Regulation of the Unfolded Protein Response by eIF2Bδ Isoforms

Cells respond to a variety of stresses, including unfolded proteins in the endoplasmic reticulum (ER), by phosphorylating a subunit of translation initiation factor eIF2, eIF2α. eIF2α phosphorylation inactivates the eIF2B complex. The inactivation of eIF2B not only suppresses the initiation of protein translation but paradoxically up-regulates the translation and expression of transcription factor ATF-4. Both of these processes are important for the cellular response to ER stress, also termed the unfolded protein response. Here we demonstrate that cellular response resulting from eIF2α phosphorylation is attenuated in several cancer cell lines. The deficiency of the unfolded protein response in these cells correlates with the expression of a specific isoform of a regulatory eIF2B subunit, eIF2Bδ variant 1 (V1). Replacement of total eIF2Bδ with V1 renders cells insensitive to eIF2α phosphorylation; specifically, they neither up-regulate ATF-4 and ATF-4 targets nor suppress protein translation. Expression of variant 2 eIF2Bδ in ER stress response-deficient cells restores the stress response. Our data suggest that V1 does not interact with the eIF2 complex, a requisite for eIF2B inhibition by eIF2α phosphorylation. Together, these data delineate a novel physiological mechanism to regulate the ER stress response with a large potential impact on a variety of diseases that result in ER stress.