Genetic alterations of cyclins, cyclin-dependent kinases, and Cdk inhibitors in human cancer

Prognostic biomarkers in ovarian cancer

Epithelial ovarian cancer (EOC) remains the most lethal gynecological malignancy despite several decades of progress in diagnosis and treatment. Taking advantage of the robust development of discovery and utility of prognostic biomarkers, clinicians and researchers are developing personalized and targeted treatment strategies. This review encompasses recently discovered biomarkers of ovarian cancer, the utility of published prognostic biomarkers for EOC (especially biomarkers related to angiogenesis and key signaling pathways), and their integration into clinical practice.

Phospho-specific Smad3 signaling: impact on breast oncogenesis

Members of the TGFβ superfamily are known to exert a myriad of physiologic and pathologic growth controlling influences on mammary development and oncogenesis. In epithelial cells, TGFβ signaling inhibits cell growth through cytostatic and pro-apoptotic activities but can also induce cancer cell EMT and, thus, has a dichotomous role in breast cancer biology. Mechanisms governing this switch are the subject of active investigation. Smad3 is a critical intracellular mediator of TGFβ signaling regulated through phosphorylation by the TGFβ receptor complex at the C terminus. Smad3 is also a substrate for several other kinases that phosphorylate additional sites within the Smad protein. This discovery has expanded the understanding of the significance and complexity of TGFβ signaling through Smads. This review highlights recent advances revealing the critical role of phospho-specific Smad3 in malignancy and illustrates the potential prognostic and therapeutic impact of Smad3 phospho-isoforms in breast cancer.

Cyclin-dependent kinases 4 and 6 control tumor progression and direct glucose oxidation in the pentose cycle

Cyclin-dependent kinases CDK4 and CDK6 are essential for the control of the cell cycle through the G(1) phase. Aberrant expression of CDK4 and CDK6 is a hallmark of cancer, which would suggest that CDK4 and CDK6 are attractive targets for cancer therapy. Herein, we report that calcein AM (the calcein acetoxymethyl-ester) is a potent specific inhibitor of CDK4 and CDK6 in HCT116 human colon adenocarcinoma cells, inhibiting retinoblastoma protein (pRb) phosphorylation and inducing cell cycle arrest in the G(1) phase. The metabolic effects of calcein AM on HCT116 cells were also evaluated and the flux between the oxidative and non-oxidative branches of the pentose phosphate pathway was significantly altered. To elucidate whether these metabolic changes were due to the inhibition of CDK4 and CDK6, we also characterized the metabolic profile of a CDK4, CDK6 and CDK2 triple knockout of mouse embryonic fibroblasts. The results show that the metabolic profile associated with the depletion of CDK4, CDK6 and CDK2 coincides with the metabolic changes induced by calcein AM on HCT116 cells, thus confirming that the inhibition of CDK4 and CDK6 disrupts the balance between the oxidative and non-oxidative branches of the pentose phosphate pathway. Taken together, these results indicate that low doses of calcein can halt cell division and kill tumor cells. Thus, selective inhibition of CDK4 and CDK6 may be of greater pharmacological interest, since inhibitors of these kinases affect both cell cycle progression and the robust metabolic profile of tumors.

Tehranolide inhibits proliferation of MCF-7 human breast cancer cells by inducing G0/G1 arrest and apoptosis

Tehranolide, a novel natural sesquiterpene lactone with an endoperoxide group, bears a structural similarity to artemisinin and has been shown to inhibit cell growth. However, the underlying mechanisms of these activities remain obscure. The purpose of this study was to investigate the fundamental mechanisms by which tehranolide inhibits growth in MCF-7 cells. Cell growth was determined by using the MTT viability assay and counting cells. Apoptosis and cell-cycle progression were evaluated by means of Hoechst 33258 staining, flow cytometry with annexin-V/propidium iodide double staining, and ROS formation. The protein expression of Bax and Bcl-2 was demonstrated by Western blotting. Moreover, to determine the molecular mechanism whereby tehranolide mediates G0/G1 arrest, the expression of PI3K, p-PI3K, Akt, p-Akt, p27kip1, cyclin D1, and CDK4 was monitored. Cell proliferation was significantly inhibited by tehranolide in a dose- and time-dependent manner. This compound inhibited cell proliferation and induced G0/G1 arrest through the PI3K/Akt/cyclin D1 pathway. It also induced apoptosis and an increase in ROS. In addition, an increase in cytochrome c and Bax, as well as a decrease in Bcl-2, was observed. Moreover, blocking the CD95 receptor with an anti-CD95 antibody (ZB4) had no effect on tehranolide-mediated apoptosis. This study has yielded promising results, which show for the first time that tehranolide does inhibit the growth of cancer cells. The selective inhibition of cancer cell growth, the apoptosis induction via the mitochondrial pathway, and the G0/G1 arrest by modulating the PI3K/AKT signaling pathway and downregulating cyclin D1, which leads to the release of p27kip1 and the association of this inhibitor with the cyclin E/CDK2 complex, ultimately preventing cell-cycle progression from G1 to S phase, all serve to provide support for further studies of tehranolide as a possible anticancer drug in the clinical treatment of cancer.

Cyclin dependent kinases in cancer: potential for therapeutic intervention

Cell cycle progression through each phase is regulated by heterodimers formed by cyclin-dependent kinases (CDKs) and their regulatory partner proteins, the cyclins. Together they coordinate the cellular events through cell cycle. De-regulation of cell-cycle control due to aberrant CDK activity is a common feature of most cancer types. Intensive research on small molecules that target cell cycle regulatory proteins has led to the identification of many candidate inhibitors that are able to arrest proliferation and induce apoptosis in neoplastic cells as a promising strategy to treat cancer. Interestingly, cyclin-dependent kinases (CDKs) have also been proposed as therapeutic targets for Multiple Myeloma (MM). Overexpression and aberrant expression of the cyclins, specifically the D cyclins is seen in the majority of MM underscoring the value of exploring CDK inhibition in MM which currently remains an incurable neoplastic plasma-cell disorder. It is characterized by clonal proliferation of malignant plasma cells in the bone marrow microenviroment and associated organ dysfunction. Recent preclinical and early clinical data explore several CDK inhibitors in the context of MM. This review will provide an overview of the main classes of CDK inhibitors with a focus on their mechanism of action and discuss clinical and pharmacological implications of CDK inhibitors as possible therapeutic approaches for the treatment of cancer with specific consideration to MM.

Immunohistochemical investigation of F344/N rat islet cell tumors from national toxicology program studies

In this study, we have investigated the immunoexpression of peptide hormones and mediators associated with human islet cell tumors in a group of proliferative islet cell lesions in F344 rats including islet cell hyperplasias, adenomas, and carcinomas, as defined by conventional histopathologic criteria. All proliferative islets expressed synaptophysin, although decreased expression intensity was observed in hyperplasias and adenomas. Most of the proliferative lesions expressed insulin, which generally decreased as lesions progressed toward malignancy. The distribution of glucagon, somatostatin, and gastrin-expressing cells was altered in proliferative islet lesions but did not comprise a large proportion of cells. Islet cell tumors were associated with increased nuclear expression of cyclin-dependent kinase 4 as well as increased proliferating cell nuclear antigen and decreased β-catenin expression. c-Myelocytomatosis oncogene expression was variable. This is the first study to describe the immunophenotype of islet cell tumors in the F344 rat and to show that islet cell tumors in the F344 rat exhibit similarities in protein expression to the human counterpart.

