DP and E2F proteins: coordinating transcription with cell cycle progression

The TFDP1 gene coding for DP1, the heterodimeric partner of the transcription factor E2F, is a target of deregulated E2F

The TFDP1 gene codes for the heterodimeric partner DP1 of the transcription factor E2F. E2F, principal target of the tumor suppressor pRB, plays central roles in cell proliferation by activating a group of growth-related genes. E2F also mediates tumor suppression by activating tumor suppressor genes such as ARF, an upstream activator of the tumor suppressor p53, when deregulated from pRB upon oncogenic changes. Among 8 E2F family members (E2F1∼E2F8), expression of activator E2Fs (E2F1∼E2F3a) is induced at the G1/S boundary of the cell cycle after growth stimulation by E2F itself. However, mechanisms regulating DP1 expression are not known. We show here that over-expression of E2F1 and forced inactivation of pRB, by adenovirus E1a, induced TFDP1 gene expression in human normal fibroblast HFFs, suggesting that the TFDP1 gene is a target of E2F. Serum stimulation of HFFs also induced TFDP1 gene expression, but with different kinetics from that of the CDC6 gene, a typical growth-related E2F target. Both over-expression of E2F1 and serum stimulation activated the TFDP1 promoter. We searched for E2F1-responsive regions by 5' and 3' deletion of the TFDP1 promoter and by introducing point mutations in putative E2F1-responsive elements. Promoter analysis identified several GC-rich elements, mutation of which reduced E2F1-responsiveness but not serum-responsiveness. ChIP assays showed that the GC-rich elements bound deregulated E2F1 but not physiological E2F1 induced by serum stimulation. These results suggest that the TFDP1 gene is a target of deregulated E2F. In addition, knockdown of DP1 expression by shRNA enhanced ARF gene expression, which is specifically induced by deregulated E2F activity, suggesting that activation of the TFDP1 gene by deregulated E2F may function as a failsafe feedback mechanism to suppress deregulated E2F and maintain normal cell growth in the event that DP1 expression is insufficient relative to that of its partner activator E2Fs. a maximum of 6 keywords: E2F, DP1, TFDP1 gene, pRB, gene expression.

Transcription factors regulating the fate and developmental potential of a multipotent progenitor in Caenorhabditis elegans

Multipotent stem and progenitor cells have the capacity to generate a limited array of related cell types. The Caenorhabditis elegans somatic gonadal precursors are multipotent progenitors that generate all 143 cells of the somatic gonad, including complex tissues and specialized signaling cells. To screen for candidate regulators of cell fate and multipotency, we identified transcription factor genes with higher expression in somatic gonadal precursors than in their differentiated sister, the head mesodermal cell. We used RNA interference or genetic mutants to reduce the function of 183 of these genes and examined the worms for defects in the somatic gonadal precursor cell fate or the ability to generate gonadal tissue types. We identify 8 genes that regulate somatic gonadal precursor fate, including the SWI/SNF chromatin remodeling complex gene swsn-3 and the Ci/GLI homolog tra-1, which is the terminal regulator of sex determination. Four genes are necessary for somatic gonadal precursors to generate the correct number and type of descendant cells. We show that the E2F homolog, efl-3, regulates the cell fate decision between distal tip cells and the sheath/spermathecal precursor. We find that the FACT complex gene hmg-4 is required for the generation of the correct number of somatic gonadal precursor descendants, and we define an earlier role for the nhr-25 nuclear hormone receptor-encoding gene, in addition to its previously described role in regulating the asymmetric division of somatic gonadal precursors. Overall, our data show that genes regulating cell fate are largely different from genes regulating developmental potential, demonstrating that these processes are genetically separable.

Effect of Upregulation of Transcription Factor TFDP1 Binding Promoter Activity Due to RBP4 g.36491960G>C Mutation on the Proliferation of Goat Granulosa Cells

Retinol-binding protein 4 (RBP4), a member of the lipocalin family, is a specific carrier of retinol (vitamin A) in the blood. Numerous studies have shown that RBP4 plays an important role in mammalian embryonic development and that mutations in RBP4 can be used for the marker-assisted selection of animal reproductive traits. However, there are few studies on the regulation of reproduction and high-prolificacy traits by RBP4 in goats. In this study, the 5′ flanking sequence of RBP4 was amplified, and a G>C polymorphism in the promoter region -211 bp (g.36491960) was detected. An association analysis revealed that the respective first, second and third kidding number and mean kidding number of nanny goats with CC and GC genotypes (2.167 ± 0.085, 2.341 ± 0.104, 2.529 ± 0.107 and 2.189 ± 0.070 for CC and 2.052 ± 0.047, 2.206 ± 0.057, 2.341 ± 0.056 and 2.160 ± 0.039 for GC) were significantly higher (p < 0.05) than those with the GG genotype (1.893 ± 0.051, 2.027 ± 0.064, 2.107 ± 0.061 and 1.74 ± 0.05). The luciferase assay showed that luciferase activity was increased in C allele individuals compared with that in G allele individuals. A competitive electrophoretic mobility shift assay (EMSA) showed that individuals with the CC genotype had a stronger promoter region binding capacity than those with the GG genotype. In addition, transcription factor prediction software showed that the RBP4 g.36491960G>C mutation added a novel binding site for transcription factor DP-1 (TFDP1). RT−qPCR results showed that the expression of TFDP1 was significantly higher in the high-prolificacy group than in the low-prolificacy group, and the expression of RBP4 was higher in both the CC and GC genotypes than that in the GG genotype. TFDP1 overexpression significantly increased the expression of RBP4 mRNA (p < 0.05) and the expression of the cell proliferation factors cyclin-D1, cyclin-D2 and CDK4 (p < 0.05). The opposite trend was observed after interference with TFDP1. Both the EdU and CCK-8 results showed that TFDP1 expression could regulate the proliferation of goat ovarian granulosa cells. In summary, our results showed that RBP4 g.36491960G>C was significantly associated with fecundity traits in goats. The g.36491960G>C mutation enhanced the transcriptional activity of RBP4 and increased the expression of RBP4, thus improving the fertility of Yunshang black goats.

The C/EBPβ-Dependent Induction of TFDP2 Facilitates Porcine Reproductive and Respiratory Syndrome Virus Proliferation

Porcine reproductive and respiratory syndrome (PRRS) is an important infectious disease caused by porcine reproductive and respiratory syndrome virus (PRRSV), leading to significant economic losses in swine industry worldwide. Although several studies have shown that PRRSV can affect the cell cycle of infected cells, it is still unclear how it manipulates the cell cycle to facilitate its proliferation. In this study, we analyzed the mRNA expression profiles of transcription factors in PRRSV-infected 3D4/21 cells by RNA-sequencing. The result shows that the expression of transcription factor DP2 (TFDP2) is remarkably upregulated in PRRSV-infected cells. Further studies show that TFDP2 contributes to PRRSV proliferation and the PRRSV nucleocapsid (N) protein induces TFDP2 expression by activating C/EBPβ. TFDP2 positively regulates cyclin A expression and triggers a less proportion of cells in the S phase, which contributes to PRRSV proliferation. This study proposes a novel mechanism by which PRRSV utilizes host protein to regulate the cell cycle to favor its infection. Findings from this study will help us for a better understanding of PRRSV pathogenesis.

