Inhibition of hypoxia-induced apoptosis by modulation of retinoblastoma protein-dependent signaling in cardiomyocytes

Multinucleated polyploid cardiomyocytes undergo an enhanced adaptability to hypoxia via mitophagy

AIMS

There is a large subpopulation of multinucleated polyploid cardiomyocytes (M*Pc CMs) in the adult mammalian heart. However, the pathophysiological significance of increased M*Pc CMs in heart disease is poorly understood. We sought to determine the pathophysiological significance of increased M*Pc CMs during hypoxia adaptation.

METHODS AND RESULTS

A model of hypoxia-induced cardiomyocyte (CM) multinucleation and polyploidization was established and found to be associated with less apoptosis and less reactive oxygen species (ROS) production. Compared to mononucleated diploid CMs (1*2c CMs), tetraploid CMs (4c CMs) exhibited better mitochondria quality control via increased mitochondrial autophagy (mitophagy). RNA-seq revealed Prkaa2, the gene for AMPKα2, was the most obviously up-regulated autophagy-related gene. Knockdown of AMPKα2 increased apoptosis and ROS production and suppressed mitophagy in 4c CMs compared to 1*2c CMs. Rapamycin, an autophagy activator, alleviated the adverse effect of AMPKα2 knockdown. Furthermore, silencing PINK1 also increased apoptosis and ROS in 4c CMs and weakened the adaptive superiority of 4c CMs. Finally, AMPKα2^-/- mutant mice exhibited exacerbation of apoptosis and ROS production via decreases in AMPKα2-mediated mitophagy in 4c CMs compared to 1*2c CMs during hypoxia.

CONCLUSIONS

Compared to 1*2c CMs, hypoxia-induced 4c CMs exhibited enhanced mitochondria quality control and less apoptosis via AMPKα2-mediated mitophagy. These results suggest that multinucleation and polyploidization allow CM to better adapt to stress via enhanced mitophagy. In addition, activation of AMPKα2 may be a promising target for myocardial hypoxia-related diseases.

Inhibition of cyclin-dependent kinase 2 protects against doxorubicin-induced cardiomyocyte apoptosis and cardiomyopathy

With the rapid increase in cancer survival because of improved diagnosis and therapy in the past decades, cancer treatment-related cardiotoxicity is becoming an urgent healthcare concern. The anthracycline doxorubicin (DOX), one of the most effective chemotherapeutic agents to date, causes cardiomyopathy by inducing cardiomyocyte apoptosis. We demonstrated previously that overexpression of the cyclin-dependent kinase (CDK) inhibitor p21 promotes resistance against DOX-induced cardiomyocyte apoptosis. Here we show that DOX exposure provokes cardiac CDK2 activation and cardiomyocyte cell cycle S phase reentry, resulting in enhanced cellular sensitivity to DOX. Genetic or pharmacological inhibition of CDK2 markedly suppressed DOX-induced cardiomyocyte apoptosis. Conversely, CDK2 overexpression augmented DOX-induced apoptosis. We also found that DOX-induced CDK2 activation in the mouse heart is associated with up-regulation of the pro-apoptotic BCL2 family member BCL2-like 11 (Bim), a BH3-only protein essential for triggering Bax/Bak-dependent mitochondrial outer membrane permeabilization. Further experiments revealed that DOX induces cardiomyocyte apoptosis through CDK2-dependent expression of Bim. Inhibition of CDK2 with roscovitine robustly repressed DOX-induced mitochondrial depolarization. In a cardiotoxicity model of chronic DOX exposure (5 mg/kg weekly for 4 weeks), roscovitine administration significantly attenuated DOX-induced contractile dysfunction and ventricular remodeling. These findings identify CDK2 as a key determinant of DOX-induced cardiotoxicity. CDK2 activation is necessary for DOX-induced Bim expression and mitochondrial damage. Our results suggest that pharmacological inhibition of CDK2 may be a cardioprotective strategy for preventing anthracycline-induced heart damage.

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.

Signaling Pathways in Cardiac Myocyte Apoptosis

Cardiovascular diseases, the number 1 cause of death worldwide, are frequently associated with apoptotic death of cardiac myocytes. Since cardiomyocyte apoptosis is a highly regulated process, pharmacological intervention of apoptosis pathways may represent a promising therapeutic strategy for a number of cardiovascular diseases and disorders including myocardial infarction, ischemia/reperfusion injury, chemotherapy cardiotoxicity, and end-stage heart failure. Despite rapid growth of our knowledge in apoptosis signaling pathways, a clinically applicable treatment targeting this cellular process is currently unavailable. To help identify potential innovative directions for future research, it is necessary to have a full understanding of the apoptotic pathways currently known to be functional in cardiac myocytes. Here, we summarize recent progress in the regulation of cardiomyocyte apoptosis by multiple signaling molecules and pathways, with a focus on the involvement of these pathways in the pathogenesis of heart disease. In addition, we provide an update regarding bench to bedside translation of this knowledge and discuss unanswered questions that need further investigation.

p21(CIP1/WAF1)-dependent inhibition of cardiac hypertrophy in response to Angiotensin II involves Akt/Myc and pRb signaling

The cyclin-dependent kinase inhibitor p21(CIP1/WAF1) (p21) is highly expressed in the adult heart. However, in response to stress, its expression is downregulated. Therefore, we investigated the role of p21 in the regulation of cardiac hypertrophic growth. At 2 months of age, p21 knockout mice (p21KO) lack an overt cardiac phenotype. In contrast, by 10 months of age, p21KO developed age-dependent cardiac hypertrophy and heart failure. After 3 weeks of trans-aortic banding (TAB), the heart/body weight ratio in 11 week old p21KO mice increased by 57%, as compared to 42% in wild type mice indicating that p21KO have a higher susceptibility to pressure overload-induced cardiac hypertrophy. We then chronically infused 8 week old wild type mice with Angiotensin II (2.0mg/kg/min) or saline subcutaneously by osmotic pumps for 14 days. Recombinant TAT conjugated p21 protein variants (10mg/kg body weight) or saline were intraperitoneally injected once daily for 14 days into Angiotensin II and saline-infused animals. Angiotensin II treated mice developed pathological cardiac hypertrophy with an average increase of 38% in heart/body weight ratios, as compared to saline-treated controls. Reconstitution of p21 function by TAT.p21 protein transduction prevented Angiotensin II-dependent development of cardiac hypertrophy and failure. Taken together, our genetic and biochemical data show an important function of p21 in the regulation of growth-related processes in the heart.

