Transient mitotic phase localization of bcl-2 oncoprotein in human carcinoma cells and its possible role in prevention of apoptosis

We used a monoclonal antibody and fluorescence immunocytochemistry to localize the product of the bcl-2 gene in two cultured human carcinoma cell lines, KB and OVCAR-3. These cells show little or no localization of bcl-2 oncoprotein in interphase cells but demonstrate a dramatic appearance of bcl- 2 in early prophase or late G₂, which persists throughout mitosis, rapidly disappearing at telophase. The pattern of bcl-2 localization shows a diffuse nuclear distribution before chromosome condensation, followed by a specific concentration of bcl-2 at the margins of condensed chromosomes in prophase, metaphase, and anaphase. Treatment of these cells with taxol, an agent that interferes with formation of the mitotic spindle, causes mitotic arrest and apoptosis after a prolonged incubation period. During mitotic arrest due to taxol, bcl-2 remains associated with condensed chromosomes but is lost after >2 days at approximately the same time as the appearance of apoptotic features in these cells. Western blotting indicates that the only extractable protein reactive with this monoclonal antibody under these conditions is a ~28 KD form of bcl-2. These results suggest a model for the role of bcl-2 in protection from apoptosis and a potentially common mechanism by which the final step of apoptosis might be mediated.