Mitogen-activated protein kinase activation and regulation in the pressure-loaded fetal ovine heart

Right ventricular remodeling in response to volume overload in fetal sheep

The fetal myocardium is known to be sensitive to hemodynamic load, responding to systolic overload with cellular hypertrophy, proliferation, and accelerated maturation. However, the fetal cardiac growth response to primary volume overload is unknown. We hypothesized that increased venous return would stimulate fetal cardiomyocyte proliferation and terminal differentiation, particularly in the right ventricle (RV). Vascular catheters and pulmonary artery flow probes were implanted in 16 late-gestation fetal sheep: a right carotid artery-jugular vein (AV) fistula was surgically created in nine fetuses, and sham operations were performed on seven fetuses. Instrumented fetuses were studied for 1 wk before hearts were dissected for component analysis or cardiomyocyte dispersion for cellular measurements. Within 1 day of AV fistula creation, RV output was 20% higher in experimental than sham fetuses ( P < 0.0001). Circulating atrial natriuretic peptide levels were elevated fivefold in fetuses with an AV fistula ( P < 0.002). On the terminal day, RV-to-body weight ratios were 35% higher in the AV fistula group ( P < 0.05). Both left ventricular and RV cardiomyocytes grew longer in fetuses with an AV fistula ( P < 0.02). Cell cycle activity was depressed by >50% [significant in left ventricle ( P < 0.02), but not RV ( P < 0.054)]. Rates of terminal differentiation were unchanged. Based on these studies, we speculate that atrial natriuretic peptide suppressed fetal cardiomyocyte cell cycle activity. Unlike systolic overload, fetal diastolic load appears to drive myocyte enlargement, but not cardiomyocyte proliferation or maturation. These changes could predispose to RV dysfunction later in life. NEW & NOTEWORTHY Adaptation of the fetal heart to changes in cardiac load allows the fetus to maintain adequate blood flow to its systemic and placental circulations, which is necessary for the well-being of the fetus. Addition of arterial-venous fistula flow to existing venous return increased right ventricular stroke volume and output. The fetal heart compensated by cardiomyocyte elongation without accelerated cellular maturation, while cardiomyocyte proliferation decreased. Even transient volume overload in utero alters myocardial structure and cardiomyocyte endowment.

Systemic arterial hypertension but not IGF-I treatment stimulates cardiomyocyte enlargement in neonatal lambs

Although cardiomyocyte terminal differentiation is nearly complete at birth in sheep, as in humans, very limited postnatal expansion of myocyte number may occur. The capacity of newborn cardiomyocytes to respond to growth stimulation by proliferation is poorly understood. Our objective was to test this growth response in newborn lambs with two stimuli shown to be potent inducers of cardiomyocyte growth in fetuses and adults: increased systolic load (Load) and insulin-like growth factor I (IGF-I). Vascular catheters and an inflatable aortic occluder were implanted in lambs. Hearts were collected for analysis at 18 days of age after a 7-day experiment and compared with control hearts. Load hearts, but not IGF-I hearts, were heavier ( P = 0.001) because of increased mass of the left ventricle (LV), septum, and left atrium (40-50%, P = 0.004). Terminal differentiation and cell cycle activity were not different between groups. Myocyte length was 7% greater in Load lamb hearts ( P < 0.05), and binucleated myocytes, which comprise ~90% of LV cells, were 25% larger in volume ( P = 0.03). Myocyte number per gram of myocardium was decreased in all ventricles of Load lambs ( P = 0.01). Cells from the IGF-I group were not different by any comparison. These results suggest that the newborn sheep LV responds to systolic stress with cardiomyocyte hypertrophy, not proliferation. Furthermore, IGF-I is ineffective at stimulating cardiomyocyte proliferation at this age (despite effectiveness when administered before birth). Thus, to expand cardiomyocyte number in the newborn heart, therapies other than systolic pressure load and IGF-I treatment need to be developed.

