Inhibition of Increased Invasiveness of Breast Cancer Cells With Acquired Tamoxifen Resistance by Suppression of CYR61

Background/Aim: Hormone sensitivity-targeted therapy with selective estrogen receptor modulators (SERMs), such as 4-hydroxytamoxifen (4-OHT), is the mainstay of treatment for breast cancers (BCs) that express estrogen receptor alpha (ER alpha). However, development of resistance limits this therapy approach. The question arises whether changes associated with 4-OHT resistance could be exploited therapeutically. Materials and Methods: First, 4-OHT-resistant sublines of ER alpha-positive breast carcinoma cell lines MCF-7 and T47D were generated. Viability was assessed by the Alamar Blue assay. Cell invasion was quantified in modified Boyden chambers with Matrigel. Changes in expression of CYR61, S100A4, and ER alpha were examined by RT-qPCR. Expression of CYR61 was suppressed by transient gene silencing using siRNA. Successful suppression was verified by western blot. Efficacy of 4-OHT treatment was analyzed by quantification of viability using Alamar Blue assay. Correlation of CYR61 levels in patients with luminal A BC to distant metastases-free survival was determined by Kaplan-Meier analysis. Results: ER alpha-positive MCF-7 and T47D BC cells exhibit an extremely weak invasion rate. Acquired tamoxifen resistance significantly increased the invasive behavior of both tamoxifen-resistant MCF-7-TR and T47D-TR sublines. In addition, expression of CYR61 and S100A4 showed significantly increased levels, whereas expression of ER alpha was decreased. Suppression of CYR61 expression resulted in a significant decreased invasion rate. In addition, expression of S100A4 was reduced, whereas expression of ER alpha was increased. Furthermore, suppression of CYR61 resulted in re sensitization to 4-OHT. High CYR61 levels in patients with luminal A BC resulted in reduced distant metastases-free survival. Conclusion: The prometastatic factor CYR61 appears play an important role in the increased invasiveness of tamoxifen-resistant ER alpha-positive BC cells. Its suppression leads to a lower invasion rate. Given the few therapeutic options available for tamoxifen-resistant BC, therapy that reduces CYR61 may improve its treatability in future.