Melanoma-associated antigen family protein-D4: clinical significance and functional relevance in breast cancer

Melanoma-associated antigen (MAGE) family genes are broadly expressed during development and are involved in the regulation of cell survival, cell cycle progression and apoptosis. MAGE family proteins are generally described as tumour-specific antigens and as representing ideal targets for cancer immunotherapy. In the current study, we identified melanoma-associated antigen protein-D4 (MAGE-D4), a recently characterised MAGE family member, as a new prognostic biomarker and potential therapeutic target for breast cancer. Specifically, in a whole genome microarray analysis of 103 cases of invasive breast tumours, MAGE-D4 expression was observed in 43.8% of tumours, while undetectable in normal breast tissue. Multivariate and univariate analyses also indicated MAGE-D4 expression to be associated with tumour grade, spread to lymph nodes and shortened times to relapse (P = 0.0369) and death (P = 0.0133) from time of cancer diagnosis; suggesting a role for MAGE-D4 in tumour progression. To further investigate the involvement of MAGE-D4 in breast cancer cell biology, the phenotypic effects of this gene were characterised in vitro. We observed a marked upregulation of MAGE-D4 expression - at both mRNA and protein levels - in the breast cancer cell line Hs578T compared with the syngenic Hs578Bst breast cell line. Interestingly, RNA interference-mediated knockdown of MAGE-D4 expression in Hs578T cells significantly reduced cell migration and invasion and correlated with inhibition of STAT3 and NF-κB p65 subunit phosphorylation, thus affecting two common signalling pathways involved in regulating cancer progression. Moreover, monolayer cell growth rate was not affected by MAGE-D4 gene knockdown, while growth in soft agar was significant compromised. Our results indicate that MAGE-D4 contributes to the tumorigenesis of breast cancer cells by regulating migration, invasion and anchorage-independent growth, and therefore may represent a novel target for the detection and treatment of breast cancer.