Endoplasmic reticulum (ER) stress is emerging as a factor for the pathogenesis of granular corneal dystrophy type 2 (GCD2). This study was designed to investigate the molecular mechanisms underlying the protective effects of melatonin on ER stress in GCD2. Our results showed that GCD2 corneal fibroblasts were more susceptible to ER stress-induced death than were wild-type cells. Melatonin significantly inhibited GCD2 corneal cell death, caspase-3 activation, and poly (ADP-ribose) polymerase 1 cleavage caused by the ER stress inducer, tunicamycin. Under ER stress, melatonin significantly suppressed the induction of immunoglobulin heavy-chain-binding protein (BiP) and activation of inositol-requiring enzyme 1α (IRE1α), and their downstream target, alternative splicing of X-box binding protein 1(XBP1). Notably, the reduction in BiP and IRE1α by melatonin was suppressed by the ubiquitin-proteasome inhibitor, MG132, but not by the autophagy inhibitor, bafilomycin A1, indicating involvement of the ER-associated protein degradation (ERAD) system. Melatonin treatment reduced the levels of transforming growth factor-β-induced protein (TGFBIp) significantly, and this reduction was suppressed by MG132. We also found reduced mRNA expression of the ERAD system components HRD1 and SEL1L, and a reduced level of SEL1L protein in GCD2 cells. Interestingly, melatonin treatments enhanced SEL1L levels and suppressed the inhibition of SEL1L N-glycosylation caused by tunicamycin. In conclusion, this study provides new insights into the mechanisms by which melatonin confers its protective actions during ER stress. The results also indicate that melatonin might have potential as a therapeutic agent for ER stress-related diseases including GCD2.
Bibliographical noteFunding Information:
This study was supported by a faculty research grant of Yonsei University College of Medicine for 2007 (6-2007-0102), by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2016R1D1A1B03934794) and by a grant of the Korea Health Technology R&D Project through the Korea Health Industry Development Institute (KHIDI), funded by the Ministry of Health & Welfare, Republic of Korea (Grant number: HI16C1009).
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