Analysis of New NAD-cleavage Enzymes Involved in Signal Transduction System

人细胞表面抗原CD 38，其氨基酸序列与Astrasia ADP-核糖基环化酶同源，是一种具有单跨膜结构域的46-kDa II型糖蛋白。我们先前证明了CD 38的胞外结构域具有NAD^+糖水解酶（NAD^+ glycohydrolase，NAD酶）活性，并且HL-60细胞中全反式维甲酸（all-transretinoic acid，RA）诱导的胞外形式NAD酶活性是由于CD 38。CD 38不仅催化NAD^+的水解，还催化环状ADP-核糖的形成和水解，后者是介导细胞内Ca ^2+库释放Ca^2+的新候选物。在本研究中，我们获得了以下发现。1.用抗CD 38单克隆抗体刺激RA分化的HL-60细胞，可诱导分子量为120，000、87，000和77，000的细胞蛋白快速酪氨酸磷酸化。其中一个显著的磷酸化蛋白被鉴定为c-cbl原癌基因产物p120^<c-cbl>[2]。抗CD 38单克隆抗体能显著增强分化HL-60细胞对甲酰-Met-Leu-Phe的超氧化物生成。3.这些激动性单克隆抗体识别的刺激蛋白酪氨酸磷酸化的表位都映射到相同的羧基端序列的CD 38，其胞外NAD酶活性也需要相同的序列。4. Fc γ-II受体似乎参与通过激动性抗CD 38 mAb诱导的细胞蛋白酪氨酸磷酸化介导的信号转导途径。5. CD 38 mRNA的表达是通过核内RA受体介导的，在CD 38基因的第一内含子中存在RA受体反应元件。

The human cell surface antigen CD38, which has an amino acid sequence homologous to Aplysia ADP-ribosyl cyclase, is a 46-kDa type II glycoprotein with a single-transmembrane domain. We previously demonstrated that the extracellular domain of CD38 exhibits NAD^+ glycohydrolase (NADase) activity and that the ecto-form NADase activity induced by all-trans retinoic acid (RA) in HL-60 cells is due to CD38. CD38 catalyzes not only the hydrolysis of NAD^+ but also the formation and hydrolysis of cyclic ADP-ribose, which is a novel candidate that mediates Ca^<2+> release from intracellular Ca^<2+> stores. In the present study, we obtained the following findings. 1. Stimulation of RA-differentiated HL-60 cells with anti-CD38 monoclonal antibodies (mAbs) induced rapid tyrosine phosphorylation of cellular proteins with the molecular weights of 120,000,87,000 and 77,000. One of the prominent phosphorylated proteins was identified as the c-cbl proto-oncogene product, p120^<c-cbl>.2. Superoxide formation in response to formyl-Met-Leu-Phe was markedly enhanced by the anti-CD38 mAbs in the differentiated HL-60 cells. 3. The epitopes recognized by these agonistic mAbs stimulating protein tyrosine phosphorylation were all mapped on the same carboxyl-terminal sequence of CD38, and the same sequence was also required for its ecto-NADase activity. 4. Fcgamma-II receptors appeared to be involved in the signal transduction pathway mediated through the agonistic anti-CD38 mAb-induced tyrosine phosphorylation of cellular proteins. 5. The expression of CD38 mRNA was mediated through nuclear RA receptors ; a RA receptor-responsive element was present in the first intron of CD38 gene.