Molecularly defined unfolded protein response subclasses have distinct correlations with fatty liver disease in zebrafish

Abstract

The unfolded protein response (UPR) is a complex network of sensors and target genes that ensure efficient folding of secretory proteins in the endoplasmic reticulum (ER). UPR activation is mediated by three main sensors, which regulate the expression of hundreds of targets. UPR activation can result in outcomes ranging from enhanced cellular function to cell dysfunction and cell death. How this pathway causes such different outcomes is unknown. Fatty liver disease (steatosis) is associated with markers of UPR activation and robust UPR induction can cause steatosis; however, in other cases, UPR activation can protect against this disease. By assessing the magnitude of activation of UPR sensors and target genes in the liver of zebrafish larvae exposed to three commonly used ER stressors (tunicamycin, thapsigargin and Brefeldin A), we have identified distinct combinations of UPR sensors and targets (i.e. subclasses) activated by each stressor. We found that only the UPR subclass characterized by maximal induction of UPR target genes, which we term a stressed-UPR, induced steatosis. Principal component analysis demonstrated a significant positive association between UPR target gene induction and steatosis. The same principal component analysis showed significant correlation with steatosis in samples from patients with fatty liver disease. We demonstrate that an adaptive UPR induced by a short exposure to thapsigargin prior to challenging with tunicamycin reduced both the induction of a stressed UPR and steatosis incidence. We conclude that a stressed UPR causes steatosis and an adaptive UPR prevents it, demonstrating that this pathway plays dichotomous roles in fatty liver disease.

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

“…Canonical fibrogenic genes, COL1A1, ACTA2, PDGFRB, MMP2 , and TIMP1 followed a mostly biphasic pattern of induction, with highest expression at 0.1 μg/mL, which was progressively attenuated at higher, and presumably more toxic doses (Fig. 2B), consistent with the knowledge that UPR-inducing drugs typically induce a level of ER stress that can be resolved, whereas high concentrations induce an ER burden that the UPR cannot overcome14, which is in line with our previous observation7. There was also upregulation of collagen 1-alpha protein at low tunicamycin doses (0.1–1 μg/mL) (Fig.…”

Zebrafish: An Important Tool for Liver Disease Research

“…Zebrafish embryos subsist on yolk for the first 5 days of development; larvae that are not fed within 2 days of yolk consumption can develop fasting-induced steatosis. 47,65,66,77 Although the incidence of FLD varies by strain 77 and the assay used to measure it, researchers who found a low incidence of fasting-induced steatosis in wild-type larvae used these to screen for mutations that increased this incidence. One mutant, red moon ( rmn ), carried a homozygous mutation in the solute carrier family 16 , member 6a gene ( slc16a6a ).…”

“…77 Disruption of atf6 , which encodes a mediator of the UPR, reduced fatty liver in foigr mutants 66 as well as in steatosis due to prolonged ER stress 66,82 or alcohol use. 82 Atf6 overexpression was sufficient to cause fatty liver in zebrafish under stress-free conditions.…”

The lure of zebrafish in liver research: regulation of hepatic growth in development and regeneration

“…At the molecular level, ethanol metabolism generates ROS and induces the unfolded protein response (UPR) in the endoplasmic reticulum (ER), which activates the lipogenic transcription factors SREBP and ATF6[83,85-88]. Ethanol promotes maladaptive regeneration by activating HSCs, which proliferate and increase the deposition of extracellular matrix components leading to scar formation[11].…”