Lateral intracerebroventricular injection of Apelin-13 inhibits apoptosis after cerebral ischemia/reperfusion injury

Apelin-13 inhibits neuronal apoptosis caused by hydrogen peroxide, yet apoptosis following cerebral ischemia-reperfusion injury has rarely been studied. In this study, Apelin-13 (0.1 μg/g) was injected into the lateral ventricle of middle cerebral artery occlusion model rats. TTC, TUNEL, and immunohistochemical staining showed that compared with the cerebral ischemia/reperfusion group, infarct volume and apoptotic cell number at the ischemic penumbra region were decreased in the Apelin-13 treatment group. Additionally, Apelin-13 treatment increased Bcl-2 immunoreactivity and decreased caspase-3 immunoreactivity. Our findings suggest that Apelin-13 is neuroprotective against cerebral ischemia/reperfusion injury through inhibition of neuronal apoptosis.

Incidence and Expression of Circulating Cell Free p53-Related Genes in Acute Myocardial Infarction Patients

AIM

The circulating RNA levels are predictive markers in several diseases. We determined the levels of circulating p53-related genes in patients with acute ST-segment elevation myocardial infarction (STEMI), indicating major heart muscle damage.

METHODS

Plasma RNA was extracted from the patients (n=45) upon their arrival to the hospital (STEMI 0h) and at four hours post-catheterization (STEMI 4h) as well as from controls (n=34).

RESULTS

Of 18 circulating p53-related genes, nine genes were detectable. A significantly lower incidence of circulating p21 (p ＜ 0.0001), Notch1 (p=0.042) and BTG2 (p ＜ 0.0001) was observed in the STEMI 0h samples in comparison to the STEMI 4h and control samples. Lower expression levels (2.1-fold) of circulating BNIP3L (p=0.011), p21 (3.4-fold, p=0.005) and BTG2 (6.3-fold, p=0.0001) were observed in the STEMI 0h samples in comparison to the STEMI 4h samples, with a 7.4-fold lower BTG2 expression (p ＜ 0.001) and 2.6-fold lower p21 expression (p=0.034) compared to the control samples. Moreover, the BNIP3L expression (borderline significance, p=0.0655) predicted the level of peak troponin, a marker of myocardial infarction. In addition, the BNIP3L levels on admission (p=0.0025), at post-catheterization (p=0.020) and the change between the groups (p=0.0079) were inversely associated with troponin. The BNIP3L (p=0.0139) and p21 levels (p=0.0447) were also associated with a longer time to catheterization.

CONCLUSIONS

Our results suggest that circulating downstream targets of p53 are inhibited during severe AMI and subsequently re-expressed after catheterization, uncovering possible novel death-or-survival decisions regarding the fate of p53 in the heart and the potential use of its target genes as prognostic biomarkers for oxygenation normalization.

Focal adhesion kinase antagonizes doxorubicin cardiotoxicity via p21(Cip1.)

Clinical application of potent anthracycline anticancer drugs, especially doxorubicin (DOX), is limited by a toxic cardiac side effect that is not fully understood and preventive strategies are yet to be established. Studies in genetically modified mice have demonstrated that focal adhesion kinase (FAK) plays a key role in regulating adaptive responses of the adult myocardium to pathological stimuli through activation of intracellular signaling cascades that facilitate cardiomyocyte growth and survival. The objective of this study was to determine if targeted myocardial FAK activation could protect the heart from DOX-induced de-compensation and to characterize the underlying mechanisms. To this end, mice with myocyte-restricted FAK knock-out (MFKO) or myocyte-specific expression of an active FAK variant (termed SuperFAK) were subjected to DOX treatment. FAK depletion enhanced susceptibility to DOX-induced myocyte apoptosis and cardiac dysfunction, while elevated FAK activity provided remarkable cardioprotection. Our mec6hanistic studies reveal a heretofore unappreciated role for the protective cyclin-dependent kinase inhibitor p21 in the repression of the pro-apoptotic BH3-only protein Bim and the maintenance of mitochondrial integrity and myocyte survival. DOX treatment induced proteasomal degradation of p21, which exacerbated mitochondrial dysfunction and cardiomyocyte apoptosis. FAK was both necessary and sufficient for maintaining p21 levels following DOX treatment and depletion of p21 compromised FAK-dependent protection from DOX. These findings identify p21 as a key determinant of DOX resistance downstream of FAK in cardiomyocytes and indicate that cardiac-restricted enhancement of the FAK/p21 signaling axis might be an effective strategy to preserve myocardial function in patients receiving anthracycline chemotherapy.

Anti-apoptotic function of the E2F transcription factor 4 (E2F4)/p130, a member of retinoblastoma gene family in cardiac myocytes

The E2F4-p130 transcriptional repressor complex is a cell-cycle inhibitor in mitotic cells. However, the role of E2F4/p130 in differentiated cells is largely unknown. We investigated the role of E2F4/p130 in the regulation of apoptosis in postmitotic cardiomyocytes. Here we demonstrate that E2F4 can inhibit hypoxia-induced cell death in isolated ventricular cardiomyocytes. As analyzed by chromatin immunoprecipitation, the E2F4-p130-repressor directly blocks transcription of essential apoptosis-related genes, E2F1, Apaf-1, and p73α through recruitment of histone deacetylase 1 (HDAC1). In contrast, diminution of the E2F4-p130-HDAC1-repressor and recruitment of E2F1 and histone acetylase activity to these E2F-regulated promoters is required for the execution of cell death. Expression of kinase-dead HDAC1.H141A or HDAC-binding deficient p130ΔHDAC1 abolishes the antiapoptotic effect of E2F4. Moreover, histological examination of E2F4(-/-) hearts revealed a markedly enhanced degree of cardiomyocyte apoptosis. Taken together, our genetic and biochemical data delineate an essential negative function of E2F4 in cardiac myocyte apoptosis.

