Chemistry & Biochemistry department/ College of medicine

/Mustansiriyah University

Dr. Ali al-bayati Cell signaling Lec. 1

Cell signaling is part of a complex system of communication that governs basic cellular
activities and coordinates cell actions. The ability of cells to perceive and correctly respond
to their microenvironment is the basis of development, tissue repair, and immunity as well
as normal tissue homeostasis. Errors in cellular information processing are responsible for
diseases such as cancer, autoimmunity, and diabetes.

There are many different ways for cells to communicate with each other and the outside
environment. They may communicate directly through juxtacrine signaling, over short
distances through paracrine signaling and over large distances through endocrine signaling.
Additionally, some cells require cell-to-cell contact in order for communication to occur.
For this there are gap junctions which connect the cytoplasm of two cells together. In most
cases, a molecule carries the signal from one cell and receptors on the other cell bind to the
signal molecule thereby allowing communication.

The Three Stages of Cell Signaling:

1. Reception:

 Signal molecules: These extracellular ligands can be diverse, including hormones,

growth factors, neurotransmitters, and cytokines.

 Receptors: These membrane-bound or intracellular proteins have specific binding sites

for the corresponding signal molecule. Binding induces conformational changes in the
receptor, leading to its activation.

2. Transduction:

 Second messengers: These small, diffusible molecules are generated by activated

receptors and relay the signal within the cell. Common second messengers include
cyclic AMP (cAMP), calcium ions, and phosphoinositides.

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  Signal transduction pathways: These are intricate networks of protein kinases,
phosphatases, and other signaling molecules that amplify and refine the signal. Each
step in the pathway provides opportunities for regulation and control.

3. Response:

 Effector molecules: These proteins, enzymes, or transcription factors are ultimately

activated by the signal transduction pathway and elicit the cellular response. This
response can involve changes in gene expression, protein activity, cell metabolism, or
cell behavior.

Signaling within, between, and amongst cells is subdivided into the following classifications:

 Intracrine signals are produced by the target cells that stay within the target cell.

 Autocrine signals are produced by the target cell, are secreted, and affect the target cell
itself via receptors. Sometimes autocrine cells can target cells close by if they are the same
type of cell as the emitting cell. An example of this is immune cells.

 Juxtacrine signaling occurs when the two type of cells are adjacent to each other so that
contact is established through gap junctions or through protein molecules on the surface of
the two cells.
 Paracrine signals target cells in the vicinity of the emitting
cell. Neurotransmitters represent an example.
 Endocrine signals target distant cells. Endocrine cells produce hormones that travel
through the blood to reach all parts of the body.

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The main difference between the different categories of signaling is the distance that the
signal travels through the organism to reach the target cell.

Signaling molecules are the molecules that are responsible for transmitting information
between cells in the body. The size, shape, and function of different types of signaling
molecules can vary greatly. Some carry signals over short distances, while others
transmit information over very long distances.

There are a diverse number of small molecules and polypeptides that serve to coordinate a
cells individual biological activity within the context of the organism as a whole.

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 These molecules have been functionally classified as:-

1- Hormones (e.g., melatonin) 4- Cytokines (e.g., interferon – gamma )

2- Growth factor (e.g., epidermal growth 5- Chemokines (e.g., RANTES)
factor) 6- Neurotransmitters (e.g., acetylcholine )
3- Extra-cellular matrix components (e.g., 7- Neurotrophins ( e.g., nerve growth
fibronectin) factor)
Signaling molecules may trigger:

1. An immediate change in the metabolism of the cell (e.g., increased glycogenolysis

when a liver cell detects adrenaline);
2. An immediate change in the electrical charge across the plasma membrane (e.g., the
source of action potentials);
3. A change in the gene expression — transcription — within the nucleus. (These
responses take more time).

Types of Receptor:-
Receptors can be divided into:-
1- Intracellular receptors. 2- Cell-surface receptors.
Ligand – gated ion channel receptors are a class of receptor that may occur both at the cell-
surface or intracellular.
Cell-surface receptors:-

There are many different class of transmembrane receptor that recognizes different
extracellular signaling molecules:-

1- G- protein – coupled receptors. e.g., chemokine receptors.

2- Receptor tyrosine Kinases. e.g., growth factor receptors.
3- Integrins.
4- Toll-like receptors.

