A Novel Protein–Protein Interaction between RSK3 and IκBα and a New Binding Inhibitor That Suppresses Breast Cancer Tumorigenesis

SIMPLE SUMMARY: Breast cancer is the most common carcinoma and the leading cause of cancer-related death among women worldwide. Many kinases play important roles in the tumorigenesis of various cancers. IκBα phosphorylation is important for the regulation of NF-κB activity and is linked to the regulation of tumorigenesis. However, the kinase signaling network regulating IκBα phosphorylation in the context of cancer is not well understood. Herein, we report that RSK3 (RPS6KA2) is a novel binding partner of IκBα. RSK3 induces IκBα phosphorylation and NF-κB activation. Further, a chemical screening approach identified an inhibitor of RSK3/IκBα binding that impairs RSK3-mediated IκBα phosphorylation and decreases breast cancer cell survival, proliferation, and migration. ABSTRACT: Multiple cancer-related biological processes are mediated by protein-protein interactions (PPIs). Through interactions with a variety of factors, members of the ribosomal S6 kinase (RSK) family play roles in cell cycle progression and cell proliferation. In particular, RSK3 contributes to cancer viability, but the underlying mechanisms remain unknown. We performed a kinase library screen to find IκBα PPI binding partners and identified RSK3 as a novel IκBα binding partner using a cell-based distribution assay. In addition, we discovered a new PPI inhibitor using mammalian two-hybrid (MTH) analysis. We assessed the antitumor effects of the new inhibitor using cell proliferation and colony formation assays and monitored the rate of cell death by FACS apoptosis assay. IκBα is phosphorylated by the active form of the RSK3 kinase. A small-molecule inhibitor that targets the RSK3/IκBα complex exhibited antitumor activity in breast cancer cells and increased their rate of apoptosis. RSK3 phosphorylation and RSK3/IκBα complex formation might be functionally important in breast tumorigenesis. The RSK3/IκBα-specific binding inhibitor identified in this study represents a lead compound for the development of new anticancer drugs.