The E2F transcription factor is a cellular target for the RB protein

Activation of RB/E2F signaling pathway is required for the modulation of hepatitis C virus core protein-induced cell growth in liver and non-liver cells

Hepatitis C virus (HCV) core protein is a multifunctional protein that affects transcription and cell growth in vitro and in vivo. Here, we confirm the proliferative activities of core protein in liver and non-liver cells and delineate part of the mechanism whereby core protein promotes cell growth. We show that core protein suppresses the expression of tumor suppressor protein p53 and cyclin-dependent kinase (CDK) inhibitor p21 and enhances the activation of cyclin-dependent kinase 2 (CDK2), the phosphorylation of retinoblastoma (Rb), the activation of the transcription factor E2F-1, and the expression of E2F-1 and S phase kinase-interacting protein 2 (SKP2) genes. Pretreatment of core protein-expressing cells with the inhibitor of CDK2, Butyrolactone I, abolished the phosphorylation of Rb, the activation of E2F-1, and inhibited the expression of E2F-1 gene and cell growth induced. Consistent with these findings, we define a new signaling pathway whereby the HCV core protein mediates cell growth in infected cells.

Prognostic significance of p16 protein levels in oropharyngeal squamous cell cancer

PURPOSE

Functional inactivation of p16 is an early and frequent event in head and neck squamous cell cancers. In this study, we sought to determine whether p16 expression is of prognostic importance in oropharyngeal squamous cell carcinoma.

EXPERIMENTAL DESIGN

p16 protein expression was evaluated by immunohistochemistry in a tissue microarray composed of 123 oropharyngeal squamous cell cancers with a mean patient follow-up time of 33 months.

RESULTS

p16 overexpression was associated with more advanced Tumor-Node-Metastasis stage and higher histologic grade. Despite this association with unfavorable features, p16 overexpression was associated with decreased 5-year local recurrence rates (11 versus 53%) and increased 5-year disease-free survival (62 versus 19%) and overall survival (60 versus 21%). In multivariate analysis, p16 expression status remained an independent prognostic factor for local recurrence, disease-free survival, and overall survival.

CONCLUSIONS

In patients with oropharyngeal squamous cell carcinoma, overexpression of p16 as determined by immunohistochemistry is associated with significantly improved prognosis and lower local recurrence rates.

4-hydroxynonenal and cell cycle

Lipid peroxidation is very low in proliferating cells and tumours and it might have a role in the regulation of cell proliferation and differentiation by acting through its products. 4-hydroxynonenal (HNE) has been proposed as a mediator of lipoperoxidation effects. It has been demonstrated that HNE can inhibit cell growth and induce differentiation in different leukemic cell lines. The onset of differentiation, induced by HNE, was accompanied by a reduction of c-myc expression. In HL-60 cells, HNE induced the accumulation of cells in the G0/G1 phase of the cell cycle. Cell cycle progression is regulated by three protein classes, the cyclins, the cyclin-dependent kinases (CDKs), and the CDK inhibitors (CKIs). In HL-60 cells, HNE decreased the expression of cyclin D1, D2 and A and caused an increase of p21 (the most important CKI) expression, whereas it did not affect CDK expressions. Since cyclins D/CDK2 and cyclin A/CDK2 phosphorylate pRB, HNE caused an increase of hypophosphorylated pRb. Hypophosphorylated pRb binds and inactivates the E2F transcription factors. Band-shift experiments demonstrated that HNE caused a decrease of "free" E2F, as well as an increase of pRb (and pRB family members) bound to E2F with consequent repression of the transcription.

p55, the Drosophila ortholog of RbAp46/RbAp48, is required for the repression of dE2F2/RBF-regulated genes

Many proteins have been proposed to be involved in retinoblastoma protein (pRB)-mediated repression, but it is largely uncertain which cofactors are essential for pRB to repress endogenous E2F-regulated promoters. Here we have taken advantage of the stream-lined Drosophila dE2F/RBF pathway, which has only two E2Fs (dE2F1 and dE2F2), and two pRB family members (RBF1 and RBF2). With RNA interference (RNAi), we depleted potential corepressors and looked for the elevated expression of groups of E2F target genes that are known to be directly regulated by RBF1 and RBF2. Previous studies have implicated histone deacetylase (HDAC) and SWI/SNF chromatin-modifying complexes in pRB-mediated repression. However, our results fail to support the idea that the SWI/SNF proteins are required for RBF-mediated repression and suggest that a requirement for HDAC activities is likely to be limited to a subset of targets. We found that the chromatin assembly factor p55/dCAF-1 is essential for the repression of dE2F2-regulated targets. The removal of p55 deregulated the expression of E2F targets that are normally repressed by dE2F2/RBF1 and dE2F2/RBF2 complexes in a cell cycle-independent manner but had no effect on the expression of E2F targets that are normally coupled with cell proliferation. The results indicate that the mechanisms of RBF regulation at these two types of E2F targets are different and suggest that p55, and perhaps p55's mammalian orthologs RbAp46 and RbAp48, have a conserved function in repression by pRB-related proteins.

Bcl-2 expression suppresses mismatch repair activity through inhibition of E2F transcriptional activity

Bcl-2 stimulates mutagenesis after the exposure of cells to DNA-damaging agents. However, the biological mechanisms of Bcl-2-mediated mutagenesis have remained largely obscure. Here we demonstrate that the Bcl-2-mediated suppression of hMSH2 expression results in a reduced cellular capacity to repair mismatches. The pathway linking Bcl-2 expression to the suppression of mismatch repair (MMR) activity involves the hypophosphorylation of pRb, and then the enhancement of the E2F-pRb complex. This is followed by a decrease in hMSH2 expression. MMR has a key role in protection against deleterious mutation accumulation and in maintaining genomic stability. Therefore, the decreased MMR activity by Bcl-2 may be an underlying mechanism for Bcl-2-promoted oncogenesis.

The E2F family: specific functions and overlapping interests

The E2F transcription factors are key regulators of cell cycle progression and the E2F field has made rapid advances since its advent in 1986. Yet, while our understanding of the roles and functions of the E2F family has made enormous progress, with each discovery new questions arise. In this review, we summarise the most recent advances in the field and discuss the remaining key questions. In particular, we will focus on how specificity is achieved among the E2Fs.

