An ATF4-Signal-Modulating Machine Other Than GADD34 Acts in ATF4-to-CHOP Signaling to Block CHOP Expression in ER-Stress-Related Autophagy.
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.
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.
Noboru
N
Iwasaki
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.
Yoshiki
Y
Sugiyama
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.
Shuichi
S
Miyazaki
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.
Hiroshi
H
Nakagawa
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.
Kazuhiko
K
Nishimura
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.
Saburo
S
Matsuo
D051701:Activating Transcription Factor 4 / Q000235:genetics*; D001343:Autophagy; D059865:Endoplasmic Reticulum Stress; D056945:Hep G2 Cells; D006801:Humans; D054645:Protein Phosphatase 1 / Q000037:antagonists & inhibitors / Q000378:metabolism; D015398:Signal Transduction; D012969:Sodium Fluoride / Q000494:pharmacology; D051742:Transcription Factor CHOP / Q000235:genetics; D014415:Tunicamycin / Q000494:pharmacology
C497109:ATF4 protein, human; C497249:DDIT3 protein, human; D014415:Tunicamycin; D051701:Activating Transcription Factor 4; D051742:Transcription Factor CHOP; D012969:Sodium Fluoride; C087519:PPP1R15A protein, human; D054645:Protein Phosphatase 1
APOPTOSIS; AUTOPHAGY; CHOP EXPRESSION; ER-STRESS; GADD34; PERK PATHWAY