Differential requirement of cyclin-dependent kinase 2 for oligodendrocyte progenitor cell proliferation and differentiation

Cyclin-dependent kinases (Cdks) and their cyclin regulatory subunits control cell growth and division. Cdk2-cyclin E complexes, phosphorylating the retinoblastoma protein, drive cells through the G1/S transition into the S phase of the cell cycle. Despite its fundamental role, Cdk2 was found to be indispensable only in specific cell types due to molecular redundancies in its function. Converging studies highlight involvement of Cdk2 and associated cell cycle regulatory proteins in oligodendrocyte progenitor cell proliferation and differentiation. Giving the contribution of this immature cell type to brain plasticity and repair in the adult, this review will explore the requirement of Cdk2 for oligodendrogenesis, oligodendrocyte progenitor cells proliferation and differentiation during physiological and pathological conditions.

Post-myocardial infarct p27 fusion protein intravenous delivery averts adverse remodelling and improves heart function and survival in rodents

AIMS

P27Kip1 (p27) blocks cell proliferation through the inhibition of cyclin-dependent kinase 2 (cdk-2). Despite robust expression in the heart, little is known about the regulation and function of p27 in this terminally differentiated tissue. Previously, we demonstrated that p27 exerts anti-apoptotic and growth-inhibitory effects through interaction with casein kinase 2 (ck2) in neonatal rat cardiomyocytes. Here, we test the hypothesis that delivery of a transactivator of transcription (TAT)-p27 fusion protein (TAT.p27) will improve cardiac function and survival in a rat model of myocardial infarction (MI).

METHODS AND RESULTS

Fisher rats underwent permanent left anterior descending ligation-induced MI followed by iv injection of TAT.p27 or TAT.LacZ (20 mg/kg) on Days 1 and 7 post-MI. Delivery of TAT.p27 was evaluated by western blot (WB) and immunofluorescence microscopy. Heart function was assessed by echocardiography and pressure-volume catheter. Apoptosis, hypertrophy, and fibrosis were detected by histochemistry and morphometry. WB confirmed gradual reduction in endogenous cardiac p27 levels following MI, with immunohistochemistry demonstrating successful delivery of TAT.p27 to the heart. At 48 h post-MI, cardiac apoptosis was decreased in rats treated with TAT.p27 when compared with saline- and TAT.LacZ-treated controls. At 28 days post-MI, rats treated with TAT.p27 manifested less cardiomyocyte hypertrophy and fibrosis, less diminished cardiac function, and greater survival. Additionally, p27KO mice undergoing experimental MI suffered an early increase in apoptosis with a larger infarct size and markedly reduced survival when compared with wild-type (WT) controls.

CONCLUSION

These gain- and loss-of-function studies reveal a critical role for p27 in cardiac remodelling post-MI.

Downregulation of doxorubicin-induced myocardial apoptosis accompanies postnatal heart maturation

Doxorubicin is a highly effective chemotherapeutic agent used for treating a wide spectrum of tumors, but its usage is limited because of its dose-dependent cardiotoxicity, especially in pediatric patients. Accumulating evidence indicates that caspase-dependent apoptosis contributes to the cardiotoxicity of doxorubicin. However, less attention has been paid to the effects of age on doxorubicin-induced apoptosis signaling in myocardium. This study focused on investigating differential apoptotic sensitivity between neonatal and adult myocardium, in particular, between neonatal and adult cardiomyocytes in vivo. Our results show that caspase-3 activity in normal mouse hearts decreased by ≥ 20-fold within the first 3 wk after birth, associated with a rapid downregulation in the expression of key proapoptotic proteins in intrinsic and extrinsic pathways. This rapid downregulation of caspase-3 activity was confirmed by immunostaining for cleaved caspase-3 and terminal deoxynucleotidyl transferase dUTP-mediated nick-end label staining. Doxorubicin treatment induced a dose-dependent increase in caspase-3 activity and apoptosis in neonatal mouse hearts, and both caspase-8 and caspase-9 activations were involved. Using transgenic mice with a nuclear localized LacZ reporter gene to label cardiomyocytes in vivo, we observed a fourfold higher level of doxorubicin-induced cardiomyocyte apoptosis in 1-wk-old mice compared with that in 3-wk-old mice. This study points to a major difference in apoptotic signaling in doxorubicin cardiotoxicity between neonatal and adult mouse hearts and reveals a critical transition from high to low susceptibility to doxorubicin-induced apoptosis during postnatal heart maturation.

The role of E2F-1 and downstream target genes in mediating ischemia/reperfusion injury in vivo

E2Fs are a family of transcription factors that regulate proliferation, differentiation and apoptosis in many cell types. E2F-1 is the prototypical E2F and the family member that has most often been implicated in also mediating apoptosis. To better understand the role of E2F-1 in mediating cardiomyocyte injury we initially analyzed E2F family member expression after ischemia/reperfusion (I/R) in vivo or simulated ischemia in vitro. I/R injury in vivo caused a 3.4-fold increase specifically in E2F-1 protein levels. Expression of other E2F family members did not change. To establish the role of E2F-1 in I/R we examined the response of germline deleted E2F-1 mice to I/R injury. Infarct size as a percentage of the area at risk was decreased 39.8% in E2F-1(-/-) mice compared to E2F-1(+/+) controls. Interestingly, expression of classic, E2F-1 apoptotic target genes was not altered in E2F-1 null cardiomyocytes after I/R. However, upregulation of the primary member of the Forkhead family of transcription factors, FoxO-1a, was attenuated. Consistent, with a role for FoxO-1a as an important target of E2F-1 in I/R, a number of proapoptotic FoxO-1a target genes were also altered. These results suggest that E2F-1 and FoxO-1a belong to a complex transcriptional network that may modulate myocardial cell death during I/R injury.

p38 phosphorylates Rb on Ser567 by a novel, cell cycle-independent mechanism that triggers Rb-Hdm2 interaction and apoptosis

The retinoblastoma protein (Rb) inhibits both cell division and apoptosis, but the mechanism by which Rb alternatively regulates these divergent outcomes remains poorly understood. Cyclin dependent kinases (Cdks) promote cell division by phosphorylating and reversibly inactivating Rb by a hierarchical series of phosphorylation events and sequential conformational changes. The stress-regulated mitogen activated protein kinase (MAPK) p38 also phosphorylates Rb, but it does so in a cell cycle-independent manner that is associated with apoptosis rather than with cell division. Here, we show that p38 phosphorylates Rb by a novel mechanism that is distinct from that of Cdks. p38 bypasses the cell cycle-associated hierarchical phosphorylation and directly phosphorylates Rb on Ser567, which is not phosphorylated during the normal cell cycle. Phosphorylation by p38, but not Cdks, triggers an interaction between Rb and the human homologue of murine double minute 2 (Hdm2), leading to degradation of Rb, release of E2F1 and cell death. These findings provide a mechanistic explanation for how Rb regulates cell division and apoptosis through different kinases, and reveal how Hdm2 may functionally link the tumor suppressors Rb and p53.

