Myocyte Proliferation Research Articles

Dehydroepiandrosterone reverses chronic hypoxia/reoxygenation-induced right ventricular dysfunction in rats

Dehydroepiandrosterone (DHEA) prevents chronic hypoxia-induced pulmonary hypertension and associated right ventricle dysfunction in rats. In this animal model, reoxygenation following hypoxia reverses pulmonary hypertension but not right ventricle dysfunction. We thus studied the effect of DHEA on the right ventricle after reoxygenation, i.e. after a normoxic recovery phase secondary to chronic hypoxia in rats. Right ventricle function was assessed in vivo by Doppler echocardiography and in vitro by the isolated perfused heart technique in three groups of animals: control, recovery (21 days of hypoxia followed by 21 days of normoxia) and recovery DHEA (30 mg · kg(-1) every 2 days during the recovery phase). Right ventricle tissue was assessed by optical and electron microscopy. DHEA abolished right ventricle diastolic dysfunction, as the echographic E wave remained close to that of controls (mean ± SD 76.5 ± 2.4 and 79.7 ± 1.7 cm · s(-1), respectively), whereas it was diminished to 40.3 ± 3.7 in the recovery group. DHEA also abolished right ventricle systolic dysfunction, as shown by the inhibition of the increase in the slope of the pressure-volume curve in isolated heart. The DHEA effect was related to cardiac myocytes proliferation. In conclusion, DHEA prevents right ventricle dysfunction in this animal model by preventing cardiomyocyte alteration.

MicroRNAs in Cardiac Development and Function

MicroRNAs (miRNAs) negatively regulate gene expression at the post‐transcriptional level, primarily by base‐pairing with the 3’‐untranslated region (3’‐UTR) of their target mRNAs. Many miRNAs are expressed in a tissue/organ‐specific manner and are associated with an increasing number of cell proliferation, differentiation and tissue development events.Cardiac muscle expresses distinct genes encoding structural proteins and a subset of signal molecules that control tissue specification and differentiation. The transcriptional regulation of cardiac and skeletal muscle development has been well established, yet only until recently has it been uncovered that miRNAs participate in the regulatory networks.A subset of miRNAs are either specifically or highly expressed in cardiac and skeletal muscle, providing an opportunity to understand how gene expression is controlled by miRNAs at the post‐transcriptional level in this muscle type. miR‐1, miR‐133, miR‐206 and miR‐208 have been found to be muscle‐specific, and thus have been called myomiRs. Our studies, using a conditional knockout strategy, show that microRNAs are essential for proper development and function of the heart. We further demonstrated that miR‐1 and miR‐133 play a vital role in myocyte proliferation and differentiation. Gain‐ and loss‐of‐function studies identify miR‐208a an important regulator of cardiac remodeling in response to stress.The discovery of myomiRs as a previously unrecognized component in the regulation of gene expression adds an entirely new layer of complexity to our understanding of cardiac, skeletal muscle development and function. Investigating myomiRs will not only reveal novel molecular mechanisms of the miRNA‐mediated regulatory network in cardiac and skeletal muscle development, but also raise new opportunities for therapeutic intervention for cardiovascular diseases.

FGF signaling is involved in physiological adaptation to pressure overload in developing heart

Fibroblast growth factors (FGFs) play an important role during embryonic induction, growth and patterning. Exogenous FGF2 as well as pressure loading were shown to induce embryonic myocyte proliferation. Here we tested whether FGF2 signaling is involved in transmission of pressure overload to increased myocyte proliferation in vivo.Pressure overload was induced by conotruncus constriction in Stage 21 chick embryos. Anti‐bromodeoxyuridine labeling was used to assess proliferation at 24, 48 and 72 h. Western blotting and RT‐PCR to measure FGF2 protein and mRNA levels was performed on ventricular extracts at 48 h interval. Blood was collected at 48 h to assess the amount of FGF2 in serum by ELISA. Proliferation of ventricular myocytes was increased significantly at 48 h. Neither western blotting, nor immunohistochemistry revealed any changes in the amount of myocardial FGF2. ELISA showed a 93% increase of FGF2 in the serum. Increased amount of FGF2 mRNA in ventricular myocardium was confirmed by real time PCR.We conclude that FGF2 synthesis is increased in embryonic ventricular cardiomyocytes in response to increased stretch due to pressure overload. Increased stretch causes its release into serum, causing it to act in endocrine, rather then paracrine manner.Supported by Ministry of Education VZ 0021620806, ASCR AV0Z50450515 and AV0Z50110509 and GACR P302/11/1308.Grant Funding Source : P302/11/1308

