Inhibitors of the Cip/Kip family

Regulation of p27 (Kip1) by Ubiquitin E3 Ligase RNF6

The cyclin-dependent kinase inhibitor p27 (Kip1) is an important regulator of the G1/S checkpoint. It is degraded by the SCF-SKP2 complex in late G1 thereby allowing cells to progress to the S phase. Here we investigated the role of the E3 ubiquitin ligase RNF6 (Ring Finger Protein 6) in cell cycle progression in prostate cancer cells. Our data demonstrate that RNF6 can promote cell cycle progression by reducing the levels of p27. Knockdown of RNF6 led to an increase in the stability of p27 and to the arrest of cells in the G1 phase. RNF6 interacted with p27 via its KIL domain and this interaction was found to be phosphorylation independent. RNF6 enhanced ubiquitination and subsequent degradation of p27 in the early G0/G1 phase of the cell cycle. Knockdown of RNF6 expression by short hairpin RNA led to inhibition of the CDK2/Cyclin E complex thereby reducing phosphorylation of Retinoblastoma protein (Rb) and to a subsequent decrease in cell cycle progression and proliferation. Our data suggest that RNF6 acts as a negative regulator for p27^kip1 leading to its proteasome-dependent degradation in the early G0/G1 phase of the cell cycle.

Derivation and Characterization of Murine and Amphibian Müller Glia Cell Lines

Purpose

Müller glia (MG) in the retina of Xenopus laevis (African clawed frog) reprogram to a transiently amplifying retinal progenitor state after an injury. These progenitors then give rise to new retinal neurons. In contrast, mammalian MG have a restricted neurogenic capacity and undergo reactive gliosis after injury. This study sought to establish MG cell lines from the regeneration-competent frog and the regeneration-deficient mouse.

Methods

MG were isolated from postnatal day 5 GLAST-CreERT; Rb^fl/fl mice and from adult (3–5 years post-metamorphic) Xlaevis. Serial adherent subculture resulted in spontaneously immortalized cells and the establishment of two MG cell lines: murine retinal glia 17 (RG17) and Xenopus glia 69 (XG69). They were characterized for MG gene and protein expression by qPCR, immunostaining, and Western blot. Purinergic signaling was assessed with calcium imaging. Pharmacological perturbations with 2’-3’-O-(4-benzoylbenzoyl) adenosine 5’-triphosphate (BzATP) and KN-62 were performed on RG17 cells.

Results

RG17 and XG69 cells express several MG markers and retain purinergic signaling. Pharmacological perturbations of intracellular calcium responses with BzATP and KN-62 suggest that the ionotropic purinergic receptor P2X7 is present and functional in RG17 cells. Stimulation of XG69 cells with adenosine triphosphate–induced calcium responses in a dose-dependent manner.

Conclusions

We report the characterization of RG17 and XG69, two novel MG cell lines from species with significantly disparate retinal regenerative capabilities.

Translational Relevance

RG17 and XG69 cell line models will aid comparative studies between species endowed with varied regenerative capacity and will facilitate the development of new cell-based strategies for treating retinal degenerative diseases.

RAS specific protease induces irreversible growth arrest via p27 in several KRAS mutant colorectal cancer cell lines

Ras-specific proteases to degrade RAS within cancer cells are under active development as an innovative strategy to treat tumorigenesis. The naturally occurring biological toxin effector called RAS/RAP1-specific endopeptidase (RRSP) is known to cleave all RAS within a cell, including HRAS, KRAS, NRAS and mutant KRAS G13D. Yet, our understanding of the mechanisms by which RRSP drives growth inhibition are unknown. Here, we demonstrate, using isogenic mouse fibroblasts expressing a single isoform of RAS or mutant KRAS, that RRSP equally inactivates all isoforms of RAS as well as the major oncogenic KRAS mutants. To investigate how RAS processing might lead to varying outcomes in cell fate within cancer cells, we tested RRSP against four colorectal cancer cell lines with a range of cell fates. While cell lines highly susceptible to RRSP (HCT116 and SW1463) undergo apoptosis, RRSP treatment of GP5d and SW620 cells induces G1 cell cycle arrest. In some cell lines, growth effects were dictated by rescued expression of the tumor suppressor protein p27 (Kip1). The ability of RRSP to irreversibly inhibit cancer cell growth highlights the antitumor potential of RRSP, and further warrants investigation as a potential anti-tumor therapeutic.

Stimulation of c-Jun/AP-1-Activity by the Cell Cycle Inhibitor p57Kip2

p57 is a member of the Cip/Kip family of cell cycle inhibitors which restrict the eukaryotic cell cycle by binding to and inhibiting cyclin/CDK complexes. They are considered as tumor suppressors and inactivating genomic mutations of p57 are associated with human overgrowth disorders. Increasing evidence suggests that p57 controls additional cellular processes beyond cell cycle control such as apoptosis, cell migration or transcription. Here we report that p57 can stimulate AP-1 promotor activity. While transactivation by c-Jun is strongly activated by p57, it did not enhance c-Fos induced transcription. This indicates that c-Jun is the target of p57 in the canonical AP-1 heterodimeric transcription factor. We could detect endogenous p57/c-Jun containing complexes in cells by co-immunoprecipitation. The strong stimulation of c-Jun activity is not the consequence of activating phosphorylation in the transactivation domain (TAD) of c-Jun, but rather due to negative interference with c-Jun repressors and positive interference with c-Jun activators. In contrast to full-length p57, the amino- and carboxy-terminal domains of p57 are insufficient for a significant activation of c-Jun induced transcription. When expressed in presence of full length p57, the p57 C-terminus abrogated and the N-terminus enhanced c-Jun activation. This indicates that the C-terminus may bind and sequester a putative activator of c-Jun, whereas the N-terminus may sequester a c-Jun repressor. Interestingly, the p57 aminoterminus is sufficient for binding to the two c-Jun repressors HDAC1 and HDAC3. These data are consistent with a model of c-Jun activation where p57 is a part of large nuclear remodeling/transcription complexes. p57 might stimulate transcription by inhibiting transcription repressor proteins like HDACs via its N-terminus and/or attracting transcription activators through its C-terminus. These data suggest that in addition to its role as a CDK inhibitor and tumor suppressor, p57 may also exert tumor promoting functions by activation of the proto-oncoprotein c-Jun.

S5, a Withanolide Isolated from Physalis Pubescens L., Induces G2/M Cell Cycle Arrest via the EGFR/P38 Pathway in Human Melanoma A375 Cells

S5 is a withanolide natural product isolated from Physalis pubescens L. Our previous experimental studies found that it has significant antitumor activity on renal cell carcinoma. In the present study, the anti-melanoma effect of S5 and the related molecular mechanism was first investigated. It was found that S5 induced an obvious growth inhibitory effect on human melanoma A375 cells with low toxicity to human peripheral blood cells. Furthermore, the results demonstrated that the cell death mode of S5 on A375 cells is not due to inducing apoptosis and autophagy. However, there was a significant time-dependent increase in G2/M phase after treatment of A375 with S5. Meanwhile, S5 could also decrease the protein expression of Cdc25c, Cdc2, and CyclinB1, and increased the expression of p-P53 and P21, suggesting that S5 inhibited A375 cell death through G2/M phase arrest. Moreover, the signal pathway factors P38, extracellular regulated protein kinases (ERK), and epidermal growth factor receptor (EGFR) were observed taking part in the S5-induced A375 cells growth inhibitory effect. In addition, suppressing P38 and EGFR reversed the cell proliferation inhibitory effect and G2/M cell cycle arrest induced by S5 and inhibition of EGFR enhanced the downregulation of the expression of P38 and p-P38, indicating that S5 induced A375 G2/M arrest through the EGFR/P38 pathway. Briefly, this study explained for the first time the mechanism of S5-induced A375 cell growth inhibition in order to provide the basis for its clinical application in melanoma.

