AP4 encodes a c-MYC-inducible repressor of p21. Proc Natl Acad Sci U S A. 2008;105:15046-15051. [PMID: 18818310 DOI: 10.1073/pnas.0801773105]

Real-time transcriptional profiling of cellular and viral gene expression during lytic cytomegalovirus infection

During viral infections cellular gene expression is subject to rapid alterations induced by both viral and antiviral mechanisms. In this study, we applied metabolic labeling of newly transcribed RNA with 4-thiouridine (4sU-tagging) to dissect the real-time kinetics of cellular and viral transcriptional activity during lytic murine cytomegalovirus (MCMV) infection. Microarray profiling on newly transcribed RNA obtained at different times during the first six hours of MCMV infection revealed discrete functional clusters of cellular genes regulated with distinct kinetics at surprising temporal resolution. Immediately upon virus entry, a cluster of NF-κB- and interferon-regulated genes was induced. Rapid viral counter-regulation of this coincided with a very transient DNA-damage response, followed by a delayed ER-stress response. Rapid counter-regulation of all three clusters indicated the involvement of novel viral regulators targeting these pathways. In addition, down-regulation of two clusters involved in cell-differentiation (rapid repression) and cell-cycle (delayed repression) was observed. Promoter analysis revealed all five clusters to be associated with distinct transcription factors, of which NF-κB and c-Myc were validated to precisely match the respective transcriptional changes observed in newly transcribed RNA. 4sU-tagging also allowed us to study the real-time kinetics of viral gene expression in the absence of any interfering virion-associated-RNA. Both qRT-PCR and next-generation sequencing demonstrated a sharp peak of viral gene expression during the first two hours of infection including transcription of immediate-early, early and even well characterized late genes. Interestingly, this was subject to rapid gene silencing by 5–6 hours post infection. Despite the rapid increase in viral DNA load during viral DNA replication, transcriptional activity of some viral genes remained remarkably constant until late-stage infection, or was subject to further continuous decline. In summary, this study pioneers real-time transcriptional analysis during a lytic herpesvirus infection and highlights numerous novel regulatory aspects of virus-host-cell interaction.

Cytomegaloviruses are large DNA viruses, which establish life-long latent infections, leaving the infected individual at risk of reactivation and disease. Here, we applied 4-thiouridine-(4sU)-tagging of newly transcribed RNA to monitor the real-time kinetics of transcriptional activity of both cellular and viral genes during lytic murine CMV (MCMV) infection. We observed a cascade of MCMV-induced signaling events including a rapid inflammatory/interferon-response, a transient DNA-damage-response and a delayed ER-stress-response. All of these were heavily counter-regulated by viral gene expression. Besides dramatically increasing temporal resolution, our approach provides the unique opportunity to study viral transcriptional activity in absence of any interfering virion-associated-RNA. Virion-associated-RNA consists of transcripts that are unspecifically incorporated into the virus particles thereby resembling the cellular RNA profile of late stage infection. A clear picture of which viral genes are expressed, particularly at very early times of infection, could thus not be obtained. By overcoming this problem, we provide intriguing insights into the regulation of viral gene expression, namely 1) a peak of viral gene expression during the first two hours of infection including the expression of well-characterized late genes and 2) remarkably constant or even continuously declining expression of some viral genes despite the onset of rapid viral DNA replication.

Decorin: a guardian from the matrix

Decorin, an archetypal member of the small leucine-rich proteoglycan gene family, has a broad binding repertoire that encompasses matrix structural components, such as collagens, and growth factors, particularly those that belong to the transforming growth factor-β ligand superfamily. Within the tumor microenvironment, stromal decorin has an inherent proclivity to directly bind and down-regulate several receptor tyrosine kinases, which are often overexpressed in cancer cells. The decorin interactome commands a powerful antitumorigenic signal by potently repressing and attenuating tumor cell proliferation, survival, migration, and angiogenesis. This collection of interacting molecules also regulates key downstream signaling processes indirectly via the sequestration of growth factors or directly via the antagonism of receptor tyrosine kinases. We propose that decorin can be considered a "guardian from the matrix" because of its innate ability to oppose pro-tumorigenic cues.

Down-regulation of AP-4 inhibits proliferation, induces cell cycle arrest and promotes apoptosis in human gastric cancer cells

Background

AP-4 belongs to the basic helix-loop-helix leucine-zipper subgroup; it controls target gene expression, regulates growth, development and cell apoptosis and has been implicated in tumorigenesis. Our previous studies indicated that AP-4 was frequently overexpressed in gastric cancers and may be associated with the poor prognosis. The purpose of this study is to examine whether silencing of AP-4 can alter biological characteristics of gastric cancer cells.

