HSP27 Is a Ubiquitin-Binding Protein Involved in I-κBα Proteasomal Degradation

Abstract

HSP27 is an ATP-independent chaperone that confers protection against apoptosis through various mechanisms, including a direct interaction with cytochrome c. Here we show that HSP27 overexpression in various cell types enhances the degradation of ubiquitinated proteins by the 26S proteasome in response to stressful stimuli, such as etoposide or tumor necrosis factor alpha (TNF-α). We demonstrate that HSP27 binds to polyubiquitin chains and to the 26S proteasome in vitro and in vivo. The ubiquitin-proteasome pathway is involved in the activation of transcription factor NF-κB by degrading its main inhibitor, I-κBα. HSP27 overexpression increases NF-κB nuclear relocalization, DNA binding, and transcriptional activity induced by etoposide, ΤNF-α, and interleukin 1β. HSP27 does not affect I-κBα phosphorylation but enhances the degradation of phosphorylated I-κBα by the proteasome. The interaction of HSP27 with the 26S proteasome is required to activate the proteasome and the degradation of phosphorylated I-κBα. A protein complex that includes HSP27, phosphorylated I-κBα, and the 26S proteasome is formed. Based on these observations, we propose that HSP27, under stress conditions, favors the degradation of ubiquitinated proteins, such as phosphorylated I-κBα. This novel function of HSP27 would account for its antiapoptotic properties through the enhancement of NF-κB activity.

ACKNOWLEDGMENTS

We thank A. P. Arrigo, C. Ducasse, and C. Diaz-Latoud (CNRS UMR 5534, Villeurbanne, France) for the HSP27-derived plasmids and helpful discussions. We thank A. Brent Carter (University of Iowa Health Care) for plasmid p4NF-κB-luc. We thank A. Hammann, F. Ghiringhelli, and C. Rébé for technical assistance.

This work was supported by the Institut National pour la Santé et la Recherche Medicale (INSERM) and by grants from the Ligue Nationale contre le Cancer and the Association pour la Recherche contre le Cancer (ARC grants 9567 and 5204). A.P. was supported by a fellowship from the Ministère de l'Education Nationale, and S.G. was supported by a poste vert from the INSERM.