Effects of Baicalin and Octreotide on the serum TNF-alpha level and apoptosis in multiple organs of rats with severe acute pancreatitis

We investigated the effects of Baicalin and Octreotide on the levels of endotoxin and TNF-alpha in blood and the effects of apoptotic changes in multiple organs of SAP rats, and explored the underlying therapeutic mechanisms of Baicalin and Octreotide. In this study, 135 SAP rats were randomly divided into model control, Baicalin treated and Octreotide treated group (n = 45), respectively, the same number of normal rats were included in sham-operated group (n = 45). The above-mentioned groups were further subdivided into 3, 6 and 12 h subgroups, respectively (15 rats in each subgroup). At 3, 6 and 12 h after operation, the mortality rate of rats, endotoxin and TNF-alpha levels in blood as well as the pathological severity scores, expression levels of Bax protein and apoptosis indexes in multiple organs were determined. Compared to model control group (1),both drugs can relieve the pathological injuries of multiple organs and decrease significantly the levels of endotoxin and TNF-alpha in blood and the mortality rate of rats in treated groups (P < 0.05 or P < 0.01); (2) the expression of Bax protein was upregulated in pancreas, lung, intestinal mucosa (P < 0.05 or P < 0.01) but downregulated in spleen and lymph nodes (P < 0.001 and P < 0.05, respectively) in Baicalin treated group; The apoptosis indexes significantly increased in pancreas, intestinal mucosa, lymph nodes and spleen (P < 0.05 or P < 0.01). (3) the expression of Bax protein was upregulated in pancreas and lung but downregulated in spleen and lymph nodes (P < 0.05 or P < 0.01) in Octreotide treated group; The apoptosis indexes significantly increased in lymph nodes and spleen in Octreotide treated group (P < 0.05 or P < 0.01). Baicalin and Octreotide share a similar therapeutic efficacy in the treatment of SAP via a mechanism that is associated with inhibiting the levels of TNF-alpha in blood and induce apoptosis in multiple organs.

Antagonism of E2F-1 regulated Bnip3 transcription by NF-kappaB is essential for basal cell survival

The transcription factor E2F-1 drives proliferation and death, but the mechanisms that differentially regulate these divergent actions are poorly understood. The hypoxia-inducible death factor Bnip3 is an E2F-1 target gene and integral component of the intrinsic mitochondrial death pathway. The mechanisms that govern Bnip3 gene activity remain cryptic. Herein we show that the transcription factor NF-kappaB provides a molecular switch that determines whether E2F-1 signals proliferation or death under physiological conditions. We show under basal nonapoptotic conditions that NF-kappaB constitutively occupies and transcriptionally silences Bnip3 gene transcription by competing with E2F-1 for Bnip3 promoter binding. Conversely, in the absence of NF-kappaB, or during hypoxia when NF-kappaB abundance is reduced, basal Bnip3 gene transcription is activated by the unrestricted binding of E2F-1 to the Bnip3 promoter. Genetic knock-down of E2F-1 or retinoblastoma gene product over-expression in cardiac and human pancreatic cancer cells deficient for NF-kappaB signaling abrogated basal and hypoxia-inducible Bnip3 transcription. The survival kinase PI3K/Akt inhibited Bnip3 expression levels in cells in a manner dependent upon NF-kappaB activation. Hence, by way of example, we show that the transcriptional inhibition of E2F-1-dependent Bnip3 expression by NF-kappaB highlights a survival pathway that overrides the E2F-1 tumor suppressor program. Our data may explain more fundamentally how cells, by selectively inhibiting E2F-1-dependent death gene transcription, avert apoptosis down-stream of the retinoblastoma/E2F-1 cell cycle pathway.

Signaling and apoptosis differences between severe hypoxia and desferoxamine treatment of human epithelial cells

The mechanisms by which cells undergo proliferation arrest or cell death in response to hypoxia are still not completely understood. Originally, we showed that HeLa and Hep3B carcinoma cells undergo different proliferation responses in hypoxia. We now show that these 2 cell lines also have different cell death responses to severe hypoxia, with HeLa showing both cell cycle arrest and apoptosis (as early as 12 h after hypoxia treatment), and Hep3B showing resistance to both. Hypoxia-induced apoptosis in Hela was associated with decreases of both phospho-S473- and -T308-AKT and loss of AKT function, whereas Hep3B cells were resistant to hypoxia-induced apoptosis and did not lose phospho-AKT or AKT function. We then decided to test if our observations were confirmed using a hypoxia mimic, desferoxamine. Desferoxamine treatment yielded cell cycle arrest in HeLa and moderate arrest in Hep3B but, surprisingly, did not induce notable apoptosis of either cell line with up to 24 h of treatment. Hypoxia-treated normal human mammary epithelial cells also showed hypoxia-induced apoptosis. Interestingly, in these cell lines, there was a complete correlation between loss of phospho-AKT and (or) total AKT, and susceptibility to hypoxia-induced apoptosis. Our data suggests a model in which regulated loss of active AKT at a precise time point in hypoxia may be associated with apoptosis in susceptible cells.

Cyclin-dependent kinase 2 signaling regulates myocardial ischemia/reperfusion injury

Ischemia/reperfusion (I/R) injury to the heart is accompanied by the upregulation and posttranslational modification of a number of proteins normally involved in regulating cell cycle progression. Two such proteins, cyclin-dependent kinase-2 (Cdk2) and its downstream target, the retinoblastoma gene product (Rb), also play a critical role in the control of apoptosis. Myocardial ischemia activates Cdk2, resulting in the phosphorylation and inactivation of Rb. Blocking Cdk2 activity reduces apoptosis in cultured cardiac myocytes. Genetic or pharmacological inhibition of Cdk2 activity in vivo during I/R injury led to a 36% reduction in infarct size (IFS), when compared to control mice, associated with a reduction in apoptotic myocytes. To confirm that Rb was the critical target in Cdk2-mediated I/R injury, we determined the consequences of I/R injury in cardiac-specific Rb-deficient mice (CRb(L/L)). IFS was increased 140% in CRb(L/L) mice compared to CRb+/+ controls. TUNEL positive nuclei and caspase-3 activity were augmented by 92% and 36%, respectively, following injury in the CRb(L/L) mice demonstrating that loss of Rb in the heart significantly exacerbates I/R injury. These data suggest that Cdk2 signaling pathways are critical regulators of cardiac I/R injury in vivo and support a cardioprotective role for Rb.