A dual role of miR-22 modulated by RelA/p65 in resensitizing fulvestrant-resistant breast cancer cells to fulvestrant by targeting FOXP1 and HDAC4 and constitutive acetylation of p53 at Lys382

Antiestrogen resistance is a major challenge encountered during the treatment of estrogen receptor alpha positive (ERα⁺) breast cancer. A better understanding of signaling pathways and downstream transcription factors and their targets may identify key molecules that can overcome antiestrogen resistance in breast cancer. An aberrant expression of miR-22 has been demonstrated in breast cancer; however, its contribution to breast cancer resistance to fulvestrant, an antiestrogen drug, remains unknown. In this study, we demonstrated a moderate elevation in miR-22 expression in the 182^R-6 fulvestrant-resistant breast cancer line we used as a model system, and this elevation was positively correlated with the expression of the miRNA biogenesis enzymes AGO2 and Dicer. The level of phosphorylated HER2/neu at Tyr877 was also upregulated in these cells, whereas the level of RelA/p65 phosphorylated at Ser536 (p-p65) was downregulated. Knockdown of HER2/neu led to an induction of p-p65 and a reduction in miR-22 levels. Luciferase assays identified two NF-κB binding motifs in the miR-22 promoter that contributed to transcriptional repression of miR-22. Activation of RelA/p65, triggered by LPS, attenuated miR-22 expression, but this expression was restored by sc-514, a selective IKKβ inhibitor. Inhibition of miR-22 suppressed cell proliferation, induced apoptosis and caused cell cycle S-phase arrest, whereas enhancing expression of p21^Cip1/Waf1 and p27^Kip1. Surprisingly, ectopic expression of miR-22 also suppressed cell proliferation, induced apoptosis, caused S-phase arrest, and promoted the expression of p21^Cip1/Waf1 and p27^Kip1. Ectopic overexpression of miR-22 repressed the expression of FOXP1 and HDAC4, leading to a marked induction of acetylation of HDAC4 target histones. Conversely, inhibition of miR-22 promoted the expression of both FOXP1 and HDAC4, without the expected attenuation of histone acetylation. Instead, p53 acetylation at lysine 382 was unexpectedly upregulated. Taken together, our findings demonstrated, for the first time, that HER2 activation dephosphorylates RelA/p65 at Ser536. This dephosphoryalted p65 may be pivotal in transactivation of miR-22. Both increased and decreased miR-22 expression cause resensitization of fulvestrant-resistant breast cancer cells to fulvestrant. HER2/NF-κB (p65)/miR-22/HDAC4/p21 and HER2/NF-κB (p65)/miR-22/Ac-p53/p21 signaling circuits may therefore confer this dual role on miR-22 through constitutive transactivation of p21.

The deficiency of CX3CL1/CX3CR1 system ameliorates high fructose diet-induced kidney injury by regulating NF-κB pathways in CX3CR1-knock out mice

Fructose, the most important functional food additive from the last century, has been widely used in industry, agriculture, light industry, food and medicine. With the improvement of people's living standard and economic level, excess intake of fructose results in metabolic symptoms, including hyperleptinemia, insulin resistance and neuroinflammation is causing high risk of chronic kidney disease development in humans and animals. However, the underlying molecular mechanism of renal injury is still not fully understood, and the development of effective drugs and treatments are delayed. Hence, we investigated the role of crosstalk of CX3CL1-CX3CR1 axis and nuclear factor-κB (NF-κB) signaling pathway in the development of renal injury. CX3CL1-knock-out C57BL/6 mice were constructed and used to analyze the influence of CX3CL1-related signaling pathways on kidney injury of wild-type (WT) mice and CXECR1 deficiency mice, which were administrated with 30% fructose water. Western blotting, quantitative RT-PCR (qRT-PCR), immunohistochemistry, ELISA, flow cytometry and biochemical indicator analysis were used to determine the levels of renal injury and key signaling pathway associated with renal damage. The results indicated that administration of high fructose intake can cause typical renal inflammatory responses in serum and tissues. Fructose enhances the CX3CL1-CX3CR1 axis and NF-κB activation, and promotes crosstalk of CX3CL1-CX3CR1 and NF-κB pathways. The phosphorylated AKT could be significantly activated in fructose-induced renal injury via CX3CL1-CX3CR1 axis. CX3CR1 expression between WT and CX3CR1^−/− mice were evaluated to establish their relationship with injury. Our results indicated that CX3CR1 may be the central and major indicator in the process of renal injury, which mediate AKT pathway and further enhance the NF-κB activation. These findings demonstrated that crosstalk of CX3CL1-CX3CR1 axis and NF-κB signaling pathway play a direct role in fructose-induced kidney injury. Inhibition of CX3CL1-CX3CR1 pathway may suppress renal-related diseases. It may be a potential treatment choice for the clinical diagnoses and treatment in the future.

Therapeutic inhibition of SGK1 suppresses colorectal cancer

Colorectal cancer (CRC) is one of the leading causes of death worldwide. Thus, the development of new therapeutic targets for CRC treatment is urgently needed. SGK1 is involved in various cellular activities, and its dysregulation can result in multiple cancers. However, little is known about its roles and associated molecular mechanisms in CRC. In present study, we found that SGK1 was highly expressed in tumor tissues compared with peri-tumor samples from CRC patients. In vitro experiments revealed that SGK1 overexpression promoted colonic tumor cell proliferation and migration and inhibited cell apoptosis induced by 5-fluorouracil (5-FU), while SGK1 shRNA and inhibitors showed the inverse effects. Using CRC xenograft mice models, we demonstrated that knockdown or therapeutic inhibition of SGK1 repressed tumor cell proliferation and tumor growth. Moreover, SGK1 inhibitors increased p27 expression and promoted p27 nuclear accumulation in colorectal cancer cells, and p27 siRNAs could attenuate the repression of CRC cell proliferation induced by SGK1 inhibitors. Collectively, SGK1 promotes colorectal cancer development via regulation of CRC cell proliferation, migration and survival. Inhibition of SGK1 represents a novel strategy for the treatment of CRC.

Disease and Carrier Isolates of Neisseria meningitidis Cause G1 Cell Cycle Arrest in Human Epithelial Cells

Microbial pathogens have developed several mechanisms to modulate and interfere with host cell cycle progression. In this study, we analyzed the effect of the human pathogen Neisseria meningitidis on the cell cycle of epithelial cells. Two pathogenic isolates, as well as two carrier isolates, were tested for their ability to adhere to and invade into the epithelial cell lines Detroit 562 and NP69 and to modulate the cell cycle. We found that all isolates adhered equally well to both Detroit 562 and NP69 cells, whereas the carrier isolates were significantly less invasive. Using propidium iodide staining and 5-ethynyl-2'-deoxyuridine pulse-labeling, we provide evidence that meningococcal infection arrested cells in the G1 phase of the cell cycle at 24 h postinfection. In parallel, a significant decrease of cells in the S phase was observed. Interestingly, G1-phase arrest was only induced after infection with live bacteria but not with heat-killed bacteria. By Western blotting we demonstrate that bacterial infection resulted in a decreased protein level of the cell cycle regulator cyclin D1, whereas cyclin E expression levels were increased. Furthermore, N. meningitidis infection induced an accumulation of the cyclin-dependent kinase inhibitor (CKI) p21(WAF1/CIP1) that was accompanied by a redistribution of this CKI to the cell nucleus, as shown by immunofluorescence analysis. Moreover, the p27(CIP1) CKI was redistributed and showed punctate foci in infected cells. In summary, we present data that N. meningitidis can interfere with the processes of host cell cycle regulation.

Synthesis, Characterization and Anti-Cancer Activity of Hydrazide Derivatives Incorporating a Quinoline Moiety

Identification of the novel (E)-N′-((2-chloro-7-methoxyquinolin-3-yl)methylene)-3-(phenylthio)propanehydrazide scaffold 18 has led to the development of a new series of biologically active hydrazide compounds. The parent compound 18 and new quinoline derivatives 19–26 were prepared from the corresponding quinoline hydrazones and substituted carboxylic acids using EDC-mediated peptide coupling reactions. Further modification of the parent compound 18 was achieved by replacement of the quinoline moiety with other aromatic systems. All the newly synthesized compounds were evaluated for their anti-cancer activity against the SH-SY5Y and Kelly neuroblastoma cell lines, as well as the MDA-MB-231 and MCF-7 breast adenocarcinoma cell lines. Analogues 19 and 22 significantly reduced the cell viability of neuroblastoma cancer cells with micromolar potency and significant selectivity over normal cells. The quinoline hydrazide 22 also induced G₁ cell cycle arrest, as well as upregulation of the p27^kip1 cell cycle regulating protein.

PRMT5-PTEN molecular pathway regulates senescence and self-renewal of primary glioblastoma neurosphere cells

Glioblastoma (GBM) represents the most common and aggressive histologic subtype among malignant astrocytoma and is associated with poor outcomes because of heterogeneous tumour cell population including mature non-stem-like cell and immature stem-like cells within the tumour. Thus, it is critical to find new target-specific therapeutic modalities. Protein arginine methyltransferase enzyme 5 (PRMT5) regulates many cellular processes through its methylation activity and its overexpression in GBM is associated with more aggressive disease. Previously, we have shown that silencing of PRMT5 expression in differentiated GBM cell lines results in apoptosis and reduced tumour growth in mice. Here, we report the critical role of PRMT5 in GBM differentiated cells (GBMDC) grown in serum and GBM neurospheres (GBMNS) grown as neurospheres in vitro. Our results uncover a very significant role for PRMT5 in GBMNS self-renewal capacity and proliferation. PRMT5 knockdown in GBMDC led to apoptosis, knockdown in GBMNS led to G1 cell cycle arrest through upregulation of p27 and hypophoshorylation of retinoblastoma protein, leading to senescence. Comparison of impact of PRMT5 on cellular signalling by the Human Phospho-Kinase Array and chromatin immunoprecipitation-PCR revealed that unlike GBMDC, PRMT5 regulates PTEN expression and controls Akt and ERk activity in GBMNS. In vivo transient depletion of PRMT5 decreased intracranial tumour size and growth rate in mice implanted with both primary tumour-derived GBMNS and GBMDC. This is the first study to identify PTEN as a potential downstream target of PRMT5 and PRMT5 is vital to support both mature and immature GBM tumour cell populations.

