Chapter 2: Drugs, Receptors, & Pharmacodynamic Mechanisms

PHRM 3310

What are the 5 major classes of endogenous receptors?

• intracellular nuclear hormone receptors

• ligand-regulated kinase receptors

• cytokine receptors

• ligan-gated ion channels

• G-protein coupled receptors (GPCRs)

Where are intracellular nuclear hormone receptors located?

inside the cell

How are intracellular nuclear hormone receptors activated?

when ligands bind to them

What are the 3 major domains of an intracellular nuclear hormone receptor?

• ligand-binding domain

• DNA-binding domain

• transcription-activating domain

What is the mechanism of an intracellular nuclear hormone receptor?

• ligand diffuses across the cell membrane (ligand is typically small & lipophilic)

• ligand binds the receptor, causing a conformational change

• the receptor dissociates from associated proteins (e.g., chaperone proteins)

• the receptor dimerizes & translocates to the nucleus

• the receptor’s DNA-binding domain binds specific DNA sequences (response elements)

• the receptor’s transcription-activating domain recruits coactivators & transcription machinery

• gene transcription is altered (either increased or decreased)

What happens when a ligan binds a ligand-regulated kinase receptor?

receptor dimerizes & autophosphorylates specific amino acid residues, activating its kinase function

What is the mechanism of a ligand-regulated kinase receptor?

• ligand binds the extracellular domain of the receptor

• the receptor dimerizes (2 monomers come together)

• each monomer phosphorylates the other on specific residues (transphosphorylation/autophosphorylation)

• the phosphorylated residues serve as docking sites for intracellular signaling proteins

• these adaptor proteins activate downstream signaling cascades

• signaling is terminated by phosphatases that remove the phosphate groups

How are cytokine receptors different from ligand-regulated kinase receptors?

cytokine receptors are NOT kinases themselves; instead, they are associated with separate kinase proteins that are activated upon ligand binding

What is the mechanism of a cytokine receptor?

• ligand binds to its receptor

• receptor dimerizes, bringing associated kinases close together

• the kinases transphosphorylate & activate each other

• activated kinases phosphorylate residues on the receptor’s cytoplasmic tail

• transcription factor proteins dock at these phosphorylated residues

• the kinases phosphorylate the transcription factors, causing them to dimerize

• dimerized transcription factors translocate to the nucleus & activate gene transcription

What is the main function of ligand-gated ion channels?

open or close in response to ligand binding, allowing ion flow across the membrane & rapidly changing membrane potential or cell activity

What is the mechanism of a ligand-gated ion channel?

• ligand binds to the extracellular domain of the channel

• binding induces a conformational change that opens the channel pore

• ions flow down their electrochemical gradient (e.g., Na⁺, K⁺, Ca²⁺, Cl^-)

• the resulting change in membrane potential or intracellular ion concentration triggers a rapid cellular response (e.g., action potential, muscle contraction)

• ligand dissociated, the channel closes, & the cell returns to baseline

What is the basic function of a G-protein coupled receptor (GPCR)?

a G-protein coupled receptor (GPCR) couples an extracellular ligand/agonist to activation of a specific G protein, which then triggers intracellular signaling

What kinds of molecules can act as G-protein coupled receptor (GPCR) agonists?

• neurotransmitters (NTs)

• growth factors (GFs)

• hormones

• drugs

What are the 2 main states of a heterotrimeric G protein?

• GDP-bound state = inactive

• GTP-bound state = active

What is the mechanism of G-protein coupled receptor (GPCR) activation of a G protein?

• agonist binds to the G-protein coupled receptor (GPCR), causing a conformational change

• the G-protein coupled receptor (GPCR) acts as a guanine nucleotide exchange factor (GEF) for the Gα, & GTP binds in its place

• the G protein dissociated into Gα-GTP & Gβγ subunits

• Both Gα-GTP & Gβγ can interact with downstream effectors (e.g., enzymes, ion channels)

• signaling is terminated when Gα hydrolyzes GTP to GDP (using its intrinsic GTPase activity), & the subunits reassociate

How is an activated G protein turned off?

the Gα subunit hydrolyzes GTP to GDP, returning the protein to its inactive, heterotrimeric state (Gα-GDP + Gβγ)

What are the 4 major classes of G proteins?

