Coordinated signaling of activating transcription factor 6a and inositolrequiring enzyme 1a regulates hepatic stellate cell-mediated fibrogenesis in mice

Liver injury and the unfolded protein response (UPR) are tightly linked, but their relationship differs with cell type and injurious stimuli. UPR initiation promotes hepatic stellate cell (HSC) activation and fibrogenesis, but the underlying mechanisms are unclear. Despite the complexity and overlap downstream of UPR transducers inositol-requiring protein 1a (IRE1a), activating transcription factor 6a (ATF6α), and protein kinase RNA-like ER kinase (PERK), previous research in HSCs primarily focused on IRE1a. Here, we investigated the fibrogenic role of ATF6α or PERK in vitro and HSC-specific UPR signaling in vivo. Overexpression of ATF6α, but not the PERK effector activating transcription factor 4 (ATF4), promoted HSC activation and fibrogenic gene transcription in immortalized HSCs. Furthermore, ATF6α inhibition through Ceapin-A7, or ATF6α deletion, disrupted transforming growth factor b (TGFb)-mediated activation of primary human hepatic stellate cells (hHSCs) or murine hepatic stellate cells (mHSCs), respectively. We investigated the fibrogenic role of ATF6α in vivo through conditional HSC-specific ATF6α deletion. ATF6αHSCD/D mice displayed reduced fibrosis and HSC activation following bile duct ligation (BDL) or carbon tetrachloride (CCl4)-induced injury. The ATF6αHSCD/D phenotype differed from HSC-specific Ire1a deletion, as Ire1aHSCD/D mice showed reduced fibrogenic gene transcription but no changes in fibrosis compared with Ire1afl/fl mice following BDL. Interestingly, ATF6α signaling increased in Ire1aD/D HSCs, whereas IRE1a signaling was upregulated in ATF6αD/D HSCs. Finally, we asked whether co-deletion of ATF6α and Ire1a additively limits fibrosis. Unexpectedly, fibrosis worsened in ATF6αHSCD/DIre1aHSCD/D mice following BDL, and ATF6αD/DIre1aD/D mHSCs showed increased fibrogenic gene transcription. ATF6α and IRE1a individually promote fibrogenic transcription in HSCs, and ATF6α drives fibrogenesis in vivo. Unexpectedly, disruption of both pathways sensitizes the liver to fibrogenesis, suggesting that fine-tuned UPR signaling is critical for regulating HSC activation and fibrogenesis.