D type cyclins associate with multiple protein kinases and the DNA replication and repair factor PCNA

Low-level laser irradiation stimulates tenocyte proliferation in association with increased NO synthesis and upregulation of PCNA and cyclins

Low-level laser therapy is commonly used to treat tendinopathy or tendon injury. Tendon healing requires tenocyte migration to the repair site, followed by proliferation and synthesis of the extracellular matrix. There are few evidence to elucidate that low-level laser promote tenocyte proliferation. This study was designed to determine the effect of laser on tenocyte proliferation. Furthermore, the association of this effect with secretion of nitric oxide (NO) and the expressions of proliferating cell nuclear antigen (PCNA) and cyclins D1, E, A, and B1 was investigated. Tenocytes intrinsic to rat Achilles tendon were treated with low-level laser (660 nm). Tenocyte proliferation was evaluated by MTT assay and immunocytochemistry with Ki-67 stain. NO in the conditioned medium was measured by ELISA. Western blot analysis was used to evaluate the protein expressions of PCNA and cyclins D1, E, A, and B1. The results revealed that tenocytes proliferation was enhanced dose dependently by laser. NO secretion was increased after laser treatment. PCNA and cyclins E, A, and B1 were upregulated by laser. In conclusion, low-level laser irradiation stimulates tenocyte proliferation in a process that is mediated by upregulation of NO, PCNA, and cyclins E, A, and B1.

Recruitment of trimeric proliferating cell nuclear antigen by G1-phase cyclin-dependent kinases following DNA damage with platinum-based antitumour agents

BACKGROUND

In cycling tumour cells, the binary cyclin-dependent kinase Cdk4/cyclin D or Cdk2/cyclin E complex is inhibited by p21 following DNA damage to induce G1 cell-cycle arrest. However, it is not known whether other proteins are also recruited within Cdk complexes, or their role, and this was investigated.

METHODS

Ovarian A2780 tumour cells were exposed to the platinum-based antitumour agent 1R,2R-diaminocyclohexane(trans-diacetato)(dichloro)platinum(IV) (DAP), which preferentially induces G1 arrest in a p21-dependent manner. The Cdk complexes were analysed by gel filtration chromatography, immunoblot and mass spectrometry.

RESULTS

The active forms of Cdk4 and Cdk2 complexes in control tumour cells have a molecular size of ~140 kDa, which increased to ~290 kDa when inhibited following G1 checkpoint activation by DAP. Proteomic analysis identified Cdk, cyclin, p21 and proliferating cell nuclear antigen (PCNA) in the inhibited complex, and biochemical studies provided unequivocal evidence that the increase in ~150 kDa of the inhibited complex is consistent with p21-dependent recruitment of PCNA as a trimer, likely bound to three molecules of p21. Although p21 alone was sufficient to inhibit the Cdk complex, PCNA was critical for stabilising p21.

CONCLUSION

G1 Cdk complexes inhibited by p21 also recruit PCNA, which inhibits degradation and, thereby, prolongs activity of p21 within the complex.

Systematic analyses of the cytotoxic effects of compound 11a, a putative synthetic agonist of photoreceptor-specific nuclear receptor (PNR), in cancer cell lines

Photoreceptor cell-specific receptor (PNR/NR2E3) is an orphan nuclear receptor that plays a critical role in retinal development and photoreceptor maintenance. The disease-causing mutations in PNR have a pleiotropic effect resulting in varying retinal diseases. Recently, PNR has been implicated in control of cellular functions in cancer cells. PNR was reported to be a novel regulator of ERα expression in breast cancer cells, and high PNR expression correlates with favorable response to tamoxifen treatment. Moreover, PNR was shown to increase p53 stability in HeLa cells, implying that PNR may be a therapeutic target in this and other cancers that retain a wild type p53 gene. To facilitate further understanding of PNR functions in cancer, we characterized compound 11a, a synthetic, putative PNR agonist in several cell-based assays. Interestingly, we showed that 11a failed to activate PNR and its cytotoxicity was independent of PNR expression, excluding PNR as a mediator for 11a cytotoxicity. Systematic analyses of the cytotoxic effects of 11a in NCI-60 cell lines revealed a strong positive correlation of cytotoxicity with p53 status, i.e., p53 wild type cell lines were significantly more sensitive to 11a than p53 mutated or null cell lines. Furthermore, using HCT116 p53+/+ and p53-/- isogenic cell lines we revealed that the mechanism of 11a-induced cytotoxicity occurred through G₁/S phase cell cycle arrest rather than apoptosis. In conclusion, we observed a correlation of 11a sensitivity with p53 status but not with PNR expression, suggesting that tumors expressing wild type p53 might be responsive to this compound.

The nuclear accumulation of alpha-synuclein is mediated by importin alpha and promotes neurotoxicity by accelerating the cell cycle

α-Synuclein (α-syn), a 14 kDa pre-synaptic protein, is widely involved in the Parkinson's disease (PD) pathogenesis. Recent studies have shown that the nuclear accumulation of α-syn might have a toxic effect. The main purpose of the present study was to explore which amino acid residues in α-syn are associated with its nuclear accumulation, the molecule(s) mediated the nuclear import of α-syn, and the role of α-syn accumulated in the nucleus. It has been noted that the nuclear import of α-syn may be mediated by importin α and that both the amino acid residues 1-60 and 103-140 of α-syn were indispensable for its nuclear import. After imported into the nucleus, the accumulated α-syn played a toxic role in both the PC12 cells and the C57 mice. Furthermore, α-syn-nuclear localization signal-injected mice showed behavioral symptoms associated with PD. Further studies performed in vitro showed that the toxicity of α-syn in the nucleus might be due to an interference of the cell cycle. Thus, it can be concluded that α-syn can accumulate in nucleus, which is mediated by importin α, and promote neurotoxicity by accelerating the cell cycle.

