Association of induction of a fully tumorigenic phenotype in murine radiation-induced T-lymphoma cells with loss of differentiation antigens, gain of CD44, and alterations in p53 protein levels

IRES-Mediated Protein Translation Overcomes Suppression by the p14ARF Tumor Suppressor Protein

Internal ribosome entry sites (IRES elements) have attracted interest in cancer gene therapy because they can be used in the design of gene transfer vectors that provide bicistronic co-expression of two transgene products under the control of a single promoter. Unlike cellular translation of most mRNAs, a process that requires a post-translational 5' modification of the mRNA known as the cap structure, IRES-mediated translation is independent of the cap structure. The cellular conditions that may intervene to modulate IRES-mediated, cap-independent versus cap-dependent translation, however, remain poorly understood, although they could be critical to the choice of gene transfer vectors. Here we have compared the effects of the p14ARF (Alternate Reading Frame) tumor suppressor, a translational suppressor frequently overexpressed in cancer, on cap-dependent translation versus cap-independent translation from the EMCV viral IRES often used in bicistronic gene transfer vectors. We find that ectopic overexpression of p14ARF suppresses endogenous and ectopic cap-dependent protein translation, consistent with other studies. However, p14ARF has little or no effect on transgene translation initiated within an IRES element. This suggests that transgenes placed downstream of an IRES element will retain efficient translation of their gene products in the presence of high levels of ectopic or endogenous p14ARF, a finding that could be particularly relevant to therapeutic gene therapy strategies for cancer.

Inhibition of T-cell acute lymphoblastic leukemia proliferation in vivo by re-expression of the p16INK4a tumor suppressor gene

T-cell acute lymphoblastic leukemia (T-ALL) is characterized by the presence of differentiation-inhibited pro- and pre-T-cell blasts. The p16INK4a tumor suppressor gene has been shown to be frequently deleted in human T-ALL cases. Deletion of p16INK4a may be associated with poor prognosis and relapse of the disease. Radiation-induced murine T-ALL in C57B1/6 mice shares pathogenetic and molecular characteristics with the human disease. We used the murine disease as a model to study the status of the INK4/ARF gene locus and to examine the effect of p16INK4a-re-expression in T-ALL cells on their leukemic potential in vivo. In 9 of 17 radiation-induced murine T-ALL cell lines, the p16INK4a protein was not expressed as determined by immunoblotting. Southern blot analysis revealed homozygous deletions of the p16INK4a gene locus in three of the nine lines, along with the genes encoding p15INK4b and p19ARF. Transduction of p16INK4a-negative T-ALL lines with retrovirus encoding p16INK4a significantly inhibited their in vitro proliferation by inducing G1-arrest. Importantly, re-expression of p16INK4a in p16INK4a-negative T-ALL cells obliterated the induction of lethal disseminated leukemia in syngeneic mice. This is the first demonstration that re-establishment of p16INK4a expression is critical for in vivo growth regulation of T-ALL cells.

A new immunobiological view of radiation-promoted lymphomagenesis

Whole-body irradiation produces T-cell leukaemias/ lymphomas (TCL) in some strains of inbred mice in an X-ray dose-related manner. Radiation biologists have related the rapid "initiation' and early appearance of preleukaemic cells in these mice to unrepaired DNA damage inflicted by radiation. Following initiation, radiation-altered thymic differentiation fosters multi-step transformation changes in proto-oncogenes and suppressor gene expression in individual clones of non-invasive preleukaemia cells as they progress to malignancy. The malignant clones arising from small numbers of initiated preleukaemia thymocytes become fully transformed only after several more months to a year after irradiation in those strains of mice which develop T-cell lymphomas. When the RFM mouse was subjected to sublethal whole-body X-ray, only 50% of the mice developed TCL by 6 months, yet nearly all developed preleukaemia thymocytes. The T-cell-mediated immune response of the irradiated host has never been substantiated to contribute to malignant TCL development. Until recently, X-ray-induced TCL were not known to carry common tumour rejection antigens TATA. However, several studies have revealed that both preleukaemia cells and fully malignant TCL express an immunogenic, common oncofoetal glycoprotein, termed 44kD OFA. OFA-activated memory CD4 Tm and CD8 Ten. T-effector cells in irradiated mice expressing OFA. As most irradiated RFM mice exhibit preleukaemia thymocytes yet only half develop tumours, this finding implicates active host T-cell effector responses in X-ray-initiated tumorigenesis. Further, the recent discovery of OFA-specific CD8 Ts clones in irradiated mice, which inhibited cytotoxicity of CD8 clones to OFA or TSTA, may explain which mice develop T-cell lymphomas.