Immunohistochemical features of 3,3',4,4'-tetrachloroazobenzene-induced rat gingival lesions

Gingival lesions of squamous hyperplasia, cystic keratinizing hyperplasia (CKH), and squamous cell carcinoma (SCC) can be induced in rats treated by chronic gavage with 10-100 mg/kg 3,3',4,4'-tetrachloroazobenzene. We evaluated gingival squamous hyperplasia (GSH), CKH, and SCC for the immunohistochemical pattern of expression of carcinogenesis-associated markers. The 3 types of lesions and controls were stained with proliferation markers (proliferating cell nuclear antigen [PCNA] and cyclin-D1), tumor-suppressor markers (β-catenin and mammary serine protease inhibitor [maspin]) and stroma-related markers (α-smooth muscle actin [SMA] and osteonectin/SPARC). The lesions had common immunohistochemical characteristics that differed in their expression patterns among the various diagnoses. PCNA and cyclin-D1 expression was higher in GSH, CKH, and SCC than in controls. The normal membranous expression of β-catenin was lower in GSH, and almost absent in CKH and SCC. Maspin expression was similar in GSH and controls, whereas both CKH and SCC showed decreased expression. SMA and/or osteonectin/SPARC were seen in stromal cells in CKH and SCC. Collectively, there appears to be a progression from hyperplastic and cystic lesions toward malignancy based on the morphological changes, supported by the expression of carcinogenesis-associated proteins. The exact sequence of events leading to SCC remains to be defined in a time-dependent manner.

Reduced levels of p15INK4b, p16INK4a, p21cip1 and p27kip1 in pancreatic carcinoma

Pancreatic carcinoma is one of the leading causes of cancer mortality worldwide, although the molecular mechanisms of this disease are poorly understood. The aim of this study was to examine the expression of cyclin-dependent kinase inhibitors (CDKIs) and the epigenetic modifications in the promoters of these genes. We also evaluated the correlation between the methylation status of CDKI genes and smoking habit in clinical pancreatic carcinoma specimens. Western blotting and real-time PCR were performed to assess CDKI expression. Methylation-specific PCR was carried out to examine the methylation status of the promoters of CDKI genes. In this study, we revealed that reduced levels of the CDKI proteins, p15INK4b, p16INK4a, p21cip1 and p27kip1, are a prominent feature of pancreatic carcinoma patients. The DNA hypermethylation of the promoter was observed in 40% (2 of 5) of the p15INK4b genes, 60% (3 of 5) of the p16INK4a genes and 60% of the p21cip1 genes, which markedly correlated with their decreased mRNA expression. No hypermethylation was detected in the p27kip1 gene promoter in 5 pancreatic carcinoma patients with markedly decreased expression of p27kip1 mRNA, suggesting an alternative mechanism of p27kip in these patients. In this study, patients with a smoking habit displayed methylation of 2 CDKI genes in their pancreatic carcinoma specimens. We concluded that epigenetic modification via hypermethylation represents a critical mechanism for the inactivation of CDKI genes in pancreatic carcinoma.

CREPT accelerates tumorigenesis by regulating the transcription of cell-cycle-related genes

Tumorigenesis is caused by an uncontrolled cell cycle and the altered expression of many genes. Here, we report a gene CREPT that is preferentially expressed in diverse human tumors. Overexpression of CREPT accelerates tumor growth, whereas depletion of CREPT demonstrates a reversed effect. CREPT regulates cyclin D1 expression by binding to its promoter, enhancing its transcription both in vivo and in vitro, and interacting with RNA polymerase II (RNAPII). Interestingly, CREPT promotes the formation of a chromatin loop and prevents RNAPII from reading through the 3' end termination site of the gene. Our findings reveal a mechanism where CREPT increases cyclin D1 transcription during tumorigenesis, through enhancing the recruitment of RNAPII to the promoter region, possibly, as well as chromatin looping.

Novel targeted therapies in head and neck cancer

INTRODUCTION

Molecularly targeted therapy, with the potential for increased selectivity and fewer adverse effects, hold promise in the treatment of HNSCC.

AREAS COVERED

Targeted agents for HNSCC expected to improve the effectiveness of current therapy including HER family, Src-family kinase, cell cycle, MET, AKT, HDAC, PARP, COX inhibitors and antiangiogenesis.

EXPERT OPINION

Epidermal growth factor receptor inhibitors are established in HNSCC and the need now is to find biomarkers for sensitivity to better select patients. Moreover, other pathway inhibitors hold significant promise and are being tested in clinical trials. Angiogenesis inhibition is likely to yield only modest efficacy alone but may augment existing standards. Lastly, one clinical arena where targeted therapies may find secure purchase is in the adjuvant or prevention setting where minimal or preneoplastic disease can be affected by inhibition of a single or few targets.

Daphnoretin induces cell cycle arrest and apoptosis in human osteosarcoma (HOS) cells

In this study antiproliferation, cell cycle arrest and apoptosis induced by daphnoretin in human osteosarcoma (HOS) cells were investigated. Antiproliferative activity was measured with the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. The IC(50) value of daphnoretin was 3.89 μM after 72 h treatment. Induction of apoptosis was evidenced by apoptotic body appearance and Annexin V-FITC/PI apoptosis detection kit. Flow cytometric analysis indicated daphnoretin arrested the cell cycle in the G2/M phase. Western-blot assay showed that the G2/M phase arrest was accompanied by down-regulation of cdc2, cyclin A and cyclin B1. Moreover, daphnoretin inhibited Bcl-2 expression and induced Bax expression to desintegrate the outer mitochondrial membrane and causing cytochrome c release. Mitochondrial cytochrome c release was associated with the activation of caspase-9 and caspase-3 cascade. Our results demonstrated that daphnoretin caused death of HOS cells by blocking cells successively in G2/M phases and activating the caspase-3 pathway.

Phase I, dose-escalation trial of the oral cyclin-dependent kinase 4/6 inhibitor PD 0332991, administered using a 21-day schedule in patients with advanced cancer

PURPOSE

To identify the dose-limiting toxicity (DLT) and maximum tolerated dose (MTD) of the first-in-class, oral CDK4/6 inhibitor PD 0332991 administered once daily for 21 of 28 days (3/1 schedule) in patients with retinoblastoma protein (Rb)-positive advanced solid tumors and to describe pharmacokinetic-pharmacodynamic relationships relative to drug effects.

EXPERIMENTAL DESIGN

This open-label phase I study (NCT00141297) enrolled patients who received PD 0332991 orally in six dose-escalation cohorts in a standard 3 + 3 design.

