Autocrine and Paracrine Regulation of Myocardial Cell Growth in Vitro The TGFβ Paradigm

Piezo1 Channel as a Potential Target for Hindering Cardiac Fibrotic Remodeling

Fibrotic tissues share many common features with neoplasms where there is an increased stiffness of the extracellular matrix (ECM). In this review, we present recent discoveries related to the role of the mechanosensitive ion channel Piezo1 in several diseases, especially in regulating tumor progression, and how this can be compared with cardiac mechanobiology. Based on recent findings, Piezo1 could be upregulated in cardiac fibroblasts as a consequence of the mechanical stress and pro-inflammatory stimuli that occurs after myocardial injury, and its increased activity could be responsible for a positive feedback loop that leads to fibrosis progression. The increased Piezo1-mediated calcium flow may play an important role in cytoskeleton reorganization since it induces actin stress fibers formation, a well-known characteristic of fibroblast transdifferentiation into the activated myofibroblast. Moreover, Piezo1 activity stimulates ECM and cytokines production, which in turn promotes the phenoconversion of adjacent fibroblasts into new myofibroblasts, enhancing the invasive character. Thus, by assuming the Piezo1 involvement in the activation of intrinsic fibroblasts, recruitment of new myofibroblasts, and uncontrolled excessive ECM production, a new approach to blocking the fibrotic progression can be predicted. Therefore, targeted therapies against Piezo1 could also be beneficial for cardiac fibrosis.

Lipoxygenase inhibitor ML351 dysregulated an innate inflammatory response leading to impaired cardiac repair in acute heart failure

The presistent increase of 12/15 lipoxygenase enzyme activity is correlated with uncontrolled inflammation, leading to organ dysfunction. ML351, a potent 12/15 lipoxygenase (12/15LOX) inhibitor, was reported to reduce infarct size and inflammation in a murine ischemic stroke model. In the presented work, we have applied three complementary experimental approaches, in-vitro, ex-vivo, and in-vivo, to determine whether pharmacological inhibition of 12/15LOX could dampen the inflammatory response in adult mice after Kdo2-Lipid A (KLA) as an endotoxin stimulator or post myocardial infarction (MI). Male C57BL/6 (8-12 weeks) mice were subjected to permanent coronary ligation thereby inducing acute heart failure (MI-d1 and MI-d5) for in-vivo studies. 12/15LOX antagonist ML351 (50 mg/kg) was subcutaneously injected 2 h post-MI, while MI-controls received saline. For ex-vivo experiments, ML351 (25 mg/kg) was injected as bolus after 5 min of inflammatory stimulus (KLA 1 μg/g) injection. Peritoneal macrophages (PMɸ) were harvested after 4 h post KLA. For in-vitro studies, PMɸ were treated with KLA (100 ng/mL), ML351 (10 µM), or KLA + ML351 for 4 h, and inflammatory response was evaluated. In-vivo, 5LOX expression was reduced after ML351 administration, inducing a compensatory increase of 12LOX that sensitized PMɸ toward a proinflammatory state. This was marked by higher inflammatory cytokines and dysregulation of the splenocardiac axis post-MI. ML351 treatment increased CD11b⁺ and Ly6C^high populations in spleen and Ly6G⁺ population in heart, with a decrease in F4/80⁺ macrophage population at MI-d1. In-vitro results indicated that ML351 suppressed initiation of inflammation while ex-vivo results suggested ML351 overactivated inflammation consequently delaying the resolution process. Collectively, in-vitro, ex-vivo, and in-vivo results indicated that pharmacological blockade of lipoxygenases using ML351 impaired initiation of inflammation thereby dysregulated acute immune response in cardiac repair.

Transforming Growth Factor Beta (TGF-β1) Induces Pro-Reparative Phenotypic Changes in Epicardial Cells in Mice

We evaluated the content of active form of TGF-β1 in the intact and post-infarction heart and the effect of this factor on the properties of epicardial cells. During the acute stage after myocardial infarction, the production of TGF-β1 in the heart increased, which closely correlated with activation of epicardial cells (appearance of a pool of Wt1+ epicardial cells entering the epithelial-mesenchymal transition). The role of TGF-β1 as the factor of epicardial activation was confirmed by the results of in vitro experiments: addition of recombinant TGF-β1 to cultured epicardial cells led to enhanced expression of genes of epithelial-mesenchymal transition and phenotypic transformation of these cells leading to the appearance of cells with markers of smooth muscle cells and fibroblasts. Our findings suggest that the regulatory axis "TGF-β1-epicardium cells" can be considered as an important link of the post-infarction reparative process and adaptive response during heart remodeling after myocardial infarction and as the target for therapeutic interventions.