NORE1A induction by membrane-bound CD40L (mCD40L) contributes to CD40L-induced cell death and G1 growth arrest in p21-mediated mechanism

Membrane-bound CD40L (mCD40L) but not soluble CD40L (sCD40L) has been implicated in direct cell death induction and apoptosis in CD40-expressing carcinomas. In this study, we show that mCD40L but not sCD40L induces NORE1A/Rassf5 expression in an NFκB-dependant mechanism. NORE1A expression appeared to contribute to mCD40L-induced cell death and enhance cell transition from G1 to S phase of the cell cycle in a p21-dependent mechanism. The upregulation of p21 protein was attributed to NORE1A expression, since NORE1A inhibition resulted in p21 downregulation. p21 upregulation was concomitant with lower p53 expression in the cytoplasmic fraction with no detectable increase at the nuclear p53 level. Moreover, mCD40L-induced cell death mediated by NORE1A expression appeared to be independent of mCD40L-induced cell death mediated by sustained JNK activation since NORE1A inhibition did not affect JNK phosphorylation and vice versa. The presented data allow better understanding of the mechanism by which mCD40L induces cell death which could be exploited in the clinical development of CD40-targeted anti-cancer therapies.

HTLV-I Tax-Mediated Inactivation of Cell Cycle Checkpoints and DNA Repair Pathways Contribute to Cellular Transformation: "A Random Mutagenesis Model"

To achieve cellular transformation, most oncogenic retroviruses use transduction by proto-oncogene capture or insertional mutagenesis, whereby provirus integration disrupts expression of tumor suppressors or proto-oncogenes. In contrast, the Human T-cell leukemia virus type 1 (HTLV-I) has been classified in a separate class referred to as "transactivating retroviruses". Current views suggest that the viral encoded Tax protein transactivates expression of cellular genes leading to deregulated growth and transformation. However, if Tax-mediated transactivation was indeed sufficient for cellular transformation, a fairly high frequency of infected cells would eventually become transformed. In contrast, the frequency of transformation by HTLV-I is very low, likely less than 5%. This review will discuss the current understanding and recent discoveries highlighting critical functions of Tax in cellular transformation. HTLV-I Tax carries out essential functions in order to override cell cycle checkpoints and deregulate cellular division. In addition, Tax expression is associated with increased DNA damage and genome instability. Since Tax can inhibit multiple DNA repair pathways and stimulate unfaithful DNA repair or bypass checkpoints, these processes allow accumulation of genetic mutations in the host genome. Given this, a "Random Mutagenesis" transformation model seems more suitable to characterize the oncogenic activities of HTLV-I.

MicroRNAs-role in lung cancer

Regulation of gene expression is essential for normal physiological functions; thus deregulation of gene expression is common in disease conditions. One level of regulation of gene expression is performed by noncoding RNAs, among which microRNAs (miRNA) are the best studied. Abnormal expression of these molecular players can lead to pathogenic processes such as heart disease, immune system abnormalities, and carcinogenesis, to name but a few. Of a length of 18–25 nucleotides miRNAs are involved in binding partial complementary sequences within the 3′-UTR (3′-untranslated region) of the target mRNAs. Depending on the type of neoplastic transformation, miRNAs can act both as oncogenes (oncomirs) or as tumor suppressors. Because of the great importance of miRNAs, most researches focus on either their role as biomarkers or their potential as therapeutic targets. Herein, we present the review of microRNA biology, function, and tumorigenic potential with emphasis on their role in lung cancer.

MicroRNA miR-24 promotes cell proliferation by targeting the CDKs inhibitors p27Kip1 and p16INK4a

Cell cycle progression is controlled by numerous mechanisms ensuring correct cell division. The transition from one cell cycle phase to another occurs in an orderly fashion and is regulated by different cellular proteins. Therefore an alteration of the regulatory mechanisms of the cell cycle results in uncontrolled cell proliferation, which is a distinctive feature of human cancers. Recent evidences suggest that microRNAs (miRs) may also control the levels of multiple cell cycle regulators and therefore control cell proliferation. In fact miRs are a class of small non-coding RNAs, which modulate gene expression. They are involved in numerous physiological cellular processes and most importantly accumulating evidence indicates that many miRs are aberrantly expressed in human cancers. In this report we describe that miR-24 directly targets p27(Kip1) and p16(Ink4a) in primary keratinocyte and in different cancer derived cell lines promoting their proliferation, suggesting that miR-24 is involved in cyclin-dependent kinase inhibitors post-transcriptional regulation and that upregulation of miR-24 may play a role in carcinogenesis.

Lapatinib induces p27(Kip1)-dependent G₁ arrest through both transcriptional and post-translational mechanisms

Lapatinib, a dual EGFR/HER2 tyrosine kinase inhibitor, has been shown to have potent antitumor effects against human breast cancer. Recent studies have shown that lapatinib upregulates p27(Kip1) (here after referred to as p27) expression and induces G₁ cell cycle arrest in various types of cancer cells. However, the regulation of p27 in lapatinib-induced cell cycle arrest is not well studied. Here we demonstrate that lapatinib-induced cell growth inhibition and G₁ cell cycle arrest in HER2-overexpressing human breast cancer cells were dependent on p27. We also show that lapatinib-induced upregulation of p27 expression was mediated through both transcriptional and post-translational mechanisms. On the one hand, lapatinib treatment led to increased FOXO3a expression and enhanced p27 transcription. On the other hand, lapatinib treatment resulted in increased DYRK1B expression, which correlated with increased p27 phosphorylation at Ser10 and decreased p27 degradation. Interestingly, we found that ERβ1 but not ERβ2 expression also upregulated p27 and enhanced lapatinib-induced cell proliferation inhibition and G₁ cell cycle arrest in HER2-overexpressing breast cancer cells. Taken together, our results suggest that lapatinib induces p27 expression via both transcriptional and post-translational upregulations, leading to cell cycle arrest and cell proliferation inhibition, and that its effect on breast cancer cells may be modified by ER expression status.

The p27-Skp2 axis mediates glucocorticoid-induced cell cycle arrest in T-lymphoma cells

Glucocorticoid therapy is an important treatment modality of hematological malignancies, especially T-cell acute lymphoblastic leukemia (T-ALL). Glucocorticoids are known to induce a cell cycle arrest and apoptosis in T-lymphoma cells. We could demonstrate that the cell cycle arrest induced by the synthetic glucocorticoid dexamethasone (Dex) clearly precedes apoptosis in human CEM T-ALL and murine S49.1 T-lymphoma cells. Cyclin D3 is strongly downregulated, whereas the CDK inhibitor p27^Kip1 (p27) is strongly upregulated in response to dexamethasone in these cells. RNAi-mediated knockdown of p27 as well as overexpression of its negative regulator Skp2 revealed the critical function of p27 in the Dex-induced G₁ arrest of CEM cells. Our studies indicate that several mechanisms contribute to the increase of p27 protein in our T-lymphoma cell lines. We found a significant upregulation of p27 mRNA in S49.1 and CEM cells. In addition, Dex treatment activated the mouse p27 promotor in reporter gene experiments, indicating a transcriptional regulation. However, the relatively moderate induction of p27 mRNA levels by Dex did not explain the strong increase of p27 protein in CEM and S49.1 cells. We found clear evidence for a posttranslational mechanism responsible for the robust increase in p27 protein. Dex treatment of S49.1 and CEM cells increases the half-life of p27 protein, which indicates that decreased protein degradation is the primary mechanism of p27 induction by glucocorticoids. Interestingly, we found that Dex treatment decreased the protein and mRNA levels of the negative regulator of p27 protein and E3 ubiquitin ligase subunit Skp2. We conclude that the cell cycle inhibitor p27 and its negative regulator Skp2 are key players in the glucocorticoid-induced growth suppression of T-lymphoma cells and should be considered as potential drug targets to improve therapies of T-cell malignancies.

