SPOP as an Emerging Key Player in Breast Cancer Oncogenesis

Abstract

Breast cancer is currently the second leading cause of cancer deaths among American women thereby highlighting the compelling need to uncover novel and superior treatment regimens for women suffering from this disease. Recent advances in genome‐wide studies have allowed for the identification of cancer‐related mutational landscapes, which allows for the identification of oncogenic alterations that are unique to particular cancer patients. Through this technology it has been uncovered that up to 37% of breast cancer patients have downregulated expression of the E3 ubiquitin ligase substrate binding subunit Speckle‐type POZ Protein (SPOP) thereby prompting our lab to study the mechanistic basis behind breast oncogenesis when SPOP expression is low. Previous findings from our lab and others have shown that SPOP targets GLI3 for ubiquitination and subsequent degradation which subsequently promotes SHH signaling in prostate cancer patients. Since SHH hedgehog signaling is often found to be hyper‐activated in advanced breast cancer, we hypothesize that downregulated SPOP promotes GLI3‐dependent Sonic Hedgehog (SHH) signaling in breast cancer patients to promote tumorigenesis and that this signaling pathway could prove to be an attractive therapeutic target for SPOP downregulated breast cancer. To study the effect of downregulated SPOP, a lentivirus carrying an shRNA against SPOP was generated and infected into MCF‐7 cells. Proliferation assays were utilized to confirm that downregulated SPOP plays a role in the increased proliferation of breast cancer cells. Next, quantitative PCR was performed to determine the effect of downregulated SPOP on the expression of GLI3‐dependent SHH target genes. Finally, a natural compound library was utilized to find a novel therapeutic strategy for SPOP downregulated breast cancer. Our findings confirmed that downregulation of SPOP increases proliferation of MCF‐7 cells in vitro. Furthermore, quantitative PCR confirmed that GLI3 target genes are upregulated in MCF‐7 cells when SPOP expression is low. Finally, our natural compound library screen identified novel therapeutic compounds that specifically target SPOP downregulated MCF‐7 cells in a manner that involves disruption of GLI3‐dependent SHH signaling. These findings therefore provide critical insight into novel treatment strategies for patients suffering from SPOP downregulated breast cancer.