Effects of p21(Cip1/Waf1) at both the G1/S and the G2/M cell cycle transitions: pRb is a critical determinant in blocking DNA replication and in preventing endoreduplication

Identification of CIP-1-associated regulator of cyclin B (CARB), a novel p21-binding protein acting in the G2 phase of the cell cycle

The cyclin-dependent kinase inhibitor p21(cip1) regulates cell cycle progression, DNA replication, and DNA repair by binding to specific cellular proteins through distinct amino- and carboxyl-terminal protein binding motifs. We have identified a novel human gene, CARB (CIP-1-associated regulator of cyclin B), whose product interacts with the p21 carboxyl terminus. Immunocytochemical analysis demonstrates that the CARB protein is perinuclear and predominantly associated with the centrosome and mitotic spindle poles. In addition, CARB is also able to associate with cyclin B1, a key regulator of mitosis. However, cyclin B1-CARB complex formation occurs preferentially in the absence of p21. Unexpectedly, overexpression of CARB is associated with a growth-inhibitory and ultimately lethal phenotype in p21(-/-) cells but not in p21(+/+) cells. These data identify a novel mechanism that may underlie the effects of p21 in the G(2)/M phases of the cell cycle.

Bone morphogenetic protein-2 induces cyclin kinase inhibitor p21 and hypophosphorylation of retinoblastoma protein in estradiol-treated MCF-7 human breast cancer cells

The biologic effects and mechanisms by which bone morphogenetic proteins (BMPs) function in breast cancer cells are not well defined. A member of this family of growth and differentiation factors, BMP-2, inhibited both basal and estradiol-induced growth of MCF-7 breast tumor cells in culture. Flow cytometric analysis showed that in the presence of BMP-2, 62% and 45% of estradiol-stimulated MCF-7 cells progressed to S-phase at 24 h and 48 h, respectively. Estradiol mediates growth of human breast cancer cells by stimulating cyclins and cyclin-dependent kinases (CDKs). BMP-2 significantly increased the level of the cyclin kinase inhibitor, p21, which in turn associated with and inactivated cyclin D1. BMP-2 inhibited estradiol-induced cyclin D1-associated kinase activity. Also estradiol-induced CDK2 activity was inhibited by BMP-2. This inhibition of CDK activity resulted in hypophosphorylation of retinoblastoma protein thus keeping it in its active form. These data provide the first evidence by which BMP-2 inhibits estradiol-induced proliferation of human breast cancer cells.

p21CIP1 is dispensable for the G2 arrest caused by genistein in human melanoma cells

We have investigated the effect of genistein on cell cycle distribution of the human choroidal melanoma cell line OCM-1. We report that this isoflavonoid arrested cells in G2. This effect was correlated with the induction of the CDK inhibitor p21CIP1. However, while CDK1 activity was markedly reduced following genistein treatment, CDK2 activity was not affected. This was in agreement with the absence of G1 arrest that we observed but caused some doubt about the functionality of p21CIP1. Attempts to demonstrate mutation or post-translational modification of p21CIP1 from OCM-1 cells were unsuccessful. In fact, the level of p21CIP1 induced by genistein was shown to be insufficient to cause CDK2 inhibition. The role of p21CIP1 in the inhibition of CDK1 was questionable, as we demonstrated that genistein impaired Tyr15 dephosphorylation of CDK1 and because CDK1-cyclin B1 complexes from treated cells could be reactivated upon exposure to CDC25 phosphatase. Finally, we report that p21CIP1 was not absolutely required for the genistein-induced G2 arrest, as the isoflavone caused at least partial G2 arrest in p21-deficient Rat-1 fibroblasts as well as in p21-/- mouse embryo fibroblasts.

Alternative product of the p16/CKDN2A locus connects the Rb and p53 tumor suppressors

Two distinct products are specified by the CDKN2A locus, the p16INK4a cyclin dependent kinase inhibitor and a protein termed ARF. ARF has been shown to bind to the Mdm2-p53 complex, resulting in stabilisation of both proteins, and a feedback loop exists through which ARF levels are negatively regulated by p53. Significantly, ARF expression is positively regulated by members of the E2F family of transcription factors. This provides a link between the Rb and p53 pathways and a mechanism whereby inactivation of Rb and release of E2F will lead to the stabilisation and functional activation of p53. The alternative exon encoding the functional amino terminal portion of ARF presumably represents an independent gene that has become co-localized with p16INK4a in order to exploit a common regulatory mechanism or purpose.

Bone morphogenetic protein-2 blocks MDA MB 231 human breast cancer cell proliferation by inhibiting cyclin-dependent kinase-mediated retinoblastoma protein phosphorylation

Bone morphogenetic protein-2 (BMP-2) has been shown to act as an antiproliferative agent for a number of different cell types. We show that BMP-2 dose-dependently inhibits growth of MDA MB 231 human breast cancer cells. Epidermal growth factor (EGF) stimulates DNA synthesis and entry of these cells into the S-phase. BMP-2 inhibits EGF-induced DNA synthesis by arresting them in G1 phase of the cell cycle. BMP-2 increases the level of cyclin kinase inhibitor p21. Furthermore, we show that exposure of MDA MB 231 cells to BMP-2 stimulates association of p21 with cyclin D1 and with cyclin E resulting in the inhibition of their associated kinase activities. Finally, BMP-2 treatment is found to cause hypophosphorylation of the retinoblastoma protein (pRb), a key regulator of cell cycle progression. Our data provide a mechanism for the antiproliferative effect of BMP-2 in the breast cancer cells.

Differential modulation of paclitaxel-mediated apoptosis by p21Waf1 and p27Kip1

The impact of the cyclin dependent kinase (CDK) inhibitors p21Waf1 and p27Kip1 on paclitaxel-mediated cytotoxicity was investigated in RKO human colon adenocarcinoma cells with the ecdysone-inducible expression of p21Waf1 or p27Kip1. Ectopic expression of p27Kip1 arrested cells at G1 phase, whereas p21Waf1 expression arrested cells at G1 and G2. Expression of p21Waf1 after paclitaxel treatment produced much greater resistance to paclitaxel than did expression of p27Kip1. We attributed this difference to the additional block at G2 induced by p21Waf1, which prevented cells from entering M phase and becoming paclitaxel susceptible. Expression of p21Waf1 inhibited p34cdc2 activity and markedly reduced paclitaxel-mediated mitotic arrest, from 87.5 to 23%. In contrast, p27Kip1 expression also inhibited p34cdc2 but reduced mitotic arrest only slightly, from 87. 4 to 74.5%. We concluded that the G2 block produced by p21Waf1, but not by p27Kip1, contributed to their unequal modulation of sensitivity to paclitaxel-mediated apoptosis in RKO cells, and there is no causal relationship between paclitaxel-mediated cytotoxicity and elevation of p34cdc2 activity.

Retinoblastoma protein enhances the fidelity of chromosome segregation mediated by hsHec1p

Retinoblastoma protein (Rb) plays important roles in cell cycle progression and cellular differentiation. It may also participate in M phase events, although heretofore only circumstantial evidence has suggested such involvement. Here we show that Rb interacts, through an IxCxE motif and specifically during G(2)/M phase, with hsHec1p, a protein essential for proper chromosome segregation. The interaction between Rb and hsHec1p was reconstituted in a yeast strain in which human hsHEC1 rescues the null mutation of scHEC1. Expression of Rb reduced chromosome segregation errors fivefold in yeast cells sustained by a temperature-sensitive (ts) hshec1-113 allele and enhanced the ability of wild-type hsHec1p to suppress lethality caused by a ts smc1 mutation. The interaction between Hec1p and Smc1p was important for the specific DNA-binding activity of Smc1p. Expression of Rb restored part of the inactivated function of hshec1-113p and thereby increased the DNA-binding activity of Smc1p. Rb thus increased the fidelity of chromosome segregation mediated by hsHec1p in a heterologous yeast system.

