Inhibition of Protein Degradation Induces Apoptosis through a Microtubule-Associated Protein 1 Light Chain 3-Mediated Activation of Caspase-8 at Intracellular Membranes

Abstract

The accumulation of damaged or misfolded proteins, if unresolved, can lead to a detrimental consequence within cells termed proteotoxicity. Since cancerous cells often display elevated protein synthesis and by-product disposal, inhibition of the protein degradation pathways is an emerging approach for cancer therapy. However, the molecular mechanism underlying proteotoxicity remains largely unclear. We show here that inhibition of proteasomal degradation results in an increased oligomerization and activation of caspase-8 on the cytosolic side of intracellular membranes. This enhanced caspase-8 oligomerization and activation are promoted through its interaction with the ubiquitin-binding protein SQSTM1/p62 and the microtubule-associated protein light chain 3 (LC3), which are enriched at intracellular membranes in response to proteotoxic stress. Silencing LC3 by shRNA, or the LC3 mutants defective in membrane localization or p62 interaction fail to induce caspase-8 activation and apoptosis. Our results unveiled a previously unknown mechanism through which disruption of protein homeostasis induces caspase-8 oligomerization, activation, and apoptosis.

ACKNOWLEDGMENTS

We thank Eileen White for the bax^−/− bak^−/− BMK cells, Erich Mackow for the polyHis-Ub construct, Michael Frohman and Huiyan Huang for the Venus and linker sequences, William Lennarz and Gang Zhao for assistance on the size exclusion chromatography, and Juei-Suei Chen for molecular cloning. We thank Susan Van Horn and Guowei Tian (Stony Brook Microscopy Imaging Center) for assistance on transmission electron microscopy and fluorescence microscopy, respectively.

E.U. is supported by the NIH training grant T32CA009176. This study was supported by NIH (CA098092 and CA129536), Susan Komen for the Cure (KG081538), and the Carol Baldwin Breast Cancer Research Foundation (W.-X.Z.).

The authors declare that they have no conflicts of interest.