Activation of cyclin A-dependent protein kinases during apoptosis

Osteogenic microenvironment affects palatal development through glycolysis

Palate development involves various events, including proliferation, osteogenic differentiation, and epithelial-mesenchymal transition. Disruption of these processes can result in the cleft palate (CP). Mouse embryonic palatal mesenchyme (MEPM) cells are commonly used to explore the mechanism of palatal development and CP. However, the role of the microenvironment in the biological properties of MEPM cells, which undergoes dynamic changes during palate development, is rarely reported. In this study, we investigated whether there were differences between the palatal shelf mesenchyme at different developmental stages. Our results found that the palatal shelves facilitate proliferation at the early palate stage at mouse embryonic day (E) 13.5 and the tendency towards osteogenesis at E15.5, the late palate development stage. And the osteogenic microenvironment, which was mimicked by osteogenic differentiation medium (OIM), affected the biological properties of MEPM cells when compared to the routine medium. Specifically, MEPM cells showed slower proliferation, shorter S phase, increased apoptosis, and less migration distance after osteogenesis. E15.5 MEPM cells were more sensitive than E13.5, showing an earlier change. Moreover, E13.5 MEPM cells had weaker osteogenic ability than E15.5, and both MEPM cells exhibited different Lactate dehydrogenase A (LDHA) and Cytochrome c (CytC) expressions in OIM compared to routine medium, suggesting that glycolysis might be associated with the influence of the osteogenic microenvironment on MEPM cells. By comparing the stemness of the two cells, we investigated that the stemness of E13.5 MEPM cells was stronger than that of E15.5 MEPM cells, and E15.5 MEPM cells were more like differentiated cells than stem cells, as their capacity to differentiate into multiple cell fates was reduced. E13.5 MEPM cells might be the precursor cells of E15.5 MEPM cells. Our results enriched the understanding of the effect of the microenvironment on the biological properties of E13.5 and E15.5 MEPM cells, which should be considered when using MEPM cells as a model for palatal studies in the future.

Identification of metabolic biomarkers for diagnosis of epithelial ovarian cancer using internal extraction electrospray ionization mass spectrometry (iEESI-MS)

BACKGROUND

Epithelial ovarian cancer (EOC) is the leading cause of death from gynecologic malignancies. The poor prognosis of EOC is mainly due to its asymptomatic early stage, lack of effective screening methods, and a late diagnosis in the advanced stages of the disease.

OBJECTIVE

This study investigated metabolomic abnormalities in epithelial ovarian cancers.

METHODS

Our study developed a novel strategy to rapidly identify the metabolic biomarkers in the plasma of the EOC patients using Internal Extraction Electrospray Ionization Mass Spectrometry (IEESI-MS) and Liquid Chromatography-mass Spectrometry (HPLC-MS), which could distinguish the differential metabolites in between plasma samples collected from 98 patients with epithelial ovarian cancer, including 78 cases with original (P), and 20 cases with self-configuration (ZP), as well as 60 healthy subjects, including 30 cases in the original sample (H), 30 cases in self-configuration (ZH), and 6 cases in a blind sample (B).

RESULTS

Our study detected 880 metabolites based on criteria variable importance in projection (VIP) > 1, among which 26 metabolites were selected for further identification. They are mainly metabolism-related lipids, amino acids, nucleic acids, and others. The metabolic pathways associated with the differential metabolites were explored by the KEGG analysis, a comprehensive database that integrates genome, chemistry, and system function information. The abnormal metabolites of EOC patients identified by IEESI-MS and HPLC-MS included Lysophosphatidylcholine (16:0) [Lyso PC (16:0)], L-Phenylalanine, L-Leucine, Phenylpyruvic acid, L-Tryptophan, and L-Histidine.

CONCLUSIONS

Identifying the abnormal metabolites of EOC patients through metabolomics analyses could provide a new strategy to identify valuable potential biomarkers for the screening and early diagnosis of EOC.

Casein kinase II activates Bik to induce death of hyperplastic mucous cells in a cell cycle-dependent manner

Extensive inflammation causes epithelial cell hyperplasia in the airways and Bcl-2-interacting killer (Bik) reduces epithelial cell and mucous cell hyperplasia without affecting resting cells to restore homeostasis. These observations suggest that Bik induces apoptosis in a cell cycle-specific manner, but the mechanisms are not understood. Mice were exposed to an allergen for 3, 14, or 30 days and Bik expression was induced in airway epithelia of transgenic mice. Bik reduced epithelial and mucous cell hyperplasia when mice were exposed to an allergen for 3 or 14 days, but not when exposure lasted for 30 days, and Ki67-positivity was reduced. In culture, Bik expression killed proliferating cells but not quiescent cells. To capture the stage of the cell cycle when Bik induces cell death, airway cells that express fluorescent ubiquitin cell cycle indicators were generated that fluoresce red or green during the G0/G1 and S/G2/M phases of the cells cycle, respectively. Regardless of the cell cycle stage, Bik expression eliminated green-fluorescent cells. Also, Bik, when tagged with a blue-fluorescent protein, was only detected in green cells. Bik phosphorylation mutants at threonine 33 or serine 35 demonstrated that phosphorylation activated Bik to induce death even in quiescent cells. Immunoprecipitation and proteomic approaches identified casein kinase IIα to be responsible for phosphorylating and activating Bik to kill cells in S/G2/M. As casein kinase 2 alpha (CKIIα) is expressed only during the G2/M phase, we conclude that Bik activation in airway epithelial cells selectively targets hyperplastic epithelial cells, while leaving resting airway cells unaffected.

Tat-p27 Ameliorates Neuronal Damage Reducing α-Synuclein and Inflammatory Responses in Motor Neurons After Spinal Cord Ischemia

p27^Kip1 (p27) regulates the cell cycle by inhibiting G1 progression in cells. Several studies have shown conflicting results on the effects of p27 against cell death in various insults. In the present study, we examined the neuroprotective effects of p27 against H₂O₂-induced oxidative stress in NSC34 cells and against spinal cord ischemia-induced neuronal damage in rabbits. To promote delivery into NSC34 cells and motor neurons in the spinal cord, Tat-p27 fusion protein and its control protein (Control-p27) were synthesized with or without Tat peptide, respectively. Tat-p27, but not Control-27, was efficiently introduced into NSC34 cells in a concentration- and time-dependent manner, and the protein was detected in the cytoplasm. Tat-p27 showed neuroprotective effects against oxidative stress induced by H₂O₂ treatment and reduced the formation of reactive oxygen species, DNA fragmentation, and lipid peroxidation in NSC34 cells. Tat-p27, but not Control-p27, ameliorated ischemia-induced neurological deficits and cell damage in the rabbit spinal cord. In addition, Tat-p27 treatment reduced the expression of α-synuclein, activation of microglia, and release of pro-inflammatory cytokines such as interleukin-1β and tumor necrosis factor-α in the spinal cord. Taken together, these results suggest that Tat-p27 inhibits neuronal damage by decreasing oxidative stress, α-synuclein expression, and inflammatory responses after ischemia.

