The role of mitogen-activated protein kinases in Listeria monocytogenes invasion

Listeria monocytogenes is a facultative intracellular gram-positive bacterium that causes severe disease in newborns, the elderly and immunocompromised people. The disease listeriosis has a high mortality rate in susceptible people and usually manifests as septicemia, meningitis or meningoencephalitis. The pathogenesis of L. monocytogenes infection depends upon the ability of the bacterium to invade and replicate inside various cell types, including macrophages and epithelial cells. Using an in vitro model of invasion, we have uncovered mitogen-activated protein (MAP) kinase signal transduction cascades that are induced in the host epithelial cells during the L. monocytogenes infection process. MAP kinase pathways are involved in the regulation of many eucaryotic cell functions including mitogenesis, differentiation, stress responses, apoptosis, and cytoskeletal rearrangements. Activation of the extracellular signal-regulated kinase-2 (ERK-2) and p38 MAP kinase seems to be specific for internalization of L. monocytogenes, while ERK-1 and c-Jun N-terminal kinase (JNK) are not. Initially, we found that ERK-1 and ERK-2 were tyrosine phosphorylated during L. monocytogenes infection of cultured epithelial cells. Subsequently, the action of a Listeria secreted cytolysin listeriolysin O(LLO) was shown to be responsible for triggering most of the MAP kinase activity, presumably through damage to the host eucaryotic cell membrane. However, LLO is not involved in the invasion process and thus not all of these signals were required for L. monocytogenes invasion. Using mutants lacking functional LLO, we found that ERK-2 and p38 activity was induced during invasion. In addition, PD98059, a specific inhibitor of MAPK/ERK kinase-1 (MEK-1) activation, blocked Listeria monocytogenes invasion into host cells but not adherence. MEK-1 is a kinase which activates ERK-1 and ERK-2. The effects of PD98059 were reversible as adherent extracellular bacteria were internalized upon removal of the drug. SB203580, a selective inhibitor of p38 activity, also decreased L. monocytogenes invasion into host cells. The invasion of Salmonella typhimurium was not affected by these drugs. These results indicate that the MEK-1/ERK-2 and p38 MAP kinase pathways are required for L. monocytogenes invasion into host epithelial cells.Science, Faculty ofMicrobiology and Immunology, Department ofGraduat