Elimination of misfolded proteins from the endoplasmic reticulum (ER) by ER-associated degradation (ERAD) involves substrate retrotranslocation from the ER lumen into the cytosol for degradation by the proteasome. For many substrates, retrotranslocation requires the action of ubiquitinating enzymes, which polyubiquitinate substrates emerging from the ER lumen, and of the p97-Ufd1-Npl4 ATPase complex, which dislocates polyubiquitinated substrates into the cytosol. Polypeptides extracted by p97 are eventually transferred to the proteasome for destruction. Eeyarestatin I (EerI) was previously identified as a potent inhibitor of retrotranslocation, but the mechanism of its action is unclear. Our work demonstrates that EerI associates with the p97 ATPase to block p97-associated deubiquitination (PAD), which is essential for the degradation of misfolded ER proteins. We further identify ataxin-3 (atx3), a p97-associated deubiquitinating enzyme previously implicated in ERAD, as one of the targets affected by EerI. In the absence of PAD, substrates carrying unprocessed polyubiquitin chains reach the proteasome, but remain bound by the proteasome without being degraded. Our analyses establish a role for a novel deubiquitinating process in proteasome dependent protein turnover. Given that the ubiquitin-proteasome system (UPS) has recently emerged as a major target for drug development in cancer therapy, we study whether EerI has anti-cancer activity. Indeed, we found that Eeyarestatin I (EerI) has anti-tumor and biologic activities similar to bortezomib, a proteasome inhibitor used in clinic to treat several types of tumors. Like bortezomib, EerI-induced cytotoxicity requires the upregulation of the BH3 only pro-apoptotic protein NOXA. We further demonstrate that both EerI and bortezomib activate NOXA via an unanticipated mechanism that requires cooperation between two processes: First, these agents elicit an integrated stress response program at the ER to activate the CREB/ATF transcription factors ATF3 and ATF4. We show that ATF3 and ATF4 form a complex capable of binding to the NOXA promoter to facilitate its transcription. Second, EerI and bortezomib also decrease the level of ubiquitinated histone H2A to relieve its inhibition on NOXA transcription. Our results identify a class of anti-cancer agents that integrate ER stress response with an epigenetic mechanism to induce cell death. We recently developed in vitro binding and cell-based functional assays to demonstrate that a nitrofuran-containing (NFC) group in EerI is the functional domain responsible for the cytotoxicity. Using both SPR and pull down assays, we show that EerI directly binds the p97 ATPase, an essential component of the ERAD machinery, via the NFC domain. An aromatic domain in EerI, although not required for p97 interaction, can localize EerI to the ER membrane, which improves its target specificity. Substitution of the aromatic module with another benzene-containing domain that maintains membrane localization generates a structurally distinct compound that nonetheless has similar biologic activities as EerI. Our findings reveal a class of bifunctional chemical agents that can preferentially inhibit membrane-bound p97 to disrupt ER homeostasis and induce tumor cell death. These results also suggest that the AAA ATPase p97 may be a potential drug target for cancer therapy.
| Wang, Qiuyan; Li, Lianyun; Ye, Yihong (2008) Inhibition of p97-dependent protein degradation by Eeyarestatin I. J Biol Chem 283:7445-54 |
