Regulation of p53-mediated apoptosis and cell cycle arrest by Steel factor.

Activation of the p53 protein can lead to apoptosis and cell cycle arrest. In contrast, activation of the signalling pathway controlled by the Kit receptor tyrosine kinase prevents apoptosis and promotes cell division of a number of different cell types in vivo. We have investigated the consequences of activating the Kit signalling pathway by its ligand Steel factor on these opposing functions of the p53 protein in Friend erythroleukemia cells. A temperature-sensitive p53 allele (Val-135) was introduced into the Friend erythroleukemia cell line (DP-16) which lacks endogenous p53 expression. At 38.5 degrees C, the Val-135 protein maintains a mutant conformation and has no effect on cell growth. At 32 degrees C, the mutant protein assumes wild-type properties and induces these cells to arrest in G1, terminally differentiate, and die by apoptosis. We demonstrate that Steel factor inhibits p53-mediated apoptosis and differentiation but has no effect on p53-mediated G1/S cell cycle arrest. These results demonstrate that Steel factor functions as a cell survival factor in part through the suppression of differentiation and apoptosis induced by p53 and suggest that cell cycle arrest and apoptosis may be separable functions of p53.     

Role for cyclin D1 in UVC-induced and p53-mediated apoptosis.

DNA damaging agents such as ultraviolet (UV) induce cell cycle arrest followed by apoptosis in cells where irreparable damage has occurred. Here we show that during early phase G1 arrest which occurs in UV-irradiated human U343 glioblastoma cells, there are (1) decreases in cyclin D1 and cdk4 levels which parallel a loss of S-phase promoting cyclin D1/cdk4 complexes, and (2) increases in p53 and p21 protein levels. We also show that the late phase UV-induced apoptosis of U343 cells occurs after cell cycle re-entry and parallels the reappearance of cyclin D1 and cdk4 and cyclin D1/cdk4 complexes. These findings suggest that cyclin D1 can abrogate UV-induced G1 arrest and that the p53-mediated apoptosis that occurs in these cells is dependent on cyclin D1 levels. We examined these possibilities using U343 cells that ectopically express cyclin D1 and found that indeed cyclin D1 can overcome the cell cycle arrest caused by UV. Moreover, the appearance of p53 protein and the induction of apoptosis in UV-irradiated cells was found to be dependent on the level of ectopically expressed cyclin D1. These findings, therefore, indicate that expression of cyclin D1 following DNA damage is essential for cell cycle re-entry and p53-mediated apoptosis.     

The amino-terminal functions of the simian virus 40 large T antigen are required to overcome wild-type p53-mediated growth arrest of cells.

High levels of the p53 tumor suppressor protein can block progression through the cell cycle. A model system for the study of the mechanism of action of wild-type p53 is a cell line (T64-7B) derived from rat embryo fibroblasts transformed by activated ras and a temperature-sensitive murine p53 gene. At 37 to 39 degrees C, the murine p53 protein is in a mutant conformation and the cells actively divide, whereas at 32 degrees C, the protein has a wild-type conformation and the cells arrest in the G1 phase of the cell cycle. Wild-type simian virus 40 large T antigen and a variety of T-antigen mutants were assayed for the ability to bypass the cell cycle block effected by the wild-type p53 protein to induce colony formation at 32 degrees C. The results indicate that two functions within the amino terminus of T antigen are essential to induce cell growth: (i) the ability to bind to the retinoblastoma protein, Rb, and (ii) the presence of a domain in the first exon that appears to interact with the cellular protein, p300. Thus, the cell cycle arrest triggered by wild-type p53 may be overcome by formation of a T-antigen complex with Rb, p300, or both that could then function to either remove p53-mediated negative growth regulatory signals or promote a positive cell growth signal. Surprisingly, T antigen-p53 complexes are not required to overcome the temperature-sensitive p53 block to the cell cycle in these cells. These data suggest that simian virus 40 T antigen associated with Rb, p300, or both proteins can communicate in a cell with the functions of the wild-type p53 protein.     

Cdk7- and Cdc25A-independent dephosphorylation of Cdk2 during phorbol ester-mediated cell cycle arrest in U937 cells

The molecular mechanism underlying protein kinase C (PKC)-mediated cell cycle arrest is poorly understood. We undertook to characterize phorbol ester-activated PKC-mediated cell cycle arrest. Treatment with phorbol ester inhibited cell growth of human histiocytic lymphoma U937 cells with 83% of the cells arrested in G1 phase. Reduced activity of cdk2 correlated with cdk2 dephosphorylation and accumulation of cdk2 inhibitor p21Waf in phorbol ester-treated cells. Dephosphorylation of cdk2 was not associated with cdk7 and cdc25A activity in phorbol ester-treated cells. Protein phosphatase inhibitor assays suggest that the dephosphorylation of cdk2 results in the activation of a specific protein tyrosine phosphatase. Thus, dephosphorylation of cdk2 as well as accumulation of cdk2 inhibitor is likely to contribute to the G1 phase arrest in phorbol ester-treated in U937 cells.     

