Cyclin E dysregulation and chromosomal instability in endometrial cancer

Molecular insights and clinical implications for the tumor suppressor role of SCFFBXW7 E3 ubiquitin ligase

FBXW7 is one of the most well-characterized F-box proteins, serving as substrate receptor subunit of SKP1-CUL1-F-box (SCF) E3 ligase complexes. SCFFBXW7 is responsible for the degradation of various oncogenic proteins such as cyclin E, c-MYC, c-JUN, NOTCH, and MCL1. Therefore, FBXW7 functions largely as a major tumor suppressor. In keeping with this notion, FBXW7 gene mutations or downregulations have been found and reported in many types of malignant tumors, such as endometrial, colorectal, lung, and breast cancers, which facilitate the proliferation, invasion, migration, and drug resistance of cancer cells. Therefore, it is critical to review newly identified FBXW7 regulation and tumor suppressor function under physiological and pathological conditions to develop effective strategies for the treatment of FBXW7-altered cancers. Since a growing body of evidence has revealed the tumor-suppressive activity and role of FBXW7, here, we updated FBXW7 upstream and downstream signaling including FBXW7 ubiquitin substrates, the multi-level FBXW7 regulatory mechanisms, and dysregulation of FBXW7 in cancer, and discussed promising cancer therapies targeting FBXW7 regulators and downstream effectors, to provide a comprehensive picture of FBXW7 and facilitate the study in this field.

FBXW7 polymorphism asserts susceptibility to colorectal cancer

FBXW7, belonging to the F-Box protein family, is considered a candidate cancer susceptibility gene. Our findings indicate that single nucleotide polymorphisms (SNPs) in the FBXW7 gene are linked to cancer risk, strengthening FBXW7's role in the pathogenesis of colorectal cancer. Our case-control study comprised of 450 patients diagnosed with colorectal cancer (CRC) and an equal number of 450 healthy subjects. FBXW7 SNPs rs2255137C>T and rs6842544C>T were genotyped using PCR-Restriction Fragment Length Polymorphism (PCR-RFLP) and Single-Stranded Conformation Polymorphism (SSCP) techniques and further cross-checked by direct sequencing. Linkage disequilibrium and haplotype analyses of these SNPs were also assessed. The in-silico approach was used to reveal the functional analysis between the nonsynonymous variation (rs6842544) and CRC followed by its validation at the protein level by western blotting and reverse transcription-PCR. A significant association of colorectal cancer was detected with rs6842544 SNP. However, there was no association between FBXW7 rs2255137 polymorphism and CRC. The homozygous individuals carrying the C variant in FBXW7 rs6842544 showed a slightly higher risk for colorectal cancer (OR = 1.590, 95%CI = 0.39 ∼ 2.89, p = 0.011). The haplotype CC identified in this study seemed to be associated with good prognosis (OR = 1.22, 95% CI = 1.00 ∼ 1.47, p = 0.0013) whereas the TT haplotype was found to reduce the CRC risk (OR = 0.642, 95%CI = 0.48 ∼ 0.84, p = 0.039). In-silico prediction proposed that the variant R133G is responsible for the lower expression of FBXW7. Additionally, the expression profiling of FBXW7 nonsynonymous SNP was significantly lower in primary CRC tissues than in the paired non-cancerous tissues at protein and mRNA levels. The study indicates that the FBXW7 rs6842544 is associated with the risk of development of CRC and could serve as a molecular biological marker to screen high-risk groups for CRC.

CCND2 and CCND3 hijack immunoglobulin light-chain enhancers in cyclin D1- mantle cell lymphoma

Mantle cell lymphoma (MCL) is characterized by the t(11;14)(q13;q32) translocation resulting in overexpression of cyclin D1. However, a small subset of cyclin D1^- MCL has been recognized, and approximately one-half of them harbor CCND2 translocations while the primary event in cyclin D1^-/D2^- MCL remains elusive. To identify other potential mechanisms driving MCL pathogenesis, we investigated 56 cyclin D1^-/SOX11⁺ MCL by fluorescence in situ hybridization (FISH), whole-genome/exome sequencing, and gene-expression and copy-number arrays. FISH with break-apart probes identified CCND2 rearrangements in 39 cases (70%) but not CCND3 rearrangements. We analyzed 3 of these negative cases by whole-genome/exome sequencing and identified IGK (n = 2) and IGL (n = 1) enhancer hijackings near CCND3 that were associated with cyclin D3 overexpression. By specific FISH probes, including the IGK enhancer region, we detected 10 additional cryptic IGK juxtapositions to CCND3 (6 cases) and CCND2 (4 cases) in MCL that overexpressed, respectively, these cyclins. A minor subset of 4 cyclin D1^- MCL cases lacked cyclin D rearrangements and showed upregulation of CCNE1 and CCNE2. These cases had blastoid morphology, high genomic complexity, and CDKN2A and RB1 deletions. Both genomic and gene-expression profiles of cyclin D1^- MCL cases were indistinguishable from cyclin D1⁺ MCL. In conclusion, virtually all cyclin D1^- MCLs carry CCND2/CCND3 rearrangements with immunoglobulin genes, including a novel IGK/L enhancer hijacking mechanism. A subset of cyclin D1^-/D2^-/D3^- MCL with aggressive features has cyclin E dysregulation. Specific FISH probes may allow the molecular identification and diagnosis of cyclin D1^- MCL.

FBXW7: a critical tumor suppressor of human cancers

The ubiquitin-proteasome system (UPS) is involved in multiple aspects of cellular processes, such as cell cycle progression, cellular differentiation, and survival (Davis RJ et al., Cancer Cell 26:455-64, 2014; Skaar JR et al., Nat Rev Drug Discov 13:889-903, 2014; Nakayama KI and Nakayama K, Nat Rev Cancer 6:369-81, 2006). F-box and WD repeat domain containing 7 (FBXW7), also known as Sel10, hCDC4 or hAgo, is a member of the F-box protein family, which functions as the substrate recognition component of the SCF E3 ubiquitin ligase. FBXW7 is a critical tumor suppressor and one of the most commonly deregulated ubiquitin-proteasome system proteins in human cancer. FBXW7 controls proteasome-mediated degradation of oncoproteins such as cyclin E, c-Myc, Mcl-1, mTOR, Jun, Notch and AURKA. Consistent with the tumor suppressor role of FBXW7, it is located at chromosome 4q32, a genomic region deleted in more than 30% of all human cancers (Spruck CH et al., Cancer Res 62:4535-9, 2002). Genetic profiles of human cancers based on high-throughput sequencing have revealed that FBXW7 is frequently mutated in human cancers. In addition to genetic mutations, other mechanisms involving microRNA, long non-coding RNA, and specific oncogenic signaling pathways can inactivate FBXW7 functions in cancer cells. In the following sections, we will discuss the regulation of FBXW7, its role in oncogenesis, and the clinical implications and prognostic value of loss of function of FBXW7 in human cancers.

