimental results not used to formulate the original model. Here, we use Monte Carlo and polynomial chaos expansion simulations to examine the effects of uncertainty in parameter values, model logic and experimental validation data on the assessed accuracy of that model. The prediction accuracy of the model was robust to parameter changes over a wide range being least sensitive to uncertainty in time constants and most affected by uncertainty in reaction weights. Quantifying epistemic uncertainty in the reaction logic of the model showed that while replacing 'OR' with 'AND' reactions greatly reduced model accuracy, replacing 'AND' with 'OR' reactions was more likely to maintain or even improve accuracy. Finally, data uncertainty had a modest effect on assessment of model accuracy. This article is part of the theme issue 'Uncertainty quantification in cardiac and cardiovascular modelling and simulation'.

The slow force response to stretch: Controversy and contradictions

When exposed to an abrupt stretch, cardiac muscle exhibits biphasic active force enhancement. The initial, instantaneous, force enhancement is well explained by the Frank-Starling mechanism. However, the cellular mechanisms associated with the second, slower phase remain contentious. This review explores hypotheses regarding this "slow force response" with the intention of clarifying some apparent contradictions in the literature. The review is partitioned into three sections. The first section considers pathways that modify the intracellular calcium handling to address the role of the sarcoplasmic reticulum in the mechanism underlying the slow force response. The second section focuses on extracellular calcium fluxes and explores the identity and contribution of the stretch-activated, non-specific, cation channels as well as signalling cascades associated with G-protein coupled receptors. The final section introduces promising candidates for the mechanosensor(s) responsible for detecting the stretch perturbation.

Acute Myocardial Response to Stretch: What We (don't) Know

Myocardial stretch, as result of acute hemodynamic overload, is one of the most frequent challenges to the heart and the ability of the heart to intrinsically adapt to it is essential to prevent circulatory congestion. In this review, we highlight the historical background, the currently known mechanisms, as well as the gaps in the understanding of this physiological response. The systolic adaptation to stretch is well-known for over 100 years, being dependent on an immediate increase in contractility—known as the Frank-Starling mechanism—and a further progressive increase—the slow force response. On the other hand, its diastolic counterpart remains largely unstudied. Mechanosensors are structures capable of perceiving mechanical signals and activating pathways that allow their transduction into biochemical responses. Although the connection between these structures and stretch activated pathways remains elusive, we emphasize those most likely responsible for the initiation of the acute response. Calcium-dependent pathways, including angiotensin- and endothelin-related pathways; and cGMP-dependent pathways, comprising the effects of nitric oxide and cardiac natriuretic hormones, embody downstream signaling. The ischemic setting, a paradigmatic situation of acute hemodynamic overload, is also touched upon. Despite the relevant knowledge accumulated, there is much that we still do not know. The quest for further understanding the myocardial response to acute stretch may provide new insights, not only in its physiological importance, but also in the prevention and treatment of cardiovascular diseases.

Liganded peroxisome proliferator-activated receptors (PPARs) preserve nuclear histone deacetylase 5 levels in endothelin-treated Sprague-Dawley rat cardiac myocytes

Ligand activation of peroxisome proliferator-activated receptors (PPARs) prevents cardiac myocyte hypertrophy, and we previously reported that diacylglycerol kinase zeta (DGKζ) is critically involved. DGKζ is an intracellular lipid kinase that catalyzes phosphorylation of diacylglycerol; by attenuating DAG signaling, DGKζ suppresses protein kinase C (PKC) and G-protein signaling. Here, we investigated how PPAR-DGKζ signaling blocks activation of the hypertrophic gene program. We focused on export of histone deacetylase 5 (HDAC5) from the nucleus, a key event during hypertrophy, since crosstalk occurs between PPARs and other members of the HDAC family. Using cardiac myocytes isolated from Sprague-Dawley rats, we determined that liganded PPARs disrupt endothelin-1 (ET1)-induced nuclear export of HDAC5 in a manner that is dependent on DGKζ. When DGKζ-mediated PKC inhibition was circumvented using a constitutively-active PKCε mutant, PPARs failed to block ET1-induced nuclear retention of HDAC5. Liganded PPARs also prevented (i) activation of protein kinase D (the downstream effector of PKC), (ii) HDAC5 phosphorylation at 14-3-3 protein chaperone binding sites (serines 259 and 498), and (iii) physical interaction between HDAC5 and 14-3-3, all of which are consistent with blockade of nucleo-cytoplasmic shuttling of HDAC5. Finally, the ability of PPARs to prevent neutralization of HDAC5 activity was associated with transcriptional repression of hypertrophic genes. This occurred by first, reduced MEF2 transcriptional activity and second, augmented deacetylation of histone H3 associated with hypertrophic genes expressing brain natriuretic peptide, β-myosin heavy chain, skeletal muscle α-actin, and cardiac muscle α-actin. Our findings identify spatial regulation of HDAC5 as a target for liganded PPARs, and to our knowledge, are the first to describe a mechanistic role for nuclear DGKζ in cardiac myocytes. In conclusion, these results implicate modulation of HDAC5 as a mechanism by which liganded PPARs suppress the hypertrophic gene program.

