Regulation of hepatic bile acid transporters Ntcp and Bsep expression
Introduction
Bile acids secreted into bile undergo enterohepatic circulation (97%). Both hepatic uptake and biliary excretion of bile acids are driven by transport proteins. Bile acid uptake from blood into liver is mediated mainly by the Na+-taurocholate cotransporting polypeptide (Ntcp), and efflux from liver into bile by the bile salt export pump (Bsep).
Ntcp (gene symbol Slc10a1) has been cloned in various species [1], [2], [3], [4], and localized to the basolateral membrane of hepatocytes. Ntcp transports all physiological bile acids [5], [6], [7]. Bsep (gene symbol Abcb11) is responsible for the canalicular excretion of bile acids [8], [9]. Targeted inactivation of the mouse Bsep gene results in mild, non-progressive, but persistent intrahepatic cholestasis [10]. In contrast, a mutation in human BSEP gene is responsible for progressive familial intrahepatic cholestasis subtype 2 (PFIC-2) [11], [12].
Ntcp mRNA and protein expression is uniformly down-regulated in all experimental models of cholestasis and liver disease [13], [14], [15], [16], [17]. In contrast, Bsep expression is only modestly impaired during cholestasis even with complete bile-duct obstruction [16], [18].
Bile acids in general are thought to decrease Ntcp expression through farnesoid X receptor (FXR)-small heterodimer partner (Shp) pathway, and increase Bsep expression through direct FXR activation [19], [20], [21].
In rats, Ntcp mRNA expression is male-predominant, which is due to the inhibitory effect of female-pattern GH secretion [22]. However, it is not known whether mouse, rat, and human Ntcp/NTCP expression share similar gender predominance and underlying regulatory mechanisms. Except for the influence of bile acids, the regulation of Ntcp and Bsep is not complete. A few studies have been performed in rats, some in humans, but less in mice. Therefore, in the present study, we determined the effects of age, gender, and various chemicals, including microsomal enzyme inducers, cholestyramine, and chenodeoxycholic acid (CDCA) on the regulation of Ntcp and Bsep mRNA expression in mouse livers and evaluate their human relevance.
Section snippets
Materials
Sodium chloride, HEPES sodium salt, HEPES free acid, lithium lauryl sulfate, EDTA, and d-(+)-glucose were purchased from Sigma–Aldrich (St. Louis, MO). Micro-O-protect was purchased from Roche Diagnostics (Indianapolis, IN). Formaldehyde, 3-(N-morpholino)propanesulfonic acid, sodium citrate, and NaHCO3 were purchased from Fischer Chemicals (Fairlawn, NJ). Chloroform, agarose, and ethidium bromide were purchased from AMRESCO Inc. (Solon, OH). 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) was a gift
Constitutive expression of Ntcp and Bsep in adult male and female mouse and human livers
Both Ntcp/NTCP and Bsep/BSEP have been shown to be highly expressed in mouse and human livers [1], [18], [29], [30]. Ntcp protein expression is 60% higher in female than male mouse liver (Fig. 1a), and Ntcp mRNA is 70% higher in female than male mouse livers (Fig. 1b). In human livers, NTCP mRNA is 43% higher in women than men; however, it is not statistically different due to high variation in individual human NTCP expression. In contrast, no gender differences exist in mouse Bsep expression
Discussion
The goal of the present study was to obtain systematic information on the regulation of Ntcp and Bsep, two critical bile acid transporters in mouse and human livers. There are numerous reports on the regulation of Ntcp [19], [32], [33], [34] and Bsep [20], [32], [34] by bile acids. The focus of this study was to characterize the ontogenic and gender-related constitutive expression of these two hepatic bile acid transporters, as well as examine the regulation of Ntcp and Bsep by microsomal
Acknowledgements
This work was supported by NIH grants RR021940, ES09649, and ES09716.
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