Role of ubiquitin ligase HRD1 in a cellular model of Parkinson's disease

[Speaker] Tomohiro Omura:1
[Co-author] Hiroki Matsuda:1, Luna Nomura:1, Masato Naito:1, Shunsaku Nakagawa:1, Satoshi Imai:1, Atsushi Yonezawa:1,2, Takayuki Nakagawa:1, Kazuo Matsubara:1
1:Department of Clinical Pharmacology and Therapeutics, Kyoto University Hospital, Japan, 2:Graduate School of Pharmaceutical Sciences, Kyoto University, Japan

[Objective] It has been reported that endoplasmic reticulum (ER) stress is possibly involved in the etiology of Parkinson's disease (PD). We have previously reported that ubiquitin ligase HRD1 involved in ER stress degrades unfolded proteins which are accumulated in the ER by the loss of Parkin protein and then cause familial PD, resulting in the suppression of cell death. This indicates that HRD1 may be involved as a compensatory mechanism for the loss of parkin in familial PD. However, the role of HRD1 in sporadic PD has not yet been identified. This study therefore aims to reveal the role of HRD1 and its associated molecules in a cellular model of sporadic PD.
[Methods] The expression levels of HRD1, SEL1L (HRD1-stabilizer) and ER stress-associated molecules were examined by exposing human dopaminergic neuroblastoma SH-SY5Y to 6-OHDA. Next, cell viability was assessed when wt-HRD1 was overexpressed in SH-SY5Y and then exposed these cells to 6-OHDA. The cell viability and the expression levels of various molecules were also examined when the expressions of HRD1, SEL1L, etc. were suppressed by RNAi.
[Results and Discussions] The expression of HRD1, SEL1L and ER stress-associated molecules increased with the exposure to 6-OHDA. 6-OHDA-induced cell death was suppressed in cells with overexpression of wt-HRD1, whereas it was enhanced in cells with suppression of HRD1 expression. This suggests that HRD1 may suppress 6-OHDA-induced cell death. On the other hand, suppression of SEL1L expression decreased the amount of HRD1 proteins, while 6-OHDA-induced cell death was enhanced more than when HRD1 expression was suppressed. Therefore, SEL1L is considered necessary for maintaining and stabilizing the amount of HRD1 proteins, and stabilizing the amount of HRD1 proteins through SEL1L may serve to protect against 6-OHDA-induced cell death. Furthermore, expression of parkin increased when HRD1 expression was suppressed in SH-SY5Y, but such change was not observed when suppressing the expression of SEL1L. This means that when the amount of mRNA of HRD1 decreases, parkin is induced as a compensatory mechanism, suggesting that it may suppress the enhancement of 6-OHDA-induced cell death caused by the loss of HRD1.

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