Imatinib activates pathological hypertrophy by altering myocyte calcium regulation

BACKGROUND

Imatinib mesylate is a selective tyrosine-kinase inhibitor used in the treatment of multiple cancers, most notably chronic myelogenous leukemia. There is evidence that imatinib can induce cardiotoxicity in cancer patients. Our hypothesis is that imatinib alters calcium regulatory mechanisms and can contribute to development of pathological cardiac hypertrophy.

METHODS AND RESULTS

Neonatal rat ventricular myocytes (NRVMs) were treated with clinical doses (low: 2 μM; high: 5 μM) of imatinib and assessed for molecular changes. Imatinib increased peak systolic Ca(2+) and Ca(2+) transient decay rates and Western analysis revealed significant increases in phosphorylation of phospholamban (Thr-17) and the ryanodine receptor (Ser-2814), signifying activation of calcium/calmodulin-dependent kinase II (CaMKII). Imatinib significantly increased NRVM volume as assessed by Coulter counter, myocyte surface area, and atrial natriuretic peptide abundance seen by Western. Imatinib induced cell death, but did not activate the classical apoptotic program as assessed by caspase-3 cleavage, indicating a necrotic mechanism of death in myocytes. We expressed AdNFATc3-green fluorescent protein in NRVMs and showed imatinib treatment significantly increased nuclear factor of activated T cells translocation that was inhibited by the calcineurin inhibitor FK506 or CaMKII inhibitors.

CONCLUSION

These data show that imatinib can activate pathological hypertrophic signaling pathways by altering intracellular Ca(2+) dynamics. This is likely a contributing mechanism for the adverse cardiac effects of imatinib.

Endogenous endothelin 1 mediates angiotensin II-induced hypertrophy in electrically paced cardiac myocytes through EGFR transactivation, reactive oxygen species and NHE-1

Emerging evidence supports a key role for endothelin-1 (ET-1) and the transactivation of the epidermal growth factor receptor (EGFR) in angiotensin II (Ang II) action. We aim to determine the potential role played by endogenous ET-1, EGFR transactivation and redox-dependent sodium hydrogen exchanger-1 (NHE-1) activation in the hypertrophic response to Ang II of cardiac myocytes. Electrically paced adult cat cardiomyocytes were placed in culture and stimulated with 1 nmol l(-1) Ang II or 5 nmol l(-1) ET-1. Ang II increased ~45 % cell surface area (CSA) and ~37 % [(3)H]-phenylalanine incorporation, effects that were blocked not only by losartan (Los) but also by BQ123 (AT1 and ETA receptor antagonists, respectively). Moreover, Ang II significantly increased ET-1 messenger RNA (mRNA) expression. ET-1 similarly increased myocyte CSA and protein synthesis, actions prevented by the reactive oxygen species scavenger MPG or the NHE-1 inhibitor cariporide (carip). ET-1 increased the phosphorylation of the redox-sensitive ERK1/2-p90(RSK) kinases, main activators of the NHE-1. This effect was prevented by MPG and the antagonist of EGFR, AG1478. Ang II, ET-1 and EGF increased myocardial superoxide production (187 ± 9 %, 149 ± 8 % and 163.7 ± 6 % of control, respectively) and AG1478 inhibited these effects. Interestingly, Los inhibited only Ang II whilst BQ123 cancelled both Ang II and ET-1 actions, supporting the sequential and unidirectional activation of AT1, ETA and EGFR. Based on the present evidence, we propose that endogenous ET-1 mediates the hypertrophic response to Ang II by a mechanism that involves EGFR transactivation and redox-dependent activation of the ERK1/2-p90(RSK) and NHE-1 in adult cardiomyocytes.

