Expression of p53, bcl-2, bax, bcl-x2 and c-myc in radiation-induced apoptosis in Burkitt's lymphoma cells

Apoptosis, a form of physiological cell death, is a genetically determined program essential for normal development and maintenance of tissues, which has been linked to a variety of gene products. We have examined the susceptibility to radiation-induced apoptosis of cell lines derived from the human B cell tumour, Burkitt's lymphoma (BL), displaying a variety of phenotypic characteristics and expressing genes implicated in apoptosis at different levels. The susceptibility to apoptosis following gamma radiation varied significantly amongst the lines. Cell lines with wild type p53 were susceptible to radiation-induced apoptosis but two of five BL lines with only mutant p53 allele also displayed similar susceptibility. Some BL cell lines that expressed bcl-2 at levels comparable with Epstein-Barr virus (EBV) transformed normal B cells were highly susceptible to gamma radiation-induced apoptosis, whereas others expressing low levels were resistant. When these lines were analysed for bax and bcl-X(L) expression again no correlation was observed with susceptibility or resistance to apoptosis. Two BL cell lines having deregulated expression of c-myc were resistant to the induction of apoptosis while two others which had regulated c-myc expression were susceptible. Thus the status of p53, c-myc, bcl-2, bcl-X(L) and bax is not sufficiently informative in BL lines to predict susceptibility to radiation-induced apoptosis.     

CD40-mediated apoptosis in murine B-lymphoma lines containing mutated p53

Crosslinking CD40 induces normal B-cells to proliferate and differentiate but causes many tumor cell lines to undergo apoptosis. As p53 is required for many apoptotic pathways, we analyzed the effects of CD40 ligation and their correlation with p53 function in four murine B-lymphoma lines. A20 and M12 cells showed complete growth inhibition and an increase in the proportion of apoptotic cells upon CD40 crosslinking, but WEHI 231 and WEHI 279 cells were unaffected. IkappaBalpha protein levels were reduced in all four lines in response to CD40 crosslinking, indicating that CD40 signaling was normal. NFkappaB activity was quantitated by reporter and gel shift assays and found to be equivalent in all four cell lines. P53 reporter constructs were activated in WEHI231 but not A20 or M12 cells, suggesting that p53 in the latter two cell lines may be mutated. These data are supported by the lack of detectable p21 mRNA in A20 and M12 cells. P21 mRNA was increased to detectable levels in M12 cells upon CD40 ligation; however, blocking this effect with the p53 inhibitor pifithrin had no effect on CD40-mediated apoptosis. Sequencing showed that p53 in A20 and M12 cells contained point mutations leading to amino acid substitutions in DNA binding regions, but was unmutated in WEHI231 and WEHI 279. These results suggest that CD40-mediated apoptosis can occur in the absence of functional p53.     

Segregational fidelity of chromosomes in human thyroid tumour cells

Using fluorescence in situ hybridisation (FISH) we have analysed the segregational fidelity of all the human chromosomes during mitotic cell division. The losses and gains of chromosomes were analysed in human polyploid cell lines derived from a well-differentiated papillary thyroid cancer. These thyroid cells can be cultured for more than 300 population doublings. For the purpose of our study the polyploid nature of the cells may act as a protective buffer against the cell-lethal effects of the loss of individual chromosomes. To evaluate the role of the p53 gene product in maintaining the fidelity of chromosome segregation we compared the frequencies of chromosome loss and gain in cultures with wild-type p53 activity (K1E7neo3) and cultures transfected with plasmids expressing a mutant p53 product (K1E7scx6). Cultures were analysed for the presence of both structurally normal and rearranged chromosomes at both early and late passages. Cell cultures with defective p53 activity showed progressive chromosome loss from a median chromosome number of 87–97 to 75–86. Cell growth in cultures with wild-type p53 activity showed the loss of chromosomes 6, 7, and 8 and the gain of 17 and 20. Cultures expressing mutant p53 activity showed the loss of chromosomes 2, 5, 14 and 17 and the gain of 4 and 22. The combination of defective p53 and growth resulted in further destabilisation with the additional losses of chromosomes 3, 11, 15, 16 and 21. Chromosomes 1, 9, 10, 12, 13, 18, 19, X and Y segregated stably under all the culture conditions as did the structurally rearranged marker chromosomes. The study has demonstrated variation in the fidelity of mitotic chromosome segregation and the influence of p53 gene activity upon the segregation of individual human chromosomes. Received: 7 August 1998; in revised form: 28 August 1998 / Accepted: 29 August 1998     

DNA damage in human B cells can induce apoptosis, proceeding from G1/S when p53 is transactivation competent and G2/M when it is transactivation defective.

