New developments in the multi-site phosphorylation and integration of stress signalling at p53

p38 MAPK-induced MDM2 degradation confers paclitaxel resistance through p53-mediated regulation of EGFR in human lung cancer cells

Paclitaxel (PTX) is a chemotherapeutic agent that is used to treat a variety of cancers, including non-small cell lung cancer (NSCLC). However, the emergence of drug resistance limits the utility of PTX. This study determined the signaling pathway that contributes to PTX resistance. We first established PTX resistant cell lines (H460/R and 226B/R) using a dose-escalating maintenance of PTX. We found that p38 MAPK and epidermal growth factor receptor (EGFR) were constitutively activated in these cell lines. The inhibition of p38 MAPK activity by SB203580 treatment or the transfection of dominant-negative p38 MAPK sensitized both cell lines to PTX treatment. Erlotinib, an EGFR inhibitor, also increased PTX-induced apoptosis in PTX resistant cells, which suggests a role for p38 MAPK and EGFR in the development of PTX resistance. We demonstrated that p38 MAPK enhanced EGFR expression via the induction of the rapid degradation of mouse double-minute 2 homolog (MDM2) and the consequent stabilization of p53, a transcription factor of EGFR. These results suggest for the first time that the p38 MAPK/p53/EGFR axis is crucial for the facilitation of PTX resistance in NSCLCs. We also propose a mechanism for the role of the tumor-suppressor p53 in drug resistance. These results provide a foundation for the future development of potential therapeutic strategies to regulate the p38 MAPK/p53/EGFR pathway for the treatment of lung cancer patients with PTX resistance.

A novel Aurora-A-mediated phosphorylation of p53 inhibits its interaction with MDM2

PURPOSE

Crosstalk between Aurora-A kinase and p53 has been proposed. While the genetic amplification of Aurora-A has been observed in many human cancers, how p53 is regulated by Aurora-A remains ambiguous. In this study, Aurora-A-mediated phosphorylation of p53 was analyzed by mass spectrometry in order to identify a new phosphorylation site. Subsequently, the functional consequences of such phosphorylation were examined.

EXPERIMENTAL DESIGN

In vitro phosphorylation of p53 by Aurora-A was performed and the phosphorylated protein was then digested with trypsin and enriched for phosphopeptides by immobilized metal affinity chromatography. Subsequently, a combination of β-elimination and Michael addition was applied to the phosphopeptides in order to facilitate the identification of phosphorylation sites by MS. The functional consequences of the novel phosphorylation of p53 on the protein-protein interactions, protein stability and transactivation activity were then examined using co-immunoprecipitation, Western blotting and reporter assays.

RESULTS

Ser-106 of p53 was identified as a novel site phosphorylated by Aurora-A. A serine-to-alanine mutation at this site was found to attenuate Aurora-A-mediated phosphorylation in vitro. In addition, phosphate-sensitive Phos-tag SDS-PAGE was used to confirm that the Ser-106 of p53 is in vivo phosphorylated by Aurora-A. Finally, co-immunoprecipitation studies suggested that Ser-106 phosphorylation of p53 decreases its interaction with MDM2 and prolongs the half-life of p53.

CONCLUSIONS

The inhibition of the interaction between p53 and MDM2 by a novel Aurora-A-mediated p53 phosphorylation was identified in this study and this provides important information for further investigations into the interaction between p53 and Aurora-A in terms of cancer biology.

Ribonuclease activity of p53 in cytoplasm in response to various stress signals

The tumor suppressor p53 protein is expressed at low levels under normal conditions. The subcellular localization and functional activation of p53 are influenced by diverse stress signals. p53 in cytoplasm exerts intrinsic 3'→5' exonuclease activity with various RNA and DNA substrates. ssRNAs containing an adenosine and uridine-rich (ARE) element are permissive targets for p53-mediated degradation. The analysis of the exonuclease activity in cytoplasm with activated p53 induced by various drug treatments or following γ-irradiation revealed that the expression of p53 exonuclease activity in response to stress signals is heterogeneous. Various genotoxic and non-genotoxic agents upregulate p53 yet have different effects on expression of exonuclease activity with ARE RNA but not with DNA substrate. Ribonuclease activity is enhanced in cytoplasmic extracts of HCT116 (p53+/+) cells exposed to γ-irradiation or treated by the non-genotoxic drug AS101 but decreased following treatment by genotoxic (e.g., doxorubicin) or non-genotoxic (e.g., DFMO) agents, thus indicating that p53 exonuclease activity is dependent on the specific stress and nature of the substrate. Apparently, the disparity in expression of p53 ribonuclease activity after each treatment is attributable to the different post-treatment response and to two posttranscriptional events: the interaction of RNA-binding HuR protein with ARE RNA protects the substrate from degradation by p53 and/or decrease in p53 ARE RNA binding capacity due to phosphorylation at Ser392 leads to reduction in p5 ribonuclease activity. Our results provide new insights into p53 exonuclease function and into the mechanisms behind the regulation ARE-RNA degradation by p53 under different cellular conditions.