Myocardial mineralocorticoid receptor activation by stretching and its functional consequences

Myocardial stretch triggers an angiotensin II-dependent autocrine/paracrine loop of intracellular signals, leading to reactive oxygen species-mediated activation of redox-sensitive kinases. Based on pharmacological strategies, we previously proposed that mineralocorticoid receptor (MR) is necessary for this stretch-triggered mechanism. Now, we aimed to test the role of MR after stretch by using a molecular approach to avoid secondary effects of pharmacological MR blockers. Small hairpin interference RNA capable of specifically knocking down the MR was incorporated into a lentiviral vector (l-shMR) and injected into the left ventricular wall of Wistar rats. The same vector but expressing a nonsilencing sequence (scramble) was used as control. Lentivirus propagation through the left ventricle was evidenced by confocal microscopy. Myocardial MR expression, stretch-triggered activation of redox-sensitive kinases (ERK1/2-p90(RSK)), the consequent Na(+)/H(+) exchanger-mediated changes in pHi (HEPES-buffer), and its mechanical counterpart, the slow force response, were evaluated. Furthermore, reactive oxygen species production in response to a low concentration of angiotensin II (1.0 nmol/L) or an equipotent concentration of epidermal growth factor (0.1 μg/mL) was compared in myocardial tissue slices from both groups. Compared with scramble, animals transduced with l-shMR showed (1) reduced cardiac MR expression, (2) cancellation of angiotensin II-induced reactive oxygen species production but preservation of epidermal growth factor-induced reactive oxygen species production, (3) cancellation of stretch-triggered increase in ERK1/2-p90(RSK) phosphorylation, (4) lack of stretch-induced Na(+)/H(+) exchanger activation, and (5) abolishment of the slow force response. Our results provide strong evidence that MR activation occurs after myocardial stretch and is a key factor to promote redox-sensitive kinase activation and their downstream consequences.

Ca(2+) influx through L-type Ca(2+) channels and transient receptor potential channels activates pathological hypertrophy signaling

Common cardiovascular diseases such as hypertension and myocardial infarction require that myocytes develop greater than normal force to maintain cardiac pump function. This requires increases in [Ca(2+)]. These diseases induce cardiac hypertrophy and increases in [Ca(2+)] are known to be an essential proximal signal for activation of hypertrophic genes. However, the source of "hypertrophic" [Ca(2+)] is not known and is the topic of this study. The role of Ca(2+) influx through L-type Ca(2+) channels (LTCC), T-type Ca(2+) channels (TTCC) and transient receptor potential (TRP) channels on the activation of calcineurin (Cn)-nuclear factor of activated T cells (NFAT) signaling and myocyte hypertrophy was studied. Neonatal rat ventricular myocytes (NRVMs) and adult feline ventricular myocytes (AFVMs) were infected with an adenovirus containing NFAT-GFP, to determine factors that could induce NFAT nuclear translocation. Four millimolar Ca(2+) or pacing induced NFAT nuclear translocation. This effect was blocked by Cn inhibitors. In NRVMs Nifedipine (Nif, LTCC antagonist) blocked high Ca(2+)-induced NFAT nuclear translocation while SKF-96365 (TRP channel antagonist) and Nickel (Ni, TTCC antagonist) were less effective. The relative potency of these antagonists against Ca(2+) induced NFAT nuclear translocation (Nif>SKF-96365>Ni) was similar to their effects on Ca(2+) transients and the LTCC current. Infection of NRVM with viruses containing TRP channels also activated NFAT-GFP nuclear translocation and caused myocyte hypertrophy. TRP effects were reduced by SKF-96365, but were more effectively antagonized by Nif. These experiments suggest that Ca(2+) influx through LTCCs is the primary source of Ca(2+) to activate Cn-NFAT signaling in NRVMs and AFVMs. While TRP channels cause hypertrophy, they appear to do so through a mechanism involving Ca(2+) entry via LTCCs.