Dual Eigen-modules of Cis-Element Regulation Profiles and Selection of Cognition-Language Eigen-direction along Evolution in Hominidae

To understand the genomic basis accounting for the phenotypic differences between human and apes, we compare the matrices consisting of the cis-element frequencies in the proximal regulatory regions of their genomes. One such frequency matrix is represented by a robust singular value decomposition. For each singular value, the negative and positive ends of the sorted motif eigenvector correspond to the dual ends of the sorted gene eigenvector, respectively, comprising a dual eigen-module defined by cis-regulatory element frequencies (CREF). The CREF eigen-modules at levels 1, 2, 3, and 6 are highly conserved across humans, chimpanzees, and orangutans. The key biological processes embedded in the top three CREF eigen-modules are reproduction versus embryogenesis, fetal maturation versus immune system, and stress responses versus mitosis. Although the divergence at the nucleotide level between the chimpanzee and human genome was small, their cis-element frequency matrices crossed a singularity point, at which the fourth and fifth singular values were identical. The CREF eigen-modules corresponding to the fourth and fifth singular values were reorganized along the evolution from apes to human. Interestingly, the fourth sorted gene eigenvector encodes the phenotypes unique to human such as long-term memory, language development, and social behavior. The number of motifs present on Alu elements increases substantially at the fourth level. The motif analysis together with the cases of human-specific Alu insertions suggests that mutations related to Alu elements play a critical role in the evolution of the human-phenotypic gene eigenvector.

The rice PLATZ protein SHORT GRAIN6 determines grain size by regulating spikelet hull cell division

Grain size is a major determinant of cereal grain yields; however, the relevant regulatory mechanisms controlling this trait have not been fully elucidated. The rice (Oryza sativa) mutant short grain6 (sg6) was identified based on its reduced grain length and weight. Here, we functionally characterized the role of SG6 in determining grain size through the regulation of spikelet hull cell division. SG6 encodes a previously uncharacterized plant AT-rich sequence and zinc-binding (PLATZ) protein that is ubiquitously localized throughout the cell and is preferentially expressed in the early developing panicles but not in the endosperm. The overexpression of SG6 resulted in significantly larger and heavier grains, as well as increased plant heights, which is consistent with its elevated spikelet hull cell division rate. Yeast two-hybrid analyses revealed that SG6 interacts with the core cell cycle machinery DP protein and several other putative cell division regulators, consistent with our transcriptomic analysis, which showed that SG6 activates the expression of many DNA replication and cell-cycle-related genes. These results confirm the crucial role of SG6 in determining grain size by regulating spikelet hull cell division and provide clues for understanding the functions of PLATZ family proteins and the network regulating cereal grain size.

Expression of the TFDP1 gene in the endometrium of women with deep infiltrating endometriosis

The field of endometriosis etiopathogenesis aims to identify the origin of disease in endometrial disorders. Changes in gene and protein expression related to cell adhesion, collagenases, and, mainly, cell cycle regulators have been identified. We set out to analyze the expression of the transcription factor DP-1 (TFDP1) gene, which encodes a protein that controls the G1/S phase passage of the cell cycle, in the endometrium of women with deep infiltrating endometriosis (DIE). Samples of endometrium from both endometriosis-affected women and healthy women were collected, cultured and maintained at the Cell Bank of the Pelvic Pain and Endometriosis Unit of the Federal University of Sao Paulo. This study analyzed five samples from the endometrium cell culture of healthy patients (i.e. no pelvic disease, as determined by means of laparoscopic tubal ligation) and six samples from women diagnosed with DIE. Samples were evaluated for TFDP1 gene expression by real-time PCR. We observed a downregulation of TFDP1 in the endometrium cells of women with DIE when compared to the control (a fold-change of -2.05, p value=.011). The TFDP1 gene is part of the cell cycle pathway, but its function is not yet clear. Additional studies are necessary to clarify the function of TFDP1 in endometriosis etiopathogenesis.

The TGF-β Signalling Network in Muscle Development, Adaptation and Disease

Skeletal muscle possesses remarkable ability to change its size and force-producing capacity in response to physiological stimuli. Impairment of the cellular processes that govern these attributes also affects muscle mass and function in pathological conditions. Myostatin, a member of the TGF-β family, has been identified as a key regulator of muscle development, and adaptation in adulthood. In muscle, myostatin binds to its type I (ALK4/5) and type II (ActRIIA/B) receptors to initiate Smad2/3 signalling and the regulation of target genes that co-ordinate the balance between protein synthesis and degradation. Interestingly, evidence is emerging that other TGF-β proteins act in concert with myostatin to regulate the growth and remodelling of skeletal muscle. Consequently, dysregulation of TGF-β proteins and their associated signalling components is increasingly being implicated in muscle wasting associated with chronic illness, ageing, and inactivity. The growing understanding of TGF-β biology in muscle, and its potential to advance the development of therapeutics for muscle-related conditions is reviewed here.

Bifurcation in Cell Cycle Dynamics Regulated by p53

We study the regulating mechanism of p53 on the properties of cell cycle dynamics in the light of the proposed model of interacting p53 and cell cycle networks via p53. Irradiation (IR) introduce to p53 compel p53 dynamics to suffer different phases, namely oscillating and oscillation death (stabilized) phases. The IR induced p53 dynamics undergo collapse of oscillation with collapse time Δt which depends on IR strength. The stress p53 via IR drive cell cycle molecular species MPF and cyclin dynamics to different states, namely, oscillation death, oscillations of periods, chaotic and sustain oscillation in their bifurcation diagram. We predict that there could be a critical Δtc induced by p53 via IRc, where, if Δt〈Δtc the cell cycle may come back to normal state, otherwise it will go to cell cycle arrest (apoptosis).

Nuclear Smad7 Overexpressed in Mesenchymal Cells Acts as a Transcriptional Corepressor by Interacting with HDAC-1 and E2F to Regulate Cell Cycle

Smad family proteins are essential intracellular mediators that regulate transforming growth factor-β (TGF-β) ligand signaling. In response to diverse stimuli, Smad7 is rapidly expressed and acts as a cytoplasmic inhibitor that selectively interferes with signals elicited from TGF-β family receptors. In addition, earlier works have indicated that retrovirally transduced Smad7 induces long-lasting cell proliferation arrest in a variety of mesenchymal cells through down-regulation of G1 cyclins. However, the molecular mechanisms underlying the cytostatic effects of Smad7 remain unknown. We show here that Smad7 can form a complex with endogenous histone deacetylase proteins HDAC-1 and HDAC-3 in NIH 3T3 mouse fibroblast cells. By contrast, forced expression of a dominant-negative variant of HDAC-1 efficiently protected cells against Smad7 proliferation inhibition, suggesting that Smad7 depends on the deacetylase activity of its associated HDAC-1 to arrest the cell cycle. Furthermore, Smad7 caused HDAC-1 bind to E2F-1 to form a ternary complex on chromosomal DNA containing an E2F-binding motif and leading to repression in the activity of the E2F target genes. Smad7 mutations that prevented its binding to either HDAC-1 or E2F-1 resulted in a significant decrease in Smad7-mediated inhibition of cell proliferation. The present results strongly suggest that nuclear Smad7 is a transcriptional corepressor for E2F, providing a molecular basis for the Smad7-induced arrest of the cell cycle.

Tumor suppressors and cell-cycle proteins in lung cancer

The cell cycle is the cascade of events that allows a growing cell to duplicate all its components and split into two daughter cells. Cell cycle progression is mediated by the activation of a highly conserved family of protein kinases, the cyclin-dependent kinases (CDKs). CDKs are also regulated by related proteins called cdk inhibitors grouped into two families: the INK4 inhibitors (p16, p15, p19, and p18) and the Cip/Kip inhibitors (p21, p27, and p53). Several studies report the importance of cell-cycle proteins in the pathogenesis and the prognosis of lung cancer. This paper will review the most recent data from the literature about the regulation of cell cycle. Finally, based essentially on the data generated in our laboratory, the expression, the diagnostic, and prognostic significance of cell-cycle molecules in lung cancer will be examined.

Role of cell cycle regulators in lung carcinogenesis

Cellular division is an ordered, tightly regulated process involving multiple checkpoints that assess extracellular growth signals, cell size and DNA integrity. Progression throughout the cell cycle is based on the activation of different CDK-cyclin complexes that prevent cells from entering into a new phase until thay have successfully complete the previous one. In addition, a series of cell cycle checkpoints are designed to preserve genome integrity and chromosomal stability. Neoplastic lung cells develop the ability to bypass several of these checkpoints, and tumor cell proliferation is frequently associated with genetic or epigenetic alterations in key regulators of the cell cycle. The goal of this review is to summarize the knowledge about the dysregulation of major cell cycle regulators in lung cancer pathogenesis and to discuss the use of these proteins as targets for therapeutic intervention.

