Regulation of the cdk inhibitor p21 gene during cell cycle progression is under the control of the transcription factor E2F

Oncogenic MORC2 in cancer development and beyond

Microrchidia CW-type zinc finger 2 (MORC2) is a member of the MORC superfamily of nuclear proteins. Growing evidence has shown that MORC2 not only participates in gene transcription and chromatin remodeling but also plays a key in human disease and tumor development by regulating the expression of downstream oncogenes or tumor suppressors. The present review provides an updated overview of MORC2 in the aspect of cancer hallmark and therapeutic resistance and summarizes its upstream regulators and downstream target genes. This systematic review may provide a favorable theoretical basis for emerging players of MORC2 in tumor development and new insight into the potential clinical application of basic science discoveries in the future.

ETV5 regulates proliferation and cell cycle genes in the INS-1 (832/13) cell line independently of the concentration of secreted insulin

The transcription factor E-twenty-six variant 5 (ETV5) regulates acute insulin secretion. Adequate insulin secretion is dependent on pancreatic β-cell size and cell proliferation, but the effects of ETV5 on proliferation, cell number, and viability, as well as its relationship with insulin secretion, have not been established yet. Here, we partially silenced ETV5 in the INS-1 (832/13) cell line by siRNA transfection and then measured secreted insulin concentration at different time points, observing similar levels to control cells. After 72 h of ETV5 silencing, we observed decreased cell number and proliferation, without any change in viability or apoptosis. Thus, partial silencing of ETV5 modulates cell proliferation in INS-1 (832/13) independently of secreted insulin levels via upregulation of E2F1 and of inhibitors of the cyclin/CDKs complexes (p21Cdkn1a , p27Cdkn1b , and p57Cdkn1c ) and downregulation of cell cycle activators (PAK3 and FOS).

The Interplay of NEAT1 and miR-339-5p Influences on Mesangial Gene Expression and Function in Various Diabetic-Associated Injury Models

Mesangial cells (MCs), substantial cells for architecture and function of the glomerular tuft, take a key role in progression of diabetic kidney disease (DKD). Despite long standing researches and the need for novel therapies, the underlying regulatory mechanisms in MCs are elusive. This applies in particular to long non-coding RNAs (lncRNA) but also microRNAs (miRNAs). In this study, we investigated the expression of nuclear paraspeckle assembly transcript 1 (NEAT1), a highly conserved lncRNA, in several diabetes in-vitro models using human MCs. These cells were treated with high glucose, TGFβ, TNAα, thapsigargin, or tunicamycin. We analyzed the implication of NEAT1 silencing on mesangial cell migration, proliferation, and cell size as well as on mRNA and miRNA expression. Here, the miRNA hsa-miR-339-5p was not only identified as a potential interaction partner for NEAT1 but also for several coding genes. Furthermore, overexpression of hsa-miR-339-5p leads to a MC phenotype comparable to a NEAT1 knockdown. In-silico analyses also underline a relevant role of NEAT1 and hsa-miR-339-5p in mesangial physiology, especially in the context of DKD.

Tumor suppressor p53 promotes ferroptosis in oxidative stress conditions independent of modulation of ferroptosis by p21, CDKs, Rb and E2F

p53 is a well-established critical cell cycle regulator. By inducing transcription of the gene encoding p21, p53 inhibits cyclin-dependent kinase (CDK)-mediated phosphorylation of cell cycle inhibitor RB proteins. Phosphorylation of RB releases E2F transcription factor proteins that transactivate cell cycle-promoting genes. Here we sought to uncover the contribution of p53, p21, CDK, RB, and E2F to the regulation of ferroptosis, an oxidative form of cell death. Our studies have uncovered unexpected complexity in this regulation. First, we showed that elevated levels of p53 enhance ferroptosis in multiple inducible and isogenic systems. On the other hand, we found that p21 suppresses ferroptosis. Elevation of CDK activity also suppressed ferroptosis under conditions where p21 suppressed ferroptosis, suggesting that the impact of p21 must extend beyond CDK inhibition. Furthermore, we showed that overexpression of E2F suppresses ferroptosis in part via a p21-dependent mechanism, consistent with reports that this transcription factor can induce transcription of p21. Finally, deletion of RB genes enhanced ferroptosis. Taken together, these results show that signals affecting ferroptotic sensitivity emanate from multiple points within the p53 tumor suppressor pathway.

PAQR3 Inhibits Non-small Cell Lung Cancer Growth by Regulating the NF-κB/p53/Bax Axis

Background

The expression of progestin and adipoQ receptor 3 (PAQR3) is generally downregulated in multiple tumors, which is associated with tumor progression and poor prognosis.

Methods

The clinical value of PAQR3 was analyzed using various databases and in 60 patients with non-small cell lung cancer (NSCLC). In addition, cell counting kit-8 (CCK-8), colony formation, and flow cytometry assays were used to evaluate the effect of PAQR3 on the growth of NSCLC cells in vitro. Gene set enrichment analysis (GSEA) was performed to investigate the possible mechanism through which PAQR3 is involved in the progression of lung cancer. Furthermore, western blotting was employed to verify the relevant mechanism.

Results

The expression of PAQR3 was decreased in 60 NSCLC patients and was related to the histological subtype, lymph node metastasis, tumor size, and diagnosis of NSCLC. Patients with lung adenocarcinoma with increased PAQR3 expression tended to have a better prognosis. Besides, PAQR3 inhibited proliferation, clone formation, and cycle transition in NSCLC cells, but induced apoptosis. The results of GSEA showed that PAQR3 regulated the progression of lung cancer by affecting cell cycle, DNA replication, and the p53 signaling pathway. We confirmed that PAQR3 overexpression inhibited the expression of NF-κB, while it increased the expression of p53, phospho-p53, and Bax. On the contrary, PAQR3 inhibition played an opposite role in these proteins.

Conclusion

PAQR3 inhibited the growth of NSCLC cells through the NF-κB/P53/Bax signaling pathway and might be a new target for diagnosis and treatment.

Differential Impacts on Host Transcription by ROP and GRA Effectors from the Intracellular Parasite Toxoplasma gondii

The intracellular parasite Toxoplasma gondii employs a vast array of effector proteins from the rhoptry and dense granule organelles to modulate host cell biology; these effectors are known as ROPs and GRAs, respectively. To examine the individual impacts of ROPs and GRAs on host gene expression, we developed a robust, novel protocol to enrich for ultrapure populations of a naturally occurring and reproducible population of host cells called uninfected-injected (U-I) cells, which Toxoplasma injects with ROPs but subsequently fails to invade. We then performed single-cell transcriptomic analysis at 1 to 3 h postinfection on U-I cells (as well as on uninfected and infected controls) arising from infection with either wild-type parasites or parasites lacking the MYR1 protein, which is required for soluble GRAs to cross the parasitophorous vacuole membrane (PVM) and reach the host cell cytosol. Based on comparisons of infected and U-I cells, the host's earliest response to infection appears to be driven primarily by the injected ROPs, which appear to induce immune and cellular stress pathways. These ROP-dependent proinflammatory signatures appear to be counteracted by at least some of the MYR1-dependent GRAs and may be enhanced by the MYR-independent GRAs (which are found embedded within the PVM). Finally, signatures detected in uninfected bystander cells from the infected monolayers suggest that MYR1-dependent paracrine effects also counteract inflammatory ROP-dependent processes.IMPORTANCE This work performs transcriptomic analysis of U-I cells, captures the earliest stage of a host cell's interaction with Toxoplasma gondii, and dissects the effects of individual classes of parasite effectors on host cell biology.

