Regulation of protein kinase C in Escherichia coli K1 invasion of human brain microvascular endothelial cells

Escherichia coli is one of the most important pathogens involved in the development of neonatal meningitis in many parts of the world. Traversal of E. coli across the blood-brain barrier is a crucial event in the pathogenesis of E. coli meningitis. Our previous studies have shown that outer membrane protein A (OmpA) expression is necessary in E. coli for a mechanism involving actin filaments in its passage through the endothelial cells. Focal adhesion kinase (FAK) and phosphatidylinositol 3-kinase (PI3K) have also been activated in host cells during the process of invasion. In an attempt to elucidate the mechanisms leading to actin filament condensation, we have focused our attention on protein kinase C (PKC), an enzyme central to many signaling events, including actin rearrangement. In the current study, specific PKC inhibitors, bisindolmaleimide and a PKC-inhibitory peptide, inhibited E. coli invasion of human brain microvascular endothelial cells (HBMEC) by more than 75% in a dose-dependent manner, indicating a significant role played by this enzyme in the invasion process. Our results further showed that OmpA+ E. coli induces significant activation of PKC in HBMEC as measured by the PepTag nonradioactive assay. In addition, we identified that the PKC isoform activated in E. coli invasion is a member of the conventional family of PKC, PKC-alpha, which requires calcium for activation. Immunocytochemical studies have indicated that the activated PKC-alpha is associated with actin condensation beneath the bacterial entry site. Overexpression of a dominant negative mutant of PKC-alpha in HBMEC abolished the E. coli invasion without significant changes in FAK phosphorylation or PI3K activity patterns. In contrast, in HBMEC overexpressing the mutant forms of either FAK or PI3K, E. coli-induced PKC activation was significantly blocked. Furthermore, our studies showed that activation of PKC-alpha induces the translocation of myristoylated alanine-rich protein kinase C substrate, an actin cross-linking protein and a substrate for PKC-alpha, from the membrane to cytosol. This is the first report of FAK- and PI3K-dependent PKC-alpha activation in bacterial invasion related to cytoskeletal reorganization.