The AMPK/p27Kip1 Pathway as a Novel Target to Promote Autophagy and Resilience in Aged Cells

Once believed to solely function as a cyclin-dependent kinase inhibitor, p27^Kip1 is now emerging as a critical mediator of autophagy, cytoskeletal dynamics, cell migration and apoptosis. During periods of metabolic stress, the subcellular location of p27^Kip1 largely dictates its function. Cytoplasmic p27^Kip1 has been found to be promote cellular resilience through autophagy and anti-apoptotic mechanisms. Nuclear p27^Kip1, however, inhibits cell cycle progression and makes the cell susceptible to quiescence, apoptosis, and/or senescence. Cellular location of p27^Kip1 is regulated, in part, by phosphorylation by various kinases, including Akt and AMPK. Aging promotes nuclear localization of p27^Kip1 and a predisposition to senescence or apoptosis. Here, we will review the role of p27^Kip1 in healthy and aging cells with a particular emphasis on the interplay between autophagy and apoptosis.

A loss-of-function mutation p.T256M in NDRG4 is implicated in the pathogenesis of pulmonary atresia with ventricular septal defect (PA/VSD) and tetralogy of Fallot (TOF)

Pulmonary atresia with ventricular septal defect (PA/VSD) is a rare congenital heart disease (CHD) characterized by a lack of luminal continuity and blood flow from either the right ventricle or the pulmonary artery, together with VSDs. The prevalence of PA/VSD is about 0.2% of live births and approximately 2% of CHDs. PA/VSD is similar to tetralogy of Fallot (TOF) in terms of structural and pathological characteristics. The pathogenesis of these two CHDs remains incompletely understood. It was previously reported that N‐myc downstream‐regulated gene (NDRG)4 is required for myocyte proliferation during early cardiac development. In the present study, we enrolled 80 unrelated patients with PA/VSD or TOF and identified a probably damaging variant p.T256M of NDRG4. The p.T256M variant impaired the proliferation ability of human cardiac myocytes (hCM). Furthermore, the p.T256M variant resulted in G1 and G2 arrest of hCM, followed by an increase in p27 and caspase‐9 expression. Our results provide evidence that the p.T256M variant in NDRG4 is a pathogenic variant associated with impaired hCM proliferation and cell‐cycle arrest and likely contributes towards the pathogenesis of PA/VSD and TOF.

Buforin IIb induced cell cycle arrest in liver cancer

The inhibitory effect of buforin IIb on different types of cancer, although not liver cancer, has been demonstrated previously. The aim of the present study was to investigate the effects of buforin IIb on the progression of liver cancer. The human liver cancer cell line HepG2 was treated with purified buforin IIb and the cell activity was determined by MTT, colony formation and transwell assays. The protein expression levels of cyclin-dependent kinases (CDKs) and cyclins were analyzed by western blotting and immunofluorescent cell staining. A tumor growth model was constructed using nude mice, and buforin IIb treatment was administered. The levels of CDK2 and cyclin A in the tumor tissues were detected by western blotting. Buforin IIb treatment depressed cell viability and colony formation and induced apoptosis significantly, and 1.0 µM concentration of buforin IIb was found to be the optimal dosage. The cell cycle was arrested at the G2/M phase following buforin IIb treatment. CDK2 and cyclin A were downregulated by treatment of the cells with 1.0 µM buforin IIb for 24 h. Treatment with buforin IIb also inhibited the migration of liver cancer cells in vitro. Furthermore, 50 nmol buforin IIb injection suppressed HepG2 cell subcutaneous tumor growth in the nude mouse model. Similar to the in vitro results, buforin IIb injection reduced the expression of CDK2 and cyclin A in the tumor tissue. these results demonstrate that buforin IIb inhibited liver cancer cell growth via the regulation of CDK2 and cyclin A expression.

Multiple functions of p27 in cell cycle, apoptosis, epigenetic modification and transcriptional regulation for the control of cell growth: A double-edged sword protein

The cell cycle is controlled by precise mechanisms to prevent malignancies such as cancer, and the cell needs these tight and advanced controls. Cyclin dependent kinase inhibitor p27 (also known as KIP₁) is a factor that inhibits the progression of the cell cycle by using specific molecular mechanisms. The inhibitory effect of p27 on the cell cycle is mediated by CDKs inhibition. Other important functions of p27 include cell proliferation, cell differentiation and apoptosis. Post- translational modification of p27 by phosphorylation and ubiquitination respectively regulates interaction between p27 and cyclin/CDK complex and degradation of p27. In this review, we focus on the multiple function of p27 in cell cycle regulation, apoptosis, epigenetic modifications and post- translational modification, and briefly discuss the mechanisms and factors that have important roles in p27 functions.

Bilateral gene interaction hierarchy analysis of the cell death gene response emphasizes the significance of cell cycle genes following unilateral traumatic brain injury

BACKGROUND

Delayed or secondary cell death that is caused by a cascade of cellular and molecular processes initiated by traumatic brain injury (TBI) may be reduced or prevented if an effective neuroprotective strategy is employed. Microarray and subsequent bioinformatic analyses were used to determine which genes, pathways and networks were significantly altered 24 h after unilateral TBI in the rat. Ipsilateral hemi-brain, the corresponding contralateral hemi-brain, and naïve (control) brain tissue were used for microarray analysis.

RESULTS

Ingenuity Pathway Analysis showed cell death and survival (CD) to be a top molecular and cellular function associated with TBI on both sides of the brain. One major finding was that the overall gene expression pattern suggested an increase in CD genes in ipsilateral brain tissue and suppression of CD genes contralateral to the injury which may indicate an endogenous protective mechanism. We created networks of genes of interest (GOI) and ranked the genes by the number of direct connections each had in the GOI networks, creating gene interaction hierarchies (GIHs). Cell cycle was determined from the resultant GIHs to be a significant molecular and cellular function in post-TBI CD gene response.

CONCLUSIONS

Cell cycle and apoptosis signalling genes that were highly ranked in the GIHs and exhibited either the inverse ipsilateral/contralateral expression pattern or contralateral suppression were identified and included STAT3, CCND1, CCND2, and BAX. Additional exploration into the remote suppression of CD genes may provide insight into neuroprotective mechanisms that could be used to develop therapies to prevent cell death following TBI.

Caffeine-Induced Premature Chromosome Condensation Results in the Apoptosis-Like Programmed Cell Death in Root Meristems of Vicia faba

We have demonstrated that the activation of apoptosis-like programmed cell death (AL-PCD) was a secondary result of caffeine (CF) induced premature chromosome condensation (PCC) in hydroxyurea-synchronized Vicia faba root meristem cells. Initiation of the apoptotic-like cell degradation pathway seemed to be the result of DNA damage generated by treatment with hydroxyurea (HU) [double-stranded breaks (DSBs) mostly] and co-treatment with HU/CF [single-stranded breaks (SSBs) mainly]. A single chromosome comet assay was successfully used to study different types of DNA damage (neutral variant–DSBs versus alkaline–DSBs or SSBs). The immunocytochemical detection of H2AXS139Ph and PARP-2 were used as markers for DSBs and SSBs, respectively. Acridine orange and ethidium bromide (AO/EB) were applied for quantitative immunofluorescence measurements of dead, dying and living cells. Apoptotic-type DNA fragmentation and positive TUNEL reaction finally proved that CF triggers AL-PCD in stressed V. faba root meristem cells. In addition, the results obtained under transmission electron microscopy (TEM) further revealed apoptotic-like features at the ultrastructural level of PCC-type cells: (i) extensive vacuolization; (ii) abnormal chromatin condensation, its marginalization and concomitant degradation; (iii) formation of autophagy-like vesicles (iv) protoplast shrinkage (v) fragmentation of cell nuclei and (vi) extensive degeneration of the cells. The results obtained have been discussed with respect to the vacuolar/autolytic type of plant-specific AL-PCD.