Rb and p53-Deficient Myxofibrosarcoma and Undifferentiated Pleomorphic Sarcoma Require Skp2 for Survival

Myxofibrosarcoma (MFS) and undifferentiated pleomorphic sarcoma (UPS) are highly genetically complex soft tissue sarcomas. Up to 50% of patients develop distant metastases, but current systemic therapies have limited efficacy. MFS and UPS have recently been shown to commonly harbor copy number alterations or mutations in the tumor suppressor genes RB1 and TP53. As these alterations have been shown to engender dependence on the oncogenic protein Skp2 for survival of transformed cells in mouse models, we sought to examine its function and potential as a therapeutic target in MFS/UPS. Comparative genomic hybridization and next-generation sequencing confirmed that a significant fraction of MFS and UPS patient samples (n = 94) harbor chromosomal deletions and/or loss-of-function mutations in RB1 and TP53 (88% carry alterations in at least one gene; 60% carry alterations in both). Tissue microarray analysis identified a correlation between absent Rb and p53 expression and positive expression of Skp2. Downregulation of Skp2 or treatment with the Skp2-specific inhibitor C1 revealed that Skp2 drives proliferation of patient-derived MFS/UPS cell lines deficient in both Rb and p53 by degrading p21 and p27. Inhibition of Skp2 using the neddylation-activating enzyme inhibitor pevonedistat decreased growth of Rb/p53-negative patient-derived cell lines and mouse xenografts. These results demonstrate that loss of both Rb and p53 renders MFS and UPS dependent on Skp2, which can be therapeutically exploited and could provide the basis for promising novel systemic therapies for MFS and UPS. SIGNIFICANCE: Loss of both Rb and p53 renders myxofibrosarcoma and undifferentiated pleomorphic sarcoma dependent on Skp2, which could provide the basis for promising novel systemic therapies.See related commentary by Lambert and Jones, p. 2437.

Knockdown of arsenic resistance protein 2 inhibits human glioblastoma cell proliferation through the MAPK/ERK pathway

It is generally known that glioblastoma is the most common primary malignant brain tumor and that it is highly aggressive and deadly. Although surgical and pharmacological therapies have made long‑term progress, glioblastoma remains extremely lethal and has an uncommonly low survival rate. Therefore, further elucidation of the molecular mechanisms of glioblastoma initiation and its pathological processes are urgent. Arsenic resistance protein 2 (Ars2) is a highly conserved gene, and it has been found to play an important role in microRNA biosynthesis and cell proliferation in recent years. Furthermore, absence of Ars2 results in developmental death in Drosophila, zebrafish and mice. However, there are few studies on the role of Ars2 in regulating tumor development, and the mechanism of its action is mostly unknown. In the present study, we revealed that Ars2 is involved in glioblastoma proliferation and we identified a potential mechanistic role for it in cell cycle control. Our data demonstrated that Ars2 knockdown significantly repressed the proliferation and tumorigenesis abilities of glioblastoma cells in vitro and in vivo. Further investigation clarified that Ars2 deficiency inhibited the activation of the MAPK/ERK pathway, leading to cell cycle arrest in the G1 phase, resulting in suppression of cell proliferation. These findings support the conclusion that Ars2 is a key regulator of glioblastoma progression.

Phenolics extracted from tartary (Fagopyrum tartaricum L. Gaerth) buckwheat bran exhibit antioxidant activity, and an antiproliferative effect on human breast cancer MDA-MB-231 cells through the p38/MAP kinase pathway

Phenolics extracted from tartary buckwheat (Fagopyrum tartaricum L. Gaerth) bran were analyzed quantitatively and qualitatively. The bioactivity of the phenolic extracts was evaluated, such as the antioxidant activity, and the inhibition capacity on the growth of cancer cells. The molecular mechanism for the inhibitive effect on cancer cells was explored. Results indicated that tartary buckwheat bran phenolics mainly exist in a free form, and free phenolics were twice as abundant as bound phenolics. Free caffeic acid (119.75 μg per 100 mg DW) and bound rutin (51.66 μg per 100 mg DW) represented the main free and bound phenolic compounds, respectively. The free phenolic extract contributed to the major (>90%) antioxidant activities including the oxygen radical antioxidant capacity (ORAC) and cellular antioxidant activity (CAA). The free phenolic extract exhibited anticancer activity for human breast cancer MDA-MB-231 cells in a dose-dependent manner. This significant inhibition effect was achieved through the p38/MAP kinase pathway by inducing cell apoptosis (up-regulating p-p38 and p-ASK1 expressions and down-regulating TRAF2 and p-p53 expressions), and negatively regulating the progression of the cell cycle from the G1 to S phase (increased expression of p21 and suppressed expressions of PCNA, cyclin D1 and CDK4). All these results indicated that tartary buckwheat bran could be a rich resource of natural antioxidants and inhibitors for the growth of MDA-MB-231 cells.

Dependence of p53-deficient cells on the DHX9 DExH-box helicase

DHX9 is a DExH-box helicase family member with key regulatory roles in a broad range of cellular processes. It participates at multiple levels of gene regulation, including DNA replication, transcription, translation, RNA transport, and microRNA processing. It has been implicated in tumorigenesis and recent evidence suggests that it may be a promising chemotherapeutic target. Previous studies have determined that DHX9 suppression elicits an apoptotic or senescence response by activating p53 signaling. Here, we show that DHX9 inhibition can also have deleterious effects in cells lacking functional p53. Loss of DHX9 led to increased cell death in p53-deficient mouse lymphomas and HCT116 human colon cancer cells, and G0/G1 cell cycle arrest in p53-deficient mouse embryonic fibroblasts. Analysis of mRNA levels for p53 transcriptional targets showed that a subset of p53 targets in the p53-null lymphomas and HCT116 cells were activated despite the absence of functional p53. This implies an alternative pathway of DHX9-mediated activation of cell death and cell cycle arrest in p53-deficient cells and supports the feasibility of targeting DHX9 in p53-deficient tumors.

Cell-derived micro-environment helps dental pulp stem cells promote dental pulp regeneration

OBJECTIVES

The function of the dental pulp is closely connected to the extracellular matrix (ECM) structure, and ECM has received significant attention due to its biological functions for regulating cells. As such, the interaction between the ECM niche and cells is worth exploring for potential clinical uses.

MATERIALS AND METHODS

In this study, dental pulp stem cell (DPSC)-derived ECM (DPM) was prepared through cell culture and decellularization to function as the cell niche, and changes in DPSC behaviour and histological analysis of dental pulp tissue regeneration were evaluated following the DPM culture. DPM promoted the replication of DPSCs and exhibited retention of their mineralization. Then, the DPM-based culture strategy under odontogenic culture medium was further investigated, and the mineralization-related markers showed that DPSCs were regulated towards odontogenic differentiation. Dental pulp-like tissue with well-arranged ECM was harvested after a 2-month subcutaneous implantation in nude mice with DPM application. Additionally, DPSCs cultured on the plastic culture surface showed the up-regulation of mineralization makers in vitro, but there was a disorder in matrix formation and mineralization when the cells were cultured in vivo.

