RANKL promotes alveolar epithelial expansion

Chronic obstructive pulmonary disease (COPD) is the third leading cause of death globally, with progressive emphysema driven by repeated epithelial damage and impaired repair. Recently, we found that secretion of the osteokine “receptor agonist of nuclear factor κB signaling ligand” (RANKL) is higher in lung fibroblasts from patients with COPD compared to control and that RANKL represses lung epithelial cell death. However, the underlying mechanism, its cross-species conservation and the involved epithelial cell types remain unclear. To investigate how RANKL affects lung epithelial cells, we used primary lung organoids from human and mouse epithelial cells and showed that RANKL supplementation increased alveolar organoid forming capacity in these cultures compared to vehicle-treated controls. In elastase-treated mice, RANKL was able to rescue the elastase-induced loss of epithelial cells compared to vehicle-treated controls and it augmented the proportion of epithelial cells in transitional states expressing Krt8 and MHCII. Using A549 epithelial cells to investigate whether RANK or the alternative receptor for RANKL, “leucine-rich repeat-containing G-protein coupled receptor 4” (LGR4) were responding to RANKL treatment, we found that RANKL most likely acts through RANK These results suggest that RANKL may enhance stem cell survival of primarily alveolar epithelial cells in both mice and humans. We therefore conclude that RANKL is another osteokine, in addition to periostin, osteopontin, osteoglycin, and osteoprotegerin, that has a role in lung tissue repair and that its signaling pathway could be explored for therapeutic applications.

慢性阻塞性肺疾病（Chronic obstructive pulmonary disease, COPD）是全球第三大死亡原因，其病理进程为反复上皮损伤与修复受损所驱动的进行性肺气肿。近期研究发现，与对照组相比，慢阻肺患者肺成纤维细胞分泌的骨源性细胞因子（osteokine）“核因子κB信号配体受体激动剂（receptor agonist of nuclear factor κB signaling ligand, RANKL）”水平显著升高，且RANKL可抑制肺上皮细胞死亡。但其具体作用机制、跨物种保守性及所涉及的上皮细胞亚型仍不明确。为探究RANKL对肺上皮细胞的调控作用，我们采用人类与小鼠上皮细胞来源的原代肺类器官开展实验，结果显示，与溶剂处理对照组相比，添加RANKL可提升此类培养体系中的肺泡类器官形成能力。在经弹性蛋白酶处理的小鼠模型中，与溶剂对照组相比，RANKL可挽救弹性蛋白酶诱导的上皮细胞丢失，并上调表达角蛋白8（Krt8）与主要组织相容性复合体II类（MHCII）的过渡状态上皮细胞的比例。我们利用A549上皮细胞探究RANKL是否通过其受体RANK或替代受体——富含亮氨酸重复序列的G蛋白偶联受体4（leucine-rich repeat-containing G-protein coupled receptor 4, LGR4）——发挥作用，结果发现RANKL极有可能通过RANK介导信号通路。上述结果表明，RANKL可增强小鼠与人类原代肺泡上皮细胞的干细胞存活能力。综上，我们得出结论：除骨膜蛋白、骨桥蛋白、骨糖蛋白与骨保护素外，RANKL是另一种参与肺组织修复的骨源性细胞因子，其信号通路有望被开发为治疗应用的潜在靶点。