Background/Purpose: Interstitial lung disease is a prevalent and worrisome complication of systemic sclerosis (SSc), which is now the leading cause of death in SSc. There is growing recognition that endoplasmic reticulum (ER) stress plays a pathogenic role in SSc-associated interstitial lung disease (SSc-ILD) and in other interstitial lung diseases. However, the nature of the response during ER stress in alveolar epithelial cells (AEC) and lung fibroblasts, two key players in pulmonary fibrosis, remains unknown. Here we demonstrate that differential ER stress response in these two cell types depends upon the expression of the pro-apoptotic C/EBP homologous protein or “CHOP”, an important ER stress marker.

Methods: Lung tissues were collected postmortem from control subjects and from SSc patients who fulfilled the ACR preliminary criteria for SSc and had evidence of lung involvement. SSc-ILD was confirmed by histological examination of postmortem lung tissue. Additionally, lung tissues were obtained from mice with bleomycin-induced pulmonary fibrosis and from control animals. AEC and lung fibroblasts were isolated using standard procedures. Lung tissues were analyzed by immunohistochemistry. Protein expression in AEC and lung fibroblasts was determined by immunoblotting; apoptosis was measured by enzyme-linked immunosorbent assay; chop promoter activity was analyzed by luciferase assay.

Results: We demonstrate for the first time that CHOP expression is profoundly increased in AEC surrounded by fibrotic tissues in SSc-ILD patients, but not in normal lung tissues. We observed staining for CHOP exclusively in AEC surrounded by fibrotic tissues. CHOP expression was also noted to be localized to thickened alveolar septae in SSc-ILD patients and in bleomycin-treated mice, but not in normal lung tissues from human or murine controls. In contrast, myofibroblasts (positively stained for α-SMA) show no significant immunoreactivity for CHOP. Thrombin, known to be elevated in SSc-ILD patients and in the bleomycin murine model of ILD, had no observable effect on CHOP expression in lung fibroblasts. However, thrombin was noted to upregulate CHOP expression in primary AEC and in A549 cells via an Ets1-dependent pathway. Importantly, we demonstrate that lung myofibroblasts from SSc-ILD patients and from mice treated with bleomycin, which are resistant to apoptosis, lose the resistance to apoptosis when transfected with CHOP.

Conclusion: AEC and lung myofibroblasts in SSc-ILD exhibit a differential response to ER stress, which may confer the differential fate of these two cell types, i.e., the susceptibility to apoptosis of AEC and the resistance to apoptosis of lung myofibroblasts. The ER stress marker CHOP is involved in regulating apoptotic mechanisms in fibrotic lung tissue downstream of two key mediators, thrombin and TGF-β, making CHOP a possible novel target for the treatment of SSc-ILD.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/differential-response-to-endoplasmic-reticulum-stress-between-alveolar-epithelial-cells-and-lung-fibroblasts-in-systemic-sclerosis/