CD44: physiological expression of distinct isoforms as evidence for organ-specific metastasis formation

Continuous progress has been achieved during recent decades in the therapy of metastasizing malignancies by improving chemotherapeutic strategies and new approaches in radiation therapy. Genetic manipulation of tumor cells and of the tumor fighting immune system is hoped to add significant contributions to curative interventions in disseminated tumors. That we are still far from eradicating death by malignant growth is due ultimately to our limited understanding of the cascade of events resulting in metastasis formation, which until recently was believed to rely on multiple rounds of mutation and selection processes. This implies an individually specific history of each metastatic tumor, which would rule out uniform diagnostic and therapeutic concepts. When it was noted in a rat tumor model that the transfer of cDNA of a single gene, a CD44 variant isoform (CD44v) covering the exons v4-v7, sufficed to initiate metastasis formation of a locally growing tumor, hope was created that a "metastogene" may have been identified. Although the idea of CD44v expression as a unifying concept for tumor progression was not sustained, the discovery of CD44v-initiated metastatic spread allowed a conceptually new hypothesis on tumor progression as a consequence of the reactivation of genetic programs of ontogeny, stem cell differentiation, and/or lymphocyte activation. Since distinct CD44 isoforms play an important role in these processes, unraveling the functions of this family of molecules can indeed provide a cornerstone in the understanding of tumor progression. This article summarizes briefly the present knowledge on known functions of CD44 isoforms with particular focus on parallels between physiological programs and tumor progression.

Dissemination and tissue invasiveness in murine acute leukemia associated with acquisition of p53 mutation and loss of wild-type p53

Approximately 60% of mice treated with split-dose radiation develop leukemias that disseminate widely through the body, whereas 40% of the treated mice incur leukemias that are contained entirely within the thymus. We studied the status of p53 in non-cultured samples of thymic leukemias and in cell lines established from these leukemias. In those mice with disseminated disease, primary samples were also obtained from visceral leukemic organs, and cell lines were established from these leukemic organs for further study. Using single-strand conformation polymorphism (SSCP), nucleic acid sequencing, and immunochemical analysis, we found that mutation of both p53 alleles occurred in leukemic cell lines developed from nine of 10 disseminated leukemias; mutation of one p53 allele with the other remaining wild-type occurred in one disseminated leukemia. A p53 mutation unique for each mouse was found in all cell lines established from the different leukemic organs of each mouse. The same mutation was also found in the non-cultured leukemic tissues of each mouse, indicating that the mutations originated in vivo and were clonal. Seven of seven non-disseminating thymomas possessed wild-type p53 only. Hence, in vivo dissemination and tissue invasiveness were associated with the loss of wild-type p53 by mutation of both alleles or by mutation and loss of heterozygosity, as revealed by studies of cell lines established from them. The selective in vivo dissemination of leukemia cells possessing p53 mutations had a parallel in vitro. Leukemia cell lines from mice harboring disseminating leukemia were established more readily (success rate greater than 80%) than lines from mice harboring thymic nondisseminating leukemia (success rate less than 10%). Additionally, while mice with disseminating leukemia harbored a mixture of wild-type and mutant p53-encoding thymoma cells, only cell lines possessing mutant p53 became established in culture. Mutations found in thymoma cell lines were always detectable by SSCP and sequencing of DNA extracted from non-cultured thymoma tissue. However, in non-cultured leukemic tissue of visceral organs, the clonal p53 mutations found in cell lines established from them were often not detectable by SSCP or sequencing but were detectable by immunochemical analysis or polymerase chain reaction amplification. This indicates an unexpected degree of masking of mutant genes by wild-type genes present in the leukemic tissue. Masking was evident even in leukemic organs that were grossly larger than normal organs. Hence, routine screening of leukemic tissue by SSCP and sequencing may result in a highly significant underestimation of the incidence of p53 mutations.(ABSTRACT TRUNCATED AT 400 WORDS)

Elevated levels of wild-type p53 induced by radiolabeling of cells leads to apoptosis or sustained growth arrest

BACKGROUND

The tumor suppressor protein p53 regulates progression through the checkpoint between the G1 and S phases of the cell cycle in response to radiation- or drug-induced DNA damage. We have examined potential p53-mediated effects of metabolically labeling cultured mammalian cells with [35S]methionine and [3H]thymidine, methods that are commonly used to study the biochemical properties, synthesis, processing and degradation of proteins and the replication of DNA in proliferating cells.

