Studies on the mechanisms of chemical leukaemogenesis

N-Ethyl-n-Nitrosourea Induced Leukaemia in a Mouse Model through Upregulation of Vascular Endothelial Growth Factor and Evading Apoptosis

Chemical carcinogens are commonly used to investigate the biology and prognoses of various cancers. This study investigated the mechanism of leukaemogenic effects of n-ethyl-n-nitrosourea (ENU) in a mouse model. A total of 14 3-week-old male Institute of Cancer Research (ICR)-mice were used for the study. The mice were divided into groups A and B with seven mice each. Group A served as the control while group B received intraperitoneal (IP) injections of 80 mg/kg ENU twice with a one-week interval and were monitored monthly for 3 months for the development of leukaemia via blood smear examination. The mice were sacrificed humanely using a CO₂ chamber. Blood, spleen, lymph nodes, liver, kidney and lung samples were collected for blood smear examination and histopathological evaluation. The expression of angiogenic protein (VEGF), and pro and anti-apoptotic proteins (BCL2 and BAX), was detected and quantified using Western blot technique. Leukaemia was confirmed by the presence of numerous blast cells in the peripheral blood smear in group B. Similarly, the VEGF and BCL2 proteins were significantly (p < 0.05) upregulated in group B compared to A. It is concluded that IP administration of 80 mg/kg ENU induced leukaemia in ICR-mice 12 weeks post administration through upregulation of angiogenic and anti-apoptotic proteins: VEGF and BCL2.

Induction of stem cell cycling in mice increases their sensitivity to a chemical leukaemogen: implications for inherited genomic instability and the bystander effect

Preconception paternal irradiation (PPI) modifies haemopoietic and stromal tissues of offspring and increases risk of generating lympho-haemopopietic malignancy if those offspring are then exposed to a leukaemogen. We hypothesised that this increased risk was related to inherited damage which had caused increased stem cell proliferation rates. To test for this link, in vivo, rapid stem cell proliferation was established by giving sub-lethal irradiation (3Gy gamma-rays) and allowing 3 days recovery. At this stage, 60% of haemopoietic spleen colony-forming units (CFU-S) were in DNA-synthesis, compared to <10% in unirradiated controls. Two groups of mice, unirradiated controls and irradiated animals, were then injected with 50mg/kg methyl nitrosourea (MNU) and observed daily for onset of lympho-haemopoietic malignancy. In a further control group of 60 mice, irradiated but not injected with MNU, only one leukaemia developed. In unirradiated controls, 20% of the mice developed malignancies between 3 and 8 months later: in the irradiated, MNU-treated groups, 95% developed malignancies between 2 and 7 months later. Thus, at least one powerful potentiating mechanism for induction of lympho-haemopoietc malignancy following inherited damage can be related to haemopoietic stem cell proliferation. Genomic instability is exposed by cell proliferation and has been implicated in this type of damage. However, a regulatory stromal microenvironment plays a part in inducing that proliferation. Thus, the microenvironment is the effective "bystander" which is thought to promote and amplify genomic instability, and thereby influence the induction of malignancy both in PPI offspring and in mice with induced stem cell proliferation.

Transgenerational effects of preconception paternal contamination with (55)Fe

The conjecture that germline mutations induced by radiation exposure before conception may predispose subsequent offspring to cancer remains contentious. Previous experimental studies have shown that preconception paternal irradiation with (239)Pu induces perturbations in the hemopoietic systems of offspring and influences sensitivity to a secondary carcinogen. In the present study, male DBA2 mice were injected intravenously with the Auger electron emitter (55)Fe (4 kBq g(-1)) 18 or 84 days before mating with normal females. Comet analysis showed an increased incidence of DNA strand breaks in sperm from contaminated animals after 84 days, but not after 18 days, indicating spermatogonial rather than spermatid damage. Offspring were either assayed for changes in bone marrow stem cells and committed progenitors or challenged with the chemical carcinogen methyl nitrosourea (MNU, 50 mg/kg) at 10 weeks of age and monitored for the onset of malignancy. Offspring from irradiated fathers had normal peripheral blood profiles, although the stem cell population was amplified in offspring arising from those exposed to (55)Fe at 84 days before conception. Exposure to MNU significantly increased the incidence of lympho-hemopoietic malignancies in offspring from the 84-day group, but not in those from the 18-day group. These findings support the hypothesis that aberrations that are potentially leukemogenic may be transmitted to offspring after radiation damage to the paternal germline.

Induction of lympho-haemopoietic malignancy: impact of preconception paternal irradiation

PURPOSE

To investigate the effects of preconception paternal irradiation (PPI) from injected 239Pu on the susceptibility to induction of lympho-haemopoietic malignancy by subsequent irradiation or exposure to a chemical carcinogen.

