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

Skip interacts with the retinoblastoma tumor suppressor and inhibits its transcriptional repression activity

Ski interacting protein (Skip) plays an important role in the transforming activity of both v-Ski and EBNA2 (Epstein-Barr virus encoded latency protein) and is involved in EBNA2 and NotchIC activation of CBF1-repressed promoters. We have previously shown that Skip acts as a transcriptional co-activator on a number of cellular and viral promoters. Here, we report that Skip also interacts with pRb and, in cooperation with Ski, can overcome pRb-induced transcriptional repression. We show a strong and direct interaction between pRb and Skip, and we map the site of interaction to amino acid residues 171-353 of the evolutionarily conserved SNW domain of Skip. Furthermore, the combination of Skip and Ski can successfully overcome the G1 arrest and flat cell phenotype induced by pRb. Taken together, these studies suggest that one potential function of the Skip-Ski complex is to overcome the growth-suppressive activities of pRb.

Rescue of embryonic epithelium reveals that the homozygous deletion of the retinoblastoma gene confers growth factor independence and immortality but does not influence epithelial differentiation or tissue morphogenesis

The ability to rescue viable prostate precursor tissue from retinoblastoma-deficient (Rb-/-) fetal mice has allowed for the isolation and characterization of the first Rb-/- prostate epithelial cell line. This cell line, designated Rb-/-PrE, was utilized for experiments examining the consequences of Rb loss on an epithelial population. These findings demonstrated that Rb deletion has no discernible effect on prostatic histodifferentiation in Rb-/-PrE cultures. When Rb-/-PrE cells were recombined with embryonic rat urogenital mesenchyme and implanted into athymic male, nude mouse hosts, the recombinants developed into fully differentiated and morphologically normal prostate tissue. The Rb-/-PrE phenotype was characterized by serum independence in culture and immortality in vivo, when compared with wild type controls. Cell cycle analysis revealed elevated S phase DNA content accompanied by increased expression of cyclin E1 and proliferating cell nuclear antigen. Rb-/-PrE cultures also exhibited a diminished ability to growth arrest under high density culture conditions. We believe that the development of Rb-/- prostate tissue and cell lines has provided a unique experimental platform with which to investigate the consequences of Rb deletion in epithelial cells under various physiological conditions. Additionally, the development of this technology will allow similar studies in other tissues and cell populations rescued from Rb-/- fetuses.

Interaction of YY1 with E2Fs, mediated by RYBP, provides a mechanism for specificity of E2F function

To explore mechanisms for specificity of function within the family of E2F transcription factors, we have identified proteins that interact with individual E2F proteins. A two-hybrid screen identified RYBP (Ring1- and YY1-binding protein) as a protein that interacts specifically with the E2F2 and E2F3 family members, dependent on the marked box domain in these proteins. The Cdc6 promoter contains adjacent E2F- and YY1-binding sites, and both are required for promoter activity. In addition, YY1 and RYBP, in combination with either E2F2 or E2F3, can stimulate Cdc6 promoter activity synergistically, dependent on the marked box domain of E2F3. Using chromatin immunoprecipitation assays, we show that both E2F2 and E2F3, as well as YY1 and RYBP, associate with the Cdc6 promoter at G(1)/S of the cell cycle. In contrast, we detect no interaction of E2F1 with the Cdc6 promoter. We suggest that the ability of RYBP to mediate an interaction between E2F2 or E2F3 and YY1 is an important component of Cdc6 activation and provides a basis for specificity of E2F function.

p27Kip1 is essential for the antiproliferative action of 1,25-dihydroxyvitamin D3 in primary, but not immortalized, mouse embryonic fibroblasts

1alpha,25-Dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) inhibits the growth of numerous cancer cell types. The intracellular proteins that mediate 1,25(OH)(2)D(3)-induced growth inhibition are poorly defined, although it is speculated that p21 and p27 are involved. We tested the requirement of p21 and p27 by treating primary wild-type, p21(-/-), and p27(-/-) mouse embryonic fibroblasts (MEFs) with 100 nm 1,25(OH)(2)D(3). In response to treatment, the wild-type and p21(-/-) MEFs exhibited 54 and 60% growth inhibition (p < 0.05), respectively, whereas the growth of p27(-/-) MEFs was unaffected. Western analyses indicated that p27 expression is induced by 1,25(OH)(2)D(3) treatment in wild-type and p21(-/-) MEFs. p21 expression is also induced by 1,25(OH)(2)D(3) treatment in wild-type and p27(-/-) MEFs, although the effect is less robust than for p27. Next, we spontaneously immortalized each MEF strain, which resulted in a gain of responsiveness to 1,25(OH)(2)D(3) by the p27(-/-) MEFs, as exhibited by 87% growth inhibition (p < 0.05). Both wild-type and p21(-/-) MEFs retained responsiveness (43 and 72% growth inhibition (p < 0.05), respectively). These data from primary and immortalized MEFs demonstrate that there are both p27-dependent and -independent pathways that mediate the antiproliferative action of 1,25(OH)(2)D(3).

Physical and functional interaction of HIV-1 Tat with E2F-4, a transcriptional regulator of mammalian cell cycle

Tat protein of the human immunodeficiency virus type-1 (HIV-1) plays a critical role in the regulation of viral transcription and replication. In addition, Tat regulates the expression of a variety of cellular genes and could account for AIDS-associated diseases including Kaposi's Sarcoma and non-Hodgkin's lymphoma by interfering with cellular processes such as proliferation, differentiation, and apoptosis. The molecular mechanisms underlying the pleiotropic activities of Tat may include the generation of functional heterodimers of Tat with cellular proteins. By screening a human B-lymphoblastoid cDNA library in the yeast two-hybrid system, we identified E2F-4, a member of E2F family of transcription factors, as a Tat-binding protein. The interaction between Tat and E2F-4 was confirmed by GST pull-down experiments performed with cellular extracts as well as with in vitro translated E2F-4. The physical association of Tat and E2F-4 was confirmed by in vivo binding experiments where Tat.E2F-4 heterodimers were recovered from Jurkat cells by immunoprecipitation and immunoblotting. By using plasmids expressing mutant forms of Tat and E2F-4, the domains involved in Tat.E2F-4 interaction were identified as the regions encompassing amino acids 1-49 of Tat and amino acids 1-184 of E2F-4. Tat x E2F-4 complexes were shown to bind to E2F cis-regions with increased efficiency compared with E2F-4 alone and to mediate the activity of E2F-dependent promoters including HIV-1 long terminal repeat and cyclin A. The data point to Tat as an adaptor protein that recruits cellular factors such as E2F-4 to exert its multiple biological activities.

Differentially expressed genes associated with mouse lung tumor progression

To detect altered gene expression associated with mouse lung tumor progression, we compared the gene expression profile of lung adenocarcinomas with that of lung adenomas and normal lungs. Autoradiographic analysis showed that among the 588 genes surveyed, 152 genes were detected and the remaining 436 genes did not give any signals. A gene-specific semiquantitative reverse transcription polymerase chain reaction method was used to confirm the expression profile. A total of 29 genes was found to be differentially expressed in mouse lung tumors when compared to normal lungs. The pattern of expression, either underexpression or overexpression, was the same for 10 genes between adenocarcinomas and adenomas. Among them, seven genes were overexpressed, two genes were underexpressed and one gene was lost. Interestingly, 19 genes showed differential expression or increased incidence or difference in level of change between lung adenomas and adenocarcinomas, including Stat1, ADAP, IGFBP-6, PDGF-A, TGF-beta2, Int-3, VEGFR2, BAX, BAG-1, c-Jun, FasL, TRAIL, YB-1, CD31, Cdc42, B-raf, Rab-2, Abi-1, and ACE. These genes can be designated as candidate 'lung tumor progression' (LTP) genes because their expression changes may specifically affect lung tumor progression in mice. Further analyses of these candidate LTP genes may provide new leads for elucidation of lung tumor progression in mice.

