Endoplasmic reticulum stress as a novel target to inhibit transdifferentiation of human retinal pigment epithelial cells

Background: The epithelial to mesenchymal transition (EMT) of retinal pigment epithelial (RPE) cells is a critical event in the pathogenesis of proliferative vitreoretinopathy and neovascular age-related macular degeneration, which are the leading causes of severe vision loss. Endoplasmic reticulum (ER) stress has been implicated in the EMT of many cell types and various ocular diseases. However, the relationship between ER stress and EMT in RPE cells remains unknown. Therefore, in the study, we explored the impact of ER stress on EMT in RPE cells. Methods: Different concentrations of tunicamycin (TM) and thapsigargin (TG) were used to induce ER stress in human RPE cells. The expression of epithelial marker, mesenchymal markers and some of genes/proteins involved in TGF- $\beta{}$ /Smad signaling were analized by qPCR, western blot or immunostaining at the condition with or without stimulation of TGF- $\beta{}$ 2 (10 ng/mL). Boyden chamber and scratch assay were used to evaluate the migration of RPE cells, while cell viability and apoptosis of RPE cells were measured by MTT and TUNEL assay, respectively. Results: Treatment of RPE cells with TM and TG (24 h) reduced the expression of $\alpha{}$ -SMA and FN, and increased the expression of Occludin in a dose dependent manner at protein level, which was highly associated with the expression of GRP78. Treatment with TGF- $\beta{}$ 2 significantly increased the expression of $\alpha{}$ -SMA and FN, and decreased the expression of Occludin both in protein and mRNA levels, which was significantly inhibited by a 4h pre-treatment with TM. In addition, the expression of TGF- $\beta{}$ RII and Smad2/3, and mRNAs of TGF- $\beta{}$ RII and Smad3 were also decreased by the TM treatment. TM-induced ER stress inhibited RPE cell migration, and high concentrations of TM and TG reduced cell viability and induced apoptosis of RPE cells. Conclusions: Chemical induction of ER stress inhibited EMT and migration in RPE cells, possibly by inactivation of TGF- $\beta{}$ signaling, suggesting that regulation of ER stress in RPE cells may be a new approach to prevent the development of intraocular fibrosis.

Keywords

Endoplasmic reticulum stress

Epithelial-mesenchymal transition

Cell movements

TGF-β signaling

Retinal pigment epithelial cells

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Fig. 1.

ER stress inhibits epithelial to mesenchymal transition in a dose-dependent manner in human RPE cells. The human RPE cells were treated with different concentrations of TM (0‒300 ng/mL) (A) or TG (0‒400 nM) (B)for 24 h, and the expression of $\alpha{}$ -smooth muscle actin (SMA), fibronectin (FN), occludin, and Grp78 was analyzed by western blot. GAPDH was used as the internal control. Data are presented as mean $\pm$ SEM. n = 4/group. *p $<$ 0.05, **p $<$ 0.01 vs. Ctrl. Densitometry results of western blotting are shown on the right of each blot; GAPDH was used as a protein-loading control.

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Front. Biosci. (Landmark Ed) Print ISSN 2768-6701 Electronic ISSN 2768-6698