Chronic obstructive pulmonary disease (COPD) is mainly caused by chronic inflammation secondary to long-term inhalation exposure of harmful substances such as cigarette smoke (CS). COPD is characterized by progressive, partially reversible airway obstruction, consisting of airway lesions (peripheral airway lesions) and emphysematous lesion (Agusti and Vogelmerier, 2023), often accompanied by thickening and fibrosis of the respiratory tract, and increased number of macrophages (Hogg and Timens, 2009). The relative progression of emphysematous lesions and respiratory tract lesions in COPD varies among patients (Vestbo et al., 2013). World Health Organization predicts that COPD will be the third leading cause of death worldwide by 2030 (https://www.emro.who.int/health-topics/chronic-obstructive-pulmonary-disease-copd/index.html). Current maintenance treatment for COPD includes anticholinergic, β-2 stimulants, and inhaled corticosteroids that reduce the airway thickening and inflammation (Agusti and Vogelmerier, 2023); however, there is no effective therapy to prevent progression of pulmonary emphysema (Barnes, 2000).

Secretoglobin (SCGB) 3A2 is a cytokine-like immunomodulatory molecule belonging to the SCGB subfamily 3 A within the SCGB gene superfamily (Reynolds et al., 2002) that consists of secreted proteins of about 10 kD (Kimura et al., 2022, Mukherjee et al., 1999). SCGBs are known to form a homodimer or heterodimer with other members. Recent structural model analysis proposed that SCGB3A2 forms a dimer and a tetramer (Yokoyama et al., 2018). Currently, the most widely studied SCGB protein is SCGB1A1, also called uteroglobin, Club cell secretory protein, or Club cell 10-kDa protein, or Club cell 16-kDa protein (Mukherjee et al., 2007). Previous reports showed that SCGB1A1 suppresses inflammation and regulates the immune system in the lung (Mukherjee et al., 2007). However, the functions of a majority of SCGB proteins are unclear, and their receptors and signaling pathways remain unknown.

SCGB3A2 is predominantly expressed in airway epithelial cells (Niimi et al., 2001) where two other SCGB proteins, SCGB1A1 and SCGB3A1 are also expressed. A recent report examining the spatiotemporal expression of SCGB1A1, SCGB3A1, and SCGB3A2 during mouse lung development, suggested that each SCGB may possess a unique biological function and SCGB3A2 may be involved in the development of ciliated cells (Naizhen et al., 2019). Previous studies using various mouse models demonstrated that SCGB3A2 ameliorates inflammation and pulmonary fibrosis in the lung (Cai et al., 2014, Cai et al., 2015, Chiba et al., 2006, Kido et al., 2014, Kurotani et al., 2011, Yoneda et al., 2016). It also exhibits growth factor activity, promoting embryonic lung development (Cai et al., 2014, Kimura et al., 2022, Kurotani et al., 2008). Studies on anti-fibrotic activity of SCGB3A2 have revealed an intracellular signaling of SCGB3A2, in which SCGB3A2 inhibits TGFβ signaling through increased phosphorylation of STAT1, increased expression of inhibitory SMAD7, and decreased phosphorylation of SMAD2/3 (Kurotani et al., 2011).

A few publications have described a receptor for SCGB3A2. One of first identified was the macrophage scavenger receptor with collagenous structure (MARCO) expressed on the surface of alveolar macrophages using a human lung cancer derived cell line (Bin et al., 2003). The presence of a SCGB3A2-specific receptor different from MARCO was suggested using primary lung fibroblast cells cultured with SCGB3A2. Positive binding of SCGB3A2 was found on the surface of fibroblast cells (Kurotani et al., 2008). Recently, syndecan-1 (SDC1), a member of the transmembrane heparan sulfate proteoglycans, was reported as a receptor for SCGB3A2 (Yokoyama et al., 2018). SDC1 is mainly expressed in epithelial cells but can also be expressed in mesenchymal cells and fibroblasts (Kotsovilis et al., 2010, Stepp et al., 2015).

STAT factors are the mediators of signaling of most cytokines (including interleukins, interferons, and growth factors) (Schindler and Darnell, 1995). There are seven members of the STAT family: STAT1, STAT2, STAT3, STAT4, STAT5a, STAT5b and STAT6 (Schindler and Darnell, 1995). All STAT family members share six domains (N-terminal domain, coiled coil domain, DNA binding domain, linker domain, Src-homology (SH) 2 domain, and transcriptional activation domain) and have a similar structure. When STAT is phosphorylated by a receptor tyrosine kinase such as Janus kinase (JAK) upon binding of cytokines and/or growth factors to their specific receptor, the mutual interaction between the phosphorylation site and SH2 domain facilitates to form a dimer (Reich, 2013). Importin-α and importin-β then bind to the nuclear localization signal of the STAT dimer. After migration into the nucleus, the STAT dimer binds to a specific DNA binding domain of responsive genes (Reich, 2013).

Alpha-1-antitrypsin (A1AT) is a serine proteinase inhibitor which rapidly deactivates the neutrophil elastase activity, preventing the development of pulmonary emphysema (Beatty et al., 1980). A1AT deficiency (AATD) is known to cause emphysema (Janoff, 1985). SERPINA1 that encodes A1AT was the first gene proven that the gene mutation results in A1AT deficiency, leading to the development of COPD (Silverman, 2020). Transcription of SERPINA1 gene was activated in human hepatocytes via the JAK2/STAT3 pathway (Mansuy-Aubert et al., 2013). A1AT replacement therapy (Prolastin®; Food and Drug Administration) has been used to treat AATD, with demonstrated clinical improvement in pulmonary emphysema (Parr et al., 2009).

Based on the facts that SCGB3A2 activates STAT1 signaling, and the SERPINA1 gene transcription is activated by STAT3 signaling as described above, we hypothesized that SCGB3A2 may regulate transcription of the SERPINA1 gene via STAT1 and/or STAT3 signaling, leading to the suppression of pulmonary emphysema. Therefore, to prove this hypothesis, this study focused on understanding a possible correlation among SCGB3A2, A1AT, and STAT signaling, and determining the role of SCGB3A2 in COPD, especially in emphysema using a mouse model.