Seiichi Oyadomari

Unfolded and malfolded client proteins impose a stress on the endoplasmic reticulum (ER), which contributes to cell death in pathophysiological conditions. The transcription factor C/EBP homologous protein (CHOP) is activated by ER stress, and CHOP deletion protects against its lethal consequences. We find that CHOP directly activates GADD34, which promotes ER client protein biosynthesis by dephosphorylating phospho-Ser 51 of the alpha-subunit of translation initiation factor 2 (eIF2alpha) in stressed cells. Thus, impaired GADD34 expression reduces client protein load and ER stress in CHOP(-/-) cells exposed to perturbations that impair ER function. CHOP(-/-) and GADD34 mutant cells accumulate less high molecular weight protein complexes in their stressed ER than wild-type cells. Furthermore, mice lacking GADD34-directed eIF2alpha dephosphorylation, like CHOP(-/-) mice, are resistant to renal toxicity of the ER stress-inducing drug tunicamycin. CHOP also activates ERO1alpha, which encodes an ER oxidase. Consequently, the ER of stressed CHOP(-/-) cells is relatively hypo-oxidizing. Pharmacological and genetic manipulations that promote a hypo-oxidizing ER reduce abnormal high molecular weight protein complexes in the stressed ER and protect from the lethal consequences of ER stress. CHOP deletion thus protects cells from ER stress by decreasing ER client protein load and changing redox conditions within the organelle.

Overload of pancreatic beta cells in conditions such as hyperglycemia, obesity, and long-term treatment with sulfonylureas leads to beta cell exhaustion and type 2 diabetes. Because beta cell mass declines under these conditions, apparently as a result of apoptosis, we speculated that overload kills beta cells as a result of endoplasmic reticulum (ER) stress. The Akita mouse, which carries a conformation-altering missense mutation (Cys96Tyr) in Insulin 2, likewise exhibits hyperglycemia and a reduced beta cell mass. In the development of diabetes in Akita mice, mRNAs for the ER chaperone Bip and the ER stress-associated apoptosis factor Chop were induced in the pancreas. Overexpression of the mutant insulin in mouse MIN6 beta cells induced Chop expression and led to apoptosis. Targeted disruption of the Chop gene delayed the onset of diabetes in heterozygous Akita mice by 8-10 weeks. We conclude that ER overload in beta cells causes ER stress and leads to apoptosis via Chop induction. Our findings suggest a new therapeutic approach for preventing the onset of diabetes by inhibiting Chop induction or by increasing chaperone capacity in the ER.

Overload of pancreatic β cells in conditions such as hyperglycemia, obesity, and long-term treatment with sulfonylureas leads to β cell exhaustion and type 2 diabetes. Because β cell mass declines under these conditions, apparently as a result of apoptosis, we speculated that overload kills β cells as a result of endoplasmic reticulum (ER) stress. The Akita mouse, which carries a conformation-altering missense mutation (Cys96Tyr) in Insulin 2, likewise exhibits hyperglycemia and a reduced β cell mass. In the development of diabetes in Akita mice, mRNAs for the ER chaperone Bip and the ER stress–associated apoptosis factor Chop were induced in the pancreas. Overexpression of the mutant insulin in mouse MIN6 β cells induced Chop expression and led to apoptosis. Targeted disruption of the Chop gene delayed the onset of diabetes in heterozygous Akita mice by 8–10 weeks. We conclude that ER overload in β cells causes ER stress and leads to apoptosis via Chop induction. Our findings suggest a new therapeutic approach for preventing the onset of diabetes by inhibiting Chop induction or by increasing chaperone capacity in the ER.