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

Phosphorylation on tyrosine-15 of p34(Cdc2) by ErbB2 inhibits p34(Cdc2) activation and is involved in resistance to taxol-induced apoptosis

ErbB2 overexpression confers resistance to taxol-induced apoptosis by inhibiting p34(Cdc2) activation. One mechanism is via ErbB2-mediated upregulation of p21(Cip1), which inhibits Cdc2. Here, we report that the inhibitory phosphorylation on Cdc2 tyrosine (Y)15 (Cdc2-Y15-p) is elevated in ErbB2-overexpressing breast cancer cells and primary tumors. ErbB2 binds to and colocalizes with cyclin B-Cdc2 complexes and phosphorylates Cdc2-Y15. The ErbB2 kinase domain is sufficient to directly phosphorylate Cdc2-Y15. Increased Cdc2-Y15-p in ErbB2-overexpressing cells corresponds with delayed M phase entry. Expressing a nonphosphorylatable mutant of Cdc2 renders cells more sensitive to taxol-induced apoptosis. Thus, ErbB2 membrane RTK can confer resistance to taxol-induced apoptosis by directly phosphorylating Cdc2.

Stage-dependent activation of cell cycle and apoptosis mechanisms in the right ventricle by pressure overload

The molecular basis of the intrinsic vulnerability of the compliant right ventricle to chronic pressure overload is poorly understood. Extensive apoptosis, possibly coupled with aberrant cell cycle reentry, in response to unrestrained biomechanical stress may account for this phenotypic flaw. To address this issue we have studied changes in expression of the cell cycle and apoptosis regulators in the right ventricle following induction of pulmonary hypertension in the rat by injection of monocrotaline. Hypertrophy, apoptosis and cell cycle events, as well as expression of their regulator genes were documented during a period of 31 days. The hypertrophy index reached 127% at day 31. At the early stage both apoptosis and cell proliferation pathways were coincidentally activated. The level of cyclin A and E transcripts steadily increased, the labeling index was 4.8% at day 31, and expression of the caspase-3 gene peaked at day 14. Until day 21 execution of apoptosis was prevented, probably by a high level of Bcl-2. At this time point Bcl-2 collapsed, cyclin D1 was upregulated, the differentiation gatekeeper p27Kip1 was downregulated, pro-caspase-3 was activated and extensive apoptosis developed. These results indicate that the right ventricle is especially vulnerable to apoptotic pressure-dependent stimuli, and that the cell cycle and apoptosis pathways were co-activated in this experimental model.

Androgen stimulated cellular proliferation in the human prostate cancer cell line LNCaP is associated with reduced retinoblastoma protein expression

To elucidate the mechanism of androgen-dependent cellular proliferation in prostate cancer, androgen-dependent alterations of individual cell cycle regulatory proteins in the androgen-sensitive prostate cancer cell line LNCaP were evaluated. LNCaP cells were deprived of androgens by culture in steroid-depleted media for 5 days, which resulted in the maximal accumulation of cells in G(0)/G(1) phase of the cell cycle. The mitogenic concentration of the synthetic androgen R1881 was established as 0.1 nM using cell proliferation assay. Protein and mRNA levels of particular cyclins, cyclin-dependent kinases (Cdks), cyclin-dependent kinase inhibitors (Ckis), and the retinoblastoma proteins (Rb) were assessed. Androgen stimulation resulted in a post-transcriptional reduction in Rb protein levels, an increase in Rb phosphorylation at serine 780 and an accumulation of high molecular weight Rb protein species. Androgen stimulation also induced the expression of the Cdk2 and Cdk1 as well as their regulatory partners, cyclin A and cyclin B, resulting in a corresponding increase in cyclin A/Cdk2 activity in vitro. Pulse-chase showed decreased Rb protein stability in androgen-treated LNCaP cells. Collectively, our findings suggest a novel mechanism of androgen-dependent prostate cancer growth in which androgen stimulation results in decreased Rb protein expression in LNCaP cells. The observation of decreased Rb protein stability in the setting of increased phosphorylation supports the concept of phosphorylation mediated protein degradation. We propose that the observed reduction in Rb protein level occurs through Rb degradation via the ubiquitin/proteasome pathway, and is preceded by selective Rb phosphorylation by cyclin A/Cdk2 and cyclin B/Cdk1.

Alternative fates of keratinocytes transduced by human papillomavirus type 18 E7 during squamous differentiation

The human papillomavirus type 18 (HPV-18) E7 protein promotes S-phase reentry in postmitotic, differentiated keratinocytes in squamous epithelium to facilitate vegetative viral DNA amplification. To examine the nature and fate of the differentiated cells that reenter S phase, organotypic cultures of primary human keratinocytes transduced with HPV-18 E7 were pulse-chase-pulse-labeled with (3)H-thymidine ((3)H-TdR) and bromodeoxyuridine (BrdU). The kinetics of the appearance of doubly labeled suprabasal cells demonstrate that E7 expression did not promote prolonged S phase. Rather, there was a considerable lag before a small percentage of the cells reentered another round of S phase. Fluorescence in situ hybridization analysis, indeed, revealed a small fraction of the cells with more than 4n chromosomes in the differentiated strata. Differentiated cells positive for (3)H-TdR, BrdU, or both often had enlarged nuclei or were binucleated. These results suggest that S phase is not followed by cell division, although nuclear division may occur. Interestingly, a significant fraction of differentiated cells that entered S phase subsequently accumulated p27kip1 protein with a kinetics preceding the accumulation of cyclin E. We conclude that E7-transduced, differentiated keratinocytes that enter S phase have two alternative fates: (i) a low percentage of cells undergoes endoreduplication, achieving higher than 4n ploidy, and (ii) a high percentage of cells accumulates the p27kip1, cyclin E, and p21cip1 proteins, resulting in arrest and preventing further S-phase reentry.

Rho regulates p21(CIP1), cyclin D1, and checkpoint control in mammary epithelial cells

The small GTPase Rho is important for cell cycle progression and Ras transformation in fibroblasts. However, it is unclear whether Rho is needed for proliferation in other cell types, and its targets in promoting normal cell cycle progression are unknown. Here, we demonstrate that Rho is required for G1 to S progression in MCF10A mammary epithelial cells, both in response to EGF and in response to oncogenic Ras. We describe two effects of Rho, the repression of p21(CIP1) and the induction of cyclin D1, that may underlie its role in promoting S phase entry. The Rho inhibitor, C3 exotransferase, induced p21(CIP1) both in EGF-stimulated and V12Ras-expressing cells. In addition, C3 blocked EGF-stimulated cyclin D1 promoter activity whereas V14RhoA induced the cyclin D1 promoter and cooperated with V12Ras in cyclin D1 induction. Finally, a high proportion of cells co-expressing V14RhoA and V12Ras displayed lobulated, polyploid nuclei that were actively synthesizing DNA. Our results demonstrate that Rho plays a fundamental role in promoting Ras-dependent S phase entry in mammary epithelial cells, whether in response to normal or oncogenic signaling, and indicate that in cells expressing oncogenic Ras, the activation of Rho diminishes p21(CIP1) expression, increases cyclin D1 promoter activity, and uncouples DNA synthesis from mitosis.

Effect of flavonoids on cell cycle progression in prostate cancer cells

The effect of some flavonoids, which are components of fruits, vegetables, and peas, on the cell cycle progression of human LNCaP prostate cancer cells has been investigated in this study. Genistein arrested the cell cycle at the G2/M phases, which is attributed to the suppression of cyclin B expression. In addition, genistein induced the cyclin-dependent kinase inhibitor p21, which does not depend on p53 activation. Apigenin and luteolin also increased p21 levels, but quercetin did not. Apigenin induced p21 production through a p53-dependent pathway, but luteolin did so in a p53-independent manner. These results suggest that flavonoids are potent regulators of cyclin B and p21 for cell cycle progression, which may play some roles in prevention of carcinogenesis.