Foxp-Mediated Suppression of N-Cadherin Regulates Neuroepithelial Character and Progenitor Maintenance in the CNS

Neuroepithelial attachments at adherens junctions are essential for the self-renewal of neural stem and progenitor cells and the polarized organization of the developing central nervous system. The balance between stem cell maintenance and differentiation depends on the precise assembly and disassembly of these adhesive contacts, but the gene regulatory mechanisms orchestrating this process are not known. Here, we demonstrate that two Forkhead transcription factors, Foxp2 and Foxp4, are progressively expressed upon neural differentiation in the spinal cord. Elevated expression of either Foxp represses the expression of a key component of adherens junctions, N-cadherin, and promotes the detachment of differentiating neurons from the neuroepithelium. Conversely, inactivation of Foxp2 and Foxp4 function in both chick and mouse results in a spectrum of neural tube defects associated with neuroepithelial disorganization and enhanced progenitor maintenance. Together, these data reveal a Foxp-based transcriptional mechanism that regulates the integrity and cytoarchitecture of neuroepithelial progenitors.

Prediction of The Risk for Hypertension Among Carrier of C825T Genetic Polymorphism of G Protein Beta 3 Gene (GNB3)

BackgroundGNB3 825T is associated with the occurrence of the splice variant Gß3s, which, despite a deletion of 41 amino acids, is functionally active in reconstituted systems. This may cause enhanced vascular reactivity and increased proliferation of smooth muscle cells and cardiac myocytes, ultimately resulting in vascular and myocardial hypertrophy.AimTo predict the risk for hypertension in individuals with C825T genetic polymorphism of G protein β3 gene.Material and MethodThe study consisted of 192 normotensive individuals and 120 hypertensive patients. Individuals were genotyped for C825T genetic polymorphism of G protein β3 gene by using polymerase chain reaction‐ restriction fragment length polymorphism. Statistical analysis was done by SPSS program.ResultsFrequency of C and T alleles were 47.5% and 52.5% in hypertensive group compared to 62% and 38% in control group. The carriers of T allele exhibited a significant greater risk for hypertension compared to carriers of C allele (OR= 1.8, 95%CI= 1.3–2.6).ConclusionT allele is a risk factor for hypertension.Source of Fund: DMCG

Electrophysiological properties of cardiac myocytes in regenerating zebrafish hearts

Zebrafish hearts fully regenerate within 30–60 days post‐amputation (dpa) of up to 20% of the ventricle. Lineage studies have shown regeneration is mediated by proliferation of existing myocytes. Genes encoding ion channel proteins undergo transcriptional changes during regeneration suggesting that electrical properties of the heart may be remodeled as part of the regenerative process. In these studies, we isolated myocytes from hearts 7 dpa, a time of peak myocyte proliferation, and characterized their electrical properties by patch clamp. Myocytes from uninjured and 30 dpa hearts had similar Na+, K+ and Ca2+ currents and action potentials. However, 7 dpa myocytes exhibited decreased T‐ and L‐type Ca2+ current amplitudes and a tendency to generate short duration action potentials. The T‐ and L‐type Ca2+ channel subunits CACNA1G, CACNA1C and CACN1B have reduced expression at 7 dpa. Our studies are the first to characterize ion channels in regenerating hearts. Reduced Ca2+ currents and shortened action potentials will reduce cardiac contractile forces, which may be required for regeneration. Changes in myocyte electrical activity and Ca2+ signaling may also play a role in regulating the regenerative process. Collectively, our studies document that dynamic changes in ion channel activity accompanies the transformation of quiescent myocytes into highly proliferative cardiac cells during regeneration.

