A central role of EGF receptor transactivation in angiotensin II -induced cardiac hypertrophy

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

AIMS

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

METHODS AND RESULTS

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

CONCLUSION

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

The biochemical, biological, and pathological kaleidoscope of cell surface substrates processed by matrix metalloproteinases

Matrix metalloproteinases (MMPs) constitute a family of more than 20 endopeptidases. Identification of specific matrix and non-matrix components as MMP substrates showed that, aside from their initial role as extracellular matrix modifiers, MMPs play significant roles in highly complex processes such as the regulation of cell behavior, cell-cell communication, and tumor progression. Thanks to the comprehensive examination of the expanded MMP action radius, the initial view of proteases acting in the soluble phase has evolved into a kaleidoscope of proteolytic reactions connected to the cell surface. Important classes of cell surface molecules include adhesion molecules, mediators of apoptosis, receptors, chemokines, cytokines, growth factors, proteases, intercellular junction proteins, and structural molecules. Proteolysis of cell surface proteins by MMPs may have extremely diverse biological implications, ranging from maturation and activation, to inactivation or degradation of substrates. In this way, modification of membrane-associated proteins by MMPs is crucial for communication between cells and the extracellular milieu, and determines cell fate and the integrity of tissues. Hence, insights into the processing of cell surface proteins by MMPs and the concomitant effects on physiological processes as well as on disease onset and evolution, leads the way to innovative therapeutic approaches for cancer, as well as degenerative and inflammatory diseases.

Differential regulation of hypoxia-inducible factor-1 through receptor tyrosine kinase transactivation in vascular smooth muscle cells

Hypoxia-inducible factor-1 (HIF-1) is a decisive element for the transcriptional regulation of many genes expressed in hypoxic conditions. In vascular smooth muscle cells, the vasoactive hormone angiotensin II (Ang II) is a very potent inducer and activator of HIF-1. As opposed to hypoxia, which induces HIF-1alpha by protein stabilization, Ang II induced HIF-1alpha through transcriptional and translational mechanisms. Interestingly, a number of intracellular signaling events triggered by Ang II are mediated by the transactivation of receptor tyrosine kinases. The major receptor tyrosine kinases shown to be transactivated by Ang II in vascular smooth muscle cells are the epidermal growth factor receptor and the IGF-I receptor. In this study, we demonstrate that the transactivation of both these receptor tyrosine kinases is involved in HIF-1 complex activation by Ang II. More interestingly, this modulation of HIF-1 is at different degrees and through different pathways. Our results show that transactivation of IGF-I receptor is essential for HIF-1alpha protein translation through phosphatidylinositol 3-kinase/p70S6 kinase pathway activation, and epidermal growth factor receptor transactivation is implicated in HIF-1 complex activation through the stimulation of the p42/p44 MAPK pathway. Our results therefore show that Ang II-induced receptor tyrosine kinase transactivation is essential in both the induction and activation of HIF-1. These findings identify novel and intricate signaling mechanisms involved in HIF-1 complex activation.

The growth-promoting effects of angiotensin II in adrenal glomerulosa cells: an interactive tale

The zona glomerulosa of the adrenal cortex is well-known for its high level of proliferation, compared to the adjacent zona fasciculata, both in in vivo and in vitro conditions. Angiotensin II (Ang II) is a potent growth factor for glomerulosa cells, appearing as a proliferative factor in vivo, under sodium-deficient diet conditions, as well as in vitro, in studies conducted with whole zona glomerulosa. However, in cells maintained in primary culture for 3 days, Ang II rather promotes cellular hypertrophy with a concomitant arrest in basal cell proliferation. The present essay aims at providing experimental arguments supporting such unexpected observations, with particular focus on the modulatory impact of the extracellular environment on Ang II action, namely AT(1) receptor-induced signaling pathways and cell responses.

Mechanisms of endothelin-1-induced MAP kinase activation in adrenal glomerulosa cells

G protein-coupled receptors (GPCRs) such as angiotensin II, bradykinin and endothelin-1 (ET-1) are critically involved in the regulation of adrenal function, including aldosterone production from zona glomerulosa cells. Whereas, substantial data are available on the signaling mechanisms of ET-1 in cardiovascular tissues, such information in adrenal glomerulosa cells is lacking. Bovine adrenal glomerulosa (BAG) cells express receptors for endothelin-1 (ET-1) and their stimulation caused phosphorylation of Src (at Tyr416), proline-rich tyrosine kinase (Pyk2 at Tyr402), extracellularly regulated signal kinases (ERK1/2), and their dependent proteins, p90 ribosomal S6 kinase (RSK-1) and CREB. ET-1 elicited these responses predominantly through activation of a G(i)-linked cascade with a minor contribution from the G(q)/PKC pathway. Whereas, selective inhibition of EGF-R kinase with AG1478 caused complete inhibition of EGF-induced ERK/RSK-1/CREB activation, it caused only partial reduction (30-40%) of such ET-1-induced responses. Consistent with this, inhibition of matrix metalloproteinases (MMPs) with GM6001 reduced ERK1/2 activation by ET-1, consistent with partial involvement of the MMP-dependent EGF-R activation in this cascade. Activation of ERK/RSK-1/CREB by both ET-1 and EGF was abolished by inhibition of Src, indicating its central role in ET-1 signaling in BAG cells. Moreover, the signaling characteristics of ET-1 in cultured BAG cells closely resembled those observed in clonal adrenocortical H295R cells. The ET-1-induced proliferation of BAG and H295 R cells was much smaller than that induced by Ang II or FGF. These data demonstrate that ET-1 causes ERK/RSK-1/CREB phosphorylation predominantly through activation of G(i) and Src, with a minor contribution from MMP-dependent EGF-R transactivation.