Hexavalent chromium-induced apoptosis of granulosa cells involves selective sub-cellular translocation of Bcl-2 members, ERK1/2 and p53

Hexavalent chromium (CrVI) has been widely used in industries throughout the world. Increased usage of CrVI and atmospheric emission of CrVI from catalytic converters of automobiles, and its improper disposal causes various health hazards including female infertility. Recently we have reported that lactational exposure to CrVI induced a delay/arrest in follicular development at the secondary follicular stage. In order to investigate the underlying mechanism, primary cultures of rat granulosa cells were treated with 10 μM potassium dichromate (CrVI) for 12 and 24h, with or without vitamin C pre-treatment for 24h. The effects of CrVI on intrinsic apoptotic pathway(s) were investigated. Our data indicated that CrVI: (i) induced DNA fragmentation and increased apoptosis, (ii) increased cytochrome c release from the mitochondria to cytosol, (iii) downregulated anti-apoptotic Bcl-2, Bcl-XL, HSP70 and HSP90; upregulated pro-apoptotic BAX and BAD, (iv) altered translocation of Bcl-2, Bcl-XL, BAX, BAD, HSP70 and HSP90 to the mitochondria, (v) upregulated p-ERK and p-JNK, and selectively translocated p-ERK to the mitochondria and nucleus, (vi) activated caspase-3 and PARP, and (vii) increased phosphorylation of p53 at ser-6, ser-9, ser-15, ser-20, ser-37, ser-46 and ser-392, increased p53 transcriptional activation, and downregulated MDM-2. Vitamin C pre-treatment mitigated CrVI effects on apoptosis and related pathways. Our study, for the first time provides a clear insight into the effect of CrVI on multiple pathways that lead to apoptosis of granulosa cells which could be mitigated by vitamin C.

The effect of cellular environment and p53 status on the mode of action of the platinum derivative LA-12

In this study, we characterized the effects of LA-12 on tumor cell lines possessing wild type p53 and on p53-deficient/mutant cell lines and the results were compared to those obtained using cisplatin. We have determined changes of p53 levels, of its transcriptional activity, of its posttranscriptional modifications and the effect of the treatment on the cell cycle, on the induction of apoptosis and on gene expression. LA-12 induces weak accumulation of both transcriptionally active p53 tumor suppressor and of p21(WAF1/CIP1) protein. LA-12 and cisplatin also significantly differ in their effects on apoptosis and cell cycle and on gene expression spectra in studied cell lines. LA-12 induces higher apoptosis levels in comparison with those induced by cisplatin, especially in p53-deficient H1299 cells and in MCF-7DD cells with transcriptionally inactive p53. We suggest that LA-12-mediated apoptosis is not fully dependent on p53. This confirms the therapeutic potential of LA-12 as a more potent cytostatic agent for both tumor cells expressing wild type p53 and for p53-deficient or mutant cells.

Exonucleolytic degradation of RNA by p53 protein in cytoplasm

p53 in cytoplasm displays an intrinsic 3'-->5' exonuclease activity. To understand the significance of p53 exonuclease activity in cytoplasm, cytoplasmic extracts of various cell lines were examined for exonuclease activity with different single-stranded RNA (ssRNA) substrates. Using an in vitro RNA degradation assay, we observed in cytoplasmic extracts of LCC2 cells, expressing high levels of endogenous wtp53, an efficient 3'-->5' exonuclease activity with RNA substrates, removing the 3'-terminal nucleotides. Interestingly, RNA containing AU-rich sequences (ARE) is the permissive substrate for exonucleolytic degradation. Evidence that exonuclease function with RNA detected in cytoplasmic extracts is attributed to the p53 is supported by several facts: (1) this activity closely parallels with status and levels of endogenous cytoplasmic p53; (2) the endogenous exonuclease exerts identical RNA substrate specificity and excision profile characteristic for purified baculovirus-or bacterially-expressed wtp53s; (3) the exonuclease activity with ARE RNA is competed out by the presence of ss or double-stranded DNA substrate utilized by p53 protein in cytoplasm; (4) immunoprecipitation by specific anti-p53 antibodies markedly reduced the exonuclease activity with both RNA and DNA substrates; and (5) transfection of the wtp53, but not exonuclease-deficient mutant p53-R175H, into p53-null H1299 or HCT116 cells induced high levels of exonuclease activity with ARE RNA substrate in cytoplasm with characteristic excision profile. The efficient ARE RNA degradation correlates with the efficient binding of p53 to ARE RNA in cytoplasm. The possible role of p53 exonuclease activity in ARE-mRNA destabilization in cytoplasm, which may be important for expression of proteins that control cell growth and/or apoptosis is discussed.

