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

Human leukemia (HL) 60 cells were differentiated by dimethylsulfoxide (DMSO ) treatment to granulocyte-like cells, leukotriene (LT) synthesizing activi ty of which was increased in response to the differentiation of the cells. Four synthesizing enzymes, cytosolic phospholipase A(2) (cPLA(2)), 5-lipoxy genase (5-LO), LTA(4) hydrolase and LTC4 synthase, and an enzyme associated protein, 5-lipoxygenase activating protein (FLAP) are involved in the gene ration of LTC4 and LTB4. We examined the expression of messenger RNA (mRNA) for these LT synthesizing enzymes and an associated protein in DMSO differ entiated HL-60 cells by reverse transcriptase polymerase chain reaction (RT -PCR). The production of LTC4 and LTB4, measured by radioimmunoassay (RIA), was increased after the incubation with DMSO for more than 3 days. Messeng er RNA abundance for 5-LO, LTC4 synthase and LTA(4) hydrolase was increased , that for FLAP was stable, but that for cPLA(2) was decreased. These resul ts indicate that DMSO induced increase of LT synthesis is associated with t he increase of mRNA expression of 5-LO, LTC4 synthase and LTA(4) hydrolase, although the precise regulatory mechanisms of the increased mRNA expressio n are not determined. We also investigated an action of dexamethasone (DEX) on DMSO-induced enhancement of LT synthesis. DEX suppressed DMSO induced i ncrease of LTC4 synthesis, but rather enhanced DMSO induced LTB4 production . The DEX attenuated the DMSO-induced increase of mRNA expression for LTC4 synthase, but showed no effect on that for LTA(4) hydrolase. The inhibition of LTC4 synthesis is associated with the suppression of mRNA expression fo r LTC4 synthase. However, increased LTB4 synthesis by DEX is regulated by t he mechanisms which are independent from mRNA level of LTA(4) hydrolase.