Pharmacodynamics : Signal Transduction - PowerPoint PPT Presentation

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Topic 10

Pharmacodynamics: Signal Transduction

713 311 PRINCIPLES OF VETERINARY PHARMACOLOGY

Dr. Korawuth Punareewattana

Faculty of Veterinary Medicine, Khon Kaen University

Topics on Signal Transduction

Cell signaling

General principles
Signaling molecules and their receptors

Signal transduction

General concepts
Mechanism of transduction

Cell Signaling: General Principles

Cell Signaling (Communication between cells)

receive a signal

Recognition of stimulus at outer cell surface by specific membrane-embedded receptor
Transfer of signal across membrane to cytoplasmic surface

transmit the signal within the cell (signal transduction)

Transmission of signal to specific effector molecules on inner membrane surface or in cytoplasm that trigger cell response

alter behavior to fitsignal

Signaling and Signal transduction

Signaling

is about communication between different groups of cells and tissues…how one group of cells informs another group of cells what to do.

Signal transduction

refers to how the presence of an extracellular signal can produce a change in the intracellular state of the cell without the initial signal crossing the membrane.

Or a step of cell signaling within target cells

Signal Transduction

Cell Signaling

General Principles; signaling molecules (Ligands, Drugs) and their receptors

Extracellular Signals

recognized by specific receptors.
2 types of Signals
Hydrophilic; unable to cross the plasma membrane - bind to cell-surface receptors

Example: growth factors

Hydrophobic; pass across the cell membrane and directly regulate the activity of an intracellular receptor.
Example: Steroid hormones, thyroid hormones, retinoids, and vitamin D bind to intracellular receptors that are ligand-activated transcription factors.

cell

surface

receptors

cell

intra-

cellular

receptors

cell

Cell surface receptors: Getting the Message Across

Extracellular ligand-binding domain

Many signals (e.g. peptide growth factors) cannot cross the plasma membrane.
Cells get round this problem by utilising transmembrane receptors.
These are proteins that sit in the membrane with the ligand* binding domain outside the cell and an intracellular domain that couples to the next step in the signalling pathway
*Ligand – a molecule that binds to, and is recognised by a receptor

Plasma membrane

outside

inside

Transmembrane domain

Intracellular domain – couples to next step (may have enzymatic activity)

Intracellular receptors

Steroid hormones such as cortisol

diffuses across the plasma membrane,
binds to a intracellular receptor protein,
enters the nucleus, and
activates specific transcription.

Signal Transduction

Signal transduction is a process that relay signal or information or instruction from cell surface receptor to effectors inside the cells.
Steps of signal transduction

Activation of intracellular domain of receptors
Signal transduction

Cascade of phosphorylation
Use 2nd messengers

Activation of effectors

Patterns of signal transduction

Depends on the types of cell-surface receptors

Signal molecule

binds

receptor

activates

Intracellular signaling pathway

Target protein

alters cell behavior

A simple intracellular signaling pathway activated by an

extracellular signal molecule influences cell behavior.

Cell-surface receptors

There are three known classes of cell-surface receptor proteins:

Ion-channel-linked receptors also known as transmitter-gated ion channels.
G-protein-linked receptors also known as serpentine receptors
Enzyme-linked receptors

enzymes; tyrosine kinases
directly associate with an enzyme that they regulate

Types of receptor & Signal transduction

Types of receptor

Cell surface receptorLigands (Drug)

Classical receptors activate signal transduction
Ion-channel receptors activate or block ion movement
Uptake carriers block transport

Intracellular receptor

Enzymes block enzyme function
Transcription factor activate or block

Cell-surface receptors

act as signal transducers: they bind the signaling ligand with high affinity and convert this extracellular event into one or more intracellular signals that alter the behavior of the target cell.
Activated cell surface receptors trigger a phosphorylation cascade which results in signal relay through the cell and to the nucleus.

Some pathways using 2nd messengers (eg. cAMP) to amplify the signals

Phosphorylation cascade

Many enzymes are regulated by covalent attachment of phosphate, in ester linkage, to the side-chain hydroxyl group of a particular amino acid residue (Ser, Thr, or Tyr).

A protein kinase transfers the terminal phosphate of ATP to a hydroxyl group on a protein.
A protein phosphatase catalyzes removal of the Pi by hydrolysis.