MATERIALS AND METHODS

The male CBA/H and DBA2 mouse was injected with 0, 128 or 256 Bqg(-1) 239Pu 12 weeks before mating with the normal CBA/H and C57B1 female respectively. CBA/H offspring were exposed to 3.3 Gy gamma-rays total body irradiation: BDF1 offspring were injected with 50 mg kg(-1) methyl nitrosourea (MNU). The offspring were assayed for changes in bone marrow progenitor cell numbers and chromosome aberrations and were followed up for subsequent induction of neoplasia.

RESULTS

While the untreated mouse showed a normal distribution for cellularity, spleen colony-forming units (CFU-S) and fibroblastoid colony-forming units (CFU-F), significant numbers of PPI offspring presented levels outside the normal range. There was a tendency for them also to show increased, dose-related, levels of chromosomal aberrations. Offspring treated with irradiation or MNU developed an increased incidence of lympho-haemopoietic malignancies.

CONCLUSIONS

These studies have shown that PPI results in offspring that are more susceptible to the induction of lymphohaemopoietic malignancy on encountering a secondary carcinogenic insult. This may be linked to inherited chromosomal instability and abnormal kinetics of haemopoiesis. The experiments indicate a potential mechanism by which an increased incidence of leukaemia may be linked to PPI.

Trisomy 15 as a regular finding in chemically induced murine T-cell leukemogenesis

Trisomy 15 is described as a common finding in all T-cell leukemias induced by a single dose of methylnitrosourea (MNU) in BDF1 mice and in the leukemias induced by 7 doses of benzo(a)pyrene. Additional trisomies were found in about half of the leukemias. The organ distribution suggests that the leukemic cells with trisomy 15 originate in the thymus. Trisomy 15 was detected in the thymus as early as 6 weeks after the application of MNU, i.e. during the latency period.

Use of multiparameter analysis to quantitate hematological damage from exposure to a chemical (ethylene oxide)

This study was designed to test the value of a multiparameter approach in evaluating perturbations in bone marrow and peripheral blood elements of mice exposed to ethylene oxide (EtO). Mice exposed to 255 ppm EtO for 5 h/d were removed for analysis after 1, 2, 8, and 14 d (sequential exposure) and 4, 6, 8, and 10 wk (5 d/wk). Prior to sacrifice, blood was removed from the orbital sinus for blood cell counts, hemoglobin determination, and hematocrit. A blood film was made for differential leukocyte counts. Bone marrow was flushed from femurs and tibias and counted, and aliquot were used for stem-cell assay (CFU-S) or flow cytometry (FCM) analysis. One aliquot of marrow was stained with propidium iodide for cell-cycle analysis and another was reacted with fluorescein-conjugated monoclonal antibody for B-cell analysis. The preparations were analyzed for forward and 90 degrees scatter and fluorescence on an Ortho 50H cytofluorograph. Perturbations of peripheral leukocytes occurred after one exposure. After multiple exposures, hematocrit, red-cell number, and hemoglobin were generally depressed, with transient compensatory bursts, and bone marrow cellularity and CFU-S were below normal. However, white-cell numbers fluctuated dramatically during the exposure period. There was a shift in differential toward granulocytes, at times resulting in severely depressed numbers of lymphocytes in the peripheral blood. The FCM analysis showed an early depletion of granulocytes in the bone marrow followed by replacement and a relative lymphocyte deficit, especially pronounced at 10 wk. The B-cell changes reflected general lymphocyte perturbations. Shifts in numbers of cells in S and G/M were observed, consistent with a moderate bone marrow response to cell loss.

The sensitivity to methylnitrosourea-induced T-cell leukemogenesis in mice is modified by the injection of hydrocortisone

T-cell leukemias were induced in adult BDF1 mice by a single i.v. injection of methylnitrosourea (MNU). Leukemogenesis was delayed by a single or repeated injections of hydrocortisone (HC) after MNU and also when HC was given one day before MNU. Enhancement of leukemogenesis was seen in experiments with 10 and 14 days' intervals between HC and MNU. The T-cell subset composition of the thymus after HC treatment was studied at these time intervals, but a specific target cell for the action of MNU, reduced one day after HC and increased in number during the thymic regeneration at 10 and 14 days could not be defined. HC did not prohibit the toxic action of MNU as measured by hemopoietic stem cell numbers in the femur.

T-cell phenotypes in chemically induced leukemia in mice

T-cell leukemias were induced in adult BDF mice by a single i.v. injection of 50 mg kg-1 of methylnitrosourea (MNU). Leukemic cells from the thymus and other organ sites showed the theta antigen and were peanut-negative, but were heterogeneous with respect to Lyt-1 and Lyt-2. The acute cytotoxic effect of the MNU showed no preferential toxicity to a special T-cell subset in the thymus. During the latency period of leukemogenesis a continuous fall in peanut-positive cells in the thymus was found.

