Physical interaction of the retinoblastoma protein with human D cyclins

TIMP-1 regulation of cell cycle in human breast epithelial cells via stabilization of p27(KIP1) protein

Increasing evidence suggests that tissue inhibitor of metalloproteinases-1 (TIMP-1) can directly regulate cell growth and apoptosis independent of its matrix metalloproteinases (MMPs)-inhibitory activity. While TIMP-1's antiapoptotic activity has been well demonstrated, conflicting data has been reported regarding TIMP-1's role in growth regulation. Here we show that TIMP-1 reduces the growth rate of human breast epithelial (MCF10A) cells by inducing cell cycle arrest at G(1). TIMP-1-mediated cell cycle arrest is associated with its downregulation of cyclin D(1) and upregulation of p27(KIP1), resulting in inhibition of cyclin-dependent kinase activity necessary for phosphorylation of the tumor suppressor retinoblastoma protein. We further show that TIMP-1 modulation of cyclin D(1) and p27(KIP1) is achieved through TIMP-1-mediated differential regulation of protein stability independent of growth factor signaling. We also show that TIMP-1-mediated differential regulation of cyclin D(1) and p27(KIP1) is independent of cell adhesion signaling. Whereas approximately 50% of MCF10A cells with reduced TIMP-1 expression underwent cell death following loss of cell adhesion (anoikis), TIMP-1 overexpressing cells remained viable with prominent cell cycle arrest without detectable cell death. Taken together, we propose that TIMP-1-mediated cell survival independent of cell adhesion is accompanied with cell cycle arrest in human breast epithelial cells, although cell cycle regulation may not be a prerequisite for TIMP-1 regulation of apoptosis in general.

INSM1 functions as a transcriptional repressor of the neuroD/beta2 gene through the recruitment of cyclin D1 and histone deacetylases

INSM1/IA-1 (insulinoma-associated 1) is a developmentally regulated zinc-finger transcription factor, exclusively expressed in the foetal pancreas and nervous system, and in tumours of neuroendocrine origin. We have identified an INSM1 binding site in the neuroD/beta2 promoter and demonstrated transcriptional repressor activity of INSM1 by transient transfection assay. A chromatin immunoprecipitation assay confirmed that in vivo INSM1 is situated on the promoter region of the neuroD/beta2 gene. In an attempt to elucidate the molecular mechanism of transcriptional repression by the INSM1 gene, cyclin D1 was identified as an interacting protein by using a 45-day-old human foetal brain cDNA library and a yeast two-hybrid screen. The physical association between INSM1 and cyclin D1 was confirmed by in vitro and in vivo pull-down assay. Cyclin D1 co-operates with INSM1 and suppresses neuroD/beta2 promoter activity. Co-immunoprecipitation of INSM1, cyclin D1 and HDACs (histone deacetylases) in mammalian cells revealed that INSM1 interacts with HDAC-1 and -3 and that this interaction is mediated through cyclin D1. Overexpression of cyclin D1 and HDAC-3 significantly enhanced the transcriptional repression activity of INSM1 on the neuroD/beta2 promoter. A further chromatin immunoprecipitation assay confirmed that HDAC-3 occupies this same region of the neuroD/beta2 promoter, by forming a transcription complex with INSM1. Thus we conclude that INSM1 recruits cyclin D1 and HDACs, which confer transcriptional repressor activity.

Role of DNA methylation in control of tumor suppressor gene and oncogene expression in colorectal carcinoma

AIM: To investigate the relationship between DNA methylation and expression of p16, Rb, and cyclin D₁ in the carcinogenesis of colorectal carcinoma.

METHODS: Methylation-specific polymerase chain reaction (MS-PCR) was used to detect the methylation status of p16, Rb, and cyclin D₁ in the specimens from colorectal carcinoma, cancer-adjacent tissues of carcinoma and adenoma, and normal colorectal mucosa, respectively. The correlations of methylation with protein expression and the clinicopathological indexes were analyzed.

RESULTS: For the levels of p16 and Rb methylation, there were increased tendencies in the carcinogenesis of colorectal cancer, while for the level of cyclin D₁ methylation, there was a decreased tendency. The expression of p16 and cyclin D₁ protein were inversely correlated with the methylation status of p16 (normal mucosa: r = -0.185, P = 0.173; adenoma: r = -0.381, P = 0.013; cancer-adjacent tissues: r = -0.419, P = 0.001; cancer tissues: r = -0.516, P = 0.000) and cyclin D₁ gene (normal mucosa: r = -0.282, P = 0.035; adenoma: r = -0.329, P = 0.033; cancer-adjacent tissues: r = -0.298, P = 0.026; cancer tissues: r = -0.618, P = 0.000). The levels of p16 and cyclin D₁ methylation were significantly correlated with the degrees of differentiation (p16: χ² = 11.232, P = 0.002, cyclin D₁: χ² = 9.144, P = 0.015), the depth of invasion (p16: χ² = 6.229, P = 0.013; cyclin D₁: χ² = 8.023, P = 0.006) and the metastasis of lymph node (p16: χ² = 5.707, P = 0.016; cyclin D₁: χ² = 7.794, P = 0.005). The methylation of Rb gene didn't play a main role in the inhibition of Rb protein expression during colorectal carcinogenesis.

CONCLUSION: Aberrant methylations of p16 and cyclin D₁ are the main mechanisms of p16 inactivation and cyclin D₁ over-expression, which play important roles in colorectal carcinogenesis. They are valuble in the early diagnosis and prognosis of colorectal carcinoma.

Cyclin D1: polymorphism, aberrant splicing and cancer risk

The cyclin D1 proto-oncogene exercises powerful control over the mechanisms that regulate the mitotic cell cycle, and excessive cyclin D1 expression and/or activity is common in human cancers. Although somatic mutations of the cyclin D1 locus are rarely observed, mounting evidence demonstrates that a specific polymorphism of cyclin D1 (G/A870) and a protein product of a potentially related alternate splicing event (cyclin D1b) may influence cancer risk and outcome. Herein, we review the epidemiological and functional literatures that link these alterations of cyclin D1 to human tumor development and progression.

Selectivity and potency of cyclin-dependent kinase inhibitors

Members of the cyclin-dependent kinase (CDK) family play key roles in various cellular processes. There are 11 members of the CDK family known till now. CDKs are activated by forming noncovalent complexes with cyclins such as A-, B-, C-, D- (D1, D2, and D3), and E-type cyclins. Each isozyme of this family is responsible for particular aspects (cell signaling, transcription, etc) of the cell cycle, and some of the CDK isozymes are specific to certain kinds of tissues. Aberrant expression and overexpression of these kinases are evidenced in many disease conditions. Inhibition of isozymes of CDKs specifically can yield beneficiary treatment modalities with minimum side effects. More than 80 3-dimensional structures of CDK2, CDK5, and CDK6 complexed with inhibitors have been published. This review provides an understanding of the structural aspects of CDK isozymes and binding modes of various known CDK inhibitors so that these kinases can be better targeted for drug discovery and design. The amino acid residues that constitute the cyclin binding region, the substrate binding region, and the area around the adenosine triphosphate (ATP) binding site have been compared for CDK isozymes. Those amino acids at the ATP binding site that could be used to improve the potency and subtype specificity have been described.