Increased fetal myocardial sensitivity to insulin-stimulated glucose metabolism during ovine fetal growth restriction

Unlike other visceral organs, myocardial weight is maintained in relation to fetal body weight in intrauterine growth restriction (IUGR) fetal sheep despite hypoinsulinemia and global nutrient restriction. We designed experiments in fetal sheep with placental insufficiency and restricted growth to determine basal and insulin-stimulated myocardial glucose and oxygen metabolism and test the hypothesis that myocardial insulin sensitivity would be increased in the IUGR heart. IUGR was induced by maternal hyperthermia during gestation. Control (C) and IUGR fetal myocardial metabolism were measured at baseline and under acute hyperinsulinemic/euglycemic clamp conditions at 128-132 days gestation using fluorescent microspheres to determine myocardial blood flow. Fetal body and heart weights were reduced by 33% (P = 0.008) and 30% (P = 0.027), respectively. Heart weight to body weight ratios were not different. Basal left ventricular (LV) myocardial blood flow per gram of LV tissue was maintained in IUGR fetuses compared to controls. Insulin increased LV myocardial blood flow by ∼38% (P < 0.01), but insulin-stimulated LV myocardial blood flow in IUGR fetuses was 73% greater than controls. Similar to previous reports testing acute hypoxia, LV blood flow was inversely related to arterial oxygen concentration (r(2 )= 0.71) in both control and IUGR animals. Basal LV myocardial glucose delivery and uptake rates were not different between IUGR and control fetuses. Insulin increased LV myocardial glucose delivery (by 40%) and uptake (by 78%) (P < 0.01), but to a greater extent in the IUGR fetuses compared to controls. During basal and hyperinsulinemic-euglycemic clamp conditions LV myocardial oxygen delivery, oxygen uptake, and oxygen extraction efficiency were not different between groups. These novel results demonstrate that the fetal heart exposed to nutrient and oxygen deprivation from placental insufficiency appears to maintain myocardial energy supply in the IUGR condition via increased glucose uptake and metabolic response to insulin, which support myocardial function and growth.

Endocrine and other physiologic modulators of perinatal cardiomyocyte endowment

Immature contractile cardiomyocytes proliferate to rapidly increase cell number, establishing cardiomyocyte endowment in the perinatal period. Developmental changes in cellular maturation, size and attrition further contribute to cardiac anatomy. These physiological processes occur concomitant with a changing hormonal environment as the fetus prepares itself for the transition to extrauterine life. There are complex interactions between endocrine, hemodynamic and nutritional regulators of cardiac development. Birth has been long assumed to be the trigger for major differences between the fetal and postnatal cardiomyocyte growth patterns, but investigations in normally growing sheep and rodents suggest this may not be entirely true; in sheep, these differences are initiated before birth, while in rodents they occur after birth. The aim of this review is to draw together our understanding of the temporal regulation of these signals and cardiomyocyte responses relative to birth. Further, we consider how these dynamics are altered in stressed and suboptimal intrauterine environments.

Thyroid hormone is required for growth adaptation to pressure load in the ovine fetal heart

Thyroid hormone exerts broad effects on the adult heart, but little is known regarding the role of thyroid hormone in the regulation of cardiac growth early in development and in response to pathophysiological conditions. To address this issue, we determined the effects of fetal thyroidectomy on cardiac growth and growth-related gene expression in control and pulmonary-artery-banded fetal sheep. Fetal thyroidectomy (THX) and/or placement of a restrictive pulmonary artery band (PAB) were performed at 126 ± 1 days of gestation (term, 145 days). Four groups of animals [n = 5-6 in each group; (i) control; (ii) fetal THX; (iii) fetal PAB; and (iv) fetal PAB + THX] were monitored for 1 week prior to being killed. Fetal heart rate was significantly lower in the two THX groups compared with the non-THX groups, while mean arterial blood pressure was similar among groups. Combined left and right ventricle free wall + septum weight, expressed per kilogram of fetal weight, was significantly increased in PAB (6.27 ± 0.85 g kg(-1)) compared with control animals (4.72 ± 0.12 g kg(-1)). Thyroidectomy significantly attenuated the increase in cardiac mass associated with PAB (4.94 ± 0.13 g kg(-1)), while THX alone had no detectable effect on heart mass (4.95 ± 0.27 g kg(-1)). The percentage of binucleated cardiomyocytes was significantly decreased in THX and PAB +THX groups (∼16%) compared with the non-THX groups (∼27%). No differences in levels of activated Akt, extracellular signal-regulated kinase or c-Jun N-terminal kinase were detected among the groups. Markers of cellular proliferation but not apoptosis or expression of growth-related genes were lower in the THX and THX+ PAB groups relative to thyroid-intact animals. These findings suggest that in the late-gestation fetal heart, thyroid hormone has important cellular growth functions in both physiological and pathophysiological states. Specifically, thyroid hormone is required for adaptive fetal cardiac growth in response to pressure overload.

