The MAGE-1 gene codes for
tumor-associated
peptides recognized by cytolytic T lymphocytes in association with MHC-class-1 molecules such as
HLA-A1 and
HLA-Cw16. In the course of a study aiming at the immunohistochemical detection of the MAGE-1 gene product in
tumor samples, 2 mouse
monoclonal antibodies (MAbs) directed against a full-length recombinant MAGE-1 fusion
protein were found to react strongly not only with the 46-kDa MAGE-1
protein, but also with a 72-kDa product in immunoblots of lysates obtained from several MAGE-1-mRNA-positive
melanoma cell lines. Pre-incubation of the
antibodies with the recombinant MAGE-1 fusion
protein abolished their reactivity both with MAGE-1
protein and with the 72-kDa product, thus confirming the occurrence of
antigenic determinant(s) shared by the 2
proteins. The 72-kDa
protein is not an alternative product of MAGE-1, since it was still detected in lysates of a MAGE-1 loss variant derived from a MAGE-1-positive
melanoma cell line. Moreover, the 72-kDa
protein does not appear to be a product of the other members of the MAGE gene family known to be expressed in
tumors (such as MAGE-2, -3, -4 and -12). Interestingly, expression of the 72-kDa
protein was found to be correlated with that of MAGE-1
protein. Thus, in 30 tumor cell lines analyzed by immunoblotting and RT-PCR, the 72-kDa
protein was never detected in MAGE-1-mRNA-negative cell lines, while it was co-expressed with MAGE-1
protein in 12 out of 15 cell lines expressing MAGE-1. Furthermore, the 72-kDa
protein was detected in lysates of human testis, the only normal tissue known to express MAGE-1. Finally, treatment of MAGE-1-mRNA-negative cell lines with
5-Aza-2'-deoxycytidine, a hypomethylating agent known to induce MAGE-1 expression, resulted in the expression of the 72-kDa
protein. Taken collectively, these findings suggest that expression of the gene encoding the 72-kDa
protein identified in this study through
antigenic determinant(s) shared with MAGE-1
protein is regulated in a way similar to that of MAGE-1.