Inhibition of phorbol ester-stimulated chemiluminescence in humai polymorphonuclear leukocytes by retinoic acid and 5,6-edoxvretinoic acid

Chemiluminescence (CL) is an index of both the generation of and reactions mediated by O₂^-and ¹O₂. The tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA), is a potent stimulator of CL by human polymorphonuclear leukocytes; treatment with TPA (100 ng/ml) provokes a CL response that peaks within five min and persists for over 30 min. The response proportional to concentration over the range of one to 100ng ml. The ability of different phorbol diesters to stimulate both CL andO2 production correlates with their relative activities as tumor promoters in vivo. Non-phorbol diester tumor promoters such as iodoacetic acid, anthralin, and Tween 60 are inactive in this system. The TPA-mediated stimulation of CL can be inhibited by retinoids; cells preincubated for 15 min with 100 µM retinoic acid show only a marginal CL response to TPA. Addition of retinoic acid to resting polymorphonuclear leukocytes results in a transient burst of CL without concomitant 02t release, observations indicative of an excitable substrate. A similar CL response is seen when retinoic acid is incubated with potassium superoxide in a cell-free system. 5,6-Epoxyretinoic acid, an even more effective inhibitor of TPA-stimulated CL than retinoic acid when added simultaneously with TPA does not undergo these two CL reactions. Thus, it appears that retinoic acid may undergo oxidative activation to a species that can exert enhanced antipromoter activities. Polymorphonuclear leukocytes provide a useful system for exploring the roles of reactive oxygen species in the mechanism of action of both TPA and retinoic acid.