Transforming Growth Factor Beta-1 (TGF-β1) Regulates Assembly of Cardiac Spheroids

Cells of all tissues in human body interact with their neighboring cells and components of the extracellular matrix thereby creating a unique 3D microenvironment. These interactions are realized through a complex network of biochemical and mechanical signals that are important in maintaining normal cellular homeostasis. Numerous attempts have been undertaken during the last two decades to develop 3D models for studying their properties and understanding the mechanisms of regulation of cell microenvironment in vivo. Cardiac spheroids (cardiospheres) are one these models of cardiac microenvironment. In this study we demonstrate that unique microenvironment formed in cardiospheres consists of stem/progenitor and mesenchymal cells surrounded by extracellular matrix proteins synthesized by these cells. TGF-β1 participates in the regulation of contraction of cells forming cardiospheres, promotes activation of the epithelial-mesenchymal transition and self-organization of cells, which leads to the formation of larger spheroids. Thereby, the effect of TGF-β1 on the cells of cardiospheres can serve as a model for studying the mechanisms of regulation of cardiac microenvironment.

Calcium Signaling in the Ventricular Myocardium of the Goto-Kakizaki Type 2 Diabetic Rat

The association between diabetes mellitus (DM) and high mortality linked to cardiovascular disease (CVD) is a major concern worldwide. Clinical and preclinical studies have demonstrated a variety of diastolic and systolic dysfunctions in patients with type 2 diabetes mellitus (T2DM) with the severity of abnormalities depending on the patients' age and duration of diabetes. The cellular basis of hemodynamic dysfunction in a type 2 diabetic heart is still not well understood. The aim of this review is to evaluate our current understanding of contractile dysfunction and disturbances of Ca²⁺ transport in the Goto-Kakizaki (GK) diabetic rat heart. The GK rat is a widely used nonobese, nonhypertensive genetic model of T2DM which is characterized by insulin resistance, elevated blood glucose, alterations in blood lipid profile, and cardiac dysfunction.

Long-term modulation of Na+ and K+ channels by TGF-β1 in neonatal rat cardiac myocytes

Previous work shows that transforming growth factor-β1 (TGF-β1) promotes several heart alterations, including atrial fibrillation (AF). In this work, we hypothesized that these effects might be associated with a potential modulation of Na(+) and K(+) channels. Atrial myocytes were cultured 1-2 days under either control conditions, or the presence of TGF-β1. Subsequently, Na(+) (I(Na)) and K(+) (I(K)) currents were investigated under whole-cell patch-clamp conditions. Three K(+) currents were isolated: inward rectifier (I(Kin)), outward transitory (I(to)), and outward sustained (I(Ksus)). Interestingly, TGF-β1 decreased (50%) the densities of I(Kin) and I(Ksus) but not of I(to). In addition, the growth factor reduced by 80% the amount of I(Na) available at -80 mV. This effect was due to a significant reduction (30%) in the maximum I(Na) recruited at very negative potentials or I(max), as well as to an increased fraction of inactivated Na(+) channels. The latter effect was, in turn, associated to a -7 mV shift in V(1/2) of inactivation. TGF-β1 also reduced by 60% the maximum amount of intramembrane charge movement of Na(+) channels or Q(max), but did not affect the corresponding voltage dependence of activation. This suggests that TGF-β1 promotes loss of Na(+) channels from the plasma membrane. Moreover, TGF-β1 also reduced (50%) the expression of the principal subunit of Na(+) channels, as indicated by western blot analysis. Thus, TGF-β1 inhibits the expression of Na(+) channels, as well as the activity of K(+) channels that give rise to I(Ksus) and I(Kin). These results may contribute to explaining the previously observed proarrhythmic effects of TGF-β1.