Disruption of the Rb--Raf-1 interaction inhibits tumor growth and angiogenesis

The retinoblastoma tumor suppressor protein (Rb) plays a vital role in regulating mammalian cell cycle progression and inactivation of Rb is necessary for entry into S phase. Rb is inactivated by phosphorylation upon growth factor stimulation of quiescent cells, facilitating the transition from G(1) phase to S phase. Although the signaling events after growth factor stimulation have been well characterized, it is not yet clear how these signals contact the cell cycle machinery. We had found previously that growth factor stimulation of quiescent cells lead to the direct binding of Raf-1 kinase to Rb, leading to its inactivation. Here we show that the Rb-Raf-1 interaction occurs prior to the activation of cyclin and/or cyclin-dependent kinases and facilitates normal cell cycle progression. Raf-1-mediated inactivation of Rb is independent of the mitogen-activated protein kinase cascade, as well as cyclin-dependent kinases. Binding of Raf-1 seemed to correlate with the dissociation of the chromatin remodeling protein Brg1 from Rb. Disruption of the Rb-Raf-1 interaction by a nine-amino-acid peptide inhibits Rb phosphorylation, cell proliferation, and vascular endothelial growth factor-mediated capillary tubule formation. Delivery of this peptide by a carrier molecule led to a 79% reduction in tumor volume and a 57% reduction in microvessel formation in nude mice. It appears that Raf-1 links mitogenic signaling to Rb and that disruption of this interaction could aid in controlling proliferative disorders.

E2f1 mutation induces early onset of diabetes and Sjögren's syndrome in nonobese diabetic mice

E2f1 is an important regulator of T cell proliferation, differentiation, and apoptosis that controls the transcription of a group of genes that are normally regulated at the G1 to S phase transition in the cell cycle. Insulin-dependent diabetes mellitus (IDDM) and Sjogren's syndrome (SS) are highly regulated autoimmune diseases that develop spontaneously in NOD mice. The aim of the present in vivo study was to explore the functional importance of the E2f1 molecule in IDDM and SS, in the context of whole animal physiology and pathophysiology, using E2f1-deficient NOD mice. For the experiment, we produced NOD mice homozygous for a nonfunctional E2f1 allele onto a NOD background. E2f1-deficient NOD mice developed an early and increased onset of diabetes as compared with their littermates. These mice also exhibited a defect in T lymphocyte development, leading to excessive numbers of mature T cells (CD4+ and CD8+), due to a maturation stage-specific defect in the apoptosis of thymocytes and peripheral T cells. We also found that they also exhibited a more rapid and increased entry into the S phase following antigenic stimulation of spleen cells and thymocytes in vitro. Furthermore, E2f1-deficient mice showed a profound decrease of immunoregulatory CD4+CD25+ T cells, while the spleen cells of NOD mice lacking E2f1 showed a significant increase of the proinflammatory cytokine IFN-gamma following antigenic stimulation in vitro. Consistent with these observations, E2f1 homozygous mutant NOD mice were highly predisposed to the development of IDDM and SS.

E2F target genes: unraveling the biology

The E2F transcription factors are downstream effectors of the retinoblastoma protein (pRB) pathway and are required for the timely regulation of numerous genes essential for DNA replication and cell cycle progression. Several laboratories have used genome-wide approaches to discover novel target genes of E2F, leading to the identification of several hundred such genes that are involved not only in DNA replication and cell cycle progression, but also in DNA damage repair, apoptosis, differentiation and development. These new findings greatly enrich our understanding of how E2F controls transcription and cellular homeostasis.

4-hydroxynonenal and regulation of cell cycle: effects on the pRb/E2F pathway

The hypothesis that 4-hydroxynonenal (HNE), a product of lipid peroxidation, might negatively affect cell proliferation, arose from the observation that lipid peroxidation is very low in tumors. In leukemic cells HNE inhibited cell growth and reduced c-myc and c-myb expression. HNE also induced differentiation in different leukemic cell lines. In HL-60 human leukemic cells, HNE induced the accumulation of cells in the G(0)/G(1) phase of the cell cycle accompanied by a decrease of cyclins D1, D2, and A. Moreover, HNE caused an increase in p21 expression. As cyclin D/CDK2 and cyclin A/CDK2 phosphorylate pRB, these findings suggested that pRb phosphorylation could be affected by HNE. Hypophosphorylated pRb binds and inactivates the E2F transcription factors. HNE induced the dephosphorylation of pRb and the increase in pRb/E2F1 complexes, whereas pRb/E2F4 complexes were reduced, because HNE downregulated E2F4 protein expression. The analysis of E2F binding to the P2 c-myc promoter revealed that HNE caused a decrease in "free" E2F, as well as an increase in pRb (and pRB family members) bound to E2F, with consequent repression of the transcription. In conclusion, HNE reduces E2F transcriptional activity by modifying a number of genes involved in regulation of the pRb/E2F pathway.

Life and death decisions by E2F-1

Deregulation of the transcription factor E2F-1 is a common event in most human cancers. Paradoxically, E2F-1 has been shown to have the ability to induce both cell cycle progression and programmed cell death, leading potentially to both tumour-promoting as well as tumour-suppressive effects. Although the pathway to cell cycle progression seems straightforward with a number of growth-promoting E2F target genes having been described, the pathways to apoptosis are less well defined and more complex. The discovery that E2F-1 'knockout' mice are highly tumour prone has caused a recent surge in the number of reports relating to programmed cell death. This review focuses on these recent findings, highlighting the way in which they have increased our understanding of E2F-1-induced cell death, as well as indicating the questions that remain. Insight gained as to the role of this intriguing molecule in cancer and its potential for targeted therapy will also be discussed.

Distinct mechanisms for repression of RNA polymerase III transcription by the retinoblastoma tumor suppressor protein

The retinoblastoma (RB) protein represses global RNA polymerase III transcription of genes that encode nontranslated RNAs, potentially to control cell growth. However, RNA polymerase III-transcribed genes exhibit diverse promoter structures and factor requirements for transcription, and a universal mechanism explaining global repression is uncertain. We show that RB represses different classes of RNA polymerase III-transcribed genes via distinct mechanisms. Repression of human U6 snRNA (class 3) gene transcription occurs through stable promoter occupancy by RB, whereas repression of adenovirus VAI (class 2) gene transcription occurs in the absence of detectable RB-promoter association. Endogenous RB binds to a human U6 snRNA gene in both normal and cancer cells that maintain functional RB but not in HeLa cells whose RB function is disrupted by the papillomavirus E7 protein. Both U6 promoter association and transcriptional repression require the A/B pocket domain and C region of RB. These regions of RB contribute to U6 promoter targeting through numerous interactions with components of the U6 general transcription machinery, including SNAP(C) and TFIIIB. Importantly, RB also concurrently occupies a U6 promoter with RNA polymerase III during repression. These observations suggest a novel mechanism for RB function wherein RB can repress U6 transcription at critical steps subsequent to RNA polymerase III recruitment.