An Angiotensin II Type 1 Receptor Mutant Lacking Epidermal Growth Factor Receptor Transactivation Does Not Induce Angiotensin II–Mediated Cardiac Hypertrophy

We have shown previously that tyrosine 319 in a conserved YIPP motif in the C terminus of angiotensin II (Ang II) type 1 receptors (AT 1 Rs) is essential for transactivation of epidermal growth factor receptor (EGFR) in vitro. We hypothesized that the signaling mechanism mediated through the specific amino acid sequence in the G protein–coupled receptor plays an important role in mediating cardiac hypertrophy in vivo. Transgenic mice with cardiac-specific overexpression of wild-type AT 1 R (Tg-WT) and an AT 1 R with a mutation in the YIPP motif (Tg-Y319F) were studied. Tg-Y319F mice developed no significant cardiac hypertrophy, in contrast to the significant development of hypertrophy in Tg-WT mice. Expression of fetal-type genes, such as atrial natriuretic factor, was also significantly lower in Tg-Y319F than in Tg-WT mice. Infusion of Ang II caused an enhancement of hypertrophy in Tg-WT mice but failed to induce hypertrophy in Tg-Y319F mice. Left ventricular myocardium in Tg-Y319F mice developed significantly less apoptosis and fibrosis than that in Tg-WT mice. EGFR phosphorylation was significantly inhibited in Tg-Y319F mice, confirming that EGFR was not activated in Tg-Y319F mouse hearts. In contrast, activation/phosphorylation of protein kinase C, STAT3, extracellular signal-regulated kinase, and Akt and translocation of Gαq/11 to the cytosolic fraction were maintained in Tg-Y319F hearts. Furthermore, a genetic cross between Tg-WT and transgenic mice with cardiac-specific overexpression of dominant negative EGFR mimicked the phenotype of Tg-Y319F mice. In conclusion, overexpression of AT 1 -Y319F in cardiac myocytes diminished EGFR transactivation and inhibited a pathological form of cardiac hypertrophy. The YIPP motif in the AT 1 R plays an important role in mediating cardiac hypertrophy in vivo.

TACE-dependent EGF receptor activation in angiotensin-II-induced kidney disease

Angiotensin II (Ang II) has been implicated in the development of cardiovascular disorders and chronic kidney disease (CKD). Ang II causes renal lesions through the activation of tumor necrosis factor (TNF)-alpha-converting enzyme (TACE, also called a disintegrin and a metalloproteinase domain 17) and the release of transforming growth factor (TGF)-alpha, which binds to and activates the epidermal growth factor receptor. Renal lesions such as glomerulosclerosis, tubular atrophy, fibrosis, mononuclear cell infiltration and proteinuria following chronic Ang II infusion are substantially reduced in mice lacking TGF-alpha and those given a specific TACE inhibitor. These findings indicate that the selective inhibition of renal TACE could have therapeutic potential in the treatment of CKD.

The AT1A receptor "gain-of-function" mutant N111S/delta329 is both constitutively active and hyperreactive to angiotensin II

The renin-angiotensin-aldosterone system (RAAS) is central to cardiovascular and renal physiology. However, there is no animal model in which the activation of the RAAS only reflects the activation of the angiotensin II (ANG II) AT1 receptor. As a first step to developing such a model, we characterized a gain-of-function mutant of the mouse AT1A receptor. This mutant carries two mutations: N111S predicted to activate the receptor constitutively and a COOH-terminal deletion, delta329, expected to reduce receptor internalization and desensitization. We expressed this double mutant (AT1A-N111S/delta329) in heterologous cells. It showed a pharmacological profile consistent with that of other constitutively active mutants. Furthermore, it increased basal production of inositol phosphates, as well as basal cytosolic and nuclear ERK activities. Basal proliferation of cells expressing the mutant was also greater than that of the wild type. The double mutant was poorly internalized and failed to recruit beta-arrestin 2 in the presence of ANG II. It also showed hypersensitive and hyperreactive responses to ANG II for both inositol phosphate production and ERK activation. The additivity of the phenotypes of the two mutations makes this mutant an appropriate candidate to test the physiological consequences of the AT1A receptor activation itself in transgenic animal models.

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

Stretch of beta1 integrins activates an outwardly rectifying, tamoxifen-sensitive Cl(-) current (Cl(-) SAC) via AT1 receptors, NADPH oxidase, and reactive oxygen species, and Cl(-) SAC resembles the volume-sensitive Cl(-) current (I(Cl,swell)). Epidermal growth factor receptor (EGFR) kinase undergoes transactivation upon stretch, integrin engagement, and AT1 receptor activation and, in turn, stimulates NADPH oxidase. Therefore, we tested whether Cl(-) SAC is regulated by EGFR kinase signaling and is volume sensitive. Paramagnetic beads coated with mAb for beta1 integrin were attached to myocytes and pulled with an electromagnet. Stretch activated a Cl(-) SAC that was 1.13 +/- 0.10 pA/pF at +40 mV. AG1478 (10 muM), an EGFR kinase blocker, inhibited 93 +/- 13% of Cl(-) SAC, and intracellular pretreatment with 1 muM AG1478 markedly suppressed Cl(-) SAC activation. EGF (3.3 nM) directly activated an outwardly rectifying Cl(-) current (0.81 +/- 0.05 pA/pF at +40 mV) that was fully blocked by 10 muM tamoxifen, an I(Cl,swell) blocker. Phosphatidylinositol 3-kinase (PI-3K) is downstream of EGFR kinase. Wortmannin (500 nM) and LY294002 (100 microM), blockers of PI-3K, inhibited Cl(-) SAC by 67 +/- 6% and 91 +/- 25% respectively, and the EGF-induced Cl(-) current also was fully blocked by LY294002. Furthermore, gp91ds-tat (500 nM), a cell-permeable, chimeric peptide that specifically blocks NADPH oxidase assembly, profoundly inhibited the EGF-induced Cl(-) current. Inactive permeant and active impermeant control peptides had no effect. Myocyte shrinkage with hyperosmotic bathing media inhibited the Cl(-) SAC and EGF-induced Cl(-) current by 88 +/- 9% and 127 +/- 11%, respectively. These results suggest that beta1 integrin stretch activates Cl(-) SAC via EGFR, PI-3K, and NADPH oxidase, and that both the Cl(-) SAC and the EGF-induced Cl(-) currents are likely to be the volume-sensitive Cl(-) current, I(Cl,swell).