The role of cyclin D1 in response to long-term exposure to ionizing radiation

The health-related hazards resulting from long-term exposure to radiation remain unknown. Thus, an appropriate molecular marker is needed to clarify these effects. Cyclin D1 regulates the cell cycle transition from the G 1 phase to the S phase. Cyclin D1 is degraded as a G1/S checkpoint after 10 Gy of single acute radiation exposure, whereas conversely, cyclin D1 is stabilized when human tumor cells are exposed to fractionated radiation (FR) with 0.5 Gy of x-rays for 31 d. In this article, we review new findings regarding cyclin D1 overexpression in response to long-term exposure to FR. Cyclin D1 overexpression is associated with induction of genomic instability in irradiated cells. Therefore, repression of cyclin D1 expression is likely to cancel the harmful effects of long-term exposure to FR. Thus cyclin D1 may be a marker of long-term exposure to radiation and is a putative molecular radioprotection target for radiation safety.

SMAD3 regulates the diverse functions of rat granulosa cells relating to the FSHR/PKA signaling pathway

The function of Smad3, a downstream signaling protein of the transforming growth factor β (TGFβ) pathway, in ovarian follicle development remains to be elucidated. The effects of Smad3 on ovarian granulosa cells (GCs) in rat were studied. Female rats (21 days of age Sprague-Dawley) received i.p. injections of pregnant mare serum gonadotropin, and GCs were harvested for primary culture 48 h later. These cells were engineered to overexpress or knockdown Smad3, which were validated by immunohistochemistry and western blot. The expression of proliferating cell nuclear antigen (PCNA), cyclin D2, TGFβ receptor II (TGFβRII), protein kinase A (PKA), and FSH receptor (FSHR) was also detected by western blotting. Cell cycle and apoptosis of GCs were assayed by flow cytometry. The level of estrogen secreted by GCs was detected by ELISA. Smad3 overexpression promoted estrogen production and proliferation while inhibiting apoptosis of GCs. Reduction in Smad3 by RNAi resulted in reduced estrogen production and proliferation and increased apoptosis of GCs. Manipulation of Smad3 expression also resulted in changes in FSHR and PKA expression, suggesting that the effects of Smad3 on follicle development are related to FSHR-mediated cAMP signaling.

Cyclin D1 cooperates with p21 to regulate TGFβ-mediated breast cancer cell migration and tumor local invasion

Introduction

Deregulation of the cell cycle machinery is often found in human cancers. Modulations in the cell cycle regulator function and expression result not only in proliferative advantages, but also lead to tumor progression and invasiveness of the cancer. In particular, cyclin D1 and p21 are often over-expressed in human cancers, correlating with high tumor grade, poor prognosis and increased metastasis. This prompted us to investigate the role of the cyclin D1/p21 signaling axis downstream of transforming growth factor beta (TGFβ) in breast cancer progression.

Methods

Cyclins mRNA and protein expressions were assessed by quantitative real-time PCR and Western blot in triple negative breast cancer cell lines. Co-localization and interaction between cyclin D1 and p21 were performed by immunocytochemistry and co-immunoprecipitation, respectively. Cell migration was assessed by wound healing and quantitative time-lapse imaging assays. In addition, the effects of cyclin D1 on cellular structure and actin organization were examined by staining with F-actin marker phalloidin and mesenchymal intermediate filament vimentin. Finally, a mammary fat pad xenograft mouse model was used to assess mammary tumor growth and local invasion.

Results

We found TGFβ to specifically up-regulate the expression of cyclin D1 in triple negative breast cancer cells. Induction of cyclin D1 is also required for TGFβ-mediated cell migration. Suppression of cyclin D1 expression not only resulted in a rounded and epithelial-like phenotype, but also prevented TGFβ-induced vimentin and F-actin co-localization at the cell edge as well as invadopodia formation. Furthermore, TGFβ promoted the nuclear co-localization and physical interaction between cyclin D1 and p21. The co-expression of cyclin D1 and p21 proteins are required for the initial steps of tumor development, as double knockdown of these two molecules prevented primary tumor formation in a Xenograft mouse model. Moreover, the in vivo studies indicated that locally advanced features of the invasive tumors, including skeletal muscle, mammary fat pad and lymphovascular invasion, as well as ulcerated skin, were attenuated in the absence of cyclin D1 and p21.

Conclusions

Thus, our findings highlight the cyclin D1/p21 signaling axis as a critical regulator of TGFβ-mediated tumor growth initiation and local tumor cell invasion, both in vitro and in vivo.

Sex sorting increases the permeability of the membrane of stallion spermatozoa

At present, the only repeatable means of selecting the sex of offspring is the Beltsville semen sorting technology using flow cytometry (FC). This technology has reached commercial status in the bovine industry and substantial advances have occurred recently in swine and ovine species. In the equine species, however, the technology is not as well developed. To better understand the changes induced in stallion spermatozoa during the sorting procedure, pooled sperm samples were sorted: sperm motility and kinematics were assessed using computer assisted sperm analysis, sperm membrane integrity was assessed using the YoPro-1 assay, while plasmalemmal stability and lipid architecture were assessed using Merocyanine 540/SYTOX green and Annexin-V, respectively. Lipid peroxidation was also investigated with the probe Bodipy(581/591)-C11. All assays were performed shortly after collection, after incubation and after sex sorting using FC. In order to characterize potential molecular mechanisms implicated in sperm damage, an apoptosis protein antibody dot plot array analysis was performed before and after sorting. While the percentage of total motile sperm remained unchanged, sex sorting reduced the percentages of progressive motile spermatozoa and of rapid spermatozoa as well as curvilinear velocity (VCL). Sperm membranes responded to sorting with an increase in the percentage of YoPro-1 positive cells, suggesting the sorted spermatozoa had a reduced energy status that was confirmed by measuring intracellular ATP content.

Signaling pathways that control cell proliferation

Cells decide to proliferate or remain quiescent using signaling pathways that link information about the cellular environment to the G1 phase of the cell cycle. Progression through G1 phase is controlled by pRB proteins, which function to repress the activity of E2F transcription factors in cells exiting mitosis and in quiescent cells. Phosphorylation of pRB proteins by the G1 cyclin-dependent kinases (CDKs) releases E2F factors, promoting the transition to S phase. CDK activity is primarily regulated by the binding of CDK catalytic subunits to cyclin partners and CDK inhibitors. Consequently, both mitogenic and antiproliferative signals exert their effects on cell proliferation through the transcriptional regulation and ubiquitin-dependent degradation of cyclins and CDK inhibitors.

