Mechanisms in fetal lung development, type II cell differentiation and function

     Developmental induction of type II cell differentiation and surfactant lipoprotein synthesis by the fetal lung are essential for newborn survival. In addition to their vital role to reduce alveolar surface tension after birth, surfactant components protect the alveolar epithelium from the inflammatory consequences of exposure to high O2 tension and inhaled pathogens. We observed that the major surfactant protein, SP-A (encoded by the SFTPA gene), an immune modulator, is developmentally upregulated in fetal lung with type II cell differentiation and surfactant glycerophospholipid synthesis. SP-A expression and type II cell differentiation in cultured human fetal lung (HFL) epithelial cells are stimulated by cAMP and inhib­ited by transforming growth factor TGF-β and hypoxia. In studies using trans­genic mice and transfected type II cells, we identified a conserved ~300-bp region upstream of the rabbit and human SFTPA genes that is critical for lung cell-specific, developmental and cAMP induced expres­sion. This region serves as an ‘enhanceosome’ through which developmental and cAMP induction of hSFTPA promoter activity are mediated by cooperative interactions of transcription factors bound to key response elements (Fig. 1). The TBE (red) binds thyroid transcription factor 1 (TTF-1/Nkx2.1) and nuclear factor (NF)-κB in a cAMP-respon­sive manner, resulting in enhanced recruitment of the histone acetylase, CREB-binding protein (CBP), and steroid receptor coactivator (SRC)-1, with increased acetylation of histone H3K9. cAMP also enhances estrogen-related receptor ∝(ERRα) binding and transcriptional activity by increasing its interaction with protein kinase-A and SRC-2. In fetal mice doubly-deficient in Src-1 and Src-2, SP-A expression was significantly decreased. Whereas, O2-dependent cAMP induction of SP-A in HFL type II cells is mediated, in part, by TTF-1 and NF-κB binding to the SFTPA promoter, mechanisms for O2 regulation of type II cell differentiation and SFTPA expression are not fully understood. We recently observed that the redox-sensitive tran­scription factor, nuclear factor erythroid 2-related factor 2 (NRF2) and its co-regulated transcription factors, C/EBPβ and PPARγ, were markedly induced by cAMP in HFL type II cells in an O2-dependent manner. Moreover, NRF2 binds to an ‘anti-oxidant response element’ (ARE) in the hSFTPA promoter in cultured HFL epi­the­lial cells (Fig. 1). In mouse fetal lung, a developmental increase in Nrf2, C/ebpβ and Pparγ, and a decrease in Nrf2 inhibitor Keap1 were observed between 14.5  and 19.5 (term) days post-coitum (dpc), with temporal induction of SP-A.

     As a component of the innate immune system, SP-A binds to and activates alveolar macrophages (MΦ) to facilitate pathogen clearance within the lung alveoli. SP-A also is an immune modulator that inhibits dendritic cell maturation and stimulates proliferation of immunosuppressive regulatory T cells (Treg, CD4+CD25+ FoxP3+). The immune functions of SP-A likely protect the postnatal lung from the inflammatory challenges imposed by exposure to reactive oxygen species (ROS) and microbial pathogens. In our ongoing research, we are testing the hypothesis that transcription factors, including NRF2, C/EBPβ and PPARγ serve crucial roles in the developing fetal lung to promote type II cell differentiation and enhance production of SP-A and other key immune modulators that act on immune cells to protect the alveolar epithelium from oxidative and inflammatory stress. We postulate that decreased production of immune modulators by lungs of premature infants increase hyperoxic and inflammatory stress and enhance susceptibility to bronchopulmonary dysplasia (BPD) and other chronic lung diseases.

     SP-A also can have proinflammatory actions. As described below, we have compelling evidence that secretion of surfactant components SP-A and the proinflammatory glycerophospholipid, platelet-activating factor (PAF), by the developing fetal lung near term, serve as fetal signals for the initiation of labor.