Dexamethasone increases expression of 5-lipoxygenase and its activating protein in human monocytes and THP-1 cells

The aim of this study was to assess the effect of dexamethasone on 5-lipoxygenase pathway expression in human peripheral blood monocytes and the acute monocytic leukemia cell line, THP-1. Cells were conditioned over a period of days with dexamethasone, at concentrations relevant in vivo, to study the effect of the glucocorticoid on calcium-ionophore-stimulated 5-lipoxygenase product and arachidonic acid release. The effect of dexamethasone on levels of immunoreactive protein and steady-state messenger RNA encoding for 5-lipoxygenase and its activating protein (5-LAP) was also assessed. Dexamethasone increased the stimulated release of 5-lipoxygenase products from both monocytes and THP-1 cells in a dose-dependent fashion. The increase in product generation was not due to changes in the availability of arachidonic acid. However, immunoreactive protein and steady-state messenger RNA encoding for 5-lipoxygenase and 5-LAP were increased by conditioning with dexamethasone. There was no apparent effect of the glucocorticoid on LTA(4)-hydrolase-immunoreactive protein levels or specific activity. We conclude that dexamethasone increases 5-lipoxygenase pathway expression in both monocytes and in THP-1 cells. This effect is due, at least in part, to increases in immunoreactive protein and steady-state messenger RNA encoding for 5-lipoxygenase and 5-LAP. These results suggest a role for glucocorticoids in the regulation of 5-lipoxygenase pathway expression in mononuclear phagocytes.