Differential Behavior of 4E‐BP1 and 4E‐BP2 Under ER Stress

Abstract

eIF4E‐binding proteins (4E‐BPs) are a family of translational repressors, when unphosphorylated or hypo‐phosphorylated, can bind to eIF4E and inhibit eukaryotic cap‐dependent translation initiation. The 3 4E‐BPs (4E‐BP1, 4E‐BP2 and 4E‐BP3) are not isoforms but are often regarded alike in terms of inhibiting translation initiation. They are not clearly differentiated in terms of function. Not only do they originate from different genes on different chromosomes, but they also have distinct mRNA structure. We hypothesized a potential for their differential regulation under cell stress. During cell stress, 4E‐BPs need to be upregulated to stop protein synthesis and conserve cellular energy. 4E‐BP1 promoter has CARE (C/Ebp‐ATF Response Element) in its promoter region, that helps it respond to ER stress in an ATF4‐dependent manner. 4E‐BP2 on the other hand, does not have any CARE elements in its promoter, hence ideally should remain unchanged under ER stress. Gene expression was observed through qPCR and protein levels were studied using western blotting. We observed a nearly 4‐fold increase in 4E‐BP1 mRNA levels under ER stress in sync with ATF4 mRNA levels, whereas 4E‐BP2 mRNA levels didn’t change significantly. 4E‐BP1 protein levels increased under ER stress whereas 4E‐BP2 protein levels decreased under ER stress. We observed selective proteasomal degradation of 4E‐BP2 under ER stress, which recovered when treated with proteasome inhibitor MG132. The reason behind this selective degradation needs to be further elucidated. These findings shed a new light on the behavior of 4E‐BP2 under cell stress which has been, until now, treated synonymous to 4E‐BP1 in the cellular landscape.