Proteasome activation by poly-ADP-ribose-polymerase in human myelomonocytic cells after oxidative stress

Cytotoxic action of a variety of antitumor drugs generate oxidatively modif ied proteins that are predominantly metabolized via the proteasome. In the present study, a differentiation-retrodifferentiation cell system was expos ed to oxidative stress by hydrogen peroxide treatment. Thus, the activity o f the nuclear proteasome in proliferating human U937 leukemic cells increas ed by 2.5-fold after hydrogen peroxide treatment. In contrast, growth-arres ted differentiated U937 cells demonstrated 40% less constitutive proteasoma l activity, which was not inducible after hydrogen peroxide exposure. After a retrodifferentiation process, however, in which differentiated U937 cell s resume autonomous growth again, the proteasomal activity was indistinguis hable from that in U937 control cells, both constitutively and after induct ion of oxidative stress. Moreover, cells of TUR, a differentiation-resistan t U937 subclone, expressed an elevated constitutive proteasomal activity th at increased by 2.5-fold after oxidative stress. Immunoblot analysis reveal ed that these differences in proteasomal activities did not correlate with proteasome protein expression but with protein levels of the nuclear enzyme poly-ADP-ribose-polymerase (PARP). Further studies using specific PARP inh ibitors revealed that the noninducible proteasome activity in differentiate d U937 cells was PARP independent, whereas the increased activity level in oxidatively stressed TUR cells was downregulated upon PARP inhibition. Immu noprecipitation experiments demonstrated a protein-protein interaction of t he functional active PARP with the proteasome in correlation with the prote asome activity. Similar results were obtained by analyzing protein carbonyl s after oxidative stress. Taken together, these data suggest that prolifera ting, rather than growth-arrested, cells metabolize oxidatively damaged nuc lear proteins via the proteasome by expressing high levels of PARP. (C) 200 0 Elsevier Science Inc.