The Interface between Cell Signaling Pathways and Pregnane X Receptor

Abstract

Highly expressed in the enterohepatic system, pregnane X receptor (PXR, NR1I2) is a well-characterized nuclear receptor (NR) that regulates the expression of genes in the liver and intestines that encode key drug metabolizing enzymes and drug transporter proteins in mammals. The net effect of PXR activation is to increase metabolism and clear drugs and xenobiotics from the body, producing a protective effect and mediating clinically significant drug interaction in patients on combination therapy. The complete understanding of PXR biology is thus important for the development of safe and effective therapeutic strategies. Furthermore, PXR activation is now known to specifically transrepress the inflammatory- and nutrient-signaling pathways of gene expression, thereby providing a mechanism for linking these signaling pathways together with enzymatic drug biotransformation pathways in the liver and intestines. Recent research efforts highlight numerous post-translational modifications (PTMs) which significantly influence the biological function of PXR. However, this thrust of research is still in its infancy. In the context of gene-environment interactions, we present a review of the recent literature that implicates PXR PTMs in regulating its clinically relevant biology. We also provide a discussion of how these PTMs likely interface with each other to respond to extracellular cues to appropriately modify PXR activity.

Keywords: PARylation; SUMOylation; acetylation; cell signaling; nuclear receptor; phosphorylation; pregnane X receptor; ubiquitination; xenobiotics.

Figure 1

A pictorial diagram of the types of post-translational modification of pregnane X receptor (PXR, NR1I2). Created with BioRender.com (10 November 2021).

Figure 2

PXR is ubiquitinated in cells. The plasmid encoding 6x-Histidine-tagged PXR (His-PXR) protein was transfected alone or together with an expression vector encoding ubiquitin (Ub) into HeLa cells. Twenty-four (24) hours post-transfection, cells were treated for an additional 24 h (VEH = 0.1% DMSO, Rif = 10 μM, MG132 = 25 μM). Total cell extract was subjected to purification using nickel-linked agarose beads, followed by SDS-PAGE and subsequent Western blotting with anti-PXR antibodies (Santa Cruz-H11).

Figure 3

Poly-ubiquitination of the PXR protein is stimulated by the MEKK signaling pathway. The plasmid encoding the 6x-His-tagged ubiquitin protein and indicated mutant Ub expression vectors were co-transfected into HeLa cells with the expression vector encoding the PXR protein together with a constitutively active form of MEKK (a.a. 380–672), which activates JNK signaling cascade, as indicated. Total cell extract was subjected to purification using nickel-linked agarose beads, followed by SDS-PAGE and Western blotting using a monoclonal antibody against the human PXR protein (Santa Cruz, H-11).

Figure 4

This scheme provides the specific sites and types of PXR PTMs and their associated references.