In normal individuals, the epithelium of the colon absorbs 1.5–2 l of water a day to generate dehydrated feces. However, in the condition of bile acid malabsorption (BAM), an excess of bile acids in the colon results in diarrhea. Several studies have attempted to address the mechanisms contributing to BAM induced by various bile acids. However, none have addressed a potential dysregulation of aquaporin (AQP) water channels, which are responsible for the majority of transcellular water transport in epithelial cells, as a contributing factor to the onset of diarrhea and the pathogenesis of BAM. In this study, we aimed to systematically analyze the expression of AQPs in colonic epithelia from rat, mouse, and human and determine whether their expression is altered in a rat model of BAM. Mass spectrometry-based proteomics, RT-PCR, and western blotting identified various AQPs in isolated colonic epithelial cells from rats (AQP1, 3, 4, 7, 8) and mice (AQP1, 4, 8). Several AQPs were also detected in human colon (AQP1, 3, 4, 7–9). Immunohistochemistry localized AQP1 to the apical plasma membrane of epithelial cells in the bottom of the crypts, whereas AQP3 (rat, human) and AQP4 (mice, human) were localized predominantly in the basolateral plasma membrane. AQP8 was localized intracellularly and at the apical plasma membrane of epithelial cells. Rats fed sodium cholate for 72 h had significantly increased fecal water content, suggesting development of BAM-associated diarrhea. Colonic epithelial cells isolated from this model had significantly altered levels of AQP3, 7, and 8, suggesting that these AQPs may be involved in the pathogenesis of bile acid-induced diarrhea.

Diarrhoea is likely a common long-term complication after colon cancer surgery. Attention to this complication and a specific diagnosis will aid the targeted treatment of patients suffering from this complication.

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Bile acid diarrhoea is a chronic condition caused by increased delivery of bile acids to the colon. The underlying mechanisms remain to be elucidated. To investigate genes involved in bile acid diarrhoea, systems-level analyses were employed on a rat bile acid diarrhoea model. Twelve male Wistar Munich rats, housed in metabolic cages, were fed either control or bile acid-mixed (1% w/w) diets for ten days. Food intake, water intake, urine volume, bodyweight and faecal output were monitored daily. After euthanasia, colonic epithelial cells were isolated using calcium-chelation and processed for systems-level analyses, i.e. RNA-sequencing transcriptomics and mass spectrometry proteomics. Bile acid-fed rats suffered diarrhoea, indicated by increased drinking, faeces weight and faecal water content compared with control rats. Urine output was unchanged. With bile acid-feeding, RNA-sequencing revealed 204 increased and 401 decreased mRNAs; mass spectrometry 183 increased and 111 decreased proteins. Among the altered genes were genes associated with electrolyte and water transport (including Slc12a7, Clca4 and Aqp3) and genes associated with bile acid transport (Slc2b1, Abcg2, Slc51a, Slc51b and Fabps). Correlation analysis showed a significant positive correlation (Pearson's r=0.28) between changes in mRNA-expression and changes in protein-expression. However, caution must be exercised in making a direct correlation between experimentally determined transcriptomes and proteomes. Genes associated with bile acid transport responded to bile acid-feeding, suggesting that colonic bile acid transport also occur by regulated protein facilitated mechanisms in addition to passive diffusion. In summary, the study provides annotated rat colonic epithelial cell transcriptome and proteome with response to bile acid-feeding.

Bile acids (BAs) affect water transport in the colon, as evident in Bile acid malabsorption (BAM). In BAM patients, an increased production of BAs or a decrease in the capacity for BA reabsorption in the ileum cause BAs to overflow into the colon, where they lead to diarrhoea through mechanisms that are yet to be determined. Although there is evidence for expression of specific water channels (AQPs) in the colon, results from different studies have been contradictory. A connection between BAs and AQPs has not been described. Here we profiled mRNA and protein levels of AQPs in the colons of rodents and examined the short and long‐term effects of naturally occurring BAs on the expression of AQPs in human colonic T84 and HT29 epithelial cells. In colonic epithelial cells isolated from rats, AQP1, ‐3, ‐4, ‐7 and ‐8 were detected at the mRNA level by RT‐PCR, with AQP1,‐3 and ‐8 expression confirmed at the protein level by immunohistochemistry (IHC). In mouse AQP1, ‐4 and ‐8 mRNA and protein were detected by RT‐PCR and IHC. Compared to the proximal segment, AQP3, ‐4, ‐7, ‐8 mRNAs were expressed at higher levels in the distal half of the rat colon as seen by RT‐qPCR. In T84 and HT‐29 cells, the BA sodium deoxycholate increased AQP3‐mRNA expression in a time‐dependent manner. Furthermore, the BA sodium ursodeoxycholate or the Farnesoid X receptor agonist GW4064 increased AQP8 mRNA in T84 cells. In conclusion, AQPs likely have a spatiotemporal expression profile in the rodent large intestine. Cell line studies suggest that specific BAs may alter AQP expression in the large intestine, and as such AQPs may play a role in BAM syndromes. The study was supported by The Danish Council for Independent Research, MEMBRANES and Gangstedfonden.