Elevated cyclin A associated kinase activity promotes sensitivity of metastatic human cancer cells to DNA antimetabolite drug

Drug resistance is a major obstacle in successful systemic therapy of metastatic cancer. We analyzed the involvement of cell cycle regulatory proteins in eliciting response to N (phosphonoacetyl)-L-aspartate (PALA), an inhibitor of de novo pyrimidine synthesis, in two metastatic variants of human cancer cell line MDA-MB-435 isolated from lung (L-2) and brain (Br-1) in nude mouse, respectively. L-2 and Br-l cells markedly differed in their sensitivity to PALA. While both cell types displayed an initial S phase delay/arrest, Br-l cells proliferated but most L-2 cells underwent apoptosis. There was distinct elevation in cyclin A, and phosphorylated Rb proteins concomitant with decreased expression of bcl-2 protein in the PALA treated L-2 cells undergoing apoptosis. Markedly elevated cyclin A associated and cdk2 kinase activities together with increased E2F1-DNA binding were detected in these L-2 cells. Induced ectopic cyclin A expression sensitized Br-l cells to PALA by activating an apoptotic pathway. Our findings demonstrate that elevated expression of cyclin A and associated kinase can activate an apoptotic pathway in cells exposed to DNA antimetabolites. Abrogation of this pathway can lead to resistance against these drugs in metastatic variants of human carcinoma cells.

Bioinformatics analysis of transcription profiling of sepsis

Sepsis is a fatal whole-body inflammatory response that complicates a serious infection. To elucidate the molecular mechanism of sepsis, transcription profile data of GSE12624 which included a total of 70 samples (34 sepsis samples and 36 non-sepsis samples) was downloaded. The t test based on Bayes method in limma package was used to identify differentially expressed genes (DEGs) between sepsis and non-sepsis samples (criterion: P value <0.05). Gene Ontology (GO) enrichment analysis was conducted to investigate the biological processes involved DEGs. Protein-protein interaction (PPI) network and sub-network analysis were conducted to investigate the interactions between DEGs. A total of 894 DEGs, including 479 downregulated DEGs and 415 upregulated DEGs, were identified in sepsis samples comparing with non-sepsis samples. GO enrichment analysis showed that DEGs mainly involved in cellular metabolic process, primary metabolic process, and response to organic cyclic compound. In the PPI network, four genes of CDC2, GTF2F2, PCNA, and SMAD4 with degrees more than 10 were identified. Subsequently, four sub-networks, in which genes of PTBP1, PSMA3, PSMA6, PSMB9, PSMB10, and GADD45 had relative high degrees were identified from the PPI network. After the discussion referring to previous studies, we suggested that CDC2, GTF2F2, PCNA, SMAD4 PSMA3, PTBP1, and GADD45 might be used as new therapeutic targets for sepsis. However, experiments should be further performed to prove the practical utility of these candidates.

A new arylbenzofuran derivative functions as an anti-tumour agent by inducing DNA damage and inhibiting PARP activity

We previously reported that 7-hydroxy-5, 4'-dimethoxy-2-arylbenzofuran (HDAB) purified from Livistona chinensis is a key active agent. The present study investigated the function and molecular mechanism of HDAB. HDAB treatment of cervical cancer cells resulted in S phase arrest and apoptosis, together with cyclin A2 and CDK2 upregulation. Cyclin A2 siRNA and a CDK inhibitor efficiently relieved S phase arrest but increased the apoptosis rate. Mechanistic studies revealed that HDAB treatment significantly increased DNA strand breaks in an alkaline comet assay and induced ATM, CHK1, CHK2 and H2A.X phosphorylation. Wortmannin (a broad inhibitor of PIKKs) and CGK733 (a specific ATM inhibitor), but not LY294002 (a phosphatidylinositol 3-kinase inhibitor) or NU7026 (a DNA-PK specific inhibitor), prevented H2A.X phosphorylation and γH2A.X-positive foci formation in the nuclei, reversed S phase arrest and promoted the HDAB-induced apoptosis, suggesting that HDAB is a DNA damaging agent that can activate the ATM-dependent DNA repair response, thereby contributing to cell cycle arrest. In addition, molecular docking and in vitro activity assays revealed that HDAB can correctly dock into the hydrophobic pocket of PARP-1 and suppress PARP-1 ADP-ribosylation activity. Thus, the results indicated that HDAB can function as an anti-cancer agent by inducing DNA damage and inhibiting PARP activity.

Effect of cell cycle phase on the sensitivity of SAS cells to sonodynamic therapy using low-intensity ultrasound combined with 5-aminolevulinic acid in vitro

Sonodynamic therapy (SDT) with 5-aminolevulinic acid (5-ALA) can effectively inhibit various types of tumor in vitro and in vivo. However, the association between the efficacy of SDT and the phase of the cell cycle remains to be elucidated. 5-ALA may generate different quantities of sonosensitizer, protoporphyrin IX (PpIX), in different phases of the cell cycle, which may result in differences in sensitivity to 5-ALA-induced SDT. The present study aimed to investigate the effect of the cell cycle on the susceptibility of SAS cells to SDT following synchronization to different cell cycle phases. These results indicates that the rates of cell death and apoptosis of the SAS cells in the S and G2/M phases were significantly higher following SDT, compared with those in the G1-phase cells and unsynchronized cells, with a corresponding increase in PpIX in the S and G2/M cells. In addition, the expression of caspase-3 increased, while that of B-cell lymphoma (Bcl)-2 decreased markedly in theS and G2/M cells following SDT. Cyclin A was also expressed at higher levels in the S and G2/M cells, compared with the G1-phase cells. SDT also caused a significant upregulation of cyclin A in all phases of the cell cycle, however this was most marked in the S and G2/M cells. It was hypothesized that high expression levels of cyclin A in the S and G2/M cells may promote the induction of caspase-3 and reduce the induction of Bcl-2 by SDT and, therefore, enhance apoptosis. Taken together, these data demonstrated that cells in The S and G2/M phases generate more intracellular PpIX, have higher levels of cyclin A and are, therefore, more sensitive to SDT-induced cytotoxicity. These findings indicate the potential novel approach to preventing the onset of cancer by combining cell-cycle regulators with SDT. This sequential combination therapy may be a simple and cost-effective way of enhancing the effects of SDT in clinical settings.