• Gs (Gαs)

• Gi (Gαi)

• Gq (Gαq)

• G12 (Gα12)

What does Gs do?

stimulates adenylyl cyclase, increasing cAMP production

What is the full signaling pathway for Gs?

• agonist binds GPCR → Gs activates

• Gαs-GTP stimulates adenylyl cyclase

• adenylyl cyclase converts ATP → cAMP

• cAMP activates PKA (protein kinase A)

• PKA phosphorylates target proteins (e.g., ion channels, transcription factors, metabolic enzymes)

• signaling is terminated by phosphodiesterase (PDEs), which degrade cAMP, & by Gαs hydrolyzing GTP to GDP

What does Gi do?

inhibits adenylyl cyclase, decreasing cAMP levels; its Gβγ subunits can also directly regulate ion channels

What is the full signaling pathway for Gi?

• agonist binds GPCR → Gi activates

• Gαi-GTP inhibits adenylyl cyclase, reducing cAMP production

• lower cAMP → less PKA (protein kinase A) activation

• meanwhile, Gβγ subunits released from Gi can directly regulate ion channels (e.g., activate certain K⁺ channels or inhibit certain Ca²⁺ channels)

• termination: Gαi hydrolyzes GTP to GDP; phosphodiesterase (PDEs) continue to degrade any remaining cAMP

What does Gq activate?

phospholipase C (PLC), which leads to production of IP₃ & DAG

What is the full signaling pathway for Gq?

• agonist binds GPCR → Gq activates

• Gαq-GTP activates phospholipase C (PLC)

• PLC cleaves PIP₂ into:

  • IP₃ → diffuses to the endoplasmic reticulum (ER)

  • DAG → stays in the membrane

• IP₃ binds IP₃ receptors on the endoplasmic reticulum, causing release of stored Ca²⁺ into the cytosol

• Ca²⁺ & DAG together activates protein kinase C (PKC)

• PKC phosphorylates target proteins (e.g., ion channels, transcription factors, other kinases?

• termination: IP₃ is dephosphorylated, Ca²⁺ is pumped back into stores, DAG is metabolized, & Gαq hydrolyzes GTP to GDP

What is the main downstream effect of G12 signaling?

it activates Rho GTPases & regulates the cytoskeleton

What are the major G-protein coupled receptor (GPCR) effector types?

• adenylyl cyclase (Gs, Gi)

• phospholipase C (Gq)

• ion channels (Gi Gβγ)

• Rho GEF /Rho GTPases (G12)

What second messengers are generated by G-protein coupled receptor (GPCR) effector pathways?

• cAMP from adenylyl cyclase

• IP₃ & DAG from phospholipase C

• Ca²⁺ (released from intracellular stores via IP₃)

What does cAMP directly activate?

cAMP-dependent protein kinase A (PKA)

How is cAMP signaling turned off?

by phosphodiesterases (PDEs), which degrade cAMP to AMP

What are some targets of cAMP/PKA signaling?

• transcription factors (via response elements like CRE)

• ion channels (various types)

• metabolic enzymes & many other diverse substrates

What does IP₃ do?

binds to IP₃ receptors on the endoplasmic reticulum (ER), causing release of stored Ca²⁺ into the cytosol

What does DAG do?

remains in the membrane &, together with Ca²⁺ , activates protein kinase C (PKC)

How is phospholipase C (PLC) signaling turned off?

• IP₃ is dephosphorylated by phosphatases

• DAG is metabolized (e.g., by DAG kinase)

• Ca²⁺ is pumped back into the endoplasmic reticulum (ER) by sarcoplasmic/endoplasmic reticulum Ca²⁺-ATPase or SR Ca²⁺-ATPase (SERCA) pumps & out of the cell by plasma membrane Ca²⁺ ATPases

• Gαq hydrolyzes GTP to GDP