RESULTS

Forty-one patients were enrolled. DLTs were observed in five patients (12%) overall; at the 75, 125, and 150 mg once daily dose levels. The MTD and recommended phase II dose of PD 0332991 was 125 mg once daily. Neutropenia was the only dose-limiting effect. After cycle 1, grade 3 neutropenia, anemia, and leukopenia occurred in five (12%), three (7%), and one (2%) patient(s), respectively. The most common non-hematologic adverse events included fatigue, nausea, and diarrhea. Thirty-seven patients were evaluable for tumor response; 10 (27%) had stable disease for ≥4 cycles of whom six derived prolonged benefit (≥10 cycles). PD 0332991 was slowly absorbed (median T(max), 5.5 hours), and slowly eliminated (mean half-life was 25.9 hours) with a large volume of distribution (mean, 2,793 L). The area under the concentration-time curve increased linearly with dose. Using an E(max) model, neutropenia was shown to be proportional to exposure.

CONCLUSIONS

PD 0332991 warrants phase II testing at 125 mg once daily, at which dose neutropenia was the sole significant toxicity.

Curcumin enhanced antiproliferative effect of mitomycin C in human breast cancer MCF-7 cells in vitro and in vivo

AIM

To investigate the efficacy of mitomycin C (MMC) in combination with curcumin in suppressing human breast cancer in vitro and in vivo.

METHODS

Human breast cancer MCF-7 cells were used. Cell viability was measured using MTT assay. The cell cycle phase was detected with flow cytometric analysis. Cell cycle-associated proteins were examined using Western blot analysis. MCF-7 breast cancer xenografts were established to monitor tumor growth and cell cycle-associated protein expression.

RESULTS

Curcumin inhibited MCF-7 breast cancer cell viability in a concentration-dependent manner (IC(50) value=40 μmol/L). Similarly, MMC inhibited the cell viability with an IC(50) value of 5 μmol/L. Combined treatment of MMC and curcumin showed a synergistic antiproliferative effect. In the presence of curcumin (40 μmol/L), the IC(50) value of MMC was reduced to 5 μmol/L. In MCF-7 xenografts, combined administration of curcumin (100 mg/kg) and MMC (1-2 mg/kg) for 4 weeks produced significantly greater inhibition on tumor growth than either treatment alone. The combined treatment resulted in significantly greater G(1) arrest than MMC or curcumin alone. Moreover, the cell cycle arrest was associated with inhibition of cyclin D1, cyclin E, cyclin A, cyclin-dependent kinase 2 (CDK2) and CDK4, along with the induction of the cell cycle inhibitor p21 and p27 both in MCF-7 cells and in MCF-7 xenografts. These proteins were regulated through p38 MAPK pathway.

CONCLUSION

The results suggest that the combination of MMC and curcumin inhibits MCF-7 cell proliferation and cell cycle progression in vitro and in vivo via the p38 MAPK pathway.

Cdk2-null mice are resistant to ErbB-2-induced mammary tumorigenesis

The concept of targeting G(1) cyclin-dependent kinases (CDKs) in breast cancer treatments is supported by the fact that the genetic ablation of Cdk4 had minimal impacts on normal cell proliferation in majority of cell types, resulting in near-normal mouse development, whereas such loss of Cdk4 completely abrogated ErbB-2/neu-induced mammary tumorigenesis in mice. In most human breast cancer tissues, another G(1)-regulatory CDK, CDK2, is also hyperactivated by various mechanisms and is believed to be an important therapeutic target. In this report, we provide genetic evidence that CDK2 is essential for proliferation and oncogenesis of murine mammary epithelial cells. We observed that 87% of Cdk2-null mice were protected from ErbB-2-induced mammary tumorigenesis. Mouse embryonic fibroblasts isolated from Cdk2-null mouse showed resistance to various oncogene-induced transformation. Previously, we have reported that hemizygous loss of Cdc25A, the major activator of CDK2, can also protect mice from ErbB-2-induced mammary tumorigenesis [Cancer Res (2007) 67(14): 6605-11]. Thus, we propose that CDC25A-CDK2 pathway is critical for the oncogenic action of ErbB-2 in mammary epithelial cells, in a manner similar to Cyclin D1/CDK4 pathway.

Cyclin D1 G870A polymorphism and colorectal cancer susceptibility: a meta-analysis of 20 populations

PURPOSE

Studies investigating the association between genetic polymorphism of cyclin D1 (CCND1) G870A and risk of colorectal cancer (CRC) reported conflicting results. In order to derive a more precise estimation of the relationship, a meta-analysis was performed.

MATERIALS AND METHODS

We performed an extensive search of relevant studies and carried out a meta-analysis, including 20 studies with 5,975 cases and 8,333 controls, to obtain a more precise estimate.

RESULTS

Overall, significantly elevated colorectal cancer risk was associated with variant allele 870A when all studies were pooled (AA vs. GG: OR = 1.23, 95% CI = 1.04-1.44; GA vs. GG: OR = 1.13, 95% CI = 1.01-1.26; dominant model: OR = 1.16, 95% CI = 1.03-1.31). In the subgroup analysis by ethnicity, significantly increased risks were detected among Caucasians (AA vs. GG: OR = 1.27, 95% CI = 1.04-1.44; dominant model: OR = 1.17, 95% CI = 1.02-1.34).We also observed sporadic CRC with an increased cancer susceptibility (AA vs. GG: OR = 1.24, 95% CI = 1.04-1.48; dominant model: OR = 1.17, 95% CI = 1.04-1.33), when colorectal cancer was stratified into sporadic CRC and hereditary nonpolyposis colorectal cancer (HNPCC). However, no significant associations were found in both Asians and HNPCC patients for all genetic models.

CONCLUSION

Result suggests that the cyclin D1 870A allele is a low-penetrant risk factor for developing sporadic colorectal cancer, especially among Caucasians.

MiR-100 regulates cell differentiation and survival by targeting RBSP3, a phosphatase-like tumor suppressor in acute myeloid leukemia

Acute myeloblastic leukemia (AML) is characterized by the accumulation of abnormal myeloblasts (mainly granulocyte or monocyte precursors) in the bone marrow and blood. Though great progress has been made for improvement in clinical treatment during the past decades, only minority with AML achieve long-term survival. Therefore, further understanding mechanisms of leukemogenesis and exploring novel therapeutic strategies are still crucial for improving disease outcome. MicroRNA-100 (miR-100), a small non-coding RNA molecule, has been reported as a frequent event aberrantly expressed in patients with AML; however, the molecular basis for this phenotype and the statuses of its downstream targets have not yet been elucidated. In the present study, we found that the expression level of miR-100 in vivo was related to the stage of the maturation block underlying the subtypes of myeloid leukemia. In vitro experiments further demonstrated that miR-100 was required to promote the cell proliferation of promyelocytic blasts and arrest them differentiated to granulocyte/monocyte lineages. Significantly, we identified RBSP3, a phosphatase-like tumor suppressor, as a bona fide target of miR-100 and validated that RBSP3 was involved in cell differentiation and survival in AML. Moreover, we revealed a new pathway that miR-100 regulates G1/S transition and S-phase entry and blocks the terminal differentiation by targeting RBSP3, which partly in turn modulates the cell cycle effectors pRB/E2F1 in AML. These events promoted cell proliferation and blocked granulocyte/monocyte differentiation. Our data highlight an important role of miR-100 in the molecular etiology of AML, and implicate the potential application of miR-100 in cancer therapy.