Effect of DTPP-mediated photodynamic therapy on cell morphology, viability, cell cycle, and cytotoxicity in a murine lung adenocarcinoma cell line

Photodynamic therapy (PDT) involves the administration and activation of photosensitizing reagents in cancer tissues to induce cytotoxicity. Here we examined the effects of 5-5- (4-N, N-diacetoxylphenyl)-10,15,20- tetraphenylporphyrin (DTPP) -mediated PDT on cell morphology, viability, cell cycle, and cytotoxicity in a murine lung adenocarcinoma cell line. LA795 murine lung adenocarcinoma cell line was used in the study, with cellular uptake of DTPP being quantified by a UV-visible spectrophotometer. The subcellular localization of DTPP was detected by confocal laser scanning microscopy, alteration of cell morphology after PDT was observed by an inverted light microscope, and late-stage apoptosis was examined by terminal dUTP nick end labeling (TUNEL) . The effects of influencing factors on cytotoxicity of PDT in LA795 cells was investigated with varying concentrations of DTPP, energy densities, power densities, and antioxidants by 3- (4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assays. Effects of PDT on cell cycle and plasma membrane integrity were studied by flow cytometry analysis. The uptake of DTPP by LA795 cells reached maximum after incubation for 24 h. Confocal laser scanning microscopy showed that DTPP was mainly in the mitochondrion, and slight localization was detected in the lysosomes. Cellular inhibitory effects increased with increased irradiation dose and DTPP concentration, while unactivated DTPP had low toxicity. Flow cytometry analysis revealed that DTPP-PDT-treated cells showed S phase arrest. Cell membrane damage initiation, repair, and irreversible damage were observed at 2, 4, and 5 h after DTPP-PDT , respectively. Together, our results demonstrated cell apoptosis, compromised viability, and cell cycle S phase arrest of LA795 in response to DTPP-PDT , while no effect on the lung cancer cells was observed with irradiation or photosensitizer treatment alone.

Polysaccharide from Phellinus linteus induces S-phase arrest in HepG2 cells by decreasing calreticulin expression and activating the P27kip1-cyclin A/D1/E-CDK2 pathway

ETHNOPHARMACOLOGY RELEVANCE

Our previous study showed that the proteoglycan P1 from Phellinus linteus (Mesima) exhibits significant anti-tumor activity against human hepatocellular carcinoma cells (HepG2); however, its molecular mechanism remains unknown. This study aims to provide insights into the mechanism of the anti-tumor activity of P1 against HepG2 cells.

METHODS

We examined the effects of P1 on HepG2 cell proliferation in vitro and in vivo. Flow cytometry was used to analyze the cell cycle distribution and apoptosis. Proteomic analysis, real-time (RT)-PCR, and Western blot were carried out to observe the expression of several cell cycle control proteins in HepG2 cells.

RESULTS

Both the volume and the weight of solid tumors were significantly decreased in P1-treated mice (200mg/kg) compared with the control. The HepG2 cells in the P1-treated tumors were significantly decreased, irregularly shaped, and smaller. P1 slightly increased the body weight of the tumor-bearing mice, which indicates that P1 is nontoxic to mammals at 200mg/kg. P1 also caused a significant dose-dependent increase in S phase arrest, but no apoptosis was observed in HepG2 cells. The results of the proteomic analysis, RT-PCR, and Western blot analysis showed that significantly downregulated expression of calreticulin, cyclin D1, cyclin E, and CDK2 and upregulated expression of P27 kip1 and cyclin A in the P1-treated HepG2 cells (200 μg/ml).

CONCLUSION

These results suggest that calreticulin expression and the P27 kip1-cyclin A/D1/E-CDK2 pathway were involved in P1-induced S-phase cell cycle arrest in HepG2 cells.

Walleye dermal sarcoma virus: expression of a full-length clone or the rv-cyclin (orf a) gene is cytopathic to the host and human tumor cells

Walleye dermal sarcoma virus (WDSV) is etiologically associated with a skin tumor, walleye dermal sarcoma (WDS), which develops in the fall and regresses in the spring. WDSV genome contains, in addition to gag, pol and env, three open reading frames (orfs) designated orf a (rv-cyclin), orf b and orf c. Unintegrated linear WDSV provirus DNA isolated from infected tumor cells was used to construct a full-length WDSV provirus clone pWDSV, while orf a was cloned into pSVK3 to construct the expression vector porfA. Stable co-transfection of a walleye cell line (W12) with pWDSV and pcDNA3 generated fewer and smaller G418-resistant colonies compared to the control. By Northern blot analysis, several small transcripts (2.8, 1.8, 1.2, and 0.8 kb) were detected using a WDSV LTR-specific probe. By RT-PCR and Southern blot analysis, three cDNAs (2.4, 1.6 and 0.8 kb) were identified, including both orf a and orf b messenger. Furthermore stable co-transfection of both a human lung adenocarcinoma cell line (SPC-A-1) and a cervical cancer cell line (HeLa) with pcDNA3 and ether porfA or pWDSV also generated fewer and smaller G418-resistant colonies. We conclude that expression of the full-length WDSV clone or the orf a gene inhibits the host fish and human tumor cell growth, and Orf A protein maybe a potential factor which contributes to the seasonal tumor development and regression. This is the first fish provirus clone that has been expressed in cell culture system, which will provide a new in vitro model for tumor research and oncotherapy study.

New insights into the kinetic resistance to anticancer agents

Kinetic resistance plays a major role in the failure of chemotherapy towards many solid tumors. Kinetic resistance to cytotoxic drugs can be reproduced in vitro by growing the cells as multicellular spheroids (Multicellular Resistance) or as hyperconfluent cultures (Confluence-Dependent Resistance). Recent findings on the cell cycle regulation have permitted a better understanding why cancer cells which arrest in long quiescent phases are poorly sensitive to cell-cycle specific anticancer drugs. Two cyclin-dependent kinase inhibitors (CDKI) seem particularly involved in the cell cycle arrest at the G1 to S transition checkpoint: the p53-dependent p21(cip1) protein which is activated by DNA damage and the p27(kip1) which is a mediator of the contact inhibition signal. Cell quiescence could alter drug-induced apoptosis which is partly dependent on an active progression in the cell cycle and which is facilitated by overexpression of oncogenes such as c-Myc or cyclins. Investigations are yet necessary to determine the influence of the cell cycle on the balance between antagonizing (bcl-2, bcl-X(L)...) or stimulating (Bax, Bcl-X(S), Fas...) factors in chemotherapy-induced apoptosis. Quiescent cells could also be protected from toxic agents by an enhanced expression of stress proteins, such as HSP27 which is induced by confluence. New strategies are required to circumvent kinetic resistance of solid tumors: adequate choice of anticancer agents whose activity is not altered by quiescence (radiation, cisplatin), recruitment from G1 to S/G2 phases by cell pretreatment with alkylating drugs or attenuation of CDKI activity by specific inhibitors.