Fbxw7 regulates renal cell carcinoma migration and invasion via suppression of the epithelial-mesenchymal transition

F-box and WD repeat domain containing 7 (Fbxw7) is an F-box protein that belongs to the SKP1-CUL1-F-box protein E3 ligase complex and is responsible for transferring the ubiquitin molecule to the substrate, which results in its recognition and subsequent degradation by proteasomes. Furthermore, it can identify a network of signaling proteins that function in cell growth, diversion and apoptosis. In the present study, Fbxw7 was downregulated in renal cell carcinoma (RCC) tissues compared with the adjacent non-tumor tissues and its expression was significantly associated with the tumor-node-metastasis stage, lymph node metastasis and distant metastasis in patients with RCC. Furthermore, multivariate Cox regression analyses indicated that Fbxw7 expression was an independent factor for the prediction of the overall survival of patients with RCC. A functional study demonstrated that downregulation of Fbxw7 facilitated tumor cell migration and invasion via the epithelial-mesenchymal transition (EMT). Therefore, the results of the current study indicted that Fbxw7 is an anti-oncogene that serves a notable function in RCC development by suppressing RCC metastasis and the EMT, indicating the potential therapeutic value of Fbxw7 in inhibiting metastasis in RCC.

NFAT1 transcription factor regulates cell cycle progression and cyclin E expression in B lymphocytes

The NFAT family of transcription factors has been primarily related to T cell development, activation, and differentiation. Further studies have shown that these ubiquitous proteins are observed in many cell types inside and outside the immune system, and are involved in several biological processes, including tumor growth, angiogenesis, and invasiveness. However, the specific role of the NFAT1 family member in naive B cell proliferation remains elusive. Here, we demonstrate that NFAT1 transcription factor controls Cyclin E expression, cell proliferation, and tumor growth in vivo. Specifically, we show that inducible expression of NFAT1 inhibits cell cycle progression, reduces colony formation, and controls tumor growth in nude mice. We also demonstrate that NFAT1-deficient naive B lymphocytes show a hyperproliferative phenotype and high levels of Cyclin E1 and E2 upon BCR stimulation when compared to wild-type B lymphocytes. NFAT1 transcription factor directly regulates Cyclin E expression in B cells, inhibiting the G1/S cell cycle phase transition. Bioinformatics analysis indicates that low levels of NFAT1 correlate with high expression of Cyclin E1 in different human cancers, including Diffuse Large B-cell Lymphomas (DLBCL). Together, our results demonstrate a repressor role for NFAT1 in cell cycle progression and Cyclin E expression in B lymphocytes, and suggest a potential function for NFAT1 protein in B cell malignancies.

Dual CCNE1/PIK3CA targeting is synergistic in CCNE1-amplified/PIK3CA-mutated uterine serous carcinomas in vitro and in vivo

Background:

Clinical options for patients harbouring advanced/recurrent uterine serous carcinoma (USC), an aggressive variant of endometrial tumour, are very limited. Next-generation sequencing (NGS) data recently demonstrated that cyclin E1 (CCNE1) gene amplification and pik3ca driver mutations are common in USC and may therefore represent ideal therapeutic targets.

Methods:

Cyclin E1 expression was evaluated by immunohistochemistry (IHC) on 95 USCs. The efficacy of the cyclin-dependent kinase 2/9 inhibitor CYC065 was assessed on multiple primary USC cell lines with or without CCNE1 amplification. Cell-cycle analyses and knockdown experiments were performed to assess CYC065 targeting specificity. Finally, the in vitro and in vivo activity of CYC065, Taselisib (a PIK3CA inhibitor) and their combinations was tested on USC xenografts derived from CCNE1-amplified/pik3ca-mutated USCs.

Results:

We found that 89.5% of the USCs expressed CCNE1. CYC065 blocked cells in the G1 phase of the cell cycle and inhibited cell growth specifically in CCNE1-overexpressing USCs. Cyclin E1 knockdown conferred increased resistance to CYC065, whereas CYC065 treatment of xenografts derived from CCNE1-amplified USCs significantly reduced tumour growth. The combination of CYC065 and Taselisib demonstrated synergistic effect in vitro and was significantly more effective than single-agent treatment in decreasing tumour growth in xenografts of CCNE1-amplified/pik3ca-mutated USCs.

Conclusions:

Dual CCNE1/PIK3CA blockade may represent a novel therapeutic option for USC patients harbouring recurrent CCNE1-amplified/pi3kca-mutated tumours.

Cyclin-Dependent Kinase Inhibitors as Anticancer Therapeutics

Cyclin-dependent kinases (CDKs) have been considered promising drug targets for a number of years, but most CDK inhibitors have failed rigorous clinical testing. Recent studies demonstrating clear anticancer efficacy and reduced toxicity of CDK4/6 inhibitors such as palbociclib and multi-CDK inhibitors such as dinaciclib have rejuvenated the field. Favorable results with palbociclib and its recent U.S. Food and Drug Administration approval demonstrate that CDK inhibitors with narrow selectivity profiles can have clinical utility for therapy based on individual tumor genetics. A brief overview of results obtained with ATP-competitive inhibitors such as palbociclib and dinaciclib is presented, followed by a compilation of new avenues that have been pursued toward the development of novel, non-ATP-competitive CDK inhibitors. These creative ways to develop CDK inhibitors are presented along with crystal structures of these agents complexed with CDK2 to highlight differences in their binding sites and mechanisms of action. The recent successes of CDK inhibitors in the clinic, combined with the potential for structure-based routes to the development of non-ATP-competitive CDK inhibitors, and evidence that CDK inhibitors may have use in suppressing chromosomal instability and in synthetic lethal drug combinations inspire optimism that CDK inhibitors will become important weapons in the fight against cancer.

FBXW7 mutation analysis and its correlation with clinicopathological features and prognosis in colorectal cancer patients

PURPOSE

This study aimed to determine the prognostic value of mutations in the tumor suppressor gene FBXW7 for clinical outcomes in colorectal cancer (CRC).

METHODS

Between January 2000 and December 2009, FBXW7 mutations in tumor tissues from 1,519 CRC patients at Taipei Veterans General Hospital were assessed using a MassArray system. We compared the clinicopathological variables and prognosis between the wild-type and mutant tumor tissue groups.