Evidence for coordinated interaction of cyclin D3 with p21 and cdk6 in directing the development of uterine stromal cell decidualization and polyploidy during implantation

Uterine decidualization, characterized by stromal cell proliferation, and differentiation into specialized type of cells (decidual cells) with polyploidy, during implantation is critical to the pregnancy establishment in mice. The mechanisms by which the cell cycle events govern these processes are poorly understood. The cell cycle is tightly regulated at two particular checkpoints, G1-S and G2-M phases. Normal operation of these phases involves a complex interplay of cyclins, cyclin-dependent kinases (cdks) and cdk inhibitors (CKIs). We previously observed that upregulation of uterine cyclin D3 at the implantation site is tightly associated with decidualization in mice. To better understand the role of cyclin D3 in this process, we examined cell-specific expression and associated interactions of several cell cycle regulators (cyclins, cdks and CKIs) specific to different phases of the cell cycle during decidualization in mice. Among the various cell cycle molecules examined, coordinate expression and functional association of cyclin D3 with cdk4 suggest a role for proliferation and, that of cyclin D3 with p21 and cdk6 is consistent with the development of polyploidy during stromal cell decidualization.

Roads to polyploidy: the megakaryocyte example

Polyploidy, recognized by multiple copies of the haploid chromosome number, has been described in plants, insects, and in mammalian cells such as, the platelet precursors, the megakaryocytes. Several of these cell types reach high ploidy via a different cell cycle. Megakaryocytes undergo an endomitotic cell cycle, which consists of an S phase interrupted by a gap, during which the cells enter mitosis but skip anaphase B and cytokinesis. Here, we review the mechanisms that lead to this cell cycle and to polyploidy in megakaryocytes, while also comparing them to those described for other systems in which high ploidy is achieved. Overall, polyploidy is associated with an orchestrated change in expression of several genes, of which, some may be a result of high ploidy and hence a determinant of a new cell physiology, while others are inducers of polyploidization. Future studies will aim to further explore these two groups of genes.

p21(WAF1) is associated with CDK2 and CDK4 protein during HL-60 cell differentiation by TPA treatment

TPA-treated HL-60 cells are mainly arrested in G1 by p21(WAF1) accumulation. We investigate the downstream changes following such accumulation. Increased p21(WAF1) is associated with CDK2 and CDK4. pRb is dephosphorylated in the presence of p21-CDK2/4 complexes, and the Rb-E2F1 complex increases after TPA treatment, whereas the Rb-HDAC1 complex decreases slightly. Our results suggest that increased p21(WAF1) is associated with CDK2/4, and that these complexes induce pRb dephosphorylation. In turn, hypophosphorylated pRb are mainly complexed with E2F1, but HDAC1 appears not to be a key component in this process.

COX-2 independent induction of cell cycle arrest and apoptosis in colon cancer cells by the selective COX-2 inhibitor celecoxib

The regular use of various nonsteroidal anti-inflammatory drugs (NSAIDs) was shown to decrease the incidence of colorectal cancer. This effect is thought to be caused predominantly by inhibition of cyclooxygenase-2 (COX-2) and, subsequently, prostaglandin synthesis. However, recent studies have suggested that COX-independent pathways may contribute considerably to these antiproliferative effects. To evaluate the involvement of COX-dependent and COX-independent mechanisms further, we assessed the effects of celecoxib (selective COX-2 inhibitor) and SC560 (selective COX-1 inhibitor) on cell survival, cell cycle distribution, and apoptosis in three colon cancer cell lines, which differ in their expression of COX-2. Both drugs induced a G0/G1 phase block and reduced cell survival independent of whether or not the cells expressed COX-2. Celecoxib was more potent than SC560. The G0/G1 block caused by celecoxib could be attributed to a decreased expression of cyclin A, cyclin B1, and cyclin-dependent kinase-1 and an increased expression of the cell cycle inhibitory proteins p21Waf1 and p27Kip1. In addition, celecoxib, but not SC560, induced apoptosis, which was also independent of the COX-2 expression of the cells. In vivo, celecoxib as well as SC560 reduced the proliferation of HCT-15 (COX-2 deficient) colon cancer xenografts in nude mice, but both substances had no significant effect on HT-29 tumors, which express COX-2 constitutively. Thus, our in vitro and in vivo data indicate that the antitumor effects of celecoxib probably are mediated through COX-2 independent mechanisms and are not restricted to COX-2 over-expressing tumors.

Role of p21(Cip1/Waf1) in cell-cycle exit of endomitotic megakaryocytes

The cyclin-dependent kinase inhibitor p21(Waf-1/Cip-1) is expressed at high level during megakaryocyte differentiation, but its precise function remains unknown. In this study, it is confirmed that p21 was expressed at a high level in hypoploid (2N and 4N) and polyploid (at least 8N) human megakaryocytes derived from CD34(+) cells. A high expression of p27(Kip1), p16, cyclin E, and cyclin D3 was also found in both populations associated with a hypophosphorylated form of retinoblastoma protein, suggesting that the majority of hypoploid and polyploid megakaryocytes are G(1)-arrested cells. As human megakaryocytes grown in vitro present a defect in their polyploidization, the study switched to the murine model. The modal ploidy of megakaryocytes derived from lineage-negative cells was 32N, and an elevated expression of p21 was found in high-ploidy megakaryocytes. In addition, p21 and p27 were coexpressed in the majority of mature polyploid megakaryocytes. The p21 was detected by immunofluorescence in megakaryocytes derived from p53(-/-) mice, demonstrating a p53-independent regulation during megakaryocyte differentiation. Megakaryocytopoiesis of p21(-/-) mice was subsequently studied. No marked abnormality in the ploidy of primary or cultured megakaryocytes was detected. Overexpression of p21 in p21(-/-) or normal murine megakaryocytes and in human megakaryocytes showed in all these cases a marked inhibition in megakaryocyte polyploidization. In conclusion, while a reciprocal relation is observed between p21 levels in megakaryocytes and the cycling state of the cells, p21 is not essential for the determination of the ploidy profile in normal megakaryocytes in vivo. However, high levels of its expression in cultured megakaryocytes arrest the endomitotic cell cycle.

Rapamycin increases the cellular concentration of the BCL-2 protein and exerts an anti-apoptotic effect

The immunosuppressant rapamycin, an immunophilin-binding antibiotic, has been studied in follicular B-cell lymphoma lines that express the highest level of the BCL-2 protein. The growth rate of human follicular B-cell lymphoma lines was slowed more efficiently than that of other human B-cell lines or non-B-cell lines. This effect was dependent on the arrest of cells in the G(1) phase; the number of apoptotic cells was not increased. Rapamycin inhibited apoptosis or caspase activation induced by cytotoxic drugs, whereas caspase activation by doxorubicin was not inhibited. The increase in the cellular concentration of BCL-2 protein was related to its concentration in the steady state and was unrelated to the amount of bcl-2 mRNA. The increase of BCL-2 level in the cells rather than its level in the steady state may be important for drug resistance. The biochemical target of rapamycin, the mTOR kinase, may be a candidate sensitising agent for chemotherapy. This effect of rapamycin shows that G(1) arrest and protection from apoptosis are combined events susceptible to regulation by pharmacological means.

p53 and p21 expression in precancerous lesions and carcinomas of the uterine cervix: overexpression of p53 predicts poor disease outcome

OBJECTIVES

Abnormal expression of the p53 and p21(waf1/cip1) tumor suppressor genes has been observed in a variety of human tumors, but little is known about its expression during cervical tumorigenesis. To identify the potential implications of both genes in the development of cervical carcinoma and explore the clinical importance of changes in gene expression, we assessed the levels of both proteins in precancerous lesions and carcinomas of the cervix.

METHODS

In our study, 10 low-grade squamous intraepithelial lesions (LSIL), 35 high-grade squamous intraepithelial lesions (HSIL), 12 microinvasive carcinomas, and 103 invasive carcinomas were evaluated. The expression of p53 and p21 was studied by immunohistochemistry using monoclonal antibodies specific for these proteins.

