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.

Background: The phosphorylation of NF-kappaB inhibitor alpha (IκBα) protein is pivotal to the regulation of NF-κB transcription factor activity in the cell. The phosphorylation of IκBα by IκB kinase family have been so many identified, but the phosphorylation sites of IκBα by other kinases remain poorly understood. We investigated the new phosphorylation site for IκBα and identified its biological function in breast cancer cells. Methods: Previously we observed that aurora kinase (AURK) binds IκBα in the cell. To identify the essential domains of IκBα for phosphorylation of IκBα by AURK, kinase assay was performed with a series of IκBα truncation mutants. AURK significantly promotes activation of IκBα at serine 32, but not serine 36 residues, unlike IκB kinase (IKK) family proteins activate both IκBα serine residues. We also confirmed phosphorylation of IκBα by the matrix-assisted laser-desorption/ionization time-of-flight mass spectrometry (MALDI-TOF/TOF MS) and nano-liquid chromatography hybrid quadrupole-Orbitrap mass spectrometer (nanoLC-MS/MS; Q Exactive). Results: We identified two novel sites of serine phosphorylation at S63 and S262. Alanine transition of S63 and S262 (S63A and S262A) of IκBα induced inhibition of cell proliferation and suppression of p65 transcription activity. Besides, S63A and/or S262A of IκBα regulated apoptotic and necroptic effects in breast cancer cells. Conclusions: Therefore, we identified novel phosphorylation site of IκBα by AURK, and its site was related to apoptosis and necroptosis pathway in breast cancer cells.