Occurrence of nuclear β_II-tubulin in cultured cells

Microtubules are cylindrical organelles that play critical roles in cell division. Their subunit protein, tubulin, is a target for various antitumor drugs. Tubulin exists as various forms, known as isotypes. In most normal cells, tubulin occurs only in the cytosol and not in the nucleus. However, we have recently reported the finding of the β_II isotype of tubulin in the nuclei of cultured rat kidney mesangial cells. Mesangial cells, unlike most normal cell lines, have the ability to proliferate rapidly in culture. In efforts to determine whether nuclear β_II-tubulin occurred in other cell lines, we examined the distribution of the β_I, β_II, and β_IV mammalian tubulin isotypes in a variety of normal and cancer human cell lines by immunofluorescence microscopy. We have found that, in the normal cell lines, all three isotypes are present only in the cytoplasm. However, the β_II isotype of tubulin is located not only in the cytoplasm, but also in the nuclei of the following cell lines: LNCaP prostate carcinoma, MCF-7, MDA-MB-231, MDA-MB-435, and Calc 18 breast carcinoma, C6 and T98G glioma, and He-La cells. In contrast, the β_I and β_IV isotypes, which are also synthesized in cancer cells, are not localized to the nucleus but are restricted to the cytoplasm. We have also seen β_II in breast cancer excisions. In most of these cells, β_II appears to be concentrated in the nucleoli. These results suggest that transformation may lead to localization of β_II-tubulin in cell nuclei, serving an as yet unknown function, and that nuclear β_II may be a useful marker for detection of tumor cells.