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

Defining the molecular underpinnings controlling cardiomyocyte proliferation

Shortly after birth, mammalian cardiomyocytes (CM) exit the cell cycle and cease to proliferate. The inability of adult CM to replicate renders the heart particularly vulnerable to injury. Restoration of CM proliferation would be an attractive clinical target for regenerative therapies that can preserve contractile function and thus prevent the development of heart failure. Our review focuses on recent progress in understanding the tight regulation of signaling pathways and their downstream molecular mechanisms that underly the inability of CM to proliferate in vivo. In this review, we describe the temporal expression of cell cycle activators e.g., cyclin/Cdk complexes and their inhibitors including p16, p21, p27 and members of the retinoblastoma gene family during gestation and postnatal life. The differential impact of members of the E2f transcription factor family and microRNAs on the regulation of positive and negative cell cycle factors is discussed. This review also highlights seminal studies that identified the coordination of signaling mechanisms that can potently activate CM cell cycle re-entry including the Wnt/Ctnnb1, Hippo, Pi3K-Akt and Nrg1-Erbb2/4 pathways. We also present an up-to-date account of landmark studies analyzing the effect of various genes such as Argin, Dystrophin, Fstl1, Meis1, Pitx2 and Pkm2 that are responsible for either inhibition or activation of CM cell division. All these reports describe bona fide therapeutically targets that could guide future clinical studies toward cardiac repair.

Induced Cardiomyocyte Proliferation: A Promising Approach to Cure Heart Failure

Unlike some lower vertebrates which can completely regenerate their heart, the human heart is a terminally differentiated organ. Cardiomyocytes lost during cardiac injury and heart failure cannot be replaced due to their limited proliferative capacity. Therefore, cardiac injury generally leads to progressive failure. Here, we summarize the latest progress in research on methods to induce cardiomyocyte cell cycle entry and heart repair through the alteration of cardiomyocyte plasticity, which is emerging as an effective strategy to compensate for the loss of functional cardiomyocytes and improve the impaired heart functions.

Impact of PCSK9 on CTRP9-Induced Metabolic Effects in Adult Rat Cardiomyocytes

The adipocytokine adiponectin and its structural homologs, the C1q/TNF-related proteins (CTRPs), increase insulin sensitivity, fatty acid oxidation and mitochondrial biogenesis. Adiponectin- and CTRP-induced signal transduction has been described to involve the adiponectin receptors and a number of co-receptors including the Low density lipoprotein receptor-related protein 1 (LRP1). LRP1 is another target of the proprotein convertase subtilisin/kexin-9 (PCSK9) in addition to the LDL-receptor (LDL-R). Here, we investigated the influence of PCSK9 on the metabolic effects of CTRP9, the CTRP with the highest homology to adiponectin. Knockdown of LRP1 in H9C2 cardiomyoblasts blunts the effects of CTRP9 on signal transduction and mitochondrial biogenesis, suggesting its involvement in CTRP9-induced cellular effects. Treatment of adult rat cardiomyocytes with recombinant PCSK9 but not knockdown of endogenous PCSK9 by siRNA results in a strong reduction in LRP1 protein expression and subsequently reduces the mitochondrial biogenic effect of CTRP9. PCSK9 treatment (24 h) blunts the effects of CTRP9-induced signaling cascade activation (AMP-dependent protein kinase, protein kinase B). In addition, the stimulating effects of CTRP9 on cardiomyocyte mitochondrial biogenesis and glucose metabolism (GLUT-4 translocation, glucose uptake) are largely blunted. Basal fatty acid (FA) uptake is strongly reduced by exogenous PCSK9, although protein expression of the PCSK9 target CD36, the key regulator of FA transport in cardiomyocytes, is not altered. In addition, only minor effects of PCSK9 were observed on CTRP9-induced FA uptake or the expression of genes involved in FA metabolism or uptake. Finally, this CTRP9-induced increase in CD36 expression occurs independent from LRP1 and LDL-R. In conclusion, PCSK9 treatment influences LRP1-mediated signaling pathways in cardiomyocytes. Thus, therapeutic PCSK9 inhibition may provide an additional benefit through stimulation of glucose metabolism and mitochondrial biogenesis in addition to the known lipid-lowering effects. This could be an important beneficial side effect in situations with impaired mitochondrial function and reduced metabolic flexibility thereby influencing cardiac function.

Effect of dietary NiCl2 on the cell cycle of cecal tonsil in the chicken broiler

Although the effects of nickel chloride (NiCl2) on the immune system have long been recognized, little is known about the effects of nickel (II) on the cell cycle and related signaling events in immune organs, such as cecal tonsil, a key immune organ of chicken. In the present study, we investigated the effect of NiCl2 on the cell cycle of cecal tonsil. The cell cycle was detected by the methods of flow cytometry (FCM), quantitative real-time PCR (qRT-PCR) and immunohistochemistry (IHC). The results showed that dietary NiCl2 in excess of 300 mg/kg caused the G2/M cell cycle arrest and the reduction of cell proportion at S phase of the cecal tonsil. The G2/M cell cycle arrest was accompanied by the up-regulation of p53, p21 protein expression and mRNA expression, and down-regulation of cyclinB and proliferating cell nuclear antigen (PCNA) protein expression and mRNA expression. The data suggested that the cells' (mainly the T lymphocytes) proliferation in the cecal tonsil was inhibited by the high dietary NiCl2.

Live cell screening platform identifies PPARδ as a regulator of cardiomyocyte proliferation and cardiac repair

Zebrafish can efficiently regenerate their heart through cardiomyocyte proliferation. In contrast, mammalian cardiomyocytes stop proliferating shortly after birth, limiting the regenerative capacity of the postnatal mammalian heart. Therefore, if the endogenous potential of postnatal cardiomyocyte proliferation could be enhanced, it could offer a promising future therapy for heart failure patients. Here, we set out to systematically identify small molecules triggering postnatal cardiomyocyte proliferation. By screening chemical compound libraries utilizing a Fucci-based system for assessing cell cycle stages, we identified carbacyclin as an inducer of postnatal cardiomyocyte proliferation. In vitro, carbacyclin induced proliferation of neonatal and adult mononuclear rat cardiomyocytes via a peroxisome proliferator-activated receptor δ (PPARδ)/PDK1/p308Akt/GSK3β/β-catenin pathway. Inhibition of PPARδ reduced cardiomyocyte proliferation during zebrafish heart regeneration. Notably, inducible cardiomyocyte-specific overexpression of constitutively active PPARδ as well as treatment with PPARδ agonist after myocardial infarction in mice induced cell cycle progression in cardiomyocytes, reduced scarring, and improved cardiac function. Collectively, we established a cardiomyocyte proliferation screening system and present a new drugable target with promise for the treatment of cardiac pathologies caused by cardiomyocyte loss.

