title: Physiological hypoxia prevents bile salt-induced apoptosis in human and rat hepatocytes.
abstract: OBJECTIVE
  Hydrophobic bile salts such as glycochenodeoxycholate (GCDC) accumulate in cholestatic liver disease and induce hepatocellular apoptosis, promoting profibrotic signalling. The tissue microenvironment is an integral player in cellular pathophysiology, but it is not routinely incorporated into laboratory studies. Tissue oxygen partial pressure (pO₂) may be an underestimated component of the microenvironment: in the liver, a pO₂ of 30-45 mmHg (approximately 6% O₂) is physiological, because of predominant portal blood supply. It was the aim of this project to investigate the impact of physiological hypoxia (i.e. 6% O₂) on hepatocellular function, namely, bile salt-induced apoptosis.
  
  
  METHODS
  Human hepatoma cells (HepG2-Ntcp) and primary rat hepatocytes were cultured at standard laboratory (hyperoxic) conditions (21% O₂) and at physiological hypoxia (6% O₂) in parallel for 1-8 days to study hepatocellular apoptosis and activation of signalling pathways. Standard laboratory analyses were applied for bile salt uptake, caspase-3/-7 activity, western blotting and gene-array analysis.
  
  
  RESULTS
  Culturing at physiological hypoxia protected both human and rat hepatocytes against GCDC-induced apoptosis: caspase-3/-7 activation was diminished by 3.1 ± 0.5-fold in human HepG2-Ntcp and completely abolished in primary rat hepatocytes. Bile salt uptake was unaffected. Induction of hypoxia-inducible factor-1α indicated adaption to physiological hypoxia. The MEK/ERK cascade was activated and anti-apoptotic mediators were induced: N-Myc down-regulated gene, gelsolin and carbonic anhydrase IX were upregulated 12.4-, 6.5- and 5.2-fold respectively.
  
  
  CONCLUSIONS
  We conclude from these data that (i) physiological hypoxia protects hepatocytes from bile salt-induced apoptosis, (ii) tissue pO₂ is a crucial, underestimated component of the microenvironment and should (iii) be considered when studying hepatocellular physiology in vitro.
authors.affiliation: Department of Medicine II, University of Munich, Munich, Germany.
authors.forename: Simon
authors.initials: S
authors.lastname: Hohenester
authors.forename: Timo
authors.initials: T
authors.lastname: Vennegeerts
authors.forename: Michaela
authors.initials: M
authors.lastname: Wagner
authors.forename: Ralf
authors.initials: R
authors.lastname: Wimmer
authors.forename: Heidrun
authors.initials: H
authors.lastname: Drolle
authors.forename: Christina
authors.initials: C
authors.lastname: Rieger
authors.forename: Gerald U
authors.initials: GU
authors.lastname: Denk
authors.forename: Christian
authors.initials: C
authors.lastname: Rust
authors.forename: Michael
authors.initials: M
authors.lastname: Fiegl
mesh_terms: D000818:Animals; D017209:Apoptosis / Q000187:drug effects / Q000502:physiology*; D001647:Bile Acids and Salts / Q000009:adverse effects; D015153:Blotting, Western; D059447:Cell Cycle Checkpoints / Q000187:drug effects / Q000502:physiology*; D015687:Cell Hypoxia / Q000502:physiology*; D005434:Flow Cytometry; D056945:Hep G2 Cells; D022781:Hepatocytes / Q000502:physiology*; D006801:Humans; D051795:Hypoxia-Inducible Factor 1, alpha Subunit / Q000378:metabolism*; D046228:Microarray Analysis; D051381:Rats; D015398:Signal Transduction / Q000502:physiology
chemical_list: D001647:Bile Acids and Salts; D051795:Hypoxia-Inducible Factor 1, alpha Subunit
keywords: apoptosis; bile salt; liver; physiological hypoxia