Molecular biology of malignant mesothelioma: a review

Malignant mesothelioma (MM) is an uncommon tumor with high mortality and morbidity rates. It arises from mesothelial cells that line the pleural, pericardial, peritoneal, and testicular cavities. This is a disease with an indolent course because tumors arise 20 to 40 years after exposure to an inciting agent. Extensive research has shown that mesothelial cells are transformed into MM cells through various chromosomal and cellular pathway defects. These changes alter the normal cells' ability to survive, proliferate, and metastasize. This article discusses the alterations that occur in transforming normal mesothelial cells into MM. It also details some of the signal transduction pathways that seem to be important in MM with the potential for novel targeted therapeutics.

Adenovirus infection and cytotoxicity of primary mantle cell lymphoma cells

Mantle cell lymphoma (MCL) is a distinct form of non-Hodgkin's lymphoma (NHL) derived from CD5+ B cells. MCL cells overexpress cyclin D1 as a consequence of translocation of the gene into the immunoglobulin heavy-chain gene locus. MCL is an aggressive form of NHL with frequent relapses after standard-dose chemotherapy. In this context, a variety of novel therapies for patients with MCL have been investigated. In this study, we use an expanded panel of attenuated adenoviruses to study adenovirus-mediated cytotoxicity of MCL cells. Our results demonstrate: 1) adenovirus infection of MCL cells despite the absence of receptor/coreceptor molecules known to be important for adenovirus infection of other cells types; 2) cytotoxicity of MCL cells after infection with specific adenovirus mutants; 3) a high degree of cytotoxicity after infection of some patient samples with viruses lacking the E1B 19k "antiapoptotic" gene; and 4) cytotoxicity after infection with viruses containing mutations in E1A pRb or p300 binding. The extent of cytotoxicity with the panel of viruses demonstrated interpatient variability, but 100% cytotoxicity, as determined by molecular analysis, was detected in some samples. These studies provide the foundation for: 1) the development of adenoviruses as cytotoxic agents for MCL and 2) analyses of key regulatory pathways operative in MCL cells.

Aberrant cell cycle regulation in cervical carcinoma

Carcinoma of the uterine cervix is one of the most common malignancies among women worldwide. Human papillomaviruses (HPV) have been identified as the major etiological factor in cervical carcinogenesis. However, the time lag between HPV infection and the diagnosis of cancer indicates that multiple steps, as well as multiple factors, may be necessary for the development of cervical cancer. The development and progression of cervical carcinoma have been shown to be dependent on various genetic and epigenetic events, especially alterations in the cell cycle checkpoint machinery. In mammalian cells, control of the cell cycle is regulated by the activity of cyclin-dependent kinases (CDKs) and their essential activating coenzymes, the cyclins. Generally, CDKs, cyclins, and CDK inhibitors function within several pathways, including the p16(INK4A)-cyclin D1-CDK4/6-pRb-E2F, p21(WAF1)- p27(KIP1)-cyclinE-CDK2, and p14(ARF)-MDM2-p53 pathways. The results from several studies showed aberrant regulation of several cell cycle proteins, such as cyclin D, cyclin E, p16(INK4A), p21(WAF1), and p27(KIP1), as characteristic features of HPV- infected and HPV E6/E7 oncogene-expressing cervical carcinomas and their precursors. These data suggested further that interactions of viral proteins with host cellular proteins, particularly cell cycle proteins, are involved in the activation or repression of cell cycle progression in cervical carcinogenesis.

Cyclin D1 gene (CCND1) polymorphism and the risk of squamous cell carcinoma of the larynx

Cyclin D1 is one of the key proteins involved in cell cycle control, and it is believed that its overexpression may be connected with tumorigenesis. A reason for cyclin D1 deregulation may be connected to a common G870A polymorphism at codon 242 in exon 4 of the CCND1 gene. This single nucleotide substitution, localized in the conserved splice donor site between exon 4 and the intron 4 boundary, might modulate the frequency of alternative splicing. It has been postulated that the A allele results in a higher level of mRNA (transcript b) encoding a protein with an altered C-terminal domain. The influence of CCND1 G-->A polymorphism for the risk of cancer and the prognosis of patients with different types of solid tumors has already been suggested. This study was conducted to investigate the association between the cyclin D1 gene polymorphism and laryngeal cancer risk, as well as the clinical outcome. We also examined the relationship between genotype/allele distributions and the cyclin D1 expression profile. The genotyping study was done using the PCR-RFLP method in 63 patients with larynx cancer and 102 healthy controls. The heterozygotic genotype GA as well as a combination of GA and AA genotypes were associated with an increased risk of larynx cancer compared to the GG genotype (OR =3.02; P =0.004 and OR =2.52; P =0.013, respectively). The A allele frequency was higher in cancer cases (0.484) than in controls (0.416) that were connected with a slightly increased risk of cancer development (OR =1.34); however, the difference was not significant. The AA genotype was associated with an early cancer onset compared to the GG genotype (median age: 51.5 and 63.0 years, respectively). We also demonstrated that the AA genotype was associated with the occurrence of lymph node metastases (OR =3.26) and a higher level of cyclin D1 overexpression. These results suggest that the CCND1 A allele may be a genetic factor that modulates the risk of larynx cancer development, and it may also have an effect on tumor biology and disease prognosis.

Molecular determinants for the complex formation between the retinoblastoma protein and LXCXE sequences

The retinoblastoma tumor suppressor protein (pRb) is a key negative regulator of cell proliferation that is frequently disregulated in human cancer. Many viral oncoproteins (for example, HPV E7 and E1A) are known to bind to the pRb pocket domain via a LXCXE binding motif. There are also some 20 cellular proteins that contain a LXCXE motif and have been reported to associate with the pocket domain of pRb. Using NMR spectroscopy and isothermal calorimetry titration, we show that LXCXE peptides of viral oncoproteins bind strongly to the pocket domain of pRb. Additionally, we show that LXCXE-like peptides of HDAC1 bind to the same site on pRb with a weak (micromolar) and transient association. Systematic substitution of residues other than conserved Leu, Cys, and Glu show that the residues flanking the LXCXE are important for the binding, whereas positively charged amino acids in the XLXCXEXXX sequence significantly weaken the interaction.

Expression of estrogen receptor-alpha and Ki67 in relation to pathological and molecular features in early-onset infiltrating ductal carcinoma

Estrogen causes breast cancer by triggering proliferation via an estrogen receptor (ER)-mediated mechanism. However, paradoxically, ER alpha, one of the two known ER subtypes, and the proliferation marker, Ki67, are not usually expressed in the same breast tumor. To explore whether ER alpha-positive tumors and proliferating (Ki67-positive) tumors have different tumorigenic characteristics, we performed an immunohistochemical study on 74 early-onset infiltrating ductal carcinomas of the breast. To test this hypothesis, we examined whether ER alpha-positive and Ki67-positive tumors showed differences in (i) pathological grade, (ii) three indices of tumor grade (tubule formation, nuclear pleomorphism, and mitotic number), and (iii) expression of important proteins implicated in breast tumorigenesis (cyclin D1, ErbB2, ATM, BRCA1, Rb, p53, and p21). The results of the multigenic analysis showed that ER alpha and Ki67 were the only two important markers significantly and independently associated with tumor grade, consistent with the above hypothesis. ER alpha-positive, Ki67-negative tumors frequently displayed a low tumor grade (i.e. being well differentiated), whereas Ki67-positive, ER alpha-negative tumors were more likely to exhibit a high tumor grade. In addition, positive ER alpha expression (46 of 74 cases, 62%) correlated well with positive cyclin D1 expression (p < 0.005), less nuclear pleomorphism (p < 0.001), and a low mitotic count (p < 0.005), whereas positive Ki67 expression (36 of 74 cases, 49%) correlated with reduced BRCA1 expression (p < 0.01) and high mitotic activity (p < 0.01). These findings suggest that the expressions of ER alpha and Ki67 might be involved in distinct pathological and molecular features during breast cancer development.

