Inhibition of proteolytic and ATPase activities of the proteasome by the BTK inhibitor CGI-1746

Bruton’s tyrosine kinase (BTK) inhibitor, ibrutinib, has been shown to synergize in vitro with proteasome inhibitors (PIs) in reducing the viability of cells derived from B cell malignancies, but the mechanism is not known. We report here that an off-target effect of ibrutinib causes synergy because not all BTK inhibitors exhibited the synergistic effect, and those that synergized did so even in cells that do not express BTK. The allosteric BTK inhibitor CGI-1746 showed the strongest synergy. Co-treatment of cells with CGI-1746 increased PI-induced accumulation of ubiquitin conjugates and expression of heat shock proteins and NOXA and decreased a ratio of reduced to oxidized glutathione. CGI-1746, but not other BTK inhibitors, inhibited ATPase activity and all three peptidase activities of the 26S proteasome. The effect demonstrates a conceptually novel mode of proteasome inhibition that may aid the development of more potent PIs. [Display omitted] •Some BTK inhibitors exhibit synergistic cytotoxicity with proteasome inhibitors•Synergy is observed in cell lines that do not express BTK•Allosteric BTK inhibitor CGI-1746 shows the strongest synergy•CGI-1746 inhibits peptidase and ATPase activities of the 26S proteasome Biological sciences; Biochemistry; Cell biology; Cancer