Amniotic membrane-derived cells inhibit proliferation of cancer cell lines by inducing cell cycle arrest

Cells derived from the amniotic foetal membrane of human term placenta have drawn particular attention mainly for their plasticity and immunological properties, which render them interesting for stem-cell research and cell-based therapeutic applications. In particular, we have previously demonstrated that amniotic mesenchymal tissue cells (AMTC) inhibit lymphocyte proliferation in vitro and suppress the generation and maturation of monocyte-derived dendritic cells. Here, we show that AMTC also significantly reduce the proliferation of cancer cell lines of haematopoietic and non-haematopoietic origin, in both cell–cell contact and transwell co-cultures, therefore suggesting the involvement of yet-unknown inhibitory soluble factor(s) in this ‘cell growth restraint’. Importantly, we provide evidence that the anti-proliferative effect of AMTC is associated with induction of cell cycle arrest in G0/G1 phase. Gene expression analyses demonstrate that AMTC can down-regulate cancer cells' mRNA expression of genes associated with cell cycle progression, such as cyclins (cyclin D2, cyclin E1, cyclin H) and cyclin-dependent kinase (CDK4, CDK6 and CDK2), whilst they up-regulate cell cycle negative regulator such as p15 and p21, consistent with a block in G0/G1 phase with no progression to S phase. Taken together, these findings warrant further studies to investigate the applicability of these cells for controlling cancer cell proliferation in vivo.

Novel role of parathyroid hormone-related protein in the pathophysiology of the diabetic kidney: evidence from experimental and human diabetic nephropathy

Parathyroid hormone-related protein (PTHrP) and its receptor type 1 (PTH1R) are extensively expressed in the kidney, where they are able to modulate renal function. Renal PTHrP is known to be overexpressed in acute renal injury. Recently, we hypothesized that PTHrP involvement in the mechanisms of renal injury might not be limited to conditions with predominant damage of the renal tubulointerstitium and might be extended to glomerular diseases, such as diabetic nephropathy (DN). In experimental DN, the overexpression of both PTHrP and the PTH1R contributes to the development of renal hypertrophy as well as proteinuria. More recent data have shown, for the first time, that PTHrP is upregulated in the kidney from patients with DN. Collectively, animal and human studies have shown that PTHrP acts as an important mediator of diabetic renal cell hypertrophy by a mechanism which involves the modulation of cell cycle regulatory proteins and TGF- β 1. Furthermore, angiotensin II (Ang II), a critical factor in the progression of renal injury, appears to be responsible for PTHrP upregulation in these conditions. These findings provide novel insights into the well-known protective effects of Ang II antagonists in renal diseases, paving the way for new therapeutic approaches.

Tissue biomarkers as prognostic variables of cervical cancer

The most important prognostic variables of cervical carcinoma are FIGO stage, lymph node status and clinical-pathological features of primary tumor. Recently, there has been increasing interest in the identification of biomarkers able to predict both response to treatment and survival. The aim of this review is to critically evaluate current published evidence on the ability of various tissue biomarkers to predict the clinical outcome of patients with cervical carcinoma. In particular, the paper takes into account DNA content, cell-cycle and apoptosis-regulatory proteins, epidermal growth factor receptor [EGFR], vascular endothelial growth factor [VEGF], cyclooxygenase [COX]-2, signal transducer and activator of transcription [Stat]3, human papilloma virus [HPV] status, tumor hypoxia, tumor infiltrating lymphocytes [TIL], microarray technology and microRNA (miRNA). The presence of HPV-18 genotype and an elevated VEGF expression appear to be poor prognostic factors in women with early disease treated with primary surgery, whereas the expression of EGFR, VEGF, COX-2 and tumor hypoxia may have a major impact on the survival of patients treated with definitive radiotherapy or chemoradiation. The data supporting the reliability of ΔNp73 and TAp73α as novel biomarkers of response to radiotherapy are interesting but still limited. DNA microarray technology could offer new laboratory tools for a rationale planning of treatment strategy, and miRNAs might represent new candidate targets to be investigated for both prognostic and therapeutic purposes. Moreover, the assessment of different types of TIL and their ligands in tumor biopsies could enable the identification of a subset of high-risk patients, paving the way to novel immune therapies aimed at blocking T-reg cell activity.

An intron-retaining splice variant of human cyclin A2, expressed in adult differentiated tissues, induces a G1/S cell cycle arrest in vitro

Background

Human cyclin A2 is a key regulator of S phase progression and entry into mitosis. Alternative splice variants of the G1 and mitotic cyclins have been shown to interfere with full-length cyclin functions to modulate cell cycle progression and are therefore likely to play a role in differentiation or oncogenesis. The alternative splicing of human cyclin A2 has not yet been studied.

Methodology/Principal Findings

Sequence-specific primers were designed to amplify various exon–intron regions of cyclin A2 mRNA in cell lines and human tissues. Intron retaining PCR products were cloned and sequenced and then overexpressed in HeLa cells. The subcellular localization of the splice variants was studied using confocal and time-lapse microscopy, and their impact on the cell cycle by flow cytometry, immunoblotting and histone H1 kinase activity. We found a splice variant of cyclin A2 mRNA called A2V6 that partly retains Intron 6. The gene expression pattern of A2V6 mRNA in human tissues was noticeably different from that of wild-type cyclin A2 (A2WT) mRNA. It was lower in proliferating fetal tissues and stronger in some differentiated adult tissues, especially, heart. In transfected HeLa cells, A2V6 localized exclusively in the cytoplasm whereas A2WT accumulated in the nucleus. We show that A2V6 induced a clear G1/S cell cycle arrest associated with a p21 and p27 upregulation and an inhibition of retinoblastoma protein phosphorylation. Like A2WT, A2V6 bound CDK2, but the A2V6/CDK2 complex did not phosphorylate histone H1.

Conclusion/Significance

This study has revealed that some highly differentiated human tissues express an intron-retaining cyclin A2 mRNA that induced a G1/S block in vitro. Contrary to full-length cyclin A2, which regulates cell proliferation, the A2V6 splice variant might play a role in regulating nondividing cell states such as terminal differentiation or senescence.

PKC-dependent phosphorylation of p27 at T198 contributes to p27 stabilization and cell cycle arrest

In this manuscript, we present experimental evidence that PKCs phosphorylate p27 at T198 in vitro and in vivo, resulting in p27 stabilization and cell cycle arrest in MCF-7 and HeLa cells. Our findings indicate that (1) recombinant PKCα, βII, δ, η and θ isoforms phosphorylate, in in vitro kinase assays, wild-type recombinant p27 protein expressed in E. coli and wild-type p27 protein immunoprecpitated from transfected HEK-293 cells but not the T198A mutant, (2) adoptive expressed PKCα and δ phosphorylate both transfected and endogenous p27 at T198 in HEK-293 cells, (3) T198 phosphorylation of transfected and endogenous p27 is increased by PKC activators [Phorbol 12-myristate 13-acetate (PMA)] and suppressed by PKC inhibitors (Rottlerin A, G06976, Calphostin C), (4) in parallel with increased T198 phosphorylation, PMA induces stabilization of p27 protein in HeLa cells, whereas PKC inhibitors induce a decrease in p27 stability and, finally, (5) PMA-induced p27 upregulation is necessary for growth arrest of HeLa and MCF-7 cells induced by PKC activation by PMA.   Overall, these results suggest that PKC-dependent upregulation of p27 induced by its phosphorylation at T198 represents a mechanism that mediates growth arrest promoted by PMA and provide novel insights on the ability of different PKC isoforms to play a role in controlling cell cycle progression.

RNAi targeting CXCR4 inhibits tumor growth through inducing cell cycle arrest and apoptosis

CXC chemokine receptor 4 (CXCR4) is involved in many human malignant tumors and plays an important role in tumor growth and metastasis. To explore the effects of CXCR4 expression on the malignant cells of oral squamous cell carcinoma (OSCC), Tca8113 and SCC-9 cell lines, as well as their xenograft models, of nude mice were used to detect cancer cell proliferation alteration. This study also examined the corresponding molecular mechanism after CXCR4 knockdown using a recombinant lentiviral vector expressing small interference RNA (siRNA) for CXCR4. RNA interference-mediated knockdown of CXCR4 in highly aggressive (Tca8113 and SCC-9) tumor cells significantly inhibited the proliferation of the two cell lines in vitro and in vivo. The expression levels of >1,500 genes involved in cell cycle, apoptosis, and multiple signaling pathways were also altered. These results provide new evidence of CXCR4 as a promising tumor gene therapeutic target.

Clinico-pathological and biological prognostic variables in squamous cell carcinoma of the vulva

Several clinical-pathological parameters have been related to survival of patients with invasive squamous cell carcinoma of the vulva, whereas few studies have investigated the ability of biological variables to predict the clinical outcome of these patients. The present paper reviews the literature data on the prognostic relevance of lymph node-related parameters, primary tumor-related parameters, FIGO stage, blood variables, and tissue biological variables. Regarding these latter, the paper takes into account the analysis of DNA content, cell cycle-regulatory proteins, apoptosis-related proteins, epidermal growth factor receptor [EGFR], and proteins that are involved in tumor invasiveness, metastasis and angiogenesis. At present, the lymph node status and FIGO stage according to the new 2009 classification system are the main predictors for vulvar squamous cell carcinoma, whereas biological variables do not have yet a clinical relevance and their role is still investigational.

