**3. Transcriptional Regulation**

BSEP expression is highly regulated by transcriptional mechanisms, and a wide interindividual variability has been described at the mRNA and protein levels [15].

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Expression of BSEP is regulated by a major ligand-activated transcription factor, farnesoid X receptor (FXR, NR1H4), which forms a signaling-competent nuclear receptor heterodimer with the retinoid X receptor (RXR) (Figure 2). Bile acids, such as chenodeoxycholic acid (CDCA), deoxycholic acid (DCA) and cholic acid (CA), are endogenous ligands of FXR with varying potential for activation [16–19]. Upon ligand binding, the FXR/RXR heterodimer binds to an FXR response element (FXRE) in the promoter region of BSEP, thereby inducing the expression of the transporter [20]. Additionally, components of the activating signal cointegrator-2-containing complex (ASCOM) interact with FXR to enhance BSEP expression. Ananthanarayanan and co-workers [21] showed that the recruitment of ASCOM to the BSEP promoter was disrupted in cholestasis, which was induced by common bile duct ligation. Furthermore, co-activator-associated arginine methyltransferase 1 (CARM1) also regulates FXR/RXR-dependent BSEP transcription [22]. Similarly, steroid receptor co-activator 2 (SRC2) knockout mice showed reduced expression of BSEP [23], indicating its involvement in transcriptional regulation of the transporter.

' ' ' ' ' ' **Figure 2.** Transcriptional regulation of BSEP via recognition sequences in the promoter region of the BSEP gene: MARE: Maf recognition element; LRHRE: liver receptor homolog-1 responsive element; FXRE: farnesoid X receptor responsive element. The farnesoid receptor FXR binds bile salts after heterodimerization with the retinoid X receptor. The ligand with the highest affinity for FXR is chenodeoxycholic acid (CDCA): however deoxycholic (DCA) and cholic acid (CA) also increase BSEP expression. The co-activators 'activating signal cointegrator-2-containing complex' (ASCOM), 'co-activator-associated arginine methyltransferase 1' (CARM1) and 'steroid receptor co-activator SRC2' (SRC2) increase BSEP expression via FXR. Liver receptor homolog 1 (LHR-1) and nuclear factor erythroid 2-related factor (Nrf-2), a sensor for oxidative stress, also increase BSEP expression.

Hepatocyte-specific liver receptor homolog-1 (LRH-1, NR5A2) is another transcription factor involved in modulation of BSEP expression. LRH-1 plays a supporting role for FXR [24]. The absence of LRH-1 is associated with reduced BSEP expression and an altered BA composition, with disappearance of CA and taurocholic acid (TCA) [25]. BSEP promoter activity is also stimulated by nuclear factor erythroid 2-related factor 2 (Nrf2), a positive transcriptional regulator, which acts as a sensor for oxidative stress. Nrf2 regulates the expression of BSEP, but also that of a number of hepatic phase I and II enzymes and other hepatic efflux transporters such as MRP3 (ABCC3) and MRP4 (ABCC4) [26].
