index
Announcements
Questions
Section 9.12: Signal Transduction
The 'Thinking' Cell
Principles of Molecular Circuits
Primary Message: Ligand Binding to Membrane Receptors
Secondary Messengers: Relay and Amplification
Responses to Signals: Kinases and Phosphorylation
Signal Termination
G Proteins
General Mechanisms of Signal Transduction
The Adenylyl Cyclase Pathway
The Inositol-Phospholipid Pathway
Receptor Tyrosine Kinases
Defects in Signaling Pathways
Traditional Cancer Therapies
Target-Specific Cancer Therapies
Gleevec
Herceptin
RTK Cancer Therapies in Development
Questions
References
Next Lecture: Sections 10.1 - 10.3

Fundamentals of Biochemistry

Biochem 380 - Fall 2006

Lecture 028

Outline

Announcements
Questions from previous lecture
Section 9.12: Signal Transduction

Announcements

Lecture 27 notes are online
Quiz on Friday on Chapter 9
Skipping Section 9.11

Questions

Any questions on the material from the previous lecture?

Section 9.12: Signal Transduction

Chapter 9 concludes with a very important section on signal transduction. This is a process where an extracellular signal is received by a cell at its surface and then transformed and amplified into some kind of biochemical response within the cell
Signal transduction allow cells to respond to conditions in the external environment. It is also used by multicellular organisms to control cell growth, differentiation and activity. Failures in signaling pathways can have serious consequences, such as cancer and other diseases
We will look at some important components of signal transduction pathways, including receptors, secondary messengers and downstream effectors. We'll also see how some of these components are used in two example pathways: the adenylyl cyclase pathway and the inositol-phospholipid pathway

The 'Thinking' Cell

Just as the nervous system of a multicellular animal is composed of sensory input, informational processing and motor outputs, we can find biochemical counterparts of such subsystems in individual cells
Influences from the outside world can be communicated to cells through signals received at the cell membrane
Ultimately, many of these signal will result in some kind of functional response such as a change in gene expression
Signal transduction pathways represent some of the informational processing steps and mechanisms between these signal inputs and functional responses

Principles of Molecular Circuits

An external signal entering a cell can be amplified, split up and fed back upon itself. The biochemical mechanisms that enable such transformations can be thought of as molecular circuits. The flow of signals through these circuits can be organized into a number of distinct stages:
- Transmission of the primary message across the membrane
- Relay and amplication of the primary signal by secondary messengers
- Production of functional responses such as activation of kinases and protein phosphorylation
- Mechanisms such as phosphatases that can bring about signal termination

Primary Message: Ligand Binding to Membrane Receptors

Some nonpolar molecules such as estrogen and other steroids can diffuse across the cell membrane and act directly as signal molecules inside the cell
However, most signal molecules are too large and polar to pass through and must act indirectly by binding as ligands to a receptor outside the membrane
This binding event is then communicated by a structural change of the receptor protein across the membrane to the inside
This structural response is the primary message that will initiate subsequent responses inside the cell

Secondary Messengers: Relay and Amplification

The physical effects of a primary message across the membrane can be quite small, especially if only a few signal molecules are present outside the cell
However, they can be used to activate enzymes or membrane channels that can amplify the signal by activating large numbers of secondary messengers
Such secondary messengers are often small molecules such as cAMP, calcium ions, inositol 1,4,5-trisphosphate (IP₃) and diacylglycerol (DAG), which can diffuse to other parts of the cell
In addition to amplification, distinct secondary messengers can flow through multiple independent signaling pathways

Responses to Signals: Kinases and Phosphorylation

Phosphorylation is a common response to extracellular signals
For example, we'll see the activation of kinases such as Protein Kinase A, activated by the secondary messenger cyclic AMP, and Protein Kinase C, which is activated by the secondary messenger diacylglycerol
The targets of these kinases are other proteins, which have phosphoryl groups transferred to specific serine and threonine residues
Phosphorylation is a cellular response to signals that can persist for longer amounts of time, depending upon reversal and degradation mechanisms

Signal Termination

The effects produced in a signal transduction cascade will persist until the molecular modifications are reversed or degraded
There are a number of mechanisms for restoring molecular circuits to their original state so that they will be able to respond again at a later time
Phosphatases can reverse the effects of phosphorylation
Components in the pathway such as G proteins, inositol 1,4,5-trisphosphate (IP₃) and diacylglycerol (DAG) can have their modifications reversed or they can be metabolized into other molecules
A delay in such restoring mechanisms can lead to desensitization, in which subsequent responses may occur more slowly or not at all
Failure of signal termination mechanisms can result in diseases such as cancer, with uncontrolled cell division resulting from growth signals that are permanently stuck in the 'on' state

G Proteins

G-proteins are widely used as components in signal transduction pathways, with multiple families of G proteins that can serve different functions. However, they are all characterized by the ability to bind and hydrolyze GTP (guanosine triphosphate)
Many G proteins are heterotrimeric, with three subunits: α, β and γ. A signal at a receptor causes the α subunit to exchange GDP for GTP, and dissociate from the β-γ part. After a certain amount of time, the α subunit will hydrolyze the GTP to GDP, causing it to return to the inactive state and reassociate with the β-γ component