The orphan receptor COUP-TFII regulates G2/M progression of breast cancer cells by modulating the expression/activity of p21(WAF1/CIP1), cyclin D1, and cdk2

The orphan receptors COUP-TFI and COUP-TFII play an important role in development and differentiation by activating specific genes and by modulating the activity of nuclear receptors including estrogen receptor alpha (ERalpha) and retinoic acid receptors (RARs). Previously, it was demonstrated that the expression and activity of ERalpha and RARs are lost or impaired in anti-estrogen-resistant breast cancers. Here we show that, similar to ERalpha and RARs, the expression of COUP-TFII but not COUP-TFI is reduced in approximately 30% of breast cancer cell lines. Introduction of COUP-TFII to MDA-MB-435 cells resulted in reduced growth and plating efficiency. Interestingly, COUP-TFII increased the expression of cyclin D1 and p21(WAF1/CIP1) in MDA-MB-435 cells. Although parental and COUP-TFII-transduced cells progressed through the G1-S phase at a similar rate, progression of COUP-TFII cells through the G2/M transition phase was delayed. The activity of cdk2 required for G2/M progression was reduced in COUP-TFII cells compared to parental cells. This property of COUP-TFII is distinct from that of ERalpha and RARs, which usually modulate the G1 phase of breast cancer cells. Furthermore, these results reveal an important physiological function of COUP-TFII, which correlates with its ability to induce gene expression rather than modulation of nuclear receptor activity.

p21Waf1/Cip1/Sdi1-induced growth arrest is associated with depletion of mitosis-control proteins and leads to abnormal mitosis and endoreduplication in recovering cells

Induction of a cyclin-dependent kinase inhibitor p21Waf1/ Cip1/Sdi1 is an integral part of cell growth arrest associated with senescence and damage response. p21 overexpression from an inducible promoter resulted in senescence-like growth arrest in a human fibrosarcoma cell line. After release from p21-induced growth arrest, cells re-entered the cell cycle but displayed growth retardation, cell death and decreased clonogenicity. The failure to form colonies was associated with abnormal mitosis and endoreduplication in the recovering cells and was correlated with the induced level of p21 and the duration of p21 induction. p21 induction was found to inhibit the expression of multiple proteins involved in the execution and control of mitosis. p21-induced depletion of the cellular pools of mitosis-control proteins was followed by asynchronous resynthesis of such proteins after release from p21, which explains the observed mitotic abnormalities. Genetic destabilization in cells recovering from p21-induced growth arrest may conceivably play a role in carcinogenesis and tumor progression.

A dominant negative RAS-specific guanine nucleotide exchange factor reverses neoplastic phenotype in K-ras transformed mouse fibroblasts

Ras proteins are small GTPases playing a pivotal role in cell proliferation and differentiation. Their activation state depends on the competing action of GTPase Activating Proteins (GAP) and Guanine nucleotide Exchange Factors (GEF). A tryptophan residue (Trp1056 in CDC25Mm-GEF), conserved in all ras-specific GEFs identified so far has been previously shown to be essential for GEF activity. Its substitution with glutamic acid results in a catalytically inactive mutant, which is able to efficiently displace wild-type GEF from p21ras and to originate a stable ras/GEF binary complex due to the reduced affinity of the nucleotide-free ras/GEF complex for the incoming nucleotide. We show here that this 'ras-sequestering property' can be utilized to attenuate ras signal transduction pathways in mouse fibroblasts transformed by oncogenic ras. In fact overexpression of the dominant negative GEFW1056E in stable transfected cells strongly reduces intracellular ras-GTP levels in k-ras transformed fibroblasts. Accordingly, the transfected fibroblasts revert to wild-type phenotype on the basis of morphology, cell cycle and anchorage independent growth. The reversion of the transformed phenotype is accompanied by DNA endoreduplication. The possible use of dominant negative ras-specific GEFs as a tool to down-regulate tumor growth is discussed.

Reversible phosphorylation at the C-terminal regulatory domain of p21(Waf1/Cip1) modulates proliferating cell nuclear antigen binding

The p53-inducible gene product p21(WAF1/CIP1) plays a critical role in regulating the rate of tumor incidence, and identifying mechanisms of its post-translational regulation will define key pathways that link growth control to p21-dependent tumor suppression. A eukaryotic cell model system has been developed to determine whether protein kinase signaling pathways that phosphorylate human p21 exist in vivo and whether such pathways regulate the binding of p21 to one of its key target proteins, proliferating cell nuclear antigen (PCNA). Although human p21 expressed in Sf9 cells is able to form a complex with human PCNA, the inclusion of cell-permeable phosphatase inhibitors renders p21 protein inactive for PCNA binding. The treatment of this inactive isoform of p21 with alkaline phosphatase restores its binding to PCNA, suggesting that p21 expressed in Sf9 cells is subject to reversible phosphorylation at a key regulatory site(s). A biochemical approach was subsequently used to map the phosphorylation sites within p21, whose modification in vitro can inhibit p21-PCNA complex formation, to the C-terminal domain at residues Thr(145) or Ser(146). A phospho-specific antibody was developed that only bound to full-length p21 protein after phosphorylation in vitro at Ser(146), and this reagent was further used to demonstrate that the inactive isoform of p21 recovered from Sf9 cells treated with phosphatase inhibitors had been phosphorylated in vivo at Ser(146). These data identify the first phosphorylation site within the C-terminal regulatory domain of p21 whose modification in vivo modulates p21-PCNA interactions and define a eukaryotic cell model that can be used to study post-translational signaling pathways that regulate p21.

Effects of p21Waf1/Cip1/Sdi1 on cellular gene expression: implications for carcinogenesis, senescence, and age-related diseases

Induction of cyclin-dependent kinase inhibitor p21(Waf1/Cip1/Sdi1) triggers cell growth arrest associated with senescence and damage response. Overexpression of p21 from an inducible promoter in a human cell line induces growth arrest and phenotypic features of senescence. cDNA array hybridization showed that p21 expression selectively inhibits a set of genes involved in mitosis, DNA replication, segregation, and repair. The kinetics of inhibition of these genes on p21 induction parallels the onset of growth arrest, and their reexpression on release from p21 precedes the reentry of cells into cell cycle, indicating that inhibition of cell-cycle progression genes is a mechanism of p21-induced growth arrest. p21 also up-regulates multiple genes that have been associated with senescence or implicated in age-related diseases, including atherosclerosis, Alzheimer's disease, amyloidosis, and arthritis. Most of the tested p21-induced genes were not activated in cells that had been growth arrested by serum starvation, but some genes were induced in both forms of growth arrest. Several p21-induced genes encode secreted proteins with paracrine effects on cell growth and apoptosis. In agreement with the overexpression of such proteins, conditioned media from p21-induced cells were found to have antiapoptotic and mitogenic activity. These results suggest that the effects of p21 induction on gene expression in senescent cells may contribute to the pathogenesis of cancer and age-related diseases.

