Sp1 and Sp3 function as key regulators of leukotriene C(4) synthase gene expression in the monocyte-like cell line, THP-1

The goal of this study was to examine the mechanisms of leukotriene C(4) (LTC(4)) synthase gene expression in mononuclear phagocytes. Transfection of the monocyte-like cell line THP-1 with LTC(4) synthase promoter-reporter constructs demonstrated that the first 1.3 kb of the promoter mediated a 21.1-fold increase in reporter activity. Deletion analysis revealed that the region between -92 and -23 bp, which contains a signal protein (Sp)1 consensus site at -42 to -37 bp, mediated an 11.5-fold increase in reporter activity. Using a probe from -56 to -17 bp, electrophoretic mobility shift assays (EMSAs) demonstrated that Sp1 and THP-1 and HeLa nuclear extracts bind to this region. Binding was eliminated by mutation of the Sp1 consensus site. Supershift EMSAs using anti-Sp1 and anti-Sp3 antibodies demonstrated that these Sp family members bind to the region. Transfection of the Sp-deficient Drosophila SL-2 cell line with a construct containing the -92 to -23 bp promoter region and Sp expression vectors revealed that Sp1 and Sp3 transactivate gene transcription. We conclude that the Sp1 site is a necessary element for LTC(4) synthase gene transcription. Sp1 and Sp3 function through this site to positively regulate transcription. Thus, we provide evidence that the LTC(4) synthase gene is transcriptionally regulated in mononuclear phagocytes.

TGF-beta increases leukotriene C4 synthase expression in the monocyte-like cell line, THP-1

The goal of this study was to determine whether cytokines modulate leukotriene C4 (LTC4) synthase expression in mononuclear phagocytes. A panel of cytokines was surveyed for changes in LTC4 synthase mRNA in THP-1 cells. TGF-beta1, -2, and -3 had significant stimulatory effects. The addition of TGF-beta resulted in a time-dependent increase in LTC4 synthase mRNA at 6 h, which persisted through 48 h. Furthermore, this conditioning resulted in an increase in immunoreactive protein for LTC4 synthase through 7 days. TGF-beta conditioning of cells resulted in a time- and dose-dependent increase in stimulated LTC4 synthase activity. Following transient transfection of THP-1 cells with a promoter-reporter construct containing 1.2 kb of the LTC4 synthase promoter, TGF-beta treatment resulted in a 2-fold increase in reporter activity. Conditioning with TGF-beta did not prolong the half-life of LTC4 synthase mRNA, as assessed by RNase protection assays in actinomycin D-treated cells. Cycloheximide exposure experiments revealed that new protein synthesis was not required for the observed stimulatory effect of TGF-beta on LTC4 synthase mRNA. We conclude that LTC4 synthase expression is increased at a transcriptional level by TGF-beta in mononuclear phagocytes.

Leukotriene B4 costimulates 5-lipoxygenase activity in neutrophils via increased 5-lipoxygenase translocation

The goal of this investigation was to assess the effect of leukotriene B4 (LTB4) on 5-lipoxygenase activity and to examine the possible mechanisms of this effect. Exogenous LTB4 significantly increased the release of endogenous LTB4 from A-23187-stimulated neutrophils. The 5-lipoxygenase product release from A-23187-stimulated neutrophils decreased in the presence of an LTB4 receptor antagonist, suggesting that LTB4 has a receptor-mediated, autocrine effect on 5-lipoxygenase activity. Neutrophil 5-lipoxygenase activity increased significantly as cell density increased. In the presence of exogenous LTB4, no significant change in [14C]arachidonic acid release from neutrophils was observed. Exogenous LTB4 increased the amount of immunoreactive 5-lipoxygenase protein detected in the nuclear fraction of disrupted cells. LTB4 receptor antagonism decreased the amount of immunoreactive 5-lipoxygenase detected in the nuclear fraction. Thus LTB4 exerts an autocrine, receptor-mediated, costimulatory effect on 5-lipoxygenase activity. This feedback appears to have biological significance and involves enhanced 5-lipoxygenase translocation to the nuclear membrane.