Cyclin E in normal and neoplastic cell cycles

Cyclin E-Cdk2 has long been considered an essential and master regulator of progression through G1 phase of the cell cycle. Although recent mouse models have prompted a rethinking of cyclin E function in mammals, it remains clear that cyclin E impacts upon many processes central to cell division. Normal cells maintain strict control of cyclin E activity, and this is commonly disrupted in cancer cells. Moreover, cyclin E deregulation is thought to play a fundamental role in tumorigenesis. In this review, we discuss the regulation and functions of cyclin E in normal and neoplastic mammalian cells.

Reactive species-mediated regulation of cell signaling and the cell cycle: the role of MAPK

Cardiovascular disease development is significantly influenced by the effects of reactive species (RS). By virtue of their controlled production, regulation, and reactive nature, RS play important roles in the modulation of cellular signaling, growth, and death in the vasculature. Concentration gradients are important in determining the effects of RS. Low to moderate concentrations of RS act as mediators in signaling cascades and gene regulation, whereas high levels of RS cause cellular damage and death. Because a dual redox regulation state seems to exist in several signaling cascades, e.g., RS often induce upstream initiating events, whereas downstream events are reliant on reductive processes, alterations in cellular redox states influence the activation/inactivation of signaling events and transcription factors. In this review, the relationships between RS, specific signal transduction pathways, and aspects of cell-cycle control are discussed.

A three-nucleotide mutation altering the Maize streak virus Rep pRBR-interaction motif reduces symptom severity in maize and partially reverts at high frequency without restoring pRBR–Rep binding

Geminivirus infectivity is thought to depend on interactions between the virus replication-associated proteins Rep or RepA and host retinoblastoma-related proteins (pRBR), which control cell-cycle progression. It was determined that the substitution of two amino acids in the Maize streak virus (MSV) RepA pRBR-interaction motif (LLCNE to LLCLK) abolished detectable RepA–pRBR interaction in yeast without abolishing infectivity in maize. Although the mutant virus was infectious in maize, it induced less severe symptoms than the wild-type virus. Sequence analysis of progeny viral DNA isolated from infected maize enabled detection of a high-frequency single-nucleotide reversion of C(601)A in the 3 nt mutated sequence of the Rep gene. Although it did not restore RepA–pRBR interaction in yeast, sequence-specific PCR showed that, in five out of eight plants, the C(601)A reversion appeared by day 10 post-inoculation. In all plants, the C(601)A revertant eventually completely replaced the original mutant population, indicating a high selection pressure for the single-nucleotide reversion. Apart from potentially revealing an alternative or possibly additional function for the stretch of DNA that encodes the apparently non-essential pRBR-interaction motif of MSV Rep, the consistent emergence and eventual dominance of the C(601)A revertant population might provide a useful tool for investigating aspects of MSV biology, such as replication, mutation and evolution rates, and complex population phenomena, such as competition between quasispecies and population turnover.

The therapeutic potential of targeting the cell cycle

With the advent of modern molecular genetics, molecular biology and biochemistry has come a revolution in oncology drug discovery research. We are rapidly developing an increased understanding in the mechanisms driving cellular proliferation, transformation, differentiation and metastasis. The hope is that from these advances will emerge novel therapeutics that are more specific, more efficacious and less toxic than their predecessors. Uncontrolled proliferation is a hallmark of a cancer cell. Over the past two decades it has become increasingly clear that molecules that directly control cell cycle progression accumulate defects during tumourigenesis. These defects can result in the loss of checkpoint control and/or the inappropriate activation of the 'drivers' of cell cycle progression, the cyclin-dependent kinases (cdks). This review will describe the recent advances in our understanding of cell cycle regulation and its relation to tumourigenesis, and highlight the potential for the development of novel anticancer therapeutics.

Sequestration of pRb by cyclin D3 causes intranuclear reorganization of lamin A/C during muscle cell differentiation

The A-type lamins that localize in nuclear domains termed lamin speckles are reorganized and antigenically masked specifically during myoblast differentiation. This rearrangement was observed to be linked to the myogenic program as lamin speckles, stained with monoclonal antibody (mAb) LA-2H10, were reorganized in MyoD-transfected fibroblasts induced to transdifferentiate to muscle cells. In C2C12 myoblasts, speckles were reorganized early during differentiation in cyclin D3-expressing cells. Ectopic cyclin D3 induced lamin reorganization in C2C12 myoblasts but not in other cell types. Experiments with adenovirus E1A protein that can bind to and segregate the retinoblastoma protein (pRb) indicated that pRb was essential for the cyclin D3-mediated reorganization of lamin speckles. Cyclin D3-expressing myoblasts displayed site-specific reduction of pRb phosphorylation. Furthermore, disruption of lamin structures by overexpression of lamins inhibited expression of the muscle regulatory factor myogenin. Our results suggest that the reorganization of internal lamins in muscle cells is mediated by key regulators of the muscle differentiation program.

Prognostic Implications of Molecular and Immunohistochemical Profiles of the Rb and p53 Cell Cycle Regulatory Pathways in Primary Non–Small Cell Lung Carcinoma

Purpose: Many studies have highlighted the aberrant expression and prognostic significance of individual proteins in either the Rb (particularly cyclin D1, p16INK4A, and pRb) or the p53 (p53 and p21Waf1) pathways in non–small cell lung cancer. We hypothesize that cumulative abnormalities within each and between these pathways would have significant prognostic potential regarding survival.

Experimental Design: Our study population consisted of 106 consecutive surgically resected cases of predominantly early-stage non–small cell lung cancer from the National Cancer Institute-Mayo Clinic series, and assessment of proteins involved both immunohistochemical (cyclin D1, p21Waf1, pRb, p16INK4A, and p53) and mutational analysis (p53) in relationship to staging and survival.

Results: Cyclin D1 overexpression was noted in 48% of the tumors, p16INK4A negative in 53%, pRb negative in 17%, p53 immunopositive in 50%, p53 mutation frequency in 48%, and p21Waf1 overexpression in 47%, none with prognostic significance. Cyclin D1 overexpression in pRb-negative tumors revealed a significantly worse prognosis with a mean survival of 2.3 years (P = 0.004). A simultaneous p53 mutation dramatically reduced the mean survival time to 0.9 years (P = 0.007). Cyclin D1 overexpression with either a p53 mutation or a p53 overexpression was also associated with a significantly poorer prognosis (P = 0.0033 and 0.0063, respectively).

Conclusions: Some cumulative abnormalities in the Rb and p53 pathways (e.g., cyclin D1 overexpression and p53 mutations) significantly cooperate to predict a poor prognosis; however, the complexity of the cell cycle protein interaction in any given tumor warrants caution in interpreting survival results when specific protein abnormalities are taken in isolation.

Are RB proteins a potential substrate of Pin1 in the regulation of the cell cycle?

RB family members are post-transductionally regulated proteins and phosphorylation at Ser/Thr residues leads to their gradual inactivation. Cyclin/cdk complexes are mainly responsible for the regulation of these pocket proteins, which is crucial for release of E2F factor. Despite the fact that E2F release is a phosphorylation-dependent process, it is still not evident how phosphorylation physically determines the shift from the active to the inactive feature of RB molecules. We would like to put forward the hypothesis that Pin1 is involved in RB proteins phosphorylation and E2F release, suggesting an additional post-translational level of control on this family of molecules.