Regulation of the latency-reactivation cycle by products encoded by the bovine herpesvirus 1 (BHV-1) latency-related gene

Like other α-herpesvirinae subfamily members, the primary site for bovine herpesvirus 1 (BHV-1) latency is ganglionic sensory neurons. Periodically BHV-1 reactivates from latency, virus is shed, and consequently virus transmission occurs. Transcription from the latency-related (LR) gene is readily detected in neurons of trigeminal ganglia (TG) of calves or rabbits latently infected with BHV-1. Two micro-RNAs and a transcript encompassing a small open reading frame (ORF-E) located within the LR promoter can also be detected in TG of latently infected calves. A BHV-1 mutant that contains stop codons near the beginning of the first open reading frame (ORF2) within the major LR transcript (LR mutant virus) has been characterized. The LR mutant virus does not express ORF2, a reading frame that lacks an initiating ATG (reading frame B), and has reduced expression of ORF1 during productive infection. The LR mutant virus does not reactivate from latency following dexamethasone treatment suggesting that LR protein expression regulates the latency-reactivation cycle. Higher levels of apoptosis occur in TG neurons of calves infected with the LR mutant viruses when compared to wild-type BHV-1 indicating that the anti-apoptotic properties of the LR gene is necessary for the latency-reactivation cycle. ORF2 inhibits apoptosis and regulates certain viral promoters, in part, because it interacts with three cellular transcription factors (C/EBP-alpha, Notch1, and Notch3). Although ORF2 is important for the latency-reactivation cycle, we predict that other LR gene products play a supportive role during life-long latency in cattle.

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 RNA binding protein Musashi1 regulates apoptosis, gene expression and stress granule formation in urothelial carcinoma cells

The RNA-binding protein Musashi1 (MSI1) is a marker of progenitor cells in the nervous system functioning as a translational repressor. We detected MSI1 mRNA in several bladder carcinoma cell lines, but not in cultured normal uroepithelial cells, whereas the paralogous MSI2 gene was broadly expressed. Knockdown of MSI1 expression by siRNA induced apoptosis and a severe decline in cell numbers in 5637 bladder carcinoma cells. Microarray analysis of gene expression changes after MSI1 knockdown significantly up-regulated 735 genes, but down-regulated only 31. Up-regulated mRNAs contained a highly significantly greater number and density of Musashi binding sites. Therefore, a much larger set of mRNAs may be regulated by Musashi1, which may affect not only their translation, but also their turnover. The study confirmed p21^CIP1 and Numb proteins as targets of Musashi1, suggesting additionally p27^KIP1 in cell-cycle regulation and Jagged-1 in Notch signalling. A significant number of up-regulated genes encoded components of stress granules (SGs), an organelle involved in translational regulation and mRNA turnover, and impacting on apoptosis. Accordingly, heat shock induced SG formation was augmented by Musashi1 down-regulation. Our data show that ectopic MSI1 expression may contribute to tumorigenesis in selected bladder cancers through multiple mechanisms and reveal a previously unrecognized function of Musashi1 in the regulation of SG formation.

Protection of cisplatin cytotoxicity by an inactive cyclin-dependent kinase

Cisplatin cytotoxicity is dependent on cyclin-dependent kinase 2 (Cdk2) activity in vivo and in vitro. A Cdk2 mutant (Cdk2-F80G) was designed in which the ATP-binding pocket was altered. When expressed in mouse kidney cells, this protein was kinase inactive, did not inhibit endogenous Cdk2, but protected from cisplatin. The mutant was localized in the cytoplasm, but when coexpressed with cyclin A, it was activated, localized to the nucleus, and no longer protected from cisplatin cytotoxicity. Cells exposed to cisplatin in the presence of the activated mutant had an apoptotic phenotype, and endonuclease G was released from mitochondria similar to that mediated by endogenous Cdk2. But unlike apoptosis mediated by wild-type Cdk2, cisplatin exposure of cells expressing the activated mutant did not cause cytochrome c release or significant caspase-3 activation. We conclude that cisplatin likely activates both caspase-dependent and -independent cell death, and Cdk2 is required for both pathways. The mutant-inactive Cdk2 protected from both death pathways, but after activation by excess cyclin A, caspase-independent cell death predominated.

Knocking-down cyclin A(2) by siRNA suppresses apoptosis and switches differentiation pathways in K562 cells upon administration with doxorubicin

Cyclin A₂ is critical for the initiation of DNA replication, transcription and cell cycle regulation. Cumulative evidences indicate that the deregulation of cyclin A₂ is tightly linked to the chromosomal instability, neoplastic transformation and tumor proliferation. Here we report that treatment of chronic myelogenous leukaemia K562 cells with doxorubicin results in an accumulation of cyclin A₂ and follows by induction of apoptotic cell death. To investigate the potential preclinical relevance, K562 cells were transiently transfected with the siRNA targeting cyclin A₂ by functionalized single wall carbon nanotubes. Knocking down the expression of cyclin A₂ in K562 cells suppressed doxorubicin-induced growth arrest and cell apoptosis. Upon administration with doxorubicin, K562 cells with reduced cyclin A₂ showed a significant decrease in erythroid differentiation, and a small fraction of cells were differentiated along megakaryocytic and monocyte-macrophage pathways. The results demonstrate the pro-apoptotic role of cyclin A₂ and suggest that cyclin A₂ is a key regulator of cell differentiation. To the best of our knowledge, this is the first report that knocking down expression of one gene switches differentiation pathways of human myeloid leukemia K562 cells.

9-bromonoscapine-induced mitotic arrest of cigarette smoke condensate-transformed breast epithelial cells

In the present investigation, we determined the chemotherapeutic efficacy of 9-bromonoscapine (Br-Nos), a more potent noscapine analog, on MCF10A, spontaneously immortalized human normal breast epithelial cells and MCF10A-CSC3, cigarette smoke condensate (CSC)-transformed cells. The results from cytogenetic analysis showed that Br-Nos induced polyploidy and telomeric association in MCF10A-CSC3 cells, while MCF10A cells remained unaffected. Our immunofluorescence data further demonstrated that MCF10A-CSC3 cells were susceptible to mitotic catastrophe on exposure to Br-Nos and failed to recover after drug withdrawal. MCF10A-CSC3 cells exhibited Br-Nos-induced aberrant multipolar spindle formation, which irreversibly impaired the alignment of replicated chromosome to the equatorial plane and finally culminated in cell death. Although MCF10A cells upon Br-Nos treatment showed bipolar spindles with some uncongressed chromosomes, these cells recovered fairly well after drug withdrawal. Our flow-cytometry analysis data reconfirmed that MCF10A-CSC3 cells were more susceptible to cell death compared to MCF10A cells. Furthermore, our results suggest that decreased levels of cdc2/cyclin B1 and cdc2 kinase activity are responsible for Br-Nos-induced mitotic cell arrest leading to cell death in MCF10A-CSC3 cells. This study thus explores the underlying mechanism of Br-Nos-induced mitotic catastrophe in CSC-transformed MCF10A-CSC3 cells and its potential usefulness as a chemotherapeutic agent for prevention of cigarette smoke-induced breast cancer growth.

