# The Nuclear Receptor PXR in Chronic Liver Disease
## 2 PXR Structure, Expression, and Activation  
PXR is also named NR1I2 (nuclear receptor subfamily 1 group I member 2) and SXR (steroid and xenobiotic receptor) since it is activated by steroid-like molecules, such as pregnane 21-carbon steroids [1]. PXR is predominantly located in the gut and in the liver, where it exerts its main role of modulating the transcription of hepatic genes encoding for phase I-II drug metabolizing enzymes (CYP3A, CYP2B, UDP-glucuronosyltransferaseases, sulfotransferase) and drug transporters, such as multidrug resistance proteins (MDRs) and multidrug resistance-associated proteins (MRPs) [7; 13; 14]. In the human liver, more than 15 splicing and transcript variants of PXR have been observed and correlated to alterations in its transcriptional activity. The predominant isoforms found in human liver are PXR1, which is the full-length isoform, and PXR2 and PXR3. PXR2 represents 6-15% of hepatic mRNA transcripts, whereas PXR3 is poorly expressed, representing 0-0.84% of all human PXR transcripts [15]. PXR variability has been shown to contribute to the interindividual variability in the expression of drug metabolizing enzymes and transporters (DMETs) [12]. For example, a small PXR variant (_sPXR_), encoding for a 37 kDa dominant negative isoform, is able to repress the function of the full-length PXR1 by competing with cofactors (e.g., SR1) for binding [16]. An inter-species variability has also been described. Two PXR isoforms, PXR1 and PXR2, are expressed in mice, the latter displaying a lower ligand activation profile with respect to PXR1 and directly repressing PXR1 activity [17]. As in other NRs, PXR is endowed with three domains, i.e., the N-terminus ligand-independent transactivation domains (AF-1), the DNA-binding domain (DBD), and a ligand-binding region (LBD) at the C-terminus, which is also responsible for the heterodimer with retinoid X receptor (RXR) (Figure 1a). An additional ligand-dependent transactivation domain, named activation function 2 (AF-2), is located within the ligand binding domain [1; 5; 18]. Beyond the presence of PXR splicing and transcript variants, the finely tuned regulation of PXR transcriptional activity is due to the great variety of its ligands, including endobiotic compounds (pregnane, steroids, bile acids, vitamins) and xenobiotics (macrolide antibiotics, antifungals, toxins, and environmental pollutants) [4; 14]. The capability of recognizing and binding a wide range of compounds is probably due to its structure, in particular to the great flexibility of its LBD [1]. As for other NRs, the first event leading to PXR activation consists in the binding of the ligand to the receptor LBD [19]. This binding site, normally characterized by an \"\(\alpha\)-helical sandwich\", has a unique five-stranded \(\beta\)-sheet that acts as a homodimerization site, forming tryptophan-zipper interactions [1; 18]. Mutations of specific tryptophan residues (Trp223 and 225) within the structure of the PXR homodimer interface reduces its ability to interact with the steroid receptor coactivator-1 (SRC-1), leading to an impaired transcriptional activity of PXR [18; 19]. The flexible and large ligand-binding pocket of PXR (from 1000 A to 1540 A, after ligand binding) is characterized by four distinct regions, i.e., a hydrophobic cage, a polar margin, a hydrophobic portion, and a polar duct [7; 18; 20]. As stated before, xenobiotics could take advantage of LBD flexibility to interact with the PXR receptor. For example, rifampicin interacts with 18 residues of the LBD, forming hydrogen bonds with Ser247, His407, and Gln285, while the HIV reverse transcriptase inhibitor PNU-142721 uses the hydrophobic cage to connect with PXR, through a \(\pi\)-\(\pi\) bond with Phe288 [18]. Some species-related differences observed in the affinity of PXR ligands could be ascribable to the low homology (73%) of the ligand-binding pocket of PXR between human and mouse [21]. Consequently, rifampicinshows a high affinity towards human but not murine PXR, while PCN is a potent activator of PXR in mice.