Inhibition of palmitate‐induced GADD34 expression augments apoptosis in mouse insulinoma cells (MIN6)

Abstract

Saturated fatty acids like palmitate induce endoplasmic reticulum (ER) stress in pancreatic beta-cells, an event linked to apoptotic loss of β-cells in type 2 diabetes. Sustained activation of the ER stress response leads to expression of growth arrest and DNA damage-inducible protein 34 (GADD34), a regulatory subunit of protein phosphatase 1. In the present study, we have used small interfering RNA in order to knockdown GADD34 expression in insulin-producing MIN6 cells prior to induction of ER stress by palmitate and evaluated its consequences on RNA-activated protein kinase-like ER-localized eIF2alpha kinase (PERK) signalling and apoptosis. Salubrinal, a specific inhibitor of eukaryotic initiation factor 2α (eIF2α) dephosphorylation, was used as a comparison. Salubrinal treatment augmented palmitate-induced ER stress and increased GADD34 levels. Both GADD34 knockdown and salubrinal treatment potentiated the cytotoxic effects of palmitate as evidenced by increased DNA fragmentation and activation of caspase 3, with the fundamental difference that the former did not involve enhanced levels of GADD34. The data from this study suggest that sustained activation of PERK signalling and eIF2α phosphorylation sensitizes insulin-producing MIN6 cells to lipoapoptosis independently of GADD34 expression levels.

Inactivation of Ppp1r15a minimises weight gain and insulin resistance during caloric excess in female mice

“…Salubrinal weakly inhibits dephosphorylation of eIF2α at high concentrations 47 , but its dramatic effects to increase P-eIF2α in cells may reflect additional effects perhaps on one or more of the eIF2α kinases. In insulinoma cells, salubrinal was found to enhance the toxicity of palmitate 48 , but our data indicate that genetic loss of PPP1R15A ameliorates the lipotoxicity of palmitate. This supports a potential therapeutic role of PPP1R15A inhibition if more potent and selective small molecules were to be generated.…”

Coordinated Regulation of the Neutral Amino Acid Transporter SNAT2 and the Protein Phosphatase Subunit GADD34 Promotes Adaptation to Increased Extracellular Osmolarity

“…Induction of this transporter is a crucial part of the TonEBP transcription program and mediates a volume increase in response to mild hyperosmotic stress (3). The induction of GADD34 has been associated with both prosurvival and proapoptotic outcomes in response to diverse stress conditions (27,41,42). Several lines of evidence support prosurvival functions of GADD34 during hyperosmotic stress, especially under mild stress condi-FIGURE 8.…”

The regulatory protein GADD34 inhibits TRAIL-induced apoptosis via TRAF6/ERK-dependent stabilization of myeloid cell leukemia 1 in liver cancer cells

“…ered as a tumor suppressor. In contrast, GADD34 knockdown enhances palmitate-induced mouse insulinoma cell apoptosis (7). Moreover, GADD34 knockout results in more liver cell apo-ptosis in lipopolysaccharide-treated mice (5).…”

“…Previous studies indicate that GADD34 has opposing effects on apoptosis. Some studies show that GADD34 may induce apoptosis (3,4), whereas other studies demonstrate that GADD34 can prevent apoptosis or tissue injury (5)(6)(7)(8). The mechanisms underlying the anti-apoptotic effect of GADD34 remain elusive.…”