OSU03012 activates Erk1/2 and Cdks leading to the accumulation of cells in the S-phase and apoptosis

OSU03012, a Celecoxib derivative, has been shown to inhibit proliferation and induce apoptosis in human cancer cell lines. However, its underlying mechanisms are not completely understood. In our study, the relationship between cell cycle inhibition and apoptosis induced by OSU03012 was investigated in human oral cancer cell lines. In the premalignant and malignant cell lines, OSU03012-induced growth inhibition, S-phase arrest, and apoptosis were accompanied by a marked increase in the activity of Erk1/2 and Cdk2/cyclin A. Inhibition of Cdks by roscovitine partially blocked OSU03012-induced growth inhibition and apoptosis. Although the activity of cdc2/cyclin B was reduced, expression of constructively active cdc2AF did not reverse OSU03012-induced S-phase arrest. When Erk1/2 was inhibited by U0126 before addition of OSU03012, growth inhibition and apoptosis induced by OSU03012 were attenuated. The levels of the Cdk2/cyclin A were reduced and cells accumulated in the G(0)/G(1) phase. When cells were allowed to accumulate in S-phase before addition of U0126, apoptosis also was attenuated suggesting that Erk1/2 is required for both progression of cells into the S-phase and apoptosis. Expression of constructively active MEK enhanced OSU03012-induced apoptosis. OSU03012 selectively inhibited the proliferation in premalignant and malignant, but not normal human oral cell lines. In conclusion, we show that OSU03012 has potent anti-proliferative and apoptotic activity against premalignant and malignant human oral cells through activation of Erk1/2, and Cdks. OSU0312 may provide unique opportunities for cancer prevention and sensitization of cancer cells to S-phase modalities.

Inhibition of caspase-3 by Survivin prevents Wee1 Kinase degradation and promotes cell survival by maintaining phosphorylation of p34Cdc2

The anti-apoptotic protein Survivin and the cyclin-dependent kinase p34Cdc2 regulate cell cycle progression and apoptosis. p34Cdc2 activation is required for its pro-apoptotic activity and phosphorylation of p34Cdc2 at Tyrosine-15 (Tyr15) maintains p34Cdc2 in an inactive state. In BaF3 IL-3-dependent murine hematopoietic cells, over-expression of wild-type (wt)-Survivin increased Tyrosine phosphorylation of p34Cdc2, while over-expression of dominant-negative (dn) T34A-Survivin decreased Tyr15 phosphorylation. The increased phospho-Tyr15 levels associated with ectopic wt-Survivin directly correlated with enhanced BaF3 cell survival upon growth factor withdrawal, while conversely, low phospho-Tyr15 levels and decreased survival were seen in BaF3 cells expressing ectopic dn-Survivin. Tyrosine-15 phosphorylation of p34Cdc2 is mediated by the Wee1 Kinase, a known target of caspase-3. In BaF3 cells over-expressing wt-Survivin, 2-fold higher levels of Wee1 protein were detected compared to cells expressing vector or dn-Survivin. Treatment of control vector-transduced BaF3 cells with the selective caspase-3 inhibitor Ac-DEVD-CHO increased p34Cdc2-Tyr15 phosphorylation and Wee1 protein levels. In a similar fashion, over-expression of wt-Survivin maintained high levels of phospho-Tyr15-p34Cdc2 and Wee1 protein. Since Survivin requires Hsp90 for stability, we treated cells with the Hsp90 inhibitors AICAR and 17-AAG to further link Survivin to blocking p34Cdc2 activation. Treatment of BaF3 cells expressing ectopic wt-Survivin with AICAR or 17-AAG significantly reduced p34Cdc2-Tyr15 phosphorylation compared to vehicle-treated controls. These results suggest that Survivin protects the p34Cdc2-Tyr15-targeting kinase Wee1 from degradation by blocking caspase-3 activation leading to inhibition of the pro-apoptotic function of p34Cdc2 and enhanced cell survival.

Post-translational modification of cyclin A1 is associated with staurosporine and TNFalpha induced apoptosis in leukemic cells

Understanding of molecular mechanisms underlying the effects of cell cycle proteins in response to the chemotherapeutic agents is of great importance for improving the efficacy of targeted therapeutics and overcoming resistance to chemotherapeutic agents. Staurosporine and tumor necrosis factor alpha (TNFalpha) are the therapeutic agents that inhibit tumor cell growth by inducing cell death. Staurosporine induces apoptosis through the intrinsic pathway, while TNFalpha trigger the cell death via the extrinsic apoptotic pathway. We have previously demonstrated that the cell cycle regulatory protein, cyclin A1 played an important role in the development of acute myeloid leukemia (AML), and cyclin A1 expression correlated with disease characteristics and patient outcome in leukemia. However, it remains unknown how cyclin A1 expression is regulated in leukemic cells treated with the therapeutic agents. Here, we demonstrate that cyclin A1 protein is regulated by proteasome-mediated ubiquitination and degradation in untreated U-937 cells. Interestingly, ubiquitination- and proteasomal-mediated degradation of cyclin A1 is prevented in cells treated with staurosporine or TNFalpha. Induction of apoptosis in U-937 cells by staurosporine or TNFalpha resulted in an increase in cyclin A1 protein expression, which correlated well with cyclin A1 protein modification and the activation of caspase-3. Blocking caspases activity by Z-VAD-FMK had no effect on the increased cyclin A1 expression, suggesting that cyclin A1 might be regulated by caspase-3 independent pathways. We further propose that CDC25C may be associated with cyclin A1 protein modification in response to staurosporine or TNFalpha treatment. Our results suggest that cyclin A1 protein is stabilized via post-transcriptional modification in response to apoptosis induced by staurosporine or TNFalpha.

Cytoplasmic initiation of cisplatin cytotoxicity

The mechanism of action of cisplatin as a chemotherapeutic agent has been attributed to DNA binding, while its mechanism of action as a nephrotoxin is unresolved. Only approximately 1% of intracellular cisplatin interacts with DNA, primarily forming intrastrand cross-linked adducts, and many studies have implicated both nuclear and cytoplasmic causes of cisplatin-induced death in cultured cells. We have demonstrated that cisplatin cytotoxicity depends on cdk2 activity, which is at least partly through the cdk2-E2F1 pathway. The mechanism of the dependency on cdk2, and whether cdk2 activation of E2F1 represents the only cell death pathway involved, is still unclear. Our previous work showed that deletion of the nuclear localization signal from p21 WAF1/CIP1, a cdk2 inhibitor, did not alter its protective action against cisplatin cytotoxicity. Active cdk2-cyclin complexes are localized in both the nucleus and cytoplasm, and it was reported that cdk2 translocated to the cytoplasm after an apoptotic stimulus. Herein, we show that cisplatin caused cell death in enucleated mouse kidney proximal tubule cells (TKPTS), which was prevented by cdk2 inhibition. Also, we localized cytoplasmic cdk2 to both the endoplasmic reticulum (ER) and Golgi compartments, and ER stress was blocked by specific cdk2 inhibition. We conclude that cisplatin can induce nuclear independent apoptosis, cisplatin cytotoxicity can be initiated by cytoplasmic events, and cytoplasmic cdk2 plays an important role in apoptosis signaling.

Differential regulation of survivin by p53 contributes to cell cycle dependent apoptosis

Recent studies indicate that cell-cycle checkpoints are tightly correlated with the regulation of apoptosis, in which p53 plays an important role. Our present works show that the expression of E6/E7 oncogenes of human papillomavirus in HeLa cells is inhibited in the presence of anti-tumor reagent tripchlorolide (TC), which results in the up-regulation of p53 in HeLa cells. Interestingly, under the same TC-treatment, the cells at the early S-phase are more susceptible to apoptosis than those at the middle S-phase although p53 protein is stabilized to the same level in both situations. Significant difference is exhibited between the two specified expression profiles. Further analysis demonstrates that anti-apoptotic gene survivin is up-regulated by p53 in the TC-treated middle-S cells, whereas it is down-regulated by p53 in the TC-treated early-S cells. Taken together, the present study indicates that the differential p53-regulated expression of survivin at different stages of the cell cycle results in different cellular outputs under the same apoptosis-inducer.