Consequences of the loss of p53, RB1, and PTEN: Relationship to gefitinib resistance in endometrial cancer

OBJECTIVE

These studies demonstrate how loss of function mutations or downregulation of key tumor suppressors missing from type I and type II endometrial cancer cells contributes to carcinogenesis and to resistance to the EGFR inhibitor gefitinib (ZD1839).

METHODS

Cell models devoid of tumor suppressors PTEN and RB1 or PTEN were studied. PTEN, RB1 and p53 expression was reinstated, and the effects on cell cycle, apoptosis, and cell cycle regulators were evaluated.

RESULTS

In Ishikawa H cells that model type I endometrial cancer in the loss of PTEN and RB1, re-expressing PTEN and RB1 increased the apoptotic and G1 phases and decreased the S and G2-M phases, which further sensitize the cells to gefitinib. Expressing p53 in Hec50co that model type II tumors by loss of this tumor suppressor arrested cells at the G1-S checkpoint, and apoptosis was also induced. Yet this did not improve sensitivity to gefitinib. Modulation of the cell cycle regulators responsible for these changes is explored, and a potential new therapeutic target, MDM2, is identified.

CONCLUSION

The downregulation of p53 expression in type II Hec50co cells is linked to gefitinib resistance. In addition, the overexpression of MDM2, the principal factor that inhibits p53 function also occurs in these resistant cells. MDM2 phosphorylation is only partially blocked by gefitinib, and high MDM2 expression may relate to drug resistance.

Involvement of the extracellular signal-regulated protein kinase pathway in phosphorylation of p53 protein and exerting cytotoxicity in human neuroblastoma cells (SH-SY5Y) exposed to acrylamide

Using human neuroblastoma SH-SY5Y cells, effects of acrylamide on p53 protein and intracellular signal transducting pathways were examined. Acrylamide increased p53, phosphorylated p53, and p53-associated protein murine double minute 2 (MDM2). The phosphorylation of p53 was specific for the Ser15 site. Among mitogen-activated protein kinases (MAPKs), acrylamide caused phosphorylation of extracellular signal-regulated protein kinase (ERK) and p38 but not c-Jun NH(2)-terminal kinase. Nevertheless, blocking p38 pathway by LL-Z1640-2 did not suppress the phosphorylation of p53 at Ser15. In contrast, a specific inhibitor of ERK kinase (U0126 or PD98059) could abolish the accumulation as well as the phosphorylation of p53 at Ser15. Elevation of MDM2 was also abolished by U0126. An inhibitor of phosphatidylinositol 3-kinase-related kinase (PIKK) pathway (wortmannin) suppressed the increase of p53 and its phosphorylation at Ser15. Hence, acrylamide increases p53 protein and its phosphorylation at Ser15 through ERK and/or PIKK pathways. On the other hand, U0126 and PD98059 suppressed to some extent the cytotoxicity of acrylamide evaluated by trypan blue exclusion and lactate dehydrogenase (LDH) leakage, whereas neither LL-Z1640-2 nor wortmannin was effective in suppressing the toxicity. Thus, ERK pathway seems to play a role both in causing the phosphorylation of p53 at Ser15 and in the cytotoxicity of acrylamide in SH-SY5Y cells.

NF-kappaB prevents cells from undergoing Cr(VI)-induced apoptosis

The transcription factor NF-kappaB has been reported to prevent cells from undergoing apoptosis as well as promote cell apoptosis. To investigate the role of NF-kappaB in Cr(VI)-induced apoptosis, two cell lines were developed from human bronchial epithelial BEAS-2B cells: IKK cells, which were stably transfected with IkappaBalpha expression vector, that have normal NF-kappaB activity, and KM cells, which were stably transfected with mutated IkappaBalpha kinase expression vector, that exhibit very little NF-kappaB activity. With Cr(VI) stimulation, KM cells, but not IKK cells, exhibited substantial cell death. Cell morphological and TUNEL analyses indicated that the KM cells showed apoptotic features. These results suggest that NF-kappaB activation is required to prevent the cells from undergoing Cr(VI)-induced apoptosis.

