Requirement of cyclin E-Cdk2 inhibition in p16(INK4a)-mediated growth suppression

Altered G1 signaling order and commitment point in cells proliferating without CDK4/6 activity

Cell-cycle entry relies on an orderly progression of signaling events. To start, cells first activate the kinase cyclin D-CDK4/6, which leads to eventual inactivation of the retinoblastoma protein Rb. Hours later, cells inactivate APC/CCDH1 and cross the final commitment point. However, many cells with genetically deleted cyclin Ds, which activate and confer specificity to CDK4/6, can compensate and proliferate. Despite its importance in cancer, how this entry mechanism operates remains poorly characterized, and whether cells use this path under normal conditions remains unknown. Here, using single-cell microscopy, we demonstrate that cells with acutely inhibited CDK4/6 enter the cell cycle with a slowed and fluctuating cyclin E-CDK2 activity increase. Surprisingly, with low CDK4/6 activity, the order of APC/CCDH1 and Rb inactivation is reversed in both cell lines and wild-type mice. Finally, we show that as a consequence of this signaling inversion, Rb inactivation replaces APC/CCDH1 inactivation as the point of no return. Together, we elucidate the molecular steps that enable cell-cycle entry without CDK4/6 activity. Our findings not only have implications in cancer resistance, but also reveal temporal plasticity underlying the G1 regulatory circuit.

p27 allosterically activates cyclin-dependent kinase 4 and antagonizes palbociclib inhibition

The p27 protein is a canonical negative regulator of cell proliferation and acts primarily by inhibiting cyclin-dependent kinases (CDKs). Under some circumstances, p27 is associated with active CDK4, but no mechanism for activation has been described. We found that p27, when phosphorylated by tyrosine kinases, allosterically activated CDK4 in complex with cyclin D1 (CDK4-CycD1). Structural and biochemical data revealed that binding of phosphorylated p27 (phosp27) to CDK4 altered the kinase adenosine triphosphate site to promote phosphorylation of the retinoblastoma tumor suppressor protein (Rb) and other substrates. Surprisingly, purified and endogenous phosp27-CDK4-CycD1 complexes were insensitive to the CDK4-targeting drug palbociclib. Palbociclib instead primarily targeted monomeric CDK4 and CDK6 (CDK4/6) in breast tumor cells. Our data characterize phosp27-CDK4-CycD1 as an active Rb kinase that is refractory to clinically relevant CDK4/6 inhibitors.

Lycopene Protects Keratinocytes Against UVB Radiation-Induced Carcinogenesis via Negative Regulation of FOXO3a Through the mTORC2/AKT Signaling Pathway

Lycopene, one of the most potent anti-oxidants, has been reported to exhibit potent anti-proliferative properties in a wide range of cancer cells through modulation of the cell cycle and apoptosis. Forkhead box O3 (FOXO3a) plays a pivotal role in modulating the expression of genes involved in cell death. Herein, we investigated the role of FOXO3a signaling in the anti-cancer effects of lycopene. Results showed that lycopene pretreatment attenuated UVB-induced cell hyper-proliferation and promoted apoptosis, accompanied by decreased cyclin-dependent kinase 2 (CDK2) and CDK4 complex in both human keratinocytes and SKH-1 hairless mice. FOXO3a is phosphorylated in response to UVB irradiation and sequestered in the cytoplasm, while lycopene pretreatment rescued this sensitization. Gene ablation of FOXO3a attenuated lycopene-induced decrease in cell hyper-proliferation, CDK2, and CDK4 complex, indicating a critical role of FOXO3a in the lycopene-induced anti-proliferative effect of keratinocytes during UVB irradiation. Transfection with FOXO3a siRNA inhibited the lycopene-induced increase in cell apoptosis, BAX and cleaved PARP expression. Moreover, loss of AKT induced further accelerated lycopene-induced FOXO3a dephosphorylation, while loss of mechanistic target of rapamycin complex 2 (mTORC2) by transfection with RICTOR siRNA induced levels of AKT phosphorylation comparable to those obtained with lycopene. In contrast, overexpression of AKT or mTORC2 decreased the effects of lycopene on the expression of FOXO3a as well as AKT phosphorylation, suggesting that lycopene depends on the negative modulation of mTORC2/AKT signaling. Taken together, our findings demonstrate that the mTORC2/AKT/FOXO3a axis plays a critical role in the anti-proliferative and pro-apoptotic effects of lycopene in UVB-induced photocarcinogenesis. J. Cell. Biochem. 119: 366-377, 2018. © 2017 Wiley Periodicals, Inc.

CDKN2A/p16 Deletion in Head and Neck Cancer Cells Is Associated with CDK2 Activation, Replication Stress, and Vulnerability to CHK1 Inhibition

Checkpoint kinase inhibitors (CHKi) exhibit striking single-agent activity in certain tumors, but the mechanisms accounting for hypersensitivity are poorly understood. We screened a panel of 49 established human head and neck squamous cell carcinoma (HNSCC) cell lines and report that nearly 20% are hypersensitive to CHKi monotherapy. Hypersensitive cells underwent early S-phase arrest at drug doses sufficient to inhibit greater than 90% of CHK1 activity. Reduced rate of DNA replication fork progression and chromosomal shattering were also observed, suggesting replication stress as a root causative factor in CHKi hypersensitivity. To explore genomic underpinnings of CHKi hypersensitivity, comparative genomic analysis was performed between hypersensitive cells and cells categorized as least sensitive because they showed drug IC₅₀ value greater than the cell panel median and lacked early S-phase arrest. Novel association between CDKN2A/p16 copy number loss, CDK2 activation, replication stress, and hypersensitivity of HNSCC cells to CHKi monotherapy was found. Restoring p16 in cell lines harboring CDKN2A/p16 genomic deletions alleviated CDK2 activation and replication stress, attenuating CHKi hypersensitivity. Taken together, our results suggest a biomarker-driven strategy for selecting HNSCC patients who may benefit the most from CHKi therapy.Significance: These results suggest a biomarker-driven strategy for selecting HNSCC patients who may benefit the most from therapy with CHK inhibitors. Cancer Res; 78(3); 781-97. ©2017 AACR.

miR-6883 Family miRNAs Target CDK4/6 to Induce G1 Phase Cell-Cycle Arrest in Colon Cancer Cells

CDK4/6 targeting is a promising therapeutic strategy under development for various tumor types. In this study, we used computational methods and The Cancer Genome Atlas dataset analysis to identify novel miRNAs that target CDK4/6 and exhibit potential for therapeutic development in colorectal cancer. The 3'UTR of CDK4/6 mRNAs are targeted by a family of miRNAs, which includes miR-6883-5p, miR-149*, miR-6785-5p, and miR-4728-5p. Ectopic expression of miR-6883-5p or miR-149* downregulated CDK4 and CDK6 levels in human colorectal cancer cells. RNA-seq analysis revealed an inverse relationship between the expression of CDK4/6 and miR-149* and intronic miRNA-6883-5p encoding the clock gene PER1 in colorectal cancer patient samples. Restoring expression of miR-6883-5p and miR-149* blocked cell growth leading to G₀-G₁ phase cell-cycle arrest and apoptosis in colorectal cancer cells. CDK4/6 targeting by miR-6883-5p and miR-149* could only partially explain the observed antiproliferative effects. Notably, both miRNAs synergized with the frontline colorectal cancer chemotherapy drug irinotecan. Further, they resensitized mutant p53-expressing cell lines resistant to 5-fluorouracil. Taken together, our results established the foundations of a candidate miRNA-based theranostic strategy to improve colorectal cancer management. Cancer Res; 77(24); 6902-13. ©2017 AACR.

