A novel candidate metastasis-associated gene, mta1, differentially expressed in highly metastatic mammary adenocarcinoma cell lines. cDNA cloning, expression, and protein analyses.

To understand the genes involved in breast cancer invasion and metastasis, we analyzed a novel candidate metastasis-associated gene, mta1, which was isolated by differential cDNA library screening using the 13762NF rat mammary adenocarcinoma metastatic system. Northern blot analyses showed that the mRNA expression level of the mta1 gene was 4-fold higher in the highly metastatic cell line MTLn3 than in the nonmetastatic cell line MTC.4. The mta1 gene was expressed in various normal rat organs, especially in the testis, suggesting its essential normal function. The mRNA expression levels of the human homologue of this gene also correlated with the metastatic potential in two human breast cancer metastatic systems. The full-length mta1 cDNA sequence contained an open reading frame encoding a protein of 703 amino acid residues, and sequence analysis by data base homology search indicated that mta1 is a novel gene. The Mta1 protein contained several possible phosphorylation sites, and a proline-rich amino acid stretch at the carboxyl-terminal end completely matched the consensus sequence for the src homology 3 domain-binding motif. Using antibodies raised against glutathione S-transferase-Mta1 fusion protein or a synthetic oligopeptide, Western blots showed that the molecular mass of the Mta1 protein was approximately 80 kDa, and the levels of the Mta1 protein also correlated with the metastatic potential, results similar to those obtained from the Northern analyses. Thus, the novel gene mta1 may encode a molecule that is functional in normal cells as well as in breast cancer invasion and metastasis.