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

Rosa S Kim,Varinder Athwal,Hsin-I Chou,Christopher L Robinson,Nicolas Goossens,Xiaochen Sun,Scott L Friedman,Daisuke Hasegawa,Dipankar Bhattacharya,Yujin Hoshida,David Y Zhang,Youngmin Lee,Bryan C Fuchs,Takuma Tsuchida

doi:10.1038/srep39342

Abstract

Autophagy and the unfolded protein response (UPR) both promote activation of hepatic stellate cells (HSC), however the link between the two stimuli remains unclear. Here we have explored the role of X-box binding protein 1 (XBP1), one of three UPR effector pathways and sought to establish the interdependence between autophagy and the UPR during HSC activation. XBP1 induction accompanied both culture-based HSC activation and ER stress induced by tunicamycin. Ectopic overexpression of XBP1 induced collagen 1-alpha expression in HSCs, which was inhibited by knockdown of ATG7, a critical autophagy mediator. Genome-wide transcriptomic profiling indicated an upregulation of collagen synthesis pathways, but not of the transforming growth factor (TGF)-b pathway, a canonical fibrogenic driver, suggesting that XBP1 activates a specific subset of fibrogenesis pathways independent of TGF-β1. XBP1 target gene signatures were significantly induced in rodent liver fibrosis models (n = 3–5) and in human samples of non-alcoholic fatty liver disease (NAFLD) (n = 72–135). Thus, XBP1-mediated UPR contributes to fibrogenic HSC activation and is functionally linked to cellular autophagy.

Highlights

We previously demonstrated that ER stress signaling through one of the three ER transmembrane proteins, IRE1, promotes hepatic stellate cells (HSC) activation and fibrogenesis by increasing the secretion of type I collagen
Primary HSCs isolated from 4 mice were cultured for 2, 7, and 14 days, and the expression of sXBP1 mRNA was quantified along with fibrosis-related genes by quantitative PCR (qPCR) (Fig. 1)
Our findings indicate that X-box binding protein 1 (XBP1), the downstream effector of IRE1, is induced during HSC activation in culture and in vivo, and we have functionally linked XBP1 to autophagy in this cell type

Summary

Introduction

We previously demonstrated that ER stress signaling through one of the three ER transmembrane proteins, IRE1, promotes HSC activation and fibrogenesis by increasing the secretion of type I collagen. Is accompanied by enhanced autophagy, a housekeeping self-degradative process induced by the UPR in many cell types to eliminate misfolded or aggregated proteins and to maintain energy homeostasis[7] Another ER stress effector pathway, PERK has been recently implicated in hepatic fibrogenesis[8]. Autophagy is a major cellular catabolic process that delivers proteins, organelles and other cellular components to the lysosome for degradation as well as fuel recycling, and we previously demonstrated that autophagy is required for stellate cell activation and hepatic fibrogenesis in mice[10] While it has been demonstrated in various biological contexts that cellular stress activates a set of well-orchestrated processes including the UPR and autophagy to restore homeostasis or commit to apoptosis, the interplay across these pathways remains to be determined in the context of hepatic fibrosis. We have explored the impact of both uXBP1 and sXBP1 on the fibrogenic phenotype of HSCs, characterized their global transcriptional consequences in human and mouse primary HSCs and HSC lines, and linked these XBP-1 associated responses to phenotypes in rodent models of liver injury as well as human fibrotic liver disease

Methods

Results

Conclusion