Methods

Two specific siRNAs targeting AP-4 were designed, synthesized, and transfected into gastric cancer cell lines and human normal mucosa cells. AP-4 expression was measured with real-time quantitative PCR and Western blot. Cell proliferation and chemo-sensitivity were detected by CCK-8 assay. Cell cycle assay and apoptosis assay were performed by flow cytometer, and relative expression of cell cycle regulators were detected by real-time quantitative PCR and Western blot, expression of the factors involved in the apoptosis pathway were examined in mRNA and protein level.

Results

The expression of AP-4 was silenced by the siRNAs transfection and the effects of AP-4 knockdown lasted 24 to 96 hrs. The siRNA-mediated silencing of AP-4 suppressed the cellular proliferation, induced apoptosis and sensitized cancer cells to anticancer drugs. In addition, the expression level of p21, p53 and Caspase-9 were increased when AP-4 was knockdown, but the expression of cyclin D1, Bcl-2 and Bcl-x_L was inhibited. It didn't induce cell cycle arrest when AP-4 was knockdown in p53 defect gastric cancer cell line Kato-III.

Conclusions

These results illustrated that gene silencing of AP-4 can efficiently inhibited cell proliferation, triggered apoptosis and sensitized cancer cells to anticancer drugs in vitro, suggesting that AP-4 siRNAs mediated silencing has a potential value in the treatment of human gastric cancer.

Regulation of the human catalytic subunit of telomerase (hTERT)

Over the past decade, there has been much interest in the regulation of telomerase, the enzyme responsible for maintaining the integrity of chromosomal ends, and its crucial role in cellular immortalization, tumorigenesis, and the progression of cancer. Telomerase activity is characterized by the expression of the telomerase reverse transcriptase (TERT) gene, suggesting that TERT serves as the major limiting agent for telomerase activity. Recent discoveries have led to characterization of various interactants that aid in the regulation of human TERT (hTERT), including numerous transcription factors; further supporting the pivotal role that transcription plays in both the expression and repression of telomerase. Several studies have suggested that epigenetic modulation of the hTERT core promoter region may provide an additional level of regulation. Although these studies have provided essential information on the regulation of hTERT, there has been ambiguity of the role of methylation within the core promoter region and the subsequent binding of various activating and repressive agents. As a result, we found it necessary to consolidate and summarize these recent developments and elucidate these discrepancies. In this review, we focus on the co-regulation of hTERT via transcriptional regulation, the presence or absence of various activators and repressors, as well as the epigenetic pathways of DNA methylation and histone modifications.

Transforming growth factor-β and atherosclerosis: interwoven atherogenic and atheroprotective aspects

Age-related progression of cardiovascular disease is by far the largest health problem in the US and involves vascular damage, progressive vascular fibrosis and the accumulation of lipid-rich atherosclerotic lesions. Advanced lesions can restrict flow to key organs and can trigger occlusive thrombosis resulting in a stroke or myocardial infarction. Transforming growth factor-beta (TGF-β) is a major orchestrator of the fibroproliferative response to tissue damage. In the early stages of repair, TGF-β is released from platelets and activated from matrix reservoirs; it then stimulates the chemotaxis of repair cells, modulates immunity and inflammation and induces matrix production. At later stages, it negatively regulates fibrosis through its strong antiproliferative and apoptotic effects on fibrotic cells. In advanced lesions, TGF-β might be important in arterial calcification, commonly referred to as "hardening of the arteries". Because TGF-β can signal through multiple pathways, namely the SMADs, a MAPK pathway and the Rho/ROCK pathways, selective defects in TGF-β signaling can disrupt otherwise coordinated pathways of tissue regeneration. TGF-β is known to control cell proliferation, cell migration, matrix synthesis, wound contraction, calcification and the immune response, all being major components of the atherosclerotic process. However, many of the effects of TGF-β are essential to normal tissue repair and thus, TGF-β is often thought to be "atheroprotective". The present review attempts to parse systematically the known effects of TGF-β on both the major risk factors for atherosclerosis and to isolate the role of TGF-β in the many component pathways involved in atherogenesis.

The c-MYC oncoprotein, the NAMPT enzyme, the SIRT1-inhibitor DBC1, and the SIRT1 deacetylase form a positive feedback loop