Exit from G1 and S phase of the cell cycle is regulated by repressor complexes containing HDAC-Rb-hSWI/SNF and Rb-hSWI/SNF

We present evidence that Rb forms a repressor containing histone deacetylase (HDAC) and the hSWI/SNF nucleosome remodeling complex, which inhibits transcription of genes for cyclins E and A and arrests cells in the G1 phase of the cell cycle. Phosphorylation of Rb by cyclin D/cdk4 disrupts association with HDAC, relieving repression of the cyclin E gene and G1 arrest. However, the Rb-hSWI/SNF complex persists and is sufficient to maintain repression of the cyclin A and cdc2 genes, inhibiting exit from S phase. HDAC-Rb-hSWI/SNF and Rb-hSWI/SNF then appear to maintain the order of cyclin E and A expression during the cell cycle, which in turn regulates exit from G1 and from S phase, respectively.

Synergistic induction of centrosome hyperamplification by loss of p53 and cyclin E overexpression

Centrosome hyperamplification and the consequential mitotic defects contribute to chromosome instability in cancers. Loss or mutational inactivation of p53 has been shown to induce chromosome instability through centrosome hyperamplification. It has recently been found that Cdk2-cyclin E is involved in the initiation of centrosome duplication, and that constitutive activation of Cdk2-cyclin E results in the uncoupling of the centrosome duplication cycle and the DNA replication cycle. Cyclin E overexpression and p53 mutations occur frequently in tumors. Here, we show that cyclin E overexpression and loss of p53 synergistically increase the frequency of centrosome hyperamplification in cultured cells as well as in tumors developed in p53-null, heterozygous, and wildtype mice. Through examination of cells derived from Waf1-null mice, we further found that Waf1, a potent inhibitor of Cdk2-cyclin E and a major target of p53's transactivation function, is involved in coordinating the initiation of centrosome duplication and DNA replication, suggesting that Waf1 may act as a molecular link between p53 and Cdk2-cyclin E in the control of the centrosome duplication cycle.

p53 mutation and mitotic infidelity

Chromosome instability (a high frequency of chromosomal loss and gain and genome doubling, often referred to as karyotypic instability) is one of the major characteristics of cancer cells. It facilitates carcinogenesis by increasing the chance of specific mutations responsible for malignant phenotypes. Chromosome instability in most cases reflects the occurrence of defective mitosis, including unequal distribution of chromosomes to daughter cells and failure to undergo cytokinesis, which leads to generation of aneuploid cells. Both in vivo and in vitro, chromosome instability has been shown to correlate with loss or mutation of the p53 tumor suppressor protein, the product of one of the most frequently mutated genes in cancer. The major function of p53 is to prevent cells from proceeding through the cell cycle when cells experience stress, insults, or errors that disturb the preprogrammed cell cycle progression. During the last several years, significant advances have been made in understanding how p53 is involved in the regulation of mitosis and how loss or mutation of p53 affects mitotic fidelity, which will be the subject of this review.

Binding domain for p21(WAF1) on the polypeptide chain of the protein kinase CK2 beta-subunit

Protein kinase CK2 is a ubiquitous serine/threonine kinase which is involved in many proliferation-related processes in the cell. It is composed of two regulatory beta-subunits and two catalytic alpha-subunits. Its regulation still remains mysterious in spite of many years of intense research. One of its regulators is the cdk inhibitory molecule p21(WAF1)-a protein which is expressed in situations of genotoxic stress. p21(WAF1) binds to the beta-subunit of CK2 and inhibits the activity of CK2. Using deletion mutants of CK2 beta as well as a peptide library consisting of 15-amino-acid-long peptides derived from the polypeptide chain of CK2 beta we mapped the binding region for p21(WAF1) on the polypeptide chain of CK2 beta. We localized an amino-terminal and a carboxy-terminal binding domain. Binding of p21(WAF1) to both regions of the CK2 beta-subunit interferes with the phosphotransferase activity of the CK2 holoenzyme.

DNA damage-related gene expression as biomarkers to assess cellular response after gamma irradiation of a human lymphoblastoid cell line

Since defects in molecular mechanisms controlling DNA repair, cell cycle checkpoint and apoptosis could modify cellular sensitivity to DNA damaging agents, we have conducted a multiparametric molecular analysis for better understanding the regulation pathways leading to cell survival or cell death after irradiation. Using a human lymphoblastoid cell line, we have analysed, following gamma irradiation (0.5, 1, 2, 4, 8, 16 and 32 Gy, at 0.5, 24, 48 and 72 h after treatment), the correlation between proliferation, cell cycle analysis, apoptosis and micronuclei frequency with the expression of TP53, WAF1, DNA LIGASE 1, PCNA, BAX, BLC-2, BAK, DAD1, ICH1-Long and -Short forms mRNAs. We have found that whereas TP53, BAK, ICH1-Short form, and DAD1 were expressed at constant levels, WAF1, PCNA, BAX were up-regulated, ICH1-Long form, DNA LIGASE 1, and BCL-2 were down-regulated. These modifications of expression were significantly correlated with doses, survival, proliferation, cell cycle delays, and apoptosis. A positive correlation of WAF1 and BAX, and a borderline negative correlation with BCL-2 expressions were observed with micronuclei frequency for doses ranging from 0.5 to 4 Gy. In conclusion, our data clearly demonstrate that gene expression profiling, which is easier and more rapid to conduct than the assessments of classical phenotypic responses, could be useful to improve knowledge concerning pathways involved in cellular response to irradiation, knowing that such biomarkers could constitute tools to assess radio-sensitivity/radio-resistance. Oncogene (2000) 19, 916 - 923.

PC-SPES: a unique inhibitor of proliferation of prostate cancer cells in vitro and in vivo

BACKGROUND

Management of prostate cancer that has spread beyond the capsule is a difficult problem. Innovative and nontoxic approaches to the disease are urgently required. Recently, a commercially available herbal mixture called PC-SPES showed potent antitumor activities on a variety of malignant cells in vitro.

METHODS

PC-SPES was evaluated for its ability to inhibit clonal growth, and to induce cell cycle arrest of three human prostate cancer cell lines (LNCaP, PC-3, and DU 145). Western blot analysis examined the effect of PC-SPES on levels of p21(waf1), p27(kip1), Bcl-2, and E-cadherin in the three cell lines; and telomerase activity was examined by telomeric repeat amplification protocol (TRAP) assay. Furthermore, the effect of oral PC-SPES (250 mg/kg/day) on growth of PC-3 and DU 145 tumors present in male BNX nu/nu triple immunodeficient mice was studied. LNCaP cells were not analyzed in mice because they grow only with difficulty in these immunodeficient mice.

RESULTS

PC-SPES markedly inhibited clonal growth of LNCaP, PC-3, and DU 145 prostate cancer cells, with a 50% inhibition (ED50) at approximately 2 microl/ml. Pulse-exposure studies showed that a 5-day pulse-exposure to PC-SPES (2 microl/ml) in liquid culture achieved a 50% inhibition of PC-3 clonal growth in soft agar, suggesting that the growth inhibition mediated by the extracts remained after removal of PC-SPES. Cell cycle analysis using the prostate cancer cell lines found that PC-SPES induced a significant increase in the number of cells in G0-G1 and G2/M, with a concomitant decrease in the number of cells in S phase. PC-SPES (2 microl/ml, 4 days) increased slightly the levels of p21(waf1) in the three cell lines, decreased by 40% the levels of Bcl-2 in PC-3, and the levels of p27(kip1) and E-cadherin and telomerase were unchanged in each of the lines. In vivo treatment with oral PC-SPES of male BNX mice having DU 145 tumors produced significant inhibition of their growth (P < 0.001), with no objective side effects including blood chemistries, weights, or autopsy analysis. The PC-SPES showed no statistical effect on the in vivo growth of PC-3 cells.

