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Session 1: Cell death Poster 17
Increase in p21 CDKN1A protein levels by protesome inhibitors does not affect the
apoptotic response after DNA damage
Micol Tillhon 1 , Nicoletta Pieri 1 , Ornella Cazzalini 2 , Lucia A. Stivala 2 , A. Ivana Scovassi 1 ,
Ennio Prosperi 1
1 Istituto di Genetica Molecolare CNR, Pavia; 2 Dipartimento di Medicina Sperimentale,
Sez. Patologia generale, Università di Pavia, Via Ferrata 1, 27100 Pavia, Italy -
tillhon@igm.cnr.it
p21 CDKN1A is a well known cell cycle inhibitor playing also important roles in transcription
regulation, apoptosis, and DNA repair. Pharmacological approaches have been developed to
contrast tumor cell proliferation, by exploiting the p21 ability to induce cell cycle arrest. In
this regard, both inhibitors of histone deacetylases (HDAC) and of the proteasomal
machinery, have been shown to increase p21 protein levels, thereby impairing tumor cell
growth. However, this type of intervention has been regarded as potentially dangerous, when
applied in concomitance with typical anticancer drugs inducing DNA damage. In fact, high
p21 levels have been shown to inhibit the apoptotic program and to allow the repair of DNA
damage, thereby rendering tumor cells less susceptible to the killing activity of the latter
drugs. Thus, p21 protein levels may greatly influence the outcome of chemotherapy. We have
previously found that HDAC inhibitors did not significantly affect initiation of nucleotide
excision repair (NER) in the presence of p21 protein. In contrast, no clear information is
available on the effect of proteasome inhibitors on DNA repair. Here, we have investigated
whether the presence of elevated p21 protein levels induced by the proteasome inhibitor
MG132, may affect the NER and the apoptotic process. We have analyzed the recruitment of
NER proteins and p21 to localized DNA damage sites, and determined NER efficiency in
normal, as well as in p21-null human fibroblasts. The results have shown that MG132 induced
the persistence of XPC, PCNA and p21 protein, at local DNA damage sites. The persistence
of p21 at DNA damage sites did not significantly affect the recruitment of other PCNAinteracting
NER factors, like DNA polymerase delta. However, NER efficiency was reduced
and cell death was increased by the proteasome inhibitor. These results suggest that MG132
affects NER efficiency and induce apototic cell death, independently of the presence of p21
protein. [Work supported by AIRC].
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