RESULTS

p21 was expressed in all samples of normal epithelium, LSIL, and HSIL, and the mean values of expression were 50.3, 42.5, and 44.5%, respectively. Conversely, the expression of p21 was significantly reduced in microinvasive (30.7%) and invasive carcinomas (9.9%). p53 nuclear staining was not detected in normal epithelium samples or LSILs, while 4 (11.4%) of 35 HSILs, 1 (8.3%) of 12 microinvasive carcinomas, and 38 (36.9%) of 103 invasive carcinomas were positive for p53. Compared with the results of the control group, precancerous lesions, and microinvasive carcinoma, the mean value of p53 expression (4.8%) in invasive carcinoma was significantly higher. Furthermore, p53 overexpression was significantly associated with advanced stage of the tumor (P < 0.001) [16/67 (23.9%) stage I, 15/28 (53.6%) stage II, and 7/8 (87.5%) stage III/IV]. In univariate analysis, p53 overexpression was a significant predictor of poor survival, whereas it had no independent influence on overall survival using the Cox regression method. Our data also revealed that no association between p53 immunostaining and p21 expression was found.

CONCLUSIONS

The trend of reduced p21 expression in microinvasive and invasive carcinomas suggests that p21 may play a tumor-suppressor function in neoplastic transformation in cervical epithelium and inactivation of p21 may be an early event in cervical carcinogenesis. Our results indicated that p53 overexpression was a significant predictor of poor disease outcome in univariate analysis. Moreover, significantly increased expression of p53 in advanced-stage cervical carcinoma implies that inactivation of p53 is associated with tumor progression. Finally, this study further supports the notion that induction of p21 expression can be regulated in a p53-independent manner.

NS1- and minute virus of mice-induced cell cycle arrest: involvement of p53 and p21(cip1)

The nonstructural protein NS1 of the autonomous parvovirus minute virus of mice (MVMp) is cytolytic when expressed in transformed cells. Before causing extensive cell lysis, NS1 induces a multistep cell cycle arrest in G(1), S, and G(2), well reproducing the arrest in S and G(2) observed upon MVMp infection. In this work we investigated the molecular mechanisms of growth inhibition mediated by NS1 and MVMp. We show that NS1-mediated cell cycle arrest correlates with the accumulation of the cyclin-dependent kinase (Cdk) inhibitor p21(cip1) associated with both the cyclin A/Cdk and cyclin E/Cdk2 complexes but in the absence of accumulation of p53, a potent transcriptional activator of p21(cip1). By comparison, MVMp infection induced the accumulation of both p53 and p21(cip1). We demonstrate that p53 plays an essential role in the MVMp-induced cell cycle arrest in both S and G(2) by using p53 wild-type (+/+) and null (-/-) cells. Furthermore, only the G(2) arrest was abrogated in p21(cip1) null (-/-) cells. Together these results show that the MVMp-induced cell cycle arrest in S is p53 dependent but p21(cip1) independent, whereas the arrest in G(2) depends on both p53 and its downstream effector p21(cip1). They also suggest that induction of p21(cip1) by the viral protein NS1 arrests cells in G(2) through inhibition of cyclin A-dependent kinase activity.

Differential roles of p21(Waf1) and p27(Kip1) in modulating chemosensitivity and their possible application in drug discovery studies

In this study, the differential role of the cyclin-dependent kinase (CDK) inhibitors p21(Waf1) and p27(Kip1) in cell cycle regulation was proposed for use in screening natural or synthetic compounds for cell cycle-dependent (particularly M phase-dependent) antineoplastic activity. p21(Waf1) or p27(Kip1) was ectopically expressed with an ecdysone-inducible mammalian expression system in a human colon adenocarcinoma cell line. Induction of p21(Waf1) or p27(Kip1) expression inhibited the activities of CDK2 and completely arrested cells at G(1) phase of the cell cycle by p27(Kip1) and at G(1) and G(2) phases by p21(Waf1). We examined the sensitivity of these cells to several antineoplastic agents known to be cell cycle-dependent or -independent. Substantially increased resistance to cell cycle-dependent antineoplastic agents was found in the cells when the expression of p21(Waf1) or p27(Kip1) was induced. In contrast, only a desensitization to cell cycle-independent antineoplastic agents was found in the cells arrested by p21(Waf1) or p27(Kip1). Because p21(Waf1) induces an additional block at G(2) phase that inhibits cell entry into M phase, we further examined the difference between p21(Waf1)- and p27(Kip1)-induced cells in their sensitivity to D-24851, a novel M phase-dependent compound. We found that induction of p21(Waf1) after exposure of the cells to D-24851 conferred stronger resistance than did induction of p27(Kip1). Taken together, our results suggest that the differential effect of p21(Waf1) and p27(Kip1) on cell cycle regulation may be advantageous for screening chemical libraries for novel antineoplastic candidates that are cell cycle-dependent, and M phase-dependent in particular.

Exogenous expression of p21(WAF1/CIP1) exerts cell growth inhibition and enhances sensitivity to cisplatin in hepatoma cells

The effects of exogenous expression of p21(WAF1/CIP1) in hepatoma cells were examined. Two stably p21(WAF1/CIP1)-transfected clones and one clone transfected with expression vector only were used for study. Introduction of p21(WAF1/CIP1) resulted in significant cell growth inhibition, and the magnitude of the cell growth inhibition in these transfected cells was proportional to the level of p21(WAF1/CIP1) protein expressed. Exogenous p21(WAF1/CIP1) expression also significantly enhanced chemosensitivity to cisplatin. In addition, apoptosis occurred earlier in cells transfected with p21(WAF1/CIP1) after cisplatin treatment. These findings raise the potential that forced upregulation of p21(WAF1/CIP1) in hepatocellular carcinoma (HCC) may reduce the doses of cisplatin to achieve similar responses and suggest the possible use of p21(WAF1/CIP1) in HCC treatment.

Sodium butyrate induces growth arrest and senescence-like phenotypes in gynecologic cancer cells

We demonstrated here the growth-suppressing effects of sodium butyrate (NaB) on human endometrial and ovarian cancer cells. The arrest of cells at the G1 checkpoint accounted for this effect. NaB-mediated p21 might arrest endometrial and ovarian cancer cells at the G0/G1 phase by eliciting pRb unphosphorylation. To demonstrate the role of pRb regulation by p21, we measured the sensitivity to NaB of cervical cancer cells in which pRb had been inactivated by HPV E7. The cervical cancer cells displayed a sensitivity in NaB-mediated G2/M arrest in addition to their sensitivity in G0/G1 arrest. Arrest at G0/G1 and G2/M accompanied induction of senescence-like phenotypes (SLPs). Most importantly, the effect of NaB on senescence induction was not coupled with the predominance of hypophosphorylated pRb forms in the cervical cancer cells. This suggested that NaB had the potential to elicit SLPs through p21-mediated withdrawal from cell cycle progression. The consequences of p21 induction were manifold. The effects of NaB on gynecologic cancer cell growth indicated its potential use in cancer treatment. NaB was effective even in the cancer cells with mutant p53 and/or Rb genes by eliciting cell senescence.

Inactivation of p21 by E1A leads to the induction of apoptosis in DNA-damaged cells

A major impediment to successful chemotherapy is the propensity for some tumor cells to undergo cell cycle arrest rather than apoptosis. It is well established, however, that the adenovirus E1A protein can sensitize these cells to the induction of apoptosis by anticancer agents. To further understand how E1A enhances chemosensitivity, we have made use of a human colon carcinoma cell line (HCT116) which typically undergoes cell cycle arrest in response to chemotherapeutic drugs. As seen by the analysis of E1A mutants, we show here that E1A can induce apoptosis in these cells by neutralizing the activities of the cyclin-dependent kinase inhibitor p21. E1A's ability to interact with p21 and thereby restore Cdk2 activity in DNA-damaged cells correlates with the reversal of G(1) arrest, which in turn leads to apoptosis. Analysis of E1A mutants failing to bind p300 (also called CBP) or Rb shows that they are almost identical to wild-type E1A in their ability to initially overcome a G(1) arrest in cells after DNA damage, while an E1A mutant failing to bind p21 is not. However, over time, this mutant, which can still target Rb, is far more efficient in accumulating cells with a DNA content greater than 4N but is similar to wild-type E1A and the other E1A mutants in releasing cells from a p53-mediated G(2) block following chemotherapeutic treatment. Thus, we suggest that although E1A requires the binding of p21 to create an optimum environment for apoptosis to occur in DNA-damaged cells, E1A's involvement in other pathways may be contributing to this process as well. A model is proposed to explain the implications of these findings.