Silent information regulator 1 (SIRT1) represents an NAD(+)-dependent deacetylase that inhibits proapoptotic factors including p53. Here we determined whether SIRT1 is downstream of the prototypic c-MYC oncogene, which is activated in the majority of tumors. Elevated expression of c-MYC in human colorectal cancer correlated with increased SIRT1 protein levels. Activation of a conditional c-MYC allele induced increased levels of SIRT1 protein, NAD(+), and nicotinamide-phosphoribosyltransferase (NAMPT) mRNA in several cell types. This increase in SIRT1 required the induction of the NAMPT gene by c-MYC. NAMPT is the rate-limiting enzyme of the NAD(+) salvage pathway and enhances SIRT1 activity by increasing the amount of NAD(+). c-MYC also contributed to SIRT1 activation by sequestering the SIRT1 inhibitor deleted in breast cancer 1 (DBC1) from the SIRT1 protein. In primary human fibroblasts previously immortalized by introduction of c-MYC, down-regulation of SIRT1 induced senescence and apoptosis. In various cell lines inactivation of SIRT1 by RNA interference, chemical inhibitors, or ectopic DBC1 enhanced c-MYC-induced apoptosis. Furthermore, SIRT1 directly bound to and deacetylated c-MYC. Enforced SIRT1 expression increased and depletion/inhibition of SIRT1 reduced c-MYC stability. Depletion/inhibition of SIRT1 correlated with reduced lysine 63-linked polyubiquitination of c-Myc, which presumably destabilizes c-MYC by supporting degradative lysine 48-linked polyubiquitination. Moreover, SIRT1 enhanced the transcriptional activity of c-MYC. Taken together, these results show that c-MYC activates SIRT1, which in turn promotes c-MYC function. Furthermore, SIRT1 suppressed cellular senescence in cells with deregulated c-MYC expression and also inhibited c-MYC-induced apoptosis. Constitutive activation of this positive feedback loop may contribute to the development and maintenance of tumors in the context of deregulated c-MYC.

A gestational low-protein diet represses p21(WAF1/Cip1) expression in the mammary gland of offspring rats through promoter histone modifications

Maternal exposure to environmental agents throughout pregnancy may change certain metabolic processes during the offspring's mammary gland development and alter the epigenome. This may predispose the offspring to breast cancer later in life. The purpose of the present study was to examine the effect of maternal protein restriction on the regulation of cyclin-dependent kinase inhibitor 1 (p21) gene expression in the mammary gland of rat offspring. Timed-mated Sprague-Dawley rats were fed one of the two isoenergetic diets, control (C, 18 % casein) or low protein (LP, 9 % casein), during gestation. Compared with the C group, LP offspring showed a decrease of p21 in the mammary gland at both the mRNA and protein levels. Chromatin immunoprecipitation assay demonstrated that the down-regulation of p21 transcription in LP offspring was associated with reduced acetylation of histone H3 and dimethylation of H3K4 within the p21 promoter region, but was not associated with acetylation of histone H4 or histone methylation. DNA methylation analysis using bisulphite sequencing did not detect differences in methylation at the p21 promoter between the offspring of the C and LP groups. We conclude that maternal protein restriction inhibits p21 gene expression in the mammary gland of offspring through histone modifications at the promoter region of the p21 gene.

Protein kinase C (PKC) delta suppresses keratinocyte proliferation by increasing p21(Cip1) level by a KLF4 transcription factor-dependent mechanism

PKCδ increases keratinocyte differentiation and suppresses keratinocyte proliferation and survival. However, the mechanism of proliferation suppression is not well understood. The present studies show that PKCδ overexpression increases p21(Cip1) mRNA and protein level and promoter activity and that treatment with dominant-negative PKCδ, PKCδ-siRNA, or rottlerin inhibits promoter activation. Analysis of the p21(Cip1) promoter upstream regulatory region reveals three DNA segments that mediate PKCδ-dependent promoter activation. The PKCδ response element most proximal to the transcription start site encodes six GC-rich DNA elements. Mutation of these sites results in a loss of PKCδ-dependent promoter activation. Gel mobility supershift and chromatin immunoprecipitation reveal that these DNA elements bind the Kruppel-like transcription factor KLF4. PKCδ increases KLF4 mRNA and protein level and KLF4 binding to the GC-rich elements in the p21(Cip1) proximal promoter. In addition, KLF4-siRNA inhibits PKCδ-dependent p21(Cip1) promoter activity. PKCδ increases KLF4 expression leading to enhanced KLF4 interaction with the GC-rich elements in the p21(Cip1) promoter to activate transcription.

Molecular basis for the actions of Hsp90 inhibitors and cancer therapy

Heat-shock protein 90 (Hsp90) inhibitor downregulates c-Myc expression and upregulates the expression of tumor repressor proteins such as p53 and pRB, inhibiting the G1/S transition and causing G2/M arrest during cell cycle progression. The cycle progression is extensively controlled by the pRB/E2F signaling pathway. E2F is released from the pRB/E2F complex with the phosphorylation of pRB by cyclin-cyclin-dependent kinase (CDK) complexes. The released E2F promotes the transcription of target genes involved in cell cycle progression. The pRB/E2F signaling pathway is controlled by DNA methyltransferase-1 (Dnmt-1). The elevated expression of Dnmt-1 has been reported in carcinomas of the colon, lung and prostate. A defect of pRB expression in Rb -/- cancer cells is caused by the aberrant methylation of CpG in the Rb promoter. The Hsp90 inhibitor disrupts the Dnmt-1/Hsp90 association and upregulates pRB expression. In this review, the Hsp90 inhibitors that show promise for cancer therapy are summarized.