Cardiac-specific deletion of the microtubule-binding protein CENP-F causes dilated cardiomyopathy

SUMMARYCENP-F is a large multifunctional protein with demonstrated regulatory roles in cell proliferation, vesicular transport and cell shape through its association with the microtubule (MT) network. Until now, analysis of CENP-F has been limited to in vitro analysis. Here, using a Cre-loxP system, we report the in vivo disruption of CENP-F gene function in murine cardiomyocytes, a cell type displaying high levels of CENP-F expression. Loss of CENP-F function in developing myocytes leads to decreased cell division, blunting of trabeculation and an initially smaller, thin-walled heart. Still, embryos are born at predicted mendelian ratios on an outbred background. After birth, hearts lacking CENP-F display disruption of their intercalated discs and loss of MT integrity particularly at the costamere; these two structures are essential for cell coupling/electrical conduction and force transduction in the heart. Inhibition of myocyte proliferation and cell coupling as well as loss of MT maintenance is consistent with previous reports of generalized CENP-F function in isolated cells. One hundred percent of these animals develop progressive dilated cardiomyopathy with heart block and scarring, and there is a 20% mortality rate. Importantly, although it has long been postulated that the MT cytoskeleton plays a role in the development of heart disease, this study is the first to reveal a direct genetic link between disruption of this network and cardiomyopathy. Finally, this study has broad implications for development and disease because CENP-F loss of function affects a diverse array of cell-type-specific activities in other organs.

Abstract 8851: Intracoronary Infusion of Cardiosphere-Derived Cells (icCDCs) Improves Cardiac Function by Stimulating Myocyte Proliferation in Non-Infarcted Hibernating Myocardium with No Effect in Normal Myocardium

Background: icCDCs have been shown to improve function and reduce fibrotic area when regionally infused into infarcted myocardium. Their ability to improve flow and function in viable dysfunctional myocardium in the absence of scar is unclear. Methods: Swine (n=12) with collateral-dependent hibernating myocardium from a chronic LAD occlusion were studied 4-months after instrumentation in the closed-chest propofol sedated state. CDCs were isolated from myocardial biopsies at the time of initial instrumentation. We assessed regional flow, LAD wall thickening (%LADWT) and global EF before and 4 weeks after icCDCs (∼40x10 6 cells) into each of the 3 major coronary arteries. Autologous (n=3) as well as non-autologous CDCs with cyclosporine treatment (200mg/day; n=3) had similar effects and results were pooled and compared to untreated swine (n=6). Histological analysis was performed to assess the frequency of Ki-67 and phospho-Histone H3 (pHH3) positive myocytes along with myocyte nuclear density and myocyte diameter. Results: Infusion of icCDCs in normal animals (icCDCs; n=5 vs. untreated; n=6) did not affect regional (%LADWT 51±1% vs. 48±3%, p-ns) or global function (EF: 70±2% vs. 70±3%, p-ns). In contrast, in hibernating myocardium, %LADWT increased from 23±6 to 51±5 % (p<0.01) and EF increased from 57±1 to 71±4 % (p<0.01) after icCDCs. There was no change in LAD coronary flow reserve (adenosine vasodilated flow/ resting flow 1.88 ± 0.20 to 2.03 ± 0.31, p-ns). icCDCs increased the number of Ki-67 and pHH3 positive myocytes (Table). This was accompanied by a prominent increase in myocyte nuclear density with reduced myocyte size suggesting the formation of new myocytes. Conclusion: We conclude that icCDCs result in a robust improvement in function in hibernating myocardium. This is not secondary to increases in myocardial perfusion but is secondary to pronounced myocyte proliferation and regeneration with newly formed myocytes in diseased but not normal myocardium.

