Spontaneously immortalized mouse mesothelial cells display characteristics of malignant transformation

Our plain-English summary of Spontaneously immortalized mouse mesothelial cells display characteristics of malignant transformation, by A.L. Sherwood, S.E. Mutsaers, V.K. Peeva, et al (Cell Proliferation, 2008).

The study

• Authors: A.L. Sherwood, S.E. Mutsaers, V.K. Peeva, C. Robinson, C.J. DeSilva, N.R. Swanson, R.A. Lake
• Journal: Cell Proliferation
• Published: 2008;41(6):894-908
• Cited by: 6 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Objectives: Mesotheliomas occur in occult serous cavities after chronic exposure of mesothelial cells to asbestos fibres. Molecular events that contribute to the development of this cancer are therefore not readily accessible for study. We have used in vitro culture systems to study and compare induced and spontaneous transformation events in primary mouse mesothelial cells.

Materials And Methods: Mouse mesothelial cells were cultivated until small populations of proliferating cells emerged from senescing cultures. Spontaneously transformed cultures of cells were characterized and compared to malignantly transformed cells.

Results: Human mesothelial cells had a finite lifespan of 10-15 population doublings when cultured in vitro; mouse mesothelial cells typically exhibit this same pattern. Here, we show that mouse mesothelial cells can be cultured for extended periods and that these cells can transform spontaneously. Lines of spontaneously transformed cells generated in this study are immortal and growth factor-independent. They display the salient characteristic features of transformation, including increased proliferation rate, lack of contact inhibition, aneuploidy and ability to grow in anchorage-independent conditions. A subset of these cell lines developed into tumours in syngeneic mice. Comparative gene expression analysis demonstrated that spontaneously transformed cell lines were more closely related to neoplastic cells than to primary cells.

Conclusion: These findings have implications for interpretation of in vitro transformation studies, demonstrating broad similarity between spontaneous and induced genetic changes.

In plain English

This laboratory study in Perth, Australia, used mesothelial cells (which line body cavities) from the abdominal lining of mice. The cells themselves were not exposed to asbestos. For comparison, the authors used mesothelioma cell lines from mice that had been injected with crocidolite asbestos. Most normal cells stopped dividing after about 9 to 13 population doublings, but small groups of dividing cells later emerged. Three such lines were grown for up to 90 population doublings. Compared with normal cells, these lines grew faster, could grow in low serum, had abnormal amounts of DNA, and formed colonies in soft agarose. Only MTMhp (40) formed tumours after injection under the skin: in four out of four genetically matched (syngeneic) mice, and also in immunocompromised (athymic) mice. The other two lines formed none. The gene activity patterns of the three lines were closer to the asbestos-induced mesothelioma lines than to normal cells. The authors concluded that changes arising spontaneously in culture resemble those that lead to mesothelioma in animals. They cautioned that laboratory transformation studies need great care in interpretation.

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: Laboratory (in vitro) cell culture study, with injection of cultured cells into mice to test tumour formation and gene expression (microarray) analysis
• Who was studied: Primary mesothelial cells from the peritoneal tissue of C57Bl/6 mice, with tissue from three female mice used for each cell line. Three spontaneously transformed lines were derived from them. Tumour tests used syngeneic C57Bl/6 mice (n = 4) and athymic nu/nu mice (n = 2). The research institutions are in Perth, Australia; study dates are not given.
• Exposure: The cultured cells were not exposed to asbestos. Transformation arose spontaneously during prolonged culture. Comparison mesothelioma cell lines came from mice that had been injected with crocidolite asbestos.
• Main results:
  • Normal (primary) mesothelial cells grew for six to eight passages (about 9 to 13 population doublings), after which most showed signs of senescence (stopped dividing).
  • The three spontaneously transformed lines were cultured for up to 90 population doublings over 250 days.
  • Unlike normal cells, the spontaneously transformed (MTMhp) lines could proliferate in low serum conditions (0.5% FCS).
  • About 35% of normal cells entered S phase within 24 hours, compared with more than 95% of the transformed line MTMhp (42).
  • About 40% of normal cells were senescent, compared with less than 5% of MTMhp (42) cells.
  • The transformed lines had abnormal DNA content: they were typically hyperdiploid, at 1.1 to 1.8 times the G1 peak of diploid cells.
  • Normal cells did not form colonies in semisolid medium, but the transformed lines did. Most of their colonies were small (fewer than 20 cells), compared with more than 50 cells for the mesothelioma control lines.
  • In syngeneic mice, MTMhp (40) formed tumours in four out of four mice, while MTMhp (41) and MTMhp (42) formed no tumours. In athymic mice, only MTMhp (40) formed tumours.
  • Hierarchical clustering showed that gene expression in the transformed lines was more closely related to the malignant (mesothelioma) cell lines than to primary mesothelial cells.
  • Compared with a published set of genes changed in human mesothelioma specimens, the transformed lines scored about three times higher than normal cells, for both up- and down-regulated genes. Their normalized enrichment scores were 1.28 and 1.61, against 0.41 and 0.54 for normal cells.
  • The tumour-forming line MTMhp (40) showed low expression of the tumour suppressor genes p16INK4a and p21Cip1, compared with normal cells and the other two transformed lines.
• The authors' conclusion: The authors concluded that the changes leading to malignant transformation of mesothelial cells in the animal are similar to the changes that occur in spontaneous transformation in culture.
• Limitations the authors state:
  • The authors stated that their study could not show whether the presence of senescing cells contributes to spontaneous transformation.
  • Antibodies used to identify human mesothelial cells do not work on mouse cells, so the mouse cells could only be identified by electron microscopy.
  • The authors cautioned that transformation studies done in culture may be contaminated by spontaneously transformed cells and should be interpreted with great care.

Topics

Medical terms in this study, explained in our Lexicon: Mesothelial Cells, Malignant Transformation, Mesothelioma, Serous Cavities.

Related studies

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• Malignant transformation of immortalized human bronchial epithelial cells by asbestos fibers — T.K. Hei, L.J. Wu, et al. (1997)
• Human mesothelial cells are unusually susceptible to simian virus 40-mediated transformation and asbestos cocarcinogenicity — M. Bocchetta, I. Di Resta, et al. (2000)
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