Proliferating cell nuclear antigen in developing and adult rat cardiac muscle cells

[Proliferation of adult mammalian ventricular cardiomyocytes: a sporadic but feasible phenomenon]

Proliferation of adult mammalian ventricular cardiomyocytes has been ruled out by some researchers, who have argued that these cells are terminally differentiated; however, this dogma has been rejected because other researchers have reported that these cells can present the processes necessary to proliferate, that is, DNA synthesis, mitosis and cytokinesis when the heart is damaged experimentally through pharmacological and surgical strategies or due to pathological conditions concerning the cardiovascular system. This review integrates some of the available works in the literature evaluating the DNA synthesis, mitosis and cytokinesis in these myocytes, when the myocardium is damaged, with the purpose of knowing if their proliferation can be considered as a feasible phenomenon. The review is concluded with a reflection about the perspectives of the knowledge generated in this area.

Detection of slipped-DNAs at the trinucleotide repeats of the myotonic dystrophy type I disease locus in patient tissues

Slipped-strand DNAs, formed by out-of-register mispairing of repeat units on complementary strands, were proposed over 55 years ago as transient intermediates in repeat length mutations, hypothesized to cause at least 40 neurodegenerative diseases. While slipped-DNAs have been characterized in vitro, evidence of slipped-DNAs at an endogenous locus in biologically relevant tissues, where instability varies widely, is lacking. Here, using an anti-DNA junction antibody and immunoprecipitation, we identify slipped-DNAs at the unstable trinucleotide repeats (CTG)n•(CAG)n of the myotonic dystrophy disease locus in patient brain, heart, muscle and other tissues, where the largest expansions arise in non-mitotic tissues such as cortex and heart, and are smallest in the cerebellum. Slipped-DNAs are shown to be present on the expanded allele and in chromatinized DNA. Slipped-DNAs are present as clusters of slip-outs along a DNA, with each slip-out having 1–100 extrahelical repeats. The allelic levels of slipped-DNA containing molecules were significantly greater in the heart over the cerebellum (relative to genomic equivalents of pre-IP input DNA) of a DM1 individual; an enrichment consistent with increased allelic levels of slipped-DNA structures in tissues having greater levels of CTG instability. Surprisingly, this supports the formation of slipped-DNAs as persistent mutation products of repeat instability, and not merely as transient mutagenic intermediates. These findings further our understanding of the processes of mutation and genetic variation.

Over 30 diseases are caused by the expansion of a trinucleotide repeat (TNR) in a specific gene, including the most common adult-onset form of muscular dystrophy, myotonic dystrophy (DM1). The mechanistic contributors to this unstable (TNR) expansion are not fully known, although since the discovery of these types of diseases over twenty years ago, the extrusion of the expanded repeats into mutagenic slipped-DNA conformations has been hypothesized. Here, we show the presence of slipped-DNA at the DM1 disease locus in various patient tissues. The allelic amounts of slipped-DNA in tissues correlate with overall levels of repeat instability. Slipped-DNA was also found to form in clusters along a tract of expanded repeats, which has been previously shown in vitro to impede DNA repair. This is the first evidence for slipped-DNA formation at an endogenous disease-causing gene in patient tissues.

Telomerase expression in the mammalian heart

While the mammalian heart has low, but functionally significant, levels of telomerase expression, the cellular population responsible remains incompletely characterized. This study aimed to identify the cell types responsible for cardiac telomerase activity in neonatal, adult, and cryoinjured adult hearts using transgenic mice expressing green fluorescent protein (GFP), driven by the promoter for murine telomerase reverse transcriptase (mTert), which is a necessary and rate-limiting component of telomerase. A rare population of mTert-GFP-expressing cells was identified that possessed all detectable cardiac telomerase RNA and telomerase activity. It was heterogeneous and included cells coexpressing markers of cardiomyocytic, endothelial, and mesenchymal lineages, putative cardiac stem cell markers, and, interestingly, cardiomyocytes with a differentiated phenotype. Quantification using both flow cytometry and immunofluorescence identified a significant decline in mTert-GFP cells in adult animals compared to neonates (∼9- and ∼20-fold, respectively). Cardiac injury resulted in a ∼6.45-fold expansion of this population (P<0.005) compared with sham-operated controls. This study identifies the cells responsible for cardiac telomerase activity, demonstrates a significant diminution with age but a marked response to injury, and, given the relationship between telomerase activity and stem cell populations, suggests that they represent a potential target for further investigation of cardiac regenerative potential.—Richardson, G. D., Breault, D., Horrocks, G., Cormack, S., Hole, N., Owens, W. A. Telomerase expression in the mammalian heart.

