Secreted Phosphoprotein 1 in Lung Diseases

Secreted phosphoprotein 1 (SPP1), also known as osteopontin (OPN) or early T lymphocyte activation protein 1 (ETA-1), is a multifunctional protein involved in numerous biological processes, including immune modulation, stress response, and tissue remodeling. The role of SPP1 in interstitial lung diseases (ILDs) has become an area of increasing interest, given its elevated expression in various ILDs such as idiopathic pulmonary fibrosis (IPF), connective tissue disease-associated ILD (CTD-ILD), and pneumoconiosis, especially with recent data derived from single-cell RNA sequencing. In addition to ILDs, SPP1 has been implicated in infectious granulomatous lung diseases, lung and pleural malignancies, airway diseases, and COVID-19. In most cases, higher SPP1 levels in serum, bronchoalveolar lavage fluid, or lung tissue carry a poor prognosis. SPP1 is expressed in multiple cells critical for fibrogenesis, including macrophages, epithelial cells, and fibroblasts, and SPP1 has emerged as a potential target for therapeutic interventions. Here, we review the proposed mechanisms by which SPP1 contributes to the development of lung disease, with an emphasis on ILD.