Iwasaki Noboru, Sugiyama Yoshiki, Miyazaki Shuichi, Nakagawa Hiroshi, Nishimura Kazuhiko, Matsuo Saburo
Laboratory of Toxicology, Course of Veterinary Bioscience, Graduate School of Life and Environmental Sciences, Osaka Prefecture University, 1-58 Rinku-Orai-Kita, Izumisano, 598-8531, Japan.
The Center for Advanced Research, Graduate School of Medical Science, Toho University of Medicine, Ota-ku Omori-Nishi, 5-21-16, Tokyo, 143-8540, Japan.
J Cell Biochem. 2015 Jul;116(7):1300-9. doi: 10.1002/jcb.25085.
Cells respond to ER-stress via ER-stress sensors, leading to the UPR and subsequent apoptosis; however, occasionally, they activate autophagy without subsequent apoptosis in response to ER-stress. We previously showed that the induction of apoptosis by ER-stress was related to the presence or absence of CHOP expression; nevertheless, how ATF4 expression is elicited without downstream CHOP expression is unknown. We studied the role of GADD34 on the induction of autophagy and/or apoptosis by NaF- or tunicamycin-induced ER-stress in HepG2 cells transfected with GADD34 siRNA. Although NaF and tunicamycin both induced PERK activation followed by eIF2α phosphorylation and ATF4 expression, CHOP expression was only induced by tunicamycin. Concomitant with the signaling change, autophagy was activated both by NaF and tunicamycin, and apoptosis was induced only by tunicamycin. After 4 h, GADD34 mRNA expression was also increased by NaF and tunicamycin. Suppression of GADD34 by GADD34 siRNA increased ATF4 expression in both NaF- and tunicamycin-treated cells. The GADD34 siRNA increased CHOP expression, which corresponded to increased ATF4 in tunicamycin-treated cells; however, the increased ATF4 did not induce CHOP expression in NaF-treated cells. In concert with signal changes, siRNA treatment additively increased the autophagic activity of both NaF- and tunicamycin-treated cells; however, apoptosis was produced and accelerated only for tunicamycin-treated cells. These findings indicate that GADD34 expression induced by ER-stress delays CHOP expression and retards apoptotic cell death, and that an ATF4-signal-modulating machine other than GADD34 acts on ATF4-to-CHOP signaling to block ATF4-induced CHOP expression in ER-stress related autophagy.
细胞通过内质网应激传感器对内质网应激作出反应，导致未折叠蛋白反应（UPR）及随后的细胞凋亡；然而，偶尔它们会在对内质网应激作出反应时激活自噬而不发生随后的细胞凋亡。我们之前表明内质网应激诱导的细胞凋亡与CHOP表达的有无有关；然而，在没有下游CHOP表达的情况下ATF4表达是如何引发的尚不清楚。我们研究了GADD34在用GADD34小干扰RNA（siRNA）转染的HepG2细胞中，对氟化钠（NaF）或衣霉素诱导的内质网应激所诱导的自噬和/或细胞凋亡中的作用。尽管NaF和衣霉素都诱导了蛋白激酶R样内质网激酶（PERK）激活，随后是真核翻译起始因子2α（eIF2α）磷酸化和ATF4表达，但CHOP表达仅由衣霉素诱导。伴随着信号变化，NaF和衣霉素都激活了自噬，而细胞凋亡仅由衣霉素诱导。4小时后，NaF和衣霉素也增加了GADD34信使核糖核酸（mRNA）表达。用GADD34 siRNA抑制GADD34增加了NaF和衣霉素处理细胞中的ATF4表达。GADD34 siRNA增加了CHOP表达，这与衣霉素处理细胞中增加的ATF4相对应；然而，增加的ATF4在NaF处理细胞中并未诱导CHOP表达。与信号变化一致，siRNA处理相加性地增加了NaF和衣霉素处理细胞的自噬活性；然而，细胞凋亡仅在衣霉素处理细胞中产生并加速。这些发现表明内质网应激诱导的GADD34表达延迟了CHOP表达并延缓了凋亡性细胞死亡，并且除GADD34之外的一种ATF4信号调节机制作用于ATF4到CHOP的信号传导，以在与内质网应激相关的自噬中阻断ATF4诱导的CHOP表达。