Parthenolide inhibits proliferation of vascular smooth muscle cells through induction of G0/G1 phase cell cycle arrest

OBJECTIVE

This study is to determine the effect of the natural product parthenolide, a sesquiterpene lactone isolated from extracts of the herb Tanacetum parthenium, on the proliferation of vascular smooth muscle cells (VSMCs).

METHODS

Rat aortic VSMCs were isolated and cultured in vitro, and treated with different concentrations of parthenolide (10, 20 and 30 mumol/L). [(3)H]thymidine incorporation was used as an index of cell proliferation. Cell cycle progression and distribution were determined by flow cytometric analysis. Furthermore, the expression of several regulatory proteins relevant to VSMC proliferation including IkappaBalpha, cyclooxygenase-2 (Cox-2), p21, and p27 was examined to investigate the potential molecular mechanism.

RESULTS

Treatment with parthenolide significantly decreased the [(3)H]thymidine incorporation into DNA by 30%~56% relative to control values in a dose-dependent manner (P<0.05). Addition of parthenolide also increased cell population at G(0)/G(1) phase by 19.2%~65.7% (P<0.05) and decreased cell population at S phase by 50.7%~84.8% (P<0.05), which is consistent with its stimulatory effects on p21 and p27. In addition, parthenolide also increased IkappaBalpha expression and reduced Cox-2 expression in a time-dependent manner.

CONCLUSION

Our results show that parthenolide significantly inhibits the VSMC proliferation by inducing G(0)/G(1) cell cycle arrest. IkappaBalpha and Cox-2 are likely involved in such inhibitory effect of parthenolide on VSMC proliferation. These findings warrant further investigation on potential therapeutic implications of parthenolide on VSMC proliferation in vivo.

Centrosome-associated regulators of the G(2)/M checkpoint as targets for cancer therapy

In eukaryotic cells, control mechanisms have developed that restrain cell-cycle transitions in response to stress. These regulatory pathways are termed cell-cycle checkpoints. The G(2)/M checkpoint prevents cells from entering mitosis when DNA is damaged in order to afford these cells an opportunity to repair the damaged DNA before propagating genetic defects to the daughter cells. If the damage is irreparable, checkpoint signaling might activate pathways that lead to apoptosis. Since alteration of cell-cycle control is a hallmark of tumorigenesis, cell-cycle regulators represent potential targets for therapy. The centrosome has recently come into focus as a critical cellular organelle that integrates G(2)/M checkpoint control and repairs signals in response to DNA damage. A growing number of G(2)/M checkpoint regulators have been found in the centrosome, suggesting that centrosome has an important role in G(2)/M checkpoint function. In this review, we discuss centrosome-associated regulators of the G(2)/M checkpoint, the dysregulation of this checkpoint in cancer, and potential candidate targets for cancer therapy.

The unfoldomics decade: an update on intrinsically disordered proteins

BACKGROUND

Our first predictor of protein disorder was published just over a decade ago in the Proceedings of the IEEE International Conference on Neural Networks (Romero P, Obradovic Z, Kissinger C, Villafranca JE, Dunker AK (1997) Identifying disordered regions in proteins from amino acid sequence. Proceedings of the IEEE International Conference on Neural Networks, 1: 90-95). By now more than twenty other laboratory groups have joined the efforts to improve the prediction of protein disorder. While the various prediction methodologies used for protein intrinsic disorder resemble those methodologies used for secondary structure prediction, the two types of structures are entirely different. For example, the two structural classes have very different dynamic properties, with the irregular secondary structure class being much less mobile than the disorder class. The prediction of secondary structure has been useful. On the other hand, the prediction of intrinsic disorder has been revolutionary, leading to major modifications of the more than 100 year-old views relating protein structure and function. Experimentalists have been providing evidence over many decades that some proteins lack fixed structure or are disordered (or unfolded) under physiological conditions. In addition, experimentalists are also showing that, for many proteins, their functions depend on the unstructured rather than structured state; such results are in marked contrast to the greater than hundred year old views such as the lock and key hypothesis. Despite extensive data on many important examples, including disease-associated proteins, the importance of disorder for protein function has been largely ignored. Indeed, to our knowledge, current biochemistry books don't present even one acknowledged example of a disorder-dependent function, even though some reports of disorder-dependent functions are more than 50 years old. The results from genome-wide predictions of intrinsic disorder and the results from other bioinformatics studies of intrinsic disorder are demanding attention for these proteins.

RESULTS

Disorder prediction has been important for showing that the relatively few experimentally characterized examples are members of a very large collection of related disordered proteins that are wide-spread over all three domains of life. Many significant biological functions are now known to depend directly on, or are importantly associated with, the unfolded or partially folded state. Here our goal is to review the key discoveries and to weave these discoveries together to support novel approaches for understanding sequence-function relationships.

CONCLUSION

Intrinsically disordered protein is common across the three domains of life, but especially common among the eukaryotic proteomes. Signaling sequences and sites of posttranslational modifications are frequently, or very likely most often, located within regions of intrinsic disorder. Disorder-to-order transitions are coupled with the adoption of different structures with different partners. Also, the flexibility of intrinsic disorder helps different disordered regions to bind to a common binding site on a common partner. Such capacity for binding diversity plays important roles in both protein-protein interaction networks and likely also in gene regulation networks. Such disorder-based signaling is further modulated in multicellular eukaryotes by alternative splicing, for which such splicing events map to regions of disorder much more often than to regions of structure. Associating alternative splicing with disorder rather than structure alleviates theoretical and experimentally observed problems associated with the folding of different length, isomeric amino acid sequences. The combination of disorder and alternative splicing is proposed to provide a mechanism for easily "trying out" different signaling pathways, thereby providing the mechanism for generating signaling diversity and enabling the evolution of cell differentiation and multicellularity. Finally, several recent small molecules of interest as potential drugs have been shown to act by blocking protein-protein interactions based on intrinsic disorder of one of the partners. Study of these examples has led to a new approach for drug discovery, and bioinformatics analysis of the human proteome suggests that various disease-associated proteins are very rich in such disorder-based drug discovery targets.

The unfoldomics decade: an update on intrinsically disordered proteins

BACKGROUND

Our first predictor of protein disorder was published just over a decade ago in the Proceedings of the IEEE International Conference on Neural Networks (Romero P, Obradovic Z, Kissinger C, Villafranca JE, Dunker AK (1997) Identifying disordered regions in proteins from amino acid sequence. Proceedings of the IEEE International Conference on Neural Networks, 1: 90-95). By now more than twenty other laboratory groups have joined the efforts to improve the prediction of protein disorder. While the various prediction methodologies used for protein intrinsic disorder resemble those methodologies used for secondary structure prediction, the two types of structures are entirely different. For example, the two structural classes have very different dynamic properties, with the irregular secondary structure class being much less mobile than the disorder class. The prediction of secondary structure has been useful. On the other hand, the prediction of intrinsic disorder has been revolutionary, leading to major modifications of the more than 100 year-old views relating protein structure and function. Experimentalists have been providing evidence over many decades that some proteins lack fixed structure or are disordered (or unfolded) under physiological conditions. In addition, experimentalists are also showing that, for many proteins, their functions depend on the unstructured rather than structured state; such results are in marked contrast to the greater than hundred year old views such as the lock and key hypothesis. Despite extensive data on many important examples, including disease-associated proteins, the importance of disorder for protein function has been largely ignored. Indeed, to our knowledge, current biochemistry books don't present even one acknowledged example of a disorder-dependent function, even though some reports of disorder-dependent functions are more than 50 years old. The results from genome-wide predictions of intrinsic disorder and the results from other bioinformatics studies of intrinsic disorder are demanding attention for these proteins.

RESULTS

Disorder prediction has been important for showing that the relatively few experimentally characterized examples are members of a very large collection of related disordered proteins that are wide-spread over all three domains of life. Many significant biological functions are now known to depend directly on, or are importantly associated with, the unfolded or partially folded state. Here our goal is to review the key discoveries and to weave these discoveries together to support novel approaches for understanding sequence-function relationships.

