Nuclear factor kappaB protects pancreatic beta-cells from tumor necrosis factor-alpha-mediated apoptosis

Abstract

Recent studies incriminating tumor necrosis factor (TNF)-α as the final effector in pancreatic β-cell death in type 1 diabetes underscore the potential role of TNF-α-dependent NF-κB activation as an important modulator of pancreatic β-cell death in autoimmune diabetes. Although nuclear factor (NF)-κB activation has been implicated in the protection of target cells against apoptosis by a variety of death effectors, its role in pancreatic islet cell death is not clear. We studied the role of NF-κB activation in pancreatic islet cell death by using a γ-interferon (IFN-γ)/TNF-α synergism model we had previously reported. TNF-α induced inhibitor of κB (IκB) degradation and p65 translocation from cytoplasm to nuclei in MIN6N8 insulinoma cells. The NF-κB DNA-binding nuclear complex activated by TNF-α contained both the p65 and p50 subunit. IFN-γ pretreatment did not affect TNF-α-induced NF-κB activation. Treatment with a proteasome inhibitor blocked p65 translocation and induced susceptibility to TNF-α in otherwise resistant insulinoma cells or primary pancreatic islet cells. Specific inhibition of NF-κB activation by adenoviral transduction of IκB “superrepressor” also sensitized insulinoma cells and primary islet β-cells to TNF-α-induced apoptosis. These results suggest the protective role of NF-κB activation against cytokine-mediated pancreatic β-cell death, contrary to previous reports implicating NF-κB as a mediator of pancreatic islet cell death.

Although apoptosis of pancreatic β-cells is a critical step in the development of type 1 diabetes (1,2), it has not yet been clearly elucidated which molecules are the real effectors of pancreatic β-cell death. We have recently published in vitro and in vivo data suggesting that γ-interferon (IFN-γ) and tumor necrosis factor (TNF)-α synergism is responsible for apoptosis of pancreatic β-cells (3). IFN-γ seems to sensitize otherwise resistant pancreatic islet cells to TNF-α-mediated apoptosis, and TNF-α is thought to exert the final apoptosis on pancreatic islet cells. The role of TNF-α as the final death effector molecule is consistent with other studies that use genetic ablation models (4,5). However, other data showing the opposite effect of TNF-α in autoimmune diabetes have been published, reflecting the complexity of the pathogenesis and probably different role of cytokines in the different stages of the disease progression (6,7).

Although TNF-α is one of the most important death effector molecules, most primary or immortalized cells are not susceptible to apoptosis by TNF-α alone because of the concomitant activation of the antiapoptotic process by TNF-α (8–10). Many studies have implicated nuclear factor (NF)-κB as an important player in the protection of target cells against TNF-α-induced apoptosis (11–13). However, other studies have reported increased cell death by NF-κB activation using neuronal cells, pancreatic islet cells/insulinoma cells, or others (14–20), reflecting a complex interplay of cytokines and transcriptional factors that could be different according to the cell types or modes of cell death.

This investigation was carried out to determine the role of NF-κB activation in cytokine-induced pancreatic β-cell death. We found evidence supporting the role of NF-κB in the protection of pancreatic β-cells against TNF-α-induced apoptosis.