Human Antigen R (HuR): A Regulator of Heme Oxygenase-1 Cytoprotection in Mouse and Human Liver Transplant Injury

Ischemia-reperfusion injury (IRI) represents a risk factor in liver transplantation (LT). We have shown that overexpression of heme oxygenase-1 (HO-1) mitigates hepatic IRI in LT recipients. Here, we hypothesized that human antigen R (HuR), the stabilizer of adenylate-uridylate (AU)-rich mRNAs, is required for hepatoprotection in LT.

In an experimental arm, HuR/HO-1 protein expression was correlated with hepatic IRI phenotype. In an in vitro inflammation mimic model of hepatic warm IRI, induction of HuR/HO-1 and cytoplasmic localization following cytokine preconditioning were detected in primary hepatocyte cultures, whereas HuR silencing caused negative regulation of HO-1, followed by enhanced cytotoxicity. Using the HuR-inhibitor, we showed that HuR likely regulates HO-1 through its 3' untranslated region and causes neutrophil activation (CD69+/lymphocyte antigen 6 complex locus G [Ly6-G]). HuR silencing in bone marrow-derived macrophages decreased HO-1 expression, leading to the induction of proinflammatory cytokines/chemokines. RNA sequencing of HuR silenced transcripts under in vitro warm IRI revealed regulation of genes thymus cell antigen 1 (THY1), aconitate decarboxylase 1 (ACOD1), and Prostaglandin E Synthase (PTGES). HuR, but not hypoxia-inducible protein alpha, positively regulated HO-1 in warm, but not cold, hypoxia/reoxygenation conditions. HuR modulated HO-1 in primary hepatocytes, neutrophils, and macrophages under reperfusion. Adjunctive inhibition of HuR diminished microtubule-associated proteins 1A/1B light chain 3B (LC3B), a marker for autophagosome, under HO-1 regulation, suggesting a cytoprotective mechanism in hepatic IR. In a clinical arm, hepatic biopsies from 51 patients with LT were analyzed at 2 hours after reperfusion. Graft HuR expression was negatively correlated with macrophage (CD80/CD86) and neutrophil (Cathepsin G) markers. Hepatic IRI increased HuR/HO-1 expression and inflammatory genes. High HuR-expressing liver grafts showed lower serum alanine aminotransferase/serum aspartate aminotransferase levels and improved LT survival.

This translational study identifies HuR as a regulator of HO-1-mediated cytoprotection in sterile liver inflammation and a biomarker of ischemic stress resistance in LT.