CONCLUSION

Intrinsically disordered protein is common across the three domains of life, but especially common among the eukaryotic proteomes. Signaling sequences and sites of posttranslational modifications are frequently, or very likely most often, located within regions of intrinsic disorder. Disorder-to-order transitions are coupled with the adoption of different structures with different partners. Also, the flexibility of intrinsic disorder helps different disordered regions to bind to a common binding site on a common partner. Such capacity for binding diversity plays important roles in both protein-protein interaction networks and likely also in gene regulation networks. Such disorder-based signaling is further modulated in multicellular eukaryotes by alternative splicing, for which such splicing events map to regions of disorder much more often than to regions of structure. Associating alternative splicing with disorder rather than structure alleviates theoretical and experimentally observed problems associated with the folding of different length, isomeric amino acid sequences. The combination of disorder and alternative splicing is proposed to provide a mechanism for easily "trying out" different signaling pathways, thereby providing the mechanism for generating signaling diversity and enabling the evolution of cell differentiation and multicellularity. Finally, several recent small molecules of interest as potential drugs have been shown to act by blocking protein-protein interactions based on intrinsic disorder of one of the partners. Study of these examples has led to a new approach for drug discovery, and bioinformatics analysis of the human proteome suggests that various disease-associated proteins are very rich in such disorder-based drug discovery targets.

S10 phosphorylation of p27 mediates atRA induced growth arrest in ovarian carcinoma cell lines

All trans retinoic acid (atRA) has been shown to inhibit the growth of CAOV3 ovarian carcinoma cells and to elevate the level of p27 cyclin-dependent kinase inhibitor. We report here that phosphorylation at S10 residue is an important event in mediating p27 role in atRA induced growth arrest. atRA treatment of atRA sensitive CAOV3 cells increases the levels of S10 phospho-p27 in both nuclear and cytoplasmic cell compartments. This increase is accompanied by a decrease in the levels of skp2 protein. This effect was not observed in SKOV3 cells which are resistant to atRA growth inhibitory effect. An A10-p27 mutant that cannot be phosphorylated at S10 induces a dominant negative effect on the atRA effect on the levels and activity of endogenous p27. Overexpression of A10-p27 mutant renders CAOV3 cells more resistant to atRA treatment and reverses the effect that atRA has on p27 binding to CDKs, on CDK activity, and on the expression of S phase genes.

Role of intrinsic flexibility in signal transduction mediated by the cell cycle regulator, p27 Kip1

p27(Kip1) (p27), which controls eukaryotic cell division through interactions with cyclin-dependent kinases (Cdks), integrates and transduces promitogenic signals from various nonreceptor tyrosine kinases by orchestrating its own phosphorylation, ubiquitination and degradation. Intrinsic flexibility allows p27 to act as a "conduit" for sequential signaling mediated by tyrosine and threonine phosphorylation and ubiquitination. While the structural features of the Cdk/cyclin-binding domain of p27 are understood, how the C-terminal regulatory domain coordinates multistep signaling leading to p27 degradation is poorly understood. We show that the 100-residue p27 C-terminal domain is extended and flexible when p27 is bound to Cdk2/cyclin A. We propose that the intrinsic flexibility of p27 provides a molecular basis for the sequential signal transduction conduit that regulates p27 degradation and cell division. Other intrinsically unstructured proteins possessing multiple sites of posttranslational modification may participate in similar signaling conduits.

The cell cycle: a critical therapeutic target to prevent vascular proliferative disease

Percutaneous coronary intervention is the preferred revascularization approach for most patients with coronary artery disease. However, this strategy is limited by renarrowing of the vessel by neointimal hyperplasia within the stent lumen (in-stent restenosis). Vascular smooth muscle cell proliferation is a major component in this healing process. This process is mediated by multiple cytokines and growth factors, which share a common pathway in inducing cell proliferation: the cell cycle. The cell cycle is highly regulated by numerous mechanisms ensuring orderly and coordinated cell division. The present review discusses current concepts related to regulation of the cell cycle and new therapeutic options that target aspects of the cell cycle.

Cellular thiol status-dependent inhibition of tumor cell growth via modulation of p27kip1 translocation and retinoblastoma protein phosphorylation by 1′-acetoxychavicol acetate

1'-Acetoxychavicol acetate (ACA) has been shown to inhibit tumor cell growth, but there is limited information on its effects on cell signaling and the cell cycle control pathway. In this study, we sought to determine how ACA alters cell cycle and its related control factors in its growth inhibitory effect in Ehrlich ascites tumor cells (EATC). ACA caused an accumulation of cells in the G1 phase and an inhibition of DNA synthesis, which were reversed by supplementation with N-acetylcysteine (NAC) or glutathione ethyl ester (GEE). Furthermore, ACA decreased hyperphosphorylated Rb levels and increased hypophosphorylated Rb levels. NAC and GEE also abolished the decease in Rb phosphorylation by ACA. As Rb phosphorylation is regulated by G1 cyclin dependent kinase and CDK inhibitor p27(kip1), which is an important regulator of the mammalian cell cycle, we estimated the amount of p27(kip1) levels by western blotting. Treatment with ACA had virtually no effect on the amount of p27(kip1) levels, but caused a decrease in phosphorylated p27(kip1) and an increase in unphosphorylated p27(kip1) as well as an increase in the nuclear localization of p27(kip1). These events were abolished in the presence of NAC or GEE. These results suggest that in EATC, cell growth inhibition elicited by ACA involves decreases in Rb and p27(kip1) phosphorylation and an increase in nuclear localization of p27(kip1), and these events are dependent on the cellular thiol status.

Oncostatin M induces growth arrest by inhibition of Skp2, Cks1, and cyclin A expression and induced p21 expression

Oncostatin M has been characterized as a potent growth inhibitor for various tumor cells. Oncostatin M-treated glioblastoma cells cease proliferation and instigate astrocytal differentiation. The oncostatin M-induced cell cycle arrest in G(1) phase is characterized by increased level of the cyclin-dependent kinase (CDK) inhibitory proteins p21(Cip1/Waf1/Sdi1) and p27(Kip1). Induction of p21 protein corresponds to increased mRNA level, whereas p27 accumulates due to increased stability of the protein. Interestingly, stabilization of p27(Kip1) occurs even in S phase, showing that p27 stabilization is a direct consequence of oncostatin M signaling and not a result of the cell cycle arrest. Degradation of p27 in late G(1) and S phase is initiated by the ubiquitin ligase complex SCF-Skp2/Cks1. Oncostatin M inhibits expression of two components of this E3 ligase complex (Skp2 and Cks1). Although combined overexpression of Skp2 and Cks1 rescues p27 degradation in S phase, it can not override p27 accumulation in G(1) phase and cell cycle arrest by oncostatin M. In addition to increasing Cdk inhibitor level, oncostatin M also impairs cyclin A expression. Cyclin A mRNA and protein level decline shortly after oncostatin M addition. The accumulation of two CDK inhibitor proteins and the repression of cyclin A expression may explain the broad and potent antiproliferative effect of the cytokine.

Cell cycle genes in ovarian cancer: steps toward earlier diagnosis and novel therapies

Human malignant tumors are characterized by abnormal proliferation resulting from alterations in cell cycle-regulatory mechanisms. The regulatory pathways controlling cell cycle phases include several oncogenes and tumor suppressor genes that display a range of abnormalities with potential usefulness as markers of evolution or treatment response in ovarian cancer. This review summarizes the current knowledge about these aberrations in malignant tumors of the ovary. We sought to divide cell cycle-regulatory genes into four subgroups on the basis of their predominant role in a specific phase or during the transition between two phases of the cell cycle.

Induction of p19INK4d in response to ultraviolet light improves DNA repair and confers resistance to apoptosis in neuroblastoma cells

The genetic instability driving tumorigenesis is fueled by DNA damage and by errors made by the DNA replication. Upon DNA damage the cell organizes an integrated response not only by the classical DNA repair mechanisms but also involving mechanisms of replication, transcription, chromatin structure dynamics, cell cycle progression, and apoptosis. In the present study, we investigated the role of p19INK4d in the response driven by neuroblastoma cells against DNA injury caused by UV irradiation. We show that p19INK4d is the only INK4 protein whose expression is induced by UV light in neuroblastoma cells. Furthermore, p19INK4d translocation from cytoplasm to nucleus is observed after UV irradiation. Ectopic expression of p19INK4d clearly reduces the UV-induced apoptosis as well as enhances the cellular ability to repair the damaged DNA. It is clearly shown that DNA repair is the main target of p19INK4d effect and that diminished apoptosis is a downstream event. Importantly, experiments performed with CDK4 mutants suggest that these p19INK4d effects would be independent of its role as a cell cycle checkpoint gene. The results presented herein uncover a new role of p19INK4d as regulator of DNA-damage-induced apoptosis and suggest that it protects cells from undergoing apoptosis by allowing a more efficient DNA repair. We propose that, in addition to its role as cell cycle inhibitor, p19INK4d is involved in maintenance of DNA integrity and, therefore, would contribute to cancer prevention.

