Increased Ectodomain Shedding of Cell Adhesion Molecule 1 from Pancreatic Islets in Type 2 Diabetic Pancreata: Correlation with Hemoglobin A1c Levels

Pulmonary emphysema and type 2 diabetes mellitus (T2DM), both caused by lifestyle factors, frequently concur. Respectively, the diseases affect lung alveolar and pancreatic islet cells, which express cell adhesion molecule 1 (CADM1), an immunoglobulin superfamily member. Protease-mediated ectodomain shedding of full-length CADM1 produces C-terminal fragments (CTFs) with proapoptotic activity. In emphysematous lungs, the CADM1 shedding rate and thus the level of CTFs in alveolar cells increase. In this study, CADM1 expression in islet cells was examined by western blotting. Protein was extracted from formalin-fixed, paraffin-embedded sections of pancreata isolated from patients with T2DM (n = 12) or from patients without pancreatic disease (n = 8) at autopsy. After adjusting for the number of islet cells present in the adjacent section, we found that full-length CADM1 decreased in T2DM islets, while ectodomain shedding increased. Hemoglobin A1c levels, measured when patients were alive, correlated inversely with full-length CADM1 levels (P = 0.041) and positively with ectodomain shedding rates (P = 0.001). In immunofluorescence images of T2DM islet cells, CADM1 was detected in the cytoplasm, but not on the cell membrane. Consistently, when MIN6-m9 mouse beta cells were treated with phorbol ester and trypsin to induce shedding, CADM1 immunostaining was diffuse in the cytoplasm. When a form of CTFs was exogenously expressed in MIN6-m9 cells, it localized diffusely in the cytoplasm and increased the number of apoptotic cells. These results suggest that increased CADM1 ectodomain shedding contributes to blood glucose dysregulation in T2DM by decreasing full-length CADM1 and producing CTFs that accumulate in the cytoplasm and promote apoptosis of beta cells. Thus, this study has identified a molecular alteration shared by pulmonary emphysema and T2DM.由生活方式因素诱发的肺气肿与2型糖尿病（type 2 diabetes mellitus, T2DM）常合并发生。二者分别累及肺泡细胞与胰岛细胞，而这两类细胞均表达免疫球蛋白超家族成员细胞黏附分子1（cell adhesion molecule 1, CADM1）。全长CADM1经蛋白酶介导的胞外域脱落可产生具有促凋亡活性的C端片段（C-terminal fragments, CTFs）。在肺气肿患者的肺组织中，CADM1的脱落速率及其在肺泡细胞中的CTFs水平均有所升高。本研究通过蛋白质印迹法检测了胰岛细胞中的CADM1表达情况。实验材料取自尸检获取的胰腺福尔马林固定石蜡包埋切片，受试对象包括12名T2DM患者与8名无胰腺疾病的对照个体。在对相邻切片中的胰岛细胞数量进行校正后，我们发现T2DM患者胰岛内的全长CADM1表达水平降低，而其胞外域脱落水平则升高。患者生前检测的糖化血红蛋白A1c（hemoglobin A1c）水平与全长CADM1表达水平呈负相关（P=0.041），与CADM1胞外域脱落速率呈正相关（P=0.001）。在T2DM患者胰岛细胞的免疫荧光成像中，CADM1仅在细胞质中被检测到，而未出现在细胞膜上。与之一致的是，当用佛波酯与胰蛋白酶处理MIN6-m9小鼠β细胞以诱导CADM1脱落时，CADM1的免疫染色在细胞质中呈弥散分布。若在MIN6-m9细胞中外源性表达CTFs的某一亚型，该蛋白会在细胞质中弥散分布，并增加凋亡细胞的数量。上述结果表明，CADM1胞外域脱落水平升高可通过降低全长CADM1的表达量，并产生在细胞质中积累、促进β细胞凋亡的CTFs，从而参与T2DM患者的血糖调节异常过程。因此，本研究明确了肺气肿与T2DM共有的一种分子改变机制。