Differential regulation of eotaxin expression by TNF-α and PMA in human monocytic U-937 cells.

Regulation of eotaxin expression was investigated in U-937 cells, a human monocyte-like cell line. Eotaxin mRNA was induced by tumor necrosis factor-α (TNF-α; 0.1-100 ng/ml) and phorbol 12-myristate 13-acetate (PMA; 0.01-1 μM). PMA-induced eotaxin mRNA expression was of greater magnitude and was maximal at a later time point than TNF-α-induced expression (16 h vs. 2 h after stimulation), which was consistent with eotaxin protein expression detected by immunocytochemistry. Dexamethasone (0.01-10 μM) decreased eotaxin mRNA expression in both TNF-α- and PMA-stimulated U-937 cells. PMA-induced eotaxin mRNA expression was inhibited by cycloheximide (10 μg/ml), whereas TNF-α-induced expression was not. The protein kinase C (PKC) inhibitor staurosporine (10-50 nM) inhibited PMA-induced eotaxin mRNA expression, whereas TNF-α-induced expression was enhanced by this reagent. These results suggest that eotaxin expression can be induced by more than one mechanism: the PMA-triggered pathway is mediated by PKC activation and requires new protein synthesis, whereas the TNF-α-triggered pathway is independent of PKC and protein synthesis. TNF-α- and PMA-induced pathways are both associated with nuclear factor-κB, because its binding activity was enhanced in the presence of these stimuli, and both pathways were limited by its inhibitor, diethyldithiocarbamate.