Mineralocorticoid receptor activation is crucial in the signalling pathway leading to the Anrep effect

The increase in myocardial reactive oxygen species after epidermal growth factor receptor transactivation is a crucial step in the autocrine/paracrine angiotensin II/endothelin receptor activation leading to the slow force response to stretch (SFR). Since experimental evidence suggests a link between angiotensin II or its AT1 receptor and the mineralocorticoid receptor (MR), and MR transactivates the epidermal growth factor receptor, we thought to determine whether MR activation participates in the SFR development in rat myocardium. We show here that MR activation is necessary to promote reactive oxygen species formation by a physiological concentration of angiotensin II (1 nmol l(-1)), since an increase in superoxide anion formation of ~50% of basal was suppressed by blocking MR with spironolactone or eplerenone. This effect was also suppressed by blocking AT1, endothelin (type A) or epidermal growth factor receptors, by inhibiting NADPH oxydase or by targeting mitochondria, and was unaffected by glucocorticoid receptor inhibition. All interventions except AT1 receptor blockade blunted the increase in superoxide anion promoted by an equipotent dose of endothelin-1 (1 nmol l(-1)) confirming that endothelin receptors activation is downstream of AT1. Similarly, an increase in superoxide anion promoted by an equipotent dose of aldosterone (10 nmol l(-1)) was blocked by spironolactone or eplerenone, by preventing epidermal growth factor receptor transactivation, but not by inhibiting glucocorticoid receptors or protein synthesis, suggesting non-genomic MR effects. Combination of aldosterone plus endothelin-1 did not increase superoxide anion formation more than each agonist separately. We found that aldosterone increased phosphorylation of the redox-sensitive kinases ERK1/2-p90RSK and the NHE-1, effects that were eliminated by eplerenone or by preventing epidermal growth factor receptor transactivation. Finally, we provide evidence that the SFR is suppressed by MR blockade, by preventing epidermal growth factor receptor transactivation or by scavenging reactive oxygen species, but it is unaffected by glucocorticoid receptor blockade or protein synthesis inhibition. Our results suggest that MR activation is a necessary step in the stretch-triggered reactive oxygen species-mediated activation of redox-sensitive kinases upstream NHE-1.

The Anrep effect requires transactivation of the epidermal growth factor receptor

Myocardial stretch elicits a biphasic contractile response: the Frank-Starling mechanism followed by the slow force response (SFR) or Anrep effect. In this study we hypothesized that the SFR depends on epidermal growth factor receptor (EGFR) transactivation after the myocardial stretch-induced angiotensin II (Ang II)/endothelin (ET) release. Experiments were performed in isolated cat papillary muscles stretched from 92 to 98% of the length at which maximal twitch force was developed (L(max)). The SFR was 123 +/- 1% of the immediate rapid phase (n = 6, P < 0.05) and was blunted by preventing EGFR transactivation with the Src-kinase inhibitor PP1 (99 +/- 2%, n = 4), matrix metalloproteinase inhibitor MMPI (108 +/- 4%, n = 11), the EGFR blocker AG1478 (98 +/- 2%, n = 6) or the mitochondrial transition pore blocker clyclosporine (99 +/- 3%, n = 6). Stretch increased ERK1/2 phosphorylation by 196 +/- 17% of control (n = 7, P < 0.05), an effect that was prevented by PP1 (124 +/- 22%, n = 7) and AG1478 (131 +/- 17%, n = 4). In myocardial slices, Ang II (which enhances ET mRNA) or endothelin-1 (ET-1)-induced increase in O(2)() production (146 +/- 14%, n = 9, and 191 +/- 17%, n = 13, of control, respectively, P < 0.05) was cancelled by AG1478 (94 +/- 5%, n = 12, and 98 +/- 15%, n = 8, respectively) or PP1 (100 +/- 4%, n = 6, and 99 +/- 8%, n = 3, respectively). EGF increased O(2)() production by 149 +/- 4% of control (n = 9, P < 0.05), an effect cancelled by inhibiting NADPH oxidase with apocynin (110 +/- 6% n = 7), mKATP channels with 5-hydroxydecanoic acid (5-HD; 105 +/- 5%, n = 8), the respiratory chain with rotenone (110 +/- 7%, n = 7) or the mitochondrial permeability transition pore with cyclosporine (111 +/- 10%, n = 6). EGF increased ERK1/2 phosphorylation (136 +/- 8% of control, n = 9, P < 0.05), which was blunted by 5-HD (97 +/- 5%, n = 4), suggesting that ERK1/2 activation is downstream of mitochondrial oxidative stress. Finally, stretch increased Ser703 Na(+)/H(+) exchanger-1 (NHE-1) phosphorylation by 172 +/- 24% of control (n = 4, P < 0.05), an effect that was cancelled by AG1478 (94 +/- 17%, n = 4). In conclusion, our data show for the first time that EGFR transactivation is crucial in the chain of events leading to the Anrep effect.