Aberrant regulation of survivin by the RB/E2F family of proteins

Survivin is a putative oncogene that is aberrantly expressed in cancer cells. It has been hypothesized to play a central role in cancer progression and resistance to therapy in diverse tumor types. Although some of the transcriptional processes regulating its expression have been established, the diversity of genes that may be controlling the levels of its expression in both normal cells as well as in cancer cells has not been fully explored. The most common genetically mutated pathways in human malignancies are the p53 tumor suppressor pathway and the RB/E2F pathway. Both of these pathways, when intact, provide essential checkpoints in the maintenance of normal cell growth and protect the cell from DNA damage. Using non-transformed embryonic fibroblasts, we provide evidence of a molecular link between the regulation of survivin transcription and the RB/E2F family of proteins. We demonstrate that both pRB and p130 can interact with the survivin promoter and can repress survivin transcription. We also show that the E2F activators (E2F1, E2F2, and E2F3) can bind to the survivin promoter and induce survivin transcription. Genetically modified cells that harbor deletions in various members of the RB/E2F family confirm our data from the wild-type cells. Our findings implicate several members of the RB/E2F pathway in an intricate mechanism of survivin gene regulation that, when genetically altered during the process of tumorigenesis, may function within cancer cells to aberrantly alter survivin levels and enhance tumor progression.

Contribution of the p38MAPK signalling pathway to proliferation in human cultured airway smooth muscle cells is mitogen-specific

We have investigated the role of p38MAPK in human airway smooth muscle (HASM) proliferation in response to thrombin and bFGF. The regulation of cyclin D1 mRNA, cyclin D1, cyclin E and p21Cip1 protein levels, and the extent of retinoblastoma protein (pRb) phosphorylation in response to activation of p38MAPK have also been examined. Two distinct inhibitors of p38MAPK, SB 203580 (10 microm) and SB 202190 (10 microm), prevented bFGF (0.3-3 nm)-stimulated cell proliferation, but had no effect on the response to thrombin (0.3-3 U ml(-1)). In cells incubated with thrombin or bFGF for 20 h, there was an increase in p38MAPK phosphorylation in response to bFGF, but not to thrombin. Thrombin and bFGF-stimulated increases in ERK phosphorylation and cyclin D1 mRNA and protein levels were not influenced by SB 203580 pre-treatment. Similarly, cyclin E and p21Cip1 protein levels, measured after 20 h incubation with mitogen, did not appear to be regulated by SB 203580 (10 microm). Although both thrombin and bFGF significantly increased levels of pRb phosphorylation, SB 203580 (10 microm) inhibited only bFGF-stimulated pRb phosphorylation. In addition, SB 203580 (10 microm) selectively inhibited bFGF-stimulated DNA synthesis, suggesting that the antimitogenic actions of SB 203580 on pRb phosphorylation cause cell cycle arrest at late G1 phase. In conclusion, these results indicate that p38MAPK is involved in bFGF-, but not in thrombin-stimulated HASM proliferation. The activation of the p38MAPK pathway by bFGF, but not by thrombin, regulates the phosphorylation of pRb without influencing cyclin D1 expression.

Retinoblastoma protein is frequently absent or phosphorylated in anaplastic large-cell lymphoma

The possible role of retinoblastoma protein (Rb) in the pathogenesis of anaplastic large-cell lymphoma (ALCL) is unknown. We investigated Rb protein expression, both total (phosphorylated and underphosphorylated) and active (underphosphorylated), in four anaplastic lymphoma kinase (ALK)-positive ALCL cell lines (Karpas 299, JB-6, SU-DHL1, and SR-786) by Western blot analysis, and in 67 ALCL tumors (30 ALK-positive, 37 ALK-negative) using immunohistochemical methods. We also used fluorescence in situ hybridization and polymerase chain reaction methods to assess for loss of heterozygosity of the rb gene. The findings were correlated with apoptotic rate assessed by the terminal dUTP nick-end labeling assay. Immunoblots showed high total Rb levels in Karpas 299, SU-DHL1 and SR-786 and relatively lower levels in and JB-6. Underphosphorylated Rb was negative or expressed at low levels in all cell lines. In ALCL tumors, total Rb was detected in 44 (66%) and absent in 23 (34%). The mean apoptotic rate was 3.2% in Rb-negative tumors compared with 2.7%, 2.2%, and 1.2% in tumors with <10%, 10 to 50%, and >50% Rb-positive cells, respectively (P = 0.2, Kruskall-Wallis test). In a subset of 25 total Rb-positive tumors we assessed for underphosphorylated Rb, which was detected in 12 tumors. The detection of only total Rb in the remaining 13 tumors suggests that Rb was phosphorylated. Fluorescence in situ hybridization showed allelic loss of the rb gene in 10 (40%) of 25 tumors analyzed and was significantly associated with absence of Rb expression (P = 0.003). Similar results were obtained for loss of heterozygosity of the 13q14 locus. Five-year progression-free survival for patients with Rb-negative ALCL was 89.4% compared with 47.7% for patients with total Rb-positive ALCL (P = 0.006, log-rank test). Similar trends for progression-free survival held true for patients with ALK-positive and ALK-negative tumors analyzed separately. In conclusion, Rb is absent or phosphorylated in most ALCL cell lines and tumors and absence of Rb expression is associated with better clinical outcome in patients with ALCL.

Molecular biology of gliomas

Gliomas are the most common primary neoplasm of the brain. Unfortunately, they are often refractory to treatment and portend a poor prognosis. However, recent discoveries have shed light on the molecular events driving glioma growth, including abnormalities of three major molecular pathways: extracellular growth factors and their receptors (eg, EGF/EGFR and PDGF/PDGFR), signal transduction cascades (eg, RAS and AKT), and cell proliferation controls (eg, INK4A-ARF). Each of these abnormalities is described in detail. Efforts to inhibit abnormally activated pathways are underway through multi-institutional clinical trials.

Direct binding of apoptosis signal-regulating kinase 1 to retinoblastoma protein: novel links between apoptotic signaling and cell cycle machinery

The retinoblastoma protein Rb has antiproliferative and antiapoptotic functions. Our previous studies have shown that certain apoptotic signals can inactivate Rb via the p38 pathway. Here we show that Rb associates with the apoptosis signal-regulating kinase ASK1 in response to specific apoptotic signals. An LXCXE motif on ASK1 was required for Rb binding; this correlated with increased E2F1 transcriptional activity and up-regulation of the proapoptotic protein p73. Overexpression of Rb inhibited ASK1-induced apoptosis; in addition, an ASK1 mutant incapable of binding Rb could not induce apoptosis, indicating that ASK1 has to overcome the antiapoptotic properties of Rb to kill cells. Chromatin immunoprecipitation assays show that in asynchronous cells the p73P1 promoter is occupied predominantly by E2F3; upon tumor necrosis factor (TNF)-alpha stimulation, E2F3 is dissociated from the promoter and replaced by E2F1. At the same time, TNF-alpha stimulation causes Rb to dissociate from the p73P1 promoter. These are promoter-specific events because Rb binds to the mitogenic cdc25A promoter upon TNF-alpha stimulation. These studies suggest that Rb acts as a link between apoptotic and proliferative pathways by interacting with distinct kinases and occupying different promoters.

