In recent years, evidence has accumulated that NAD+ serves as a precursor of metabolites that are involved in a number of regulatory processes. In this work we show that extracellularly added NAD+ was rapidly degraded by intact human monocytes to nicotinamide and ADP-ribose. Besides these main products, minor amounts of AMP, ADP and cADP-ribose were formed. Expression of CD38, which has been identified as NAD+-glycohydrolase (EC 3.2.2.6) degrading NAD+ into nicotinamide and ADP-ribose, was determined on freshly isolated human monocytes by flow cytometry and RT-PCR. Upon ligation with anti-CD38 mAb, CD38 underwent internalization, shedding and new expression. As monocytes possess an intracellular CD38 pool, it could serve as a source for newly expressed CD38. Differentiation of monocytes to macrophages resulted in down-regulation of surface expression of CD38. This decrease correlates with a reduction in NADase activity, indicating that the amount of functional active CD38 molecules decrease during differentiation. As CD38 mRNA was found to be diminished in macrophages, regulation of the gene product seems to occur at the level of transcription or mRNA stability.

In recent years, evidence has accumulated that NAD 1 serves as a precursor of metabolites that are involved in a number of regulatory processes. In this work we show that extracellularly added NAD 1 was rapidly degraded by intact human monocytes to nicotinamide and ADP-ribose. Besides these main products, minor amounts of AMP, ADP and cADP-ribose were formed. Expression of CD38, which has been identified as NAD 1 -glycohydrolase (EC 3.2.2.6) degrading NAD 1 into nicotinamide and ADP-ribose, was determined on freshly isolated human monocytes by flow cytometry and RT-PCR. Upon ligation with anti-CD38 mAb, CD38 underwent internalization, shedding and new expression. As monocytes possess an intracellular CD38 pool, it could serve as a source for newly expressed CD38. Differentiation of monocytes to macrophages resulted in down-regulation of surface expression of CD38. This decrease correlates with a reduction in NADase activity, indicating that the amount of functional active CD38 molecules decrease during differentiation. As CD38 mRNA was found to be diminished in macrophages, regulation of the gene product seems to occur at the level of transcription or mRNA stability.Keywords: NAD; CD38; cADP-ribose; human monocytes; macrophages.CD38 is a 45-kDa type II integral transmembrane glycoprotein that catalyzes the hydrolysis of NAD 1 and to a minor extent the formation and hydrolysis of cyclic ADP-ribose (cADP-ribose) [1 -4]. Although cADP-ribose is known as a potent second messenger for Ca 21 release from ryanodine-sensitive intracellular stores in many cell types [5-9], it is not clear which enzyme(s) are involved in its synthesis inside the cell. Formation of cADP-ribose by CD38 occurs at the cell surface, but not at the site of cADPribose action. CD38 not only displays enzyme activity, but also serves as an adhesion and signal transduction molecule [10,11]. It has been shown that ligation of CD38 by monoclonal antibodies triggers a number of responses including proliferation, lymphopoiesis, apoptosis, adhesion, cytokine production and tyrosine phosphorylation of proteins [12 -18]. Moreover, CD38 was found to have a rather peculiar distribution pattern in hematopoietic cells. It is strongly expressed on lymphocyte precursors, declines once the cells differentiate and is up-regulated again on lymphocytes and mature plasma cells [11,19,20].While the vast body of data on CD38 is derived from studies carried out with lymphocytes, very little is known about the function and regulation of CD38 present on monocytes/macrophages.Recently CD38 ligation on monocytes was found to induce calcium mobilization, phosphorylation of several intracellular proteins and CD38 was identified as a coaccessory molecule in a superantigen-induced proliferation [21]. Furthermore up-regulation of CD38 was observed upon treatment with .To elucidate the function of CD38 in human monocyte metabolism and differentiation we investigated whether CD38 has a role in degradation of extracellular NAD 1 controlling the level of NAD 1 and/or using the nucle...