Signalling Mechanism In Cell
Dr. D. Datta
Professor & HOD
Department of Pathology
Silchar Medical College
Speaker : Dr. Bijita Dutta
The term cell growth is used in the contexts of cell development
and cell division.
• When used in the context of cell development, the term refers
to increase in cytoplasmic and organelle volume (G1 phase) as
well as increase in genetic material before replication (G2
• When used in the context of cell division, it refers to growth
of cell populations, where one cell (the "mother cell") grows
and divides to produce two "daughter cells” (M phase).
The cell cycle is broken up into four stages:
• G1, S, G2 and M
• S : DNA replication occurs during this S(“synthesis”) phase.
• M : DNA packaging, chromosome segregation and cell
division (cytokinesis) occur in M(mitosis).
• S phase and M phase are separated by Gap phases.
1.G1 is the gap between M and S.
Cell growth is one of the important events of G1.The
transition from G1 to S is the critical control point in the cell
2. G2 is the gap between S and M, and provides time for
proofreading to ensure that DNA is properly replicated and
packaged prior to the cell division.
• G0 or quiescence occurs when cells exit the cell cycle due to
the absence of growth-promoting signals or presence of
• The G1, S and G2 phases comprise interphase, which
accounts for most of the time in each cell cycle.
• The M phase, mitosis, is relatively short (approximately 1 hour
of a 24 hour cell cycle).
• Mitosis is itself divided into several
phases,i.e,prophase,metaphase,telophase and anaphase.
The mechanisms of regulation can be broken down into two
• First, how is the cell cycle regulated so that the different
phases occur in the correct order?
• Second, how do extracellular signals activate or inhibit the cell
The orderly progression of cells through the various phases of
the cell cycle is orchestrated by cyclin-dependent kinases
(CDKs), which are activated by binding to cyclins, so called
because of the cyclic nature of their production and
degradation. The CDK-cyclin complexes phosphorylate
crucial target proteins that drive the cell through the cell cycle.
The four key cyclin-Cdks that drive the cell
The G1 phase of the cell cycle is unique in that it represents
the only time where cells are sensitive to signals from their
• Cells require growth factor-dependent signals up to a point
in late G1, referred to as the “restriction point” or “start”, after
which the transition is made into S phase.
• In order to move from early G1 to late G1, the cell must
synthesize cyclin E.
• Transcription of the cyclin E gene requires a transcription
factor called E2F.
Types of cell signalling
According to the source of the ligand and the location of its
receptors (i.e., in the same, adjacent, or distant cells) there are
three general modes of signalling1. Autocrine
2. Paracrine and
Cells respond to the signalling molecules that they themselves
secrete, thus establishing an autocrine loop.
Proliferation of antigen-stimulated lymphocytes
2. Pathological –
• Tumors frequently overproduce growth factors and their
receptors, thus stimulating their own proliferation through an
• One cell type produces the ligand, which then acts on adjacent
target cells that express the appropriate receptor.
• The responding cells are in close proximity to the ligandproducing cell and are generally of a different type.
• Example1.Connective tissue repair of healing wounds, in which a factor
produced by one cell type (e.g., a macrophage) has a growth
effect on adjacent cells (e.g., a fibroblast).
2.Hepatocyte replication during liver regeneration.
3.Notch effects in embryonic development, wound healing, and
• Hormones synthesized by cells of endocrine organs act on
target cells distant from their site of synthesis, being usually
carried by the blood.
• Growth factors may also circulate and act at distant sites, as
is the case for HGF.
• Several cytokines, such as those associated with the systemic
aspects of inflammation, also act as endocrine agents.
• The binding of a ligand to its receptor triggers a series of
events by which extracellular signals are transduced into the
cell resulting in changes in gene expression.
• So the components of signal transduction pathway are1. Ligands
3. 2nd messengers
4. Transcription factors
Ligands (Primary Messengers)
• Growth factors : The proliferation of many cell types is driven by
polypeptides known as growth factors.
• These factors can have restricted or multiple cell targets. All growth
factors function as ligands that bind to specific receptors, which
deliver signals to the target cells. These signals stimulate the
transcription of genes that may be silent in resting cells, including
genes that control cell cycle entry and progression.
Epidermal Growth Factor (EGF) and
Transforming Growth Factor α (TGF-α)
These two factors belong to the EGF family and share a common receptor (EGFR).
