Expression of the nuclear coactivator AIB1 in normal and malignant breast tissue

The gene of the nuclear receptor coactivator AIB1 (amplified in breast canc er 1) is amplified in breast cancer cell lines as well as in breast tumor t issues. AIB1 mRNA is often highly expressed (> 60) in primary breast tumors and it has been shown that AIB1 enhances estrogen and progesterone depende nt transcription invitro. Therefore, it has been postulated that AIB1 contr ibutes to the development of breast cancer. However, to date, it has not be en shown that AIB1 amplification and overexpression correlates with elevate d protein levels in breast cancer tissues. In this study we analyzed protei n levels of AIB1 in normal and breast tumor tissues by immunohistochemistry . We compared 41 human breast tumor tissues with 24 normal breast tissue sa mples and found that AIB1 stained in the nuclei of approximately 46% of the tumors and 30% of the normal tissues. Overall, AIB1 protein levels were si gnificantly higher in tumor tissue than in normal tissue and the highest le vels of nuclear staining were found exclusively in breast tumor tissues in 9.8% of the cases. These data suggest that increased AIB1 mRNA expression d oes not always translate into elevated protein levels and that AIB1 most li kely will be relevant to the etiology of a subset of about 10% of breast ca rcinomas.