Costameres, focal adhesions, and cardiomyocyte mechanotransduction

Mechanotransduction refers to the cellular mechanisms by which load-bearing cells sense physical forces, transduce the forces into biochemical signals, and generate appropriate responses leading to alterations in cellular structure and function. This process affects the beat-to-beat regulation of cardiac performance but also affects the proliferation, differentiation, growth, and survival of the cellular components that comprise the human myocardium. This review focuses on the experimental evidence indicating that the costamere and its structurally related structure the focal adhesion complex are critical cytoskeletal elements involved in cardiomyocyte mechanotransduction. Biochemical signals originating from the extracellular matrix-integrin-costameric protein complex share many common features with those signals generated by growth factor receptors. The roles of key regulatory kinases and other muscle-specific proteins involved in mechanotransduction and growth factor signaling are discussed, and issues requiring further study in this field are outlined.

Inhibition of Histone Deacetylation Blocks Cardiac Hypertrophy Induced by Angiotensin II Infusion and Aortic Banding

Background— A number of distinct stress signaling pathways in myocardium cause cardiac hypertrophy and heart failure. Class II histone deacetylases (HDACs) antagonize several stress-induced pathways and hypertrophy. However, cardiac hypertrophy induced by transgenic overexpression of the homeodomain only protein, HOP, can be prevented by the nonspecific HDAC inhibitors trichostatin A and valproic acid, suggesting that alternate targets that oppose class II HDAC function might exist in myocardium. We tested the effects of several HDAC inhibitors, including a class I HDAC-selective inhibitor, SK-7041, on cardiac hypertrophy induced by angiotensin II (Ang II) treatment or aortic banding (AB).

Methods and Results— Cardiac hypertrophy was induced by chronic infusion of Ang II or by AB in mice or rats and evaluated by determining the ratio of heart weight to body weight or to tibia length, cross-sectional area, or echocardiogram. Cardiac hypertrophy induced by Ang II or AB for 2 weeks was significantly reduced by simultaneous administration of trichostatin A, valproic acid, or SK-7041. Echocardiogram revealed that exaggerated left ventricular systolic dimensions were relieved by HDAC inhibitors. HDAC inhibitors partially reversed preestablished cardiac hypertrophy and improved survival of AB mice. The expressions of atrial natriuretic factor, α-tubulin, β-myosin heavy chain, and interstitial fibrosis were reduced by HDAC inhibition.

Conclusions— These results suggest that the predominant effect of HDAC inhibition, mainly mediated by class I HDACs, is to prevent cardiac hypertrophy in response to a broad range of agonist and stretch stimuli.

Dependence of GnRH-induced phosphorylation of CREB and BAD on EGF receptor transactivation in GT1-7 neuronal cells

The hypothalamic neuropeptide, gonadotropin releasing hormone (GnRH), is a primary regulatory factor in the neuroendocrine control of reproduction. The GnRH decapeptide is released in an episodic manner from hypothalamic GnRH neurons, which are known to express GnRH receptors. Here we examined the signaling pathways by which autocrine GnRH stimulation generates cell survival and proliferative signals in hypothalamic GT1-7 cells. Both GnRH and epidermal growth factor (EGF) caused rapid phosphorylation of cyclic AMP response element binding protein (CREB) and BAD. The selective epidermal growth factor receptor (EGF-R) antagonist, AG1478, attenuates the phosphorylation of these proteins by GnRH and EGF. Inhibition of PKC and Src abolished the stimulatory effects of GnRH, but not that of EGF, consistent with a critical role of these signaling molecules upstream of the EGF-R. All of these effects of GnRH were mimicked by phorbol 12 myristate 13-acetate (PMA). Consistent with the prosurvival and mitogenic effects of phosphoinositide 3-kinase/Akt (P13-K/Akt) downstream of the EGF-R, inhibition of P13-K diminished the activation of these proteins following stimulation with GnRH, EGF, and PMA. Overexpression of dominant negative Akt attenuated agonist-induced phosphorylation of BAD, but not that of ERK1/2 and CREB. Moreover, overexpression of wild-type RSK-1 resulted in enhanced basal as well as agonist-induced phosphorylation of CREB and BAD, indicating a critical role of RSK-1 in activating cytosolic as well as nuclear proteins. These data reveal novel signaling mechanisms of GnRH-induced phosphorylation of CREB and BAD in GT1-7 neurons through transactivation of the EGF-R.

Role of metalloproteinase-dependent EGF receptor activation in α1-adrenoceptor-stimulated MAP kinase phosphorylation in GT1-7 neurons

Adrenoceptors (ARs) are involved in the regulation of gonadotropin-releasing hormone (GnRH) release from native and immortalized hypothalamic (GT1-7) neurons. However, the AR-mediated signaling mechanisms and their functional significance in these cells are not known. Stimulation of GT1-7 cells with the alpha1-AR agonist, phenylephrine (Phe), causes phosphorylation of extracellular signal-regulated kinases 1 and 2 (ERK1/2) mitogen-activated protein (MAP) kinases that is mediated by protein kinase C (PKC)-dependent transactivation of the epidermal growth factor receptor (EGF-R). Phe stimulation causes shedding of the soluble ligand, heparin-binding EGF (HB-EGF), as a consequence of matrix metalloproteinase (MMP) activation. Phe-induced phosphorylation of the EGF-R, and subsequently of Shc and ERK1/2, was attenuated by inhibition of MMP or HB-EGF with the selective inhibitor, CRM197, or by a neutralizing antibody. In contrast, phosphorylation of the EGF-R, Shc and ERK1/2 by EGF and HB-EGF was independent of PKC and MMP activity. Moreover, inhibition of Src attenuated ERK1/2 responses by Phe, but not by HB-EGF and EGF, indicating that Src acts upstream of the EGF-R. Consistent with a potential role of reactive oxygen species (ROS), Phe-induced phosphorylation of EGF-R was attenuated by the antioxidant, N-acetylcysteine. These data suggest that activation of the alpha1-AR causes phosphorylation of ERK1/2 through activation of PKC, ROS and Src, and shedding of HB-EGF, which binds to and activates the EGF-R.