Extensive and coordinated transcription of noncoding RNAs within cell-cycle promoters

Transcription of long noncoding RNAs (lncRNAs) within gene regulatory elements can modulate gene activity in response to external stimuli, but the scope and functions of such activity are not known. Here we use an ultra-high density array that tiles the promoters of 56 cell cycle genes to interrogate 108 samples representing diverse perturbations. We identify 216 transcribed regions that encode putative lncRNAs--many with RT-PCR-validated periodic expression during the cell cycle, show altered expression in human cancers, and are regulated in expression by specific oncogenic stimuli, stem cell differentiation, or DNA damage. DNA damage induces five lncRNAs from the CDKN1A promoter, and one such lncRNA, named PANDA, is induced in a p53- dependent manner. PANDA interacts with the transcription factor NF-YA to limit expression of pro-apoptotic genes; PANDA depletion markedly sensitized human fibroblasts to apoptosis by doxorubicin. These findings suggest potentially widespread roles for promoter lncRNAs in cell growth control.

Discovery of a potential allosteric ligand binding site in CDK2

Cyclin-dependent kinases (CDKs) are key regulatory enzymes in cell cycle progression and transcription. Aberrant activity of CDKs has been implicated in a number of medical conditions, and numerous small molecule CDK inhibitors have been reported as potential drug leads. However, these inhibitors exclusively bind to the ATP site, which is largely conserved among protein kinases, and clinical trials have not resulted in viable drug candidates, attributed in part to the lack of target selectivity. CDKs are unique among protein kinases, as their functionality strictly depends on association with their partner proteins, the cyclins. In an effort to identify potential target sites for disruption of the CDK-cyclin interaction, we probed the extrinsic fluorophore 8-anilino-1-naphthalene sulfonate (ANS) with human CDK2 and cyclin A using fluorescence spectroscopy and protein crystallography. ANS interacts with free CDK2 in a saturation-dependent manner with an apparent K(d) of 37 μM, and cyclin A displaced ANS from CDK2 with an EC(50) value of 0.6 μM. Co-crystal structures with ANS alone and in ternary complex with ATP site-directed inhibitors revealed two ANS molecules bound adjacent to one another, away from the ATP site, in a large pocket that extends from the DFG region above the C-helix. Binding of ANS is accompanied by substantial structural changes in CDK2, resulting in a C-helix conformation that is incompatible for cyclin A association. These findings indicate the potential of the ANS binding pocket as a new target site for allosteric inhibitors disrupting the interaction of CDKs and cyclins.

Clinicopathological and molecular characterization of nine cases of columnar cell variant of papillary thyroid carcinoma

The majority of papillary thyroid carcinoma is indolent and associated with long-term survival. The columnar cell variant, however, is a rare subtype that is variable in biological behavior; some are clinically aggressive, whereas others are more clinically indolent. Tumor size, tumor circumscription, and encapsulation may influence the behavior of columnar cell carcinomas. Other variables including genetic changes and putative biomarkers associated with malignant growth have not been thoroughly examined in these neoplasms. In this study, nine cases of columnar cell variant of papillary thyroid carcinoma from three institutions were classified as clinically indolent or aggressive based on pathological features, clinical history, and outcome. Indolent tumors were typically small, circumscribed or encapsulated, and from younger female patients, whereas aggressive tumors were large, locally aggressive, associated with regional and distant metastasis, and from older male patients. The missense mutation, V600E in the BRAF oncogene (BRAF(V600E)), was detected in three of nine of cases, of which two were clinically aggressive. Immunohistochemical evaluation of neoplasia-associated markers showed increased nuclear cyclin D1 expression, elevated Ki-67 proliferation indices, and predominantly weak nuclear p53 staining in both indolent and aggressive tumors. Expression of β-catenin was largely restricted to a membranous pattern in both tumor types. Cytoplasmic expression of bcl-2 was overall mildly reduced in indolent neoplasms. Nuclear expression of estrogen and progesterone receptors was increased in both indolent and aggressive neoplasms, but was without sex- or age-related differences; however, whereas progesterone receptor expression was diffuse and strong in clinically indolent carcinomas, its expression was diminished in aggressive neoplasms. Recognition of the clinicopathological characteristics and the molecular and immunophenotypic features of the columnar cell variant of papillary thyroid carcinoma may aid in characterizing neoplasms that behave indolently or aggressively.

Synergistic anticancer effects of combined γ-tocotrienol with statin or receptor tyrosine kinase inhibitor treatment

Systemic chemotherapy is the only current method of treatment that provides some chance for long-term survival in patients with advanced or metastatic cancer. γ-Tocotrienol is a natural form of vitamin E found in high concentrations in palm oil and displays potent anticancer effects, but limited absorption and transport of by the body has made it difficult to obtain and sustain therapeutic levels in the blood and target tissues. Statins are inhibitors of 3-hydroxy-3-methylglutaryl-coenzyme A (HMGCoA) reductase and are an example of a promising cancer chemotherapeutic agent whose clinical usefulness has been limited due to high-dose toxicity. Similarly, erlotinib and gefitinib are anticancer agents that inhibit the activation of individual HER/ErbB receptor subtypes, but have shown limited clinical success because of heterodimerization between different EGF receptor family members that can rescue cancer cells from agents directed against a single receptor subtype. Recent studies have investigated the anticancer effectiveness of low-dose treatment of various statins or EGF receptor inhibitors alone and in combination with γ-tocotrienol on highly malignant +SA mouse mammary epithelial cells in vitro. Combined treatment with subeffective doses of γ-tocotrienol with these other chemotherapeutic agents resulted in a synergistic inhibition of +SA cell growth and viability. These findings strongly suggest that combined treatment of γ-tocotrienol with other anticancer agents may not only provide an enhanced therapeutic response but also provide a means to avoid the toxicity, low bioavailability, or limited therapeutic action associated with high-dose monotherapy.

Dysregulation of the cell cycle and chromosomal imbalances in juxtaglomerular cell tumors - a comparative study with endocrine tumors of the pancreas

Two juxtaglomerular cell tumors (JGCTs) were investigated in comparison with 14 endocrine tumors of the pancreas (ETPs), focusing on the cell cycle, apoptosis, and cytogenetic changes. JGCTs revealed nuclear accumulation of Cyclin D(1), together with the cyclin-dependent kinase inhibitors p21(Cip1/Waf1) and p27(Kip1). In contrast, no accumulation of Cyclin D(3), p53, p16(INK4a), or Mdm-2 was seen. Bcl-2 protein was intensively, but Rb only moderately, expressed. This immunoreactive profile was not found in the ETPs, which were negative for Bcl-2, p27(Kip1), p21(Cip1/Waf1), and - with one exception - for Cyclin D(1) (1/14) but expressed Cyclin D(3) in 7/14 cases. JGCTs displayed characteristic genetic alterations with combined losses of chromosomes 9, 11, 15, and 21 and gains of chromosome 18. In contrast, no characteristic pattern of genetic alterations was found in ETPs. In both, the amount of chromosomal aberrations correlated with tumor size. In small ETPs and JGCTs, genetic losses dominated over gains of chromosomes, whereas in large/malignant ETPs, gains and losses were equally affected. Thus, JGCTs represent a special type of renal endocrine neoplasm characterized by deregulation of cell cycle components and a typical profile of chromosomal aberrations. Since only two JCTs were investigated, further studies for validation of these results are, however, necessary.

