Antagonistic roles of CD14 and TLR4 in LPS‐induced colonic epithelial cell apoptosis

Abstract

Luminal exposure to bacterial LPS induces apoptosis in intestinal epithelial cells via unknown mechanism. Recent studies in monocytes showed that after LPS binding to CD14, phosphatidylcholine‐specific phospholipase C (PC‐PLC), SMase and ceramide activate PKCζ, and recruits TLR4 for complex formation and downstream MAPK and NFκB signals.AimTo investigate the receptors and signaling pathways involved in LPS‐induced epithelial apoptosis.ResultsApical expression of CD14, low levels of MD2, and absence of TLR4 were demonstrated in primary human colonocytes and Caco‐2 cells. Luminal LPS challenge increased cell apoptosis, which may be inhibited by anti‐CD14 or gene silencing of CD14 and PKCζ, but not anti‐TLR4. Inhibitors to PC‐PLC, SMase and PKCζ decreased apoptotic levels. CD14‐mediated ceramide and phospho‐PKCζ levels were increased after LPS challenge, whereas TLR4 signals were not. TLR4 overexpression by plasmid transfection inhibited LPS/CD14‐mediated apoptosis. Moreover, both CD14 and TLR4 were found on mouse colonocytes. Colonic tissues of TLR4‐mutant mice exposed to luminal LPS or bacterial overgrowth display increased mucosal apoptosis and phospho‐PKCζ compared to wild types.ConclusionsLPS‐induced epithelial apoptosis was dependent on CD14‐mediated lipid secondary messengers, whereas TLR4 plays an antagonistic role. Funding support by National Science Council and NTU.