Targeted expression of cyclin D2 results in cardiomyocyte DNA synthesis and infarct regression in transgenic mice

Restriction point transit and commitment to a new round of cell division is regulated by the activity of cyclin-dependent kinase 4 and its obligate activating partners, the D-type cyclins. In this study, we examined the ability of D-type cyclins to promote cardiomyocyte cell cycle activity. Adult transgenic mice expressing cyclin D1, D2, or D3 under the regulation of the alpha cardiac myosin heavy chain promoter exhibited high rates of cardiomyocyte DNA synthesis under baseline conditions. Cardiac injury in mice expressing cyclin D1 or D3 resulted in cytoplasmic cyclin D accumulation, with a concomitant reduction in the level of cardiomyocyte DNA synthesis. In contrast, cardiac injury in mice expressing cyclin D2 did not alter subcellular cyclin localization. Consequently, cardiomyocyte cell cycle activity persisted in injured hearts expressing cyclin D2, ultimately resulting in infarct regression. These data suggested that modulation of D-type cyclins could be exploited to promote regenerative growth in injured hearts.

The activities of cyclin D1 that drive tumorigenesis

The proto-oncogene cyclin D1 has been implicated in the genesis of a large proportion of human tumors from diverse histological origins. It has long been assumed that the action of cyclin D1, as an activator of cdk4 and cdk6 and leading to progression through the G1 phase of the cell cycle, underlies its pathological activity. But, more recently, analyses of the patterns of gene expression in human cancer have revealed a previously unappreciated mechanism of action for cyclin D1, suggesting that both cdk-dependent and cdk-independent activities might contribute to tumorigenesis. The development of therapeutics designed to target the aberrant activity of cyclin D1 in human cancers will rely upon an intimate molecular understanding of these distinct mechanisms of actions and their relative importance. Here, we describe the known functions of the cyclin D1 oncogene and delineate the evidence that cdk-independent actions are important for cyclin D1-mediated oncogenesis.

Trichostatin A activates p18INK4c gene: differential activation and cooperation with p19INK4d gene

We have reported that histone deacetylase (HDAC) inhibitors activate a member of the INK4 family, the p19INK4d gene, causing G1 phase arrest. We report here that HDAC inhibitor, Trichostatin A, activates another member of the INK4 family, the p18INK4c gene, through its promoter in Jurkat cells. Interestingly, the activation patterns of the p18INK4c gene were different from those of p19INK4d. Furthermore, mouse embryo fibroblasts lacking p18Ink4c or p18Ink4c/p19Ink4d were resistant to the growth inhibitory effects of TSA as compared to their wild-type counterpart. Our findings suggest that p18INK4c is involved in TSA-mediated cell growth inhibition and cooperates with p19INK4d.

Genetic evidence for functional dependency of p18Ink4c on Cdk4

The INK4 family of cyclin-dependent kinase (CDK) inhibitors negatively regulates cyclin D-dependent CDK4 and CDK6 and induces the growth-suppressive function of Rb family proteins. Mutations in the Cdk4 gene conferring INK4 resistance are associated with familial and sporadic melanoma in humans and result in a wide spectrum of tumors in mice, suggesting that INK4 is a major regulator of CDK4. Mice lacking the Cdk4 gene exhibit various defects in many organs associated with hypocellularity, whereas loss of the p18(Ink4c) gene results in widespread hyperplasia and organomegaly. To genetically test the notion that the function of INK4 is dependent on CDK4, we generated p18; Cdk4 double-mutant mice and examined the organs and tissues which developed abnormalities when either gene is deleted. We show here that, in all organs we have examined, including pituitary, testis, pancreas, kidney, and adrenal gland, hyperproliferative phenotypes associated with p18 loss were canceled. The double-mutant mice exhibited phenotypes very close to or indistinguishable from that of Cdk4 single-mutant mice. Mice lacking p27(Kip1) develop widespread hyperplasia and organomegaly similar to those developed by p18-deficient mice. The p27; Cdk4 double-mutant mice, however, displayed phenotypes intermediate between those of p27 and Cdk4 single-mutant mice. These results provide genetic evidence that in mice p18(Ink4c) and p27(Kip1) mediate the transduction of different cell growth and proliferation signals to CDK4 and that p18(Ink4c) is functionally dependent on CDK4.

Antizyme targets cyclin D1 for degradation. A novel mechanism for cell growth repression

Overproduction of the ornithine decarboxylase (ODC) regulatory protein ODC-antizyme has been shown to correlate with cell growth inhibition in a variety of different cell types. Although the exact mechanism of this growth inhibition is not known, it has been attributed to the effect of antizyme on polyamine metabolism. Antizyme binds directly to ODC, targeting ODC for ubiquitin-independent degradation by the 26 S proteasome. We now show that antizyme induction also leads to degradation of the cell cycle regulatory protein cyclin D1. We demonstrate that antizyme is capable of specific, noncovalent association with cyclin D1 and that this interaction accelerates cyclin D1 degradation in vitro in the presence of only antizyme, cyclin D1, purified 26 S proteasomes, and ATP. In vivo, antizyme up-regulation induced either by the polyamine spermine or by antizyme overexpression causes reduction of intracellular cyclin D1 levels. The antizyme-mediated pathway for cyclin D1 degradation is independent of the previously characterized phosphorylation- and ubiquitination-dependent pathway, because antizyme up-regulation induces the degradation of a cyclin D1 mutant (T286A) that abrogates its ubiquitination. We propose that antizyme-mediated degradation of cyclin D1 by the proteasome may provide an explanation for the repression of cell growth following antizyme up-regulation.

Evidence for Cyclin D3 as a Novel Target of Rapamycin in Human T Lymphocytes

The immunosuppressant rapamycin has been shown to inhibit G(1)/S transition of the cell cycle. This inhibition is thought to be mediated by maintenance of the threshold levels of cyclin-dependent kinase (CDK) inhibitor p27(Kip1) (p27) and inhibition of p70 s6 kinase (p70(s6k)). However, recent evidence suggests that cells still remain sensitive to rapamycin in the absence of functional p27 or p70(s6k). Here, we show that rapamycin represses cyclin D3 levels in activated human T lymphocytes with no inhibitory effects on cyclin D2. Furthermore, rapamycin elicits similar cyclin D3 modulatory effects in B lymphocytes. The overall effect of rapamycin on cyclin D3 leads to impaired formation of active complexes with Cdk4 or Cdk6 and subsequent inhibition of cyclin D3/CDK kinase activity. Decrease in cyclin D3 protein levels is due to translational repression and not due to attenuated transcription of the cyclin D3 gene. Importantly, stable overexpression of cyclin D3 (2-2.5 fold) in Jurkat T cell transfectants renders them resistant to lower doses (1-10 ng/ml) of rapamycin. These results point to a critical role of cyclin D3 in rapamycin-mediated immunosuppressive effects in T cells and cell cycle regulation in lymphocytes in general.