P21 suppresses cell proliferation and down-regulates POLD1 expression in human gastric cancer cell line MGC-803

AIM: To investigate whether and how P21 regulates POLD1 expression in human gastric cancer cell line MGC-803 and to find new clues to blocking the malignant proliferation of cancer cells.

METHODS: MGC-803 cells were divided into three groups: blank control group (untransfected cells), negative control group (cells transfected with an empty vector pXJ41-neo), and experimental group (cells transfected with a eukaryotic expression plasmid pXJ41-p21). After transfection, cell proliferation was detected by MTT assay; cell apoptosis was detected by flow cytometry; and the mRNA and protein expression was detected by quantitative real-time PCR and Western blot, respectively.

RESULTS: Compared to the two control groups, cell proliferation was significantly inhibited, cell apoptosis was increased (11.36 ± 0.51 vs 7.39 ± 0.17, 7.69 ± 0.47, F = 85.338, P < 0.05), and the mRNA and protein expression of POLD1 was inhibited in the experimental group. In addition, the relative expression levels of cyclin E and Rb1 increased, that of CDK2 decreased, and that of c-myc showed little change in the experimental group when compared to the two control groups.

CONCLUSION: P21 can suppress cell proliferation and promote apoptosis in human gastric cancer cell line MGC-803. P21 can also suppress POLD1 expression possibly by regulating the expression of CDK2, cyclin E, Rb1 or other cell cycle factors in MGC-803 cells.

p21(Cip1) regulates cell-substrate adhesion and interphase microtubule dynamics in untransformed human mammary epithelial cells

Despite its frequent inactivation in human breast cancers, the role of p21(Cip1) (p21) in morphological plasticity of normal mammary epithelial cells is still poorly understood. To address this question, we have investigated the consequences of p21 silencing in two-dimensional (2D) morphogenesis of untransformed human mammary epithelial cells. Here we show that p21 inactivation causes a reduction of 2D cell spreading and suppresses focal adhesion. In order to investigate the cytoskeletal modifications associated with this altered morphology, we have analyzed the microtubule dynamics in interphase p21-depleted cells. Our results demonstrate that interphase microtubule dynamic instability is strongly increased by p21 silencing. This alteration correlates with severe microtubule hypoacetylation. Next, we show that these microtubule defects in p21-depleted cells can be reversed by the use of the small molecule tubacin, a specific inhibitor of the α-tubulin deacetylase HDAC6. Tubacin-induced microtubule dynamics decrease also correlates with a partial recovery of cell spreading and focal adhesion in those cells. Collectively, these data indicate that p21 regulates the morphological plasticity of normal mammary epithelial cells by modulating dynamics of key cytoskeletal components.

Proteoglycans in cancer biology, tumour microenvironment and angiogenesis

Proteoglycans, key molecular effectors of cell surface and pericellular microenvironments, perform multiple functions in cancer and angiogenesis by virtue of their polyhedric nature and their ability to interact with both ligands and receptors that regulate neoplastic growth and neovascularization. Some proteoglycans such as perlecan, have pro- and anti-angiogenic activities, whereas other proteoglycans, such as syndecans and glypicans, can also directly affect cancer growth by modulating key signalling pathways. The bioactivity of these proteoglycans is further modulated by several classes of enzymes within the tumour microenvironment: (i) sheddases that cleave transmembrane or cell-associated syndecans and glypicans, (ii) various proteinases that cleave the protein core of pericellular proteoglycans and (iii) heparanases and endosulfatases which modify the structure and bioactivity of various heparan sulphate proteoglycans and their bound growth factors. In contrast, some of the small leucine-rich proteoglycans, such as decorin and lumican, act as tumour repressors by physically antagonizing receptor tyrosine kinases including the epidermal growth factor and the Met receptors or integrin receptors thereby evoking anti-survival and pro-apoptotic pathways. In this review we will critically assess the expanding repertoire of molecular interactions attributed to various proteoglycans and will discuss novel proteoglycan functions modulating cancer progression, invasion and metastasis and how these factors regulate the tumour microenvironment.

