Pharmcodynamics II

Pharmacodynamics II

Signal Transduction Mechanisms

1. G-Protein Coupled Receptors (GPCRs)

When a ligand, such as a hormone or a neurotransmitter, binds to a GPCR on the cell surface, it triggers a series of events inside the cell that leads to a specific response.

Upon ligand binding, the GPCR undergoes conformational changes that result in the activation of a G protein located on the intracellular face of the plasma membrane. The activated G protein consists of three subunits (α, β, and γ), which dissociate upon activation and interact with downstream effector molecules to initiate intracellular signaling cascades. The α subunit is responsible for the interaction with the effector enzyme or ion channel, while the βγ subunit complex serves as a signal transducer.

The type of G protein activated by a GPCR and the downstream signaling pathway initiated by the G protein depends on the specific type of GPCR and the ligand that binds to it. For example, the activation of Gαs by a GPCR leads to the stimulation of adenylate cyclase, which increases intracellular levels of cyclic AMP (cAMP) and activates protein kinase A (PKA), ultimately leading to downstream signaling events. In contrast, activation of Gαi by a GPCR leads to the inhibition of adenylate cyclase, decreasing intracellular cAMP levels and inhibiting PKA activity.

Other G proteins, such as Gαq, activate phospholipase C (PLC) upon activation, leading to the generation of inositol triphosphate (IP3) and diacylglycerol (DAG). These second messengers regulate calcium mobilization and activate protein kinase C (PKC), leading to downstream signaling events.

 Table 1: The main G-protein subtypes and their functions

G α Subtypes	Associated Receptors	Main Effectors
Gαs	Many amine and other receptors (e.g. catecholamines, histamine, serotonin)	Stimulates adenylyl cyclase, causing increased cAMP formation
Gαi	As for Gαs, also opioid, cannabinoid receptors	Inhibits adenylyl cyclase, decreasing cAMP formation
Gαo	As for Gαs, also opioid, cannabinoid receptors	Limited effects of α subunit (effects mainly due to βγ subunits)
Gαq	Amine, peptide and prostanoid receptors	Activates phospholipase C, increasing production of second messengers inositol trisphosphate (IP3) and diacylglycerol (DAG)
Gβγ	All GPCRs	·       Activate potassium channels ·       Inhibit voltage-gated calcium channels ·       Activate GPCR kinases (GRKs) ·       Activate mitogen-activated protein kinase cascade

2. Ion-channel receptors

Ion-channel receptors are proteins that span the cell membrane and act as gates for ions such as Na+, K+, Ca2+, and Cl-. When these receptors bind to their specific ligand, they open or close the channel, allowing ions to flow in or out of the cell. This causes a quick change in the cell's electrical charge, which triggers electrical signals that travel along the nerve or muscle fiber. 

Some examples of such channels include the nicotinic cholinergic receptor, the GABA A receptor, and the receptors for glutamate, aspartate, and glycine.

The way ion-channel receptors work is simple: the ligand binds to the receptor, and the receptor changes its shape, which opens or closes the ion channel. Depending on the direction of ion movement across the channel, the electrical signal can either increase or decrease. The number and properties of open ion channels also affect the strength and duration of the signal.

Fig. GABAA Receptor (Chloride ion-channel)

Ion-channel receptors can also be controlled by other mechanisms, like phosphorylation or interaction with other cellular proteins, which can modulate their activity. Phosphorylation of specific parts of the receptor can change its function, leading to changes in ion-channel activity. Additionally, the interaction with other proteins can influence the localization and activity of the receptor.

3. Transmembrane Enzyme-linked Receptors

Transmembrane enzyme-linked receptors are a type of cell surface receptor that, upon ligand binding, undergo a conformational change that activates their intracellular enzymatic domain. These receptors play critical roles in many cellular processes, including growth and differentiation, immune responses, and metabolism.

The signal transduction mechanism of transmembrane enzyme-linked receptors involves several steps. First, ligand binding induces receptor dimerization or oligomerization, bringing the intracellular enzymatic domains in close proximity to each other. This proximity facilitates the trans-autophosphorylation of specific tyrosine residues on the receptor, leading to the activation of the enzymatic domain.

The activated enzymatic domain can then phosphorylate specific tyrosine residues on downstream signaling proteins, leading to their activation and the initiation of intracellular signaling cascades. These downstream signaling pathways can involve numerous signaling proteins, such as adaptor proteins, enzymes, and transcription factors, leading to a diverse range of cellular responses.

Fig. Transmembrane enzyme-linked receptors

The signal transduction of transmembrane enzyme-linked receptors can also be regulated by several mechanisms, including dephosphorylation by phosphatases, receptor internalization, and degradation. Additionally, cross-talk with other signaling pathways and modulation by extracellular factors, such as cytokines and growth factors, can also regulate the activity of these receptors.

4. Transmembrane JAK-STAT Binding Receptor

The Janus kinase (JAK) - signal transducer and activator of transcription (STAT) pathway is a signaling pathway utilized by many cytokines, hormones, and growth factors. Transmembrane receptors that bind to these ligands belong to the family of receptor tyrosine kinases (RTKs) and cytokine receptors. In the case of cytokine receptors, many of them lack intrinsic kinase activity, so they rely on the non-receptor tyrosine kinase JAK to phosphorylate tyrosine residues on their cytoplasmic domains.

Upon ligand binding, JAKs associated with the receptor are activated and phosphorylate specific tyrosine residues on the receptor. This creates a docking site for STAT proteins, which are then recruited to the receptor and subsequently phosphorylated by JAKs. Phosphorylated STATs form homo- or heterodimers and translocate to the nucleus, where they bind to specific DNA sequences and regulate gene transcription.