RESULTS AND CONCLUSIONS

DPM-based cultivation could serve as a cell niche and modulate DPSC behaviour, and this method also provided an alternative to harvest tissue-specific ECM and provided a strategy for ECM-cell interaction.

By recruiting HDAC1, MORC2 suppresses p21 Waf1/Cip1 in gastric cancer

Microrchidia (MORC) family CW-type zinc-finger 2 (MORC2) regulates chromatin remodeling during the DNA-damage response, represses gene transcription, promotes lipogenesis. Here, we found that MORC2 down-regulated p21 by recruiting HDAC1 to the p21 promoter, in a p53-independent manner. MORC2-mediated down-regulation of p21 in turn promoted cell cycle progression in gastric cancer cells. Furthermore, MORC2 expression correlated negatively with p21 expression in gastric tumors in patients. We suggest that MORC2 may be a potential therapeutic target in cancer.

PAK1-mediated MORC2 phosphorylation promotes gastric tumorigenesis

To date, microrchidia (MORC) family CW-type zinc-finger 2 (MORC2), has been found to be involved in p21-activated kinase1 (PAK1) pathway to maintain genomic integrity. Here, we explore its novel role in cancer. We demonstrate that PAK1-mediated MORC2 phosphorylation promotes cell cycle progression, defective phosphorylation of MORC2-S677A results in attenuated cell proliferation and tumorigenicity of gastric cancer cells, which is significantly enhanced in overexpression of phospho-mimic MORC2-S677E form, suggesting the importance of MORC2 phosphorylation in tumorigenesis. More importantly, phosphorylation of MORC2 correlates positively with PAK1 expression in clinical gastric cancer. Furthermore, high expression of PAK1 and phosphorylation of MORC2 appear to be associated with poor prognosis of clinical gastric cancer. Collectively, these findings revealed a novel function of MORC2 phosphorylation in promoting gastric cell proliferation in vitro and tumorigenesis in vivo, suggesting that blocking PAK1-mediated MORC2 phosphorylation might be a potential therapeutic strategy for gastric tumorigenesis.

2α-Hydroxyursolic Acid Inhibited Cell Proliferation and Induced Apoptosis in MDA-MB-231 Human Breast Cancer Cells through the p38/MAPK Signal Transduction Pathway

The mechanisms of action of 2α-hydroxyursolic acid in inhibiting cell proliferation and inducing apoptosis in MDA-MB-231 human breast cancer cells were investigated. The antiproliferative activity and cytotoxicity were determined by the methylene blue assay. The expression of proteins was determined using Western blot. 2α-Hydroxyursolic acid significantly inhibited MDA-MB-231 cell proliferation, and no cytotoxicity was observed at concentrations below 30 μM. 2α-Hydroxyursolic acid significantly down-regulated expressions of TRAF2, PCNA, cyclin D1, and CDK4 and up-regulated the expressions of p-ASK1, p-p38, p-p53, and p-21. 2α-Hydroxyursolic acid induced apoptosis in MDA-MB-231 cells by significantly increasing the Bax/Bcl-2 ratio and inducing the cleaved caspase-3. Additionally, treatment of SB203580, a p38 MAPK specific inhibitor, reversed the inhibition of PCNA, cyclin D1, and Bcl-2 expression induced by 2α-hydroxyursolic acid in MDA-MB-231 cells. These results suggested that 2α-hydroxyursolic acid exhibited anticancer activity through the inhibition of cell proliferation and the induction of apoptosis by regulating the p38/MAPK signal transduction pathway.

Motif co-regulation and co-operativity are common mechanisms in transcriptional, post-transcriptional and post-translational regulation

A substantial portion of the regulatory interactions in the higher eukaryotic cell are mediated by simple sequence motifs in the regulatory segments of genes and (pre-)mRNAs, and in the intrinsically disordered regions of proteins. Although these regulatory modules are physicochemically distinct, they share an evolutionary plasticity that has facilitated a rapid growth of their use and resulted in their ubiquity in complex organisms. The ease of motif acquisition simplifies access to basal housekeeping functions, facilitates the co-regulation of multiple biomolecules allowing them to respond in a coordinated manner to changes in the cell state, and supports the integration of multiple signals for combinatorial decision-making. Consequently, motifs are indispensable for temporal, spatial, conditional and basal regulation at the transcriptional, post-transcriptional and post-translational level. In this review, we highlight that many of the key regulatory pathways of the cell are recruited by motifs and that the ease of motif acquisition has resulted in large networks of co-regulated biomolecules. We discuss how co-operativity allows simple static motifs to perform the conditional regulation that underlies decision-making in higher eukaryotic biological systems. We observe that each gene and its products have a unique set of DNA, RNA or protein motifs that encode a regulatory program to define the logical circuitry that guides the life cycle of these biomolecules, from transcription to degradation. Finally, we contrast the regulatory properties of protein motifs and the regulatory elements of DNA and (pre-)mRNAs, advocating that co-regulation, co-operativity, and motif-driven regulatory programs are common mechanisms that emerge from the use of simple, evolutionarily plastic regulatory modules.

Organ size control is dominant over Rb family inactivation to restrict proliferation in vivo

In mammals, a cell's decision to divide is thought to be under the control of the Rb/E2F pathway. We previously found that inactivation of the Rb family of cell cycle inhibitors (Rb, p107, and p130) in quiescent liver progenitors leads to uncontrolled division and cancer initiation. Here, we show that, in contrast, deletion of the entire Rb gene family in mature hepatocytes is not sufficient for their long-term proliferation. The cell cycle block in Rb family mutant hepatocytes is independent of the Arf/p53/p21 checkpoint but can be abrogated upon decreasing liver size. At the molecular level, we identify YAP, a transcriptional regulator involved in organ size control, as a factor required for the sustained expression of cell cycle genes in hepatocytes. These experiments identify a higher level of regulation of the cell cycle in vivo in which signals regulating organ size are dominant regulators of the core cell cycle machinery.

The C-terminal domain of Brd2 is important for chromatin interaction and regulation of transcription and alternative splicing

This study determines genes that are regulated by Brd2 and finds that, in addition to expression control, Brd2 modulates the alternative splicing of several hundred genes. The in vivo interaction of Brd2 with chromatin is analyzed, and the contributions of individual Brd2 domains to the chromatin interaction are determined.

Brd2 is a member of the bromodomain extra terminal (BET) protein family, which consists of four chromatin-interacting proteins that regulate gene expression. Each BET protein contains two N-terminal bromodomains, which recognize acetylated histones, and the C-terminal protein–protein interaction domain. Using a genome-wide screen, we identify 1450 genes whose transcription is regulated by Brd2. In addition, almost 290 genes change their alternative splicing pattern upon Brd2 depletion. Brd2 is specifically localized at promoters of target genes, and our data show that Brd2 interaction with chromatin cannot be explained solely by histone acetylation. Using coimmunoprecipitation and live-cell imaging, we show that the C-terminal part is crucial for Brd2 association with chromatin. Live-cell microscopy also allows us to map the average binding time of Brd2 to chromatin and quantify the contributions of individual Brd2 domains to the interaction with chromatin. Finally, we show that bromodomains and the C-terminal domain are equally important for transcription and splicing regulation, which correlates with the role of these domains in Brd2 binding to chromatin.