Regulation of glioblastoma progression by cord blood stem cells is mediated by downregulation of cyclin D1

Background

The normal progression of the cell cycle requires sequential expression of cyclins. Rapid induction of cyclin D1 and its associated binding with cyclin-dependent kinases, in the presence or absence of mitogenic signals, often is considered a rate-limiting step during cell cycle progression through the G₁ phase.

Methodology/Principal Findings

In the present study, human umbilical cord blood stem cells (hUCBSC) in co-cultures with glioblastoma cells (U251 and 5310) not only induced G₀-G₁ phase arrest, but also reduced the number of cells at S and G₂-M phases of cell cycle. Cell cycle regulatory proteins showed decreased expression levels upon treatment with hUCBSC as revealed by Western and FACS analyses. Inhibition of cyclin D1 activity by hUCBSC treatment is sufficient to abolish the expression levels of Cdk 4, Cdk 6, cyclin B1, β-Catenin levels. Our immuno precipitation experiments present evidence that, treatment of glioma cells with hUCBSC leads to the arrest of cell-cycle progression through inactivation of both cyclin D1/Cdk 4 and cyclin D1/Cdk 6 complexes. It is observed that hUCBSC, when co-cultured with glioma cells, caused an increased G₀-G₁ phase despite the reduction of G₀-G₁ regulatory proteins cyclin D1 and Cdk 4. We found that this reduction of G₀-G₁ regulatory proteins, cyclin D1 and Cdk 4 may be in part compensated by the expression of cyclin E1, when co-cultured with hUCBSC. Co-localization experiments under in vivo conditions in nude mice brain xenografts with cyclin D1 and CD81 antibodies demonstrated, decreased expression of cyclin D1 in the presence of hUCBSC.

Conclusions/Significance

This paper elucidates a model to regulate glioma cell cycle progression in which hUCBSC acts to control cyclin D1 induction and in concert its partner kinases, Cdk 4 and Cdk 6 by mediating cell cycle arrest at G₀-G₁ phase.

Small-molecule blocks malignant astrocyte proliferation and induces neuronal gene expression

In the central nervous system (CNS), neural stem cells (NSCs) differentiate into neurons, astrocytes, and oligodendrocytes--these cell lineages are considered unidirectional and irreversible under normal conditions. The introduction of a defined set of transcription factors has been shown to directly convert terminally differentiated cells into pluripotent stem cells, reinforcing the notion that preserving cellular identity is an active process. Indeed, recent studies highlight that tumor suppressor genes (TSGs) such as Ink4a/Arf and p53, control the barrier to efficient reprogramming, leaving open the question whether the same TSGs function to maintain the differentiated state. During malignancy or following brain injury, mature astrocytes have been reported to re-express neuronal genes and re-gain neurogenic potential to a certain degree, yet few studies have addressed the underlying mechanisms due to a limited number of cellular models or tools to probe this process. Here, we use a synthetic small-molecule (isoxazole) to demonstrate that highly malignant EGFRvIII-expressing Ink4a/Arf(-/-); Pten(-/-) astrocytes downregulated their astrocyte character, re-entered the cell cycle, and upregulated neuronal gene expression. As a collateral discovery, isoxazole small-molecules blocked tumor cell proliferation in vitro, a phenotype likely coupled to activation of neuronal gene expression. Similarly, histone deacetylase inhibitors induced neuronal gene expression and morphologic changes associated with the neuronal phenotype, suggesting the involvement of epigenetic-mediated gene activation. Our study assesses the contribution of specific genetic pathways underlying the de-differentiation potential of astrocytes and uncovers a novel pharmacological tool to explore astrocyte plasticity, which may bring insight to reprogramming and anti-tumor strategies.

Curcumin: A review of anti-cancer properties and therapeutic activity in head and neck squamous cell carcinoma

Curcumin (diferuloylmethane) is a polyphenol derived from the Curcuma longa plant, commonly known as turmeric. Curcumin has been used extensively in Ayurvedic medicine for centuries, as it is nontoxic and has a variety of therapeutic properties including anti-oxidant, analgesic, anti-inflammatory and antiseptic activity. More recently curcumin has been found to possess anti-cancer activities via its effect on a variety of biological pathways involved in mutagenesis, oncogene expression, cell cycle regulation, apoptosis, tumorigenesis and metastasis. Curcumin has shown anti-proliferative effect in multiple cancers, and is an inhibitor of the transcription factor NF-κB and downstream gene products (including c-myc, Bcl-2, COX-2, NOS, Cyclin D1, TNF-α, interleukins and MMP-9). In addition, curcumin affects a variety of growth factor receptors and cell adhesion molecules involved in tumor growth, angiogenesis and metastasis. Head and neck squamous cell carcinoma (HNSCC) is the sixth most common cancer worldwide and treatment protocols include disfiguring surgery, platinum-based chemotherapy and radiation, all of which may result in tremendous patient morbidity. As a result, there is significant interest in developing adjuvant chemotherapies to augment currently available treatment protocols, which may allow decreased side effects and toxicity without compromising therapeutic efficacy. Curcumin is one such potential candidate, and this review presents an overview of the current in vitro and in vivo data supporting its therapeutic activity in head and neck cancer as well as some of the challenges concerning its development as an adjuvant chemotherapeutic agent.

Cyclin-dependent kinases (cdks) and the DNA damage response: rationale for cdk inhibitor-chemotherapy combinations as an anticancer strategy for solid tumors

IMPORTANCE OF THE FIELD

The eukaryotic cell division cycle is a tightly regulated series of events coordinated by the periodic activation of multiple cyclin-dependent kinases (cdks). Small-molecule cdk-inhibitory compounds have demonstrated preclinical synergism with DNA-damaging agents in solid tumor models. An improved understanding of how cdks regulate the DNA damage response now provides an opportunity for optimization of combinations of cdk inhibitors and DNA damaging chemotherapy agents that can be translated to clinical settings.

AREAS COVERED IN THIS REVIEW

Here, we discuss novel work uncovering multiple roles for cdks in the DNA-damage-response network. First, they activate DNA damage checkpoint and repair pathways. Later their activity is turned off, resulting in cell cycle arrest, allowing time for DNA repair to occur. Recent clinical data on cdk inhibitor-DNA-damaging agent combinations are also discussed.

WHAT THE READER WILL GAIN

Readers will learn about novel areas of cdk biology, the complexity of DNA damage signaling networks and clinical implications.

TAKE HOME MESSAGE

New data demonstrate that cdks are 'master' regulators of DNA damage checkpoint and repair pathways. Cdk inhibition may therefore provide a means of potentiating the clinical activity of DNA-damaging chemotherapeutic agents for the treatment of cancer.