Coordinated regulation of mitochondrial topoisomerase IB with mitochondrial nuclear encoded genes and MYC

Mitochondrial DNA (mtDNA) is entirely dependent on nuclear genes for its transcription and replication. One of these genes is TOP1MT, which encodes the mitochondrial DNA topoisomerase IB, involved in mtDNA relaxation. To elucidate TOP1MT regulation, we performed genome-wide profiling across the 60-cell line panel (the NCI-60) of the National Cancer Institute Developmental Therapeutics Program. We show that TOP1MT mRNA expression varies widely across these cell lines with the highest levels in leukemia (HL-60, K-562) and melanoma (SK-MEL-28), intermediate levels in breast (MDA-MB-231), ovarian (OVCAR) and colon (HCT-116, HCT-15, KM-12), and lowest levels in renal (ACHN, A498), prostate (PC-3, DU-145) and central nervous system cell lines (SF-539, SF-268, SF-295). Genome-wide analyses show that TOP1MT expression is significantly correlated with the other mitochondrial nuclear-encoded genes including the mitochondrial nucleoid genes, and demonstrate an overall co-regulation of the mitochondrial nuclear-encoded genes. We also find very high correlation between the expression of TOP1MT and the proto-oncogene MYC (c-myc). TOP1MT contains E-boxes (c-myc binding sites) and TOP1MT transcription follows MYC up- and down-regulation by MYC promoter activation and siRNA against MYC. Our finding implicates MYC as a novel regulator of TOP1MT and confirms its role as a master regulator of MNEGs and mitochondrial nucleoids.

The overexpression of AP-4 as a prognostic indicator for gastric carcinoma

As a transcription factor belonging to the basic helix-loop-helix leucine-zipper subgroup, AP-4 can control target gene expression by altering cell signal transduction, and regulate growth, development, and cell apoptosis. Under pathological circumstances, it is involved in tumorigenesis. Herein, immunohistochemistry and real-time PCR were used to detect the transcription factor AP-4 expression in gastric cancer, and these data were examined for correlation with histology, pTNM stage, and prognosis. The AP-4 expression rate was 83.67% in a total of 98 gastric cancer tissues, which was significantly higher than 40.91% in non-neoplastic tissues; AP-4 mRNA relative expression shows a significant difference between gastric cancer and normal tissues, and AP-4 expression has a significantly positive correlation with the depth of tumor invasion (P < 0.0001), degree of tumor differentiation (P = 0.0058), lymph node metastasis (P = 0.0255), and pTNM stage (P = 0.001). Survival analysis showed that AP-4-positive patients' median survival time (12.60 months) was significantly shorter than that (41.40 months) of AP-4-negative patients. AP-4 expression in gastric cancer is associated with clinicopathological parameters of gastric cancer, such as differentiation, lymph node metastasis, depth of invasion (P = 0.0010), and pTNM stage. What's more, AP-4 overexpression indicated a worse prognosis for patients. So AP-4 may be a molecular marker to predict the progression and prognosis of the tumor.

Transcriptional and post-translational regulation of the quiescence factor and putative tumor suppressor p150(Sal2)

The evolutionarily conserved SALL genes encode transcription factors with roles in embryonic development. The product of the SALL2 gene was first identified as a binding partner of the mouse polyoma virus large T antigen and later shown to possess tumor suppressor-like functions. Independent studies identified SALL2 as a factor regulating the quiescent state in human fibroblasts. Here, we investigate factors that regulate the expression of SALL2 and turnover of p150(Sal2) in growing vs. resting cells. The transcription factor AP4 increases along with SALL2 in quiescent cells and positively regulates SALL2 expression. TGFβ effectively inhibits expression of SALL2 and its regulator AP4 when added to quiescent fibroblasts. TGFβ repression of SALL2 and AP4 is independent of the induction of connective tissue growth factor (CTGF) by TGFβ. p150(Sal2) disappears rapidly on restoration of serum. In both growing fibroblasts and established ovarian surface epithelial cells, p150(Sal2) undergoes polyubiquitination and proteosomal degradation. A CUL4/DDB1 E3 ligase containing RBBP7 as the p150(Sal2) receptor has been identified as mediating the destruction of p150(Sal2) as cells transition from a quiescent to an actively growing state.

Decorin antagonizes Met receptor activity and down-regulates {beta}-catenin and Myc levels

A theme emerging during the past few years is that members of the small leucine-rich proteoglycan gene family affect cell growth by interacting with multiple receptor tyrosine kinases (RTKs), mostly by a physical down-regulation of the receptors, thereby depriving tumor cells of pro-survival signals. Decorin binds and down-regulates several RTKs, including Met, the receptor for hepatocyte growth factor. Here we demonstrate that decorin blocks several biological activities mediated by the Met signaling axis, including cell scatter, evasion, and migration. These effects were mediated by a profound down-regulation of noncanonical β-catenin levels. In addition, Myc, a downstream target of β-catenin, was markedly down-regulated by decorin, whereas phosphorylation of Myc at threonine 58 was markedly induced. The latter is known to destabilize Myc and target it for proteasomal degradation. We also discovered that systemic delivery of decorin using three distinct tumor xenograft models caused down-regulation of Met and a concurrent suppression of β-catenin and Myc levels. We found that decorin protein core labeled with the near infrared dye IR800 specifically targeted the tumor cells expressing Met. Even 68-h post-injection, decorin was found to reside within the tumor xenografts with little or no binding to other tissues. Collectively, our results indicate a role for a secreted proteoglycan in suppressing the expression of key oncogenic factors required for tumor progression.