Regulation of expression by promoters versus internal ribosome entry site in the 5'-untranslated sequence of the human cyclin-dependent kinase inhibitor p27kip1

p27^kip1 regulates cell proliferation by binding to and inhibiting the activity of cyclin-dependent kinases and its expression oscillates with cell cycle. Recently, it has been suggested from studies using the traditional dicistronic DNA assay that the expression of p27^kip1 is regulated by internal ribosome entry site (IRES)-mediated translation initiation, and several RNA-binding protein factors were thought to play some role in this regulation. Considering the inevitable drawbacks of the dicistronic DNA assay, which could mislead a promoter activity or alternative splicing to IRES as previously demonstrated, we decided to reanalyze the 5′-untranslated region (5′-UTR) sequence of p27^kip1 and test whether it contains an IRES element or a promoter using more stringent methods, such as dicistronic RNA and promoterless dicistronic and monocistronic DNA assays. We found that the 5′-UTR sequence of human p27^kip1 does not have any significant IRES activity. The previously observed IRES activities are likely generated from the promoter activities present in the 5′-UTR sequences of p27^kip1. The findings in this study indicate that transcriptional regulation likely plays an important role in p27^kip1 expression, and the mechanism of regulation of p27 expression by RNA-binding factors needs to be re-examined. The findings in this study also further enforce the importance that more stringent studies, such as promoterless dicistronic and monocistronic DNA and dicistronic RNA tests, are required to safeguard any future claims of cellular IRES.

Polyacetylenes function as anti-angiogenic agents

PURPOSE

To investigate the antiangiogenic effects of plant extracts and polyacetylenes isolated from Bidens pilosa Linn., which is a popular nutraceutical herbal tea and folk medicine in anti-inflammatory, antitumor, and other medications worldwide.

METHODS

Anti-cell proliferation, anti-tube formation, and cell migration assays were used for the valuation of bioactivities of target plant extracts and phytocompounds against angiogenesis. Bioactivity-guided fractionation, HPLC, and various spectral analyses were used to identify active fraction and phytocompounds for anti-angiogenesis.

RESULTS

We show that an ethyl acetate (EA) fraction of B. pilosa exhibited significant anti-cell proliferation and anti-tube formation activities against human umbilical vein endothelium cells (HUVEC). Bioassay-guided fractionation led to isolation of one new and one known polyacetylenes, 1,2-dihydroxytrideca-5,7,9,11-tetrayne (1) and 1,3-dihydroxy-6(E)- tetradecene-8,10,12-triyne (2), respectively, from the EA fraction. Compounds 1 and 2 manifested highly specific and significant activities against HUVEC proliferation with IC50 values of 2.5 and 0.375 microg/ml, respectively, however, compound 1 had a more potent effect on preventing tube formation of HUVEC than compound 2 at a dose of 2.5 microg/ml. Western blot analysis showed that both compounds upregulated p27(Kip) or p21(Cip1), cyclin-dependent kinase inhibitors, in HUVEC.

CONCLUSIONS

This is the first report to demonstrate that polyacetylenes possess significant anti-angiogenic activities and the ability to regulate the expression of cell cycle mediators, for example, p27(Kip1), p21(Cip1), or cyclin E.

Basic fibroblast growth factor exhibits dual and rapid regulation of cyclin D1 and p27 to stimulate proliferation of rat cerebral cortical precursors

While extracellular signals play a major role in brain neurogenesis, little is known about the cell cycle machinery underlying mitogen stimulation of precursor proliferation. Current models suggest that the D cyclins function as primary sensors of extracellular mitogens. Here we define the mechanisms by which basic fibroblast growth factor (bFGF) stimulates cortical precursors, with particular attention to the responses of cell cycle promitogenic and antimitogenic regulators. bFGF produced a 4-fold increase in DNA synthesis and a 3-fold rise in bromodeoxyuridine labeling, suggesting that the factor promotes the G1/S transition. There was also a 3-fold increase in cyclin-dependent kinase 2 (CDK2) kinase activity, which is critical for S phase entry. CDK2 activation was apparently cyclin E dependent, since only its protein and mRNA levels were elevated at 24 h, whereas CDK2, p27KIP1 and p57KIP2 levels were unaltered. Late G1 phase CDK2/cyclin E activity depends on early G1 D cyclin function. Indeed, cyclin D1, but not cyclin D3, was upregulated selectively at 8 h after bFGF treatment, a time when cyclin E was unchanged. The sequential activation of cyclin D1 and cyclin E supports the idea that cyclin E gene transcription is regulated by cyclin-D/CDK4/6-mediated pRb phosphorylation and subsequent E2F transcription factor release. However, in addition to increased D1 cyclin, we unexpectedly detected a 75% reduction in p27KIP1 protein at 8 h, suggesting that both pro- and antimitogenic regulators are targets of extracellular mitogens during brain development.

p107 inhibits G1 to S phase progression by down-regulating expression of the F-box protein Skp2

Cell cycle progression is negatively regulated by the pocket proteins pRb, p107, and p130. However, the mechanisms responsible for this inhibition are not fully understood. Here, we show that overexpression of p107 in fibroblasts inhibits Cdk2 activation and delays S phase entry. The inhibition of Cdk2 activity is correlated with the accumulation of p27, consequent to a decreased degradation of the protein, with no change of Thr187 phosphorylation. Instead, we observed a marked decrease in the abundance of the F-box receptor Skp2 in p107-overexpressing cells. Reciprocally, Skp2 accumulates to higher levels in p107^−/− embryonic fibroblasts. Ectopic expression of Skp2 restores p27 down-regulation and DNA synthesis to the levels observed in parental cells, whereas inactivation of Skp2 abrogates the inhibitory effect of p107 on S phase entry. We further show that the serum-dependent increase in Skp2 half-life observed during G1 progression is impaired in cells overexpressing p107. We propose that p107, in addition to its interaction with E2F, inhibits cell proliferation through the control of Skp2 expression and the resulting stabilization of p27.

Vitamin D3 receptor/Sp1 complex is required for the induction of p27Kip1 expression by vitamin D3

1alpha,25-dihydroxyvitamin D3 (vitamin D3) has been shown to upregulate p27Kip1 expression via Sp1 and NF-Y binding sites in the p27Kip1 promoter. However, whether vitamin D3 receptor (VDR) involves in this process is unclear. In this study, we demonstrated that expression of VDR in SW620 cells, which exhibited low level of endogenous VDR, increased vitamin D3-stimulated p27Kip1 promoter activity. On the contrary, suppression of Sp1 expression by small interference RNA reduced the stimulation of p27Kip1 promoter activity by vitamin D3 in LNCaP cells. DNA affinity precipitation assay and chromatin immunoprecipitation assay showed that VDR bound to the p27Kip1 promoter in vitro and in vivo. In addition, we also demonstrated that VDR interacted with Sp1 in vitro and in cells. Collectively, our results suggest that VDR is involved in the induction of p27Kip1 by vitamin D3 and may interact with Sp1 to modulate the expression of target genes that lack VDR response element (VDRE) in their promoters.

Local cyclin-dependent kinase inhibition by flavopiridol inhibits coronary artery smooth muscle cell proliferation and migration: Implications for the applicability on drug-eluting stents to prevent neointima formation following vascular injury

In-stent restenosis is a hyperproliferative disease which can be successfully treated by drug-eluting stents releasing compounds that exhibit cell-cycle inhibitory properties to inhibit coronary smooth muscle cell (CASMC) proliferation and migration, resembling the key pathomechanisms of in-stent restenosis. Cyclin-dependent kinases (CDK) are key regulators of the eukaryotic cell cycle. CDK activity may be blocked by novel compounds such as flavopiridol. Therefore, CDK inhibitors are attractive drugs to be used for the local prevention of in-stent restenosis. In this study, we demonstrate that flavopiridol leads to potent inhibition of CASMC proliferation and migration. Molecular effects on cell-cycle regulatory mechanisms and distribution were evaluated by post-transcriptional assessment of distinct cyclins and cyclin-dependent kinase inhibitor (CKI) levels and flow cytometry. Cellular necrosis and apoptosis was assessed in CASMC and coronary endothelial cells. Flavopiridol induced a potent antiproliferative effect by cell-cycle inhibition in G1 and G2/M and led to increased protein levels of CKIs p21cip1 and p27kip1 as well as p53 in CASMC. Hyperphosphorylation of retinoblastoma protein was abrogated and mitogen-mediated smooth muscle cell migration significantly reduced. No accelerated cytotoxicity or increased apoptosis was detectable. Flavopiridol-coated stents, implanted in rat carotid arteries, led to significant decrease of neointima formation. As proof of principle, our results demonstrate that stents eluting CDK inhibitors such as flavopiridol effectively inhibit neointima formation. Therefore, this new class of therapeutics may be suitable for further clinical investigations on drug-eluting stents to prevent in-stent restenosis.

