Differential regulation of cyclo-oxygenase-2 and 5-lipoxygenase-activating protein (FLAP) expression by glucocorticoids in monocytic cells

1 The objective of the present study was to determine the effects of dexamethasone on key constituents of prostaglandin and leukotriene biosynthesis, cyclo-oxygenase-2 (COX-2) and 5-lipoxygenase activating protein (FLAP). The human monocytic cell line THP-1 was used as a model system, mRNA and protein levels of COX-2 and FLAP were determined by Northern and Western blot analyses, respectively. 2 Low levels of COX-2 and FLAP mRNA were expressed in undifferentiated THP-1 cells, but were induced upon differentiation of the cells along the monocyclic pathway by treatment with phorbol ester (TPA, 5 nM). Maximal expression was observed after two days. 3 Coincubation of the undifferentiated cells with dexamethasone (10(-9)-10(-6) M) and phorbol ester prevented induction of COX-2 mRNA, but did not affect the induction of FLAP mRNA. 4 Dexamethasone downregulated COX-2 mRNA and protein in differentiated, monocyte-like THP-1 cells. In contrast, FLAP mRNA and protein were upregulated by dexamethasone in differentiated THP-1 cells. After 24 h, FLAP mRNA levels were increased more than 2 fold. Dexamethasone did not change 5-lipoxygenase mRNA expression. 5 Release of prostaglandin E-2 (PGE(2)) and peptidoleukotrienes was determined in cell culture supernatants of differentiated THP-1 cells by ELISA. Calcium ionophore-dependent PGE(2) synthesis was associated with COX-2 expression, whereas COX-1 and COX-2 seemed to participate in arachidonic acid-dependent PGE(2) synthesis. Very low levels of peptidoleukotrienes were released from differentiated THP-1 cells upon incubation with ionophore. Treatment with dexamethasone did not significantly affect leukotriene release. 6 These data provide evidence that prostaglandin synthesis is consistently downregulated by glucocorticoids. However, the glucocorticoid-mediated induction of FLAP may provide a mechanism to maintain leukotriene biosynthesis through more efficient transfer of arachidonic acid to the 5-lipoxygenase reaction, in spite of inhibitory effects on other enzymes of the biosynthetic pathway.