Cyclin D1 overexpression perturbs DNA replication and induces replication-associated DNA double-strand breaks in acquired radioresistant cells

Fractionated radiotherapy (RT) is widely used in cancer treatment, because it preserves normal tissues. However, repopulation of radioresistant tumors during fractionated RT limits the efficacy of RT. We recently demonstrated that a moderate level of long-term fractionated radiation confers acquired radioresistance to tumor cells, which is caused by DNA-PK/AKT/GSK3β-mediated cyclin D1 overexpression. The resulting cyclin D1 overexpression leads to forced progression of the cell cycle to S-phase, concomitant with induction of DNA double-strand breaks (DSBs). In this study, we investigated the molecular mechanisms underlying cyclin D1 overexpression-induced DSBs during DNA replication in acquired radioresistant cells. DNA fiber data demonstrated that replication forks progressed slowly in acquired radioresistant cells compared with corresponding parental cells in HepG2 and HeLa cell lines. Slowly progressing replication forks were also observed in HepG2 and HeLa cells that overexpressed a nondegradable cyclin D1 mutant. We also found that knockdown of Mus81 endonuclease, which is responsible for resolving aberrant replication forks, suppressed DSB formation in acquired radioresistant cells. Consequently, Mus81 created DSBs to remove aberrant replication forks in response to replication perturbation triggered by cyclin D1 overexpression. After treating cells with a specific inhibitor for DNA-PK or ATM, apoptosis rates increased in acquired radioresistant cells but not in parental cells by inhibiting the DNA damage response to cyclin D1-mediated DSBs. This suggested that these inhibitors might eradicate acquired radioresistant cells and improve fractionated RT outcomes.

Proliferating cell nuclear antigen (PCNA) interactions in solution studied by NMR

PCNA is an essential factor for DNA replication and repair. It forms a ring shaped structure of 86 kDa by the symmetric association of three identical protomers. The ring encircles the DNA and acts as a docking platform for other proteins, most of them containing the PCNA Interaction Protein sequence (PIP-box). We have used NMR to characterize the interactions of PCNA with several other proteins and fragments in solution. The binding of the PIP-box peptide of the cell cycle inhibitor p21 to PCNA is consistent with the crystal structure of the complex. A shorter p21 peptide binds with reduced affinity but retains most of the molecular recognition determinants. However the binding of the corresponding peptide of the tumor suppressor ING1 is extremely weak, indicating that slight deviations from the consensus PIP-box sequence dramatically reduce the affinity for PCNA, in contrast with a proposed less stringent PIP-box sequence requirement. We could not detect any binding between PCNA and the MCL-1 or the CDK2 protein, reported to interact with PCNA in biochemical assays. This suggests that they do not bind directly to PCNA, or they do but very weakly, with additional unidentified factors stabilizing the interactions in the cell. Backbone dynamics measurements show three PCNA regions with high relative flexibility, including the interdomain connector loop (IDCL) and the C-terminus, both of them involved in the interaction with the PIP-box. Our work provides the basis for high resolution studies of direct ligand binding to PCNA in solution.

Tbx5-hedgehog molecular networks are essential in the second heart field for atrial septation

The developmental mechanisms underlying human congenital heart disease (CHD) are poorly understood. Atrial septal defects (ASDs) can result from haploinsufficiency of cardiogenic transcription factors including TBX5. We demonstrated that Tbx5 is required in the second heart field (SHF) for atrial septation in mice. Conditional Tbx5 haploinsufficiency in the SHF but not the myocardium or endocardium caused ASDs. Tbx5 SHF knockout embryos lacked atrial septum progenitors. We found that Tbx5 mutant SHF progenitors demonstrated cell-cycle progression defects and that Tbx5 regulated cell-cycle progression genes including Cdk6. Activated hedgehog (Hh) signaling rescued ASDs in Tbx5 mutant embryos, placing Tbx5 upstream or parallel to Hh in cardiac progenitors. Tbx5 regulated SHF Gas1 and Osr1 expression, supporting both pathways. These results describe a SHF Tbx5-Hh network required for atrial septation. A paradigm defining molecular requirements in SHF cardiac progenitors for cardiac septum morphogenesis has implications for the ontogeny of CHD.

Targeting cytosolic proliferating cell nuclear antigen in neutrophil-dominated inflammation

New therapeutic approaches that can accelerate neutrophil apoptosis under inflammatory conditions to enhance the resolution of inflammation are now under study. Neutrophils are deprived of proliferative capacity and have a tightly controlled lifespan to avoid their persistence at the site of injury. We have recently described that the proliferating cell nuclear antigen (PCNA), a nuclear factor involved in DNA replication and repair of proliferating cells is a key regulator of neutrophil survival. The nuclear-to-cytoplasmic relocalization occurred during granulocytic differentiation and is dependent on a nuclear export sequence thus strongly suggesting that PCNA has physiologic cytoplasmic functions. In this review, we will try to put into perspective the physiologic relevance of PCNA in neutrophils. We will discuss key issues such as molecular structure, post-translational modifications, based on our knowledge of nuclear PCNA, assuming that similar principles governing its function are conserved between nuclear and cytosolic PCNA. The example of cystic fibrosis that features one of the most intense neutrophil-dominated pulmonary inflammation will be discussed. We believe that through an intimate comprehension of the cytosolic PCNA scaffold based on nuclear PCNA knowledge, novel pathways regulating neutrophil survival can be unraveled and innovative agents can be developed to dampen inflammation where it proves detrimental.

Expression of cyclin DI in human prostate cancer cell lines

Deregulation of cyclin expression has been found in many tumors. In this report, we studied expression of cyclin DI in three human prostate cancer cell lines: the androgen-dependent LNCaP and the androgen-independent PC3 and DU 145 cell lines. Northern blot analysis showed that DU145 and PC3 cells expressed more abundant cyclin DI than LNCaP cells. Southern blot analysis showed no evident gene amplification or rearrangement of cyclin DI in any of these cell lines. Serum starvation and replenishment were used in the cell culture to study the regulation of expression of cyclin DI. Cyclin DI mRNA expression was detected by Northern blot analysis when LNCaP cells grew in medium with serum but was not detected after serum withdrawal; however, cyclin DI mRNA was induced after serum was added. Cyclin DI mRNA expression by PC3 and DU 145 cells was detected both when they grew in medium with serum and after serum withdrawal, although expression decreased greatly after 24 hours in the PC3 cell line. Immunoprecipitation and immunohistochemical staining also showed that cyclin D I protein was always expressed in PC3 and DU 145 cells under different growth factor environment, whereas it decreased significantly in LNCaP cells deprived of serum and the level resumed again when serum was re-added. This suggests that expression of cyclin DI is regulated by exogenous growth factors in LNCaP cell line and becomes constitutive in PC3 and DU 145 cell lines.