Heart tissue of harlequin (hq)/Big Blue mice has elevated reactive oxygen species without significant impact on the frequency and nature of point mutations in nuclear DNA

Age is a major risk factor for heart disease, and cardiac aging is characterized by elevated mitochondrial reactive oxygen species (ROS) with compromised mitochondrial and nuclear DNA integrity. To assess links between increased ROS levels and mutations, we examined in situ levels of ROS and cII mutation frequency, pattern and spectrum in the heart of harlequin (hq)/Big Blue mice. The hq mouse is a model of premature aging with mitochondrial dysfunction and increased risk of oxidative stress-induced heart disease with the means for in vivo mutation detection. The hq mutation produces a significant downregulation in the X-linked apoptosis-inducing factor gene (Aif) impairing both the antioxidant and oxidative phosphorylation functions of AIF. Brain and skin of hq disease mice have elevated frequencies of point mutations in nuclear DNA and histopathology characterized by cell loss. Reports of associated elevations in ROS in brain and skin have mixed results. Herein, heart in situ ROS levels were elevated in hq disease compared to AIF-proficient mice (p<0.0001) yet, mutation frequency and pattern were similar in hq disease, hq carrier and AIF-proficient mice. Heart cII mutations were also assessed 15 days following an acute exposure to an exogenous ROS inducer (10 mg paraquat/kg). Acute paraquat exposure with a short mutant manifestation period was insufficient to elevate mutation frequency or alter mutation pattern in the post-mitotic heart tissue of AIF-proficient mice. Paraquat induction of ROS requires mitochondrial complex I and thus is likely compromised in hq mice. Results of this preliminary survey and the context of recent literature suggest that determining causal links between AIF deficiency and the premature aging phenotypes of specific tissues is better addressed with assay of mitochondrial ROS and large-scale changes in mitochondrial DNA in specific cell types.

Suppression of right ventricular hypertrophy after extensive pulmonary resection in rats by granulocyte colony-stimulating factor

BACKGROUND

The objective of the present study was to investigate the effects of granulocyte colony-stimulating factor (G-CSF) on right ventricular hypertrophy following extensive pulmonary resection in rats.

MATERIALS AND METHODS

Adult rats were divided into four groups: (1) Group S (right thoracotomy only); (2) Group L (right three lobectomy); (3) Group LG10 (Group L+G-CSF [10microg/kg/d]); and (4) Group LG100 (Group L+G-CSF [100microg/kg/d]). At postoperative day 21, weight ratio of the right ventricular to the left ventricle plus septum (RV/LV+S, indicator of right ventricular hypertrophy) were measured, and a histopathological study was conducted to determine percentage wall thickness of peripheral pulmonary arteries and proliferating cell nuclear antigen labeling index (indicator of oxidative DNA damage) of right ventricles.

RESULTS

Mean RV/LV+S for Group S was 0.27+/-0.02, significantly smaller than that for the lobectomy groups (Group L, LG10, LG100; 0.47+/-0.05, 0.35+/-0.02, 0.38+/-0.05). G-CSF significantly suppressed right ventricular hypertrophy. Mean medial wall thickness of peripheral pulmonary arteries for Group S was 13.6% +/- 4.9%, significantly smaller than that for Group L (22.9% +/- 9.6%). Compared with Group L, G-CSF reduced medial wall thickness (LG10, 17.6% +/- 9.5%; LG100, 18.0% +/- 11.2%). Incidence of proliferating cell nuclear antigen positive nuclei for Group S was 1.07% +/- 0.49%, significantly smaller than that for Group L (13.77% +/- 5.87%). G-CSF significantly reduced the incidence of proliferating cell nuclear antigen positive nuclei (LG10, 4.04% +/- 2.14%; LG100, 3.18% +/- 1.66%).

CONCLUSIONS

G-CSF administration not only reduce medial wall thickness of peripheral pulmonary arteries but also directly protect cardiomyocytes of the right ventricle, thus suppressing right ventricular hypertrophy. These results suggest that low-dose G-CSF administration can prevent right heart failure following extensive pulmonary resection.

Cardiomyocyte death and renewal in the normal and diseased heart

During post-natal maturation of the mammalian heart, proliferation of cardiomyocytes essentially ceases as cardiomyocytes withdraw from the cell cycle and develop blocks at the G0/G1 and G2/M transition phases of the cell cycle. As a result, the response of the myocardium to acute stress is limited to various forms of cardiomyocyte injury, which can be modified by preconditioning and reperfusion, whereas the response to chronic stress is dominated by cardiomyocyte hypertrophy and myocardial remodeling. Acute myocardial ischemia leads to injury and death of cardiomyocytes and nonmyocytic stromal cells by oncosis and apoptosis, and possibly by a hybrid form of cell death involving both pathways in the same ischemic cardiomyocytes. There is increasing evidence for a slow, ongoing turnover of cardiomyocytes in the normal heart involving death of cardiomyocytes and generation of new cardiomyocytes. This process appears to be accelerated and quantitatively increased as part of myocardial remodeling. Cardiomyocyte loss involves apoptosis, autophagy, and oncosis, which can occur simultaneously and involve different individual cardiomyocytes in the same heart undergoing remodeling. Mitotic figures in myocytic cells probably represent maturing progeny of stem cells in most cases. Mitosis of mature cardiomyocytes that have reentered the cell cycle appears to be a rare event. Thus, cardiomyocyte renewal likely is mediated primarily by endogenous cardiac stem cells and possibly by blood-born stem cells, but this biological phenomenon is limited in capacity. As a consequence, persistent stress leads to ongoing remodeling in which cardiomyocyte death exceeds cardiomyocyte renewal, resulting in progressive heart failure. Intense investigation currently is focused on cell-based therapies aimed at retarding cardiomyocyte death and promoting myocardial repair and possibly regeneration. Alteration of pathological remodeling holds promise for prevention and treatment of heart failure, which is currently a major cause of morbidity and mortality and a major public health problem. However, a deeper understanding of the fundamental biological processes is needed in order to make lasting advances in clinical therapeutics in the field.

