DIFFERENTIAL REGULATION OF EOTAXIN EXPRESSION BY TNF-ALPHA AND PMA INHUMAN MONOCYTIC U-937 CELLS

Regulation of eotaxin expression was investigated in U-937 cells, a hu man monocytelike cell line. Eotaxin mRNA was induced by tumor necrosis factor-alpha (TNF-alpha; 0.1-100 ng/ml) and phorbol 12-myristate 13-a cetate (PMA; 0.01-1 mu M). PMA-induced eotaxin mRNA expression was of greater magnitude and was maximal at a later time point than TNF-alpha -induced expression (16 h vs. 2 h after stimulation), which was consis tent with eotaxin protein expression detected by immunocytochemistry. Dexamethasone (0.01-10 mu M) decreased eotaxin mRNA expression in both TNF-alpha- and PMA-stimulated U-937 cells. PMA-induced eotaxin mRNA e xpression was inhibited by cycloheximide (10 mu g/ml), whereas TNF-alp ha-induced expression was not. The protein kinase C (PKC) inhibitor st aurosporine (10-50 nM) inhibited PMA-induced eotaxin mRNA expression, whereas TNF-alpha-induced expression was enhanced by this reagent. The se results suggest that eotaxin expression can be induced by more than one mechanism: the PMA-triggered pathway is mediated by PKC activatio n and requires new protein synthesis, whereas the TNF-alpha-triggered pathway is independent of PKC and protein synthesis. TNF-alpha- and PM A-induced pathways are both associated with nuclear factor-kappa B, be cause its binding activity was enhanced in the presence of these stimu li, and both pathways were limited by its inhibitor, diethyldithiocarb amate.