Aurora B kinase regulates the postmitotic endoreduplication checkpoint via phosphorylation of the retinoblastoma protein at serine 780

The phenotypic change characteristic of Aurora B inhibition is the induction of polyploidy. Utilizing specific siRNA duplexes and a selective small molecule inhibitor (AZD1152) to inhibit Aurora B activity in tumor cells, we sought to elucidate the mechanism by which Aurora B inhibition results in polyploidy. Cells treated with AZD1152 progressed through mitosis with misaligned chromosomes and exited without cytokinesis and subsequently underwent endoreduplication of DNA despite activation of a p53-dependent pseudo G1 checkpoint. Concomitant with polyploid cell formation, we observed the appearance of Rb hypophosphorylation, an event that occurred independently of cyclin-dependent kinase inhibition. We went on to discover that Aurora B directly phosphorylates Rb at serine 780 both in vitro and in vivo. This novel interaction plays a critical role in regulating the postmitotic checkpoint to prevent endoreduplication after an aberrant mitosis. Thus, we propose for the first time that Aurora B determines cellular fate after an aberrant mitosis by directly regulating the Rb tumor suppressor protein.

HuR contributes to cyclin E1 deregulation in MCF-7 breast cancer cells

Many cancers overexpress cyclin E1 and its tumor-specific low molecular weight (LMW) isoforms. However, the mechanism of cyclin E1 deregulation in cancers is still not well understood. We show here that the mRNA-binding protein HuR increases cyclin E1 mRNA stability in MCF-7 breast carcinoma cells. Thus, mRNA stabilization may be a key event in the deregulation of cyclin E1 in MCF-7 cells. Compared with MCF10A immortalized breast epithelial cells, MCF-7 cells overexpress full-length cyclin E1 and its LMW isoforms and exhibit increased cyclin E1 mRNA stability. Increased mRNA stability is associated with a stable adenylation state and an increased ratio of cytoplasmic versus nuclear HuR. UV cross-link competition and UV cross-link immunoprecipitation assays verified that HuR specifically bound to the cyclin E1 3'-untranslated region. Knockdown of HuR with small interfering RNA (siRNA) in MCF-7 cells decreased cyclin E1 mRNA half-life (t(1/2)) and its protein level: a 22% decrease for the full-length isoforms and 80% decrease for the LMW isoforms. HuR siRNA also delayed G(1)-S phase transition and inhibited MCF-7 cell proliferation, which was partially recovered by overexpression of a LMW isoform of cyclin E1. Overexpression of HuR in MCF10A cells increased cyclin E1 mRNA t(1/2) and its protein level. Taken together, our data show that HuR critically contributes to cyclin E1 overexpression and its growth-promoting function, at least in part by increasing cyclin E1 mRNA stability, which provides a new mechanism of cyclin E1 deregulation in breast cancer.

p27Kip1 and p130 cooperate to regulate hematopoietic cell proliferation in vivo

To investigate the potential functional cooperation between p27Kip1 and p130 in vivo, we generated mice deficient for both p27Kip1 and p130. In p27Kip1-/-; p130-/- mice, the cellularity of the spleens but not the thymi is significantly increased compared with that of their p27Kip1-/- counterparts, affecting the lymphoid, erythroid, and myeloid compartments. In vivo cell proliferation is significantly augmented in the B and T cells, monocytes, macrophages, and erythroid progenitors in the spleens of p27Kip1-/-; p130-/- animals. Immunoprecipitation and immunodepletion studies indicate that p130 can compensate for the absence of p27Kip1 in binding to and repressing CDK2 and is the predominant CDK-inhibitor associated with the inactive CDK2 in the p27Kip1-/- splenocytes. The finding that the p27Kip1-/-; p130-/- splenic B cells are hypersensitive to mitogenic stimulations in vitro lends support to the concept that the hyperproliferation of splenocytes is not a result of the influence of their microenvironment. In summary, our findings provide genetic and molecular evidence to show that p130 is a bona fide cyclin-dependent kinase inhibitor and cooperates with p27Kip1 to regulate hematopoietic cell proliferation in vivo.

Myostatin regulation of muscle development: Molecular basis, natural mutations, physiopathological aspects

Since its identification in 1997, myostatin has been considered as a novel and unique negative regulator of muscle growth, as mstn-/- mice display a dramatic and widespread increase in skeletal muscle mass. Myostatin also appears to be involved in muscle homeostasis in adults as its expression is regulated during muscle atrophy. Moreover, deletion of the myostatin gene seems to affect adipose tissue mass in addition to skeletal muscle mass. Natural myostatin gene mutations occur in cattle breeds such as Belgian Blue, exhibiting an obviously increased muscle mass, but also in humans, as has recently been demonstrated. Here we review these natural mutations and their associated phenotypes as well as the physiological influence of the alterations in myostatin expression and the physiopathological consequences of changes in myostatin expression, especially with regard to satellite cells. Interestingly, studies have demonstrated some rescue effects of myostatin in muscular pathologies such as myopathies, providing a novel pharmacological strategy for treatment. Furthermore, the myostatin pathway is now better understood thanks to in vitro studies and it consists of inhibition of myoblast progression in the cell cycle, inhibition of myoblast terminal differentiation, in both cases associated to protection from apoptosis. The molecular pathway driving the myogenic myostatin influence is currently under extensive study and many molecular partners of myostatin have been identified, suggesting novel potent muscle growth enhancers for both human and agricultural applications.

Role of cell-cycle regulators in lung cancer

Lung cancer is the leading cause of cancer death worldwide. Histologically, 80% of lung cancers are classified as non-small-cell lung cancer (NSCLC), and the remaining 20% as small-cell lung cancer (SCLC). Lung carcinoma is the result of molecular changes in the cell, resulting in the deregulation of pathways controlling normal cellular growth, differentiation, and apoptosis. This review summarizes some of the most recent findings about the role of cell-cycle proteins in lung cancer pathogenesis and progression.

Involvement of protein kinase C and E2F-5 in euxanthone-induced neurite differentiation of neuroblastoma

Euxanthone, a neuritogenic agent isolated from the medicinal herb Polygala caudata, has been shown to induce morphological differentiation and neurite outgrowth in murine neuroblastoma Neuro 2a cells (BU-1 subclone). In order to elucidate the underlying mechanisms of euxanthone-induced neurite outgrowth, a proteomic approach was employed. In the present study, two dimensional (2-D) gel electrophoresis and matrix-assisted laser desorption/ionization-time of flight (MALDI-ToF) mass spectrometry were performed to investigate the alterations in protein expression profile of euxanthone-treated BU-1 cells. Fourteen identified proteins were changed in expression levels after induction of neurite growth. These proteins included participants in transcription and cell cycle regulation, calcium influx and calcium signaling, fatty acid metabolism, cytoskeleton reorganization, casein kinase signal transduction, putative transbilayer amphipath transport and protein biosynthesis. Among the 14 identified proteins, E2F transcription factor 5 (E2F-5) was significantly up-regulated after euxanthone treatment. Go6976, a protein kinase C (PKC) alpha/betaI inhibitor, was found to inhibit neuritogenesis and expression of E2F-5 in the euxanthone-treated BU-1 cells, while SH-6, the Akt/PKB inhibitor, had no inhibitory effect. The gene silencing of E2F-5 by small interfering RNA (siRNA) was found to abolish the euxanthone-induced neurite outgrowth. In conclusion, these results indicated that the transcription factor E2F-5 was actively involved in the regulation of euxanthone-induced neurite outgrowth via PKC pathway.