Inhibition of transforming growth factor-beta signaling induces left ventricular dilation and dysfunction in the pressure-overloaded heart

This study utilized a transgenic mouse model that expresses an inducible dominant-negative mutation of the transforming growth factor (TGF)-beta type II receptor (DnTGFbetaRII) to define the structural and functional responses of the left ventricle (LV) to pressure-overload stress in the absence of an intact TGF-beta signaling cascade. DnTGFbetaRII and nontransgenic (NTG) control mice (male, 8-10 wk) were randomized to receive Zn(2+) (25 mM ZnSO(4) in drinking H(2)O to induce DnTGFbetaRII gene expression) or control tap H(2)O and then further randomized to undergo transverse aortic constriction (TAC) or sham surgery. At 7 days post-TAC, interstitial nonmyocyte proliferation (Ki67 staining) was greatly reduced in LV of DnTGFbetaRII+Zn(2+) mice compared with the other TAC groups. At 28 and 120 days post-TAC, collagen deposition (picrosirius-red staining) in LV was attenuated in DnTGFbetaRII+Zn(2+) mice compared with the other TAC groups. LV end systolic diameter and end systolic and end diastolic volumes were markedly increased, while ejection fraction and fractional shortening were significantly decreased in TAC-DnTGFbetaRII+Zn(2+) mice compared with the other groups at 120 days post-TAC. These data indicate that interruption of TGF-beta signaling attenuates pressure-overload-induced interstitial nonmyocyte proliferation and collagen deposition and promotes LV dilation and dysfunction in the pressure-overloaded heart, thus creating a novel model of dilated cardiomyopathy.

Role of TGF-beta on cardiac structural and electrical remodeling

The type β transforming growth factors (TGF-βs) are involved in a number of human diseases, including heart failure and myocardial arrhythmias. In fact, during the last 20 years numerous studies have demonstrated that TGF-β affects the architecture of the heart under both normal and pathological conditions. Moreover, TGF-β signaling is currently under investigation, with the aim of discovering potential therapeutic roles in human disease. In contrast, only few studies have investigated whether TGF-β affects electrophysiological properties of the heart. This fact is surprising since electrical remodeling represents an important substrate for cardiac disease. This review discusses the potential role of TGF-β on cardiac excitation-contraction (EC) coupling, action potentials, and ion channels. We also discuss the effects of TGF-β on cardiac development and disease from structural and electrophysiological points of view.

Impaired angiotensin II-extracellular signal-regulated kinase signaling in failing human ventricular myocytes

Angiotensin II was reported to induce insulin-like growth factor-I and endothelin-1 gene expression and peptide release by ventricular cardiomyocytes. However, the progression from cardiac hypertrophy to failure in humans is characterized by a reduced myocyte expression of insulin-like growth factor-I and endothelin-1, notwithstanding the enhanced cardiac generation of angiotensin II. In the present study we investigated the functional status of the signaling pathways responsible for angiotensin II-induced endothelin-1 and insulin-like growth factor-I formation in human ventricular myocytes isolated from patients with dilated (n = 19) or ischemic (n = 14) cardiomyopathy and nonfailing donor hearts (n = 6).In human nonfailing ventricular myocytes, angiotensin II (100 nmol/l) induced insulin-like growth factor-I and endothelin-1 gene expression, and peptide release was mediated by extracellular signal-regulated kinase activation and inhibited by extracellular signal-regulated kinase antagonism (PD98059, 30 micromol/l), endothelin-1 formation being partially reduced also by c-Jun N-terminal kinase inhibition (SP600125, 10 micromol/l); insulin-like growth factor-I and endothelin-1 formations were unaffected by the inhibition of p38 mitogen-activated protein kinase (SB203580, 10 micromol/l) and Janus tyrosine kinase 2 (AG490, 10 micromol/l). In failing myocytes, angiotensin II failed to induce insulin-like growth factor-I and endothelin-1 formation; angiotensin II-induced extracellular signal-regulated kinase activation was significantly impaired (-88% vs. controls) although c-Jun NH2-terminal kinase activation was preserved. The impaired extracellular signal-regulated kinase phosphorylation in failing myocytes was associated with increased myocyte levels of mitogen-activated protein kinase phosphatases.Therefore, the altered growth factor production in failing myocytes is associated with a significant derangement in intracellular signaling.

