Intracellular estrogen receptor-binding fragment-associated antigen 9 exerts in vivo tumor-promoting effects via its coiled-coil region.

Estrogen receptor-binding fragment-associated antigen 9 (EBAG9) is a tumor-promoting factor of largely unknown function. To assess a causative role of EBAG9 in advanced malignancies, we generated the EG7-OVA and MethA murine tumor cell lines that stably express full-length or truncated EBAG9 protein, using retroviral-mediated gene transduction. Upon subcutaneous inoculation into immunocompetent mice, both cell lines showed marked acceleration of in vivo tumor growth when full-length EBAG9 was overexpressed. Interestingly, deletion of the coiled-coil region, thereby producing truncated EBAG9 protein, abolished the tumor-acceleration effect, establishing the importance of this domain in EBAG9-mediated tumor promotion. However, there was no alteration in in vitro cell proliferation or expression levels of MHC class I and co-stimulatory molecules believed to play a role in immune evasion of tumor cells in these tumor cell lines expressing full-length or truncated EBAG9 protein. Furthermore, both full-length and truncated EBAG9 proteins showed a predominantly cytoplasmic localization in the tumor cells. Collectively, these results suggest that EBAG9 overexpression can be causative in enhancing the malignant properties of tumor cells, and that tumor promotion likely requires EBAG9 intracellular association with as yet unidentified binding partners via the coiled-coil region.