CONCLUSIONS

PC-SPES inhibits clonal proliferation of human prostate cancer cells both in vitro and in vivo, using a murine model.

p53-dependent G2 arrest associated with a decrease in cyclins A2 and B1 levels in a human carcinoma cell line

In vivo transfer of wild-type (wt) p53 gene via a recombinant adenovirus has been proposed to induce apoptosis and increase radiosensitivity in several human carcinoma models. In the context of combining p53 gene transfer and irradiation, we investigated the consequences of adenoviral-mediated wtp53 gene transfer on the cell cycle and radiosensitivity of a human head and neck squamous cell carcinoma line (SCC97) with a p53 mutated phenotype. We showed that ectopic expression of wtp53 in SCC97 cells resulted in a prolonged G1 arrest, associated with an increased expression of the cyclin-dependent kinase inhibitor WAF1/p21 target gene. A transient arrest in G2 but not in G1 was observed after irradiation. This G2 arrest was permanent when exponentially growing cells were transduced by Ad5CMV-p53 (RPR/INGN201) immediately after irradiation with 5 or 10 Gy. Moreover, levels of cyclins A2 and B1, which are known to regulate the G2/M transition, dramatically decreased as cells arrived in G2, whereas maximal levels of expression were observed in the absence of wtp53. In conclusion, adenoviral mediated transfer of wtp53 in irradiated SCC97 cells, which are mutated for p53, appeared to increase WAF1/p21 expression and decrease levels of the mitotic cyclins A2 and B1. These observations suggest that the G2 arrest resulted from a p53-dependent premature inactivation of the mitosis promoting factor.

IGF-II enhances trichostatin A-induced TGFbeta1 and p21(Waf1,Cip1, sdi1) expression in Hep3B cells

Cell growth and division are controlled through the actions of cyclin-dependent kinases (CDKs) and cyclin dependent kinase inhibitors (CKIs). Treatment of cell lines with Trichostatin A leads to induction of one of these CKIs, p21, and growth arrest. Induction of p21 can also occur through the actions of TGFbeta1. Latent TGFbeta1 can be activated by the M6P/IGF2R. In the present study we have examined the effect of TSA on members of the IGF axis, the CKIs p21 and p27, and also TGFbeta1 in Hep3B cells. The only member of the IGF axis to be affected by treatments was IGF2. Expression of another gene from the same chromosomal location, H19, was also affected. TGFbeta1 expression was greatly enhanced by TSA. In addition, both CKIs, p21 and p27, were upregulated by TSA. Effects of adding IGF-II or TGFbeta1 to TSA-treated cells on p21 induction were examined. The results show that the induction of p21 by TSA can be modulated by additions of IGF-II whereas addition of TGFbeta1 affects its own expression but not p21. In conclusion, the results indicate that the induction of p21 and cell growth arrest caused by Trichostatin A may involve multiple signaling pathways.

CSF-1 activates MAPK-dependent and p53-independent pathways to induce growth arrest of hormone-dependent human breast cancer cells

The CSF-1 receptor (CSF-1R) is expressed in >50% of human breast cancers. To investigate the consequence of CSF-1R expression, hormone-dependent human breast cancer cell lines, MCF-7 and T-47D, were transfected with CSF-1R. Unexpectedly, CSF-1 substantially inhibited estradiol (E2) and insulin-dependent proliferation of MCF-7 transfectants (MCF-7fms) and prevented cyclin E/cdk2 and cyclin A/cdk2 activation, consistent with a G1 arrest. In contrast, CSF-1 increased DNA synthesis in T-47D transfectants (T-47Dfms) alone and with E2 or insulin. In response to CSF-1, there was a marked and sustained upregulation of the cyclin-dependent kinase inhibitor, p21Waf1/Cip1, in MCF-7fms but not T-47Dfms. CSF-1 also markedly upregulated cyclin D1 in MCF-7fms. The coordinate increase in cyclin D1 and p21 had the effect of decreasing the specific but not absolute activity of cyclin D1/cdk4. p53 was not involved since CSF-1 induction of p21 was unaffected by dominant-negative p53 expression. ERK activation by CSF-1 was robust and sustained in MCF-7fms and to a much lesser extent in T-47Dfms. Using pharmacological and transient transfection approaches, we showed that ERK activation was necessary and sufficient for p21 induction in MCF-7fms. Moreover, activated MEK inhibited E2-stimulated cdk2 activity. Our findings indicate that the consequence of CSF-1R-mediated signals in human breast cancer cells is dependent on the genetic background of the particular tumor.

Pleiotropic cell-division defects and apoptosis induced by interference with survivin function

Here we investigate the role of the control of apoptosis in normal cell division. We show that interference with the expression or function of the apoptosis inhibitor survivin causes caspase-dependent cell death in the G2/M phase of the cell cycle, and a cell-division defect characterized by centrosome dysregulation, multipolar mitotic spindles and multinucleated, polyploid cells. Use of a dominant-negative survivin mutant or antisense survivin complementary DNA disrupts a supramolecular assembly of survivin, caspase-3 and the cyclin-dependent-kinase inhibitor p21Waf1/Cip1 within centrosomes, and results in caspase-dependent cleavage of p21. Polyploidy induced by survivin antagonists is accentuated in p21-deficient cells, and corrected by exogenous expression of p21. These findings show that control of apoptosis and preservation of p21 integrity within centrosomes by survivin are required for normal mitotic progression.

SIAH-1 inhibits cell growth by altering the mitotic process

SIAH-1, the human homologue of the drosophila seven in absentia gene, is a p53-p21Waf-1 inducible gene. We report that stable transfection with SIAH-1 of the epithelial breast cancer cell line MCF-7 blocks its growth process. The transfectants show a redistribution of SIAH-1 protein within the nucleus, more specifically to the nuclear matrix, associated to dramatic changes in cell morphology and defective mitosis. Multinucleated giant cells (2-12 nuclei in more than 50% cells) were a most striking observation associated with tubulin spindle disorganization and defective cytokinesis. There were also present at high frequency abortive mitotic figures, DNA bridges and persistance of intercellular bridges and midbodies, along with an increased expression of p21Waf-1. These results indicate that the mechanism of growth arrest induced by SIAH-1 in MCF-7 cells involves disorganization of the mitotic program, mainly during nuclei separation and cytokinesis.

Cytokine response gene 6 induces p21 and regulates both cell growth and arrest

Cytokine response gene #6 (CR6), cloned from interleukin 2-stimulated T lymphocytes, is homologous to GADD45 and MyD118, genes which promote cell cycle arrest and apoptosis. To determine how this gene family could possibly mediate both cell survival/proliferation and cell cycle arrest/death, transfectants were generated so that the genes could be expressed ectopically, independently from their normal inducing agents. In cycling retinoblastoma protein-negative (pRb-) cells, ectopic CR6 expression blocked G2/M transition, but did not prevent G1/S transition so that endoreduplication resulted. By comparison, when CR6, GADD45, and MyD118 genes were expressed ectopically in proliferating pRb+ cells, either G1/S or G2/M transition was effectively blocked, so that there was no endoreduplication. Consistent with these findings, in proliferating pRb-cells, ectopic expression of CR6 promoted the expression of both G1 and G2/M cyclins. By comparison, in pRb+ cells, the expression of G1 cyclins was increased, while expression of the mitotic cyclins was decreased. However, in pRb+ cells, cyclin-dependent kinase activities associated with both G1 and G2/M cyclins were decreased. Moreover, ectopic expression of all three genes resulted in the expression of the CKI, p21, both in pRb- and pRb+ cells. The physiologic induction of CR6 expression by IL2 in quiescent normal human T cells occurs transiently in the first half of G1, coordinately with the expression of p21. Therefore, this gene family regulates G1 and G2, and promotes either cell growth or arrest by a common mechanism.