Emerging therapies for breast cancer

HER2 and CDK4/6 are undoubted two most important biological targets for breast cancer. Anti-HER2 treatments enhance objective response and progression-free survival/disease-free survival as well as overall survival. Three CDK4/6 inhibitors consistently improve objective response and progression-free survival; however, overall survival data are waited. Optimization of chemotherapy and endocrine strategies remains an unmet need. Check point inhibitor-based immunotherapy combined with chemotherapy is a promising field, especially for triple-negative breast cancer.

Loss of Cyclin-dependent Kinase 2 in the Pancreas Links Primary β-Cell Dysfunction to Progressive Depletion of β-Cell Mass and Diabetes

The failure of pancreatic islet β-cells is a major contributor to the etiology of type 2 diabetes. β-Cell dysfunction and declining β-cell mass are two mechanisms that contribute to this failure, although it is unclear whether they are molecularly linked. Here, we show that the cell cycle regulator, cyclin-dependent kinase 2 (CDK2), couples primary β-cell dysfunction to the progressive deterioration of β-cell mass in diabetes. Mice with pancreas-specific deletion of Cdk2 are glucose-intolerant, primarily due to defects in glucose-stimulated insulin secretion. Accompanying this loss of secretion are defects in β-cell metabolism and perturbed mitochondrial structure. Persistent insulin secretion defects culminate in progressive deficits in β-cell proliferation, reduced β-cell mass, and diabetes. These outcomes may be mediated directly by the loss of CDK2, which binds to and phosphorylates the transcription factor FOXO1 in a glucose-dependent manner. Further, we identified a requirement for CDK2 in the compensatory increases in β-cell mass that occur in response to age- and diet-induced stress. Thus, CDK2 serves as an important nexus linking primary β-cell dysfunction to progressive β-cell mass deterioration in diabetes.

Evidence for a CDK4-dependent checkpoint in a conditional model of cellular senescence

Cellular senescence, the stable cell cycle arrest elicited by various forms of stress, is an important facet of tumor suppression. Although much is known about the key players in the implementation of senescence, including the pRb and p53 axes and the cyclin dependent kinase inhibitors p16^INK4a and p21^CIP1, many details remain unresolved. In studying conditional senescence in human fibroblasts that express a temperature sensitive SV40 large T-antigen (T-Ag), we uncovered an unexpected role for CDK4. At the permissive temperature, where pRb and p53 are functionally compromised by T-Ag, cyclin D-CDK4 complexes are disrupted by the high p16^INK4a levels and reduced expression of p21^CIP1. In cells arrested at the non-permissive temperature, p21^CIP1 promotes reassembly of cyclin D-CDK4 yet pRb is in a hypo-phosphorylated state, consistent with cell cycle arrest. In exploring whether the reassembled cyclin D-CDK4-p21 complexes are functional, we found that shRNA-mediated knockdown or chemical inhibition of CDK4 prevented the increase in cell size associated with the senescent phenotype by allowing the cells to arrest in G1 rather than G2/M. The data point to a role for CDK4 kinase activity in a G2 checkpoint that contributes to senescence.

Cyclin D activates the Rb tumor suppressor by mono-phosphorylation

The widely accepted model of G₁ cell cycle progression proposes that cyclin D:Cdk4/6 inactivates the Rb tumor suppressor during early G₁ phase by progressive multi-phosphorylation, termed hypo-phosphorylation, to release E2F transcription factors. However, this model remains unproven biochemically and the biologically active form(s) of Rb remains unknown. In this study, we find that Rb is exclusively mono-phosphorylated in early G₁ phase by cyclin D:Cdk4/6. Mono-phosphorylated Rb is composed of 14 independent isoforms that are all targeted by the E1a oncoprotein, but show preferential E2F binding patterns. At the late G₁ Restriction Point, cyclin E:Cdk2 inactivates Rb by quantum hyper-phosphorylation. Cells undergoing a DNA damage response activate cyclin D:Cdk4/6 to generate mono-phosphorylated Rb that regulates global transcription, whereas cells undergoing differentiation utilize un-phosphorylated Rb. These observations fundamentally change our understanding of G₁ cell cycle progression and show that mono-phosphorylated Rb, generated by cyclin D:Cdk4/6, is the only Rb isoform in early G₁ phase.

DOI:http://dx.doi.org/10.7554/eLife.02872.001

Cells go through a tightly controlled, multi-step procedure before they divide. This cell division program—the cell cycle—is necessary for preventing unrestrained cellular growth, which may lead to cancer. Proteins called cyclins control the progression through each of the phases of the cell cycle, with different cyclins working during different phases.

During the G1 phase of the cell cycle, cells grow in size and produce the proteins that are required to copy DNA. Once a cell passes a checkpoint called the 'restriction point' at the end of the G1 phase, it is committed to dividing. It is therefore particularly important to keep events during G1 phase in check.

The Retinoblastoma tumor suppresor protein (Rb) is a key player in regulating the G1 phase. Rb sequesters transcription factors that are essential for the cell cycle to progress. Previously, it was thought that a complex called cyclin D added more and more phosphates to the Rb protein during the G1 phase. This process predicted a slow release of transcription factors, which attach to DNA and start the process of DNA replication. While many studies have presented data that is consistent with this model, direct biochemical evidence of these events is lacking.

Narasimha, Kaulich, Shapiro et al. now present biochemical analyses of Rb proteins that show—completely unexpectedly—that the cyclin D complex adds just one phosphate group to Rb during the G1 phase, although this group can be added to one of fourteen different sites. The resulting 'mono-phosphorylated' Rb varieties can each sequester different transcription factors and stop them working.

At the restriction point, many more phosphate groups are then rapidly added, and the Rb protein is inactivated by a different cyclin. This cyclin—called Cyclin E—then drives cells into the next phase of the cell cycle. Establishing how cyclin E is activated is a priority for future research.

DOI:http://dx.doi.org/10.7554/eLife.02872.002

The molecular balancing act of p16(INK4a) in cancer and aging

p16(INK4a), located on chromosome 9p21.3, is lost among a cluster of neighboring tumor suppressor genes. Although it is classically known for its capacity to inhibit cyclin-dependent kinase (CDK) activity, p16(INK4a) is not just a one-trick pony. Long-term p16(INK4a) expression pushes cells to enter senescence, an irreversible cell-cycle arrest that precludes the growth of would-be cancer cells but also contributes to cellular aging. Importantly, loss of p16(INK4a) is one of the most frequent events in human tumors and allows precancerous lesions to bypass senescence. Therefore, precise regulation of p16(INK4a) is essential to tissue homeostasis, maintaining a coordinated balance between tumor suppression and aging. This review outlines the molecular pathways critical for proper p16(INK4a) regulation and emphasizes the indispensable functions of p16(INK4a) in cancer, aging, and human physiology that make this gene special.

Lack of prognostic significance of p16 and p27 after radical prostatectomy in hormone-naïve prostate cancer

BACKGROUND

Loss of normal cell cycle control is an early event in the evolution of cancer. The expression of cyclin-dependent kinase (CDK) inhibitors p16 and p27 has been previously associated with progression of prostate cancer (PC). 70 patients diagnosed with early stage PCwere treated with radical prostatectomy (RP) at our institution and their tumor specimens were immunohistochemically evaluated for expression of p16 and p27. Available clinical data of time to PSA recurrence were correlated with the examined parameters and combined with pre-operative PSA level, Gleason score and pathological TNM (pT) stage assessment.

RESULTS

Nuclear overexpression of p16 was not associated with time to biochemical failure (BF) (p = 0.572). Same was the case for nuclear p27 overexpression (p = 1.000). Also, no significant correlations were found between either p16 or p27, and pre-operative PSA level, pT stage and Gleason grade. pT stage emerged as the only independent prognostic factor for biochemical recurrence (p = 0.01).