The Giant Danio (D. Aequipinnatus) as A Model of Cardiac Remodeling and Regeneration

The paucity of mammalian adult cardiac myocytes (CM) proliferation following myocardial infarction (MI) and the remodeling of the necrotic tissue that ensues, result in non-regenerative repair. In contrast, zebrafish (ZF) can regenerate after an apical resection or cryoinjury of the heart. There is considerable interest in models where regeneration proceeds in the presence of necrotic tissue. We have developed and characterized a cautery injury model in the giant danio (GD), a species closely related to ZF, where necrotic tissue remains part of the ventricle, yet regeneration occurs. By light and transmission electron microscopy (TEM), we have documented four temporally overlapping processes: (1) a robust inflammatory response analogous to that observed in MI, (2) concomitant proliferation of epicardial cells leading to wound closure, (3) resorption of necrotic tissue and its replacement by granulation tissue, and (4) regeneration of the myocardial tissue driven by 5-EDU and [(3) H]thymidine incorporating CMs. In conclusion, our data suggest that the GD possesses robust repair mechanisms in the ventricle and can serve as an important model of cardiac inflammation, remodeling and regeneration.

Is a deceptive role of IGF-1 in Sirt1–PARP1 interactions the primary step of postnatal regression of hypertrophic cardiomyopathy in infants of diabetic mothers?

Is a deceptive role of IGF-1 in Sirt1–PARP1 interactions the primary step of postnatal regression of hypertrophic cardiomyopathy in infants of diabetic mothers?

Induced cardiac myocytes apoptosis in vitro by microRNA-122 over expression

ObjectiveThe role of microRNA-122 (miR122) in myocardial cells is still unknown. Thus we investigated the involvement of miRNAs in the knockout Pax8 mice and study the function of miR122 during...

New Perspectives on Regeneration of the Heart

### De novo cardiomyocytes from within the activated adult heart after injury Smart et al Nature . 2011. doi:10.1038/nature10188 Can the adult mammalian heart regenerate and replace lost or damaged tissue? Text books suggest it cannot but recent studies suggest we may have to revisit this issue. Until recently, regeneration of the heart was thought to be restricted to fish and amphibians. For instance, hearts of zebrafish can regenerate completely within just 2 months of surgical amputation of up to 20% of the ventricle.1 This occurs by proliferation and dedifferentiation of mature cardiac myocytes.2 By contrast, severe injury of the adult mammalian heart leads to the formation of scar tissue, which replaces damaged and lost cardiac myocytes. Interestingly, neonatal mammalian hearts were recently shown to have some regenerative capacity, not unlike zebrafish, although this was lost beyond 7 days of age.3 In the last decade, several studies have described evidence for cardiac stem cell populations within the postnatal mammalian heart, which appear able to form cells with cardiac myocyte properties in culture and after transplantation back into the heart (reviewed in Rasmussen et al4). This has encouraged the search for ways to stimulate these endogenous cardiac stem cells to proliferate and differentiate into cardiac myocytes and vascular cells in situ to regenerate the injured heart even though their exact identity is unknown. In a recent study for example, Loffredo et al cleverly used genetically engineered mice to demonstrate at least some degree of endogenous myocardial replacement after an experimentally induced heart attack (myocardial infarction).5 Cardiac myocytes were …

Autologous Mesenchymal Stem Cells Mobilize cKit + and CD133 + Bone Marrow Progenitor Cells and Improve Regional Function in Hibernating Myocardium