Chronic right ventricular pressure overload results in a hyperplastic rather than a hypertrophic myocardial response

Myocardial hyperplasia is generally considered to occur only during fetal development. However, recent evidence suggests that this type of response may also be triggered by cardiac overload after birth. In congenital heart disease, loading conditions are frequently abnormal, thereby affecting ventricular function. We hypothesized that chronic right ventricular pressure overload imposed on neonatal hearts initiates a hyperplastic response in the right ventricular myocardium. To test this, young lambs (aged 2-3 weeks) underwent adjustable pulmonary artery banding to obtain peak right ventricular pressures equal to left ventricular pressures for 8 weeks. Transmural cardiac tissue samples from the right and left ventricles of five banded and five age-matched control animals were studied. We found that chronic right ventricular pressure overload resulted in a twofold increase in right-to-left ventricle wall thickness ratio. Morphometric right ventricular myocardial tissue analysis revealed no changes in tissue composition between the two groups; nor were right ventricular myocyte dimensions, relative number of binucleated myocytes, or myocardial DNA concentration significantly different from control values. In chronic pressure overloaded right ventricular myocardium, significantly (P < 0.01) more myocyte nuclei were positive for the proliferation marker proliferating cellular nuclear antigen than in control right ventricular myocardium. Chronic right ventricular pressure overload applied in neonatal sheep hearts results in a significant increase in right ventricular free wall thickness which is primarily the result of a hyperplastic myocardial response.

Concise review: stem cells, myocardial regeneration, and methodological artifacts

This review discusses the current controversy about the role that endogenous and exogenous progenitor cells have in cardiac homeostasis and myocardial regeneration following injury. Although great enthusiasm was created by the possibility of reconstituting the damaged heart, the opponents of this new concept of cardiac biology have interpreted most of the findings supporting this possibility as the product of technical artifacts. This article challenges this established, static view of cardiac growth and favors the notion that the mammalian heart has the inherent ability to replace its cardiomyocytes through the activation of a pool of resident primitive cells or the administration of hematopoietic stem cells.

Downregulation of PCNA potentiates p21-mediated growth inhibition in response to hyperoxia

Prolonged exposure to hyperoxia inhibits cell proliferation in G1 via increased expression of p21. While p21 inhibits proliferating cell nuclear antigen (PCNA)-dependent DNA synthesis, it can also directly lower PCNA abundance; however, it is unclear whether loss of PCNA contributes to growth arrest. Here, we investigate how PCNA loss affects ability of p21 to exert G1 growth arrest of lung epithelial cells exposed to hyperoxia. In A549 cells that express p21 and growth arrest in G1 during hyperoxia, small interfering RNA (siRNA) knockdown of p21 led to G1 checkpoint bypass, increased cell death, and restoration of PCNA expression. Conditional overexpression of the PCNA binding domain of p21 in H1299 cells that do not normally express p21, or exposure to hyperoxia, caused a time-dependent loss of PCNA. Titrating PCNA levels using siRNA to approximate the low amount observed in cells expressing p21 resulted in S phase arrest. While lowering PCNA by itself caused S phase arrest, the combination of hyperoxia and siRNA against PCNA dramatically reduced PCNA abundance resulting in G1 arrest. G1 growth arrest was markedly enhanced upon the addition of p21 to these cells. Our findings suggest a model in which reducing expression of the abundant protein PCNA allows the less abundant protein p21 to be more effective at suppressing the processivity functions of remaining PCNA, thereby fully exerting the G1 checkpoint. Given that high p21 expression is often associated with lower PCNA abundance, our findings are suggestive of a global growth inhibitory mechanism involving p21-mediated PCNA suppression.

Novel 3D culture system for study of cardiac myocyte development

Insufficient myocardial repair after pathological processes contributes to cardiovascular disease, which is a major health concern. Understanding the molecular mechanisms that regulate the proliferation and differentiation of cardiac myocytes will aid in designing therapies for myocardial repair. Models are needed to delineate these molecular mechanisms. Here we report the development of a model system that recapitulates many aspects of cardiac myocyte differentiation that occur during early cardiac development. A key component of this model is a novel three-dimensional tubular scaffold engineered from aligned type I collagen strands. In this model embryonic ventricular myocytes undergo a transition from a hyperplastic to a quiescent phenotype, display significant myofibrillogenesis, and form critical cell-cell connections. In addition, embryonic cardiac myocytes grown on the tubular substrate have an aligned phenotype that closely resembles in vivo neonatal ventricular myocytes. We propose that embryonic cardiac myocytes grown on the tube substrate develop into neonatal cardiac myocytes via normal in vivo mechanisms. This model will aid in the elucidation of the molecular mechanisms that regulate cardiac myocyte proliferation and differentiation, which will provide important insights into myocardial development.

p21(CIP1) Controls proliferating cell nuclear antigen level in adult cardiomyocytes

