The pregnane X receptor (PXR) is the molecular target for catatoxic steroids such as pregnenolone 16␣-carbonitrile (PCN), which induce cytochrome P450 3A (CYP3A) expression and protect the body from harmful chemicals. In this study, we demonstrate that PXR is activated by the toxic bile acid lithocholic acid (LCA) and its 3-keto metabolite. Furthermore, we show that PXR regulates the expression of genes involved in the biosynthesis, transport, and metabolism of bile acids including cholesterol 7␣-hydroxylase (Cyp7a1) and the Na ؉ -independent organic anion transporter 2 (Oatp2). Finally, we demonstrate that activation of PXR protects against severe liver damage induced by LCA. Based on these data, we propose that PXR serves as a physiological sensor of LCA, and coordinately regulates gene expression to reduce the concentrations of this toxic bile acid. These findings suggest that PXR agonists may prove useful in the treatment of human cholestatic liver disease.

Steroid hormones exert profound effects on differentiation, development, and homeostasis in higher eukaryotes through interactions with nuclear receptors. We describe a novel orphan nuclear receptor, termed the pregnane X receptor (PXR), that is activated by naturally occurring steroids such as pregnenolone and progesterone, and synthetic glucocorticoids and antiglucocorticoids. PXR exists as two isoforms, PXR.1 and PXR.2, that are differentially activated by steroids. Notably, PXR.1 is efficaciously activated by pregnenolone 16alpha-carbonitrile, a glucocorticoid receptor antagonist that induces the expression of the CYP3A family of steroid hydroxylases and modulates sterol and bile acid biosynthesis in vivo. Our results provide evidence for the existence of a novel steroid hormone signaling pathway with potential implications in the regulation of steroid hormone and sterol homeostasis.

Synaptic clustering of neurotransmitter receptors is crucial for efficient signal transduction and integration in neurons. PDZ domain-containing proteins such as PSD-95/SAP90 interact with the intracellular C termini of a variety of receptors and are thought to be important in the targeting and anchoring of receptors to specific synapses. Here, we show that PICK1 (protein interacting with C kinase), a PDZ domain-containing protein, interacts with the C termini of alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA) receptors in vitro and in vivo. In neurons, PICK1 specifically colocalizes with AMPA receptors at excitatory synapses. Furthermore, PICK1 induces clustering of AMPA receptors in heterologous expression systems. These results suggest that PICK1 may play an important role in the modulation of synaptic transmission by regulating the synaptic targeting of AMPA receptors.

Localizing cell surface receptors to specific subcellular positions can be critical for their proper functioning, as most notably demonstrated at neuronal synapses. PDZ proteins apparently play critical roles in such protein localizations. Receptor tyrosine kinases have not been previously shown to interact with PDZ proteins in vertebrates. We report that Eph receptors and their membrane-linked ligands all contain PDZ recognition motifs and can bind and be clustered by PDZ proteins. In addition, we find that Eph receptors and ligands colocalize with PDZ proteins at synapses. Thus, PDZ proteins may play critical roles in localizing vertebrate receptor tyrosine kinases and/or their ligands and may be particularly important for Eph function in guidance or patterning or at the synapse.