The RelA NF-kappa B subunit and the aryl hydrocarbon receptor (AhR) cooperate to transactivate the c-myc promoter in mammary cells

NF-kappaB/Rel transcription factors regulate many genes involved in control of cellular proliferation, neoplastic transformation, and apoptosis, inclu ding the c-myc oncogene, Recently, we have observed that levels of NF-kappa B and aryl hydrocarbon receptor (AhR), which mediates malignant transformat ion by environmental carcinogens, are highly elevated and appear constituti vely active in breast cancer cells. ReI factors have been found to function ally interact with other transcription factors. Here we demonstrate a physi cal and functional association between the RelA subunit of NF-kappaB and Ah R resulting in the activation of c-myc gene transcription in breast cancer cells. RelA and AhR proteins were coimmunoprecipitated from cytoplasmic and nuclear extracts of non-malignant MCF-10F breast epithelial and malignant Hs578T breast cancer cells. In transient cotransfection, RelA and AhR gene products demonstrated cooperation in transactivation of the c-myc promoter, which was dependent on the NF-kappaB elements, and in induction of endogen ous c-Myc protein levels. A novel AhR/RelA-containing NF-kappaB element bin ding complex was identified by electrophoretic mobility shift analysis of n uclear extracts from RelA and AhR co-transfected Hs578T cells. Thus, the Re lA and AhR proteins functionally cooperate to bind to NF-kappaB elements an d induce c-myc gene expression. These findings suggest a novel signaling me chanism whereby the Ah receptor can stimulate proliferation and tumorigenes is of mammary cells.