Cell cycle withdrawal associated with terminal differentiation is responsible for the incapability of many organs to regenerate after injury. Here, we employed a cell-free system to analyze the molecular mechanisms underlying cell cycle arrest in cardiomyocytes. In this assay, incubation of S phase nuclei mixed with cytoplasmic extract of S phase cells and adult primary cardiomyocytes results in a dramatic reduction of proliferating cell nuclear antigen (PCNA) protein levels. This effect was blocked by the proteasome inhibitors MG132 and lactacystin, whereas actinomycin D and cycloheximide had no effect. Immunodepletion and addback experiments revealed that the effect of cardiomyocyte extract on PCNA protein levels is maintained by p21 but not p27. In serum-stimulated cardiomyocytes PCNA expression was reconstituted, whereas the protein level of p21 but not that of p27 was reduced. Cytoplasmic extract of serum-stimulated cardiomyocytes did not influence the PCNA protein level in S phase nuclei. Moreover, the hypertrophic effect of serum stimulation was blocked by ectopic expression of p21 and the PCNA protein level was found to be upregulated in adult cardiomyocytes derived from p21 knockout mice. Our data provide evidence that p21 regulates the PCNA protein level in adult cardiomyocytes, which has implications for cardiomyocyte growth control.

Apoptosis of myocytes and proliferation markers as prognostic factors in end-stage dilated cardiomyopathy

INTRODUCTION

The aim of the study was to evaluate the role of apoptosis, proliferation markers, volume density of interstitium, and myofibril volume fraction for the prognosis in patients with end-stage dilated cardiomyopathy (DCM).

METHODS

Endomyocardial biopsy was performed during open-heart surgery in 56 patients with end-stage DCM. Patients were divided into two groups, one group with shorter survival (24+/-9 months, mean+/-S.D.) and another group with survival of more than 7 years after operation. The TUNEL method was used for the detection of apoptosis, and immunohistochemical methods were used for the evaluation of inhibitor of apoptosis (bcl-2) and proliferation markers (PCNA and Ki-67).

RESULTS

The increased percentage of apoptotic myocytes, decreased expression of bcl-2, and decreased expression of PCNA and Ki-67 antigen was found in the group with early mortality compared to that with longer survival. Myofibril volume fraction was lower and volume density of interstitium was higher in the group with early mortality compared to that with longer survival.

CONCLUSION

Apoptosis, bcl-2 expression, and proliferation activity of myocytes, myofibril volume fraction, and volume density of interstitial tissue might be useful in predicting the prognosis (progressive vs. nonprogressive form) of patients with heart failure due to DCM.

L-arginine protects human heart cells from low-volume anoxia and reoxygenation

Protective effects of L-arginine were evaluated in a human ventricular heart cell model of low-volume anoxia and reoxygenation independent of alternate cell types. Cell cultures were subjected to 90 min of low-volume anoxia and 30 min of reoxygenation. L-Arginine (0-5.0 mM) was administered during the preanoxic period or the reoxygenation phase. Nitric oxide (NO) production, NO synthase (NOS) activity, cGMP levels, and cellular injury were assessed. To evaluate the effects of the L-arginine on cell signaling, the effects of the NOS antagonist N(G)-nitro-L-arginine methyl ester, NO donor S-nitroso-N-acetyl-penicillamine, guanylate cyclase inhibitor methylene blue, cGMP analog 8-bromo-cGMP, and ATP-sensitive K+ channel antagonist glibenclamide were examined. Our data indicate that low-volume anoxia and reoxygenation increased NOS activity and facilitated the conversion of L-arginine to NO, which provided protection against cellular injury in a dose-dependent fashion. In addition, L-arginine cardioprotection was achieved by the activation of guanylate cyclase, leading to increased cGMP levels in human heart cells. This action involves a glibenclamide-sensitive, NO-cGMP-dependent pathway.

Evidence of an altered in vivo vascular cell turnover in spontaneously hypertensive rats and its modulation by long-term antihypertensive treatment

The aims of this study were to measure in vivo cell turnover in the thoracic aorta from spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY) and to investigate how it could be modulated by chronic antihypertensive treatment. Cell turnover was estimated in rats in which DNA had been prelabeled in utero with [ 3 H]-thymidine, by the rate of disappearance of total [ 3 H]-DNA from birth to 20 weeks of age. In SHR compared with WKY, neonatal relative aortic mass was transiently elevated and was reversed to hypotrophy at 8 weeks. At 20 weeks of age, aortic hypertrophy reappeared. Aortic DNA content reflected the morphologic changes observed with age. In both SHR and WKY, the decline with time in [ 3 H]-prelabeled aortic DNA coupled with the increase in total organ DNA demonstrated that cells prelabeled in utero died and were replaced. Decline in [ 3 H]-DNA from birth to 8 weeks of age was approximately threefold faster in the aorta from SHR than in WKY. In older SHR, the decrease in [ 3 H]-DNA was then slower and similar to that of WKY. Chronic treatment of SHR for 15 weeks from the age of 5 weeks, with hydralazine, enalapril, or nifedipine prevented the rise in systolic blood pressure, aortic mass, and DNA content. This was associated with an unchanged residual radioactivity of [ 3 H]-prelabeled aortic DNA over time, suggesting that the treatment did not stimulate cumulative cell death. We propose that the altered cell turnover is a component of aortic remodeling observed in hypertension. Our data also suggest that it is possible to modulate in vivo cell turnover and affect vascular remodeling by pharmacologic therapy.

