title: Oxidative stress impairs HIF1α activation: a novel mechanism for increased vulnerability of steatotic hepatocytes to hypoxic stress.
abstract: Steatosis increases the sensitivity of hepatocytes to hypoxic injury. Thus, this study was designed to elucidate the role of hypoxia-inducible factor-1α (HIF1α) in steatotic hepatocytes during hypoxia. AML12 hepatocytes and isolated rat hepatocytes were treated with a free fatty acid mixture of oleate and palmitate (2:1, 1 mM) for 18 h, which generated intrahepatocyte fat accumulation. The cells were then exposed to hypoxia (1% oxygen, 6-24 h). After hypoxia, a further increase in cellular fat accumulation was seen. In steatotic hepatocytes, a decreased HIF1α activation by hypoxia was observed. The capacity of these cells to express HIF1α-dependent genes responsible for the utilization of nutrients for energy was also impaired. This resulted in significantly lower intracellular ATP levels and greater cell death in steatotic hepatocytes compared with control hepatocytes. In contrast, overexpression of constitutively active HIF1α significantly increased cell viability as well as ATP and GLUT1 mRNA levels in steatotic hepatocytes under hypoxia. Hypoxia significantly enhanced HIF1α mRNA levels in control but not in steatotic hepatocytes. Concomitantly, an increase in oxidative stress was found in steatotic hepatocytes under hypoxic conditions compared with control cells. This included higher reactive oxygen species generation, lower cellular and nuclear GSH levels, and higher accumulation of 4-hydroxynonenal protein adducts. Hypoxia-mediated oxidative stress was accompanied by inactivation of basal nuclear factor-κB (NF-κB) DNA binding. Treatment with N-acetyl-l-cysteine, a reducing agent, improved NF-κB DNA-binding capacity and restored HIF1α induction. Conversely, overexpression of an NF-κB super-suppressor in control hepatocytes (IκBαΔN-transfected cells) resulted in complete inhibition of HIF1α expression, confirming that indeed NF-κB regulates HIF1α expression in hypoxic hepatocytes. In conclusion, hypoxia in combination with hepatic steatosis was shown to promote augmented oxidative stress, leading to NF-κB inactivation and impaired HIF1α induction and thereby increased susceptibility to hypoxic injury.
authors.affiliation: The Robert H. Smith Faculty of Agriculture, Food and Environment, Institute of Biochemistry, Food Science and Nutrition, The Hebrew University of Jerusalem, Israel.
authors.forename: Sarit
authors.initials: S
authors.lastname: Anavi
authors.forename: Noga Budick
authors.initials: NB
authors.lastname: Harmelin
authors.forename: Zecharia
authors.initials: Z
authors.lastname: Madar
authors.forename: Oren
authors.initials: O
authors.lastname: Tirosh
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D001483:Base Sequence; D015153:Blotting, Western; D053148:Caspase 3 / Q000378:metabolism; D015687:Cell Hypoxia; D002460:Cell Line; D002478:Cells, Cultured; D017931:DNA Primers; D024202:Electrophoretic Mobility Shift Assay; D005230:Fatty Acids, Nonesterified / Q000008:administration & dosage; D005234:Fatty Liver / Q000378:metabolism* / Q000473:pathology; D051272:Glucose Transporter Type 1 / Q000235:genetics; D051795:Hypoxia-Inducible Factor 1, alpha Subunit / Q000378:metabolism*; D051379:Mice; D018384:Oxidative Stress; D012333:RNA, Messenger / Q000378:metabolism; D051381:Rats; D017382:Reactive Oxygen Species / Q000378:metabolism; D060888:Real-Time Polymerase Chain Reaction
chemical_list: D017931:DNA Primers; D005230:Fatty Acids, Nonesterified; D051272:Glucose Transporter Type 1; C497443:Hif1a protein, mouse; D051795:Hypoxia-Inducible Factor 1, alpha Subunit; D012333:RNA, Messenger; D017382:Reactive Oxygen Species; C495678:Slc2a1 protein, mouse; D000255:Adenosine Triphosphate; D053148:Caspase 3