REGULATION OF DISTINCT CYCLIC AMP-SPECIFIC PHOSPHODIESTERASE (PHOSPHODIESTERASE TYPE-4) ISOZYMES IN HUMAN MONOCYTIC CELLS

Many functions of the immune and inflammatory responses are inhibited by agents that increase intracellular levels of cAMP. Recent investiga tions have revealed that cAMP levels in inflammatory cells are regulat ed by cyclic nucleotide phosphodiesterases (PDEs) belonging to the PDE 4 family (cAMP-specific PDEs). At least four different genes are known to encode PDE4 isozymes, which are characterized by their selectivity for cAMP over cGMP and their sensitivity to the antidepressant drug r olipram. The aim of our studies was to investigate whether monocytic c ells could regulate PDE4 activity and whether certain PDE4 isozymes we re expressed preferentially over others. Our results showed that treat ment of peripheral blood monocytes or closely related Mono Mac 6 cells with dibutyryl-cAMP or other cAMP-elevating agents transiently increa sed rolipram-sensitive PDE4 activity 2-3-fold, without concomitant inc reases in cGMP-inhibited PDE (PDE3) activity. PDE4 activity was predom inantly cytosolic, whereas PDE3 activity was localized to the particul ate fraction. Our Northern and Western blot studies with reagents reco gnizing three distinct PDE4 gene products (PDE4A, PDE4B, and PDE4D) re vealed that their expression is transcriptionally regulated in monocyt ic cells. Although none of the three isozymes was detectable under nor mal culture conditions, all of these were up-regulated when Mono Mac 6 cells were exposed to dibutyryl-cAMP. Distinct differences were obser ved in their temporal patterns of expression. Endotoxin lipopolysaccha ride, a potent monocyte stimulus, also enhanced PDE4 activity in monoc ytic cells. These data indicate that monocytic cells may express diffe rent PDE4 isozymes, depending on their state of activation or differen tiation. These isozymes could thus regulate intracellular cAMP levels at various stages of monocyte activation and could thereby be importan t in limiting the inflammatory response.