Cisplatin treatment of Epstein-Barr virus-immortalized human B lymphoblastoid cell lines (LCLs) results in p53-mediated apoptosis which occurs largely in a population of cells at the G1/S boundary of the cell cycle. Cell cycle progression appears to be required for this apoptosis because arresting cells earlier in G1 inhibited apoptosis despite the accumulation of p53. Overexpression of wild-type p53 also induces apoptosis in an LCL. Therefore six mutant genes derived from Burkitt's lymphoma (BL) cells were assayed for their ability to induce apoptosis when similarly overexpressed. The same genes were analysed in transient transfection assays for their ability to transactivate appropriate reporter plasmids. A correlation between the ability of p53 to transactivate and induce apoptosis was revealed. The only mutant capable of transactivation also induced apoptosis. Further analysis of the BL lines in which p53 had been characterized showed that whereas some lines were essentially resistant to cisplatin, three were rapidly induced to undergo apoptosis. All three have a single p53 allele encoding a mutant which is incapable of transactivation or (for two tested) mediating apoptosis when expressed in an LCL. Cell cycle analysis revealed that this apparently p53-independent apoptosis did not follow G1 arrest but in fact occurred largely in cells distributed in the G2/M phase of the cell cycle. These data suggest the existence of a second checkpoint in the G2 or M phase which, in the absence of a functional p53, is the primary point of entry into the apoptosis programme following DNA damage.     

Karyotypic abnormalities associated with haemopoietic lineage switching are not linked with mutations to p53

Leukemic cells can undergo lineage switching to display the phenotypic features of another haemopoietic pathway, as exemplified by B lymphoma and erythroleukemic cell lines generating variants with a monocytic appearance. Unlike the diploid parental lines, the vast majority of myeloid derivative lines examined (12 of 13 lines) were aneuploid. As p53 is involved in the maintenance of chromosomal stability, we investigated the role of p53 in the emergence of abnormal karyotypes in cells which had undergone lineage switching. Single strand conformation polymorphism and sequence analysis of cDNA, together with protein immunoprecipitations, were used to assess the p53 status of parental and variant cell lines. Unexpectedly, four or five monocytic lines with chromosomal alterations contained wild type p53. Conversely, a p53 point mutation found in one aneuploid monocytic line was also present in the diploid parental pre-B cell. These results provide strong evidence that mechanisms other than p53 mutations are responsible for karyotypic abnormalities seen in cells that have undergone lineage switching.     

Reconstitution of p53 expression in a nonproducer Ab-MuLV-transformed cell line by transfection of a functional p53 gene

L12 are Ab-MuLV-transformed cells that express the abl p120 oncogene product but lack the cellularly encoded p53. The functional p53 gene in these cells has been inactivated by the insertion of Moloney virus-like sequences into the first p53 intron. Transfection of L12 cells with a functional p53 gene, contained in a 16 kb Eco RI genomic cloned fragment gave rise to L12-derived cell lines with novel p53 sequences of various sizes and copy number. A high percentage of L12-derived clones efficiently transcribed p53 mRNA and synthesized the p53 protein. Whereas injection of L12 parental cells into syngeneic mice caused the development of local tumors that later regressed, L12-derived clones that expressed p53 caused lethal tumors in syngeneic mice, thus behaving similarly to other Ab-MuLV-transformed p53-producer cell lines. These results suggest that the expression of p53 is essential for tumor cells to exhibit a fully transformed phenotype, manifested in lethal tumors in syngeneic mice.     

Characterization of CD8+ cytotoxic T lymphocyte/tumor cell interactions reflecting recognition of an endogenously expressed murine wild-type p53 determinant

p53 mutations are frequently found in human cancers and are often associated with the overexpression of wild-type (WT) protein or peptide sequences, supporting the notion that WT p53 epitopes may serve as potential targets for tumor immunotherapy. We have developed a cytotoxic T lymphocyte (CTL)/p53 tumor-associated antigen (TAA) model, based on immune recognition of a WT p53 determinant. WT p53-peptide-specific, major histocompatibility complex (MHC) classI-restricted CTL were produced from immunocompetent C57BL/6 (H-2b) mice after immunization with a previously defined WT p53 peptide (p53(232-240)) Epitope-specific CTL were then employed to identify syngeneic tumor cell populations expressing that antigenic determinant. Two syngeneic tumor cell lines, MC38 colon carcinoma and MC57G fibrosarcoma, were demonstrated to express the endogenous WT p53(232-240) determinant naturally, as defined by CD8 + CTL recognition. Cold-target inhibition assays confirmed that CTL-mediated lysis was due to immune recognition of the p53(232-240) peptide epitope. The p53(232-240)-specific CTL line did not lyse syngeneic normal cells (i.e., mitogen-activated splenocytes) in the absence of exogenous peptide, suggesting that the WT-p53-specific CTL could distinguish between tumor cells expressing self-TAA and normal host cells. We have demonstrated, for the first time, that the adoptive transfer of WT-p53-specific CTL to mice with established pulmonary metastasis resulted in antitumor activity in vivo. The ability to generate MHC-class-I-restricted CD8- CTL lines specific for a non-mutated p53 determinant from normal, immunocompetent mice, which display antitumor activity both in vitro and in vivo (by adoptive transfer), may have implications for the immunotherapy of certain p53-expressing malignancies.     

