Glycogen synthase kinase-3β promotes radiation-induced lung fibrosis by regulating β-catenin/lin28 signaling network to determine type II alveolar stem cell transdifferentiation state

The role of type II alveolar epithelial stem cells (AEC II) for alveolar repair in radiation-induced lung fibrosis (RILF) remains largely unknown, mainly because of AEC II phenotype's spontaneous change in vitro. Cell differentiation status is determined by Lin28 and let-7 miRNAs in see-saw-pattern. Lin28, a repressor of let-7 and a stem cell marker, is activated by β-catenin. The expression of β-catenin is regulated by GSK-3β/TGF-β1 signaling. To understand the true role of AEC II in RILF, we freshly isolated primary AEC II directly from thoracically irradiated lungs. We then explored the expressions of cell phenotype markers and differentiation regulators in these isolated AEC II to analyze the correlation between GSK-3β/TGF-β1/β-catenin signaling pathway, lin28/let-7 balance, and AEC II phenotypes at different injury phases following irradiation. Results showed that isolated single primary cells displayed AEC II ultrastructural features and proSP-C positive. The gene expressions of prosp-c (an AEC II biomarker) and hopx (an AEC I marker) significantly increased in isolated AEC II during injury repair phase (P < .001 and P < .05) but decreased at end-stage of injury, while mesenchymal markers increased in both isolated AEC II and irradiated lungs. mRNA levels of gsk-3β, tgf-β1, and β-catenin increased in all irradiated AEC II, but more pronounced in the second half of injury phase (P < .05-P < .001). Similarly, the expression of lin28 was also significantly elevated in isolated AEC II at the late phase (P < .05-P < .001). Four let-7 miRNAs were significantly upregulated in all irradiated AEC II groups (P < .05-P < .001). The time-dependent and highly consistent uptrends for four lin28/let-7 ratios in sorted AEC II contrasted to downtrends in irradiated lungs. In conclusion, RILF occurred when GSK-3β/TGF-β1 signaling increased β-catenin levels, which led to the augmentation of AEC II population by elevated lin28/let-7 ratio and the transcription of profibrotic cytokines and factors, thereby inducing AEC II to undergo transdifferentiation into mesenchymal cells.