Investigating Chalcones and their Ability to Inhibit the Human HSP60/10 Chaperonin System and Colorectal Cancer Cells

Abstract

Colorectal cancer is the fourth commonly diagnosed cancer worldwide. Despite the different therapeutic strategies, the five-year survival rate of Stage IV colorectal cancer is 10%. Hence, there is an urgent need to develop novel therapies. Heat shock proteins (HSPs) have attracted attention as anti-cancer targets since they are involved in cancer development. We are interested in targeting the HSP60/10 chaperonin system, a.k.a GroEL/ES in bacteria. In healthy cells, HSP60/10 is in the mitochondrial matrix and assists in protein folding. However, recent studies demonstrate an aberrant localization of HSP60 in cytosol, which is hypothesized to promote tumor cell survival and proliferation. This mis-localization may make it possible to selectively target cancer cells. We recently reported results from two high-throughput screens (HTS) that identified several hundred inhibitors of the prototypical GroEL/ES chaperonin system from Escherichia coli, most of which also inhibited human HSP60/10. Several hits contained the “chalcone” core scaffold, which consists of two aromatic rings joined by an α,β-unsaturated ketone linker. We hypothesize that chalcone-based HSP60/10 inhibitors could be developed that will exhibit selective cytotoxicity. The main objective was to generate structure-activity relationships (SAR) that identify the key substructures that allow chalcone analogs to inhibit HSP60/10 biochemical functioning and selectively target colorectal cancer cells. Three sub-structures of typical chalcones – the α,β-unsaturated ketone linker and the two aryl rings – were varied and evaluated in a panel of chaperonin-mediated biochemical assays and cell viability assays using cancerous and non-cancerous colon and intestine cells. While our results indicated that the linker was highly important for generating potent inhibition in HSP60/10 biochemical assays and cell viability assays, we found that analogs bearing a 2-nitro substituent on the B-ring were most selective for targeting cancer cells. While these lead compounds exhibited weak inhibition in a wound healing assay, they were nearly as equipotent in a clonogenic assay as they were in the cell viability assay. This study identified key SAR that allow chalcone analogs to inhibit the HSP60/10 chaperonin system and selectively target colorectal cancer cells. These findings will help future studies to optimize the pharmacological properties of this series of HSP60/10-targeting colorectal cancer chemotherapeutics.