Tumor suppressor p53 restricts Ras stimulation of RhoA and cancer cell motility

Many features of the cancer cell phenotype emerge as a result of cooperation between multiple oncogenic mutations. Here we show that activated Ras(V12) and loss of p53 function can cooperate to promote cell motility, a feature closely associated with cancer progression to malignancy. Our analysis indicates that Ras(V12) and loss of p53 synergistically induce RhoA activity, revealing a previously unknown role for p53 in tumor suppression. p53 prevents activation of RhoA and thus induction of cell motility by Ras(V12) through a simple signaling circuit, which integrates multiple inputs that converge on RhoA. Our data suggest that p53 suppresses cancer progression to malignancy by modulating the quality of Ras signaling.     

Modulation of the level of expression of cellular genes in adenovirus 12-infected and transformed human cells.

The level of expression of thymidine kinase (TK), heat shock protein 70 (HSP70), beta-tubulin and p53 was assessed in human embryo kidney cells (HEKs) infected with adenovirus type 12 (Ad 12) and Ad 12 early region 1 (E1) mutants. HSP70, beta-tubulin and p53 levels were unchanged but TK activity was dramatically increased following wild-type infection. The initial activation of TK required the expression of the product of the E1A 13S mRNA but sustained expression only occurred with those viruses expressing the E1B proteins as well. A number of human cell lines transformed with either Ad 12 or Ad 5 E1 DNA were also assessed for the level of expression of HSP70, beta-tubulin and p53. Both HSP70 and beta-tubulin levels were greatly increased compared with primary human cells although there was considerable variation between lines. p53 was only expressed at high levels in Ad 12-transformed lines expressing E1A and E1B proteins.     

Human melanoma cell line UV responses show independency of p53 function.

UV radiation-induced mutation of the p53 gene is suggested as a causative event in skin cancer, including melanoma. We have analyzed here p53 mutations in melanoma cell lines and studied its stabilization, DNA-binding activity, and target gene activation by UVC. p53 was mutated in three of seven melanoma cell lines. However, high levels of p53 were detected in all cell lines, including melanoma cells with wild-type p53, with the exception of one line with a truncated form. Upon UV induction, p53 accumulated in lines with wild-type p53, and p53 target genes p21Cip1/Waf1, GADD45, and mdm2 were induced, but the induction of p21Cip1/Waf1 was significantly delayed as compared with the increase in p53 DNA-binding activity. However, despite p53 target gene induction, p53 DNA-binding activity was absent in one melanoma line with wild-type p53, and p53 target genes were induced also in cells with mutant p53. In response to UV, DNA replication ceased in all cell lines, and apoptosis ensued in four lines independently of p53 but correlated with high induction of GADD45. The results suggest that in melanoma, several p53 regulatory steps are dislodged; its basal expression is high, its activation in response to UV damage is diminished, and the regulation of its target genes p21Cip1/Waf1 and GADD45 are dissociated from p53 regulation.     

Characterization of newly established colorectal cancer cell lines: correlation between cytogenetic abnormalities and allelic deletions associated with multistep tumorigenesis

