Smoking can cause lung cancer, which commonly coexists with chronic obstructive pulmonary disease (COPD) [
1,
2]. The people with COPD are much more of developing lung cancer than those without COPD, and are worse treatment expectations after diagnosis and treatment. Lung cancer and COPD are closely related and may share common characteristics, such as an underlying genetic predisposition, epithelial and endothelial cell plasticity, dysfunctional inflammatory mechanisms including the deposition of excessive extracellular matrix, angiogenesis, susceptibility to DNA damage and cellular mutagenesis [
2‐
5]. The epithelial mesenchymal transition (EMT) is a highly plastic process in which epithelial cells change into a mesenchymal phenotype, and it has been found to be associated with an invasive or metastatic phenotype during cancer progression [
2‐
4]. The active compounds found in cigarette smoke, such as nicotine and reactive oxygen species (ROS), can induce inflammation and EMT through various signaling pathways [
6‐
8]. Cigarette smoking induces a higher expression of vimentin and other mesenchymal markers and a decrease in E-cadherin expression, which are key indicators of EMT production [
2‐
5]. Several key matrix metalloproteinases (MMPs) and signaling pathways such as MMP-9 production that drive an EMT are also aberrantly activated in lung cancer [
2,
9,
10]. We previously showed that cigarette smoke extract (CSE) and cigarette smoke (CS) induced oxidative stress, inflammation, EMT and fibrosis in a lung cancer cell culture and in the lungs of mice through the activation of SH2 domain-containing phosphatase (Shp) 2 and Rac1 signaling pathways, respectively, which further activate MMP-9 production [
11‐
13]. Recently, studies further demonstrated that pulmonary epithelial cells induced by cigarette smoke release MMP-2 and MMP-9, which contribute to the progressions of EMT [
14]. Among all MMPs, MMP-9 is thought to play a key role in mediating EMT by tissue remodeling through the degradation of basement membrane collagens and extracellular matrix proteins in COPD and lung cancer patients [
2,
9,
10]. MMP-9 level is elevated in peripheral blood, bronchoalveolar lavage fluid (BALF) and exhaled breath condenses in COPD and lung cancer patients [
15,
16]. The activity of MMP-9 in non-small cell lung cancer (NSCLC) was positively correlated with advanced T category and distant metastasis. Moreover, the meta-analysis revealed that over-expression of MMP-9 in tissue was a risk factor of advanced T category, tumor stage and poor outcome [
17]. However, its role in mediating airway damage and remodeling is still controversial, given that constitutional knockout (KO) of MMP-9 in mice does not affect the pathological outcomes of CS-induced emphysema [
18]. MMP-9 is produced mainly by macrophages and neutrophils [
18], but also by epithelial cells, mast cells, and fibroblasts in the lung [
19,
20]. It has been reported that pulmonary macrophages in COPD patients express similar level of MMP-9 compared to normal controls [
18]. Further, stimulation of human airway epithelial cells by TGF-β1, a key proinflammatory factor contributing to the generation of EMT and COPD, resulted in MMP-9 elevation [
11], indicating pulmonary epithelial cells could be a significant source of MMP-9 production in COPD and lung cancer. Adding to the complexity, the activity, besides the absolute amount of MMP-9, is proposed to be essential for determining its function in COPD [
19]. Studies showed controversial results regarding the relationship between the activity of MMP-9 and lung function measured using the Tiffeneau-Pinelli index (FEV1/FEV ratio) [
21,
22], suggesting the urgent need to understand the mechanisms underlying MMP-9 expression, activation and its regulation of airway remodeling in COPD.
The non-receptor protein-tyrosine phosphatase (PTP) Shp2 is encoded by the proto-oncogene PTPN11 and is a ubiquitously expressed key regulator of cell signaling, which regulates physiological and pathological processes, including cell development, growth, inflammation, chemotaxis, oxidative stress, and the Ras/Raf/Erk, PI3K/Akt, and JAK/STAT pathways and immune checkpoint receptors [
23]. Our group previously demonstrated that CS stimulates pulmonary epithelial cells to release IL-8 through Shp2 activation [
11]. Selective inhibition or conditional knockout (KO) of Shp2 in lung epithelia reduced airway inflammation in a CS-induced mouse model [
11]. In the same experiments, we noticed that inhibition or conditional KO of Shp2 in lung epithelia ameliorated small airway epithelia lesion, which led us consider the role of epithelial Shp2 in regulating small airway fibrosis during COPD and its underlying mechanisms. Given that Shp2 promotes tumor EMT that is concurrent with the elevation of MMP-9 secretion [
24,
25], we hypothesized that CS-induces Shp2 activation in lung epithelial cells promotes MMP-9 production, and contributes to the progression of COPD-related EMT.
In this work, we employed a mouse model of CS-induced COPD, and a pulmonary epithelial cell culture model of cigarette smoke extract (CSE)-induced EMT to study the expression of MMP-9 during CS exposure, as well as the role of MMP-9 in regulating EMT. We found MMP-9 was upregulated following CS or CSE exposure in both in vivo and in vitro models at least partially through the activation of the Shp2/ERK1/2/JNK/Smad pathway. Selective inhibition of Shp2 or Shp2 KO in lung epithelial cells significantly suppressed the expression of MMP-9 activity, which prevented the CS exposure induced EMT progression. Our study contributes to understanding the underlying mechanisms of pulmonary epithelial structural remodeling, which may provide novel therapeutic solutions for treating its related diseases, such as COPD and lung cancer.