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 line U937. After CTx treatment, the initial inhibition of the oxidative burst induced by N-formylmethionyl-leucyl-phenylalanine (FMLP) was followed by a progressive increase over 20 h, resulting in 4-6-fold potentiation of the initial burst. Various cyclic-AMP-elevating agents produced 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 in arachidonate release and intracellular Ca2+ triggered by FMLP were amplified after CTx treatment, ADP-ribosylation of G(i) alpha subunits catalysed by pertussis toxin was slightly increased after CTx treatment, despite similar immunoreactivity of the alpha subunit of G(i)2. FMLP binding sites present in CTx-treated membranes were 3-6 times more abundant than in control membranes. Expression of mRNAs encoding the FMLP receptor and one of its related receptors were enhanced after CTx treatment of both differentiated and undifferentiated U937 cells. In parallel, after undifferentiated cells were treated with CTx, they were able to increase intracellular Ca2+, but not the oxidative burst, in response to FMLP. These data demonstrate that CTx, by increasing cyclic AMP, enhances the expression of chemotactic receptors independently of U937 cell differentiation.