The nucleocytoplasmic shuttling of E2F4 is involved in the regulation of human intestinal epithelial cell proliferation and differentiation

The specific mechanisms controlling the transition from proliferation to terminal differentiation in human intestinal epithelial cells (HIEC) remain largely undefined. Herein, we analyzed the expression and localization of Rb and E2F proteins in well-established normal intestinal epithelial cell models which allow for the re-enactment of the crypt-villus axis in vitro as well as in intact epithelium and in colon cancer cells. We report that (1) expression of E2F1 is down-regulated while E2F4 protein is sequestered in the cytoplasm during G(0) arrest associated with serum deprivation, confluency, and terminal differentiation of intestinal cells; (2) concurrently, there is an accumulation of the hypophosphorylated form of the pocket proteins into the nucleus with an increased association of E2F4 with pRb and p130; (3) cells which expressed high levels of nuclear E2F4 are all positive for Ki67 staining in human fetal intestine; (4) activation of HIEC crypt cells by growth factors leads to an increase in the nuclear localization of E2F4 which may be attributable to a decrease in the serine/threonine phosphorylation of this transcription factor; (5) inhibition of p38 MAP kinase with alpha/beta inhibitor SB203580 induces E2F4 translocation into the nucleus and its transcriptional activity. In conclusion, our data suggest a key role for E2F4 in proliferation of human intestinal crypt cells and that its cytoplasmic retention as well as its sequestration by Rb proteins may represent a critical step in initiating cell-cycle exit.

Up-regulation of transcriptional factor E2F1 in papillary and anaplastic thyroid cancers

Expression of genes in the Rb-E2F signaling pathway is controlled by E2F transcriptional factors originally defined as molecules that bind to the promoter of E2 adenovirus. The E2F gene family consists of six members and is designated E2F1-6. The Rb-E2F signaling pathway is among the main regulators of the cell cycle, hence its importance in differentiation and oncogenesis. We document here up-regulation of E2F1, but not other members of the E2F gene family, in 15 of 18 primary papillary thyroid cancers examined (83%) in comparison to corresponding noncancerous thyroid tissues and in all of 11 anaplastic thyroid cancer (ATC) cell lines (100%). The E2F4 gene, however, was down-regulated in 12 of the papillary thyroid cancers (67%). Immunohistochemical analysis with antibody to E2F1 revealed prominent intracellular E2F1 protein in most of the primary papillary cancers (16 of 18; 89%) but was not detectable in normal thyroid tissues. These data indicated that increased expression of the E2F1 gene might play a significant role in human thyroid carcinogenesis through derangement of the Rb-E2F signaling pathway.

Transcriptional regulation by the retinoblastoma protein

The retinoblastoma protein (RB) plays a key role in the control of cell proliferation and mediates the terminal differentiation of certain cell types. Increasing evidence suggests that RB functions by contacting and modifying the behaviour of transcription factors. RB can form complexes with E2F and MyoD in vivo, and complexes with a number of other transcription factors have also been demonstrated in vitro. The interaction of regulatory transcription factors with RB may be explained by sequence similarity between RB and two general transcription factors: TBP and TFIIB. Here I review the evidence for a role of RB in the regulation of transcription and highlight some of the likely mechanisms of RB function.

The retinoblastoma protein as a transcriptional repressor

The retinoblastoma protein (pRB) is one of the best-studied tumour suppressor gene products. Its loss during the genesis of many human tumours, its inactivation by several DNA tumour virus oncoproteins, and its ability to inhibit cell growth when introduced into dividing cells all suggest that pRB negatively regulates some aspect of normal cell growth. The discovery that pRB associates with transcription factors such as E2F has provided the first model for pRB function. In this review, we discuss how pRB may regulate cell growth by repressing transcription of genes essential for cell proliferation.

Distinction of Endocervical and Endometrial Adenocarcinomas

Determining the origin of uterine adenocarcinomas can be difficult in biopsy and curettage specimens because the morphologic spectrum of endocervical and endometrial adenocarcinomas overlaps. In hysterectomy specimens, the primary site is often equivocal for tumors that involve the lower uterine segment and endocervix and lack identifiable precursor lesions. Most endocervical adenocarcinomas (ECAs) contain high-risk human papillomavirus (HPV) DNA, whereas endometrial adenocarcinomas (EMAs) rarely do. p16 is an inhibitor ofcyclin-dependent kinases, and overexpression of p16 has been observed in cervical intraepithelial lesions and invasive carcinomas associated with high-risk HPV types. We evaluated the utility of immunohistochemistry for p16 in the distinction of ECAs and EMAs. p16 expression was assessed in 24 unequivocal EMAs and 19 unequivocal ECAs and correlated with HPV DNA detection by in situ hybridization and polymerase chain reaction. These assays were then used to assist in the classification of four lower uterine segment/upper endocervical adenocarcinomas (LUS/EC-A) of equivocal origin. p16 expression was moderate-strong and diffuse in 18 ECAs (median 90% of tumor cells positive, range 90%-100%), and weak and diffuse in one. Fourteen of these were positive for HPV DNA, whereas 5 lacked detectable HPV DNA by in situ hybridization; one of these 5 was positive by polymerase chain reaction. In contrast, EMAs displayed weaker staining with patchy distribution (median 30% of tumor cells positive, range 5%-70%) and none contained HPV DNA by in situ hybridization. Two LUS/EC-As, which were positive for HPV, exhibited strong, diffuse p16 expression, consistent with endocervical origin of the tumors. The remaining 2 LUS/EC-As showed patchy p16 staining and did not contain detectable HPV DNA, consistent with the endometrial origin of the tumors. The p16 expression pattern can distinguish ECAs from EMAs. Compared with HPV DNA detection by in situ hybridization, p16 immunohistochemistry appears to be a more sensitive and easier to perform method for distinguishing ECAs from EMAs, can be used to assist in the classification of LUS/EC-As of equivocal origin, and should be evaluated for its utility in the prospective classification of uterine adenocarcinomas in curettage specimens prior to hysterectomy.

Mouse polyomavirus large T antigen inhibits cell growth and alters cell and colony morphology in Saccharomyces cerevisiae

The gene for mouse polyomavirus large tumor (LT) antigen, a potent oncoprotein, was expressed in Saccharomyces cerevisiae from the inducible GAL1 promoter. Substantial cell growth inhibition as well as colony and cell morphology changes dependent on cyclic adenosine monophosphate (cAMP) were observed. In contrast to cell and colony morphology alterations, the growth inhibition appeared to be transient, thus indicating the existence of an active adaptation of yeast cells to the LT antigen presence.