Angiotensin II promotes the proliferation of activated pancreatic stellate cells by Smad7 induction through a protein kinase C pathway

Activated pancreatic stellate cells (PSCs) play major roles in promoting pancreatic fibrosis. We previously reported that angiotensin II (Ang II) enhances activated PSC proliferation through EGF receptor transactivation. In the present study, we elucidated a novel intracellular mechanism by which Ang II stimulates cellular proliferation. TGF-beta1 inhibits activated PSC proliferation via a Smad3 and Smad4-dependent pathway in an autocrine manner. We demonstrated that Ang II inhibited TGF-beta1-induced nuclear accumulation of Smad3 and Smad4. Furthermore, Ang II rapidly induced inhibitory Smad7 mRNA expression. Adenovirus-mediated Smad7 overexpression inhibited TGF-beta1-induced nuclear accumulation of Smad3 and Smad4, and potentiated activated PSC proliferation. PKC inhibitor Go6983 blocked the induction of Smad7 mRNA expression by Ang II. In addition, 12-O-tetradecanoyl-phorbol 13-acetate, a PKC activator, increased Smad7 mRNA expression. These results suggest that Ang II enhances activated PSC proliferation by blocking autocrine TGF-beta1-mediated growth inhibition by inducing Smad7 expression via a PKC-dependent pathway.

Type 2 diabetes causes remodeling of cerebrovasculature via differential regulation of matrix metalloproteinases and collagen synthesis: role of endothelin-1

The risk of cerebrovascular disease is four- to sixfold higher in patients with diabetes. Vascular remodeling, characterized by extracellular matrix deposition and an increased media-to-lumen ratio, occurs in diabetes and contributes to the development of complications. However, diabetes-induced changes in the cerebrovascular structure remain unknown. Endothelin-1 (ET-1), a potent vasoconstrictor with profibrotic properties, is chronically elevated in diabetes. To determine diabetes-mediated changes in the cerebrovasculature and the role of ET-1 in this process, type 2 diabetic Goto-Kakizaki (GK) rats were administered an ET(A) receptor antagonist for 4 weeks. Middle cerebral arteries were harvested and studies were performed to determine vascular structure. Tissue and plasma ET-1 levels were increased in GK rats compared with controls. Significant medial hypertrophy and collagen deposition resulted in an increased wall-to-lumen ratio in diabetic rats that was reduced by ET(A) receptor antagonism. Vascular matrix metalloproteinase (MMP)-2 activity was higher, but MMP-1 levels were significantly reduced in GK rats, and MMP levels were restored to control levels by ET(A) receptor antagonism. We conclude that ET-1 promotes cerebrovascular remodeling in type 2 diabetes through differential regulation of MMPs. Augmented cerebrovascular remodeling may contribute to an increased risk of stroke in diabetes, and ET(A) receptor antagonism may offer a novel therapeutic target.

Pleiotropic AT1 receptor signaling pathways mediating physiological and pathogenic actions of angiotensin II

Angiotensin II (Ang II) activates a wide spectrum of signaling responses via the AT1 receptor (AT1R) that mediate its physiological control of blood pressure, thirst, and sodium balance and its diverse pathological actions in cardiovascular, renal, and other cell types. Ang II-induced AT1R activation via Gq/11 stimulates phospholipases A2, C, and D, and activates inositol trisphosphate/Ca2+ signaling, protein kinase C isoforms, and MAPKs, as well as several tyrosine kinases (Pyk2, Src, Tyk2, FAK), scaffold proteins (G protein-coupled receptor kinase-interacting protein 1, p130Cas, paxillin, vinculin), receptor tyrosine kinases, and the nuclear factor-kappaB pathway. The AT1R also signals via Gi/o and G11/12 and stimulates G protein-independent signaling pathways, such as beta-arrestin-mediated MAPK activation and the Jak/STAT. Alterations in homo- or heterodimerization of the AT1R may also contribute to its pathophysiological roles. Many of the deleterious actions of AT1R activation are initiated by locally generated, rather than circulating, Ang II and are concomitant with the harmful effects of aldosterone in the cardiovascular system. AT1R-mediated overproduction of reactive oxygen species has potent growth-promoting, proinflammatory, and profibrotic actions by exerting positive feedback effects that amplify its signaling in cardiovascular cells, leukocytes, and monocytes. In addition to its roles in cardiovascular and renal disease, agonist-induced activation of the AT1R also participates in the development of metabolic diseases and promotes tumor progression and metastasis through its growth-promoting and proangiogenic activities. The recognition of Ang II's pathogenic actions is leading to novel clinical applications of angiotensin-converting enzyme inhibitors and AT1R antagonists, in addition to their established therapeutic actions in essential hypertension.

Multi-tasking RGS proteins in the heart: the next therapeutic target?

Regulator of G-protein-signaling (RGS) proteins play a key role in the regulation of G-protein-coupled receptor (GPCR) signaling. The characteristic hallmark of RGS proteins is a conserved approximately 120-aa RGS region that confers on these proteins the ability to serve as GTPase-activating proteins (GAPs) for G(alpha) proteins. Most RGS proteins can serve as GAPs for multiple isoforms of G(alpha) and therefore have the potential to influence many cellular signaling pathways. However, RGS proteins can be highly regulated and can demonstrate extreme specificity for a particular signaling pathway. RGS proteins can be regulated by altering their GAP activity or subcellular localization; such regulation is achieved by phosphorylation, palmitoylation, and interaction with protein and lipid-binding partners. Many RGS proteins have GAP-independent functions that influence GPCR and non-GPCR-mediated signaling, such as effector regulation or action as an effector. Hence, RGS proteins should be considered multifunctional signaling regulators. GPCR-mediated signaling is critical for normal function in the cardiovascular system and is currently the primary target for the pharmacological treatment of disease. Alterations in RGS protein levels, in particular RGS2 and RGS4, produce cardiovascular phenotypes. Thus, because of the importance of GPCR-signaling pathways and the profound influence of RGS proteins on these pathways, RGS proteins are regulators of cardiovascular physiology and potentially novel drug targets as well.