RESULTS

FBXW7 mutations were present in 114/1,519 CRC patients (7.5%). In stage I/II CRC patients, mutant FBXW7 was more common than wild-type FBXW7 (62.3% vs. 50.8%). CRC patients with FBXW7 mutations did not differ significantly in their 5-year overall survival (OS). Stage I/II CRC patients with FBXW7 mutations had lower OS, but this difference was not significant (71.6% vs. 78.2%). Among FBXW7 tumors, S582L was the most frequent mutation type (19.3%), followed by R465H (16.6%), R505C (14.9%) and R479Q (14.9%). Subgroup analysis of FBXW7 mutants showed that R465H/R465C/R479Q had better 5-year OS than other mutant types (76.9% vs. 56.0%; p=0.012).

CONCLUSIONS

There was no strong association between patient prognosis and FBXW7 mutations in our large-scale study.

Expression of cyclin A, cyclin E and p27 in normal, hyperplastic and frankly malignant endometrial samples

Cellular growth is under the control of certain molecules such as cyclins and cyclin dependent kinases. Dysregulation of these proteins disrupt cell cycle and may trigger malignant transformation. Cyclins and kinase inhibitors also play essential roles in endometrial cellular proliferation. But the exact roles of these mediators in the disease process is not clear. We evaluated expression of cyclin A, cyclin E and p27 in normal, hyperplastic and malignant endometrial samples assuming different expression patterns in physiological and pathological processes. A total of 75 patients with histopathological diagnosis of normal proliferative, hyperplastic or malignant endometrial samples were evaluated with different cellular proliferation markers, cyclin A, cyclin E and p27. For cyclin E, endometrial cancer samples had higher rate of immunoreactivity than normal proliferative and hyperplastic endometrial samples. Staining properties for cyclin A were comparable for three groups. However, p27 immunoreactivity decreased progressively as lesions progress from proliferative benign endometrium to frank carcinoma. Further large-scale studies with clinical follow-up will reveal the exact role of cyclins on endometrial carcinogenesis.

FBXW7 mutations in patients with advanced cancers: clinical and molecular characteristics and outcomes with mTOR inhibitors

PURPOSE

FBXW7 is a tumor suppressor gene responsible for the degradation of several proto-oncogenes. Preclinical data suggest that FBXW7 mutations sensitize cells to mTOR inhibitors. Clinicopathologic characteristics of cancer patients with FBXW7 mutations and their responses to mTOR inhibitors remain unknown.

METHODS

Using multiplex gene panels we evaluated how the FBXW7 mutation affected the cancer phenotype of patients referred to a phase I clinic starting January 2012. Whenever possible patients positive for FBXW7 mutation were treated with regimens containing an mTOR inhibitors and their outcomes were reviewed.

RESULTS

FBXW7 mutations were detected in 17 of 418 patients (4.0%). Among tumor types with more than 10 patients tested, FBXW7 mutations occurred in colorectal cancer (7/49; 14.3%), squamous cell cancer of head and neck (2/18; 11.1%), liver (1/13; 7.7%), and ovarian cancers (1/40; 2.5%). No one clinical, pathological or demographic feature was characteristic of the FBXW7-mutated patient population. The mutation occurred in isolation in only 2/17 (12%) patients, and KRAS was frequently found as a concomitant mutation, especially in patients with colorectal cancer (6/7; 86%). Ten patients were treated on a protocol containing an mTOR inhibitor, with a median time to treatment failure of 2.8 months (range, 1.3-6.8). One patient with liver cancer (fibrolamellar subtype) continues to have a prolonged stable disease for 6.8+ months.

CONCLUSION

In patients with advanced cancers, somatic mutations in FBXW7 usually occur with other simultaneous molecular aberrations, which can contribute to limited therapeutic efficacy of mTOR inhibitors.

MiR-495 regulates proliferation and migration in NSCLC by targeting MTA3

Our previous studies have showed that metastasis-associated protein 3 (MTA 3) is overexpressed in non-small cell lung cancer (NSCLC) tissue, and increased MTA3 mRNA levels is a risk factor of lymph node metastasis. Using bioinformatics analyses, we found that MTA3 was a potential target of miR-495. However, the pathophysiological role of miR-495 and its relevance to the growth and development of NSCLC have yet to be investigated. The purpose of this study was to elucidate the molecular mechanisms by which miR-495 acts as a tumor suppressor in NSCLC. qRT-PCR data showed significant downregulation of miR-495 in 56 NSCLC tissue samples and 5 lung cancer cell lines, compared with their adjacent normal tissue; furthermore, western blotting analysis revealed MTA3 protein was overexpressed in the tumor samples compared with the matched adjacent normal tissue. MiR-495 was shown to not only inhibit the proliferation of lung cancer cells (A549 and Calu-3) but also to inhibit cell migration in vitro. Using western blotting and luciferase assays, MTA3 was identified as a target of miR-495. These findings suggest the importance of miR-495 targeting of MTA3 in the regulation of lung cancer growth and migration.

Overexpression of TROP2 predicts poor prognosis of patients with cervical cancer and promotes the proliferation and invasion of cervical cancer cells by regulating ERK signaling pathway

Overwhelming evidence has demonstrated that the aberrant expression of the human trophoblast cell-surface antigen (TROP2) was associated with tumor aggressiveness and poor prognosis in a variety of human cancers, however the roles of TROP2 in cervical cancer have not been investigated. The purpose of our study was to elucidate the prognostic significance of TROP2 expression in patients with cervical cancer and determine its effect on tumor progression. Immunohistochemistry assay showed that 88.7% (94/106 cases) of cervical cancer specimens were positively stained with TROP2, and the overexpression of TROP2 was closely related with FIGO stage, histological grades, lymphatic metastasis, invasive interstitial depth and high expression of Ki-67. Patients with TROP2-positive staining exhibited a significantly decreased overall survival and progression free survival; it was also an independent predictor for prognosis according to multivariate analysis. Moreover, down-regulation of TROP2 mediated by siRNA in Siha and CaSki cells resulted in a strong inhibition of proliferation and invasion, TROP2 abrogation also elevated the apoptotic ratio and caused G1 arrest. Conversely, enforced expression of TROP2 in HeLa and C33A cells remarkably promoted cell growth, migration and invasion. In addition, the tumorigenic function of TROP2 was associated with the increased expressions of cyclin D1, cyclin E, CDK2 and CDK4 but reduced expression of p27 and E-cadherin via the activation of Erk1/2 signaling pathway. Furthermore, the inhibition of TROP2 expression in cervical cancer cell lines enhances sensitivity to cisplatin. The present study suggest that overexpression of TROP2 may play crucial roles in the development and pathogenesis of human cervical cancer, therefore, TROP2 may represent a prospective prognostic indicator and a potential therapeutic target of cervical cancer.

