For β adrenoceptors, and for many other GPCRs, β-arrestin can produce other effects. One effect is to accelerate endocytosis of β adrenoceptors from the plasma membrane. This can down-regulate β adrenoceptors if receptors subsequently travel to lysosomes, similar to down-regulation of EGF receptors, but it can also help reverse the desensitized state for those receptors returned to the plasma membrane by exposing receptors to phosphatase enzymes in endosomes (Figure 2–12B). In some cases, β-arrestin can itself act as a positive signal transducer, analogous to G proteins but through a different mechanism, by serving as a molecular scaffold to bind other signaling proteins (rather than through binding GTP). In this way, β-arrestin can confer on GPCRs a great deal of flexibility in signaling and regulation. This flexibility is still poorly understood but is presently thought to underlie the ability of some drugs to produce a different spectrum of downstream effects from other drugs, despite binding to the same GPCR. Current drug development efforts are exploring the potential of this phenomenon, called functional selectivity or agonist bias, as a means to achieve specificity in drug action beyond that presently possible using conventional agonists and antagonists. Functionally selective agonists are thought to occupy the orthosteric ligand-binding site, making their binding competitive with conventional orthosteric agonists, but differ from conventional agonists in effects on receptor conformation after binding. Allosteric ligands may also stabilize different conformational states of the receptor, but differ from functionally selective ligands by binding noncompetitively to a different site.
A. Cyclic Adenosine Monophosphate (cAMP)
Acting as an intracellular second messenger, cAMP mediates such hormonal responses as the mobilization of stored energy (the breakdown of carbohydrates in liver or triglycerides in fat cells stimulated by β-adrenomimetic catecholamines), conservation of water by the kidney (mediated by vasopressin), Ca2+ homeostasis (regulated by parathyroid hormone), and increased rate and contractile force of heart muscle (β-adrenomimetic catecholamines). It also regulates the production of adrenal and sex steroids (in response to corticotropin or follicle-stimulating hormone), relaxation of smooth muscle, and many other endocrine and neural processes.
cAMP exerts most of its effects by stimulating cAMP-dependent protein kinases (Figure 2–13). These kinases are composed of a cAMP-binding regulatory (R) dimer and two catalytic (C) chains. When cAMP binds to the R dimer, active C chains are released to diffuse through the cytoplasm and nucleus, where they transfer phosphate from ATP to appropriate substrate proteins, often enzymes. The specificity of the regulatory effects of cAMP resides in the distinct protein substrates of the kinases that are expressed in different cells. For example, the liver is rich in phosphorylase kinase and glycogen synthase, enzymes whose reciprocal regulation by cAMP-dependent phosphorylation governs carbohydrate storage and release.
When the hormonal stimulus stops, the intracellular actions of cAMP are terminated by an elaborate series of enzymes. cAMP-stimulated phosphorylation of enzyme substrates is rapidly reversed by a diverse group of specific and nonspecific phosphatases. cAMP itself is degraded to 5′-AMP by several cyclic nucleotide phosphodiesterases (PDEs; Figure 2–13). Milrinone, a selective inhibitor of type 3 phosphodiesterases that are expressed in cardiac muscle cells, has been used as an adjunctive agent in treating acute heart failure. Competitive inhibition of cAMP degradation is one way that caffeine, theophylline, and other methylxanthines produce their effects (see Chapter 20).
B. Phosphoinositides and Calcium