The β3-Adrenergic Receptor: Structure, Physiopathology of Disease, and Emerging Therapeutic Potential

Abstract

The discovery and characterization of the signal cascades of the β-adrenergic receptors have made it possible to effectively target the receptors for drug development. β-Adrenergic receptors are a class A rhodopsin type of G protein-coupled receptors (GPCRs) that are stimulated mainly by catecholamines and therefore mediate diverse effects of the parasympathetic nervous system in eliciting "fight or flight" type responses. They are detectable in several human tissues where they control a plethora of physiological processes and therefore contribute to the pathogenesis of several disease conditions. Given the relevance of the β-adrenergic receptor as a molecular target for many pathological conditions, this comprehensive review aims at providing an in-depth exploration of the recent advancements in β₃-adrenergic receptor research. More importantly, we delve into the prospects of the β₃-adrenergic receptor as a therapeutic target across a variety of clinical domains.

Keywords: adrenaline; cancer; cardiovascular disease; discovery; eye diseases; metabolic disorders; pathophysiology; pharmacology; therapeutics; β1, β2, β3-adrenoceptors.

Figure 1

Classification of adrenergic receptors.

Figure 2

Major transduction mechanisms of the adrenoceptors. Different adrenoceptor types interact with different G protein types and therefore induce specific intracellular effects. Stimulation of an α₁-adrenoceptor leads to conformational changes in the receptor that result in interaction with and subsequent dissociation of the G_αq subunit from the βγ subunits of the heterotrimeric G protein which in turn interacts with and activates phospholipase C (PLC) (left). Active PLC catalyzes the hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP₂) into inositol 1,4,5-triphosphate (IP₃) and diacylglycerol (DAG). Binding of ligands to β-adrenoceptors which are normally coupled to G_αs results in interaction with and subsequent activation of adenylyl cyclase (AC) (right panel). Active AC then synthesizes cyclic adenosine monophosphate (cAMP) from adenosine triphosphate (ATP). However, when α₂-adrenoceptor is stimulated (middle panel), it is the G_αi/o subunit that dissociates from the βγ subunits to inhibit AC activity, thus leading to reduced intracellular cAMP concentration. Both β₂- and β₃-subtypes may also engage the G_αi/o protein.

Figure 3

β ₃-Adrenoceptor signaling and desensitization. Stimulation of the β₃-adrenergic receptor through catecholamine binding results in a rise in intracellular cAMP concentration which binds and activates PKA. Activated PKA phosphorylates and activates lipases (e.g., HSL, PLIN, and CGI-58) to increase adipocyte lipolysis releasing free fatty acids (FFAs) and glycerol. PKA also phosphorylates CREB which translocates into the nucleus. CREB and ATF-2 (also known as CRE; cAMP response element) together form a transcription factor that regulates expression of thermogenic genes. In homologous desensitization, cAMP also activates the EPAC/RAP pathway which is synergized by tumor necrosis factor receptor (TNFR) activation (i.e., heterologous desensitization) through TNF-α binding. Activated RAP2A in turn activates PLC which hydrolyzes PIP₂ into IP₃ and DAG. IP₃'s action on endoplasmic reticulum increases intracellular Ca²⁺ concentration which induces transcription of TRIB1 gene. Resulting TRIB1 protein recruits COP-1, an E3 Ub ligase whose activity leads to the degradation of CEBPα and subsequent downregulation of ADRB3 gene expression.

Figure 4

β-Adrenoceptor activation and regulation. When intracellular cAMP concentration rises resulting from receptor activation, cAMP molecules bind the regulatory subunits of PKA resulting in the release of active catalytic subunits of PKA which migrate into the nucleus where they catalyze phosphorylation of CREB. Once phosphorylated, CREB, a signal-regulated transcription factor, recruits a coactivator CBP which together induce gene transcription upon binding CRE. Activated PKA subunits may phosphorylate the receptor directly on Ser and Thr residues of intracellular loops or indirectly via phosphorylation of β-ARK. Phosphorylation activates β-ARK which in turn phosphorylates Ser and Thr residues of the β-adrenoceptor upon recognizing its stimulated configuration which decouples the Gs subunit from the receptor by steric exclusion. β-Arrestins bind the phosphorylated receptor. This prevents the receptor from further interaction with the G_s subunit of the trimeric G protein and acts as an adapter to initiate receptor endocytosis, subsequently sequestering and desensitizing the receptor (right panel).

