Bile Acid‐Mediated Accumulation of Brain Cholesterol Contributes to Hepatic Encephalopathy Due to Acute Liver Failure
Authors: DeMorrow Sharon , Grant Stephanie , Frampton Gabriel , McMillin Matthew 
Published: November 2023

During acute liver failure (ALF), the enterohepatic circulation becomes disrupted leading to bile acid accumulation in the blood stream. We have previously shown that increased serum bile acids induce permeability of the blood brain barrier and contribute to the development of hepatic encephalopathy (HE) following ALF via activation of farnesoid X receptor (FXR) and subsequent upregulation of small heterodimer partner (SHP) expression in neurons. The events downstream of FXR activation contributing to HE are unknown. However, in the liver FXR activation is known to decrease liver X receptor beta (LXRβ) expression. Highly expressed in neurons in the brain, LXRβ has a role in regulation of Cyp46A1, an enzyme responsible for converting cholesterol to 24(S)‐hydroxycholesterol—the main mechanism of cholesterol clearance in the brain. We hypothesize that increased levels of cholesterol in the brain during ALF and HE result from bile acid‐mediated activation of the FXR/SHP pathway and LXRβ suppression leading to a dysregulation of cholesterol clearance.

Methods

C57Bl/6 (WT) mice or cytochrome p450 7A1 knockout (Cyp7A1−/−) mice were injected with azoxymethane (AOM) to induce ALF and HE. In parallel, WT mice were given an intracerebroventricular infusion of FXR Vivo morpholino sequences to reduce FXR protein expression, or β‐cyclodextrin to reduce cholesterol accumulation, prior to AOM injection. Cognitive impairment was monitored by assessment of reflex responses and ataxia. Liver histology was assessed with hematoxylin and eosin staining. Liver function was assessed by measuring serum enzymes. Cyp46A1, SHP, and LXRβ expression were assessed by immunoblotting and qPCR. Brain and serum levels of cholesterol and 24(S)‐hydroxycholesterol were measured using commercially available kits.

Results

AOM‐treated mice had elevated cholesterol and decreased 24(S)‐hydroxycholesterol in the brain, compared to saline‐injected controls. This result was not observed in AOM‐treated Cyp7A1−/− mice that have reduced circulating bile acid content, nor in mice infused with FXR vivo morpholino. Cortical Cyp46A1 was downregulated after AOM injection to WT mice but not Cyp7A1−/− mice or mice infused with FXR vivo morpholino. SHP was upregulated while LXRβ was downregulated in the brain following AOM injection, and these effects were not observed in AOM‐treated mice that were infused with FXR Vivo morpholino sequences. Lastly, attenuation of cholesterol accumulation by β‐cyclodextrin infusion, delayed the neurological decline after AOM injection.

Conclusion

During AOM‐induced ALF and HE, increased cholesterol levels in the brain are mediated by bile acid activation of FXR and SHP, which together inhibit Cyp46A1 expression by inhibiting LXRβ expression. FXR/SHP‐mediated signaling may be a possible therapeutic target for the management of HE through its interaction with proteins controlling cholesterol homeostasis.

Support or Funding Information

This study was funded by a VA merit award (BX002638‐01) from the United States Department of Veterans Affairs Biomedical Laboratory Research and Development Service (BLR&D) and an NIH R01 award (DK082435) to Dr. DeMorrow. This material is the result of work supported with resources and the use of facilities at the Central Texas Veterans Health Care System, Temple, Texas. The content is the responsibility of the author(s) alone and does not necessarily reflect the views or policies of the Department of Veterans Affairs or the United States Government.