RESULTS

Wild-type p53 protein concentrations rapidly increased to high levels following metabolic radiolabeling of cells, as determined by four distinct assays. The increased concentration of wild-type p53 resulted in apoptosis of normal human peripheral blood lymphocytes and of murine T-cell acute lymphoblastic leukemia cells. In leukemia cells containing no p53, or only mutant p53 alleles, p53 protein levels were not increased and the cells did not undergo apoptosis in response to radioactive labeling. Radiolabeling of human diploid fibroblasts resulted in a prolonged growth arrest that was maintained for nearly three weeks.

CONCLUSIONS

The results of experiments employing radiolabeling techniques to characterize various aspects of cellular physiology may be seriously influenced by the induction of aberrant cell-cycle arrest and/or apoptosis mediated by wild-type p53. Furthermore, our observations suggest that stabilization of wild-type p53 in response to irradiation may not act primarily to facilitate the repair of DNA damage by inducing a transient G1-phase arrest, but rather to ensure genetic stability through sustained cell-cycle arrest or apoptotic death of the damaged cells.

Radiation response of mouse lymphoid and myeloid cell lines. Part I. Sensitivity to killing by ionizing radiation, rate of loss of viability, and cell type of origin

The sensitivity of 10 mouse lymphoid or myeloid cell lines to gamma-ray- and DNA-associated 125I-decay-induced clonogenic cell killing have been compared with their rate of loss of viability (membrane integrity) and with their putative cell type of origin. The pseudodiploid haematopoietic cell lines showed D0 values for 125I-induced DNA double-strand breakage (dsb) that ranged from 7.7 +/- 0.7 to 40.8 +/- 2.8 decays. These lines generally appeared to be more sensitive to killing by radiation-induced DNA dsb than are fibroblast-like cell lines. The increased sensitivity of haematopoietic cell lines to killing by DNA dsb may be related to their mode of death (apoptosis versus necrosis). Mode of cell death may thus be an important factor in determining the 'inherent radiosensitivity' of normal cells/tissues. Haematopoietic cell lines that undergo rapid interphase apoptotic death showed extreme sensitivity to DNA dsb. The latter cell lines were found to have derived from immature lymphoid cells, and it is speculated that their high radiosensitivity might reflect the action of a mechanism that normally eliminates cells containing illegitimate V(D)J recombinase-induced DNA dsb.

p53 protein overexpression identifies a group of central primitive neuroectodermal tumours with poor prognosis

Primitive neuroectodermal tumours (PNET's) or medulloblastomas are common primary brain tumours of childhood. Current treatment protocols achieve 50-60% cures. However, it has proved difficult to develop better treatment for the remaining patients because prognostic factors are not established. We have investigated the prognostic value of p53 protein expression in 87 PNET's using immunohistochemistry with DO-7 and CM-1 antibodies on biopsy paraffin sections. Eight patients (9%) had intensely reactive tumour cell nuclei, and a significantly reduced survival (P = 0.002); only one survives and this with a recurrent tumour 50 months following diagnosis. Sixty eight per cent of patients had faintly reactive tumour cell nuclei, a reduced survival up to 4 years but a long term survival not significantly different (P = 0.41) from 23% of patients with p53 negative PNET's; the 10 year survival rates were 37% and 40%, respectively. Males had a reduced survival (P = 0.04) with a 2-fold relative risk of death compared to females. Multivariate analysis showed that intense overexpression of p53 protein identifies a group of PNET patients with a 7-fold relative risk of death compared to all other cases, irrespective of sex. This marked difference suggests the involvement of p53 in the pathogenesis of PNET's which have a particularly poor response to treatment, and should help to develop new therapies for this group of patients.