Major signaling mechanisms share common features

In both receptor tyrosine kinase signaling and G-protein-mediated signaling,

a signaling protein is activated by the addition of a phosphate group (phosphorylation) and inactivated by the removal of the phosphate.

Signaling by

phosphorylation

Signaling by phosphorylation of

GTP-binding protein

Second Messengers

Several different molecules act as second messengers including:

Cyclic AMP (cAMP)
Cyclic GMP (cGMP)
Ca2+ ions
Inositol phosphates (IP3)
Lipids; eg. Diacylglycerol (DAG)

Signal amplification is an important feature of signal cascades:

One hormone molecule can lead to formation of many cAMP molecules.
Each catalytic subunit of Protein Kinase A catalyzes phosphorylation of many proteins during the life-time of the cAMP.

G-protein-linked receptors

Receptor

The interaction between the G-protein-linked receptor and the target is mediated by a trimeric GTP-binding regulator protein(G-protein).
All of the G-protein-linked receptors belong to a large superfamily of homologous, seven-pass transmembrane proteins.

Trimeric GTP-binding

protein

Enzymes activated by G-proteins catalyze the synthesis of second messengers.

Receptor tyrosine kinases

single-pass transmembrane proteins

ligand-binding site outside the cell
catalytic site inside

The binding of growth factors such as insulin, epidermal growth factor, and platelet-derived growth factor to the extracellular domain of their receptors switches on the kinase activity of their catalytic domain and triggers phosphate group additions to a network of intracellular proteins to transduce the signal.

Examples of Signal Transduction Systems

Receptor tyrosine kinase or Enzyme-link receptor

Kinase cascade
or Phosphorylation cascade

G-protein link receptor

using 2nd messenger (cAMP)

Steroid hormone receptor

Simple signaling system

Signal Integration

Complex cell behaviors are generally regulated by specific combinations of signals rather than by a single signal acting alone.

ways to integrate combinations of signals

previous example of

complex cell behavior

How are multiple signals

understood by the cell?

Fast and slow responses

Signal Transduction: General Principles Summary

Cell signaling requires both

extracellular signaling molecules and
a complementary set of receptor proteins in each cell.

Most signaling molecules are hydrophilic and

activate cell surface receptors.

There are three main families of cell surface receptors.
The highly regulated phosphorylation cascade stimulated by activated receptors

relays the signal through the cytoplasm and to the nucleus to alter cellular behavior

Signaling molecules (ligands)

- hydrophobic

- hydrophilic

Receptors

- specific

- intracellular

- cell surface

Ion-channel-linked receptors

G-protein-linked receptors

enzyme-linked receptors

Intracellular signaling proteins

- receive and transduce the signal

- short half-life

- molecular switches

On and Off states

Phosphate group additions and loss relays signal

2 types of signal transduction

1) Phosphorelation cascade

2) Phosphorelation and 2nd messengers

P

Nucleus

A Simple Signalling System

INFg receptor

Interferon (IFN) g

Activated IFNg receptor recruits JAK kinase

Phosphorylation causes STATs to dimerise and migrate to nucleus…

…where they initiate transcription

ADP

ATP

JAK phosphorylates STAT monomer

STAT transcription factor

JAK tyrosine kinase

Cascading Kinases

Binding of epidermal growth factor to its receptor activates ras

Ras activates the serine/threonine kinase raf

Erk-1 phosphorylates the transcription factor myc and activates transcription

ras

raf

ras

GTP

GDP

ADP

ATP

Raf phosphorylates and activates the dual-specificity kinase Mek-1

P

Mek1

Nucleus

ATP

P

Erk1

ATP

ADP

P

Erk1

Mek-1 phosphorylates the serine/threonine kinase Erk-1 which migrates to the nucleus

MLK3

raf

Mekk1

ATP

ADP

Mek1

MKK4

MKK7

ATP

ADP

Erk1

Jnk2

Jnk1

Cascading Kinases

The raf Mek-1 Erk-1 cascade is one example of a MAP kinase cascade. A number of these cascades have been described, and although they utilise specific kinases, the pathways are very similar…

N.B. Mode of activation of the first kinase in the cascade is variable and depende on the signal

Growth Factor

Stress

Cytokines

MAP kinase

kinase kinase

Serine/threonine kinase

MAP kinase

kinase

Dual-specificity kinase

Serine/threonine kinase

MAP kinase