Bronchopulmonary dysplasia (BPD), the most common chronic pulmonary disease in preterm infants, is primarily characterized by alveolar developmental arrest with shortened secondary septation and fewer, larger alveoli [1,2]. Systemic inflammatory response syndrome in preterm infants initiated by pre-and postnatal factors such as chorioamnionitis, oxygen toxicity, barotrauma/volutrauma, and postnatal infections play a central role in the pathogenesis of BPD. Accumulating evidence suggests that inflammation via pattern recognition receptors, such as Toll-like receptor 4 (TLR4), may disrupt critical pulmonary developmental processes such as angiogenesis, extracellular matrix (ECM) deposition, and alveologenesis [3,4]. Thus, a better understanding of how inflammation disrupts alveologenesis may provide insight into BPD pathogenesis.

During the phase of alveologenesis, saccules are subdivided into alveoli by the secondary septa. A critical event required for secondary septa elongation is the deposition of elastin-based ECM [5]. We have previously used intra-amniotic lipopolysaccharide (LPS)-induced alveolar developmental arrest to mimic the lung morphology of BPD. Studies have shown that LPS may lead to abnormal abundance and localization of elastin in the lungs of newborn rats, mostly in the primary septa and a few in the secondary septa [6,7]. However, the mechanism through which LPS perturbs elastin deposition remains unclear. Lysyl oxidase (LOX) contributes to ECM functions by promoting intra- and intermolecular covalent cross-linking of insoluble ECM substrates such as collagen and elastin [[8], [9], [10], [11]]. A previous study showed that LOX expression and activity are upregulated in injured lungs of newborn mice or human infants with BPD [8]. We also found that LOX activity was increased in the lungs of an LPS-induced rat model of BPD [7].

Activated epidermal growth factor receptor (EGFR) regulates LOX expression in vitro and in vivo [12]. We previously found that LPS induced EGFR activation in cultured myofibroblasts and in the lungs of rat pups with BPD [13,14]. EGFR activation occurs in response to its ligands including transforming growth factor-α (TGF-α) [[13], [14], [15]]. TGF-α is synthesized as a precursor membrane-bound protein pro-TGF-α, which is cleaved from the cell surface to allow its binding to EGFR [16,17]. Tumor necrosis factor α-converting enzyme (TACE) has been implicated in the cleavage of pro-TGF-α into mature soluble TGF-α in various epithelial tissues [[16], [17], [18], [19]]. LPS is a potent proinflammatory stimulus that induces cytokine expression through recognition of toll-like receptor 4 (TLR4) [20]. Zhang et al. [21] reported that EGFR plays a vital role in the LPS/TLR4 signaling pathway.

Alveolar myofibroblasts, which are thought to migrate to the septal tips during alveolarization, are mainly responsible for the production and deposition of elastin [[22], [23], [24]]. Our previous in vitro experiments demonstrated that LPS could disturb the directional migration of myofibroblasts via TGF-a/EGFR signaling [13]. Cell division cycle 42 (Cdc42) induces reorganization of actin and microtubule cytoskeletons, which causes the nucleus to move to the back of the cell and the microtubule organizing center, together with the Golgi, to orient towards the leading edge [25,26]. Golgi reorientation towards the leading edge is a critical process in directional cell migration [27]. Therefore, it is conceivable that LPS inhibits Cdc42 via TGF-a/EGFR signaling.

In the present study, we report that LPS induces EGFR activation through TLR4/TACE/TGF-α signaling, leading to LOX overactivation and Cdc42 inhibition, and disrupts elastin deposition and localization, eventually resulting in alveolar developmental arrest. In addition, EGFR inhibition partially restored alveolar developmental arrest induced by LPS, indicating that this signaling pathway may serve as a novel therapeutic target for BPD.