Figure 5

Mechanism by which β₃-adrenoceptor mediates regulation of L-TCC Function. (a) Stimulation of L-TCC via cAMP/PKA pathway. (b) Inhibition of L-TCC via eNOS/PKA pathway. β₃-Adrenoceptor activation may upregulate intracellular Ca²⁺ depolarization through positive regulation of L-TCC function to allow Ca²⁺ entry into myocytes (a). This is achieved via activation of the canonical pathway leading to intracellular increase in cAMP, subsequent activation of PKA, and phosphorylation of L-TCC by activated PKA. However, the β₃-adrenoceptor is negatively coupled to eNOS apparently through G_αi/o subunit of the G protein (b). Interaction of G_αi/o with eNOS leads to activation of eNOS and synthesis of NO. NO then binds and activates sGC resulting in the synthesis of cGMP. PKG is activated upon binding cGMP and phosphorylates L-TCC to inhibit Ca²⁺ influx.

Figure 6

Mechanism of nitric oxide-mediated vasodilation and pathogenesis of endothelial dysfunction in vascular disease. In response to agonist/bradykinin/catecholamine stimulation of the β-adrenergic receptor which results in intracellular rise in cAMP and Ca²⁺ concentrations, cAMP and Ca²⁺/calmodulin bind and activate eNOS by disrupting its interaction with caveolin. Activated dimer of eNOS (coupled) then catalyzes the synthesis of NO by electron transfer through electron carriers to reduce O₂. The critical cofactor, tetrahydrobiopterin (BH₄), acts as the electron donor to reduce and activate O₂ which enables oxidation of L-Arg to NO releasing citrulline as a by-product under physiological conditions. NO diffuses into the smooth muscle cells to activate cGS initiating a cascade of events leading to vascular smooth muscle relaxation. Uncoupling of eNOS results in the synthesis of O₂⁻ a harmful free radical rather than antiatherosclerotic NO which is helpful. Uncoupling of eNOS is attributable to the oxidative depletion of BH₄ to dihydrobiopterin (BH₂), depletion of L-Arg (substrate), or accumulation of its analog (asymmetrical dimethylarginine) as well as eNOS S‐glutathionylation. Under oxidative stress conditions, BH₄ is oxidized by O₂⁻ and even more strongly by NOO− to BH₂. NOO− is produced by scavenging of NO by O₂⁻. Unfortunately, BH₂ promotes functional dimeric eNOS uncoupling, thereby resulting in a vicious cycle of increasing harmful oxidants and oxidative stress. This phenomenon of excessive O₂⁻ and oxidative stress leading to high ratio of BH₂:BH₄ is a major risk factor of cardiovascular disease pathophysiology.

Figure 7

Hypothesized molecular mechanism by which green tea extract ameliorates weight loss. All β-adrenoceptors mediate catecholamine-stimulated lipolysis, beiging, and thermogenesis of adipocytes. Inhibition of COMT which degrades norepinephrine and epinephrine which are the natural ligands of β-adrenoceptors by catechins results in an increased concentration of ligands and therefore prolonged stimulation of the β-adrenoceptors especially of the β₃-subtype. The resulting downstream cascade leads to upregulation of lipolytic genes Pnpla2 (Atgl) and Lipe (HSL), and thermogenesis-associated genes such as SIRT, Ppargc1a, and UCP1. The outcome is increased browning, energy expenditure, and fat oxidation of white adipocytes, thermogenesis in brown adipose tissues and loss in body weight. GT, therefore, employs majorly the β₃-adrenoceptor pathway activation to achieve therapeutic effects.