Analysis of sense and naturally occurring antisense transcripts of myosin heavy chain in the human myocardium

Naturally occurring antisense RNA has the potential to form a duplex with its complementary sense mRNA, thereby regulating protein expression. Previously, we demonstrated considerable amounts of endogenous antisense RNA for both alpha- and beta-myosin heavy chain (MHC) in rat heart suggesting a role in posttranscriptional MHC-regulation (Luther et al. [1997] J Mol Cell Cardiol 29(1):27-35). To evaluate whether antisense RNA is also involved in MHC regulation in human heart we analyzed ventricular myocardium transcripts in nonfailing hearts (n=3) and hearts from patients undergoing heart transplantation (n=5). Investigation of RNA by reverse transcription polymerase chain reaction (RT-PCR) detected an antisense RNA transcript for beta-MHC but none for alpha-MHC. Northern blot analysis of normal and failing hearts detected sense mRNA for beta-MHC, but not alpha-MHC suggesting no functionally relevant levels of alpha-MHC mRNA exist in the human ventricle. The results describe-for the first time-the existence of endogenous polyadenylated MHC antisense transcripts in the human heart. The potential effect of attenuating translation was shown in an in vitro translation assay using a synthetic antisense-oligonucleotide derived from the sequence of the naturally occurring antisense RNA.

ENU induces mutations in the heart of lacZ transgenic mice

The use of transgenic mouse models as somatic mutation assays allows determination of mutation in all tissues of the mouse, including non-dividing tissues. In this regard, these models can be used to study the possibility that mutations can be induced in mitotically quiescent organs such as the heart. Mutations are generally thought to be associated with mitotic processes of DNA replication. Mutations, however, are also postulated to occur in the absence of mitosis as the result of DNA repair. In order to determine whether or not mutations could be induced in the heart, we analyzed the mutant frequency in the hearts of F(1) (Muta Mouse X SWR) mice that had been treated acutely with 250 mg/kg ENU and sampled at days 10, 35, and 70 post-treatment. A significant increase in mutant frequency at day 70 shows that mutations can be induced in the heart. Since the heart contains small numbers of non-muscle cells, additional mechanisms that could explain these results were also considered. The effect of ENU-induced cell proliferation or a sub-population of rapidly dividing cells is ruled out by C(14)-thymidine uptake studies which showed minimal proliferation. By the same token, the influence of ex vivo mutations (i.e., DNA adducts fixed as mutations during replication in the bacteria) is ruled out by the observed time course of mutations, as well as experimental evidence showing that such mutations are not detected in the lacZ assay.

Apoptosis and proliferation of cardiomyocytes in heart failure of different etiologies

Apoptosis and proliferation of myocytes were studied in human heart failure (HF). Endomyocardial samples from the right ventricle of 38 patients with terminal HF were compared with 10 traffic accident victims without a history of cardiovascular disease. The TUNEL method was used for the detection of apoptosis, and immunohistochemical methods were used for the evaluation of p53, bcl-2, proliferation cell nuclear antigen (PCNA), and proliferation marker MIB-1. Apoptosis of cardiomyocytes, which was not p53-dependent, was present in 0.07 % of myocytes in HF, whereas no apoptotic myocytes were found in the control group (p < 0.01). An increased expression of bcl-2 was found in HF compared to controls (p < 0.01), yet bcl-2 failed to protect myocytes from apoptosis. Increased expression of proliferation markers was found in myocytes in HF compared to controls (PCNA labeling: 3.7% vs. 1.2%, p < 0.01; MIB-1 labeling: 0.1% vs. 0%, p< 0.01). Nevertheless, no mitotic figures in cardiomyocytes were found in our specimens. The volume density of interstitium was 22% in HF vs. 10% in the control group (p < 0.01). In conclusion, apoptosis of cardiomyocytes and fibrosis play an important role in HF, whereas clinical importance and the rate of myocyte proliferation remain to be determined.

Cell cycle profiles and expressions of p21CIP1 AND P27KIP1 during myocyte development

The ability of the cardiac myocyte to divide ceases shortly after birth. Thus, following severe injury, e.g., during myocardial infarction, the mature heart is unable to regenerate new tissue to replace the dead or damaged tissue. The identification of the molecules controlling the cessation of myocyte cell division may lead to therapeutic strategies which aim to re-populate the damaged myocardial area. Hence, we have determined the cell cycle profile, expressions and activities of the cyclin-dependent kinase inhibitors (CDKIs), p21CIP1 and p27KIP1, during rat ventricular myocyte development. Fluorescent activated cell sorting (FACS) analyses showed the percentage of S phase myocytes to be decreased significantly throughout development, concomitant with a significant increase in the percentage of G0/G1 and G2/M phase cells. The expression of p21CIP1 and p27KIP1 increased significantly throughout cardiac development and complexed differentially with a number of cyclins and CDKs. Furthermore, an adult myocyte extract reduced neonatal myocyte CDK2 kinase activity significantly (>30%, p<0.05) whereas immunodepletion of p21CIP1 from adult lysates restored CDK2 kinase activity. Thus, p21CIP1 and p27KIP1 may be important for the withdrawal of cardiac myocytes from the cell cycle and for maintaining the G0/G1 and G2/M phase blockades.

