HuR/HIF – SIRT1 Signaling Axis in Liver Transplant Rejuvenation

University Of California Los Angeles, Los Angeles CA

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Abstract

ABSTRACT Orthotopic liver transplantation (OLT) is the gold standard of care in patients with end-stage liver disease and those with tumors of hepatic origin. However, the scarcity of donor organs prompted the use of extended criteria “marginal” livers, which are particularly susceptible to ischemia-reperfusion injury (IRI), which predispose to acute/chronic rejection, and may require re-transplantation. We have introduced the concept of organ “rejuvenation”, i.e., conversion of the donor liver from the state of IRI-hypersensitivity to the homeostatic state of IRI-resistance. We have also proposed that SIRT1 deacetylase serves as a rheostat linking IR-stress with liver rejuvenation in both, mouse and human OLT. We have recently identified new regulators of hepatic resistance against warm vs. cold ischemia stress responses, i.e., Human Antigen R (HuR) and Hypoxia-Inducible Factor (HIF-1α). We have also discovered Ikaros (IKZF1), acts as a macrophage activation marker and exacerbates liver IRI. Importantly, we also found that preserved hepatocellular function/improved clinical outcomes in human OLT patients were associated with increased HuR/HIF-1α but depressed Ikaros levels in the liver biopsy samples. We hypothesize that crosstalk between hepatocyte HuR / HIF-1α and macrophage Ikaros provides a new means to regulate the adaptation of donor livers to IR-stress and reperfusion-mediated hepatic damage. Specific Aim 1: Determine mechanisms of hepatocyte HuR / HIF-1α crosstalk with SIRT1 in IRI-OLT. Hypothesis: Hepatocyte SIRT1 activation, controlled by distinct hypoxia/reoxygenation (H/R) requirements for HuR (warm H/R) vs. HIF-1α (cold H/R), provide new means to regulate adaptation of donor livers to IR-stress. 1.1: SIRT1 function is dependent on HuR signaling for anti-inflammatory responses in oxidative stress. 1.2: HuR/HIF-1α signaling in a mouse model of hepatic ischemia is temperature stress-dependent. 1.3: HIF-1α controls cold-induced IRI-OLT. 1.4: HuR controls warm-induced IRI-OLT. Specific Aim 2: Delineate mechanisms of macrophage Ikaros crosstalk with SIRT1 in IRI-OLT. Hypothesis: Macrophage Ikaros signaling exacerbates IRI-OLT by repressing SIRT1 transcription and M2 macrophage polarization. 2.1: Ikaros-SIRT1 myeloid axis influences hepatic HuR/HIF1α hypoxia sensing circuit in IRI-OLT. 2.2: Macrophage Ikaros signaling depends on SIRT1 transcription for M2 polarization. Specific Aim 3: Define mechanism of human liver rejuvenation under hypothermic machine preservation. Hypothesis: Manipulation of HIF-1α / SIRT1 axis during ex-vivo HMP improves hepatocellular function to rejuvenate human livers declined for transplantation due to preexisting poor quality. 3.1: Pharmacological stabilizer of HIF-1α protein synergizes with SIRT1 to improve human liver function. 3.2: Preconditioning with PHD-inhibitor, which activates/stabilizes HIF-1α, synergizes with enhanced SIRT1 signaling to ameliorate inflammation, promote cytoprotection, and rejuvenate human livers. These experiments are of high relevance to refine donor liver donation (DBD and DCD) as well as to improve quality/size of the current organ supply.