Objective: To examine whether lipopolysaccharide (LPS)-induced apoptosis correlates with TNF-alpha release by type II alveolar epithelial cells (AEC II), whether TNF-alpha knockout (TNF KO) abrogates the induction of apoptosis by LPS and whether TNF-alpha is sufficient to induce apoptosis in this cell type.
Methods: AEC II were isolated from wild type mice and TNF KO mice. Cells were stimulated with LPS or recombinant murine TNF-alpha for 24 h. TNF-alpha in culture supernatant was determined by ELISA following LPS stimulation. Apoptosis was determined by the terminal deoxynucleotidyl transferase end-labeling (TUNEL) assay after treatment with either LPS or TNF-alpha.
Results: LPS induced apoptosis in wild type AEC II in a concentration-dependent manner. LPS-induced AEC II apoptosis was accompanied by an 11-fold increase (from 0.073 +/- 0.065 ng/ml in control to 0.94 +/- 0.14 ng/ml in 50 micro g/ml of LPS, P < 0.01) in TNF-alpha release. However, increasing concentrations (5 or 25 ng/ml) of recombinant murine TNF-alpha failed to induce AEC II apoptosis. In addition, apoptosis did occur in AEC II isolated from TNF KO mice following LPS stimulation.
Conclusions: This study confirms that LPS induces TNF-alpha release and apoptosis in murine AEC II in vitro. Exogenous TNF-alpha failed to induce AEC II apoptosis, and apoptosis occurred following LPS stimulation in cells lacking the ability to produce TNF-alpha. Taken together, these results suggest that LPS-induced AEC II apoptosis occurs by a TNF-alpha-independent mechanism.