Coordinated signaling of activating transcription factor 6α and inositol-requiring enzyme 1α 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 1α (IRE1α), activating transcription factor 6α (ATF6α), and protein kinase RNA-like ER kinase (PERK), previous research in HSCs primarily focused on IRE1α. 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 deletion, disrupted transforming growth factor β (TGFβ)-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 deletion. mice displayed reduced fibrosis and HSC activation following bile duct ligation (BDL) or carbon tetrachloride (CCl)-induced injury. The phenotype differed from HSC-specific deletion, as mice showed reduced fibrogenic gene transcription but no changes in fibrosis compared with mice following BDL. Interestingly, ATF6α signaling increased in HSCs, whereas IRE1α signaling was upregulated in HSCs. Finally, we asked whether co-deletion of and additively limits fibrosis. Unexpectedly, fibrosis worsened in mice following BDL, and mHSCs showed increased fibrogenic gene transcription. ATF6α and IRE1α 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. ATF6α is a critical driver of hepatic stellate cell (HSC) activation in vitro. HSC-specific deletion of limits fibrogenesis in vivo despite increased IRE1α signaling. Conditional deletion of Ire1α from HSCs limits fibrogenic gene transcription without impacting overall fibrosis. This could be due in part to observed upregulation of the ATF6α pathway. Dual loss of and from HSCs worsens fibrosis in vivo through enhanced HSC activation.