Second messengers are one of the initiating components of intracellular signal

transduction cascades ,they are released by the cell to trigger physiological changes such
as proliferation, differentiation, migration, survival, and apoptosis.

Changes in the concentration of small molecules (second messengers), constitute the next
step in the molecular information circuit. Particularly important second messengers include

4
cyclic AMP and cyclic GMP, calcium ion, inositol 1, 4, 5-trisphosphate, (IP3), and
diacyglycerol.

The use of second messengers has several consequences. First, second messengers are often
free to diffuse to other compartments of the cell, such as the nucleus, where they can
influence gene expression and other processes.

Second, the signal may be amplified significantly in the generation of second messengers.

Third, the use of common second messengers in multiple signaling pathways creates both
opportunities and potential problems.

1- G-Protein – coupled Receptors.

G protein-coupled receptor (GPCR), also called seven-transmembrane receptor or
heptahelical receptor, protein located in the cell membrane that binds extracellular
substances and transmits signals from these substances to an intracellular molecule called a
G protein (guanine nucleotide-binding protein).

There are numerous different types of GPCRs some 1,000 types are encoded by the human
genome alone, and as a group they respond to a diverse range of substances, including light,
hormones, amines, neurotransmitters, and lipids. Some examples of GPCRs include beta-
adrenergic receptors, which bind epinephrine; prostaglandin E2 receptors, which bind
inflammatory substances called prostaglandins; and rhodopsin, which contains a
photoreactive chemical called retinal that, responds to light signals received by rod cells in
the eye.

Structure; These receptors all have a similar structure with seven transmembrane domains.
GPCRs associate with heterotrimeric G-proteins (green), that is, GTP-binding proteins
composed of three different subunits: alpha, beta, and gamma. The subunits are tethered at
the membrane surface by covalently attached lipid molecules.

Activation; When a ligand binds, the receptor activates the attached G-protein by causing
the exchange of GTP (yellow) for GDP (red). The activated G-protein then dissociates into
an alpha (G-alpha) and a beta-gamma complex. G-alpha bound to GTP is active, and can
diffuse along the membrane surface to activate (and sometimes inhibit) target proteins,

5
often enzymes that generate second messengers. Likewise, the beta-gamma complex is
also able to diffuse along the inner membrane surface and affect protein activity.

Inactivation occurs because G-alpha has intrinsic GTPase activity. After GTP hydrolysis,
G-alpha bound to GDP will reassociate with a beta-gamma complex to form an inactive G-
protein that can again associate with a receptor.

A specific example of a receptor that couples to this type of G-protein is the beta-1
adrenergic receptor found in the heart. Beta 1 receptors are the principal type of adrenergic
receptor found in the heart. The ligand for this receptor is norepinephrine, the
neurotransmitter that is released by sympathetic postganglionic neurons. (As well, the
hormone epinephrine, released from the adrenal medulla, is also a ligand for these
receptors.) Stimulation of beta-1 receptors causes increased cAMP and PKA activation.
PKA phosphorylates various target proteins in cardiac cells to cause an increase in both the
heart rate and the strength of cardiac muscle contraction. Beta-1 receptors are the
targets of drugs (beta blockers) that are used to treat heart failure and hypertension.

The total strength of signal amplification by a G-protein – coupled receptor

(GPCR) is determined by:-

1- The lifetime of the ligand-receptor complex .If the ligand-receptor complex is stable, it
takes longer for the ligand to dissociate from its receptor, thus the receptor will remain
active for longer and will activate more effector proteins.

2- The amount and lifetime of the receptor-effector protein – complex.

The more effector protein is available to be activated by the receptor, and the faster the

6
 activated by the receptor, and the faster the activated effector protein can dissociates
from the receptor, the more effector protein will be activated in the same amount of time.

3- Deactivation of the activated receptor. A receptor that is engaged in a hormone –

receptor-complex can be deactivated either by covalent modification (for complex,
phosphorylation) or by internalization.

4- Deactivation of effectors through intrinsic enzymatic activity. Either small or large

G-proteins possess GTPase activity, which controls the duration of the triggered signal.
This activity may be increased through the action of other proteins such as GTPase –
activating proteins. (GAPS).