Understanding cardiomyocyte proliferation: an insight into cell cycle activity

Cardiomyocyte proliferation and regeneration are key to the functional recovery of myocardial tissue from injury. In the recent years, studies on cardiomyocyte proliferation overturned the traditional belief that adult cardiomyocytes permanently withdraw from the cell cycle activity. Hence, targeting cardiomyocyte proliferation is one of the potential therapeutic strategies for myocardial regeneration and repair. To achieve this, a deep understanding of the fundamental mechanisms involved in cardiomyocyte cell cycle as well as differences between neonatal and adult cardiomyocytes' cell cycle activity is required. This review focuses on the recent progress in understanding of cardiomyocyte cell cycle activity at different life stages viz., gestation, birth, and adulthood. The temporal expression/activities of major cell cycle activators (cyclins and CDKs), inhibitors (p21, p27, p57, p16, and p18), and cell-cycle-associated proteins (Rb, p107, and p130) in cardiomyocytes during gestation and postnatal life are described in this review. The influence of different transcription factors and microRNAs on the expression of cell cycle proteins is demonstrated. This review also deals major pathways (PI3K/AKT, Wnt/β-catenin, and Hippo-YAP) associated with cardiomyocyte cell cycle progression. Furthermore, the postnatal alterations in structure and cellular events responsible for the loss of cell cycle activity are also illustrated.

Antitumor effect of forbesione isolated from Garcinia hanburyi on cholangiocarcinoma in vitro and in vivo

Cholangiocarcinoma (CCA) is a malignancy with no effective therapy and poor prognosis. Forbesione, a caged xanthone isolated from Garcinia hanburyi, has been reported to inhibit proliferation and to induce apoptosis in human CCA cell lines. The present study aimed to further explore the potential anticancer properties of forbesione by testing its effects against the hamster CCA cell line Ham-1 in vitro and in vivo. It was observed that forbesione inhibited the growth of Ham-1 cells in vitro and suppressed Ham-1 growth as allograft in hamsters by inducing cell cycle arrest at the S phase. This was mediated by decreasing the protein expression of cyclin E, cyclin A and cyclin-dependent kinase 2. In addition, increased expression of p21 and p27 was detected, which could possibly explain the reduced expression of proliferating cell nuclear antigen and of the bile duct cell marker cytokeratin 19 observed in forbesione-treated Ham-1 cells in vitro and in tumor tissues of forbesione-treated hamsters. Furthermore, forbesione induced apoptosis through multiple pathways. The death receptor pathway was activated by increased expression of Fas, Fas-associated death domain and activated caspase-3, along with decreased expression of procaspase-8 and procaspase-3. The mitochondrial pathway was driven by increased expression of B-cell lymphoma (Bcl)-2-like protein 4, activated caspase-9 and inhibitor of κB-α, along with decreased expression of Bcl-2, survivin, procaspase-9 and nuclear factor-κB/p65. The endoplasmic reticulum pathway was stimulated by increased expression of activated caspase-12 and decreased expression of procaspase-12. No side effects or toxicity were observed in forbesione-treated hamsters. Thus, forbesione is a potential drug candidate for cancer therapy that deserves further investigation.

GSK3ß-dependent dysregulation of neurodevelopment in SPG11-patient induced pluripotent stem cell model

Objective

Mutations in the spastic paraplegia gene 11 (SPG11), encoding spatacsin, cause the most frequent form of autosomal‐recessive complex hereditary spastic paraplegia (HSP) and juvenile‐onset amyotrophic lateral sclerosis (ALS5). When SPG11 is mutated, patients frequently present with spastic paraparesis, a thin corpus callosum, and cognitive impairment. We previously delineated a neurodegenerative phenotype in neurons of these patients. In the current study, we recapitulated early developmental phenotypes of SPG11 and outlined their cellular and molecular mechanisms in patient‐specific induced pluripotent stem cell (iPSC)‐derived cortical neural progenitor cells (NPCs).

Methods

We generated and characterized iPSC‐derived NPCs and neurons from 3 SPG11 patients and 2 age‐matched controls.

Results

Gene expression profiling of SPG11‐NPCs revealed widespread transcriptional alterations in neurodevelopmental pathways. These include changes in cell‐cycle, neurogenesis, cortical development pathways, in addition to autophagic deficits. More important, the GSK3ß‐signaling pathway was found to be dysregulated in SPG11‐NPCs. Impaired proliferation of SPG11‐NPCs resulted in a significant diminution in the number of neural cells. The decrease in mitotically active SPG11‐NPCs was rescued by GSK3 modulation.

Interpretation

This iPSC‐derived NPC model provides the first evidence for an early neurodevelopmental phenotype in SPG11, with GSK3ß as a potential novel target to reverse the disease phenotype. Ann Neurol 2016;79:826–840

Neuregulin stimulation of cardiomyocyte regeneration in mice and human myocardium reveals a therapeutic window

Therapies developed for adult patients with heart failure have been shown to be ineffective in pediatric clinical trials, leading to the recognition that new pediatric-specific therapies for heart failure must be developed. Administration of the recombinant growth factor neuregulin-1 (rNRG1) stimulates regeneration of heart muscle cells (cardiomyocytes) in adult mice. Because proliferation-competent cardiomyocytes are more abundant in growing mammals, we hypothesized that administration of rNRG1 during the neonatal period might be more effective than in adulthood. If so, neonatal rNRG1 delivery could be a new therapeutic strategy for treating heart failure in pediatric patients. To evaluate the effectiveness of rNRG1 administration in cardiac regeneration, newborn mice were subjected to cryoinjury, which induced myocardial dysfunction and scar formation and decreased cardiomyocyte cell cycle activity. Early administration of rNRG1 to mice from birth to 34 days of age improved myocardial function and reduced the prevalence of transmural scars. In contrast, administration of rNRG1 from 4 to 34 days of age only transiently improved myocardial function. The mechanisms of early administration involved cardiomyocyte protection (38%) and proliferation (62%). We also assessed the ability of rNRG1 to stimulate cardiomyocyte proliferation in intact cultured myocardium from pediatric patients. rNRG1 induced cardiomyocyte proliferation in myocardium from infants with heart disease who were less than 6 months of age. Our results identify an effective time period within which to execute rNRG1 clinical trials in pediatric patients for the stimulation of cardiomyocyte regeneration.

