SV40 oncoproteins enhance asbestos-induced DNA double-strand breaks and abrogate senescence in murine mesothelial cells

Our plain-English summary of SV40 oncoproteins enhance asbestos-induced DNA double-strand breaks and abrogate senescence in murine mesothelial cells, by J.R. Pietruska, A.B. Kane (Cancer Research, 2007).

The study

• Authors: J.R. Pietruska, A.B. Kane
• Journal: Cancer Research
• Published: 2007;67(8):3637-45
• Cited by: 34 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

SV40 virus has emerged as a potential cofactor with asbestos in the development of diffuse malignant mesothelioma, but its precise role in the pathogenesis of this tumor is unclear. SV40 large T antigen is known to inactivate cellular proteins involved in DNA damage and senescence, including p53 and pRb. We hypothesize that SV40 oncoproteins will sensitize mesothelial cells to DNA damage induced by asbestos or chemotherapeutic agents. SV40 oncoprotein expression in murine mesothelial cell lines enhanced spontaneous and asbestos-induced double-strand breaks, indicated by gamma-H2AX foci, and potentiated micronucleus formation. Mesothelial cells exposed to asbestos or bleomycin for 96 h acquired senescent-like morphology and displayed elevated senescence-associated beta-galactosidase activity, reduced bromodeoxyuridine (BrdUrd) incorporation, and reduced colony formation. SV40 oncoprotein expression abrogated the senescent phenotype, and transfected cell lines showed an increase in both BrdUrd incorporation and colony formation after prolonged DNA damage. Murine mesothelial cell lines lacking wild-type p53 due to a point mutation or gene rearrangement also failed to senesce in response to asbestos or chemotherapeutic agents. In addition, stress-induced senescence in human mesothelial cell lines was impaired by SV40 oncoprotein expression (MeT-5A), p53 small interfering RNA, or spontaneous p53 mutation (REN). These studies suggest that exposure to DNA-damaging agents can induce senescence in both murine and human mesothelioma cell lines and suggest a major, although not exclusive, role for p53 in this response. SV40 virus may contribute to mesothelioma progression by impairing stress-induced senescence, in part through p53 inactivation, thereby favoring survival and proliferation of mesothelial cells that have sustained DNA damage.

