Regulation of taurocholate transport in freshly isolated proximal tubular cells of the rat kidney by protein kinases

Combined Untargeted and Targeted Metabolomics Approaches Reveal Urinary Changes of Amino Acids and Energy Metabolism in Canine Babesiosis With Different Levels of Kidney Function

Canine babesiosis is a tick-borne disease with a worldwide distribution, caused by the haemoprotozoan parasites of the genus Babesia. One of the most prevalent complication is acute kidney injury, and an early diagnosis of altered kidney function remains a challenge for veterinary practice. The aim of this study was to assess the urine metabolic profile from dogs with babesiosis and different degree of kidney function using untargeted and targeted MS-based metabolomics approaches. In this study, 22 dogs naturally infected with Babesia canis and 12 healthy dogs were included. Untargeted metabolomics approach identified 601 features with a differential abundance between the healthy group and groups of dogs with babesiosis and different level of kidney function, with 27 of them identified as a match to known standards; while targeted approach identified 17 metabolites with significantly different concentrations between the groups. A pattern of significantly altered metabolites referring to the inflammatory host response, oxidative stress, and energy metabolism modulation in babesiosis was presented. Our findings have demonstrated that kidney dysfunction accompanying canine babesiosis was associated with changes in amino acid metabolism, energy metabolism, fatty acid metabolism, and biochemical pathways such as urea cycle and ammonia detoxication. These findings will enable the inclusion of urinary markers for the detection and monitoring of renal damage in babesiosis, as well as in other similar diseases.

Modulation of ileal bile acid transporter (ASBT) activity by depletion of plasma membrane cholesterol: association with lipid rafts

Apical sodium-dependent bile acid transporter (ASBT) represents a highly efficient conservation mechanism of bile acids via mediation of their active transport across the luminal membrane of terminal ileum. To gain insight into the cellular regulation of ASBT, we investigated the association of ASBT with cholesterol and sphingolipid-enriched specialized plasma membrane microdomains known as lipid rafts and examined the role of membrane cholesterol in maintaining ASBT function. Human embryonic kidney (HEK)-293 cells stably transfected with human ASBT, human ileal brush-border membrane vesicles, and human intestinal epithelial Caco-2 cells were utilized for these studies. Floatation experiments on Optiprep density gradients demonstrated the association of ASBT protein with lipid rafts. Disruption of lipid rafts by depletion of membrane cholesterol with methyl-beta-cyclodextrin (MbetaCD) significantly reduced the association of ASBT with lipid rafts, which was paralleled by a decrease in ASBT activity in Caco-2 and HEK-293 cells treated with MbetaCD. The inhibition in ASBT activity by MbetaCD was blocked in the cells treated with MbetaCD-cholesterol complexes. Kinetic analysis revealed that MbetaCD treatment decreased the V(max) of the transporter, which was not associated with alteration in the plasma membrane expression of ASBT. Our study illustrates that cholesterol content of lipid rafts is essential for the optimal activity of ASBT and support the association of ASBT with lipid rafts. These findings suggest a novel mechanism by which ASBT activity may be rapidly modulated by alterations in cholesterol content of plasma membrane and thus have important implications in processes related to maintenance of bile acid and cholesterol homeostasis.

Bile acid transporters: structure, function, regulation and pathophysiological implications

Specific transporters expressed in the liver and the intestine, play a critical role in driving the enterohepatic circulation of bile acids. By preserving a circulating pool of bile acids, an important factor influencing bile flow, these transporters are involved in maintaining bile acid and cholesterol homeostasis. Enterohepatic circulation of bile acids is fundamentally composed of two major processes: secretion from the liver and absorption from the intestine. In the hepatocytes, the vectorial transport of bile acids from blood to bile is ensured by Na+ taurocholate co-transporting peptide (NTCP) and organic anion transport polypeptides (OATPs). After binding to a cytosolic bile acid binding protein, bile acids are secreted into the canaliculus via ATP-dependent bile salt excretory pump (BSEP) and multi drug resistant proteins (MRPs). Bile acids are then delivered to the intestinal lumen through bile ducts where they emulsify dietary lipids and cholesterol to facilitate their absorption. Intestinal epithelial cells reabsorb the majority of the secreted bile acids through the apical sodium dependent bile acid transporter (ASBT) and sodium independent organic anion transporting peptide (OATPs). Cytosolic ileal bile acid binding protein (IBABP) mediates the transcellular movement of bile acids to the basolateral membrane across which they exit the cells via organic solute transporters (OST). An essential role of bile acid transporters is evident from the pathology associated with their genetic disruption or dysregulation of their function. Malfunctioning of hepatic and intestinal bile acid transporters is implicated in the pathophysiology of cholestatic liver disease and the depletion of circulating pool of bile acids, respectively. Extensive efforts have been recently made to enhance our understanding of the structure, function and regulation of the bile acid transporters and exploring new potential therapeutics to treat bile acid or cholesterol related diseases. This review will highlight current knowledge about structure, function and molecular characterization of bile acid transporters and discuss the implications of their defects in various hepatic and intestinal disorders.

Gender differences in renal tubular taurocholate transport

The bile acids filtered through the glomeruli nearly completely escape urinary excretion due to an efficient tubular reabsorption process. Reabsorption is mediated by the sodium-dependent bile acid transporter ASBT, which is localized in the brush border membranes of proximal tubular cells. The purpose of the present study was to assess whether tubular taurocholate transport is regulated by sex hormones. Clearance studies and studies on proximal tubular cells freshly isolated from rat kidneys were performed. The studies with the isolated proximal tubular cells revealed a cell to bath 3H-taurocholate accumulation ratio of 5.63+/-0.28 in male and of 3.67+/-0.43 in female rats (p<0.01). This difference in cellular taurocholate uptake was corroborated by the clearance studies, which showed a 3H-taurocholate clearance of 133.9+/-28.1 in male rats and of 262.0+/-45.4 microl/min x 100 g b.w. in female rats (p<0.05). Testosterone treatment of female rats did not significantly alter the cell to bath 3H-taurocholate accumulation ratio. However, the cellular taurocholate accumulation significantly decreased, by 61.6+/-10.1%, following ethinylestradiol treatment of male rats. Ovariectomy, chemical castration of female rats with buserelin or treatment of female rats with the estrogen receptor antagonist ICI 182780 did not affect taurocholate uptake, but treatment of ovariectomized rats with ethinylestradiol decreased the taurocholate accumulation ratio by 53.7+/-15.8%. By determination of serum bile acids the possibility was excluded that this change was an indirect effect of cholestasis induced by ethinylestradiol. This study demonstrates gender differences in the renal handling of taurocholate in rats that may be related to an inhibitory effect of estrogens on taurocholate transport in proximal tubular cells. Since the ASBT protein content of the proximal tubular cells was found not to be different between male and female rats, a nongenomic mechanism may underly this estrogen effect.