The EGF receptor activates ERK but not JNK Ras-dependently in basal conditions but ERK and JNK activation pathways are predominantly Ras-independent during cardiomyocyte stretch

Myocardial stretch is a major determinant of ventricular hypertrophy, a physiological adaptational process that can be detrimental, leading to heart failure. Therapies aimed to limit the development of cardiac hypertrophy are thus currently evaluated. Among possible targets, the small G protein Ras and the epidermal growth factor receptor (EGFR) have been shown to be involved during stretch but their precise role in the activation of the major actors of hypertrophy, the mitogen activated protein kinases (MAPK) ERK and JNK is not well known. Our goal was thus was to evaluate precisely the activation pathways of ERK and JNK during stretch, with an emphasis on the role of the EGFR. For this purpose, neonatal rat cardiomyocytes in culture were stretched for different time durations. As measured by Western blot of their phosphorylated forms, ERK and JNK were activated by stretch. Ras inhibition decreased basal ERK phosphorylation but had no effect on stretch-induced ERK activation. Under basal conditions, EGFR activated ERK in a classical Ras-dependent manner. Upon stretch, EGFR transactivation activated ERK through both Ras-dependent and Ras-independent pathways. Interestingly, we also show that the Akt pathway participates in stretch-induced ERK activation with an involvement of EGFR. Unlike ERK, JNK activation is independent of either EGFR or PI3 kinase but dependent on other tyrosine kinases. In conclusion these data show different Ras-dependent and Ras-independent pathways in basal conditions and during stretch with a previously unrecognized role of Akt in the activation of ERK.

Suppression of cardiac myocyte hypertrophy by conjugated linoleic acid: role of peroxisome proliferator-activated receptors alpha and gamma

Conjugated linoleic acid (CLA) refers to a naturally occurring mixture of positional and geometric isomers of linoleic acid. Evidence suggests that CLA is a dietary constituent and nutraceutical with anti-cancer, insulin-sensitizing, immunomodulatory, weight-partitioning, and cardioprotective properties. The aim of this study was to evaluate the effects of intervention with CLA on cardiac hypertrophy. In vitro, CLA prevented indicators of cardiomyocyte hypertrophy elicited by endothelin-1, including cell size augmentation, protein synthesis, and fetal gene activation. Similar anti-hypertrophic effects of CLA were observed in hypertrophy induced by angiotensin II, fibroblast growth factor, and mechanical strain. CLA may inhibit hypertrophy through activation of peroxisome proliferator-activated receptors (PPARs). CLA stimulated PPAR activity in cardiomyocytes, and the anti-hypertrophic effects of CLA were blocked by genetic and pharmacological inhibitors of PPAR isoforms alpha and gamma. CLA may disrupt hypertrophic signaling by stimulating diacylglycerol kinase zeta, which decreases availability of diacylglycerol and thereby inhibits the protein kinase Cepsilon pathway. In vivo, dietary CLA supplementation significantly reduced blood pressure and cardiac hypertrophy in spontaneously hypertensive heart failure rats. These data suggest that dietary supplementation with CLA may be a viable strategy to prevent pathological cardiac hypertrophy, a major risk factor for heart failure.

Interaction and inhibitory cross-talk between endothelin and ErbB receptors in the adult heart

