The gene encoding human transmembrane secretory component (locus PIGR) is linked to D1S58 on chromosome 1

Polymeric immunoglobulin receptor

The surface of mucosal sites, such as the intestinal tract, are covered by epithelial cells. To protect the intestinal environment from invading pathogens and maintain homeostasis, the human body developed an exquisite acquired immune system, referred to as the mucosal immune system, in which epithelial cells and lymphocytes function cooperatively. The main player in this immune system is the polymeric immunoglobulins (pIgs), in particular dimeric IgA (dIgA). To exert its protective effect, dIgA produced in the lamina propria must be transported to the intestinal lumen across epithelial cells. This process is called transcytosis and is mediated by polymeric immunoglobulin receptor (pIgR), which is exclusively produced by intestinal epithelial cells (IECs). DIgA is captured by pIgR on the basolateral surface of IECs and transcytosed to the opposite side of IECs. The dIgA-pIgR complex is expressed on the apical surface of IECs and proteolytically cleaved to generate secretory IgA (SIgA). This review describes the current understanding and recent progress in this research field.

Transcriptional regulation of the human polymeric Ig receptor gene: analysis of basal promoter elements

Secretory Igs provide the first line of adaptive immune defense against ingested, inhaled, and sexually transmitted pathogens at mucosal surfaces. The polymeric Ig receptor regulates transport of dimeric IgA and pentameric IgM into external secretions. The level of expression of polymeric Ig receptor is controlled to a large extent by transcription of the PIGR gene in mucosal epithelial cells. Here we present a detailed analysis of the promoter of the PIGR gene by transient transfection of luciferase reporter plasmids into cultured cell lines. Comparisons of the human and mouse PIGR promoters in human and mouse intestinal and liver cell lines demonstrated that the human PIGR promoter was 4- to 5-fold more active than the mouse PIGR promoter in all cell types, and that both the human and mouse PIGR promoters were more active in intestinal than in liver cell lines. Targeted deletions of 22-bp segments of the human PIGR promoter revealed that the region from nt -63 to -84 is crucial for basal transcription, and that two upstream regions can act as positive or negative regulators. Point mutations within the region from nt -63 to -84 demonstrated that an E box motif, which binds the basic helix-loop-helix protein upstream stimulatory factor, is required for PIGR promoter activity. Two additional regulatory motifs were identified in the proximal promoter region: a binding site for AP2, and an inverted repeat motif that binds an unidentified protein. These findings suggest that cooperative binding of multiple transcription factors regulates basal activity of the human PIGR promoter.

A composite DNA element in the promoter of the polymeric immunoglobulin receptor regulates its constitutive expression

The polymeric immunoglobulin receptor (pIgR), which is constitutively expressed on the basolateral surface of secretory epithelial cells, mediates external translocation of polymeric IgA and pentameric IgM (collectively called pIg) to exocrine secretions. A high level of synthesis must be maintained because the receptor is continuously cleaved to release bound secretory component (SC) in secretory IgA and secretory IgM, as well as free SC from unoccupied receptor. We have isolated the promoter of the pIgR gene and identified a short activating region that is required for the expression of pIgR promoter-driven reporter genes. This region contained an E-box and an inverted repeat sequence (IRS). Gel electrophoresis mobility shift assays with nuclear extracts from different pIgR-expressing epithelial cell lines demonstrated proteins that bind independently to both the E-box and the IRS sequence of the pIgR promoter. In addition, a DNA probe that contained both the E-box and the IRS gave rise to a larger complex that could not be competed by either element on its own. Binding was confirmed by DNase I footprinting of the E-box and IRS sequences with nuclear extracts, and by dimethyl sulfide footprinting in living HT-29 epithelial cells. Finally, a mutation in the pIgR promoter that inhibited protein binding to the E-box and the formation of the larger complex, abolished activated transcription from the reporter gene.