The glomuvenous malformation protein Glomulin binds Rbx1 and regulates cullin RING ligase-mediated turnover of Fbw7

Fbw7, a substrate receptor for Cul1-RING-ligase (CRL1), facilitates the ubiquitination and degradation of several proteins, including Cyclin E and c-Myc. In spite of much effort, the mechanisms underlying Fbw7 regulation are mostly unknown. Here, we show that Glomulin (Glmn), a protein found mutated in the vascular disorder glomuvenous malformation (GVM), binds directly to the RING domain of Rbx1 and inhibits its E3 ubiquitin ligase activity. Loss of Glmn in a variety of cells, tissues, and GVM lesions results in decreased levels of Fbw7 and increased levels of Cyclin E and c-Myc. The increased turnover of Fbw7 is dependent on CRL and proteasome activity, indicating that Glmn modulates the E3 activity of CRL1(Fbw7). These data reveal an unexpected functional connection between Glmn and Rbx1 and demonstrate that defective regulation of Fbw7 levels contributes to GVM.

RNA interference-mediated silencing of NANOG reduces cell proliferation and induces G0/G1 cell cycle arrest in breast cancer cells

Since the processes of normal embryogenesis and neoplasia share many of similar pathways, tumor development has been interpreted as an abnormal form of organogenesis. NANOG is a homeodomain-containing transcription factor that functions to maintain self-renewal and proliferation of embryonic stem cells (ESCs). Aberrant expression of NANOG has been observed in many types of human malignancies. However, its potential implication in tumorigenesis has not been fully clarified. In this study, we have employed small interference RNA (RNAi) technology to silence endogenous NANOG expression in breast cancer cells and successfully selected three independent clones with stably inhibited NANOG expression of MCF-7 cells. Functional analysis revealed that down-regulation of NANOG reduced cell proliferation, colony formation and migration ability of MCF-7 cells. Consistently, proliferation of breast cancer MDA-MB-231 cells was also significantly inhibited after the knockdown of NANOG expression. Interestingly, we found that the expression levels of cyclinD1 and c-myc were markedly down-regulated and the cell cycle were blocked at the G0/G1 phases after the knockdown of NANOG, while the expression of cyclinE and signal transducers and activators of transcription3 (STAT3) remained unaffected. In addition, the expression of NANOG and cyclinD1 can be rescued after the transfection of pcDNA3.1 (-)-NANOG expression vector into the three clones. Finally, our chromatin immunoprecipitation (ChIP) experiment showed that NANOG protein can bind to the promoter region of cyclinD1 and regulate cells cycle. Taken together, our findings may not only establish a molecular basis for the role of NANOG in modulating cell cycle progression of breast cancer cells but also suggest a potential target for the treatment of at least some subtypes of breast cancer.

Fbxw7 regulates lipid metabolism and cell fate decisions in the mouse liver

E3 ubiquitin ligase complexes of the SCF type consist of ring-box 1 (Rbx1), cullin 1 (Cul1), S-phase kinase-associated protein 1 (Skp1), and a member of the F-box family of proteins. The identity of the F-box protein determines the substrate specificity of the complex. The F-box family member F-box- and WD repeat domain-containing 7 (Fbxw7; also known as Fbw7, SEL-10, hCdc4, and hAgo) targets for degradation proteins with wide-ranging functions, and uncovering its in vivo role has been difficult, because Fbxw7-/- embryos die in utero. Using two different Cre-loxP systems (Mx1-Cre and Alb-Cre), we generated mice with liver-specific null mutations of Fbxw7. Hepatic ablation of Fbxw7 resulted in hepatomegaly and steatohepatitis, with massive deposition of triglyceride, a phenotype similar to that observed in humans with nonalcoholic steatohepatitis. Both cell proliferation and the abundance of Fbxw7 substrates were increased in the Fbxw7-deficient liver. Long-term Fbxw7 deficiency resulted in marked proliferation of the biliary system and the development of hamartomas. Fbxw7 deficiency also skewed the differentiation of liver stem cells toward the cholangiocyte lineage rather than the hepatocyte lineage in vitro. This bias was corrected by additional loss of the Notch cofactor RBP-J, suggesting that Notch accumulation triggered the abnormal proliferation of the biliary system. Together, our results suggest that Fbxw7 plays key roles, regulating lipogenesis and cell proliferation and differentiation in the liver.

Indirect participation of Hsp90 in the regulation of the cyclin E turnover

Cyclin E is the Cdk2-regulatory subunit required for the initiation of DNA replication at the G1/S transition. It accumulates in late G1 phase and gets rapidly degraded by the ubiquitin/proteasome pathway during S phase. The degradation of cyclin E is a consequence of its phosphorylation and subsequent isomerization by the peptidyl-prolyl isomerase Pin1. We show that in the colon cancer cells HT-29 the inhibition of the chaperone function of Hsp90 by geldanamycin (GA) enhances the ubiquitinylation of cyclin E and triggers active degradation via the proteasome pathway. As Hsp90 forms multiprotein complexes with and regulates the function and cell contents of numerous signaling proteins, this observation suggests a direct interaction between Hsp90 and cyclin E. However, experiments using cell lysate fractionation did not reveal the presence of complexes containing both Hsp90 and cyclin E. Coupled transcription/translation experiments also failed to detect the formation of complexes between newly synthesized cyclin E and Hsp90. We conclude that Hsp90 can regulate the degradation of cellular proteins without binding to them, by an indirect mechanism. This conclusion postulates a new category of proteins that are affected by the inactivation of Hsp90. Our observations do not support the possible involvement of a PPIase in this indirect mechanism. Besides, we did not observe active geldanamycin-dependent degradation of cyclin E in the prostate cancer-derived cell line DU-145, indicating that the Hsp90-dependent stabilization of cyclin E requires specific regulatory mechanism which may be lost in certain types of cancer cells.

Aneuploidy predicts outcome in patients with endometrial carcinoma and is related to lack of CDH13 hypermethylation

PURPOSE

Many investigators have reported that aneuploidy detected by flow cytometry is a useful prognostic marker in patients with endometrial cancer. Laser scanning cytometry (LSC) is a technology similar to flow cytometry but is more feasible for clinical laboratory use. We evaluated the usefulness of DNA ploidy detected by LSC as a prognostic marker in patients with endometrial cancer and investigated genetic and epigenetic factors related to aneuploidy.

EXPERIMENTAL DESIGN

Endometrial cancer specimens from 106 patients were evaluated. The methylation status of CDH13, Rassf1, SFRP1, SFRP2, SFRP4, SFRP5, p16, hMLH1, MGMT, APC, ATM, and WIF1 and mutations in the p53 and CDC4 genes were investigated. LSC was carried out to determine DNA ploidy. Fluorescence in situ hybridization was done with chromosome-specific centromeric probes to assess chromosomal instability.

RESULTS

Univariate and multivariate analyses revealed that p53 mutation and lack of CDH13 hypermethylation associated positively with aneuploidy. Univariate analysis showed that aneuploidy, chromosomal instability, and lack of CDH13 hypermethylation as well as surgical stage were significantly predictive of death from endometrial cancer. Furthermore, multivariate analysis revealed that stage in combination with either DNA aneuploidy or lack of CDH13 hypermethylation was an independent prognostic factor.