Common and reversible regulation of wild-type p53 function and of ribosomal biogenesis by protein kinases in human cells

Two specific inhibitors of cyclin-dependent kinase 2 (Cdk2), roscovitine and olomoucine, have been shown recently to induce nuclear accumulation of wt p53 and nucleolar unravelling in interphase human untransformed IMR-90 and breast tumor-derived MCF-7 cells. Here, we show that the early response of MCF-7 cells to roscovitine is fully reversible since a rapid restoration of nucleolar organization followed by an induction of p21(WAF1/CIP1), a downregulation of nuclear wt p53 and normal cell cycle resumption occurs if the compound is removed after 4 h. Interestingly, similar reversible effects are also induced by the casein kinase II (CKII) inhibitor, 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole. Upon short-term treatment also, both compounds significantly, but reversibly, reduce the level of 45S precursor ribosomal RNA. Cells exposed to the two types of protein kinase inhibitors for longer times keep exhibiting altered nucleolar and wt p53 features, yet they strikingly differentiate in that most roscovitine-treated cells fail to ever accumulate high levels of p21(WAF1/CIP1) in contrast with DRB-treated ones. In both cases, however, the cells eventually fall into an irreversible state and die. Moreover, we found that constitutive overexpression of p21(WAF1/CIP1) alters the nucleolar unravelling process in the presence of DRB, but not of roscovitine, suggesting a role for this physiological Cdk inhibitor in the regulation of nucleolar function. Our data also support the notion that both roscovitine- and DRB-sensitive protein kinases, probably including Cdk2 and CKII, via their dual implication in the p53-Rb pathway and in ribosomal biogenesis, would participate in coupling cell growth with cell division.

Expression of P21(WAF1/CIP1/SID1) cyclin-dependent kinase inhibitor in hematopoietic progenitor cells

P21(Waf1/Cip1/Sid1) is a critical component of biomolecular pathways leading to the G(1) arrest evoked in response to DNA damage, growth arrest signals and differentiation commitment. It belongs to the Cip/Kip class of cyclin-dependent kinase inhibitors and is at least partly regulated by p53. P21(Waf1/Cip1/Sid1) functional inactivation possibly resulting from mutations of the gene itself or, more likely, from p53 mutations may be critical for either the cell fate following DNA-damaging insults or clonal evolution toward malignancy. In the study presented here we describe a competitive polymerase chain reaction (PCR) strategy whose sensitivity and reproducibility enable us to attain a precise quantitation of p21(Waf1/Cip1/Sid1) expression levels in hematopoietic progenitors, the cell compartment which mostly suffers from the side effects of genotoxic drugs in use for cancer cure. The strategy was set in the M07 factor-dependent hematopoietic progenitor cell line. We confirmed that its p21(waf1/cip1/sid1) constitutive expression level is very low and up-modulated by DNA-damaging agents: ionizing radiations and ultraviolet light. Gene up-modulation resulted in checkpoint activation and, in particular, in a significant G(1) arrest, required for either the repair of damaged DNA sequences or apoptotic cell death. Our competitive PCR strategy was further validated in CD34(+) purified hematopoietic progenitors from healthy donors mobilized into the peripheral blood by granulocyte colony-stimulating factor and intended for allogeneic bone marrow transplantation. The constitutive p21(WAF1/CIP1/SID1) expression levels, measured in three separate harvests, were very low and no significant differences were apparent. Our results support the use of a competitive PCR strategy as a useful tool for clinical purposes, to assess the individual biomolecular response of early hematopoietic progenitors to antiblastic drugs.

Replicative senescence of human endothelial cells in vitro involves G1 arrest, polyploidization and senescence-associated apoptosis

Human ageing is characterized by a progressive loss of physiological functions, increased tissue damage and defects in various tissue renewal systems. Age-related decreases of the cellular replicative capacity can be reproduced by in vitro assays of cellular ageing. When diploid human fibroblasts reach their finite lifespan, they enter an irreversible G1 growth arrest status referred to as replicative senescence. While deregulation of programmed cell death (apoptosis) is a key feature of age-related pathology in several tissues, this is not reflected in the standard in vitro senescence model of human fibroblasts, and the role of apoptosis during cellular ageing remains unclear. We have analyzed replicative senescence of human umbilical vein endothelial cells (HUVEC) in vitro and found that senescent HUVEC also arrest in the G1 phase of the cell cycle but, unlike fibroblasts, accumulate with a 4N DNA content, indicative of polyploidization. In contrast to human fibroblasts, senescent endothelial cells display a considerable increase in spontaneous apoptosis. The data imply that age-dependent apoptosis is a regular feature of human endothelial cells and suggest cell type specific differences in human ageing.

Change in gene expression subsequent to induction of Pnn/DRS/memA: increase in p21(cip1/waf1)

Pnn (PNN) is a nuclear and cell adhesion-related protein. Previous work has suggested that Pnn/DRS/memA is a potential tumor suppressor involved in the regulation of cell adhesion and cell migration. Using the ecdysone-inducible mammalian expression system, a stable inducible GFP-tagged human Pnn gene (PNNGFP) expressing 293 cell line was created (EcR293-PNNGFP). Cells induced to express PNNGFP not only exhibited increased cell-cell adhesion but also exhibited changes in cell growth and cell cycle progression. cDNA array analyses, together with real time PCR, revealed that the effects of exogenously expressed Pnn on cellular behavior may be linked to the regulation of the expression of specific subset genes. This subset includes cell cycle-related genes such as p21(cip1/waf1), CDK4, CPR2; cell migration and invasion regulatory genes such as RhoA, CDK5, TIMP-1, MMP-7, and EMMPRIN; and MIC-1. Concordant with previous observations of Pnn-induced phenotype changes, genes coding for epithelial associated processes and cell division controls were elevated, while those coding for increased cell motility and cellular reorganizations were downregulated. We utilized p21 promoter-luciferase reporter constructs and demonstrated that a marked stimulation of p21 promoter activity in 293 cells correlated with increased Pnn expression. Taken together, these data indicate that Pnn may participate in the regulation of gene expression, thereby, positively promoting cell-cell adhesion, and negatively affecting cell migration and cell proliferation.

Cell-autonomous and non-cell-autonomous functions of the Rb tumor suppressor in developing central nervous system

The retinoblastoma tumor suppressor (RB) plays an important role in the regulation of cell cycle progression and terminal differentiation of many cell types. Rb(-/-) mouse embryos die at midgestation with defects in cell cycle regulation, control of apoptosis and terminal differentiation. However, chimeric mice composed of wild-type and Rb-deficient cells are viable and show minor abnormalities. To determine the role of Rb in development more precisely, we analyzed chimeric embryos and adults made with marked Rb(-/-) cells. Like their germline Rb(-/-) counterparts, brains of midgestation chimeric embryos exhibited extensive ectopic S-phase entry. In Rb-mutants, this is accompanied by widespread apoptosis. However, in chimeras, the majority of Rb-deficient cells survived and differentiated into neuronal fates. Rescue of Rb(-/-) neurons in the presence of wild-type cells occurred after induction of the p53 pathway and led to accumulation of cells with 4n DNA content. Therefore, the role of Rb during development can be divided into a cell-autonomous function in exit from the cell cycle and a non-cell-autonomous role in the suppression of apoptosis and induction of differentiation.

p21cip1 Degradation in differentiated keratinocytes is abrogated by costabilization with cyclin E induced by human papillomavirus E7

The human papillomavirus (HPV) E7 protein promotes S-phase reentry in a fraction of postmitotic, differentiated keratinocytes. Here we report that these cells contain an inherent mechanism that opposes E7-induced DNA replication. In organotypic raft cultures of primary human keratinocytes, neither cyclin E nor p21cip1 is detectable in situ. However, E7-transduced differentiated cells not in S phase accumulate abundant cyclin E and p21cip1. We show that normally p21cip1 protein is rapidly degraded by proteasomes. In the presence of E7 or E6/E7, p21cip1, cyclin E, and cyclin E2 proteins were all up-regulated. The accumulation of p21cip1 protein is a posttranscriptional event, and ectopic cyclin E expression was sufficient to trigger it. In constract, cdk2 and p27kip1 were abundant in normal differentiated cells and were not significantly affected by E7. Cyclin E, cdk2, and p21cip1 or p27kip1 formed complexes, and relatively little kinase activity was found associated with cyclin E or cdk2. In patient papillomas and E7 raft cultures, all p27kip1-positive cells were negative for bromodeoxyuridine (BrdU) incorporation, but only some also contained cyclin E and p21cip1. In contrast, all cyclin E-positive cells also contained p27kip1. When the expression of p21cip1 was reduced by rottlerin, a PKC delta inhibitor, p27kip1- and BrdU-positive cells remained unchanged. These observations show that high levels of endogenous p27kip1 can prevent E7-induced S-phase reentry. This inhibition then leads to the stabilization of cyclin E and p21cip1. Since efficient initiation of viral DNA replication requires cyclin E and cdk2, its inhibition accounts for heterogeneous viral activities in productively infected lesions.

