INDUCTION OF CYCLOOXYGENASE AND SUPPRESSION OF 12-LIPOXYGENASE IN HUMAN ERYTHROLEUKEMIA-CELLS UPON PHORBOL ESTER-INDUCED DIFFERENTIATION

When human erythroleukemia (HEL) cells were incubated with arachidonic acid, both fatty acid cyclooxygenase and arachidonate 12-lipoxygenase activities were exhibited. Subcellular localization of these enzymes were examined by differential centrifugation, and both the cyclooxygenase and the 12-lipoxygenase were present predominantly in the microsomes rather than the cytosol of the cells. The cyclooxygenase activity was stimulated by 12-O-tetradecanoylphorbol 13-acetate (TPA) in a dose-dependent and time-dependent manner. Actual increase in the cyclooxygenase protein by TPA was demonstrated by immunoprecipitation and Western blot analysis of the enzyme with the aid of an anti-cyclooxygenase antibody. Furthermore, the cyclooxygenase mRNA level increased as assessed by RNA blot analysis using a cDNA probe for the enzyme. In contrast, the TPA treatment reduced the activity of 12-lipoxygenase. By RNA blot analysis using a cDNA probe of HEL cell 12-lipoxygenase, the mRNA level of the enzyme was shown to decline by the TPA treatment. Taken together, the TPA treatment of HEL cells brought about the induction of cyclooxygenase and the suppression of 12-lipoxygenase.