The RB family is required for the self-renewal and survival of human embryonic stem cells

The mechanisms ensuring the long-term self-renewal of human embryonic stem cells are still only partly understood, limiting their use in cellular therapies. Here we found that increased activity of the RB cell cycle inhibitor in human embryonic stem cells induces cell cycle arrest, differentiation and cell death. Conversely, inactivation of the entire RB family (RB, p107 and p130) in human embryonic stem cells triggers G2/M arrest and cell death through functional activation of the p53 pathway and the cell cycle inhibitor p21. Differences in E2F target gene activation upon loss of RB family function between human embryonic stem cells, mouse embryonic stem cells and human fibroblasts underscore key differences in the cell cycle regulatory networks of human embryonic stem cells. Finally, loss of RB family function promotes genomic instability in both human and mouse embryonic stem cells, uncoupling cell cycle defects from chromosomal instability. These experiments indicate that a homeostatic level of RB activity is essential for the self-renewal and the survival of human embryonic stem cells.

Acetylation dynamics of human nuclear proteins during the ionizing radiation-induced DNA damage response

Genotoxic insults, such as ionizing radiation (IR), cause DNA damage that evokes a multifaceted cellular DNA damage response (DDR). DNA damage signaling events that control protein activity, subcellular localization, DNA binding, protein-protein interactions, etc. rely heavily on time-dependent posttranslational modifications (PTMs). To complement our previous analysis of IR-induced temporal dynamics of nuclear phosphoproteome, we now identify a range of human nuclear proteins that are dynamically regulated by acetylation, and predominantly deacetylation, during IR-induced DDR by using mass spectrometry-based proteomic approaches. Apart from cataloging acetylation sites through SILAC proteomic analyses before IR and at 5 and 60 min after IR exposure of U2OS cells, we report that: (1) key components of the transcriptional machinery, such as EP300 and CREBBP, are dynamically acetylated; (2) that nuclear acetyltransferases themselves are regulated, not on the protein abundance level, but by (de)acetylation; and (3) that the recently reported p53 co-activator and methyltransferase MLL3 is acetylated on five lysines during the DDR. For selected examples, protein immunoprecipitation and immunoblotting were used to assess lysine acetylation status and thereby validate the mass spectrometry data. We thus present evidence that nuclear proteins, including those known to regulate cellular functions via epigenetic modifications of histones, are regulated by (de)acetylation in a timely manner upon cell's exposure to genotoxic insults. Overall, these results present a resource of temporal profiles of a spectrum of protein acetylation sites during DDR and provide further insights into the highly dynamic nature of regulatory PTMs that help orchestrate the maintenance of genome integrity.

Inhibition of platelet-derived growth factor receptor tyrosine kinase and downstream signaling pathways by Compound C

AMP-activated protein kinase (AMPK) has been implicated in anti-proliferative actions in a range of cell systems. Recently, it was observed that Compound C, an inhibitor of AMPK, also reduced the cell viability in human diploid fibroblasts (HDFs). Compound C-induced growth arrest was associated with a decrease in the cell cycle regulatory proteins, such as proliferating cell nuclear antigen, phosphorylated pRB, cyclin-dependent protein kinases (Cdk 2 and 4), cyclins (D and E), and the Cdk inhibitors (p21, p16, and p27). Therefore, the present study examined the molecular mechanism of the antiproliferative effects of Compound C. Although Compound C inhibited serum-induced phosphorylation of Akt and its substrate, glycogen synthase kinase-3β, it did not affect the Akt activity in vitro. Compound C significantly inhibited the receptor tyrosine phosphorylation and the activity of downstream signaling molecules, such as p85 phosphoinositide 3-kinase, phospholipase C-γ1, and extracellular signal-regulated kinase 1/2, induced by platelet-derived growth factor (PDGF) but not by epidermal growth factor- and insulin-like growth factor. In vitro growth factor receptor tyrosine kinase activity profiling revealed the IC50 for PDGF receptor-β (PDGFRβ) to be 5.07μM, whereas the IC50 for the epidermal growth factor receptor and insulin-like growth factor receptor was ≥100μM. The inhibitory effect of Compound C on PDGFRβ and Akt was also observed in AMPKα1/α2-knockout mouse embryonic fibroblasts, indicating that its inhibitory effect is independent of the AMPK activity. The inhibitory effect of Compound C on cell proliferation and PDGFRβ tyrosine phosphorylation was also demonstrated in various PDGFR-expressing cells, including MRC-5, BEAS-2B, rat aortic vascular smooth muscle cells, and A172 glioblastoma cells. These results indicate that Compound C can be used as a potential antiproliferative agent for PDGF- or PDGFR-associated diseases, such as cancer, atherosclerosis, and fibrosis.

RBM38 is a direct transcriptional target of E2F1 that limits E2F1-induced proliferation

The E2F family of transcription factors plays a pivotal role in the regulation of cell proliferation in higher eukaryotes and is a critical downstream target of the tumor suppressor pRB. The pRB/E2F pathway is defective in most human tumors, resulting in deregulated E2F activity that induces uncontrolled cell proliferation, a hallmark of tumor cells. The RNA-binding protein RBM38, also named RNPC1, induces cell-cycle arrest in G(1), at least in part, via binding to and stabilizing the mRNA of the cyclin-dependent kinase inhibitor p21. RBM38 levels are altered in human cancer. Generally, RBM38 is overexpressed in various tumors; however, RBM38 mRNA levels are reduced in some breast tumors due to increased methylation of its promoter region. We show here that expression of RBM38 is regulated by E2F1. Specifically, RBM38 mRNA and protein levels are elevated upon activation of either exogenous E2F1 or endogenous E2Fs. Moreover, endogenous E2F1 binds the human RBM38 promoter and E2F1 knockdown reduces RBM38 levels. Our data raise the possibility that E2F1 together with E2F1-regulated RBM38 constitute a negative feedback loop that modulates E2F1 activity. In support of this, inhibition of RBM38 expression increases E2F1-mediated cell-cycle progression. Moreover, in human ovarian cancer, high correlation between expression of E2F1 and RBM38 is associated with increased survival. Overall, our data identify RBM38 as novel transcriptional target of E2F1 that restricts E2F1-induced proliferation. Furthermore, this negative feedback loop seems to restrict tumor aggressiveness, thereby promoting survival of patients with cancer.

miR than meets the eye

Retinoblastoma is a rare pediatric cancer that has served as a paradigm to investigate the mechanisms of tumorigenesis. In this issue of Genes & Development, Conkrite and colleagues (pp. 1734-1745) found high levels of the miR-17~92 and miR-106b-25 microRNAs in primary retinoblastomas and show that overexpression of miR-17~92 accelerates retinoblastoma development in mice by promoting proliferation, in part by reducing expression of the cell cycle inhibitor p21. These experiments identify the RB/miR-17~92/p21 axis as a critical regulator of retinoblastoma tumorigenesis and potentially many other cancers.

PAK4 is required for regulation of the cell-cycle regulatory protein p21, and for control of cell-cycle progression

The serine/threonine kinase PAK4 regulates cytoskeletal architecture, and controls cell proliferation and survival. In most adult tissues PAK4 is expressed at low levels, but overexpression of PAK4 is associated with uncontrolled proliferation, inappropriate cell survival, and oncogenic transformation. Here we have studied for the first time, the role for PAK4 in the cell cycle. We found that PAK4 levels peak dramatically but transiently in the early part of G1 phase. Deletion of Pak4 was also associated with an increase in p21 levels, and PAK4 was required for normal p21 degradation. In serum-starved cells, the absence of PAK4 led to a reduction in the amount of cells in G1, and an increase in the amount of cells in G2/M phase. We propose that the transient increase in PAK4 levels at early G1 reduces p21 levels, thereby abrogating the activity of CDK4/CDK6 kinases, and allowing cells to proceed with the cell cycle in a precisely coordinated way.