Loss of androgen receptor transcriptional activity at the G(1)/S transition

Androgens are essential for the differentiation, growth, and maintenance of male-specific organs. The effects of androgens in cells are mediated by the androgen receptor (AR), a member of the nuclear receptor superfamily of transcription factors. Recently, transient transfection studies have shown that overexpression of cell cycle regulatory proteins affects the transcriptional activity of the AR. In this report, we characterize the transcriptional activity of endogenous AR through the cell cycle. We demonstrate that in G0, AR enhances transcription from an integrated steroid-responsive mouse mammary tumor virus promoter and also from an integrated androgen-specific probasin promoter. This activity is strongly reduced or abolished at the G(1)/S boundary. In S phase, the receptor regains activity, indicating that there is a transient regulatory event that inactivates the AR at the G(1)/S transition. This regulation is specific for the AR, since the related glucocorticoid receptor is transcriptionally active at the G(1)/S boundary. Not all of the effects of androgens are blocked, however, since androgens retain the ability to increase AR protein levels. The transcriptional inactivity of the AR at the G(1)/S junction coincides with a decrease in AR protein level, although activity can be partly rescued without an increase in receptor. Inhibition of histone deacetylases brings about this partial restoration of AR activity at the G(1)/S boundary, demonstrating the involvement of acetylation pathways in the cell cycle regulation of AR transcriptional activity. Finally, a model is proposed that explains the inactivity of the AR at the G(1)/S transition by integrating receptor levels, the action of cell cycle regulators, and the contribution of histone acetyltransferase-containing coactivators.

Proliferation-dependent expression and phosphorylation of pRB in B cell non-Hodgkin's lymphomas: dependence on RB1 copy number

Some studies have suggested that a significant fraction of non-Hodgkin's lymphomas (NHL) do not express pRB protein, possibly due to deletions of RB1. We examined RB1/centromere 17 copy number by fluorescent in situ hybridisation, and pRB expression/phosphorylation by immunohistochemistry (IHC) and immunoblotting (IB) in 66 cases of B cell NHL. Thirteen cases had lost one RB1 copy relative to centromere 17 copy number and total DNA content. Case 458/88 had no RB1 copies. pRB levels were heterogeneous as assessed by IB (0.04-1.12 relative units), but all tumours, except for case 458/88, expressed pRB localised to the nucleus in >75% of the tumour cells by IHC. The fraction of phosphorylated pRB was correlated with pRB expression (r(2)= 0.56, P < 0.001). The 14 cases with loss of RB1 had lower pRB expression (median 0.25) than those without (median 0.48, P < 0.001), but a correlation with S phase fraction (r(2) = 0.43, P < 0.001; previously published data for tumour-specific S phase and apoptotic fractions) indicated that the variation in pRB expression was due to differences in proliferative activity. Furthermore, the regression lines for pRB expression vs S phase fraction were not different for the cases with or without loss of one RB1 copy (P = 0.5). Cases 154/88 (one RB1 copy) and 258/88 (two RB1 copies), in addition to case 458/88, had low expression of (hypophosphorylated) pRB (0.04, 0.08 and 0.04), despite their high S phase fractions (21%, 17% and 21%). There was no association between pRB expression/RB1 copy number and apoptotic fraction. Neither pRB expression nor loss of RB1 had prognostic value, but cases 154/88, 258/88, and 458/88 had short survival times (5, 3 and 46 months, respectively) compared to the others (median survival: 44 months, P = 0.03). It is suggested that pRB expression and function are normal in 63 of 66 NHL cases, including 12 of 13 lymphomas with loss of one RB1 allele.

Cancer genetics

Cancer is a genetic disease of somatic cells. Tumor karyotypes are rarely normal, and most show multiple abnormalities of both number and structure. The first direct evidence for this concept of cancer came from studies of tumor-specific translocations in leukemias and lymphomas, revealing the importance of oncogenes and the regulation of gene transcription in cancer. A second major source of information about human cancer genes is hereditary cancer. Genetic predisposition of the autosomal dominant type imposes a high relative risk for one or more kinds of cancer. In the past decade or so, more than 30 mutant genes for such hereditary cancers have been cloned. Penetrance depends upon additional, somatic, mutations. A few of the genes are oncogenes or DNA repair genes, but most are tumor suppressor genes. Some tumor suppressors regulate transcription, while others operate in signal transduction pathways that are involved in regulating processes of cell birth, differentiation, and death. The knowledge gained is stimulating new approaches to the treatment and prevention of cancer.

4-Hydroxynonenal affects pRb/E2F pathway in HL-60 human leukemic cells

4-Hydroxynonenal (HNE), a highly reactive product of lipid peroxidation, has an antiproliferative effect in several tumor cell lines and provokes alteration of cell cycle progression in HL-60 cells. HNE down-regulates c-myc expression in K562, HL-60, and MEL cells. This prompted us to study the cascade of phenomena that, starting from the CKIs expression and the phosphorylation of pRb, arrives at the E2F binding to consensus sequence in the P2 promoter of the c-myc gene. Treatment of HL-60 cells with HNE (1 microM) causes a p53-independent increase of p21(WAF1/CIP1) expression, pRb dephosphorylation, a decrease of low molecular weight E2F complexes and an increase of high molecular weight E2F complexes bound to P2 c-myc promoter. E2F4 expression is reduced by HNE treatment as well as the amount of pRb/E2F4 complexes, whereas the amount of pRb/E2F1 complexes is increased. In conclusion, HNE can affect the pRb/E2F pathway by modifying the expression of several genes involved in the control of cell proliferation.

Prognostic factors in betel and tobacco related oral cancer

Paucity of well-defined prognostic molecular markers severely hampers prediction of the clinical course of squamous cell carcinoma (SCC) of oral cavity. The aim of the study was to evaluate the prognostic significance of impairments in the expression of proteins involved in cell cycle regulation and locoregional spread in oral SCC of habitual betel and tobacco chewers. A prospective study was performed in 105 betel and tobacco consumers with oral SCCs during the period 1988-1999, to evaluate the prognostic relevance of impairments in the expression of proteins involved in cell cycle regulation and locoregional spread. Alterations in the expression of p53, pRb, p16, MDM2, p21, and Ets-1 proteins were determined by immunohistochemical analysis in formalin fixed, paraffin embedded tissue sections from oral SCCs. Analysis of multiple molecular biological factors showed overexpression of p53 in 69/105 (66%) cases, MDM2 in 72/105 (69%) cases, p21 in 57/105 (54%) cases and Ets-1 in 64/105 (61%) cases. Loss of pRb was observed in 58/105 (55%) cases and p16 loss was observed in 72/105 (69%) cases. Interestingly, multivariate analysis revealed loss of pRb as the most significant predictor of advanced tumour stage [P=0.001; Odd's Ratio (OR)=3.5] and overexpression of Ets-1 protein was an independent risk factor for lymph node metastasis (OR: 10.1; P<10(-6)). Multiple logistic regression models showed that pRb loss [Hazard's Ratio (HR): 3.93] and p53 overexpression (HR: 2.97) may serve as adverse prognosticators for disease free survival of the patients. The data demonstrate multiple impairments in p53/MDM2/p21/Ets-1 and p16/pRb pathways in betel and tobacco related oral tumourigenesis.