Mesenchymal stem cells (MSCs) improve function after infarction, but their mechanism of action remains unclear, and the importance of reduced scar volume, cardiomyocyte proliferation, and perfusion is uncertain. The present study was conducted to test the hypothesis that MSCs mobilize bone marrow progenitor cells and improve function by stimulating myocyte proliferation in collateral-dependent hibe rnating myocardium. Swine with chronic hibernating myocardium received autologous intracoronary MSCs (icMSCs; ≈44 ×10(6) cells, n = 10) 4 months after instrumentation and were studied up to 6 weeks later. Physiological and immunohistochemical findings were compared with untreated hibernating animals (n = 7), sham-normal animals (n = 5), and icMSC-treated sham-normal animals (n = 6). In hibernating myocardium, icMSCs increased function (percent wall thickening of the left anterior descending coronary artery 24 ± 4% to 43 ± 5%, P < 0.05), although left anterior descending coronary artery flow reserve (adenosine/rest) remained critically impaired (1.2 ± 0.1 versus 1.2 ± 0.1). Circulating cKit+ and CD133+ bone marrow progenitor cells increased transiently after icMSC administration, with a corresponding increase in myocardial cKit+/CD133+ and cKit+/CD133- bone marrow progenitor cells (total cKit+ from 223 ± 49 to 4415 ± 866/10(6) cardiomyocytes, P < 0.05). In hibernating hearts, icMSCs increased Ki67+ cardiomyocytes (from 410 ± 83 to 2460 ± 610/10(6) nuclei, P < 0.05) and phospho-histone H3-positive cardiomyocytes (from 9 ± 5 to 116 ± 12/10(6) nuclei, P < 0.05). Myocyte nuclear number (from 75 336 ± 5037 to 114 424 ± 9564 nuclei/mm3, P < 0.01) and left ventricular mass (from 2.5 ± 0.1 to 2.8 ± 0.1 g/kg, P < 0.05) increased, yet myocytes were smaller (14.5 ± 0.4 versus 16.5 ± 0.4 μm, P < 0.05), which supports endogenous cardiomyocyte proliferation. In sham-normal animals, icMSCs increased myocardial bone marrow progenitor cells with no effect on myocyte proliferation or regional function. Our results indicate that icMSCs improve function in hibernating myocardium independent of coronary flow or reduced scar volume. This arises from stimulation of myocyte proliferation with increases in cKit+/CD133+ bone marrow progenitor cells and cKit+/CD133- resident stem cells, which increase myocyte number and reduce cellular hypertrophy.

Modern views on amlodipine and new S-amlodipine medications

The review presents the latest evidence on the calcium antagonist amlodipine, summarizing its mechanisms of action, its pleiotropic, endothelial function-related effects, and its anti-atherogenic activity. Amlodipine suppresses the proliferation of vascular smooth myocytes and extracellular matrix and improves endothelial vasodilatation, despite the absence of L-type calcium channels in these cells. This mechanism is related to an increase in endothelial nitric oxide (NO) release. The results of experimental studies on the role of S and R amlodipine isomers in its hemodynamic and pleiotropic activity are presented. While S-amlodipine is a pharmacologically active blocker of L-type calcium channels, R-amlodipine increases endothelial NO release. New medications have been developed, based on S-amlodipine. It has been shown that S-amlodipine 5 mg/d is bioequivalent to amlodipine 10 mg/d. The pharmacodynamics analysis demonstrated that S-amlodipine 5 mg/d and amlodipine 10 mg/d did not differ significantly in terms of mean levels of systolic and diastolic blood pressure, or mean heart rate. S-amlodipine was better tolerated and characterised by a lower incidence of peripheral edema than amlodipine. However, the effects of S-amlodipine on hard end-points should be investigated in the long-term prospective studies.

Evaluation of Myocyte Proliferation in Alcoholic Cardiomyopathy: Telomerase Enzyme Activity (TERT) Compared with Ki-67 Expression

Although the human heart was classically considered a terminal organ, recent studies have reported a myocyte proliferation response versus some aggressions. Excessive ethanol consumption induces development of cardiomyopathy (CMP) through myocyte apoptosis. We evaluated myocyte proliferation response in the heart of chronic alcoholic donors with telomerase activity (telomerase reverse transcriptase (TERT)) compared with Ki-67 nuclear expression. Heart samples were prospectively obtained from organ donors on life support. We included donors with (1) high lifetime alcohol consumption (n = 15), (2) longstanding hypertension (n = 14), (3) other causes of CMP (valve, coronary or idiopathic) (n = 8) and (4) previously healthy donors (n = 6). Groups 2 and 3 were subdivided according to the presence of CMP. Evaluation comprised parameters of ethanol consumption, left ventricular function by chest X-ray and 2D echocardiography, and histology and immunohistochemical studies. Myocyte proliferation was evaluated using an assay for Ki-67 expression and measuring telomerase gene activity by real-time PCR. Forty-three donors were included in the study, 35 having CMP. Nuclear Ki-67 activity was low in healthy controls and significantly increased in the other groups, mainly in those with CMP. Alcoholics with CMP had a non-significantly lower proliferation response than the other CMP groups. No proliferation activity was detected with TERT in any case. Heart Ki-67 proliferation activity increases in organ donors with CMP, independently of its origin. Alcoholics presented non-significant lower myocyte proliferation capacity compared with the other groups of CMP. TERT activity was not a useful marker of proliferation in this model. Ki-67 is a better procedure to evaluate proliferation than TERT expression in alcohol-induced heart damage.