Cell cycle regulators during human atrial development

OBJECTIVES

The molecular mechanisms that regulate cardiomyocyte cell cycle and terminal differentiation in humans remain largely unknown. To determine which cyclins, cyclin dependent kinases (CDKs) and cyclin kinase inhibitors (CKIs) are important for cardiomyocyte proliferation, we have examined protein levels of cyclins, CDKs and CKIs during normal atrial development in humans.

METHODS

Atrial tissues were obtained in the fetus from inevitable abortion and in the adult during surgery. Cyclin and CDK proteins were determined by Western blot analysis. CDK activities were determined by phosphorylation amount using specific substrate.

RESULTS

Most cyclins and CDKs were high during the fetal period and their levels decreased at different rates during the adult period. While the protein levels of cyclin D1, cyclin D3, CDK4, CDK6 and CDK2 were still detectable in adult atria, the protein levels of cyclin E, cyclin A, cyclin B, cdc2 and PCNA were not detectable. Interestingly, p27KIP1 protein increased markedly in the adult period, while p21CIP1 protein in atria was detectable only in the fetal period. While the activities of CDK6, CDK2 and cdc2 decreased markedly, the activity of CDK4 did not change from the fetal period to the adult period.

CONCLUSION

These findings indicate that marked reduction of protein levels and activities of cyclins and CDKs, and marked induction of p27KIP1 in atria, are associated with the withdrawal of cardiac cell cycle in adult humans.

Preconditioning human cardiomyocytes and endothelial cells

BACKGROUND

The effects of simulated "ischemia" and "reperfusion" were evaluated in cell cultures of human ventricular cardiomyocytes and human saphenous vein endothelial cells.

METHODS

Myocyte and endothelial cell cultures were exposed to a low volume (1.5 ml) of either hypoxic (oxygen tension = 16 mm Hg) or anoxic (oxygen tension = 0 mm Hg) phosphate-buffered saline solution for 90 minutes ("ischemia") followed by 30 minutes of simulated "reperfusion." Cell injury was evaluated by trypan blue exclusion. Next, the effects of a preconditioning stimulus were evaluated by a brief (10 minute) exposure to hypoxic or anoxic ischemia and 10 minutes of reperfusion before prolonged (90 minutes) anoxic ischemia. Finally, the effects of anoxic preconditioning on intracellular lactate accumulation and extracellular lactate and acid release were assessed.

RESULTS

"Ischemia" and "reperfusion" resulted in greater injury to endothelial cells than to cardiomyocytes. In both cell types, anoxic ischemia resulted in greater injury than hypoxic ischemia. Preconditioning reduced cell injury in myocytes but not in endothelial cells. Endothelial cells produced more lactate than cardiomyocytes under normoxic conditions. Ischemia increased lactate accumulation and release in cardiomyocytes but not endothelial cells. Preconditioning reduced lactate accumulation and release in cardiomyocytes but not endothelial cells.

CONCLUSIONS

Endothelial cells were more susceptible to the same period of simulated ischemia than cardiomyocytes. Preconditioning protected cardiomyocytes but not endothelial cells from a subsequent prolonged period of ischemia and reperfusion.

Significance of the DNA synthesis in hypertrophic cardiomyopathies

DNA content and proliferating cell nuclear antigen (PCNA) expression were investigated in normal hearts, in hypertrophic from hemodynamic overload hearts and in hypertrophic cardiomyopathy (HCM). The aim of this study was mainly to determine whether the hyperdiploid myocardial cells in all cases are in dynamic or static phase. The percentage of PCNA positive cells only in the HCM group was significantly higher (mean value = 25.4%) than the percentage of hyperdiploid cells (mean value = 9.3%). Therefore, the DNA replication occurs through a different process from that of normal cell cycle which lead to an increase in ploidy and eventually mitosis. These data should be interpreted not only as the result of a periodic amitotic DNA renewal and not even as the result of an increased apoptosis, but especially as a repair process of the DNA molecules affected by a various types of damages in HCMs.

Localization of proliferating cell nuclear antigen in the developing and mature rat heart cell

BACKGROUND

The cardiac muscle cell ceases to divide shortly after birth; this cessation is followed by a limited period when DNA synthesis and karyokinesis occur without cytokinesis. The regulation of this process is not known. The purpose of this study is to explore the possible events that could lead to the cessation of cardiac muscle cell division. One protein requisite for DNA synthesis is proliferating cell nuclear antigen (PCNA). This protein is the auxiliary protein of DNA polymerase delta.

METHODS

Rats of fetal age day 18 or days 0, 4, 8, 12, and 16 after birth were obtained. In addition, adult hearts were used for this study. Hearts from the fetal day-18 rats and the day-0 neonatal rats were digested. Cardiac myocytes were isolated and placed in culture for an analysis of DNA synthesis by using tridiated thymidine. Ventricular muscle tissue was isolated from hearts of all ages and frozen in liquid nitrogen for Northern and Western blot analyses.

