Amplification of WHSC1L1 regulates expression and estrogen‐independent activation of ERα in SUM­‐44 cells and is associated with ERα overexpression in breast cancer. Estrogen Receptor Alpha biology in SUM44 Breast Cancer Cells

The 8p11-p12 amplicon occurs in approximately 15% of breast cancers and occurs in aggressive luminal B-type tumors. We have identified the WHSC1L1 oncogene as a driving oncogene from this region. Here we demonstrate that over-expression of WHSC1L1 is linked to over-expression of ERα in SUM-44 breast cancer cells, and in primary human breast cancers. Knockdown of WHSC1L1, particularly WHSC1L1-short, had a dramatic effect on ESR1 mRNA and ERα protein levels. SUM-44 cells do not require exogenous estrogen for growth in vitro; however they are dependent on ERα expression as ESR1 knockdown or exposure to the selective estrogen receptor degrader (SERD) fulvestrant resulted in growth inhibition. ChIP-seq experiments utilizing ERα antibodies demonstrated extensive ERα binding to chromatin in SUM-44 cells under estrogen-free conditions. ERα bound ERE and FOXA1 binding motifs under estrogen-free conditions and regulated expression of estrogen responsive genes. Short-term treatment with estradiol enhanced binding of ERα to chromatin and influenced expression of many of the same genes to which ERα was bound under estrogen-free conditions. Finally, knockdown of WHSC1L1 in SUM-44 cells resulted in loss of ERα binding to chromatin under estrogen-free conditions, which was restored upon exposure to estradiol. These results indicate that SUM-44 cells are a good model for a subset of luminal B breast cancers that have 8p11-p12 amplification, over-express WHSC1L1, and over-express ERα that is independent of estrogen for binding chromatin and regulating gene expression. Breast cancers that are dependent on ERα activity but independent of estradiol are a major cause of breast cancer mortality.