Impaired localisation and transport function of canalicular Bsep in taurolithocholate induced cholestasis in the rat

Abstract

Background: Taurolithocholate induced cholestasis is a well established model of drug induced cholestasis with potential clinical relevance. This compound impairs bile salt secretion by an as yet unclear mechanism. Aims: To evaluate which step/s of the hepatocellular bile salt transport are impaired by taurolithocholate, focusing on changes in localisation of the canalicular bile salt transporter, Bsep, as a potential pathomechanism. Methods: The steps in bile salt hepatic transport were evaluated in rats in vivo by performing pharmacokinetic analysis of 14 C taurocholate plasma disappearance. Bsep transport activity was determined by assessing secretion of 14 C taurocholate and cholyl-lysylfluorescein in vivo and in isolated rat hepatocyte couplets (IRHC), respectively. Localisation of Bsep and F-actin were assessed both in vivo and in IRHC by specific fluorescent staining. Results: In vivo pharmacokinetic studies revealed that taurolithocholate (3 µmol/100 g body weight) diminished by 58% canalicular excretion and increased by 96% plasma reflux of 14 C taurocholate. Analysis of confocal images showed that taurolithocholate induced internalisation of Bsep into a cytosolic vesicular compartment, without affecting F-actin cytoskeletal organisation. These effects were reproduced in IRHC exposed to taurolithocholate (2.5 µM). Preadministration of dibutyryl-cAMP, which counteracts taurolithocholate induced impairment in bile salt secretory function in IRHC, restored Bsep localisation in this model. Furthermore, when preadministered in vivo, dibutyryl-cAMP accelerated recovery of both bile flow and bile salt output, and improved by 106% the cumulative output of 14 C taurocholate. Conclusions: Taurolithocholate impairs bile salt secretion at the canalicular level. Bsep internalisation may be a causal factor which can be prevented by dibutyryl-cAMP.

The Role of the Sodium-Taurocholate Cotransporting Polypeptide (NTCP) and of the Bile Salt Export Pump (BSEP) in Physiology and Pathophysiology of Bile Formation

“…Supporting this assumption, relocation of Bsep into subapical vesicles has been observed in rats with estradiol-17β-glucuronide induced cholestasis [53]. Internalization of Bsep was also observed in rats treated with lithocholate and taurolithocholate, which can be prevented by preadministration of cAMP [52,54] or of tauroursodeoxycholate [62]. A modest down-regulation of Bsep is also observed in primary cultured rat hepatocytes, which display a cholestatic expression pattern of organic anion transporters [261].…”

Role of microtubules in estradiol-17β-d-glucuronide-induced alteration of canalicular Mrp2 localization and activity

“…Thus it is possible that inhibition of recovery of bile acid-dependent bile flow by colchicine also contributed to the substantial delay observed in restoration of total bile flow. Finally, the current data suggest that therapeutic strategies based on stimulation of microtubuledependent vesicular transport could be effective for treatment of cholestasis induced by drugs, such as monohydroxylated bile salts (3,10) and antidepressants (30), which promote retrieval of canalicular transporters.…”

The bile salt export pump

“…If these rats are treated with 6-ethyl chenodeoxycholate, a potent ligand for FXR, Bsep is induced and the cholestasis induced by estradiol is reversed [27]. And finally, treatment of rats with the cholestatic bile acids lithocholate or taurolithocholate leads to a retrieval of Bsep from the canalicular plasma membrane [19], which can be prevented by the administration of cAMP [19], silibinin [17], or tauroursodeoxy cholic acid [21].…”