E2F-3B is a physiological target of cyclin A

The E2F family of transcription factors controls the expression of numerous genes that are required for the G(1)/S transition. Among the mechanisms that modulate the activity of the E2F proteins, cyclin A has been found to be important for the down-regulation of E2F-1, -2, and -3A activity after cells have progressed through G(1)/S. Specifically, phosphorylation of these E2F proteins by cyclin A/Cdk2 ultimately results in their necessary degradation as cells progress through S phase. E2F-3B was recently identified as an alternatively spliced form of E2F-3A that was predicted to lack a functional cyclin A binding domain. In this paper, we present considerable evidence that contradicts this prediction. First, we demonstrate binding of cyclin A to E2F-3B as bacterially expressed proteins in vitro. Second, we demonstrate binding of cyclin A to E2F-3B in mammalian cells in vivo. Third, we show that co-expression of cyclin A with E2F-3B significantly reduces E2F-3B-mediated transcriptional activity. Finally, in synchronized cells, we observe down-regulation of E2F-3B protein expression coincident with the up-regulation of cyclin A. We conclude that E2F-3B is a physiological target of cyclin A.

Prohibitin requires Brg-1 and Brm for the repression of E2F and cell growth

E2F transcription factors play a major role in controlling mammalian cell cycle progression. We recently reported that a potential tumor suppressor, prohibitin, which interacts with retinoblastoma protein (Rb), regulates E2F function and this activity correlates with its growth-suppressive activity. We show here that prohibitin recruits Brg-1/Brm to E2F-responsive promoters, and that this recruitment is required for the repression of E2F-mediated transcription by prohibitin. Expression of a dominant-negative Brg-1 or Brm releases prohibitin-mediated repression of E2F and relieves prohibitin-mediated growth suppression. Although prohibitin associates with, and recruits, Brg-1 and Brm independently of Rb, prohibitin/Brg-1/Brm-mediated transcriptional repression requires Rb. A viral oncoprotein, SV40 large T antigen, can reverse prohibitin-mediated suppression of E2F-mediated gene transcription, and targets prohibitin through interruption of the association between prohibitin and Brg-1/Brm without affecting the prohibitin-E2F interaction.

Attenuated retinoblastoma gene product and associated E2F/retinoblastoma imbalance in anastomotic intimal hyperplasia

OBJECTIVE

The retinoblastoma gene product is a cell cycle control protein that when inhibited allows cell proliferation to progress by releasing E2F. Retinoblastoma manipulation has been attempted to prevent intimal hyperplasia (IH) in injured native vessels by arresting vascular smooth muscle cell proliferation. However, no studies have identified the role, if any, of retinoblastoma in anastomotic IH formation after prosthetic arterial grafting. The goal of this study was to describe the relation of retinoblastoma and E2F to anastomotic IH with analyzing retinoblastoma/E2F levels, retinoblastoma phosphorylation, and transcription of retinoblastoma and E2F in prosthetic arterial grafting.

METHODS

Six-mm-diameter expanded polytetrafluoroethylene carotid interposition grafts (n = 12) were implanted in 25-kg mongrel dogs. The intervening arterial segments were harvested as controls. The distal anastomoses were harvested at 14 and 30 days after implantation for immunoblot, messenger RNA (mRNA), and immunohistochemistry analyses. Tissue homogenate was separated with sodium dodecylsulfate-polyacrylamide gel electrophoresis and probed with antibody to total retinoblastoma, phosphorylated retinoblastoma at serine 795, serine 780, and serine 807/811, and E2F-1. Bands at each time point were quantitated and compared with control artery (n = 12). Each lane was standardized with reprobing with antibody to beta-tubulin. Immunohistochemistry was performed with antibody to retinoblastoma. Retinoblastoma and E2F mRNA expression levels in anastomotic IH and control artery were analyzed with an oligonucleotide microarray.

RESULTS

Total retinoblastoma, from immunoblot analysis, was decreased at the 14-day and 30-day distal anastomoses by 35.7% and 33.6%, respectively, compared with control (P <.01). Furthermore, retinoblastoma at these time points was unphosphorylated at phosphorylation sites serine 795, serine 780, and serine 807/811. E2F-1 levels at 14 days and 30 days were unchanged compared with control. Positive staining for retinoblastoma was seen in endothelial and vascular smooth muscle cell from control, 14-day, and 30-day tissue. A qualitative decrease appeared to be seen in retinoblastoma in the neointima at 14 and 30 days compared with the native wall. No differential expression of retinoblastoma and E2F mRNA was seen in anastomotic IH compared with control.

CONCLUSION

This study showed that total retinoblastoma levels are decreased and E2F-1 levels remain unchanged in anastomotic IH. Attenuated retinoblastoma is a novel concept to anastomotic IH after prosthetic arterial grafting. Retinoblastoma/E2F imbalance may not be the result of transcriptional regulation and may increase unbound E2F to promote cell proliferation. Hypophosphorylation of remaining retinoblastoma may minimize uncontrolled proliferation by preventing further increases in unbound E2F. Therefore, retinoblastoma/E2F imbalance may lead to the early but limited increase in cell proliferation seen after prosthetic arterial grafting and appears to contribute to the development and progression of anastomotic IH.

Multiple cell cycle regulator alterations in Richter's transformation of chronic lymphocytic leukemia

To investigate the role of the cell cycle regulators p21(Waf1), p27(Kip1), retinoblastoma (Rb), and cyclin D1 in Richter's transformation of chronic lymphocytic leukemia (CLL), we analyzed 19 CLL and eight Richter's syndrome (RS) tumors, previously characterized for p53 and ARF/INK4a abnormalities. p21(Waf1)immunohistochemical expression was negative in 12 of 15 CLL (80%), whereas it was moderate or strong in three of seven RS (43%). p21(Waf1) gene was in germline configuration in all the tumors analyzed. Four immunohistochemical patterns of p53 and p21(Waf1) expression were observed: (1) p53-/p21- in 10 of 15 CLL (67%), but only in two of six RS (33%); (2) p53+/p21+ in three CLL (20%) and two RS (33%); (3) p53-/p21+ in one RS; and (4) p53++/p21- in two CLL and one RS. Two p53+/p21+ CLL evolved into RS. p53 mutations clustered around the p53++/p21- (two CLL and one RS) and p53-/p21- (one CLL and one RS) tumors. While the majority of CLL displayed strong p27 immunoreactivity, RS tumors were constantly p27-negative. p27(Kip1) gene was in germline configuration in all the tumors analyzed. Most CLL cases were negative for Rb expression. In contrast, all RS exhibited strong Rb expression. Cyclin D1 overexpression was only detected in one CLL evolving into RS and one RS. In conclusion, a p53+/p21- immunohistochemical pattern is shown exclusively by p53-mutated CLL/RS. Additionally, our results suggest a possible implication of moderate/strong p21(Waf1) expression, loss of p27 expression, and cyclin D1 overexpression in the Richter's transformation of CLL.

Oxidized low density lipoprotein induces the cyclin-dependent kinase inhibitor p21(waf1) and the tumor suppressor Rb

Oxidized low density lipoprotein (OxLDL) is known to be cytotoxic towards different cell types of the arterial wall, leading to progression of an atherosclerotic plaque. We previously reported that OxLDL activates the tumor suppressor p53 in human fibroblasts [Biochem. Biophys. Res. Commun. 276 (2000) 718]. In the present work, we demonstrate that OxLDL increased intracellular levels of the kinase inhibitor p21(waf1) (p21) and of the tumor suppressor Rb. Concomitantly, level of the hypophosphorylated active form of Rb (HypoP-Rb) was also enhanced. Cycloheximide prevented the OxLDL-induced increase in p21, Rb, and HypoP-Rb, whereas okadaic acid had no effect. This increase was also prevented by the antioxidant vitamin E. In addition, the lipid extract of OxLDL, which includes the lipid peroxidation products, reproduced the action of the OxLDL particle itself. OxLDL and its lipid extract induced an oxidative stress, as assessed by the intracellular levels of reactive oxygen species and lipid peroxidation products. Finally, OxLDL induced a dose-dependent inhibition of DNA synthesis as assessed by thymidine incorporation. These results demonstrate that OxLDL or its lipid peroxidation products, by generation of an oxidative stress, enhances the expression of p21 and Rb genes, leading to an accumulation of the Hypo-P active form of the tumor suppressor Rb. This phenomenon is in accordance with the fact that p21 is a mediator of p53-dependent cell-cycle arrest in G1 and is most probably involved in the cytotoxicity of OxLDL.

