Beta-2 adrenergic receptor

The beta-2 adrenergic receptor (β2 adrenoreceptor), also known as ADRB2, is a cell membrane-spanning beta-adrenergic receptor that binds epinephrine (adrenaline), a hormone and neurotransmitter whose signaling, via adenylate cyclase stimulation through trimeric Gs proteins, increases cAMP, and, via downstream L-type calcium channel interaction, mediates physiologic responses such as smooth muscle relaxation and bronchodilation.[5]

ADRB2
Available structures
PDBOrtholog search: PDBe RCSB
Identifiers
AliasesADRB2, ADRB2R, ADRBR, B2AR, BAR, BETA2AR, adrenoceptor beta 2
External IDsOMIM: 109690 MGI: 87938 HomoloGene: 30948 GeneCards: ADRB2
Orthologs
SpeciesHumanMouse
Entrez

154

11555

Ensembl

ENSG00000169252

ENSMUSG00000045730

UniProt

P07550

P18762

RefSeq (mRNA)

NM_000024

NM_007420

RefSeq (protein)

NP_000015

NP_031446

Location (UCSC)Chr 5: 148.83 – 148.83 MbChr 18: 62.31 – 62.31 Mb
PubMed search[3][4]
Wikidata
View/Edit HumanView/Edit Mouse

Robert J.Lefkowitz[6] and Brian Kobilka[7] studied beta 2 adrenergic receptor as a model system which rewarded them the 2012 Nobel Prize in Chemistry[8] “for groundbreaking discoveries that reveal the inner workings of an important family of such receptors: G-protein-coupled-receptors”.

The official symbol for the human gene encoding the β2 adrenoreceptor is ADRB2.[9]

Gene

The ADRB2 gene is intronless. Different polymorphic forms, point mutations, and/or downregulation of this gene are associated with nocturnal asthma, obesity and type 2 diabetes.[10]

Structure

The 3D crystallographic structure (see figure and links to the right) of the β2-adrenergic receptor has been determined[11][12][13] by making a fusion protein with lysozyme to increase the hydrophilic surface area of the protein for crystal contacts. An alternative method, involving production of a fusion protein with an agonist, supported lipid-bilayer co-crystallization and generation of a 3.5 Å resolution structure.[14]

The crystal structure of the β2Adrenergic Receptor-Gs protein complex was solved in 2011. The largest conformational changes in the β2AR include a 14 Å outward movement at the cytoplasmic end of transmembrane segment 6 (TM6) and an alpha helical extension of the cytoplasmic end of TM5.[15]

Mechanism

This receptor is directly associated with one of its ultimate effectors, the class C L-type calcium channel CaV1.2. This receptor-channel complex is coupled to the Gs G protein, which activates adenylyl cyclase, catalysing the formation of cyclic adenosine monophosphate (cAMP) which then activates protein kinase A, and counterbalancing phosphatase PP2A. Protein kinase A then goes on to phosphorylate (and thus inactivate) myosin light-chain kinase, which causes smooth muscle relaxation, accounting for the vasodilatory effects of beta 2 stimulation. The assembly of the signaling complex provides a mechanism that ensures specific and rapid signaling. A two-state biophysical and molecular model has been proposed to account for the pH and REDOX sensitivity of this and other GPCRs.[16]

Beta-2 adrenergic receptors have also been found to couple with Gi, possibly providing a mechanism by which response to ligand is highly localized within cells. In contrast, Beta-1 adrenergic receptors are coupled only to Gs, and stimulation of these results in a more diffuse cellular response.[17] This appears to be mediated by cAMP induced PKA phosphorylation of the receptor.[18] Interestingly, Beta-2 adrenergic receptor was observed to localize exclusively to the T-tubular network of adult cardiomyocytes, as opposed to Beta-1 adrenergic receptor, which is observed also on the outer plasma membrane of the cell [19]

Function
Function Tissue Biological Role
Smooth muscle relaxation in: GI tract (decreases motility) Inhibition of digestion
Bronchi[20] Facilitation of respiration.
Detrusor urinae muscle of bladder wall[21][22] This effect is stronger than the alpha-1 receptor effect of contraction. Inhibition of need for micturition
Uterus Inhibition of labor
Seminal tract[23]
Increased perfusion and vasodilation Blood vessels and arteries to skeletal muscle including the smaller coronary arteries[24] and hepatic artery Facilitation of muscle contraction and motility
Increased mass and contraction speed Striated muscle[23]
Insulin and glucagon secretion Pancreas[25] Increased blood glucose and uptake by skeletal muscle
Glycogenolysis[23]
Tremor Motor nerve terminals.[23] Tremor is mediated by PKA mediated facilitation of presynaptic Ca2+ influx leading to acetylcholine release.
Legend
  The function facilitates the fight-or-flight response.

Musculoskeletal system

Activation of the β2 adrenoreceptor with long-acting agents such as oral clenbuterol and intravenously-infused albuterol results in skeletomuscular hypertrophy and anabolism.[26][27] The comprehensive anabolic, lipolytic, and ergogenic effects of long-acting β2 agonists such as clenbuterol render them frequent targets as performance-enhancing drugs in athletes.[28] Consequently, such agents are monitored for and generally banned by WADA (World Anti-Doping Agency) with limited permissible usage under therapeutic exemptions; clenbuterol and other β2 adrenergic agents remain banned not as a beta-agonist, but rather an anabolic agent. These effects are largely attractive within agricultural contexts insofar that β2 adrenergic agents have seen notable extra-label usage in food-producing animals and livestock. While many countries including the United States have prohibited extra-label usage in food-producing livestock, the practice is still observed in many countries. [29][30]

Circulatory system

Eye

In the normal eye, beta-2 stimulation by salbutamol increases intraocular pressure via net:

In glaucoma, drainage is reduced (open-angle glaucoma) or blocked completely (closed-angle glaucoma). In such cases, beta-2 stimulation with its consequent increase in humour production is highly contra-indicated, and conversely, a topical beta-2 antagonist such as timolol may be employed.

Digestive system

Other
  • Inhibit histamine-release from mast cells.
  • Increase protein content of secretions from lacrimal glands.
  • Receptor also present in cerebellum.
  • Bronchiole dilation (targeted while treating asthma attacks)
  • Involved in brain - immune - communication [31]

Ligands

Agonists
Beta-2 adrenergic receptor
Transduction mechanismsPrimary: Gs
Secondary: Gi/o
Primary endogenous agonistsepinephrine, norepinephrine
Agonistsisoprenaline, salbutamol, salmeterol, others
Antagonistscarvedilol, propranolol, labetalol, others
Inverse agonistsN/A
Positive allosteric modulatorsZn2+ (low concentrations)
Negative allosteric modulatorsZn2+ (high concentrations)
External resources
IUPHAR/BPS29
DrugBankP07550
HMDBHMDBP01634
Main article: Beta2-adrenergic agonist

Spasmolytics used in asthma and COPD

Tocolytic agents

β2 agonists used for other purposes

Antagonists

(Beta blockers)

* denotes selective antagonist to the receptor.

Allosteric modulators
  • compound-6FA,[33] PAM at intracellular binding site

Interactions

Beta-2 adrenergic receptor has been shown to interact with:

See also

References
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Further reading
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