A Novel NF-κB/Rel Site in Intron 1 Cooperates with Proximal Promoter Elements to Mediate TNF-α-Induced Transcription of the Human Polymeric Ig Receptor

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Abstract
Secretory Abs constitute the first line of specific immune defense at mucosal surfaces. Such Abs are generated by the active transport of polymeric Ig (pIg) across secretory epithelia mediated by the pIgR, also known as transmembrane secretory component (SC). The proinflammatory cytokine TNF-α is a key mediator of host responses to infections, and it can stimulate protein synthesis-dependent transcriptional up-regulation of pIgR/SC in the HT-29 intestinal adenocarcinoma cell line. By reporter gene assay we identified a novel TNF-α-responsive region located within a 748-bp fragment in intron 1 of the human pIgR/SC gene which depended on an NF-κB/Rel site for full responsiveness. EMSAs demonstrated preferential binding of the NF-κB/Rel family member p65 (RelA) to this DNA element after TNF-α stimulation, with weaker and more delayed binding of p50. Furthermore, the TNF-α-responsive region in intron 1 required cooperation with DNA elements located in the proximal promoter region of the gene. Mutational analysis demonstrated that an IFN-stimulated response element near the transcriptional start site in exon 1 was involved in the TNF-α responsiveness. Thus, DNA elements located >4 kb apart were found to cooperate in TNF-α-induced pIgR/SC up-regulation. The intronic TNF-α-responsive enhancer overlapped with a recently identified IL-4-responsive enhancer. Several intronic DNA elements found to be functionally important in the human gene are highly conserved between the human and mouse pIgR/SC genes, suggesting the presence of a conserved cytokine-responsive enhancer region.