EGF is mitogenic for a variety of epithelial cells, hepatocytes and fibroblasts,
Widely distributed in tissue secretions and fluids.
TGF-α has homology with EGF, binds to EGFR, and shares most of the biologic
activities of EGF.
The “EGF receptor” is actually a family of four membrane receptors with
intrinsic tyrosine kinase activity.
The best-characterized EGFR is referred to as EGFR1, ERB B1, or simply EGFR.
It responds to EGF, TGF-α, and other ligands of the EGF family.
EGFR1 mutations and amplification have been detected in cancers of the lung,
head and neck, breast, glioblastomas and other cancers, leading to the
development of new types of treatments for these conditions.
The ERB B2 receptor (also known as HER-2 or HER2/Neu), whose main ligand
has not been identified, has received great attention because it is overexpressed in a
subset of breast cancers and is an important therapeutic target.
Hepatocyte Growth Factor (HGF).
• HGF was originally isolated from platelets and serum.
Subsequent studies demonstrated that it is identical to a
previously identified growth factor isolated from fibroblasts
known as scatter factor. .
• HGF has mitogenic effects on hepatocytes and most epithelial
cells, including cells of the biliary epithelium, and epithelial
cells of the lungs, kidney, mammary gland, and skin.
• The receptor for HGF, c-MET, is often highly expressed or
mutated in human tumors, especially in renal and thyroid
• Several HGF and c-MET inhibitors are presently being
evaluated in cancer therapy clinical trials.
Platelet-Derived Growth Factor (PDGF).
• PDGF is a family of several closely related proteins, each consisting
of two chains. Three isoforms of PDGF (AA, AB, and BB) are
secreted as biologically active molecules. The more recently
identified isoforms PDGF-CC and PDGF-DD require extracellular
proteolytic cleavage to release the active growth factor.All PDGF
isoforms exert their effects by binding to two cell surface receptors,
designated PDGFR α and β, which have different ligand
specificities. PDGF is stored in platelet granules and is released on
platelet activation. It is produced by a variety of cells, including
activated macrophages, endothelial cells, smooth muscle cells, and
many tumor cells. PDGF causes migration and proliferation of
fibroblasts, smooth muscle cells, and monocytes to areas of
inflammation and healing skin wounds, as demonstrated by defects
in these functions in mice deficient in either the A or the B chain of
PDGF. PDGF-B and C participate in the activation of hepatic
stellate cells in the initial steps of liver fibrosis and stimulate wound
Transforming Growth Factor β (TGF-β)
and Related Growth Factors
• Native TGF-β is synthesized as a precursor protein, which is
secreted and then proteolytically cleaved to yield the biologically
active growth factor and a second latent component.
• Active TGF-β binds to two cell surface receptors (types I and II)
with serine/threonine kinase activity and triggers the
phosphorylation of cytoplasmic transcription factors called
Smads (of which there are several forms, e.g., Smad 1, 2, 3, 5, and
8). These phosphorylated Smads in turn form heterodimers with
Smad 4, which enter the nucleus and associate with other DNAbinding proteins to activate or inhibit gene transcription. TGF-β has
multiple and often opposing effects depending on the tissue and
the type of injury. Agents that have multiple effects are called
pleiotropic; because of the large diversity of TGF-β effects, it has
been said that TGF-β is pleiotropic with a vengeance.
• TGF-β is a growth inhibitor for most epithelial cells.
• It blocks the cell cycle by increasing the expression of cell
cycle inhibitors of the Cip/Kip and INK4/ARF families.
• The effects of TGF-β on mesenchymal cells depend on the
tissue environment, but it can promote invasion and
metastasis during tumor growth.
• Loss of TGF-β receptors frequently occurs in human tumors,
providing a proliferative advantage to tumor cells.
• At the same time TGF-β expression may increase in the tumor
microenvironment, creating stromal-epithelial interactions that
enhance tumor growth and invasion.
1. Receptors with intrinsic tyrosine kinase activity
2. Receptors lacking intrinsic tyrosine kinase activity that
3. G protein–coupled receptors
4. Steroid hormone receptors
Receptors with intrinsic tyrosine kinase
• Ligands for these receptors : Include most growth factors
such as EGF, TGF-α, HGF, PDGF, VEGF, FGF, c-KIT ligand
Structure: Receptors belonging to this family have
an extracellular ligand-binding domain,
a transmembrane region, and
a cytoplasmic tail that has intrinsic tyrosine kinase activity.