Reconstructing transcription factor activities in hierarchical transcription network motifs

MOTIVATION

A knowledge of the dynamics of transcription factors is fundamental to understand the transcriptional regulation mechanism. Nowadays, an experimental measure of transcription factor activities in vivo represents a challenge. Several methods have been developed to infer these activities from easily measurable quantities such as mRNA expression of target genes. A limitation of these methods is represented by the fact that they rely on very simple single-layer structures, typically consisting of one or more transcription factors regulating a number of target genes.

RESULTS

We present a novel statistical inference methodology to reverse engineer the dynamics of transcription factors in hierarchical network motifs such as feed-forward loops. The approach we present is based on a continuous time representation of the system where the high-level master transcription factor is represented as a two state Markov jump process driving a system of differential equations. We solve the inference problem using an efficient variational approach and demonstrate our method on simulated data and two real datasets. The results on real data show that the predictions of our approach can capture biological behaviours in a more effective way than single-layer models of transcription, and can lead to novel biological insights.

AVAILABILITY

http://homepages.inf.ed.ac.uk/gsanguin/software.html

CONTACT

g.sanguinetti@ed.ac.uk

SUPPLEMENTARY INFORMATION

Supplementary data are available at Bioinformatics online.

The novel tryptamine derivative JNJ-26854165 induces wild-type p53- and E2F1-mediated apoptosis in acute myeloid and lymphoid leukemias

The development of small-molecule activators of p53 is currently focused on malignancies containing a wild-type p53 genotype, which is present in most leukemias. JNJ-26854165 is one such p53-activating agent, but its mechanism of action remains to be elucidated. Here, we report the effects of JNJ-26854165 in acute leukemias. JNJ-26854165 treatment induced p53-mediated apoptosis in acute leukemia cells with wild-type p53, in which p53 rapidly drives transcription-independent apoptosis followed by activation of a transcription-dependent pathway. JNJ-26854165 accelerated the proteasome-mediated degradation of p21 and antagonized the transcriptional induction of p21 by p53. Interestingly, JNJ-26854165 induced S-phase delay and upregulated E2F1 expression in p53 mutant cells, resulting in apoptosis preferentially of S-phase cells. E2F1 knockdown blocked apoptosis induced by JNJ-26854165 in p53 mutant cells. Apoptotic activity of JNJ-26854165 against primary acute leukemia cells was maintained in leukemia/stroma cocultures, unlike doxorubicin, which has reduced cytrotoxicity in coculture systems. JNJ-26854165 synergizes with 1-β-arabinofuranosylcytosine or doxorubicin to induce p53-mediated apoptosis. Our data suggest that JNJ-26854165 may provide a novel therapeutic approach for the treatment of acute leukemias. The presence of p53-independent apoptotic activity in addition to p53-mediated apoptosis induction, if operational in vivo, may prevent the selection of p53 mutant subclones during therapy.

E2F1 mediates DNA damage and apoptosis through HCF-1 and the MLL family of histone methyltransferases

E2F1 is a key positive regulator of human cell proliferation and its activity is altered in essentially all human cancers. Deregulation of E2F1 leads to oncogenic DNA damage and anti-oncogenic apoptosis. The molecular mechanisms by which E2F1 mediates these two processes are poorly understood but are important for understanding cancer progression. During the G1-to-S phase transition, E2F1 associates through a short DHQY sequence with the cell-cycle regulator HCF-1 together with the mixed-lineage leukaemia (MLL) family of histone H3 lysine 4 (H3K4) methyltransferases. We show here that the DHQY HCF-1-binding sequence permits E2F1 to stimulate both DNA damage and apoptosis, and that HCF-1 and the MLL family of H3K4 methyltransferases have important functions in these processes. Thus, HCF-1 has a broader role in E2F1 function than appreciated earlier. Indeed, sequence changes in the E2F1 HCF-1-binding site can modulate both up and down the ability of E2F1 to induce apoptosis indicating that HCF-1 association with E2F1 is a regulator of E2F1-induced apoptosis.

p21 in cancer: intricate networks and multiple activities

One of the main engines that drives cellular transformation is the loss of proper control of the mammalian cell cycle. The cyclin-dependent kinase inhibitor p21 (also known as p21WAF1/Cip1) promotes cell cycle arrest in response to many stimuli. It is well positioned to function as both a sensor and an effector of multiple anti-proliferative signals. This Review focuses on recent advances in our understanding of the regulation of p21 and its biological functions with emphasis on its p53-independent tumour suppressor activities and paradoxical tumour-promoting activities, and their implications in cancer.

The molecular programme of tumour reversion: the steps beyond malignant transformation

How cells become malignant has preoccupied scientists for over a century. However, the converse question is also valid: are tumour cells capable of reverting from their malignant state? Askanazy's studies in 1907 indicated that teratoma cells could differentiate into normal somatic tissues and current evidence indicates that some tumour cells have acquired the molecular circuitry that results in the negation of chromosomal instability, translocations, oncogene activation and loss of tumour suppressor genes. Studying these extremely rare events of tumour reversion and deciphering these pathways, which involve SIAH1, presenilin 1, TSAP6 and translationally controlled tumour protein (TCTP), could lead to new avenues in cancer treatment.

Tumor-specific induction of apoptosis by a p53-reactivating compound

The tumor suppressor function of p53 is disabled in the majority of tumors, either by a point mutation of the p53 gene, or via MDM2-dependent proteasomal degradation. We have screened a chemical library using a cell-based assay and identified a low molecular weight compound named MITA which induced wild-type p53-dependent cell death in a variety of different types of human tumor cells, such as lung, colon and breast carcinoma cells, as well as in osteosarcoma and fibrosarcoma-derived cells. MITA inhibited p53-MDM2 interaction in vitro and in cells, which in turn prevented MDM2-mediated ubiquitination of p53 and resulted in a prolonged half-life and accumulation of p53 in tumor cells. Notably, p53 induction by MITA resulted in upregulated expression of p53 target genes MDM2, Bax, Gadd45 and PUMA, on protein and mRNA level. Importantly, neither p53 nor these target genes were induced in normal human fibroblasts (HDFs), which correlated with the absence of growth suppression in fibroblasts after treatment with MITA. However, upon activation of oncogenes in fibroblasts an induction and activation of p53 was observed, suggesting that activation of p53 by MITA occurs predominantly in tumor cells.