Deviating the level of proliferating cell nuclear antigen in Trypanosoma brucei elicits distinct mechanisms for inhibiting proliferation and cell cycle progression

The DNA replication machinery is spatially and temporally coordinated in all cells to reproduce a single exact copy of the genome per division, but its regulation in the protozoan parasite Trypanosoma brucei is not well characterized. We characterized the effects of altering the levels of proliferating cell nuclear antigen, a key component of the DNA replication machinery, in bloodstream form T. brucei. This study demonstrated that tight regulation of TbPCNA levels was critical for normal proliferation and DNA replication in the parasite. Depleting TbPCNA mRNA reduced proliferation, severely diminished DNA replication, arrested the synthesis of new DNA and caused the parasites to accumulated in G2/M. Attenuating the parasite by downregulating TbPCNA caused it to become hypersensitive to hydroxyurea. Overexpressing TbPCNA in T. brucei arrested proliferation, inhibited DNA replication and prevented the parasite from exiting G2/M. These results indicate that distinct mechanisms of cell cycle arrest are associated with upregulating or downregulating TbPCNA. The findings of this study validate deregulating intra-parasite levels of TbPCNA as a potential strategy for therapeutically exploiting this target in bloodstream form T. brucei.

FGF1-mediated cardiomyocyte cell cycle reentry depends on the interaction of FGFR-1 and Fn14

Fibroblast growth factors (FGFs) signal through FGF receptors (FGFRs) mediating a broad range of cellular functions during embryonic development, as well as disease and regeneration during adulthood. Thus, it is important to understand the underlying molecular mechanisms that modulate this system. Here, we show that FGFR-1 can interact with the TNF receptor superfamily member fibroblast growth factor-inducible molecule 14 (Fn14) resulting in cardiomyocyte cell cycle reentry. FGF1-induced cell cycle reentry in neonatal cardiomyocytes could be blocked by Fn14 inhibition, while TWEAK-induced cell cycle activation was inhibited by blocking FGFR-1 signaling. In addition, costimulation experiments revealed a synergistic effect of FGF1 and TWEAK in regard to cardiomyocyte cell cycle induction via PI3K/Akt signaling. Overexpression of Fn14 with either FGFR-1 long [FGFR-1(L)] or FGFR-1 short [FGFR-1(S)] isoforms resulted after FGF1/TWEAK stimulation in cell cycle reentry of >40% adult cardiomyocytes. Finally, coimmunoprecipitation and proximity ligation assays indicated that endogenous FGFR-1 and Fn14 interact with each other in cardiomyocytes. This interaction was strongly enhanced in the presence of their corresponding ligands, FGF1 and TWEAK. Taken together, our data suggest that FGFR-1/Fn14 interaction may represent a novel endogenous mechanism to modulate the action of these receptors and their ligands and to control cardiomyocyte cell cycle reentry.

Macrophage migration inhibitory factor antagonizes pressure overload-induced cardiac hypertrophy

Macrophage migration inhibitory factor (MIF) functions as a proinflammatory cytokine when secreted from the cell, but it also exhibits antioxidant properties by virtue of its intrinsic oxidoreductase activity. Since increased production of ROS is implicated in the development of left ventricular hypertrophy, we hypothesized that the redox activity of MIF protects the myocardium when exposed to hemodynamic stress. In a mouse model of myocardial hypertrophy induced by transverse aortic coarctation (TAC) for 10 days, we showed that growth of the MIF-deficient heart was significantly greater by 32% compared with wild-type (WT) TAC hearts and that fibrosis was increased by fourfold (2.62 ± 0.2% vs. 0.6 ± 0.1%). Circulating MIF was increased in TAC animals, and expression of MIF receptor, CD74, was increased in the hypertrophic myocardium. Gene expression analysis showed a 10-fold increase (P < 0.01) in ROS-generating mitochondrial NADPH oxidase and 2- to 3-fold reductions (P < 0.01) in mitochondrial SOD2 and mitochondrial aconitase activities, indicating enhanced oxidative injury in the hypertrophied MIF-deficient ventricle. Hypertrophic signaling pathways showed that phosphorylation of cytosolic glycogen synthase kinase-3α was greater (P < 0.05) at baseline in MIF-deficient hearts than in WT hearts and remained elevated after 10-day TAC. In the hemodynamically stressed MIF-deficient heart, nuclear p21(CIP1) increased sevenfold (P < 0.01), and the cytosolic increase of phospho-p21(CIP1) was significantly greater than in WT TAC hearts. We conclude that MIF antagonizes myocardial hypertrophy and fibrosis in response to hemodynamic stress by maintaining a redox homeostatic phenotype and attenuating stress-induced activation of hypertrophic signaling pathways.

REGγ is a strong candidate for the regulation of cell cycle, proliferation and the invasion by poorly differentiated thyroid carcinoma cells

REGγ is a proteasome activator that facilitates the degradation of small peptides. Abnormally high expression of REGγ has been observed in thyroid carcinomas. The purpose of the present study was to explore the role of REGγ in poorly differentiated thyroid carcinoma (PDTC). For this purpose, small interfering RNA (siRNA) was introduced to down-regulate the level of REGγ in the PDTC cell line SW579. Down-regulation of REGγ at the mRNA and protein levels was confirmed by RT-PCR and Western blot analyses. FACS analysis revealed cell cycle arrest at the G₁/S transition, the MTT assay showed inhibition of cell proliferation, and the Transwell assay showed restricted cell invasion. Furthermore, the expression of the p21 protein was increased, the expression of proliferating cell nuclear antigen (PCNA) protein decreased, and the expression of the p27 protein was unchanged as shown by Western blot analyses. REGγ plays a critical role in the cell cycle, proliferation and invasion of SW579 cells. The alteration of p21 and PCNA proteins related to the down-regulation of REGγ suggests that p21 and PCNA participate in the process of REGγ regulation of cell cycle progression and cell proliferation. Thus, targeting REGγ has a therapeutic potential in the management of PDTC patients.

Degradation of p21Cip1 through anaphase-promoting complex/cyclosome and its activator Cdc20 (APC/CCdc20) ubiquitin ligase complex-mediated ubiquitylation is inhibited by cyclin-dependent kinase 2 in cardiomyocytes

Cyclin-dependent kinase inhibitor p21Cip1 plays a crucial role in regulating cell cycle arrest and differentiation. It is known that p21Cip1 increases during terminal differentiation of cardiomyocytes, but its expression control and biological roles are not fully understood. Here, we show that the p21Cip1 protein is stabilized in cardiomyocytes after mitogenic stimulation, due to its increased CDK2 binding and inhibition of ubiquitylation. The APC/CCdc20 complex is shown to be an E3 ligase mediating ubiquitylation of p21Cip1 at the N terminus. CDK2, but not CDC2, suppressed the interaction of p21Cip1 with Cdc20, thereby leading to inhibition of anaphase-promoting complex/cyclosome and its activator Cdc20 (APC/CCdc20)-mediated p21Cip1 ubiquitylation. It was further demonstrated that p21Cip1 accumulation caused G2 arrest of cardiomyocytes that were forced to re-enter the cell cycle. Taken together, these data show that the stability of the p21Cip1 protein is actively regulated in terminally differentiated cardiomyocytes and plays a role in inhibiting their uncontrolled cell cycle progression. Our study provides a novel insight on the control of p21Cip1 by ubiquitin-mediated degradation and its implication in cell cycle arrest in terminal differentiation.

