Tumour necrosis factor‐α‐mediated human polymeric immunoglobulin receptor expression is regulated by both mitogen‐activated protein kinase and phosphatidylinositol‐3‐kinase in HT‐29 cell line

Human polymeric immunoglobulin receptor (pIgR) is present on the surface of glandular epithelium, and it plays a crucial role in the mucosal immune defence. pIgR expression in HT-29 cells is up-regulated by one of the proinflammatory cytokines, tumour necrosis factor (TNF)-α. However, the mechanism used by the TNF-α-mediated signalling pathway has not been examined exclusively. To elucidate this mechanism in detail, HT-29 cells were cotreated with TNF-α and mitogen-activated protein kinase kinase (MAPKK, also called MEK1) inhibitor, PD98059, and the amount of free secretory component (SC) secreted into the culture medium was measured. The amount of free SC stimulated by TNF-α was increased by addition of PD98059. This up-regulation occurred at the transcriptional level. The amount of SC was also up-regulated by addition of TNF-α with U0126, an inhibitor of MEK1 and MEK2. Nuclear factor (NF)-κB activity and NF-κB binding to the κB2 site localized upstream of the pIgR gene did not change after coincubation of HT-29 cells with TNF-α and PD98059. The expression level of pIgR by TNF-α was decreased by LY294002, an inhibitor of phosphatidylinositol-3-kinase (PI3K), at the transcriptional level. Extracellular signal-regulated kinase (ERK)1/2 phosphorylation and NF-κB binding to the κB2 site were not affected by LY294002 treatment. These data suggest that TNF-α-mediated pIgR expression is negatively regulated by ERK pathway, which is independent of NF-κB. In addition, decrease of SC production by Ly294002 suggests that the presence of PI3K mediated regulation of SC production.