Tumor suppressor genes in normal and malignant hematopoiesis

Over the last decade, a growing number of tumor suppressor genes have been discovered to play a role in tumorigenesis. Mutations of p53 have been found in hematological malignant diseases, but the frequency of these alterations is much lower than in solid tumors. These mutations occur especially as hematopoietic abnormalities become more malignant such as going from the chronic phase to the blast crisis of chronic myeloid leukemia. A broad spectrum of tumor suppressor gene alterations do occur in hematological malignancies, especially structural alterations of p15(INK4A), p15(INK4B) and p14(ARF) in acute lymphoblastic leukemia as well as methylation of these genes in several myeloproliferative disorders. Tumor suppressor genes are altered via different mechanisms, including deletions and point mutations, which may result in an inactive or dominant negative protein. Methylation of the promoter of the tumor suppressor gene can blunt its expression. Chimeric proteins formed by chromosomal translocations (i.e. AML1-ETO, PML-RARalpha, PLZF-RARalpha) can produce a dominant negative transcription factor that can decrease expression of tumor suppressor genes. This review provides an overview of the current knowledge about the involvement of tumor suppressor genes in hematopoietic malignancies including those involved in cell cycle control, apoptosis and transcriptional control.

Expression of Cdk5 and its activators in NT2 cells during neuronal differentiation

We have recently developed a rapid protocol involving NT2 cell aggregation and treatment with retinoic acid (RA) to produce terminally differentiated CNS neurons. As a first step to explore the functional roles of cell-cycle regulatory proteins in the process of neuronal differentiation, the expression profiles of cyclin-dependent kinases (Cdks) and their regulators were examined in NT2 cells following treatment with RA. One of the Cdks, Cdk5, has been demonstrated to affect the process of neuronal differentiation and suggested to play an important role in development of the nervous system. We found that the expression of Cdk5 was gradually increased, while its activators (p35 and p39) as well as Cdk5 kinase activity were induced in NT2 cells during the process of neuronal differentiation. Moreover, both p35 and p39 were localized along the axons and varicosity-like structures of differentiated NT2 neurons. Taken together, our results demonstrated that NT2 cells provide a good in vitro model system to examine signaling pathways involved in the regulation of Cdk5 activators and to elucidate the functional roles of Cdk5 in neuronal differentiation.

Mutant mouse models reveal the relative roles of E2F1 and E2F3 in vivo

The E2F1, -2, and -3 transcription factors are key downstream targets of the retinoblastoma protein (pRB) tumor suppressor that drive expression of proliferation-associated genes. Here we use mutant mouse strains to investigate E2F3's role in vivo. We show that E2F3 is essential for embryonic viability in the pure 129/Sv background but the presence of C57BL/6 alleles yields some adult survivors. Although growth retarded, surviving E2f3(-/-) animals are initially healthy. However, they die prematurely, exhibiting no obvious tumor phenotype but with the typical signs of congestive heart failure. The defects are completely distinct from those arising in E2f1 mutant mice (S. J. Field et al., Cell 85:549-561; 1996; L. Yamasaki et al., Cell 85:537-548, 1996), supporting the prevailing view that these E2Fs must have some unique biological functions in vivo. To test this model, we examined the phenotypes of E2f1 E2f3 compound mutant mice. Almost all of the developmental and age-related defects arising in the individual E2f1 or E2f3 mice were exacerbated by the mutation of the other E2f. Thus, E2F1 and E2F3 appear to play critical, overlapping roles in the development and maintenance of a variety of tissues. Importantly, this study did identify one major difference in the properties of E2F1 and E2F3: either alone or in combination with E2F1 loss, E2f3 mutation did not increase the incidence of tumor formation. These data strongly suggest that tumor suppression is a specific property of E2F1 and not E2F3.

Exogenous cysteine and cystine promote cell proliferation in CaCo-2 cells

Previous studies have shown that intracellular glutathione, a ubiquitous intracellular thiol, is related to cell proliferation and that cysteine or its disulphide form, cystine, also induces cell proliferation. Cysteine is a thiol containing amino acid and a rate-limiting precursor of glutathione. Therefore, it is still unresolved as to whether the proliferative effect of cysteine or cystine is entirely mediated by a change in the intracellular glutathione status. The objective of this study was to delineate the relationship among cysteine/cystine (thereafter referred to as cyst(e)ine), intracellular glutathione and cell proliferation in the human colon cancer CaCo-2 cell line. CaCo-2 cells were cultured in cyst(e)ine-free Dulbecco's Modified Eagle Medium without serum, and treated with 200 microm cysteine and/or 200-400 microm cystine for 24 h. In the presence of DL-buthionine-[S, R]-sulfoximine (BSO), a glutathione synthesis inhibitor, exogenously administered cyst(e)ine did not change the intracellular glutathione content, but increased the intracellular cysteine as well as cystine level. Addition of exogenous cyst(e)ine following 5 mm BSO treatment significantly increased cell proliferation as measured by 3H-thymidine incorporation and protein content. Cell cycle analyses revealed that cyst(e)ine promoted cell progression from the G1 phase to the S phase. Correspondingly, cyst(e)ine treatment induced expression of cyclin D1 and phosphorylation of retinoblastoma protein (Rb). In conclusion, these data indicate that both cysteine and cystine have proliferative effects in CaCo-2 cells independent of an increase in intracellular glutathione. Induction of cyclin D1, phosphorylation of Rb, and subsequent facilitation of G1-to-S phase transition were involved in the proliferative effect of exogenous cyst(e)ine.

Cell cycle proteins exhibit altered expression patterns in lentiviral-associated encephalitis

Cell cycle proteins regulate processes as diverse as cell division and cell death. Recently their role in neuronal death has been reported in several models of neurodegeneration. We have reported previously that two key regulators of the cell cycle, the retinoblastoma susceptibility gene product (pRb) and transcription factor E2F1, exhibit altered immunostaining patterns in simian immunodeficiency virus encephalitis (SIVE). Here we show that E2F1 and the inactivated, hyperphosphorylated form of pRb (ppRb) also exhibit altered immunostaining patterns in human immunodeficiency virus encephalitis (HIVE). Quantification of E2F1 and ppRb staining by immunofluorescent confocal microscopy confirms a significant increase in E2F1 and ppRb in both HIVE and the simian model. This increase in E2F1 and ppRb staining correlates with an increase in the presence of activated macrophages, suggesting a link between changes in cell cycle proteins and the presence of activated macrophages. Changes in ppRb and E2F1 staining in SIVE also correlate with alterations in E2F/DNA binding complexes present in the nuclear and cytoplasmic fractions from both midfrontal cortex and basal ganglia. These findings suggest that changes in cell cycle proteins occur in both HIVE and the simian model and that these changes have functional implications for gene expression in neural cells under encephalitic conditions mediated by macrophage activation or infiltration.

The retinoblastoma family of proteins directly represses transcription in Saccharomyces cerevisiae

The retinoblastoma family of proteins are key cell cycle regulatory molecules important for the differentiation of various mammalian cell types. The retinoblastoma protein regulates transcription of a variety of genes either by blocking the activation domain of various activators or by active repression via recruitment to appropriate promoters. We show here that the retinoblastoma family of proteins functions as direct transcriptional repressors in a heterologous yeast system when fused to the DNA binding domain of Gal4. Mapping experiments indicate that either the A or the B domain of the pocket region is sufficient for repression in vivo. As is the case in mammalian cells, a phosphorylation site mutant of the retinoblastoma protein is a stronger transcriptional repressor than the wild type protein. We show that transcriptional repression by pRb is dependent on CLN3 in vivo. Furthermore, the yeast histone deacetylase components, RPD3 and SIN3, are required for transcriptional repression.

