Friend leukemia virus infection enhances DNA damage-induced apoptosis of hematopoietic cells, causing lethal anemia in C3H hosts

Subcellular localization of MCM2 correlates with the prognosis of ovarian clear cell carcinoma

Highly malignant tumors overexpress the minichromosome maintenance 2 (MCM2) protein in the nucleus, which is associated with advanced tumor grade, advanced stage, and poor prognosis. In this study, we showed that MCM2 is highly expressed in clinical samples of ovarian clear cell carcinoma. Although MCM2 expression was mainly localized to the nuclei as in other cancers, a few cases exhibited cytoplasmic localization of MCM2. Surprisingly, tumor samples with cytoplasmic MCM2 demonstrated excellent prognosis, showing 100% survival during the observation period of more than 200 months. However, cases with nuclear expression of MCM2 exhibited approximately 78% 5-year-survival rate. In a previous study, we showed that Friend leukemia virus (FLV) envelope protein gp70 bound to MCM2, impaired its nuclear translocation, and enhanced DNA damage-induced apoptosis in FLV-infected hematopoietic cells with high levels of MCM2. As expected, clear cell carcinoma cells with cytoplasmic expression of MCM2 exhibited significantly higher apoptotic cell ratio than that of cells with nuclear MCM2 expression. In vitro experiments using ovarian cancer cells with cytoplasmic expression of MCM2 demonstrated that transfection of MCM2-ΔN enhanced DNA damage-induced apoptosis. Therefore, cytoplasmic localization of MCM2 significantly correlated with increased apoptosis in clear cell carcinoma cells, resulting in improved prognosis.

Targeting MCM2 function as a novel strategy for the treatment of highly malignant breast tumors

Highly malignant tumors express high levels of the minichromosome maintenance 2 (MCM2) protein, which is associated with advanced tumor grade, advanced stage, and poor prognosis. In a previous study, we showed that Friend leukemia virus (FLV) envelope protein gp70 bound MCM2, impaired its nuclear translocation, and enhanced DNA-damage-induced apoptosis in FLV-infected hematopoietic cells when the cells expressed high levels of MCM2. Here, we show that MCM2 is highly expressed in clinical samples of invasive carcinoma of the breast, especially triple-negative breast cancer (TNBC), and in cancer stem cell (CSC) marker-positive breast cancer cells. To generate a cancer therapy model using gp70, we introduced the gp70 protein into the cytoplasm of murine breast cancer cells that express high levels of MCM2 by conjugating the protein transduction domain (PTD) of Hph-1 to gp70 (Hph- 1-gp70). Hph-1-gp70 was successfully transduced into the cytoplasm of breast cancer cells. The transduced protein enhanced the DNA damage-induced apoptosis of cancer cells in vitro and in vivo. Therefore, an MCM2-targeted strategy using Hph-1-gp70 treatment to induce DNA damage might be a successful therapy for highly malignant breast cancers such as TNBC and for the eradication of CSC-like cells from breast cancer tissue.

Minichromosome maintenance 2 bound with retroviral Gp70 is localized to cytoplasm and enhances DNA-damage-induced apoptosis

The interaction of viral proteins with host-cellular proteins elicits the activation of cellular signal transduction pathways and possibly leads to viral pathogenesis as well as cellular biological events. Apoptotic signals induced by DNA-damage are remarkably up-regulated by Friend leukemia virus (FLV) exclusively in C3H hosts; however, the mechanisms underlying the apoptosis enhancement and host-specificity are unknown. Here, we show that C3H mouse-derived hematopoietic cells originally express higher levels of the minichromosome maintenance (MCM) 2 protein than BALB/c- or C57BL/6-deriverd cells, and undergo more frequent apoptosis following doxorubicin-induced DNA-damage in the presence of the FLV envelope protein gp70. Dual transfection with gp70/Mcm2 reproduced doxorubicin-induced apoptosis even in BALB/c-derived 3T3 cells. Immunoprecipitation assays using various deletion mutants of MCM2 revealed that gp70 bound to the nuclear localization signal (NLS) 1 (amino acids 18–24) of MCM2, interfered with the function of NLS2 (amino acids 132–152), and suppressed the normal nuclear-import of MCM2. Cytoplasmic MCM2 reduced the activity of protein phosphatase 2A (PP2A) leading to the subsequent hyperphosphorylation of DNA-dependent protein kinase (DNA-PK). Phosphorylated DNA-PK exhibited elevated kinase activity to phosphorylate P53, thereby up-regulating p53-dependent apoptosis. An apoptosis-enhancing domain was identified in the C-terminal portion (amino acids 703–904) of MCM2. Furthermore, simultaneous treatment with FLV and doxorubicin extended the survival of SCID mice bearing 8047 leukemia cells expressing high levels of MCM2. Thus, depending on its subcellular localization, MCM2 plays different roles. It participates in DNA replication in the nucleus as shown previously, and enhances apoptosis in the cytoplasm.