Proliferating cell nuclear antigen (PCNA): a key factor in DNA replication and cell cycle regulation

BACKGROUND

PCNA (proliferating cell nuclear antigen) has been found in the nuclei of yeast, plant and animal cells that undergo cell division, suggesting a function in cell cycle regulation and/or DNA replication. It subsequently became clear that PCNA also played a role in other processes involving the cell genome.

SCOPE

This review discusses eukaryotic PCNA, with an emphasis on plant PCNA, in terms of the protein structure and its biochemical properties as well as gene structure, organization, expression and function. PCNA exerts a tripartite function by operating as (1) a sliding clamp during DNA synthesis, (2) a polymerase switch factor and (3) a recruitment factor. Most of its functions are mediated by its interactions with various proteins involved in DNA synthesis, repair and recombination as well as in regulation of the cell cycle and chromatid cohesion. Moreover, post-translational modifications of PCNA play a key role in regulation of its functions. Finally, a phylogenetic comparison of PCNA genes suggests that the multi-functionality observed in most species is a product of evolution.

CONCLUSIONS

Most plant PCNAs exhibit features similar to those found for PCNAs of other eukaryotes. Similarities include: (1) a trimeric ring structure of the PCNA sliding clamp, (2) the involvement of PCNA in DNA replication and repair, (3) the ability to stimulate the activity of DNA polymerase δ and (4) the ability to interact with p21, a regulator of the cell cycle. However, many plant genomes seem to contain the second, probably functional, copy of the PCNA gene, in contrast to PCNA pseudogenes that are found in mammalian genomes.

Proliferating cell nuclear antigen (PCNA): a key factor in DNA replication and cell cycle regulation

BACKGROUND

PCNA (proliferating cell nuclear antigen) has been found in the nuclei of yeast, plant and animal cells that undergo cell division, suggesting a function in cell cycle regulation and/or DNA replication. It subsequently became clear that PCNA also played a role in other processes involving the cell genome.

SCOPE

This review discusses eukaryotic PCNA, with an emphasis on plant PCNA, in terms of the protein structure and its biochemical properties as well as gene structure, organization, expression and function. PCNA exerts a tripartite function by operating as (1) a sliding clamp during DNA synthesis, (2) a polymerase switch factor and (3) a recruitment factor. Most of its functions are mediated by its interactions with various proteins involved in DNA synthesis, repair and recombination as well as in regulation of the cell cycle and chromatid cohesion. Moreover, post-translational modifications of PCNA play a key role in regulation of its functions. Finally, a phylogenetic comparison of PCNA genes suggests that the multi-functionality observed in most species is a product of evolution.

CONCLUSIONS

Most plant PCNAs exhibit features similar to those found for PCNAs of other eukaryotes. Similarities include: (1) a trimeric ring structure of the PCNA sliding clamp, (2) the involvement of PCNA in DNA replication and repair, (3) the ability to stimulate the activity of DNA polymerase δ and (4) the ability to interact with p21, a regulator of the cell cycle. However, many plant genomes seem to contain the second, probably functional, copy of the PCNA gene, in contrast to PCNA pseudogenes that are found in mammalian genomes.

Knockdown of cyclin-dependent kinase inhibitors induces cardiomyocyte re-entry in the cell cycle

Proliferation of mammalian cardiomyocytes stops rapidly after birth and injured hearts do not regenerate adequately. High cyclin-dependent kinase inhibitor (CKI) levels have been observed in cardiomyocytes, but their role in maintaining cardiomyocytes in a post-mitotic state is still unknown. In this report, it was investigated whether CKI knockdown by RNA interference induced cardiomyocyte proliferation. We found that triple transfection with p21(Waf1), p27(Kip1), and p57(Kip2) siRNAs induced both neonatal and adult cardiomyocyte to enter S phase and increased the nuclei/cardiomyocyte ratio; furthermore, a subpopulation of cardiomyocytes progressed beyond karyokynesis, as assessed by the detection of mid-body structures and by straight cardiomyocyte counting. Intriguingly, cardiomyocyte proliferation occurred in the absence of overt DNA damage and aberrant mitotic figures. Finally, CKI knockdown and DNA synthesis reactivation correlated with a dramatic change in adult cardiomyocyte morphology that may be a prerequisite for cell division. In conclusion, CKI expression plays an active role in maintaining cardiomyocyte withdrawal from the cell cycle.

Cardiac deletion of Smyd2 is dispensable for mouse heart development

Chromatin modifying enzymes play a critical role in cardiac differentiation. Previously, it has been shown that the targeted deletion of the histone methyltransferase, Smyd1, the founding member of the SET and MYND domain containing (Smyd) family, interferes with cardiomyocyte maturation and proper formation of the right heart ventricle. The highly related paralogue, Smyd2 is a histone 3 lysine 4- and lysine 36-specific methyltransferase expressed in heart and brain. Here, we report that Smyd2 is differentially expressed during cardiac development with highest expression in the neonatal heart. To elucidate the functional role of Smyd2 in the heart, we generated conditional knockout (cKO) mice harboring a cardiomyocyte-specific deletion of Smyd2 and performed histological, functional and molecular analyses. Unexpectedly, cardiac deletion of Smyd2 was dispensable for proper morphological and functional development of the murine heart and had no effect on global histone 3 lysine 4 or 36 methylation. However, we provide evidence for a potential role of Smyd2 in the transcriptional regulation of genes associated with translation and reveal that Smyd2, similar to Smyd3, interacts with RNA Polymerase II as well as to the RNA helicase, HELZ.

TWEAK is a positive regulator of cardiomyocyte proliferation

AIMS

Proliferation of mammalian cardiomyocytes stops during the first weeks after birth, preventing the heart from regenerating after injury. Recently, several studies have indicated that induction of cardiomyocyte proliferation can be utilized to regenerate the mammalian heart. Thus, it is important to identify novel factors that can induce proliferation of cardiomyocytes. Here, we determine the effect of TNF-related weak inducer of apoptosis (TWEAK) on cardiomyocytes, a cytokine known to regulate proliferation in several other cell types.

