Involvement of nuclear factor-kB and Nurr-1 in cytokine-induced transcription of proopiomelanocortin gene in AtT20 corticotroph cells.

OBJECTIVE: The precise mechanism whereby proinflammatory cytokines activate the hypothalamo-pituitary-adrenal axis is still unclear. We examined whether transcription factors nuclear factor (NF)-kappaB and Nurr-1 are involved in the cytokine-induced proopiomelanocortin (POMC) gene expression.

METHODS: The mouse corticotropinoma cell line AtT20 was treated with tumor necrosis factor-alpha (TNF-alpha) or interleukin-1beta (IL-1beta). Real-time PCR, luciferase assay and Western blotting were conducted to assess the gene expression, promoter activity and protein expression in various conditions.

RESULTS: Intrinsic expression of NF-kappaB was confirmed by RT-PCR. An active component of NF-kappaB (p65) was upregulated in the nuclear fraction by both TNF-alpha and IL-1beta treatment in a dose- and time-related manner. These cytokines potently stimulated the promoter activity of NF-kappaB and Nurr-1. We also found rapid upregulation of the Nurr-1 gene and protein after treatment with these cytokines. Cotreatment of the cells with either of the cytokines and corticotropin-releasing hormone resulted in additive effects. Cytokine-induced Nurr-1 transcription and Nurr-1 transcription induced by overexpression of NF-kappaB were both blunted by mutagenesis within the NF-kappaB responsive element, which implies that Nurr-1 upregulation specifically requires NF-kappaB binding to its own DNA-binding site. Proinflammatory cytokines exert positive effects on POMC gene expression, which were inhibited by pretreatment with a specific NF-kappaB inhibitor.

CONCLUSION: These results together imply that Nurr-1 expression is a connecting point between neuroendocrine and immune systems in mediating cytokine-induced POMC gene expression.