Background and Aims The unfolded protein response (UPR) is a coordinated cellular response to endoplasmic reticulum (ER) stress that functions to maintain cellular homeostasis. When ER stress is unresolved, the UPR can trigger apoptosis. Pathways within the UPR influence bile acid metabolism in adult animal models and adult human liver diseases, however, the UPR has not been studied in young animal models or pediatric liver diseases. In this study we sought to determine whether weanling age mice had altered UPR activation compared with adult mice, which could lead to increased bile acid–induced hepatic injury. Approach and Results We demonstrate that after 7 days of cholic acid (CA) feeding to wild‐type animals, weanling age mice have a 2‐fold greater serum alanine aminotransferase (ALT) levels compared with adult mice, with increased hepatic apoptosis. Weanling mice fed CA have increased hepatic nuclear X‐box binding protein 1 spliced (XBP1s) expression, but cannot increase expression of its protective downstream target’s ER DNA J domain‐containing protein 4 and ER degradation enhancing α‐mannoside. In response to tunicamycin induced ER stress, young mice have blunted expression of several UPR pathways compared with adult mice. CA feeding to adult liver‐specific XBP1 knockout (LS‐XBP1−/−) mice, which are unable to resolve hepatic ER stress, leads to increased serum ALT and CCAAT/enhancer binding homologous protein, a proapoptotic UPR molecule, expression to levels similar to CA‐fed LS‐XBP1−/− weanlings. Conclusions Weanling mice have attenuated hepatic XBP1 signaling and impaired UPR activation with resultant increased susceptibility to bile acid–induced injury.

Small heterodimer partner (Shp) regulates several metabolic processes, including bile acid levels, but lacks the conserved DNA binding domain. Phylogenetic analysis revealed conserved genetic evolution of SHP, FXR, CYP7A1 and CYP8B1. Shp, although primarily studied as a downstream target of Farnesoid X Receptor (Fxr), has a distinct hepatic role that is poorly understood. Here we report that liver-specific Shp knockout (LShpKO) mice have impaired negative feedback of Cyp7a1 and Cyp8b1 upon bile acid challenge and demonstrate that a single copy of the Shp gene is sufficient to maintain this response. LShpKO mice also exhibit elevated total bile acid pool with ileal bile acid composition mimicking that of CA-fed control mice. Agonistic activation of Fxr (GW4064) in the LShpKO did not alter the elevated basal expression of Cyp8b1 but lowered Cyp7a1 expression. We found that deletion of Shp led to an enrichment of distinct motifs and pathways associated with circadian rhythm, copper ion transport, and DNA synthesis. We confirmed increased expression of metallothionein genes that can regulate copper levels in the absence of SHP. LShpKO livers also displayed a higher basal proliferation that was exacerbated specifically with bile acid challenge either with cholic acid or 3,5-Diethoxycarbonyl-1,4-Dihydrocollidine but not with another liver mitogen, TCPOBOP (TC). Overall, our data indicate that hepatic SHP uniquely regulates certain proliferative and metabolic cues.

Background Increased intrahepatic bile acids cause endoplasmic reticulum (ER) stress and the unfolded protein response (UPR) is activated to maintain homeostasis. UPR dysregulation, including the inositol-requiring enzyme 1α/X-box protein 1 (IRE1α/XBP1) pathway, is associated with adult liver diseases but has not been characterized in pediatric liver diseases. We evaluated hepatic UPR expression in pediatric cholestatic liver disease (CLD) explants and hypothesize that an inability to appropriately activate the hepatic IRE1α/XBP1 pathway is associated with the pathogenesis of CLD. Methods We evaluated 34 human liver explants, including: pediatric CLD (Alagille, ALGS, and progressive familial intrahepatic cholestasis, PFIC), pediatric non-cholestatic liver disease controls (autoimmune hepatitis, AIH), adult CLD, and normal controls. We performed RNA-seq, quantitative PCR, and western blotting to measure expression differences of the hepatic UPR and other signaling pathways. Results Pathway analysis demonstrated that the KEGG ‘protein processing in ER’ pathway was downregulated in pediatric CLD compared to normal controls. Pediatric CLD had decreased hepatic IRE1α/XBP1 pathway gene expression and decreased protein expression of phosphorylated IRE1α compared to normal controls. IRE1α/XBP1 pathway gene expression was also decreased in pediatric CLD compared to AIH disease controls. Conclusions Pediatric CLD explants have decreased expression of the protective IRE1α/XBP1 pathway and down-regulated KEGG protein processing in the ER pathways. IRE1α/XBP1 pathway expression differences occur when compared to both normal and non-cholestatic disease controls. Attenuated expression of the IRE1α/XBP1 pathway is associated with cholestatic diseases and may be a target for future therapeutics.