CONCLUSION

These results suggest that analysis of DNA ploidy and methylation status of CDH13 may help predict clinical outcome in patients with endometrial cancer. Prospective randomized trials are needed to confirm the validity of an individualized approach, including determination of tumor ploidy and methylation status of CDH13, to management of endometrial cancer patients.

Fbw7 regulates the activity of endoreduplication mediators and the p53 pathway to prevent drug-induced polyploidy

Fbw7 is a tumor suppressor that is mutated in numerous cancers. It encodes an E3 ubiquitin ligase, whose ability to decrease the levels of pivotal regulators of cell growth and proliferation underlies its tumor suppressor function. Here, we explore the consequences of Fbw7 inactivation on the outcome of chemotherapeutic treatments. When exposed to spindle toxins such as vinblastine and taxol, Fbw7-deficient cells undergo extensive mitotic slippage and endoreduplication, rendering them polyploid. A combined deregulation of several Fbw7 target proteins is required for this phenotype. Specifically, elevated expression of cyclin E and Aurora A in Fbw7-deficient cells is required for drug-induced polyploidy. However, overexpression of either cyclin E or Aurora A alone is not sufficient for drug-induced polyploidy. In addition, we demonstrate that Fbw7 deficiency limits the ability of p53 to respond to mitotic toxins but not to DNA damage. Furthermore, Fbw7 expression regulates the p53-dependent induction of genes such as Lats2 and p21 in response to vinblastine. Hence, we suggest that Fbw7 serves as a master regulator of the mitotic and tetraploidy checkpoints.

FBW7 ubiquitin ligase: a tumour suppressor at the crossroads of cell division, growth and differentiation

FBW7 (F-box and WD repeat domain-containing 7) is the substrate recognition component of an evolutionary conserved SCF (complex of SKP1, CUL1 and F-box protein)-type ubiquitin ligase. SCF(FBW7) degrades several proto-oncogenes that function in cellular growth and division pathways, including MYC, cyclin E, Notch and JUN. FBW7 is also a tumour suppressor, the regulatory network of which is perturbed in many human malignancies. Numerous cancer-associated mutations in FBW7 and its substrates have been identified, and loss of FBW7 function causes chromosomal instability and tumorigenesis. This Review focuses on structural and functional aspects of FBW7 and its role in the development of cancer.

Adenovirus E1B55K region is required to enhance cyclin E expression for efficient viral DNA replication

Adenoviruses (Ads) with E1B55K mutations can selectively replicate in and destroy cancer cells. However, the mechanism of Ad-selective replication in tumor cells is not well characterized. We have shown previously that expression of several cell cycle-regulating genes is markedly affected by the Ad E1b gene in WI-38 human lung fibroblast cells (X. Rao, et al., Virology 350:418-428, 2006). In the current study, we show that the Ad E1B55K region is required to enhance cyclin E expression and that the failure to induce cyclin E overexpression due to E1B55K mutations prevents viral DNA from undergoing efficient replication in WI-38 cells, especially when the cells are arrested in the G(0) phase of the cell cycle by serum starvation. In contrast, cyclin E induction is less dependent on the function encoded in the E1B55K region in A549 and other cancer cells that are permissive for replication of E1B55K-mutated viruses, whether the cells are in the S phase or G(0) phase. The small interfering RNA that specifically inhibits cyclin E expression partially decreased viral replication. Our study provides evidence suggesting that E1B55K may be involved in cell cycle regulation that is important for efficient viral DNA replication and that cyclin E overexpression in cancer cells may be associated with the oncolytic replication of E1B55K-mutated viruses.

Cyclin E overexpression impairs progression through mitosis by inhibiting APC(Cdh1)

Overexpression of cyclin E, an activator of cyclin-dependent kinase 2, has been linked to human cancer. In cell culture models, the forced expression of cyclin E leads to aneuploidy and polyploidy, which is consistent with a direct role of cyclin E overexpression in tumorigenesis. In this study, we show that the overexpression of cyclin E has a direct effect on progression through the latter stages of mitotic prometaphase before the complete alignment of chromosomes at the metaphase plate. In some cases, such cells fail to divide chromosomes, resulting in polyploidy. In others, cells proceed to anaphase without the complete alignment of chromosomes. These phenotypes can be explained by an ability of overexpressed cyclin E to inhibit residual anaphase-promoting complex (APC^Cdh1) activity that persists as cells progress up to and through the early stages of mitosis, resulting in the abnormal accumulation of APC^Cdh1 substrates as cells enter mitosis. We further show that the accumulation of securin and cyclin B1 can account for the cyclin E–mediated mitotic phenotype.

Structure of a Fbw7-Skp1-cyclin E complex: multisite-phosphorylated substrate recognition by SCF ubiquitin ligases

The ubiquitin-mediated proteolysis of cyclin E plays a central role in cell-cycle progression, and cyclin E accumulation is a common event in cancer. Cyclin E degradation is triggered by multisite phosphorylation, which induces binding to the SCF(Fbw7) ubiquitin ligase complex. Structures of the Skp1-Fbw7 complex bound to cyclin E peptides identify a doubly phosphorylated pThr380/pSer384 cyclin E motif as an optimal, high-affinity degron and a singly phosphorylated pThr62 motif as a low-affinity one. Biochemical data indicate that the closely related yeast SCF(Cdc4) complex recognizes the multisite phosphorylated Sic1 substrate similarly and identify three doubly phosphorylated Sic1 degrons, each capable of high-affinity interactions with two Cdc4 phosphate binding sites. A model that explains the role of multiple cyclin E/Sic1 degrons is provided by the findings that Fbw7 and Cdc4 dimerize, that Fbw7 dimerization enhances the turnover of a weakly associated cyclin E in vivo, and that Cdc4 dimerization increases the rate and processivity of Sic1 ubiquitination in vitro.

A jekyll and hyde role of cyclin E in the genotoxic stress response: switching from cell cycle control to apoptosis regulation

Cyclin E protein levels and associated kinase activity rise in late G(1) phase, reach a peak at the G(1)/S transition, and quickly decline during S phase. The cyclin E/Cdk2 complex has a well-established function in regulating two fundamental biological processes: cell cycle progression and DNA replication. However, cyclin E expression is deregulated in a wide range of tumors. Our recent reports have uncovered a critical role for cyclin E, independent of Cdk2, in the cell death of hematopoietic tumor cells exposed to genotoxic stress. An 18-kD C-terminal fragment of cyclin E, p18-cyclin E, which is generated by caspase-mediated cleavage in hematopoietic cells during genotoxic stress-induced apoptosis has a critical role in the amplification of the intrinsic apoptotic pathway. By interacting with Ku70, p18-cyclin E liberates Bax, which participates in the amplification of apoptosis by sustaining a positive feedback loop targeting mitochondria. This process is independent of p53 function and new RNA or protein synthesis. Therefore, cyclin E emerges as an arbiter of the genotoxic stress response by regulating a finite physiological balance between cell proliferation and death in hematopoietic cells.

