Signal Transduction Pathway Interference

Cells convert signals at their surface into internal responses through signal-transduction pathways: a receptor detects a chemical messenger and triggers a cascade of intracellular events that amplifies and relays the signal. Many drugs act by entering these cascades — as agonists or antagonists at cell-surface receptors such as G-protein-coupled receptors, or as inhibitors of signalling enzymes such as protein kinases. By interfering with transduction, a drug can reshape a cell's response to its environment over seconds to minutes.

Definition

Signal transduction pathway interference is the alteration of a cell's intracellular signalling cascade by a drug that acts at a signalling receptor or signalling enzyme, changing the second messengers and downstream effectors that relay an extracellular signal into a cellular response.

Scope

This topic covers how drugs interfere with intracellular signalling: action at G-protein-coupled receptors and the second messengers they control, inhibition of receptor and intracellular tyrosine and serine/threonine kinases, and the broader idea of cascade amplification and feedback. It treats signal-transduction interference as a molecular mechanism of drug action for reference, not as guidance on the clinical use of any signalling-directed drug.

Core questions

• At which node of the pathway does the drug act — the receptor, a transducer (G protein), or a downstream enzyme such as a kinase?
• Does the drug initiate, amplify, or block the signal?
• Which second messengers and effectors carry the signal once the drug has acted?
• How does cascade amplification shape the dose-response relationship and the time course of effect?

Key concepts

• G-protein-coupled receptor (GPCR)
• Receptor tyrosine kinase
• Second messenger (cAMP, calcium, IP3)
• Signal amplification
• Kinase inhibition
• Agonism and antagonism
• Biased signalling
• Negative feedback

Mechanisms

A surface receptor that binds its messenger changes shape and engages intracellular partners, launching a cascade. For G-protein-coupled receptors, the activated receptor switches on heterotrimeric G proteins that regulate effector enzymes and ion channels, generating second messengers such as cyclic AMP, inositol trisphosphate, and calcium; one activated receptor can switch on many G proteins, so the signal is amplified. Receptor tyrosine kinases instead dimerize and autophosphorylate, recruiting adaptors that launch phosphorylation cascades. Drugs interfere at several points: agonists and antagonists at GPCRs initiate or block the first step; small molecules can inhibit the catalytic activity of receptor or intracellular kinases, halting the cascade downstream of the receptor. Because cascades amplify and are shaped by feedback, the relationship between drug occupancy and cellular response is often non-linear, and some ligands selectively engage particular branches of a pathway (biased signalling) (Pierce 2002; Cohen 2002; Niswender 2010).

Clinical relevance

Drugs acting on signal transduction span much of pharmacology, from receptor agonists and antagonists used across many specialties to kinase inhibitors used in oncology and inflammation. Understanding where a drug enters a signalling cascade helps explain its spectrum of effects and the basis of some adverse effects. This topic describes the molecular basis of signalling-directed drugs for reference and education and does not provide dosing or treatment guidance.

Evidence & guidelines

G-protein-coupled receptors are among the most heavily exploited drug targets, and their signalling architecture is reviewed in the molecular pharmacology literature (Pierce 2002; Niswender 2010). The rise of protein kinases as a major target class is documented in target-focused reviews (Cohen 2002), and target-class surveys quantify the share of drugs that act through signalling receptors and enzymes (Overington 2006).

History

The concept of intracellular signal transduction emerged with the discovery of cyclic AMP as a second messenger in the late 1950s and the subsequent elucidation of G proteins and receptor-coupled cascades. The molecular characterization of seven-transmembrane receptors and of protein kinases turned these cascades into defined drug targets, culminating in the era of receptor-directed and kinase-directed therapeutics (Pierce 2002; Cohen 2002).

Debates

Can biased signalling produce safer drugs?

Some ligands preferentially activate one branch of a receptor's signalling (for example, G-protein versus arrestin pathways), raising the hope of separating beneficial from harmful effects; whether bias reliably translates into a better therapeutic profile remains debated.

Seminal works

• pierce-2002
• cohen-2002

Frequently asked questions

What is a second messenger?

It is an intracellular molecule, such as cyclic AMP or calcium, whose concentration changes when a surface receptor is activated; it carries and amplifies the signal from the receptor to downstream effectors inside the cell.

How do kinase inhibitors interfere with signalling?

Protein kinases relay signals by phosphorylating target proteins. A kinase inhibitor blocks this catalytic step, so the signalling cascade is interrupted downstream of the receptor that would normally activate the kinase.