Expression of the VRK (vaccinia-related kinase) gene family of p53 regulators in murine hematopoietic development

The vaccinia-related kinase (VRK) proteins are a new group of three Ser-Thr kinases in the human kinome. VRK proteins are upstream regulators of several transcription factors. VRK1 phosphorylates p53 in Thr-18 within the region of binding to mdm2 preventing their interaction. The tissue distribution of three genes is still largely unknown. In the present report the expression of these genes was analyzed during murine hematopoietic development. The three genes are expressed in fetal liver and peripheral blood, with higher levels between days 11.5 and 13.5, a time when there is a massive expansion of liver cells, and thereafter their expression falls significantly. VRK genes are expressed, particularly at mid-gestation, in embryo thymus and spleen, but in adult thymus and spleen their levels are very low. VRK2 is expressed at lower levels than VRK1 and VRK3 in the mouse embryo. VRK genes play a role during embryonic development of hematopoiesis.

p53-associated 3'-->5' exonuclease activity in nuclear and cytoplasmic compartments of cells

The tumor suppressor protein p53 plays an important role in maintenance of the genomic integrity of cells. p53 possesses an intrinsic 3'-->5' exonuclease activity. p53 was found in the nucleus and in the cytoplasm of the cell. In order to evaluate the subcellular location and extent of p53-associated 3'--> 5' exonuclease activity, we established an in vitro experimental system of cell lines with different nuclear/cytoplasmic distribution of p53. Nuclear and cytoplasmic extracts obtained from LCC2 cells (expressing a high level of cytoplasmic wild-type p53), MCF-7 cells (expressing a high level of wild-type nuclear p53), MDA cells (expressing mutant p53) and H1299 cells (p53-null) were subjected to the analysis of exonuclease activity. Interestingly, 3'-->5' exonuclease was predominantly cytoplasmic; the nuclear extracts derived from all cell lines tested, exerted a low level of exonuclease activity. Cytoplasmic extracts of LCC2 cells, with a high level of wild-type p53, showed an enhanced exonuclease activity in comparison to those expressing either a low level of wild-type p53 (in MCF-7 cells) or the mutant p53 (in MDA cells). Evidence that exonuclease function detected in cytoplasmic extracts is attributed to the p53 is supported by several facts: First, this activity closely parallels with levels and status of endogenous cytoplasmic p53. Second, immunoprecipitation of p53 from cytoplasmic extracts of LCC2 cells markedly reduced the exonuclease activity. Third, the observed 3'-->5' exonuclease in cytoplasmic fraction of LCC2 cells displays identical biochemical properties characteristic of recombinant wild-type p53. The biochemical functions include: (a) substrate specificity; exonuclease hydrolyzes single-stranded DNA in preference to double-stranded DNA and RNA/DNA template-primers, (b) efficient excision of 3'-terminal mispairs from DNA/DNA and RNA/DNA substrates, (c) the preferential excision of purine-purine mispairs over purine-pyrimidine mispairs and (d) functional interaction with exonuclease-deficient DNA polymerase, for example, murine leukemia virus reverse transcriptase (representing a relatively low fidelity enzyme), thus enhancing the fidelity of DNA synthesis by excision of mismatched nucleotides from the nascent DNA strand. Taken together, the data demonstrate that wild-type p53 in cytoplasm, in its noninduced state, is functional; it displays intrinsic 3'-->5' exonuclease activity. The possible role of p53-associated 3'-->5' exonuclease activity in DNA repair in nucleus and cytoplasm is discussed.

Radiation-induced apoptosis in scid mice spleen after low dose irradiation

To assess the radioadaptive response of the whole body system in mice, we examined the temporal effect of low dose priming as an indicator of challenging irradiation-induced apoptosis through a p53 tumor suppressor protein- mediated signal transduction pathway. The p53 protein also plays an important role both in cell cycle control and DNA repair through cellular signal transduction. Using severe combined immunodeficiency mice defective in DNA-dependent protein kinase catalytic subunit, we examined the role of DNA-dependent protein kinase activity in radioadaptation induced by low dose irradiation. Specific pathogen free 5-week-old female severe combined immunodeficiency mice and the parental mice (CB- 17 Icr +/+) were irradiated with X-ray at 3.0 Gy at 1, 2, 3 or 4 weeks after the conditioning irradiation at 0.15, 0.30, 0.45 or 0.60 Gy. The mice spleens were fixed for immunohistochemistry 12 h after the challenging irradiation. The p53-dependent apoptosis related Bax proteins on formalin-fixed paraffin-embedded sections were stained by the avidin-biotin peroxidase complex method. The apoptosis incidence in the sections was measured by hematoxylin-eosin staining. The frequency of Bax- and apoptosis-positive cells increased up to 12 h after the challenging irradiation in the spleen of both mice. However, these cells were not observed after a low dose irradiation at 0.15-0.60 Gy. When pre-irradiation at 0.45 Gy 2 weeks before the challenging irradiation at 3.0 Gy was performed, Bax accumulation and apoptosis induced by challenging irradiation were depressed in the spleens of CB-17 Icr +/+ mice, but not in severe combined immunodeficiency mice. These data suggest that DNA-dependent protein kinase might play a major role in radioadaptation induced by pre-irradiation with a low dose in mice spleen. We expect that the present findings will provide useful information in the health care of space crews.

