Exploring possible mechanisms for primary bile acid malabsorption: evidence for different regulation of ileal bile acid transporter transcripts in chronic diarrhoea.

OBJECTIVES: Chronic diarrhoea resulting from primary idiopathic bile acid malabsorption (IBAM) is common, but its aetiology is largely unknown. We investigated possible mechanisms, first looking for common sequence variations in the cytoplasmic ileal bile acid-binding protein (IBABP, gene symbol FABP6), and secondly, determining the expression of ileal mucosal transcripts for the apical sodium-linked bile acid transporter (ASBT), IBABP, the putative basolateral transporters, OSTalpha and OSTbeta, and regulatory factors. METHODS: Genomic DNA was prepared from two cohorts of patients and two control groups; the promoter and exonic regions of FABP6 were sequenced. In intestinal biopsies, transcript expression was measured by quantitative real time-PCR, using ileum from 17 patients and 21 controls. RESULTS: Sequence variations were identified in FABP6, but overall frequencies were similar in patients and controls. Transcripts of ASBT and IBABP, but not OSTalpha and OSTbeta, were expressed at higher levels in ileum than duodenum. The transcription factors farnesoid-X-receptor (FXR) and liver-receptor-homologue (LRH-1) were also more abundant in ileum, as was fibroblast growth factor 19 (FGF19), unlike short heterodimer partner (SHP), c-Fos, or CDX2. No significant differences in mean or median values were found between the groups for any of these transcripts. However, findings on regression analysis suggested that these transporters differ in their regulation, particularly in the relationships of CDX2, LRH-1 and FXR with OSTalpha. CONCLUSION: Most cases of IBAM are unlikely to be caused by genetic variation in FABP6 or by major differences in transporter transcript expression. Our evidence indicates that other factors, such as regulation of expression of the basolateral bile acid transporter, should be considered as possible causes.

Calcium channel TRPV6 expression in human duodenum: different relationships to the vitamin D system and aging in men and women.

UNLABELLED: Intestinal absorption of calcium affects bone mineralization and varies greatly. In human duodenum, expression of the calcium channel TRPV6 was directly related to blood 1,25-dihydroxyvitamin D in men, but effects of age with lower median vitamin D receptor levels were more significant in women. INTRODUCTION: The TRPV6 calcium channel/transporter is implicated in animal studies of intestinal calcium absorption, but in humans, its role and relationship to differences in mineral metabolism is unclear. We aimed to characterize TRPV6 expression in human intestine including defining relationships to the vitamin D endocrine system. MATERIALS AND METHODS: TRPV6 transcript expression was determined in endoscopic mucosal biopsies obtained from normal duodenum. Expression was compared with that in ileum and with in situ hybridization in archival tissues and related to sequence variants in genomic DNA. TRPV6 expression was related in 33 subjects to other transcripts involved in calcium absorption including the vitamin D receptor (VDR) and to blood vitamin D metabolites including 1,25-dihydroxyvitamin D [1,25(OH)(2)D]. RESULTS: TRPV6 transcripts were readily detected in duodenum but not in ileum. Expression was highest in villous epithelial cells. Sequence variants in the coding and upstream regions of the gene did not affect TRPV6 expression. The relationship between duodenal TRPV6 expression and 1,25(OH)(2)D differed in men and women. In men, linear regression showed a strong association with 1,25(OH)(2)D (r = 0.87, p < 0.01), which was unaffected by age. In women, there was no significant overall relationship with 1,25(OH)(2)D, but there was a significant decrease with age (r = -0.69, p < 0.001). Individual expression of TRPV6 and VDR was significantly correlated. The group of older women (>50) had lower median levels of both TRPV6 and VDR transcripts than younger women (p < 0.001 and 0.02, respectively). CONCLUSIONS: Duodenal TRPV6 expression is vitamin D dependent in men, but not in older women, where expression of TRPV6 and VDR are both reduced. These findings can explain, at least in part, the lower fractional calcium absorption seen in older postmenopausal women.

Duodenal expression of the epithelial calcium transporter gene TRPV6: is there evidence for Vitamin D-dependence in humans?

Intestinal absorption of dietary calcium is regulated by 1,25-dihydroxycholecalciferol (1,25(OH)(2)D(3)) in humans and in experimental animals but interspecies differences in responsiveness to 1,25(OH)(2)D(3) are found, possibly due to differences in the promoters of genes for intestinal calcium transport proteins or of the Vitamin D receptor (VDR). The epithelial calcium transporter, known as ECAC2 or CAT1, the product of the TRPV6 gene expressed in proximal intestinal enterocytes, is the first step in calcium absorption and studies in mice have shown that its expression is Vitamin D-dependent. In contrast in man, we showed that duodenal TRPV6 mRNA expression was independent of blood 1,25(OH)(2)D(3), although in Caco-2 cells, 1,25(OH)(2)D(3)-dependent changes have been demonstrated. We sought to explain these findings. A consensus Vitamin D response element in the mouse gene is absent in the human gene. We re-analysed our duodenal expression data according to a CDX2-site polymorphism in the VDR promoter. Mean TRPV6 expression was the same, but there was evidence of different responsiveness to 1,25(OH)(2)D(3). In the GG genotype group, but not the AG, duodenal TRPV6 expression increased with 1,25(OH)(2)D(3). We postulate that lower levels of expression of VDR in the GG group produce greater sensitivity to 1,25(OH)(2)D(3).

Human ileal bile acid-binding protein promoter and the effects of CDX2.

The ileal bile acid-binding protein (IBABP) is important for the reabsorption of bile salts in the distal small intestine. Studies with the human IBABP gene (FABP6, on chromosome 5q33.3-q34) defined the major transcription start site and identified conserved elements. A consensus element for the caudal-related homeobox factor CDX2 was functional in gel-shift assays and in transfection experiments.