Rapid myeloid recovery as a possible mechanism of whole-body radioadaptive response

We investigated the mechanism underlying the radioadaptive response that rescues mice from hematopoietic failure. C57BL/6 mice were irradiated with low-dose acute X rays (0.5 Gy) for priming 2 weeks prior to a high-dose (6 Gy) challenge irradiation. Bone marrow cells, erythrocytes and platelets in low-dose-preirradiated mice showed earlier recovery after the challenge irradiation than those in mice subjected only to the challenge irradiation. This suggests that hematopoiesis is enhanced after a challenge irradiation in preirradiated mice. The rapid recovery of bone marrow cells after the challenge irradiation was consistent with the proliferation of hematopoietic progenitors expressing the cell surface markers Lin-, Sca-1- and c-Kit+ in low-dose-preirradiated mice. A subpopulation of myeloid (Mac-1+/Gr-1+) cells, which were descendants of Lin-, Sca-1- and c-Kit+ cells, rapidly recovered in the bone marrow of low-dose-preirradiated mice, whereas the number of B-lymphoid (CD19+/B220+) cells did not show a statistically significant increase. Plasma cytokine profiles were analyzed using antibody arrays, and results indicated that the concentrations of several growth factors for myelopoiesis after the challenge irradiation were considerably increased by low-dose preirradiation. The rapid recovery of erythrocytes and platelets but not leukocytes was observed in the peripheral blood of preirradiated mice, suggesting that low-dose preirradiation triggered the differentiation to myelopoiesis. Thus the adaptive response induced by low-dose preirradiation in terms of the recovery kinetics of the number of hematopoietic cells may be due to the rapid recovery of the number of myeloid cells after high-dose irradiation.

Rapid induction of IAP family proteins and Smac/DIABLO expression after proapoptotic stimulation with doxorubicin in RPMI 8226 multiple myeloma cells

We studied the expression dynamics of inhibitor of apoptosis protein (IAP) family members and Smac/DIABLO after treatment with doxorubicin in human multiple myeloma cell line RPMI 8226 and its doxorubicin-resistant variant DRR. Proapoptotic stimulation with doxorubicin rapidly induced the overexpression of mRNA as well as protein for IAPs in RPMI 8226 cells followed by a gradual decrease of their expression. Smac/DIABLO, which is known to neutralize IAPs, showed increased expression at the mRNA level after treatment; however, Western blot analysis revealed a slight decrease of the amount of protein. Immunoprecipitation analysis revealed the association of Smac/DIABLO with cIAP1 or XIAP after treatment with doxorubicin. In contrast to the RPMI 8226 cells, DRR cells did not undergo apoptosis in response to doxorubicin treatment. The DRR cells had higher levels of IAPs expression at the mRNA level and did not show a remarkable peak or decrease in the expression of mRNAs for cIAP1, cIAP2, XIAP, and survivin after treatment with doxorubicin. Furthermore, the expression of Smac/DIABLO mRNA was not up-regulated after treatment. These findings indicate that the suppression of IAPs expression by Smac/DIABLO shortly after proapoptotic stimulation might play a role in the mechanisms of apoptotic induction, and that the maintenance of high IAPs expression and low Smac/DIABLO expression after treatment might lead to the doxorubicin-resistance of multiple myeloma cells.