Cytokines regulate matrix metalloproteinases and migration in cardiac fibroblasts

We sought to define the relationship between cytokine stimulated release of matrix metalloproteinases (MMPs) and cell migration using adult rat cardiac fibroblasts. Interleukin-1beta (IL-1beta) increased release of MMP-2, -3, and -9, and TIMP-1, by 3-6-fold, measured by immunoblotting and gel zymography. Tumor necrosis factor-alpha (TNFalpha) augmented IL-1beta stimulated release of MMP-9, but not MMP-2 or -3. Transforming growth factor-beta1 (TGFbeta1) attenuated all the responses to IL-1beta. IL-1beta was also the most robust stimulus of adult rat cardiac fibroblast migration, measured in Boyden chamber assays. The combination of IL-1beta plus TNFalpha substantially enhanced migration, whereas TGFbeta1 strongly inhibited the migratory response to IL-1beta. The pan-selective MMP inhibitor GM 6001 effectively blocked IL-1beta stimulated migration. Pharmacologic inhibitors selective for ERK, JNK, and p38 MAP kinase pathways inhibited the IL-1beta regulation of individual MMPs. Increased MMP activity associated with migration of cardiac fibroblasts may be important determinants of cytokine-directed remodeling of injured myocardium.

Transforming growth factor-β1 decreases cardiac muscle L-type Ca2+ current and charge movement by acting on the Cav1.2 mRNA

Transforming growth factors-β (TGF-βs) are essential to the structural remodeling seen in cardiac disease and development; however, little is known about potential electrophysiological effects. We hypothesized that chronic exposure (6–48 h) of primary cultured neonatal rat cardiomyocytes to the type 1 TGF-β (TGF-β1, 5 ng/ml) may affect voltage-dependent Ca2+ channels. Thus we investigated T- ( ICaT) and L-type ( ICaL) Ca2+ currents, as well as dihydropyridine-sensitive charge movement using the whole cell patch-clamp technique and quantified CaV1.2 mRNA levels by real-time PCR assay. In ventricular myocytes, TGF-β1 did not exert significant electrophysiological effects. However, in atrial myocytes, TGF-β1 reduced both ICaL and charge movement (55% at 24–48 h) without significantly altering ICaT, cell membrane capacitance, or channel kinetics (voltage dependence of activation and inactivation, as well as the activation and inactivation rates). Reductions of ICaL and charge movement were explained by concomitant effects on the maximal values of L-channels conductance ( Gmax) and charge movement (Qmax). Thus TGF-β1 selectively reduces the number of functional L-channels on the surface of the plasma membrane in atrial but not ventricular myocytes. The TGF-β1-induced ICaL reduction was unaffected by supplementing intracellular recording solutions with okadaic acid (2 μM) or cAMP (100 μM), two compounds that promote L-channel phosphorylation. This suggests that the decreased number of functional L-channels cannot be explained by a possible regulation in the L-channels phosphorylation state. Instead, we found that TGF-β1 decreases the expression levels of atrial CaV1.2 mRNA (70%). Thus TGF-β1 downregulates atrial L-channel expression and may be therefore contributing to the in vivo cardiac electrical remodeling.

Transforming growth factor-beta function blocking prevents myocardial fibrosis and diastolic dysfunction in pressure-overloaded rats

BACKGROUND

Excessive myocardial fibrosis impairs cardiac function in hypertensive hearts. Roles of transforming growth factor (TGF)-beta in myocardial remodeling and cardiac dysfunction were examined in pressure-overloaded rats.

METHODS AND RESULTS

Pressure overload was induced by a suprarenal aortic constriction in Wistar rats. Fibroblast activation (proliferation and phenotype transition to myofibroblasts) was observed after day 3 and peaked at days 3 to 7. Thereafter, myocyte hypertrophy and myocardial fibrosis developed by day 28. At day 28, echocardiography showed normal left ventricular fractional shortening, but the decreased ratio of early to late filling velocity of the transmitral Doppler velocity and hemodynamic measurement revealed left ventricular end-diastolic pressure elevation, indicating normal systolic but abnormal diastolic function. Myocardial TGF-beta mRNA expression was induced after day 3, peaked at day 7, and remained modestly increased at day 28. An anti-TGF-beta neutralizing antibody, which was administered intraperitoneally daily from 1 day before operation, inhibited fibroblast activation and subsequently prevented collagen mRNA induction and myocardial fibrosis, but not myocyte hypertrophy. Neutralizing antibody reversed diastolic dysfunction without affecting blood pressure and systolic function.

CONCLUSIONS

TGF-beta plays a causal role in myocardial fibrosis and diastolic dysfunction through fibroblast activation in pressure-overloaded hearts. Our findings may provide an insight into a new therapeutic strategy to prevent myocardial fibrosis and diastolic dysfunction in pressure-overloaded hearts.

