Autocrine growth and progression of murine X-ray-induced T cell lymphomas

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.

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.

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

We investigated the mechanism of radiation induction of murine thymic lymphomas by studying the characteristics of primary x-ray-induced thymic lymphoma (PXTL) cell lines and of their oncogene-induced, progressed progeny. It is widely thought that proto-oncogene alterations are associated with the induction of murine lymphomas; however, few, if any primary murine radiation-induced lymphomas possess (proto-)oncogene alterations. Independently derived cell lines grown directly (i.e., without in vivo transplantation) from thymic lymphomas of irradiated C57BL/6 mice possess the properties of immortalized pre-T cells and lack many of the characteristics of "tumor cells". PXTL cells are poorly tumorigenic upon transplantation, do not clone in methylcellulose cultures, are growth factor dependent and autocrine, and lack consistent chromosome and oncogene abnormalities. However, the thymic lymphomas are malignant and cause the death of each afflicted mouse. PXTL cells expressed two immunologically distinct forms of the tumor suppressor protein p53 that have moderately increased stability (t1/2 = 1 h) when compared with p53 of normal splenic T lymphocytes. Early PXTL cells could progress in vitro to a fully tumorigenic phenotype after infection with retroviruses encoding the c-myc and v-ras oncogenes. Progressed T-lymphoma cells acquired growth factor independence, a highly transplantable and tumorigenic phenotype, and the ability to quantitatively clone in methylcellulose cultures. Progressed lymphoma cells coordinately downregulated the expression of five T-cell differentiation markers, upregulated the expression of CD44 (Pgp-1), and harbored vastly elevated levels of two immunologically distinct forms of p53. Our results suggest that the early thymic lymphomas consist of differentiation-inhibited, immortal pre-T cells that are precursors to progressed, fully tumorigenic T-lymphoma cells.

Effects of fractionated x-irradiation on the Ly-6--Ril-1--Pol-5 region

Our laboratory has focused on defining, localizing, and understanding the mode of action of genes involved in fractionated x-irradiation (FXI) leukemia in susceptible and restraint mouse strains. We have described the genetic and molecular evidence suggesting the existence of multiple independent loci involved in FXI-induced leukemogenesis. These studies indicated that one of these, Ril-1, a locus on the distal portion of chromosome 15, is the major locus influencing susceptibility to the disease. Our data unequivocally place Ril-1 in the gene complex Ly-6--Ril-1--Sis--H-30--Pol-5. Ril-1 appears to be closest to Ly-6 and Sis. We report that in FXI-induced leukemias there are hypomethylation changes in the Ly-6 region as compared to normal thymocytes. In contrast, Sis was found to be hypermethylated and not expressed. In addition, we have noted DNA rearrangements in the Ly-6--Pol-5 region in the majority of tumors examined using the Ly-6 and spleen focus-forming virus (SFFLV) molecular probes. Increased expression of Ly-6 and other surface markers encoded in this region has been noted in FXI-induced thymomas.

Assignment of the Ly-6--Ril-1--Sis--H-30--Pol-5/Xmmv-72--Ins-3--Krt-1--Int-1 --Gdc-1 region to mouse chromosome 15

Previous work has demonstrated linkage between Ly-6, H-30, and a locus, Ril-1, that affects susceptibility to radiation-induced leukemia. Results or preliminary linkage analyses suggested further that the cluster might be linked to Ly-11 on the proximal portion of mouse chromosome 2. Using molecular probes to examine somatic cell lines and recombinant inbred and congenic strains of mice, we have re-evaluated these linkage relationships. A cloned genomic DNA fragment derived from a retroviral site has been used to define a novel locus, Pol-5, that is tightly linked to both H-30 and Ril-1 as shown by analysis of the B6.C-H-30c congenic mouse strain. Following the segregation of the Pol-5 mouse-specific DNA fragment in a series of somatic cell hybrids carrying various combinations of mouse chromosomes on a rat or Chinese hamster background mapped Pol-5 to mouse chromosome 15. During the course of these studies, restriction fragment length polymorphisms were defined associated with several loci, including Pol-5, Ly-6, Sis, Ins-3, Krt-1, Int-1, and Gdc-1. Three of these loci, Sis, Int-1, and Gdc-1, have been previously mapped to chromosome 15 by others using somatic cell hybrids or isoenzyme analyses. Following the inheritance of these eight loci in recombinant inbred strains of mice allowed the definition of a linkage group on the chromosome with the order Ly-6--Ril-1--Sis--H-30--Pol-5--Ins-3--Krt-1--Int-1--Gdc-1. Analyses of alleles inherited as passengers in B6.C-H-30c, C3H.B-Ly-6b, and C57BL/6By-Eh/+ congenic mouse strains and in situ hybridization experiments support the above gene order and indicate further that the cluster is located on distal chromosome 15, with Ly-6 and Sis near Eh.

Transfection of a factor-dependent cell line with the murine interleukin-3 (IL-3) cDNA results in autonomous growth and tumorigenesis

The factor dependent murine myeloid line 32D c15 was transfected by electroporation with a murine interleukin-3 (IL-3) cDNA expression plasmid bearing the murine metallothionein-I promoter element. Factor-independent cell growth was readily obtained, and was shown to be accompanied by the production of biologically active IL-3. Three cell lines, growing autonomously and secreting IL-3 activity into their supernatants were established. S-1 nuclease analysis was employed to demonstrate that the introduced plasmid and not the endogenous IL-3 gene was the source of the IL-3 in one of these lines. The transfected IL-3 secreting cell lines, but not the parental factor-dependent 32D c15 cells, were uniformly able to induce tumors in syngeneic mice. These results indicate that the conversion to a malignant phenotype of a "partially transformed" cell line may be achieved by one additional dominant genetic event, such as the acquisition of autocrine growth factor secretion.