JAKs also activate other signaling pathways, including the mitogen-activated protein kinase (MAPK) and phosphatidylinositol-3 kinase (PI3K) pathways. These pathways are involved in a range of cellular processes such as proliferation, differentiation, survival, and apoptosis.

The JAK-STAT pathway is critical for normal immune function, and defects in this pathway can lead to various immunodeficiency disorders. Dysregulation of the pathway has also been implicated in the pathogenesis of autoimmune diseases, as well as cancer and other diseases. Therefore, the JAK-STAT pathway has emerged as an important target for therapeutic intervention, and several drugs that target JAKs or STATs have been developed for the treatment of autoimmune diseases and cancers.

5. Receptors that Regulate Transcription Factors

Receptors that regulate transcription factors are a class of receptors that are involved in the regulation of gene expression by controlling the activity of transcription factors. These receptors are typically found in the nucleus and act as transcriptional regulators by binding to specific DNA sequences in the promoter regions of genes.

Fig. Signal Transduction of Estrogen Receptor

Signal transduction in these receptors typically begins with ligand binding, which induces a conformational change in the receptor and results in the recruitment of co-activator or co-repressor proteins that regulate transcription factor activity. Once bound to the receptor, the co-activator or co-repressor proteins interact with the transcription factor and either enhance or suppress its activity.

The activated transcription factor then binds to the promoter region of the target gene and activates or represses its transcription, leading to changes in gene expression. The downstream effects of this transcriptional regulation can be diverse and can include changes in cell proliferation, differentiation, and survival.

Examples of receptors that regulate transcription factors include the nuclear hormone receptors, such as the estrogen and the androgen receptors, and the Notch receptor. 

Dose Response Relationship

The dose-response relationship refers to the relationship between the amount (dose) of a drug or chemical that is administered to an organism and the magnitude of the response it produces. 

In pharmacology, this relationship is typically graphed as a dose-response curve, which plots the concentration or dose of the drug on the x-axis and the biological response such as the response rate, the enzyme activity, or the cell viability on the y-axis.

A typical dose-response curve starts with a threshold dose, below which no effect is observed, and as the dose of the drug increases, the response also increases. 

 

A dose-response curve has a sigmoidal shape, with three main phases: the initial or baseline phase, the linear or steep phase, and the plateau or saturation phase. 

 

In the initial phase, the biological effect is minimal or absent, even at low doses of the drug or compound. In the linear phase, the effect increases linearly or exponentially with the dose, reaching a maximum or optimal effect at a certain dose or concentration. In the plateau phase, the effect remains constant or saturated, even at higher doses of the drug or compound. Beyond this point, further increases in dose do not produce any additional response, and may even result in toxicity or adverse effects.

The slope and the shape of the dose-response curve depend on several factors, including the potency, efficacy, and variability of the drug or compound, as well as the sensitivity and variability of the biological system. 

 

Some drugs have steep dose-response curves, meaning that even small changes in dose can produce a large change in response, while others have flatter curves, indicating that a larger change in dose is required to produce the same magnitude of response.

The dose-response relationship helps researchers determine the optimal dose range for therapeutic effects while minimizing the risk of adverse effects. It also allows for the calculation of important parameters such as the effective dose 50 (ED50), lethal dose 50 (LD50), and therapeutic index.

ED50

ED50 refers to the dose of a drug that produces a specific effect in 50% of the population or experimental subjects. The ED50 value is often used to compare the potency of different drugs, as a drug with a lower ED50 is more potent than a drug with a higher ED50.

 

LD50

LD50 refers to the dose of a toxicant or drug that causes death in 50% of the population or experimental subjects. The LD50 value is used to measure the toxicity of substances and is often used to determine safe exposure levels for humans and animals.

Potency and Efficacy

The potency refers to the dose or concentration of the drug that produces a certain level of effect, such as the half-maximal effective concentration (EC50) or the lethal dose (LD50). 

The efficacy refers to the maximum or optimal effect that the drug or compound can produce, regardless of the dose or concentration.

In above fig., the dose-response curve of Drug A shows a steep increase in response with increasing dose, indicating high potency. The maximum response of Drug A is 95%.

On the other hand, the dose-response curve of Drug B shows a more gradual increase in response with increasing dose, indicating lower potency compared to Drug A. However, the maximum response of Drug B (98%) is higher than Drug A, indicating higher efficacy.

In summary, Drug A is more potent but less efficacious than Drug B, while Drug B is less potent but more efficacious than Drug A.

Therapeutic Index

Therapeutic index is a measure of the safety of a drug, defined as the ratio of the dose of the drug that produces toxicity in 50% of a population (LD50) to the dose that produces a therapeutic effect in 50% of the same population (ED50).

In other words, the therapeutic index reflects the range between the dose that is effective for the majority of patients and the dose that produces toxic effects. The higher the therapeutic index, the safer the drug is considered to be, as the effective dose is farther away from the toxic dose. Therefore, drugs with a high therapeutic index are preferred for clinical use, as they are less likely to cause harm to patients.

Drug	LD50 (mg/kg)	ED50 (mg/kg)
A	200	20
B	100	5

To calculate the therapeutic index, we use the formula:

Therapeutic index = LD50 / ED50

For drug A, the therapeutic index would be:

Therapeutic index for Drug A = 200 / 20 = 10

For drug B, the therapeutic index would be:

Therapeutic index for Drug B = 100 / 5 = 20

Thus, drug B has a higher therapeutic index than drug A, indicating that it is a safer drug with a wider margin of safety between its therapeutic and toxic doses.