Reactivation of developmental pathways holds promise as regenerative therapy for parenchymal lung diseases. Retinoic acid (RA) is a small molecule with pleiotropic effects, which is essential for correct alveologenesis during lung development. All-trans RA (ATRA) and RA receptor (RAR) specific agonists can induce alveolar regeneration in animal models of alveolar insufficiency. However, encouraging preclinical data from animal models have translated poorly in clinical trials.
The human lung is difficult to study because of its location and indispensable function for life. To overcome this limitation, we have developed a novel ex vivo human model using precision-cut lung slice (PCLS) culture. Lung parenchyma can be maintained and manipulated, generating a tractable model, which greatly facilitate the study of lung injury and repair in man.
Using in vitro human cell culture and PCLS we investigated the role of RA in human repair and regeneration. Lipofibroblasts were identified in human lung sections and PCLS as a potential RA storage cell.
We have shown that ATRA induced microvascular angiogenesis via RAR, but has little direct effect on alveolar epithelial cells or fibroblasts. Here we demonstrate that ATRA induces lung microvascular cell migration and angiogenesis via VEGF-Receptor 2, with induction of AKT signalling and suppression of ERK1/2.
Preliminary data suggest that in PCLS, ATRA increased total cell proliferation, as well as potentially increasing protein markers of AT2 (Surfactant protein C), AT1 (HOPX) and endothelial cells (PECAM-1, VEGF-R2). ATRA also increased angiogenesis factors in primary microvascular endothelial cells and PCLS, including MCP-1 and HB-EGF. Additionally, we show that HB-EGF treatment increased alveolar epithelial cell migration (A549), suggesting that ATRA may act in a paracrine manner to induce alveolar epithelial regeneration.