Expression of class II β-tubulin by proliferative myoepithelial cells in canine mammary mixed tumors

Benign mammary mixed tumors in dogs resemble human salivary pleomorphic adenomas with regard to their histogenesis, including the occurrence of cartilaginous or bony metaplasia as well as the expression pattern of cytoskeletal proteins in proliferative myoepithelial cells. Recently, a monoclonal antibody specific for class 11 beta-tubulin has been developed. The epitope it recognizes was determined to be the heptapeptide Glu-Glu-Glu-Glu-Gly-Glu-Asp, which is the common sequence found among the canine, rat, mouse, and human class 11 beta-tubulin-specific regions. We carried out immunohistochemical studies on mammary mixed tumors obtained from three female dogs using this the monoclonal antibody. The antibody to class 11 P-tubulin reacted intensely with proliferative myoepithelial cells in canine mammary mixed tumors, whereas staining was barely detectable in normal myoepithelial cells surrounding alveoli and alveolar ducts within the tumor and adjacent normal tissue. Proliferative myoepithelial cells also expressed vimentin, but a-smooth muscle actin (alphaSMA) staining was barely detectable. Immunoblot analysis showed that class 11 beta-tubulin and vimentin were expressed in myoepithelial cell lines prepared from the three mammary mixed tumors. On the other hand, only one cell line, which was negative for aSMA, produced cartilage-specific type 11 collagen. These results suggest that class 11 beta-tubulin could be a new molecular marker of proliferating myoepithelial cells in canine mammary mixed tumors and that differential expression of cytoskeletal components is associated with cartilaginous metaplasia of proliferative myoepithelial cells in mixed mammary tumors.