Binuclear cells induced by acridine orange in giant cell tumor of bone

We have recently found the presence of many binuclear cells among isolated and smeared cells in giant cell tumor of the bone (GCT). These binuclear cells are possibly associated with the formation of multinuclear cells. Therefore, this study was undertaken to clarify the mechanism of binucleation in GCT, using primary culture cells exposed to acridine orange (AO) which is a fluorescent vital staining dye for the cytoplasm and nucleus and which inhibits mitosis. The cells were isolated from explants of fresh tumor materials obtained from two GCT patients (GCT1 and GCT2). These cells were cultured in Dulbecco's modified Eagle medium (DMEM) with 10% Fetal calf serum (FCS). After exposure to 0.5 microgram/ml AO, for 0, 6, 24, 48, 96 and 144 hours the following parameters were investigated: 1) cell growth rate (GR); 2) frequency of hyperdiploid cells (%HDC) by DNA cytofluorometry; 3) mitotic index (MI); 4) BrdU labeling index (LI); 5) frequency of binuclear cells (%BNC). Compared to the control cells which were cultured in AO-free medium, the GR of both GCT cells exposed to AO was remarkably inhibited. The MI was 0 from 24 to 144 hours. The %HDC was increased at 24 hours and was maintained high until 144 hours. The LI was temporarily increased at 6 hours, but was decreased at 48 hours. The %BNC was gradually increased. AO inhibited DNA synthesis and cell mitotic activity in cultured GCT cells and it finally caused inhibition of cell growth. However, the frequencies of G2 arrest cells and binuclear cells were increased. These results suggested that the binuclear cells in GCT may be formed from G2 arrest cells by amitotic nuclear division, but not by mitosis without cytoplasmic division, or by cell fusion.