Total protein of 1 × 106 ARPE-19 incubated 24 hours with the different stimuli in serum-free and serum-containing media was isolated according to a standard protocol. Briefly, the cells were washed twice with PBS, detached mechanically with ice-cold PBS and a cell scraper and were pelleted by centrifugation. The cell pellet was resuspended in pre-cooled RIPA buffer (2% nonidet-P40, 50 mM Tris-HCl, 150 mM sodium chloride, 1 mM EDTA, 0.5% sodium desoxycholate, 0.1% sodium dodecylsulfate, 1 mM sodium orthovanadate, 1 mM phenyl-methane-sulfonyl fluoride, 10 μg/mL leupeptin and 10 μg/mL pepstatin adjusted to pH 7.4) and sonicated on ice. After centrifugation at 14,000g, the soluble protein was aliquoted and frozen at −20°C. The protein concentration was analyzed by protein assay kit (DC Protein Assay kit; Bio-Rad Laboratories, Munich, Germany) before usage in downstream applications.