Proteomic identification of a direct role for cyclin d1 in DNA damage repair

The human CCND1 gene, which encodes the cell-cycle protein cyclin D1, is one of the most frequently amplified genes in human cancers. Cyclin D1 activates the cyclin-dependent kinases CDK4 and CDK6 and drives cell proliferation. Beyond the cell-cycle role, the full repertoire of cyclin D1 functions in cancer cells is still unclear. Emerging evidence indicates that cyclin D1 may play a role in DNA damage response. In this review, we discuss observations linking cyclin D1 to DNA damage repair and summarize our recent findings, which show a cyclin D1 function in homologous recombination-mediated DNA repair.

Interplay between MEK-ERK signaling, cyclin D1, and cyclin-dependent kinase 5 regulates cell cycle reentry and apoptosis of neurons

In response to neurotoxic signals, postmitotic neurons make attempts to reenter the cell cycle, which results in their death. Although several cell cycle proteins have been implicated in cell cycle-related neuronal apoptosis (CRNA), the molecular mechanisms that underlie this important event are poorly understood. Here, we demonstrate that neurotoxic agents such as β-amyloid peptide cause aberrant activation of mitogen-activated kinase kinase (MEK)-extracellular signal-regulated kinase (ERK) signaling, which promotes the entry of neurons into the cell cycle, resulting in their apoptosis. The MEK-ERK pathway regulates CRNA by elevating the levels of cyclin D1. The increase in cyclin D1 attenuates the activation of cyclin-dependent kinase 5 (cdk5) by its neuronal activator p35. The inhibition of p35-cdk5 activity results in enhanced MEK-ERK signaling, leading to CRNA. These studies highlight how neurotoxic signals reprogram and alter the neuronal signaling machinery to promote their entry into the cell cycle, which eventually leads to neuronal cell death.

Extraneuronal activities and regulatory mechanisms of the atypical cyclin-dependent kinase Cdk5

Cyclin-dependent kinase, Cdk5, is an atypical but essential member of the Cdk family of proline-directed serine/threonine kinases with no evident role in cell cycle progression. Cdk5 is present in post-mitotic and terminally differentiated neuronal/glial cells and is also known to arrest cell cycle. Also atypical is the activation of Cdk5 by binding of a non-cyclin activator protein, namely, the Cdk5 regulatory proteins Cdk5R1 (p35), truncated Cdk5R1 (p25), or Cdk5R2 (p39). Despite its ubiquitous presence in all cells and tissues, Cdk5 is often referred to as a neuron-specific kinase largely due to the abundant presence of the activator proteins in neuronal cells. Recently, this concept of a canonical neuronal function of Cdk5 has been extended, if not challenged, by the observation of p35 and p39 expression, as well as Cdk5 activity, in multiple non-neuronal cells. Extraneuronal Cdk5 regulates critical biological processes including transcript-selective translation control for regulation of macrophage gene expression, glucose-inducible insulin secretion, hematopoietic cell differentiation, vascular angiogenesis, cell migration, senescence, and wound-healing, among others. Recent advances in the extraneuronal functions of Cdk5 are reviewed and discussed here in the context of their physiological activities and pathophysiological implications with some speculative comments on the endogenous control mechanisms that might "turn on" Cdk5 activity. The potential importance of targeted inhibition of Cdk5 as therapeutic agents against glucotoxicity, diabetes, cardiovascular diseases, and cancer is also discussed.

TRIP-1: a regulator of osteoblast function

Transforming growth factor β (TGFβ) receptor interacting protein-1 (TRIP-1) is an intracellular protein expressed in osteoblasts with high affinity for type 5b tartrate resistant acid phosphatase (TRAP). It is suggested that through this interaction, TRIP-1 serves as a positive regulator of TGFβ signaling and osteoblast differentiation during bone remodeling. We show here that TRIP-1 is abundant in osteoblasts in vivo and in vitro. TRIP-1 mRNA and protein expression were increased at early stages and decreased at later stages during osteoblast differentiation, suggesting a predominant role during early maturation. To investigate a role during bone remodeling, primary osteoblasts were treated with different hormones and factors that are known to affect remodeling. TRIP-1 levels were decreased with dexamethasone and increased with vitamin D(3) , dihydrotestosterone (DHT), TGFβ1, and bone morphogenic protein 2 (BMP-2). Treatment with parathyroid hormone (PTH) and β-estradiol did not affect TRIP-1 levels. Transfected small interfering RNA (siRNA) against TRIP-1 inhibited osteoblast differentiation as characterized by a decrease in alkaline phosphatase staining and enzyme activity, and decrease in the expression of collagen I, alkaline phosphatase, Runx2, osteopontin, and osteocalcin. The proliferation of osteoblasts was also affected by TRIP-1 siRNA. This particular effect was defined by decreased cell number, marked reduction of cyclin D1, a 38% decrease of cells in S phase (p < 0.001) and a 97% increase of cells in the G2/M phase (p < 0.01) of the cell cycle. However, TRIP-1 siRNA did not induce an effect in apoptosis. Using a TGFβ luciferase reporter we found that knocking down TRIP-1 decreased the activation of TGFβ signaling by 40% percent (p < 0.001). In conclusion, our characterization of TRIP-1 in osteoblasts provides the first evidence of its key role as a positive regulator of osteoblast function.