Leukemia induction by methylnitrosourea (MNU) in selected mouse strains. Effects of MNU on hemopoietic stem cells, the immune system and natural killer cells

T cell leukemias were induced by a single dose of methylnitrosourea (MNU) in DBA/2, C57/Bl/6, NMRI, BDF1, and CBA mice. The latency period in the CBA strain was much longer than in the others. Studies on the pluripotent stem cells in the bone marrow and T cell reactions of thymus and spleen cells showed a toxicity of MNU for these parameters but no significant differences between the strains. The activity of the natural killer (NK) cells in the spleen and peritoneal exudate cells, studied also after additional stimulation by injection of Corynebacterium parvum, was influenced by MNU, but again no relation to leukemogenesis could be established. The first leukemic (transplantable) cells were found in the thymus. The presence of leukemic cells could be responsible for low NK cell activities found in BDF1 and DBA/2 mice late after MNU.

Studies on natural killer cell activity and the influence of Corynebacterium parvum on murine T-cell leukemogenesis induced by butylnitrosourea

Butylnitrosourea (BNU) was used to induce thymic lymphomas in BDF1 mice. During and after the 12-week BNU exposure the spontaneous NK cell activity against YAC-1 cells and that arising 4 days after stimulation with Corynebacterium parvum (CP) were measured, as were the mitogen responses of splenic T and B cells. In addition to BNU, groups of mice received multiple injections of the interferon inducer CP during or after the BNU exposure period. The results show a slight impairment of the NK cell activity by BNU and also after the injections of CP depending heavily on the treatment protocol. After the multiple injections of CP, either into BNU-treated mice or into controls, no further stimulation by CP was possible. The mitogen responses, reduced after BNU, were further reduced after the additional treatment. Both effects can be explained by the known induction of suppressor cells by CP. Although these treatments had pronounced effects on the parameters tested in vitro there was no significant influence on the development of thymic lymphomas in vivo.

Residual toxicity in hematopoietic cells following a single dose of methylnitrosourea

The residual injury to the proliferation capability of hemopoietic stem cells (CFU-S) which results from their exposure to leukemogenic agents was evaluated in mice given a single leukemogenic dose of methyl nitrosourea (MNU 50 mg/kg body weight, i.v.). Bone marrow cellularity, splenic weight, number of CFU-S and the proportion of cycling to noncycling CFU-S were measured in an effort to detect and acute and residual injury to the CFU-S from mice given MNU 21 and 3 days earlier. Marrow cells were also transferred into lethally irradiated mice to observe the self-renewal capability of the CFU-S in the recipient spleen and bone marrow. The results of these measurements show that the CFU-S in marrow from mice given 50 mg/kg of MNU 21 days earlier still have a defective ability for self-renewal, although the total cellularity, number of CFU-S and proportion of cycling and noncycling CFU-S in the donor have returned to the normal range. The relationship of this self-renewal defect to the development of leukemia after this leukemogenic dose of MNU is not known.

Two cases of acute leukemia following treatment of malignant glioma

Two female patients, 42 and 30 years old, respectively, died of acute nonlymphocytic leukemia 43 and 38 months, respectively, after a subsequent treatment: chemotherapy for one and irradiation and chemotherapy for the other, following excision of a malignant glioma. At the time of death, both seemed to be in complete remission of their brain tumor. Both had been treated with procarbazine and nitrosoureas. The latter were responsible for severe myelosuppressive episodes and seem to have played an essential role in the induction of the leukemia. In one case, a myelodysplasia was observed before the onset of the AL and the diagnosis of refractory anemia with excess of blasts seemed warranted. Secondary acute leukemias are rare in the evolution of malignant gliomas and the usefulness of subsequent radiochemotherapy cannot be questioned at the present time. The risks involved in this therapy are minor when compared to the short-term fatal prognosis of this type of tumor.

Autoimmune haemolytic disease in mice after exposure to a methylating carcinogen

N-Methyl-N-nitrosourea (MNU) but not methyl methanesulphonate (MMS) induced both autoimmune haemolytic anaemia and thymic lymphoma in susceptible strains of mice, particularly the C57BL/6. These effects could be positively correlated with the formation of O6-methylguanine in target DNA. All murine lymphoid cells showed lack of ability to remove O6-methylguanine from their DNA, therefore the variation of responses between different mouse strains indicated that other host factors, probably genetic, must be involved. The results do indicate however that a potent pre-mutagenic DNA base modification can initiate the events leading to autoimmune disease in susceptible mice.