Attenuation of gene expression by a trinucleotide repeat-rich tract from the terminal exon of the rat hepatic polymeric immunoglobulin receptor gene

A 359 bp terminal exon fragment of the rat polymeric immunoglobulin receptor gene has been tested for biological effects. The fragment contains an S1 nuclease-sensitive microsatellite with d(GGA) and d(GAA) trinucleotide repeats that are expressed discordantly in the 3'UTRs of liver mRNAs encoded by the single copy gene. When human A293 cells are transfected with expression plasmids carrying this fragment in forward orientations, flanking or replacing poly(A) cassettes in the 3' ends of the transcription units, luciferase reporter gene expression is attenuated 47 to 59% or 98.5%, respectively. In contrast, when the fragment is tested similarly in reverse orientation, there is significantly less or no attenuation of gene expression. These observations, and computer models of partial triplet repeat DNA tertiary and RNA secondary structures, suggest that this fragment might regulate gene expression by orientation and position-dependent mechanisms at transcriptional and post-transcriptional levels.

Secretory component mRNA and protein expression in colorectal adenomas and carcinomas

Secretary component (SC) is expressed basolaterally as a transmembrane protein (pIg receptor) on secretory epithelial cells. As pIg receptor it plays a central role in humoral immunity by mediating the external translocation of dimeric IgA and pentameric IgM. A few case reports have suggested that reduced or absent SC protein expression is associated with diarrhoeal disease, but there is no convincing evidence that a primary pIg receptor deficiency can occur. In this study the relative presence of SC mRNA was determined by Northern blot analysis and related to immunohistochemically determined SC protein expression in 33 colorectal adenomas (31 patients) with increased risk of developing sporadic colorectal cancer, as well as in 19 colorectal carcinomas from 19 patients with such sporadic tumours. In the adenomas, SC mRNA levels were positively related to SC protein expression; both mRNA and SC protein were negatively related to histological grade. Similarly, SC mRNA levels tended to be related to the SC protein expression in the carcinomas. SC mRNA was detected in all adenomas, and only two of ten carcinomas (10.5%) deemed to be SC deficient by immunohistochemistry also lacked SC mRNA expression, suggesting diallelic alterations in the SC-encoding gene (locus PIGR). This possibility agreed with Southern blot analysis performed on a separate sample of 32 other colonic carcinomas in which the diallelic loss of D1S58 (which exhibits a close linkage centromerically to PIGR) was calculated to be 6.4%. Together these findings suggested that reduced SC protein expression in colorectal adenomas might be a transcriptional defect reflecting the degree of cellular dysplasia, whereas absent SC protein expression in colorectal carcinomas might also involve post-transcriptional defects and occasional diallelic gene deletions representing late events in carcinogenesis.

Discordant expression and variable numbers of neighboring GGA- and GAA-rich triplet repeats in the 3' untranslated regions of two groups of messenger RNAs encoded by the rat polymeric immunoglobulin receptor gene

An unusual S1-nuclease sensitive microsatellite (STMS) has been found in the single copy, rat polymeric immunoglobulin receptor gene (PIGR) terminal exon. In Fisher rats, elements within or beyond the STMS are expressed variably in the 3' untranslated regions (3'UTRs) of two 'Groups' of PIGR-encoded hepatic mRNAs (pIg-R) during liver regeneration. STMS elements include neighboring constant regions (a 60-bp d[GA]-rich tract with a chi-like octamer, followed by 15 tandem d[GGA] repeats) that merge directly with 36 or 39 tandem d[GAA] repeats (Fisher or Wistar strains, respectively) interrupted by d[AA] between their 5th-6th repeat units. The Wistar STMS is flanked upstream by two regions of nearly contiguous d[CA] or d[CT] repeats in the 3' end of intron 8; and downstream, by a 283 bp 'unit' containing several inversions at its 5' end, and two polyadenylation signals at its 3' end. The 283 nt unit is expressed in Group 1 pIg-R mRNAs; but it is absent in the Group 2 family so that their GAA repeats merge with their poly A tails. In contrast to genomic sequence, GGA triplet repeats are amplified (n > or = 24-26), whereas GAA triplet repeats are truncated variably (n < or = 9-37) and expressed uninterruptedly in both mRNA Groups. These results suggest that 3' end processing of the rat PIGR gene may involve misalignment, slippage and premature termination of RNA polymerase II. The function of this unusual processing and possible roles of chi-like octamers in quiescent or extrahepatic tissues are discussed.