Characterization of cells resistant to the potent histone deacetylase inhibitor spiruchostatin B (SP-B) and effect of overexpressed p21waf1/cip1 on the SP-B resistance or susceptibility of human leukemia cells

We previously showed that the B cell leukemia cell line NALM-6 had the highest susceptibility among a number of leukemia cell lines to spiruchostatin B (SP-B), a potent histone deacetylase (HDAC) inhibitor. We also showed that SP-B-induced cytotoxicity depended on induction of apoptosis that was mediated by p21^waf1/cip1 expression. In the present study, we generated and characterized a stable, SP-B-resistant NALM-6 cell line (NALM-6/SP-B) by continuous exposure to SP-B, starting with a low SP-B concentration. NALM-6/SP-B cells were also more resistant to FK228, which has a similar chemical structure to SP-B, and were slightly more resistant to the P-gp substrates doxorubicin and vincristine than parental cells, but displayed similar susceptibility to other HDAC inhibitors and to paclitaxel as the parental cells. There was little change in the basal mRNA expression of HDAC1, p53, Bax, Bcl-2, Fas, caspase-3, c-Myc and MDR1 in NALM-6/SP-B compared to parental cells, but the mRNA expression of p21^waf1/cip1 was decreased. The introduction of an exogenous p21^waf1/cip1 expression vector restored SP-B induction of NALM-6/SP-B cell apoptosis. Moreover, overexpressed p21^waf1/cip1 enhanced SP-B induction of the apoptosis of the human erythroleukemia leukemia cell line K562 which is less susceptible to SP-B than NALM-6 cells. These results suggest that downregulation of p21^waf1/cip1, which is a characteristic feature of NALM-6/SP-B cells, was important for their resistance to SP-B, and that this SP-B resistance could be overcome by the introduction of exogenous p21^waf1/cip1. Furthermore, introduction of p21^waf1/cip1 to other leukemia cells such as K562 may enhance their susceptibility to SP-B. This is the first report of the characterization of SP-B-resistant cells and of the effect of overexpressed p21^waf1/cip1 on the resistance or susceptibility of human leukemia cells to SP-B.

Cell cycle control by anchorage signaling

Virtually all the cells constituting solid organs in adult animals require anchorage to the extracellular matrix for their proliferation and survival. When deprived of anchorage, those cells arrest in G(1) phase of the cell cycle and die of apoptosis known as anoikis. However, if malignantly transformed, cells no longer require such an anchorage to proliferate and survive, and it is generally thought that the acquirement of this ability underlies the tumorigenic and metastatic capability of malignant cells. Therefore, for the past two decades, great efforts have been devoted to uncovering the nature of the anchorage signal and the mechanism by which this signal controls the G(1)-S transition in the cell cycle with little progress. However, several critical findings were recently made on anchorage signaling and the control of the cell cycle and cell death by this signaling. This review focuses on the newly emerging understanding and perspective of this highly important cell cycle and cell death regulation.

Immunolocalization of PCNA, Ki67, p27 and p57 in normal and dexamethasone-induced intrauterine growth restriction placental development in rat

Intrauterine growth restriction (IUGR) is a major clinical problem which causes perinatal morbidity and mortality. Although fetuses with IUGR form a heterogeneous group, a major etiological factor is abnormal placentation. Despite the fact that placental development requires the coordinated action of trophoblast proliferation and differentiation, there are few studies on cell cycle regulators, which play the main roles in the coordination of these events. Moreover it is still not determined how mechanisms of coordination of proliferation and differentiation are influenced by dexamethasone-induced IUGR in the placenta. The aim of the study was to investigate the spatial and temporal immunolocalization of proliferating cell nuclear antigen (PCNA), Ki67, p27 and p57 in normal and IUGR placental development in pregnant Wistar rats. The study demonstrated altered expressions of distinct cell cycle proteins and cyclin dependent kinase inhibitors (CKIs) in IUGR placental development compared to control placental development. We found reduced immunostaining of PCNA and Ki67 and increased immunostaining of p27 and p57 in the dexamethasone-induced IUGR placental development compared to control placental development. In conclusion, our data show that the cell populations in the placenta stain for a number of cell cycle related proteins and that these staining patterns change as a function of both gestational age and abnormal placentation.

Antiapoptotic and proliferative activity of curcumin on ovarian follicles in mice exposed to whole body ionizing radiation

The aim of this study was to evaluate the antiapoptotic and proliferative activity of curcumin (Cur) on the ovarian follicles in mice exposed to whole body ionizing radiation (Rd). The mice were exposed to 8.3 gray whole body Rd, and Cur groups were given as a daily dose of 100 mg/kg of Cur for 10 days (10 days before Rd). The ovaries were collected 3 and 12 h after irradiation. To date, no such studies have been performed on antiapoptotic and proliferative activity of Cur on the ovarian follicles in mice exposed to whole body Rd. Analysis of mice ovary after exposure to Rd by terminal-deoxynucleotidyl-transferase-mediated dUTP nick end labeling showed that there were apoptotic cells both in the follicular wall and the antrum, and that the number of follicles showing early atresic features was high 3 h after Rd. On the other hand, analysis of mice ovary 12 h after exposure to Rd showed that the number of follicles containing apoptotic cells with advanced atresic features was significantly higher when compared to the 3-h Rd exposure group. The proliferating cell nuclear antigen -positive granulosa cells were decreased in association with follicular atresia. The groups given treatment were observed to have some benefit from Cur against the damage caused by Rd. The results of this study demonstrate that Cur prevents follicular atresia in Rd-induced apoptosis in ovarian follicles.

A novel tyrosine kinase inhibitor restores chondrocyte differentiation and promotes bone growth in a gain-of-function Fgfr3 mouse model

Activating germline fibroblast growth factor receptor 3 (FGFR3) mutations cause achondroplasia (ACH), the most common form of human dwarfism and a spectrum of skeletal dysplasias. FGFR3 is a tyrosine kinase receptor and constitutive FGFR3 activation impairs endochondral ossification and triggers severe disorganization of the cartilage with shortening of long bones. To decipher the role of FGFR3 in endochondral ossification, we analyzed the impact of a novel tyrosine kinase inhibitor (TKI), A31, on both human and mouse mutant FGFR3-expressing cells and on the skeleton of Fgfr3(Y367C/+) dwarf mice. We found that A31 inhibited constitutive FGFR3 phosphorylation and restored the size of embryonic dwarf femurs using an ex vivo culture system. The increase in length of the treated mutant femurs was 2.6 times more than for the wild-type. Premature cell cycle exit and defective chondrocyte differentiation were observed in the Fgfr3(Y367C/+) growth plate. A31 restored normal expression of cell cycle regulators (proliferating cell nuclear antigen, KI67, cyclin D1 and p57) and allowed pre-hypertrophic chondrocytes to properly differentiate into hypertrophic chondocytes. Our data reveal a specific role for FGFR3 in the cell cycle and chondrocyte differentiation and support the development of TKIs for the treatment of FGFR3-related chondrodysplasias.