Cytokines inhibit p53-mediated apoptosis but not p53-mediated G1 arrest.

Murine erythroleukemia cells that lack endogenous p53 expression were transfected with a temperature-sensitive p53 allele. The temperature-sensitive p53 protein behaves as a mutant polypeptide at 37 degrees C and as a wild-type polypeptide at 32 degrees C. Three independent clones expressing the temperature-sensitive p53 protein were characterized with respect to p53-mediated G1 cell cycle arrest, apoptosis, and differentiation. Clone ts5.203 responded to p53 activation at 32 degrees C by undergoing G1 arrest, apoptosis, and differentiation. Apoptosis was seen in cells representative of all phases of the cell cycle and was not restricted to cells arrested in G1. The addition of a cytokine (erythropoietin, c-kit ligand, or interleukin-3) to the culture medium of ts5.203 cells blocked p53-mediated apoptosis and differentiation but not p53-mediated G1 arrest. These observations indicate that apoptosis and G1 arrest can be effectively uncoupled through the action of cytokines acting as survival factors and are consistent with the idea that apoptosis and G1 arrest represent separate functions of p53. Clones ts15.15 and tsCB3.4 responded to p53 activation at 32 degrees C by undergoing G1 arrest but not apoptosis. We demonstrate that tsCB3.4 secretes a factor with erythropoietin-like activity and that ts15.15 secretes a factor with interleukin-3 activity and suggest that autocrine secretion of these cytokines blocks p53-mediated apoptosis. These data provide a framework in which to understand the variable responses of cells to p53 overexpression.     

Caffeine induces G2/M arrest and apoptosis via a novel p53-dependent pathway in NB4 promyelocytic leukemia cells

Methylxantine derivative, caffeine, is known to prevent the p53-dependent apoptosis pathway via inhibition of ATM (ataxia telangiectasia mutated) kinase, which activates p53 by phosphorylation of the Ser-15 residue. In contrast, it has been reported that caffeine induces p53-mediated apoptosis through Bax protein in non-small-cell lung cancer cells. Therefore, the effects of caffeine on cellular growth in malignant cells are controversial. We investigated the effects of caffeine on cell proliferation, cell cycle progression, and induction of apoptosis in NB4 promyelocytic leukemia cells containing wild-type p53. Caffeine suppressed the cellular growth of NB4 cells in a dose- and time-dependent manner. Caffeine induced G(2)/M phase cell cycle arrest in NB4 cells in association with the induction of phosphorylation at the Ser-15 residue of p53 and induction of tyrosine phosphorylation of cdc2. Expression of Bax protein was increased in NB4 cells after treatment with caffeine. Interestingly, the antisense oligonucleotides for p53 significantly reduced p53 expression and caffeine-induced G(2)/M phase cell cycle arrest in NB4 cells. These results suggest that caffeine induces cell cycle arrest and apoptosis in association with activation of p53 by a novel pathway to phosphorylate the Ser-15 residue and induction of phosphorylation of cdc 2 in leukemic cells with normal p53.     

Inhibition of G1 cyclin-dependent kinase activity during growth arrest of human breast carcinoma cells by prostaglandin A2.

Prostaglandin A2 (PGA2) potently inhibits cell proliferation and suppresses tumor growth in vivo, but little is known regarding the molecular mechanisms mediating these effects. Here we demonstrate that treatment of breast carcinoma MCF-7 cells with PGA2 leads to G1 arrest associated with a dramatic decrease in the levels of cyclin D1 and cyclin-dependent kinase 4 (cdk4) and accompanied by an increase in the expression of p21. We further show that these effects occur independent of cellular p53 status. The decline in cyclin D and cdk4 protein levels is correlated with loss in cdk4 kinase activity, cdk2 activity is also significantly inhibited in PGA2-treated cells, an effect closely associated with the upregulation of p21. Immunoprecipitation experiments verified that p21 was indeed complexed with cdk2 in PGA2-treated cells. Additional experiments with synchronized MCF-7 cultures stimulated with serum revealed that treatment with PGA2 prevents the progression of cells from G1 to S. Accordingly, the kinase activity associated with cdk4, cyclin E, and cdk2 immunocomplexes, which normally increases following serum addition, was unchanged in PGA2-treated cells. Furthermore, the retinoblastoma protein (Rb), a substrate of cdk4 and cdk2 whose phosphorylation is necessary for cell cycle progression, remains underphosphorylated in PGA2-treated serum-stimulated cells. These findings indicate that PGA2 exerts its growth-inhibitory effects through modulation of the expression and/or activity of several key G1 regulatory proteins. Our results highlight the chemotherapeutic potential of PGA2, particularly for suppressing growth of tumors lacking p53 function.     

