ESTROGEN-DEPENDENT SUSHI DOMAIN CONTAINING 3 REGULATES CYTOSKELETON ORGANIZATION AND MIGRATION IN BREAST CANCER CELLS

Aromatase inhibitors (AI) are the standard endocrine therapy for postmenopausal breast cancer; however, currently used biomarkers, i.e., estrogen receptor-alpha/progesterone receptor (ERα/PR), predict only slightly more than half of the potential responders to AI treatment. To identify novel markers of AI responsiveness, a genome-wide microarray analysis was performed using primary breast tumor samples from 50 postmenopausal women (PMW) who later developed metastatic breast cancer. Sushi domain containing 3 (SUSD3) was significantly differentially expressed gene, with 3.38-fold higher mRNA levels in AI-responsive breast tumors versus non-responders (p<0.001). SUSD3 was highly expressed in ERα-positive breast tumors and treatment with estradiol increased SUSD3 expression in ERα-positive breast cancer cells. Treatment with an antiestrogen or ERα knockdown abolished basal and estradiol-dependent SUSD3 expression. Recruitment of ERα upstream of the transcription start site of SUSD3 was demonstrated by chromatin immunoprecipitation (ChIP)-PCR. Flow cytometric analysis of SUSD3 knockdown cells revealed blunted estradiol effects on progression into S and M phases. SUSD3 was localized to the plasma membrane of breast cancer cells. SUSD3 knockdown decreased the appearance of actin-rich protrusions, stress fibers and large basal focal adhesions, while increasing the presence of cortical actin concomitant with a decrease in Rho and FAK activity. SUSD3-deficient cells demonstrated diminished cell spreading, cell-cell adhesion, and motility. In conclusion, SUSD3 is a novel promoter of estrogen-dependent cell proliferation and regulator of cell-cell and cell-substrate interactions and migration in breast cancer. It may serve as a novel predictor of response to endocrine therapy and potential therapeutic target.