Phosphatidylinositol 3-kinase is required for the transcriptional activation of cyclin D2 in BCR activated primary mouse B lymphocytes

Induction of cyclin D2 is essential for mediating cell cycle entry in B cells activated by BCR cross-linking. In the present study we show that, like B lymphocytes lacking cyclin D2, the p85alpha subunit of phosphatidylinositol 3-kinase (PI3K) or other components of the B cell signalosome, p110delta-null B cells fail to induce cyclin D2 and enter early G1 but not S phase of the cell cycle. The inhibitors of PI3K activity, LY294002 and Wortmannin, also abrogate cyclin D2 induction by BCR cross-linking, confirming that the class IA PI3K is necessary for cyclin D2 induction in response to BCR stimulation. Furthermore, using both p85alpha-null and p110delta-null B cells and inhibitors of PI3K, this study demonstrates for the first time, that BCR cross-linking induces cyclin D2 mRNA expression via transcriptional activation of the cyclin D2 promoter and that this transcriptional activation of cyclin D2 requires PI3K activity. Moreover, we identify a region between nucleotides -1624 and -1303 of the cyclin D2 promoter containing elements responsive to anti-IgM, which are PI3K dependent. Further characterisation of signalling intermediates downstream of the BCR revealed a perturbation of MAPK signalling pathways in p85alpha-null and p110delta-null B cells, and our data suggests that cross-talk exists between the PI3K and JNK pathways.

A novel mitochondrial protein DIP mediates E2F1-induced apoptosis independently of p53

The transcription factor E2F1 does not only induce cell proliferation but also shows the strongest proapoptotic effect of all E2F family members as part of an antitumor safeguard mechanism. We have recently identified KIAA0767 as a novel p53-independent target of E2F1. Here, we investigated the biological function of interaction. Overexpression studies of KIAA0767, termed D(eath)-I(nducing)-P(rotein), revealed its strong proapoptotic effect. DIP greatly reduced cell viability in several in vitro systems accompanied by typical apoptotic features such as caspase-3 activation and cleavage of poly(ADP-ribose)-polymerase. Endogenous DIP levels increased following E2F1 activation. Yet, inhibition of endogenous DIP function by small interfering RNA rescued p53-negative cells from E2F1-induced apoptosis, indicating that DIP is an essential mediator of the p53-independent E2F1 death pathway. Localization studies showed that DIP localizes to the mitochondria, where endogenous DIP is upregulated following E2F1 induction. These results provide new insights to the incompletely understood regulatory mechanisms of E2F1-induced apoptosis.

EAPP, a novel E2F binding protein that modulates E2F-dependent transcription

E2F transcription factors play an essential role in cell proliferation and apoptosis and their activity is frequently deregulated in human cancers. In a yeast two-hybrid screen we identified a novel E2F-binding protein. Due to its strong phosphorylation we named it EAPP (e2F-associated phosphoprotein). EAPP is localized in the nucleus and interacts with E2F-1, E2F-2, and E2F-3, but not with E2F-4. Examination of a number of human cell lines revealed that EAPP levels are elevated in most transformed cells. Moreover, EAPP mRNA was detected in all investigated human tissues in varying amounts. EAPP is present throughout the cell cycle but disappears during mitosis. In transfection assays with reporters controlled by either an artificial E2F-dependent promoter or the murine thymidine kinase promoter, EAPP increased the activation caused by E2F-1 but not by E2F-4. Surprisingly, the promoter of the p14(ARF) gene, which was also activated by E2F-1, became repressed by EAPP. Overexpression of EAPP in U2OS cells resulted in a significant increase of cells in S-phase, whereas RNAi-mediated knock down of EAPP reduced the fraction of cells in S-phase. Taken together, these data suggest that EAPP modulates E2F-regulated transcription, stimulates proliferation, and may be involved in the malignant transformation of cells.

p55, the Drosophila ortholog of RbAp46/RbAp48, is required for the repression of dE2F2/RBF-regulated genes

Many proteins have been proposed to be involved in retinoblastoma protein (pRB)-mediated repression, but it is largely uncertain which cofactors are essential for pRB to repress endogenous E2F-regulated promoters. Here we have taken advantage of the stream-lined Drosophila dE2F/RBF pathway, which has only two E2Fs (dE2F1 and dE2F2), and two pRB family members (RBF1 and RBF2). With RNA interference (RNAi), we depleted potential corepressors and looked for the elevated expression of groups of E2F target genes that are known to be directly regulated by RBF1 and RBF2. Previous studies have implicated histone deacetylase (HDAC) and SWI/SNF chromatin-modifying complexes in pRB-mediated repression. However, our results fail to support the idea that the SWI/SNF proteins are required for RBF-mediated repression and suggest that a requirement for HDAC activities is likely to be limited to a subset of targets. We found that the chromatin assembly factor p55/dCAF-1 is essential for the repression of dE2F2-regulated targets. The removal of p55 deregulated the expression of E2F targets that are normally repressed by dE2F2/RBF1 and dE2F2/RBF2 complexes in a cell cycle-independent manner but had no effect on the expression of E2F targets that are normally coupled with cell proliferation. The results indicate that the mechanisms of RBF regulation at these two types of E2F targets are different and suggest that p55, and perhaps p55's mammalian orthologs RbAp46 and RbAp48, have a conserved function in repression by pRB-related proteins.

Acquisition of essential somatic cell cycle regulatory protein expression and implied activity occurs at the second to third cell division in mouse preimplantation embryos

It is clear that G1-S phase control is exerted after the mouse embryo implants into the uterus 4.5 days after fertilization (E4.5); null mutants of genes that control cell cycle commitment such as max, rb (retinoblastoma), and dp1 are embryonic lethal after implantation with proliferation phenotypes. But, a number of studies of genes mediating proliferation control in the embryo after fertilization-implantation have yielded confusing results. In order to understand when embryos might first exert G1-S phase regulatory control, we assayed preimplantation mouse embryos for the acquisition of expression of mRNA, protein, and phospho-protein for max, Rb, and DP-1, and for the proliferation-promoting phospho-protein forms of mycC (thr58/ser62) and Rb (ser795). The key findings are that: (1) DP-1 protein was present in the nucleus as early as the four-cell stage onwards, (2) max protein was in the nucleus, suggesting function from the four-cell stage onwards, (3) both mycC and Rb all form protein was present at increasing quantities in the cytoplasm from the 2 cell and 4/8 cell stage, respectively, (4) the phosphorylated form of mycC phospho was present in the nucleus at high levels from the two-cell stage through blastocyst-stage, and (5) the phosphorylated form of Rb was detected at low levels in the two-cell stage embryo and was highly expressed at the 4/8-cell stage through the blastocyst stage. Taken together, these data suggest that activation of mycC phospho/max dimer pairs, (E2F)/DP-1 dimer pairs, and repression of Rb inhibition of cell cycle progression via phosphorylation at ser795 occurs at the earliest stages of embryonic development. In addition, the presence of max, mycC phospho, DP-1, and Rb phospho in the nuclei of embryonic and placental lineage cells in the blastocyst and in trophoblast stem cells suggests that a similar type of cell cycle regulation is present throughout preimplantation development and in both embryonic and extra-embryonic cell lineages.