METHODS AND RESULTS

Stimulation of neonatal rat cardiomyocytes with TWEAK resulted in increased DNA synthesis, increased expression of the proliferative markers Cyclin D2 and Ki67, and downregulation of the cell cycle inhibitor p27KIP1. Importantly, TWEAK stimulation resulted also in mitosis (H3P), cytokinesis (Aurora B), and increased cardiomyocyte numbers. Loss of function experiments revealed that re-induction of proliferation was dependent on tumour necrosis factor receptor superfamily member 12A (FN14) signalling. Downstream signalling was mediated through activation of extracellular signal-regulated kinases and phosphatidylinositol 3-kinase as well as inhibition of glycogen synthase kinase-3beta. In contrast to neonatal cardiomyocytes, TWEAK had no effect on adult rat cardiomyocytes due to developmental downregulation of its receptor FN14. However, adenoviral expression of FN14 enabled efficient induction of cell cycle re-entry in adult cardiomyocytes after TWEAK stimulation.

CONCLUSION

Our data establish TWEAK as a positive regulator of cardiomyocyte proliferation.

Proliferating cell nuclear antigen is protected from degradation by forming a complex with MutT Homolog2

Proliferating cell nuclear antigen (PCNA) has been demonstrated to interact with multiple proteins involved in several metabolic pathways such as DNA replication and repair. However, there have been fewer reports about whether these PCNA-binding proteins influence stability of PCNA. Here, we observed a physical interaction between PCNA and MutT homolog2 (MTH2), a new member of the MutT-related proteins that hydrolyzes 8-oxo-7,8-dihydrodeoxyguanosine triphosphate (8-oxo-dGTP). In several unstressed human cancer cell lines and in normal human fibroblast cells, PCNA and MTH2 formed a complex and their mutual binding fragments were confirmed. It was intriguing that PCNA and MTH2 were dissociated dependent on acetylation of PCNA, which in turn induced degradation of PCNA in response to UV irradiation, but not in response to other forms of DNA-damaging stress. To further explore the link between dissociation of PCNA-MTH2 and degradation of PCNA, RNAi against MTH2 was performed to mimic the dissociated status of PCNA to evaluate changes in the half-life of PCNA. Knockdown of MTH2 significantly promoted degradation of PCNA, suggesting that the physiological interaction of PCNA-MTH2 may confer protection from degradation for PCNA, whereas UV irradiation accelerates PCNA degradation by inducing dissociation of PCNA-MTH2. Moreover, secondary to degradation of PCNA, UV-induced inhibition of DNA synthesis or cell cycle progression was enhanced. Collectively, our data demonstrate for the first time that PCNA is protected by this newly identified partner molecule MTH2, which is related to DNA synthesis and cell cycle progression.

E2F2 expression induces proliferation of terminally differentiated cardiomyocytes in vivo

AIMS

In previous experiments we have demonstrated that expression of the transcription factors E2F2 and E2F4 is sufficient to induce proliferation of isolated primary cardiomyocytes from newborn rats and mice. We now wanted to analyse whether E2F2 or E2F4 are also able to promote cell cycle progression of adult cardiomyocytes in vivo, which unlike cardiomyocytes from newborn rodents lack the ability to undergo cell proliferation.

METHODS AND RESULTS

E2F2 or E2F4 was expressed in hearts of mice at different developmental stages using adenoviral vectors. Effects regarding proliferation, hypertrophy, and apoptosis were analysed on histological sections, and quantitative assessment of cell cycle regulatory genes was performed by real-time PCR (polymerase chain reaction) and western blot. We found that both E2F2 and E2F4 can stimulate hypertrophic cell growth of cardiomyocytes. However, only directed expression of E2F2 but not of E2F4 was sufficient to induce proliferation of cardiomyocytes. Expression of E2F2 in vivo did not increase the percentage of apoptotic cardiomyocytes but down-regulated the expression of the pro-apoptotic genes caspase-6 and apaf-1. Further analysis of the cell cycle regulatory machinery revealed that expression of E2F2 caused a strong induction of cyclin A and E while the expression of cyclin-dependent kinase inhibitors (CKIs) such as p21 was not affected.

CONCLUSION

We conclude that a limited induction of cardiomyocyte cell proliferation can be achieved by E2F2-mediated stimulation of cyclin A and E expression without a reduction of CKIs.

hSMG-1 and ATM sequentially and independently regulate the G1 checkpoint during oxidative stress

Genotoxic stress activates the phosphatidylinositol 3-kinase-like kinases (PIKKs) that phosphorylate proteins involved in cell cycle arrest, DNA repair and apoptosis. Previous work showed that the PIKK ataxia telangiectasia mutated (ATM) but not ATM and Rad3 related phosphorylates p53 (Ser15) during hyperoxia, a model of prolonged oxidative stress and DNA damage. Here, we show hSMG-1 is responsible for the rapid and early phosphorylation of p53 (Ser15) and that ATM helps maintain phosphorylation after 24 h. Despite reduced p53 phosphorylation and abundance in cells depleted of hSMG-1 or ATM, levels of the p53 target p21 were still elevated and the G(1) checkpoint remained intact. Conditional overexpression of p21 in p53-deficient cells revealed that hyperoxia also stimulates wortmannin-sensitive degradation of p21. siRNA depletion of hSMG-1 or ATM restored p21 stability and the G(1) checkpoint during hyperoxia. These findings establish hSMG-1 as a proximal regulator of DNA damage signaling and reveal that the G(1) checkpoint is tightly regulated during prolonged oxidative stress by both PIKK-dependent synthesis and proteolysis of p21.

TCAP knockdown by RNA interference inhibits myoblast differentiation in cultured skeletal muscle cells

Null mutation of titin-cap (TCAP) causes limb-girdle muscular dystrophy type 2G (LGMD2G). LGMD2G patients develop muscle atrophy, and lose the ability to walk by their third decade. Previous findings suggest that TCAP regulates myostatin, a key regulator of muscle growth. We tested the hypothesis that TCAP knockdown with RNA interference will lead to differential expression of genes involved in muscle proliferation and differentiation, impairing muscle cell growth. mRNA from cultured cells treated with TCAP siRNA duplex constructs was analyzed using Northern blots and real-time RT-PCR. siRNA treatment decreased TCAP mRNA expression in differentiating muscle cells. Significant (p<0.05) decreases in mRNA were observed for myogenic regulatory factors. siRNA treatment also prevented development of the normal phenotype of muscle cells. Our findings suggest that TCAP knockdown with RNA interference alters normal muscle cell differentiation.

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.

