Effect of dexamethasone on leukotriene synthesis in DMSO-stimulated HL-60 cells

Abstract

Human leukemia (HL) 60 cells were differentiated by dimethylsulfoxide (DMSO) treatment to granulocyte-like cells, leukotriene (LT) synthesizing activity of which was increased in response to the differentiation of the cells. Four synthesizing enzymes, cytosolic phospholipase A 2 (cPLA 2), 5-lipoxygenase (5-LO), LTA 4 hydrolase and LTC 4 synthase, and an enzyme associated protein, 5-lipoxygenase activating protein (FLAP) are involved in the generation of LTC 4 and LTB 4. We examined the expression of messenger RNA (mRNA) for these LT synthesizing enzymes and an associated protein in DMSO differentiated HL-60 cells by reverse transcriptase polymerase chain reaction (RT-PCR). The production of LTC 4 and LTB 4, measured by radioimmunoassay (RIA), was increased after the incubation with DMSO for more than 3 days. Messenger RNA abundance for 5-LO, LTC 4 synthase and LTA 4 hydrolase was increased, that for FLAP was stable, but that for cPLA 2 was decreased. These results indicate that DMSO induced increase of LT synthesis is associated with the increase of mRNA expression of 5-LO, LTC 4 synthase and LTA 4 hydrolase, although the precise regulatory mechanisms of the increased mRNA expression are not determined. We also investigated an action of dexamethasone (DEX) on DMSO-induced enhancement of LT synthesis. DEX suppressed DMSO induced increase of LTC 4 synthesis, but rather enhanced DMSO induced LTB 4 production. The DEX attenuated the DMSO-induced increase of mRNA expression for LTC 4 synthase, but showed no effect on that for LTA 4 hydrolase. The inhibition of LTC 4 synthesis is associated with the suppression of mRNA expression for LTC 4 synthase. However, increased LTB 4 synthesis by DEX is regulated by the mechanisms which are independent from mRNA level of LTA 4 hydrolase.