The flavonoid quercetin induces cell cycle arrest and mitochondria-mediated apoptosis in human cervical cancer (HeLa) cells through p53 induction and NF-κB inhibition

With increasing use of plant-derived cancer chemotherapeutic agents, exploring the antiproliferative effects of phytochemicals has gained increasing momentum for anticancer drug design. The dietary phytochemical quercetin, modulates several signal transduction pathways associated with cell proliferation and apoptosis. The present study was undertaken to examine the effect of quercetin on cell viability, and to determine the molecular mechanism of quercetin-induced cell death by investigating the expression of Bcl-2 family proteins (Bcl-2, Bcl-xL, Mcl1, Bax, Bad, p-Bad), cytochrome C, Apaf-1, caspases, and survivin as well as the cell cycle regulatory proteins (p53, p21, cyclin D1), and NF-κB family members (p50, p65, IκB, p-IκB-α, IKKβ and ubiquitin ligase) in human cervical cancer (HeLa) cells. The results demonstrate that quercetin suppressed the viability of HeLa cells in a dose-dependent manner by inducing G2/M phase cell cycle arrest and mitochondrial apoptosis through a p53-dependent mechanism. This involved characteristic changes in nuclear morphology, phosphatidylserine externalization, mitochondrial membrane depolarization, modulation of cell cycle regulatory proteins and NF-κB family members, upregulation of proapoptotic Bcl-2 family proteins, cytochrome C, Apaf-1 and caspases, and downregulation of antiapoptotic Bcl-2 proteins and survivin. Quercetin that exerts opposing effects on different signaling networks to inhibit cancer progression is a classic candidate for anticancer drug design.

Cell-cycle markers and biosensors

Since the first schematic illustrations of dividing cells, we have come a long way in characterising eukaryotic cells and defining their cell-cycle status thanks to a number of complementary approaches. Although most of these approaches rely on cell-fixation procedures to identify molecular components in cell lysates, cultured cells or tissues, the development of GFP technology has enabled visualisation of virtually any fusion protein in cellulo and in vivo, and the exploitation of functional elements with well-defined spatiotemporal characteristics has enabled the development of genetically encoded fluorescent markers of cell-cycle phases, thus providing novel means of characterising the status of living cells in real time with high resolution. Together with technological advances in fluorescence chemistry and imaging approaches, the more recent development of fluorescent biosensors has provided direct means of probing cell-cycle regulators and of studying their dynamics with high spatial and temporal resolution. Here we review classical approaches that rely on cell fixation to characterise the cell-cycle status and its regulatory enzymes, and we describe the more recent development of cell-cycle markers based on genetically encoded fusions of fluorescent proteins with characteristic cell-cycle features, and of fluorescent biosensor technology to probe cell-cycle regulators in living cells. Biosensors not only provide a means of characterising the behaviour of cell-cycle regulators in their natural environment, they are also very useful for comparative studies of biological processes in healthy and pathological conditions, and can be further applied to diagnostic approaches to assess the status of a specific target, and to monitor response to therapeutic intervention.

Gene regulation by SMAR1: Role in cellular homeostasis and cancer

Changes in the composition of nuclear matrix associated proteins contribute to alterations in nuclear structure, one of the major phenotypes of malignant cancer cells. The malignancy-induced changes in this structure lead to alterations in chromatin folding, the fidelity of genome replication and gene expression programs. The nuclear matrix forms a scaffold upon which the chromatin is organized into periodic loop domains called matrix attachment regions (MAR) by binding to various MAR binding proteins (MARBPs). Aberrant expression of MARBPs modulates the chromatin organization and disrupt transcriptional network that leads to oncogenesis. Dysregulation of nuclear matrix associated MARBPs has been reported in different types of cancers. Some of these proteins have tumor specific expression and are therefore considered as promising diagnostic or prognostic markers in few cancers. SMAR1 (scaffold/matrix attachment region binding protein 1), is one such nuclear matrix associated protein whose expression is drastically reduced in higher grades of breast cancer. SMAR1 gene is located on human chromosome 16q24.3 locus, the loss of heterozygosity (LOH) of which has been reported in several types of cancers. This review elaborates on the multiple roles of nuclear matrix associated protein SMAR1 in regulating various cellular target genes involved in cell growth, apoptosis and tumorigenesis.

Discovery of Dinaciclib (SCH 727965): A Potent and Selective Inhibitor of Cyclin-Dependent Kinases

Inhibition of cyclin-dependent kinases (CDKs) has emerged as an attractive strategy for the development of novel oncology therapeutics. Herein is described the utilization of an in vivo screening approach with integrated efficacy and tolerability parameters to identify candidate CDK inhibitors with a suitable balance of activity and tolerability. This approach has resulted in the identification of SCH 727965, a potent and selective CDK inhibitor that is currently undergoing clinical evaluation.

Distribution of CCND1 A870G polymorphism in patients with advanced uterine cervical carcinoma

We examined the distribution of the CCND1 A870G (rs9344) polymorphic variant in patients with cervical cancer (n = 129) and healthy individuals (n = 288) in a sample of a Polish cohort. We showed that patients with advanced cervical cancer bearing the CCND1 A/A and A/G genotypes displayed a 1.811-fold increased risk of cervical cancer (95% CI = 1.150-2.852, p = 0.0098). We also found a significantly higher frequency of the CCND1 870A allele in patients with cancer than in controls, p = 0.0116. Our investigation confirmed that the CCND1 870A gene variant may be a genetic risk factor in the incidence of advanced cervical cancer.

Dinaciclib (SCH 727965), a novel and potent cyclin-dependent kinase inhibitor

Cyclin-dependent kinases (CDK) are key positive regulators of cell cycle progression and attractive targets in oncology. SCH 727965 inhibits CDK2, CDK5, CDK1, and CDK9 activity in vitro with IC(50) values of 1, 1, 3, and 4 nmol/L, respectively. SCH 727965 was selected as a clinical candidate using a functional screen in vivo that integrated both efficacy and safety parameters. Compared with flavopiridol, SCH 727965 exhibits superior activity with an improved therapeutic index. In cell-based assays, SCH 727965 completely suppressed retinoblastoma phosphorylation, which correlated with apoptosis onset and total inhibition of bromodeoxyuridine incorporation in >100 tumor cell lines of diverse origin and background. Moreover, short exposures to SCH 727965 were sufficient for long-lasting cellular effects. SCH 727965 induced regression of established solid tumors in a range of mouse models following intermittent scheduling of doses below the maximally tolerated level. This was associated with modulation of pharmacodynamic biomarkers in skin punch biopsies and rapidly reversible, mechanism-based effects on hematologic parameters. These results suggest that SCH 727965 is a potent and selective CDK inhibitor and a novel cytotoxic agent.

Helicobacter pylori gamma-glutamyltranspeptidase induces cell cycle arrest at the G1-S phase transition

In our previous study, we showed that Helicobacter pylori gamma-glutamyltranspeptidase (GGT) is associated with H. pylori-induced apoptosis through a mitochondrial pathway. To better understand the role of GGT in apoptosis, we examined the effect of GGT on cell cycle regulation in AGS cells. To determine the effect of recombinant GGT (rGGT) on cell cycle distribution and apoptosis, rGGT-treated and untreated AGS cells were analyzed in parallel by flow cytometry using propidium iodide (PI). We found that rGGT inhibited the growth of AGS cells in a time-dependent manner, and that the pre-exposure of cells to a caspase-3 inhibitor (z-DEVD-fmk) effectively blocked GGT-induced apoptosis. Cell cycle analysis showed G1 phase arrest and apoptosis in AGS cells following rGGT treatment. The rGGT-mediated G1 phase arrest was found to be associated with down-regulation of cyclin E, cyclin A, Cdk 4, and Cdk 6, and the up-regulation of the cyclin-dependent kinase (Cdk) inhibitors p27 and p21. Our results suggest that H. pylori GGT induces cell cycle arrest at the G1-S phase transition.

