Increased expression of tight junctions in ARPE-19 cells under endoplasmic reticulum stress

To investigate the effects of endoplasmic reticulum (ER) stress on the tight junctions of the retinal pigment epithelial (RPE) cells in vitro.

ER stress was induced in cultured ARPE-19 cells, a human RPE cell line, by exposure to tunicamycin (TM) or to thapsigargin (TG). After 6, 12, 24 and 48 hours of exposure, the expressions of GRP78/Bip (Bip), C/EBP-homologous protein (CHOP), vascular endothelial growth factor (VEGF), zonula occludens (ZO)-1, occludin and claudin-1 were determined by real-time RT-PCR. Immunoblot analysis and/or immunohistochemistry for proteins of tight junctions and ER stress markers, viz., Bip, activating transcription factor (ATF) 6, CHOP, and caspase-4, were performed at 48 hours after the exposure. Enzyme-linked immunosorbent assay was used to determine the concentration of VEGF165. Transepithelial electrical resistance (TER) of the ARPE-19 cells was determined to measure the permeability.

The expressions of the mRNAs and/or proteins of Bip, CHOP, ATF6 and caspase-4 were significantly increased in ARPE-19 cells under ER stress induced by TM and TG. The mRNAs of VEGF were also increased by both TM and TG. However, the concentration of VEGF165 was not significantly increased after 48 hours exposure to TM and TG compared to that of the control in the apical chamber medium. The proteins and mRNAs of occludin and claudin-1 were significantly increased by TM and TG, and that of ZO-1 was significantly increased by TG. Immunohistochemistry showed that the staining of ZO-1, occludin and claudin-1 under ER stress was stronger than that of the control. A significant increase of TER was observed after exposure to TM and TG.

The increased expressions of tight junction molecules by TM- or TG-exposed ARPE-19 cells indicate that ER stress can alter the function of RPE cells and may be involved in the pathogenesis of age-related macular degeneration.