CD38 is a type II transmembrane glycoprotein found on both hematopoietic and non-hematopoietic cells. It is known for its involvement in the metabolism of cyclic ADP-ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate, two nucleotides with calcium mobilizing activity independent of inositol trisphosphate. It is generally believed that CD38 is an integral protein with ectoenzymatic activities found mainly on the plasma membrane. Here we show that enzymatically active CD38 is present intracellularly on the nuclear envelope of rat hepatocytes. CD38 isolated from rat liver nuclei possessed both ADP-ribosyl cyclase and NADase activity. Immunofluorescence studies on rat liver cryosections and isolated nuclei localized CD38 to the nuclear envelope of hepatocytes. Subcellular localization via immunoelectron microscopy showed that CD38 is located on the inner nuclear envelope. The isolated nuclei sequestered calcium in an ATP-dependent manner. cADPR elicited a rapid calcium release from the loaded nuclei, which was independent of inositol trisphosphate and was inhibited by 8-amino-cADPR, a specific antagonist of cADPR, and ryanodine. However, nicotinic acid adenine dinucleotide phosphate failed to elicit any calcium release from the nuclear calcium stores. The nuclear localization of CD38 shown in this study suggests a novel role of CD38 in intracellular calcium signaling for non-hematopoietic cells.CD38 is a 42-45-kDa type II transmembrane glycoprotein (1) found in various mammalian tissues and cell types and is capable of converting NAD ϩ into cyclic ADP-ribose (cADPR) 1 (2). This ability is due to the ADP-ribosyl cyclase activity found on the extracellular carboxyl domain of the enzyme. The product, cADPR, possesses calcium mobilizing activity independent of inositol 1,4,5-trisphosphate (IP 3 ) (3). Instead, cADPR seems to regulate calcium mobilization through the calcium-inducedcalcium release mechanism (4, 5). In addition to cyclizing NAD ϩ to cADPR, CD38 is also able to use NADP ϩ as a substrate and catalyze the exchange of its nicotinamide group with nicotinic acid to produce NAADP (5), another potent calciummobilizing metabolite.CD38 is generally believed to be an important surface immunoregulatory molecule; its myriad of possible functions include the induction of B and T cell proliferation (6), regulation of the humoral immune response (7), apoptosis (8), tyrosine phosphorylation of various proteins (9), activation of certain kinases (10), and cytokine release (11). CD38 also displays adhesion properties and might possibly mediate a selectin-type adhesion between different blood populations and human vascular endothelial cells via its putative ligand, CD31 (12). In addition to its immuno-functions, CD38 has been shown to play an important role in insulin secretion via the cADPR-dependent calcium signaling pathway (13). Indeed, results suggest the appearance of autoantibodies to CD38 in patients may contribute to the development of noninsulin-dependent diabetes (14).The calcium stores mobilized by c...
SUMMARYPurification and characterization of CD38tADP-ribosyl cyctase in the rat lung tissue was performed with microsomes solubilized in Triton X-100 and the ADPribosyl cyclase was then purified using sequential column chromatography. Partially purified rat lung ADP-ribosyl cyclase was analyzed by immunoblotting using an antibody raised against a recombinant rat CD38 and showed the presence of monomer (42 kDa) and dimer (85 kDa) under non-reducing conditions but under reducing conditions, only the monomer was detected. Both the monomer and dimer could be eluted out in a stable manner from SDS-PAGE and the enzymatic activity was retained by the two different forms of CD38/ADPribosyl cyclase. Immunohistochemical staining showed the presence of CD38 on the bronchial epithelium and the alveoli.
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