Going out of the brain: non-nervous system physiological and pathological functions of Cdk5

Cyclin-dependent kinase 5 (Cdk5) is a proline-directed serine/threonine kinase that is mostly active in the nervous system, where it regulates several processes such as neuronal migration, actin and microtubule dynamics, axonal guidance, and synaptic plasticity, among other processes. In addition to these known functions, in the past few years, novel roles for Cdk5 outside of the nervous system have been proposed. These include roles in gene transcription, vesicular transport, apoptosis, cell adhesion, and migration in many cell types and tissues such as pancreatic cells, muscle cells, neutrophils, and others. In this review, we will summarize the recently studied non-neuronal functions of Cdk5, with a thorough analysis of the biological consequences of these novel roles.

shRNA targeting PLCε inhibits bladder cancer cell growth in vitro and in vivo

OBJECTIVES

To investigate the role of phospholipase Cε (PLCε) by silencing PLCε with short hairpin RNA (shRNA) in human bladder cancer cells BIU-87 in vitro and in vivo.

METHODS

A PLCε shRNA expression vector was transfected into BIU-87 cells, and the expression of PLCε protein was detected by Western blotting. Cell proliferation was determined using the MTT assay, and the cell cycle was detected using flow cytometry. A tumor xenograft experiment was established to evaluate the tumor growth under the condition of PLCε knockdown, and the expression of PLCε, proliferating cell nuclear antigen, and cyclin D1 were detected by Western blotting or immunohistrochemistry.

RESULTS

PLCε shRNA reduced the protein level of PLCε, leading to marked proliferation inhibition and significant cell cycle arrest. Furthermore, PLCε shRNA reduced the tumor xenograft growth implanted with BIU-87 cells. The protein expression of PLCε, proliferating cell nuclear antigen, and cyclin D1 were downregulated in the bladder tumor xenograft.

CONCLUSIONS

The knockdown of PLCε by shRNA could inhibit bladder tumor growth and might be an alternative approach for human bladder cancer therapy.

Cdk5: mediator of neuronal development, death and the response to DNA damage

In the central nervous system, cyclin-dependent kinase 5 (Cdk5), an unusual member of the Cdk family, is implicated in the regulation of various physiological processes ranging from neuronal survival, migration and differentiation, to synaptogenesis, synaptic plasticity and neurotransmission. Dysregulation of this kinase has been demonstrated to play a critical role in the pathogenic process of neurodegenerative disorders. DNA damage is emerging as an important pathological component in various neurodegenerative conditions. In this review, we discuss the recent progress regarding the regulation and roles of Cdk5 under physiological conditions, and its dysregulation under pathological conditions, especially in neuronal death mediated by DNA damage.

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.

Cdk2-dependent phosphorylation of p21 regulates the role of Cdk2 in cisplatin cytotoxicity

Cisplatin cytotoxicity is dependent on cyclin-dependent kinase 2 (Cdk2) activity in vivo and in vitro. We found that an 18-kDa protein identified by mass spectrometry as p21(WAF1/Cip1) was phosphorylated by Cdk2 starting 12 h after cisplatin exposure. The analysis showed it was phosphorylated at serine 78, a site not previously identified. The adenoviral transduction of p21 before cisplatin exposure protects from cytotoxicity by inhibiting Cdk2. Although cisplatin causes induction of endogenous p21, the protection is inefficient. We hypothesized that phosphorylation of p21 at serine 78 could affect its role as a Cdk inhibitor, and thereby lessen its ability to protect from cisplatin cytotoxicity. To investigate the effect of serine 78 phosphorylation on p21 activity, we replaced serine 78 with aspartic acid, creating the phosphomimic p21(S78D). Mutant p21(S78D) was an inefficient inhibitor of Cdk2 and was inefficient at protecting TKPTS cells from cisplatin-induced cell death. We conclude that phosphorylation of p21 by Cdk2 limits the effectiveness of p21 to inhibit Cdk2, which is the mechanism for continued cisplatin cytotoxicity even after the induction of a protective protein.

The G1 phase Cdks regulate the centrosome cycle and mediate oncogene-dependent centrosome amplification

Because centrosome amplification generates aneuploidy and since centrosome amplification is ubiquitous in human tumors, a strong case is made for centrosome amplification being a major force in tumor biogenesis. Various evidence showing that oncogenes and altered tumor suppressors lead to centrosome amplification and aneuploidy suggests that oncogenes and altered tumor suppressors are a major source of genomic instability in tumors, and that they generate those abnormal processes to initiate and sustain tumorigenesis. We discuss how altered tumor suppressors and oncogenes utilize the cell cycle regulatory machinery to signal centrosome amplification and aneuploidy.

Role of Cell Division Autoantigen 1 (CDA1) in Cell Proliferation and Fibrosis

Cell Division Autoantigen 1 (CDA1) was discovered following screening a human expression library with serum from a patient with Discoid Lupus Erythematosus. CDA1, encoded by TSPYL2 on the X chromosome, shares anti-proliferative, pro‑fibrotic properties with TGF-β. It inhibits cell growth through p53, pERK1/2, p21‑mediated pathways, is implicated in tumorigenesis, the DNA damage response. Its pro-fibrotic property is mediated through cross-talk with TGF-β that results in upregulation of extracellular matrix proteins. The latter properties have identified a key role for CDA1 in diabetes associated atherosclerosis. These dual properties place CDA1 as an attractive molecular target for treating tumors, vascular fibrosis including atherosclerosis, other vascular disorders associated with enhanced TGF-β action, tissue scarring.

