Increased ectodomain shedding of cell adhesion molecule 1 as a cause of type II alveolar epithelial cell apoptosis in patients with idiopathic interstitial pneumonia

Abstract

Background: Lung alveolar epithelial cell (AEC) apoptosis has attracted attention as an early pathogenic event in the development of idiopathic interstitial pneumonia (IIP); however, the causative mechanism remains unclear. Cell adhesion molecule 1 (CADM1) is an AEC adhesion molecule in the immunoglobulin superfamily. It generates a membrane-associated C-terminal fragment, aαCTF, through protease-mediated ectodomain shedding, termed aα-shedding. Increased CADM1 aα-shedding contributes to AEC apoptosis in emphysematous lungs. Methods: Formalin-fixed, paraffin-embedded lung lobes (n=39) from 36 autopsied patients with IIP were classified as acute IIP (n=10), fibrosing-type nonspecific IIP (f-NSIP, n=10), cryptogenic organizing IIP (n=9), and usual IIP (n=10). CADM1 expression in the lung sections was examined by western blotting and compared with control lungs (n=10). The rate of CADM1 aα-shedding was calculated as the relative amount of aαCTF to full-length CADM1, and the full-length CADM1 level was estimated per epithelial cell by normalization to cytokeratin 7, a lung epithelial marker. Apoptotic AECs were detected by immunohistochemistry for single-stranded DNA (ssDNA). NCI-H441 and A549 human lung epithelial cells were transfected with small interfering RNA (siRNA) to silence CADM1 expression and analyzed by terminal nucleotide nick end labeling assays. Results: The rate of CADM1 aα-shedding was higher in all IIP subtypes than in the control (P≤0.019), and the full-length CADM1 level was lower in f-NSIP (P=0.007). The aα-shedding rate and full-length CADM1 level were correlated with each other (P=0.015) and with the proportion of ssDNA-positive AECs (P≤0.024). NCI-H441 cells transfected with siRNA exhibited a 61% lower rate of expression of full-length CADM1 and a 17-fold increased proportion of apoptotic cells. Similar results were obtained with A549 cells. Conclusions: CADM1 aα-shedding appeared to be increased in all four IIP subtypes and consequently contributed to AEC apoptosis by decreasing the full-length CADM1 level. This mechanism particularly impacted f-NSIP. The molecular mechanism causing AEC apoptosis may be similar between IIP and emphysema.