Activation of NF-κB by nontypeable Hemophilus influenzae is mediated by toll-like receptor 2-TAK1-dependent NIK-IKKα/β-IκBα and MKK3/6-p38 MAP kinase signaling pathways in epithelial cells

Nontypeable Hemophilus influenzae (NTHi) is an important human pathogen in both children and adults. In children, it causes otitis media, the most common childhood infection and the leading cause of conductive hearing loss in the United States. In adults, it causes lower respiratory tract infections in the setting of chronic obstructive pulmonary disease, the fourth leading cause of death in the United States. The molecular mechanisms underlying the pathogenesis of NTHi-induced infections remain undefined, but they may involve activation of NF-kappaB, a transcriptional activator of multiple host defense genes involved in immune and inflammatory responses. Here, we show that NTHi strongly activates NF-KB in human epithelial cells via two distinct signaling pathways, NF-kappaB translocation-dependent and -independent pathways. The NF-K kappaB translocation-dependent pathway involves activation of NF-kappaB inducing kinase (NIK)-IKK alpha/beta complex leading to I kappaB alpha phosphorylation and degradation, whereas the NF-kappaB translocation-independent pathway involves activation of MKK3/6-p38 mitogen-activated protein (MAP) kinase pathway. Bifurcation of NTHi-induced NIK-IKK alpha/beta -I kappaB alpha and MKK3/6-p38 MAP kinase pathways may occur at transforming growth factor-beta activated kinase 1 (TAK1). Furthermore, we show that toll-like receptor 2 (TLR2) is required for NTHi-induced NF-kappaB activation. In addition, several key inflammatory mediators including IL-1 beta, IL-8, and tumor necrosis factor-cu are up-regulated by NTHi. Finally, P6, a 16-kDa lipoprotein highly conserved in the outer membrane of all NTHi and H. influenzae type b strains, appears to also activate NF-kappaB via similar signaling pathways. Taken together, our results demonstrate that NTHi activates NF-kappaB via TLRZ-TAK1-dependent NIK-IKK alpha/beta -I kappaB alpha and MKK3/6-p38 MAP kinase signaling pathways. These studies may bring new insights into molecular pathogenesis of NTHi-induced infections and open up new therapeutic targets for these diseases.