The epidermal growth factor receptors and their family of ligands: their putative role in atherogenesis

The epidermal growth factor receptor is a member of type-I growth factor receptor family with tyrosine kinase activity that is activated following the binding of multiple cognate ligands. Several members of the EGF family of ligands are expressed by cells involved in atherogenesis. EGF receptor mediated processes have been well characterised within epithelial, smooth muscle and tumour cell lines in vitro, and the EGF receptor has been identified immunocytochemically on intimal smooth muscle cells within atherosclerotic plaques. There is also limited evidence for the expression of the EGF receptor family on leukocytes, although their function has yet to be clarified. In this review, we will discuss the biological functions of this receptor and its ligands and their potential to modulate the function of cells involved in the atherosclerotic process.

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.

Local overexpression of HB-EGF exacerbates remodeling following myocardial infarction by activating noncardiomyocytes

Insulin-like growth factor (IGF), hepatocyte growth factor (HGF), and heparin-binding epidermal growth factor-like growth factor (HB-EGF) are cardiogenic and cardiohypertrophic growth factors. Although the therapeutic effects of IGF and HGF have been well demonstrated in injured hearts, it is uncertain whether natural upregulation of HB-EGF after myocardial infarction (MI) plays a beneficial or pathological role in the process of remodeling. To answer this question, we conducted adenoviral HB-EGF gene transduction in in vitro and in vivo injured heart models, allowing us to highlight and explore the HB-EGF-induced phenotypes. Overexpressed HB-EGF had no cytoprotective or additive death-inducible effect on Fas-induced apoptosis or oxidative stress injury in primary cultured mouse cardiomyocytes, although it significantly induced hypertrophy of cardiomyocytes and proliferation of cardiac fibroblasts. Locally overexpressed HB-EGF in the MI border area in rabbit hearts did not improve cardiac function or exhibit an angiogenic effect, and instead exacerbated remodeling at the subacute and chronic stages post-MI. Namely, it elevated the levels of apoptosis, fibrosis, and the accumulation of myofibroblasts and macrophages in the MI area, in addition to inducing left ventricular hypertrophy. Thus, upregulated HB-EGF plays a pathophysiological role in injured hearts in contrast to the therapeutic roles of IGF and HGF. These results imply that regulation of HB-EGF may be a therapeutic target for treating cardiac hypertrophy and fibrosis.

Mechanisms of extracellularly regulated kinases 1/2 activation in adrenal glomerulosa cells by lysophosphatidic acid and epidermal growth factor

The regulation of adrenal function, including aldosterone production from adrenal glomerulosa cells, is dependent on a variety of G protein-coupled receptors (GPCRs) and receptor tyrosine kinases (RTKs). In many cell types, GPCR-mediated MAPK activation is mediated through transactivation of RTKs, in particular the epidermal growth factor (EGF) receptor (EGF-R). However, the extent to which this cross-communication between GPCRs and RTKs is operative in the adrenal glomerulosa has not been defined. Bovine adrenal glomerulosa cells express receptors for lysophosphatidic acid (LPA) and EGF. In cultured bovine adrenal glomerulosa cells, LPA, which is predominantly coupled to Gi and partially to Gq/protein kinase C alpha and epsilon, caused phosphorylation of Src (at Tyr416), proline-rich tyrosine kinase (Pyk2 at Tyr402), EGF-R, protein kinase B/Akt, extracellularly regulated signal kinases 1/2, and their dependent protein, p90 ribosomal S6 kinase. Overexpression of dominant negative mutants of Ras or EGF-R, and selective inhibition of EGF-R kinase with AG1478, significantly reduced LPA-induced ERK1/2 phosphorylation. However, this was not impaired by inhibition of matrix metalloproteinase (MMP) and heparin-binding EGF. LPA-induced ERK1/2 activation occurs predominantly through EGF-R transactivation by Gi/Src and partly through activation of protein kinase C, which acts downstream of EGF-R and Ras. In contrast, LPA-induced phosphorylation of Shc and ERK1/2 in clonal hepatocytes (C9 cells) was primarily mediated through MMP-dependent transactivation of the EGF-R. These observations in adrenal glomerulosa and hepatic cells demonstrate that LPA phosphorylates ERK1/2 through EGF-R transactivation in a MMP-dependent or -independent manner in individual target cells. This reflects the ability of GPCRs expressed in cell lines and neoplastic cells to utilize distinct signaling pathways that can elicit altered responses compared with those of native tissues.

Role of EGF receptor transactivation in phosphoinositide 3-kinase-dependent activation of MAP kinase by GPCRs

Many G protein coupled receptors (GPCRs) cause phosphorylation of MAP kinases through transactivation of the epidermal growth factor receptor (EGF-R), leading to increased cell survival and growth, motility, and migration. Phosphoinositide 3-kinase (PI3K) is one of the important cell survival signaling molecules activated by EGF-R stimulation. However, the extent to which EGF-R transactivation is essential for GPCR agonist-stimulated PI3K activation is not known. Here we examined the mechanism of PI3K activation that elicits GPCR-mediated ERK1/2 activation by pathways dependent and/or independent of EGF-R transactivation in specific cell types. Immortalized hypothalamic neurons (GT1-7 cells) express endogenous gonadotropin-releasing hormone receptors (GnRH-R) and their stimulation causes marked phosphorylation of ERK1/2 and Akt (Ser 473) through transactivation of the EGF-R and recruitment of PI3K. In C9 hepatocytes, agonist activation of AT1 angiotensin II (AT1-R), lysophosphatidic acid (LPA), and EGF receptors caused phosphorylation of Akt through activation of the EGF-R in a PI3K-dependent manner. However, ERK1/2 activation by these agonists in these cells was independent of PI3K activation. In contrast, agonist stimulation of HEK 293 cells stably expressing AT1-R caused ERK1/2 phosphorylation that was independent of EGF-R transactivation but required PI3K activation. LPA signaling in these cells showed partial and complete dependence on EGF-R and PI3K, respectively. These data indicate that GPCR-induced ERK1/2 phosphorylation is dependent or independent of PI3K in specific cell types, and that the involvement of PI3K during ERK1/2 activation is not dependent solely on agonist-induced transactivation of the EGF-R.

