Repetitively hypoxic preconditioning attenuates ischemia/reperfusion-induced liver dysfunction through upregulation of hypoxia-induced factor-1 alpha-dependent mitochondrial Bcl-xl in rat

Repetitive hypoxic preconditioning (HP) enforces protective effects to subsequently severe hypoxic/ischemic stress. We hypothesized that HP may provide protection against ischemia/reperfusion (I/R) injury in rat livers via hypoxia-induced factor-1 alpha (HIF-1α)/reactive oxygen species (ROS)-dependent defensive mechanisms. Female Wistar rats were exposed to hypoxia (15 h/day) in a hypobaric hypoxic chamber (5500 m) for HP induction, whereas the others were kept in sea level. These rats were subjected to 45 min of hepatic ischemia by portal vein occlusion followed by 6 h of reperfusion. We evaluated HIF-1α in nuclear extracts, MnSOD, CuZnSOD, catalase, Bad/Bcl-xL/caspase 3/poly-(ADP-ribose)-polymerase (PARP), mitochondrial Bcl-xL, and cytosolic cytochrome C expression with Western blot and nitroblue tetrazolium/3-nitrotyrosine stain. Kupffer cell infiltration and terminal deoxynucleotidyl transferase-mediated nick-end labeling method apoptosis were determined by immunocytochemistry. The ROS value from liver surface and bile was detected by an ultrasensitive chemiluminescence-amplification method. Hepatic function was assessed with plasma alanine aminotransferase (ALT) and aspartate aminotransferase (AST) levels. HP increased nuclear translocation of HIF-1α and enhanced Bcl-xL, MnSOD, CuZnSOD, and catalase protein expression in a time-dependent manner. The response of HP enhanced hepatic HIF-1α, and Bcl-xL expression was abrogated by a HIF-1α inhibitor YC-1. Hepatic I/R increased ROS levels, myeloperoxidase activity, Kupffer cell infiltration, ALT and AST levels associated with the enhancement of cytosolic Bad translocation to mitochondria, release of cytochrome C to cytosol, and activation of caspase 3/PARP-mediated apoptosis. HP significantly ameliorated hepatic I/R-enhanced oxidative stress, apoptosis, and mitochondrial and hepatic dysfunction. In summary, HP enhances HIF-1α/ROS-dependent cascades to upregulate mitochondrial Bcl-xL protein expression and to confer protection against I/R injury in the livers.