RNA oncovirus expression in N-methyl-n-nitrosourea-induced lymphomas in mice

RNA oncovirus expression was investigated in thymic lymphomas induced by N-methyl-N-nitrosourea (MNU) in (C57BL/6 x DBA/2)F1(BDF1) mice. Out of 15 newborn mice injected with 20 mg/kg MNU, five developed thymic tumours to T-cell origin. Virus production of thymic cells from tumorous and normal animals was studied by an infectious center assay. All five tumours released ecotropic (mouse tropic) virus. No xenotropic or dual-tropic mink cell focus-inducing virus was found. No virus was obtained from thymus cells taken from six normal mice killed between 83 and 124 days; however, an ecotropic virus was cultured from the thymus of a 134-day-old normal mouse. According to these results, the MNU-induced thymic lymphoma development in BDF1 mice is accompanied by activation of endogenous ecotropic virus, whereas spontaneous activation also occurs in older mice.

T- and B-cell determination in various lymphoid tissues of mice during N-nitrosobutylurea (NBU) leukemogenesis

T- and B-cell distribution in thymus, spleen, and lymph nodes has been investigated during chemical lymphomagenesis. N-nitrosobutylurea (0.04 g/100 ml) in drinking water was administered to 35-day-old male C57BL mice for a period of 60 days. As of week 11, 21 of 22 Nbu mice developed thymic lymphomas of lymphoblastic cell type which showed a general leukemic spread to spleen, lymph nodes, and several extralymphatic organs at week 16. Immunocytologic studies revealed the T-cell origin of these thymic type lymphomas. Alteration of the T-cell system during latency period was represented by reduced thymic weight, cell numbers, as well as absolute and relative T-cell reduction. Absolute T-cell reduction was also observed in the spleen, although the reduction of splenic lymphoid cells was mainly caused by a decrease of B cells. In lymph node subnormal variations of theta-positive cells took place, whereas B-cell distribution was not altered. An impaired T- and B-cell function was suspected from preleukemic changes of lymphoid subpopulations. The results are compared with other murine lymphomas and discussed with regard to a functional hypothesis of lymphomagenesis.

Haemopoiesis and immune status during BNU-induced leukaemogenesis in mice

Female BDF1 mice were exposed to 0.04% BNU in the drinking water for 15 weeks. All animals except one observed for more than 3 weeks died of leukaemia. After 1, 2, 3, 5, 7, 10, and 15 weeks of exposure marrow smears, pluripotent (CFU-S) and ganuloid committed (CFU-C) stem cells from bone marrow and spleen, and the lymphocyte response to different T- and B-cell mitogens and to alloantigens were determined. There was a progressive fall of the CFU-S and CFU-C content of the femur and even more in the spleen, and a progressive decrease in the number of normal marrow lymphocytes. After 10 and 15 weeks new "lymphoid" cells, presumably leukaemic cells, were seen in marrow. The lymphocyte stimulation assays showed a depressed reactivity, rather uniform in the thymus after more than 1 week and also, with a partial recovery after 3 weeks, in the spleen. The implications of these results for the chemically induced leukaemogenesis are discussed.

Leukaemogenic action of phorbol in intact and thymectomized mice of different strains

Phorbol, the unesterified parent alcohol of the skin promoter TPA, was administered i.p., twice weekly, throughout the lifetime of mice of 7 inbred strains: males and females of AKR/J, C3Heb and BALB/c, and females of SJL/J, DBA/2, SWR and C57BL. A striking difference in strain response was observed, with a pronounced leukaemogenic effect in SWR, a signficiant shortening of the latent period for spontaneous reticulum cell sarcomas (RCNB) in SJL/J, and no demonstrable effect in the other strains. When mice of 3 of the above-mentioned strains (SWR, SJL/J and AKR/J) were thymectomized prior to the beginning of phorbol treatment, different patterns of response were again observed. Thymectomy did not influence the leukaemia incidence in SWR mice, slightly inhibited RCNB development in SJL/J mice and converted phorbol into a leukaemogenic agent for AKR/J mice.

The presence of terminal deoxynucleotidyl transferase in the N-methyl-N-nitrosourea induced leukaemia in BDF1 mice and its effect on the accuracy of the DNA polymerases

DNA polymerases have been prepared from leukaemic and normal spleens and their fidelity in copying a polyd AT).polyd(AT) template assessed. The leukaemic cytoplasmic DNA polymerases were less accurate than the controls whereas no difference in accuracy was observed in the nuclear DNA polymerases. The preparations of leukaemic cytoplasmic DNA polymerase also contained the enzyme terminal deoxynucleotidyl transferase. When this enzyme was removed by further purification the accuracy of the cytoplasmic DNA polymerases increased to that of the controls.