Loss of myoepithelial cell characteristics in metastasizing rat mammary tumors relative to their nonmetastasizing counterparts.

A series of WF rat mammary tumors comprising one transplantable nonmetastasizing line (MT-W9), two predominantly lymphatic (SMT-2A and SMT-077) and one lymphatic and hematogenous (MT-450) metastasizing transplantable lines, and 7,12-dimethylbenz[a]anthracene (DMBA)-induced nonmetastasizing primary tumors was examined for the presence of epithelial and myoepithelial cell characteristics with the use of immunocytochemical techniques. Tumor cells staining for myosin were only occasionally observed in a basal orientation in glandular structures in sections of DMBA-induced and MT-W9 tumors; anti-laminin serum stained the peripheries of the glandular structures in the DMBA-induced and MT-W9 tumors but failed to stain the SMT-2A and SMT-077 tumor cells. In the nonmetastasizing tumors immunologically detectable keratin occurred mainly in the outer cellular layer of glandlike structures, whereas milk fat globule membrane immunoreactivity occurred primarily in the luminal cells. Both these types of immunoreactivity were observed in MT-450 tumor cells, but the pattern of keratin staining was random. No such immunoreactivity occurred in SMT-2A or SMT-077 tumors. No tumor cell-associated staining for fibronectin was seen in any of the tumors examined, although host stromal components stained intensely. The nonmetastasizing tumors contained cuboidal epithelial cells with lumen formation, surface microvilli, and intercellular junctional complexes, together with a relatively undifferentiated elongated cell component. Other elongated cells showed hemidesmosomes, pinocytotic vesicles, tonofilaments, and small bundles of cytoplasmic filaments, suggesting gradations toward a myoepithelial phenotype. The MT-450 tumors were ultrastructurally similar to the nonmetastasizing tumors, but no features of myoepithelial cells were seen, although some cuboidal epithelial cells exhibited prominent tonofilaments. The SMT-2A and SMT-077 tumors consisted of nests of cuboidal-like cells with highly pleomorphic nuclei and much intercellular collagen. The results indicate a progressive loss of cellular differentiation characteristics, particularly those of the myoepithelial cell with increasing malignancy in this system.