Synthesis of complex phosphopeptides as mimics of p53 functional domains

A complete set of mono-, di- and triphosphorylated peptides comprising amino acids 10-27, the Mdm2 and p300 binding site(s) of p53, with and without a fluorescein label at the N-terminus, was synthesized by step-by-step solid phase synthesis. Fluorescence polarization analysis revealed that phosphorylation at Thr18 decreased binding to recombinant Mdm2 protein compared with the unphosphorylated and the two other single phosphorylated analogues. Unlabelled multiply phosphorylated peptides corresponding to this amino-terminal transactivation domain proved to be powerful tools in analysing the phosphate specificity of existing anti-p53 monoclonal and polyclonal antibodies using direct ELISA. The tetramerization domain of human p53 protein was modelled with a 53 residue-long unlabelled unphosphorylated and Ser315-phosphorylated peptide pair. CD analysis showed similar alpha-helical structures for both peptides and no major difference in the secondary structure could be observed upon phosphorylation. Size-exclusion HPLC indicated that these synthetic oligomerization domain mimics underwent a pH-dependent tetramerization process, but the presence of a phosphate group at Ser315 did not modify the oligomeric state of the 308-360 p53 fragments. Nevertheless, the fluorescein-labelled Ser315 phosphorylated peptide bound to the downstream signalling ligand DNA topoisomerase I protein with slightly higher affinity than did the unphosphorylated analogue.

Measurement of UVB-Induced DNA damage and its consequences in models of immunosuppression

Exposure to UVB results in formation of cyclobutane pyrimidine dimers (CPDs) and 6-4 photoproducts in DNA. These can be quantified by a variety of techniques including alkaline gel electrophoresis, ELISAs, Southwestern blotting, and immunohistochemistry. Damage to DNA results in activation of damage response pathways, as indicated by Western blotting using antibodies specific for p53 and breast cancer-associated gene 1 (BRCA1) phosphorylation. The signal from DNA damage to activation of these response pathways appears to be mediated by FKBP12-rapamycin-associated protein (FRAP), since these phosphorylation events are blocked by rapamycin. UVB-induced DNA damage also leads to induction of immunosuppressive cytokines including tumor necrosis factor alpha (TNF-alpha) and interleukin (IL)-10 in skin. Induction of TNF-alpha by UVB is readily detectable in cultured normal human epidermal keratinocytes (NHEKs) using ELISA, while induction of IL-10 is readily detectable in cultured mouse keratinocytes but not in NHEKs. Induction of DNA damage by liposome-encapsulated HindIII results in induction of immunosuppressive responses similar to UVB. Clinical testing shows that liposome-encapsulated T4 endonuclease V or photolyase stimulates repair of CPDs in the skin of human subjects, and prevents UVB-induced immunosuppression. Stimulation of repair and prevention of immunosuppression have been linked to prevention of skin cancer by liposome-encapsulated T4 endonuclease V in repair-deficient xeroderma pigmentosum patients.

A role for p53 in the frequency and mechanism of mutation

The tumor suppressor protein, p53, is often referred to as the guardian of the genome. When p53 function is impaired, its ability to preserve genomic integrity is compromised. This may result in an increase in mutation on both a molecular and chromosomal level and contribute to the progression to a malignant phenotype. In order to study the effect of p53 function on the acquisition of mutation, in vitro and in vivo models have been developed in which both the frequency and mechanism of mutation can be analyzed. In human lymphoblastoid cells in which p53 function was impaired, both the spontaneous and induced mutant frequency increased at the autosomal thymidine kinase (TK) locus. The mutant frequency increased to a greater extent in cell lines in which p53 harbored a point mutation than in those lines in which a "null" mutation had been introduced by molecular targeting or by viral degradation indicating a possible "gain-of-function" associated with the mutant protein. Further, molecular analysis revealed that the loss of p53 function was associated with a greater tendency towards loss-of-heterozygosity (LOH) within the TK gene that was due to non-homologous recombination than that found in wild-type cells. Most data obtained from the in vivo models uses the LacI reporter gene that does not efficiently detect mutation that results in LOH. However, studies that have examined the effect of p53 status on mutation in the adenine phosphoribosyl transferase (APRT) gene in transgenic mice also suggest that loss of p53 function results in an increase in mutation resulting from non-homologous recombination. The results of these studies provide clear and convincing evidence that p53 plays a role in modulating the mutant frequency and the mechanism of mutation. In addition, the types of mutation that occur within the p53 gene are also of importance in determining the mutant frequency and the pathways leading to mutation.

