Abstract
In addition to its well-characterized role in the lens, αB-crystallin performs other functions. Methylglyoxal (MGO) can alter the function of the basement membrane of retinal pigment epithelial (RPE) cells. Thus, if MGO is not efficiently detoxified, it can induce adverse reactions in RPE cells. In this study, we examined the mechanisms underlying the anti-apoptotic activity of αB-crystallin in the human retinal pigment epithelial cell line ARPE-19 following MGO treatment using various assays, including nuclear staining, flow cytometry, DNA electrophoresis, pulse field gel electrophoresis, western blot analysis, confocal microscopy and co-immunoprecipitation assays. To directly assess the role of phosphorylation of αB-crystallin, we used site-directed mutagenesis to convert relevant serine residues to alanine residues. Using these techniques, we demonstrated that MGO induces apoptosis in ARPE-19 cells. Silencing αB-crystallin sensitized ARPE-19 cells to MGO-induced apoptosis, indicating that αB-crystallin protects ARPE-19 cells from MGO-induced apoptosis. Furthermore, we found that αB-crystallin interacts with the caspase subtypes, caspase-2L, -2S, -3, -4, -7, -8, -9 and -12 in untreated control ARPE-19 cells and that MGO treatment caused the dissociation of these caspase subtypes from αB-crystallin; transfection of S19A, S45A or S59A mutants caused the depletion of αB-crystallin from the nuclei of untreated control RPE cells leading to the release of caspase subtypes. Additionally, transfection of these mutants enhanced MGO-induced apoptosis in ARPE-19 cells, indicating that phosphorylation of nuclear αB-crystallin on serine residues 19, 45 and 59 plays a pivotal role in preventing apoptosis in ARPE-19 cells. Taken together, these results suggest that αB-crystallin prevents caspase activation by physically interacting with caspase subtypes in the cytoplasm and nucleus, thereby protecting RPE cells from MGO-induced apoptosis.
Highlights
Crystallins found in non-lens tissues were predicted to be entirely different from those in the lens [1,2,3]
Methylglyoxal Induces Apoptosis in ARPE-19 Cells We found that MGO efficiently reduced the viability of ARPE-19 cells at 1.5–3.5 mMconcentrations ina dose-dependent manner (Fig. S1A)
Because the viability of ARPE-19 cells treated with 2 mM MGO for 48 h was approximately 50% of control values, this concentration was used in all further studies
Summary
Crystallins found in non-lens tissues were predicted to be entirely different from those in the lens [1,2,3]. Previous studies on in vivo and in vitro expressed aB-crystallin suggest that it may function as an anti-apoptotic protein in human RPE cells [4,5]. MGO contributes to the formation of advanced glycation end products (AGEs), reacts rapidly with RNA and denatured DNA, has both mutagenic and clastogenic activities [8]. The accumulation of AGEs in RPE basement membrane is an acknowledged contributor to AMD [9]. MGO can induce numerous adverse reactions if it is not efficiently detoxified [10,11], and it is known to induce apoptosis in various cell types [12,13,14,15]
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