Mechanisms of G2 arrest in response to overexpression of p53

Overexpression of p53 causes G2 arrest, attributable in part to the loss of CDC2 activity. Transcription of cdc2 and cyclin B1, determined using reporter constructs driven by the two promoters, was suppressed in response to the induction of p53. Suppression requires the regions -287 to -123 of the cyclin B1 promoter and -104 to -74 of the cdc2 promoter. p53 did not affect the inhibitory phosphorylations of CDC2 at threonine 14 or tyrosine 15 or the activity of the cyclin-dependent kinase that activates CDC2 by phosphorylating it at threonine 161. Overexpression of p53 may also interfere with the accumulation of CDC2/cyclin B1 in the nucleus, required for cells to enter mitosis. Constitutive expression of cyclin B1, alone or in combination with the constitutively active CDC2 protein T14A Y15F, did not reverse p53-dependent G2 arrest. However, targeting cyclin B1 to the nucleus in cells also expressing CDC2 T14A Y15F did overcome this arrest. It is likely that several distinct pathways contribute to p53-dependent G2 arrest.

BRCA1-associated growth arrest is RB-dependent

BRCA1 is a susceptibility gene for breast and ovarian cancer with growth-inhibitory activity for which the mechanism of action remains unclear. When introduced into cells, BRCA1 inhibits growth of some but not all cell lines. In an attempt to uncover the mechanism of growth suppression by BRCA1, we examined a panel of cell lines for their ability to reduce colony outgrowth in response to BRCA1 overexpression. Of all variables tested, only those cells with wild-type pRb were sensitive to BRCA1-induced growth suppression. In cells with an intact rb gene, inactivation of pRb by HPV E7 abrogates the growth arrest imposed by BRCA1. In accordance with these observations, we found that BRCA1 could not suppress BrdUrd uptake in primary fibroblasts from rb-/- mice and exhibited an intermediate ability to inhibit DNA synthesis in rb+/- as compared with rb+/+ cells. We further found that the BRCA1 protein complexes with the hypophosphorylated form of pRb. This binding is localized to amino acids 304-394 of BRCA1 protein and requires the ABC domain of pRb. In-frame deletion of BRCA1 fragment involved in interaction with pRb completely abolished the growth-suppressive property of BRCA1. Although it has been reported that BRCA1 interacts with p53, we find the p53 status did not affect the ability of BRCA1 to suppress colony formation. Our data suggest that the growth suppressor function of BRCA1 depends, at least in part, on Rb.

Deregulation of p53/p21Cip1/Waf1 pathway contributes to polyploidy and apoptosis of E1A+cHa-ras transformed cells after gamma-irradiation

The p53/p21Cip1/Waf1-dependent checkpoint control of G1/S and G2/M phases of the cell cycle in response to DNA damage is an important mechanism of genome stability maintenance in normal cells. In many tumor cells, due to frequent point mutations and deletions of p53, the stringent control of the cell cycle and apoptosis is compromised. We have examined the cell cycle control and cell death of the rat embryo fibroblast cells (REF) transformed by E1A+cHa-ras oncogenes and expressing wild type p53. Gamma-irradiation at a dosage of 6 Gy has been used to analyse the p53-dependent trans-activation of the target p21cip1/waf1 gene and the levels of activity of cyclin-dependent kinases. Our results show that the cell cycle inhibitors p21Cip1/Waf1 and p27KIP accumulate in response to irradiation both in REF and E1A+cHa-ras cells. In contrast to normal REF cells, the accumulation of p21Cip1/Waf1 and p27KIP inhibitors, however, does not lead to inhibition of Cdk2 and cyclins E, A-associated kinase activities and to a G1/S block in E1A+cHa-ras cells. It is unlikely that the lack of inhibitory function of p21Cip1/Waf1 can be explained by its inability to bind Cdk2 and Cdk4 kinases or PCNA. Moreover, the p21Cip1/Waf1-associated kinase activity is increased upon gamma-irradiation of E1A+cHa-ras cells. We suggest that inactivation of p21Cip1/Waf1 may be accounted for by its interaction with E1A oncoproducts as the inhibitor is detected in immunoprecipitates using E1A-specific antibodies. During a temporary G2/M delay induced by gamma-irradiation, E1A+cHa-ras transformants continue DNA replication, which leads to accumulation of polyploid cells with lobulated nuclei and micronuclei. Thus, DNA damage of E1A+cHa-ras transformed cells, with a combination of functionally active wild type p53 and inactive p21Cip1/Waf1, contributes to formation of polyploid cells which then die due to apoptosis.

Control of E2F activity by p21Waf1/Cip1

Cyclin-dependent kinase inhibitors (cdkis), such as p21, are believed to control proliferation through an ability to function as stoichiometric antagonists of cyclin-dependent kinases (cdks). The p21 gene is a direct transcriptional target for the p53 protein, and its activation is likely to be important in effecting the p53 response. It is widely accepted that p21 can influence cell cycle progression by controlling the activity of cdks that act on the retinoblastoma tumour suppressor protein (pRb) which, in a hypophosphorylated state, associates with E2F transcription factors to prevent the activation of genes required for progression into S phase. Phosphorylation of pRb by G1 cdk complexes releases E2F and thereby enables progress through the cell cycle. Here, we describe results which suggest a p21-dependent mechanism that facilitates the regulation of E2F through a pathway that is independent of the cdk control of pRb activity. As p21 can associate with E2F subunits, it is possible that these effects are exerted through a complex with E2F. Furthermore, we find that p21 can regulate transcription in vitro. The results suggest that p21 may control E2F activity through a pathway that acts independently of pRb.

Regulation of endoreduplication in maize (Zea mays L.) endosperm. Isolation of a novel B1-type cyclin and its quantitative analysis

To investigate the involvement of cyclin in mitotic and endoreduplicative cell cycle control, we have isolated a mitotic cyclin clone from a maize endosperm cDNA library. The deduced amino acid sequence of this clone identifies a novel B1-type cyclin with distinctly different sequence in regions with putative involvement in intracellular localization. This cyclin, designated Zeama;CycB 1;3 (CycZme1), was shown by RNA gel blots and quantitative RT-PCR to be specific for tissues engaging in cell proliferation. It accumulated in metaphase-arrested cells and declined rapidly upon release into G1 phase. During the transition from mitosis to endoreduplication in maize endosperm, CycZme1 transcript declined precipitously while transcripts associated with S phase (histone-H3 and PCNA) and multiple phases of the cell cycle (Cdc2, alpha-tubulin) remained at moderate to high levels. We conclude that CycZme1 down-regulation is involved in the cellular transition to endoreduplication.

p53 and chemosensitivity

BACKGROUND

Although hematologic malignancies and some solid tumors such as germ cell tumors and pediatric malignancies can be cured by cytotoxic treatment, the most prevalent solid tumors are relatively resistant to these interventions. Apoptosis is involved in the cell kill of anticancer drugs and p53 is believed to be of principal importance in this process. However p53 also plays a role in cell cycle arrest and DNA repair, cellular processes that can decrease the sensitivity to chemotherapy. Therefore, p53 may play a dual role after exposure to cytotoxic treatment, activating either mechanisms that lead to apoptosis or launching processes directing to DNA repair and survival of the cell.

