Upstream Stimulatory Factor but Not C-Myc Enhances Transcription of the Human Polymeric Immunoglobulin Receptor Gene

Secretory antibodies protect mucosal surfaces from ingested, inhaled and sexually transmitted pathogens. The polymeric immunoglobulin receptor (pIgR) transports antibodies across mucosal epithelia into external secretions. We and others have identified a region of the human polymeric immunoglobulin receptor gene (locus PIGR) that is sufficient for basal transcriptional activity. An E-Box motif, which binds transcription factors of the basic helix-loop-helix/leucine zipper (bHLH/zip) family, was identified as a major regulatory element in the PIGR gene promoter. Transient transfection of PIGR promoter reporter plasmids in intestinal epithelial cell (IEC) lines suggested that the transcription factors upstream stimulatory factor (USF) and c-Myc may exert opposing effects on PIGR promoter activity. Mutations within and flanking the E-Box that favored USF binding enhanced promoter activity, while mutations that favored c-Myc binding reduced promoter activity. Ectopic expression of USF1 or USF2 enhanced PIGR promoter activity, while exogenous c-Myc did not. Electrophoretic mobility shift assays (EMSA) demonstrated that USF1 and USF2 bound to the E-Box motif as homo- and heterodimers. Chromatin immunoprecipitation (ChIP) demonstrated that USF proteins bind the PIGR promoter in vivo, which is enriched in acetylated histones. E-Box motifs are commonly observed in promoters of genes that are highly expressed in the human colon. Genes that are down-regulated in colorectal cancer, including PIGR, frequently have non-canonical E-Boxes, while genes that are up-regulated in colorectal cancer generally have canonical E-Boxes. The results of our experiments may shed light on the mechanisms of dysregulated expression of pIgR in inflammatory bowel disease and colorectal cancer, diseases associated with aberrant expression of c-Myc.