Human PXR Forms a Tryptophan Zipper-Mediated Homodimer †

The human nuclear receptor pregnane X receptor (PXR) responds to a wide variety of potentially harmful chemicals and coordinates the expression of genes central to xenobiotic and endobiotic metabolism. Structural studies reveal that the PXR ligand binding domain (LBD) uses a novel sequence insert to form a homodimer unique to the nuclear receptor superfamily. Terminal β-strands from each monomeric LBD interact in an ideal antiparallel fashion to bury potentially exposed surface β-strands, generating a ten-stranded intermolecular β-sheet. Conserved tryptophan and tyrosine residues lock across the dimer interface and provide the first tryptophan-zipper (Trp-Zip) interaction observed in a native protein. We show using analytical ultracentrifugation that the PXR LBD forms a homodimer in solution. We further find that removal of the interlocking aromatic residues eliminates dimer formation but does not affect PXR's ability to interact with DNA, RXRα, or ligands. Disruption of the homodimer significantly reduces receptor activity in transient transfection experiments, however, and effectively eliminates the receptor's recruitment of the transcriptional coactivator SRC-1 both in vitro and in vivo. Taken together, these results suggest that the unique Trp-Zip-mediated PXR homodimer plays a role in the function of this nuclear xenobiotic receptor

Insulin-like growth factor I binding in hepatocytes from human liver human hepatoma and normal regenerating and fetal rat liver.

Insulin-like growth factor-I (IGF-I) in human hepatoma cells (HEP-G2) has in addition to its effect on cell growth shortterm metabolic effects acting through its own receptor. We have demonstrated that normal human hepatocytes compared with HEP-G2 cells have virtually no IGF-I binding sites. Because the rate of growth is the major difference between the hepatoma and the normal liver we asked if normal liver might express IGF-I binding sites under physiologic growth conditions. Indeed whereas adult rat hepatocytes have low IGF-I binding sites similar to those in human liver hepatocytes from regenerating liver after 3 d subtotal hepatectomy have an approximately sixfold increase (P < 0.005) and those from fetal rat liver a - 12-fold increase (P < 0.005) to levels comparable to those in the HEP-G2 cells. The specificity of 125I IGF-I binding to its receptor was demonstrated by competition studies with monoclonal antibodies directed toward the IGF-I and the insulin receptors with unlabeled IGF-I and insulin and by affinity labeling experiments. Thus if IGF-I has any shortterm metabolic functions in the adult human liver it is not through interaction with its own receptor. Autocrine regulation by IGF-I of liver growth appears possible since IGF-I binding sites are expressed under pathological and physiological conditions of growth. The mechanism that couples these two phenomena remains to be elucidated. Originally published Journal of Clinical Investigation Vol. 81 No. 4 Apr 198