Discovery of novel effectors of the proteasome pathway: cyclopentenones as inhibitors of ubiquitin isopeptidase activity

I discovered that certain electrophilic prostaglandins inhibit the ubiquitin-specific protease (USP) activity of the proteasome pathway. Herein, evidence is presented that supports the hypothesis that the cross-conjugated α,β-unsaturated dienone is a molecular determinant for the potency of this activity, and that this chemical feature causes an alteration in cellular ubiquitin dynamics, resulting in decreased free ubiquitin and decreased protein degradation. I show that this decrease in protein degradation activates the unfolded-protein response (UPR) of both the cytoplasm and the endoplasmic reticulum, likely due to the accumulation of deranged/misfolded proteins. I make the novel observation that as an attempt to compensate for the loss in protein degradation by the proteasome pathway, the lysosomal degradation pathway is activated in USP inhibitor treated cells. Lastly I show that, ultimately, cell death occurs due to the build-up of toxic levels of cellular protein. These data reconcile previously known effects of prostaglandin treatment, namely that heat-shock proteins are up-regulated and that a number of short-lived proteins are stabilized, and in so doing, establish a cohesive model for prostaglandin-induced apoptosis. The potential that components of the ubiquitin-proteasome pathway may be useful targets for cancer chemotherapy has been realized only recently with the success, in human clinical trials, of the proteasome inhibitor, VELCADE™ (PS-341). Given that several hundred other potential molecular targets reside within the proteasome pathway, there is intense interest in discovering novel points for drug intervention. Our data suggest that inhibition of USP activity represents a legitimate target for chemotherapeutic development.