A global genomic view on LNX siRNA-mediated cell cycle arrest

LNX protein is the first described PDZ domain-containing member of the RING finger-type E3 ubiquitin ligase family. Studies have approved that LNX could participate in signal transduction, such as Notch pathway, and play an important role in tumorigenesis. In this study, we found that down-regulation of LNX resulted in G0/G1 cell cycle arrest in G0/G1 phase in HEK293 cells. To explore the molecular mechanism of this phenomenon, we employed expression microarray to comparatively analyze the genome-wide expression between the LNX-knockdown cells and the normal cells. We also used quantitative real-time PCR to further confirm the differential expression patterns of 25 transcripts involved in cell cycle. Combined with known information about genic functions, signal pathways and cell cycle machinery, we analyzed the role of endogenous LNX in cell cycle. The results suggest that down-regulation of LNX could result in cell cycle arrest in G0/G1 phase through inhibition of β-catenin, MAPK, NFκB, c-Myc-dependent pathway and activation of p53, TGF-β-dependent pathway. This study provides new perspectives on LNX's pleiotropic activities, especially its essential role in cell proliferation and cell cycle.

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

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

A PIN1 polymorphism that prevents its suppression by AP4 associates with delayed onset of Alzheimer's disease

Alzheimer's disease (AD), the most common form of dementia, is characterized by the presence of neurofibrillary tangles composed of tau and senile plaques of amyloid-beta peptides (Aβ) derived from amyloid precursor protein (APP). Pin1 is a unique prolyl isomerase that has been shown to protect against age-dependent neurodegeneration by acting on phosphorylated tau and APP to suppress tangle formation and amyloidogenic APP processing. Here we report a functional polymorphism, rs2287839, in the Pin1 promoter that is significantly associated with a 3-year delay in the average age at onset (AAO) of late-onset AD in a Chinese population. More significantly, the Pin1 polymorphism rs2287839 is located within the consensus binding motif for the brain-selective transcription factor, AP4 (CAGCTG) and almost completely abolishes the ability of AP4 to bind and suppress the Pin1 promoter, as shown by chromatin immunoprecipitation, electrophoretic mobility shift assay, and promoter luciferase assay. Moreover, overexpression or knockdown of AP4 resulted in an 80% reduction or 2-fold increase in endogenous Pin1 levels, respectively. Thus, AP4 is a novel transcriptional repressor of Pin1 expression and the Pin1 promoter single nucleotide polymorphism (SNP) identified in this study that prevents such suppression is associated with delayed onset of AD. These results indicate that regulation of Pin1 by AP4 plays a critical role in determining age at onset of AD and might be a novel therapeutic target to delay the onset of AD.

Kaempferol enhances cisplatin's effect on ovarian cancer cells through promoting apoptosis caused by down regulation of cMyc

Background

Ovarian cancer is one of the most significant malignancies in the western world. Studies showed that Ovarian cancers tend to grow resistance to cisplatin treatment. Therefore, new approaches are needed in ovarian cancer treatment. Kaempferol is a dietary flavonoid that is widely distributed in fruits and vegetables, and epidemiology studies have revealed a protective effect of kaempferol against ovarian cancer risk. Our early studies also found that kaempferol is effective in reducing vascular endothelial growth factor (VEGF) expression in ovarian cancer cells. In this study, we investigated kaempferol's effects on sensitizing ovarian cancer cell growth in response to cisplatin treatment.

Results

Ten chemicals were screened for sensitizing OVCAR-3 ovarian cancer cell growth in response to cisplatin treatment. For kaempferol, which shows a significant synergistic interaction with cisplatin, expression of ABCC1, ABCC5, ABCC6, NFkB1, cMyc, and CDKN1A genes was further examined. For cisplatin/kaempferol treatments on OVCAR-3 cancer cells, the mRNA levels of ABCC1, ABCC5, and NFkB1 did not change. However, significant inhibition of ABCC6 and cMyc mRNA levels was observed for the cisplatin/kaempferol combined treatment. The CDKN1A mRNA levels were significantly up-regulated by cisplatin/kaempferol treatment. A plot of CDKN1A mRNA levels against that of cMyc gene further revealed a reverse, linear relationship, proving cMyc's regulation on CDKN1A gene expressions. Our work found that kaempferol works synergistically with cisplatin in inhibiting ovarian cancer cell viability, and their inhibition on cell viabilities was induced through inhibiting ABCC6 and cMyc gene transcription. Apoptosis assay showed the addition of 20 μM kaempferol to the cisplatin treatment induces the apoptosis of the cancer cells.

