O. Ullrich et al., Proteasome activation by poly-ADP-ribose-polymerase in human myelomonocytic cells after oxidative stress, FREE RAD B, 29(10), 2000, pp. 995-1004
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.