L16 - Pharmacogenomics of Drug Receptors: Beta-Two Adrenergic Receptor

Polygenic Effects and Drug Response

Inter-individual variability in drug response is shaped by polygenic effects, considering multiple genes rather than single genes.
Focus on the combined impact of variability in metabolism and drug receptors on overall drug responsiveness.
Example of genotype combinations in metabolism and drug receptor efficacy shows varying AUC (Area Under the Curve) and dose-response profiles.
Wild-type genotypes exhibit typical drug response, while heterozygous and homozygous variant genotypes show altered AUC and drug efficacy.
The overall individual response is a composite effect of metabolism and receptor genotypes, influencing both drug efficacy and toxicity.

Drug Receptors and Pharmacogenomics

Drug receptors are highly druggable targets, with more than half of prescribed drugs interacting with them.
G protein-coupled receptors (GPCRs) are the most common drug targets.
Different types of receptors include:
- Ion channels (e.g., acetylcholine).
- G protein-coupled receptors (e.g., adrenergic).
- Tyrosine kinase receptors (e.g., insulin)
- Intracellular receptors (e.g., nuclear receptors for steroids).
GPCRs span the cell membrane and activate intracellular G proteins upon agonist binding, leading to downstream signaling and pharmacological response.

Beta-Two Adrenergic Receptor

Focus of the lecture: the beta-two adrenergic receptor, a G protein-coupled receptor.
The receptor is polymorphic, with genetic variations affecting bronchodilator responsiveness.
Beta-two agonists: Adrenergic receptors are cell surface receptors coupled to G proteins.

Adrenergic Receptors and Their Function

Adrenergic receptors bind and are activated by endogenous ligands called catecholamines (adrenaline and noradrenaline).
Located on many cells and binding of an agonist to these receptors causes a sympathetic response (fight or flight) which leads to increased heart rate, dilated pupils, energy mobilization, and diversion of blood flow to skeletal muscles rather than non-essential organs.
Two main groups of adrenergic receptors exist:
- Alpha
- Beta (with subtypes beta-one, beta-two, and beta-three).
  - Beta one: associated with heart muscle contraction.
  - Beta two: associated with smooth muscle contraction in the lung.
  - Beta three: associated with lipolysis.
All three are linked to G proteins, which are linked to adenylate cyclase as the second messenger.

Structure of Beta-Two Adrenergic Receptor

Comprises seven transmembrane helices forming a ligand-binding pocket.
Extracellular region recognizes and binds ligands.
N-terminal tail and three extracellular loops.
Intracellular region (including C-terminal tail) communicates with the inside of the cell.
Site that binds the G protein sends the signal into the cell.
Specific action in smooth muscle relaxation in the bronchi, relevant to asthma.

Asthma and Beta-Two Adrenergic Receptor

Asthma is a common chronic airway disorder characterized by:
- Variable airway obstruction.
- Bronchial hyper-responsiveness/bronchospasm.
- Inflammation.
Treatment includes:
- Short and long-acting beta agonists
- Inhaled corticosteroids (to dampen inflammation).
Responses to therapy are bronchodilation, reduced inflammation, decreased bronchial responsiveness, and fewer exacerbations, BUT responses vary among individuals.
Beta-two adrenergic receptor variants do not contribute to asthma susceptibility but are associated with medication response.

Beta-Two Adrenergic Receptor Polymorphisms

At least 17 SNPs exist in regulatory and coding regions.
Regulatory regions: affect protein expression levels.
Coding regions: impact amino acid sequence.
Five polymorphisms in the coding region are degenerate (no amino acid change).
Four polymorphisms cause amino acid substitutions at positions 16, 27, 164 and 304.
Common SNPs occur at nucleotide positions 46 (guanine to adenine) and 79 (cytosine to guanine).
Resulting in:
- Glycine for arginine at codon 16.
- Glutamic acid for glutamine at position 27.

Population Frequencies of Genotypes:

Homozygous for arginine: 20%.
Heterozygous: 38%.
Homozygous for glycine: 45%.
Homozygous for glutamic acid: 25%.
Heterozygous: 49%.
Homozygous for glutamine: 26%.
Isoleucine substitution for threonine at position 164:
- Results in decreased binding affinity.
- Rare variant (potentially lethal when homozygous in animal models).

Agonist-Induced Downregulation

SNPs alter the degree of agonist-induced downregulation.
Chronic exposure to an agonist can cause:
- Uncoupling of the G protein.
- Internalization of the receptor i.e removing the receptor from the surface of the cell, where it is no longer available for the drug to act on.
Homozygous glycine individuals are more susceptible to agonist-induced downregulation.
Downregulation manifests as a poor response to beta-two agonists.
Heterozygous state shows an intermediate effect.
Glycine variant is associated with enhanced Receptor down regulation leading to variable responses to beta-two agonists.
Glutamic acid variant is resistant to down regulation.
Isoleucine to threonine substitution can severely alter receptor function and is linked to poor treatment outcomes.

Population Frequencies and Molecular Mechanisms

Glycine to arginine substitution: mainly in African American populations (60-70%).
Glutamine to glutamic acid substitution: mainly in European/Caucasian populations.
Isoleucine to threonine: rare across all ethnicities.
Molecular mechanisms of receptor down regulation:
- Uncoupling of G protein from receptor.
- Internalization and degradation of receptor.
- Inhibition of downstream transcription.
- Agonist induced receptor down regulation occurs when pro-longed exposure to an agonist leads to a decrease in the number of available receptors on the surface, thus decreasing receptor density and making the cell less sensitive to the agonist.

Endocytosis

The receptor is internalized via a vesicle where that vesicle will ultimately lies with a lysosome and then that internalized receptor is degraded.
Downstream Inhibition of transciption.
Agonists can also impact the trafficking of receptors within a cell:
- Newly made proteins are made in the nucleus and traffic to the cell surface where they will ultimately sit. Over time, the agonist in the cell can also alter the trafficking process within the cell impacting the availability of the receptors on the cell surface.
Pharmacological consequences:
- Cell becomes less sensitive to agonist (higher doses or concentrations needed).
- Tolerance develops (gradual decrease in responsiveness).
- Changes in signaling pathways.

Clinical Impact of Beta-Two Adrenergic Receptor Polymorphisms

Lima et al. (1999) study: impact of genetic polymorphism on albuterol bronchodilator pharmacodynamics.
Moderate asthmatics (18-50 years) given 8 mg oral albuterol.
Measurements: genotype, plasma concentration, and forced expiratory volume (FEV1).
Individuals that are homozygous for arginine have an excellent pharmacodynamic response. Individuals that are either heterozygous, or homozygous variant have a significantly reduced FEV1 volume, i.e aren't achieving the same level a bronchodialation with the administration of albuterol, and the same dose.
Plasma concentration of albuterol in all populations remains the same so results are not the result of pharmacokinetic response but rather a response that is happening at the level of the receptor. I.e it is a pharmacodynamic response.
Individuals are achieving the same level of drug disposition in the body, it is the response that varies.
Receptor genotype:
- Homozygous for arginine: value of 1.42
- Heterozygous genotype: value drops down to .55.
- The area under the pharmacodynamic curve is greater for those that are homozygous for arginine relative to those that have at least one variant.
Conclusion:
- Beta-two adrenergic receptor polymorphism: a major determinant of bronchodilator response to albuterol.
- Arginine 16 homozygotes: more responsive to agonist-induced bronchodilation due to less agonist-induced downregulation.