Development of transgenic mice expressing a conditionally active form of the eIF2α kinase PKR

Control of oncogenesis by eIF2α phosphorylation: implications in PTEN and PI3K-Akt signaling and tumor treatment

mRNA translation plays an important role in tumor development and represents a valid target of pharmaceutical intervention in cancer. A key step in mRNA translation involves the regulation of initiation by the eukaryotic initiation factor eIF2. Eukaryotic cells respond to various forms of stress by inducing the phosphorylation of the α-subunit of eIF2 at S51, a modification that leads to protein synthesis inhibition. Phosphorylated eIF2α can act either as a promoter of cell survival or an inducer of cell death in response to distinct stimuli. Increased eIF2α phosphorylation has a cytoprotective function in response to genetic or pharmacological inhibition of the PI3K-Akt pathway but also exhibits a proapoptotic function downstream of the PTEN tumor suppressor, independent of PI3K-Akt signaling inhibition. The functional interplay between the PI3K-Akt and eIF2α phosphorylation pathways may have important implications in the design of anti-tumor therapies that depend on the cell fate decisions of phosphorylated eIF2α.

Evidence for eIF2α phosphorylation-independent effects of GSK2656157, a novel catalytic inhibitor of PERK with clinical implications

The endoplasmic reticulum (ER)-resident protein kinase PERK is a major component of the unfolded protein response (UPR), which promotes the adaptation of cells to various forms of stress. PERK phosphorylates the α subunit of the translation initiation factor eIF2 at serine 51, a modification that plays a key role in the regulation of mRNA translation in stressed cells. Several studies have demonstrated that the PERK-eIF2α phosphorylation pathway maintains insulin biosynthesis and glucose homeostasis, facilitates tumor formation and decreases the efficacy of tumor treatment with chemotherapeutic drugs. Recently, a selective catalytic PERK inhibitor termed GSK2656157 has been developed with anti-tumor properties in mice. Herein, we provide evidence that inhibition of PERK activity by GSK2656157 does not always correlate with inhibition of eIF2α phosphorylation. Also, GSK2656157 does not always mimic the biological effects of the genetic inactivation of PERK. Furthermore, cells treated with GSK2656157 increase eIF2α phosphorylation as a means to compensate for the loss of PERK. Using human tumor cells impaired in eIF2α phosphorylation, we demonstrate that GSK2656157 induces ER stress-mediated death suggesting that the drug acts independent of the inhibition of eIF2α phosphorylation. We conclude that GSK2656157 might be a useful compound to dissect pathways that compensate for the loss of PERK and/or identify PERK pathways that are independent of eIF2α phosphorylation.