Luteolin inhibits subretinal fibrosis and epithelial-mesenchymal transition in laser-induced mouse model via suppression of Smad2/3 and YAP signaling

Background: Subretinal fibrosis (SF) accounts for vision loss in patients with neovascular age-related macular degeneration (nAMD) even treated with adequate intravitreal injection of anti-vascular endothelial growth factor (anti-VEGF) drugs. Currently, there is no treatment available to prevent or treat SF caused by nAMD. Purpose: This study aims to investigate the potential effects of luteolin on SF and epithelial-mesenchymal transition (EMT) as well as the underlying molecular pathways both in vivo and in vitro. Methods: Seven-week-old male C57BL/6J mice were employed to establish laser-induced choroidal neovascularization (CNV) and SF. One day after the laser induction, luteolin was administered intravitreally. SF and CNV were assessed with the immunolabeling of collagen type I (collagen I) and isolectin B4 (IB4), respectively. RPE65 and alpha-SMA colocalization in the lesions was used to evaluate the extent of EMT in retinal pigment epithelial (RPE) cells by using immunofluorescence. In vitro, luteolin was administered to TGF beta 1-treated primary human RPE (phRPE) cells. RT-qPCR, Western blot and immunofluorescence were employed to evaluate the change of EMT-related molecules, epithelial markers, and relevant signaling pathways. The functional changes associated with EMT were investigated using the scratch assay, Transwell migration assay, and collagen gel contraction assay. CCK-8 was used to determine the cell viability of phRPE cells. Results: On day 7 and 14 after laser induction in mice, intravitreal injection of luteolin dramatically decreased the immunolabeled sizes of both collagen I and IB4, as well as the amount of colocalized double immunostaining of alpha-SMA and RPE65 in laser-induced SF lesions. In vitro, TGF beta 1-treated phRPE cells demonstrated increased cell migration and contraction capacity, accompanied with considerable overexpression of fibronectin, alpha-SMA, N-cadherin and vimentin, as well as downregulation of E-cadherin and ZO-1. The above changes were largely inhibited by luteolin co-incubation. Mechanistically, luteolin could evidently decrease the phosphorylation of Smad2/3, whereas increase the phosphorylation of YAP in TGF beta 1-treated phRPE cells. Conclusion: This study demonstrates that luteolin exhibits the anti-fibrotic effect in a laser-induced mouse model by inhibiting EMT of RPE cells via deactivating Smad2/3 and YAP signaling, which provides a potential natural compound for the prevention and treatment of SF and fibrosis-related diseases.