DESIGN

In this article, we review in details the p53 functions involved in the mediation of chemosensitivity. The preclinical and clinical data published in the recent years about the relation between p53 and chemosensitivity are discussed and the potential pitfalls associated to most of these studies, and that may account for the contradictory results produced so far are also mentioned.

Differentiation of human malignant melanoma cells that escape apoptosis after treatment with 9-nitrocamptothecin in vitro

After in-vitro exposure to 0.05 micromol/L 9-nitrocamptothecin (9NC) for periods of time longer than 5 days, 65% to 80% of the human malignant melanoma SB1B cells die by apoptosis, whereas the remaining cells are arrested at the G2-phase of the cell cycle. Upon discontinuation of exposure to 9NC the G2-arrested cells resume cell cycling or remain arrested depending on the duration of 9NC exposure. In contrast to cycling malignant cells, the cells irreversibly arrested at G2 exhibit features of normal-like cells, the melanocytes, as assessed by the appearance of dendrite-like structures; loss of proliferative activity; synthesis of the characteristic pigment, melanin; and, particularly, loss of tumorigenic ability after xenografting in immunodeficient mice. Further, the expression of the cyclin-dependent kinase inhibitor p16 is upregulated in the 9NC-treated, G2-arrested, but downregulated in density G1-arrested cells, whereas the reverse is observed in the expression of another cyclin-dependent kinase inhibitor, p21. These results suggest that malignant melanoma SB1B cells that escape 9NC-induced death by apoptosis undergo differentiation toward nonmalignant, normal-like cells.

Cell cycle regulators: mechanisms and their role in aetiology, prognosis, and treatment of cancer

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Collaborative role of E2F transcriptional activity and G1 cyclindependent kinase activity in the induction of S phase

A considerable body of evidence points to a role for both cyclin E/cyclin-dependent kinase (cdk)2 activity and E2F transcription activity in the induction of S phase. We show that overexpression of cyclin E/cdk2 in quiescent cells induces S phase, that this coincides with an induction of E2F activity, and that coexpression of E2F enhances the cyclin E/cdk2-mediated induction of S phase. Likewise, E2F overexpression can induce S phase and does so in the apparent absence of cyclin E/cdk2 activity. In addition, although the inhibition of cyclin E/cdk2 activity blocks the induction of S phase after growth stimulation of normal mouse embryo fibroblasts, inhibition of cyclin E/cdk2 does not block S phase induction in Rb-/- cells where E2F activity is deregulated. These results point to the important roles for E2F and cyclin E/cdk2 in the induction of S phase. Moreover, the nature of the E2F targets and the suspected targets for cyclin E/cdk2 suggests a potential molecular mechanism for the collaborative action of cyclin E/cdk2 and E2F in the induction of S phase.

Involvement of p21(WAF1/Cip1) and p27(Kip1) in intestinal epithelial cell differentiation

Using the conditionally immortalized human cell line tsFHI, we have investigated the role of cyclin-dependent kinase inhibitors (CKIs) in intestinal epithelial cell differentiation. Expression of cyclins, cyclin-dependent kinases (Cdk), and CKIs was examined under conditions promoting growth, growth arrest, or expression of differentiated traits. Formation of complexes among cell cycle regulatory proteins and their kinase activities were also investigated. The tsFHI cells express three CKIs: p16, p21, and p27. With differentiation, p21 and p27 were strongly induced, but with different kinetics: the p21 increase was rapid but transient and the p27 increase was delayed but sustained. Our results suggest that the function of p16 is primarily to inhibit cyclin D-associated kinases, making tsFHI cells dependent on cyclin E-Cdk2 for pRb phosphorylation and G1/S progression. Furthermore, they indicate that p21 is the main CKI involved in irreversible growth arrest during the early stages of cell differentiation in association with D-type cyclins, cyclin E, and Cdk2, whereas p27 may induce or stabilize expression of differentiated traits acting independently of cyclin-Cdk function.

Hypophosphorylated retinoblastoma protein is associated with G2 arrest in esophageal squamous cell carcinoma

Hypophosphorylated retinoblastoma (Rb) gene product binds critical transcription factors, leading to G1 arrest in a number of conditions, including following DNA damage. We have previously shown that irradiated esophageal squamous cell carcinoma (ESSC) cells undergo predominantly G2 arrest, with increases in inhibitors of Rb phosphorylation. We thus hypothesized that this G2 arrest would be accompanied by increases in hypophosphorylated Rb protein (pRb). We sequenced the Rb genes of three human ESSC lines (KYSE) following reverse transcription polymerase chain reaction of exons A-E. Western gels were performed on protein extracts for pRb. Cells were irradiated at 6 Gy, and protein was extracted at 6 h. ELISA was used to measure hypophosphorylated pRb in radiated versus control cells. Student's t test was used to compare results. All lines had wild-type Rb genes. Western gels confirmed the presence of pRb. There were significant increases in hypophosphorylated pRb in all three lines following irradiation (no line with less than a 100% increase). We have thus shown that irradiation-induced G2 arrest occurs in association with wild-type Rb genes and that there is associated hypophosphorylation of pRb. This supports our data describing a further role for other G1 mediators, such as p21, in G2 arrest. Further investigations into therapies to expoit this cell cycle checkpoint are warranted and planned.

Differentiation-related changes in the cell cycle traverse

This review examines recent developments relating to the interface between cell proliferation and differentiation. It is suggested that the mechanism responsible for this transition is more akin to a "dimmer" than to a "switch," that it is more useful to refer to early and late stages of differentiation rather than to "terminal" differentiation, and examples of the reversibility of differentiation are provided. An outline of the established paradigm of cell cycle regulation is followed by summaries of recent studies that suggest that this paradigm is overly simplified and should be interpreted in the context of different cell types. The role of inhibitors of cyclin-dependent kinases in differentiation is discussed, but the data are still inconclusive. An increasing interest in the changes in G2/M transition during differentiation is illustrated by examples of polyploidization during differentiation, such as megakaryocyte maturation. Although the retinoblastoma protein is currently maintaining its prominent role in control of proliferation and differentiation, it is anticipated that equally important regulators will be discovered and provide an explanation at the molecular level for the gradual transition from proliferation to differentiation.

p21Waf1/Cip1/Sdi1 induces permanent growth arrest with markers of replicative senescence in human tumor cells lacking functional p53

We have shown previously that wild type p53 can rapidly induce replicative senescence in EJ human bladder carcinoma cells lacking functional p53. A major effector of p53 functions is p21Waf1/Cip1/Sdi1, a potent cyclin-dependent kinase inhibitor. p21Waf1/Cip1/Sdi1 has been shown to be involved in both p53 dependent and independent control of cell proliferation, differentiation and death. To directly investigate the effects of p21Waf1/Cip1/Sdi1 in the p53 response observed in EJ tumor cells, we established p21Waf1/Cip1/Sdi1 inducible lines using the tetracycline-regulatable vector system. p21Waf1/Cip1/Sdi1 induction caused irreversible cell cycle arrest in both G1 and G2/M, and diminished Cdk2 kinase activity. In addition, p21Waf1/Cip1/Sdi1 induction led to morphological alterations characteristic of cells undergoing replicative senescence with morphological, biochemical and ultrastructural markers of the senescent phenotype. Furthermore, sustained p21Waf1/Cip1/Sdi1 induction sensitized EJ cells to apoptotic cell death induced by mitomycin C, a cross-linking DNA damaging agent. These findings support the function of p21Waf1/Cip1/Sdi1 as an inducer of replicative senescence and a major mediator of this phenomenon in response to p53. Moreover, our results imply that therapeutic intervention in human cancers might be aimed at sustained elevation of p21Waf1/Cip1/Sdi1 expression.

