Ileal apical Na⁺-dependent bile acid transporter ASBT is upregulated in rats with diabetes mellitus induced by low doses of streptozotocin

Abstract

bile acid transporter ASBT is upregulated in rats with diabetes mellitus induced by low doses of streptozotocin. Am J Physiol Gastrointest Liver Physiol 299: G898 -G906, 2010. First published July 22, 2010; doi:10.1152/ajpgi.00139.2010.-Increased intestinal bile acid absorption and expansion of the bile acid pool has been implicated in the hypercholesterolemia associated with diabetes mellitus. However, the molecular basis of the increase in bile acid absorption in diabetes mellitus is not fully understood. The ileal apical Na ϩ -dependent bile acid transporter (ASBT) is primarily responsible for active reabsorption of the majority of bile acids. Current studies were designed to investigate the modulation of ASBT function and expression in streptozotocin (STZ)-induced diabetes mellitus in rats and to examine the effect of insulin on rat ASBT promoter by insulin. Diabetes mellitus was induced in Sprague-Dawley rats by intraperitoneal injection of low doses of STZ (20 mg/kg body wt) on five consecutive days. Human insulin (10 U/day) was given to a group of diabetic rats for 3 days before euthanasia. RNA and protein were extracted from mucosa isolated from the small intestine and ASBT expression was assessed by real-time quantitative RT-PCR and Western blotting. Our data showed that ASBT mRNA and protein expression were significantly elevated in diabetic rats. Insulin treatment of diabetic rats reversed the increase in ASBT protein expression to control levels. Consistently, ileal Na ϩ -dependent [ 3 H]taurocholic uptake in isolated intestinal epithelial cells was significantly increased in diabetic rats. In vitro studies utilizing intestinal epithelial Caco-2 cells demonstrated that ASBT expression and promoter activity were significantly decreased by insulin. These studies demonstrated that insulin directly influences ASBT expression and promoter activity and that ASBT function and expression are increased in rats with STZinduced diabetes mellitus. The increase in ASBT expression may contribute to disturbances in cholesterol homeostasis associated with diabetes mellitus. hypercholesterolemia; insulinopenia; enterohepatic circulation DIABETES MELLITUS IS A COMPLEX disorder resulting from insulin imbalance and characterized by dyslipidemia and hypercholesterolemia (18,34

N-glycosylation is essential for ileal ASBT function and protection against proteases

“…Interestingly, high levels of glucose increased the glycosylation of ASBT and enhanced its function. In this regard, we have previously shown that ASBT mRNA and protein expression were significantly elevated in rat model of diabetes mellitus (5). Our current studies provide novel evidence for a posttranslational mechanism that contributes to an increase in ASBT protein levels in response to hyperglycemia associated with diabetes mellitus.…”

“…Deglycosylation led to a decrease in ASBT function without affecting its targeting to plasma membrane (34); however, the exact mechanism by which the deglycosylation of ASBT alters its function remains elusive. Also, bile acid absorption was increased in diabetic patients and animal models of diabetes mellitus, and whether hyperglycemia affects ASBT function and N-glycosylation is not known (5,8,29).…”

The solute carrier family 10 (SLC10): Beyond bile acid transport

“…In the knock-out mice, this effect does not seem to be compensated by alternative mechanisms of BA absorption, such as passive absorption in the jejunum or in the colon (Dawson et al, 2003). While extrapolation from mice to humans should be done with caution due to fundamental differences between the two species, these observations support the notion that ASBT is the chief and rate-limiting step of bile acid absorption in the intestine (Annaba et al, 2010b). In contrast, we find a higher number of SLC10A1 SNPs, possibly because the resulting phenotype may be partly compensated for by other transporters capable of shuttling bile acids in the liver, such as OATP1B1 and OATP1B3.…”

“…In rat primary hepatocytes, TCA improved activity of the insulin receptor and activated the insulin signaling pathway PI3K/PDK-1/AKT, with a consequent stimulation of GPCR-dependent activity of glycogen synthase (Cao et al, 2010). In a rat model of streptozotocin-induced diabetes mellitus, TGR5-mediated insulin secretion in the ileum reduced ASBT at mRNA, protein and functional levels, and decreased Slc10a2 promoter activity in Caco-2 cells (Annaba et al, 2010b). Recently, Chen and collaborators at GlaxoSmithKline showed that ASBT inhibition with 264W94 elevated GLP-1, decreased hemoglobin A1c (HbA1c) (a marker for abnormally high sugar levels) and reduced plasma glucose in Zucker Diabetic Fatty (ZDF) rats.…”

S-acylation modulates the function of the apical sodium-dependent bile acid transporter in human cells

“…We next investigated whether ASBT is acylated in HEK 293 cells stably transfected with a human ASBT-V5 fusion protein (2BT cells). We have previously shown that ASBT-V5 in 2BT cells is functional and regulated in a similar manner to native ASBT (26,36). When acyl-RAC was performed in these cells, ~80% of ASBT-V5 bound to the resin, similar to what was seen in native tissues ( Figure 1D).…”