Endothelin-1 (ET-1) regulates contractility and growth of the mammalian heart by binding endothelin receptor type A (ET(A)) and endothelin receptor type B (ET(B)) G-protein-coupled receptors. To identify growth signaling pathways associated with ET-1 receptors in adult myocardium, a combined immunoprecipitation/proteomic analysis was performed. Signaling proteins believed to function downstream of ET(A) such as Galpha(q), phospholipase C-beta1, protein kinase C (PKC) epsilon, and PKCdelta were identified in immunoprecipitates of ET(A) by matrix-assisted laser desorption ionization/time of flight mass spectrometry. Also prominent were the growth factor receptor tyrosine kinases erbB2 and erbB4 and their downstream growth signaling effectors phosphoinositide-3 kinase (PI3 kinase), Akt, Raf-1, mitogen-activated protein kinase kinase (MEK), and extracellular signal-regulated kinase (Erk). Western blot analysis confirmed coimmunoprecipitation of erbB2/4, PI3 kinase, and Akt with ET(A), and confocal microscopy revealed their colocalization in cardiac transverse tubules (T-tubules). The erbB4 receptor ligand neuregulin-1beta (NRG1beta) promoted erbB2/4 tryosine phosphorylation and Akt serine phosphorylation in ventricular myocytes, whereas treatment with ET-1 did not. This observation argues against ET-1 growth signaling occurring via erbB2/4 transactivation in adult myocardium. ET-1 did, however, stimulate Erk1/2 phosphorylation and substantially blunted several NRG1beta-mediated actions, including erbB2/4 phosphorylation, serine phosphorylation of Akt, and negative inotropy. This inhibitory cross-talk between ET(A) and erbB2/4-Akt pathways was mimicked by a phorbol ester and blocked by pharmacological inhibition of PKC or MEK/Erk. The proteomic analysis and subsequent investigation of receptor cross-talk indicate that growth signaling between ET(A) and erbB pathways is fundamentally different in adult versus neonatal cardiac myocytes. The results may be relevant to cardiomyopathies associated with 1) prolonged exposure to ET-1; 2) degeneration of T-tubules; and 3) therapies targeted at erbB2 inhibition.

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.

Global gene expression analysis in the bones reveals involvement of several novel genes and pathways in mediating an anabolic response of mechanical loading in mice

To identify the genes and signal pathways responsible for mechanical loading-induced bone formation, we evaluated differential gene expression on a global basis in the tibias of C57BL/6J (B6) mice after four days of four-point bending. We applied mechanical loads to the right tibias of the B6 mice at 9 N, 2 Hz for 36 cycles per day, with the left tibias used as unloaded controls. RNA from the tibias was harvested 24 h after last stimulation and subjected to microarray. Of the 20,280 transcripts hybridized to the array, 346 were differentially expressed in the loaded bones compared to the controls. The validity of the microarray data was established with the increased expression of bone-related genes such as pleiotrophin, osteoglycin, and legumain upon four-point bending and confirmation of increased expression of selected genes by real-time PCR. The list of differentially expressed genes includes genes involved in cell growth, differentiation, adhesion, proteolysis, as well as signaling molecules of receptors for growth factors, integrin, Ephrin B2, endothelin, and adhesion G protein coupled receptor. Pathway analyses suggested that 28 out of the 346 genes exhibited a direct biological association. Among the biological network, fibronectin and pleitrophin function as important signaling molecules in regulating periosteal bone formation and resorption in response to four-point bending. Furthermore, some expressed sequence tags (ESTs) with no prior known function have been identified as potential mediators of mechanotransduction signaling pathways. Further studies on these previously unknown genes will improve our understanding of the molecular pathways and mechanisms involved in bone's response to mechanical stress.

Cardiomyocyte hypertrophy and degradation of connexin43 through spatially restricted autocrine/paracrine heparin-binding EGF

Growth factor signaling can affect tissue remodeling through autocrine/paracrine mechanisms. Recent evidence indicates that EGF receptor transactivation by heparin-binding EGF (HB-EGF) contributes to hypertrophic signaling in cardiomyocytes. Here, we show that HB-EGF operates in a spatially restricted circuit in the extracellular space within the myocardium, revealing the critical nature of the local microenvironment in intercellular signaling. This highly localized microenvironment of HB-EGF signaling was demonstrated with 3D morphology, consistent with predictions from a computational model of EGF signaling. HB-EGF secretion by a given cardiomyocyte in mouse left ventricles led to cellular hypertrophy and reduced expression of connexin43 in the overexpressing cell and in immediately adjacent cells but not in cells farther away. Thus, HB-EGF acts as an autocrine and local paracrine cardiac growth factor that leads to loss of gap junction proteins within a spatially confined microenvironment. These findings demonstrate how cells can coordinate remodeling with their immediate neighboring cells with highly localized extracellular EGF signaling.