The effect of adrenalectomy on the cardiac response to subacute fetal anemia

The mechanisms that stimulate fetal heart growth during anemia are unknown. To examine the hypothesis that adrenal hormones contribute to this process, we determined the effects of adrenalectomy (Adx) on heart growth and the activation of cardiac mitogen-activated protein kinases (MAPKs) in the presence and absence of fetal anemia. To identify mechanisms contributing to the initiation of cardiac growth, the duration of anemia was limited to a period shorter than that previously described to result in increased cardiac mass. Four groups of fetal sheep were studied (Adx-Anemic, Adx-Control, Intact-Anemic, Intact-Control). Anemia was created by daily controlled hemorrhage for 5 days; hearts were collected for analysis at 133 d gestation (term 145 d). Cardiomyocyte morphometry, immunohistochemistry for Ki-67 (proliferation marker), and Western blotting for protein levels of MAPKs and proliferating cell nuclear antigen (PCNA) were performed. Blood pressure, heart rate, heart weight-to-body weight ratio, and cardiomyocyte length and width remained similar among groups throughout the study. PCNA levels in the Adx-Anemic group were twice as high as in any other group (both ventricles, p < 0.05). Levels of phosphorylated extracellular signal-regulated kinase (ERK) were ~60% higher in the Intact-Anemic and Adx-Anemic groups, compared with the Intact-Control and Adx-Control groups (p < 0.02). These results suggest that adrenal hormones may attenuate fetal cardiomyocyte proliferation in response to anemia (as evidenced by the increased PCNA in Adx-Anemic fetuses) and that phosphorylation of myocardial ERK results from fetal anemia, irrespective of the status of the fetal adrenal gland.

Cardioprotection with postconditioning: loss of efficacy in murine models of type-2 and type-1 diabetes

Postconditioning (PostC), or relief of myocardial ischemia in a stuttered manner, has been shown to reduce infarct size, due in part to upregulation of survival kinase signaling. Virtually all of these data have, however, been obtained in healthy adult cohorts; the question of whether PostC-induced cardioprotection is maintained in the setting of clinically relevant comorbidities has remained largely unexplored. Accordingly, our aim was to assess the consequences of a major risk factor-diabetes-on the infarct-sparing effect of stuttered reflow. Isolated buffer-perfused hearts were obtained from normoglycemic C57BL/6J mice, BKS.Cg-m+/+Lepr(db)/J (db/db) mice (model of type-2 diabetes), C57BL/6J mice injected with streptozotocin (model of type-1 diabetes), and streptozotocin-injected mice in which normoglycemia was re-established by islet cell transplantation. All hearts underwent 30  min of ischemia and, within each cohort, hearts received either standard (control) reperfusion or three to six 10-s cycles of stuttered reflow. PostC reduced infarct size via upregulation of extracellular signal-regulated kinase 1/2 in normoglycemic mice. In contrast, diabetic hearts were refractory to PostC-induced cardioprotection-an effect that, in the type-1 model, was reversed by restoration of normoglycemia. We provide novel evidence for a profound-but potentially reversible-diabetes-induced defect in the cardioprotective efficacy of PostC.

Atrial natriuretic peptide inhibits angiotensin II-stimulated proliferation in fetal cardiomyocytes

The role of atrial natriuretic peptide (ANP) in regulating fetal cardiac growth is poorly understood. Angiotensin II (Ang II) stimulates proliferation in fetal sheep cardiomyocytes when growth is dependent on the activity of the mitogen-activated protein kinase (MAPK) and phosphoinositol-3-kinase (PI3K) pathways. We hypothesized that ANP would suppress near-term fetal cardiomyocyte proliferation in vitro and inhibit both the MAPK and PI3K pathways. Forty-eight hour 5-bromodeoxyuridine (BrdU) uptake (used as an index of proliferation) was measured in cardiomyocytes isolated from fetal sheep (135 day gestational age) in response to 100 nm Ang II with or without ANP (0.003-100 nm) or 1 microm 8-bromo-cGMP. The effects of these compounds on the MAPK and PI3K pathways were assessed by measuring extracellular signal-regulated kinase (ERK) and AKT phosphorylation following 10 min of treatment with Ang II, ANP or 8-bromo-cGMP. In right ventricular myocytes (RV), the lowest dose of ANP (0.003 nm) inhibited Ang II-stimulated BrdU uptake by 68%. Similarly, 8-bromo-cGMP suppressed Ang II-stimulated proliferation by 62%. The same effects were observed in left ventricular (LV) cardiomyocytes but the RV was more sensitive to the inhibitory effects of ANP than the LV (P < 0.0001). Intracellular cGMP was increased by 4-fold in the presence of 100 nm ANP. Ang II-stimulated ERK and Akt phosphorylation was inhibited by 100 nm ANP. The activity of ANP may in part be cGMP dependent, as 8-bromo-cGMP had similar effects on the cardiomyocytes.

