Data from Hippo/Mst1 Stimulates Transcription of the Proapoptotic Mediator <i>NOXA</i> in a FoxO1-Dependent Manner

Abstract

<div>Abstract<p>The proapoptotic protein Noxa, a member of the BH3-only Bcl-2 protein family, can effectively induce apoptosis in cancer cells, although the relevant regulatory pathways have been obscure. Previous studies of the cytotoxic effects of α-tocopheryl succinate (α-TOS) on cancer cells identified a mechanism whereby α-TOS caused apoptosis requiring the Noxa-Bak axis. In the present study, <i>ab initio</i> analysis revealed a conserved FoxO-binding site (DBE; DAF-16 binding element) in the <i>NOXA</i> promoter, and specific affinity of FoxO proteins to this DBE was confirmed by fluorescence anisotropy. FoxO1 and FoxO3a proteins accumulated in the nucleus of α-TOS–treated cells, and the drug-induced specific FoxO1 association with the <i>NOXA</i> promoter and its activation were validated by chromatin immunoprecipitation. Using siRNA knockdown, a specific role for the FoxO1 protein in activating <i>NOXA</i> transcription in cancer cells was identified. Furthermore, the proapoptotic kinase Hippo/Mst1 was found to be strongly activated by α-TOS, and inhibiting Hippo/Mst1 by specific siRNA prevented phosphorylation of FoxO1 and its nuclear translocation, thereby reducing levels of <i>NOXA</i> transcription and apoptosis in cancer cells exposed to α-TOS. Thus, we have demonstrated that anticancer drugs, exemplified by α-TOS, induce apoptosis by a mechanism involving the Hippo/Mst1-FoxO1-Noxa pathway. We propose that activation of this pathway provides a new paradigm for developing targeted cancer treatments. <i>Cancer Res; 71(3); 946–54. ©2011 AACR</i>.</p></div>