Organic copper complexes as a new class of proteasome inhibitors and apoptosis inducers in human cancer cells

Here we report that organic copper complexes can potently and selectively inhibit the chymotrypsin-like activity of the proteasome in vitro and in vivo. Several copper compounds, such as NCI-109268 and bis-8-hydroxyquinoline copper(II) [Cu(8-OHQ) 2], can inhibit the chymotrypsin-like activity of purified 20S proteasome. In human leukemia cells, proteasome inhibition occurs within 15 min after treatment, followed by apoptosis. Neither proteasome inhibition nor apoptosis occurs in non-transformed, immortalized human natural killer cells under the same treatment. Furthermore, proteasome inhibition and apoptosis induction were detected in prostate cancer cells treated with the ligand 8-OHQ alone following pre-treatment with copper(II) chloride. None of these events occurred in cells treated with copper(II) chloride alone, 8-OHQ alone (without growth in copper-enriched media), or nickel(II) chloride pre-treatment followed by 8-OHQ. Furthermore, we found that copper-mediated inhibition of purified 20S proteasome cannot be blocked by a reducing agent and that organic copper compounds do not generate hydrogen peroxide in the cells, suggesting that proteasome inhibition and apoptosis induction are not due to copper-mediated oxidative damage of proteins. Our results suggest that certain types of organic ligands could bind to tumor cellular copper, forming potent proteasome inhibitors and apoptosis inducers at copper concentrations found in tumor tissues.