Structural insights into glucocorticoid receptor function

Abstract

The glucocorticoid receptor (GR) is a steroid hormone-activated transcription factor that binds to various glucocorticoid response elements to up- or down- regulate the transcription of thousands of genes involved in metabolism, development, stress and inflammatory responses. GR consists of two domains enabling interaction with glucocorticoids, DNA response elements and coregulators, as well as a large intrinsically disordered region that mediates condensate formation. A growing body of structural studies during the past decade have shed new light on GR interactions, providing a new understanding of the mechanisms driving context-specific GR activity. Here, we summarize the established and emerging mechanisms of action of GR, primarily from a structural perspective. This minireview also discusses how the current state of knowledge of GR function may guide future glucocorticoid design with an improved therapeutic index for different inflammatory disorders.

Keywords: DNA binding domain; glucocorticoid receptor; ligand binding domain; transactivation; transrepression.

Figure 1.. Molecular architecture and mechanism of action of glucocorticoid receptor (GR).

(A) Domain architecture of GR highlighting the N-terminal domain (NTD), DNA-binding domain (DBD), hinge region and ligand binding domain (LBD). (B) In the absence of ligand, GR associates with chaperones in the cytosol. Upon ligand binding, GR translocates into nucleus, interacts with its response elements, and recruits coregulators and histone-modifying enzymes to drive gene transactivation and transrepression.

Figure 2.. GR DBD interacting with different DNAs.

(A) Overall structure of the apo GR DBD with key subdomains highlighted (PDB: 6CFN). (B) GR DBD bound to conventional GRE as a dimer. Three key residues in the DNA helix form base-specific interaction with nucleotides in both strands (shown in the dark and light orange) from the consensus GRE sequence (middle). Key interactions in the D loop mediate the dimer formation (right) (PDB: 3FYL). (C) GR DBD bound to the TSLP nGRE without a dimer formation (left). Key interactions in the high affinity binding site are highlighted (right) (PDB: 4HN5). (D) GR DBD bound to the IL-11 AP1 response element (left) with only one monomer participating in base-specific interactions (right) (PDB: 5VA7). (E) GR DBD bound to the PLAU NF-kB response element complex(left) with the zoomed base-specific interaction (right) (PDB: 5E6A). (F) GR DBD bound to a pre-GBS (left) and methylated pre-GBS (middle) show the key ‘side-on’ hydrophobic interaction from methylated C (PDB: 6X6D and 6X6E, respectively). The conserved methyl-Arg-G triad is highlighted (right) (PDB: 3C2I). Hydrogen bonds are shown in red while the hydrophobic interactions are shown in black.

Figure 3.. GR LBD interacting with ligands and coregulators.

(A). Chemical structures of common GR ligands with the canonical numbering shown on the structure of cortisol. (B). Overall structure of GR LBD with cortisol (green) bound to Tif2 (light purple), with α-helices shown in light blue, β-strands in orange, and loops in gray (PDB: 1M2Z). (C) Extensive hydrogen bonds (dark blue residues) and hydrophobic interactions (light blue residues) are formed to accommodate cortisol binding. (D) Different GR coregulator binding modes. In the presence of cortisol (agonist), the LxxLL motif from the co-activator forms hydrophobic contacts with GR residues in the AF-2 site. Primary charge clamps mediated by Lys579 (from H3) and Glu755 (from AF-H) hold the peptide in place. In the presence of mifepristone (antagonist), the AF-H is displaced from the agonist-bound position, disrupting interaction with the charge clamp residues Glu755 (PDB: 3H52).

Figure 4.. Vamorolone reduces unwanted GR activity by disrupting a key hydrogen bond network that supports transactivation.

(A) A critical hydrogen bond formed between GR Asn564 and C11-OH of prednisolone (top) is lacking when vamorolone (bottom) binds. (B) Ligand-H12 allostery is rewired and weakened by Vamolone relative to a glucocorticoid bypassing the critical and conserved residue Asn564. (C) HDX shows that the GR conformation is more dynamic in solution when in complex with Vamorolone relative to the strong agonist prednisolone. The AF-2 site in particular is highly destabilized due to an inability of Vamorolone to support strong communication from the ligand binding pocket to this co-regulator interaction surface.