CENP-F expression is associated with poor prognosis and chromosomal instability in patients with primary breast cancer

DNA microarrays have the potential to classify tumors according to their transcriptome. Tissue microarrays (TMAs) facilitate the validation of biomarkers by offering a high-throughput approach to sample analysis. We reanalyzed a high profile breast cancer DNA microarray dataset containing 96 tumor samples using a powerful statistical approach, between group analyses. Among the genes we identified was centromere protein-F (CENP-F), a gene associated with poor prognosis. In a published follow-up breast cancer DNA microarray study, comprising 295 tumour samples, we found that CENP-F upregulation was significantly associated with worse overall survival (p<0.001) and reduced metastasis-free survival (p<0.001). To validate and expand upon these findings, we used 2 independent breast cancer patient cohorts represented on TMAs. CENP-F protein expression was evaluated by immunohistochemistry in 91 primary breast cancer samples from cohort I and 289 samples from cohort II. CENP-F correlated with markers of aggressive tumor behavior including ER negativity and high tumor grade. In cohort I, CENP-F was significantly associated with markers of CIN including cyclin E, increased telomerase activity, c-Myc amplification and aneuploidy. In cohort II, CENP-F correlated with VEGFR2, phosphorylated Ets-2 and Ki67, and in multivariate analysis, was an independent predictor of worse breast cancer-specific survival (p=0.036) and overall survival (p=0.040). In conclusion, we identified CENP-F as a biomarker associated with poor outcome in breast cancer and showed several novel associations of biological significance.

hCDC4 variation in osteosarcoma

The hCDC4 gene (also known as Fbw7 or Archipelago) encodes an F-box protein that is responsible for targeting cyclin E for Skp1-cullin-F box protein (SCF) ubiquitination and proteosomal degradation. Disruption of this pathway has been associated with chromosomal instability and aneuploidy in several cancer cell lines and primary tumors. This study aimed to examine whether hCDC4 mutations contribute to aneuploidy in osteosarcoma. We analyzed 147 primary high-grade osteosarcoma specimens and 6 osteosarcoma cell lines. The protein truncation test (PTT) and single-strand conformation polymorphism (SSCP) analysis with subsequent sequencing were performed to detect alterations of the hCDC4 gene. All specimens exhibited the same PTT pattern of normal bands with less intense common bands. Two shifts were detected by SSCP, and subsequent DNA analysis identified one in-frame three-base GAG (424-426) deletion and one silent nucleotide substitution (C1261T). We conclude that somatic hCDC4 mutations are infrequent in osteosarcoma, and are unlikely to play an important role in aneuploidy of this tumor.

Deregulation of Cyclin E induces chromosomal instability in human colon cancer

AIM: To study the influence of stably inducible expression of Cyclin E on the chromosomal instability in human colon cancer cell line DLD1.

METHODS: Tetracycline-responsive gene-inducible cell line DLD1tTA-cyclin E was generated. Western blot was used to examine the induction of Cyclin E expression upon removal of doxycycline. 4'-6-Diamidino-2-phenylindole (DAPI) staining was performed to detect the percentage of cells with chromosomal instability 1, 3, 5, 7 and 14 d after the induction of Cyclin E expression.

RESULTS: Western blot showed that the peak of Cyclin E expression appeared 96 h after induction. The percentage of cells with chromosomal instability in tet-off DLD1tTA-Cyclin E cells ranged from 0.97% to 1.22% (t = 3.81, P > 0.01). However, the percentages of cells with chromosomal instability were 2.41%, 3.63%, 3.92%, 6.17% and 8.34%, respectively, 1, 3, 5, 7 and 14 d after the induction of Cyclin E expression (t = 4.77, P < 0.01).

CONCLUSION: Deregulation of cyclin E can induce the formation of aneuploidy in human colon cancer cell line DLD1, and it also plays an important role in the pathway of chromosomal instability.

Diffuse large B-cell lymphoma with overexpression of cyclin e substantiates poor standard treatment response and inferior outcome

PURPOSE

Gold standard to predict survival and stratify patients for risk-adapted therapy in diffuse large B-cell lymphoma (DLBCL) is the international prognostic index, although it does not consider the molecular heterogeneity of DLBCL. Deregulation of cyclin E (CCNE) is a strong predictor of poor prognosis in some neoplastic diseases. In tumor cells, it induces chromosomal instability with an increased rate of aneuploidy/polyploidy.

EXPERIMENTAL DESIGN

We analyzed in this retrospective study the prognostic value of immunohistochemical CCNE expression on a validated tissue microarray containing 101 de novo DLBCLs and, in 9 cases, the CCNE-induced chromosomal instability as assessed by cytometry.

RESULTS

Forty-six of 98 evaluable DLBCLs expressed CCNE in a mean proportion of 20 +/- 29% of tumor cells; 38 cases expressed CCNE in >/=20% of tumor cells. CCNE-positive samples were aneuploid compared with near tetraploidy in CCNE-negative cases. Multivariate analysis showed CCNE expression in >/=20% of tumor cells to be an international prognostic index-independent, Adriamycin-based treatment-independent, and BCL2-independent prognostic factor for poor disease-specific survival. CCNE expression in >/=80% of tumor cells was associated with dismal short-term prognosis. CCNE expression in >/=50% of tumor cells emerged as an independent predictive factor for standard CHOP treatment resistance.

CONCLUSIONS

CCNE expression assessment is easy on paraffin-embedded tissue. The high prognostic value of CCNE expression in DLBCL may be the basis for future prospective trials. In addition, a high CCNE expression hints at the presence of a possible target for individualized cancer therapy.

Deregulated cyclin E promotes p53 loss of heterozygosity and tumorigenesis in the mouse mammary gland

Deregulation of cyclin E expression and/or high levels have been reported in a variety of tumors and have been used as indicators of poor prognosis. Although the role that cyclin E plays in tumorigenesis remains unclear, there is evidence that it confers genomic instability when deregulated in cultured cells. Here we show that deregulated expression of a hyperstable allele of cyclin E in mice heterozygous for p53 synergistically increases mammary tumorigenesis more than that in mice carrying either of these markers individually. Most tumors and tumor-derived cell lines demonstrated loss of p53 heterozygosity. Furthermore, this tumor susceptibility is related to the number of times the transgene is induced indicating that it is directly attributable to the expression of the cyclin E transgene. An indirect assay indicates that loss of p53 function is an early event occurring in the mammary epithelia of midlactation mammary glands in which cyclin E is deregulated long before evidence of malignancy. These data support the hypothesis that deregulated expression of cyclin E stimulates p53 loss of heterozygosity by promoting genomic instability and provides specific evidence for this in vivo. Cyclin E deregulation and p53 loss are characteristics often observed in human breast carcinoma.