Cell cycle arrest and apoptosis induced by the coronavirus infectious bronchitis virus in the absence of p53

Manipulation of the cell cycle and induction of apoptosis are two common strategies used by many viruses to regulate their infection cycles. In cells infected with coronaviruses, cell cycle perturbation and apoptosis were observed in several reports. However, little is known about how these effects are brought out, and how manipulation of the functions of host cells would influence the replication cycle of coronavirus. In this study, we demonstrate that infection with coronavirus infectious bronchitis virus (IBV) imposed a growth-inhibitory effect on cultured cells by inducing cell cycle arrest at S and G₂/M phases in both p53-null cell line H1299 and Vero cells. This cell cycle arrest was catalyzed by the modulation of various cell cycle regulatory genes and the accumulation of hypophosphorylated RB, but was independent of p53. Proteasome inhibitors, such as lactacystin and NLVS, could bypass the IBV-induced S-phase arrest by restoring the expression of corresponding cyclin/Cdk complexes. Our data also showed that cell cycle arrest at both S- and G₂/M-phases was manipulated by IBV for the enhancement of viral replication. In addition, apoptosis induced by IBV at late stages of the infection cycle in cultured cells was shown to be p53-independent. This conclusion was drawn based on the observations that apoptosis occurred in both IBV-infected H1299 and Vero cells, and that IBV infection did not affect the expression of p53 in host cells.

Nuclear colocalization and interaction between bcl-xL and cdk1(cdc2) during G2/M cell-cycle checkpoint

In response to cancer chemotherapeutic drugs, cells rapidly trigger the apoptotic program or undergo growth arrest and senescence at specific phases of the cell cycle. Mitochondrial bcl-xL plays a central role in preventing alteration of mitochondrial dysfunction, cytochrome c release, caspase activation, DNA fragmentation and apoptosis. However, its pleitropic function depends on its subcellular localization. Here, we show that in addition to its mitochondrial effect that delays apoptosis, bcl-xL colocalizes and binds to cdk1(cdc2) during G(2)/M cell-cycle checkpoint and its overexpression stabilizes a G(2)/M-arrest senescence program in surviving cells after DNA damage. Bcl-xL potently inhibits cdk1(cdc2) kinase activity, which is reversible by a synthetic peptide between the 41st amino acid and 60th amino acid surrounding of the Thr47 and Ser62 phosphorylation sites, and Asn52 deamidation site, within the flexible loop domain of bcl-xL. A mutant deleted of this region does not alter the antiapoptotic function of bcl-xL, but impedes its effect on cdk1(cdc2) activity and on the G(2)/M-arrest senescence program after DNA damage. The nuclear interaction of bcl-xL and cdk1(cdc2) suggests that bcl-xL is coupled to the stabilization of a cell-cycle checkpoint induced by DNA damage, and this effect is genetically distinct from its function on apoptosis.

Cell-specific responses to loss of cyclin-dependent kinases

Inactivation of cyclin-dependent kinases (Cdks) and/or cyclins in mice has changed our view of cell cycle regulation. In general, cells are far more resistant to the loss of Cdks than originally anticipated, suggesting widespread compensation among the Cdks. Early embryonic cells are, so far, not sensitive to the lack of multiple Cdks or cyclins. In contrast, differentiated cells are more dependent on Cdk/cyclin complexes and the functional redundancy is more limited. Our challenge is to better understand these cell-type specific differences in cell cycle regulation that can be used to design efficient cancer therapy. Indeed, tumor cells seem to respond to inhibition of Cdk activities, however, with different outcome depending on the tumor cell type. Tumor cells share some proliferation features with stem cells, but appear more sensitive to loss of Cdk activity, somewhat resembling differentiated cells. We summarize the current knowledge of cell cycle regulation in different cell types and highlight their sensitivity to the lack of Cdk activities.

Decreased cyclin-dependent kinase activity promotes thyroid hormone-dependent tail regression in Rana catesbeiana

The thyroid hormone (TH), 3,5,3'-triiodothyronine (T(3)), is an important regulator of diverse cellular processes including cell proliferation, differentiation, and apoptosis, with increasing evidence that the modulation of the phosphoproteome is an important factor in the TH-mediated response. However, little is understood regarding the mechanisms whereby phosphorylation may contribute to T(3)-mediated cellular outcomes during development. The cyclin-dependent kinases (Cdks) and mitogen-activated protein kinases (MAPK/ERK) have been implicated in TH signaling in mammalian cells. In this study, we have investigated, in frogs, the possible role that these kinases may have in the promotion of tail regression during tadpole metamorphosis, an important postembryonic process that is completely TH-dependent. Cdk2 steady state levels and activity increase in the tail concurrent with progression through the growth phase of metamorphosis, followed by a precipitous decrease coinciding with tail regression. Cyclin-A-associated kinase activity also follows a similar trend except that its associated kinase activity is maintained longer before a decrease in activity. Protein steady state levels of ERK1 and ERK2 remain relatively constant, and their kinase activities do not decrease until much later during tail regression. Tail tips cultured in serum-free medium in the presence of T(3) undergo regression, which is accelerated by coincubation with a specific Cdk2 inhibitor. Coincubation with PD098059, a MAPK inhibitor, has no effect. Thus, T(3)-dependent tail regression does not require MAPKs, but a decrease in Cdk2 activity promotes tail regression.

c-Jun induces apoptosis of starved BM2 monoblasts by activating cyclin A-CDK2

c-Jun is one of the major components of the activating protein-1 (AP-1), the transcription factor that participates in regulation of proliferation, differentiation, and apoptosis. In this study, we explored functional interactions of the c-Jun protein with several regulators of the G1/S transition in serum-deprived v-myb-transformed chicken monoblasts BM2. We show that the c-Jun protein induces expression of cyclin A, thus up-regulating activity of cyclin A-associated cyclin-dependent kinase 2 (CDK2), and causing massive programmed cell death of starved BM2cJUN cells. Specific inhibition of CDK2 suppresses frequency of apoptosis of BM2cJUN cells. We conclude that up-regulation of cyclin A expression and CDK2 activity can represent important link between the c-Jun protein, cell cycle machinery, and programmed cell death pathway in leukemic cells.