Normal human fibroblasts are resistant to RAS-induced senescence

Oncogenic stimuli are thought to induce senescence in normal cells in order to protect against transformation and to induce proliferation in cells with altered p53 and/or retinoblastoma (Rb) pathways. In human fibroblasts, RAS initiates senescence through upregulation of the cyclin-dependent kinase inhibitor p16INK4A. We show here that in contrast to cultured fibroblast strains, freshly isolated normal fibroblasts are resistant to RAS-induced senescence and instead show some characteristics of transformation. RAS did not induce growth arrest or expression of senescence-associated beta-galactosidase, and Rb remained hyperphosphorylated despite elevated levels of p16. Instead, RAS promoted anchorage-independent growth of normal fibroblasts, although expression of hTert with RAS increased colony formation and allowed normal fibroblasts to bypass contact inhibition. To test the hypothesis that p16 levels determine how cells respond to RAS, we expressed RAS in freshly isolated fibroblasts that expressed very low levels of p16, in hTert-immortalized fibroblasts that had accumulated intermediate levels of p16, and in IMR90 fibroblasts with high levels of p16. RAS induced growth arrest in cells with higher p16 levels, and this effect was reversed by p16 knockdown in the hTert-immortalized fibroblasts. These findings indicate that culture-imposed stress sensitizes cells to RAS-induced arrest, whereas early passage cells do not arrest in response to RAS.

14-3-3 suppresses the nuclear localization of threonine 157-phosphorylated p27(Kip1)

p27(Kip1) (p27), a CDK inhibitor, migrates into the nucleus, where it controls cyclin-CDK complex activity for proper cell cycle progression. We report here that the classical bipartite-type basic amino-acid cluster and the two downstream amino acids of the C-terminal region of p27 function as a nuclear localization signal (NLS) for its full nuclear import activity. Importin alpha3 and alpha5, but not alpha1, transported p27 into the nucleus in conjunction with importin beta, as evidenced by an in vitro transport assay. It is known that Akt phosphorylates Thr 157 of p27 and this reduces the nuclear import activity of p27. Using a pull-down experiment, 14-3-3 was identified as the Thr157-phosphorylated p27NLS-binding protein. Although importin alpha5 bound to Thr157-phosphorylated p27NLS, 14-3-3 competed with importin alpha5 for binding to it. Thus, 14-3-3 sequestered phosphorylated p27NLS from importin alpha binding, resulting in cytoplasmic localization of NLS-phosphorylated p27. These findings indicate that 14-3-3 suppresses importin alpha/beta-dependent nuclear localization of Thr157-phosphorylated p27, suggesting implications for cell cycle disorder in Akt-activated cancer cells.

RanGAP1*SUMO1 is phosphorylated at the onset of mitosis and remains associated with RanBP2 upon NPC disassembly

The RanGTPase activating protein RanGAP1 has essential functions in both nucleocytoplasmic transport and mitosis. In interphase, a significant fraction of vertebrate SUMO1-modified RanGAP1 forms a stable complex with the nucleoporin RanBP2/Nup358 at nuclear pore complexes. RanBP2 not only acts in the RanGTPase cycle but also is a SUMO1 E3 ligase. Here, we show that RanGAP1 is phosphorylated on residues T409, S428, and S442. Phosphorylation occurs before nuclear envelope breakdown and is maintained throughout mitosis. Nocodazole arrest leads to quantitative phosphorylation. The M-phase kinase cyclin B/Cdk1 phosphorylates RanGAP1 efficiently in vitro, and T409 phosphorylation correlates with nuclear accumulation of cyclin B1 in vivo. We find that phosphorylated RanGAP1 remains associated with RanBP2/Nup358 and the SUMO E2–conjugating enzyme Ubc9 in mitosis, hence mitotic phosphorylation may have functional consequences for the RanGTPase cycle and/or for RanBP2-dependent sumoylation.

The cell cycle: a review of regulation, deregulation and therapeutic targets in cancer

The cell cycle is controlled by numerous mechanisms ensuring correct cell division. This review will focus on these mechanisms, i.e. regulation of cyclin-dependent kinases (CDK) by cyclins, CDK inhibitors and phosphorylating events. The quality checkpoints activated after DNA damage are also discussed. The complexity of the regulation of the cell cycle is also reflected in the different alterations leading to aberrant cell proliferation and development of cancer. Consequently, targeting the cell cycle in general and CDK in particular presents unique opportunities for drug discovery. This review provides an overview of deregulation of the cell cycle in cancer. Different families of known CDK inhibitors acting by ATP competition are also discussed. Currently, at least three compounds with CDK inhibitory activity (flavopiridol, UCN-01, roscovitine) have entered clinical trials.

Novel p27(kip1) C-terminal scatter domain mediates Rac-dependent cell migration independent of cell cycle arrest functions

Hepatocyte growth factor (HGF) signaling via its receptor, the proto-oncogene Met, alters cell proliferation and motility and has been associated with tumor metastasis. HGF treatment of HepG2 human hepatocellular carcinoma cells induces cell migration concomitant with increased levels of the p27(kip1) cyclin-cdk inhibitor. HGF signaling resulted in nuclear export of endogenous p27 to the cytoplasm, via Ser-10 phosphorylation, where it colocalized with F-actin. Introduction of transducible p27 protein (TATp27) was sufficient for actin cytoskeletal rearrangement and migration of HepG2 cells. TATp27 mutational analysis identified a novel p27 C-terminal domain required for cell migration, distinct from the N-terminal cyclin-cyclin-dependent kinase (cdk) binding domain. Loss or disruption of the p27 C-terminal domain abolished both actin rearrangement and cell migration. The cell-scattering activity of p27 occurred independently of its cell cycle arrest functions and required cytoplasmic localization of p27 via Ser-10 phosphorylation. Furthermore, Rac GTPase was necessary for p27-dependent migration but alone was insufficient for HepG2 cell migration. These results predicted a migration defect in p27-deficient cells. Indeed, p27-deficient primary fibroblasts failed to migrate, and reconstitution with TATp27 rescued the motility defect. These observations define a novel role for p27 in cell motility that is independent of its function in cell cycle inhibition.

ELAV/Hu proteins inhibit p27 translation via an IRES element in the p27 5'UTR

p27Kip1 restrains cell proliferation by binding to and inhibiting cyclin-dependent kinases. To investigate the mechanisms of p27 translational regulation, we isolated a complete p27 cDNA and identified an internal ribosomal entry site (IRES) located in its 5'UTR. The IRES allows for efficient p27 translation under conditions where cap-dependent translation is reduced. Searching for possible regulators of IRES activity we have identified the neuronal ELAV protein HuD as a specific binding factor of the p27 5'UTR. Increased expression of HuD or the ubiquitously expressed HuR protein specifically inhibits p27 translation and p27 IRES activity. Consistent with an inhibitory role of Hu proteins in p27 translation, siRNA mediated knockdown of HuR induced endogenous p27 protein levels as well as IRES-mediated reporter translation and leads to cell cycle arrest in G1.

Targeting the cell cycle for cancer therapy

Most if not all neoplasias show a directly or indirectly deregulated cell cycle. Targeting its regulatory molecules, the cyclin-dependent kinases, as a therapeutic mode to develop new anticancer drugs, is being currently explored in both academia and pharmaceutical companies. The development of new compounds is being focused on the many features of the cell cycle with promising preclinical data in most fields. Moreover, a few compounds have entered clinical trials with excellent results maintaining the high hopes. Thus, although too early to provide a cell cycle target based new commercial drug, there is no doubt that it will be an excellent source of new anticancer compounds.

Inhibition of p21/Waf1/Cip1/Sdi1 expression by hepatitis C virus core protein

The possibility of interaction between hepatitis C virus (HCV) core protein and the cell cycle regulator protein p21/Waf1/Cip1/Sdi1 (p21/Waf1) in cultured cells was analyzed. Although colocalization of HCV core protein and p21/Waf1 was not clearly observed, p21/Waf1 expression was much weaker in HCV core protein-expressing cells than in the control. A Northern blot analysis showed nearly the same level of p21/Waf1 mRNA in both cells, suggesting that HCV core protein inhibited p21/Waf1 expression post-transcriptionally. The degradation patterns of p21/Waf1 did not differ significantly in HCV core protein-expressing cells and in the control, suggesting that the stability of p21/Waf1, once it was accumulated in the cell, was not significantly affected by HCV core protein. But this does not necessarily exclude the possibility that synthesis, maturation, and nuclear transport of p21/Waf1 is impaired, or that the degradation of newly synthesized, improperly processed p21/Waf1 is promoted by HCV core protein. The decrease in p21/Waf1 accumulation was partially inhibited by proteasome inhibitors and a calpain inhibitor in both HCV core protein-expressing cells and the control. In vitro kinase assay revealed that a p21/Waf1-mediated inhibition of cyclin-dependent kinase 2 activity was partially negated by HCV core protein. Taken together, the present results suggest that HCV core protein inhibits p21/Waf1 expression post-transcriptionally and impairs the function of p21/Waf1 in the cell.