RESULTS

Tridiated thymidine analysis revealed that, although serum stimulation significantly increased the number of labeled fetal cardiac muscle cells, it did not have that effect on neonatal cardiac muscle cells in culture. Northern blot analysis revealed that the steady state levels of mRNA for PCNA remained constant from fetal day 18 through day 4 after birth. Steady state levels declined during the second postnatal week and then reached basal levels by day 16. PCNA message was still present in adult heart tissue. By using indirect immunofluorescence and Western blotting, PCNA protein could be located in the nucleus of cardiac muscle cells during the first 2 weeks after birth. At 16 days after birth, the protein was found in the cytoplasm in very low amounts but was not found in the nucleus. The protein was barely detectable by Western blotting in the cytoplasmic fraction from the adult myocardium.

CONCLUSIONS

The results of this study suggest that the PCNA message and protein product declined after birth, but both were present at low levels in the adult myocardium. However, the PCNA protein was not translocated to the nucleus in adult myocardial cells. The events involving PCNA correlated closely with the time period when cell division and then DNA synthesis ceased in these cells.

Angiotensin converting enzyme inhibition prevents polyploidization of cardiomyocytes in spontaneously hypertensive rats with left ventricular hypertrophy

Polyploidization of cardiomyocyte nuclei is a physiological phenomenon that increases in pathological conditions such as myocardial hypertrophy. The purpose of this study was to evaluate the potential benefit of the angiotensin converting enzyme (ACE) inhibitor quinapril in reversing the polyploidization of cardiomyocyte nuclei in spontaneously hypertensive rats (SHR) with established left ventricular hypertrophy (LVH). Sixteen week-old male SHR were treated with oral quinapril (average dose 10 mg/kg per day) for 20 weeks. Sixteen- and 36-week-old untreated SHR and 16- and 36-week-old normotensive Wistar-Kyoto (WKY) rats were used as controls. Nuclear polyploidization was determined by DNA flow cytometry of frozen tissues from the left ventricle, at least 20,000 nuclei being measured in each sample. The rates of tetraploidy in the 16- and 36-week-old SHR groups were 2.8 per cent (range 2.16-3 per cent) and 5.4 per cent (range 4.9-5.9 per cent), respectively. Treated SHR had a similar rate of DNA tetraploidy to the 16- and 36-week-old WKY rat groups: 1.8 per cent (range 1.5-2.3 per cent), 1.55 per cent (range 1.5-1.6 per cent), and 1.5 per cent (range 1.4-1.6 per cent), respectively. The differences in the percentage of tetraploid cardiomyocytes between the SHR untreated groups and the SHR treated group were statistically significant (P < 0.05). Regression of LVH and normalization of blood pressure were observed in treated rats. These results indicate that DNA tetraploidy in the myocardium of SHR increases with hypertrophy and decreases on quinapril treatment. It is suggested that ACE inhibition modifies nuclear processes involved in myocyte growth in arterial hypertension.

Molecular remodelling in hypertrophied hearts from polyomavirus large T-antigen transgenic mice

Polyomavirus large T-antigen transgenic mice develop cardiac hypertrophy characterized by an increase in atrial natriuretic factor and beta-myosin heavy chain isoform expression. The aim of this study was to examine changes in proto-oncogene expression in hypertrophied hearts from the transgenic mice. Expression of early growth response-1 (Egr-1) mRNA was detected in hearts from all 15 transgenic mice, but was not detectable in 13 control mice. Reverse transcriptase-polymerase chain reaction experiments using Egr-1-specific primers confirmed the increase in Egr-1 mRNA in enlarged hearts from the transgenic mice. Expression of c-jun, junD and Ha-ras mRNAs was increased in the transgenic hearts 3, 17 and 2.8-fold respectively. Western blots showed an increase in c-myc, c-jun and ras protein in hypertrophied transgenic hearts. Immunofluorescence analyses confirmed an increase in Egr-1 and c-jun protein in transgenic cardiomyocytes. Proliferating cell nuclear antigen, Ki-ras and HSP 90 mRNAs were decreased 22, 2.7 and 3-fold, respectively in the transgenic hearts. Not altered in most hypertrophied hearts was expression of c-fos, junB, p53, c-neu, c-myc, HSP70, HSP27, TGF-beta or IGF 1 mRNAs. Proto-oncogene and growth factor gene expression in hypertrophy induced by PVLT expression is modulated with some proto-oncogenes increased and others decreased in expression.

Disappearance of cyclin A correlates with permanent withdrawal of cardiomyocytes from the cell cycle in human and rat hearts

The regulated expression of cyclins controls the cell cycle. Because cardiomyocytes in adult mammals withdraw permanently from the cell cycle and thus cannot regenerate after injury, we examined cyclin expression during development by comparing cyclin A-E mRNA levels in fetal and adult human hearts. Cyclin B mRNA was detectable in adult hearts, although at a level markedly lower than that in fetal hearts. Levels of cyclin C, D1, D2, D3, and E mRNA were essentially identical in the two groups. In contrast, cyclin A mRNA was undetectable in adult hearts whereas cyclin A mRNA and protein were readily detectable in fetal hearts and cardiomyocytes, respectively. We then measured cyclin A mRNA and protein levels in rat hearts at four stages of development (fetal and 2, 14, and 28 d). Cyclin A mRNA and protein levels decreased quickly after birth (to 37% at day 2) and became undetectable within 14 d, an observation consistent with reports that cardiomyocytes stop replicating in rats by the second to third postnatal week. This disappearance of cyclin A gene expression in human and rat hearts at the time cardiomyocytes become terminally differentiated suggests that cyclin A downregulation is important in the permanent withdrawal of cardiomyocytes from the cell cycle.

