Tumor necrosis factor-alpha and interleukin-1beta inhibit apolipoprotein B secretion in CaCo-2 cells via the epidermal growth factor receptor signaling pathway

In inflammatory conditions of the gut, cytokines are released into the mucosa and submucosa propagating and sustaining the inflammatory response. In CaCo-2 cells, we have shown that various inflammatory cytokines interfere with the secretion of lipids, an effect that is likely caused by the release of a ligand to the epidermal growth factor (EGF) receptor. In the present study, the role of the EGF receptor signaling pathway and the effects of the cytokines tumor necrosis factor-alpha (TNF-alpha) and and interleukin 1beta (IL-1beta) on triacylglycerol-rich lipoprotein secretion were investigated. CaCo-2 cells were incubated with oleic acid to enhance triacylglycerol-rich lipoprotein secretion. TNF-alpha and IL-1beta significantly decreased the basolateral secretion of apolipoprotein B (apoB) mass, with IL-1beta being more potent. Tyrphostin, an inhibitor of the EGF receptor intrinsic tryosine kinase, prevented or markedly attenuated the decrease in apoB secretion by TNF-alpha or IL-1beta. Both cytokines increased the phosphorylation of the EGF receptor by 30 min. Moreover, phosphotyrosine immunoblots of the EGF receptor demonstrated an increase in tyrosine residues phosphorylated by 0.5 and 6.5 h. At both these time points, TNF-alpha and IL-1beta also decreased the binding of EGF to its cell surface receptor. At 6.5 h, activation of the EGF receptor was sustained. In contrast, the early activation of the receptor was only transient as receptor phosphorylation and binding of EGF to its receptor returned to basal levels by 2 h. Preventing ligand binding to the EGF receptor by a receptor-blocking antibody attenuated receptor activation observed after 6.5 h. This did not occur at 0.5 h, suggesting that early activation of the EGF receptor was non-ligand-mediated. Similarly, apoB secretion was inhibited by an early non-ligand-mediated process; whereas at the later time, inhibition of apoB secretion was ligand-mediated. Thus, the inflammatory cytokines TNF-alpha and IL-1beta interfere with the secretion of triacylglycerol-rich lipoproteins by both early and delayed signaling events mediated by the EGF receptor signaling pathway.