Basic research and clinical application of SAP-1, a protein tyrosine phosphatase expressed in gastrointestinal cancers.

由于酪氨酸磷酸化水平是由蛋白酪氨酸激酶和蛋白酪氨酸磷酸酶（PTPases）的作用平衡决定的，因此除了PTKs的不调节激活外，PTPases的失活也可能参与了胃肠道细胞的恶性转化。SAP-1是最近被分子克隆的人跨膜型PTPase。该酶在胞质区含有一个PTPase结构域，在胞外区含有8个iii型纤维连接蛋白结构域，具有多个潜在的n -糖基化位点。SAP-1在人类结直肠癌细胞系和胰腺癌细胞系中含量丰富，而在相应的正常组织中含量不高。在本研究中，我们探讨了SAP-1的生理作用和临床应用。(1) SAP-1过表达抑制了培养细胞的生长。SAP-1与EGF受体结合。(2)克隆了SAP-1启动子区域的基因组DNA，并观察到多个转录因子结合元件。(3)利用免疫组织化学技术，我们现在已经检测了手术或内镜切除的结直肠癌、腺瘤和正常结肠粘膜中SAP-1的表达。正常结肠组织或轻度发育不良的腺瘤未检测到SAP-1的表达。相比之下，48例腺癌中有19例（40%）表达SAP-1。K-RAS基因测序显示，15例（67%）sap -1阳性癌症中有10例包含密码子12突变。这些结果表明，SAP-1在人类结直肠癌中经常过表达，并且这种过表达可能在腺瘤-癌序列中相对较晚发生。我们将进一步研究可溶性形式的SAP-1在cob-直肠癌或胰腺癌患者血清中的存在。

Since the level of tyrosine phosphorylation is determined by the balance between the actions of both protein tyrosine kinases and protein tyrosine phosphatases (PTPases), not only the unregulated activation of PTKs but also the inactivation of PTPases may be involved in the malignant transformation of gastrointestinal cells. SAP-1 is a human transmembrane-type PTPase that has recently been molecularly cloned. This enzyme contains a single PTPase domain in the cytoplasmic region and eight fibronectin type III-like domains with multiple potential N-glycosylation sites in the extracellular region. SAP-1 is abundant in human colorectal cancer cell lines and pancreatic cancer cell lines but not in the corresponding normal tissues. In this study, we investigated the physiological roles of SAP-1 and clinical applications of SAP-1. (1) Overexpression of SAP-1 inhibited growth of cultured cells. SAP-1 binds to EGF receptors. (2) Genomic DNA of SAP-1 promoter region was cloned and several transcription factor binding elements were observed. (3) With the use of immunohistochemistry, we have now examined the expression of SAP-1 in surgically or endoscopically excised colorectal cancers, adenoma and normal colon mucosa. Normal colon tissue or adenomas with mild dysplasia showed no detectable expression of SAP-1. In contrast, 19 of 48 (40%) adenocarcinomas expressed SAP-1. Sequencing of the K-RAS gene revealed that 10 of 15 (67%) SAP-1-positive cancers contained a mutation in codon 12. These results suggest that SAP-1 is frequently overexpressed in human colorectal cancers and that such overexpression may occur relatively late in the adenoma-carcinoma sequence. We will further investigate the existance of soluble form of SAP-1 in serm of patients with cob-rectal or pancreatic cancers.