The liver is sensitive to many toxic insults and diseases, in which hepatocellular injury is mediated by neutrophil (PMN)-mediated inflammation and hypoxia (HX). PMNs cause hepatocyte (HPC) injury through the release of the toxic protease elastase (EL). Primary HPCs are sensitized to the cytotoxic effects of EL when exposed to HX, indicating an interaction between EL and HX. The mechanism of this interaction is not understood, and is the focus of this dissertation.I tested the hypothesis that the interaction of HX and EL requires reactive oxygen species (ROS), mitogen activated protein kinases (MAPK) and HIF-1α signaling. Results indicated that ROS were not involved; however lipid peroxidation is an important mediator of cell death in this interaction. Furthermore, HX increased HIF-1α activation, which was enhanced by cotreatment with EL and resulted in transcription of the cell death proteins Nix and BNIP3. The transactivation of HIF-1α was mediated in part by p38 MAPK signaling in HX/EL-cotreatment. The results indicated that HIF-1α is a critical mediator of HPC injury in vitro, however it is not known if HIF-1α can contribute to liver injury in an in vivo model of drug-induced liver injury (DILI). Therefore, I tested the hypothesis that HIF-1α contributes to HPC injury in a well-studied model of DILI, acetaminophen overdose. To test this, conditional HIF-1α knockout mice were utilized. Results indicated that HIF-1α contributes to early cell death signaling in HPCs, and to the production of pro-inflammatory cytokines, PMN infiltration, and coagulation system activation, all of which contribute to early acetaminophen hepatotoxicity at 6 hours after overdose. However, HIF-1α knockout mice eventually developed severe liver injury by 24 hours, indicating that HIF-1α might have a protective role at later times. In summary, the mechanism of the cytotoxic interaction of HX and EL was elucidated, and lipid peroxidation, p38 MAPK and HIF-1α were identified to have critical roles in hepatocellular injury. Furthermore, I have demonstrated a dual role for HIF-1α in a model of DILI. This indicates that HIF-1α is a complex transcription factor and could be a central mediator of both hepatocyte damaging and protective pathways in models of toxic liver injury and disease when both PMNs and tissue HX are involved.
Read
