CYCLOSPORINE-A DOWN-REGULATES THE LTA(4) HYDROLASE LEVEL IN HUMAN KERATINOCYTE CULTURES

Leukotriene A(4) hydrolase is a key enzyme in the biosynthesis of leuk otriene B-4, a potent pro-inflammatory compound, The purpose of this s tudy was to determine the capacity of antiinflammatory and anti-prolif erative compounds to regulate the levels and activity of leukotriene A (4) hydrolase in cultured human keratinocytes. The content of leukotri ene A(4) hydrolase was determined by Western blot analysis, and the ac tivity of leukotriene A(4) hydrolase was expressed as the leukotriene B-4 formation after incubation of keratinocyte cultures with leukotrie ne A(4). Leukotriene B-4 was measured by reversed-phase high performan ce liquid chromatography, Preincubation for 10 min of the cultured ker atinocytes with the leukotriene A(4) hydrolase inhibitor RP 64699 (0.1 -10 mu M) caused a significant dose-dependent inhibition of leukotrien e B-4 formation (IC50=0.7 mu M). Cyclosporin A (0.1 mu g/ml and 1.0 mu g/ml) had no direct effect on leukotriene A(4) hydrolase activity, bu t after incubation for 72 h there was a decrease in the mean leukotrie ne B-4 formation per culture dish (35% and 48%, respectively), The dec reased leukotriene B-4 formation was caused mainly by a decrease in th e mean leukotriene A(4) hydrolase content per mg protein (30.1% at 0.1 mu g/ml cyclosporin A(4)and 40.0% at 1.0 mu g/ml cyclosporin A), alth ough keratinocyte proliferation was also slightly decreased, Incubatio ns with 1,25-dihydroxyvitamin D-3 (10(-7)-10(-10) M), all-trans retino ic acid (10(-6)-10(-10) M), eicosatrienoic acid (10(-6)-10(-8) M), dex amethasone (10(-5)-10(-7) M), interferon-gamma (10 and 100 units/ml) o r methotrexate (0.1-10 mu g/ml) had no effect on either the leukotrien e B-4 formation or the amount of leukotriene A(4) hydrolase in keratin ocyte cultures, These results show that cyclosporin A, in contrast to other anti-inflammatory and anti-proliferative compounds, inhibits the level of leukotriene A(4) hydrolase in keratinocyte cultures. Since s imilar cyclosporin A concentrations are obtained during treatment of p soriasis with cyclosporin A, the effect on leukotriene A(4) hydrolase may play a role in the anti-inflammatory action of cyclosporin A.