FoxO3a and BCR-ABL regulate cyclin D2 transcription through a STAT5/BCL6-dependent mechanism

Cell cycle arrest by FoxO transcription factors involves transcriptional repression of cyclin D, although the exact mechanism remains unclear. In this study, we used the BCR-ABL-expressing cell line BV173 as a model system to investigate the mechanisms whereby FoxO3a regulates cyclin D2 expression. Inhibition of BCR-ABL by STI571 results in down-regulation of cyclin D2 expression, activation of FoxO3a activity, and up-regulation of BCL6 expression. Using reporter gene assays, we demonstrate that STI571, FoxO3a, and BCL6 can repress cyclin D2 transcription through a STAT5/BCL6 site located within the cyclin D2 promoter. We propose that BCR-ABL inhibition leads to FoxO3a activation, which in turn induces the expression of BCL6, culminating in the repression of cyclin D2 transcription through this STAT5/BCL6 site. This process was verified by mobility shift and chromatin immunoprecipitation analyses. We find that conditional activation of FoxO3a leads to accumulation of BCL6 and down-regulation of cyclin D2 at protein and mRNA levels. Furthermore, silencing of FoxO3a and BCL6 in BCR-ABL-expressing cells abolishes STI571-mediated effects on cyclin D2. This report establishes the signaling events whereby BCR-ABL signals are relayed to cyclin D2 to mediate cell cycle progression and defines a potential mechanism by which FoxO proteins regulate cyclin D2 expression.

ICI182,780 induces p21Waf1 gene transcription through releasing histone deacetylase 1 and estrogen receptor alpha from Sp1 sites to induce cell cycle arrest in MCF-7 breast cancer cell line

We used the estrogen-responsive MCF-7 breast cancer cell line as a relevant model to study the anti-proliferative effects of ICI182,780 and identified the negative cell cycle regulator p21Waf1 as a specific target of ICI182,780. Furthermore, silencing of the p21Waf1 expression by small interfering RNA overcame the G0/G1 cell cycle arrest induced by ICI182,780, suggesting that the induction of p21Waf1 expression has a direct role in mediating the ICI182,780-induced G0/G1 arrest. We further demonstrated that the induction of p21Waf1 by ICI182,780 is mediated at transcriptional and gene promoter levels through the proximal Sp1 sites located near the transcription start site. Co-immunoprecipitation, DNA "pull-down," and chromatin immunoprecipitation experiments together showed that in cycling cells, estrogen receptor alpha and histone deacetylase 1 (HDAC1) are recruited to the proximal Sp1 sites of the promoter to repress p21Waf1 expression. In the presence of ICI182,780, estrogen receptor alpha and HDACs are dissociated from Sp1, resulting in increased histone acetylation and de-repression of the p21Waf1 promoter and induction of p21Waf1 expression. The fact that p21Waf1 expression is normally repressed by HDAC activity in cycling cells is further demonstrated by the finding that p21Waf1 transcription can be induced by the silencing of HDACs with small interfering RNA or treatment with HDAC inhibitors.

Cell cycle regulation: Repair and regeneration in acute renal failure

Research into mechanisms of acute renal failure has begun to reveal molecular targets for possible therapeutic intervention. Much useful knowledge into the causes and prevention of this syndrome has been gained by the study of animal models. Most recently, investigation of the effects on acute renal failure of selected gene knock-outs in mice has contributed to our recognition of many previously unappreciated molecular pathways. Particularly, experiments have revealed the protective nature of two highly induced genes whose functions are to inhibit and control the cell cycle after acute renal failure. By use of these models we have started to understand the role of increased cell cycle activity after renal stress, and the role of proteins induced by these stresses that limit this proliferation.

Induction of cell cycle regulatory proteins by murine B cell proliferating pectic polysaccharide from the roots of Bupleurum falcatum L

Bupleuran 2IIc, a pectic polysaccharide isolated from the roots of Bupleurum falcatum L., was characterized as a T-cell-independent B cell mitogen, that activates, proliferates and differentiates B cells in vivo and in vitro (Immunology 97 (1999) 540). Studies were focused on elucidating the mechanism by which bupleuran 2IIc causes proliferation of B cells and expression of cell cycle regulatory proteins. B cells showed slower rates of entry into the S and G2/M phases of the cell cycle when stimulated with bupleuran 2IIc versus anti-IgM. However, the Stimulation Index continued up to two times longer with bupleuran 2IIc over anti-IgM. Although both bupleuran 2IIc and anti-IgM induced similar expressions of cell cycle regulatory proteins, cyclins D2, A, and B1, in B cells, those cells stimulated with bupleuran 2IIc appeared to sustain expressions of these protein for longer periods of time. Stimulation of B cells with bupleuran 2IIc induced phosphorylation of retinoblastoma protein, pRB, an important gene product regulating the restriction point, R, which is responsible for the transition from the G0/G1 to the S phases of the cell cycle. The results of this study demonstrate that both bupleuran 2IIc and anti-IgM interact with B cells, thus, leading to expressions of cell cycle regulatory proteins. However, the respective modes of binding and proximity of interactions with the B cell membrane may differ.

Discrete signaling pathways participate in RB-dependent responses to chemotherapeutic agents

The retinoblastoma (RB) tumor suppressor has been proposed to function as a key mediator of cell cycle checkpoints induced by chemotherapeutic agents. However, these prior studies have relied on embryonic fibroblasts harboring chronic loss of RB, a condition under which compensation of RB functions is known to occur. Here we utilized primary adult fibroblasts derived from mice harboring loxP sites flanking exon 3 of the Rb gene to delineate the action of RB in the chemotherapeutic response. In this system we find that targeted disruption of Rb leads to little overt change in cell cycle distribution. However, these cells exhibited deregulation of RB/E2F target genes and became aneuploid following culture in the absence of RB. When challenged with both DNA damaging and antimetabolite chemotherapeutics, RB was required for primary adult cells to undergo DNA damage checkpoint responses and loss of RB resulted in enhanced aneuploidy following challenge. In contrast, following spontaneous immortalization and the loss of functional p53 signaling, the antimetabolite 5-fluorouracil (5-FU) failed to induce arrest despite the presence of RB. In these immortal cultures RB/E2F targets were deregulated in a complex, gene-specific manner and RB was required for the checkpoint response to camptothecin (CPT). Mechanistic analyses of the checkpoint responses in primary cells indicated that loss of RB leads to increased p53 signaling and decreased viability following both CPT and 5-FU treatment. However, the mechanism through which these agents act to facilitate cell cycle inhibition through RB were distinct. These studies underscore the critical role of RB in DNA-damage checkpoint signaling and demonstrate that RB mediates chemotherapeutic-induced cell cycle inhibition in adult fibroblasts by distinct mechanisms.

Cell cycle regulation: repair and regeneration in acute renal failure

Research into mechanisms of acute renal failure has begun to reveal molecular targets for possible therapeutic intervention. Much useful knowledge into the causes and prevention of this syndrome has been gained by the study of animal models. Most recently, investigation of the effects on acute renal failure of selected gene knock-outs in mice has contributed to our recognition of many previously unappreciated molecular pathways. Particularly, experiments have revealed the protective nature of 2 highly induced genes whose functions are to inhibit and control the cell cycle after acute renal failure. By use of these models we have started to understand the role of increased cell cycle activity after renal stress and the role of proteins induced by these stresses that limit this proliferation.

The myelodysplastic syndromes: a matter of life or death

Apoptosis is upregulated in early myelodysplastic syndromes (MDS) and may contribute to the peripheral cytopenias commonly observed. Conversely, leukemic progression is associated with abrogation of programmed cell death (PCD). The stage of hematopoietic cell maturation at which defects in PCD arise and the underlying causes of apoptosis dysregulation remain unknown. This paper outlines the apoptotic process in normal hematopoietic cells and summarizes current data regarding the role, potential causes and clinical implications of altered apoptosis in MDS.