CONCLUSIONS

These data question previously reported data supporting the prognostic relevance of both p16 and p27 proteins in early PC.

Dual loss of rb1 and Trp53 in the adrenal medulla leads to spontaneous pheochromocytoma

Using a Cre/loxP system, we have determined the phenotypic consequences attributable to in vivo deletion of both Rb1 and Trp53 in the mouse adrenal medulla. The coablation of these two tumor suppressor genes during embryogenesis did not disrupt adrenal gland development but resulted in the neoplastic transformation of the neural crest-derived adrenal medulla, yielding pheochromocytomas (PCCs) that developed with complete penetrance and were inevitably bilateral. Despite their typically benign status, these PCCs had profound ramifications on mouse vitality, with effected mice having a median survival of only 121 days. Evaluation of these PCCs by both immunohistochemistry and electron microscopy revealed that most Rb1(-/-):Trp53(-/-) chromaffin cells possessed atypical chromagenic vesicles that did not seem capable of appropriately storing synthesized catecholamines. The structural remodeling of the heart in mice harboring Rb1(-/-):Trp53(-/-) PCCs suggests that the mortality of these mice may be attributable to the inappropriate release of catecholamines from the mutated adrenal chromaffin cells. On the basis of the collective data from Rb1 and Trp53 knockout mouse models, it seems that the conversion of Rb1 loss-driven adrenal medulla hyperplasia to PCC can be greatly enhanced by the compound loss of Trp53, whereas the loss of Trp53 alone is generally ineffectual on adrenal chromaffin cell homeostasis. Consequently, the Trp53 tumor suppressor gene is an efficient genetic modifier of Rb1 loss in the development of PCC, and their compound loss in the adrenal medulla has a profound impact on both cellular homeostasis and animal vitality.

Bmi1 regulates stem cells and proliferation and differentiation of committed cells in mammary epithelium

PolycombGroup (PcG) proteins are epigenetic silencers involved in maintaining cellular identity, and their deregulation can result in cancer [1]. Mice without the PcG gene Bmi1 are runted and suffer from progressive loss of hematopoietic and neural stem cells [2-4]. Here, we assess the effects of Bmi1 on stem cells and differentiation of an epithelial tissue in vivo. We chose the mammary gland because it allows limiting dilution transplantations [5, 6] and because Bmi1 is overexpressed in breast cancer [7, 8]. Our analyses show that Bmi1 is expressed in all cells of the mouse mammary gland and is especially high in luminal cells. Loss of Bmi1 results in a severe mammary-epithelium growth defect, which can be rescued by codeletion of the Ink4a/Arf locus or pregnancy. Even though mammary stem cells are present in the absence of Bmi1, their activity is reduced, and this is only partially due to Ink4a/Arf expression. Interestingly, loss of Bmi1 causes premature lobuloalveolar differentiation, whereas overexpression of Bmi1 inhibits lobuloalveolar differentiation induced by pregnancy hormones. Because Bmi1 affects not only mammary stem cells but also more committed cells, our data warrant a more detailed analysis of the different roles of Bmi1 in breast-cancer etiology.

Molecular mechanism underlying the functional loss of cyclindependent kinase inhibitors p16 and p27 in hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is one of the most common human cancers, and its incidence is still increasing in many countries. The prognosis of HCC patients remains poor, and identification of useful molecular prognostic markers is required. Many recent studies have shown that functional alterations of cell-cycle regulators can be observed in HCC. Among the various types of cell-cycle regulators, p16 and p27 are frequently inactivated in HCC and are considered to be potent tumor suppressors. p16, a G1-specific cell-cycle inhibitor that prevents the association of cyclindependent kinase (CDK) 4 and CDK6 with cyclin D1, is frequently inactivated in HCC via CpG methylation of its promoter region. p16 may be involved in the early steps of hepatocarcinogenesis, since p16 gene methylation has been detected in subsets of pre-neoplastic liver cirrhosis patients. p27, a negative regulator of the G1-S phase transition through inhibition of the kinase activities of Cdk2/cyclin A and Cdk2/cyclin E complexes, is now considered to be an adverse prognostic factor in HCC. In some cases of HCC with increased cell proliferation, p27 is overexpressed but inactivated by sequestration into cyclin D1-CDK4-containing complexes. Since loss of p16 is closely related to functional inactivation of p27 in HCC, investigating both p16 and p27 may be useful for precise prognostic predictions in individuals with HCC.

A CDKN2A mutation in familial melanoma that abrogates binding of p16INK4a to CDK4 but not CDK6

The CDKN2A locus encodes two distinct proteins, p16INK4a and p14ARF, both of which are implicated in replicative senescence and tumor suppression in different contexts. Here, we describe the characterization of a novel strain of human diploid fibroblasts (designated Milan HDFs) from an individual who is homozygous for the R24P mutation in p16INK4a. As this mutation occurs in the first exon of INK4a (exon 1alpha), it has no effect on the primary sequence of p14(ARF). Based on both in vitro and in vivo analyses, the R24P variant is specifically defective for binding to CDK4 but remains able to associate with CDK6. Nevertheless, Milan HDFs behave as if they are p16INK4a deficient, in terms of sensitivity to spontaneous and oncogene-induced senescence, and the R24P variant has little effect on proliferation when ectopically expressed in normal fibroblasts. It can, however, impair the proliferation of U20S cells, presumably because they express more CDK6 than primary fibroblasts. These observations suggest that CDK4 and CDK6 are not functionally redundant and underscore the importance of CDK4 in the development of melanoma.

p21Waf1/Cip1/Sdi1 mediates retinoblastoma protein degradation

Damage-induced G1 checkpoint in mammalian cells involves upregulation of p53, which activates transcription of p21(Waf1) (CDKN1A). Inhibition of cyclin-dependent kinase (CDK)2 and CDK4/6 by p21 leads to dephosphorylation and activation of Rb. We now show that ectopic p21 expression in human HT1080 fibrosarcoma cells causes not only dephosphorylation but also depletion of Rb; this effect was p53-independent and susceptible to a proteasome inhibitor. CDK inhibitor p27 (CDKN1B) also caused Rb dephosphorylation and depletion, but another CDK inhibitor p16 (CDKN2A) induced only dephosphorylation but not depletion of Rb. Rb depletion was observed in both HT1080 and HCT116 colon carcinoma cells, where p21 was induced by DNA-damaging agents. Rb depletion after DNA damage did not occur in the absence of p21, and it was reduced when p21 induction was inhibited by p21-targeting short hairpin RNA or by a transdominant inhibitor of p53. These results indicate that p21 both activates Rb through dephosphorylation and inactivates it through degradation, suggesting negative feedback regulation of damage-induced cell-cycle checkpoint arrest.

CDK4 and CDK6 delay senescence by kinase-dependent and p16INK4a-independent mechanisms

Replicative senescence of human diploid fibroblasts (HDFs) is largely implemented by the cyclin-dependent kinase (CDK) inhibitors p16(INK4a) and p21(CIP1). Their accumulation results in a loss of CDK2 activity, and cells arrest with the retinoblastoma protein (pRb) in its hypophosphorylated state. It has become standard practice to bypass the effects of p16(INK4a) by overexpressing CDK4 or a variant form that is unable to bind to INK4 proteins. Although CDK4 and CDK6 and their INK4-insensitive variants can extend the life span of HDFs, they also cause a substantial increase in the levels of endogenous p16(INK4a). Here we show that CDK4 and CDK6 can extend the life span of HDFs that have inactivating mutations in both alleles of INK4a or in which INK4a levels are repressed, indicating that overexpression of CDK4/6 is not equivalent to ablation of p16(INK4a). However, catalytically inactive versions of these kinases are unable to extend the replicative life span, suggesting that the impact of ectopic CDK4/6 depends on their ability to phosphorylate as yet unidentified substrates rather than to sequester CDK inhibitors. Since p16(INK4a) deficiency, CDK4 expression, and p53 or p21(CIP1) ablation have additive effects on replicative life span, our results underscore the idea that senescence is an integrated response to diverse signals.

