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 metasta sis, we analyzed a novel candidate metastasis-associated gene, mta1, w hich was isolated by differential cDNA library screening using the 137 62NF rat mammary adenocarcinoma metastatic system. Northern blot analy ses showed that the mRNA expression level of the mta1 gene was 4-fold higher in the highly metastatic cell line MTLn3 than in the nonmetasta tic cell line MTC.4. The mta1 gene was expressed in various normal rat organs, especially in the testis, suggesting its essential normal fun ction. The mRNA expression levels of the human homologue of this gene also correlated with the metastatic potential in two human breast canc er 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 phosphoryla tion sites, and a proline-rich amino acid stretch at the carboxyl-term inal end completely matched the consensus sequence for the src homolog y 3 domain-binding motif. Using antibodies raised against glutathione S-transferase-Mta1 fusion protein or a synthetic oligopeptide, Western blots showed that the mo lecular mass of the Mta1 protein was similar to 80 kDa, and the levels of the Mta1 protein also correlated with th e metastatic potential, results similar to those obtained from the Nor thern analyses. Thus, the novel gene mta1 may encode a molecule that i s functional in normal cells as well as in breast cancer invasion and metastasis.