SHS-induced alterations in cell turnover, including instances of apoptosis, are complicated by compromised extracellular matrix. Cells necessary for the normal physiology of the lung such as endothelium, respiratory epithelium, and conducting airway epithelium require spatial integrity stabilized by collagen and other matrix molecules. A significant determinant of the architectural matrix between cells is due to type collagen IV synthesis and secretion [
29],[
30]. In fact, the importance of type IV collagen as a stabilizing molecule is confirmed in research centering on COPD [
31] and other adult inflammatory diseases [
32]. Our qualitative and quantitative data revealing diminished type IV collagen identifies an important concept relating to SHS exposure. First of all, extracellular matrixes are targeted by smoke exposure and secondly, RAGE expressed by epithelial cells plausibly functions to signal and regulate cells that secrete matrix in the developing lung.
RAGE-ligand interactions initiate cellular communication via the activation of signaling intermediates prior to the activation of NF-κB [
24]. The current research sought to determine to what extent MMP-9, a matrix metalloprotease (MMP) secreted by fibroblasts, alveolar macrophages, and epithelial cells, functions in the SHS exposed pups. MMPs are endopeptidases that can destroy components of the extracellular matrix and MMP-9 is an NF-κB target [
33] that specifically targets type IV collagen [
34]. Because of their destructive capabilities, MMPs are recognized as not only central players in cases of disease, but during remodeling events observed during development as well [
35]. For example, MMPs are also considered potent effectors of normal lung morphogenesis that assist in the orchestration of definitive lung parenchyma [
36],[
37]. Notably, MMP-9 has been directly implicated in the progression of bronchopulmonary dysplasia (BPD), a developmental anomaly characterized by inflammation, lack of alveolar septation, and abnormal pulmonary vascular development [
38]. Known for perpetuating inflammatory axes, TNF-α and IL-1β induce the release of numerous inflammatory cytokines, enhance leukocyte adhesion during chemotactic transmigration, and coordinate MMP-9 elaboration [
39]-[
41]. Interestingly, mouse models of inflammatory diseases have demonstrated a link between the availability of TNF-α and IL-1β and the direct effects of MMP-9 on cell survival, inflammation status, and matrix durability [
42]. Our work builds upon these discoveries by identifying MMP-9 as a SHS target that likely effectuates end points via RAGE-mediated pathways.
In summary, RAGE expression and matrix destabilization are probable byproducts of pulmonary SHS exposure during embryogenesis. This study suggests that protection from damaging SHS-induced effects such as fetal weight decreases, matrix abundance, and MMP imbalances is possible when RAGE is inhibited. Despite notable advancements in SHS research provided by the current research, additional work is still necessary that focuses on RAGE signaling during pulmonary branching morphogenesis and to what extent RAGE alone is capable of inducing SHS related lung phenotypes.