Molecular Mechanisms of Hormonal Activity. II. Kinase Systems. Systems with Intracellular Receptors. Transactivation of STS

Hormone receptors and other components, functional mechanisms, and biological role of analyzed signal transduction systems (STS) are described. The recently revealed module principle of the structure and STS transactivation providing diversity and plasticity of regulation are highlighted. STS activities are significantly changed in many diseases. Novel promising pharmaceuticals targeted to certain components of STS increase in number from year to year. The data published by the beginning of January 2004 are summarized in this review.

Transactivation of the EGF receptor and a PI3 kinase-ATF-1 pathway is involved in the upregulation of NOX1, a catalytic subunit of NADPH oxidase

We previously reported that hypertrophy of vascular smooth muscle cells caused by prostaglandin (PG) F2alpha is mediated by the induction of NOX1, a catalytic subunit of NADPH oxidase that generates superoxide. The signal transduction pathway(s) involved in this process, however, remained unresolved. PGF2alpha enhanced the phosphorylation of the epidermal growth factor (EGF) receptor, and a selective inhibitor of EGF receptor kinase, tyrphostin AG1478, significantly suppressed PGF2alpha-induced NOX1 expression. AG1478 also blunted the PGF2alpha-induced phosphorylation of extracellular signal-regulated protein kinase (ERK)1/2 and Akt. Phosphoinositide 3 (PI3) kinase inhibitors not only reduced PGF2alpha-induced NOX1 expression, but also suppressed the phosphorylation of ATF-1, a transcription factor previously shown to play a key role in the induction of NOX1. Accordingly, the transactivation of the EGF receptor and the activation of ERK1/2, PI3 kinase, and ATF-1 constitute the signaling pathways involved in the upregulation of NOX1.

Epidermal Growth Factor Receptor-Dependent and -Independent Pathways in Hydrogen Peroxide-Induced Mitogen-Activated Protein Kinase Activation in Cardiomyocytes and Heart Fibroblasts

Mild doses of oxidative stress in the heart correlate with the induction of apoptosis or hypertrophy in cardiomyocytes (CMCs) and fibrosis or proliferation of fibroblasts. Three branches of mitogen-activated protein kinases (MAPKs), i.e., c-Jun N-terminal kinases (JNKs), extracellular signal-related kinases 1 and 2 (ERK1/2), and p38, are activated by oxidants in a variety of cell types, including CMCs. However, the initiation process of these signaling pathways remains unsolved. We explored the role of the epidermal growth factor (EGF) receptor in H(2)O(2)-induced MAPK activation using two different cell types from the same organ: CMCs and heart fibroblasts (HFs). Pretreatment of each cell type with EGF revealed differences in how CMCs and HFs responded to subsequent treatment with H(2)O(2): in CMCs, the second treatment resulted in little further activation of JNKs and ERK1/2, whereas HFs retained the full response of JNKs and ERK1/2 activation by H(2)O(2) regardless of EGF pretreatment. AG-1478 [4-(3'-chloroanilino)-6,7-dimethoxy-quinazoline], a pharmacologic inhibitor of the EGF receptor tyrosine kinase, inhibited JNK and ERK1/2 activations but not p38 in both cell types. The data using the Src inhibitor PP2 [4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine] resemble those found when using AG-1478 in either cell type. Pharmacologic inhibitors of matrix metalloproteinases (MMPs) further illustrated the difference between the two cell types. In HFs, MMP inhibitors GM6001 [N-[(2R)-2-(hydroxamidocarbonylmethyl)-4-methylpentanoyl]-l-tryptophan methylamide] and BB2516 [[2S-[N4(R(*)),2R(*),3S(*)]]-N4-[2,2-dimethyl-1-[(methylamino)carbonyl]propyl]-N1,2-dihydroxy-3-(2-methylpropyl)butanediamide, marimastat] inhibited JNKs and ERK1/2 activation without affecting p38 activation by H(2)O(2) inhibitors. In contrast, these MMP failed to significantly inhibit the activation of JNKs, ERKs, or p38 in CMCs. These data suggest the complexity of the cell type-dependent signaling web initiated by oxidants in the heart.

The role of the AT1 angiotensin receptor in cardiac hypertrophy: angiotensin II receptor or stretch sensor?

Activation of the AT(1) angiotensin receptor is a clinically important maladaptive response during cardiac hypertrophy. Autocrine and paracrine effects of locally generated angiotensin II, are believed to be the main mediators of these responses. However, a recent report has suggested that mechanical stress can activate AT(1) receptors independently of angiotensin II generation. This finding, as well as recent studies on intracrine effects and the pharmacological consequences of receptor hetero-oligomerization, suggest that unexpected mechanisms could contribute to the role of the renin-angiotensin system during cardiac hypertrophy.

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.