Receptors lacking intrinsic tyrosine kinase
activity that recruit kinases
Ligands for these receptors:
• many cytokines, such as IL-2, IL-3, and other interleukins;
• interferons α, β and γ;
• granulocyte colony stimulating factor;
• growth hormone and
• prolactin .
G protein–coupled receptors
• epinephrine and norepinephrine,
• parathyroid hormone,
• corticotropin, and
• an enormous number of common pharmaceutical drugs
• Structure of G protein-coupled receptors :
They contain seven transmembrane α-helices and
constitute the largest family of plasma membrane
Steroid hormone receptors
Ligands other than steroid hormones include
• thyroid hormone,
• vitamin D, and
These receptors are generally located in the nucleus and
function as ligand-dependent transcription factors.
• Signal transduction systems used by growth factors transfer
information to the nucleus and modulate gene transcription
through the activity of transcription factors.
• Among the transcription factors that regulate cell
proliferation are products of several
a) growth-promoting genes, such as c-MYC and c-JUN, and
b) cell cycle–inhibiting genes, such as p53.
Structure of transcription factors :
Transcription factors are modular in structure and contain
the following domains :
• DNA-binding domain (DBD): attaches to specific sequences
of DNA adjacent to regulated genes.
DNA sequences that bind transcription factors are often
referred to as response elements.
• Trans-activating domain (TAD) : contains binding sites for
other proteins such as transcription coregulators.
• An optional signal sensing domain (SSD) (e.g., a ligand
binding domain), which senses external signals.
• Growth factors induce the synthesis or activity of transcription
• Cellular events requiring rapid responses depend on posttranslational modifications for activation.
• These modifications include
(a) Heterodimerization ,e.g, the dimerization of the products of
the protooncogenes c-FOS and c-JUN to form the
transcription factor activator protein-1 (AP-1), which is
activated by MAP kinase signaling pathways,
(b) Phosphorylation , as for STATs in the JAK/STAT pathway,
(c) Release of inhibition to permit migration into the nucleus, as
for NF-κB, and
(d) Release from membranes by proteolytic cleavage, as for
Role Of Extracellular Matrix In Cell
The ECM regulates the growth, proliferation, movement, and
differentiation of the cells living within it.
It is constantly remodeling.
• Control of cell growth : ECM components can regulate cell
proliferation by signaling through cellular receptors of the
• Establishment of tissue microenvironments.
• Storage and presentation of regulatory molecules: For
example, growth factors like FGF and HGF are secreted and
stored in the ECM in some tissues. This allows the rapid
deployment of growth factors in the time of need.
• Integrins and proteoglycans are the major ECM adhesion
receptors which cooperate in signalling events, determining
the signalling outcomes and thus the cell fate.
Integration of cell signalling
At any given point of time, every single cell of our body is
receiving multiple signals. All these signals work in
combinations to regulate the behaviour of the cell, with each of
the hundreds of thousands of different cell types in our bodies
responding to this babble of signals differently. So cells
integrate the many signals that they receive in deciding
whether to survive, grow and divide (proliferate), differentiate,
or die (apoptosis).
Signal Transduction Pathways In
In cancer cells, these signalling pathways are often altered via
mutations, gene amplifications or deletions and results in a
phenotype characterized by uncontrolled growth and
increased capability to invade surrounding tissues.
Therefore, these crucial transduction molecules represent
attractive targets for cancer therapy.
The most advanced targeted agents currently under
development interfere with function and expression of several
Growth Factor Overexpression
• Growth factors are frequently found overexpressed in a variety
The result is that the respective receptors are stimulated at
a higher rate.
• Often tumors are found to secrete growth factors
such as EGF,IGF-I and PDGF.
• These factors bind to their receptors and initiate growth and
proliferative signals establishing an autocrine loop that leads
to tumor growth.
• Receptors can be mutated in a way that they transmit the signal
without ligand binding.
• For instance, tyrosine kinase receptors dimerize or
oligomerize following ligand binding and carry out the signal
transduction cascade. In various tumors tyrosine kinase
receptors can be constitutively activated by mutations that
render them active independent of ligand binding. Such
mutations were found on NEU/c-erbB-2.
Mutation Of Non-receptor Tyrosine Kinase
• There are several such tyrosine kinases that are activated in
tumors via mutations. Most of these mutations result from
chromosomal translocations that give rise to hybrid gene
products. A major example is the BCR-ABL in CML.