Polyoma virus disrupts ARF signaling to p53

Polyoma virus (Py) differs from other small DNA tumor viruses in not encoding a protein that inactivates p53. The complete Py early region encoding the large T-antigen (PyLT), middle T-antigen (PyMT) and small T-antigen (PyST) will transform primary rodent cells and REF52 cells, but PyMT, the main Py oncogene, by itself will only transform these cells when p53 or ARF is inactivated. We have related Py oncogene cooperation with the effects of the Py T-antigens on the ARF-p53 signaling pathway. PyMT activates an ARF-induced p53-mediated block to cell division explaining the inability of PyMT alone to generate dividing transformed cells. In contrast, in REF52 cells transformed by the whole Py early region (PyREF52), ARF is upregulated but p53 is not activated. Thus PyLT and/or PyST negates the PyMT-induced ARF-mediated block to cell division by disrupting the signaling pathway from ARF to p53. Although there is no detectable interaction or co-localization of endogenous ARF (nucleoli) and MDM2 (nucleoplasm) in PyREF52 cells, expression of transfected ectopic ARF results in an MDM2/ARF interaction and sequestration of MDM2 into the nucleoli. Sequestration of MDM2 by ARF in the nucleoli is not essential for a p53 response in REF52 cells as activation of Raf in REF52Raf-ER cells results in an ARF-induced p53-mediated cell cycle block in the absence of a detectable ARF-MDM2 interaction. Py may provide new insights into the cellular ARF-p53 signaling pathway.

Novel human p53 mutations that are toxic to yeast can enhance transactivation of specific promoters and reactivate tumor p53 mutants

Since highly expressed human p53 can inhibit human and yeast cell growth, we predicted that p53 mutants could be generated with increased growth inhibition of the yeast Saccharomyces cerevisiae and that these would be useful for characterizing p53 functions and tumor p53 mutants. A random mutagenesis screen led to the isolation of mutations in the DNA binding domain that result in p53 being lethal even at moderate expression levels in yeast. Three independent mutants had an alanine change at the evolutionary invariant V122 in the L1 loop. The other toxic mutations affected codons 277 (C277R, C277W) and 279 (G279R). This latter amino acid change was also reported in tumors, while all the other mutations are novel. A recently developed rheostatable GALI promoter system that provides graded increases in expression of p53 was used to examine the transactivation function of the toxic mutations when expression was greatly reduced and cells were viable. At low expression levels the toxic mutants lacked transactivation from a 3xRGC responsive element (RE). Surprisingly some exhibited enhanced transactivation with p21 and bax REs. The V122A mutant was able to re-activate transactivation of various p53 tumor mutants and retained growth inhibition when co-expressed with dominant-negative tumor mutations. Upon expression in human Saos-2 cells the V122A p53 mutant caused growth suppression, was capable of transactivation and exhibited higher than wild type activity with the bax promoter in luciferase assays. A non-functional p53 tumor mutant was partially reactivated by V122A for both transactivation and growth suppression. Thus, the screen for toxic p53 mutants in yeast can identify novel p53 variants that may be useful in dissecting p53 regulated cellular responses and in developing p53-based cancer therapies.

Cadmium induces phosphorylation of p53 at serine 15 in MCF-7 cells

When MCF-7 cells were incubated with 10 or 20 microM CdCl(2), p53 protein level increased after 18 h. Among serines in p53 protein immunoprecipitated from cells treated with CdCl(2), only Ser 15 was phosphorylated. No clear phosphorylation was found on Ser 6, 9, 20, 37, and 392. Accumulation of p53 protein phosphorylated at Ser 15 was also found after 18 h exposure. While phosphorylation of extracellular signal-regulated protein kinase, c-Jun NH2-terminal kinase and p38 was found in cells treated with CdCl(2), treatment with U0126, LL-Z1640-2, or SB203580 did not suppress Ser 15 phosphorylation. On the other hand, treatment with wortmannin or caffeine suppressed CdCl(2)-induced Ser 15 phosphorylation and accumulation of p53 protein. The present results showed that cadmium induces phosphorylation of p53 at Ser 15 in MCF-7 cells depending on phosphatidylinositol 3-kinase related kinases, but not on mitogen-activated protein kinases.