Comprehensive assessment of P21 polymorphisms and lung cancer risk

The purpose of this study is to comprehensively evaluate potential functional polymorphisms in the P21 gene in relation to the risk of lung cancer. We first determined the frequencies of P21 polymorphisms in 27 healthy Koreans, and then examined three polymorphisms (-2266G > A, S31R, and IVS2 + 16G > C), based on their frequencies and haplotype-tagging status, in a case-control study. Individuals with at least one -2266A allele were at a significantly decreased risk of lung cancer compared with those harboring the -2266 GG genotype [adjusted odds ratio (OR) = 0.71, 95% confidence interval (CI) = 0.53-0.95, P = 0.02). The haplotypes (ht2-4) carrying 31R or IVS2 + 16C alleles were associated with a significantly decreased risk of lung cancer compared with the haplotype 31S/IVS2 + 16G, which carried wild-type alleles at both loci (adjusted OR = 0.65, 95% CI = 0.50-0.83, P = 0.007)]. When the -2266A allele and ht2-4 were considered to be protective alleles, the risk of lung cancer decreased in a dose-dependent manner as the number of protective alleles increased (P = 0.0002). These results suggest that a combined analysis of these three P21 polymorphisms might better predict the risk of lung cancer than the analysis of a single polymorphism.

Expression of the high mobility group A family member p8 is essential to maintaining tumorigenic potential by promoting cell cycle dysregulation in LbetaT2 cells

The mechanism by which the HMGA protein p8 facilitates tumorigenesis may be cell cycle dysregulation. Control- (C) LbetaT2 cells, which express p8, form tumors at a rate five-times faster than p8-knockdown (p8-KD)-LbetaT2 cells. In association with this heightened tumorigenic potential, p8-expressing C-LbetaT2 cells avoid G(0)/G(1) arrest and become genetically unstable while p8-KD-LbetaT2 cells arrest in G(0)/G(1), become senescent upon overgrowth, and maintain a diploid population. These phenotypic changes correspond to altered cell cycle regulation at the G(1)-to-S transition that may be due to p8-mediated changes in expression of the Cip/Kip family members of cell cycle inhibitors, p21, p27, and p57.

Human TopBP1 participates in cyclin E/CDK2 activation and preinitiation complex assembly during G1/S transition

Human TopBP1 with eight BRCA1 C terminus domains has been mainly reported to be involved in DNA damage response pathways. Here we show that TopBP1 is also required for G(1) to S progression in a normal cell cycle. TopBP1 deficiency inhibited cells from entering S phase by up-regulating p21 and p27, resulting in down-regulation of cyclin E/CDK2. Although co-depletion of p21 and p27 with TopBP1 restored the cyclin E/CDK2 kinase activity, however, cells remained arrested at the G(1)/S boundary, showing defective chromatin-loading of replication components. Based on these results, we suggest a dual role of TopBP1 necessary for the G(1)/S transition: one for activating cyclin E/CDK2 kinase and the other for loading replication components onto chromatin to initiate DNA synthesis.

Iressa strengthens the cytotoxic effect of docetaxel in NSCLC models that harbor specific molecular characteristics

Non-small cell lung cancer (NSCLC) is the most lethal malignant tumor and is also considered one of the most chemoresistant cancers. Despite the benefits obtained from platinum-based therapy, the majority of patients treated will progress and die. In the continuing quest for personalized therapy based on the biomolecular characteristics of each single patient, clinical practice now seems to be oriented towards combining conventional drugs with molecular-targeted agents. In the present study, we evaluated the antitumor activity of docetaxel, one of the most widely used drugs for second-line treatment, and Iressa, an EGFR-targeting tyrosine kinase inhibitor, administered singly or in sequence. The study was performed on three human NSCLC cell lines (ChaGo-K1, CAEP and RAL) that exhibit different expression of proliferation and apoptosis-related markers, and do not harbor EGFR mutations. The efficacy of docetaxel and Iressa differed in the three cell lines and an important synergistic interaction was observed with the sequence 1-h docetaxel --> 72-h Iressa during which Iressa doubled the fraction of docetaxel-induced apoptotic cells, amplifying a caspase-dependent apoptosis and inhibiting docetaxel-induced p21 hyperexpression. Moreover, the important role of MAPK-dependent modulation of this molecular marker was shown using a specific inhibitor. The results from the present preclinical study demonstrate the cytotoxic activity of Iressa and its ability to increase taxane activity in a model that does not harbor EGFR-specific mutations, thus highlighting the importance of focusing on alternative molecular targets of Iressa activity.

Cyclin D1/cdk4 can interact with E2F4/DP1 and disrupts its DNA-binding capacity

The E2F family of transcription factors regulate the expression of many growth-related genes in a cell cycle-dependent manner. These transcription factors can activate or, in conjunction with an Rb-related protein, repress transcription. E2F transcriptional activity is regulated at several different levels that are each linked to cell cycle progression. In many cell types, E2F4 and E2F5 are the predominant E2F species during G(0) and early G(1) and function primarily as repressors of E2F-regulated genes. In this study, co-immunoprecipitation techniques were used to demonstrate that cyclins D1, D2, and D3 are capable of interacting with E2F4, E2F5, and DP1. Overexpression of cyclin D1/cdk4 reduced E2F4-mediated transcription in a simple reporter gene assay and electrophoretic mobility shift analyses using nuclear extracts from transfected cells indicated that cyclin D1/cdk4 disrupts the DNA-binding ability of E2F4. Cell cycle analysis following stimulation of serum-starved 3T3 cells indicated that E2F4 undergoes changes in its phosphorylation pattern coincident with the synthesis of cyclin D1. Examination of a series of E2F4 deletion mutants indicated that a cyclin D1-binding site located close to the carboxyl terminus of E2F4 was critical for the disruption of DNA binding by cyclin D1/cdk4. These data support a model in which E2F4 DNA binding is abolished during mid-G(1) at the same time when E2F interactions with pRb-related proteins are disrupted by cyclin D1/cdk4.

Transcriptional regulation of the cyclin-dependent kinase inhibitor 1A (p21) gene by NFI in proliferating human cells

The cyclin-dependent kinase inhibitor 1A (CDKN1A), also known as p21 (WAF1/CIP1) modulates cell cycle, apoptosis, senescence and differentiation via specific protein-protein interactions with the cyclins, cyclin-dependent kinase (Cdk), and many others. Expression of the p21 gene is mainly regulated at the transcriptional level. By conducting both ligation-mediated PCR (LMPCR) and chromatin immunoprecipitation (ChIP) in vivo, we identified a functional target site for the transcription factor, nuclear factor I (NFI), in the basal promoter from the p21 gene. Transfection of recombinant constructs bearing mutations in the p21 NFI site demonstrated that NFI acts as a repressor of p21 gene expression in various types of cultured cells. Inhibition of NFI in human skin fibroblasts through RNAi considerably increased p21 promoter activity suggesting that NFI is a key repressor of p21 transcription. Over-expression of each of the four NFI isoforms in HCT116 cells established that each of them contribute to various extend to the repression of the p21 gene. Most of all, over-expression of NFI-B in doxorubicin, growth-arrested HCT116 increased the proportion of cells in the S-phase of the cell cycle whereas NFI-A and NFI-X reduced it, thereby establishing a role for NFI in the cell cycle dependent expression of p21.