Anti-mitotic properties of indirubin-3'-monoxime, a CDK/GSK-3 inhibitor: induction of endoreplication following prophase arrest

The bis-indole indirubin is the active ingredient of the Traditional Chinese Medicine recipe Danggui Longhui Wan used against chronic myelocytic leukemia. We have previously shown that indirubins are potent inhibitors of cyclin-dependent kinases and glycogen synthase kinase-3. We here investigated the anti-mitotic properties of this class of compounds using the cell permeable indirubin-3'-monoxime and the HBL-100 cell line. Indirubin-3'-monoxime reversibly arrests asynchronous HBL-100 cells in G2. This arrest is not accompanied by any significant change in expression of the major cell cycle regulators. However indirubin-3'-monoxime inhibits the phosphorylation of consensus CDK phosphorylation sites as well as of nucleolin at a specific CDK1/cyclin B phosphorylation site, suggesting a direct action on the mitotic CDK1/cyclin B. When indirubin-3'-monoxime is added to HBL-100 cells synchronized in M phase by nocodazole, cells undergo an endoreplication leading to an 8n DNA content. As soon as indirubin-3'-monoxime is washed away, these polyploid cells become aneuploid and later die from necrosis. This mechanism of endoreplication followed by cell death may contribute to the anti-tumour properties of indirubins.

p53 Displacement from Centrosomes and p53-mediated G1 Arrest following Transient Inhibition of the Mitotic Spindle

Growing evidence indicates a central role for p53 in mediating cell cycle arrest in response to mitotic spindle defects so as to prevent rereplication in cells in which the mitotic division has failed. Here we report that a transient inhibition of spindle assembly induced by nocodazole, a tubulin-depolymerizing drug, triggers a stable activation of p53, which can transduce a cell cycle inhibitory signal even when the spindle-damaging agent is removed and the spindle is allowed to reassemble. Cells transiently exposed to nocodazole continue to express high levels of p53 and p21 in the cell cycle that follows the transient exposure to nocodazole and become arrested in G(1), regardless of whether they carry a diploid or polyploid genome after mitotic exit. We also show that p53 normally associates with centrosomes in mitotic cells, whereas nocodazole disrupts this association. Together these results suggest that the induction of spindle damage, albeit transient, interferes with the subcellular localization of p53 at specific mitotic locations, which in turn dictates cell cycle arrest in the offspring of such defective mitoses.

Checking out the G(2)/M transition

Tight regulation of cell cycle progression is essential for the maintenance of genomic integrity. The orderly progression from one cell cycle phase to the other is mediated by timed activation of distinct cyclin/cdk complexes. For example, onset of mitosis is regulated by the activation of cyclin B/cdc2 and this event is controlled by several cell cycle checkpoints. Such checkpoints ensure that chromosome segregation does not occur in the case of unreplicated or damaged DNA, or misaligned chromosomes. Recently, new insights into the targets of the DNA damage checkpoint help to unravel more of the complex mechanisms of cell cycle checkpoints. This review focuses on the factors controlling the transition from G(2) phase to mitosis. Also, the pathways contributing to the DNA damage checkpoints in these phases of the cell cycle will be discussed.

Contrasting roles of p57(KIP2) and p21(WAF1/CIP1/SDI1) in transplanted human and bovine adrenocortical cells

Cell transplantation provides a way to compare the regulation of cell proliferation in the same cell type in cell culture and in a vascularized tissue structure in a host animal. The cyclin-dependent kinase inhibitors p57(KIP2), p21(WAF1/CIP1/SDI1) and p27(KIP1) have been extensively studied in cell culture but their role in growth control in tissues is less well understood. In the present experiments we compared the behavior of cell cycle inhibitors in human and bovine adrenocortical cells in culture and following cell transplantation in scid mice. p57 was expressed in the majority of cells in the intact human adrenal cortex. However, double immunofluorescence showed that cells that are in the cell cycle are p57(-) adrenocortical cells, p57 and p27 levels were not affected by inhibition of growth at high cell density, whereas p21 was higher in dividing than growth-inhibited cells. However, p21 was also high in senescent adrenocortical cells. After transplantation of human adrenocortical cells in scid mice, p57 and p27 were observed in most cells in the transplant tissue. Over time the number of p21(+) cells decreased greatly in human adrenocortical cells, but not in bovine adrenocortical cells. This difference correlated with lower levels of cell division (assessed by Ki-67 or incorporation of bromodeoxyuridine) in the human cells in transplant tissues in comparison to bovine cells. The differences between human and bovine cells were observed both when cells were transplanted beneath the kidney capsule and when cells were injected subcutaneously in collagen gel. We conclude that the behavior of p57, but not p21, is consistent with a role as a physiological mediator of proliferative quiescence in the adrenal cortex. The high level of p21 in dividing adrenocortical cells in culture, and in bovine adrenocortical cells in transplant tissues, may be a response to conflicting positive and negative growth influences. Cells may enter the cell cycle under the influence of a strong positive mitogenic signal, but coexisting negative growth stimuli trigger a p21-dependent block to further progression through the cell cycle. This model suggests that bovine adrenocortical cells respond to positive growth stimuli in transplant tissues but human cells lack this response.

The cytotoxic effect of E1B 55-kDa mutant adenovirus on human hepatocellular carcinoma cell lines

It has been suggested the E1B 55 kDa mutant adenovirus dl1520 can selectively kill p53-deficient human tumor cells. In this study, we examined the cytotoxic effect of dl1520 on nine human hepatocellular carcinoma (HCC) cell lines with different p53 genetic and functional status. The results showed that HCC cell lines with deleted or mutant p53 gene and reduced p53 transcriptional activities were more susceptible to dl1520-induced cytolysis. Hep3B (p53-null) and HepG2 (p53-wt) cells were arrested at G2/M phase when cytolysis occurred. Cyclin-dependent kinase inhibitor (CDKI) p21(Waf-1/Cip-1) was downregulated 24 hours after dl1520 infection in HepG2 cells and increased when cytolysis occurred. No p21 expression was detected in Hep3B cells. DNA fragmentation was found in both Hep3B and HepG2 cells after dl1520 infection. Bax expression increased in dl1520-infected HepG2 cells but not in Hep3B cells. Notably, three Bax-like proteins, molecular mass around 40 to 80 kDa, accumulated 48 hours after adenovirus infection in Hep3B cells but not in HepG2 cells. These results suggest that the susceptibility of HCC cells to dl1520-induced cytolysis is related to both p53 genotype and functional status, and is mediated by both cell cycle disturbance and apoptosis.

Bcl-2 exerts a pRb-mediated cell cycle inhibitory function in HEC1B endometrial carcinoma cells

OBJECTIVE

In various human tumors the expression of Bcl-2 appears to vary significantly during the transformation process. Indeed, in several glandular systems, Bcl-2 levels appear to be sustained in premalignant lesions and rather low after the malignant change. Since we recently reported that transformed human endometrial cells display constitutively low levels of Bcl-2, we set out to investigate the biological meaning of this down-regulation. To this end we analyzed the effects of Bcl-2 forced overexpression in a moderately differentiated endometrial cell line of human origin.