CDC4 mutations occur in a subset of colorectal cancers but are not predicted to cause loss of function and are not associated with chromosomal instability

CDC4/FBXW7 is part of a ubiquitin ligase complex which targets molecules such as cyclin E, c-myc, and c-jun for destruction. CDC4 mutations occur in several cancer types and are best described in colorectal tumors. Knockout of CDC4 in vitro in colorectal cancer cells causes changes suggestive of chromosomal instability (CIN). In p53(+/-) mice, radiation-induced lymphomas show deletion or mutation of one copy of CDC4 and knockdown of CDC4 leads to increased aneuploidy in mouse fibroblasts. We screened 244 colorectal tumors and 40 cell lines for CDC4 mutations and allelic loss. Six percent (18 of 284) of tumors, including near-diploid (CIN-) lesions, harbored CDC4 mutations and there was no association between mutation and CIN (polyploidy). The CDC4 mutation spectrum in colorectal tumors was heavily biased towards C:G > T:A changes, either missense mutations at critical arginine residues or nonsense changes in the 5' half of the gene. The reasons for this odd mutation spectrum were unclear but C:G > T:A changes were not found more often than expected at APC, K-ras, or p53 in the same tumors and we found no specific defects in DNA repair to account for the observations. No colorectal tumor was found to carry two CDC4 mutations predicted to abolish protein function; partial loss of CDC4 function may therefore cause tumorigenesis. The in vitro studies, therefore, did not assess the functional effects of mutant alleles which are found in vivo. CDC4 mutations may be selected primarily to drive progression through the cell cycle although CIN might be an important secondary effect in some cancers.

Traffic safety for the cell: influence of cyclin-dependent kinase activity on genomic stability

Genomic instability has long been considered a key factor in tumorigenesis. Recent evidence suggests that DNA damage may be widespread in early pre-neoplastic states, with deregulation of cyclin-dependent kinase (Cdk) activity a driving force. Increased Cdk activity may critically reduce licensing of origins of DNA replication, drive re-replication, or mediate overexpression of checkpoint proteins, inducing deleterious cell cycle delay. Conversely, inhibition of Cdk activity may compromise replication efficiency, expression of checkpoint proteins, or activation of DNA repair proteins. These vital functions point to the impact of Cdk activity on the stability of the genome. Insight into these pathways may improve our understanding of tumorigenesis and lead to more rational cancer therapies.

The importance of being big

The ultimate stem cell, the oocyte, is frequently very large. For example, Drosophila and Xenopus oocytes are approximately 10(5) times larger than normal somatic cells. Importantly, once the large oocytes are fertilized, the resulting embryonic cells proliferate rapidly. Moreover, these divisions occur in the absence of cell growth and are not governed by normal cell cycle controls. Observations suggest that mitogens and cell growth signals modulate proliferation by upregulating G1-phase cyclins, which in turn promote cell division. Like embryonic cells, the proliferation of cancer cells is largely independent of mitogens and growth factors. This occurs, in part, because many proteins that are known to modulate G1-phase cyclin activity are frequently mutated in cancer cells. Interestingly, we have found that both the expression and the activity of G1-phase cyclins is modulated by growth rate and cell size in yeast. These and other data suggest that proliferative capacity correlates with cell size. Thus, a major goal of our laboratory is to use yeast to investigate the relationship between proliferation rate, G1-phase cyclins, growth rate, and cell size. The elucidation of this relationship will help clarify the role of cell size in promoting proliferation in both normal and cancer cells.

The loss of PIN1 deregulates cyclin E and sensitizes mouse embryo fibroblasts to genomic instability

During the G0/G1-S phase transition, the timely synthesis and degradation of key regulatory proteins is required for normal cell cycle progression. Two of these proteins, c-Myc and cyclin E, are recognized by the Cdc4 E3 ligase of the Skp1/Cul1/Rbx1 (SCF) complex. SCF(Cdc4) binds to a similar phosphodegron sequence in c-Myc and cyclin E proteins resulting in ubiquitylation and degradation of both proteins via the 26 S proteosome. Since the prolyl isomerase Pin1 binds the c-Myc phosphodegron and participates in regulation of c-Myc turnover, we hypothesized that Pin1 would bind to and regulate cyclin E turnover in a similar manner. Here we show that Pin1 regulates the turnover of cyclin E in mouse embryo fibroblasts. Pin1 binds to the cyclin E-Cdk2 complex in a manner that depends on Ser384 of cyclin E, which is phosphorylated by Cdk2. The absence of Pin1 results in an increased steady-state level of cyclin E and stalling of the cells in the G1/S phase of the cell cycle. The cellular changes that result from the loss of Pin1 predispose Pin1 null mouse embryo fibroblasts to undergo more rapid genomic instability when immortalized by conditional inactivation of p53 and sensitizes these cells to more aggressive Ras-dependent transformation and tumorigenesis.

The retinoblastoma protein--from bench to bedside

The retinoblastoma tumour suppressor protein (Rb) has come a long way since its initial discovery in 1986. Encoded by the first candidate tumour suppressor gene it has emerged a versatile and context-dependent modulator of cell behaviour. Its activity is managed by signalling networks sensing intra- and extracellular cues. These cues are relayed to hold or permit inactivation of Rb by phosphorylation. Loss or mutation of the retinoblastoma gene is rare in sporadic cancers but defects in the pathways that license inactivation of Rb are found in the majority of them, suggesting that loss of Rb control is central to tumour development and arguing that its reinstatement might reverse tumour formation. Furthermore, mouse models with engineered defects in the Rb-phosphorylating kinases provide evidence that moderation of Rb inactivation may be a strategy for the prevention of tumour formation. The rationale behind these arguments, their underlying molecular concepts and strategies towards therapeutic application will be discussed.

Ras activity regulates cyclin E degradation by the Fbw7 pathway

The Skp1-Cullin1 F-box protein-Fbw7 ubiquitin ligase regulates phosphorylation-dependent cyclin E degradation, and disruption of this pathway is associated with genetic instability and tumorigenesis. Fbw7 is a human tumor suppressor that is targeted for mutation in primary cancers. However, mechanisms other than mutation of Fbw7 may also disrupt cyclin E proteolysis in cancers. We show that oncogenic Ha-Ras activity regulates cyclin E degradation by the Fbw7 pathway. Activated Ras impairs Fbw7-driven cyclin E degradation, and, conversely, inhibition of normal Ras activity decreases cyclin E abundance. Moreover, activation of the mitogen-activated protein kinase pathway is the essential Ras function that inhibits cyclin E turnover, and activated Ha-Ras expression inhibits both the binding of cyclin E to Fbw7 and cyclin E ubiquitination. Last, we found that oncogenic Ras activity potentiates cyclin E-induced genetic instability but only when cyclin E is susceptible to degradation by Fbw7. Thus, we conclude that Ras activity regulates Fbw7-mediated cyclin E proteolysis and suggest that impaired cyclin E proteolysis is a mechanism through which Ras mutations promote tumorigenesis.