Suppression of cardiomyocyte hypertrophy by inhibition of the ubiquitin-proteasome system

Inhibitors of the proteasome interfere with transcriptional regulation of growth signaling pathways and block cell cycle progression of mitotic cells. As growth signaling pathways are highly conserved between mitotic and postmitotic cells, we hypothesized that proteasome inhibition might also be a valuable approach to interfere with hypertrophic growth of postmitotic cardiomyocytes. To test this hypothesis, we analyzed the effects of proteasome inhibition on hypertrophic growth of neonatal rat cardiomyocytes. Partial inhibition of the proteasome effectively suppressed cardiomyocyte hypertrophy as determined by reduced cell size, inhibition of hypertrophy-mediated induction of RNA and protein synthesis, reduced expression of several hypertrophic marker genes, and diminished transcriptional activation of the BNP promotor. Importantly, suppression of hypertrophic growth was independent of the hypertrophic agonist used. Expressional profiling and subsequent Western blot and kinase assays revealed that proteasome inhibition induced a cellular stress response with reduced expression of conserved growth signaling mediators and impaired G1/S phase transition of cardiomyocytes. In hypertensive Dahl-salt sensitive rats, inhibition of the proteasome with low doses of the FDA approved proteasome inhibitor Velcade significantly reduced hypertrophic heart growth. Our data provide important insight into the suppressive effects of proteasome inhibitors on hypertrophic growth of cardiomyocytes and establish low-dose proteasome inhibition as a new and broad-spectrum approach to interfere with cardiac hypertrophy.

Lung specific expression of a human mutant p53 affects cell proliferation in transgenic mice

The human mutant p53(273H) has been shown in vitro to have both dominant-negative and gain-of-function properties, as well as to retain partial DNA-binding and transcriptional activation functions. We have developed a line of transgenic mice in which the human mutant p53(273H) is expressed in a lung specific manner (p53 (+/+/TG)). Crossing of the transgenic mice with p53 knockout mice led to generate mice with various genetic backgrounds. To evaluate the influence of p53 mutants in cell proliferation in mice lung tissue, we analyzed cell proliferation rate by Bromodeoxyuridine (BrdU) labeling and by expression of proliferating cell nuclear antigen (PCNA). BrdU analysis showed a 3.7-fold increase in the number of BrdU positive cells in the (p53 (-/+/TG)) mice compared to the (p53 (-/+)) mice, whereas no difference was observed in proliferation rate in the p53 (-/-/TG) lungs as compared to p53 (-/-) lungs. After the mice were treated with gamma-irradiation, BrdU positive cells were absent from both the p53 (-/+/TG) and p53 (-/+) mice, whereas a decrease in the rate of cell proliferation occurred in p53 (-/-/TG) lungs as compared to p53 (-/-) lungs. Real time PCR results indicated that the p53(273H) mutant did not retain the function to activate expression of p21 (WAF1/CIP1) in the transgenic mice. The above results indicate that overexpression of the human mutant p53(273H) in vivo results in an increase in basal proliferation rate which requires the presence of wild type p53. Mutant p53(273H) may affect cell proliferation by interrupting murine endogenous p53 function.

Nox2 regulates endothelial cell cycle arrest and apoptosis via p21cip1 and p53

Endothelial cells (EC) express constitutively two major isoforms (Nox2 and Nox4) of the catalytic subunit of NADPH oxidase, which is a major source of endothelial reactive oxygen species. However, the individual roles of these Noxes in endothelial function remain unclear. We have investigated the role of Nox2 in nutrient deprivation-induced cell cycle arrest and apoptosis. In proliferating human dermal microvascular EC, Nox2 mRNA expression was low relative to Nox4 (Nox2:Nox4 approximately 1:13), but was upregulated 24 h after starvation and increased to 8+/-3.5-fold at 36 h of starvation. Accompanying the upregulation of Nox2, there was a 2.28+/-0.18-fold increase in O2.- production, a dramatic induction of p21cip1 and p53, cell cycle arrest, and the onset of apoptosis (all p<0.05). All these changes were inhibited significantly by in vitro deletion of Nox2 expression and in coronary microvascular EC isolated from Nox2 knockout mice. In Nox2 knockout cells, although there was a 3.8+/-0.5-fold increase in Nox4 mRNA expression after 36 h of starvation (p<0.01), neither O2.- production nor the p21cip1 or p53 expression was increased significantly and only 0.46% of cells were apoptotic. In conclusion, Nox2-derived O2.-, through the modulation of p21cip1 and p53 expression, participates in endothelial cell cycle regulation and apoptosis.

Identification of Proteomic Changes during Differentiation of Adult Mouse Subventricular Zone Progenitor Cells

The use of neural precursor cells (NPCs) represents a promising repair strategy for many neurological disorders. However, the molecular events and biological features that control NPC proliferation and their differentiation into neurons, astrocytes, and oligodendrocytes are unclear. In the present study, we used a comparative proteomics approach to identify proteins that were differentially regulated in NPCs after short-term differentiation. We also used a subcellular fractionation technique for enrichment of nuclei and other dense organelles to identify proteins that were not readily detected in whole cell extracts. In total, 115 distinct proteins underwent expression changes during NPC differentiation. Forty one of these were only identified following subcellular fractionation. These included transcription factors, RNA-processing factors, cell cycle proteins, and proteins that translocate between the nucleus and cytoplasm. Biological network analysis showed that the differentiation of NPCs was associated with significant changes in cell cycle and protein synthesis machinery. Further characterization of these proteins could provide greater insight into the mechanisms involved in regulation of neurogenesis in the adult central nervous system (CNS) and potentially identify points of therapeutic intervention.

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.

FGF1/p38 MAP kinase inhibitor therapy induces cardiomyocyte mitosis, reduces scarring, and rescues function after myocardial infarction

Mammalian cardiomyocytes have limited proliferation potential, and acutely injured mammalian hearts do not regenerate adequately. Instead, injured myocardium develops fibrosis and scarring. Here we show that FGF1/p38 MAP kinase inhibitor treatment after acute myocardial injury in 8- to 10-week-old rats increases cardiomyocyte mitosis. At 3 months after injury, 4 weeks of FGF1/p38 MAP kinase inhibitor therapy results in reduced scarring and wall thinning, with markedly improved cardiac function. In contrast, p38 MAP kinase inhibition alone fails to rescue heart function despite increased cardiomyocyte mitosis. FGF1 improves angiogenesis, possibly contributing to the survival of newly generated cardiomyocytes. Our data indicate that FGF1 and p38 MAP kinase, proteins involved in cardiomyocyte proliferation and angiogenesis during development, may be delivered therapeutically to enhance cardiac regeneration.

