Abstract
Nonalcoholic fatty liver disease (steatosis) is the most prevalent liver disease in the Western world. One of the advanced pathologies is nonalcoholic steatohepatitis (NASH), which is associated with induction of the unfolded protein response (UPR) and disruption of autophagic flux. However, the mechanisms by which these processes contribute to the pathogenesis of human diseases are unclear. Herein, we identify the α isoform of the inhibitor of Bruton's tyrosine kinase (IBTKα) as a member of the UPR, whose expression is preferentially translated during endoplasmic reticulum (ER) stress. We found that IBTKα is located in the ER and associates with proteins LC3b, SEC16A, and SEC31A and plays a previously unrecognized role in phagophore initiation from ER exit sites. Depletion of IBTKα helps prevent accumulation of autophagosome intermediates stemming from exposure to saturated free fatty acids and rescues hepatocytes from death. Of note, induction of IBTKα and the UPR, along with inhibition of autophagic flux, was associated with progression from steatosis to NASH in liver biopsies. These results indicate a function for IBTKα in NASH that links autophagy with activation of the UPR.
Highlights
Nonalcoholic fatty liver disease (NAFLD)3 is the most prevalent liver disease in the Western world, and its associated pathologies range from simple steatosis to nonalcoholic steatohepatitis (NASH); hallmarks of NASH include hepatic inflammation, increased liver enzymes, and fibrosis that increases the potential to progress to cirrhosis, liver failure, and hepatocellu
To determine whether IBTK␣ is preferentially translated in human hepatocytes following metabolic stress, we treated human hepatoma HepG2 cells with palmitate or thapsigargin, a pharmacological agent that potently induces endoplasmic reticulum (ER) stress
We conclude that PERK is required for repression of global protein synthesis, coincident with preferential translation of IBTK␣ and unfolded protein response (UPR) members in human hepatocytes in response to ER stress triggered by lipotoxicity
Summary
To determine whether IBTK␣ is preferentially translated in human hepatocytes following metabolic stress, we treated human hepatoma HepG2 cells with palmitate or thapsigargin, a pharmacological agent that potently induces ER stress. HepG2 cells deleted for PERK (PERK-KO) by using CRISPR/Cas retained high levels of translation as viewed by heavy polysomes independent of stress (Fig. 1, D and E) and showed only modest changes in fraction distributions of IBTK␣, ATF4, or CHOP mRNAs (Fig. 1, F and G). IBTK␣ mRNA and protein were induced only in WT HepG2 cells treated with palmitate, whereas basal levels remained unchanged between WT and CHOP-KO cells These results indicate that PERK activation and its downstream effector CHOP are required for induced IBTK␣ mRNA expression in the UPR. The low levels of caspase 3 activation determined during treatment with saturated FFAs were coincident with caspase 3 being retained in the cytoplasm These results indicate that apoptosis is not the predominant. Oleate had only a modest effect on the UPR and did not alter the autophagic markers, whereas treatment with tunica-
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
