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

Abstract

Edited by Eric R. FearonTransforming growth factor ␤ (TGF␤) 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 TGF␤ is associated with endoplasmic reticulum (ER) stress and the unfolded protein response (UPR). TGF␤ 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 TGF␤ treatment of immortalized HSCs (i.e. LX-2 cells) induces phosphorylation of the UPR sensor inositol-requiring enzyme 1␣ (IRE1␣) in a SMAD2/3-procollagen I-dependent manner. We further show that IRE1␣ mediates HSC activation downstream of TGF␤ 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 ␤ (C/EBP␤), which is crucial for TGF␤-or IRE1␣-mediated LX-2 activation. Pharmacological inhibition of C/EBP␤ expression with the antiviral drug adefovir dipivoxil attenuated TGF␤-mediated activation of LX-2 or primary rat HSCs in vitro and hepatic fibrogenesis in vivo. Finally, we identified a critical relationship between C/EBP␤ and the transcriptional regulator p300 during HSC activation. p300 knockdown disrupted TGF␤-or UPR-induced HSC activation, and pharmacological inhibition of the C/EBP␤-p300 complex decreased TGF␤-induced HSC activation. These results indicate that TGF␤-induced IRE1␣ signaling is critical for HSC activation through a C/EBP␤-p300 -dependent mechanism and suggest C/EBP␤ as a druggable target for managing fibrosis.

Metabolic Hallmarks of Hepatic Stellate Cells in Liver Fibrosis

“…The unfolded protein response (UPR) branch involved in fibrosis activation is protein kinase R (PKR)-like endoplasmic reticulum kinase (PERK), which indirectly induces SMAD2. In TGFβ-treated HSCs induction of another UPR branch, IRE1α occurs [169]. In line with this, treatment of cells with hydrogen peroxide or ethanol feeding in vivo led to UPR activation, while abrogation of the IRE1α branch of the UPR inhibited HSC activation and autophagy [170].…”

Cyp1b1 directs Srebp-mediated cholesterol and retinoid synthesis in perinatal liver; Association with retinoic acid activity during fetal development

“…Srebp 1c contributes to Hamp expression [81]. Activation of stellate cells is also integrated with Srebp regulation [82,83]. Srebp regulation may in some way connection to Cyp1b1 and retinol.…”

“…These same genes are substantially affected by a retinol-depleted maternal diet (GVAD) that lowers liver retinol. At birth, GVAD and Cyp1b1 deletion each suppress the iron regulator, Hamp, while also increasing multiple stellate activation markers, each a likely target of Srebp control [82,83]. The low presence of Lrat and retinyl esters and enrichment of activated stellate markers suggests an altered basal state that may be more sensitive to these changes [14].…”

Transcriptomic Profiling of DAF-7/TGFβ Pathway Mutants in C. elegans

“…Some of the GO terms that were identified in this study are not associated with any reported roles of the DAF-7 pathway, such as Response to Topologically Incorrect Protein. However, TGFβ has been reported to interact with the unfolded protein response in other model systems, such as hepatic stellate cells and corneal endothelial cells, suggesting this function is evolutionarily conserved [41,42]. Regardless of the novelty of the identified GO terms, the DEGs generated from these datasets provide specific gene targets that can be further studied to provide insight into the multifunctional TGFβ pathway.…”