Endoplasmic reticulum stress induces fibrogenic activity in hepatic stellate cells through autophagy.

Metabolic stress during liver injury enhances autophagy and provokes stellate cell activation, with secretion of scar matrix. Conditions that augment protein synthesis increase demands on the endoplasmic reticulum (ER) folding capacity and trigger the unfolded protein response (UPR) to cope with resulting ER stress. Generation of reactive oxygen species (ROS) is a common feature of hepatic fibrogenesis, and crosstalk between oxidant stress and ER stress has been proposed. The aim of our study was to determine the impact of oxidant and ER stress on stellate cell activation. Oxidant stress was induced in hepatic stellate cells using H2O2 in culture or by ethanol feeding in vivo, and the UPR was analyzed. Because the branch of the UPR mainly affected was IREα, we blocked this pathway in stellate cells and analyzed the fibrogenic response, together with autophagy and downstream MAPK signaling. The Nrf2 antioxidant response was also evaluated in stellate cells under oxidant stress conditions. H2O2 treatment in culture or ethanol feeding in vivo increased the UPR based on splicing of XBP1 mRNA, which triggered autophagy. The Nrf2-mediated antioxidant response, as measured by qRT-PCR of its target genes was also induced under ER stress conditions. Conversely, blockade of the IRE1α pathway in stellate cells significantly decreased both their activation and autophagic activity in a p38 MAPK-dependent manner, leading to a reduced fibrogenic response. These data implicate mechanisms underlying protein folding quality control in regulating the fibrogenic response in hepatic stellate cells.

Hernández-Gea V ，Hilscher M ，Rozenfeld R ，Lim MP ，Nieto N ，Werner S ，Devi LA ，Friedman SL ... -  《-》

Coordinated signaling of activating transcription factor 6α and inositol-requiring enzyme 1α regulates hepatic stellate cell-mediated fibrogenesis in mice.

Xue F ，Lu J ，Buchl SC ，Sun L ，Shah VH ，Malhi H ，Maiers JL ... -  《-》

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

Kim RS ，Hasegawa D ，Goossens N ，Tsuchida T ，Athwal V ，Sun X ，Robinson CL ，Bhattacharya D ，Chou HI ，Zhang DY ，Fuchs BC ，Lee Y ，Hoshida Y ，Friedman SL ... -  《Scientific Reports》

Unfolded protein response induced by Brefeldin A increases collagen type I levels in hepatic stellate cells through an IRE1α, p38 MAPK and Smad-dependent pathway.

Unfolded protein response (UPR) triggered as a consequence of ER stress has been shown to be involved in the development of different pathologies, including fibrotic disorders. In the present paper we explore the role played by UPR on a key fibrogenic parameter in the liver: collagen type I levels in activated hepatic stellate cells (HSC). Using Brefeldin A (BFA) as an ER stress inducer we found that UPR correlated with enhanced mRNA and protein levels of collagen type I in a cell line of immortalized non-tumoral rat HSC. Analysis of the three branches of UPR revealed the activation of IRE1α, PERK and ATF6 in response to BFA, although PERK activation was shown not to be involved in the fibrogenic action of BFA. BFA also activated p38 MAPK in an IRE1α-dependent way and the p38 MAPK inhibitor SB203580 prevented the increase in collagen type I mRNA and protein levels caused by BFA, suggesting the involvement of this kinase on this effect. Analysis of Smad activation showed that phosphorylated nuclear levels of Smad2 and 3 were increased in response to BFA treatment. Inhibition of Smad3 phosphorylation by SIS3 prevented the enhancement of collagen type I levels caused by BFA. Pretreatment with IRE1α and p38 MAPK inhibitors also prevented the increased p-Smad3 accumulation in the nucleus, suggesting an IRE1α-p38 MAPK-Smad pathway to be responsible for the fibrogenic action of BFA on HSC.

de Galarreta MR ，Navarro A ，Ansorena E ，Garzón AG ，Mòdol T ，López-Zabalza MJ ，Martínez-Irujo JJ ，Iraburu MJ ... -  《-》

Fibroblast growth factor 21 is regulated by the IRE1α-XBP1 branch of the unfolded protein response and counteracts endoplasmic reticulum stress-induced hepatic steatosis.

Endoplasmic reticulum (ER) stress activates the adaptive unfolded protein response (UPR) and represents a critical mechanism that underlies metabolic dysfunctions. Fibroblast growth factor 21 (FGF21), a hormone that is predominantly secreted by the liver, exerts a broad range of effects upon the metabolism of carbohydrates and lipids. Although increased circulating levels of FGF21 have been documented in animal models and human subjects with obesity and nonalcoholic fatty liver disease, the functional interconnections between metabolic ER stress and FGF21 are incompletely understood. Here, we report that increased ER stress along with the simultaneous elevation of FGF21 expression were associated with the occurrence of nonalcoholic fatty liver disease both in diet-induced obese mice and human patients. Intraperitoneal administration of the ER stressor tunicamycin in mice resulted in hepatic steatosis, accompanied by activation of the three canonical UPR branches and increased the expression of FGF21. Furthermore, the IRE1α-XBP1 pathway of the UPR could directly activate the transcriptional expression of Fgf21. Administration of recombinant FGF21 in mice alleviated tunicamycin-induced liver steatosis, in parallel with reduced eIF2α-ATF4-CHOP signaling. Taken together, these results suggest that FGF21 is an integral physiological component of the cellular UPR program, which exerts beneficial feedback effects upon lipid metabolism through counteracting ER stress.

Jiang S ，Yan C ，Fang QC ，Shao ML ，Zhang YL ，Liu Y ，Deng YP ，Shan B ，Liu JQ ，Li HT ，Yang L ，Zhou J ，Dai Z ，Liu Y ，Jia WP ... -  《-》