IN-VITRO DIFFERENTIATION OF HUMAN MONOCYTES TO MACROPHAGES - CHANGE OF PDE PROFILE AND ITS RELATIONSHIP TO SUPPRESSION OF TUMOR-NECROSIS-FACTOR-ALPHA RELEASE BY PDE INHIBITORS

1 During ill vitro culture in 10% human AB serum, human peripheral blo od monocytes acquire a macrophage-like phenotype. The underlying diffe rentiation was characterized by increased activities of the macrophage marker enzymes unspecific esterase (NaF-insensitive form) and acid ph osphatase, as well as by a down-regulation in surface CD14 expression. 2 In parallel, a dramatic change in the phosphodiesterase (PDE) profi le became evident within a few days that strongly resembled that previ ously described for human alveolar macrophages. Whereas PDE1 and PDE3 activities were augmented, PDE4 activity, which represented the major cyclic AMP-hydrolysing activity of peripheral blood monocytes, rapidly declined. 3 Monocytes and monocyte-derived macrophages responded to l ipopolysaccharide (LPS) with the release of tumour necrosis factor-alp ha (TNF). In line with the change in CD14 expression, the EC50 value o f LPS for induction of TNF release increased from approximately 0.1 ng ml(-1) in peripheral blood monocytes to about 2 ng ml(-1) in macropha ges. 4 Both populations of cells were equally susceptible towards inhi bition of TNF release by cyclic AMP elevating agents such as dibutyryl cyclic AMP, prostaglandin E-2 (PGE(2)) or forskolin, which all led to a complete abrogation of TNF production in a concentration-dependent manner and which were more efficient than the glucocorticoid dexametha sone. 5 In monocytes, PDE4 selective inhibitors (rolipram, RP73401) su ppressed TNF formation by 80%, whereas motapizone, a PDE3 selective co mpound, exerted a comparatively weak effect (10-15% inhibition). Combi ned use of PDE3 plus PDE4 inhibitors resulted in an additive effect an d fully abrogated LPS-induced TNF release as did the mixed PDE3/4 inhi bitor tolafentrine. 6 In monocyte-derived macrophages, neither PDE3- n or PDE4-selective drugs markedly affected TNF generation when used alo ne (<15% inhibition), whereas in combination, they led to a maximal in hibition of TNF formation by about 40-50%. However, in the presence of PGE(2) (10 nM), motapizone and rolipram or RP73401 were equally effec tive and blocked TNF release by 40%. Tolafentrine or motapizone in the presence of either PDE4 inhibitor, completely abrogated TNF formation in the presence of PGE(2). Thus, an additional cyclic AMP trigger is necessary for PDE inhibitors to become effective in macrophages. 7 Fin ally, the putative regulatory role for PDE1 in the regulation of TNF p roduction in macrophages was investigated. Zaprinast, at a concentrati on showing 80% inhibition of PDE1 activity (100 mu mol l(-1)), did not influence TNF release. At higher concentrations (1 mmol l(-1)), zapri nast became effective, but this inhibition of TNF release can be attri buted to a significant inhibitory action of this drug on PDE3 and PDE4 isoenzymes. 8 In summary, the in vitro differentiation of human perip heral blood monocytes to macrophages is characterized by a profound ch ange in the PDE isoenzyme pattern. The change in the PDE4 to PDE3 rati o is functionally reflected by an altered susceptibility towards selec tive PDE inhibitors. under appropriate stimulating conditions.