Relationship between cell kinetics and apoptotic effects in TNF-alpha and IFN-gamma-treated human tumour cell lines

TNF-alpha is a potent inducer of apoptosis and also affects the transit of cells through the phases of the cell cycle. It is thought that the proliferation signalling pathway is related to the apoptosis pathway, the details of this cross-talk are not yet fully understood. In this report, the effects of tumour necrosis factor-alpha (TNF-alpha) and interferon-gamma (IFN-gamma) on human tumour cell lines with respect to proliferation and apoptosis are examined. The TNF-alpha-sensitive cell lines Me-180 and MCF-7, the resistant cell line TCC-Sup, and the intermediate line 5637 were used. After a one day treatment, the transit through all phases of the cell cycle slowed down and after 3 days stopped completely, as measured with the BrdU-assay and flow cytometry. During the same time however, the levels of c-Myc and Ki-67 expression and the number of cells becoming apoptotic increased. Combined treatment with TNF-alpha and IFN-gamma augmented both the effects on the cell cycle and on apoptosis in the sensitive lines, and had only a minor additional effect on the resistant cell line as compared to single TNF-alpha treatment. The cells becoming apoptotic detached from the culture flask bottom and floated in the medium. Several apoptosis assays were used to prove that the floating cells were indeed apoptotic. As a subsidiary result of receptor measurement, we observed complexes of TNF-alpha receptors I with TNF-alpha receptors II using the related blocking antibodies and I125-TNF-alpha as ligand. The association of proliferative parameters and apoptosis became obvious by plotting the levels of c-Myc expression versus remaining live cells after apoptotic cells were detached. Our data revealed a good linear correlation indicating that high levels of c-Myc render cells sensitive to apoptosis, independent of the treatment, TNF-alpha alone or TNF-alpha in combination with IFN-gamma. The quantitative linear correlation may point to a threshold mechanism.