Resistance against Friend leukemia virus-induced leukemogenesis in DNA-dependent protein kinase (DNA-PK)-deficient scid mice associated with defective viral integration at the Spi-1 and Fli-1 site

Retroviral DNA integration is mediated by the viral protein integrase. However, elements of the host DNA repair machinery such as the phosphatidylinositol 3-kinase (PI-3K)-related protein kinase family system would play a role in the integration of viral DNA into the host DNA. Here, we show that a host PI-3K-related protein kinase, DNA-dependent protein kinase (DNA-PK), plays a role in the specific integration of retroviral DNA and induction of retroviral diseases in vivo. DNA-PK-deficient scid mice inoculated with Friend leukemia virus (FLV) exhibited a random integration into their genomic DNA and expressed the viral envelope protein gp70. However, the specific integration of FLV at Spi-1 or Fli-1 sites did not occur in association with the significant resistance of scid mice to FLV-induced leukemogenesis. In contrast, the knockout of another member of the PI-3K-related protein kinase family, encoded by the ataxia telangiectasia mutated (ATM) gene, resulted in mice as sensitive to FLV-induced leukemogenesis as the wild type mice. FLV was specifically integrated into the DNA at Spi-1 and Fli-1 sites with significant expression of these transcription factors. These findings indicated that DNA-PK would be essential for controlling the in vivo integration of FLV at specific sites as well as the susceptibility to FLV-induced leukemogenesis.

DNA-dependent protein kinase enhances DNA damage-induced apoptosis in association with Friend gp70

Friend leukemia virus (FLV) infection strongly enhances gamma-irradiation-induced apoptosis of hematopoietic cells of C3H hosts leading to a lethal anemia. Experiments using p53 knockout mice with the C3H background have clarified that the apoptosis is p53-dependent and would not be associated with changes of cell populations caused by the infection with FLV. In bone marrow cells of FLV + total body irradiation (TBI)-treated C3H mice, the p53 protein was prominently activated to overexpress p21 and bax suggesting that apoptosis-enhancing mechanisms lay upstream of p53 protein in the signaling pathway. Neither of DNA-dependent protein kinase (DNA-PK)-deficient SCID mice nor ataxia telangiectasia mutated (ATM) gene knockout mice with the C3H background exhibited a remarkable enhancement of apoptosis or p53 activation on FLV + TBI-treatment indicating that DNA-PK and ATM were both essential. ATM appeared necessary for introducing DNA damage-induced apoptosis, while DNA-PK enhanced p53-dependent apoptosis under FLV-infection. Surprisingly, viral envelope protein, gp70, was co-precipitated with DNA-PK but not with ATM in FLV + TBI-treated C3H mice. These results indicated that FLV-infection enhances DNA damage-induced apoptosis via p53 activation and that DNA-PK, in association with gp70, might play critical roles in modulating the signaling pathway.

Cytological basis for enhancement of radiation-induced mortality by Friend leukaemia virus infection

PURPOSE

To analyse the cytological basis for enhancement of radiation-induced mortality by Friend leukaemia virus infection.

MATERIALS AND METHODS

Cellularity in haematopoietic tissues of C3H mice infected with FLV and/or whole-body irradiation was examined.

RESULTS

When mice were treated with a sublethal dose (3 Gy) of irradiation at 1 week after virus infection, most manifested a severe loss of cellularity in the spleen, bone marrow and peripheral blood 2 weeks after irradiation. More than 90% of the mice died within 1 month post-irradiation. However, this deleterious effect of virus infection on the survival of irradiated mice was observed only when they were irradiated at around 1 week after virus inoculation. Strain differences in the sensitivity to this effect were observed among virus-sensitive strains of mice.

CONCLUSIONS

The results indicate that Friend leukaemia virus infection can cause enhancement of radiation sensitivity of haematopoietic cells in host animals in a restricted manner in terms of genetic background and the interval between infection and irradiation.