Effect of interleukin-1 beta on cardiac hypertrophy and production of natriuretic peptides in rat cardiocyte culture

This study was designed to examine the effects of interleukin-1 beta (IL-1 beta) on myocyte (MC) hypertrophy and the production of A-type natriuretic peptide (ANP) and B-type natriuretic peptide (BNP) in rat ventricular cardiocyte culture, and to investigate the role of nonmyocyte (NMC) in this process. We examined the effects of IL-1 beta on the production of ANP and BNP in comparison with the effects of endothelin-1 (ET-1) by using two types of neonatal rat cardiocyte culture; MC-enriched culture and MC-NMC coculture. In the MC-enriched culture, the increase in secretion of ANP and BNP was small in treatment with IL-1 beta (1000 pg/ml), while ET-1 (10 nM) markedly augmented the secretion of ANP and BNP. In the MC-NMC coculture, IL-1 beta and ET-1 each significantly augmented the secretion of ANP and BNP. The degree of the increase of ANP and BNP was equivalent between IL-1 beta and ET-1. As for the morphological changes of MCs, IL-1 beta induced the star-shaped MC hypertrophy characterized by elongation and pointed edges only in the MC-NMC coculture, while ET-1 induced the MC hypertrophy characterized by shapes of squares, triangles or circles in both cultures. This study shows that IL-1 beta induces unique cardiac hypertrophy and the marked secretion of ANP and BNP, and that NMC is indispensable when treated with IL-1 beta.

Involvement of receptor-type tyrosine kinase gene families in cardiac hypertrophy

OBJECTIVE

The activation of protein tyrosine kinases (PTKs) has been postulated to be involved in cell differentiation and proliferation. To elucidate the involvement of tyrosine kinase genes in normal and pathological conditions, we analysed the expression patterns of receptor-type PTKs in the normal and hypertensive hypertrophied heart in rats.

MATERIALS AND METHODS

Hypertrophied and normal rat hearts were obtained from hypertensive rats; deoxycorticosterone acetate (DOCA)-salt and 2 kidney-1 clip (2K-1C), and their sham-operated rats, respectively. A reverse transcription-polymerase chain reaction (RT-PCR) was performed using degenerated primers which were designed from highly conserved regions in the catalytic domains of receptor-type PTKs. The PCR products were ligated into a sequence vector, and subcloned by transforming bacteria. To compare the expression level of these PTK mRNAs in the normal and hypertrophied heart, we performed semi-competetive RT-PCR and immunohistochemical and Western blot analyses.

RESULTS

Nucleotide sequencing of approximately 80 clones of PTKs revealed 10 receptor-type, five nonreceptor-type and two unknown types in the rat heart. Tie-2/Tek, Ryk, insulin-like growth factor-I receptor were abundantly expressed in the rat heart as members of receptor-type PTKs. Immunohistochemistry and RT-PCR demonstrated the presence of platelet-derived growth factor (PDGF)-alpha receptor, PDGF-beta receptor and fibroblast growth factor-3 receptor in both normal and hypertrophied hearts. We also confirmed the presence of Flt-1, KDR/FIk-1, and their ligand vascular endothelial growth factor, c-Met and its ligand hepatocyte growth factor (HGF), and Tie-1, Tie-2/Tek by immunohistochemistry and RT-PCR. The coexpression of cardiac HGF and c-Met in hypertrophied hearts, especially in 2K-1 C rats, was induced more intensively than that in DOCA-salt rats.

CONCLUSION

These findings suggest that HGF/c-Met interactions may play an important role in cardiac hypertrophy and remodeling, probably as a result of the activation of the local renin-angiotensin system.

Catecholamines in cardiac hypertrophy

There has been intense interest in the roles catecholamines may play in compensatory myocardial hypertrophy. This article reviews the following: (1) chronic infusions of catecholamines in experimental animals result in cardiac hypertrophy, but in many of the studies mechanical factors have played a role; (2) experiments using isolated papillary muscles and isolated hearts, stretched isolated myocytes, and denervated hearts in vivo demonstrate that mechanical activity is sufficient to cause increased protein synthesis and cell growth; (3) in neonatal myocyte cell cultures, alpha-adrenergic agonists are powerful stimulants for protein synthesis and cell growth. Beta-adrenergic stimulation of nonmyocyte myocardial cells causes release of a factor that promotes protein synthesis in neonatal myocytes. Either alpha or beta stimulation, probably through different mechanisms, appears to have growth-promoting effects on isolated adult myocytes in culture; (4) alpha stimulation is transduced through the Gq pathway and its activation of phospholipase C, cleavage of phosphatidylinositol (4,5)-bisphosphate, and then further through the ras/raf, mitogen-activated protein (MAP) kinase system; (5) transgenic mice with upregulation of catecholamine-related systems have not clarified the independent role of either the alpha- or beta-adrenergic pathway; and (6) observations in humans suggest that mechanical factors predominate in the development and regression of cardiac hypertrophy. Humoral mechanisms, including catecholamines, may play a role, but their quantitative importance has not been determined. It is hypothesized that catecholamines may play a role in transition from the adaptive to the maladaptive state.