E2F6 inhibits cobalt chloride-mimetic hypoxia-induced apoptosis through E2F1

E2F6, a potent transcriptional repressor, plays important roles in cell cycle regulation. However, roles of E2F6 in hypoxia-induced apoptosis are unknown. Here, we demonstrated biological functions of E2F6 in hypoxia-induced apoptosis and regulatory pathways. During hypoxia (CoCl(2), 800 microM)-induced human embryonic kidney 293 cell apoptosis, E2F6 expression was down-regulated with concurrent increases in E2F1 expression and transactivation. E2F6 overexpression abrogated hypoxia-induced apoptosis and alteration of E2F1. Conversely, specific knockdown of E2F6 by small interfering RNA had opposite effects. Chromatin immunoprecipitation assay confirmed that E2F6 regulated E2F1 expression through the transrepression of E2F1 promoter. Interestingly, E2F1 transactivation and apoptosis induced by hypoxia in cells stably expressing E2F1 were inhibited by E2F6 overexpression, suggesting that the inhibitory effects of E2F6 are not only mediated by the repression of E2F1 promoter. This was confirmed by E2F6-inhibited transactivation of E2F1 and apoptosis via competing with E2F1 for DNA binding sites evidenced by the different behaviors of E2F6DeltaC (C-terminal deletion) and E2F6.E68 (mutant DNA binding site) and by the lack of association of E2f6 with E2F1 protein. Moreover, hypoxia up-regulated expression of E2F1-responsive proapoptotic gene apoptosis protease-activating factor 1 was repressed by E2F6 overexpression. Together, these findings demonstrate a novel role of E2F6 in control of hypoxia-induced apoptosis through regulation of E2F1.

Proteomic insights into cardiac cell death and survival

Cardiovascular disease is the leading cause of death and disability in the developed world. To design novel therapeutic strategies to treat and prevent this disease, better understanding of cardiac cell function is necessary. In addition to (and, indeed, in combination with) genetics, physiology and molecular biology, proteomics plays a critical role in our understanding of cardiovascular systems at multiple scales. The purpose of this review is to examine recent developments in the field of myocardial injury and protection, examining how proteomics has informed investigations into organelles, signaling complexes, and cardiac phenotype.

Grifolin, a potential antitumor natural product from the mushroom Albatrellus confluens, induces cell-cycle arrest in G1 phase via the ERK1/2 pathway

Grifolin, a natural product isolated from the mushroom Albatrellus confluens, has been shown to inhibit the growth of some cancer cell lines and induce significant apoptosis. However, the molecular targets and the signaling mechanism underlying the anticancer effect of this compound are not completely understood. Here, we undertook a gene expression profiling study to identify novel targets of grifolin. We found that the effect of grifolin on the human nasopharyngeal carcinoma cell line CNE1 occurs primarily via the ERK1/2 pathway. At high doses, both the ERK1/2 and the ERK5 pathways may be involved in the inhibition. Because inhibition of the ERK1/2 or the ERK5 pathway has been associated with cell-cycle arrest and growth inhibition, we evaluated the cell cycle distribution after grifolin treatment. We found that grifolin significantly caused cell-cycle arrest in G1 phase. To investigate the underlying mechanisms, G1-related proteins were assayed by Western blotting. Following grifolin treatment, a concomitant inhibition of cyclin D1, cyclin E, CDK4 expression, and subsequent reduction in pRB phosphorylation occurred. Meanwhile, grifolin treatment also resulted in a significant upregulation of CKI (p19INK4D). These results suggest that the inhibition of the ERK1/2 or the ERK5 pathway is responsible for at least part of the induction of cell-cycle arrest in G1 phase by grifolin. These results are significant in that they provide a mechanistic framework for further exploring the use of grifolin as a novel antitumor agent.

Forced expression of the cell cycle inhibitor p57Kip2 in cardiomyocytes attenuates ischemia-reperfusion injury in the mouse heart

Background

Myocardial hypoxic-ischemic injury is the cause of significant morbidity and mortality worldwide. The cardiomyocyte response to hypoxic-ischemic injury is known to include changes in cell cycle regulators. The cyclin-dependent kinase inhibitor p57^Kip2 is involved in cell cycle control, differentiation, stress signaling and apoptosis. In contrast to other cyclin-dependent kinase inhibitors, p57^Kip2 expression diminishes during postnatal life and is reactivated in the adult heart under conditions of cardiac stress. Overexpression of p57^Kip2 has been previously shown to prevent apoptotic cell death in vitro by inhibiting stress-activated kinases. Therefore, we hypothesized that p57^Kip2 has a protective role in cardiomyocytes under hypoxic conditions. To investigate this hypothesis, we created a transgenic mouse (R26loxpTA-p57^k/+) that expresses p57^Kip2 specifically in cardiac tissue under the ventricular cardiomyocyte promoter Mlc2v.

Results

Transgenic mice with cardiac specific overexpression of p57^Kip2 are viable, fertile and normally active and their hearts are morphologically indistinguishable from the control hearts and have similar heart weight/body weight ratio. The baseline functional parameters, including left ventricular systolic pressure (LVSP), left ventricular end diastolic pressure (LVEDP), LVdp/dt_max, heart rate (HR) and rate pressure product (RPR) were not significantly different between the different groups as assessed by the Langendorff perfused heart preparation. However, after subjecting the heart ex vivo to 30 minutes of ischemia-reperfusion injury, the p57^Kip2 overexpressing hearts demonstrated preserved cardiac function compared to control mice with higher left ventricular developed pressure (63 ± 15 vs 30 ± 6 mmHg, p = 0.05), rate pressure product (22.8 ± 4.86 vs 10.4 ± 2.1 × 10³bpm × mmHg, p < 0.05) and coronary flow (3.5 ± 0.5 vs 2.38 ± 0.24 ml/min, p <0.05).