Cell cycle control of embryonic stem cells

Embryonic stem cells have the capacity for unlimited proliferation while retaining their potential to differentiate into a wide variety of cell types. Murine, primate and human embryonic stem cells (ESCs) exhibit a very unusual cell cycle structure, characterized by a short G1 phase and a high proportion of cells in S-phase. In the case of mESCs, this is associated with a unique mechanism of cell cycle regulation, underpinned by the precocious activity of cyclin dependent protein kinase (Cdk) activities. As ES cells differentiate, their cell cycle structure changes dramatically so as to incorporate a significantly longer G1 phase and their mechanism of cell cycle regulation changes to that typically seen in other mammalian cells. The unique cell cycle structure and mechanism of cell cycle control indicates that the cell cycle machinery plays a role in establishment or maintenance of the stem cell state. This idea is supported by the frequent involvement of cell cycle regulatory molecules in cell immortalization.

p16 and p27 are functionally correlated during the progress of hepatocarcinogenesis

The molecular mechanism of the cell-cycle machinery in hepatocellular carcinoma (HCC) has not yet been fully elucidated. Among the various types of cell-cycle regulators, p16 and p27 are now considered to be potent tumor suppressors. p16 is a G1-specific cell-cycle inhibitor that prevents the association of cyclin-dependent kinase (CDK) 4 and CDK6 with cyclin D(1). Many studies have reported that p16 is inactivated not only in aggressive types of HCC but also in preneoplastic liver cirrhosis. In many cases of HCC, p16 is mainly inactivated by extensive CpG methylation, suggesting that epigenetic changes in the p16 gene may be important events during hepatocarcinogenesis. p27, an inhibitor of CDK2, is presently regarded as a potent adverse prognostic factor in many aggressive cancers. It should be noted that some cases of HCC show increased cell proliferation despite the expression of considerable amounts of p27. In these cases, p27 is inactivated by sequestration into cyclin D(1)-CDK4-containing complexes. Although the reason for the compositional changes in the p27-containing complexes is unclear, our experimental results indicate that loss of p16 following DNA methylation is closely related to the functional inactivation of p27 in HCC. We suggest that assessment of the p16 status may be useful for a precise prognostic prediction for individuals with HCCs expressing high levels of p27.

The status of CDKN2A alpha (p16INK4A) and beta (p14ARF) transcripts in thyroid tumour progression

CDKN2A locus on chromosome 9p21 encodes two tumour suppressor proteins pl6^INK4A, which is a regulator of the retinoblastoma (RB) protein, and p14^ARF, which is involved in the ARF–Mdm2–p53 pathway. The aim of this study was to determine if CDKN2A gene products are implicated in differentiated thyroid carcinogenesis and progression. We used real-time quantitative RT–PCR and immunohistochemistry to assess both transcripts and proteins levels in 60 tumours specimens. Overexpression of p14^ARF and pl6^INK4A was observed in follicular adenomas, follicular carcinomas and papillary carcinomas, while downregulation was found in oncocytic adenomas compared to nontumoral paired thyroid tissues. These deregulations were statistically significant for pl6^INK4a (P=0.006) in follicular adenomas and close to statistical significance for p14^ARF in follicular adenomas (P=0.06) and in papillary carcinomas (P=0.05). In all histological types, except papillary carcinomas, we observed a statistically significant relationship between p14^ARF and E2F1 (r=0.64 to 1, P<0.05). Our data are consistent with involvement of CDKN2A transcript upregulation in thyroid follicular tumorigenesis as an early event. However, these deregulations do not appear to be correlated to the clinical outcome and they could not be used as potential prognostic markers.

Distinct action of the retinoblastoma pathway on the DNA replication machinery defines specific roles for cyclin-dependent kinase complexes in prereplication complex assembly and S-phase progression

The retinoblastoma (RB) and p16ink4a tumor suppressors are believed to function in a linear pathway that is functionally inactivated in a large fraction of human cancers. Recent studies have shown that RB plays a critical role in regulating S phase as a means for suppressing aberrant proliferation and controlling genome stability. Here, we demonstrate a novel role for p16ink4a in replication control that is distinct from that of RB. Specifically, p16ink4a disrupts prereplication complex assembly by inhibiting mini-chromosome maintenance (MCM) protein loading in G1, while RB was found to disrupt replication in S phase through attenuation of PCNA function. This influence of p16ink4a on the prereplication complex was dependent on the presence of RB and the downregulation of cyclin-dependent kinase (CDK) activity. Strikingly, the inhibition of CDK2 activity was not sufficient to prevent the loading of MCM proteins onto chromatin, which supports a model wherein the composite action of multiple G1 CDK complexes regulates prereplication complex assembly. Additionally, p16ink4a attenuated the levels of the assembly factors Cdt1 and Cdc6. The enforced expression of these two licensing factors was sufficient to restore the assembly of the prereplication complex yet failed to promote S-phase progression due to the continued absence of PCNA function. Combined, these data reveal that RB and p16ink4a function through distinct pathways to inhibit the replication machinery and provide evidence that stepwise regulation of CDK activity interfaces with the replication machinery at two discrete execution points.

Pocket protein function in melanocyte homeostasis and neoplasia

Melanoma is the most lethal of human skin cancers and its incidence is increasing worldwide [L.K. Dennis (1999). Arch. Dermatol. 135, 275; C. Garbe et al. (2000). Cancer 89, 1269]. Melanomas often metastasize early during the course of the disease and are then highly intractable to current therapeutic regimens [M.F. Demierre and G. Merlino (2004). Curr. Oncol. Rep. 6, 406]. Consequently, understanding the factors that maintain melanocyte homeostasis and prevent their neoplastic transformation into melanoma is of utmost interest from the perspective of therapeutic interdiction. This review will focus on the role of the pocket proteins (PPs), Rb1 (retinoblastoma protein), retinoblastoma-like 1 (Rbl1 also known as p107) and retinoblastoma-like 2 (Rbl2 also known as p130), in melanocyte homeostasis, with particular emphasis on their functions in the cell cycle and the DNA damage repair response. The potential mechanisms of PP deregulation in melanoma and the possibility of PP-independent pathways to melanoma development will also be considered. Finally, the role of the PP family in ultraviolet radiation (UVR)-induced melanoma and the precise contribution that each PP family member makes to melanocyte homeostasis will be discussed in the context of a number of genetically engineered mouse models.

