The unfolded protein response mediates fibrogenesis and collagen I secretion through regulating TANGO1 in mice.

Fibrogenesis encompasses the deposition of matrix proteins, such as collagen I, by hepatic stellate cells (HSCs) that culminates in cirrhosis. Fibrogenic signals drive transcription of procollagen I, which enters the endoplasmic reticulum (ER), is trafficked through the secretory pathway, and released to generate extracellular matrix. Alternatively, disruption of procollagen I ER export could activate the unfolded protein response (UPR) and drive HSC apoptosis. Using a small interfering RNA screen, we identified Transport and Golgi organization 1 (TANGO1) as a potential participant in collagen I secretion. We investigated the role of TANGO1 in procollagen I secretion in HSCs and liver fibrogenesis. Depletion of TANGO1 in HSCs blocked collagen I secretion without affecting other matrix proteins. Disruption of secretion led to procollagen I retention within the ER, induction of the UPR, and HSC apoptosis. In wild-type (WT) HSCs, both TANGO1 and the UPR were induced by transforming growth factor β (TGFβ). As the UPR up-regulates proteins involved in secretion, we studied whether TANGO1 was a target of the UPR. We found that UPR signaling is responsible for up-regulating TANGO1 in response to TGFβ, and this mechanism is mediated by the transcription factor X-box binding protein 1 (XBP1). In vivo, murine and human cirrhotic tissue displayed increased TANGO1 messenger RNA levels. Finally, TANGO1+/- mice displayed less hepatic fibrosis compared to WT mice in two separate murine models: CCl4 and bile duct ligation. Loss of TANGO1 leads to procollagen I retention in the ER, which promotes UPR-mediated HSC apoptosis. TANGO1 regulation during HSC activation occurs through a UPR-dependent mechanism that requires the transcription factor, XBP1. Finally, TANGO1 is critical for fibrogenesis through mediating HSC homeostasis. The work reveals a unique role for TANGO1 and the UPR in facilitating collagen I secretion and fibrogenesis. (Hepatology 2017;65:983-998).

Maiers JL ，Kostallari E ，Mushref M ，deAssuncao TM ，Li H ，Jalan-Sakrikar N ，Huebert RC ，Cao S ，Malhi H ，Shah VH ... -  《-》

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 ... -  《-》

Transforming growth factor β (TGFβ) cross-talk with the unfolded protein response is critical for hepatic stellate cell activation.

Transforming growth factor β (TGFβ) potently activates hepatic stellate cells (HSCs), which promotes production and secretion of extracellular matrix (ECM) proteins and hepatic fibrogenesis. Increased ECM synthesis and secretion in response to TGFβ is associated with endoplasmic reticulum (ER) stress and the unfolded protein response (UPR). TGFβ and UPR signaling pathways are tightly intertwined during HSC activation, but the regulatory mechanism that connects these two pathways is poorly understood. Here, we found that TGFβ treatment of immortalized HSCs (i.e. LX-2 cells) induces phosphorylation of the UPR sensor inositol-requiring enzyme 1α (IRE1α) in a SMAD2/3-procollagen I-dependent manner. We further show that IRE1α mediates HSC activation downstream of TGFβ and that its role depends on activation of a signaling cascade involving apoptosis signaling kinase 1 (ASK1) and c-Jun N-terminal kinase (JNK). ASK1-JNK signaling promoted phosphorylation of the UPR-associated transcription factor CCAAT/enhancer binding protein β (C/EBPβ), which is crucial for TGFβ- or IRE1α-mediated LX-2 activation. Pharmacological inhibition of C/EBPβ expression with the antiviral drug adefovir dipivoxil attenuated TGFβ-mediated activation of LX-2 or primary rat HSCs in vitro and hepatic fibrogenesis in vivo Finally, we identified a critical relationship between C/EBPβ and the transcriptional regulator p300 during HSC activation. p300 knockdown disrupted TGFβ- or UPR-induced HSC activation, and pharmacological inhibition of the C/EBPβ-p300 complex decreased TGFβ-induced HSC activation. These results indicate that TGFβ-induced IRE1α signaling is critical for HSC activation through a C/EBPβ-p300-dependent mechanism and suggest C/EBPβ as a druggable target for managing fibrosis.

Liu Z ，Li C ，Kang N ，Malhi H ，Shah VH ，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》

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 ... -  《-》