Conclusions

Kaempferol enhances the effect of cisplatin through down regulation of cMyc in promoting apoptosis of ovarian cancer cells. As a dietary component, kaempferol sensitizes ovarian cancer cells to cisplatin treatment and deserves further studies for possible applications in chemotherapy of ovarian cancers.

Identification of an E-box DNA binding protein, activated protein 4, and its function in regulating the expression of the gene encoding diapause hormone and pheromone biosynthesis-activating neuropeptide in Helicoverpa armigera

Activated protein 4 (AP-4), an E-box DNA-binding protein, was cloned from the cotton bollworm, Helicoverpa armigera (Har). The expression of Har-AP-4 mRNA and the protein that it encodes are significantly higher in nondiapause pupae than in diapause pupae. In vitro-translated Har-AP-4 can bind specifically to the E-box motif on the promoter of the diapause hormone and pheromone biosynthesis-activating neuropeptide (DH-PBAN). Har-AP-4, fused with the green fluorescent protein (GFP), is localized to the nucleus, and overexpression of Har-AP-4 can significantly activate the promoter of the DH-PBAN gene that is involved in nondiapause pupal development in H. armigera. These results suggest that Har-AP-4, which binds to the promoter of DH-PBAN, may play a role in regulating pupal development in H. armigera.

p21(WAF1) gene promoter is epigenetically silenced by CTIP2 and SUV39H1

Mainly regulated at the transcriptional level, the cellular cyclin-dependent kinase inhibitor, CDKN1A/p21(WAF1) (p21), is a major cell cycle regulator of the response to DNA damage, senescence and tumor suppression. Here, we report that COUP-TF-interacting protein 2 (CTIP2), recruited to the p21 gene promoter, silenced p21 gene transcription through interactions with histone deacetylases and methyltransferases. Importantly, treatment with the specific SUV39H1 inhibitor, chaetocin, repressed histone H3 lysine 9 trimethylation at the p21 gene promoter, stimulated p21 gene expression and induced cell cycle arrest. In addition, CTIP2 and SUV39H1 were recruited to the silenced p21 gene promoter to cooperatively inhibit p21 gene transcription. Induction of p21(WAF1) gene upon human immunodeficiency virus 1 (HIV-1) infection benefits viral expression in macrophages. Here, we report that CTIP2 further abolishes Vpr-mediated stimulation of p21, thereby indirectly contributing to HIV-1 latency. Altogether, our results suggest that CTIP2 is a constitutive p21 gene suppressor that cooperates with SUV39H1 and histone methylation to silence the p21 gene transcription.

Complementary quantitative proteomics reveals that transcription factor AP-4 mediates E-box-dependent complex formation for transcriptional repression of HDM2

Transcription factor activating enhancer-binding protein 4 (AP-4) is a basic helix-loop-helix protein that binds to E-box elements. AP-4 has received increasing attention for its regulatory role in cell growth and development, including transcriptional repression of the human homolog of murine double minute 2 (HDM2), an important oncoprotein controlling cell growth and survival, by an unknown mechanism. Here we demonstrate that AP-4 binds to an E-box located in the HDM2-P2 promoter and represses HDM2 transcription in a p53-independent manner. Incremental truncations of AP-4 revealed that the C-terminal Gln/Pro-rich domain was essential for transcriptional repression of HDM2. To further delineate the molecular mechanism(s) of AP-4 transcriptional control and its potential implications, we used DNA-affinity purification followed by complementary quantitative proteomics, cICAT and iTRAQ labeling methods, to identify a previously unknown E-box-bound AP-4 protein complex containing 75 putative components. The two labeling methods complementarily quantified differentially AP-4-enriched proteins, including the most significant recruitment of DNA damage response proteins, followed by transcription factors, transcriptional repressors/corepressors, and histone-modifying proteins. Specific interaction of AP-4 with CCCTC binding factor, stimulatory protein 1, and histone deacetylase 1 (an AP-4 corepressor) was validated using AP-4 truncation mutants. Importantly, inclusion of trichostatin A did not alleviate AP-4-mediated repression of HDM2 transcription, suggesting a previously unidentified histone deacetylase-independent repression mechanism. In contrast, the complementary quantitative proteomics study suggested that transcription repression occurs via coordination of AP-4 with other transcription factors, histone methyltransferases, and/or a nucleosome remodeling SWI.SNF complex. In addition to previously known functions of AP-4, our data suggest that AP-4 participates in a transcriptional-regulating complex at the HDM2-P2 promoter in response to DNA damage.