Forskolin-mediated G1 arrest in acute lymphoblastic leukaemia cells: phosphorylated pRB sequesters E2Fs

Increased intracellular levels of cAMP, induced by forskolin, lead to permanent G1 arrest of Reh cells. As expected, we observed a rapid dephosphorylation of the retinoblastoma protein (pRB) within 2 hours of forskolin treatment concomitant with reduced activity of the pRB-specific kinases. Interestingly, however, the dephosphorylation of pRB, as well as the inhibition of the kinase activities, was only transient, despite the permanent arrest of cells in G1. Importantly, although the pRB-specific kinases were fully active after 48 hours, pRB became only partially rephosphorylated.

The transient dephosphorylation of pRB could be explained by the transient decrease in the activities of the pRB-specific kinases, but to understand why pRB became only partially rephosphorylated, despite fully activated kinases,we postulated that cAMP could activate a pRB-directed phosphatase. It was therefore interesting to find that the phosphatase inhibitor, tautomycin, was able to abolish the forskolin-mediated dephosphorylation of pRB, without increasing the activities of the pRB-specific kinases.

To understand how Reh cells expressing hyperphosphorylated forms of pRB can remain arrested in G1, we used three different methods to test for the ability of pRB to form functional complexes with the family of E2F transcription factors. As expected, we observed an increased complex formation between E2F-1, E2F-4 and pRB after 2 hours when pRB was in its most dephosphorylated state. Suprisingly, however, prolonged treatment with forskolin, which induced partial rephosphorylation of pRB, in fact further increased the complex formation between the E2Fs and pRB, and this also resulted in reduced E2F-promoter activity in vivo. These data imply that in Reh cells, partially phosphorylated forms of pRB retain the ability to inhibit E2F-promoter activity, and thereby prevent cells from entering into S-phase.

Cyclin-dependent kinases as potential targets to improve stroke outcome

Increasing evidence suggests that cyclin-dependent kinases (CDKs), enzymes that normally regulate cell cycle progression, may also participate in the death of neurons. This has led to the proposal that CDKs may serve as a therapeutic target for neuropathological conditions such as stroke. This brief review will serve to examine the evidence supporting the role of CDKs in neuronal death, and will evaluate the potential of CDK inhibitors as a neuroprotective strategy for ischemic injury.

Enzyme-linked immunosorbent assay for distinct cyclin-dependent kinase activities using phosphorylation-site-specific anti-pRB monoclonal antibodies

Phosphorylation of retinoblastoma protein (pRB) by cyclin-dependent kinases (CDKs) at multiple sites leads to activation of transcription of cell-cycle-related genes. Cyclin/CDK complexes thus play a pivotal role in the regulation of progression from G1 to S phase. In the present study, we developed a nonradioactive, sandwich enzyme-linked immunosorbent assay (ELISA) system for measuring activities of cyclin/CDK complexes, in which the immobilized monoclonal antibody works as a trap for phosphorylated pRB containing phosphorylated amino acids at specific sites. For this purpose, we raised monoclonal antibodies that are highly specific to ppRB phosphorylated at Ser780, Thr356, or Ser612 and used them as detectors for the individual reaction products by cyclin/CDK complexes. In particular, this approach proved useful for cyclin D1/CDK4 that specifically recognizes Ser780 in pRB with only very limited phosphorylation of a conventional substrate, histone H1. The study revealed the newly developed sandwich ELISA system to have advantages over the current radioisotope assay in terms of sensitivity, precision, and rapidity. It should find application for inhibitor screening and drug discovery related to CDKs.

Growth inhibition and differentiation of pancreatic cancer cell lines by PPAR gamma ligand troglitazone

INTRODUCTION

Ligand activation of peroxisome proliferator-activated receptor gamma (PPAR gamma) results in growth inhibition and differentiation of various cancer cells.

AIMS

We determined whether the PPAR gamma ligand, troglitazone, inhibits the growth of pancreatic cancer cells and clarified the underlying mechanisms with a special focus on restriction point control of the late G1 phase of the cell cycle.

METHODOLOGY

Nine pancreatic cancer cell lines were used to study a variety of troglitazone effects on cell growth by MTT assay, on cell cycle by flow cytometry, on cell cycle regulating factors of late G1 phase by Western and Northern blotting and CDK2 kinase assay, and on morphology by collagen gel culture and electron-microscopy.

RESULTS

Troglitazone showed a potent dose-response effect on the growth inhibition of six pancreatic cancer cell lines, which were suppressed to less than 50% of control at the concentration of 10 microM. The growth inhibition was linked to the G1 phase cell cycle arrest through the upregulation of p21 mRNA and protein expression simultaneously with the inhibition of CDK2 kinase activity and the hypophosphorylation of Rb protein. The upregulation of expression of p21 mRNA was mainly due to stabilization of mRNA. Troglitazone induced significant morphologic changes of duct structure with apoptotic cells in the lumen.

CONCLUSION

Troglitazone had growth inhibitory and differentiation induction effects on the pancreatic cancer cell lines through the upregulation of p21 expression, suggesting that ligand activation of PPAR gamma is a new molecular target for effective therapy against pancreatic cancer.

Involvement of p38 mitogen-activated protein kinase in the cell growth inhibition by sodium arsenite

It is well-known that p38 mitogen-activated protein kinase (p38MAPK) participates in cellular responses to mitogenic stimuli, environmental and genotoxic stresses, and apoptotic agents. Although there are several reports on p38MAPK in relation to cell growth and apoptosis, the exact mechanism of p38MAPK-mediated cell growth regulation remains obscure. Here, we examined possible roles of p38MAPK in the sodium arsenite-induced cell growth inhibition in NIH3T3 cells. Sodium arsenite induced transient cell growth delay with marked activation of p38MAPK. In addition, arsenite induced CDK inhibitor p21(CIP1/WAF1) and enhanced its binding to the CDK2, which resulted in inhibition of CDK2 activity. The levels of cyclin D1 expression and the CDK4 kinase activity were also significantly reduced. pRB was hypophosphorylated by sodium arsenite. SB203580, a specific inhibitor of p38MAPK, blocked arsenite-induced growth inhibition as well as the arsenite-induced p21(CIP1/WAF1) expression. Expression of dominant negative p38MAPK also blocked arsenite-induced p21(CIP1/WAF1) expression. Inhibited-CDK2 activity was also completely reversed by SB203580 or expression of dominant negative p38MAPK, while the decreased-cyclin D1 protein by the compound was not restored. These data demonstrate a possible link between the activation of p38MAPK and induction of p21(CIP1/WAF1), suggesting that the activation of p38MAPK is, at least in part, related to the cell growth inhibition by sodium arsenite.

Okadaic acid suppresses TPA-induced differentiation by stimulating G1/S transition in human myeloblastic leukaemia ML-1 cells

The association between the phosphorylation status of the retinoblastoma protein, pRb and changes in cell cycle control caused by either protein kinase C (PKC) or protein kinase A (PKA) stimulation was evaluated in human myeloblastic leukaemia ML-1 cells. TPA-induced PKC activation resulted in dephosphorylation of pRb and subsequently induced ML-1 differentiation based on morphological changes and CD14 expression. In the present study, we showed that inhibition of protein phosphatases (PP-1 and PP-2a) prevented the TPA-induced differentiation in ML-1 cells. Preinhibition of PP-1 and PP-2a activities with 1-100 nM okadaic acid dose-dependently blunted the decrease in the phosphorylation status of pRb obtained with TPA and overrode cell cycle arrest. PKA stimulation with 8-chlorophenylthio-cAMP (100 microM) decreased cell proliferation by 65% and the distribution of cells in the G1 phase significantly increased from 38% to 83% concomitant with a 34% decline in the number of cells present in the S phase. In addition, PKA stimulation significantly decreased the pRb phosphorylation status but did not elicit CD14 expression, indicating that cAMP-induced dephosphorylation of pRb cannot by itself trigger differentiation in ML-1 cells.

