Drugs produce effects in the body mainly in the following ways:
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By acting on receptors
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By inhibiting carriers (molecules that transport one or more ions or molecules across the plasma membrane)
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By modulating or blocking ion channels
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By inhibiting enzymes
Receptors as Targets for Drug Action
Receptors are protein molecules in or on cells whose function is to interact with the body’s endogenous chemical messengers (hormones, neurotransmitters, the chemical mediators of the immune system, etc.) and thus initiate cellular responses. They enable the responses of the body’s cells to be coordinated. Drugs used in medicine make use of these chemical sensors – either stimulating them (drugs that do this are termed agonists ) or preventing endogenous mediators or agonists from stimulating them (drugs that do this are termed antagonists ).
There are four major types of receptor:
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Type 1: Receptors linked to ion channels, also termed ionotropic receptors or ligand-gated ion channels.
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Type 2: Receptors coupled to G-proteins (GPCRs): that are guanine nucleotide-binding proteins, also termed metabotropic receptors.
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Type 3: Receptors linked to enzymes (e.g. kinases, guanylate cyclase, etc.); these mostly initiate a kinase cascade within the cell (e.g. tyrosine kinase-linked receptors).
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Type 4: Receptors that affect gene transcription (receptors for steroids).
Within these classes are also so-called orphan receptors, which currently have no well-defined ligands.
Why has evolution provided the body with these many different ways to provide cellular signalling? One answer is the timing of responses, as removing your hand from a hot surface requires an immediate response, whereas the control of cell division requires more subtle biological control over a longer period of time. This is represented in Fig. 4.1 .
The structure and mechanisms of receptor activation are summarized below, adapted from detailed discussion from Rang and Dale’s Pharmacology .
Type 1: Receptors linked to ion channels (i.e. ionotropic receptors)
Receptors linked to ion channels are located in the cell membrane and respond in milliseconds. The channel forms part of the receptor, which are assembled from four to five membrane spanning units named α, β, γ, δ. Ion channels that have a pentameric structure (rather than a quatromeric) will have one repeated subunit. The nicotinic receptor for acetylcholine (see Chapter 11 ) is an example, which is composed of α2, β, γ and δ ( Fig. 4.2 ). This receptor type controls the fastest synaptic events in the nervous system. As an example, the action of acetylcholine acting on nicotinic receptors at the neuromuscular junction causes an increase in sodium ion (Na + ) and potassium ion (K + ) permeability and sometimes calcium ion (Ca 2+ ) permeability to result in net inward Na + currents at negative membrane potentials to depolarize a cell and raise the probability of an action potential. In contrast to other receptor types, no other biochemical steps are required for the immediate transduction event, and therefore this provides for a very fast cellular response.
Type 2: Receptors coupled to G-proteins
G-protein-coupled receptors (GPCRs) occur in the cell membrane and respond in seconds. They have a single polypeptide chain that has seven transmembrane helices. Signal transduction occurs by activation of particular G-proteins that are membrane resident and recognize GPCRs in their active state to then generate signalling pathways to modulate enzyme activity or ion channel function and thus produce a cellular response ( Figs. 4.3 to 4.5 ).
The mechanics of G-protein assembly and disassembly are highly orchestrated, and the cyclical activation and inactivation of G-proteins is best represented in a diagram ( Fig. 4.5 ). G-proteins consist of three subunits: α, β and γ. Guanine nucleotides bind to Gα, which has enzymic (GTPase) activity. Receptor activation leads to interaction with the Gα subunit and bound guanine diphosphate (GDP) is displaced by cytoplasmic guanine triphosphate (GTP). The Gα-GTP unit then interacts with a target protein to induce its GTPase activity to catalyse GTP to GDP and activate the target protein. The β and γ subunits remain together as a βγ complex (Gβγ). Specificity of GPCR response is achieved with different classes of G-protein that show selectivity with regards to receptors and effector targets (enzymes) they couple to. Thus, there are four main classes of Gα-protein that are pharmacologically relevant: Gα s , Gα i , Gα o and Gα q along with Gβγ. Targets for G-proteins include the following:
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Adenyl cyclase (cyclic adenosine monophosphate (cAMP) production).
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Phospholipase C (inositol phosphate and diacylglycerol (DAG)) formation.
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Rho A/Rho kinases (these regulate signalling pathways controlling cell growth and proliferation, contraction and motility).
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Mitogen-activated protein (MAP) kinases (these regulate many cellular functions, including cell division).
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Ion channels (direct G-protein-channel interaction through Gβγ of G i and G o proteins).
In some instances, GPCR activity can be affected by membrane proteins called receptor activity-modifying-proteins (RAMPs). Furthermore, G-protein-independent signalling can sometimes occur with GPCRs. In this regard, signalling mediated by arrestins, rather than G-proteins, is important since arrestins can act as a link to MAP kinase cascades.
It is important to note that understanding GPCR activity can no longer be represented by the dogma that one GPCR acts on one G-protein to create one distinct response via a canonical pathway. Rather, there are various processes that can lead to qualitatively different actions:
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One GPCR protein can associate with others (GPCRs) to produce more than one type of functional receptor complex (oligomerization).
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Different agonists may affect GPCRs in different ways and elicit different responses (biased signalling or selective functionality), whether this be through G-protein, RAMP or arrestin signalling.
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The signal transduction pathway can be independent of G-proteins and can cross talk with tyrosine kinase-linked receptors (see below).
Type 3: Receptors linked to enzymes
These receptors are single transmembrane proteins with a large extracellular portion that contains the binding sites for ligands (e.g. growth factors, cytokines and some hormones) and an intracellular portion that has integral enzyme activity – usually tyrosine kinase activity ( Fig. 4.6 ). Activation initiates an intracellular pathway involving cytosolic and nuclear transducers and eventually gene transcription. They are important for cell division, growth, differentiation, inflammation, tissue repair, apoptosis and immune responses. There are three main types:
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Receptor tyrosine kinases: these incorporate a tyrosine kinase moiety in the intracellular region.
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Receptor serine/threonine kinases: these phosphorylate serine or threonine residues.
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Cytokine receptors: these have no intrinsic enzyme activity but will activate various tyrosine kinases such as Janus kinase (Jak), named after the Greek god Janus because they act in a double-faced manner, whereby they contain two near-identical phosphate-transferring domains. One domain regulates kinase activity but is ultimately opposed by the other ( Fig. 4.6 ).