Functional impact of sequence variation in the promoter region of TGFB1

Pathological deregulation of the transforming growth factor, beta 1 (TGFB1) pathway has been implicated in the development of several major diseases, including cancers. Regulatory variation in the TGFB1 gene may lead to altered TGFB1 expression and activity, and thus, modulate an individual's susceptibility to disease. Here, we performed a study of the functional relevance of cis-acting regulatory variation in the proximal promoter region of the TGFB1 gene. In a previous study, 9 promoter polymorphisms were identified in the 2kb region upstream of the transcription start site and 9 distinct promoter haplotypes were inferred from a panel of individuals from 5 distinct continental population groups. Following experimental validation, we found that the 2 major haplotypes significantly influenced TGFB1 transcriptional activity in an allele-specific manner and that 3 of the SNPs (-1886G>A, -509C>T and -1550DEL/AGG) altered DNA-protein complex formation. Though the biological relevance of these findings remains to be verified, our study suggests that polymorphisms in the TGFB1 promoter could indeed influence gene expression and potentially contribute to the pathogenesis of TGFB1 related diseases.

Acute overexpression of Myc in intestinal epithelium recapitulates some but not all the changes elicited by Wnt/beta-catenin pathway activation

The Myc transcription factor is a potent inducer of proliferation and is required for Wnt/beta-catenin signaling in intestinal epithelium. Since deregulation of the Wnt/beta-catenin pathway is a prerequisite for nonhereditary intestinal tumorigenesis, we asked whether activation of Myc recapitulates the tumorigenic changes that are driven by constitutive Wnt/beta-catenin pathway signaling following adenomatous polyposis coli (APC) inactivation. Using mice in which expression of MycER(TAM), a reversibly switchable form of Myc, is expressed transgenically in intestinal epithelium, we define the acute changes that follow Myc activation as well as subsequent deactivation. Myc activation reversibly recapitulates many, but not all, aspects of APC inactivation, including increased proliferation and apoptosis and loss of goblet cells. However, whereas APC inactivation induces redistribution of Paneth cells, direct Myc activation triggers their rapid attrition. Moreover, direct Myc activation engages the ARF/p53/p21(cip1) tumor suppressor pathway, whereas deregulation of Wnt/beta-catenin signaling does not. These observations illustrate key differences in oncogenic impact in intestinal epithelium of direct Myc activation and indirect Myc activation via the Wnt/beta-catenin pathway. Furthermore, the in situ dedifferentiation of mature goblet cells that Myc induces indicates a novel cross talk between the Wnt/beta-catenin and Notch signaling pathways.

Regulation of BIRC5 and its isoform BIRC5-2B in neuroblastoma

We analysed the expression of BIRC5 and BIRC5-2B in primary neuroblastoma (NB) tumors and NB model systems. In tumors, overexpression of BIRC5 correlated closely with its isoform BIRC5-2B. Expression of both transcripts was stage-dependent, associated with poor prognosis and with the expression of the transcription factor E2F1. In cell culture, we identified BIRC5 as a direct transcriptional target of activating E2Fs, primarily when p21(Cip1) and p27(Kip1), two other E2F1 targets, are strongly suppressed. Deregulated MYCN indirectly induces BIRC5 through suppression of CDKN1A/p21(Cip1) and induction of Skp2, which in turn favors the degradation of p27(Kip1). In addition, increased BIRC5 protein stability via phosphorylation is mediated by expression of E2F targets such as CDC2. In line with this, selective knock down of CDC2 inhibited BIRC5 abundance and suppressed its anti-apoptotic activities. We conclude that BIRC5 is induced via a functional cooperation between MYCN and E2F1.

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.

Evidence that human blastomere cleavage is under unique cell cycle control

Purpose

To understand the molecular pathways that control early human embryo development.

Methods

Improved methods of linear amplification of mRNAs and whole human genome microarray analyses were utilized to characterize gene expression in normal appearing 8-Cell human embryos, in comparison with published microarrays of human fibroblasts and pluripotent stem cells.

Results

Many genes involved in circadian rhythm and cell division were over-expressed in the 8-Cells. The cell cycle checkpoints, RB and WEE1, were silent on the 8-Cell arrays, whereas the recently described tumor suppressor, UHRF2, was up-regulated >10-fold, and the proto-oncogene, MYC, and the core element of circadian rhythm, CLOCK, were elevated up to >50-fold on the 8-Cell arrays.

Conclusions

The canonical G1 and G2 cell cycle checkpoints are not active in totipotent human blastomeres, perhaps replaced by UHRF2, MYC, and intracellular circadian pathways, which may play important roles in early human development.

Electronic supplementary material

The online version of this article (doi:10.1007/s10815-009-9306-x) contains supplementary material, which is available to authorized users.