Coronary endothelial function and vascular smooth muscle proliferation are programmed by early-gestation dexamethasone exposure in sheep

Exposure of the early-gestation ovine fetus to exogenous glucocorticoids induces changes in postnatal cardiovascular physiology. We sought to characterize coronary artery vascular function in this model by elucidating the contribution of nitric oxide and reactive oxygen species to altered coronary vascular reactivity and examining the proliferative potential of coronary artery vascular smooth muscle cells. Dexamethasone (dex, 0.28 mg x kg(-1) x day(-1) for 48 h) was administered to pregnant ewes at 27-28-day gestation (term 145 days). Coronary arteries were isolated from 1- to 2-wk-old dex-exposed offspring and aged-matched controls. Compared with controls, coronary arteries from dex-exposed lambs demonstrated enhanced vasoconstriction to endothelin-1 and ACh that was abolished by endothelial removal or preincubation with the nitric oxide synthase inhibitor L-NNA, membrane-permeable superoxide dismutase + catalase, or apamin + charybdotoxin, but not indomethacin. The rate of coronary vascular smooth muscle cell (VSMC) proliferation was also significantly greater in dex-exposed lambs. Protein levels of the proliferating cell nuclear antigen were increased and alpha-smooth muscle actin decreased in dex-exposed coronary VSMC, consistent with a proliferative state. Finally, expression of the NADPH oxidase Nox 4, but not Nox 1, mRNA was also decreased in coronary VSMC from dex-exposed lambs. These findings suggest an important interaction exists between early-gestation glucocorticoid exposure and reactive oxygen species that is associated with alterations in endothelial function and coronary VSMC proliferation. These changes in coronary physiology are consistent with those associated with the development of atherosclerosis and may provide an important link between an adverse intrauterine environment and increased risk for coronary artery disease.

Engineered early embryonic cardiac tissue increases cardiomyocyte proliferation by cyclic mechanical stretch via p38-MAP kinase phosphorylation

Cardiomyocyte (CM) transplantation is one therapeutic option for cardiac repair. Studies suggest that fetal CMs display the best cell type for cardiac repair, which can finitely proliferate, integrate with injured host myocardium, and restore cardiac function. We have recently developed an engineered early embryonic cardiac tissue (EEECT) using embryonic cardiac cells and have shown that EEECT contractile properties and cellular proliferative response to cyclic mechanical stretch stimulation mimic developing fetal myocardium. However, it remains unknown whether cyclic mechanical stretch-mediated high cellular proliferation activity within EEECT reflects CM or non-CM population. Studies have shown that p38-mitogen-activated protein kinase (p38MAPK) plays an important role in both cyclic mechanical stretch stimulation and cellular proliferation. Therefore, in the present study, we tested the hypothesis that cyclic mechanical stretch (0.5 Hz, 5% strain for 48 h) specifically increases EEECT CM proliferation mediated by p38MAPK activity. Cyclic mechanical stretch increased CM, but not non-CM, proliferation and increased p38MAPK phosphorylation. Treatment of EEECT with the p38MAPK inhibitor, SB202190, reduced CM proliferation. The negative CM proliferation effects of SB202190 were not reversed by concurrent stretch stimulation. Results suggest that immature CM proliferation within EEECT can be positively regulated by mechanical stretch and negatively regulated by p38MAPK inhibition.