Helicobacter pylori gamma-glutamyltranspeptidase induces cell cycle arrest at the G1-S phase transition

In our previous study, we showed that Helicobacter pylori gamma-glutamyltranspeptidase (GGT) is associated with H. pylori-induced apoptosis through a mitochondrial pathway. To better understand the role of GGT in apoptosis, we examined the effect of GGT on cell cycle regulation in AGS cells. To determine the effect of recombinant GGT (rGGT) on cell cycle distribution and apoptosis, rGGT-treated and untreated AGS cells were analyzed in parallel by flow cytometry using propidium iodide (PI). We found that rGGT inhibited the growth of AGS cells in a time-dependent manner, and that the pre-exposure of cells to a caspase-3 inhibitor (z-DEVD-fmk) effectively blocked GGT-induced apoptosis. Cell cycle analysis showed G1 phase arrest and apoptosis in AGS cells following rGGT treatment. The rGGT-mediated G1 phase arrest was found to be associated with down-regulation of cyclin E, cyclin A, Cdk 4, and Cdk 6, and the up-regulation of the cyclin-dependent kinase (Cdk) inhibitors p27 and p21. Our results suggest that H. pylori GGT induces cell cycle arrest at the G1-S phase transition.

Cdk2 inhibition prolongs G1 phase progression in mouse embryonic stem cells

Embryonic stem cells (ESCs) proliferate rapidly and have a unique cell-cycle structure with a very short G1 phase. Previous reports suggested that the rapid G1 phase progression of ESCs might be underpinned by high and precocious Cdk2 activity and that Cdk2 activity might be crucial for both cell-cycle regulation and cell-fate decisions in human ESCs. However, the actual role of Cdk2 in cell-cycle progression of mouse ESCs (mESCs) has not been elucidated. In this study, we investigated the effects of down-regulation of Cdk2 activity by olomoucine II in 2 mESC lines. Olomoucine II treatment significantly increased the G1 phase cell numbers, decreased the S phase cell numbers, and inhibited DNA replication in mESCs. In nocodazole-synchronized mESCs, we show that specific down-regulation of Cdk2 activity prolongs G1 phase progression. In addition, down-regulation of Cdk2 activity in mESCs established a somatic cell-like cell cycle and induced expression of differentiation markers. Our results suggest that high Cdk2 activity is essential for rapid G1 phase progression and establishment of ESC-specific cell-cycle structure in mESCs and support the hypothesis of a link between cell-cycle regulation and pluripotency maintenance in ESCs. This study reveals olomoucine II to be an effective tool for manipulation of the cell cycle and pluripotency in ESCs and very likely also for the manipulation of other stem cell types, including cancer stem cells.

The zinc-finger protein KCMF1 is overexpressed during pancreatic cancer development and downregulation of KCMF1 inhibits pancreatic cancer development in mice

Potassium channel modulatory factor 1 (KCMF1) was found upregulated in a differential screen in the metaplastic epithelium in the pancreas of transforming growth factor (TGF)-alpha transgenic mice. Expression analysis indicated broad overexpression in human cancer tissues. Therefore, we investigated the hypothesis that KCMF1 promotes metaplastic changes and tumor development. KCMF1 represents an evolutionarily highly conserved protein with a 95% identity between human and zebrafish. KCMF1 is expressed during embryonic development and in the majority of adult tissues investigated. Upregulation of nuclear KCMF1 expression is evident in preneoplastic lesions and in several epithelial malignancies, such as pancreatic cancer in mice and humans. In cell culture and in the chicken chorioallantoic membrane model, KCMF1 enhances proliferation, migration and invasion of HEK-293 and Panc1 cells. In crossbreeding experiments, KCMF1-knockdown gene trap mice showed a reduced number and size of premalignant lesions and absence of pancreatic cancer formation in TGF-alpha transgenic mice. This effect is related to the decreased expression of G1 to S cell-cycle regulators such as cyclin D and cyclin-dependent kinase (CDK) 4. Our data support the hypothesis that KCMF1 mediates pro-oncogenic functions in vitro and in vivo and downregulation of KCMF1 results in the inhibition of pancreatic cancer formation in mice. These effects are mediated through downregulation of cell-cycle control genes such as cyclin D and CDK4.

Identification of potent pyrazolo[4,3-h]quinazoline-3-carboxamides as multi-cyclin-dependent kinase inhibitors

Abnormal proliferation mediated by disruption of the mechanisms that keep the cell cycle under control is a hallmark of virtually all cancer cells. Compounds targeting complexes between cyclin-dependent kinases (CDKs) and cyclins (Cy) and inhibiting their activity are regarded as promising antitumor agents to complement the existing therapies. An expansion of pyrazolo[4,3-h]quinazoline chemical class oriented to the development of three points of variability was undertaken leading to a series of compounds able to inhibit CDKs both in vitro and in vivo. Starting from the CDK selective but poorly soluble hit compound 1, we succeeded in obtaining several compounds showing enhanced inhibitory activity both on CDKs and on tumor cells and displaying improved physical properties and pharmacokinetic behavior. Our study led to the identification of compound 59 as a highly potent, orally bioavailable CDK inhibitor that exhibited significant in vivo efficacy on the A2780 ovarian carcinoma xenograft model. The demonstrated mechanisms of action of compound 59 on cancer cell lines and its ability to inhibit tumor growth in vivo render this compound very interesting as potential antineoplastic agent.

Design, synthesis, and evaluation of 2-methyl- and 2-amino-N-aryl-4,5-dihydrothiazolo[4,5-h]quinazolin-8-amines as ring-constrained 2-anilino-4-(thiazol-5-yl)pyrimidine cyclin-dependent kinase inhibitors

Following the recent discovery and development of 2-anilino-4-(thiazol-5-yl)pyrimidine cyclin dependent kinase (CDK) inhibitors, a program was initiated to evaluate related ring-constrained analogues, specifically, 2-methyl- and 2-amino-N-aryl-4,5-dihydrothiazolo[4,5-h]quinazolin-8-amines for inhibition of CDKs. Here we report the rational design, synthesis, structure-activity relationships (SARs), and cellular mode-of-action profile of these second generation CDK inhibitors. Many of the analogues from this chemical series inhibit CDKs with very low nanomolar K(i) values. The most potent compound reported in this study inhibits CDK2 with an IC(50) of 0.7 nM ([ATP] = 100 microM). Furthermore, an X-ray crystal structure of 2-methyl-N-(3-(nitro)phenyl)-4,5-dihydrothiazolo[4,5-h]quinazolin-8-amine (11g), a representative from the chemical series in complex with cyclin A-CDK2, is reported, confirming the design rationale and expected binding mode within the CDK2 ATP binding pocket.

