Phenobarbital in Nuclear Receptor Activation: An Update

Abstract

Phenobarbital (PB) is a commonly prescribed anti-epileptic drug that can also benefit newborns from hyperbilirubinemia. Being the first drug demonstrating hepatic induction of cytochrome P450 (CYP), PB has since been broadly used as a model compound to study xenobiotic-induced drug metabolism and clearance. Mechanistically, PB-mediated CYP induction is linked to a number of nuclear receptors, such as the constitutive androstane receptor (CAR), pregnane X receptor (PXR), and estrogen receptor α, with CAR being the predominant regulator. Unlike prototypical agonistic ligands, PB-mediated activation of CAR does not involve direct binding with the receptor. Instead, dephosphorylation of threonine 38 in the DNA-binding domain of CAR was delineated as a key signaling event underlying PB-mediated indirect activation of CAR. Further studies revealed that such phosphorylation sites appear to be highly conserved among most human nuclear receptors. Interestingly, while PB is a pan-CAR activator in both animals and humans, PB activates human but not mouse PXR. The species-specific role of PB in gene regulation is a key determinant of its implication in xenobiotic metabolism, drug-drug interactions, energy homeostasis, and cell proliferation. In this review, we summarize the recent progress in our understanding of PB-provoked transactivation of nuclear receptors with a focus on CAR and PXR. SIGNIFICANCE STATEMENT: Extensive studies using PB as a research tool have significantly advanced our understanding of the molecular basis underlying nuclear receptor-mediated drug metabolism, drug-drug interactions, energy homeostasis, and cell proliferation. In particular, CAR has been established as a cell signaling-regulated nuclear receptor in addition to ligand-dependent functionality. This mini-review highlights the mechanisms by which PB transactivates CAR and PXR.

Fig. 1.

Effects of PB on the brain and liver. In the brain, PB enhances GABA responses in the neurons by binding to the GABAA-receptor in the postsynaptic membrane, which increases synaptic inhibition and elevates seizure threshold. In the liver, PB transactivates a number of nuclear receptors through both direct and indirect mechanisms to alter the expression of genes associated with drug metabolism and disposition, lipid and glucose metabolism, and cell proliferation.

Fig. 2.

Mechanism of PB activation of CAR. PB antagonizes EGFR activity by directly binding to the receptor and competing with EGF, leading to decreased activity of the Scr kinase 1 which in turn reduces the phosphorylation of ERK1/2 and RACK1. This signaling alteration results in dissociation of the nonactive CAR homodimer and release of ERK1/2. The dephosphorylated RACK1 recruits PP2A to the CAR monomer to remove phosphor from Thr-38. Subsequently, the dephosphorylated CAR translocates into the nucleus, forms a heterodimer with RXR, binds to response elements containing DR4, DR5, or ER6, and triggers the expression of target genes, such as CYP2B6 and CYP3A4.

Fig. 3.

Mechanism of PB activation of PXR. PB can directly bind to hPXR through close interaction with the W299 residue within the ligand-binding pocket of hPXR. This interaction facilitates the recruitment of SRC-1 to the hPXR–RXR heterodimer and induces target gene expression (This figure was adopted with minor modification from Li et al., 2019, Mol. Pharmocol).