bcl-2 gene expression confers a survival advantage by preventing cells from entering apoptosis. In contrast to the previously described cytoplasmic localization of Bcl-2 in epithelial cells in vivo, in this study we have demonstrated, in a series of human epithelial cell lines, that Bcl-2 also localizes to mitotic nuclei. Both immunocytochemical and immunoelectron microscopical examinations localize this protein to nuclei and in particular to chromosomes. Nuclear Bcl-2 expression in these cell lines is correlated with the cell cycle. There is relatively strong expression during mitosis, most intense during prophase and metaphase, declining in telophase and then the protein becomes undetectable soon after separation of the two daughter cells. The expression and distribution of Bcl-2 is influenced by treatment with excessive thymidine. These results indicate that Bcl-2 may protect the cells from apoptosis occurring during mitosis and suggest a possible role for the protein in cell immortalization.

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