Background Alveolar epithelial-mesenchymal interactions play a critical role in perinatal pulmonary transition, which is characterized by surfactant maturation and alveolar wall thinning. This alveolar wall thinning has been attributed to apoptosis of interstitial lung fibroblasts (LFs), normally in abundance perinatally. The mechanism underlying this is unclear. Although 1α,25(OH)2D3 (1,25D) has been suggested as a local paracrine/autocrine effector for fetal lung maturation, its effect on alveolar intersitial LFs survival/apoptosis is unknown. We have previously shown that 1,25D is metabolized to its C-3 epimer (3-epi 1,25D) in rat alveolar type II cells (ATII). Relatively high circulating concentrations of 3-epi vitamin D metabolites have recently been reported in neonates.
Objective To investigate the role of 3-epi 1,25D in lung development during the perinatal period.
Methods Embryonic (e) day 19 and day 21 fetal rat lung ATII and LFs were isolated and cultured using standard methods. At near-confluence, cells were treated with 1,25D or 3-epi 1,25D (1 × 10−11 to 1 × 10−8M) for up to 48 hours. Cell proliferation (thymidine incorporation assay) and apoptosis (TUNEL assay and Western blotting for Bcl and Bax expression in the mitochondrial and cytoplasmic fractions) were studied. Markers of alveolar epithelial-mesenchymal interactions (parathyroid hormone-related protein receptor, peroxisome proliferator-activated receptorγ, adipocyte differentiation-related protein, leptin, leptin receptor, and surfactant protein B) were assessed by Western blotting.
Results Both 1,25D and its 3 epimer exhibited dose-dependent increases in expression of the key homeostatic epithelial-mesenchymal markers, an increase in interstitial fibroblast proliferation, and decreased in interstitial fibroblast apoptosis, with 3-epi 1,25D having a more robust antiapoptotic effect compared with its parent compound. Furthermore, both 1,25D and its 3 epimer inhibited the actinomycin D-induced apoptosis of LFs-again more robustly by 3-epi 1,25D.
Conclusions Vitamin D plays a significant role in fetal lung maturation by stimulating the key alveolar epithelial-mesenchymal interactions and LFs' survival. We speculate that perinatal alterations in vitamin D metabolism, which result in significant decreases in C-3 epimer levels, may be a key factor in alveolar interstitial LFs' apoptosis and alveolar wall thinning.
Supported by grants from TRDRP ((14RT-0073 and 15IT-0250), Philip Morris USA Inc. and Philip Morris International, and NIH (HL55268 and HL075405).
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