Cardiac fibroblasts arrest at the G1/S restriction point in response to interleukin (IL)-1beta. Evidence for IL-1beta-induced hypophosphorylation of the retinoblastoma protein

Although responsible for only approximately one-third of the overall myocardial mass, the interstitial fibroblasts of the heart serve a fundamental role in establishing the functional integrity of myocardium and are the major source of myocardial extracellular matrix production. Their importance in clinical medicine is underscored by the observation that fibroblast numbers increase in response to several pathologic circumstances that are associated with an increase in extracellular matrix production, such as long standing hypertension and myocardial injury/infarction. Up to the present time, however, there has been little information available on either the kinetics of the cardiac fibroblast cell cycle, or the fundamental mechanisms that regulate its entry into and exit from the cell cycle. Previous work from our laboratory examining the effects of interleukin (IL)-1beta on myocardial growth and gene expression in culture indicated that cardiac fibroblasts have a diminished capacity to synthesize DNA in response to mitogen in the presence of this cytokine. The mechanism of IL-1beta action was not clear, however, and could have resulted from action at several different points in the cell cycle. The investigations described in this report indicate that IL-1beta exerts its effect on the fibroblast cell cycle at multiple levels through altering the expression of cardiac fibroblast cyclins, cyclin-dependent kinases, and their inhibitors, which ultimately affect the phosphorylation of the retinoblastoma gene product.

Cytokine expression increases in nonmyocytes from rats with postinfarction heart failure

Growing evidence suggests that cardiac nonmyocyte cells may play an important regulatory role in the response to myocardial overload and injury via altered expression of paracrine products, such as cytokines and growth factors, but information concerning the cell-specific changes in the expression of these substances in heart-failure models is limited. Therefore, cardiac nonmyocytes were isolated from rats 1 day and 1 and 6 wk after left coronary artery ligation with resulting hemodynamic evidence of heart failure and in sham-operated control animals. mRNAs for tumor necrosis factor-alpha (TNF-alpha), interleukin (IL)-1beta, IL-6, transforming growth factors (TGF)-beta1 and TGF-beta3, and type I and type III collagen were measured by Northern analyses. The temporal and quantitative relationships between the expression of these cytokines and collagen and myocyte hypertrophy were determined. mRNA expression of IL-1beta was increased by 1.3-fold at 1 day and 1 wk, and expression of TNF-alpha, IL-1beta, IL-6, TGF-beta1, and TGF-beta3 were increased by 1.4- to 2.1-fold at the 1-wk time point before returning toward baseline at 6 wk. There were significant correlations between the expression of these cytokines and the expression of types I and III collagen, which also peaked at 1 wk. Myocyte hypertrophy was seen first at 6 wk. These observations are consistent with a hypothesis that nonmyocyte cells play a regulatory role in the extracellular matrix changes during postinfarction remodeling and highlight the importance of examining cell-specific changes in gene expression and elucidating the role of cell-to-cell interactions within the myocardium.

gp130 is involved in stretch-induced MAP kinase activation in cardiac myocytes

We have recently reported that mitogen activated protein kinase (MAP kinase) is activated by the stretch of the cultured cardiac myocytes in the angiotensin II deficient state in the angiotensinogen-deficient mice (Atg-/-), suggesting that factors other than the cardiac renin-angiotensin system are involved in the stretch-induced MAP kinase activation. We examined the contribution of cytokines using RX435, an anti-gp130 antibody. Leukemia inhibitory factor, which is one of the cytokines and has the common receptor subunit gp130, activated MAP kinase and the response was completely blocked by pretreatment of the Atg-/- cardiac myocytes with RX435. RX435 pretreatment greatly reduced stretch-induced activation of MAP kinase in Atg-/- cardiac myocytes. Interestingly, the same results were obtained in the cardiac myocytes of control mice. These results suggest that cytokine-gp130 may play a role in the stretch-induced MAP kinase activation independently of Ang II in cardiac myocytes.