NF-κB signaling regulates functional expression of the MHC class I-related neonatal Fc receptor for IgG via intronic binding sequences

The neonatal Fc receptor for IgG (FcRn) functions to transport maternal IgG to a fetus or newborn and to protect IgG from degradation. Although FcRn is expressed in a variety of tissues and cell types, the extent to which FcRn expression is regulated by immunological and inflammatory events remains unknown. Stimulation of intestinal epithelial cell lines, macrophage-like THP-1, and freshly isolated human monocytes with the cytokine TNF-alpha rapidly up-regulated FcRn gene expression. In addition, the TLR ligands LPS and CpG oligodeoxynucleotide enhanced the level of FcRn expression in THP-1 and monocytes. Treatment of TNF-stimulated THP-1 cells with the NF-kappa B-specific inhibitor or overexpression of a dominant negative mutant inhibitory NF-kappa B (I kappa B alpha; S32A/S36A) resulted in down-regulation of FcRn expression. By using chromatin immunoprecipitation we identified three NF-kappa B binding sequences within introns 2 and 4 of the human FcRn gene. An EMSA confirmed the p50/p50 and/or p65/p50 complex (s) bound to intron 2- or 4-derived oligonucleotides containing putative NF-kappa B binding sequences, respectively. The intronic NF-kappa B sequences in combination with the promoter or alone regulated the expression of a luciferase reporter gene in response to TNF-a stimulation or overexpression of NF-kappa B p65 and p50. DNA looping interactions potentially occurred after the stimulation between intronic NF-kappa B sequences and the FcRn promoter as shown by a chromosome conformation capture assay. Finally, TNF-alpha stimulations enhanced IgG transport across an intestinal Caco-2 epithelial monolayer. Together, these data provide the first evidence that NF-kappa B signaling via intronic sequences regulates FcRn expression and function.