Inhibition of topoisomerase II overrides the G2/M check points of the cell cycle in EBV-lymphocytes

Epstein-Barr virus (EBV) is associated with a number of human malignancies. In vitro EBV infection transforms human lymphocytes into proliferating cell lines (EBV-lymphocytes). Etoposide, topoisomerase II inhibitor, induced apoptosis in EBV-lymphocytes as shown by expression of phosphatidylserine, loss of DNA and mitochondrial membrane potential, and cell shrinkage. In contrast, those cells, which had yet to display signs of apoptosis, grew to exceed their normal size. These EBV-lymphocytes had unusual cellular and nuclear morphology, higher mitochondrial membrane potential, increased expression of proteins and an amount of DNA that exceeded the maximum DNA content in normal EBV-lymphocytes by more than two-fold. Application of the caspase inhibitor Z-VAD-FMK in the presence of etoposide increased the numbers of such large cells. This data suggests that inhibition of topoisomerase II by etoposide does not inhibit DNA synthesis but rather overrides the G₂/M check points of the cell cycle, resulting in cells growth over their genetically determined size. This may trigger apoptosis to eliminate cells, which failed to complete mitosis.