New Biological Evaluation of Thienoquinolines as Disruptors of the PKCε/RACK2 Protein–Protein Interaction

The superfamily of the protein kinase C (PKC) comprises ten isozymes and is widely known for its key role in signal transduction. Protein kinase Cε (PKCε) is known to play key roles in tumor suppression. PKCε requires activation to interact with RACK2, and the adaptor protein then translocates activated PKCε to subcellular sites within the proximity of their substrates. An EAVSLKPT peptide interferes with the interaction of PKCε and its adaptor protein RACK2. Since signaling in the malignant cells are sufficiently changed then the scope and limitations of PKCe as a anticancer drug target has to be estimated more clearly. Acquiring isozyme-selective inhibitors is a difficult task due to the high sequence similarity within the ten PKCs. Small molecule-disruptors of the PKCε/RACK2 protein–protein interaction could suppress PKCε signaling and reduce malignant properties. The EAVSLKPT peptide was used as a base of a pharmacophore model. Thieno[2,3-b]quinolines as a wide cluster of specific small-molecule inhibitors of the PKCε/RACK2 protein–protein interaction and PKCε signaling were revealed. The structural features of active thieno[2,3-b]quinolines were expanded on the basis of this pharmacophore model. The interaction between PKCε and RACK2 was measured using an ELISA-based assay. It was found that N-(4-acetylphenyl)-3-amino-6,7-ethelendioxy-thieno[2,3-b]quinoline-2-carboxamide (1b) shows promising inhibitory activities on the interaction of PKCε with its adaptor protein, the receptor for activated C-kinase 2 (RACK2), hence interfering with PKCε signaling. Both 1a and 1b did not show some cytotoxic properties on susceptible PC-3 cell line but both active compounds showed a significant antisprouting activity. The quinolines without thiophene ring as “open” analogs of 1b were inactive in primary assays. A structural isomer of (1a meta-acetyl), compound (1b para-acetyl) was found to exhibit, in addition to strong inhibitory activity on PKCε signaling with an IC₅₀ of 4.25 µM, also anti-angiogenic activities. Thus thieno[2,3-b]quinolines 1a and 1b could be reliable and selective biochemical tools to investigate of PKCe/RACK2 effects.