Docking-dependent regulation of the Rb tumor suppressor protein by Cdk4

Phosphorylation of target proteins by cyclin D1-Cdk4 requires both substrate docking and kinase activity. In addition to the ability of cyclin D1-Cdk4 to catalyze the phosphorylation of consensus sites within the primary amino acid sequence of a substrate, maximum catalytic activity requires the enzyme complex to anchor at a site remote from the phospho-acceptor site. A novel Cdk4 docking motif has been defined within a stretch of 19 amino acids from the C-terminal domain of the Rb protein that are essential for Cdk4 binding. Mutation or deletion of the docking motif prevents Cdk4-dependent phosphorylation of full-length Rb protein or C-terminal Rb fragments in vitro and in cells, while a peptide encompassing the Cdk4 docking motif specifically inhibits Cdk4-dependent phosphorylation of Rb. Cyclin D1-Cdk4 can overcome the growth-suppressive activity of Rb in both cell cycle progression and colony formation assays; however, while mutants of Rb in which the Cdk4 docking site has been either deleted or mutated retain growth suppressor activity, they are resistant to inactivation by cyclin D1-Cdk4. Finally, binding of Cdk4 to its docking site can inhibit cleavage of exogenous and endogenous Rb in response to distinct apoptotic signals. The Cdk4 docking motif in Rb gives insight into the mechanism by which enzyme specificity is ensured and highlights a role for Cdk4 docking in maintaining the Rb protein in a form that favors cell survival rather than apoptosis.

Cyclin C/cdk3 promotes Rb-dependent G0 exit

G0 is a physiological state occupied by resting or terminally differentiated cells that have exited the cell cycle. In contrast to the well-characterized cyclin/cdk-mediated inactivation of pRb that controls the G1/S transition, little is known about regulation of the G0/G1 transition. However, pRb is likely to participate in this process because its acute somatic inactivation is sufficient for G0-arrested cells to re-enter the cell cycle. One physiological regulator of this event may be cyclin C because its highest mRNA levels occur during G0 exit. Here we show that a non-cdk8-associated cellular pool of cyclin C combines with cdk3 to stimulate pRb phosphorylation at S807/811 during the G0/G1 transition, and that this phosphorylation is required for cells to exit G0 efficiently. Thus, G1 entry is regulated in an analogous fashion to S phase entry, but involves a distinct cyclin/cdk combination.

Histone deacetylase inhibitors activate INK4d gene through Sp1 site in its promoter

Histone deacetylase (HDAC) inhibitors are known to arrest human tumor cells at the G1 phase of the cell cycle and activate the cyclin-dependent kinase inhibitor, p21(WAF1/Cip1). However, several studies have suggested the existence of a p21(WAF1/Cip1)-independent molecular pathway. We report here that HDAC inhibitors activate a member of the INK4 family, the INK4d gene, causing G1 phase arrest, in the human T cell leukemia cell line, Jurkat. One of the major Trichostatin A (TSA)-responsive elements is a specific Sp1 binding site in the INK4d promoter. Electrophoretic mobility-shift assay revealed that Sp1 and Sp3 can specifically interact with this Sp1 binding site. Furthermore, using chromatin immunoprecipitation assay, we demonstrated that HDAC2 was present in the INK4d proximal promoter region in the absence, but not the presence, of TSA. Taken together, these results suggest that treatment with TSA transcriptionally activates INK4d by releasing HDAC2 from the histone-DNA complex at the INK4d promoter. Using a p21(WAF1/Cip1)-deleted human colorectal carcinoma cell line, HCT116 p21 (-/-), we show that upregulation of p19(INK4d) by TSA is associated with inhibition of cell proliferation. Moreover, mouse embryo fibroblasts lacking Ink4d were resistant to the growth inhibitory effects of TSA as compared to their wild-type counterpart. Our findings suggest that p19(INK4d) in addition to p21(WAF1/Cip1) is an important molecular target of HDAC inhibitors inducing growth arrest.

The cooperation of B-Myb with the coactivator p300 is orchestrated by cyclins A and D1

B-Myb is a highly conserved member of the Myb family of transcription factors whose activity is regulated during the cell cycle. Previous work has shown that the activity of B-Myb is stimulated by cyclin A/Cdk2-dependent phosphorylation whereas interaction of B-Myb with cyclin D1 inhibits its activity. Here, we have investigated the role of p300 as a coactivator for B-Myb. We show that B-Myb-dependent transactivation is stimulated by p300 as a result of interaction between B-Myb and p300. We have mapped the sequences responsible for the interaction of B-Myb and p300 to the E1A-binding region of p300 and the transactivation domain of B-Myb, respectively. Furthermore, our data suggest that phosphorylation of B-Myb stimulates its acetylation by p300 and that the acetylation of B-Myb is necessary for the full stimulation of its transactivation potential by p300. We have also studied the effect of cyclin D1 on the cooperation of B-Myb and p300. Based on our results we propose that cyclin D1 inhibits the activity of B-Myb by interfering with the interaction of B-Myb and p300. The data reported here provide novel insight into the mechanisms by which the activity of B-Myb is regulated during the cell cycle. Taken together they suggest that the coactivator p300 plays an important role in this regulation and that the cooperation of B-Myb and p300 is orchestrated by cyclins A and D1.

The ins and outs of RB: coupling gene expression to the cell cycle clock

Extracellular growth-stimulatory and -inhibitory signals govern the subunit assembly and activity of G1 cyclin-dependent kinases (cdks), which in turn can phosphorylate the retinoblastoma gene product, pRb, to cancel its growth-suppressive function. Hypophosphorylated forms of pRb, present only during the G1 phase, sequester target proteins including known transcription factors, but pRb phosphorylation late in G1 prevents these interactions and thus frees factors to alter the expression of genes required for entry into S phase. Although pRb can act as a regulator of the G1-S transition, its loss is tolerated by most cells, suggesting that its functions overlap with those of other regulators or are restricted to special circumstances under which cells exit the division cycle.

Combined superoxide dismutase/catalase mimetics alter fetal pulmonary arterial smooth muscle cell growth

Reactive oxygen species (ROS) are known to play an important role in the proliferation and viability of vascular smooth muscle cells. We have shown previously that treatment of fetal pulmonary arterial smooth muscle cells (FPASMC) with concentrations of 25 microM and higher of EUK-134, a superoxide dismutase/catalase mimetic, decreased cell viability via the induction of apoptosis. Here we demonstrate a dose-dependent decrease in serum-induced FPASMC growth at lower doses of EUK-134. This was due to the attenuation of FPASMC proliferation rather than the induction of apoptosis. Moreover, we found that the inhibition of FPASMC proliferation was observed using EUK-134 at concentrations as low as 5 microM. This inhibition of proliferation correlated with a 31% decrease in superoxide levels, as estimated using the oxidation of dihydroethidium. Flow cytometry revealed an increase in FPASMC in G2 after 24 h of exposure to 10 microM EUK-134. This was associated with a twofold increase in levels of the cell-cycle regulatory protein p21. This, together with our previous data, suggests that ROS levels determine the rate of FPASMC proliferation and, when below a threshold level, trigger apoptosis. Titration of ROS with antioxidants may help to prevent, or reverse, the vascular remodeling manifest in many cardiovascular disease states.

The role of E-cadherin/beta-catenin complex and cyclin D1 in head and neck squamous cell carcinoma

PURPOSE

To determine the relationship between expression pattern of E-cadherin, beta-catenin and cyclin D1, and clinicopathologic parameters in patients with head and neck squamous cell carcinomas (HNSCC).

MATERIALS AND METHODS

The authors evaluated the immunohistochemical expression pattern of E-cadherin, beta-catenin in relationship with cyclin D1 overexpression, degree of histologic differentiation, clinical stage, and nodal status in 146 head and neck squamous cell carcinomas (HNSCC). The authors also evaluated the expression of E-cadherin/beta-catenin complex, E-cadherin/cyclin D1, and beta-catenin/cyclin D1 double staining with confocal laser scanning microscope.

