HRI-mediated translational repression reduces proteotoxicity and sensitivity to bortezomib in human pancreatic cancer cells

Human cancer cells display extensive heterogeneity in their sensitivities to the proteasome inhibitor bortezomib (Velcade). The molecular mechanisms underlying this heterogeneity remain unclear, and strategies to overcome resistance are limited. Here, we discover that inherent differences in eIF2 alpha phosphorylation among a panel of ten human pancreatic cancer cell lines significantly impacts bortezomib sensitivity, and implicate the HRI (heme-regulated inhibitor) eIF2 alpha kinase as a novel therapeutic target. Within our panel, we identified a subset of cell lines with defective induction of eIF2 alpha phosphorylation, conferring a high degree of sensitivity to bortezomib. These bortezomib-sensitive cells exhibited impaired translation attenuation followed by toxic accumulation of protein aggregates and reactive oxygen species (ROS), whereas the bortezomib-resistant cell lines displayed increased phosphorylation of eIF2 alpha, decreased translation, few protein aggregates, and minimal ROS production. Importantly, we identified HRI as the primary bortezomib-activated eIF2 alpha kinase, and demonstrated that HRI knockdown promoted cell death in the bortezomib-resistant cells. Overall, our data implicate inducible HRI-mediated phosphorylation of eIF2 alpha as a central cytoprotective mechanism following exposure to bortezomib and provide proof-of-concept for the development of HRI inhibitors to overcome proteasome inhibitor resistance.