Abstract
<div>Abstract<p><b>Purpose:</b> Proteasome inhibition disrupts protein homeostasis and induces apoptosis. Up to 50% of patients with relapsed mantle cell lymphoma (MCL) respond to bortezomib. We used gene expression profiling to investigate the connection between proteasome inhibition, cellular response, and clinical efficacy.</p><p><b>Experimental Design:</b> We assessed transcriptional changes in primary tumor cells from five patients during treatment with bortezomib <i>in vivo</i>, and in 10 MCL cell lines exposed to bortezomib <i>in vitro</i>, on Affymetrix microarrays. Key findings were confirmed by western blotting.</p><p><b>Results:</b> MCL cell lines exposed to bortezomib <i>in vitro</i> showed upregulation of endoplasmic reticulum and oxidative stress response pathways. Gene expression changes were strongest in bortezomib-sensitive cells and these cells were also more sensitive to oxidative stress induced by H<sub>2</sub>O<sub>2</sub>. Purified tumor cells obtained at several timepoints during bortezomib treatment in 5 previously untreated patients with leukemic MCL showed strong activation of the antioxidant response controlled by NRF2. Unexpectedly, activation of this homeostatic program was significantly stronger in tumors with the best clinical response. Consistent with its proapoptotic function, we found upregulation of NOXA in circulating tumor cells of responding patients. In resistant cells, gene expression changes in response to bortezomib were limited and upregulation of NOXA was absent. Interestingly, at baseline, bortezomib-resistant cells displayed a relatively higher expression of the NRF2 gene-expression signature than sensitive cells (<i>P</i> < 0.001).</p><p><b>Conclusion:</b> Bortezomib triggers an oxidative stress response <i>in vitro</i> and <i>in vivo</i>. High cellular antioxidant capacity contributes to bortezomib resistance. <i>Clin Cancer Res; 17(15); 5101–12. ©2011 AACR</i>.</p></div>
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