The majority of human breast cancers express estrogen receptor alpha (ER), which is important for therapy with anti-estrogens. Here we describe the role of BCL9-2, a proto-oncogene previously characterized as co-activator of Wnt/ß-catenin signaling, for mammary tumorigenesis in mice and human. ER positive human breast cancers showed overexpression of BCL9-2 and tamoxifen treated patients with high BCL9-2 demonstrated a better survival. BCL9-2 was upregulated during puberty and pregnancy in normal mammary epithelia, but downregulated in the involuted gland. BCL9-2 overexpression in vivo delayed the mammary involution and induced alveolar hyperplasia. Moreover, aged BCL9-2 transgenic mice developed ductal-like mammary tumors with high nuclear ER expression. We found, that primary cell cultures of BCL9-2 breast tumors responded to tamoxifen treatment. Moreover, BCL9-2 regulated the expression of ER and the proliferation of human breast cancer cells independently of ß-catenin. Finally, we describe a novel mechanism, how BCL9-2 regulates ER transcription by interaction with Sp1 through the proximal ESR1 gene promoter. In summary, BCL9-2 induces ER positive breast cancers in vivo, regulates ER expression by a novel ß-catenin independent mechanism in breast cancer cells, and might predict the therapy response to tamoxifen treatment.

Abstract. Three-dimensional (3D) multicellular spheroids (MCS) are considered suitable models in cancer research and anticancer drug development. Although studying the complex tumour characteristics from all different degrees of malignancy is vital, MCS generation has only been described in a few moderately-and poorly differentiated oral squamous cell carcinoma (OSCC) cell lines. No previous study has demonstrated the MCS formation in a highly differentiated OSCC cell line. For the first time, the present study aimed to generate MCS from the highly differentiated OSCC cell line BHY. BHY spheroids were grown in three independent experiments in 96-well plates through the use of the liquid overlay technique. Although BHY cells are grow slowly and are difficult to culture, they formed compact MCS within 24 h. After 3 days of incubation, no further increase in spheroid size was observed. MCS were harvested, paraffin-embedded and 2 µm tissue sections were prepared for further analysis. The diameter and volume of each spheroid were determined. BHY MCS diameter ranged between 46.76 and 233.26 µm, with a volume range from 5.35x10 4 -6.65x10 6 µm³. In conclusion, using the liquid overlay technique, the highly differentiated OSCC cell line BHY forms different sized spheroids, which may be used for further investigations.