Cyclin-dependent kinase pathways as targets for cancer treatment

Cyclin-dependent kinases (cdks) are critical regulators of cell cycle progression and RNA transcription. A variety of genetic and epigenetic events cause universal overactivity of the cell cycle cdks in human cancer, and their inhibition can lead to both cell cycle arrest and apoptosis. However, built-in redundancy may limit the effects of highly selective cdk inhibition. Cdk4/6 inhibition has been shown to induce potent G1 arrest in vitro and tumor regression in vivo; cdk2/1 inhibition has the most potent effects during the S and G2 phases and induces E2F transcription factor-dependent cell death. Modulation of cdk2 and cdk1 activities also affects survival checkpoint responses after exposure to DNA-damaging and microtubule-stabilizing agents. The transcriptional cdks phosphorylate the carboxy-terminal domain of RNA polymerase II, facilitating efficient transcriptional initiation and elongation. Inhibition of these cdks primarily affects the accumulation of transcripts with short half-lives, including those encoding antiapoptosis family members, cell cycle regulators, as well as p53 and nuclear factor-kappa B-responsive gene targets. These effects may account for apoptosis induced by cdk9 inhibitors, especially in malignant hematopoietic cells, and may also potentiate cytotoxicity mediated by disruption of a variety of pathways in many transformed cell types. Current work is focusing on overcoming pharmacokinetic barriers that hindered development of flavopiridol, a pan-cdk inhibitor, as well as assessing novel classes of compounds potently targeting groups of cell cycle cdks (cdk4/6 or cdk2/1) with variable effects on the transcriptional cdks 7 and 9. These efforts will establish whether the strategy of cdk inhibition is able to produce therapeutic benefit in the majority of human tumors.

Hepatocyte Growth Factor Induces Redistribution of p21CIP1 and p27KIP1 through ERK-dependent p16INK4a Up-regulation, Leading to Cell Cycle Arrest at G1 in HepG2 Hepatoma Cells

Hepatocyte growth factor (HGF) has an anti-proliferative effect on many types of tumor cell lines and tumors in vivo. We found previously that inhibition of HGF-induced proliferation in HepG2 hepatoma cells is caused by cell cycle arrest at G1 through a high intensity ERK signal, which represses Cdk2 activity. To examine further the mechanisms of G1 arrest by HGF, we analyzed the Cdk inhibitor p16(INK4a), which has an anti-proliferative function through cell cycle arrest at G1. We found that HGF treatment drastically increased endogenous p16 levels. Knockdown of p16 with small interfering RNA reversed the arrest, indicating that the induction of p16 is required for G1 arrest by HGF. Analysis of the promoter of the human p16 gene identified the proximal Ets-binding site as a responsive element for HGF, and this responded to the high intensity ERK signal. HGF treatment of the cells led to a redistribution of p21(CIP1) and p27(KIP1) from Cdk4 to Cdk2. The redistribution was blocked by the knockdown of p16 with small interfering RNA, which restored the Cdk2 activity repressed by HGF, demonstrating the requirement of p16 induction for the redistribution and eventual repression of Cdk2 activity. Our results reveal a signaling pathway for G1 arrest induced by HGF.

Induction of p21 by p53 following DNA damage inhibits both Cdk4 and Cdk2 activities

DNA damage often activates the p53-p21 pathway and causes G(1)-phase arrest in mammalian cells. Although there is ample evidence that p21 induction by p53 leads to Cdk2 inhibition, it is unclear whether this checkpoint event also leads to Cdk4 inhibition. Diaminocyclohexane(trans-diacetato)(dichloro) platinum(IV) (DAP), a platinum-based coordination complex, is a DNA-damaging agent that is effective against a variety of tumor cells resistant to the parental drug cisplatin. Our previous studies established that treatment of human cancer cells with low effective concentrations of DAP specifically activates the G(1)-phase checkpoint and simultaneously inhibit Cdk4 and Cdk2 activities. Here we demonstrate that DAP treatment of human cancer cells activates the p53-p21 pathway without activating other known mechanisms that inhibit Cdk4 and Cdk2 activities. The induced p21 binds to both the Cdk4/cyclin D and Cdk2/cyclin E complexes and inhibits both of their kinase activities. Conversely, inhibition of p21 induction by cycloheximide or by p21 gene deletion prevents DAP-induced inhibition of Cdk4 and Cdk2 activities. Attenuated p53 expression and p21 induction also eliminates DAP-induced G(1)-phase arrest and inhibition of Cdk4 and Cdk2 activities. Together, these findings establish that activation of the p53-p21 pathway is responsible for the DAP-induced G(1)-phase checkpoint response and provide the first solid evidence that p21 induction by p53 during a DNA damage-induced G(1)-phase checkpoint response inhibits both Cdk4 and Cdk2 activities.

Biochemical and cellular mechanisms of mammalian CDK inhibitors: a few unresolved issues

p21 and p16, first identified as two small molecular weight proteins in CDK and cyclin immunocomplexes, represent two distinct families constituting a total of seven CDK inhibitors in mammalian cells. The physiological functions of these genes are believed to be broadly involved in connecting various cellular pathways to cell cycle control. Extensive studies over the past 10 years have led to a fairly clear understanding of their biochemical and cellular mechanisms and have also left some unresolved and controversial issues.

A common set of gene regulatory networks links metabolism and growth inhibition

Using genome-wide analysis of transcription factor occupancy, we investigated the mechanisms underlying three mammalian growth arrest pathways that require the pRB tumor suppressor family. We found that p130 and E2F4 cooperatively repress a common set of genes under each growth arrest condition and showed that growth arrest is achieved through repression of a core set of genes involved not only in cell cycle control but also mitochondrial biogenesis and metabolism. Motif-finding algorithms predicted the existence of nuclear respiratory factor-1 (NRF1) binding sites in E2F target promoters, and genome-wide factor binding analysis confirmed our predictions. We showed that NRF1, a factor known to regulate expression of genes involved in mitochondrial function, is a coregulator of a large number of E2F target genes. Our studies provide insights into E2F regulatory circuitry, suggest how factor occupancy can predict the expression signature of a given target gene, and reveal pathways deregulated in human tumors.

An Rb-Skp2-p27 pathway mediates acute cell cycle inhibition by Rb and is retained in a partial-penetrance Rb mutant

It is believed that Rb blocks G1-S transition by inhibiting expression of E2F regulated genes. Here, we report that the effects of E2F repression lag behind the onset of G1 cell cycle arrest in timed Rb reexpression experiments. In comparison, kinase inhibitor p27Kip1 protein accumulates with a faster kinetics. Conversely, Rb knockout leads to faster p27 degradation. Rb interacts with the N terminus of Skp2, interferes with Skp2-p27 interaction, and inhibits ubiquitination of p27. Disruption of p27 function or expression of the Skp2 N terminus prevents Rb from causing G1 arrest. A full-penetrance, inactive Rb mutant fails to interfere with Skp2-p27 interaction but, interestingly, a partial-penetrance Rb mutant that is defective for E2F binding retains full activity in inhibiting Skp2-p27 interaction and can induce G1 cell cycle arrest with wild-type kinetics. These results identify an Rb-Skp2-p27 pathway in Rb function, which may be involved in inhibition of tumor progression.

Modifications in cell cycle kinetics and in expression of G1 phase-regulating proteins in human amniotic cells after exposure to electromagnetic fields and ionizing radiation

Low-frequency electromagnetic fields are suspected of being involved in carcinogenesis, particularly in processes that could be related to cancer promotion. Because development of cancer is associated with deregulated cell growth and we previously observed a magnetic field-induced decrease in DNA synthesis [Lange et al. (2002) Alterations in the cell cycle and in the protein level of cyclin D1p, 21CIP1, and p16INK4a after exposure to 50 HZ. MF in human cells. Radiat. Environ. Biophys.41, 131], this study aims to document the influence of 50 Hz, 1 mT magnetic fields (MF), with or without initial gamma-ionizing radiation (IR), on the following cell proliferation-relevant parameters in human amniotic fluid cells (AFC): cell cycle distribution, expression of the G1 phase-regulating proteins Cdk4, cyclin D1, p21CIP1 and p16INK4a, and Cdk4 activity. While IR induced a G1 delay and a dose-dependent G2 arrest, no discernible changes in cell cycle kinetics were observed due to MF exposure. However, a significant decrease in the protein expression of cyclin D1 and an increase in p21CIP1- and p16INK4a-expression could be detected after exposure to MF alone. IR-exposure caused an augmentation of p21CIP1- and p16INK4a- levels as well, but did not alter cyclin D1 expression. A slight diminution of Cdk4 activity was noticed after MF exposure only, indicating that Cdk4 appears not to act as a mediator of MF- or IR-induced changes in the cell cycle of AFC cells. Co-exposure to MF/IR affected neither cell cycle distribution nor protein expression or kinase activity additionally or synergistically, and therefore MF seems not to modify the mutagenic potency of IR.

