PURPOSE: p53 is inactivated in many human malignancies through missense 
mutations or overexpression of the human homologue of Mdm2 (Hdm2), an E3 
ubiquitin ligase that ubiquitinates p53, thereby promoting its proteasomal 
degradation. The cis-imidazoline nutlin-3 can disrupt the p53-Hdm2 interaction 
and activate p53, inducing apoptosis in vitro in many malignancies, including 
multiple myeloma (MM).
EXPERIMENTAL DESIGN: We hypothesized that suppression of Hdm2-mediated p53 
ubiquitination may augment sequelae of p53 accumulation caused by proteasomal 
inhibition. We compared the response of MM cells versus several epithelial 
cancer models to the proteasome inhibitor bortezomib in combination with 
nutlin-3.
RESULTS: The combination of sublethal concentrations of bortezomib plus nutlin-3 
induced additive cytotoxicity against bortezomib-sensitive MM cell lines. 
Importantly, however, in breast, prostate, colon, and thyroid (papillary, 
follicular, anaplastic, and medullary) carcinoma cell lines, this combination 
triggered synergistic cytotoxicity, and increased expression of p53, p21, Hdm2, 
Bax, Noxa, PUMA, and cleavage of caspase-3 and poly ADP ribose polymerase. 
Coculture with bone marrow stromal cells attenuated MM cell sensitivity to 
nutlin-3 monotherapy and was associated with evidence of suppression of p53 
activity in MM cells, whereas combined bortezomib-nutlin-3 treatment maintained 
cytotoxicity even in the presence of bone marrow stromal cells.
CONCLUSIONS: This differential response of MM versus epithelial carcinomas to 
combination of nutlin-3 with bortezomib sheds new light on the role of p53 in 
bortezomib-induced apoptosis. Concurrent Hdm2 inhibition with bortezomib may 
extend the spectrum of bortezomib applications to malignancies with currently 
limited sensitivity to single-agent bortezomib or, in the future, to MM patients 
with decreased clinical responsiveness to bortezomib-based therapy.