Specific deletion of CMF1 nuclear localization domain causes incomplete cell cycle withdrawal and impaired differentiation in avian skeletal myoblasts

CMF1 is a protein expressed in embryonic striated muscle with onset of expression preceding that of contractile proteins. Disruption of CMF1 in myoblasts disrupts muscle-specific protein expression. Preliminary studies indicate both nuclear and cytoplasmic distribution of CMF1 protein, suggesting functional roles in both cellular compartments. Here we examine the nuclear function of CMF1, using a newly characterized antibody generated against the CMF1 nuclear localization domain and a CMF1 nuclear localization domain-deleted stable myocyte line. The antibody demonstrates nuclear distribution of the CMF1 protein both in vivo and in cell lines, with clustering of CMF1 protein around chromatin during mitosis. In more differentiated myocytes, the protein shifts to the cytoplasm. The CMF1 NLS-deleted cell lines have markedly impaired capacity to differentiate. Specifically, these cells express less contractile protein than wild-type or full-length CMF1 stably transfected cells, and do not fuse properly into multinucleate syncytia with linear nuclear alignment. In response to low serum medium, a signal to differentiate, CMF1 NLS-deleted cells enter G0, but continue to express proliferation markers and will reenter the cell cycle when stimulated by restoring growth medium. These data suggest that CMF1 is involved in regulation the transition from proliferation to differentiation in embryonic muscle.

p21Cip1 protection against hyperoxia requires Bcl-XL and is uncoupled from its ability to suppress growth

The cyclin-dependent kinase inhibitor p21Cip1/Waf1/Sdi1 protects the lung against hyperoxia, but the mechanism of protection remains unclear because loss of p21 does not lead to aberrant cell proliferation. Because some members of the Bcl-2 gene family have been implicated in hyperoxia-induced cell death, the current study investigated their expression as well as p21-dependent growth suppression and cytoprotection. Conditional overexpression of full-length p21, its amino-terminal cyclin-binding (p211-82NLS) domain or its carboxy-terminal PCNA-binding (p2176-164) domain inhibited growth of human lung adenocarcinoma H1299 cells, but only the full-length protein was cytoprotective. Low levels of p21 inhibited cell proliferation, whereas higher levels were required for protection. Expression of the anti-apoptotic protein Bcl-XL declined during hyperoxia but was maintained in cells expressing p21. RNA interference (RNAi) knockdown of Bcl-XL enhanced hyperoxic death of cells expressing p21, whereas overexpression of Bcl-XL increased cell survival. Consistent with growth suppression and cytoprotection requiring different levels of p21, hyperoxia inhibited PCNA expression in p21+/+ and p21+/- mice but not in p21-/- mice. In contrast, p21 was haplo-insufficient for maintaining expression of Bcl-XL and protection against hyperoxia. Taken together, these data show that p21-mediated cytoprotection against hyperoxia involves regulation of Bcl-XL and is uncoupled from its ability to inhibit proliferation.

Genistein-induced neuronal differentiation is associated with activation of extracellular signal-regulated kinases and upregulation of p21 and N-cadherin

Neuronal differentiation in the mammalian CNS is driven by multiple events. When treated with retinoic acid (RA), hNTera-2 (NT-2) cells undergo postmitotic neuronal differentiation. Here, we show that a prolonged exposure of NT-2 cells with non-cytotoxic doses of genistein, a protein tyrosine kinase (PTK) inhibitor, induced differentiation of NT-2 cells. Additionally, genistein enhanced RA-induced neuronal differentiation by increasing the activation of extracellular signal-related kinase 1/2 (ERK1/2) via phosphorylation at Thr183 and Tyr185 in 3-7 days. Meanwhile, genistein also upregulated N-cadherin and p21 (a Cdk inhibitor), but downregulated proliferating cell nuclear antigen protein (PCNA). MEK1/2 inhibitors, such as PD98059 and U0126, reduced RA-induced ERK1/2 activity, but could not block the genistein effects. Our observations indicate that genistein-induced neuronal differentiation is not dependent of the MEK-ERK signaling cascade. Instead, genistein-upregulated ERK activation is likely due to this chemical's direct effect on chromosome and gene transcription, rather than its inhibition on tyrosine kinases. Failure of inhibition of ERK1/2 activation by the MEK1/2 inhibitors PD98059 and U0126 suggests presence of an unknown activator for ERK1/2 in neuronal cells.

Location of cell cycle regulators cyclin B1, cyclin A, PCNA, Ki67 and cell cycle inhibitors p21, p27 and p57 in human first trimester placenta and deciduas

Although placental development and implantation depend on the coordination of trophoblast proliferation, differentiation and invasion, little is known about the cell cycle regulators that govern the control of these events. The hypothesis that the coordinated expression of cell cycle progression and inhibition factors will determine whether cytotrophoblasts proliferate or undergo cell cycle arrest or cell cycle exit allowing subsequent differentiation was tested. The cell cycle promotors cyclin A, cyclin B1, PCNA, Ki67 and the cell cycle inhibitors p21, p27 and p57 were immunolocalized in tissue sections of first trimester pregnancies (weeks 6 and 9-12). Double staining with cytokeratin 7 allowed unambiguous identification of extravillous cytotrophoblast (EVT) in the decidua. Villous cytotrophoblasts were immunolabelled for Ki67 and cyclin A but only few were stained with anti-cyclin B1. The syncytiotrophoblast was devoid of immunoreactivity for any of the cell cycle progression factors. It expressed especially p21, whereas p27 and p57 were predominantly found in villous cytotrophoblasts. PCNA, Ki67, cyclin A and cyclin B1 were immunolocalized in proximal and distal EVTs of anchoring villi and in EVT which had invaded the upper decidual segments. All EVTs strongly expressed p27 and p57, but not p21. These data clearly suggest different functions for p21, p27 and p57 in placental development with distinct roles for p21 and p57 in syncytiotrophoblast and EVT differentiation, respectively. p27 appears to be involved in both the processes. The results may also challenge the concept of differential mitotic activity in the proximal and distal parts of the first trimester cytotrophoblast cell column, but more functional studies are clearly needed. The presence of p27 and p57 in EVT cells, which invade the deciduas deeply, may account for the loss of mitogenic potential of these cells.

Stem Cells for Cardiomyocyte Regeneration: State of the Art

A wide range of stem/progenitor cell types have been used for the regeneration of the infarcted heart. This review details the current status of progress concerning different strategies that have been used to manipulate cardiomyocyte cell growth in vitro, for their use in heart failure. The current status of this field involves different types of regenerating cells. Embryonic stem (ES) cells, hematopoietic stem and progenitor cells, mesenchymal stem and progenitor cells, and resident cardiac "stem" cells are discussed here.

Requirement of Nuclear Factor-κB in Angiotensin II– and Isoproterenol-Induced Cardiac Hypertrophy In Vivo

Background— In vitro experiments have proposed a role of nuclear factor-κB (NF-κB), a transcription factor, in cardiomyocyte hypertrophy and protection against apoptosis. Currently, the net effect on cardiac remodeling in vivo under common stress stimuli is unclear.

