Chemical inhibition of the integrated stress response impairs the ubiquitin-proteasome system

The Integrated Stress Response Inhibitor (ISRIB) is an experimental compound that has been used to explore the potential beneficial effects of reducing the activation of the integrated stress response (ISR). As the ISR is a protective response, there is, however, a risk that its inhibition may compromise the cell’s ability to restore protein homeostasis. Here, we show that ISRIB treatment impairs degradation of proteins by the ubiquitin-proteasome system (UPS) during proteotoxic stress in the cytosolic, but not nuclear, compartment. Degradation of proteins intercepted by ribosome quality control (RQC) was particularly affected as accumulation of a UPS reporter substrate for ribosome quality control (RQC) was comparable to the level observed after proteasome inhibition. Consistent with impaired RQC, ISRIB treatment caused an accumulation of polyubiquitylated and detergent insoluble defective ribosome products (DRiPs) in the presence of puromycin. As depletion of the RQC ubiquitin ligase listerin partially restored ubiquitin-dependent proteasomal degradation, these data suggest that the persistent protein translation during proteotoxic stress in ISRIB-treated cells increases the pool of newly synthesized proteins targeted by RQC, which aggravates UPS dysfunction by overloading the cytosolic UPS.