PROLONGED ELEVATION OF INTRACELLULAR CYCLIC-AMP LEVELS IN U937 CELLS INCREASES THE NUMBER OF RECEPTORS FOR AND THE RESPONSES TO FORMYL-METHIONYL-LEUCYLPHENYLALANINE, INDEPENDENTLY OF THE DIFFERENTIATION PROCESS

The effects of elevated levels of cyclic AMP induced by cholera toxin (CTx) were investigated on the differentiated promyelomonocytic cell l ine U937. After CTx treatment, the initial inhibition of the oxidative burst induced by N-formylmethionyl-leucyl-phenylalanine (FMLP) was fo llowed by a progressive increase over 20 h, resulting in 4-6-fold pote ntiation of the initial burst. Various cyclic-AMP-elevating agents pro duced similar potentiation of the FMLP- or C5a-induced oxidative burst , but the phorbol 12-myristate 13-acetate-induced oxidative burst was not affected by CTx pretreatment of cells. Furthermore, the increase i n arachidonate release and intracellular Ca2+ triggered by FMLP were a mplified after CTx treatment, ADP-ribosylation of G(i) alpha subunits catalysed by pertussis toxin was slightly increased after CTx treatmen t, despite similar immunoreactivity of the alpha subunit of G(i)2. FML P binding sites present in CTx-treated membranes were 3-6 times more a bundant than in control membranes. Expression of mRNAs encoding the FM LP receptor and one of its related receptors were enhanced after CTx t reatment of both differentiated and undifferentiated U937 cells. In pa rallel, after undifferentiated cells were treated with CTx, they were able to increase intracellular Ca2+, but not the oxidative burst, in r esponse to FMLP. These data demonstrate that CTx, by increasing cyclic AMP, enhances the expression of chemotactic receptors independently o f U937 cell differentiation.