Permanent growth arrest in irradiated human fibroblasts

Exposure of human fibroblasts to doses of ionizing radiation sufficient to cause a permanent growth arrest repressed the expression of genes induced late during G(0)/G(1)-phase traverse, including both cyclin A and cyclin E. In addition, radiation prevented the cell cycle-dependent activation of cyclin D1-associated kinase activity and the subsequent phosphorylation of the RB tumor suppressor protein. Exposure to radiation did not alter the cellular levels of cyclin D1 protein, nor did it alter the formation of cyclin D1-CDK4 complexes. Surprisingly, the repression of cyclin D1-associated kinase activity in damaged mitogen-stimulated quiescent cells could not be accounted for by a relative increase in the association of CDKN1A (also known as p21(Cip1)) with cyclin D1 complexes, nor was cyclin D1 activity targeted by increased levels of CDKN1A in irradiated, logarithmically growing cultures under conditions where cyclin A activity was acutely repressed. Therefore, a radiation-induced permanent growth arrest is mediated by pathways that are distinct from those that cause cell cycle delay in damaged cells involving repression of cyclin-dependent kinase activity by CDKN1A.

p53 mutants exhibiting enhanced transcriptional activation and altered promoter selectivity are revealed using a sensitive, yeast-based functional assay

Changes in promoter specificity and binding affinity that may be associated with p53 mutations or post-translational modifications are useful in understanding p53 structure/function relationships and categorizing tumor mutations. We have exploited variable expression of human p53 in yeast to identify mutants with novel phenotypes that would correspond to altered promoter selectivity and affinity. The p53 cDNA regions coding for the DNA binding and tetramerization domains were subjected to random PCR mutagenesis and were cloned directly by recombination in yeast into a vector with a GAL1 promoter whose level of expression could be easily varied. p53 variants exhibiting higher than wild type levels of transactivation (supertrans) for the RGC responsive element were identified at low level of p53 protein expression. All the p53 mutants obtained with this screen were located in the DNA binding domain. Two out of 17 supertrans mutants have been found in tumors. Six mutations were in the L1 loop region between amino acids 115 and 124. The transactivation potential of a panel of supertrans p53 mutants on different promoters was evaluated using the p53 responsive elements, RGC, PIG3, p21 and bax. Although all mutants retained some activity with all promoters, we found different patterns of induction based on strength and promoter specificity. In particular none of the mutants was supertrans for the p21 responsive element. Interestingly, further analysis in yeast showed that the transactivation function could be retained even in the presence of dominant-negative p53 tumor mutations that could inhibit wild type p53. Five mutants were also characterized in human cells in terms of growth suppression and transactivation of various promoters. These novel supertrans p53 mutants may be useful in studies aimed at dissecting p53 downstream pathways, understanding specific interactions between p53 and the DNA, and could replace wild type p53 in cancer gene therapy protocols. The approach may also prove useful in identifying p53 tumor mutations.

p53-inducible wip1 phosphatase mediates a negative feedback regulation of p38 MAPK-p53 signaling in response to UV radiation

The stress-responsive p38 MAPK, when activated by genotoxic stresses such as UV radiation, enhances p53 activity by phosphorylation and leads to cell cycle arrest or apoptosis. Here we report that a member of the protein phosphatase type 2C family, Wip1, has a role in down-regulating p38-p53 signaling during the recovery phase of the damaged cells. Wip1 was originally identified as a gene whose expression is induced following gamma or UV radiation in a p53-dependent manner. We found that Wip1 is also inducible by other environmental stresses, such as anisomycin, H(2)O(2) and methyl methane sulfonate. UV-induction of Wip1 requires p38 activity in addition to the wild-type p53. Wip1 selectively inactivates p38 by specific dephosphorylation of its conserved threonine residue. Furthermore, Wip1 expression attenuates UV-induced p53 phosphorylation at Ser33 and Ser46, residues previously reported to be phosphorylated by p38. Wip1 expression also suppresses both p53-mediated transcription and apoptosis in response to UV radiation. These results suggest that p53-dependent expression of Wip1 mediates a negative feedback regulation of p38-p53 signaling and contributes to suppression of the UV-induced apoptosis.