The protein kinase C inhibitor Go6976 [12-(2-cyanoethyl)-6,7,12,13-tetrahydro-13-methyl-5-oxo-5H-indolo(2,3-a)pyrrolo(3,4-c)-carbazole] potentiates agonist-induced mitogen-activated protein kinase activation through tyrosine phosphorylation of the epidermal growth factor receptor

Protein kinase C (PKC) isoforms are important transducers of signals from G protein-coupled receptors (GPCRs) to diverse cellular targets, including extracellular signal-regulated kinases 1 and 2 (ERK1/2). Clone 9 rat hepatocytes (C9 cells) express receptors for angiotensin II (Ang II) type 1, lysophosphatidic acid (LPA), and epidermal growth factor (EGF), and their stimulation causes transient ERK1/2 phosphorylation through transactivation of the epidermal growth factor receptor (EGF-R). Inhibition of PKC by Go6983 [2-[1-(3-dimethylaminopropyl)-5-methoxyindol-3-yl]-3-(1H-indol-3-yl)maleimide], or PKC depletion by prolonged phorbol 12-myristate 13-acetate (PMA) treatment, attenuated ERK1/2 activation by Ang II and PMA, but not by LPA and EGF. In contrast, another PKC inhibitor, Go6976 [12-(2-cyanoethyl)-6,7,12,13-tetrahydro-13-methyl-5-oxo-5H-indolo(2,3-a)pyrrolo(3,4-c)-carbazole], enhanced basal and agonist-stimulated phosphorylation of ERK1/2, which was not caused by alteration in receptor binding and internalization, stimulation of inositol phosphate production, or activation of Pyk2 and Src tyrosine kinases. However, Go6976 enhanced agonist-induced tyrosine phosphorylation of the EGF receptor, possibly through inhibition of protein tyrosine phosphatase (PTP), because the PTP inhibitor sodium orthovanadate mimicked the effects of Go6976. Selective blockade of EGF-R kinase by AG1478 [4-(3-chloroanilino)6,7-dimethoxyquinazoline] abolished the ERK1/2 activation induced by Go6976. Similar experiments were conducted in human embryonic kidney 293 cells, which express receptors for LPA and EGF but exhibit no significant cross-communication between them. Although Go6976 caused a significant increase in EGF-induced tyrosine phosphorylation of the EGF-R and subsequent ERK1/2 activation, it had no such effects on LPA-induced responses. In Chinese hamster ovary cells, which express receptors for LPA but not for EGF, Go6976 also had no significant effect on LPA-induced ERK1/2 activation. These data indicate that Go6976 potentiates agonist-induced ERK1/2 activation through stimulation of tyrosine phosphorylation of the EGF-R.

Urotensin II Promotes Hypertrophy of Cardiac Myocytes via Mitogen-Activated Protein Kinases

Urotensin II and its receptor are coexpressed in the heart and up-regulated during cardiac dysfunction. In cultured neonatal cardiomyocytes, we mimicked this up-regulation using an adenovirus to increase expression of the urotensin receptor. In this model system, urotensin II promoted strong hypertrophic growth and phenotypic changes, including cell enlargement and sarcomere reorganization. Urotensin II potently activated the MAPKs, ERK1/2 and p38, and blocking these kinases with PD098059 and SB230580, respectively, significantly inhibited urotensin II-mediated hypertrophy. In contrast, urotensin II did not activate JNK. The activation of ERK1/2 and p38 as well as cellular hypertrophy was independent of protein kinase C, and calcium and phosphoinositide 3-kinase, yet dependent on the capacity of the urotensin receptor to trans-activate the epidermal growth factor receptor. Urotensin II promoted the tyrosine phosphorylation of epidermal growth factor receptors, which was inhibited by the selective epidermal growth factor receptor kinase inhibitor, AG1478. These data indicate that perturbations in cardiac homeostasis, which lead to up-regulation of urotensin II receptors, promote urotensin II-mediated cardiomyocyte hypertrophy via ERK1/2 and p38 signaling pathways in an epidermal growth factor receptor-dependent manner.

Emerging roles of urotensin-II in cardiovascular disease

Urotensin-II (UII) is a highly potent endogenous peptide within the cardiovascular system. Through stimulation of Galphaq-coupled UT receptors, UII mediates contraction of vascular smooth muscle and endothelial-dependent vasorelaxation, and positive inotropy in human right atrium and ventricle. A pathogenic role of the UT receptor system is emerging in cardiovascular disease states, with evidence for up-regulation of the UT receptor system in patients with congestive heart failure (CHF), pulmonary hypertension, cirrhosis and portal hypertension, and chronic renal failure. In vitro and in vivo studies show that under pathophysiological conditions, UII might contribute to cardiomyocyte hypertrophy, extracellular matrix production, enhanced vasoconstriction, vascular smooth muscle cell hyperplasia, and endothelial cell hyper-permeability. Single nucleotide polymorphisms of the UII gene may also impart a genetic predisposition of patients to diabetes. Therefore, the UT receptor system is a potential therapeutic target in the treatment of cardiac, pulmonary, and renal diseases. UT receptor antagonists are currently being developed to prevent and/or reverse the effects of over-activated UT receptors by the endogenous ligand. This review describes UII peptide and converting enzymes, and UT receptors in the cardiovascular system, focusing on pathophysiological roles of UII in the heart and blood vessels.

Differential effects of tyrosine kinase inhibitors on volume-sensitive chloride current in human atrial myocytes: evidence for dual regulation by Src and EGFR kinases