METHODS

Bcl-2 overexpression was obtained by transfecting HEC1B human endometrial adenocarcinoma cells with a suitable bcl-2 vector. The effects of Bcl-2 overepression were evaluated in several transfectants (cell clones and mixed populations) by FACS, growth rates, cloning efficiencies, and modification of the phosphorylation status of the pRb protein. Accompanying changes in the expression of the CDK inhibitor p21(WAF1/CIP1) were evaluated as well.

RESULTS

Bcl-2 overexpression resulted in a reduced cell proliferation rate, decreased cloning efficiency, appreciable cell morphology changes, G2/M cell cycle arrest, remarkable accumulation of the dephosphorylated form of retinoblastoma protein, and a significant rise in p21(WAF1/CIP1).

CONCLUSIONS

From these observation it may be deduced that the observed loss and down-regulation of the antiapoptotic protein in endometrial glandular human tumors is not random but possibly related to the cellular transformation process. It may be also inferred that the coincidence of a progressive fading of both Bcl-2 and cyclin inhibitor p21(WAF1/CIP1) expressions, together with accumulation of the hyperphosphorylated form of the retinoblastoma protein, may be seen as a potential indicator of ongoing malignant changes.

Inhibition of cell proliferation by the PCNA-binding region of p21 expressed as a GFP miniprotein

p21 (WAF1/Cip1) is the only member of the CIP/KIP family which has a well-characterized PCNA-binding domain. p21 is known to have an important function in the coordination of the cellular pathways which are activated in response to DNA damage, though the significance of the p21-PCNA interaction is not completely clear. We have analyzed the effects of expressing a miniprotein containing the PCNA-binding domain of p21 upon the cell cycle and upon the proliferation of various cell types. We have compared this with the effect of expressing a mutant form which is defective in PCNA-binding, but which retains the secondary cyclin-CDK-inhibitory site. No PCNA-dependent effects were seen in the short term upon cell cycle distribution. However, clonogenic assays show that the GFP-peptide miniprotein can significantly suppress proliferation in a PCNA-dependent manner. In some cell types, however, the suppression of proliferation was not PCNA-dependent, suggesting that cellular environment is a contributory factor to the effect of this miniprotein. The capacity of this peptide sequence to suppress cell proliferation in vivo is of interest as the basis for the design of potential antiproliferative therapeutic agents.

Regulation of the G2/M transition by p53

p53 protects mammals from neoplasia by inducing apoptosis, DNA repair and cell cycle arrest in response to a variety of stresses. p53-dependent arrest of cells in the G1 phase of the cell cycle is an important component of the cellular response to stress. Here we review recent evidence that implicates p53 in controlling entry into mitosis when cells enter G2 with damaged DNA or when they are arrested in S phase due to depletion of the substrates required for DNA synthesis. Part of the mechanism by which p53 blocks cells at the G2 checkpoint involves inhibition of Cdc2, the cyclin-dependent kinase required to enter mitosis. Cdc2 is inhibited simultaneously by three transcriptional targets of p53, Gadd45, p21, and 14-3-3 sigma. Binding of Cdc2 to Cyclin B1 is required for its activity, and repression of the cyclin B1 gene by p53 also contributes to blocking entry into mitosis. p53 also represses the cdc2 gene, to help ensure that cells do not escape the initial block. Genotoxic stress also activates p53-independent pathways that inhibit Cdc2 activity, activation of the protein kinases Chk1 and Chk2 by the protein kinases Atm and Atr. Chk1 and Chk2 inhibit Cdc2 by inactivating Cdc25, the phosphatase that normally activates Cdc2. Chk1, Chk2, Atm and Atr also contribute to the activation of p53 in response to genotoxic stress and therefore play multiple roles. p53 induces transcription of the reprimo, B99, and mcg10 genes, all of which contribute to the arrest of cells in G2, but the mechanisms of cell cycle arrest by these genes is not known. Repression of the topoisomerase II gene by p53 helps to block entry into mitosis and strengthens the G2 arrest. In summary, multiple overlapping p53-dependent and p53-independent pathways regulate the G2/M transition in response to genotoxic stress.

The large subunit of replication factor C promotes cell survival after DNA damage in an LxCxE motif- and Rb-dependent manner

Retinoblastoma (Rb) protein promotes cell survival after DNA damage. We show here that the LxCxE binding site in Rb mediates both cell survival and cell-cycle arrest after DNA damage. Replication factor C (RF-C) complex plays an important role in DNA replication. We describe a novel function of the large subunit of RF-C in promoting cell survival after DNA damage. RF-Cp145 contains an LxCxE motif, and mutation of this motif abolishes the protective effect of RF-Cp145. The inability of wild-type RF-Cp145 to promote cell survival in Rb-null cells is rescued by Rb but not by Rb mutants defective in binding LxCxE proteins. RF-C thus enhances cell survival after DNA damage in an Rb-dependent manner.

Histone deacetylase inhibitors promote apoptosis and differential cell cycle arrest in anaplastic thyroid cancer cells

Little information exists concerning the response of anaplastic thyroid carcinoma (ATC) cells to histone deacetylase inhibitors (HDAIs). In this study, the cellular response to the histone deacetylase inhibitors, sodium butyrate and trichostatin A, was analyzed in cell lines derived from primary anaplastic thyroid carcinomas. HDAIs repress the growth (proliferation) of ATC cell lines, independent of p53 status, through the induction of apoptosis and differential cell cycle arrest (arrested in G1 and G2/M). Apoptosis increases in response to drug treatment and is associated with the appearance of the cleaved form of the caspase substrate, poly-(ADP-ribose) polymerase (PARP). Cell cycle arrest is associated with the reduced expression of cyclins A and B, the increased expression of the cyclin-dependent kinase inhibitors, p21(Cip1/WAF1) and p27Kip1, the reduced phosphorylation of the retinoblastoma protein (pRb), and a reduction in cdk2 and cdk1-associated kinase activities. In ATC cells overexpressing cyclin E, drug treatment failed to replicate these events. These results suggest that growth inhibition of ATC cells by HDAIs is due to the promotion of apoptosis through the activation of the caspase cascade and the induction of cell cycle arrest via a reduction in cdk2- and cdk1-associated kinase activities.

Cytokine response gene 8 (CR8) regulates the cell cycle G1-S phase transition and promotes cellular survival

Cellular proliferation and survival are modulated by the expression of specific genes. Cytokine response gene 8 (CR8), which was originally cloned as an IL-2-induced gene in human T lymphocytes, encodes a basic helix--loop--helix (bHLH) transcription factor. The CR8 gene product is highly conserved among human, mouse and rat, and contains sequence motifs that distinguish it from other bHLH families. The CR8 gene is ubiquitously expressed, and CR8 gene expression is induced by both growth-promoting as well as growth-inhibitory stimuli. As bHLH proteins have been found to regulate both the G1-S phase cell cycle transition, as well as cellular survival, the effects of CR8 on these processes were investigated. Ectopic CR8 expression in asynchronous U2OS cell cultures reduces the percentage of cells in the cell cycle S phase, and also slows the entry of G1-synchronized cells into S phase. The prolonged G1 interval correlates with impaired elevation of cyclin E protein and prolonged p21 protein expression in G1. CR8 expression also protects U2OS cells from serum-withdrawal induced apoptosis. These results indicate that CR8 is an important modulator of both the G1-S phase cell cycle transition, and cellular survival.

The retinoblastoma gene: a prototypic and multifunctional tumor suppressor

Genome instability has been implicated in the generation of multiple somatic mutations that underlie cancer. Germline mutation in the retinoblastoma (RB) gene leads to tumor formation in both human and experimental animal models, and reintroduction of wild-type RB is able to suppress neoplastic phenotypes. Rb governs the passage of cells through the G1 phase-restriction point and this control is lost in most cancer cells. Rb has also been shown to promote terminal differentiation and prevent cell cycle reentry. Recent studies implicate Rb in mitotic progression, faithful chromosome segregation, checkpoint control, and chromatin remodeling, suggesting that Rb may function in the maintenance of genome integrity. It is likely that Rb suppresses tumor formation by virtue of its multiple biological activities. A single protein capable of performing multiple antioncogenic functions may be a common characteristic of other tumor suppressors including p53 and BRCA1/2.

