Endotoxin-Induced Hepatic Effects during Sepsis - Mediated by Apoptosis or Necrosis?

Abstract 1014 Poster Session IV, Tuesday, 5/4 (poster 127)

Hepatic damage is a serious consequence of gram-negative sepsis. The mechanism(s) of lipopolysaccharide (LPS)-mediated damage is controversial and has been attributed to either necrosis or apoptosis. Apoptotic-mediated hepatic effects usually require co-mediators. We investigated the direct effects of LPS on the human hepatoblastoma cell line HepG2. HepG2 cells were cultured at 37C with 5% CO₂ using DMEM with 10% fetal bovine serum (FBS) and non-essential amino acids (NEAA). Following growth to 60-70% confluency, the cells were incubated for 24h with LPS at concentrations ranging from 3-100 ug/ml under either serum-free conditions, or following the addition of 2% or 10% FBS. Additional HepG2 cells were exposed to LPS, which had been pre- incubated for 1h with either the cationic liposome DMRIE-C or LipofectAMINE. LPS-liposome complex-exposed HepG2 cells were incubated with Opti-MEM plus 10% FBS and NEAA for 24h. Apoptosis was determined utilizing TUNEL (TdT-mediated dUTP Nick-End Labeling) technique, determining fluorescence of fixed cells with fluorescein-labeled DNA strand breaks by microscopy. Detection of necrotic cell toxicity by membrane disruption was determined by LDH quantitation of the cell-free supernatant and compared to negative controls. Incubation of HepG2 cells with LPS alone, in concentrations up to 100ug/ml, with or without FBS, revealed no significant apoptosis after 24, 48 and 72h. There was a minimal increase in cell toxicity compared to controls, indepen- dent of the LPS concentration. Cell toxicity was higher when cells were pre-incubated for 4h under serum-free conditions. Incubation of HepG2 cells with LPS/liposome complexes induced apoptosis, in a concentration dependent manner. Cell toxicity (LDH release) was minimally increased, comparable to the amount following incubation with LPS or liposome alone. We conclude that HepG2 cells are resistant to LPS- mediated toxicity unless cell delivery is facilitated with cationic liposomes, in which case the primary effect is apoptosis. Liposome-mediated LPS toxicity is concentration dependent, with little evidence of necrosis. Preliminary results suggest that liposome-mediated LPS induces apoptosis in HepG2 cells in a Bcl-2 independent manner. We speculate that LPS hepatotoxicity in vivo may require co-mediators to facilitate LPS entry into hepatocytes, which subsequently results in the induction of apoptosis. (Table)

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Supported by the Children's Medical Center Resident Research Grant