The osmoregulatory and the amino acid-regulated responses of system A are mediated by different signal transduction pathways

The osmotic response of system A for neutral amino acid transport has been related to the adaptive response of this transport system to amino acid starvation. In a previous study (Ruiz-Montasell, B., M. Gómez-Angelats, F.J. Casado, A. Felipe, J.D. McGivan, and M. Pastor-Anglada. 1994. Proc. Natl. Acad. Sci. USA. 91:9569-9573), a model was proposed in which both responses were mediated by different mechanisms. The recent cloning of several isoforms of system A as well as the elucidation of a variety of signal transduction pathways involved in stress responses allow to test this model. SAT2 mRNA levels increased after amino acid deprivation but not after hyperosmotic shock. Inhibition of p38 activity or transfection with a dominant negative p38 did not alter the response to amino acid starvation but partially blocked the hypertonicity response. Inhibition of the ERK pathway resulted in full inhibition of the adaptive response of system A and no increase in SAT2 mRNA levels, without modifying the response to hyperosmolarity. Similar results were obtained after transfection with a dominant negative JNK1. The CDK2 inhibitor peptide-II decreased the osmotic response in a dose-dependent manner but did not have any effect on the adaptive response of system A. In summary, the previously proposed model of up-regulation of system A after hypertonic shock or after amino acid starvation by separate mechanisms is now confirmed and the two signal transduction pathways have been identified. The involvement of a CDK-cyclin complex in the osmotic response of system A links the activity of this transporter to the increase in cell volume previous to the entry in a new cell division cycle.

Genetic models in applied physiology: invited review: effect of oxygen deprivation on cell cycle activity: a profile of delay and arrest

One of the most fascinating fields that have emanated in the past few decades is developmental biology. This is not only the case from a research point of view but also from the angle of clinical care and treatment strategies. It is now well demonstrated that there are many diseases (some believe all diseases) that have their roots in embryogenesis or in early life, where nature and environment often team up to facilitate the genesis of disease. There is probably no better example to illustrate the interactions between nature and environment than in early life, as early as in the first several cell cycles. As will be apparent in this review, the cell cycle is a very regulated activity and this regulation is genetic in nature, with checkpoint proteins playing an important role in controlling the timing, the size, and the growth of daughter cells. However, it is also very clear, as will be discussed in this work, that the microenvironment of the first dividing cells is so important for the outcome of the organism. In this review, we will focus on the effect of one stress, that of hypoxia, on the young embryo and its cell division and growth. We will first review some of the cell cycle definitions and stages and then review briefly our current knowledge and its gaps in this area.

BCR targets cyclin D2 via Btk and the p85alpha subunit of PI3-K to induce cell cycle progression in primary mouse B cells

The p85alpha subunit of PI3-K and Btk are two crucial components of the B-cell receptor (BCR) signalling pathway. In the present study, we showed that primary splenic B cells from p85alpha null and xid (Btk-deficient) mice fail to induce cyclin D2 expression and enter early G1, but not S phase of the cell cycle in response to BCR engagement. Furthermore, these Btk or p85alpha null B cells displayed increased cell death compared with wild type following BCR engagement. These findings are further confirmed by studies showing that specific pharmacological inhibitors of Btk (LFM-A13), PI3-K (LY294002 and Wortmannin) and PLCgamma (U73122) also block cyclin D2 expression and S phase entry following BCR stimulation, as well as triggering apoptosis. Collectively, these data provide evidence for the concept that the B-cell signalosome (p85alpha, Btk, BLNK and PLCgamma) is involved in regulating cyclin D2 expression in response to BCR engagement. PKC and intracellular calcium are two major downstream effectors of the B-cell signalosome and can be activated by PMA and ionomycin, respectively. In small resting (G0) B cells, costimulation with PMA and ionomycin, but not PMA or ionomycin alone, induces cyclin D2 expression and cell-cycle progression. Consistent with this, we also showed that the BCR-mediated cyclin D2 induction could be abolished by pretreatment of resting B cells with specific inhibitors of capacitative Ca(2+) entry (SK&F 96365) or PKC (Gö6850). Our present results lead us to propose a model in which the B-cell signalosome targets cyclin D2 via the Ca(2+) and PKC-dependent signalling cascades to mediate cell-cycle progression in response to BCR engagement.

Telomerase accelerates osteogenesis of bone marrow stromal stem cells by upregulation of CBFA1, osterix, and osteocalcin

Telomerase activity can prevent telomere shortening and replicative senescence in human somatic cells. We and others have previously demonstrated that forced expression of telomerase in human bone marrow stromal stem cells (BMSSCs) was able to extend their life-span and enhance their bone-forming capability, without inducing malignant transformation. In this study, we determined that telomerase was able to accelerate calcium accumulation of human BMSSCs under osteogenic inductive conditions. Similarly, xenogeneic transplantation of telomerase-expressing BMSSCs (BMSSC-Ts) yielded ectopic bone formation at 2 weeks post-transplantation, 2-4 weeks earlier than typically seen with BMSSCs transfected with empty vector (BMSSC-Cs). Low-density DNA array analysis revealed that telomerase activity increases the expression of G1 regulating genes including cyclin D3, cyclin E1, E2F-4, and DP2, associated with hyperphosphorylation of retinoblastoma (pRb), leading to the extended proliferative capacity of BMSSC-Ts. Importantly, BMSSC-T transplants showed a higher number of human osteogenic cells at 8 weeks post transplantation compared with the BMSSC-C transplants, coupled with a significantly increased osteogenic capacity. One possible mechanism leading to accelerated osteogenesis by BMSSC-Ts may be attributed, at least in part, to the upregulation of the important osteogenic genes such as CBFA1, osterix, and osteocalcin in vitro. Taken together, these findings show that telomerase can accelerate cell cycle progression from G1-to-S phase and enhance osteogenic differentiation of BMSSCs, because of the upregulation of CBFA1, osterix, and osteocalcin.

Differential regulation of E2F1, DP1, and the E2F1/DP1 complex by ARF

The tumor suppressor protein ARF inhibits MDM2 to activate and stabilize p53. Recent studies provided evidence for p53-independent tumor suppression functions of ARF. For example, it has been shown that ARF induces proteolysis of certain E2F species, including E2F1. In addition, ARF relocalizes E2F1 from the nucleoplasm to nucleolus and inhibits E2F1-activated transcription. Because DP1 is a functional partner of the E2F family of factors, we investigated whether DP1 is also regulated by ARF. Here we show that DP1 associates with ARF. Coexpression of ARF relocalizes DP1 from the cytoplasm to the nucleolus, suggesting that DP1 is also a target of the ARF regulatory pathways. Surprisingly, however, the E2F1/DP1 complex is refractory to ARF regulation. Coexpression of E2F1 and DP1 blocks ARF-induced relocalization of either subunit to the nucleolus. The E2F1/DP1 complex localizes in the nucleoplasm, whereas ARF is detected in the nucleolus, suggesting that ARF does not interact with the E2F1/DP1 complex. Moreover, we show that E2F1 is more stable in the presence of ARF when coexpressed with DP1. These results suggest that ARF differentially regulates the free and heterodimeric forms of E2F1 and DP1. DP1 is a constitutively expressed protein, whereas E2F1 is mainly expressed at the G(1)/S boundary of the cell cycle. Therefore, the E2F1/DP1 complex is abundant only between late G(1) and early S phase. Our results on the differential regulation E2F1, DP1, and the E2F1/DP1 complex suggest the possibility that ARF regulates the function of these cell cycle factors by altering the dynamics of their heterodimerization during progression from G(1) to S phase.

Gene array identification of Epstein Barr virus-regulated cellular genes in EBV-converted Burkitt lymphoma cell lines

Epstein Barr virus (EBV) is associated with various B-cell neoplasms such as post-transplant lymphoproliferative disease or Burkitt lymphoma. B-lymphocyte reprogramming by EBV involves the control of numerous cellular genes. To identify such EBV-deregulated genes, we have compared the gene expression profile of EBV-negative Burkitt lymphoma cell lines (BL) (BL2, BL30, BL70) with their EBV-converted counterpart (BL2-B95, BL30-B95, BL70-B95) by cDNA array. Statistical analysis of the results was made using Ward's cluster analysis method. Results showed that the expression of up to 26% of the 1176 cellular genes analyzed may be modified in EBV-converted BL cells. Within this set of genes, a subset of genes markedly regulated in EBV-converted BL cells was defined as those for which expression in EBV+ cells was increased or decreased more than 2-fold. Expression of various genes was modulated in agreement with their previously reported regulation by EBV or by transcription factors activated by EBV. Numerous genes were newly identified as modulated in EBV-converted BL cells. Some of these results were verified by both semiquantitative RT-PCR and Western blotting, and were consistent with functional studies. Functional classification of EBV-regulated genes gave a comprehensive picture of cellular reprogramming by EBV in BL, by pointing out cellular modules such as cell cycle, apoptosis, and signal transduction pathways, including BCR and TNF receptor family and interferon pathways. Furthermore, and perhaps most importantly, cDNA array results point to three families of transcription factors, Rel/NF-kappaB, STAT1, and Ets-related proteins Spi-B, Elf-1, and Ets-1 as putative cellular targets of EBV.