Infrequent mutations of Archipelago (hAGO, hCDC4, Fbw7) in primary ovarian cancer

OBJECTIVE

Archipelago (AGO, also known as hCdc4, Fbw7, or Sel-10) is an F-box containing component of the SCF complex implicated in the ubiquitination and proteolysis of cyclin E and c-Myc, and found to be mutated in 16% of endometrial carcinomas. We have previously reported somatic mutations in AGO in 3/10 ovarian cancer cell lines, but the frequency of such mutations in primary ovarian cancer is unknown.

METHODS

The coding sequence of AGO was analyzed in 95 primary sporadic ovarian tumors and 16 cases of familial ovarian cancer, and correlated with levels of cyclin E and c-Myc protein expression. Constructs encoding mutations in AGO were transfected into an AGO-null cell line to directly test their ability to regulate cyclin E and c-Myc levels.

RESULTS

Mutations were present in only 2 of 95 sporadic cases: a premature stop within the WD domain (471 Ter) and a missense change near the F-box (S245T). Both primary tumor specimens containing these mutations showed high levels of cyclin E and c-Myc, but reconstitution of an AGO-null cell line with constructs encoding these mutations showed 471 Ter to be inactive in regulating endogenous cyclin E and c-Myc levels, while the S245T mutant was indistinguishable from wild-type. No germ-line mutations were found in familial cases of ovarian cancer.

CONCLUSION

Somatic AGO mutations are infrequent in primary ovarian cancers and are unlikely to contribute to familial ovarian cancer. Reconstitution experiments, rather than measuring tumor levels of cyclin E and c-Myc, provide an effective approach to determine the functional significance of AGO mutations identified in human cancers.

Mechanisms leading to chromosomal instability

Chromosomal instability is a common feature of cancer cells. Several cellular mechanisms lead to numerical and structural chromosomal instability in cancer cells, including defects in chromosomal segregation, cellular checkpoints that guard against reproduction of abnormal cells, telomere stability, and the DNA damage response. Human papillomavirus interferes with these processes, causing chromosomal instability and tumor formation in some of the epithelial cells which it infects. The rate of discoveries about the mechanisms leading to chromosomal instability in cancer cells is increasing rapidly. Although these mechanisms were thought to be unrelated, they are intimately intertwined, connecting the complex network of cellular pathways. Since chromosomal instability is undoubtedly a major cause of tumor evasion of therapy, understanding the mechanisms leading to chromosomal instability has major translational significance.

Cell cycle, proteolysis and cancer

Research in the past 15 years has shown that the mammalian cell cycle is controlled by the action of cyclin-dependent kinases (CDKs). A crucial substrate of the CDKs in G1-phase is the retinoblastoma tumor suppressor (pRB), which restrains proliferation largely by repressing the activity of the E2F transcription factors. More recent work has shown that the cell cycle is also a tale of two classes of ubiquitin ligases, referred to as SCF and APC/C ligases. CDKs, E2F and ubiquitin ligases reciprocally regulate each other, resulting in complex feedback loops. Perturbation of this network of molecular machines is associated with proliferative diseases, including cancer.

Aberrant expression of cell cycle regulators in Hodgkin and Reed-Sternberg cells of classical Hodgkin's lymphoma

The characteristic Hodgkin and Reed-Sternberg cells of classical Hodgkin's lymphoma, although highly positive for proliferation markers, do not accumulate to excessive cell numbers. These cells are characterized by abortive mitotic cycles, leading to multinucleation or cell death in mitosis. We have previously described high expression of G1-phase cyclins in classical Hodgkin's lymphoma, which could explain the high percentage of cells staining for proliferation markers. To further our understanding of proliferation control in classical Hodgkin's lymphoma, we extended our immunohistochemical analysis to the main S-phase cyclin, cyclin A, and its regulators p21CIP1 and p27KIP1. Expression of proliferating cell nuclear antigen (PCNA) was used as an additional marker for cells being in either S- or G2-phase. In 47% (112/239) of classical Hodgkin's lymphoma cases p21CIP1 was detected within a mean frequency of 15% positive Hodgkin's and Reed-Sternberg cells per case. Similarly, 47% (116/249) of the cases stained positively for p27KIP1 with a mean frequency of expression in Hodgkin's and Reed-Sternberg cells of 12%. In contrast, 90% of the cells in all 246 evaluable classical Hodgkin's lymphoma cases were positive for PCNA. In addition, 98% of Hodgkin's and Reed-Sternberg cells in 99% (250/253) of the cases stained strongly positive for cyclin A. These findings further corroborate the hypothesis that Hodgkin and Reed-Sternberg cells exhibit a disturbed cell cycle with an abnormally short or even absent G1-phase. In contrast to other tumors, expression of PCNA or cyclin A had no prognostic value for patient survival.

Evidence for a p27 tumor suppressive function independent of its role regulating cell proliferation in the prostate

Reduced p27 levels correlate with poor prognosis in a wide spectrum of human tumors and can accelerate tumorigenesis in mouse tissues. To determine whether p27 deficiency can accelerate tumorigenesis in tissues with inactive Rb and p53 pathways, we examined the effect of p27 status on prostate tumorigenesis in mice expressing simian virus 40 large T antigen (LT). In p27-deficient mice expressing LT, tumors progressed from high-grade prostatic intraepithelial neoplasia to poorly differentiated carcinoma at a greatly accelerated rate. p27 deficiency could not collaborate with a mutant of LT that fails to inactivate the Rb pathway alone. Furthermore, p27 deficiency does not increase the proliferation index, reduce the apoptotic index, or affect the expression of E2F-dependent genes in cells expressing LT at any stage of the disease. Expression of LT alone leads to maximal proliferation, but p27 deficiency still increases the amount of cyclin A and cyclin-dependent kinase 2-associated kinase activity in tissues. Interestingly, this model recapitulates an important feature of the human disease, specifically a high frequency of allelic loss of chromosome 16q, which is syntenic to mouse chromosome 8. Loss of heterozygosity may accelerate the inactivation of other tumor suppressors, such as E-cadherin, which are located in this interval. These experiments provide direct physiological and causal evidence that p27 has tumor suppressive functions independent of its role regulating cell proliferation.