Roles of Bcl-2 family members, PI3K and NF-κB pathways in Escherichia coli-induced apoptosis in human monocytic U937 cells

Abstract

We previously showed that infection of human monocytic U937 cells with nonpathogenic Escherichia coli (E. coli) induced rapid apoptosis in a dose- and time-dependent manner. We also found that E. coli increase p38 mitogen-activated protein Kinase (p38 MAPK) and c-Jun N-terminal kinase (JNK), and decrease extracellular-Regulated Kinase1/2 (ERK1/2) phosphorylation and increase caspase-3 and -9 activity in U937 cells. The current study determines if Bcl-2, Bax, the phosphatidylinositol 3-kinase (PI3K)/Akt and nuclear factor kappa B (NF-κB) regulates E. coli–induced U937 cell apoptosis. Studying the underlying mechanisms we found that the E. coli-induced apoptosis in U937 cells was associated with a more prominent reduction in expression of Bcl-2, levels of P-Akt and NF-κB. Because levels of inhibition of apoptosis protein (cIAP), and X-chromosomelinked inhibitor of apoptosis protein (XIAP) are regulated by NF-κB, E. coli decreased the levels of these proteins in U937 cells through inhibition of NF-κB. Moreover, E. coli markedly elevated Bax expression and cytochrome c redistribution. LY294002, PDTC and Embelin, specific inhibitors of PI3K, NF-κB and XIAP, induced U937 cell apoptosis and the apoptosis is dependent on activity of caspase-3 and -9 in E. coli-treated U937 cells. Through using LY294002 and western blotting, we identified NF-κB was the downstream Akt target regulated by E. coli. Taken together, these results clearly indicate reduced activation of NF-κB via impaired PI3K/Akt activation could result in increased apoptosis of U937 cells infected by E. coli. Moreover, E. coli can induce apoptosis with an increased expression of Bax and a reduced expression of Bcl-2, which resulted in increased levels of cytochrome c release and increase caspase-3 and -9 in U937 cells.