Abstract
One of the hallmarks of neoplastic cells is aberrant properties of their cell surfaces. Progressive changes in the phenotypes of tumor cells are often associated with alterations in composition as well as abundance of surface molecules. Highly metastatic acites sublines of 13762 rat mammary adenocarcinoma cells express abundantly a large glycoprotein complex called sialomucin complex (SMC). However, in spite of the fact that the molecule had been suggested to play significant roles in tumor progression and malignancy, the functional characterization of the glycoprotein has been limited. Recent advances in molecular cloning and gene expression technologies, such as tetracycline-regulated inducible expression, enabled us to examine the roles that SMC plays in cancer and in normal tissues. To investigate the role of SMC in tumor metastasis at the molecular level, recombinant cDNAs for SMC were transfected and expressed in mammalian cell lines. To examine the biological consequences of this multifunctionality, we tested whether the metastasis of cancer cells in the animal can be modulated by regulating SMC levels. For this purpose, SMC-transfected A375 melanoma cells were injected i.v. into the tail vein of nude mice, and the SMC expression level in these cancer cells was regulated by the oral administration of tetracycline. In this experimental metastasis assay, lung metastases, were found to be substantially greater in both number and colony size in mice bearing SMC-overexpressing cancer cells than in mice under tetracycline therapy to repress SMC expression. These observations confirmed that overexpression of SMC can contribute significantly to the metastatic ability of the tumor cells. We therefore propose that SMC is a multifunctional complex which confers anti-adhesive properties and immune resistance as well as altered cellular signaling, all of which may be critical characteristics of metastatic cancer.SMC is also expressed in normal secretory epithelial cells. Immunological analyses using monoclonal antibodies revealed the presence of the glycoprotein in many glands/tissues of human and rat, suggesting a diversity in its physiological roles. In contrast to the membrane association in 13762 ascites tumor cells, SMC is produced in normal tissues as both membrane-associated and secreted/soluble forms. The soluble SMC is a truncated isoform that lacks the transmembrane and cytoplasmic domains. Since the membrane association is a critical feature of tumor SMC for its contribution to malignancy, it is of interest to investigate how SMC is produced as a secreted form in normal tissues. Analysis of SMC mRNA by RT-PCR argues against alternative splicing as a mechanism for production of the secreted isoform. Here, we tested a mechanism involving specific proteolytic cleavage of membrane-bound isoform to release the extracellular portion of the SMC molecule. (Abstract shortened by UMI.)