In plain English

This was a laboratory study in cultured cells. The authors expressed SV40 large and small T antigens in a preneoplastic murine mesothelial cell line (D7) and a mesothelioma line (7), creating D7T and 7T, and exposed parental and transfected cells to crocidolite asbestos (10 micrograms per square centimetre) or bleomycin; topotecan was used separately in p53-deficient murine lines and human cell lines. SV40 oncoprotein expression shortened doubling time, increased spontaneous and asbestos-induced DNA double-strand breaks (gamma-H2AX foci) and micronuclei, but did not further increase bleomycin-induced breaks. After 96 hours of asbestos or bleomycin, a subpopulation of parental cells became enlarged and flattened with raised senescence-associated beta-galactosidase activity, and parental lines showed reduced BrdUrd incorporation and colony formation; transfected lines showed no senescent morphology or raised beta-galactosidase and had higher BrdUrd incorporation and colony formation than parental lines. Murine lines lacking wild-type p53 (D9, 40), the SV40-immortalised human line MeT-5A, the p53-deficient human line REN, and H28 cells with p53 knockdown also showed impaired senescence. The authors concluded SV40 may act as a cofactor with asbestos by bypassing stress-induced senescence.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: in vitro experimental study in murine and human mesothelial and mesothelioma cell lines, using SV40 oncoprotein transfection, p53-deficient lines and p53 siRNA knockdown
• Who was studied: Cultured cell lines: murine immortalized (D9), preneoplastic (D7) and mesothelioma (7, 40) mesothelial lines, SV40-transfected subclones D7T and 7T, plus human lines MeT-5A (SV40-immortalized), H28 and REN mesothelioma cells
• Exposure: Crocidolite asbestos fibres (10 micrograms per square centimetre) and the chemotherapeutic agents bleomycin sulfate (1 microgram per millilitre) and topotecan, for 3 to 96 hours or up to 6 days; plus SV40 large and small T antigen expression
• Main results:
  • SV40 oncoprotein expression shortened doubling time: 17.5 to 14.0 hours in preneoplastic mesothelial cells and 17.1 to 8.9 hours in mesothelioma cells
  • Spontaneous double-strand breaks within 24 hours were increased in the transfected preneoplastic line D7T and in both neoplastic lines compared with D7
  • In mesothelioma cells, SV40 oncoproteins significantly increased the proportion of cells with 11 to 20 and with more than 20 gamma-H2AX foci after asbestos exposure
  • SV40 oncoproteins did not increase bleomycin-induced double-strand breaks in either cell line, although bleomycin produced breaks faster than asbestos
  • Micronuclei after asbestos were increased by SV40 oncoproteins in both preneoplastic and neoplastic cells; for bleomycin the Results section reports an increase only in preneoplastic cells
  • SV40 oncoproteins protected preneoplastic and, more so, neoplastic cells from asbestos-induced apoptosis over 96 hours, while increasing bleomycin-induced apoptosis in transfected lines
  • Prolonged DNA damage in parental murine lines produced a subpopulation of enlarged, viable but nonproliferating senescent cells
  • Parental murine lines, but not the SV40-transfected lines, showed senescent morphology and raised senescence-associated beta-galactosidase activity after prolonged asbestos or bleomycin exposure
  • Transfected lines showed significantly higher BrdUrd incorporation after 96 hours of asbestos or bleomycin, and increased colony formation compared with parental lines, particularly after asbestos
  • Murine lines with a p53 point mutation (D9) or p53 gene rearrangement (40) showed a drastic reduction in senescence-associated beta-galactosidase activity after asbestos, bleomycin or topotecan compared with D7 (wild-type p53)
  • In human cells, the SV40-immortalized line MeT-5A showed only slight changes in senescence-associated beta-galactosidase and BrdUrd incorporation after topotecan or bleomycin, unlike the SV40-negative H28 line, which showed senescent morphology, raised beta-galactosidase and decreased DNA synthesis
  • p53 siRNA in H28 cells reduced p53 protein by more than 80% on topotecan exposure and was associated with decreased senescence-associated beta-galactosidase activity
• The authors' conclusion: The authors concluded that SV40 oncoproteins bypass stress-induced senescence while promoting survival and growth after DNA damage, suggesting a mechanism by which SV40 virus could act as a cofactor with asbestos in mesothelioma development and progression, with a major but not exclusive role for p53
• Limitations the authors state:
  • Because T antigen inactivates both p53 and pRb, the relative contribution of each pathway could not be determined in SV40-positive lines
  • The findings do not exclude a role for other pathways targeted by T antigen, such as pRb
  • Prosurvival signalling pathways were not specifically examined

Topics

Medical terms in this study, explained in our Lexicon: SV40, Mesothelial Cells, Malignant Mesothelioma, Mesothelioma.

Related studies

• Effects of asbestos on initiation of DNA damage, induction of DNA-strand breaks, P53-expression and apoptosis in primary, SV40-transformed and malignant human mesothelial cells — B. Burmeister, T. Schwerdtle, et al. (2004)
• Effect of simian virus large T antigen expression on cell cycle control and apoptosis in rat pleural mesothelial cells exposed to DNA damaging agents — V. Levresse, S. Moritz, et al. (1998)
• Increased incidence of DNA double-strand breaks and anti-ds DNA antibodies in blood of workers occupationally exposed to asbestos — B. Marczynski, A.B. Czuppon, et al. (1994)
• Modulation of DNA single-strand breaks by intracellular glutathione in human lung cells exposed to asbestos fibers — A. Puhakka, T. Ollikainen, et al. (2002)
• Iron mobilization from crocidolite asbestos greatly enhances crocidolite-dependent formation of DNA single-strand breaks in phi X174 RFI DNA — L.G. Lund, A.E. Aust (1992)
• Asbestos causes DNA strand breaks in cultured pulmonary epithelial cells: role of iron-catalyzed free radicals — D.W. Kamp, V.A. Israbian, et al. (1995)

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