Conclusion

These data suggest that forced cardiac expression of p57^Kip2 does not affect myocardial growth, differentiation and baseline function but attenuates injury from ischemia-reperfusion in the adult mouse heart.

The Cell Cycle Factor E2F-1 Activates Bnip3 and the Intrinsic Death Pathway in Ventricular Myocytes

The cell cycle factor E2F-1 is known to regulate a variety of cellular processes including apoptosis. Previously we showed that disruption of Rb–E2F-1 complexes provoked apoptosis of postmitotic adult and neonatal ventricular myocytes; however, the underlying mechanism was undetermined. In this report, we show that E2F-1 provokes cell death of ventricular myocytes through a mechanism that directly impinges on the intrinsic death pathway. Furthermore, we show mechanistically that the hypoxia-inducible death factor Bnip3 is a direct transcriptional target of E2F-1 that is necessary and sufficient for E2F-1–induced cell death. Expression of E2F-1 resulted in a 4.9-fold increase ( P <0.001) in nucleosomal DNA fragmentation and cell death by Hoechst 33258 dye and vital staining. E2F-1 provoked mitochondrial perturbations that were consistent with permeability transition pore opening. As determined by quantitative real-time PCR analysis, a 6.2-fold increase ( P <0.001) in endogenous Bnip3 gene transcription was observed in cells expressing wild-type E2F-1 but not in cells expressing a mutation of E2F-1 defective for DNA binding. Rb, the principle regulator of cellular E2F-1 activity, was proteolytically cleaved and inactivated in ventricular myocytes during hypoxia. Consistent with the proteolytic cleavage of Rb, chromatin immunoprecipitation analysis revealed increased binding of E2F-1 to the Bnip3 promoter during hypoxia, a finding concordant with the induction of Bnip3 gene transcription. The Bnip3 homolog Nix/Bnip3L was unaffected in ventricular myocytes by either E2F-1 or hypoxia. Genetic knockdown of E2F-1 or expression of a caspase-resistant form of Rb suppressed basal and hypoxia-inducible Bnip3 gene transcription. Loss-of-function mutations of Bnip3 defective for mitochondrial membrane insertion or small interference RNA directed against Bnip3 suppressed cell death signals elicited by E2F-1. To our knowledge, the data provide the first direct evidence that activation of the intrinsic mitochondrial death pathway by E2F-1 is mutually dependent on and obligatorily linked to the transcriptional activation of Bnip3.

Phosphatidylinositol 3-Akt-kinase-dependent phosphorylation of p21(Waf1/Cip1) as a novel mechanism of neuroprotection by glucocorticoids

The role of glucocorticoids in the regulation of apoptosis remains incongruous. Here, we demonstrate that corticosterone protects neurons from apoptosis by a mechanism involving the cyclin-dependent kinase inhibitor p21(Waf1/Cip1). In primary cortical neurons, corticosterone leads to a dose- and Akt-kinase-dependent upregulation with enhanced phosphorylation and cytoplasmic appearance of p21(Waf1/Cip1) at Thr 145. Exposure of neurons to the neurotoxin ethylcholine aziridinium (AF64A) results in activation of caspase-3 and a dramatic loss of p21(Waf1/Cip1) preceding apoptosis in neurons. These effects of AF64A are reversed by pretreatment with corticosterone. Corticosterone-mediated upregulation of p21(Waf1/Cip1) and neuroprotection are completely abolished by glucocorticoid and mineralocorticoid receptor antagonists as well as inhibitors of PI3- and Akt-kinase. Both germline and somatically induced p21(Waf1/Cip1) deficiency abrogate the neuroprotection by corticosterone, whereas overexpression of p21(Waf1/Cip1) suffices to protect neurons from apoptosis. We identify p21(Waf1/Cip1) as a novel antiapoptotic factor for postmitotic neurons and implicate p21(Waf1/Cip1) as the molecular target of neuroprotection by high-dose glucocorticoids.

Desferrioxamine (DFX) has genotoxic effects on cultured human lymphocytes and induces the p53-mediated damage response

Desferrioxamine (DFX), which is an iron chelator, mimics hypoxia by enhancing HIF1-alpha accumulation and upregulating inflammatory mediators. DFX is usually beneficial, with preventive effects related primarily to its ability to scavenge reactive oxygen species. However, toxic effects on skeletal and ocular organs have been reported. The cytokinesis block micronucleus test and alkaline single-cell gel (Comet) assay were used to evaluate the genotoxic effects of DFX on human blood lymphocytes. Cultured human lymphocytes treated with 130microM DFX for various periods of time showed significant differences in the incidence of micronucleated binucleate cells, as well as in the length and moment of the comet tail. Western blot analysis using antibodies to proteins involved in the p53-mediated response to DNA damage revealed that p53 was accumulated and DNA damage checkpoint kinases were activated in lymphocytes treated with DFX. On the other hand, the p53 downstream target proteins p21 and bax were not affected, which indicates that DFX does not promote the transactivational activity of p53. Apoptosis assays demonstrated DFX-induced apoptosis of lymphocytes via the caspase cascade. The observed increase in the sub-G1 fraction and enhanced caspase-3 activity indicate that DFX can promote apoptosis in human lymphocytes, and these results were confirmed by protein immunoblot analysis. As apoptotic cell death is preceded by the collapse of the mitochondrial membrane potential, we also measured the mitochondrial membrane potential (Deltapsi(m)) using DiOC6, which is a fluorescent membrane potential probe. The fluorescence intensity of DiOC6 in lymphocytes was significantly reduced in a time-dependent manner after DFX treatment. Taken together, these results indicate that DFX activates p53-mediated checkpoint signals and induces apoptosis via mitochondrial damage in human peripheral blood lymphocytes.

