Page 46 - Basic _ Clinical Pharmacology ( PDFDrive )

32 SECTION I Basic Principles

Many high-resolution structures of GPCRs are available from the orthosteric agonists, but differ from conventional agonists in
Protein Data Bank (www.rcsb.org). An animated model depicting effects on receptor conformation after binding. Allosteric ligands
the conformational change associated with activation is available may also stabilize different conformational states of the receptor,
from the Protein Data Bank in Europe (http://www.ebi.ac.uk/ but differ from functionally selective ligands by binding noncom-
pdbe/quips?story=B2AR). petitively to a different site.

Receptor Regulation Well-Established Second Messengers

G protein-mediated responses to drugs and hormonal agonists A. Cyclic Adenosine Monophosphate (cAMP)
often attenuate with time (Figure 2–12A). After reaching an Acting as an intracellular second messenger, cAMP mediates such
initial high level, the response (eg, cellular cAMP accumulation, hormonal responses as the mobilization of stored energy (the break-
+
Na influx, contractility, etc) diminishes over seconds or minutes, down of carbohydrates in liver or triglycerides in fat cells stimulated
even in the continued presence of the agonist. In some cases, this by β-adrenomimetic catecholamines), conservation of water by the
2+
desensitization phenomenon is rapidly reversible; a second expo- kidney (mediated by vasopressin), Ca homeostasis (regulated by
sure to agonist, if provided a few minutes after termination of the parathyroid hormone), and increased rate and contractile force of
first exposure, results in a response similar to the initial response. heart muscle (β-adrenomimetic catecholamines). It also regulates
Multiple mechanisms contribute to desensitization of GPCRs. the production of adrenal and sex steroids (in response to corti-
One well-understood mechanism involves phosphorylation of cotropin or follicle-stimulating hormone), relaxation of smooth
the receptor. The agonist-induced change in conformation of muscle, and many other endocrine and neural processes.
the β-adrenoceptor causes it not only to activate G protein, but cAMP exerts most of its effects by stimulating cAMP-depen-
also to recruit and activate a family of protein kinases called G dent protein kinases (Figure 2–13). These kinases are composed
protein-coupled receptor kinases (GRKs). GRKs phosphorylate of a cAMP-binding regulatory (R) dimer and two catalytic (C)
serine and threonine residues in the receptor’s cytoplasmic tail chains. When cAMP binds to the R dimer, active C chains are
(Figure 2–12B), diminishing the ability of activated β adrenocep- released to diffuse through the cytoplasm and nucleus, where they
tors to activate G and also increasing the receptor’s affinity for transfer phosphate from ATP to appropriate substrate proteins,
s
binding a third protein, β-arrestin. Binding of β-arrestin to the often enzymes. The specificity of the regulatory effects of cAMP
receptor further diminishes the receptor’s ability to interact with resides in the distinct protein substrates of the kinases that are
, attenuating the cellular response (ie, stimulation of adenylyl expressed in different cells. For example, the liver is rich in phos-
G s
cyclase as discussed below). Upon removal of agonist, phosphory- phorylase kinase and glycogen synthase, enzymes whose reciprocal
lation by the GRK is terminated, β-arrestin can dissociate, and regulation by cAMP-dependent phosphorylation governs carbo-
cellular phosphatases remove the phosphorylations, reversing the hydrate storage and release.
desensitized state and allowing activation to occur again upon When the hormonal stimulus stops, the intracellular actions
another encounter with agonist. of cAMP are terminated by an elaborate series of enzymes.
For β adrenoceptors, and for many other GPCRs, β-arrestin can cAMP-stimulated phosphorylation of enzyme substrates is rapidly
produce other effects. One effect is to accelerate endocytosis of reversed by a diverse group of specific and nonspecific phos-
β adrenoceptors from the plasma membrane. This can down-regulate phatases. cAMP itself is degraded to 5′-AMP by several cyclic
β adrenoceptors if receptors subsequently travel to lysosomes, nucleotide phosphodiesterases (PDEs; Figure 2–13). Milrinone, a
similar to down-regulation of EGF receptors, but it can also help selective inhibitor of type 3 phosphodiesterases that are expressed
reverse the desensitized state for those receptors returned to the in cardiac muscle cells, has been used as an adjunctive agent in
plasma membrane by exposing receptors to phosphatase enzymes treating acute heart failure. Competitive inhibition of cAMP deg-
in endosomes (Figure 2–12B). In some cases, β-arrestin can itself radation is one way that caffeine, theophylline, and other methyl-
act as a positive signal transducer, analogous to G proteins but xanthines produce their effects (see Chapter 20).
through a different mechanism, by serving as a molecular scaffold
to bind other signaling proteins (rather than through binding B. Phosphoinositides and Calcium
GTP). In this way, β-arrestin can confer on GPCRs a great deal Another well-studied second messenger system involves hormonal
of flexibility in signaling and regulation. This flexibility is still stimulation of phosphoinositide hydrolysis (Figure 2–14). Some
poorly understood but is presently thought to underlie the ability of the hormones, neurotransmitters, and growth factors that
of some drugs to produce a different spectrum of downstream trigger this pathway bind to receptors linked to G proteins,
effects from other drugs, despite binding to the same GPCR. Cur- whereas others bind to receptor tyrosine kinases. In all cases, the
rent drug development efforts are exploring the potential of this crucial step is stimulation of a membrane enzyme, phospholi-
phenomenon, called functional selectivity or agonist bias, as a pase C (PLC), which splits a minor phospholipid component
means to achieve specificity in drug action beyond that presently of the plasma membrane, phosphatidylinositol-4,5-bisphosphate
possible using conventional agonists and antagonists. Functionally (PIP ), into two second messengers, diacylglycerol (DAG) and
2
selective agonists are thought to occupy the orthosteric ligand- inositol-1,4,5-trisphosphate (IP or InsP ). Diacylglycerol is
3
3
binding site, making their binding competitive with conventional confined to the membrane, where it activates a phospholipid- and