Mammalian cells transfected with the listeriolysin gene exhibit enhanced proliferation and focus formation

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Abstract
Mouse 3T6 and 3T3 fibroblasts and rat epithelial L2 cells were transfected with recombinant plasmids containing the listeriolysin gene (hly) of Listeria monocytogenes. This bacterial gene (with and without the 5' signal sequence) was cloned under the control of a murine metallothionein promoter, resulting in elevated transcription of both forms of the hly gene after induction with ZnSO4. However, the gene product could be observed only when the listeriolysin gene lacking the 5' signal sequence was used. Intact listeriolysin could not be detected in the cytoplasm or in the supernatant of the hly-transfected cells. 3T6 and L2 cells transfected with the intact hly gene exhibited significantly increased cell proliferation and increased formation of actin microfilaments upon induction of hly expression with ZnSO4. Both cell types are not contact inhibited and formed large piles of spherical cells after transfection with hly. In contrast, contact-inhibited 3T3 cells transfected with the hly gene showed increased proliferation but no formation of such cell aggregates. When 3T6 fibroblasts were transfected with the hly gene without the 5' signal sequence, inhibition of growth, lack of cell layer confluency, and altered (spherical) cell morphology were observed.