Thallium acetate induces C6 glioma cell apoptosis

Thallium acetate is a known neurotoxic agent. In this study, we investigated the mechanisms by which thallium acetate induces cell cycle arrest and cell apoptosis in cultured LC6 glioma cells. Exposure of C6 glioma cells to thallium acetate decreased cell viability as demonstrated by the MTT assay. Incubation of thallium acetate arrested cell cycle progression at the G2/M phase and caused cellular apoptosis at 300 microM as determined by trypan blue exclusion and flow cytometric analysis. The G2/M arrest was associated with a decrease in expression of CDK2 protein and an upregulation of p53 and the CDK inhibitor p21(Cip1), but not p27(Kip1). Thallium acetate did not alter the protein levels of cyclin A and B; cyclin D1, D2, and D3; and CDK4 expression in C6 glioma cells. Incubation of C6 glioma cells with thallium acetate upregulated the expression of proapoptotic proteins Bad and Apaf and downregulated the expression of anti-apoptotic proteins Bcl-xL and Bcl-2. In conclusion, these data suggest that thallium acetate inhibits cell cycle progression at G2/M phase by suppressing CDK activity through the p53-mediated induction of the CDK inhibitor p21(Cip1). Impairment of cell cycle progression may trigger the activation of a mitochondrial pathway and shifts the balance in the Bcl-2 family toward the proapoptotic members, promoting the formation of the apoptosome and, consequently, apoptosis.

Absent STAT-1 expression perturbs adaptation and apoptosis after massive intestinal resection

BACKGROUND

We have previously established the significance of epidermal growth factor receptor (EGFR) activity and the cyclin-dependent kinase inhibitor p21waf1/cip1 (p21) for the adaptive response of the intestine to massive small bowel resection (SBR). In this study, we tested the role of the signal transducer and activator of transcription 1 (STAT-1) as this transcription factor is activated by the EGFR and known to induce p21 expression.

METHODS

Control (n = 40; C57/Bl6) and STAT-1-null mice (n = 40) underwent 50% proximal SBR or sham operation. After 3 days, the remnant ileum was harvested and the villus and crypt morphology was measured along with changes in rates of enterocyte proliferation and apoptosis.

RESULTS

The magnitude of resection-induced adaptation was greater in STAT-1-null animals as verified by taller villi and deeper crypts. The expected increase in enterocyte apoptosis did not occur after SBR in the background of STAT-1 deficiency. Western blotting revealed elevated expression of p21 protein in both STAT-1-null and controls after SBR.

CONCLUSION

Increased p21 expression after SBR in the absence of STAT-1 suggests an alternate mechanism for resection-induced regulation of p21. Enhanced adaptation in STAT-1-null animals suggests that this transcription factor serves an inhibitor to the process of adaptation, perhaps via regulation of enterocyte apoptosis.

cGMP-independent nitric oxide signaling and regulation of the cell cycle

BACKGROUND

Regulatory functions of nitric oxide (NO*) that bypass the second messenger cGMP are incompletely understood. Here, cGMP-independent effects of NO* on gene expression were globally examined in U937 cells, a human monoblastoid line that constitutively lacks soluble guanylate cyclase. Differentiated U937 cells (>80% in G0/G1) were exposed to S-nitrosoglutathione, a NO* donor, or glutathione alone (control) for 6 h without or with dibutyryl-cAMP (Bt2cAMP), and then harvested to extract total RNA for microarray analysis. Bt2cAMP was used to block signaling attributable to NO*-induced decreases in cAMP.

RESULTS

NO* regulated 110 transcripts that annotated disproportionately to the cell cycle and cell proliferation (47/110, 43%) and more frequently than expected contained AU-rich, post-transcriptional regulatory elements (ARE). Bt2cAMP regulated 106 genes; cell cycle gene enrichment did not reach significance. Like NO*, Bt2cAMP was associated with ARE-containing transcripts. A comparison of NO* and Bt2cAMP effects showed that NO* regulation of cell cycle genes was independent of its ability to interfere with cAMP signaling. Cell cycle genes induced by NO* annotated to G1/S (7/8) and included E2F1 and p21/Waf1/Cip1; 6 of these 7 were E2F target genes involved in G1/S transition. Repressed genes were G2/M associated (24/27); 8 of 27 were known targets of p21. E2F1 mRNA and protein were increased by NO*, as was E2F1 binding to E2F promoter elements. NO* activated p38 MAPK, stabilizing p21 mRNA (an ARE-containing transcript) and increasing p21 protein; this increased protein binding to CDE/CHR promoter sites of p21 target genes, repressing key G2/M phase genes, and increasing the proportion of cells in G2/M.

CONCLUSION

NO* coordinates a highly integrated program of cell cycle arrest that regulates a large number of genes, but does not require signaling through cGMP. In humans, antiproliferative effects of NO* may rely substantially on cGMP-independent mechanisms. Stress kinase signaling and alterations in mRNA stability appear to be major pathways by which NO* regulates the transcriptome.

Examination of the pRb-dependent and pRb-independent functions of E7 in vivo

High-risk human papillomaviruses encode two oncogenes, E6 and E7, expressed in nearly all cervical cancers. Although E7 protein is best known for its ability to inactivate the retinoblastoma tumor suppressor protein, pRb, many other activities for E7 have been proposed in in vitro studies. Herein, we describe studies that allowed us to define unambiguously the pRb-dependent and -independent activities of E7 for the first time in vivo. In these studies, we crossed mice transgenic for human papillomavirus 16 E7 to knock-in mice genetically engineered to express a mutant form of pRb (pRb(DeltaLXCXE)) that is selectively defective for binding E7. pRb inactivation was necessary for E7 to induce DNA synthesis and to overcome differentiation-dependent cell cycle withdrawal and DNA damage-induced cell cycle arrest. While most of E7's effects on epidermal differentiation were found to require pRb inactivation, a modest delay in terminal differentiation with resulting hyperplasia was observed in E7 mice on the Rb(DeltaLXCXE) mutant background. E7-induced p21 upregulation was also pRb dependent, and genetic Rb inactivation was sufficient to reproduce this effect. While E7-mediated p21 induction was partially p53 dependent, neither p53 nor p21 induction by E7 required p19(ARF). These data show that E7 upregulates the expression of p53 and p21 via pRb-dependent mechanisms distinct from the proposed p19-Mdm2 pathway. These results extend our appreciation of the importance of pRb as a relevant target for high-risk E7 oncoproteins.

Rb/E2F: a two-edged sword in the melanocytic system

Rb is a tumor suppressor that represses the expression of E2F regulated genes required for cell cycle progression. It is inactivated in melanomas and other cancer cells by phosphorylation catalyzed by persistent cyclin dependent kinase (CDK) activity. CDK activity is sustained in melanoma cells mostly by the elimination of the CDK inhibitor p16INK4A and by high levels of cyclins whose expression is maintained by stimuli emanating from activated cell surface receptors and/or mutated intracellular intermediates, such as N-Ras and B-Raf. However, Rb also suppresses the expression of apoptosis genes, and its presence protects normal melanocytes from cell death. Its high expression in human melanoma cells and tumors suggests a similar role in malignant cells as well. The differential release and suppression of E2F transcriptional activity is likely to depend on promoter-specific E2F/Rb interaction. Phosphorylated Rb is displaced from cell cycle genes but not from others. In addition, Rb gene repression is dependent on the nature of Rb-E2F interaction and the activity of the Rb-bound proteins recruited to the promoter. Deciphering the differences in Rb/E2F complex formation in normal and malignant melanocytes is likely to shed light on the mechanism by which Rb can exert tumor suppressing and promoting activities in this cellular system. The Rb/E2F pathway provides opportunities for efficient therapy at multiple levels. Novel drugs can reactivate Rb potential to suppress growth cycle promoting genes. In addition, the high E2F transcriptional activity in melanoma cells can be exploited to deliver cytotoxic molecules specifically to tumors, sparing the normal tissues.