To determine whether protein tyrosine kinase (PTK) modulates volume-sensitive chloride current (I_Cl.vol) in human atrial myocytes and to identify the PTKs involved, we studied the effects of broad-spectrum and selective PTK inhibitors and the protein tyrosine phosphatase (PTP) inhibitor orthovanadate (VO₄⁻³). I_Cl.vol evoked by hyposmotic bath solution (0.6-times isosmotic, 0.6T) was enhanced by genistein, a broad-spectrum PTK inhibitor, in a concentration-dependent manner (EC₅₀ = 22.4 μM); 100 μM genistein stimulated I_Cl.vol by 122.4 ± 10.6%. The genistein-stimulated current was inhibited by DIDS (4,4′-diisothiocyanostilbene-2,2′-disulfonic acid, 150 μM) and tamoxifen (20 μM), blockers of I_Cl.vol. Moreover, the current augmented by genistein was volume dependent; it was abolished by hyperosmotic shrinkage in 1.4T, and genistein did not activate Cl⁻ current in 1T. In contrast to the stimulatory effects of genistein, 100 μM tyrphostin A23 (AG 18) and A25 (AG 82) inhibited I_Cl.vol by 38.2 ± 4.9% and 40.9 ± 3.4%, respectively. The inactive analogs, daidzein and tyrphostin A63 (AG 43), did not alter I_Cl.vol. In addition, the PTP inhibitor VO₄⁻³ (1 mM) reduced I_Cl.vol by 53.5 ± 4.5% (IC₅₀ = 249.6 μM). Pretreatment with VO₄⁻³ antagonized genistein-induced augmentation and A23- or A25-induced suppression of I_Cl.vol. Furthermore, the selective Src-family PTK inhibitor PP2 (5 μM) stimulated I_Cl.vol, mimicking genistein, whereas the selective EGFR (ErbB-1) kinase inhibitor tyrphostin B56 (AG 556, 25 μM) reduced I_Cl.vol, mimicking A23 and A25. The effects of both PP2 and B56 also were substantially antagonized by pretreatment with VO₄⁻³. The results suggest that I_Cl.vol is regulated in part by the balance between PTK and PTP activity. Regulation is complex, however. Src and EGFR kinases, distinct soluble and receptor-mediated PTK families, have opposing effects on I_Cl.vol, and multiple target proteins are likely to be involved.

Matrix metalloproteinases in reproductive endocrinology

One of the most common mechanisms for transactivation of epidermal growth factor receptor (EGF-R) by G protein-coupled receptors (GPCRs) is through the release of local EGF-like ligands from transmembrane precursors by the proteolytic action of matrix metalloproteinases (MMPs). These enzymes are crucial factors in the normal physiology of the reproductive system and also participate in neuroendocrine regulation through mediation of gonadotropin-releasing hormone (GnRH) action. Recent studies by Roelle et al. showed that GnRH-induced activation of the EGF-R and extracellular signal-regulated kinases 1 and 2 (ERK1/2) in pituitary gonadotrophs occurs through ectodomain shedding of heparin binding-EGF (HB-EGF) by MMP2 and MMP9, indicating a crucial role for MMPs in GnRH signalling.

Urocortin-II and urocortin-III are cardioprotective against ischemia reperfusion injury: an essential endogenous cardioprotective role for corticotropin releasing factor receptor type 2 in the murine heart

Corticotropin-releasing factor (CRF) receptor type 2beta (CRFR2beta) is expressed in the heart. Urocortin (Ucn)-I activation of CRFR2beta is cardioprotective against ischemic reperfusion (I/R) injury by stimulation of the ERKs1/2 p42, 44. However, by binding CRF receptor type 1, Ucn-I can also activate the hypothalamic stress axis. Ucn-II/stresscopin related peptide and Ucn-III/stresscopin are two new members of the CRF/Ucn-I gene family and are selective for CRFR2beta. We propose that CRFR2beta selective Ucn-II or Ucn-III will protect cardiomyocytes and the ex vivo Langendorff perfused rat heart from I/R injury by activation of ERK1/2-p42, 44. Ucn-II is expressed in mouse cardiomyocytes, and Ucn-II or Ucn-III can bind to CRFR2beta, resulting in ERK1/2-p42, p-44 phosphorylation and cAMP stimulation. Phosphorylation of ERK1/2-p42, p-44 is regulated by the Ras/Raf-1 kinase pathway, independent of adenylate cyclase and, therefore, cAMP activation. Ucn-II and Ucn-III protect cardiomyocytes from I/R injury and reduce the percentage of infarct size:risk ratio in Langendorff perfused rat hearts exposed to regional I/R (P<0.001). The CRFR2 selective antagonist astressin2-B and an ERK1/2-p42, 44 inhibitor abolish the cardioprotective actions of Ucn-II and Ucn-III in reperfusion. Cardiomyocytes isolated from CRFR2-null mice are less resistant to I/R injury, compared with wild-type cardiomyocytes. We propose the use of CRFR2 selective agonists, Ucn-II and Ucn-III, to treat ischemic heart disease because of their potent cardioprotective effects in the murine heart and their minimal impact on the hypothalamic stress axis. We emphasize an important endogenous cardioprotective role for CRFR2beta in the murine heart.

Ras, Akt, and Mechanotransduction in the Cardiac Myocyte

The Ras subfamily of 21-kDa ("small") guanine nucleotide binding proteins [which includes Ha-Ras, Ki(A)-Ras, Ki(B)-Ras, and N-Ras] is universally important in regulating intracellular signaling events in mammalian cells and controls their growth, proliferation, senescence, differentiation, and survival. These Ras isoforms act as membrane-associated biological switches that transduce signals from transmembrane receptors, thus potentially activating a variety of downstream signaling proteins. These include ultimately two Ser/Thr protein kinase families, the extracellular signal-regulated kinases 1/2 (ERK1/2) and Akt (or protein kinase B). Activation of ERK1/2 has been associated with cardiac myocyte hypertrophy (ie, increased cell size and myofibrillogenesis, with concurrent transcriptional changes to a fetal pattern of gene expression), whereas activation of Akt is associated with the increased protein accretion in hypertrophy. Both ERK1/2 and Akt may promote myocyte survival. In the intact heart in vivo and in primary cultures of cardiac myocytes, mechanical strain induces hypertrophy, a process known as mechanotransduction, which may involve Ras, ERK1/2, and Akt. In this study, general and cardiospecific aspects of the regulation of Ras and Akt will be described. The various mechanisms through which mechanical strain might initiate Ras- or Akt-dependent signaling will be discussed. The overall conclusion is that although an involvement of Ras and Akt in mechanotransduction is likely, more work (particularly focusing on mechanoreception) needs to be undertaken before it is unequivocally established.