Upregulation of IGF1, IGF1-receptor, and late growth related genes in ventricular myocytes acutely after infarction in rats

To determine the effects of acute myocardial infarction on the expression of insulin-like growth factor1 (IGF1) and insulin-like growth factor1 receptors (IGF-1R) on the surviving myocytes of the left and right ventricles, large infarcts were produced in rats and the animals sacrificed 2 days later. Hemodynamic measurements of left and right ventricular pressures, +dP/dt and -dP/dt, and central venous pressure documented that coronary occlusion was associated with a severe impairment of cardiac function. By employing reverse transcriptase polymerase chain reaction (RTPCR), a low level of expression of IGF-1R mRNA was detected in myocytes from sham-operated rats. Acute myocardial infarction was found to enhance by nearly twofold the message for IGF-1R in viable myocytes biventricularly. Moreover, IGF1 mRNA increased 4.3-fold and 9.4-fold in left and right myocytes, respectively. In order to establish whether the upregulation of IGF1 and IGF-1R with infarction was coupled with induction of late growth related genes, which are known to be implicated in DNA replication and mitotic division, proliferating cell nuclear antigen (PCNA) and histone-H3 expression was assessed by Northern blot and RTPCR. The level of expression of PCNA mRNA was found to be increased 3.9-fold and 2.4-fold in left and right myocytes, respectively from infarcted hearts. Corresponding increments in histone-H3 mRNA were 25.5-fold and 5.3-fold, respectively. However, PCNA protein as detected by immunoperoxidase staining was restricted to a limited number of myocyte nuclei adjacent to the necrotic myocardium of the left ventricle. In conclusion, acute myocardial infarction is associated with enhanced expression of IGF1 and IGF-1R on stressed myocytes, and this phenomenon may activate genes essential for DNA synthesis, possibly affecting myocyte growth. These processes may be fundamental for the reconstitution of tissue mass and amelioration of function after infarction.

Expression of proliferating cell markers in normal and diseased human hearts

Proliferating cell nuclear antigen (PCNA) myocyte expression and histopathologic features related to its occurrence were investigated in normal and diseased hearts of adult humans using both immunohistochemical and Western blotting techniques. Ki67 Western blotting was also performed in the same samples used for PCNA blotting. Two hundred seventy-one endomyocardial biopsies, and 15 adult, 1 embryonic and 2 fetal hearts were studied. The biopsies were from normal donor hearts (n = 71), patients with cardiomyopathy and myocarditis (n = 64), and patients with transplantation with (n = 106) and without (n = 30) acute rejection of any grade. The 15 hearts were from 1 heart donor, and from patients with cardiomyopathy (n = 5), valvular heart disease (n = 2), ischemic heart disease (n = 4), amyloidosis (n = 1) and transplantation with acute rejection (n = 2). The PCNA labeling index was plotted against myocyte hypertrophy, inflammatory infiltrates and binucleation index. The PCNA labeling index ranged from 2 to 9% in embryonic and fetal hearts. PCNA was expressed by 1 to 2% of myocyte nuclei in 12% of normal heart biopsies, 1 to 5% of myocyte nuclei in 28% of cardiomyopathy and myocarditis biopsies, and by up to 8% of myocyte nuclei in 53% of biopsies of patients with transplantation, independently of the presence and degree of acute rejection. In the latter biopsies and in myocarditis, some inflammatory cells also showed PCNA expression. PCNA positive myocytes were both mono- and binucleated, and there was no correlation between binucleation and PCNA labeling indexes. Ki67 and PCNA blotting confirmed immunohistochemical results.(ABSTRACT TRUNCATED AT 250 WORDS)

Myocyte DNA synthesis with aging: correlation with ventricular loading in rats

To determine whether the detrimental mechanical and anatomical changes that occur biventricularly with aging are associated with activation of DNA synthesis, flow cytometric analysis was performed on myocyte nuclei prepared from the left and right ventricles of rats at 4, 12, 20, and 29 months of age. Heart weight increased significantly with age, and this growth adaptation was associated with the development of left ventricular failure and right ventricular dysfunction. These phenomena were coupled with marked elevations in diastolic wall stress and increases in the percentage of myocyte nuclei in S+G2M in both ventricles. Linear regression analyses revealed a direct correlation between the fraction of myocytes that entered the cell cycle and diastolic pressure and wall stress. An inverse relation was found between the percentage of myocyte nuclei in S+G2M and +dP/dt and systolic wall stress. Thus the depression of hemodynamic performance coupled with alterations in the loading conditions contributes, at least in part, to increased DNA synthesis in cardiac myocytes with age.