PGE2-induced hypertrophy of cardiac myocytes involves EP4 receptor-dependent activation of p42/44 MAPK and EGFR transactivation

Upon induction of cyclooxygenase-2 (COX-2), neonatal ventricular myocytes (VMs) mainly synthesize prostaglandin E2 (PGE2). The biological effects of PGE2 are mediated through four different G protein-coupled receptor (GPCR) subtypes (EP1–4). We have previously shown that PGE2 stimulates cAMP production and induces hypertrophy of VMs. Because the EP4 receptor is coupled to adenylate cyclase and increases in cAMP, we hypothesized that PGE2 induces hypertrophic growth of cardiac myocytes through a signaling cascade that involves EP4-cAMP and activation of protein kinase A (PKA). To test this, we used primary cultures of VMs and measured [3H]leucine incorporation into total protein. An EP4 antagonist was able to partially block PGE2 induction of protein synthesis and prevent PGE2-dependent increases in cell surface area and activity of the atrial natriuretic factor promoter, which are two other indicators of hypertrophic growth. Surprisingly, a PKA inhibitor had no effect. In other cell types, G protein-coupled receptor activation has been shown to transactivate the epidermal growth factor receptor (EGFR) and result in p42/44 mitogen-activated protein kinase (MAPK) activation and cell growth. Immunoprecipitation of myocyte lysates demonstrated that the EGFR was rapidly phosphorylated by PGE2 in VMs, and the EP4 antagonist blocked this. In addition, the selective EGFR inhibitor AG-1478 completely blocked PGE2-induced protein synthesis. We also found that PGE2 rapidly phosphorylated p42/44 MAPK, which was inhibited by the EP4 antagonist and by AG-1478. Finally, the p42/44 MAPK inhibitor PD-98053 (25 μmol/l) blocked PGE2-induced protein synthesis. Altogether, we believe these are the first data to suggest that PGE2 induces protein synthesis in cardiac myocytes in part via activation of the EP4 receptor and subsequent activation of p42/44 MAPK. Activation of p42/44 MAPK is independent of the common cAMP-PKA pathway and involves EP4-dependent transactivation of EGFR.

Identification of transcripts by macroarrays, RT–PCR and in situ hybridization in human ejaculate spermatozoa

Round spermatids contain high levels of extremely varied mRNAs that are synthesized either throughout early spermatogenesis or during spermiogenesis from the haploid genome. Concomitantly, with major changes in the chromatin organization, arrest of transcription occurs at midspermiogenesis. However, previous investigations using RT-PCR have revealed the persistence of numerous and different transcripts in ejaculated spermatozoa. In the present study, a step-by-step analysis by means of macroarray hybridization, RT-PCR and in situ hybridization was performed to identify more accurately the different mRNA species found in the human ejaculated spermatozoa. The data showed an extended pattern of various transcripts encoding a diverse range of proteins involved in signal transduction and cell proliferation. For the first time, they demonstrated that mRNAs coding for the transcription factors NFkappaB, HOX2A, ICSBP, protein kinase JNK2, growth factor HBEGF and receptors RXRbeta and ErbB3 accumulate within the sperm nucleus. The origin and fate of the sperm transcripts remain subject to discussion.

Assembly and Signaling of Adhesion Complexes

Cell-extracellular matrix (ECM) adhesion is crucial for control of cell behavior. It connects the ECM to the intracellular cytoskeleton and transduces bidirectional signals between the extracellular and intracellular compartments. The subcellular machinery that mediates cell-ECM adhesion and signaling is complex. It consists of transmembrane proteins (e.g., integrins) and at least several dozens of membrane-proximal proteins that assemble into a network through multiple protein interactions. Furthermore, despite sharing certain common components, cell-ECM adhesions exhibit considerable heterogeneity in different types of cells (e.g., the cell-ECM adhesions in cardiac myocytes are considerably different from those in fibroblasts). Here, we will first briefly describe the general properties of the integrin-mediated cell-ECM adhesion and signal transduction. Next, we will focus on one of the recently discovered cell-ECM adhesion protein complexes consisting of PINCH, integrin-linked kinase (ILK), and Parvin and use it as an example to illustrate the molecular basis underlying the assembly and functions of cell-ECM adhesions. Finally, we will discuss in detail the structure and regulation of cell-ECM adhesion complexes in cardiac myocytes, which illustrate the importance and complexity of the cell-ECM adhesion structures in organogenesis and diseases.