Methods and Results— We have generated mice with cardiomyocyte-restricted expression of the NF-κB super-repressor IκBαΔN (ΔN MHC ) using the Cre/lox technique. ΔN MHC mice displayed an attenuated hypertrophic response compared with control mice on infusion of angiotensin II (Ang II) or isoproterenol by micro-osmotic pumps, as determined by echocardiography (left ventricular wall dimensions: control plus Ang II, ×1.5±0.1 versus sham; ΔN MHC plus Ang II, ×1.1±0.1 versus sham; P <0.05; n≥9), heart weight, and histological analysis. Real-time reverse-transcriptase polymerase chain reaction showed significantly reduced expression of hypertrophy markers β-myosin heavy chain and atrial natriuretic peptide in Ang II–treated ΔN MHC mice ( P <0.05 versus control plus Ang II; n=4). Neither cardiomyocyte apoptosis nor left ventricular dilatation was observed. In cultured adult rat cardiomyocytes, NF-κB DNA binding activity was increased by both Ang II– and interleukin-6–related cytokines. The latter are known to be released by cardiac fibroblasts on Ang II stimulation and thus could locally increase the NF-κB response of cardiomyocytes. Finally, results from in vitro and in vivo experiments suggest a role for NF-κB in the regulation of prohypertrophic interleukin-6 receptor gp130 on mRNA levels.

Conclusions— These results indicate that targeted inhibition of NF-κB in cardiomyocytes in vivo is sufficient to impair Ang II– and isoproterenol-induced hypertrophy without increasing the susceptibility to apoptosis.

p38 MAP kinase inhibition enables proliferation of adult mammalian cardiomyocytes

Adult mammalian cardiomyocytes are considered terminally differentiated and incapable of proliferation. Consequently, acutely injured mammalian hearts do not regenerate, they scar. Here, we show that adult mammalian cardiomyocytes can divide. One important mechanism used by mammalian cardiomyocytes to control cell cycle is p38 MAP kinase activity. p38 regulates expression of genes required for mitosis in cardiomyocytes, including cyclin A and cyclin B. p38 activity is inversely correlated with cardiac growth during development, and its overexpression blocks fetal cardiomyocyte proliferation. Activation of p38 in vivo by MKK3bE reduces BrdU incorporation in fetal cardiomyocytes by 17.6%. In contrast, cardiac-specific p38alpha knockout mice show a 92.3% increase in neonatal cardiomyocyte mitoses. Furthermore, inhibition of p38 in adult cardiomyocytes promotes cytokinesis. Finally, mitosis in adult cardiomyocytes is associated with transient dedifferentiation of the contractile apparatus. Our findings establish p38 as a key negative regulator of cardiomyocyte proliferation and indicate that adult cardiomyocytes can divide.

Cardiomyocytes fuse with surrounding noncardiomyocytes and reenter the cell cycle

The concept of the plasticity or transdifferentiation of adult stem cells has been challenged by the phenomenon of cell fusion. In this work, we examined whether neonatal cardiomyocytes fuse with various somatic cells including endothelial cells, cardiac fibroblasts, bone marrow cells, and endothelial progenitor cells spontaneously in vitro. When cardiomyocytes were cocultured with endothelial cells or cardiac fibroblasts, they fused and showed phenotypes of cardiomyocytes. Furthermore, cardiomyocytes reentered the G2-M phase in the cell cycle after fusing with proliferative noncardiomyocytes. Transplanted endothelial cells or skeletal muscle–derived cells fused with adult cardiomyocytes in vivo. In the cryoinjured heart, there were Ki67-positive cells that expressed both cardiac and endothelial lineage marker proteins. These results suggest that cardiomyocytes fuse with other cells and enter the cell cycle by maintaining their phenotypes.

A methionine aminopeptidase-2 inhibitor, PPI-2458, for the treatment of rheumatoid arthritis

The hallmark of rheumatoid arthritis (RA) is the progressive destruction of articular joints, characterized by invasive synovial hyperplasia and pathological neovascularization. Here we report that PPI-2458, a member of the fumagillin class of irreversible methionine aminopeptidase-2 (MetAP-2) inhibitors, potently inhibits the proliferation of human fibroblast-like synoviocytes (HFLS-RA), derived from RA patients, with a growth inhibitory concentration 50 (GI(50)) of 0.04 nM and a maximum inhibition of >95% at 1 nM. Human umbilical vein endothelial cells (HUVEC) are similarly inhibited in proliferation by PPI-2458 (GI(50), 0.2 nM). We developed a method to measure the level of MetAP-2 enzyme inhibition after exposure to PPI-2458 and demonstrate that growth inhibition of PPI-2458-sensitive HFLS-RA and HUVEC is linked to MetAP-2 enzyme inhibition, in a dose-dependent fashion. The secretion of several inflammatory mediators such as IL-6 and vascular endothelial growth factor from activated HFLS-RA was not inhibited by PPI-2458. The CNS toxicity profile of PPI-2458, determined by the incidence of seizures, is significantly improved over that of the parental compound TNP-470. In the rat model of peptidoglycan-polysaccharide-induced arthritis, PPI-2458 significantly attenuated paw swelling when therapeutically administered after the onset of chronic disease. We suggest that the mechanism of PPI-2458 action, highly selective and potent anti-proliferative activity on HFLS-RA and HUVEC in vitro, a significantly improved CNS toxicity profile, and marked attenuation of chronic disease in the rat peptidoglycan-polysaccharide arthritis model in vivo, positions this compound as a drug for the treatment of RA.

Regenerative capacity of the myocardium: implications for treatment of heart failure

Research into myocardial regeneration has an exciting future, shown by the results of experimental and clinical work challenging the dogma that the heart is a postmitotic non-regenerating organ. Such studies have initiated a lively debate about the feasibility of novel treatment approaches leading to the recovery of damaged myocardial tissue. The possibility of reconstituting dead myocardium by endogenous cardiomyocyte replication, transplantation, or activation of stem cells--or even cloning of an artificial heart--is being advanced, and will be a major subject of future research. Although health expenditure for heart failure in the industrial world is high, we are still a long way from being able to treat the cause of reduced myocardial contractility. Despite the hopes of some people, conventional treatment for heart failure does not achieve myocardial regeneration. We present a virtual case report of a patient with acute myocardial infarction; we discuss treatment options, including strategies aimed at organ regeneration.

Proliferating cell nuclear antigen (PCNA): a dancer with many partners

Proliferating cell nuclear antigen (PCNA) was originally characterised as a DNA sliding clamp for replicative DNA polymerases and as an essential component of the eukaryotic chromosomal DNA replisome. Subsequent studies, however, have revealed its striking ability to interact with multiple partners, which are involved in several metabolic pathways, including Okazaki fragment processing, DNA repair, translesion DNA synthesis, DNA methylation, chromatin remodeling and cell cycle regulation. PCNA in mammalian cells thus appears to play a key role in controlling several reactions through the coordination and organisation of different partners. Two major questions have emerged: how do these proteins access PCNA in a coordinated manner, and how does PCNA temporally and spatially organise their functions? Structural and biochemical studies are starting to provide a first glimpse of how both tasks can be achieved.