Fetal programming alters reactive oxygen species production in sheep cardiac mitochondria

Exposure to an adverse intrauterine environment is recognized as an important risk factor for the development of cardiovascular disease later in life. Although oxidative stress has been proposed as a mechanism for the fetal programming phenotype, the role of mitochondrial O(2)(*-) (superoxide radical) production has not been explored. To determine whether mitochondrial ROS (reactive oxygen species) production is altered by in utero programming, pregnant ewes were given a 48-h dexamethasone (dexamethasone-exposed, 0.28 mg.kg(-1) of body weight.day(-1)) or saline (control) infusion at 27-28 days gestation (term=145 days). Intact left ventricular mitochondria and freeze-thaw mitochondrial membranes were studied from offspring at 4-months of age. AmplexRed was used to measure H(2)O(2) production. Activities of the antioxidant enzymes Mn-SOD (manganese superoxide dismutase), GPx (glutathione peroxidase) and catalase were measured. Compared with controls, a significant increase in Complex I H(2)O(2) production was found in intact mitochondria from dexamethasone-exposed animals. The treatment differences in Complex I-driven H(2)O(2) production were not seen in mitochondrial membranes. Consistent changes in H(2)O(2) production from Complex III in programmed animals were not found. Despite the increase in H(2)O(2) production in intact mitochondria from programmed animals, dexamethasone exposure significantly increased mitochondrial catalase activity, whereas Mn-SOD and GPx activities were unchanged. The results of the present study point to an increase in the rate of release of H(2)O(2) from programmed mitochondria despite an increase in catalase activity. Greater mitochondrial H(2)O(2) release into the cell may play a role in the development of adult disease following exposure to an adverse intrauterine environment.

Blockade of calcineurin reverses cardiac hypertrophy and induces the down-regulation of JNK mRNA expression in renovascular hypertensive rats

INTRODUCTION

Recently, calcineurin has been shown to induce cardiac hypertrophy. Mitogen-activated protein kinases (MAPK), including the extracellular-signal regulated kinases (ERK), the c-Jun NH2-terminal kinases (JNK) and the p38 MAPK (p38), have also been shown to be important in the transduction of trophic signals. The objective of this study was to investigate possible cross-talk between calcineurin and MAPK pathways in controlling renovascular hypertension-induced cardiac hypertrophy.

METHODS

Renovascular hypertension was induced by the two kidney-one clip method. The left ventricular weight (LVW) and the ratio of LVW to tibial length were measured to assay the degree of cardiac hypertrophy. Calcineurin activity and MAPK mRNA expression were measured.

RESULTS

In the left ventricle of rats with renovascular hypertension, calcineurin activity and JNK mRNA expression were increased while cardiac hypertrophy developed. Treatment with the calcineurin blocker ciclosporin A induced calcineurin inhibition and regression of cardiac hypertrophy with an improvement of cardiac diastolic function. The treatment also resulted in down-regulation of JNK mRNA expression, but the mRNA expressions of ERK and p38 were unchanged.

CONCLUSIONS

There is cross-talk between the calcineurin and JNK pathway in controlling renovascular hypertension-induced cardiac hypertrophy. Inhibition of the calcineurin and JNK pathways may be the basis of reversal of cardiac hypertrophy by calcineurin blockers.

Activation of the mitogen-activated protein kinases and Akt in response to pulmonary artery banding in the fetal sheep heart is developmentally regulated

BACKGROUND

The postnatal heart responds to biomechanical stress with hypertrophy. The fetal heart may also undergo hyperplasia, but the percentage of mitotically active cardiomyocytes decreases throughout gestation. The signaling pathways controlling growth and proliferation in the fetal heart are poorly understood.

OBJECTIVE

To determine whether activation of the mitogen-activated protein kinases and Akt in the acute response to pressure loading in the fetal heart is developmentally regulated.

METHODS

Pulmonary artery banding (PAB) was performed in 100- or 128-day fetuses of twin gestation (n = 6 per group) for 6 h. One twin served as a control. Right ventricular (RV) and left ventricular (LV) mitogen-activated protein kinase, Akt, and cyclin D1 protein levels were determined by Western blot.

RESULTS

Within each gestational age group, hemodynamic and arterial blood gas values were similar between PAB and control fetuses. The total mitogen-activated protein kinase and Akt protein levels were unchanged by PAB at both gestational ages, as were active p38 and JNK levels. The RV levels of active ERK tended to decrease in 128-day PAB fetuses as compared with controls, and LV active ERK normalized to total ERK was significantly decreased. At gestational age (GA) 100 days, RV active ERK levels were significantly higher in PAB animals as compared with controls, with a trend towards increased active Akt levels. No differences were seen in the 100-day LV. The levels of the cell cycle promoter cyclin D1 were unchanged in all animals.

CONCLUSIONS

Pressure loading of the fetal sheep heart leads to developmentally regulated cell signaling profiles. ERK and possibly Akt may be important regulators of in vivo cardiomyocyte hyperplasia.