• The pathways that this protein uses to cause transformation are
not clearly defined. It is known that it binds and activates
GRB-2 which in turn, activates the Ras pathway, a key
pathway for triggering MAPK activation and cell proliferation.
• Another example of fusion proteins is TEL-ABL, present in
acute lymphoblastic leukemia (ALL),
acute lymphoblastic leukemia (AML) and
chronic myeloblastic leukemia(CML)
with a reciprocal t (9; 12) translocation.
• The MAP Kinase and the PI3Kinase cascades play a central
role during cell activation and proliferation.
Several oncogenes are known to act on these pathways and
several molecules that participate on these cascades when
deregulated they become oncogenic.
• Ras, a well-studied family of oncogenes, structurally altered in
about 25% of all human tumors, functions on activating the
MAPK cascade .
• Raf1,a serine threonine kinase that is activated by Ras, is also
activated in some myeloid leukemias
• Serine threonine kinases are another important group of
oncogenes. This family of oncogenes includes the Akt family
(Akt1, Akt2, Akt3).
• Akt2 is activated in pancreatic adenocarcinomas, small cell
lung cancer, and ovarian cancers.
Raf and Akt are kinases that contribute to the oncogenic
phenotype through divergent mechanisms,e.g, they can
I. induce transcription of genes that are normally not expressed
in these cells .
II. directly interfere with cell cycle machinery and promote
progression through the cell cycle.
III. inhibit programmed cell death and, therefore, allow the
survival of a cell that carries other defects and would
• In tumor cells a transcription factor can be mutated and activated
independent of extracellular or cytoplasmic signals.
• NFkB is a transcription factor that regulates expression of several
genes and was activated in a series of tumors such as breast
tumors, pancreatic adenocarcinomas, lung cancers and acute T
• C-myc is a transcription factor implicated in a variety of human
tumors. When overexpressed it dimerizes with Max, a complex that
elicits growth signals, while the Mad-Max complex promotes
• Overexpression of c-myc has been involved in a series of human
tumors including colon, stomach, cervix, breast and
Cell Cycle Control Proteins
• Deregulation of the cell cycle control is crucial for the
development of a cancer cell since it has to proliferate at a
faster than the normal rate. This effect can be direct,involving
mutations of the cell cycle control proteins or indirect when
an oncogenic protein targets the cell cycle regulators.
• Oncogenic processes exert their greatest effect by targeting
particular regulators of the G1 to S phase progression.
• Inactivation of the Rb gene is a primary event in
• Inherited loss of INK4a gene that encodes p16 confers
susceptibility to melanoma.
• Although cell cycle transition depends on the underlying CDK
cycle, superimposed checkpoint controls help ensure that
certain processes are completed before others begin. The role
of such mechanisms is to act as a brake on the cell cycle in the
face of stress and damage and allowing repair to take place.
• The best-studied checkpoint regulator is the p53 gene and is
most frequently mutated in human cancer.
• The p53 protein acts as a transcription factor and cancer
related mutations cluster in its binding domain.
Apoptosis Related Proteins
• In cancer cells an anti-apoptotic mechanism is often activated
to rescue the transformed cell from programmed cell death.
• The most common mechanism is activation of the bcl-2
family of proteins (Bcl-2, Bcl-xL, Bcl-W) that are able to
inhibit cytochrome c release from the mitochondria and rescue
the cell from apoptosis.
• Inactivation of the pro-apoptotic molecules Bax, Bak, Bid
or Bim also contributes to rescuing the cell from apoptosis.
• Activation of oncogenic kinases such as Akt-1 protects cells
from apoptosis by inhibiting the pro-apoptotic molecule Bad.
Signal Transduction Modulation
• The elucidation of signal transduction pathways in cancer cells, has
fueled the design of drug molecules intended to act at specific
proteins of the signal transduction cascade, often referred to as
signal transduction modulators (STMs).
• STMs may interfere with signal transduction processes by blocking
cell surface receptors, inhibiting growth factor receptor tyrosine
kinases, or inhibiting the effects of further downstream genes, such
as the mitogen-activated protein kinases.
• Many drug molecules directed against a wide range of signal
transduction elements are being evaluated worldwide as potential
anticancer therapies. Several STMs are currently in clinical trials;
others are still in preclinical research and development.
Signal Transduction Modulators For Cancer Therapy