Regulation of neuronal survival and death by E2F-dependent gene repression and derepression

Neuronal death induced by a variety of means requires participation of the E2F family of transcription factors. Here, we show that E2F acts as a gene silencer in neurons and that repression of E2F-responsive genes is required for neuronal survival. Moreover, neuronal death evoked by DNA damaging agents or trophic factor withdrawal is characterized by derepression of E2F-responsive genes. Such derepression, rather than direct E2F-promoted gene activation, is required for death. Among the genes that are derepressed in neurons subjected to DNA damage or trophic factor withdrawal are the transcription factors B- and C-myb. Overexpression of B- and C-myb is sufficient to evoke neuronal death. These findings support a model in which E2F-dependent gene repression and derepression play pivotal roles in neuronal survival and death, respectively.

High intensity ERK signal mediates hepatocyte growth factor-induced proliferation inhibition of the human hepatocellular carcinoma cell line HepG2

Hepatocyte growth factor (HGF) induces growth stimulation of a variety of cell types, but it also induces growth inhibition of several types of tumor cell lines. The molecular mechanism of the HGF-induced growth inhibition of tumor cells remains obscure. We have investigated the intracellular signaling pathway involved in the antiproliferative effect of HGF on the human hepatocellular carcinoma cell line HepG2. HGF induced strong activation of ERK in HepG2 cells. Although the serum-dependent proliferation of HepG2 cells was inhibited by the MEK inhibitor PD98059 in a dose-dependent manner, 10 microM PD98059 reduced the HGF-induced strong activation of ERK to a weak activation; and as a result, the proliferation inhibited by HGF was completely restored. Above or below this specific concentration, the restoration was incomplete. Expression of constitutively activated Ha-Ras, which induces strong activation of ERK, led to the proliferation inhibition of HepG2 cells, as was observed in HGF-treated HepG2 cells. This inhibition was suppressed by the MEK inhibitor. Furthermore, HGF treatment and expression of constitutively activated Ha-Ras changed the hyperphosphorylated form of the retinoblastoma tumor suppressor gene product pRb to the hypophosphorylated form. This change was inhibited by the same concentration of MEK inhibitor needed to suppress the proliferation inhibition. These results suggest that ERK activity is required for both the stimulation and inhibition of proliferation of HepG2 cells; that the level of ERK activity determines the opposing proliferation responses; and that HGF-induced proliferation inhibition is caused by cell cycle arrest, which results from pRb being maintained in its active hypophosphorylated form via a high-intensity ERK signal in HepG2 cells.

The role of acetylation in rDNA transcription

Treatment of NIH 3T3 cells with trichostatin A (TSA), an inhibitor of histone deacetylase (HDAC), resulted in a dose-dependent increase in transcription from a rDNA reporter and from endogenous rRNA genes. Chromatin immunoprecipitation using anti-acetyl-histone H4 antibodies demonstrated a direct effect of TSA on the acetylation state of the ribosomal chromatin. TSA did not reverse inhibition of transcription from the rDNA reporter by retinoblastoma (Rb) protein, suggesting that the main mechanism by which Rb blocks rDNA transcription may not involve recruitment of deacetylases to rDNA chromatin. Overexpression of histone transacetylases p300, CBP and PCAF stimulated transcription in transfected NIH 3T3 cells. Recombinant p300, but not PCAF, stimulated rDNA transcription in vitro in the absence of nucleosomes, suggesting that the stimulation of rDNA transcription by TSA might have a chromatin-independent component. We found that the rDNA transcription factor UBF was acetylated in vivo. Finally, we also demonstrated the nucleolar localization of CBP. Our results suggest that the organization of ribosomal chromatin of higher eukaryotes is not static and that acetylation may be involved in affecting these dynamic changes directly through histone acetylation and/or through acetylation of UBF or one of the other components of rDNA transcription.

Loss of heterozygosity at the RB-1 locus and pRB immunostaining in epithelial ovarian tumors: a molecular, immunohistochemical, and clinicopathologic study

Alterations in the retinoblastoma gene (RB-1) are common in human neoplasia. The frequency of loss of heterozygosity (LOH) at the RB-1 locus on chromosome 13q14 was studied in a series of 51 epithelial ovarian tumors (10 benign, 7 borderline, and 34 malignant). LOH was scored by the absence or reduction of the signal to < 50% of one of the alleles in tumor DNA compared with normal DNA. LOH results were correlated with retinoblastoma protein (pRB) immunostaining. LOH at the RB-1 locus was observed in 9 tumors (17.6%), specifically in 1 of 7 borderline tumors and 8 of 34 ovarian carcinomas (23.5%). Among the malignant tumors, LOH occurred more frequently in carcinomas with serous differentiation (7/23; 30%). A heterogeneous (10% to 70% cells) or diffuse (> 70% cells) pRB immunostaining was less frequent in benign (1/10; 10%) and borderline (2/7; 28%) tumors than in ovarian carcinomas (15/34; 44%), an observation that correlated with the higher proliferative index in carcinomas than in benign and borderline tumors. However, lack or only focal (< 10% cells) pRB immunostaining occurred in the vast majority of tumors with LOH at the RB-1 locus (7/9; 77%), a finding that may suggest a tumor suppressor role for RB-1 in these tumors. The results suggest that RB-1 may play a role in a subset of ovarian carcinomas, particularly those exhibiting serous differentiation.

Linking the Rb and polycomb pathways

Polycomb group (PcG) proteins associate to form complexes that repress Hox genes, thereby imposing the patterning of Hox expression required for development. However, these proteins have a second Hox-independent role in regulating cell proliferation. Our results suggest that association between Rb and PcG proteins forms a repressor complex that blocks entry of cells into mitosis. Also, we provide evidence that Rb colocalizes with nuclear PcG complexes and is important for association of PcG complexes with nuclear targets. The Rb-PcG complex may provide a means to link cell cycle arrest to differentiation events leading to embryonic pattern formation.

Expression of p16, nm23-H1, E-cadherin, and CD44 gene products and their significance in nasopharyngeal carcinoma

OBJECTIVE

The present study was aimed to determine whether p16/MTS1, nm23-H1, E-cadherin, and CD44 proteins were expressed in nasopharyngeal carcinoma (NPC) and whether those expressions were pathologically significant in the progress of NPC.

METHOD

We examined non-cancerous nasopharyngeal mucosa (20 cases) and NPC (80 cases) using immunohistochemistry with six different types of monoclonal antibodies against p16, nm23-H1, E-cadherin, CD44H, CD44v3, and CD44v6 proteins.

RESULTS

The results showed that 1) the rates of positive p16 protein expression and of preserved E-cadherin protein expression in NPC were significantly lower than those in non-cancerous tissue (P <.01); 2) no significant difference in the rate of positive expression of nm23-H1, CD44H, CD44v3, and CD44v6 proteins were observed between non-cancerous nasopharyngeal mucosa and NPC; 3) no significant difference in the expression of those proteins were found by respective correlation analyses of sex, stage, and size of primary tumor in NPC; and 4) no significant difference in the rates of positive expression of CD44H, CD44v3, and CD44v6 proteins were observed in NPC between with and without lymph node metastasis, indicating that those gene products did not correlate with lymph node metastasis in NPC. However, there were inverse correlations between the expression of p16, nm23-H1, or E-cadherin protein and lymph node metastasis (P <.05), indicating that the expression of p16, nm23-H1, and E-cadherin gene were related to the carcinogenesis and tumor progression of NPC.

CONCLUSION

Detecting the expressions of those gene products may provide clinically valuable information for therapeutic strategy and for predicting the prognosis of patients with NPC.