SV40 17KT antigen complements dnaj mutations in large T antigen to restore transformation of primary human fibroblasts

Transformation of human cells requires both SV40 large T and small t antigens. Plasmids that contained mutations in the amino-terminal dnaJ domain of the early region fail to transform human diploid fibroblasts. However, large T dnaJ mutants can be rescued by plasmids that express early region products other than large T antigen. The protein found to be responsible for such complementation was the third early region product, 17KT. Similar to large T, this protein reduces levels of the retinoblastoma-related protein, p130, and stimulates cell-cycle progression of quiescent fibroblasts, two activities of large T that are disrupted by dnaJ mutations.

Histone deacetylation of RB-responsive promoters: requisite for specific gene repression but dispensable for cell cycle inhibition

The retinoblastoma tumor suppressor protein (RB) is targeted for inactivation in the majority of human tumors, underscoring its critical role in attenuating cellular proliferation. RB inhibits proliferation by repressing the transcription of genes that are essential for cell cycle progression. To repress transcription, RB assembles multiprotein complexes containing chromatin-modifying enzymes, including histone deacetylases (HDACs). However, the extent to which HDACs participate in transcriptional repression and are required for RB-mediated repression has not been established. Here, we investigated the role of HDACs in RB-dependent cell cycle inhibition and transcriptional repression. We find that active RB mediates histone deacetylation on cyclin A, Cdc2, topoisomerase IIalpha, and thymidylate synthase promoters. We also demonstrate that this deacetylation is HDAC dependent, since the HDAC inhibitor trichostatin A (TSA) prevented histone deacetylation at each promoter. However, TSA treatment blocked RB repression of only a specific subset of genes, thereby demonstrating that the requirement of HDACs for RB-mediated transcriptional repression is promoter specific. The HDAC-independent repression was not associated with DNA methylation or gene silencing but was readily reversible. We show that this form of repression resulted in altered chromatin structure and was dependent on SWI/SNF chromatin remodeling activity. Importantly, we find that cell cycle inhibitory action of RB is not intrinsically dependent on the ability to recruit HDAC activity. Thus, while HDACs do play a major role in RB-mediated repression, they are dispensable for the repression of critical targets leading to cell cycle arrest.

Expression of cyclin E renders cyclin D-CDK4 dispensable for inactivation of the retinoblastoma tumor suppressor protein, activation of E2F, and G1-S phase progression

The activation of CDK2-cyclin E in late G1 phase has been shown to play a critical role in retinoblastoma protein (pRb) inactivation and G1-S phase progression of the cell cycle. The phosphatidylinositol 3-OH-kinase inhibitor LY294002 has been shown to block cyclin D1 accumulation, CDK4 activity and, thus, G1 progression in alpha-thrombin-stimulated IIC9 cells (Chinese hamster embryonic fibroblasts). Our previous results show that expression of cyclin E rescues S phase progression in alpha-thrombin-stimulated IIC9 cells treated with LY294002, arguing that cyclin E renders CDK4 activity dispensable for G1 progression. In this work we investigate the ability of alpha-thrombin-induced CDK2-cyclin E activity to inactivate pRb in the absence of prior CDK4-cyclin D1 activity. We report that in the absence of CDK4-cyclin D1 activity, CDK2-cyclin E phosphorylates pRb in vivo on at least one residue and abolishes pRb binding to E2F response elements. We also find that expression of cyclin E rescues E2F activation and cyclin A expression in cyclin D kinase-inhibited, alpha-thrombin-stimulated cells. Furthermore, the rescue of E2F activity, cyclin A expression, and DNA synthesis by expression of E can be blocked by the expression of either CDK2(D145N) or RbDeltaCDK, a constitutively active mutant of pRb. However, restoring four known cyclin E-CDK2 phosphorylation sites to RbDeltaCDK renders it susceptible to inactivation in late G1, as assayed by E2F activation, cyclin A expression, and S phase progression. These data indicate that CDK2-cyclin E, without prior CDK4-cyclin D activity, can phosphorylate and inactivate pRb, activate E2F, and induce DNA synthesis.

The expression of retinoblastoma and Sp1 is increased by low concentrations of cyclin-dependent kinase inhibitors

We examined the effect of suboptimal concentrations of cyclin-dependent kinase inhibitors, which do not interfere with cell proliferation, on retinoblastoma expression in hamster (Chinese hamster ovary K1) and human (K562 and HeLa) cells. To achieve this, we used the chemical inhibitors roscovitine and olomoucine (which inhibit CDK2 preferentially), UCN-01 (which also inhibits CDK4/6) and p21 (as an intrinsic inhibitor). All chemical inhibitors and overexpression of p21 strongly induced retinoblastoma protein expression. UCN-01-mediated retinoblastoma expression was caused by an increase in both the levels of retinoblastoma mRNA and the stability of the protein. The expression of the transcription factor Sp1, a retinoblastoma-interacting protein, was also enhanced by all the cyclin-dependent kinase inhibitors tested. However, Sp1 expression was caused by an increase in the levels of Sp1 mRNA without modification in the stability of the protein. By using luciferase experiments, the transcriptional activation of both retinoblastoma and Sp1 promoters by UCN-01 was confirmed. Bisindolylmaleimide I, at concentrations causing a similar or higher inhibition of protein kinase C than UCN-01, provoked a lower activation of retinoblastoma and Sp1 expression. Finally, the effects of cyclin-dependent kinase inhibitors on dihydrofolate reductase gene expression were evaluated. Treatment with UCN-01 increased cellular dihydrofolate reductase mRNA levels, and dihydrofolate reductase enzymatic activity was enhanced by UCN-01, roscovitine, olomoucine and p21, in transient transfection experiments. These results support a mechanism for the self-regulation of retinoblastoma expression, and point to the need to establish the appropriate dose of cyclin-dependent kinase inhibitors as antiproliferative agents in anticancer treatments.

pRB contains an E2F1-specific binding domain that allows E2F1-induced apoptosis to be regulated separately from other E2F activities

The interaction between pRB and E2F is critical for control of the cell cycle and apoptosis. Here we report that pRB contains two distinct E2F binding sites. The previously identified E2F binding site on pRB is necessary for stable association with E2Fs on DNA. A second E2F interaction site is located entirely within the C-terminal domain of pRB and is specific for E2F1. E2F1/pRB complexes formed through this site have low affinity for DNA, but the interaction is sufficient for pRB to regulate E2F1-induced apoptosis, and E2F1 loses the ability to interact with this site following DNA damage. These results show that pRB interacts with individual E2F proteins in different ways and suggest that pRB's regulation of E2F1-induced apoptosis is physically separable from its transcriptional control of other E2F proteins.

