1. ¹Department of Cancer Biology, The University of Texas MD Anderson Cancer Center, Houston, TX, USA
2. ²Department of Urology, The University of Texas MD Anderson Cancer Center, Houston, TX, USA

Correspondence: Dr DJ McConkey, Departments of Urology and Cancer Biology, University of Texas MD Anderson Cancer Center, Box 173, 1515 Holcombe Boulevard, Houston, TX 77030, USA. E-mail: dmcconke@mdanderson.org

Received 19 February 2009; Revised 28 August 2009; Accepted 4 September 2009; Published online 2 November 2009.

Abstract

The ubiquitin-proteasome and lysosome-autophagy pathways are the two major intracellular protein degradation systems that work cooperatively to maintain homeostasis. Proteasome inhibitors (PIs) have clinical activity in hematological tumors, and inhibitors of autophagy are also being evaluated as potential antitumor therapies. In this study, we found that chemical PIs and small interfering RNA-mediated knockdown of the proteasome's enzymatic subunits promoted autophagosome formation, stimulated autophagic flux, and upregulated expression of the autophagy-specific genes (ATGs) (ATG5 and ATG7) in some human prostate cancer cells and immortalized mouse embryonic fibroblasts (MEFs). Upregulation of ATG5 and ATG7 only occurred in cells displaying PI-induced phosphorylation of the eukaryotic translation initiation factor 2 alpha (eIF2), an important component of the unfolded protein responses. Furthermore, PIs did not induce autophagy or upregulate ATG5 in MEFs expressing a phosphorylation-deficient mutant form of eIF2. Combined inhibition of autophagy and the proteasome induced an accumulation of intracellular protein aggregates reminiscent of neuronal inclusion bodies and caused more cancer cell death than blocking either degradation pathway alone. Overall, our data show that proteasome inhibition activates autophagy through a phospho-eIF2-dependent mechanism to eliminate protein aggregates and alleviate proteotoxic stress.