Cdc25-dependent activation of cyclin A/cdk2 is blocked in G2 phase arrested cells independently of ATM/ATR

Cyclin A/cdk2 is active during S and G2 phases of the cell cycle, but its regulation and function during G2 phase is poorly understood. In this study we have examined the regulation of cyclin A/cdk2 activity during normal G2 phase progression and in genotoxin-induced G2 arrest. We show that cyclin A/cdk2 is activated in early G2 phase by a cdc25 activity. In the G2 phase checkpoint arrest initiated in response to various forms of DNA damage, the cdc25-dependent activation of both cyclin A/cdk2 and cyclin B1/cdc2 is blocked. Ectopic expression of cdc25B, but not cdc25C, in G2 phase arrested cells efficiently activated both cyclin A/cdk2 and cyclin B1/cdc2. Finally, we demonstrate that the block in cyclin A/cdk2 activation in the G2 checkpoint arrest is independent of ATM/ATR. We speculate that the ATM/ATR-independent block in G2 phase cyclin A/cdk2 activation may act as a further layer of checkpoint control, and that blocking G2 phase cyclin A/cdk2 activation contributes to the G2 phase checkpoint arrest.

p53 down-regulates CHK1 through p21 and the retinoblastoma protein

Both fission yeast and mammalian cells require the function of the checkpoint kinase CHK1 for G2 arrest after DNA damage. The tumor suppressor p53, a well-studied stress response factor, has also been shown to play a role in DNA damage G2 arrest, although in a manner that is probably independent of CHK1. p53, however, can be phosphorylated and regulated by both CHK1 as well as another checkpoint kinase, hCds1 (also called CHK2). It was therefore of interest to determine whether reciprocally, p53 affects either CHK1 or CHK2. We found that induction of p53 either by diverse stress signals or ectopically using a tetracycline-regulated promoter causes a marked reduction in CHK1 protein levels. CHK1 downregulation by p53 occurs as a result of reduced CHK1 RNA accumulation, indicating that repression occurs at the level of transcription. Repression of CHK1 by p53 requires p21, since p21 alone is sufficient for this to occur and cells lacking p21 cannot downregulate CHK1. Interestingly, pRB is also required for CHK1 downregulation, suggesting the possible involvement of E2F-dependent transcription in the regulation of CHK1. Our results identify a new repression target of p53 and suggest that p53 and CHK1 play interdependent and complementary roles in regulating both the arrest and resumption of G2 after DNA damage.

Cellular transformation by SV40 large T antigen: interaction with host proteins

SV40 large T antigen (TAg) is a powerful oncoprotein capable of transforming a variety of cell types. The transforming activity of TAg is due in large part to its perturbation of the retinoblastoma (pRB) and p53 tumor suppressor proteins. In addition, TAg binds to several other cellular factors, including the transcriptional co-activators p300 and CBP, which may contribute to its transformation function. Several other features of TAg that appear to contribute to its full transformation potential are yet to be completely understood. Study of TAg therefore continues to provide new insights into the mechanism of cellular transformation.

Investigating the hows and whys of DNA endoreduplication

Endoreduplication is a form of nuclear polyploidization that results in multiple, uniform copies of chromosomes. This process is common in plants and animals, especially in tissues with high metabolic activity, and it generally occurs in cells that are terminally differentiated. In plants, endoreduplication is well documented in the endosperm and cotyledons of developing seeds, but it also occurs in many tissues throughout the plant. It is thought that endoreduplication provides a mechanism to increase the level of gene expression, but the function of this process has not been thoroughly investigated. Numerous observations have been made of endoreduplication, or at least extra cycles of S-phase, as a consequence of mutations in genes controlling several aspects of cell cycle regulation. However, until recently there were few studies directed at the molecular mechanisms responsible for this specialized cell cycle. It is suggested that endoreduplication requires nothing more elaborate than a loss of M-phase cyclin-dependent kinase activity and oscillations in the activity of S-phase cyclin-dependent kinase.

Dynamic forces in the cell cycle affecting fibroblasts in pressure ulcers

Utilizing specific cell cycle markers of gene activity, temporal changes in the equilibrium of proliferating and non proliferating fibroblasts were shown in pressure ulcers after 36 days of quality care. Average cell counts from multiple tissue sections showed that fibroblast nuclei were stained in decreasing order by antibodies to p21, p21/proliferating cell nuclear antigen (PCNA) and PCNA. P21 labeling suggested that the majority of ulcer fibroblasts were senescent. Fibroblast nuclei showing PCNA staining identified those fibroblasts that were capable of synthesizing DNA and contributing to pressure ulcer repair. Increased rates of wound closure were correlated with a decreasing number of p21 positive cells and an increasing portion of PCNA labeled cells. While the proportion of antigens appeared to correlate with the status of wound closure after 36 days of quality care, they did not always appear to reflect the final outcome of the pressure ulcer. No significant differences were observed in ulcer fibroblasts labeled with p21 at 0 and 10 days, however, the differences were significant after 36 days of quality care (p = 0.05, analysis of variance, post hoc Tukey test). The cellular contribution to pressure ulcer repair appeared to occur from ulcer fibroblasts that were capable of division, of emerging from quiescence, and that were successful in repairing their DNA.

Myostatin, a negative regulator of muscle growth, functions by inhibiting myoblast proliferation

Myostatin, a member of the transforming growth factor-beta (TGF-beta) superfamily, has been shown to be a negative regulator of myogenesis. Here we show that myostatin functions by controlling the proliferation of muscle precursor cells. When C(2)C(12) myoblasts were incubated with myostatin, proliferation of myoblasts decreased with increasing levels of myostatin. Fluorescence-activated cell sorting analysis revealed that myostatin prevented the progression of myoblasts from the G(1)- to S-phase of the cell cycle. Western analysis indicated that myostatin specifically up-regulated p21(Waf1, Cip1), a cyclin-dependent kinase inhibitor, and decreased the levels and activity of Cdk2 protein in myoblasts. Furthermore, we also observed that in myoblasts treated with myostatin protein, Rb was predominately present in the hypophosphorylated form. These results suggests that, in response to myostatin signaling, there is an increase in p21 expression and a decrease in Cdk2 protein and activity thus resulting in an accumulation of hypophosphorylated Rb protein. This, in turn, leads to the arrest of myoblasts in G(1)-phase of cell cycle. Thus, we propose that the generalized muscular hyperplasia phenotype observed in animals that lack functional myostatin could be as a result of deregulated myoblast proliferation.

The mammalian UV response: c-Jun induction is required for exit from p53-imposed growth arrest

The mammalian UV response results in rapid and dramatic induction of c-jun. Induction of a protooncogene, normally involved in mitogenic responses, by a genotoxic agent that causes growth arrest seems paradoxical. We now provide an explanation for the role of c-Jun in the UV response of mouse fibroblasts. c-Jun is necessary for cell-cycle reentry of UV-irradiated cells, but does not participate in the response to ionizing radiation. Cells lacking c-Jun undergo prolonged cell-cycle arrest, but resist apoptosis, whereas cells that express c-Jun constitutively do not arrest and undergo apoptosis. This function of c-Jun is exerted through negative regulation of p53 association with the p21 promoter. Cells lacking c-Jun exhibit prolonged p21 induction, whereas constitutive c-Jun inhibits UV-mediated p21 induction.