The cell cycle: accelerators, brakes, and checkpoints

PROLIFERATIVE CUES TRIGGER a complex series of molecular signaling events in cells. Early in the cell cycle, cells are faced with an important decision that affects their fate. They either initiate a round of replication or they withdraw from cell division. Passage through the restriction point, or "point of no return," marks cellular commitment to a new round of division. Genetic mutations that predispose individuals to tumorigenesis often affect pathways that influence cellular proliferation. Many of the mutated genes give rise to molecules that are no longer able to appropriately regulate the mammalian cell cycle; the end result is neoplasia. In this review, the critical elements that permit cell cycle progression and the positive and negative regulators that affect the process are reviewed.

Cyclin-dependent kinase inhibition by the KLF6 tumor suppressor protein through interaction with cyclin D1

Kruppel-like factor 6 (KLF6) is a tumor suppressor gene inactivated in prostate and colon cancers, as well as in astrocytic gliomas. Here, we establish that KLF6 mediates growth inhibition through an interaction with cyclin D1, leading to reduced phosphorylation of the retinoblastoma protein (Rb) at Ser(795). Furthermore, introduction of KLF6 disrupts cyclin D1-cyclin-dependent kinase (cdk) 4 complexes and forces the redistribution of p21(Cip/Kip) onto cdk2, which promotes G(1) cell cycle arrest. Our data suggest that KLF6 converges with the Rb pathway to inhibit cyclin D1/cdk4 activity, resulting in growth suppression.

Induction of Androgen Receptor-Dependent Apoptosis in Prostate Cancer Cells by the Retinoblastoma Protein

Re-expression of a tumor suppressor in tumor cells that lack it is an effective way to study its functional activities. However, because tumor cells contain multiple mutations, tumor suppressor functions that are dependent on (an)other regulators are unlikely to be identified by its re-expression alone if the other regulators are also mutated. In this study, we show that re-expression of retinoblastoma (RB) together with the androgen receptor (AR) in RB- and AR-deficient prostate cancer DU-145 cells resulted in an apoptotic activity, acting through the mitochondria damage-initiated caspase activation pathway, which was not present when RB, or the AR, was re-expressed alone. The ability of RB + AR to induce mitochondria damage was dependent on the proapoptotic proteins Bax and Bak and could be blocked by the antiapoptotic protein Bcl-x(L). Coexpressed AR did not detectably change RB's regulation of E2F and cell cycle progression in culture. On the other hand, coexpressed RB could activate the transactivation activity of the AR in an androgen-depleted media. Although androgen induced greater AR transactivation activity in this condition, it did not induce apoptosis in the absence of coexpressed RB. Analysis of mutants of RB and the AR indicated that intact pocket function of RB and the transactivation activity of the AR were required for RB + AR-induced apoptosis. These results provide direct functional data for an AR-dependent apoptosis-inducing activity of RB and highlight the importance of cell type-specific regulators in obtaining a more complete understanding of RB.

NC381, a novel anticancer agent, arrests the cell cycle in G0-G1 and inhibits lung tumor cell growth in vitro and in vivo

Although clotrimazole (CLT), an antifungal drug, inhibits tumor cell proliferation and angiogenesis, its clinical application is hampered by significant hepatotoxicity due to the presence of an imidazole moiety. In our attempts to develop CLT analogs that are devoid of imidazole and are as efficacious as CLT, one pharmacophore designated NC381 was generated and shown to inhibit tumor cell growth via a mechanism similar to that of CLT. In vitro, treatment of NCI-H460 nonsmall cell lung cancer (NSCLC) cells with NC381 inhibited growth in a time-dependent manner. Flow cytometric analysis demonstrated that the decrease in cell growth was associated with inhibition of cell cycle progression at the G(1)-S phase transition, resulting in G(0)-G(1) arrest. There was a concomitant inhibition of cyclin D1 expression and subsequent reduction in the formation of the cyclin D1-CDK4 complex. Consistent with a decrease in the cyclin D1-CDK4 complex, NC381 treatment resulted in significant inhibition of pRb phosphorylation. There also were changes in the activity of cell cycle-related proteins, including p16(Ink4) and p27(Kip1). Together, these results are consistent with a model in which NC381 arrests cell cycle progression via inhibition of the pathway that promotes exit from the G(1) phase of the cell cycle. Furthermore, the clinical applicability of NC381 was evaluated in an in vivo murine xenograft model of human NSCLC (NCI-H460). NC381 treatment resulted in significant inhibition of tumor growth. Given the poor prognosis and the limited treatment options available, the present results underscore the potential of NC381 in the treatment of human NSCLC.

EZH2 is downstream of the pRB-E2F pathway, essential for proliferation and amplified in cancer

Recent experiments have demonstrated that the Polycomb group (PcG) gene EZH2 is highly expressed in metastatic prostate cancer and in lymphomas. EZH2 is a component of the PRC2 histone methyltransferase complex, which also contains EED and SUZ12 and is required for the silencing of HOX gene expression during embryonic development. Here we demonstrate that both EZH2 and EED are essential for the proliferation of both transformed and non-transformed human cells. In addition, the pRB-E2F pathway tightly regulates their expression and, consistent with this, we find that EZH2 is highly expressed in a large set of human tumors. These results raise the question whether EZH2 is a marker of proliferation or if it is actually contributing to tumor formation. Significantly, we propose that EZH2 is a bona fide oncogene, since we find that ectopic expression of EZH2 is capable of providing a proliferative advantage to primary cells and, in addition, its gene locus is specifically amplified in several primary tumors.

Identification of target genes of the p16INK4A-pRB-E2F pathway

Deregulation of the retinoblastoma protein (pRB) pathway is a hallmark of human cancer. The core members of this pathway include the tumor suppressor protein, pRB, which through binding to a number of cellular proteins, most notably members of the E2F transcription factor family, regulates progression through the cell division cycle. With the aim of identifying transcriptional changes provoked by deregulation of the pRB pathway, we have used cell lines that conditionally express a constitutively active phosphorylation site mutant of pRB (pRBDeltaCDK) or p16INK4A (p16). The expression of pRBDeltaCDK and p16 resulted in significant repression and activation of a large number of genes as measured by high density oligonucleotide array analysis. Transcriptional changes were found in genes that are essential for DNA replication and cell proliferation. In agreement with previous results, we found a high degree of overlap between genes regulated by p16 and pRB. Data we have obtained previously for E2F family members showed that 74 of the genes repressed by pRB and p16 were induced by the E2Fs and 23 genes that were induced by pRB and p16 were repressed by the E2Fs. Thus, we have identified 97 genes as physiological targets of the pRB pathway, and the further characterization of these genes should provide insights into how this pathway controls proliferation. We show that Gibbs sampling detects enrichment of several sequence motifs, including E2F consensus binding sites, in the upstream regions of these genes and use this enrichment in an in silico filtering process to refine microarray derived gene lists.