The p21(Cip1) protein, a cyclin inhibitor, regulates the levels and the intracellular localization of CDC25A in mice regenerating livers

Liver cells from p21(Cip1-/-) mice subjected to partial hepatectomy (PH) progress into DNA synthesis faster than those from wild-type mice. These cells also show a premature induction of cyclin E/cyclin-dependent kinase (CDK) 2 activity. We studied the mechanisms whereby cells lacking p21(Cip1) showed a premature induction of this activity. Whereas the levels of CDK2, cyclin E, and p27(Kip1) were similar in both wild-type and p21(Cip1-/-) mice, those of the activator CDC25A were much higher in p21(Cip1-/-) quiescent and regenerating livers than in wild-type animals. Moreover, p21(Cip1-/-) cells also showed a premature translocation of CDC25A from cytoplasm into the nucleus. The ectopic expression of p21(Cip1) into mice embryo fibroblasts from p21(Cip1-/-) mice decreased the levels of CDC25A and delayed its nuclear translocation. The levels of CDC25A messenger RNA in p21(Cip1-/-) cells were higher than in wild-type cells, suggesting that this increase might be responsible, at least in part, for the high levels of CDC25A protein in these cells. Thus, the results reported here indicate that p21(Cip1) regulates the levels and the intracellular localization of CDC25A. We also found a good correlation between CDC25A nuclear translocation and cyclin E/CDK2 activation. In conclusion, premature translocation of CDC25A to the nucleus might be involved in the advanced induction of cyclin E/CDK2 activity and DNA replication in cells from animals lacking p21(Cip1).

The human p73 promoter: characterization and identification of functional E2F binding sites

p73, a member of the p53 family, is overexpressed in many cancers. To understand the mechanism(s) underlying this overexpression, we have undertaken a detailed characterization of the human p73 promoter. The promoter is strongly activated in cells expressing exogenous E2F1 and suppressed by exogenous Rb. At least three functional E2F binding sites, located immediately upstream of exon 1 (at -284, -155 and -132) mediate this induction. 5' serially deleted promoter constructs and constructs harboring mutated E2F sites were analyzed for their response to exogenously expressed E2F1 or Rb to establish functionality of these sites. Authenticity of E2F sites was further confirmed by electrophoretic mobility shift assay (EMSA) using E2F1/DP1 heterodimers synthesized in vitro, followed by competition assays with unlabeled wild-type or mutant oligonucleotides and supershift analysis using anti-E2F1 antibodies. In vivo binding of E2F1 to the p73 promoter was demonstrated using nuclear extracts prepared from E2F1-inducible Saos2 cells. The region conferring the highest promoter activity was found to reside between -113 to -217 of the p73 gene. Two of the three functional E2F sites (at -155 and -132) reside within this region. Our results suggest that regulation of p73 expression is primarily mediated through binding of E2F1 to target sites at -155 and -132.

H2O2 induces a transient multi-phase cell cycle arrest in mouse fibroblasts through modulating cyclin D and p21Cip1 expression

To defend against the potential damages induced by reactive oxygen species, proliferating cells enter a transient cell cycle arrest. We treated mouse fibroblasts with H(2)O(2) and found that sublethal doses of H(2)O(2) induced a transient multi-phase cell cycle arrest at the G(1), S, and G(2) phases but not the M phase. Western blot analysis demonstrated that this transient cell cycle arrest is associated with the down-regulation of cyclins D1 and D3 and up-regulation of the CKI p21(Cip1) expression. We also demonstrate that the induction in p21(Cip1) expression by H(2)O(2) is at least partially mediated at the transcriptional level and can occur in the absence of p53 function. Further immunoprecipitation kinase and immunodepletion assays indicated that in response to H(2)O(2) treatment, the down-regulation of cyclin Ds expression are associated with repression of cyclin D-CDK4, whereas the accumulation of p21(Cip1) is responsible for the inhibition of cyclin E and A-CDK2 activity and associated with the down-regulation of cyclin B-CDC2 activity. These data could account for the cell cycle arrest at the G(1), S, and G(2) phases following H(2)O(2) stimulation. Deletion of p21(Cip1), restoration of cyclin D expression, or overexpression of cyclin E alone is insufficient to effectively overcome the cell cycle arrest caused by sublethal doses of H(2)O(2). By contrast, overexpression of the human Herpesvirus 8 K cyclin, which can mimic the function of cyclin D and E, is enough to override this transient cell cycle arrest. On the basis of our findings, we propose a model in which moderate levels of H(2)O(2) induce a transient multi-phase cell cycle arrest at least partially through up-regulation of p21(Cip1) and down-regulation of cyclin D expression.

Gene expression changes in response to E2F1 activation

The p16/RB/E2F regulatory pathway, which controls transit through the G1 restriction point of the cell cycle, is one of the most frequent targets of genetic alterations in human cancer. Any of these alterations results in the deregulated expression of the transcription factor E2F, one of the key mediators of cell cycle progression. Under these conditions, E2F1 also participates in the induction of apoptosis by a p53-dependent pathway, and independently of p53. Recently, we identified the p53-homolog p73 as a first direct target of p53-independent apoptosis. Here, we used a cDNA microarray to screen an inducible E2F1-expressing Saos-2 cell line for E2F1 target genes. Expression analysis by cDNA microarray and RT-PCR revealed novel E2F1 target genes involved in E2F1-regulated cellular functions such as cell cycle control, DNA replication and apoptosis. In addition, the identification of novel E2F1 target genes participating in the processes of angiogenesis, invasion and metastasis supports the view that E2F1 plays a central role in many aspects of cancer development. These results provide new insight into the role of E2F1 in tumorigenesis as a basis for the development of novel anti-cancer therapeutics.

Ectopic pro-opiomelanocortin syndrome

Ectopic POMC syndrome remains one of the most challenging differential diagnoses in endocrinology. Recent progress in the understanding of the tissue specific regulation of POMC gene expression and new insights into the processing of the POMC peptide in nonpituitary tissues has helped elucidate some of the molecular events leading to ectopic expression and secretion of POMC peptides. Corticotropin and other POMC-derived peptides have diverse effects on adrenal steroidogenesis, growth, and extra-adrenal tissues. Differences in POMC gene regulation in the corticotrope versus ectopic POMC-producing tumors provides a scientific framework for the clinical distinction between eutopic and ectopic Cushing's syndrome. In an attempt to revisit recent basic and clinical advances in the diagnosis of ectopic POMC syndrome the authors undertook an extensive literature review of 530 cases in 197 published papers and provided a molecular biologic, demographic and diagnostic update. According to this review, the four most common causes of ectopic POMC syndrome are the small cell carcinoma of the lung (27%), bronchial carcinoids (21%), islet cell tumor of the pancreas (16%), and thymic carcinoids (10%). Although the clinical features of patients with ectopic POMC syndrome are similar to those with Cushing's disease, subgroup analysis reveals a broad spectrum of severity and progression of signs and symptoms of hypercortisolism. The endocrine workup of a patient with suspected ectopic POMC syndrome includes the establishment of pathologic hypercortisolism, diagnosis of corticotropin dependency, and the differential diagnosis of corticotropin-dependent Cushing's syndrome. The use of a variety of baseline endocrine values, dynamic endocrine testing, and invasive procedures leads to the correct diagnosis in the majority of patients with ectopic POMC syndrome. Diagnostic imaging, including conventional radiological techniques and somatostatin receptor scintigraphy, aids in the correct localization and eventual treatment of ectopic POMC production.

