The Forkhead Transcription Factor Foxo1 Bridges the JNK Pathway and the Transcription Factor PDX-1 through Its Intracellular Translocation*

It has been shown that oxidative stress and activation of the c-Jun N-terminal kinase (JNK) pathway induce the nucleocytoplasmic translocation of the pancreatic transcription factor PDX-1, which leads to pancreatic β-cell dysfunction. In this study, we have shown that the forkhead transcription factor Foxo1/FKHR plays a role as a mediator between the JNK pathway and PDX-1. Under oxidative stress conditions, Foxo1 changed its intracellular localization from the cytoplasm to the nucleus in the pancreatic β-cell line HIT-T15. The overexpression of JNK also induced the nuclear localization of Foxo1, but in contrast, suppression of JNK reduced the oxidative stress-induced nuclear localization of Foxo1, suggesting the involvement of the JNK pathway in Foxo1 translocation. In addition, oxidative stress or activation of the JNK pathway decreased the activity of Akt in HIT cells, leading to the decreased phosphorylation of Foxo1 following nuclear localization. Furthermore, adenovirus-mediated Foxo1 overexpression reduced the nuclear expression of PDX-1, whereas repression of Foxo1 by Foxo1-specific small interfering RNA retained the nuclear expression of PDX-1 under oxidative stress conditions. Taken together, Foxo1 is involved in the nucleocytoplasmic translocation of PDX-1 by oxidative stress and the JNK pathway.

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