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

Abstract

Transforming 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.