The PERK Pathway of the Unfolded Protein Response (UPR): Its Role in Hypoxia Tolerance and as a Target for Anti-tumor Strategies

Hypoxia is a dynamic feature of the tumor microenvironment that contributes to cancer progression. In order to adapt and overcome hypoxic stress, tumor cells activate survival pathways that attempt to couple metabolic processes to reduced energy availability due to oxygen deprivation. While hypoxia-inducible factors HIF-1 and HIF-2 are critical for the cellular response to hypoxia, HIF-independent pathways are also known to contribute to this adaptation. Mounting evidence demonstrates that hypoxia also activates components of the Unfolded Protein Response (UPR), a coordinated program that regulates cellular adaptation to increased levels of unfolded proteins in the endoplasmic reticulum (ER). Previous work from our lab established that hypoxia activates the UPR kinase PERK, which leads to the phosphorylation of eIF2alpha and provides the basis for the work outlined in this thesis project. Here we report that the accumulation of downstream UPR targets ATF4 and CHOP in hypoxic cells is dependent on PERK-mediated eIF2alpha phosphorylation. Moreover, inhibiting PERK activity in human tumor cells sensitizes them to hypoxia in vitro and compromises their ability to grow tumors in vivo. Preliminary findings suggest that the mechanism of increased apoptosis in UPR-defective cells under hypoxia may involve Bax activation and that ER-targeted Bcl-2 can protect UPR-defective cells from hypoxic stress. Interestingly, the mechanism of hypoxia-induced UPR activation may involve the ER oxidative protein folding machinery, which requires molecular oxygen as a terminal electron acceptor. Lastly, we provide evidence that hypoxic tumor cells having an active UPR are more sensitive to agents that produce additional ER stress, such as the proteasome inhibitor bortezomib, compared to well-oxygenated tumor cells. This enhanced cytotoxicity towards hypoxic tumor cells is due to the overactivation of ER stress pathways, and leads to increased cell death via both apoptosis and enhanced autophagy that proceeds to necrosis. Altogether, these results provide further insight into the molecular signaling and the biologic importance of UPR activation in hypoxia tolerance and tumor growth, and offer the UPR as a critical target for new anti-tumor therapies.