The Rb pathway in neurogenesis

Cell division during embryogenesis plays a crucial role in the formation of the nervous system. During this developmental process, proliferating neural precursor cells commit to a neuronal fate and, as a consequence, undergo terminal mitosis and adopt a neuronal phenotype. A key cell cycle regulator, the tumor suppressor protein, retinoblastoma (Rb), is involved in both terminal mitosis and neuronal differentiation. Neural development is a complex process involving cell proliferation, cell fate determination and differentiation, as well as programmed cell death. In this review, we will examine each of these processes in turn, focussing on the role of the Rb family proteins to examine their many influences on these events.

Alterations in expression of E2F-1 and E2F-responsive genes by RB, p53 and p21(Sdi1/WAF1/Cip1) expression

RB, p53 and p21(Sdi1/WAF1/Cip1) interact in the induction of G1 arrest. We established osteosarcoma cell lines in which a tetracycline-regulatable promoter controls the induction of RB, p53 and p21. By using these cell lines, we investigated whether RB, p53 or p21 regulates, in the same manner or differently, expression and function of E2F-1 and its responsive genes. E2F-1 gene products and transcripts of the E2F-responsive genes decreased in response to RB. Similar changes occurred to p53 and p21 when RB is present. However, in the absence of RB, some of the E2F-responsive genes decreased in response to p53 but not to p21. Thus, RB is a critical component for regulating the E2F-responsive genes, while p53 alone affects only a subset of these genes.

TEL contacts multiple co-repressors and specifically associates with histone deacetylase-3

TEL (Translocation-ETS-Leukemia or ETV 6) is disrupted by multiple chromosomal translocations in acute leukemia. The loss of heterozygosity at the TEL locus in leukemias and the hemizygous deletion of TEL that is observed in various tumors, suggests that TEL is a tumor suppressor. Overexpression of TEL alters cellular morphology and represses the expression of the matrix metalloproteinase stromelysin-1. Based on these studies, deletion analysis was used to define the minimal repression domains of TEL. TEL-mediated repression required both the N-terminal pointed domain and a central region composed of amino acids 268-303. The mSin3A and N-CoR corepressors bind to the pointed domain and the central repression domain of TEL, respectively. Unexpectedly, histone deacetylase-3, but not other histone deacetylases, also associates with the central region of TEL. Histone deacetylase-3 interacts with a TEL mutant that cannot bind N-CoR, suggesting that this is a direct interaction with TEL. In addition, histone H3 was under-acetylated near the TEL-binding sites in the endogenous stromelysin-1 promoter when TEL was expressed. Furthermore, trichostatin A, a potent histone deacetylase inhibitor, impaired TEL-dependent repression of the stromelysin-1 promoter. Finally, while TEL-expression induced cellular aggregation of Ras-transformed cells, Trichostatin A reversed the TEL-induced cellular aggregation phenotype. Thus, the cumulative data suggests that histone deacetylase-3 activity is required for the transcriptional functions of TEL.

Interdependence of cdk2 activation and interleukin-2Ralpha accumulation in T cells

We have shown previously that serum promotes T cell proliferation by acting with T cell receptor (TCR) agonists to efficiently down-regulate p27(Kip1) and activate cdk2-containing complexes. In the studies described here, the effect of serum on the expression of the alpha subunit of the interleukin-2 receptor (IL-2Ralpha) was examined. We found that serum was required for maximal and sustained IL-2Ralpha protein expression and consequent IL-2 signaling in TCR-activated splenocytes. Serum had no effect on IL-2Ralpha mRNA levels and thus modulates IL-2Ralpha expression post-transcriptionally. Unlike wild-type splenocytes, splenocytes exhibiting serum-independent cdk2 activation due to loss of p27(Kip1) efficiently expressed IL-2Ralpha in serum-deficient medium. Conversely, serum did not promote IL-2Ralpha accumulation in conditions in which cdk2 activity was blocked. These findings demonstrate that cdk2 activation is necessary and sufficient for IL-2Ralpha accumulation in TCR-stimulated splenocytes. On the other hand, IL-2 signaling was required (at least in part) for cdk2 activation in these cells. Thus, cdk2 activation, IL-2Ralpha expression, and IL-2 signaling are interdependent events, and we suggest that this feed-forward regulatory loop plays a key role in T cell mitogenesis.

p27Kip1 regulates T cell proliferation

Our studies addressed the mechanism by which serum acts in conjunction with T cell receptor (TCR) agonists to promote the proliferation of primary splenic T cells. When added to resting splenocytes, TCR agonists initiated G(0)/G(1) traverse and activated cyclin D3-cdk6 complexes in a serum-independent manner. On the other hand, both TCR agonists and 10% serum were required for the activation of cyclin E-cdk2 and cyclin A-cdk2 complexes and the entry of cells into S phase. Serum facilitated cdk2 activation by maximizing the extent and extending the duration of the TCR-initiated down-regulation of the cdk2 inhibitor, p27(Kip1). Although p27(Kip1) levels were reduced (albeit submaximally) in cells stimulated in serum-deficient medium, nearly all of the cdk2 complexes in these cells contained p27(Kip1). In contrast, in cells receiving TCR agonist and 10% serum, little if any p27(Kip1) was present in cyclin-cdk2 complexes. Unlike wild-type splenocytes, p27(Kip1)-null splenocytes did not require serum for cdk2 activation or S phase entry whereas loss of the related cdk2 inhibitor, p21(Cip1), did not override the serum dependence of these responses. We also found that cdk2 activation was both necessary and sufficient for maximal expression of cdk2 protein. These studies provide a mechanistic basis for the serum dependence of T cell mitogenesis.

Deregulated expression of DP1 induces epidermal proliferation and enhances skin carcinogenesis

E2F transcription factors have been implicated in several cellular processes, including proliferation, apoptosis, and oncogenic transformation. A functional E2F factor consists of a heterodimer containing an E2F polypeptide (E2F1-E2F6) and a DRTF1-polypeptide (DRTF1-polypeptide-1 (DP1) or DRTF1-polypeptide-2). It is the E2F subunit that supplies the transcriptional activation domain and the motif involved in binding to members of the retinoblastoma tumor suppressor family. The role of the DP subunit in regulating E2F-dependent activities is not completely understood. To examine the properties of DP1 in vivo, we generated transgenic mouse lines expressing DP1 under the control of a keratin 5 (K5) promoter. Overexpression of DP1 in basal layer keratinocytes caused mild hyperplasia and hyperproliferation of the epidermis but did not result in increased apoptosis or spontaneous tumor development. Coexpression of DP1 with E2F1 or E2F4 in the epidermis of bigenic mice modestly enhanced proliferation and apoptosis over the levels induced by E2F1 or E2F4 expression alone. In a two-stage chemical carcinogenesis assay, more and larger skin tumors developed in K5 DP1 transgenic mice than in nontransgenic mice. These findings show that in this in vivo model, deregulated expression of DP1 on its own induced proliferation and enhanced carcinogenesis.

Sequential two-step cleavage of the retinoblastoma protein by caspase-3/-7 during etoposide-induced apoptosis

During cellular apoptosis, retinoblastoma protein (RB) is subjected to cleavage near the carboxyl terminus by a caspase-3-like protease. In addition, an heretofore unidentified protease cleaves RB internally, generating fragments of 68 and 48 kDa. Internal cleavage abrogates the ability of RB to associate with E2F. To investigate the mechanism of RB internal cleavage, we developed and employed an in vitro cleavage assay. Incubation of in vitro translated (35)S-RB with apoptotic cell extracts led to RB cleavage at the C-terminus, followed by internal cleavage. The caspase peptide inhibitors z-VAD-FMK or z-DEVD-FMK blocked both cleavage events. Rapid C-terminal and internal cleavage were also observed when recombinant caspase-3 was added to (35)S-RB. Moreover, when caspase-3 was added to nonapoptotic cell extract, efficient internal cleavage of cellular RB was observed. Caspase-mediated internal cleavage occurred following RB residue aspartate(349) in the sequence DSID(349). This sequence is consistent with a DXXD recognition motif for caspase-3-like enzymes. Interestingly, we also observed RB internal cleavage in caspase-3-deficient MCF-7 cells, indicating that other caspases are capable of cleaving RB internally. Indeed, caspase-7, a member of the caspase-3 subfamily, was found to cleave (35)S-RB at both the carboxyl terminus, and following aspartate(349). By contrast, caspases that are not members of the caspase-3 subfamily failed to cleave RB. Taken together, our findings demonstrate that during apoptosis, a caspase-3-like protease is responsible for degradation and functional inactivation of RB by cleaving the protein internally following aspartate(349).

