The Molecular Mechanism of Endoplasmic Reticulum Stress-Induced Apoptosis in PC-12 Neuronal Cells: The Protective Effect of Insulin-Like Growth Factor I

Abstract

Endoplasmic reticulum (ER) stress has been implicated in several neurodegenerative diseases. Although CCAAT/enhancer-binding protein homologous protein (CHOP) has been shown to play a critical role in ER stress, the precise apoptosis cascade downstream of CHOP is unknown. In this report, we investigated the mechanism of ER stress-mediated apoptosis as well as the action of IGF-I in PC-12 neuronal cells. Our results demonstrated that tribbles-related protein 3 (TRB3), which is a target gene of CHOP, was responsible for tunicamycin (an ER stress inducer)-induced apoptosis. TRB3 could promote dephosphorylation of Akt in PC-12 cells. IGF-I inhibited ER stress-induced apoptosis by restoring the phosphorylation level of Akt. Both wortmannin (a phosphatidylinositide 3-kinase inhibitor) and SB 212090 (a p38 MAPK inhibitor) suppressed the protective effect of IGF-I on ER stress-induced apoptosis. Interestingly, IGF-I attenuated ER stress-mediated expression of TRB3 but not CHOP. This action of IGF-I was abolished by SB 212090 but not by wortmannin. Immunoprecipitation analysis revealed that IGF-I promoted the phosphorylation of CHOP by activating p38 MAPK, probably leading to a decrease in the transcriptional activity of CHOP. The dephosphorylation of Akt resulted in increased expression of a proapoptotic protein, p53 up-regulated modulator of apoptosis (PUMA), in a forkhead box O3a-dependent manner. Knockdown of PUMA by short hairpin RNA attenuated ER stress-mediated apoptosis. Thus, our current study indicates that both TRB3 and PUMA are critical molecules in ER stress-induced apoptosis. IGF-I effectively protects PC-12 neuronal cells against ER stress-induced apoptosis through the phosphatidylinositide 3-kinase/Akt and p38 MAPK pathways.

A feed-forward loop involving Trib3, Akt and FoxO mediates death of NGF-deprived neurons

“…31,32,37 In agreement, we found that Trib3 overexpression reduced neuronal phospho-Akt levels and that silencing Trib3 partially reversed the decrease in phospho-Akt caused by NGF deprivation. Moreover, Trib3 overexpression produced dephosphorylation of the Akt substrate FoxO1a, and dephosphorylation of FoxO1a caused by NGF deprivation was suppressed by silencing Trib3.…”

“…31,32,36 In PC12 cells, Trib3 knockdown partially reduced tunicamycininduced dephosphorylation of Akt at Ser473. 37 Active/ phosphorylated Akt is an important regulator of neuron survival; NGF promotes Akt phosphorylation and NGF deprivation causes Akt dephosphorylation, which in turn contributes to death. 1,2 We therefore explored whether neuronal Akt phosphorylation is affected by Trib3.…”

Effect of rosemary polyphenols on human colon cancer cells: transcriptomic profiling and functional enrichment analysis

“…In contrast, induction of TRIB3 would be more robust during severe or persistent ER stress, promoting apoptosis through inhibition (dephosphorylation) of Akt (Ohoka et al 2005). This feedback mechanism could facilitate ER stress-mediated apoptosis in severely ER stressed cells that have successfully reached pro-apoptotic threshold levels of DDIT3 (Zou et al 2009). Rom4 extract seems to initiate the DDIT3 pro-apoptotic transcription activity since the expression of its immediate inducible target genes, BAK and BIM, was also over-expressed in SW480.…”

Developmental roles of tribbles protein family members

“…Thus, their role as either oncogene or tumor suppressor of AML remains unresolved. Trib3 mediates the tumor suppressing effects homocystein in endothelial cell model (Zou et al, 2009), cannabinoid in hepatocellular carcinomas (Vara et al, 2011), tetradecylthioacetic acid (TTA) in colon cancer cells (Lundemo et al, 2011), and dehydroxymethylepoxyquinomicin (DHMEQ) in liver cancer cells (Lampiasi et al, 2009). …”