Molecular mechanisms underlying interferon-alpha-induced G0/G1 arrest: CKI-mediated regulation of G1 Cdk-complexes and activation of pocket proteins

One prominent effect of IFNs is their cell growth-inhibitory activity. The mechanism behind this inhibition of proliferation is still not fully understood. In this study, the effect of IFN-alpha treatment on cell cycle progression has been analysed in three lymphoid cell lines, Daudi, U-266 and H9. Examination of the growth-arrested cell populations shows that Daudi cells accumulate in a G0-like state, whereas U-266 cells arrest later in G1. H9 cells are completely resistant to IFN-alpha's cell growth-inhibitory effects. The G0/G1-phase arrest is preceded by a rapid induction of the cyclin-dependent kinase inhibitors (CKIs), p21 and p15. In parallel, the activities of the G1 Cdks are significantly reduced. In addition to p21/p15 induction, IFN-alpha regulates the expression of another CKI, p27, presumably by a post-transcriptional mechanism. In the G1 Cdk-complexes, there is first an increased binding of p21 and p15 to their respective kinases. At longer exposure times, when Cdk-bound p15 and p21 decline, p27 starts to accumulate. Furthermore, we found that IFN-alpha not only suppresses the phosphorylation of pRb, but also alters the phosphorylation and expression of the other pocket proteins p130 and p107. These data suggest that induction of p21/p15 is involved in the primary IFN-alpha response inhibiting G1 Cdk activity, whereas increased p27 expression is part of a second set of events which keep these Cdks in their inactive form. Moreover, elevated levels of p27 correlated with a dissociation of cyclin E/Cdk2-p130 or p107 complexes to yield cyclin E/Cdk2-p27 complexes. In resistant H9 cells, which possess a homozygous deletion of the p15/p16 genes and lack p21 protein expression, IFN-alpha causes no detectable changes in p27 expression and, furthermore, no effects are observed on either pocket proteins in this cell line. Taken together, these data suggest that the early decline in G1 Cdk activity, subsequent changes in phosphorylation of pocket proteins, and G1/G0 arrest following IFN-alpha treatment, is not primarily due to loss of the G1 kinase components, but result from the inhibitory action of CKIs on these complexes.

Differentiation-related mechanisms which suppress DNA replication

Differentiation of mammalian cells implies cessation of DNA replication and cell proliferation; the potential controls of this coupling are examined here. It is clear that the known or proposed mechanisms of down-regulation of replicative cellular activities vary in different lineages of cell differentiation, and occur in all phases of the cell cycle. In G1 these regulators include p21/Cip1 or p27/Kip1, pRb, and p53; the novel, recently reported mechanisms of their action are summarized. In S phase the availability of nucleotide precursors, the origin recognition complex (ORC), and other replication proteins may be important in differentiation, and in G2 phase the cdc2/cyclin B complex and replication licensing factors determine normal G2 traverse versus an arrest or polyploidisation. Other replication-related mechanisms include transcription factors, e.g., Sp1, telomerase, and nuclear matrix changes. Thus, differentiation alters the activity not only of the various checkpoint proteins, but also of the components of the replicative machinery itself.

Relationship of p21(WAF1/CIP1/SDI1) to cell proliferation in primary cultures of adrenocortical cells

p21(WAF1/CIP1/SDI1) was originally described as a protein expressed at high levels in senescent human fibroblasts. We have studied the expression of p21 in adrenocortical cells, p21 is not expressed under most circumstances in the intact adrenal gland in vivo, except when the gland is damaged. When human and bovine adrenocortical cells are isolated and placed in both short-term and long-term culture, p21 levels are much higher. These levels did not show a large increase when the cells senesce after long-term proliferation. Thus, these observations raise the question of whether the elevated p21 in primary cultures of adrenocortical cells is caused by damage or whether p21 is elevated because the cells are dividing rather than quiescent, because it has been reported that p21 levels peak in G1 and G2 in dividing cells. In the present experiments on bovine and human adrenocortical cells in primary culture, labeling techniques that correlated nuclear p21 with measures of cell proliferation (bromodeoxyuridine incorporation and nuclear Ki-67 antigen) supported the hypothesis that p21 is associated with cell division and not with damage. This is consistent with recent data showing that, when adrenocortical cells are transplanted into immunodeficient mice, p21 is associated with healthy dividing cells in the transplant, p21 is not a unique marker for senescence, and more studies are required both to clarify its role in cell biology and to determine molecular features which characterize the senescent state of cells both in vitro and in vivo.

Significance of Fas and retinoblastoma protein expression during the progression of Barrett's metaplasia to adenocarcinoma

BACKGROUND

Barrett's esophagus (BE) is a premalignant lesion characterized by replacement of normal squamous epithelium with columnar epithelium. This lesion can progress to dysplasia and adenocarcinoma. Recently, the Fas receptor and retinoblastoma (Rb) protein have been described as important mediators of apoptosis and tumor suppression, respectively. This study was undertaken to examine their expression during the progression of metaplasia to adenocarcinoma in BE.

METHODS

In a review of 56 adenocarcinomas arising in BE, the specimen blocks were examined using the immunohistochemical avidin-biotin-peroxidase complex technique. For each specimen, areas of normal epithelium were compared with areas of metaplasia, dysplasia, or carcinoma (when present). Monoclonal mouse anti-human antibodies were used to identify Rb protein (Rb-Ab5, 1/50 dilution; Oncogene Science) and the 40-50-kDa cell membrane Fas protein (APO-1/Fas, 1/5 dilution; DAKO Corp.).

RESULTS

Loss of Rb staining was observed as the metaplasia progressed to dysplasia and carcinoma, indicating accumulation of unstainable aberrant protein. Conversely, Fas protein staining was undetectable or weak in normal or metaplastic epithelium, increasing in the areas of high-grade dysplasia and carcinoma. These differences were statistically significant (P < .001).

CONCLUSIONS

The accumulation of abnormal Rb protein during the progression of Barrett's metaplasia to carcinoma leads to unsuppressed tumor growth. Fas overexpression may represent a cellular attempt to balance the uncontrolled tumor proliferation by promoting apoptosis.

Regulation of endogenous E2F1 stability by the retinoblastoma family proteins

Certain E2F transcription factor species play a pivotal role in regulating cell-cycle progression. The activity of E2F1, a protein with neoplastic transforming activity when unregulated, is tightly controlled at the transcriptional level during G0 exit. In addition, during this interval, the stability of endogenous E2F1 protein increased markedly. E2F1 stability also was dynamically regulated during myogenic differentiation and in response to gamma irradiation. One or more retinoblastoma family proteins likely participate in the stability process, because simian virus 40 T antigen disrupted E2F1 stability regulation during G1 exit in a manner dependent on its ability to bind to pocket proteins. Thus, endogenous E2F1 function is regulated by both transcriptional and posttranscriptional control mechanisms.