LEK1 is a potential inhibitor of pocket protein-mediated cellular processes

LEK1, a member of the LEK family of proteins, is ubiquitously expressed in developing murine tissues. Our current studies are aimed at identifying the role of LEK1 during cell growth and differentiation. Little is known about the function of LEK proteins. Recent studies in our laboratory have focused on the characterization of the LEK1 atypical Rb-binding domain that is conserved among all LEK proteins. Our findings suggest that LEK1 potentially functions as a universal regulator of pocket protein activity. Pocket proteins exhibit distinct expression patterns during development and function to regulate cell cycle, apoptosis, and tissue-specific gene expression. We show that LEK1 interacts with all three pocket proteins, p107, p130, and pRb. Additionally, this interaction occurs specifically between the LEK1 Rb-binding motif and the "pocket domain" of Rb proteins responsible for Rb association with other targets. Analyses of the effects of disruption of LEK1 protein expression by morpholino oligomers demonstrate that LEK1 depletion decreases cell proliferation, disrupts cell cycle progression, and induces apoptosis. Given its expression in developing cells, its association with pocket proteins, and its effects on proliferation, cell cycle, and viability of cells, we suggest that LEK1 functions in a similar manner to phosphorylation to disrupt association of Rb proteins with appropriate binding targets. Thus, the LEK1/Rb interaction serves to retain cells in a pre-differentiative, actively proliferative state despite the presence of Rb proteins during development. Our data suggest that LEK1 is unique among LEK family members in that it specifically functions during murine development to regulate the activity of Rb proteins during cell division and proliferation. Furthermore, we discuss the distinct possibility that a yet unidentified splice variant of the closely related human CENP-F, serves a similar function to LEK1 in humans.

Chemoresistance in human ovarian cancer: the role of apoptotic regulators

Ovarian cancer is among the most lethal of all malignancies in women. While chemotherapy is the preferred treatment modality, chemoresistance severely limits treatment success. Recent evidence suggests that deregulation of key pro- and anti-apoptotic pathways is a key factor in the onset and maintenance of chemoresistance. Furthermore, the discovery of novel interactions between these pathways suggests that chemoresistance may be multi-factorial. Ultimately, the decision of the cancer cell to live or die in response to a chemotherapeutic agent is a consequence of the overall apoptotic capacity of that cell. In this review, we discuss the biochemical pathways believed to promote cell survival and how they modulate chemosensitivity. We then conclude with some new research directions by which the fundamental mechanisms of chemoresistance can be elucidated.

Forced expression of cyclin D1 does not compensate for Id2 deficiency in the mammary gland

Id2 and cyclin D1 share several biological activities, including inhibition of differentiation, stimulation of the G1-S transition in the cell cycle and stimulation of tumorigenesis. Mammary glands of Id2(-/-) mice display severely impaired lobulo-alveolar development during pregnancy, similarly to those of cyclin D1 null females. We investigated the functional relationship between Id2 and cyclin D1 in the mammary gland. Id2(-/-) mammary glands expressed a normal level of cyclin D1. No direct interaction of Id2 with cyclin D1 or its binding partner cdk4 was detected in mammalian two-hybrid assays. Ectopic expression of a cyclin D1 transgene did not rescue the mammary phenotype of Id2(-/-) mice. These results suggest that Id2 acts downstream or independently of cyclin D1 in the control of mammary cell proliferation during pregnancy.

Apoptotic and mitogenic stimuli inactivate Rb by differential utilization of p38 and cyclin-dependent kinases

Inactivation of the retinoblastoma (Rb) tumor suppressor protein is essential for the G1/S transition during mammalian cell cycle progression. Although Rb is inactivated by phosphorylation by cyclins D and E and their associated kinases during cell cycle progression, we find that Rb is inactivated upon apoptotic stimulation by Fas through the mediation of p38 kinase, independent of cyclins and cyclin-dependent kinases (cdks). Inactivation by p38 kinase coincided with increased phosphorylation of Rb leading to dissociation of E2F and increased transcriptional activity; such p38-mediated changes in Rb function occurred only during Fas stimulation but not mitogenic progression. p38 kinase targets Rb preferentially and had minimal effects on p107 and had no effect on p130 function. We also find that phosphorylation site mutants of Rb (PSM7LP and PSM9-Rb) that cannot be inactivated by cdks can be targeted by Fas and p38 kinase, suggesting that Rb inactivation by these kinases is biochemically and functionally distinct. It appears that Rb inactivation is achieved by different kinase cascades in response to mitogenic and apoptotic signals.

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.

The retinoblastoma protein interacts with Bag-1 in human colonic adenoma and carcinoma derived cell lines

Although the retinoblastoma susceptibility gene RB1 is inactivated in a wide range of human tumours, overexpression in colonic carcinomas has been linked to the antiapoptotic function of the protein. In the current study we show that the Retinoblastoma susceptibility protein (Rb) protein interacts with Bag-1, an apoptotic regulator, in human colonic adenoma- and carcinoma-derived cell lines. Coimmunoprecipitation demonstrated that endogenous Rb and Bag-1 interact in both adenoma- and carcinoma-derived cell lines. The specificity of the interaction was demonstrated by expression of human Papillomavirus E7 oncoprotein, an inhibitor of Rb protein interactions, which disrupted the Rb/Bag-1 complex. We report that Bag-1 is predominantly localised in the nucleus of colorectal adenoma- and carcinoma-derived epithelial cells. Disruption of the Rb/Bag-1 complex through expression of E7 changes the subcellular distribution of Bag-1, decreasing nuclear localised Bag-1. Our work establishes that the Rb protein interacts with the Bag-1 apoptotic regulator protein, and introduces a novel function for Rb, involving modulation of the subcellular localisation of Bag-1 in human colonic epithelial cells.

Involvement of the transcription factor E2F1/Rb in kainic acid-induced death of murine cerebellar granule cells

The full mechanisms underlying neuronal death following excitotoxic insult remain unclear, despite many in vivo and in vitro studies. Recent work has focused on various signaling molecules and pathways, normally strictly regulated, that can trigger death if perturbed. The transcription factor, E2F1 is pivotal in controlling cell death under stress situations. The current study aimed to investigate the role of this transcription factor in modulating neuronal death following kainic acid (KA) treatment of cultured mouse cerebellar granule cells (CGCs). KA-induced death of CGCs was attenuated by the selective KA/AMPA receptor antagonist CNQX, but not MK-801. Such neuronal death was caspase-3-independent and did not activate many known death genes, such as Fas receptor, caspase-8 and p38. However, hyperphosphorylation of Rb showed a transient increase which may lead to activation of E2F1. Indeed E2F1 +/+ and -/- CGCs showed a differential response to KA-mediated toxicity, in that E2F1 -/- neurons were significantly less susceptible to KA compared to E2F1 +/+ neurons, albeit both E2F1 +/+ and -/- neurons expressed similar levels of KA receptors and responded similarly to kainate antagonist, CNQX. Using selective inhibitors to CDKs, such as olomoucine, roscovitine and flavopiridol, and the inhibitor SB203580 to p38 MAPK, we ruled out the possibility that Rb inactivation through hyperphosphorylation was due to either upstream kinases. Therefore activation of Rb/E2F1 pathway appears to involve novel interactions yet to be elucidated.

