Muscle-specific protein expression in normal salivary glands and pleomorphic adenomas: an immunocytochemical study with biochemical confirmation.

Normal salivary gland myoepithelia are contractile cells with hybrid epithelial/myogenic ultrastructural features. It is known that these cells co-express the intermediate filaments cytokeratin, vimentin, and occasionally GFAP. This complex cytoskeletal immunophenotype is also reflected in multiple morphologic cell types of pleomorphic adenoma. At present, the myofilament complement of normal and neoplastic myoepithelium is not well defined. We have evaluated the expression of desmin and smooth and sarcomeric muscle actins in 11 normal salivary glands (six snap-frozen and five methacarn fixed) and 26 pleomorphic adenomas (11 snap-frozen and 15 methacarn fixed) by ABC-immunoperoxidase method. Two of 11 frozen pleomorphic adenomas contained the muscle-specific intermediate filament desmin, which is not found in the normal glands. This novel finding was confirmed by gel electrophoresis and immunoblot. Using specific antibodies, normal gland myoepithelial cells consistently contained muscle actin isoforms of the smooth muscle type but not sarcomeric muscle actins. Muscle actin expression by the neoplastic cells of pleomorphic adenoma was found in 13 of 26 tumors (six of 11 frozen tumors (desmin negative) and seven of 15 methacarn fixed tumors). In comparison to the normal myoepithelial cell, the transformed myoepithelial-like cells of pleomorphic adenoma are not always characterized by a muscle actin cytoskeleton. Expression of desmin intermediate filaments in pleomorphic adenomas appears to be a rare event that is independent of a muscle actin cytoskeleton.