Mutant p53 can delay growth arrest and loss of CDK2 activity in senescing human fibroblasts without reducing p21(WAF1) expression

Functional wild-type p53 is required for human diploid fibroblasts (HDF) to enter an irreversible growth arrest known as replicative senescence. Experimentally, abrogation of p53 function by expression of human papillomavirus type 16 E6 or disruption of a key downstream effector p21 by homologous recombination both extended HDF life span. However, although sufficient to extend life span, p21 down-regulation is not necessary, because expression of a dominant-negative mutant p53 (143(ala)) extends life span without apparently decreasing p21 expression. Given the importance of p53 in cellular senescence and the general assumption that p21 may be the sole mediator of its action in this process, we have investigated how abrogation of p53 function can overcome senescence without lowering expression of p21. We have found up-regulated levels of the cyclin-dependent kinase 2 (cdk2) protein in HDF expressing 143(ala) mutant p53 as compared to senescent controls, together with an increase in p21-free cdk2 which, in conjunction with cyclin E, is able to form an active kinase which can phosphorylate the retinoblastoma protein. However, forced overexpression of cdk2 in near-senescent HDF failed to restore cdk2-associated kinase activity. Our data suggest that p53-mediated senescence depends on factor(s) other than p21 which modulate formation of cyclin E-cdk2 complexes.     

p16INK4A Participates in a G1 Arrest Checkpoint in Response to DNA Damage

Members of the INK4 protein family specifically inhibit cyclin-dependent kinase 4 (cdk4) and cdk6-mediated phosphorylation of the retinoblastoma susceptibility gene product (Rb). p16^INK4A, a prototypic INK4 protein, has been identified as a tumor suppressor in many human cancers. Inactivation of p16^INK4A in tumors expressing wild-type Rb is thought to be required in order for many malignant cell types to enter S phase efficiently or to escape senescence. Here, we demonstrate another mechanism of tumor suppression by implicating p16^INK4A in a G₁ arrest checkpoint in response to DNA damage. Calu-1 non-small cell lung cancer cells, which retain Rb and lack p53, do not arrest in G₁ following DNA damage. However, engineered expression of p16^INK4A at levels compatible with cell proliferation restores a G₁ arrest checkpoint in response to treatment with γ-irradiation, topoisomerase I and II inhibitors, and cisplatin. A similar checkpoint can be demonstrated in p53^−/− fibroblasts that express p16^INK4A. DNA damage-induced G₁ arrest, which requires the expression of pocket proteins such as Rb, can be abrogated by overexpression of cdk4, kinase-inactive cdk4 variants capable of sequestering p16^INK4A, or a cdk4 variant incapable of binding p16^INK4A. After exposure to DNA-damaging agents, there was no change either in overall levels of p16^INK4A or in amounts of p16^INK4A found in complex with cdks 4 and 6. Nonetheless, p16^INK4A expression is required for the reduction in cdk4- and cdk6-mediated Rb kinase activity observed in response to DNA damage. During tumor progression, loss of p16^INK4A expression may be necessary for cells with wild-type Rb to bypass this G₁ arrest checkpoint and attain a fully transformed phenotype.     

Bovine papillomavirus E2 protein activates a complex growth-inhibitory program in p53-negative HT-3 cervical carcinoma cells that includes repression of cyclin A and cdc25A phosphatase genes and accumulation of hypophosphorylated retinoblastoma protein.

The bovine papillomavirus E2 protein can inhibit the proliferation of HT-3 cells, a p53-negative cervical carcinoma cell line containing integrated human papillomavirus type 30 DNA. Here, we analyzed HT-3 cells to explore the mechanism of p53-independent E2-mediated growth inhibition. Expression of the E2 protein repressed expression of the endogenous human papillomavirus type 30 E6/E7 genes. This was accompanied by hypophosphorylation and increased accumulation of p105Rb and repression of E2F1 expression. The E2 protein also caused reduced cyclin-dependent kinase (cdk) 2 activity, but this did not appear to be due to increased expression of cdk inhibitors. Rather, expression of cyclin A, which regulates cdk2 activity, and the cdc25A and cdc25B phosphatases, which are thought to activate cdk2, was significantly reduced at both the RNA and protein levels in response to E2 expression. The E2 protein reduced expression of cdc25A and cdc25B in both HT-3 and HeLa cells, but not in cells that were not growth-inhibited by the E2 protein. E2 point mutants unable to inhibit cell growth did not repress cdc25A and cdc25B expression, nor did the cell cycle inhibitors hydroxyurea and mimosine. Based on these results and the known properties of cell cycle components, we propose a model to account for E2-induced growth inhibition of cervical carcinoma cell lines.