Human papillomaviruses: targeting differentiating epithelial cells for malignant transformation

Human papillomavirus (HPV) infections play a crucial role in the pathogenesis of cervical neoplasia. Insights into the mechanisms by which HPV infection can, in a small numbers of cases, result in malignancy, comes from the observation that three proteins encoded by high-risk genital HPVs, E6, E7 and to a lesser extent E5, target factors that control the cell cycle and proliferation. These interactions result in abrogation of cell cycle control, chromosomal alterations, telomerase activation, and eventual cell immortalization. In this review, we discuss the functions of E6, E7, and E5 proteins that are most relevant to the malignant progression of HPV-transformed cells.

Inhibition of cell proliferation and induction of apoptosis by novel tetravalent peptides inhibiting DNA binding of E2F

We have isolated several peptides from random peptide phage display libraries that specifically recognize the cell cycle regulatory transcription factor E2F and inhibit DNA binding of E2F/DP heterodimers (E2F-1, E2F-2, E2F-3, E2F-4 or E2F-5, and DP-1). The inhibitory efficiency could be strongly enhanced by generating branched tetravalent molecules. To analyse the biological consequences of peptide-mediated E2F inhibition, we fused two of these branched molecules to a cell-penetrating peptide derived from the HTV-Tat protein. Incubation of human tumor cells with these branched Tat-containing peptides led to an inhibition of cell proliferation and induction of apoptosis. These results provide new insights into the function of E2F and further validate E2F as a potential therapeutic target in proliferative diseases.

Ca2+/calmodulin-dependent modulation of cell cycle elements pRb and p27kip1 involved in the enhanced proliferation of lymphoblasts from patients with Alzheimer dementia

Failure of cell cycle regulation in neurons might be critically involved in the process of neurodegeneration in Alzheimer's disease (AD). We present here evidence to support the hypothesis that cell cycle alterations occur in cells other than neurons in AD sufferers. Lymphocytes from AD patients immortalized with Epstein-Barr virus showed an enhanced rate of proliferation and increased phosphorylation of the retinoblastoma protein (pRb) and other members of the family of pocket proteins compared with cell lines derived from normal age-matched controls. The calmodulin antagonist calmidazolium, as well as W-7 and W-13, abrogated the enhanced activity of AD cells without altering the normal basal rate of proliferation. The effect of calmidazolium was accompanied by partially dephosphorylation of pRb. No changes were found in the expression levels of the G1 cyclin/Cdks complexes. However, lymphoblasts derived from AD patients showed reduced levels of the Cdk inhibitor p27(kip1), which were restored after anti-calmodulin treatment of the cultures. These observations suggest that in AD cells the enhanced rates of cell proliferation and phosphorylation of pRb and the intracellular content of p27(kip1) may be interrelated events controlled by a mechanism dependent on the Ca(2+)/calmodulin signaling pathway. The distinct functional features of lymphoblastoid cells from AD patients offer an invaluable, noninvasive tool to investigate the etiopathogenesis, and eventually, for the early diagnosis and prognosis of this devastating disease.

Molecular pathogenesis of thyroid cancer

A number of molecular abnormalities have been described in association with the progression from normal thyroid tissue to benign adenomas to well-differentiated and finally anaplastic epithelial thyroid cancer. These include upregulation of proliferative factors, such as growth hormones and oncogenes, downregulation of apoptotic and cell-cycle inhibitory factors, such as tumor suppressors, disruption of normal cell-to-cell interactions, and cellular immortalization. The progression model for thyroid carcinoma has not been proven, but evidence suggests that an evolutionary molecular process is involved, especially in the development of follicular thyroid cancers for which there are distinct intermediate phenotypes. We present a comprehensive evaluation of factors involved in thyroid tumorigenesis and attempt to describe preliminary attributes of a progression model. The organization of this model should also provide a template for the incorporation of new information as it is derived from large-scale genomic studies.

Functional studies of the MEN1 gene

Multiple endocrine neoplasia type 1 is an autosomal dominant cancer syndrome affecting primarily parathyroid, enteropancreatic endocrine and pituitary tissues. The inactivating germline and somatic mutations spread throughout the gene and the accompanying loss of the second allele in tumours show that the MEN1 gene is a tumour suppressor. The MEN1-encoded protein, menin, is a novel nuclear protein. Menin binds and alters JunD-, NF-kappaB-, Smad3-mediated transcriptional activation. The mouse Men1 knockout model mimicks the human MEN1 condition contributing to the understanding of tumorigenesis in MEN1.

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.

The E2F-Cdc2 cell-cycle pathway specifically mediates activity deprivation-induced apoptosis of postmitotic neurons

Neuronal apoptosis plays a critical role in the normal development of the mammalian brain and is thought to contribute to the pathogenesis of several neurologic disorders. However, the intracellular mechanisms underlying apoptosis of neurons remain incompletely understood. In the present study, we characterized a cell-cycle-based mechanism by which neuronal activity deprivation induces apoptosis of postmitotic neurons. Activity deprivation, but not growth factor withdrawal, was found to induce Cdc2 expression and consequent Cdc2-mediated apoptosis in granule neurons of the developing rat cerebellum. We found that activity deprivation induces cdc2 transcription in neurons via an E2F-binding element (EBE) within the cdc2 promoter. The transcription factor E2F1 that is expressed in granule neurons was found in DNA binding assays to bind to the EBE of the cdc2 gene. In chromatin immunoprecipitation analysis, endogenous E2F1 forms a complex with the promoter of the endogenous cdc2 gene in granule neurons, indicating that endogenous E2F1 is poised to activate transcription of the endogenous cdc2 gene in neurons. Consistent with this conclusion, a dominant interfering form of E2F, when expressed in granule neurons, blocked activity deprivation-induced cdc2 transcription. In other experiments, we found that the expression of E2F1 in granule neurons induces Cdc2 expression and promotes neuronal apoptosis via the activation of Cdc2. Remarkably, in contrast to inducing the E2F-mediated expression and activation of Cdc2 in granule neurons, activity deprivation fails to stimulate the expression of E2F-target genes that trigger DNA synthesis and replication. Together, our findings define a novel apoptotic mechanism whereby E2F selectively couples an activity deprivation-induced signal to cdc2 transcription in the absence of stimulating DNA synthesis and thus culminating in Cdc2-mediated apoptosis of postmitotic neurons.

