Laser scanning cytometric analysis of cyclin b1 in primary human malignancies

Cyclins are key components of the cell cycle progression machinery. They activate their partner-dependent kinases (CDKs) and target them to respective substrate proteins within the cell. CDK-mediated phosphorylation of specific sets of proteins drives the cell through particular phases or checkpoints of the cell cycle. During unperturbed growth of normal cells, the timing of expression of several cyclins is discontinuous, occurring at discrete and well-defined periods of the cell cycle. Immunocytochemical detection of cyclins in relation to cell cycle position (DNA content) by multiparameter flow cytometric techniques has provided a new approach to cell cycle studies. This approach, like no other method, can be used to detect the 'unscheduled' expression of cyclins, namely, the presentation of G₁ cyclins by cells in G₂/M and of G₂/M cyclins by G₁ cells, without the need for cell synchronization. By use of multiparameter flow cytometric and laser scanning cytometric analysis, we correlated the expression of cyclin B1 with cell cycle position in normal lymphocytes stimulated to proliferate by the mitogen phytohemagglutinin and in 28 primary human tumors of different organ and type. Eighteen of the 28 tumors expressed the cyclin B1 in more than 5% of cells (B1 positive), and the rest showed cyclin expression from 2.1 to 5% (B1 negative). In normal lymphocytes, the expression of cyclin B1 was restricted to very late S and G₂ + M phases of the cell cycle. In 15 of 18 primary tumors studied, the expression of cyclin B1 was 'unscheduled' (unrestricted to particular phases of the cycle). The data suggest that the 'unscheduled' expression of cyclin B1 might be a common defect in neoplasia.