Redox stress and the contributions of BH3-only proteins to infarction

Ischemia followed by reperfusion is the primary cause of tissue injury and infarction during heart attack and stroke. The initiating stimulus is believed to involve reactive oxygen species that are produced during reperfusion when electron transport resumes in the mitochondria after suppression by ischemia. Programmed death has been shown to be a significant component of infarction, and evidence indicates that multiple pathways are initiated during both ischemia and reperfusion phases. Major infarction is preceded by severe ischemia that includes hypoxia, intracellular acidosis, glucose depletion, loss of ATP, and elevation of cytoplasmic calcium. The superimposition of a reactive oxygen surge on the latter condition provides the impetus for maximal damage. Compelling evidence implicates mitochondria not only as the source of initiating ROS but also as the focal sensors that translate the redox stress signal into a cellular-death response. Pivotal to this response are the BH3-only proteins that are activated by death signals and regulate mitochondrial communication with executioner proteins in the cytoplasm. The BH3-only proteins do this by controlling the activity of pores and channels in the outer mitochondrial membrane. To date at least six BH3-only proteins have been shown to contribute to ischemia-reperfusion death pathways in heart and/or brain; these include Bnip3, PUMA, Bid, Bad, HGTD-P, and Noxa. Here we review the evidence for these cell-death pathways and discuss their relevance to ischemic disease and infarction.

Involvement of polyamines in apoptosis of cardiac myoblasts in a model of simulated ischemia

Apoptotic cell death of cardiomyocytes is involved in several cardiovascular diseases including ischemia, hypertrophy, and heart failure. The polyamines putrescine, spermidine, and spermine are polycations absolutely required for cell growth and division. However, increasing evidence indicates that polyamines, cell growth, and cell death can be tightly connected. In this paper, we have studied the involvement of polyamines in apoptosis of H9c2 cardiomyoblasts in a model of simulated ischemia. H9c2 cells were exposed to a condition of simulated ischemia, consisting of hypoxia plus serum deprivation, that induces apoptosis. The activity of ornithine decarboxylase, the rate limiting enzyme of polyamine biosynthesis that synthesizes putrescine, is rapidly and transiently induced in ischemic cells, reaching a maximum after 3 h, and leading to increased polyamine levels. Pharmacological inhibition of ornithine decarboxylase by alpha-difluoromethylornithine (DFMO) depletes H9c2 cardiomyoblasts of polyamines and protects the cells against ischemia-induced apoptosis. DFMO inhibits several of the molecular events of apoptosis that follow simulated ischemia, such as the release of cytochrome c from mitochondria, caspase activation, downregulation of Bcl-xL, and DNA fragmentation. The protective effect of DFMO is lost when exogenous putrescine is provided to the cells, indicating a specific role of polyamine synthesis in the development of apoptosis in this model of simulated ischemia. In cardiomyocytes obtained from transgenic mice overexpressing ornithine decarboxylase in the heart, caspase activation is dramatically increased following induction of apoptosis, with respect to cardiomyocytes from control mice, confirming a proapoptotic effect of polyamines. It is presented for the first time evidence of the involvement of polyamines in apoptosis of ischemic cardiac cells and the beneficial effect of DFMO treatment. In conclusion, this finding may suggest novel pharmacological approaches for the protection of cardiomyocytes injury caused by ischemia.

Gene expression of cyclin-dependent kinase inhibitors and effect of heparin on their expression in mice with hypoxia-induced pulmonary hypertension

The balance between cell proliferation and cell quiescence is regulated delicately by a variety of mediators, in which cyclin-dependent kinases (CDK) and CDK inhibitors (CDKI) play a very important role. Heparin which inhibits pulmonary artery smooth muscle cell (PASMC) proliferation increases the levels of two CDKIs, p21 and p27, although only p27 is important in inhibition of PASMC growth in vitro and in vivo. In the present study we investigated the expression profile of all the cell cycle regulating genes, including all seven CDKIs (p21, p27, p57, p15, p16, p18, and p19), in the lungs of mice with hypoxia-induced pulmonary hypertension. A cell cycle pathway specific gene microarray was used to profile the 96 genes involved in cell cycle regulation. We also observed the effect of heparin on gene expression. We found that (a) hypoxic exposure for two weeks significantly inhibited p27 expression and stimulated p18 activity, showing a 98% decrease in p27 and 81% increase in p18; (b) other CDKIs, p21, p57, p15, p16, and p19 were not affected significantly in response to hypoxia; (c) heparin treatment restored p27 expression, but did not influence p18; (d) ERK1/2 and p38 were mediators in heparin upregulation of p27. This study provides an expression profile of cell cycle regulating genes under hypoxia in mice with hypoxia-induced pulmonary hypertension and strengthens the previous finding that p27 is the only CDKI involved in heparin regulation of PASMC proliferation and hypoxia-induced pulmonary hypertension.

Differential regulation of cardiomyocyte survival and hypertrophy by MDM2, an E3 ubiquitin ligase

MDM2 is an E3 ubiquitin ligase that regulates the proteasomal degradation and activity of proteins involved in cell growth and apoptosis, including the tumor suppressors p53 and retinoblastoma and the transcription factor E2F1. Although the effect of several MDM2 targets on cardiomyocyte survival and hypertrophy has already been investigated, the role of MDM2 in these processes has not yet been established. We have, therefore, analyzed the effect of overexpression as well as inhibition of MDM2 on cardiac ischemia/reperfusion injury and hypertrophy. Here we show that isolated cardiac myocytes overexpressing MDM2 acquired resistance to hypoxia/reoxygenation-induced cell death. Conversely, inactivation of MDM2 by a peptide inhibitor resulted in elevated p53 levels and promoted hypoxia/reoxygenation-induced apoptosis. Consistent with this, decreased expression of MDM2 in a genetic mouse model was accompanied by reduced functional recovery of the left ventricles determined with the Langendorff ex vivo model of ischemia/reperfusion. In contrast to cell survival, cell hypertrophy induced by the alpha-agonists phenylephrine or endothelin-1 was inhibited by MDM2 overexpression. Collectively, our studies indicate that MDM2 promotes survival and attenuates hypertrophy of cardiac myocytes. This differential regulation of cell growth and cell survival is unique, because most other survival factors are prohypertrophic. MDM2, therefore, might be a potential therapeutic target to down-regulate both cell death and pathologic hypertrophy during remodeling upon cardiac infarction. In addition, our data also suggest that cancer treatments with MDM2 inhibitors to reactivate p53 may have adverse cardiac side effects by promoting cardiomyocyte death.

