PHLDA3 overexpression in hepatocytes by endoplasmic reticulum stress via IRE1–Xbp1s pathway expedites liver injury

Abstract

ObjectiveEndoplasmic reticulum (ER) stress is involved in liver injury, but molecular determinants are largely unknown. This study investigated the role of pleckstrin homology-like domain, family A, member-3 (PHLDA3), in hepatocyte death caused by ER stress and the regulatory basis.DesignHepatic PHLDA3 expression was assessed in HCV patients with hepatitis and in several animal models with ER stress. Immunoblottings, PCR, reporter gene, chromatin immunoprecipitation (ChIP) and mutation analyses were done to explore gene regulation. The functional effect of PHLDA3 on liver injury was validated using lentiviral delivery of shRNA.ResultsPHLDA3 was overexpressed in relation to hepatocyte injury in patients with acute liver failure or liver cirrhosis or in toxicant-treated mice. In HCV patients with liver injury, PHLDA3 was upregulated in parallel with the induction of ER stress marker. Treatment of mice with tunicamycin (Tm) (an ER stress inducer) increased PHLDA3 expression in the liver. X box-binding protein-1 (Xbp1) was newly identified as a transcription factor responsible for PHLDA3 expression. Inositol-requiring enzyme 1 (IRE1) (an upstream regulator of Xbp1) was required for PHLDA3 induction by Tm, whereas other pathways (c-Jun N-terminal kinase (JNK), protein kinase RNA-like endoplasmic reticulum kinase (PERK) and activating transcription factor 6 (ATF6)) were not. PHLDA3 overexpression correlated with the severity of hepatocyte injury in animal or cell model of ER stress. In p53-deficient cells, ER stress inducers transactivated PHLDA3 with a decrease in cell viability. ER stress-induced hepatocyte death depended on serine/threonine protein kinase B (Akt) inhibition by PHLDA3. Lentiviral delivery of PHLDA3 shRNA to mice abrogated p-Akt inhibition in the liver by Tm, attenuating hepatocyte injury.ConclusionsER stress in hepatocytes induces PHLDA3 via IRE1–Xbp1s pathway, which facilitates liver injury by inhibiting Akt.

Intracellular XBP1-IL-24 axis dismantles cytotoxic unfolded protein response in the liver

“…Activated IRE1α splices XBP1 mRNA, which encodes transcription factor to increase the protein-folding capacity and degrade misfolded proteins. While IRE1α engages STAT3 pathway to promote liver regeneration upon liver injury 26 , XBP1 switches prosurvival to proapoptotic signal cascades through multiple gene regulation 27,28 . It has been reported that ATF6 exerts a pro-inflammatory effect on ischemiareperfusion liver injury 29 .…”

FXR Inhibits Endoplasmic Reticulum Stress-Induced NLRP3 Inflammasome in Hepatocytes and Ameliorates Liver Injury

“…In our recent study, ER stress marker levels were enhanced in patients with liver injury (Han et al, 2016). To confirm inflammasome activation in an animal model of liver injury, we examined liver samples from mice treated with the drug acetaminophen (APAP), which induces ER stress and liver damage (Han et al, 2016). In these liver samples, Fxr transcript levels were decreased, and Nlrp3 levels were increased relative to untreated controls ( Figure 1B).…”

“…Intriguingly, the levels of NLRP3 and other inflammasome-associated gene transcripts (i.e., thioredoxin-interacting protein [TXNIP], apoptosis-associated speck-like caspase activation and recruitment domain (CARD)-containing protein [ASC], and caspase-1 [CASP1]) increased markedly as hepatic necrosis became severe, although FXR levels decreased ( Figure 1A). In our recent study, ER stress marker levels were enhanced in patients with liver injury (Han et al, 2016). To confirm inflammasome activation in an animal model of liver injury, we examined liver samples from mice treated with the drug acetaminophen (APAP), which induces ER stress and liver damage (Han et al, 2016).…”

The XBP1 Arm of the Unfolded Protein Response Induces Fibrogenic Activity in Hepatic Stellate Cells Through Autophagy

“…Each of the three proteins is linked to a distinct downstream cascade that together mitigate cellular stress and maintain homeostasis. While UPR in hepatocytes has been well studied, especially in the context of metabolic dysregulation56, its involvement in liver fibrogenesis and HSC activation is not as well-characterized789.…”