Neuregulin in heart failure

Trastuzumab is a monoclonal antibody to the ErbB2 (Her2nue) receptor over-expressed in Her2(+) breast cancer. Trastuzumab-related cardiotoxicity has revealed the importance of ErbB2 signaling in the heart. Neuregulin (NRG-1) is an important stress-mediated paracrine growth factor that signals through the family of ErbB receptors to promote cardioprotection (myocyte cell survival, proliferation, differentiation, hypertrophy, and angiogenesis). Animal models with disrupted NRG/ErbB signaling fail to develop normally or result in impaired cardiac function post-natally. Pre-clinical animal studies and early-phase human studies suggest that recombinant NRG-1 holds promise as a new therapy for the treatment of various forms of heart failure. Much work is needed to further understand the exact mechanisms of cardiac repair and to find a safe mode of application for recombinant NRG-1 in heart failure.

Hypoxia and nitric oxide exposure promote apoptotic signaling in contractile pulmonary arterial smooth muscle but not in pulmonary epithelium

Neonatal pulmonary hypertension is characterized by hypoxia, abnormal vascular remodeling, and impaired alveolarization. Nitric oxide (NO) regulates cell replication and activation of apoptosis. Our objective was to examine cell phenotype-specific effects of hypoxia and NO exposure on cumulative apoptotic signal in neonatal pulmonary epithelial cells and arterial smooth muscle. Primary cultured newborn porcine pulmonary arterial myocytes and epithelial cells were grown in normoxic (21% O2) or hypoxic conditions (10% O2). Myocyte phenotype was predetermined by serum-supplementation or -deprivation. Cells were exposed to sodium nitroprusside (10(-7) -10(-4) M) or diluent for 3 days. Cell survival was estimated by MTT assay; BAX, Bcl-2, and cleaved caspase-3 by Western blot; cell cycle entry by laser scanning cytometry. Hypoxic epithelial cells exhibited a small increase in anti-apoptotic Bcl2, and decrease in BAX. Cell survival and active caspase-3 were unchanged. Exposure to NO had no impact on epithelial apoptosis, but initiated necrosis. In contractile myocytes, pro-apoptotic BAX abundance and caspase-3 activation were increased by hypoxia, augmented by NO exposure promoting apoptosis. Hypoxia decreased BAX/Bcl-2 ratio and promoted survival of synthetic myocytes; NO increased apoptosis of normoxic synthetic myocytes, but decreased apoptosis of hypoxic synthetic myocytes. The effect of NO on pulmonary apoptosis is phenotype-dependent. A cumulative apoptotic effect of hypoxia and NO in vitro exerted on contractile myocytes may lead to contraction of this subpopulation, while synthetic myocyte survival and proliferation is enhanced by hypoxia and NO. Epithelial survival is unaffected. We speculate that alveolar rarefaction reported after neonatal hypoxia may arise from growth arrest in the vascular rather than the epithelial compartment.

Myocyte proliferation in the developing heart.

