#3032 The p160 coactivators AIB1 and SRC-1 are known to play a critical role in modulating transcription in breast cancer cells in conjunction with ligand-bound estrogen receptor. The interactions of the p160 proteins with this nuclear receptor are also important in the development of resistance to endocrine treatments. Using quantitative coassociation immunofluorescent microscopy, the colocalisation of the p160s and ERα was increased in the LY2 endocrine resistant cell line following treatment with the anti-estrogen tamoxifen in comparison to the endocrine sensitive MCF-7 cell line. In cell cultures derived from patient tumours at the time of primary surgery prior to treatment, there was an increase in association of the coactivators with ERα following treatment with estrogen but dissociation was evident in the presence of tamoxifen. Immunohistochemical staining of a tissue microarray, constructed from 560 breast cancer patients, revealed that SRC-1 was a strong predictor of reduced disease-free survival, both in patients receiving adjuvant tamoxifen treatment and untreated patients (p<0.0001 and p=0.0111 respectively). AIB1 was not a significant independent predictor of disease recurrence. SRC-1 was assigned a hazard ratio of 2.12 when survival analysis using a Cox proportional hazards model was applied. Quantitative coassociation analysis of the p160 coactivators with ERα in the patient TMA revealed significantly stronger colocalisation of SRC-1 and ERα in patients who are known to have relapsed than those patients who did not recur (p=0.00001). This data suggests SRC-1 is pivotal in tumour aggressiveness and is a powerful predictor of progression of disease in breast cancer patients. Citation Information: Cancer Res 2009;69(2 Suppl):Abstract nr 3032.

Breast cancer affects one woman in ten in the western world and despite the phenomenal advances in recent years, the mortality rate still remains at around 35%. Resistance to current endocrine therapies, including tamoxifen and aromatase inhibitors, is common with recurrence of disease in 30-40% of patients. The estrogen receptor (ER) coactivator protein SRC-1 plays an important role in this resistance. We examined SRC-1 protein expression in a large cohort of breast cancer patients (n=560) and found expression to be a strong predictor of disease recurrence in patients treated with adjuvant tamoxifen (hazard ratio: 5.032, p<0.0001). Identification of SRC-1 interacting proteins will provide key insights into the role of SRC-1 in the development of tumour recurrence. We employed a mass spectrometry-based screen to identify proteins which could differentially interact with SRC-1 in endocrine resistant compared with endocrine sensitive breast cancer cells. Selected SRC-1 interacting proteins relevant to endocrine resistance include the high mobility proteins HMGB1 and HMGB2, which have previously been reported to enhance binding of the estrogen receptor to its response elements in target gene promoters. Immunoprecipitation assays confirmed increased interactions between HMGB2 and SRC-1 following treatment of the endocrine resistant cell line LY2 with tamoxifen and estrogen. Steroid treatment also increased trafficking of the coactivator and the SRC-1 interacting protein to the nucleus of tumour epithelial cells. Treatment enhanced recruitment of SRC-1 to the promoter of the oncogene c-Myc and enhanced target gene expression. HMGB2 expression in the breast cancer patient TMA correlated with SRC-1 and poor disease-free survival. These findings would indicate an important role for HMGB2 in the development of endocrine resistance in breast cancer patients. Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr 5138.