RESULTS

Aberrant expressions in 78% of E-cadherin, 77% of beta-catenin, and 69% of cyclin D1 in the HNSCC were observed. There was correlation of aberrant expression of E-cadherin and nodal status. Cyclin D1 overexpression was also correlated to clinical stage and nodal status. Significant relation was observed between E-cadherin and beta-catenin expression patterns. Co-expression of E-cadherin and beta-catenin was significantly detected. However, there was no correlation of cyclin D1 overexpression with E-cadherin or beta-catenin expression patterns.

CONCLUSION

These results suggest that aberrant expression of E-cadherin, E-cadherin/beta-catenin complex, and cyclin D1 may be involved in clinical stage and/or nodal status, and analysis of the pattern of E-cadherin, cyclin D1, and E-cadherin/beta-catenin complex may be good prognostic marker of HNSCC.

A Dual Role of Cyclin E in Cell Proliferation and Apotosis May Provide a Target for Cancer Therapy

Cyclin E is essential for progression through the G1-phase of the cell cycle and initiation of DNA replication by interacting with and activating its catalytic partner, the cyclin dependent kinase 2 (Cdk2). Rb, as well as Cdc6, NPAT, and nucleophosmin, critical components of cell proliferation and DNA replication, respectively, are targets of Cyclin E/Cdk2 phosphorylation. There are a number of putative binding sites for E2F in the cyclin E promoter region, suggesting an E2F-dependent regulation. Skp2 and Fbw7 are novel proteins, responsible for ubiquitin-dependent proteolysis of Cyclin E. The tight regulation of cyclin E expression, both at the transcriptional level and by ubiquitin-mediated proteolysis, indicates that it has a major role in the control of the G1- and S-phase transitions. Cyclin E is also transcriptionally regulated during radiation-induced apoptosis of hematopoietic cells. In addition to its biological roles, deregulated cyclin E expression has an established role in tumorigenesis. Cell cycle regulatory molecules, such as cyclin E, are frequently deregulated in different types of cancers, where overexpressed native or low molecular weight forms of Cyclin E have a significant role in oncogenesis. During apoptosis of hematopoietic cells, caspase-dependent proteolysis of Cyclin E generates a p18-Cyclin E variant. Understanding the role of Cyclin E in apoptosis may provide a novel target, which may be effective in cancer therapy. This review summarizes what is known about the biological role of cyclin E, its deregulation in cancer, and the opportunities it may provide as a target in clinical therapy.

SV40 large T antigen promotes dephosphorylation of p130

SV40 large T antigen (Ag) binds to all members of the retinoblastoma (RB) tumor suppressor family including pRb, p107, and p130. Although the LXCXE motif of T Ag binds directly to the RB proteins, it is not sufficient to fully inactivate their function. The N-terminal DNA J domain of T Ag cooperates with the LXCXE motif to override RB-mediated repression of E2F-dependent transcription. In addition, T Ag can reduce the overall phosphorylation state of p107 and p130 that is dependent on an intact J domain and LXCXE motif. However, the mechanism of this activity has not been described. Here we describe the use of a cell-free system to characterize the effect of T Ag on p130 phosphorylation. When incubated in extracts prepared from S phase cells, p130 undergoes specific phosphorylation. Addition of T Ag to S phase extracts leads to a reduction of p130 phosphorylation in vitro. The ability of T Ag to reduce the phosphorylation of p130 in vitro is dependent on an intact DNA J domain and can be inhibited by okadaic acid and PP2A-specific inhibitors. These results suggest that T Ag recruits a phosphatase activity in a DNA J domain-dependent manner to reduce the phosphorylation of p130.

p53, Ki67 and cyclin D1 as prognosticators of lymph node metastases in laryngeal carcinoma

The prognosis in patients suffering from head and neck squamous cell carcinomas depends on many factors. However, regional lymph node metastases are the most important parameter in determining the cure and survival of patients with head and neck cancers. The evaluation of cancer cell biology enables differentiation of their proliferation and tendency of metastases. Immunohistochemical examinations complement the well-established routine histological examination. The aim of this study was to evaluate the prognostic importance of the level of immunoproliferating proteins such as cyclin D1, nuclear antigen Ki67 and suppressor gene p53 for regional lymph node metastases in laryngeal carcinoma. The research was carried out on 73 patients treated for squamous cancer of the larynx in the Department of Otolaryngology University School of Medical Sciences in Poznan in the years 1994-1999. The group was comprised of 4 female and 69 male patients. Their ages ranged from 37 to 79 years, with a mean of 59 years. Clinical data included sex, age, localization and local and regional extent of the tumor, presence or lack of distant metastases, treatment, histological examination as well as immunohistochemical evaluation of suppressor gene p53, proliferative proteins Ki67 and cyclin D1. No statistically significant correlation was found between staining intensity of suppressor gene p53, cyclin D1 and the degree of local advancement (T). There was no correlation between the level of immunoproliferative markers and regional lymph node metastases. Statistically significant correlation was found between T stage and staining for Ki67 (P=0.017) as well as between cyclin D1 level and Ki67 (P<0.05). In conclusion, (1) no significant correlation was found between Ki67 and cyclin D1, p53 and TNM classification; (2) lack of correlation was confirmed between N+, p53, Ki67, cyclin D1 and Jacobsson classification; (3) the degree of histological grading correlated, however, with Jacobsson classification and cyclin D1 expression.

Retinoblastoma gene expression in human non-melanoma skin cancer

BACKGROUND

The aberrant expression of both the retinoblastoma and p53 tumor suppressor genes has been associated with more aggressive tumors, metastasis and lower survival.

METHODS

We have evaluated immunohistochemically the expression of pRB in a panel of non-melanoma skin cancers containing p53 somatic mutations.

RESULTS

Nuclear anti-p53 staining was detected in 18 (72%) differentiated squamous cell carcinomas, six (100%) undifferentiated squamous cell carcinomas and seven (28%) basal cell carcinomas. A correlation was observed between p53 expression and the proliferative activity of differentiated squamous cell carcinomas (P < 0.066), undifferentiated squamous cell carcinomas (P < 0.05) and basal cell carcinomas (P < 0.01). Tumors were selected for mutant p53 expression by PCR-directed DNA sequencing and pRB expression measured immunohistochemically. Anti-pRB reactivity was detected in the nuclei of basal and suprabasal layer cells of normal epidermis, and in the proliferative compartment of all the differentiated squamous cell carcinomas, and basal cell carcinomas. A correlation was observed between pRB expression and the proliferative activity of the differentiated squamous cell carcinomas (P < 0.01) and basal cell carcinomas (P < 0.025). However, anti-pRB reactivity was not detected in the six anti-p53 reactive undifferentiated squamous cell carcinomas.

Cyclin D1 splice variants. Differential effects on localization, RB phosphorylation, and cellular transformation

Cyclin D1 is a proto-oncogene that functions by inactivation of the retinoblastoma tumor suppressor protein, RB. A common polymorphism in the cyclin D1 gene is associated with the production of an alternate transcript of cyclin D1, termed cyclin D1b. Both the polymorphism and the variant transcript are associated with increased risk for multiple cancers and the severity of a given cancer; however, the underlying activities of cyclin D1b have not been elucidated relative to the canonical cyclin D1a. Because cyclin D1b does not possess the threonine 286 phosphorylation site required for nuclear export and regulated degradation, it has been hypothesized to encode a stable nuclear protein that would constitutively inactivate the RB pathway. Surprisingly, we find that cyclin D1b protein does not inappropriately accumulate in cells and exhibits stability comparable to cyclin D1a. As expected, the cyclin D1b protein was constitutively localized in the nucleus, whereas cyclin D1a was exported to the cytoplasm in S-phase. Despite enhanced nuclear localization, we find that cyclin D1b is a poor catalyst of RB phosphorylation/inactivation. However, cyclin D1b potently induced cellular transformation in contrast to cyclin D1a. In summary, we demonstrate that cyclin D1b specifically disrupts contact inhibition in a manner distinct from cyclin D1a. These data reveal novel roles for d-type cyclins in tumorigenesis.

