Abstract
BackgroundAutophagy and ER stress are involved in maintaining some well-orchestrated mechanisms aimed at either restoring cellular homeostasis or performing cell death. Autophagy is a well-defined process which governs overall cellular stress outcomes. Selective degradation of the ER mediated by autophagy occurs through a specific type of autophagy called ER-phagy, which ensures ER protein homeostasis.MethodsImmunoblotting and RT-PCR were used to evaluate the expression of ATG5 and ATG7 in chondrocyte. Western blotting, Flow cytometry,immunofluorescence cell staining and confocal microscope were used to examine the effect of ATG5 and ATG7 on autophagy, ER stress, cell apoptosis and cell proliferation. Transmission electron microscope and confocal microscope were performed to visualize the autophagy flux and autolysosome formation. The role of ATG5 and ATG7 overexpression on the PERK pathway inhibitor were detected by immunoblotting and treatment with inhibitors.ResultsIn current study, we demonstrated that Tm-induced ER stress can activate autophagy while Rapamycin-induced autophagy can inhibit ER stress in chondrocyte. Autophagy related protein ATG5 or ATG7 can promote autophagy and inhibit ER stress individually, and their combined effect can further improve the autophagy enhancement and the ER stress repression. Moreover, ATG5, ATG7 and ATG5 + ATG7 lead cells into more S phase, increase the number of S phase and inhibit apoptosis as well. ATG5, ATG7 and ATG5 + ATG7 regulate autophagy, ER stress, apoptosis and cell cycle through PERK signaling, a vital UPR branch pathway.ConclusionsATG5 and ATG7 connect autophagy with ER stress through PERK signaling. The protective effect of ATG5/7 overexpression on chondrocyte survival relys on PERK signaling. The effect of siPERK and siNrf2 on the cytoprotective effect of ATG5/7 are of synergism, while the effect of siPERK and siATF4 are of antagonism. PERK signal may be the pivot for autophagy, ER homeostasis and ER-phagy in chondrocyte.
Highlights
Autophagy and endoplasmic reticulum (ER) stress are involved in maintaining some well-orchestrated mechanisms aimed at either restoring cellular homeostasis or performing cell death
ER stress interplays with autophagy in human chondrocyte It is well known that when unfolded protein response (UPR) is triggered in ER stress, the activation of protein kinase RNA-like ER kinase (PERK) signaling pathway is initiated upon its dimerization and autophosphorylation [20,21,22]
The effect of siPERK and siNrf2 on the cytoprotective effect of ATG5/7 are of synergism, while the effect of siPERK and siATF4 are of antagonism
Summary
Autophagy and ER stress are involved in maintaining some well-orchestrated mechanisms aimed at either restoring cellular homeostasis or performing cell death. Autophagy is a well-defined process which governs overall cellular stress outcomes. Selective degradation of the ER mediated by autophagy occurs through a specific type of autophagy called ER-phagy, which ensures ER protein homeostasis. It’s well known that autophagy in mammalian systems occurs under basal conditions and can be stimulated by stresses like hypoxia, starvation, rapamycin etc. ER-phagy exists after selective degradation of the ER by autophagy,and play a key role in the physiology of secretory cells in vivo. Smith et al identify ER membrane protein CCPG1, as an ER-phagy receptor that interacts with autophagy-related components LC3, GABARAPs and FIP200, maintains ER homeostasis during both physiological and stress conditions [15,16,17]
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