In this study, GRP78, sXBP1, CHOP, and GADD34 were used to reflect ER stress response. GRP78 reflects an overall level of ER stress response. Spliced XBP1 mRNA acts as a transcriptional activator of UPR genes involved in protein folding and degradation; thus, sXBP1 serves as a protective effector to alleviate ER stress.
12,34 CHOP is regulated under the PERK-eIF2α-ATF4 pathway and serves as a mediator of apoptosis.
35 Our data revealed that TMSCs presented a lower level of activation of the UPR but experienced a similar death rate compared with TM cells. It has been reported that overexpression of CHOP decreases the expression levels of the antiapoptotic gene Bcl-2,
36–38 but in our results, a higher expression of
CHOP in TM cells did not cause greather cell death. Other researchers found that PERK
−/− and eIF2α knock-in cells failed to induce CHOP but were hypersensitive to ER stress-induced apoptosis.
39,40 Han et al.
41 reported that coexpression of ATF4 and CHOP, but not CHOP alone, decreased cell survival by increasing protein synthesis. All these suggest that CHOP may require cooperation with other effectors to induce cell death. On the other hand, a higher expression of
sXBP1 in TM cells demonstrated that these cells activated a more effective adaptive response to ER stress. Zode et al.
21 and Peters et al.
22 found that human glaucomatous TM tissues had chronic ER stress with significantly increased expression of GRP78 and CHOP. The starved glaucomatous TM cells also presented a higher level of ER stress response compared with normal TM cells. Contradictorily, Chai et al.
42 reported a downregulation of GRP78 in stressed glaucomatous TM cells when using TUN as ER stress inducer. Considering TUN is a much stronger ER stress inducer than starvation, it is highly possible that glaucomatous TM cells cannot revoke the adaptive response or revoke an unbalanced response to ER stress. In the experiment of Peters et al.,
22 glaucomatous TM cells lacked XBP1 splicing without eIF-2α increasing after dexamethasone treatment.
22 Our data show that normal TMSCs and TM cells upregulated
sXBP1 in response to TUN. Sal upregulated
sXBP1 and
CHOP expression and partially rescued cells from TUN treatment. All these support the idea that UPR is protective in response to ER stress, but the balance of UPR is critical for cell survival. Another interesting phenomenon is that Sal presented longer and stronger effects in TM cells than TMSCs, which needs further investigation.