Inhibition of cell division by the human cytomegalovirus IE86 protein: role of the p53 pathway or cyclin-dependent kinase 1/cyclin B1

The human cytomegalovirus (HCMV) IE86 protein induces the human fibroblast cell cycle from G(0)/G(1) to G(1)/S, where cell cycle progression stops. Cells with a wild-type, mutated, or null p53 or cells with null p21 protein were transduced with replication-deficient adenoviruses expressing HCMV IE86 protein or cellular p53 or p21. Even though S-phase genes were activated in a p53 wild-type cell, IE86 protein also induced phospho-Ser(15) p53 and p21 independent of p14ARF but dependent on ATM kinase. These cells did not enter the S phase. In human p53 mutant, p53 null, or p21 null cells, IE86 protein did not up-regulate p21, cellular DNA synthesis was not inhibited, but cell division was inhibited. Cells accumulated in the G(2)/M phase, and there was increased cyclin-dependent kinase 1/cyclin B1 activity. Although the HCMV IE86 protein increases cellular E2F activity, it also blocks cell division in both p53(+/+) and p53(-/-) cells.

Variation in transcriptional regulation of cyclin dependent kinase inhibitor p21waf1/cip1 among human bronchogenic carcinomas

Background

Cell proliferation control depends in part on the carefully ordered regulation of transcription factors. The p53 homolog p73, contributes to this control by directly upregulating the cyclin dependent kinase inhibitor, p21^waf1/cip1. E2F1, an inducer of cell proliferation, directly upregulates p73 and in some systems upregulates p21 directly. Because of its central role in controlling cell proliferation, upregulation of p21 has been explored as a modality for treating bronchogenic carcinoma (BC). Improved understanding of p21 transcriptional regulation will facilitate identification of BC tissues that are responsive to p21-directed therapies. Toward this goal, we investigated the role that E2F1 and p73 each play in the transcriptional regulation of p21.

Results

Among BC samples (N = 21) p21 transcript abundance (TA) levels varied over two orders of magnitude with values ranging from 400 to 120,000 (in units of molecules/10⁶ molecules β-actin). The p21 values in many BC were high compared to those observed in normal bronchial epithelial cells (BEC) (N = 18). Among all BC samples, there was no correlation between E2F1 and p21 TA but there was positive correlation between E2F1 and p73α (p < 0.001) TA. Among BC cell lines with inactivated p53 and wild type p73 (N = 7) there was positive correlation between p73α and p21 TA (p < 0.05). Additionally, in a BC cell line in which both p53 and p73 were inactivated (H1155), E2F1 TA level was high (50,000), but p21 TA level was low (470). Transiently expressed exogenous p73α in the BC cell line Calu-1, was associated with a significant (p < 0.05) 90% increase in p21 TA and a 20% reduction in E2F1 TA. siRNA mediated reduction of p73 TA in the N417 BC cell line was associated with a significant reduction in p21 TA level (p < 0.01).

Conclusion

p21 TA levels vary considerably among BC patients which may be attributable to 1) genetic alterations in Rb and p53 and 2) variation in TA levels of upstream transcription factors E2F1 and p73. Here we provide evidence that p73 upregulates p21 TA in BC tissues and upregulated p21 TA may result from E2F1 upregulation of p73 but not from E2F1 directly.

Eugenol Causes Melanoma Growth Suppression through Inhibition of E2F1 Transcriptional Activity

Metastatic malignant melanoma is an extremely aggressive cancer, with no currently viable therapy. 4-Allyl-2-methoxyphenol (eugenol) was tested for its ability to inhibit proliferation of melanoma cells. Eugenol but not its isomer, isoeugenol (2-methoxy-4-propenylphenol), was found to be a potent inhibitor of melanoma cell proliferation. In a B16 xenograft study, eugenol treatment produced a significant tumor growth delay (p = 0.0057), an almost 40% decrease in tumor size, and a 19% increase in the median time to end point. More significantly, 50% of the animals in the control group died from metastatic growth, whereas none in the treatment group showed any signs of invasion or metastasis. Eugenol was well tolerated as determined by measurement of bodyweights. Examination of the mechanism of the antiproliferative action of eugenol in the human malignant melanoma cell line, WM1205Lu, showed that it arrests cells in the S phase of the cell cycle. Flow cytometry coupled with biochemical analyses demonstrated that eugenol induced apoptosis. cDNA array analysis showed that eugenol caused deregulation of the E2F family of transcription factors. Transient transfection assays and electrophoretic mobility shift assays showed that eugenol inhibits the transcriptional activity of E2F1. Overexpression of E2F1 restored about 75% of proliferation ability in cultures. These results indicate that deregulation of E2F1 may be a key factor in eugenol-mediated melanoma growth inhibition both in vitro and in vivo. Since the E2F transcription factors provide growth impetus for the continuous proliferation of melanoma cells, these results suggest that eugenol could be developed as an E2F-targeted agent for melanoma treatment.

Cell cycle and developmental regulations of replication factors in mouse embryonic stem cells

Embryonic stem (ES) cells can grow rapidly and permanently while maintaining their differentiation capacity. To gain insight into how the cell cycle progression of undifferentiated murine ES cells is regulated, we have examined the expression patterns of various replication and cell cycle regulators. Most factors including cyclins, Cdc6, and geminin are rather constitutively expressed during the cell cycle of ES cells. Furthermore, the transcript levels of almost all the cell cycle regulators we investigated except for p21 and p27 are higher in undifferentiated ES cells than in murine embryonic fibroblasts (MEFs), and the increased stability of mRNA in ES cells may be partially responsible for this at least with some of the factors. More strikingly, the transcriptional levels of these factors are strongly correlated with the acetylated state of histone H3 at their promoter regions. However, the methylation state of histone or CpG methylation of the promoter region is not generally correlated significantly with the expression pattern of these factors in both cell types. On the protein level, Cdc6, ASK, cyclin A2, and cyclin B1 are extremely abundant in ES cells compared with MEFs. Furthermore, they are rapidly down-regulated upon induction of differentiation of ES cells. The significance of these findings is discussed in relation to the unusual proliferative properties of ES cells in an undifferentiated state.

4-hydroxynonenal and cell cycle

Lipid peroxidation is very low in proliferating cells and tumours and it might have a role in the regulation of cell proliferation and differentiation by acting through its products. 4-hydroxynonenal (HNE) has been proposed as a mediator of lipoperoxidation effects. It has been demonstrated that HNE can inhibit cell growth and induce differentiation in different leukemic cell lines. The onset of differentiation, induced by HNE, was accompanied by a reduction of c-myc expression. In HL-60 cells, HNE induced the accumulation of cells in the G0/G1 phase of the cell cycle. Cell cycle progression is regulated by three protein classes, the cyclins, the cyclin-dependent kinases (CDKs), and the CDK inhibitors (CKIs). In HL-60 cells, HNE decreased the expression of cyclin D1, D2 and A and caused an increase of p21 (the most important CKI) expression, whereas it did not affect CDK expressions. Since cyclins D/CDK2 and cyclin A/CDK2 phosphorylate pRB, HNE caused an increase of hypophosphorylated pRb. Hypophosphorylated pRb binds and inactivates the E2F transcription factors. Band-shift experiments demonstrated that HNE caused a decrease of "free" E2F, as well as an increase of pRb (and pRB family members) bound to E2F with consequent repression of the transcription.