Differential Cell Killing by Lymphomagenic Murine Leukemia Viruses Occurs Independently of p53 Activation and Mitochondrial Damage

Abstract

Upon inoculation into AKR mice, mink cell focus-forming murine leukemia virus (MCF MLV) accelerates thymic lymphoma formation. During the preleukemic phase of disease, we observed the induction of apoptosis in thymic lymphocytes. A similar induction of apoptosis was observed for cultured mink epithelial cells after MCF13 MLV infection. In this study, the relevance of viral pathogenicity to cell killing was determined by testing the susceptibility of various cell types from different species to lymphomagenic MLVs. We observed that the cytopathic effect of lymphomagenic MLVs was restricted to mink cells. Southern blot analysis of MLVinfected cells revealed an accumulation of the linear form of unintegrated viral DNA, particularly in mink cells after MCF13 MLV infection. Thus, a strong correlation was observed between viral superinfection, which results in the accumulation of high levels of unintegrated viral DNA, and cell killing. Immunoblot analysis for MCF13 MLV-infected mink epithelial cells did not show a significant change in total p53 levels or its phosphorylated form at Ser-15 compared with that in mock-treated cells. Moreover, a time course analysis for mink epithelial cells infected with MCF13 MLV did not reveal mitochondrial depolarization or a significant change in Bax levels. These results demonstrate that MCF13 MLV induces apoptosis preferentially in cells in which superinfection occurs, and the mechanism involved is independent of p53 activation and mitochondrial damage.

Mink Epithelial Cell Killing by Pathogenic Murine Leukemia Viruses Involves Endoplasmic Reticulum Stress

“…We have observed a similar phenomenon for mink cell focus-forming murine leukemia virus (MCF13 MLV), which induces apoptosis in thymic lymphocytes of inoculated AKR mice during the preleukemic period (31). Upon testing of cultured cell lines, we observed that mink cells were similarly susceptible to MCF13 MLV-induced cell killing (18). It has been shown that cell killing by pathogenic retroviruses such as FeLV-FAIDS, ts-1 MoMLV, MuLV-NT40, and FrCas E MLV strongly correlates with the inefficient processing of the precursor Env glycoprotein, resulting in its intracellular accumulation in a cell type-specific manner (11,15,21,25).…”

“…We previously showed that mink epithelial cells were susceptible to MCF13 MLV-induced cell killing, whereas other types of cells, such as Mus dunni fibroblasts, were resistant (18). To determine whether there is a correlation between Env precursor accumulation and virus-induced cell killing, we compared intracellular levels of gPr80 env in mink epithelial cells and M. dunni fibroblasts after infection with MCF13 MLV at a multiplicity of infection (MOI) of 3.…”

Retroviral DNA integration and the DNA damage response

“…5,63 However, other experiments have shown that the accumulation of unintegrated viral DNA and the reduced formation of circles can be uncoupled from cell killing. 29,59,62 It should also be noted that high concentrations of unintegrated DNA would normally be associated with high frequency of integration, which may be toxic to cells. The effects of high DNA concentration and highfrequency integration on cell survival can be tested by manipulating experimental conditions, that is, by using a high m.o.i., or infection methods that increase the effective m.o.i.…”

“…Several lines of research, beginning with the early studies of Temin and colleagues, 60 have led to the idea that unintegrated viral DNA may induce cell killing. 58,59,62 It was recently suggested that high concentrations of unintegrated viral DNA, containing free ends, might be sensed as irreparable DNA damage in the cell, and that the NHEJ system may suppress an apoptotic response by joining such ends to form circles, which are a dead-end product. 5,63 However, other experiments have shown that the accumulation of unintegrated viral DNA and the reduced formation of circles can be uncoupled from cell killing.…”

“…[56][57][58][59][60][61] Two different mechanisms have been considered. Several lines of research, beginning with the early studies of Temin and colleagues, 60 have led to the idea that unintegrated viral DNA may induce cell killing.…”