Activation of the kynurenine pathway potentially underlies neurodegeneration in patients with covert hepatic encephalopathy

Background and aims: Hepatic encephalopathy (HE) is a neuropsychiatric complication of liver disease, characterised by elevated systemic concentrations of ammonia and pro-inflammatory cytokines. These neurotoxins can cross the blood-brain barrier and act synergistically to cause neuroinflammation, which can activate the kynurenine pathway (KP). This results in depletion of local tryptophan (TRP) reserves and the production of neuroactive KP metabolites. Specifically, 3-hydroxykynurenine (3-HK) and quinolinic acid (QUIN) cause astrocyte and neuronal death, while kynurenic acid (KYNA) is a NMDA receptor antagonist with neuroprotective effects. The aim of our study was to compare
systemic KP activity between patients with covert HE (CHE), nonencephalopathic cirrhosis patients (NHE) and healthy controls.
Method: This was a single-centre prospective cohort study conducted between 2018–2021 at St Vincent’s Hospital in Sydney. Overall, there were 13 CHE patients, 10 NHE patients and 12 healthy controls. Patients with cirrhosis were diagnosed with CHE if they scored ≤−4 on the validated Psychometric Hepatic Encephalopathy Score. Plasma samples were obtained to determine the expression levels of KP enzymes, calculated by a ratio of metabolite product divided by its substrate. TRP, alongside upstream KP metabolites such as kynurenine (KYN), anthranilic acid (AA), KYNA, 3-HK and 3-hydroxyanthranilic acid (3-HAA) were quantified using high-performance liquid chromatography (HPLC) and ultra-HPLC. Downstream KP metabolites, QUIN and picolinic acid (PIC) were quantified using gas-chromatography/mass spectrometry.
Results: KPwas highly activated in cirrhosis patients, especially those with CHE, in comparison to healthy controls, as demonstrated by higher KYN/TRP ratios representing elevation of the rate-limiting enzymes, indoleamine-2,3-dioxygenase (IDO-1) and tryptophan-2,3-dioxygenase (TDO). Following catabolism of TRP to KYN, the subsequent kynurenine 3-onooxygenase (KMO) enzyme demonstrated elevation in CHE patients only, skewing the pathway towards production of 3-HK. Levels of KYNA were higher in CHE patients compared to NHE patients and healthy controls. The activation of downstream KP pathways was evident through the higher production of QUIN and PIC metabolites at the final node of the pathway in cirrhosis patients, especially those with CHE.
Conclusion: The activation of KP in patients with cirrhosis results in the dysregulated production of neurotoxic metabolites. The higher levels in the CHE cohort suggest this is potentially contributing to neurodegeneration and clinical course. Dysregulation of the KP pathway appears to already be underway in cirrhosis patients who do not yet show any clinical signs of neurocognitive impairment. Therapeutic agents that modulate KP activity may be able to alleviate symptoms of patients with CHE.