The cyclin D1-dependent kinase associates with the pre-replication complex and modulates RB.MCM7 binding

The capacity of the cyclin D-dependent kinase to promote G(1) progression through modulation of RB.E2F is well documented. We now demonstrate that the cyclin D1/CDK4 kinase binds to components of the MCM complex. MCM7 and MCM3 were identified as cyclin D1-binding proteins. Catalytically active cyclin D1/CDK4 complexes were incorporated into chromatin-bound protein complexes with the same kinetics as MCM7 and MCM3, where they associated specifically with MCM7. Although the cyclin D1-dependent kinase did not phosphorylate MCM7, active cyclin D1/CDK4, but not cyclin E/CDK2, did catalyze the dissociation of an RB.MCM7 complex. Finally, expression of an active D1/CDK4 kinase but not cyclin E/CDK2 promoted the removal of RB from chromatin-bound protein complexes. Our data suggest that D1/CDK4 complexes play a direct role in altering an inhibitory RB.MCM7 complex possibly allowing for setting of the origin in preparation for DNA replication.

Alterations in G(1) to S phase cell-cycle regulators during amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is characterized by progressive degeneration of the motor neurons in the cerebral cortex, brain stem, and spinal cord. However, the mechanisms that regulate the initiation and/or progression of motor neuron loss in this disease remain enigmatic. Cell-cycle proteins and transcriptional regulators such as cyclins, cyclin-associated kinases, the retinoblastoma gene product (pRb), and E2F-1 function during cellular proliferation, differentiation, and cell death pathways. Recent data has implicated increased expression and activation of various cell-cycle proteins in neuronal cell death. We have examined the expression and subcellular distribution of G(1) to S phase cell-cycle regulators in the spinal cord, motor cortex, and sensory cortex from clinically and neuropathologically diagnosed sporadic ALS cases and age-matched controls. Our results indicate hyperphosphorylation of the retinoblastoma protein in motor neurons during ALS, concurrent with increased levels of cyclin D, and redistribution of E2F-1 into the cytoplasm of motor neurons and glia. These data suggest that G(1) to S phase activation occurs during ALS and may participate in molecular mechanisms regulating motor neuron death.

Reduced retinoblastoma gene protein to Ki-67 ratio is an adverse prognostic indicator for ovarian adenocarcinoma patients

OBJECTIVE

Alterations in the retinoblastoma gene (RB-1) are common in human neoplasia. However, the clinical significance of the deregulated expression of RB-1 in ovarian cancer remains undefined. We therefore conducted a retrospective investigation to clarify the relationships of RB-1 gene protein (pRb) to the percentage of cycling cells, clinicopathologic variables, other G1 interacting proteins and prognosis of nonbenign epithelial ovarian tumors.

METHODS

Paraffin-embedded tissue from 127 nonbenign epithelial ovarian tumors, including 44 of low malignant potential (LMP) and 83 primary ovarian adenocarcinomas, was stained immunohistochemically for pRb, p21(Cip1), p27(Kip1), p53, and Ki-67 antigen (a cell proliferation associated marker). Expression of these markers was correlated with clinicopathologic features and with overall survival of patients with adenocarcinomas.

RESULTS

pRb levels were significantly lower in LMP tumors than in carcinomas (P = 0.027). In the latter group, pRb expression decreased with increasing grade (I-II vs III) (P = 0.010), advancing stage (I-II vs III) (P < 0.001), and bulk residual disease (P = 0.014). pRb was not related to Ki-67 expression (P > 0.10) or to overall survival (P > 0.10) but a low pRb to Ki-67 ratio emerged as an important indicator of poor survival in univariate analysis in the entire cohort (P = 0.0076) and in the platinum-treated patients (P = 0.0162) as well as in multivariate analysis, along with histologic type and FIGO stage.

CONCLUSIONS

Diminished pRb levels are related to several clinicopathologic indicators of aggressiveness in ovarian adenocarcinomas. More importantly, pRb expression coupled with the percentage of Ki-67 positive cells is a better prognostic marker than pRb, Ki-67, or other G1 interacting proteins and supplements the information gained from traditional prognosticators.

Aberrant ubiquitin-mediated proteolysis of cell cycle regulatory proteins and oncogenesis

The ubiquitin pathway plays a central role in the regulation of cell growth and cell proliferation by controlling the abundance of key cell cycle proteins. Increasing evidence indicates that unscheduled proteolysis of many cell cycle regulators contributes significantly to tumorigenesis and is indeed found in many types of human cancers. Aberrant proteolysis with oncogenic potential is elicited by two major mechanisms: defective degradation of positive cell cycle regulators (i.e., proto-oncoproteins) and enhanced degradation of negative cell cycle regulators (i.e., tumor suppressor proteins). In many cases, increased protein stability is a result of mutations in the substrate that prevent the recognition of the protein by the ubiquitin-mediated degradation machinery. Alternatively, the specific recognition proteins mediating ubiquitination (ubiquitin ligases) are not expressed or harbor mutations rendering them inactive. In contrast, the overexpression of a ubiquitin ligase may result in the enhanced degradation of a negative cell cycle regulator. This chapter aims to review the involvement of the ubiquitin pathway in the scheduled destruction of some important cell cycle regulators and to discuss the implications of their aberrant degradation for the development of cancer.

Expression patterns of retinoblastoma protein in Parkinson disease

Cellular mechanisms implicated in Parkinson disease (PD) include oxidative stress, inflammatory response, excess dopamine, DNA damage, and loss of trophic support. These stimuli have been observed to induce changes in cell cycle proteins in several cell types. One of the key regulators of cell cycle progression is the retinoblastoma protein (pRb); therefore, we assessed the staining for pRb and its inactive hyperphosphorylated isoform, ppRb, in autopsy tissue from patients with PD. In PD we found abundant pRb staining in neuronal cytoplasm of the substantia nigra, mid-frontal cortex, and hippocampus by immunohistochemistry. In controls, pRb weakly stained nucleoli of neurons in the substantia nigra and exhibited no detectable staining in mid-frontal cortex and hippocampus. Staining for ppRb resulted in a shift from weak cytoplasmic staining in neurons from control cases to strong nuclear staining in PD cases, especially within the substantia nigra, mid-frontal cortex, and hippocampus. In the substantia nigra, ppRb also co-localized to Lewy bodies, which are a pathologic feature of PD. Lewy bodies are also found in diffuse Lewy body disease (DLBD) that do not consistently exhibit changes in pRb or ppRb. These results indicate that there are changes in pRb and its inactive phospho-isoform in neurons responding to neurodegenerative stimuli associated with PD.

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.