Cyclic AMP-dependent phosphorylation of cyclin D3-bound CDK4 determines the passage through the cell cycle restriction point in thyroid epithelial cells

According to current concepts, the cell cycle commitment after restriction (R) point passage requires the sustained stimulation by mitogens of the synthesis of labile d-type cyclins, which associate with cyclin-dependent kinase (CDK) 4/6 to phosphorylate pRb family proteins and sequester the CDK inhibitor p27kip1. In primary cultures of dog thyroid epithelial cells, the cAMP-dependent cell cycle induced by a sustained stimulation by thyrotropin or forskolin differs from growth factor mitogenic pathways, as cAMP does not upregulate d-type cyclins but increases p27 levels. Instead, cAMP induces the assembly of required cyclin D3-CDK4 complexes, which associate with nuclear p27. In this study, the arrest of forskolin stimulation rapidly slowed down the entry of dog thyrocytes into S phase and the phosphorylation of pRb family proteins. The pRb kinase activity, but not the formation, of the cyclin D3-CDK4-p27 complex was strongly reduced. Using two-dimensional gel electrophoresis, a phosphorylated form of CDK4 was separated. It appeared in response to forskolin and was bound to both cyclin D3 and p27, presumably reflecting the activating Thr-172 phosphorylation of CDK4. Upon forskolin withdrawal or after cycloheximide addition, this CDK4 phosphoform unexpectedly persisted in p27 complexes devoid of cyclin D3 but it disappeared from the more labile cyclin D3 complexes. These data demonstrate that the assembly of the cyclin D3-CDK4-p27 holoenzyme and the subsequent phosphorylation and activation of CDK4 depend on distinct cAMP actions. This provides a first example of a crucial regulation of CDK4 phosphorylation by a mitogenic cascade and a novel mechanism of cell cycle control at the R point.

The RRASK motif in Xenopus cyclin B2 is required for the substrate recognition of Cdc25C by the cyclin B-Cdc2 complex

The FLRRXSK sequence is conserved in the second cyclin box fold of B-type cyclins. We show that this conserved sequence in Xenopus cyclin B2, termed the RRASK motif, is required for the substrate recognition by the cyclin B-Cdc2 complex of Cdc25C. Mutations to charged residues of the RRASK motif of cyclin B2 abolished its ability to activate Cdc2 kinase without affecting its capacity to bind to Cdc2. Cdc2 bound to the cyclin B2 RRASK mutant was not dephosphorylated by Cdc25C, and as a result, the complex was inactive. The cyclin B2 RRASK mutants can form a complex with the constitutively active Cdc2, but a resulting active complex did not phosphorylate a preferred substrate Cdc25C in vitro, although it can phosphorylate the non-specific substrate histone H1. The RRASK mutations prevented the interaction of Cdc25C with the cyclin B2-Cdc2 complex. Consistently, the RRASK mutants neither induced germinal vesicle breakdown in Xenopus oocyte maturation nor activated in vivo Cdc2 kinase during the cell cycle in mitotic extracts. These results suggest that the RRASK motif in Xenopus cyclin B2 plays an important role in defining the substrate specificity of the cyclin B-Cdc2 complex.

Genetic models in applied physiology: invited review: effect of oxygen deprivation on cell cycle activity: a profile of delay and arrest

One of the most fascinating fields that have emanated in the past few decades is developmental biology. This is not only the case from a research point of view but also from the angle of clinical care and treatment strategies. It is now well demonstrated that there are many diseases (some believe all diseases) that have their roots in embryogenesis or in early life, where nature and environment often team up to facilitate the genesis of disease. There is probably no better example to illustrate the interactions between nature and environment than in early life, as early as in the first several cell cycles. As will be apparent in this review, the cell cycle is a very regulated activity and this regulation is genetic in nature, with checkpoint proteins playing an important role in controlling the timing, the size, and the growth of daughter cells. However, it is also very clear, as will be discussed in this work, that the microenvironment of the first dividing cells is so important for the outcome of the organism. In this review, we will focus on the effect of one stress, that of hypoxia, on the young embryo and its cell division and growth. We will first review some of the cell cycle definitions and stages and then review briefly our current knowledge and its gaps in this area.

NF-kappa B-dependent induction of cyclin D1 by retinoblastoma protein (pRB) family proteins and tumor-derived pRB mutants

The retinoblastoma protein (pRB) and its homologues, p107 and p130, prevent cell cycle progression from G(0)/G(1) to S phase by forming complexes with E2F transcription factors. Upon phosphorylation by G(1) cyclin-cyclin-dependent kinase (Cdk) complexes such as cyclin D1-Cdk4/6 and cyclin E-Cdk2, they lose the ability to bind E2F, and cells are thereby allowed to progress into S phase. Functional loss of one or more of the pRB family members, as a result of genetic mutation or deregulated phosphorylation, is considered to be an essential prerequisite for cellular transformation. In this study, we found that pRB family proteins have the ability to stimulate cyclin D1 transcription by activation of the NF-kappaB transcription factor. The cyclin D1-inducing activity of pRB is abolished by adenovirus E1A oncoprotein but not by the deletion of the A-box, the B-box, or the C-terminal region of the pocket, indicating that multiple pocket sequences are independently involved in cyclin D1 activation. Intriguingly, tumor-derived pRB pocket mutants retain the cyclin D1-inducing activity. Our results reveal a novel role of pRB family proteins as potential activators of NF-kappaB and inducers of G(1) cyclin. Certain pRB pocket mutants may give rise to a cellular situation in which deregulated E2F and cyclin D1 cooperatively promote abnormal cell proliferation.

Statistical optimization for immobilized metal affinity purification of secreted human erythropoietin from Drosophila S2 cells

We used a novel approach to affinity purify human erythropoietin (hEPO) following its secretion from Drosophila melanogaster S2 cells. Immobilized metal affinity purification of hEPO was optimized using a two-step serial statistical optimization strategy. After determining the elution conditions (based on preliminary batch-type purification experiments), the first optimization step considered three purification factors; resin, equilibrium, and washing. The results of this analysis showed that the resin amount was the major factor influencing yield and purity in both model equations and the washing factor lowered the confidence limits of the acquired model equations. The washing conditions were then set based on the results of the first step optimization and the second step then optimized three factors; resin, equilibrium, and elution. The yield and purity of hEPO were then compared following purification using three different approaches; batch-type purification based upon the conditions determined by serial statistical optimization, batch-type purification performed in preliminary experiments, and FPLC column chromatography-type purification. We found that the serial statistical optimization approach provided the best combination of yield and purity. These findings indicate that serial statistical optimization strategies can be successfully employed for immobilized metal affinity protein purification using either batch-type or column approaches.