A proposed conserved role for an avocado FW2.2-like gene as a negative regulator of fruit cell division

Previous studies using 'Hass' avocado and its small fruit (SF) phenotype as a model showed that SF is limited by cell number, not by cell size. In an attempt to explore the molecular mechanisms regulating avocado fruit cell division, we isolated four distinct avocado cell proliferation-related genes and investigated their expression characteristics, comparing normal fruit (NF) and SF developmental patterns. Three cDNAs termed PaCYCA1, PaCYCB1 and PaPCNA, encoding two mitotic cyclins and a proliferating cell nuclear antigen (PCNA), were first isolated from young NF tissues. The accumulation of their transcripts was predominant in mitotically active organs, including young fruitlets, leaves and roots. Furthermore, a fourth full-length cDNA, designated Pafw2.2-like, encoding a FW2.2 (fruit-weight)-like protein, was isolated from SF tissues. FW2.2 is postulated to function as a negative regulator of cell division in tomato fruit. Remarkably, northern analysis revealed that the accumulation of the mitotic cyclins and of PCNA transcripts gradually decreased in NF tissues during growth, whereas in SF, their levels had already decreased at earlier stages of fruit development, concomitant with an earlier arrest of fruit cell division activity. In contrast, parallel sq-RT-PCR analysis showed that Pafw2.2-like mRNA accumulation was considerably higher in SF tissues than in the same NF tissues essentially at all examined stages of fruit growth. Together, our data suggest essential roles for the two mitotic cyclins genes and the PCNA gene in regulating avocado fruit development. Furthermore, the possibility that Pafw2.2-like acts as does fw2.2 in tomato, is discussed.

Cdk5 phosphorylates non-genotoxically overexpressed p53 following inhibition of PP2A to induce cell cycle arrest/apoptosis and inhibits tumor progression

Background

p53 is the most studied tumor suppressor and its overexpression may or may not cause cell death depending upon the genetic background of the cells. p53 is degraded by human papillomavirus (HPV) E6 protein in cervical carcinoma. Several stress activated kinases are known to phosphorylate p53 and, among them cyclin dependent kinase 5 (Cdk5) is one of the kinase studied in neuronal cell system. Recently, the involvement of Cdk5 in phosphorylating p53 has been shown in certain cancer types. Phosphorylation at specific serine residues in p53 is essential for it to cause cell growth inhibition. Activation of p53 under non stress conditions is poorly understood. Therefore, the activation of p53 and detection of upstream kinases that phosphorylate non-genotoxically overexpressed p53 will be of therapeutic importance for cancer treatment.

Results

To determine the non-genotoxic effect of p53; Tet-On system was utilized and p53 inducible HPV-positive HeLa cells were developed. p53 overexpression in HPV-positive cells did not induce cell cycle arrest or apoptosis. However, we demonstrate that overexpressed p53 can be activated to upregulate p21 and Bax which causes G2 arrest and apoptosis, by inhibiting protein phosphatase 2A. Additionally, we report that the upstream kinase cyclin dependent kinase 5 interacts with p53 to phosphorylate it at Serine20 and Serine46 residues thereby promoting its recruitment on p21 and bax promoters. Upregulation and translocation of Bax causes apoptosis through intrinsic mitochondrial pathway. Interestingly, overexpressed activated p53 specifically inhibits cell-growth and causes regression in vivo tumor growth as well.

Conclusion

Present study details the mechanism of activation of p53 and puts forth the possibility of p53 gene therapy to work in HPV positive cervical carcinoma.

Examination of the expanding pathways for the regulation of p21 expression and activity

p21(Waf1/Cip1/Sdi1) was originally identified as an inhibitor of cyclin-dependent kinases, a mediator of p53 in growth suppression and a marker of cellular senescence. p21 is required for proper cell cycle progression and plays a role in cell death, DNA repair, senescence and aging, and induced pluripotent stem cell reprogramming. Although transcriptional regulation is considered to be the initial control point for p21 expression, there is growing evidence that post-transcriptional and post-translational regulations play a critical role in p21 expression and activity. This review will briefly discuss the activity of p21 and focus on current knowledge of the determinants that control p21 transcription, mRNA stability and translation, and protein stability and activity.

Cdk5 suppresses the neuronal cell cycle by disrupting the E2F1-DP1 complex

Neurons that reenter a cell cycle after maturation are at increased risk for death, yet the mechanisms by which a normal neuron suppresses the cycle remain mostly unknown. Our laboratory has shown that cyclin-dependent kinase 5 (Cdk5) is a potent cell cycle suppressor, and we report here on the molecular basis of this activity. Cell cycle suppression by Cdk5 requires its binding to the p35 activator protein. The related p39 and p25 proteins cannot serve as substitutes. Unexpectedly, Cdk5 enzymatic activity is not required to perform this function. Rather, the link to cell cycle regulation is made through the formation of a previously unknown complex consisting of the p35-Cdk5 dimer and E2F1. Formation of this complex excludes the E2F1 cofactor, DP1, thus inhibiting E2F1 binding to the promoters of various cell cycle genes. This anti-cell cycle activity is most likely a neuroprotective function of Cdk5.

Anti-proliferative effect of extremely low frequency electromagnetic field on preneoplastic lesions formation in the rat liver

Background

Recently, extremely low frequency electromagnetic fields (ELF-EMF) have been studied with great interest due to their possible effects on human health. In this study, we evaluated the effect of 4.5 mT - 120 Hz ELF-EMF on the development of preneoplastic lesions in experimental hepatocarcinogenesis.

Methods

Male Fischer-344 rats were subjected to the modified resistant hepatocyte model and were exposed to 4.5 mT - 120 Hz ELF-EMF. The effects of the ELF-EMF on hepatocarcinogenesis, apoptosis, proliferation and cell cycle progression were evaluated by histochemical, TUNEL assay, caspase 3 levels, immunohistochemical and western blot analyses.

Results

The application of the ELF-EMF resulted in a decrease of more than 50% of the number and the area of γ-glutamyl transpeptidase-positive preneoplastic lesions (P = 0.01 and P = 0.03, respectively) and glutathione S-transferase placental expression (P = 0.01). The number of TUNEL-positive cells and the cleaved caspase 3 levels were unaffected; however, the proliferating cell nuclear antigen, Ki-67, and cyclin D1 expression decreased significantly (P ≤ 0.03), as compared to the sham-exposure group.

Conclusion

The application of 4.5 mT - 120 Hz ELF-EMF inhibits preneoplastic lesions chemically induced in the rat liver through the reduction of cell proliferation, without altering the apoptosis process.