2- Receptor Tyrosine Kinases(RTK):-

Receptor tyrosine kinases (RTKs) are transmembrane proteins; they are enzymes that that
transfer phosphate groups from ATP to the amino acid Tyrosine on a protein. These cell
surface receptors bind and respond to growth factors and other some hormones like insulin.
RTKs play important roles in the regulation of cell growth, differentiation, and survival.

Activation; when signaling molecules bind to RTKs, they cause neighboring RTKs to
associate with each other, forming cross-linked dimers. Cross-linking activates the tyrosine
kinase activity in these RTKs through phosphorylation specifically; each RTK in the dimer
phosphorylates multiple tyrosines on the other RTK. Once activated, the cytoplasmic tails
of RTKs serve as docking platforms for various intracellular proteins involved in signal
transduction. These proteins have a particular domain that binds to phosphorylated tyrosine
in the cytoplasmic RTK receptor tails. More than one binding domains were containing
protein can bind at the same time to an activated RTK, allowing simultaneous activation of
multiple intracellular signaling pathways. Ultimately, RTK activation brings about changes
in gene transcription. Signaling becomes complex as signals travel from the membrane to
the nucleus, due to crosstalk between intermediates in various signaling pathways in the

cell.
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 Best example of RTKs is The Insulin Receptor

The insulin receptor is a tyrosine kinase receptor meaning its activation depends mainly on
activation of tyrosine residues. Structurally, the insulin receptor is a cell membrane receptor
consisting of four subunits, two α and two β subunits. α subunits lie totally outside the cell
membrane and are linked together, and with the β subunits, by disulfide bonds. α subunits
contain the ligand-binding domains whereas, the β subunits have both intracellular and
extracellular with a trans membranous domains. The β subunits contain the tyrosine
phosphorylation kinase domain that activates intracellular signaling pathways inside the
cell. Binding of insulin to the binding domains of α subunits leads to a conformational
changes in the insulin receptor resulting in an auto-phosphorylation of the tyrosine kinase
domain of the β subunit. This tyrosine phosphorylation in turn phosphorylates and activates
multiple intracellular enzymes that facilitate different cellular responses.

3- Integrins:-

Integrins are transmembrane receptors that mediate cell-adhesion. Integrins are produced by
a wide variety of cell types, and play a role in the attachment of a cell to the extracellular
matrix (ECM) and to other cells, and in the signal transduction of signals received from
extracellular matrix components such as fibronectin, collagen. Important differences were
between integrin signaling in circulating blood cells and that in non-circulating cells
such as epithelial cells. Integrins at the cell-surface of circulating cells are inactive under
normal physiological conditions. For example; cell-surface integrins on circulating
leukocytes are maintained in an inactive state in order to avoid epithelial cell attachment.
Only in response to appropriate stimuli are leukocyte integrins converted into an active
form, such as those received at the site of an inflammatory response.

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In a similar manner, it is important that integrins at the cell surface of circulating platelets
are kept in an inactive state under normal conditions, in order to avoid thrombosis.
Epithelial cells in contrast, have active integrins at their cell surface under normal
condition , which help to maintain their stable adhesion to underlying stromal cells.

4- Toll like receptors (TLRs):-

TLRs are membrane glycoproteins, homology in the

cytoplasmic receptors region-Interleukin-1 receptors
(IL-1Rs) superfamily.

The extracellular region of TLRs contains leucine-rich

repeat (LRR) motifs & IL1Rs contains three
immunoglobulin-like domains.

They are a type of pattern recognition receptor and

recognize molecules that are shared by pathogens but
distinguishable from host molecules , once they have
reached physical barriers such as the skin or intestinal
tract mucosa and activate immune cell responses , they
are play a key role in the innate immune system.

TLRs seem only to be involved in the cytokine production and cellular activation
in response to microbes, and do not play a significant role in the adhesion and
phagocytosis of microorganisms.

5- Ligand – gated ion channel receptors:-

A ligand – activated ion channel will recognize its ligand , and then undergo a structural
change that opens a gap (channel)in the plasma membrane through which ions can pass
these ions will then relay the signal.

Signal transduction
It occurs when an extracellular signaling molecule activates a cell surface receptor. In turn,
this receptor alters intracellular molecules creating a response. There are two stages in this
process:
1. A signaling molecule activates a specific receptor protein on the cell membrane.

9
 2. A second messenger transmits the signal into the cell, eliciting a physiological
response.