Inhibition of Cdk4 activity enhances translation of p27kip1 in quiescent Rb-negative cells

We show in this work that the inhibition of Cdk4 (6) in Rb(-/-) 3T3 cells enhances the accumulation of the p27(kip1) cyclin-dependent kinase inhibitor when these cells are induced into quiescence. Two different forms of inhibition of Cdk4 (6), namely overexpression of the Cdk4 (6) inhibitor p16 and overexpression of a dominant negative mutant of Cdk4 (Cdk4(N158)), result in this effect. This suggests that the relevant activity of Cdk4 (6) that has to be inactivated in this setting is its kinase activity. The accumulation of p27(kip1) is due to enhanced translation of the protein, mediated by the 3'-untranslated region of the p27(kip1) mRNA. Moreover, the cells that overexpress p16(ink4a) or Cdk4(N158) show a delay in G(1) when made quiescent and restimulated to proliferate. This delay is overcome by transfection of a plasmid expressing antisense p27(kip1), which shows that the accumulation of p27(kip1) in these cells is related to their G(1) delay. In summary, we report a new functional link between two important cell cycle regulators, Cdk4 and p27(kip1), and provide a mechanistic explanation to the previously reported epistatic relations between these two proteins.

Proliferation of cancer cells despite CDK2 inhibition

We have investigated the contribution of CDK4 and CDK2 inhibition to G1 arrest in colon cancers following inhibition of the MEK/MAP kinase pathway. CDK4 inhibition is sufficient to cause arrest, but inhibition of CDK2 by p27 Kip1 redistribution or ectopic expression has no effect on proliferation. Likewise, inhibition of CDK2 through expression of dominant-negative (DN) CDK2 or antisense oligonucleotides did not prevent cell proliferation in these cells. We therefore tested whether CDK2 activity is dispensable in other cells. Surprisingly, osteosarcomas and Rb-negative cervical cancers continued to proliferate after depletion of CDK2 through antisense oligonucleotides or small interfering (si) RNA. Here we report of sustained cell proliferation in the absence of CDK2, and we suggest that CDK2 is not a suitable target for cancer therapy.

Human cytomegalovirus immediate-early protein 2 (IE2)-mediated activation of cyclin E is cell-cycle-independent and forces S-phase entry in IE2-arrested cells

In human cytomegalovirus (HCMV) infection, the isolated expression of the viral immediate-early protein 2 (IE2) 86 kDa regulatory protein coincides with an up-regulation of cyclin E gene expression, both in fibroblasts and U373 cells. Since IE2 also interferes with cell-cycle progression, it is unclear whether IE2 is a genuine activator of cyclin E or whether IE2-arrested cells contain elevated levels of cyclin E primarily as a consequence of them being arrested at the beginning of S phase. It is important to distinguish between these possibilities in order to define and analyse at a mechanistic level the proliferative and anti-proliferative capacities of IE2. Here we have shown that IE2 can activate cyclin E independent of the cell-cycle state and can therefore function as a genuine activator of cyclin E gene expression. A mutant of IE2 that failed to activate cyclin E also failed to promote G1/S transition. Instead, cells became arrested in G1. S-phase entry could be rescued in these cells by co-expression of cyclin E, but these cells still arrested in early S phase, as is the case with wild-type IE2. Our data demonstrate that IE2 can promote two independent cell-cycle functions at the same time: (i) the induction of G1/S transition via up-regulation of cyclin E, and (ii) a block in cell-cycle progression in early S phase. In G1, the proliferative activity of IE2 appears to be dominant over the anti-proliferative force, whereas after G1/S transition, this situation is reversed.

RB and cyclin dependent kinase pathways: defining a distinction between RB and p16 loss in lung cancer

The genetic components of the RB:CDK:cyclin:p16 tumor suppressor pathway undergo mutational and epigenetic alterations in a wide range of human cancers and serve as critical targets for inactivation by the transforming oncoproteins of several DNA tumor viruses. Lung cancer has been a useful model system for these studies as it was the first tumor to demonstrate an important role for RB in the genesis of a common adult malignancy and was also the first human cancer to demonstrate genetic evidence for a multi-component RB:p16 tumor suppressor pathway. Lung tumorigenesis, however, is a complex disease process that requires longstanding carcinogen exposure in order to acquire somatic alterations at many distinct genetic loci. Understanding the multifunctional properties of RB to regulate cell proliferation, differentiation, and apoptosis and how they relate to the sequential accumulation of other clonal gene defects will be essential in order to understand the specific patterns of gene inactivation observed in different subtypes of lung cancer and to fulfill the promise of 'molecular target' therapeutics.

Transcriptional activation by the oestrogen receptor alpha is modulated through inhibition of cyclin-dependent kinases

We have investigated the interaction between the expression of p21(WAF1/CIP1/SDI1), a stoichiometric inhibitor of Cdk, and the transcriptional activity of the oestrogen receptor alpha (ER(alpha). Transient transfection experiments demonstrated that the expression of p21(WAF1/CIP1/SDI1) amplified the transcriptional activation by ER(alpha). A dominant negative mutant of Cdk2 also enhanced the ER(alpha) transcriptional activity, indicating that the underlying mechanism relies on the inhibition of Cdk2 activity and cell cycle arrest. In agreement with this conclusion, experiments with p21(WAF1/CIP1/SDI1) mutants demonstrated that the domain involved in the binding of p21(WAF1/CIP1/SDI1) to Cdks was indispensable for the modulation of ER(alpha) activity. In addition, we show that expression of p21(WAF1/CIP1/SDI1) alleviates the block on CBP function mediated by Cdk2 and in turn stimulates transcriptional activation by ER(alpha) in a CBP-histone acetyltransferase (HAT)-dependent manner. These results suggest a novel mechanism by which p21(WAF1/CIP1/SDI1) functions as an enhancer of ER(alpha) activity through the modulation of CBP function.

DNA methylation and breast carcinogenesis

Knowledge about breast carcinogenesis has accumulated during the last decades but has barely been translated into strategies for early detection or prevention of this common disease. Changes in DNA methylation have been recognized as one of the most common molecular alterations in human neoplasia and hypermethylation of gene-promoter regions is being revealed as one of the most frequent mechanisms of loss of gene function. The heritability of methylation states and the secondary nature of the decision to attract or exclude methylation support the idea that DNA methylation is adapted for a specific cellular memory. According to Hanahan and Weinberg, there are six novel capabilities a cell has to acquire to become a cancer cell: limitless replicative potential, self-sufficiency in growth signals, insensitivity to growth-inhibitory signals, evasion of programmed cell death, sustained angiogenesis and tissue invasion and metastasis. This review highlights how DNA-methylation contributes to these features and offers suggestions about how these changes could be prevented, reverted or used as a 'tag' for early detection of breast cancer or, preferably, for detection of premalignant changes.

Active RB elicits late G1/S inhibition

The retinoblastoma tumor suppressor protein (RB) is activated/dephosphorylated to mediate cell cycle inhibition in response to antimitogenic signals. To elucidate the mode of RB action at this critical transition, we utilized cell lines that can be induced to express a constitutively active allele of RB (PSM-RB). As expected, induction of PSM-RB, but not wild-type protein (WT), inhibited progression into S phase. It has been well documented that active RB inhibits E2F reporter activity, and this observation was confirmed upon induction of PSM-RB. Additionally, active RB inhibited E2F-2-mediated stimulation of cyclin E. By contrast, PSM-RB did not affect the mRNA or protein levels of endogenous cyclin E when mediating cell cycle inhibition. Similarly, there was no observable effect on cyclin E protein levels when p16ink4a was utilized to activate endogenous RB. CDK2/cyclin E complex formation was not disrupted and cyclin E-associated kinase activity was retained in the presence of PSM-RB. Additionally, centrosome duplication, a CDK2/cyclin E-dependent event, was not altered in the presence of active RB. Together, these data indicate that active RB does not block the G1/S transition through inhibition of cyclin E expression or activity. In contrast, PSM-RB leads to a dramatic reduction in cyclin A protein levels by coordinate transcriptional repression and degradation. This attenuation of cyclin A protein correlates with cell cycle inhibition. These studies indicate that RB inhibits cell cycle progression by targeting CDK2/cyclin A-dependent events at the G1/S transition to inhibit cell cycle progression.