Role of the RB tumor suppressor in cancer

Apart from their coordinated inactivation by DNA tumor viral oncoproteins, the pRB and p53 tumor suppressor pathways were not known to be connected ten years ago. Within the last decade, our appreciation of how these pathways are interconnected has grown substantially. The checks and balances that exist between pRB and p53 involve the regulation of the G1/S transition and its checkpoints, and much of this is under the control of the E2F transcription factor family. Following DNA damage, the p53-dependent induction of p21CIP1 regulates cyclin E/Cdk2 and cyclin A/Cdk2 complexes both of which phosphorylate pRB, leading to E2F-mediated activation. Similarly, E2F1-dependent induction of p19ARF antagonizes the ability of mdm2 to degrade p53, leading to p53 stabilization and potentially p53-mediated apoptosis or cell cycle arrest. From the existing mouse models discussed above, we also know that proliferation, cell death and differentiation of distinct tissues are also intimately linked through entrance and exit from the cell cycle, and thus through pRB and p53 pathways. Virtually all human tumors deregulate either the pRB or p53 pathway, and often times both pathways simultaneously, which is critical for crippling cellular defense against neoplasia. The next decade of cancer research will likely see these two tumor suppressor pathways only merge even more.

Gris1, a new common integration site in Graffi murine leukemia virus-induced leukemias: overexpression of a truncated cyclin D2 due to alternative splicing

The Graffi murine leukemia virus is a nondefective ecotropic retrovirus that was originally reported to induce myeloid leukemia in some strains of mice (A. Graffi, Ann. N.Y. Acad. Sci. 68:540-558, 1957). Using provirus-flanking sequences as DNA probes, we identified a new common retroviral integration site called Gris1 (for Graffi integration site 1). Viral integrations in Gris1 were detected in 13% of the tumors analyzed. The Gris1 locus was mapped to the distal region of mouse chromosome 6, 85 kb upstream of the cyclin D2 gene. Such viral integration in Gris1 causes overexpression of the normal 6.5-kb major transcript of cyclin D2 but also induces the expression of a new, alternatively spliced 1.1-kb transcript from the cyclin D2 gene that encodes a truncated cyclin D2 of 17 kDa. The expression of this 1.1-kb transcript is specific to tumors in which Gris1 is rearranged but is also detected at low levels in normal tissue.

Lamin A/C binding protein LAP2alpha is required for nuclear anchorage of retinoblastoma protein

The phosphorylation-dependent anchorage of retinoblastoma protein Rb in the nucleus is essential for its function. We show that its pocket C domain is both necessary and sufficient for nuclear anchorage by transiently expressing green fluorescent protein (GFP) chimeras of Rb fragments in tissue culture cells and by extracting the cells with hypotonic solutions. Solid phase binding assays using glutathione S-transferase-fusion of Rb pockets A, B, and C revealed a direct association of lamin C exclusively to pocket C. Lamina-associated polypeptide (LAP) 2alpha, a binding partner of lamins A/C, bound strongly to pocket C and weakly to pocket B. When LAP2alpha was immunoprecipitated from soluble nuclear fractions, lamins A/C and hypophosphorylated Rb were coprecipitated efficiently. Similarly, immunoprecipitation of expressed GFP-Rb fragments by using anti-GFP antibodies coprecipitated LAP2alpha, provided that pocket C was present in the GFP chimeras. On redistribution of endogenous lamin A/C and LAP2alpha into nuclear aggregates by overexpressing dominant negative lamin mutants in tissue culture cells, Rb was also sequestered into these aggregates. In primary skin fibroblasts, LAP2alpha is expressed in a growth-dependent manner. Anchorage of hypophosphorylated Rb in the nucleus was weakened significantly in the absence of LAP2alpha. Together, these data suggest that hypophosphorylated Rb is anchored in the nucleus by the interaction of pocket C with LAP2alpha-lamin A/C complexes.

The ARF-p16 gene locus in carcinogenesis and therapy of head and neck squamous cell carcinoma

OBJECTIVES/HYPOTHESIS

We have identified families with a high incidence of tumors including head and neck squamous cell carcinoma (HNSCC). The occurrence of melanoma in these kindreds suggested that the ARF-p16 gene may be involved in carcinogenesis. We wished to determine the gene defect associated with the familial predisposition to HNSCC and to determine whether restoration of the gene may have therapeutic benefit.

STUDY DESIGN

Translational molecular research.

METHODS

Molecular techniques were used to identify mutations of the ARF-p16 gene from the affected families and to test the activity of p16 and ARF mutants. In additional, HNSCC tumor tissue was analyzed to determine whether the wild-type p16 allele was lost or maintained. ARF-expressing adenoviruses were created, and their effect on HNSCC cell lines and normal head and neck epithelial cells was determined.

RESULTS

Mutation of the ARF-p16 gene was found in two families with predisposition to develop HNSCC. Independent mutations detected in the germline DNA of both families inactivated p16, but not ARF, and the inactive mutant p16 allele segregated with disease within both families. The wild-type p16 allele was lost in HNSCC tumor tissue from both families. The efficacy of ARF in treatment of HNSCC was found to depend on retention of p53 activity within HNSCC tumor cells. Remarkably, ARF expression was found to kill cells, depending on loss of retinoblastoma activity. Because loss of retinoblastoma activity is nearly universal in tumors, ARF killed tumor cells that retained p53, but ARF spared normal cells.

CONCLUSIONS

Our results support the recognition of a new clinical entity of familial head and neck cancer. We have shown that this syndrome is associated with inactivating mutations of the p16 gene that these mutations segregate with disease in two described families. Loss of the wild-type p16 allele in HNSCC tissue from both families strongly supports the role of the mutant p16 in carcinogenesis. We have also investigated the therapeutic utility of the alternate reading frame product of the p16 gene, ARF. The finding that ARF kills cells depending on loss of retinoblastoma activity and retention of p53 suggests that ARF may be effective in treatment of roughly 50% of head and neck cancers while sparing normal cells. Recognition of p16 mutations as an etiological factor in familial HNSCC provides an accessible tool for diagnosis of this syndrome. Clinical acceptance of familial head and neck cancer will ensure that patients are appropriately diagnosed and managed.

Alterations of the p16/Rb/cyclin-D1 pathway in vulvar carcinoma, vulvar intraepithelial neoplasia, and lichen sclerosus

Three different alterations in the p16/pRb/cyclin-D1 pathway (p16(INK4a)-promoter hypermethylation and expression of pRb and cyclin-D1) were investigated in a series of 38 cases of vulvar carcinoma (VC), 13 cases of vulvar intraepithelial neoplasia (VIN), and 21 cases of lichen sclerosus (LS). Paraffin blocks from 72 patients were selected for investigation of DNA methylation patterns in the CpG island of p16(INK4a) by methylation-specific polymerase chain reaction. Immunohistochemical studies for pRb and cyclin-D1 were performed using the standard avidin-biotin-peroxidase complex method. Epigenetic silencing of p16(INK4a) was detected in 68% of VC, 69.2% of VIN, and 42.8% of LS cases. Lack of pRb protein was found in 21% of VC, 0% of VIN, and 0% of LS cases. Overexpression of cyclin-D1 was found in 21% of VC, 30.8% of VIN, and 0% of LS cases. We conclude (1) that p16(INK4a) epigenetic inactivation most likely represents an early event, insufficient for malignant transformation, that may occur in clinically benign lesions such as LS; (2) that lack of pRb was only detected in fewer than one quarter of the carcinomas and could be considered a late secondary event; and (3) that cyclin-D1, which was overexpressed in VC and VIN, could contribute to the malignant transformation in association with p16 hypermethylation.