Regulation of organ growth is critical during embryogenesis. At the cellular level, mechanisms controlling the size of individual embryonic organs include cell proliferation, differentiation, migration, and attrition through cell death. All these mechanisms play a role in cardiac morphogenesis, but experimental studies have shown that the major determinant of cardiac size during prenatal development is myocyte proliferation. As this proliferative capacity becomes severely restricted after birth, the number of cell divisions that occur during embryogenesis limits the growth potential of the postnatal heart. We summarize here current knowledge concerning regional control of myocyte proliferation as related to cardiac morphogenesis and dysmorphogenesis. There are significant spatial and temporal differences in rates of cell division, peaking during the preseptation period and then gradually decreasing toward birth. Analysis of regional rates of proliferation helps to explain the mechanics of ventricular septation, chamber morphogenesis, and the development of the cardiac conduction system. Proliferation rates are influenced by hemodynamic loading, and transduced by autocrine and paracrine signaling by means of growth factors. Understanding the biological response of the developing heart to such factors and physical forces will further our progress in engineering artificial myocardial tissues for heart repair and designing optimal treatment strategies for congenital heart disease.

LIF is a contraction-induced myokine stimulating human myocyte proliferation

The cytokine leukemia inhibitory factor (LIF) is expressed by skeletal muscle and induces proliferation of myoblasts. We hypothesized that LIF is a contraction-induced myokine functioning in an autocrine fashion to activate gene regulation of human muscle satellite cell proliferation. Skeletal muscle LIF expression, regulation, and action were examined in two models: 1) young men performing a bout of heavy resistance exercise of the quadriceps muscle and 2) cultured primary human satellite cells. Resistance exercise induced a ninefold increase in LIF mRNA content in skeletal muscle, but LIF was not detectable in plasma of the subjects. However, electrically stimulated cultured human myotubes produced and secreted LIF, suggesting that LIF is a myokine with local effects. The well established exercise-induced signaling molecules PI3K, Akt, and mTor contributed to the regulation of LIF in cultured human myotubes as chemical inhibition of PI3K and mTor and siRNA knockdown of Akt1 were independently sufficient to downregulate LIF. Human myoblast proliferation was increased by recombinant exogenous LIF and decreased by siRNA knockdown of the endogenous LIF receptor. Finally, the transcription factors JunB and c-Myc, which promote myoblast proliferation, were induced by LIF in cultured human myotubes. Indeed, both JunB and c-Myc were also increased in skeletal muscle following resistance exercise. Our data suggest that LIF is a contraction-induced myokine, potentially acting in an autocrine or paracrine fashion to promote satellite cell proliferation.

Increased Myostatin Activity and Decreased Myocyte Proliferation in Chronic Alcoholic Cardiomyopathy

Apoptosis mediates in alcohol-induced heart damage leading to cardiomyopathy (CMP). Myocyte proliferation may compensate for myocyte loss. Myostatin is upregulated after cardiac damage and by alcohol consumption thereby decreasing myocyte renewal. We assess the potential role of alcohol in inducing myocyte apoptosis as well as in inhibiting myocyte proliferation. Heart samples were obtained from organ donors, including 22 high alcohol consumers, 22 with hypertension, 8 with other causes of CMP, and 10 healthy donors. Evaluation included medical record with data on daily, recent and lifetime ethanol consumption, chest X-ray, left ventricular (LV) function assessed by two-dimensional echocardiography, and LV histology and immunohistochemistry. Apoptosis was evaluated by TUNEL, BAX, and BCL-2 assays. Myocyte proliferation was evaluated with Ki-67 assay. Myostatin activity was measured with a specific immunohistochemical assay. CMP was assessed by functional and histological criteria. Alcoholic and hypertensive donors with CMP showed higher apoptotic indices than did their partners without CMP. Myostatin activity was higher in alcoholics than in controls, mainly in those with CMP. The increase in myostatin expression in alcoholic CMP was higher than in other groups. The Ki-67 proliferation index increased in all groups with CMP compared to those without CMP, with alcoholics showing a lower increase in this proliferation response. Alcohol produces cardiac myocyte loss through apoptosis but also partially inhibits myocyte proliferation through myostatin up-regulation. The final result may suppose an imbalance in myocyte homeostasis, with a net loss in total ventricular myocyte mass and progressive ventricular dysfunction.