Lecture 6: Receptor Signaling - Glucocorticoid Receptors

Module Overview

• Biochem 384, Module 8, Topic 6: Nuclear Receptor Signaling
• Focus on glucocorticoid receptors
• Presenter has over 20 years of experience studying glucocorticoid receptors.
  • First cloned the glucocorticoid receptor during postdoctoral research.
  • Background in cancer and asthma research involving glucocorticoid signaling.

Introduction to Glucocorticoids

• Glucocorticoids are steroid hormones derived from cholesterol.
  • General structure: Four-ring lipophilic molecule.
  • Other steroids include: Estrogen, androgens, aldosterone.
• Unique properties:
  • Have many physiological effects in the human body.
  • Heavily prescribed therapeutics, primarily due to anti-inflammatory properties.
  • Play roles in various diseases based on inflammation (e.g., autoimmune diseases).
• Physiological Roles:
  • Regulate homeostasis in the body.
  • Levels rise during stress as a defense mechanism, produced by adrenal glands.
  • Must be kept in check to maintain homeostasis.

Mechanism of Action of Glucocorticoid Receptors

Nuclear Receptors

• Type of transcription factors not located in cell membranes; reside in the nucleus or cytoplasm.
• Function by binding to ligands and interacting with DNA to regulate gene expression.
• Ligands activate nuclear receptors leading to gene activation or inhibition.

Structure of Nuclear Receptors

• Inactive form has three primary domains:
  • C Terminal Domain: Ligand binding domain.
  • N Terminal Domain: Interacts with co-regulatory proteins.
  • DNA Binding Domain: Binds to specific DNA sequences that regulate gene expression.
• Activation mechanism:
  • Ligand binds to ligand-binding domain, inducing conformational changes.
  • Nuclear receptor recruits co-regulatory proteins, which can remodel chromatin and interact with transcription machinery (e.g., RNA polymerase).

Families of Nuclear Receptors

• Two major families:
  1. Steroid Receptors:
     • Family includes glucocorticoids, estrogen, androgens, progesterones, and aldosterone.
     • Bind to palindromic nucleotide sequences, commonly as homodimers.
  2. Metabolite Receptors:
     • Includes receptors for retinoic acid, thyroid hormone, and vitamin D (hormones).
     • Bind to direct repeats and are often heterodimers.

Steroid Receptor Binding

Palindromic Sequences

• Co-regulatory protein interaction illustrated through a nucleotide frequency chart.
• Examples of palindromic sequences:
  • $ext{AGA ACA}$ (one side) and its complement $ext{TGT TCT}$ (reverse side).

Metabolite Receptor Binding

• Differences from steroid receptors include:
  • Ligand type (not steroid hormones).
  • Often form heterodimers, typically with RXR (Retinoid X Receptor).
  • Bind distinct gene regulatory sequences, known as direct repeats.

Examples of Nuclear Receptors and Co-regulatory Proteins

Glucocorticoid Receptor

• Located in the cytoplasm, associated with the heat shock protein HSP90.
  • HSP90 keeps the receptor in an inactive state.
• When glucocorticoids such as prednisone or dexamethasone bind, they dissociate HSP90.
• The activated glucocorticoid receptor enters the nucleus to regulate gene expression involving anti-inflammatory responses.

Metabolite Receptors

• PPAR (Peroxisome Proliferator-Activated Receptor) is pivotal in metabolism.
• RXR often partners with metabolite receptors, binding to various ligands like 9-cis retinoic acid.
• Importance of the heterodimer mechanism in ligand regulation and DNA interaction.

Protein Structures

• Structure of ligand-binding domain shows a specific interaction with co-regulatory peptides.
• Importance of zinc fingers:
  • Stabilize protein interactions and enable sequence-specific DNA binding.
• Co-regulatory protein contribution:
  • Bind close to ligand sites, linking transcription activation to ligand binding accurately.

Glucocorticoid Action and Therapeutic Implications

Physiological Interactions

• Natural glucocorticoid cortisol synthesized in adrenal glands; serves anti-inflammatory functions.
  • Cortisol
    • Relative binding affinity: 1
• Pharmaceutical drugs include:
  1. Triamcinolone: Five times the binding affinity of cortisol.
     • Administered for asthma as an anti-inflammatory.
  2. Prednisone: Commonly used, fivefold binding affinity enhancement.
  3. Dexamethasone: Highly potent, 50 times binding affinity; used in cancer treatments.
• Modifications to ligands:
  • Fluorine and methyl groups increase binding affinity, stability, and overall efficacy of glucocorticoids as drugs.

Mechanism of Anti-Inflammatory Action

• Process of glucocorticoid receptor signaling:
  • Glucocorticoids diffuse across cell membranes and bind inactive glucocorticoid receptors linked to HSP90 in the cytoplasm.
  • Activated receptors move to the nucleus and interact with genetic elements to affect expressions of inflammatory and anti-inflammatory genes.
    • Example genes include:
      • Annexin 1 gene (upregulated by glucocorticoid).
      • Cyclooxygenase-2 (COX-2) gene (downregulated by glucocorticoid).
• This dual regulation contributes to the reduction of inflammation in various disease contexts.