Drosophila RB proteins repress differentiation-specific genes via two different mechanisms

The RB and E2F proteins play important roles in the regulation of cell division, cell death, and development by controlling the expression of genes involved in these processes. The mechanisms of repression by the retinoblastoma protein (pRB) have been extensively studied at cell cycle-regulated promoters. However, little is known about developmentally regulated E2F/RB genes. Here, we have taken advantage of the simplicity of the E2F/RB pathway in flies to inspect the regulation of differentiation-specific target genes. These genes are repressed by dE2F2/RBF and a recently identified RB-containing complex, dREAM/MMB, in a cell type- and cell cycle-independent manner. Our studies indicate that the mechanism of repression differs from that of cell cycle-regulated genes. We find that two different activities are involved in their regulation and that in proliferating cells, both are required to maintain repression. First, dE2F2/RBF and dREAM/MMB employ histone deacetylase (HDAC) activities at promoter regions. Remarkably, we have also uncovered an unconventional mechanism of repression by the Polycomb group (PcG) protein Enhancer of zeste [E(Z)], which is involved in silencing of these genes through the dimethylation of histone H3 Lys27 at nucleosomes located downstream of the transcription start sites (TSS).

Human kinome drug discovery and the emerging importance of atypical allosteric inhibitors

IMPORTANCE OF THE FIELD

Protein kinases are important targets for drug discovery because they possess critical roles in many human diseases. Several protein kinase inhibitors have entered clinical development with others having already been approved for treating a host of diseases. However, many kinase inhibitors suffer from non-selectivity because they interact with the ATP binding region which has similar structures amongst the protein kinases and this non-selectivity sometimes can cause side effects. As a consequence, there is much interest in developing drugs that inhibit kinases through non-classical mechanisms with the hope of avoiding the side effects of previous kinase drugs.

AREAS COVERED IN THIS REVIEW

This review covers emerging information on kinase biology and discusses new approaches to design selective inhibitors that do not compete with ATP.

WHAT THE READER WILL GAIN

The reader will gain a better understanding of the importance of the field of allosteric inhibitor drug discovery and how this has required the adoption of a new generation of high-throughput screening techniques.

TAKE HOME MESSAGE

Discovery and development of allosteric modulators will result in a family of novel kinase therapies with greater selectivity and more varied ways to control activity of disease causing kinase targets.

Functional analysis of CDKN2A/p16INK4a 5'-UTR variants predisposing to melanoma

Germline CDKN2A mutations are observed in 20-50% of melanoma-prone families. We identified melanoma patients that were heterozygous for non-coding germline variants in the 5'-UTR of CDKN2A (c.-21C > T; c.-25C > T&c.-180G > A; c.-56G > T; c.-67G > C) and examined their impact on the p16(INK4a) 5'-UTR activity using two luciferase-based reporter vectors that differ in basal transcription level and that were transfected into the melanoma-derived WM266-4 and in the breast cancer-derived MCF7 cells. The wild-type 5'-UTR sequence, containing a reported SNP (c.-33G > C) and a known melanoma-predisposing mutation (c.-34G > T), was included as controls. Results revealed that the variants at -21 and -34 severely reduced the reporter activity. The variants at -56 and at -25&-180 exhibited a milder impact, while results with c.-67G > C were dependent on the plasmid type. Quantification of the luciferase mRNA indicated that the effects of the variants were mainly post-transcriptional. Using a bicistronic dual-luciferase reporter plasmid, we confirmed that c.-21C > T and c.-34G > T had a severe negative impact in both cell lines. We also applied a polysomal profiling technique to samples heterozygous for the 5'-UTR variants, including patient-derived lymphoblasts. Analysis of allelic imbalance indicated that in addition to the c.-21C > T variant, the c.-56T > G and c.-67G > C variants also reduced mRNA translation efficiency. Overall, our results suggest that the c.-21C > T sequence variant is a melanoma-predisposing mutation. The c.-25C > T&c.-180G > A and particularly the c.-56G > T variants showed a range of intermediate functional defects in the different assays, and were not observed in the control population. We propose that these variants should be considered as potential mutations.

Potent anti-tumor activity of a macrocycle-quinoxalinone class pan-Cdk inhibitor in vitro and in vivo

Deregulation of cell-cycle control is a hallmark of cancer. Thus, cyclin-dependent kinases (Cdks) are an attractive target for the development of anti-cancer drugs. Here, we report the biological characterization of a highly potent pan-Cdk inhibitor with a macrocycle-quinoxalinone structure. Compound M inhibited Cdk1, 2, 4, 5, 6, and 9 with equal potency in the nM range and was selective against kinases other than Cdks. This compound inhibited multiple events in the cell cycle in vitro, including retinoblastoma protein (pRb) phosphorylation, E2F-dependent transcription, DNA replication (determined by bromodeoxyuridine incorporation), and mitosis completion (assayed by flow cytometry) in the 10 nM range. Moreover, this compound induced cell death, as determined by induction of the subG1 fraction, activated caspase-3, and anexin V. In vivo, Compound M showed anti-tumor efficacy at a tolerated dose. In a nude rat xenograft tumor model, an 8-h constant infusion of Compound M inhibited pRb phosphorylation and induced apoptosis in tumor cells at ~ 30 nM, which led to the inhibition of tumor growth. Immunosuppression was the only liability observed at this dose, but immune function returned to normal after 10 days. Suppression of pRb phosphorylation in tumor cells was clearly correlated with tumor cell growth inhibition and cell death in vitro and in vivo. In vivo, Compound M inhibited pRb phosphorylation in both tumor and gut crypt cells. Rb phosphorylation may be a suitable pharmacodynamic biomarker in both tumors and normal tissues for monitoring target engagement and predicting the efficacy of Compound M.

Anti-proliferative effects of gamma-tocotrienol on mammary tumour cells are associated with suppression of cell cycle progression

OBJECTIVES

Previous studies have shown that gamma-tocotrienol induces potent anti-proliferative effects on +SA mammary tumour cells in culture; here, investigations have been conducted to determine its effects on intracellular signalling proteins involved in regulating cell cycle progression.

MATERIALS AND METHODS

+SA cells were maintained in mitogen-free defined media containing 0 or 4 micromgamma-tocotrienol, for 48 h to synchronize cell cycle in G(0) phase, and then they were exposed to 100 ng/ml EGF to initiate cell cycle progression. Whole cell lysates were collected at various time points from each treatment group and were prepared for Western blot analysis.

RESULTS AND CONCLUSIONS

Treatment with 4 micromgamma-tocotrienol significantly inhibited +SA cell proliferation over a 4-day culture period. Moreover, this treatment resulted in a relatively large reduction in cyclin D1, cyclin dependent kinase (CDK)4, CDK2 and CDK6 levels, between 4 and 24 h after EGF exposure. Tocotrienol treatment also resulted in a relatively large increase in CDK inhibitor (CKI) p27, prior to and after EGF exposure, but had little effect on levels of CKIs, p21 and p15. Tocotrienol treatment also induced a large relative reduction in retinoblastoma (Rb) protein phosphorylation at ser780 and ser807/811. These findings strongly suggest that anti-proliferative effects of gamma-tocotrienol are associated with reduction in cell cycle progression from G(1) to S, as evidenced by increased p27 levels, and a corresponding decrease in cyclin D1, CDK2, CDK4, CDK6 and phosphorylated Rb levels.