Abstract
32D cells, a murine myeloid cell line, rapidly undergo apoptosis upon withdrawal of interleukin-3 (IL-3) supplement in culture. We found that 32D cells, if infected by several species of human mycoplasmas that rapidly activated NF-kappaB, would live and continue to grow in IL-3-depleted culture. Mycoplasma-infected cells showed no evidence of autocrine production of IL-3. Pyrrolidine dithiocarbamate (PDTC) blocked activation of NF-kappaB and led to prominent cell death. Heat-killed mycoplasmas or mycoplasmal membrane preparations alone could support continued growth of 32D cells in culture without IL-3 supplement for a substantial period of time. However, upon removal of heat-inactivated mycoplasmas, 32D cells quickly became apoptotic. In comparison, live Mycoplasma fermentans or M. penetrans infection for 4 to 5 weeks induced malignant transformation of 32D cells. Transformed 32D cells grew autonomously and no longer required support of growth-stimulating factors including IL-3 and mycoplasmas. The transformed 32D cells quickly formed tumors when injected into nude mice. Karyotyping showed that development of chromosomal changes and trisomy 19 was often associated with malignant transformation and tumorigenicity of 32D cells. Mycoplasmal infections apparently affected the fidelity of genomic transmission in cell division as well as checkpoints coordinating the progression of cell cycle events.
Highlights
Mycoplasmas are a heterogeneous group of the smallest organisms capable of self-replication
Since the lipid-associated membrane proteins (LAMPs) of M. fermentans and M. penetrans were potent inducers of NF-B and AP-1 in murine macrophages [8, 25], we examined if the Triton X-114 preparation of LAMPs could support the IL-3-independent growth of 32D cells
IL-3-dependent 32D cells transfected with various oncogenes have served as a model system to study oncogenesis [15]
Summary
Mycoplasmas are a heterogeneous group of the smallest organisms capable of self-replication. Using a murine embryonic (C3H) cell system, we demonstrated that chronic infection by mycoplasmas induced chromosomal instability as well as malignant transformation of mammalian cells. After a period of 4 to 5 weeks of infection by M. fermentans or M. penetrans, 32D cells gradually underwent malignant transformation and no longer required the continued presence of mycoplasmas for growth in the IL-3-free culture. These 32D cells grew autonomously and became highly. This in vitro model system allowed us to explore mycoplasma-mediated molecular mechanisms that rescue cells from apoptosis and induce continuous cell growth. We studied the machinery that could lead to malignant transformation in 32D cells chronically infected by mycoplasmas
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