We have established a series of 20 colorectal cancer cell lines and performed cytogenetic and RFLP analyses to show that the recurrent genetic abnormalities of chromosomes 1, 5, 17 and 18 associated with multistep tumorigenesis in colorectal cancer, and frequently detected as recurrent abnormalities in primary tumours, are also retained in long-term established cell lines. Earlier studies by us and other investigators showed that allelic losses of chromosomes 1 and 17 in primary colorectal cancers predicted poorer survival for the patients (P = 0.03). We utilized the cell lines to identify specific chromosomal sites or gene(s) on chromosomes 1 and 17 which confer more aggressive phenotype. Cytogenetic deletions of chromosome 1p were detected in 14 out of the 20 (70%) cell lines, whereas allelic deletions for 1p using polymorphic markers were detected in 13 out of 18 (72%) informative cell lines for at least one polymorphic marker. We have performed Northern blotting, immunohistochemical staining (p53 mRNA, protein) and RFLP analysis using several probes including p53 and nm23. RFLP analysis using a total of seven polymorphic markers located on 17p and 17q arms showed allelic losses aroundthe p53 locus in 16 out of the 20 cell lines (80%), four of which were losses of thep53 locus itself. In addition, seven cell lines (out of nine informative cases) also showed losses of thenm23 gene, four with concurrent losses of thep53 locus, while the remaining three were homozygous. In addition, five out of seven cell lines withnm23 deletions were derived from hepatic metastatic tumours, and one cell line was obtained from recurrent tumour. A comparison between allelic deletions of 1p and functional loss ofnm23 gene revealed a close association between these two events in cell lines derived from hepatic metastasis. Following immunohistochemical staining, nine out of the twenty cell lines showed high levels (25–80%) of mutant p53, four showed intermediate levels (>20%), and seven had undetectable levels of the protein. Of these seven, four showed complete absence of mRNA. Of the remaining three cell lines one showed aberrant mRNA due to germline rearrangement of thep53 gene, whereas in two cell lines normal levels of mRNA were present. Nineteen of the 20 cell lines had normal germline configurations for thep53 gene, while one showed a rearrangement. These data suggest that functional loss ofp53 andnm23 genes accomplished by a variety of mechanisms may be associated with poor prognosis and survival. In addition, concurrent deletions of chromosome regions 17p, 17q and 1p were closely associated with high-stage hepatic metastatic disease. These cell lines with well-characterized genetic alterations and known clinical history provide an invaluable source of material for various biological and clinical studies relating to multistep colorectal tumorigenesis.     

Mutant p53 tumor suppressor alleles release ras-induced cell cycle growth arrest.

Overexpression of an activated ras gene in the rat embryo fibroblast line REF52 results in growth arrest at either the G1/S or G2/M boundary of the cell cycle. Both the DNA tumor virus proteins simian virus 40 large T antigen and adenovirus 5 E1a are able to rescue ras induced lethality and cooperate with ras to fully transform REF52 cells. In this report, we present evidence that the wild-type activity of the tumor suppressor gene p53 is involved in the negative growth regulation of this model system. p53 genes encoding either a p53Val-135 or p53Pro-193 mutation express a highly stable p53 protein with a conformation-dependent loss of wild-type activity and the ability to eliminate any endogenous wild-type p53 activity in a dominant negative manner. In cotransfection assays, these mutant p53 genes are able to rescue REF52 cells from ras-induced growth arrest, resulting in established cell lines which express elevated levels of the ras oncoprotein and show morphological transformation. Full transformation, as assayed by tumor formation in nude mice, is found only in the p53Pro-193-plus-ras transfectants. These cells express higher levels of the ras protein than do the p53Val-135-plus-ras-transfected cells. Transfection of REF52 cells with ras alone or a full-length genomic wild-type p53 plus ras results in growth arrest and lethality. Therefore, the selective event for p53 inactivation or loss during tumor progression may be to overcome a cell cycle restriction induced by oncogene overexpression (ras). These results suggest that a normal function of p53 may be to mediate negative growth regulation in response to ras or other proliferative inducing signals.     

p53-degradation by HPV-16 E6 preferentially affects the removal of cyclobutane pyrimidine dimers from non-transcribed strand and sensitizes mammary epithelial cells to UV-irradiation

Nucleotide excision repair (NER), the most versatile and ubiquitous mechanism for DNA repair, operates to remove many types of DNA base lesions. We have studied the role of p53 function in modulating the repair of DNA damage following UV irradiation in normal and p53-compromised human mammary epithelial cells (HMEC). The effect of UV-induced DNA damage on cellular cytotoxicity and apoptosis was determined in conjunction with global, gene- and strand-specific repair. Cytotoxicity studies, using clonogenic survival and MTT assays, showed that HPV-16 E6-expressing HMEC were more UV sensitive than p53-WT cell lines. High apoptotic index obtained with p53-compromised cells was in conformity to both the low clonogenic survival and the low cellular viability. No discernible differences in the formation of initial UV-induced cyclobutane pyrimidine dimers (CPD) were observed in the cell lines of varying p53 functional status. However, the extent and the rate of damage removal from genome overall were highest for p53-WT cells. Further examination of strand-specific repair in the p53 gene revealed that the removal of CPD in the non-transcribed strand (NTS) was slower in p53-compromised cells compared to the normal p53-WT cell lines. These results suggest that loss of p53 function, in the absence of other genetic alterations, decreased both overall amount of CPD repaired and their removal rate from the genome. Additionally, normal function of p53 is required for the repair of the NTS, but not of the transcribed strand (TS) in genomic DNA in human epithelial cells. Thus, failure of quantitative removal of CPD by global genomic repair (GGR), due to loss of p53 function, causes the enhanced UV sensitivity and increased damage-induced apoptosis via a p53-independent pathway. Nevertheless, recovery of cells from UV damage requires normal p53 function and efficient GGR.