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

ABSTRACT

Transforming growth factor ß (TGFß) 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 TGFß is associated with endoplasmic reticulum (ER) stress and the unfolded protein response (UPR). TGFß 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 TGFß 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 TGFß 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 ß (C/EBPß), which is crucial for TGFß- or IRE1α-mediated LX-2 activation. Pharmacological inhibition of C/EBPß expression with the antiviral drug adefovir dipivoxil attenuated TGFß-mediated activation of LX-2 or primary rat HSCs in vitro and hepatic fibrogenesis in vivo Finally, we identified a critical relationship between C/EBPß and the transcriptional regulator p300 during HSC activation. p300 knockdown disrupted TGFß- or UPR-induced HSC activation, and pharmacological inhibition of the C/EBPß-p300 complex decreased TGFß-induced HSC activation. These results indicate that TGFß-induced IRE1α signaling is critical for HSC activation through a C/EBPß-p300-dependent mechanism and suggest C/EBPß as a druggable target for managing fibrosis.