The human vaccinia-related kinase 1 (VRK1) phosphorylates threonine-18 within the mdm-2 binding site of the p53 tumour suppressor protein

The tumour suppressor p53 protein integrates multiple signals regulating cell cycle progression and apoptosis. This regulation is mediated by several kinases that phosphorylate specific residues in the different functional domains of the p53 molecule. The human VRK1 protein is a new kinase related to a poxvirus kinase, and more distantly to the casein kinase 1 family. We have characterized the biochemical properties of human VRK1 from HeLa cells. VRK1 has a strong autophosphorylating activity in several Ser and Thr residues. VRK-1 phosphorylates acidic proteins, such as phosvitin and casein, and basic proteins such as histone 2b and myelin basic protein. Because some transcription factors are regulated by phosphorylation, we tested as substrates the N-transactivation domains of p53 and c-Jun fused to GST. Human c-Jun is not phosphorylated by VRK1. VRK1 phosphorylates murine p53 in threonine 18. This threonine is within the p53 hydrophobic loop (residues 13-23) required for the interaction of p53 with the cleft of its inhibitor mdm-2. The VRK1 C-terminus domain (residues 268-396) that contains a nuclear localization signal targets the protein to the nucleus, as determined by using fusion proteins with the green fluorescent protein. We conclude that VRK1 is an upstream regulator of p53 that belongs to a new signalling pathway.

Interaction of casein kinase 1 delta (CK1delta) with post-Golgi structures, microtubules and the spindle apparatus

Members of the casein kinase 1 family of serine/threonine kinases are highly conserved from yeast to mammals and seem to play an important role in vesicular trafficking, DNA repair, cell cycle progression and cytokinesis. We here report that in interphase cells of various mammalian species casein kinase 1 delta (CK1delta) specifically interacts with the trans Golgi network and cytoplasmic, granular particles that associate with microtubules. Furthermore, at mitosis CK1delta is recruited to the spindle apparatus and the centrosomes in cells, which have been exposed to DNA-damaging agents like etoposide or gammairradiation. In addition, determination of the affinity of CK1delta to different tubulin isoforms in immunoprecipitation-Western analysis revealed a dramatically enhanced complex formation between CK1delta and tubulins from mitotic extracts after introducing DNA damage. The high affinity of CK1delta to the spindle apparatus in DNA-damaged cells and its ability to phosphorylate several microtubule-associated proteins points to a regulatory role of CK1delta at mitosis.

Coordinate modulation of Sp1, NF-kappa B, and p53 in confluent human malignant melanoma cells after ionizing radiation

Regulation of transcriptional responses in growth-arrested human cells under conditions that promote potentially lethal damage repair after ionizing radiation (IR) is poorly understood. Sp1/retinoblastoma control protein (RCP) DNA binding increased within 30 min and peaked at 2-4 h after IR (450-600 cGy) in confluent radioresistant human malignant melanoma (U1-Mel) cells. Increased phosphorylation of Sp1 directly corresponded to Sp1/RCP binding and immediate-early gene induction, whereas pRb remained hypophosphorylated. Transfection of U1-Mel cells with the human papillomavirus E7 gene abrogated Sp1/RCP induction and G(0)/G(1) cell cycle checkpoint arrest responses, increased apoptosis and radiosensitivity, and augmented genetic instability (i.e., increased polyploidy cells) after IR. Increased NF-kappaB DNA binding in U1-Mel cells after IR treatment lasted much longer (i.e., >20 h). U1-Mel cells overexpressing dominant-negative IkappaBalpha S32/36A mutant protein were significantly more resistant to IR exposure and retained both G(2)/M and G(0)/G(1) cell cycle checkpoint responses without significant genetic instability (i.e., polyploid cell populations were not observed). Nuclear p53 protein levels and DNA binding activity increased only after high doses of IR (>1200 cGy). Disruption of p53 responses in U1-Mel cells by E6 transfection also abrogated G(0)/G(1) cell cycle checkpoint arrest responses and increased polyploidy after IR, but did not alter radiosensitivity. These data suggest that abrogation of individual components of this coordinate IR-activated transcription factor response may lead to divergent alterations in cell cycle checkpoints, genomic instability, apoptosis, and survival. Such coordinate transcription factor activation in human cancer cells is reminiscent of prokaryotic SOS responses, and further elucidation of these events should shed light on the initial molecular events in the chromosome instability phenotype.-Yang, C.-R., Wilson-Van Patten, C., Planchon, S. M., Wuerzberger-Davis, S. M., Davis, T. W., Cuthill, C., Miyamoto, S., Boothman, D. A. Coordinate modulation of Sp1, NF-kappa B, and p53 in confluent human malignant melanoma cells after ionizing radiation.