Sodium butyrate induces G2 arrest in the human breast cancer cells MDA-MB-231 and renders them competent for DNA rereplication

When exposed to sodium butyrate (NaBut), exponentially growing cells accumulate in G1 and G2 phases of the cell cycle. In the human breast cancer cell line MDA-MB-231, an arrest in G2 phase was observed when the cells were released from hydroxyurea block (G1/S interface) in the presence of NaBut. The inhibition of G2 progression was correlated with increased contents both of total p21(Waf1) and of p21(Waf1) associated with cyclin A and with an inhibition of cyclin A- and B1-associated histone H1 kinase activities measured in cell lysates, as well as with dephosphorylation of the RB protein. A decrease in the cell contents of cyclins A and B1 was also observed but this decrease was preceded by p21(Waf1) accumulation. When NaBut was removed from the culture medium of cells blocked in G2 phase, p21(Waf1) level decreased and, instead of proceeding to mitosis, these cells resumed a progression toward DNA rereplication. These results suggest that the induction of p21(Waf1) by NaBut leads to the inhibition of the sequential activation of cyclin A- and B1-dependent kinases in this cell line, resulting in the inhibition of G2 progression and rendering the cells competent for a new cell division cycle.

Hydrogen peroxide induces nuclear translocation of p53 and apoptosis in cells of oligodendroglia origin

The observation that apoptosis is an inherent pathway in oligodendrocytes development coupled with the notion that wild-type p53 is expressed in these cells, prompted us to investigate the interrelationship between the two phenomena. Using a permanent oligodendroglia-like cell line (OLN 93), we examined the role of p53 protein in apoptosis following a DNA insult induced by a brief exposure to H2O2. A marked translocation of p53 from the cytosolic to the nuclear compartment was notable by 20 min, following a 5 min treatment with 1 mM H2O2 as identified by cell immunostaining. By 48 h following H2O2 addition, nearly 60% of the cells exhibited p53 in the nuclei. At this time, a large proportion of the cells underwent apoptosis as identified by DAPI nuclear staining. The genotoxic-induced p53 relocalization appeared to be cell cycle phase specific; thus OLN 93 cultures enriched for cells in the G0/G1 stage by serum starvation, and abundant in nuclear-associated p53, were more susceptible to H2O2-induced apoptosis than their untreated counterparts and than double thymidine block, G1/S enriched, cultures. Analysis of the expression of p53 downstream genes indicated that p21 and mdm2 were upregulated following p53 nuclear translocation. From the kinetics of protein accumulation, it appears that mdm2 enhancement accelerated the exit of p53 from the nucleus to the cytosol. Our results suggest that following stress, oligodendroglia-like cells are induced to undergo p53-dependent apoptosis, an event that coincides with p53 nuclear translocation and is cell-cycle related.

Comparison of the effectiveness of adenovirus vectors expressing cyclin kinase inhibitors p16INK4A, p18INK4C, p19INK4D, p21(WAF1/CIP1) and p27KIP1 in inducing cell cycle arrest, apoptosis and inhibition of tumorigenicity

Cell cycle regulatory proteins are important candidates for therapeutic tumour suppressors. Adenovirus vectors were constructed to overexpress cyclin kinase inhibitors p16INK4A, p18INK4C, p19INK4D, p21(WAF1/CIP1) and p27KIP1 under the control of the murine cytomegalovirus immediate early gene promoter. These vectors directed the efficient expression of each of the cyclin kinase inhibitors and induced growth arrest, inhibited DNA synthesis, and prevented phosphorylation of the retinoblastoma protein (pRb) in cell lines expressing functional pRb. In pRb-deficient cells, expression of the cyclin kinase inhibitors was not effective in inhibiting DNA replication or growth arrest. Interestingly, three of the cyclin kinase inhibitors, p16, p18 and p27 were found to induce apoptotic death in transduced HeLa and A549 cells. When the vectors were tested for their ability to inhibit tumorigenicity in a polyomavirus middle T antigen model of murine breast carcinoma, expression of the cyclin kinase inhibitors resulted in a delay in tumour formation that varied from several weeks for the p19 expressing vector to greater than 25 weeks for the p27 expressing vector. When tumours were injected directly with the adenovirus vectors expressing the cyclin kinase inhibitors, only treatment with the vector expressing p16 resulted in a delay in tumour growth.

Overexpression of proliferating cell nuclear antigen in mammalian cells negates growth arrest by serum starvation and cell contact

Proliferating cell nuclear antigen (PCNA) functions as a processivity factor for DNA polymerase delta, and is expressed at high levels in growing normal and tumor cells. To clarify the relationship between cell proliferation and PCNA expression, we generated NIH-3T3 cells that overexpress PCNA and analyzed the phenotype of these cells. The resulting 3T3-PCNA cells, which overexpressed PCNA, were found to proliferate beyond the saturation density of the parental NIH-3T3 cells. Although NIH-3T3 cell proliferation is arrested under serum starvation conditions, 3T3-PCNA cell proliferation is not arrested by serum starvation. The expression levels of cdk2, cdk4 and cdk6 were the same in 3T3-PCNA and NIH-3T3 cells. The activity of cdk4 was identical for both cell types. However, the activity of cdk2 was higher in serum-starved 3T3-PCNA cells than in NIH-3T3 cells, although the expression of cyclin E decreased in both types of cells, suggesting that increases in cdk2 activity are related to negation of growth arrest in 3T3-PCNA cells. These results indicate that increases in PCNA expression lead to the disruption of growth control and may lead to malignant transformation.

Simultaneous detection of cyclin B1, p105, and DNA content provides complete cell cycle phase fraction analysis of cells that endoreduplicate

BACKGROUND

DNA analysis of endoreduplicating cells is difficult because of the overlap between stem-line G2 + M cells and 4C G1 cells. Simultaneous flow cytometry of DNA and cyclin B1 analytically separates these populations. The objective here was to develop simultaneous flow cytometry of DNA, cyclin B1, and p105 (highly expressed in mitosis) for improved, complete cell cycle phase fraction analysis of endoreduplicating cell populations.

METHODS

Monoclonal antibody, GNS-1, reactive with human cyclin B1, was conjugated with fluorescein at three different fluorochrome-to-protein (F/P) ratios and tested for optimal sensitivity in a flow cytometric assay. A formaldehyde-methanol fixation procedure was optimized for retention of p105 within mitotic cells by analytic titration of formaldehyde. p105 was stained indirectly with Cy5-conjugated secondary antibody, followed by GNS-1, and DNA was stained with Hoechst 33342. The specificity of p105 in this assay was tested by comparison of manual and flow cytometric mitotic indices and by sorting and microscopic inspection.

RESULTS

F/P 4.1 provided optimal fluorescein labeling of GNS-1. Formaldehyde (0.5%), followed by methanol permeabilization, fixed cells sufficiently to quantify stem-line and endoreduplicated G1, S, G2, and M phase fractions. Kinetic measurements of these fractions for both populations were demonstrated.

CONCLUSIONS

The fluorochrome-to-protein ratio is important and can be optimized objectively for these assays. A permeabilization-sensitive antigen (p105), previously requiring formaldehyde/detergent-fixed cell preparations, was shown to work equally well with formaldehyde/ methanol fixation. Three-laser, two-parameter intracellular antigen analysis can be successfully coupled with DNA content analysis. Cell cycle kinetic analysis of endoreduplicating populations should be improved.