Endoplasmic Reticulum Stress in Metabolic Liver Diseases and Hepatic Fibrosis

Abstract

Endoplasmic reticulum (ER) stress is a major contributor to liver disease and hepatic fibrosis, but the role it plays varies depending on the cause and progression of the disease. Furthermore, ER stress plays a distinct role in hepatocytes versus hepatic stellate cells (HSCs), which adds to the complexity of understanding ER stress and its downstream signaling through the unfolded protein response (UPR) in liver disease. Here, the authors focus on the current literature of ER stress in nonalcoholic and alcoholic fatty liver diseases, how ER stress impacts hepatocyte injury, and the role of ER stress in HSC activation and hepatic fibrosis. This review provides insight into the complex signaling and regulation of the UPR, parallels and distinctions between different liver diseases, and how ER stress may be targeted as an antisteatotic or antifibrotic therapy to limit the progression of liver disease.

Fig. 1. UPR sensors and signaling pathways.

ER stress is sensed by three ER transmembrane proteins, ATF6α, IRE1α, and PERK, which are crucial for mediating the adaptive and apoptotic signaling of the UPR. ATF6α translocates to the Golgi upon sensing ER stress, where it is cleaved and subsequently trafficked to the nucleus where it upregulates chaperones and proteins involved in ERAD. IRE1α oligomerizes and autophosphorylates in response to ER stress, and acts through several mechanisms. The endonuclease domain is involved in RIDD, as well as transcription through activating the transcription factor XBP1. IRE1α also promotes apoptosis through activation of ASK and subsequent phosphorylation of JNK. PERK oligomerizes upon sensing ER stress, and autophosphorylates. The canonical target of PERK kinase activity is eIF2α, which acts through ATF4 to promote expression of chaperones, and proteins involved in ERAD, amino acid metabolism, the oxidative stress response, and UPR-mediated apoptosis. eIF2α also serves to attenuate nonessential mRNA translation. CHOP, a stress-induced transcription factor, is upregulated downstream of ATF4 and mediates ER stress-induced apoptosis. Gad34, another transcriptional target of this pathway, dephosphorylates eIF2α, thus resuming translation which can lead to apoptosis by increasing oxidative protein folding. ASK, apoptosis-signal-regulating kinase; CHOP, CCAAT-enhancer-binding protein homologous protein; ER, endoplasmic reticulum; ERAD, endoplasmic reticulum-associated degradation; JNK, c-Jun N-terminal kinase; PERK, protein kinase RNA-like endoplasmic reticulum kinase; RIDD, regulated IRE1-dependent decay; UPR, unfolded protein response.

Fig. 2. IRE1α activation states in NAFLD.

In this model we propose two alternative activation states of IRE1α: (A) unconstrained IRE1α snitrosylation as observed in ob/ob mice, which may increase over time. Initial activation of XBP1 (in pink) and RIDD (in green) may favor adaptive UPR. Over time a loss of endoribonuclease activity (in purple) would occur with increased s-nitrosylation (in blue) leading to a loss of adaptive signaling. (B) In the absence of s-nitrosylation IRE1α activation increases over time and shifts from adaptive to maladaptive. However, whether this switch is a function of time; regulated by the activation of alternative signaling pathways such as PERK; a cell- and tissue-specific activation of IRE1α; or specific to accumulated toxic lipids, remain to be determined. NAFLD, nonalcoholic fatty liver disease; PERK, protein kinase RNA-like endoplasmic reticulum kinase; RIDD, regulated IRE1-dependent decay; UPR, unfolded protein response.

Fig. 3. Lipotoxicity and ER stress in NASH.

The saturated free fatty acid palmitate, and other toxic lipids such as lysophosphatidylcholine (LPC) are known to activate the three UPR sensors. IRE1α, via phosphorylation and activation of the stress kinase c-Jun N-terminal kinase (JNK) targets the insulin receptor substrate 1 (IRS1) for inhibitory phosphorylation leading to insulin resistance. IRE1α-dependent RNA degradation (RIDD) negatively regulates hepatic steatosis via degradation of lipogenic mRNAs and regulatory microRNA. IRE1α s-nitrosylation inhibits its endoribonuclease activity with a resultant decrease in XBP1 splicing and an increase in microRNAs that are degraded via RIDD. The PERK pathway is implicated in the regulation of hepatic steatosis; CHOP mediates palmitate-induced hepatocyte apoptosis, though its in vivo role in NASH is less clear. ATF6α is activated in NASH; however, its contribution is not well defined. Lipid perturbation-induced UPR activation activates a subset of genes distinct from misfolded protein-induced UPR; however, the specific ATF4 and XBP1 induced lipotoxic target genes have not been defined. CHOP, CCAAT-enhancer-binding protein homologous protein; ER, endoplasmic reticulum; NASH, nonalcoholic steatohepatitis; PERK, protein kinase RNA-like endoplasmic reticulum kinase; UPR, unfolded protein response.

Fig. 4. UPR signaling pathways during HSC activation and potential strategies for antifibrotic targeting.

(A) HSC activation through signals such as inflammation, TGFβ, or oxidative stress leads to ER stress and induction of the UPR. It is yet unclear how activation signals lead to UPR induction, whether through upregulation of ECM proteins or a separate mechanism independent of increased gene transcription. UPR signaling downstream of PERK, ATF6α, and IRE1α further promotes HSC activation and fi(A) HSC ac through increased SMAD2/3 phosphorylation, ER expansion, and enhanced protein secretion, as well as additional, unexplored mechanisms. (B) Targeting the UPR provides potential strategies for either limiting HSC activation or promoting apoptosis of activated HSCs, both favorable for fibrosis resolution. General UPR inhibition through chemical chaperones or BiP overexpression has already shown to be antifibrotic in animal models, and could be pursued further. In addition, preferential targeting of activated HSCs for apoptosis, whether through general activation of UPR signaling (etoposide, caffeine, quercetin, or azithromycin), increased CHOP expression, ASK1-JNK signaling, or disrupted protein export from the ER (inhibition of TANGO1), could prove to be a useful strategy for antifibrotic therapies. ASK1, ASK, apoptosis-signal-regulating kinase 1; BiP, immunoglobulin-binding protein; CHOP, CCAAT-enhancer-binding protein homologous protein; ECM, extracellular matrix; ER, endoplasmic reticulum; HSC, hepatic stellate cell; JNK, c-Jun N-terminal kinase; PERK, protein kinase RNA-like endoplasmic reticulum kinase; UPR, unfolded protein response.