Novel role for cyclin-dependent kinase 2 in neuregulin-induced acetylcholine receptor epsilon subunit expression in differentiated myotubes

Cyclin-dependent kinases (CDKs) are a family of evolutionarily conserved serine/threonine kinases. CDK2 acts as a checkpoint for the G(1)/S transition in the cell cycle. Despite a down-regulation of CDK2 activity in postmitotic cells, many cell types, including muscle cells, maintain abundant levels of CDK2 protein. This led us to hypothesize that CDK2 may have a function in postmitotic cells. We show here for the first time that CDK2 can be activated by neuregulin (NRG) in differentiated C2C12 myotubes. In addition, this activity is required for expression of the acetylcholine receptor (AChR) epsilon subunit. The switch from the fetal AChRgamma subunit to the adult-type AChRepsilon is required for synapse maturation and the neuromuscular junction. Inhibition of CDK2 activity with either the specific CDK2 inhibitory peptide Tat-LFG or by RNA interference abolished neuregulin-induced AChRepsilon expression. Neuregulin-induced activation of CDK2 also depended on the ErbB receptor, MAPK, and PI3K, all of which have previously been shown to be required for AChRepsilon expression. Neuregulin regulated CDK2 activity through coordinating phosphorylation of CDK2 on Thr-160, accumulation of CDK2 in the nucleus, and down-regulation of the CDK2 inhibitory protein p27 in the nucleus. In addition, we also observed a novel mechanism of regulation of CDK2 activity by a low molecular weight variant of cyclin E in response to NRG. These findings establish CDK2 as an intermediate molecule that integrates NRG-activated signals from both the MAPK and PI3K pathways to AChRepsilon expression and reveal an undiscovered physiological role for CDK2 in postmitotic cells.

Gene expression profiles in end-stage human idiopathic dilated cardiomyopathy: altered expression of apoptotic and cytoskeletal genes

Dilated cardiomyopathy is now the leading cause of cardiovascular morbidity and mortality. While the molecular basis of this disease remains uncertain, evidence is emerging that gene expression profiles of left ventricular myocardium isolated from failing versus nonfailing patients differ dramatically. In this study, we use high-density oligonucleotide microarrays with approximately 22000 probes to characterize differences in the expression profiles further. To facilitate interpretation of experimental data, we evaluate algorithms for normalization of hybridization data and for computation of gene expression indices using a control spike-in data set. We then use these methods to identify statistically significant changes in the expression levels of genes not previously implicated in the molecular phenotype of heart failure. These regulated genes take part in diverse cellular processes, including transcription, apoptosis, sarcomeric and cytoskeletal function, remodeling of the extracellular matrix, membrane transport, and metabolism.

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.

Gene expression profiles in children undergoing cardiac surgery for right heart obstructive lesions

BACKGROUND

The global myocardial stress response during cardiac surgery has not been systematically studied, nor is it known whether the response of the neonatal myocardium is intrinsically different from that of older children. To determine the age-related molecular basis of this response, we conducted microarray-based differential gene expression profiling on right ventricular tissue samples acquired in patients of varying ages with right ventricular outflow tract obstruction.

METHODS

We studied gene expression profiles in 24 patients during operations for lesions involving right ventricular outflow tract obstruction age stratified into group I (7 patients, aged 5 to 66 days; mean, 30 days) and group II (17 patients, aged 4 months to 12.5 years; mean, 2.8 years). Myocardial samples were taken from the right ventricular outflow tract after aortic occlusion and archived in liquid nitrogen. RNA isolation, fluorescence labeling of complementary DNA, hybridization to spotted arrays containing 19,008 characterized or unknown human complementary DNAs, and quantitative fluorescence scanning of gene-expression intensity were performed at the University of Toronto Health Network Microarray Centre. Data were analyzed with the Significance Analysis for Microarrays program. Minimum Information About Microarray Experiments-compliant, log2-normalized data sets were compared to ascertain potential statistical differences in gene expression between patient groups.

RESULTS

There were no hospital deaths or major postoperative morbid events. We identified 50 transcripts differentially expressed in the neonatal group (the predicted false discovery rate was <0.8 transcripts). The neonatal pattern of gene expression (group I) was dominated by genes with literature-validated cardioprotective, antihypertrophic, and antiproliferative properties, including increases in atrial natriuretic peptide, protein phosphatase 2A, small GTPase rap1, and protein inhibitor of activated STAT protein, PIASy. Several transcripts have not been previously reported in heart.

CONCLUSIONS

Neonatal myocardium has a unique pattern of gene expression, which may result from developmental (age-related) differences or reflect a more severe disease phenotype independent of age effects per se. The neonatal transcript profile seems to reflect a stress-induced protective program composed of genes with functions diametrically opposed to those expected to be related to the pathogenesis of critical right ventricular outflow tract obstruction, thus revealing a novel and compensatory antidisease transcriptional response in the neonatal heart.

p21Cip1 levels differentially regulate turnover of mature endothelial cells, endothelial progenitor cells, and in vivo neovascularization

p21(Cip1) (p21) controls cell cycle progression and apoptosis in mature endothelial cells (ECs) and regulates size and cycling of the hematopoietic progenitor cell pool. Because circulating endothelial progenitor cells (EPCs) contribute to postnatal neovascularization in addition to mature ECs, we investigated the regulation of ECs and EPCs in p21-deficient mice. Mature aortic EC proliferation was increased in homozygous p21(-/-) and heterozygous p21(+/-) mice, in which p21 protein levels are reduced to one third of wild-type (WT). In contrast, apoptosis sensitivity was increased by 3.5-fold only in p21(-/-), but not in p21(+/-) mice. Consistently, in vivo apoptosis of ECs within areas of neovascularization was elevated in p21(-/-) but not in p21(+/-) mice. EPC numbers were elevated 2-fold in p21(-/-) mice compared with WT (P<0.001), and clonal expansion capacity of EPCs was increased from 25+/-4 (WT) to 57+/-8 colony-forming units in p21(-/-) mice (P<0.005). EPC numbers and expansion were likewise increased in p21(+/-) mice. As the integrative endpoint, in vivo neovascularization reflecting all p21-affected parameters was increased over WT only in p21(+/-) (P<0.001), but not in p21(-/-) mice. In conclusion, reduced p21 protein levels of mice lacking one p21 allele are associated with increased proliferation of ECs and EPCs, whereas survival of ECs to apoptotic stimuli in vitro and in vivo is not impaired. Under these conditions, neovascularization was increased. In contrast, complete p21 deficiency did not result in an increased neovascularization despite increased mature EC and EPC proliferation. This may be due to the sensitization of ECs against apoptosis.