Rb dephosphorylation and suppression of E2F activity in human breast tumor cells exposed to a pharmacological concentration of estradiol

This report characterizes the influence of a pharmacological concentration of estradiol on growth arrest and cell death in MCF-7 breast tumor cells, with a focus on elements of the Rb-E2F cell-cycle regulatory pathway. Continuous exposure of MCF-7 breast tumor cells to 100 microM estradiol produces a marked reduction in the G1 and S phase populations and a corresponding increase in the G2/M population within 24 h; after 48 h, accumulation of cells in G1 becomes evident while after 72 h the cells appear to be equally distributed between the G1 and G2/M phases. The accumulation of cells in G1 is temporally associated with dephosphorylation of the Rb protein and suppression of E2F activity. Estradiol also produces an initial burst of cell death with loss of approximately 40% of the tumor cell population within 24 h; however, there is no tangible evidence for the occurrence of apoptosis based on terminal transferase end-labeling of DNA, DNA fragmentation analysis by alkaline unwinding, cell-cycle analysis or cell morphology. In addition to the lack of caspase-3 in MCF-7 cells, the absence of apoptosis could be related, at least in part, to the fact that estradiol promotes a rapid reduction in levels of the E2F-1 and Myc proteins. Overall, these studies are consistent with the concept that alterations in the levels and/or activity of the E2F family of proteins as well as proteins interacting with the E2F family may influence the nature of the antiproliferative and cytotoxic responses of the breast tumor cell.

Status of p53, p21, mdm2, pRb proteins, and DNA methylation in gonocytes of control and gamma-irradiated rats during testicular development

In fetal and newborn rat testes, gonocytes, which stop cycling for about 8 days, become highly radiosensitive. The presence of p53, p21, mdm2, and pRb, which are involved in cell cycle, apoptosis control, or both, were studied by immunohistochemistry to determine if their expression is related to this radiosensitivity. A strong cytoplasmic expression of p53 and p21 was detected. Cytoplasmic expression of p53 occurred only in arrested gonocytes, whereas that of p21 was observed before and after the block. P21 was found to colocalize with mitochondria. No expression of mdm2 was detected and pRb was present only when the gonocytes started cycling again. In animals exposed to 1.5 Gy of gamma-irradiation at Day 19 postcoitum, p53 expression was prolonged in time, whereas no change was observed in p21 amounts and localization, compared with controls. Using antibodies against 5-methyl cytosine, it was shown that gonocyte DNA passed from a hypomethylated to a methylated status 1 day after gonocytes stopped cycling. A prolonged survival of gonocytes after exposure to radiation was followed by their progressive apoptosis, which finally involved the entire gonocyte population between Days 6 and 12 postpartum. The elevated but delayed sensitivity of gonocytes to genotoxic stress may be related to the unusual expression of p53 and p21, which may itself be related to the large DNA methylation changes.

Collective inhibition of pRB family proteins by phosphorylation in cells with p16INK4a loss or cyclin E overexpression

The activity of the retinoblastoma protein pRB is regulated by phosphorylation that is mediated by G(1) cyclin-associated cyclin-dependent kinases (CDKs). Since the pRB-related pocket proteins p107 and p130 share general structures and biological functions with pRB, their activity is also considered to be regulated by phosphorylation. In this work, we generated phosphorylation-resistant p107 and p130 molecules by replacing potential cyclin-CDK phosphorylation sites with non-phosphorylatable alanine residues. These phosphorylation-resistant mutants retained the ability to bind E2F and cyclin. Upon introduction into p16(INK4a)-deficient U2-OS osteosarcoma cells, in which cyclin D-CDK4/6 is dysregulated, the phosphorylation-resistant mutants, but not wild-type p107 or p130, were capable of inhibiting cell proliferation. Furthermore, when ectopically expressed in pRB-deficient SAOS-2 osteosarcoma cells, the wild-type as well as the phosphorylation-resistant pRB family proteins were capable of inducing large flat cells. The flat cell-inducing activity of the wild-type proteins, but not that of the phosphorylation-resistant mutants, was abolished by coexpressing cyclin E. Our results indicate that the elevated cyclin D- or cyclin E-associated kinase leads to systemic inactivation of the pRB family proteins and suggest that dysregulation of the pRB kinase provokes an aberrant cell cycle in a broader range of cell types than those induced by genetic inactivation of the RB gene.

Long-term stabilization of vein graft wall architecture and prolonged resistance to experimental atherosclerosis after E2F decoy oligonucleotide gene therapy

OBJECTIVE

We tested the hypothesis that a single intraoperative transfection of rabbit vein grafts with a decoy oligonucleotide that blocks cell-cycle gene transactivation by the transcription factor E2F induces long-term stable adaptation that involves medial hypertrophy and a resistance to neointimal hyperplasia and atherosclerosis.

METHODS

Jugular vein to carotid artery interposition vein grafts in hypercholesterolemic rabbits were treated, using pressure-mediated delivery, with either E2F decoy oligonucleotide, scrambled oligonucleotide, or vehicle alone. E2F decoy inhibition of cell-cycle gene expression was determined by measuring proliferating cell nuclear antigen upregulation and bromodeoxyuridine incorporation in vascular smooth muscle cells. Neointimal hyperplasia and atherosclerosis were compared between groups at 6 months after operation. Wall stress was derived from the ratio of luminal radius to wall thickness. Normal rabbits exposed to 6 weeks of diet-induced hypercholesterolemia starting 6 months after operation were analyzed in the same manner.

RESULTS

The E2F decoy oligonucleotide, but not scrambled oligonucleotide or vehicle alone, inhibited proliferating cell nuclear antigen expression and smooth muscle cell proliferation. Furthermore, this manipulation of cell-cycle gene expression yielded an inhibition of neointimal hyperplasia and atherosclerotic plaque formation throughout the 6 months of cholesterol feeding. In normocholesterolemic rabbits, vehicle-treated and scrambled oligonucleotide-treated vein grafts remain susceptible to diet-induced atherosclerosis as well, whereas resistance to this disease induction remained stable in genetically engineered grafts.

CONCLUSION

A single intraoperative pressure-mediated delivery of E2F decoy effectively provides vein grafts with long-term resistance to neointimal hyperplasia and atherosclerosis. These findings suggest that long-term reduction in human vein graft failure rates may be feasible with this ex vivo gene therapy approach.

Interaction between YY1 and the retinoblastoma protein. Regulation of cell cycle progression in differentiated cells

Overexpression of the transcription factor YY1 activates DNA synthesis in differentiated primary human coronary artery smooth muscle cells. Overexpression of the retinoblastoma protein together with YY1 blocked this effect. In growth-arrested cells, YY1 resides in a complex with the retinoblastoma protein, but the complex is not detected in serum-stimulated S phase cultures, indicating that the interaction of the retinoblastoma protein and YY1 is cell cycle-regulated. Recombinant retinoblastoma protein directly interacts with YY1, destabilizing the interaction of YY1 with DNA and inhibiting its transcription initiator function in vitro. We conclude that in differentiated cells elevation of the nuclear level of YY1 protein favors progression into the S phase, and we propose that this activity is regulated by its interaction with the retinoblastoma protein.