Role of the retinoblastoma (pRb)-E2F/DP pathway in cancer chemopreventive effects of green tea polyphenol epigallocatechin-3-gallate

Because of the demonstrated role of green tea polyphenol epigallocatechin-3-gallate (EGCG) in cancer chemoprevention, there is considerable emphasis in understanding its mechanism of action. In this study, we assessed the involvement of the retinoblastoma (pRb)-E2F/DP pathway as an important contributor in the antiproliferative effects of EGCG. As shown by immunoblot analysis, EGCG treatment of A431 cells resulted in a dose- as well as time-dependent decrease in the total pRb with a relative increase in the hypophosphorylated form of pRb. EGCG also resulted in serine-780 phosphorylation of pRb in these cells. Further, EGCG was found to downregulate the protein expression of other members of the pRb family, viz. p130 and p107, in a dose- as well as time-dependent manner. This response was accompanied by downregulation in the protein expression of the E2F (1 through 5) family of transcription factors and their heterodimeric partners DP1 and DP2. Taken together, our study suggests that EGCG causes a downregulation of hyperphosphorylated pRb protein with a relative increase in hypophosphorylated pRb that, in turn, compromises with the availability of "free" E2F. This series of events leads to stoppage of cell cycle progression at the G1-->S phase transition thereby causing G0/G1 arrest and subsequent apoptotic cell death. This, to our knowledge, is the first study showing the involvement of the pRb-E2F/DP pathway in antiproliferative and apoptotic effects of EGCG.

Novel function of the cyclin A binding site of E2F in regulating p53-induced apoptosis in response to DNA damage

We demonstrate here that the E2F1 induced by DNA damage can bind to and promote the apoptotic function of p53 via the cyclin A binding site of E2F1. This function of E2F1 does not require its DP-1 binding, DNA binding, or transcriptional activity and is independent of mdm2. All the cyclin A binding E2F family members can interact and cooperate with p53 to induce apoptosis. This suggests a novel role for E2F in regulating apoptosis in response to DNA damage. Cyclin A, but not cyclin E, prevents E2F1 from interacting and cooperating with p53 to induce apoptosis. However, in response to DNA damage, cyclin A levels decrease, with a concomitant increase in E2F1-p53 complex formation. These results suggest that the binding of E2F1 to p53 can specifically stimulate the apoptotic function of p53 in response to DNA damage.

Involvement of the retinoblastoma (pRb)-E2F/DP pathway during antiproliferative effects of resveratrol in human epidermoid carcinoma (A431) cells

Resveratrol (trans-3,4',5-trihydroxystilbene), a polyphenolic phytoalexin found in grapes, nuts, many other fruits, and red wine, is a potent antioxidant with anti-inflammatory and cancer-preventive properties. The mechanism(s) by which resveratrol imparts cancer chemopreventive effects has not been clearly defined. Earlier, we have shown that resveratrol treatment results in an induction of the cyclin kinase inhibitor WAF1/CIP1/p21 which, by inhibiting cyclin (E, D1, and D2) and cyclin-dependent kinases (cdk2, cdk4, and cdk6), results in a G0/G1-phase arrest followed by apoptosis of A431 human epidermoid carcinoma cells (Ahmad et al., Clin. Cancer Res. 7, 1466-1473, 2001). Retinoblastoma (pRb) and the E2F family of transcription factors are important proteins, which regulate the progression of the cell cycle at and near the G1-->S phase transition. Here we provide evidence for the involvement of the pRb-E2F/DP pathway as an important contributor of resveratrol-mediated cell cycle arrest and apoptosis. Immunoblot analysis demonstrated that resveratrol treatment of A431 cells results in a dose- as well as time-dependent decrease in the hyperphosphorylated form of pRb with a relative increase in hypophosphorylated pRb. This response was accompanied by downregulation of protein expression of all five E2F ( 1 - 5 ) family members of transcription factors studied and their heterodimeric partners DP1 and DP2. This suggests that resveratrol causes a downregulation of hyperphosphorylated pRb protein with a relative increase in hypophosphorylated pRb that, in turn, compromises with the availability of free E2F. We suggest that this series of events results in a stoppage of the cell cycle progression at the G1-->S phase transition thereby leading to a G0/G1 arrest and subsequent apoptotic cell death. To our knowledge, this is the first study showing the involvement of the pRb-E2F/DP pathway as a mechanism of the cancer-chemopreventive effects of resveratrol.

BCR signals target p27(Kip1) and cyclin D2 via the PI3-K signalling pathway to mediate cell cycle arrest and apoptosis of WEHI 231 B cells

Cross-linking of the B cell antigen receptor (BCR) on immature WEHI 231 B cells results in G1 cell cycle arrest and apoptosis. Here we investigated the molecular mechanisms that are necessary and sufficient for these changes to occur. We show that BCR stimulation of WEHI 231 cells results in down-regulation of cyclin D2 and up-regulation of p27(Kip1), which are associated with pocket protein hypophosphorylation and E2F inactivation. Ectopic expression of p27(Kip1) by TAT-fusion protein or retroviral transduction is sufficient to cause G1 cell cycle arrest, followed by apoptosis. In contrast, over-expression of cyclin D2 overcomes the cell cycle arrest and apoptosis induced by anti-IgM, indicating that down-regulation of cyclin D2 is necessary for the cell cycle arrest and apoptosis activated by BCR stimulation. Thus, cyclin D2 and p27(Kip1) have opposing roles in these pathways and our data also suggest that cyclin D2 functions upstream of p27(Kip1) and the pRB pathway and therefore plays an essential part in integrating the signals from BCR with the cell cycle machinery. We next investigated which signal transduction pathways triggered by the BCR regulate cell proliferation and apoptosis via cyclin D2 and p27(Kip1). Inhibition of PI3-K signalling by LY294002 down-regulated cyclin D2 and up-regulated p27(Kip1) expression at both protein and RNA levels, mimicking the effects of BCR-stimulation. Furthermore, ectopic expression of a constitutively active form of AKT blocked the cell cycle arrest and apoptosis triggered by anti-IgM and also abrogated down-regulation of cyclin D2 and up-regulation of p27(Kip1) expression induced by BCR-engagement. These results indicate that BCR activation targets p27(Kip1) and cyclin D2 to mediate cell cycle arrest and apoptosis and that down-regulation of PI3-K/AKT activity post BCR stimulation is necessary for these to occur.

Nuclear membrane protein LAP2β mediates transcriptional repression alone and together with its binding partner GCL (germ-cell-less)

LAP2β is an integral membrane protein of the nuclear envelope involved in chromatin and nuclear architecture. Using the yeast two-hybrid system, we have cloned a novel LAP2β-binding protein, mGCL, which contains a BTB/POZ domain and is the mouse homologue of the Drosophila germ-cell-less (GCL) protein. In Drosophila embryos, GCL was shown to be essential for germ cell formation and was localized to the nuclear envelope. Here, we show that, in mammalian cells, GCL is co-localized with LAP2β to the nuclear envelope. Nuclear fractionation studies reveal that mGCL acts as a nuclear matrix component and not as an integral protein of the nuclear envelope. Recently, mGCL was found to interact with the DP3α component of the E2F transcription factor. This interaction reduced the transcriptional activity of the E2F-DP heterodimer, probably by anchoring the complex to the nuclear envelope. We demonstrate here that LAP2β is also capable of reducing the transcriptional activity of the E2F-DP complex and that it is more potent than mGCL in doing so. Co-expression of both LAP2β and mGCL with the E2F-DP complex resulted in a reduced transcriptional activity equal to that exerted by the pRb protein.