Poly-ADP ribose polymerase activates nuclear proteasome to degrade oxidatively damaged histones

The 20S proteasome has been shown to be largely responsible for the degrada tion of oxidatively modified proteins in the cytoplasm. Nuclear proteins ar e also subject to oxidation, and the nucleus of mammalian cells contains pr oteasome. In human beings, tumor cells frequently are subjected to oxidatio n as a consequence of antitumor chemotherapy, and K562 human myelogenous le ukemia cells have a higher nuclear proteasome activity than do nonmalignant cells, Adaptation to oxidative stress appears to be one element in the dev elopment of long-term resistance to many chemotherapeutic drugs and the mec hanisms of inducible tumor resistance to oxidation are of obvious importanc e, After hydrogen peroxide treatment of K562 cells, degradation of the mode l proteasome peptide substrate suc-LLVY-MCA and degradation of oxidized his tones in nuclei increases significantly within minutes. Both increased prot eolytic susceptibility of the histone substrates (caused by modification by oxidation) and activation of the proteasome enzyme complex occur independe ntly during oxidative stress. This rapid upregulation of 20S proteasome act ivity is accompanied by, and depends on, poly-ADP ribosylation of the prote asome, as shown by inhibitor experiments, C-14-ADP ribose incorporation ass ays, immunoblotting, in vitro reconstitution experiments, and immunoprecipi tation of (activated) proteasome with anti-poly-ADP ribose polymerase antib odies, The poly-ADP ribosylation-mediated activated nuclear 20S proteasome is able to remove oxidatively damaged histones more efficiently and therefo re is proposed as an oxidant-stimulatable defense or repair system of the n ucleus in K562 leukemia cells.