Ras triggers ataxia-telangiectasia-mutated and Rad-3-related activation and apoptosis through sustained mitogenic signaling

Genetic evidence indicates that Ras plays a critical role in the initiation and progression of human thyroid tumors. Paradoxically, acute expression of activated Ras in normal rat thyroid cells induced deregulated cell cycle progression and apoptosis. We investigated whether cell cycle progression was required for Ras-stimulated apoptosis. Ras increased CDK-2 activity following its introduction into quiescent cells. Apoptotic cells exhibited a sustained increase in CDK-2 activity, accompanied by the loss of CDK-2-associated p27. Blockade of Ras-induced CDK-2 activity and S phase entry via overexpression of p27 inhibited apoptosis. Inactivation of the retinoblastoma protein in quiescent cells through expression of HPV-E7 stimulated cell cycle progression and apoptosis, indicating that deregulated cell cycle progression is sufficient to induce apoptosis. Ras failed to induce G1 phase growth arrest in normal rat thyroid cells. Rather, Ras-expressing thyroid cells progressed into S and G2 phases and evoked a checkpoint response characterized by the activation of ATR. Ras-stimulated ATR activity, as evidenced by Chk1 and p53 phosphorylation, was blocked by p27, suggesting that cell cycle progression triggers checkpoint activation, likely as a consequence of replication stress. These data reveal that Ras is capable of inducing a DNA damage response with characteristics similar to those reported in precancerous lesions. Our findings also suggest that the frequent mutational activation of Ras in thyroid tumors reflects the ability of Ras-expressing cells to bypass checkpoints and evade apoptosis rather than to simply increase proliferative potential.

Analysis of cyclin B1 and CDK activity during apoptosis induced by camptothecin treatment

We have studied the role of cyclins and cyclin-dependent kinase (CDK) activity in apoptosis induced by camptothecin (CPT). In this model, 22% of the cells stain for annexin-V at 24 h and then proceed to be 93% positive by 72 h. This time window permits the analysis of cyclins in cells that are committed to apoptosis but not yet dead. We provide evidence that cyclin protein levels and then associated kinase levels increase after CPT treatment. Strikingly, cyclin B1 and cyclin E1 proteins are present at the same time in CPT treated HT29 cells. Although cyclin B1 and E1 CDK complexes are activated in CPT treated cells, only the cyclin B1 complex is required for apoptosis since reduction of cyclin B1 by RNAi or roscovitine treatment reduces the number of annexin-V-stained cells. We have detected poorly organized chromosomes and phosphorylated histone H3 epitopes at the time of maximum cyclin B1/CDK kinase activity in CPT-treated cells, which suggests that these cells enter a mitotic catastrophe. Understanding which CDKs are required for apoptosis may allow us to better adapt CDK inhibitors for use as anti-cancer compounds.

BCL2 family in DNA damage and cell cycle control

Individual BCL2 family members couple apoptosis regulation and cell cycle control in unique ways. Antiapoptotic BCL2 and BCL-x(L) are antiproliferative by facilitating G0. BAX is proapoptotic and accelerates S-phase progression. The dual functions in apoptosis and cell cycle are coordinately regulated by the multi-domain BCL2 family members (MCL-1) and suggest that survival is maintained at the expense of proliferation. The role of BH3-only molecules in cell cycle is more variable. BAD antagonizes both the cell cycle and antiapoptotic functions of BCL2 and BCL-x(L) through BH3 binding. BID has biochemically separable functions in apoptosis and S-phase checkpoint, determined by post-translational modification. p53-induced PUMA is known only to have apoptotic function. Inhibition of apoptosis is oncogenic, whereas promotion of cell cycle arrest is tumor suppressive. Paradoxically, selected BCL2 family members can be both oncogenic and tumor suppressive. Which of the dual functions predominates is lineage specific and context dependent.

Increased p21 expression and complex formation with cyclin E/CDK2 in retinoid-induced pre-B lymphoma cell apoptosis

Cip/Kip family protein p21, a cyclin-dependent kinase (CDK) inhibitor, is directly transactivated by retinoic acid receptor alpha (RARalpha) upon retinoic acid (RA):RARalpha binding. Yet the role of p21 upregulation by RA in lymphoma cells remains unknown. Here, we show that, in human pre-B lymphoma Nalm6 cells, RA-induced proliferation inhibition results from massive cell death characterized by apoptosis. Upregulated p21 by RA accompanies caspase-3 activation and precedes the occurrence of apoptosis. p21 induction leads to increased p21 complex formation with cyclin E/CDK2, which occurs when cyclin E and CDK2 levels remain constant. CDK2 can alternatively promote apoptosis, but the mechanisms remain unknown. Data presented here suggest a novel RA-signaling, by which RA-induced p21 induction and complex formation with cyclin E/CDK2 diverts CDK2 function from normally driving proliferation to alternatively promoting apoptosis.

Localization of sequences within the latency-related gene of bovine herpesvirus 1 that inhibit mammalian cell growth

The latency-related (LR) RNA of bovine herpes virus 1 (BHV-1) is abundantly expressed in sensory neurons of latently infected cattle. Wild-type expression of LR gene products is required for the latency-reactivation cycle. LR gene products inhibit apoptosis, bICP0 expression, and mammalian cell growth. The cell growth inhibitory function of the LR gene maps to a 463-bp XbaI-PstI fragment. Introduction of stop codons into the XbaI-PstI fragment had no effect on inhibiting growth. Expression of a LR strand-specific transcript correlates with growth inhibition in bovine fibroblasts and mouse neuroblastoma cells.

Functional analysis of bovine herpesvirus 1 (BHV-1) genes expressed during latency

Bovine herpes virus 1 (BHV-1) establishes latency in sensory neurons of trigeminal ganglia (TG), and germinal centers of pharyngeal tonsil. Periodically BHV-1 reactivates from latency, virus is shed, and consequently virus transmission occurs. Two transcripts, the latency related (LR) RNA and ORF-E, are abundantly expressed in TG of latently infected cattle. A LR mutant strain of BHV-1 was constructed that contains stop codons near the beginning of the LR-RNA. The LR mutant virus does not express two proteins encoded by the LR gene, or reactivate from latency suggesting that LR protein expression regulates the latency-reactivation cycle. Higher levels of apoptosis occur in TG of calves infected with the LR mutant versus wild type BHV-1 indicating that the anti-apoptotic properties of the LR gene regulate the latency-reactivation cycle. The LR gene also inhibits bICP0 expression and mammalian cell growth, but these functions do not require LR protein expression. In contrast, the ability of the LR gene to inhibit apoptosis appears to require LR protein expression. A small open reading frame (ORF-E) that is located within the LR promoter is expressed in the nucleus of neuroblastoma cells. We predict that the LR gene and ORF-E regulate the BHV-1 latency-reactivation cycle.

PKCdelta modulates p21WAF1/CIP1 ability to bind to Cdk2 during TNFalpha-induced apoptosis

Cyclin-dependent kinase 2 (Cdk2) activity is thought to be involved in cell death-associated chromatin condensation and other manifestations of apoptotic death. Here we show that during TNFalpha-induced apoptosis, PKCdelta is activated in a caspase-3-dependent manner and phosphorylates p21(WAF1/CIP1), a specific cyclin-dependent kinase inhibitor, on (146)Ser. This residue is located near a cyclin-binding motif (Cy2) that plays an important role in the interaction between p21(WAF1/CIP1) and Cdk2, and its phosphorylation modulates the ability of p21(WAF1/CIP1) to associate with Cdk2. The phosphorylation of p21(WAF1/CIP1) is temporally related to the activation kinetics of Cdk2 activity during the apoptosis. We propose that during TNFalpha-induced apoptosis, PKCdelta-mediated phosphorylation of p21(WAF1/CIP1) at (146)Ser attenuates the Cdk2 binding of p21(WAF1/CIP1) and thereby upregulates Cdk2 activity.