Towards a systems biology approach to mammalian cell cycle: modeling the entrance into S phase of quiescent fibroblasts after serum stimulation

Background

The cell cycle is a complex process that allows eukaryotic cells to replicate chromosomal DNA and partition it into two daughter cells. A relevant regulatory step is in the G₀/G₁ phase, a point called the restriction (R) point where intracellular and extracellular signals are monitored and integrated.

Subcellular localization of cell cycle proteins is increasingly recognized as a major factor that regulates cell cycle transitions. Nevertheless, current mathematical models of the G₁/S networks of mammalian cells do not consider this aspect. Hence, there is a need for a computational model that incorporates this regulatory aspect that has a relevant role in cancer, since altered localization of key cell cycle players, notably of inhibitors of cyclin-dependent kinases, has been reported to occur in neoplastic cells and to be linked to cancer aggressiveness.

Results

The network of the model components involved in the G₁ to S transition process was identified through a literature and web-based data mining and the corresponding wiring diagram of the G₁ to S transition drawn with Cell Designer notation. The model has been implemented in Mathematica using Ordinary Differential Equations. Time-courses of level and of sub-cellular localization of key cell cycle players in mouse fibroblasts re-entering the cell cycle after serum starvation/re-feeding have been used to constrain network design and parameter determination. The model allows to recapitulate events from growth factor stimulation to the onset of S phase. The R point estimated by simulation is consistent with the R point experimentally determined.

Conclusion

The major element of novelty of our model of the G₁ to S transition is the explicit modeling of cytoplasmic/nuclear shuttling of cyclins, cyclin-dependent kinases, their inhibitor and complexes. Sensitivity analysis of the network performance newly reveals that the biological effect brought about by Cki overexpression is strictly dependent on whether the Cki is promoting nuclear translocation of cyclin/Cdk containing complexes.

PTH and PTHrP signaling in osteoblasts

The striking clinical benefit of PTH in osteoporosis began a new era of skeletal anabolic agents. Several studies have been performed, new studies are emerging out and yet controversies remain on PTH anabolic action in bone. This review focuses on the molecular aspects of PTH and PTHrP signaling in light of old players and recent advances in understanding the control of osteoblast proliferation, differentiation and function.

The cell cycle and acute kidney injury

Acute kidney injury (AKI) activates pathways of cell death and cell proliferation. Although seemingly discrete and unrelated mechanisms, these pathways can now be shown to be connected and even to be controlled by similar pathways. The dependence of the severity of renal-cell injury on cell cycle pathways can be used to control and perhaps to prevent acute kidney injury. This review is written to address the correlation between cellular life and death in kidney tubules, especially in acute kidney injury.

Isolation and characterization of cytoplasmic cyclin D1 mutants

To elucidate the mechanism governing the subcellular distribution of cyclin D1 protein, we randomly mutagenized human cyclin D1 cDNA and isolated mutants that encode the protein predominantly located in the cytoplasm. Experiments with Leptomycin B suggested a defect in transportation from the cytoplasm to the nucleus rather than enhanced nuclear exportation. Sequencing revealed that the mutations responsible for the cytoplasmic localization of cyclin D1 resided in the vicinity of the cyclin box, which affected interaction with a catalytic partner, Cdk4. We propose that interaction between cyclin D1 and Cdk4 triggers the mechanism controlling the nuclear transportation of this kinase complex.

Ionizing radiation induces ATM-independent degradation of p21Cip1 in transformed cells

The cyclin-dependent kinase inhibitor p21(Cip1) plays an important role in the cellular response to DNA damage. In normal cells, genotoxic stress activates the ATM-p53 pathway that up-regulates the expression of p21(Cip1) leading to cell cycle arrest. However, we have found that in several neoplastic cell lines, ionizing radiation (IR) induces ubiquitin-dependent degradation of p21(Cip1). This process is independent of the ATM pathway as it occurs in immortalized A-T fibroblasts. Knockdown of Skp2, an F-box protein capable of regulating the normal turnover of p21(Cip1), does not prevent the IR-induced degradation. Instead, this process requires the Cul4-DDB1(Cdt2) E3 ligase as knockdown of either DDB1 or Cdt2 rescues p21(Cip1) degradation after IR. Mutating the proliferating cell nuclear antigen-binding site of p21(Cip1) also prevents its IR-induced degradation suggesting that the p21(Cip1)-proliferating cell nuclear antigen interaction is critical for this event. Although ectopic expression of a nondegradable p21(Cip1) did not by itself affect the clonogenic survival of HEK293 cells after IR, the degradation of p21(Cip1) and other targets of the Cul4-DDB1(Cdt2) E3 ligase may collectively contribute to the survival of neoplastic cells after ionizing radiation.

Antisense MDM2 enhances E2F1-induced apoptosis and the combination sensitizes androgen-sensitive [corrected] and androgen-insensitive [corrected] prostate cancer cells to radiation

We have previously shown in separate studies that MDM2 knockdown via antisense MDM2 (AS-MDM2) and E2F1 overexpression via adenoviral-mediated E2F1 (Ad-E2F1) sensitized prostate cancer cells to radiation. Because E2F1 and MDM2 affect apoptosis through both common and independent pathways, we hypothesized that coupling these two treatments would result in increased killing of prostate cancer cells. In this study, the effect of Ad-E2F1 and AS-MDM2 in combination with radiation was investigated in three prostate cancer cell lines: LNCaP cells, LNCaP-Res cells [androgen insensitive with functional p53 and androgen receptor (AR)], and PC3 cells (androgen insensitive, p53(null), and AR(null)). A supra-additive radiosensitizing effect was observed in terms of clonogenic inhibition and induction of apoptosis (caspase-3 + caspase-7 activity) in response to Ad-E2F1 plus AS-MDM2 treatments in all three cell lines. In LNCaP and LNCaP-Res, these combination treatments elevated the levels of phospho-Ser(15) p53 with significant induction of p21(waf1/cip1), phospho-gammaH2AX, PUMA, and Bax levels and reduction of AR and bcl-2 expression. Similarly, AR(null) and p53(null) PC-3 cells showed elevated levels of Bax and phospho-gammaH2AX expression. These findings show that the combination of Ad-E2F1 and AS-MDM2 significantly increases cell death in prostate cancer cells exposed to radiation and that this effect occurs in the presence or absence of AR and p53.