Qingdi Q. Li

Detection of Cyclin B1 Expression in G1-Phase Cancer Cell Lines and Cancer Tissues by Postsorting Western Blot Analysis

Protein complex of cyclin B1 and cyclin-dependent protein kinase 1 induces phosphorylation of key substrates that mediate cell cycle transition during the G2-M phase. It is believed that cyclin B1 accumulates in the S phase of the cell cycle and reaches the maximal level at mitosis but is absent in G1-phase cells. In the present study, we demonstrated that cyclin B1 was expressed in the arrested G1-phase MOLT-4 lymphocyte leukemia cells and in G1 phase T-7 transitional tumor cells, as determined by flow cytometry. In addition, we showed that cyclin B1 was detected in the G1 phase in breast cancer cells from patient tissues and in lymphocytes from patients with leukemia. These findings were confirmed for the first time by postsorting Western blot analysis and by confocal microscopy. Furthermore, by using postsorting Western blotting, we found that cyclin B1 was expressed in different time-window sections of the G1 phase under different conditions. For the asynchronously growing T-7 cells, cyclin B1 was detected in early G1 phase, whereas in MOLT-4 cells arrested in G1-S phase, cyclin B1 was mainly detected in late G1 phase. We propose that the cyclin B1 expressed in the G1 phase may differ from that expressed in the G2-M phase, and that this unscheduled type of cyclin B1 may play an important role in tumorigenesis and apoptosis.