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.