Removal of Cdk inhibitors through both sequestration and downregulation in zearalenone-treated MCF-7 breast cancer cells

Treatment of MCF 7 cells with the fungal estrogen zearalenone induced cyclin E-associated kinase activity transiently within 9-12 h; total cyclin-dependent kinase (Cdk) 2 activity was elevated for 24 h and beyond. This increased cyclin E/Cdk2 activity was associated with sequestration of the Cdk inhibitor p27 Cdk inhibitor 1B (p27(KIP1)) by newly formed cyclin D1/Cdk4 complexes and with downregulation of p27(KIP1) expression. The activation of cyclin A/Cdk2 activity corresponded with virtual elimination of p27(KIP1). The activity of cyclin E/Cdk2 complexes from zearalenone-treated lysates was inhibited in vitro by recombinant p27(KIP1), and this inhibition was relieved by the addition of recombinant cyclin D1/Cdk4 complexes. Thus, sequestration of p27(KIP1) by cyclin D1/Cdk4 resulted in activation of Cdk2 in vitro. Cdk inhibitory activity in lysates of zearalenone-treated cells was depleted by anti-p27(KIP1) and anti-Cdc2 interacting protein (p21(CIP1)) antibodies. Overexpression of the Cdk4/6-specific Cdk inhibitor of Cdk4 p16(INK4A) was associated with increased association of p27(KIP1) with Cdk2, concomitant with disruption of D cyclin/Cdk4 complexes. The proteasome inhibitor 2-leu-leu-leu-H aldehyde (MG-132) was relatively ineffective in inhibiting the initial, sequestration-dependent activation of cyclin E/Cdk2 yet was as effective as p16(INK4A) in inhibiting activation of cyclin A/Cdk2 later in G(1). Downregulation of p27(KIP1) proceeded in p16(INK4A)-expressing cells after zearalenone treatment, and G(1) arrest afforded by p16(INK4A) expression was reversible upon prolonged treatment with zearalenone. Zearalenone treatment of MCF-7 cells elicited expression of F-box protein S phase kinase-associated protein 2 (p45(SKP2)), a substrate-specific component of the ubiquitin-ligase complex that targets p27(KIP1) for degradation in the proteasome. These studies suggest that both sequestration of Cdk inhibitors by cyclin D1/Cdk4 complexes and downregulation of p27(KIP1) play major roles in the induction of Cdk2 activity and S phase entry elicited by estrogens in MCF-7 cells.

Rb localization and phosphorylation kinetics correlate with the cellular phenotype of cultured breast adenocarcinoma cells

Retinoblastoma protein (Rb) expression has been correlated with state of differentiation, proliferation rate, and metastatic potential in breast adenocarcinomas and established cell lines. These observations, based on immunoreactivity of total Rb rather than hypophosphorylated protein, do not address the relationship between functional Rb and indicators of an aggressive transformed cellular phenotype. We hypothesized that the distribution of functional Rb and the kinetics of Rb phosphorylation would differ between cell lines representing immortalized mammary epithelium (MCF10A), differentiated nonmetastatic mammary adenocarcinoma (MCF-7), and poorly differentiated, highly metastatic mammary adenocarcinoma (MDA-MB-231) and that these differences would be informative of the cellular phenotype. Direct immunofluorescence microscopy was used to compare qualitatively the subcellular localization of total and hypophosphorylated Rb protein in synchronized and asynchronous cells. This technique was also used to quantitatively assess the amounts of hypophosphorylated Rb throughout the cell cycle in these representative cell lines. Total Rb stained more prominently than hypophosphorylated Rb in the nucleus of all asynchronous cells. Rb phosphorylation was more rapid in MCF-7 cells than in MCF10A cells, whereas Rb dephosphorylation appeared deregulated in MDA-MB-231 cells. We conclude that assessment of hypophosphorylated Rb may be more useful than assessment of total Rb for the evaluation of transformed breast adenocarcinoma phenotypes.

Cyclin D-dependent kinases, INK4 inhibitors and cancer

The Cyclin D-Cdk4,6/INK4/Rb/E2F pathway plays a key role in controlling cell growth by integrating multiple mitogenic and antimitogenic stimuli. The components of this pathway are gene families with a high level of structural and functional redundancy and are expressed in an overlapping fashion in most tissues and cell types. Using classical transgenic technology as well as gene-targeting in ES cells, a series of mouse models have been developed to study the in vivo function of individual components of this pathway in both normal homeostasis and tumor development. These models have proven to be useful to define specific as well as redundant roles among members of these cell cycle regulatory gene families. This pathway is deregulated in the vast majority of human tumors by genetic and epigenetic alterations that target at least some of its key members such as Cyclin D1, Cdk4, INK4a and INK4b, pRb etc. As a consequence, some of these molecules are currently being considered as targets for cancer therapy, and several novel molecules, such as Cdk inhibitors, are under development as potential anti-cancer drugs.

DNA damage invokes mismatch repair-dependent cyclin D1 attenuation and retinoblastoma signaling pathways to inhibit CDK2

DNA-damage evokes cell cycle checkpoints, which function to maintain genomic integrity. The retinoblastoma tumor suppressor (RB) and mismatch repair complexes are known to contribute to the appropriate cellular response to specific types of DNA damage. However, the signaling pathways through which these proteins impact the cell cycle machinery have not been explicitly determined. RB-deficient murine embryo fibroblasts continued a high degree of DNA replication following the induction of cisplatin damage, but were inhibited for G(2)/M progression. This damage led to RB dephosphorylation/activation and subsequent RB-dependent attenuation of cyclin A and CDK2 activity. In both Rb+/+ and Rb -/- cells, cyclin D1 expression was attenuated following DNA damage. As cyclin D1 is a critical determinant of RB phosphorylation and cell cycle progression, we probed the pathway through which cyclin D1 degradation occurs in response to DNA damage. We found that attenuation of endogenous cyclin D1 is dependent on multiple mismatch repair proteins. We demonstrate that the mismatch repair-dependent attenuation of endogenous cyclin D1 is critical for attenuation of CDK2 activity and induction of cell cycle checkpoints. Together, these studies couple the activity of the retinoblastoma and mismatch repair tumor suppressor pathways through the degradation of cyclin D1 and dual attenuation of CDK2 activity.

Expression of P16(INK4a) in testis of rhesus monkey during heat stress and testosterone undecanoate induced azoospermia or oligozoospermia

Previous studies on azoospermia or oligozoospermia induced by heat stress or high doses of testosterone mainly focused on germ cell apoptosis; no data regarding their possible effect on spermatogonia mitosis are available. We have established unilateral cryptorchid and testosterone undecanoate (TU)-treated monkey models and examined expression of P16(INK4a) in the testis to look at its possible role in azoospermia or oligozoospermia induced by the heat stress or the TU treatment. The results showed that both heat stress and TU were capable of inducing expression of P16(INK4a) mainly in spermatogonia and other types of germ cells as well as Sertoli cells at the later stage of germ cell apoptosis, namely on Day 10 after operation or on Day 60 after TU injection. It is, therefore, suggested for the first time that P16(INK4a) protein may inhibit the spermatogonia mitosis in the testis at the later stage of the germ cell apoptosis, resulting in arrest of spermatogenesis.