General Mechanisms of Signal Transduction

Before looking at some specific examples, we can consider some of the common features shared by many different signal transduction pathways
The general mechanism of the beginning of signal transduction can be broken down into three sequential parts:
- A receptor (receives external signal by binding ligand)
- A transducer (converts binding to activation of effector)
- An effector (usually an enzyme such as a kinase that activates downstream proteins)

The Adenylyl Cyclase Pathway

The Adenylyl cyclase (AC) pathway is seen in many signal transduction systems such as hormone regulation of metabolism, including the β-adrenergic receptor pathway, which ultimately produces free glucose in response to the hormone adrenaline
The process begins with binding of a hormone such as adrenaline to the receptor, producing a conformational change in the α subunit of a G protein (the transducer)
This causes it to exchange GDP for GTP and bind to adenylyl cyclase (the effector) which then makes cAMP (secondary messenger)
cAMP in turn activates the enzyme protein kinase A, which goes on to affect downstream proteins through phosphorylation

The Inositol-Phospholipid Pathway

The inositol-phospholipid pathway is a somewhat more complex one with two secondary messengers. Like the AC pathway, it begins with activation of a G-protein on ligand-binding at a receptor
Next, the α-subunit of the G protein binds the membrane-bound enzyme phospholipase C, which then cleaves phosphatidylinositol 4,5-bisphosphate (PIP₂)
This gives two secondary messengers, inositol 1,4,5-trisphosphate (IP₃) and diacylglycerol (DAG). IP₃ stimulates Ca²⁺ release and DAG activates protein kinase C, which goes on to activate downsteam proteins by phosphorylation

Receptor Tyrosine Kinases

Receptor Tyrosine Kinases (RTKs) are components with a completely different mechanism of signal transduction. They are found in many pathways regulating cell activity, growth and division
RTKs combine receptor, transducer and effector all in a single complex. Upon ligand binding, a conformational change is produced that enables dimerization (binding of two RTKs)
Next, each RTK in the dimer cross-phosphorylates the other kinase, which activates it and enables it to phosphorylate downstream proteins

Defects in Signaling Pathways

Defects in signaling pathways are the cause of many disease, especially cancer
Cancer is characterized by uncontrolled cell growth that occurs when one or more components in a signaling pathway have become damaged or mutated
Normal genes that have mutated to versions that can cause cancer are called oncogenes
Normal genes with the potential to become cancerous, such as the genes for components in signaling pathways, are called proto-oncogenes

Traditional Cancer Therapies

Effective methods for treating cancer have been fairly limited to date
There have been three traditional approaches for treating cancer:
- Surgery
- Radiation therapy
- Chemotherapy
All of these approaches have drawbacks including harmful side effects on healthy cells and an inability to deal with aggressive/spreading cancers

Target-Specific Cancer Therapies

The crude slash, burn and poison methods of treating cancer are finally giving way to more targeted forms of therapy
In these targeted therapies, drugs are being developed to act upon specific molecular targets in the signal transduction pathways associated with cancer
This stands in contrast to the traditional methods, which mostly work to inhibit rapidly growing and dividing cells, which include both cancerous and healthy cells
By acting upon particular components in signaling pathways, the target-specific therapies tend to produce much fewer harmful side-effects
Target-specfic agents can be divided into two classes: small-molecule drugs and antibody-based molecules

Gleevec

Gleevec (Imatinib) is a small-molecule tyrosine kinase inhibitor
Gleevec acts upon an non-receptor tyrosine kinase expressed in Chronic Myelogenous Leukemia (CML)
CML results from a chromosomal defect involving the translocation of a gene from chromosome 9, c-Abl, into a gene on chromosome 22, Bcr, to produce an oncogene, Bcr-Abl, which expresses a kinase that cannot be regulated properly
The specific activity of Gleevec upon the Bcr-Abl kinase produces benefits in 90% of patients

Herceptin

HER2 (Human EGFR-Related 2) receptor is associated with certain types of breast cancer
Herceptin (Trastuzumab) is a monoclonal antibody that binds to the HER2 receptor, interfering with its signal transduction pathways, which are involved in cell growth
Herceptin became the first targeted anti-kinase therapeutic agent based on genomic research
Recent data indicate that herceptin may reduce relapses of breast cancer by 50%

RTK Cancer Therapies in Development

Questions

Questions about the material covered today?

References

RTKs and Cancer

The discovery of receptor tyrosine kinases: targets for cancer therapy,
Andreas Gschwind, Oliver M. Fischer and Axel Ullrich,
Nature Reviews, Cancer, Volume 4, May 2004, pg. 361-370

Next Lecture: Sections 10.1 - 10.3

Read Sections 10.1 - 10.3

Table of Contents

Fundamentals of Biochemistry

Biochem 380 - Fall 2006

Lecture 028

Outline

Announcements

Questions

Section 9.12: Signal Transduction

The 'Thinking' Cell

Principles of Molecular Circuits

Primary Message: Ligand Binding to Membrane Receptors

Secondary Messengers: Relay and Amplification

Responses to Signals: Kinases and Phosphorylation

Signal Termination

G Proteins

General Mechanisms of Signal Transduction

The Adenylyl Cyclase Pathway

The Inositol-Phospholipid Pathway

Receptor Tyrosine Kinases

Defects in Signaling Pathways

Traditional Cancer Therapies

Target-Specific Cancer Therapies

Gleevec

Herceptin

RTK Cancer Therapies in Development

Questions

References

RTKs and Cancer

Next Lecture: Sections 10.1 - 10.3