Physiological functions of a novel family of nucleotide-metabolizing enzymes

维甲酸(RA)诱导HL-60细胞胞外型NADase活性源于CD38，其氨基酸序列与Aplysia ADP-核糖环化酶同源。CD38不仅催化NAD2+的水解，还催化环状ADP核糖(CADPR)的形成和水解，cADPR是细胞内钙离子释放的一种新的介体或调节剂。在本研究中，我们研究了CD38及其相关胞外酶PC-1.1的功能和性质。用抗CD38单抗刺激RA分化的HL-60细胞，诱导细胞蛋白的快速酪氨酸磷酸化，包括c-CBL原癌基因产物PL20^&lt；C-Cbl&gt；抗CD38单抗可显著增强甲酰-甲硫氨酸-亮氨酸-苯丙氨酸诱导的超氧化物生成。FcGamma-II受体可能参与抗CD38单抗诱导的酪氨酸磷酸化所介导的信号转导通路。2.除淋巴细胞外，CD38也广泛存在于大鼠脑内。在原代培养的大鼠神经胶质细胞和神经细胞中，CD38在星形胶质细胞表面表达最为丰富。激光共聚焦显微镜分析表明，CD38免疫反应阳性物质和酶活性定位于星形胶质细胞的细胞表面。在培养的星形胶质细胞中加入酶底物后，细胞表面CD38迅速失活。一个RA反应元件(REARE)位于CD38基因的第一内含子上，该元件由两个直接重复的类似TGACCT的六聚体基序和一个5核苷酸间隔区组成。这种罕见的与RA受体和维甲酸X受体组成的异源二聚体相互作用。因此，RA诱导的人CD38基因的表达被证明是通过位于第一内含子的稀有基因来介导的。

Ecto-form NADase activity induced by retinoic acid(RA)in HL-60 cells is due to CD38, which has an amino acid sequence homologous to Aplysia ADP-ribosyl cyclase. CD38 catalyzes not only the hydrolysis of NAD^+, but also the formation and hydrolysis of cyclic ADP-ribose (cADPR), that is a novel mediator or modulator of Ca^<2+> release from intracellular Ca^<2+> stores. In the present study, we investigated the functions and properties of CD38 and its related ecto-enzyme, PC-1.1. Stimulation of RA-differentiated HL-60 cells with anti-CD38 monoclonal antibodies induced rapid tyrosine phosphorylation of cellular proteins including the c-cbl proto-oncogene product, pl20^<C-Cbl>. Superoxide formation in response to formyl-Met-Leu-Phe was markedly enhanced by the anti-CD38 mAbs. Fcgamma-II receptors appeared to be involved in the signal transduction pathway mediated through the anti-CD38 mAb-induced tyrosine phosphorylation. 2. CD38 was abundantly present in rat brain in addition to lymphocytes. In primary culture of rat glial and neuronal cells, CD38 was most abundantly observed in astrocyte cell surface. Confocal laser microscopic analysis revealed that immunoreactive CD38 and the enzyme activity were localized on the cell surface of astrocytes. The cell-surface CD38 was rapidly inactivated upon the addition of the enzyme substrates to the cultured astrocytes.3. An RA response element (RARE) consisting of two direct-repeated TGACCT-like hexamer motifs with a 5-nucleotide spacer was found to be located in the first intron of CD38 gene. This RARE interacted with heterodimer composed of RA receptor and retinoid X receptor. Thus, the RA-induced expression of human CD38 gene was demonstrated to be mediated through the RARE located in the first intron.