Bacteroides fragilis Enterotoxin Induces Autophagy through an AMPK and FoxO3-Pathway, Leading to the Inhibition of Apoptosis in Intestinal Epithelial Cells

Macroautophagy/autophagy is essential for preserving cellular homeostasis by recycling nutrients and removing spoiled or aged proteins and organelles. It also has an essential role in defense mechanisms against microbial infections. However, the role of autophagy in enterotoxigenic Bacteroides fragilis infection remains largely unknown. In this study, we explored the role of B. fragilis enterotoxin (BFT) in the autophagic process of intestinal epithelial cells (IECs). The LC3-I of human HCT-116 IECs was converted to LC3-II by BFT stimulation. In addition, BFT-exposed cells showed the decreased expression of p62 in a time-dependent manner and increased levels of ATG5 and ATG12 gradually. Evidence of an enhanced autophagic process was supported by autophagosomes co-localized with LC3-lysosome-associated protein 2 in BFT-stimulated cells. The AMP-activated protein kinase (AMPK) and Forkhead box O3 (FoxO3a) axis were required for BFT-induced autophagy activation. In contrast with the activation of autophagy at 3–6 h after BFT exposure, IECs induced apoptosis-related signals at 12–48 h. HCT-116 IECs suppressing the formation of autophagosomes significantly activated apoptosis signals instead of autophagy early after BFT exposure. These data suggest that BFT can activate autophagy through the AMPK-FoxO3a pathway and the autophagy may suppress apoptosis during early exposure of IECs to BFT.