Cyclin A down-regulation in TGFbeta1-arrested follicular lymphoma cells

Transforming growth factor beta1 (TGFbeta1) induces growth arrest in many cell types, including B lymphocytes. We examined the effect of TGF on cell cycle progression of a non-Hodgkin lymphoma cell line of follicular lymphoma subtype (FL). After 48 h of TGFbeta1 (10 ng/ml) treatment, a significantly increased number of DoHH2 cells was retained in G(0)/G(1) phase. We examined the level of cell cycle components, cyclins, cyclin-dependent kinases (cdk), and their inhibitors. We found that the expression of cyclin A and p21(WAF1) molecules was primarily modulated by TGFbeta1 treatment while the expression of other regulatory components, like cyclins D, cyclin E, cdk2, cdk4, and cdk6 or p15(INK4B), p16(INK4A), and p27(KIP1) was not significantly affected. We further examined expression and activity of CREB/ATF family members to examine their roles in cyclin A inhibition. The binding activity of CREB-1 and ATF-2 to the CRE region of the cyclin A promoter was almost completely abolished due to the treatment. The total level of CREB-1, ATF-2, and ATF-3 was notably reduced. Moreover, CREB-1 was dephosphorylated due to the treatment as revealed by immunoblotting. We assume that down-regulation of cyclin A was mediated by the absence of CREB/ATF activation dimers. The profound effect on the ATF family of transcription factors indicates the complexity of TGFbeta1 action on FL B malignant cells.

E2F1 and p53 are dispensable, whereas p21(Waf1/Cip1) cooperates with Rb to restrict endoreduplication and apoptosis during skeletal myogenesis

We describe temporal and genetic analyses of partially rescued Rb mutant fetuses, mgRb:Rb-/-, that survive to birth and reveal specific defects in skeletal muscle differentiation. We show that in the absence of Rb, these fetuses exhibit increased apoptosis, bona fide endoreduplication, and incomplete differentiation throughout terminal myogenesis. These defects were further augmented in composite mutant fetuses, mgRb:Rb-/-:p21-/-, lacking both Rb and the cyclin-dependent kinase inhibitor p21(Waf1/Cip1). Although E2F1 and p53 mediate ectopic DNA synthesis and cell death in several tissues in Rb mutant embryos, both endoreduplication and apoptosis persisted in mgRb:Rb-/-:E2F1-/- and mgRb:Rb-/-:p53-/- compound mutant muscles. Thus, combined inactivation of Rb and p21(Waf1/Cip1) augments endoreduplication and apoptosis, whereas E2F1 and p53 are dispensable during aberrant myogenesis in Rb-deficient fetuses.

A ribonucleotide reductase gene is a transcriptional target of p53 and p73

Many p53-inducible genes have been identified that might play a role in mediating the various downstream activities of p53. We have identified a close relative of ribonucleotide reductase, recently named p53R2, as a p53-inducible gene, and show that this gene is activated by several stress signals that activate a p53 response, including DNA damaging agents and p14(ARF). p53R2 expression was induced by p53 mutants that are defective for the activation of apoptosis, but retain cell cycle arrest function, although no induction of p53R2 was seen in response to p21(WAF1/CIP1)-mediated cell cycle arrest. Several isoforms of the p53 family member p73 were also shown to induce p53R2 expression. Transient ectopic expression of either wild type p53R2 or p53R2 targeted to the nucleus, did not significantly alter cell cycle progression in unstressed cells. The identification of this gene as a p53 target supports a direct role for p53 in DNA repair, in addition to inhibition of growth of damaged cells. Oncogene (2000) 19, 4283 - 4289

Inhibition of S-phase progression by adeno-associated virus Rep78 protein is mediated by hypophosphorylated pRb

Adeno-associated virus (AAV) has an antiproliferative action on cells. We investigated the effect of the AAV replication proteins (Rep) on the cell division cycle using retroviral vectors. Rep78 and Rep68 inhibited the growth of primary, immortalized and transformed cells, while Rep52 and Rep40 did not. Rep68 induced cell cycle arrest in phases G(1) and G(2), with elevated CDK inhibitor p21 and reduced cyclin E-, A- and B1-associated kinase activity. Rep78-expressing cells were also impaired in S-phase progression and accumu lated almost exclusively with hypophosphorylated retinoblastoma protein (pRb). The differences between Rep78 and Rep68 were mapped to the C-terminal zinc finger domain of Rep78. Rep78-induced S-phase arrest could be bypassed by adenoviral E1A or papillomaviral E7 proteins but not by E1A or E7 mutants unable to bind pRb. Rb(-/-) primary mouse embryonic fibroblasts displayed a strongly reduced S-phase arrest when challenged with Rep78, compared with matched Rb(+/+) controls. These results suggest that physiological levels of active pRb can interfere with S-phase progression. We propose that the AAV Rep78 protein arrests cells within S-phase by a novel mechanism involving the ectopic accumulation of active pRb.

Switch from p53 to MDM2 as differentiating human keratinocytes lose their proliferative potential and increase in cellular size

p53 transcription factor is mutated in most skin cell carcinomas and in more than 50% of all human malignancies. One of its transcriptional targets is MDM2, which in turn down-regulates p53. The role of the p53/MDM2 regulatory loop upon genotoxic stress is well documented, but less is known about its role in normal tissue homeostasis. We have explored this pathway during the different transitions of the human epidermal differentiation programme and after isolating stem cells, transit amplifying cells or differentiating cells from epidermis. Maximum expression of p53 was found in proliferating keratinocytes. A striking and transient induction of MDM2 and a down-modulation of p53 characterized the transition from proliferation to differentiation in primary human keratinocytes. These changes were delayed in late differentiating carcinoma cells, and were clearly different in suspended primary fibroblasts. Interestingly, these changes correlated with an increase in cell size, at the time of irreversible commitment to differentiation. Induction of MDM2 was also associated with suppression of proliferation in normal, or hyperproliferative, psoriatic epidermis. Moreover, both proteins were induced as keratinocytes were driven to leave the stem cell compartment by c-Myc activation. Overall, our results show a critical regulation of the p53/MDM2 pathway at the epidermal transition from proliferation to differentiation.

Different impact of p53 and p21 on the radiation response of mouse tissues

Mammalian tissues differ dramatically in their sensitivity to genotoxic stress, although the mechanisms determining these differences remain largely unknown. To analyse the role of p53 and p21 in determination of tissue specificity to DNA damage in vivo, we compared the effects of gamma radiation on DNA synthesis on whole-body sections of wild type, p53-deficient and p21-deficient mice. A dramatic reduction in 14C-thymidine incorporation after gamma irradiation was observed in the majority of rapidly proliferating tissues of wild type and p21-/- but not in p53-/- mice, confirming the key role of p53 in determination of tissue response to genotoxic stress in vivo and suggesting that p53-mediated inhibition of DNA synthesis does not depend on p21. Rapid radiation induced p53-dependent apoptosis was mapped to the areas of high levels of p53 mRNA in radiation sensitive tissues analysed (white pulp in the spleen and bases of crypts in small intestine), indicating that p53 regulation at the mRNA level is a determinant of cellular sensitivity to genotoxic stress. High p53 mRNA expression is inherited as a recessive trait in cell-cell hybrids suggesting the involvement of a negative control mechanism in the regulation of p53 gene expression.

MCG10, a novel p53 target gene that encodes a KH domain RNA-binding protein, is capable of inducing apoptosis and cell cycle arrest in G(2)-M

p53, a tumor suppressor, inhibits cell proliferation by inducing cellular genes involved in the regulation of the cell cycle. MCG10, a novel cellular p53 target gene, was identified in a cDNA subtraction assay with mRNA isolated from a p53-producing cell line. MCG10 can be induced by wild-type but not mutant p53 and by DNA damage via two potential p53-responsive elements in the promoter of the MCG10 gene. The MCG10 gene contains 10 exons and is located at chromosome 3p21, a region highly susceptible to aberrant chromosomal rearrangements and deletions in human neoplasia. The MCG10 gene locus encodes at least two alternatively spliced transcripts, MCG10 and MCG10as. The MCG10 and MCG10as proteins contain two domains homologous to the heterogeneous nuclear ribonucleoprotein K homology (KH) domain. By generating cell lines that inducibly express either wild-type or mutated forms of MCG10 and MCG10as, we found that MCG10 and MCG10as can suppress cell proliferation by inducing apoptosis and cell cycle arrest in G(2)-M. In addition, we found that MCG10 and MCG10as, through their KH domains, can bind poly(C) and that their RNA-binding activity is necessary for inducing apoptosis and cell cycle arrest. Furthermore, we found that the level of the poly(C) binding MCG10 protein is increased in cells treated with the DNA-damaging agent camptothecin in a p53-dependent manner. These results suggest that the MCG10 RNA-binding protein is a potential mediator of p53 tumor suppression.