Principles of Signal Transduction

An environmental signal, such as a hormone, is first

received by interaction with a cellular component,
most often a cell-surface receptor. The information
that the signal has arrived is then converted into other
chemical. One signaling molecule can cause many
responses, the signal must be amplified so that the
response is carried out multiple times rather than just
be a single molecule.

Amplification is built into the system. Any molecule

that catalyzes a reaction can do so multiple times
producing more than one product molecule. So
each step in the signaling chain has the potential for
amplification.

General Features of Signal Transduction

Signal transductions are remarkably specific and exquisitely sensitive. Specificity is
achieved by precise molecular complementarity between the signal and receptor
molecules.

Cellular Responses:-

Activation of genes, alteration in metabolism, the continued proliferation death of the cell
and the stimulation or suppression, is some of the cellular signal transduction.

Most mammalian cells require stimulation to control not only cell division but also
survival.

Cellular response

The binding of chemical signals to their corresponding receptors induces events within the
cell that ultimately change its behavior. The nature of these intracellular events differs

10
 according to the type of receptor. Also, the same chemical signal can trigger different
responses in different types of cell.

Cell surface receptors work in several ways when they are occupied. Some receptors enter
the cell still bound to the chemical signal. Others activate membrane enzymes, which
produce certain intracellular chemical mediators. Still other receptors open membrane
channels, allowing a flow of ions that causes either a change in the electrical properties of
the membrane or a change in the ion concentration in the cytoplasm. This regulation of
enzymes or membrane channels produces changes in the concentration of intracellular
signaling molecules, which are often called second messengers (the first messenger being
the extracellular chemical signal bound to the receptor).

Two common intracellular signaling molecules are cyclic AMP and the calcium ion. Cyclic
AMP is a derivative of adenosine triphosphate, the ubiquitous energy-carrying molecule
of the cell. The intracellular concentrations of both cyclic AMP and calcium ions are
normally very low. The binding of an extracellular chemical signal to a cell surface
receptor stimulates an enzyme complex in the membrane to produce cyclic AMP. This
second messenger then diffuses into the cytoplasm and acts on intracellular enzymes
called kinases that modify the behavior of the cell, culminating in the activation of target
genes that increase the synthesis of certain proteins. The action of cyclic AMP is brief
because it is rapidly degraded by specific enzymes.

The end

11
 Chemistry and Biochemistry department/ College of medicine
/ Mustansiriyah University

Dr. Ali Al-bayati biochemistry of hormones Lec. 2

Endocrinology is the study of medicine that relates to the endocrine system, which is the
system that controls hormones. An endocrinologist will deal with diseases that are caused by
problems with hormones.

The endocrine system is a series of glands that produce and secrete hormones that the body
uses for a wide range of functions. These control many different bodily functions, including:
Respiration, Metabolism, Reproduction, Sensory perception, Movement, Sexual
development and Growth.

The endocrine system is one of the two coordinating and integrating systems of the body. It
acts through chemical messengers - hormones –carried in the circulation.
Two systems control all physiologic processes:
1. The nervous system exerts point-to-point control through nerves, similar to sending
messages by conventional telephone. Nervous control is electrical in nature and fast.
2. The endocrine system broadcasts its hormonal messages to essentially all cells by
secretion into blood and extracellular fluid. Like a radio broadcast, it requires a
receiver to get the message - in the case of endocrine messages, cells must bear a
receptor for the hormone being broadcast in order to respond.

The endocrine system consists of

several glands, in different parts of the
body that secrete hormones directly into
the blood rather than into a duct system.
Hormones have many different
functions and modes of action; one
hormone may have several effects on
different target organs, and, conversely,
one target organ may be affected by
more than one hormone. Hormones are
produced by glands and sent into the
bloodstream to the various tissues in the body. They send signals to those tissues to tell them
what they are supposed to do. When the glands do not produce the right amount of
hormones, diseases develop that can affect many aspects of life.

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Characteristics of the Endocrine System;
1. Composed of glands that secrete hormones into the circulatory system.
2. Hormones are secreted in minute amounts into the interstitial space.
3. Hormones eventually enter the circulatory system and arrive at specific target tissues.
4. Functions are similar to the nervous system. Differences;
 Amplitude-modulated vs. Frequency-modulated
 Response of target tissue to hormones is usually slower and of longer duration
than that to neurons.

Hormones
The term hormone (hormao G = to excite) was first used by William M. Bayliss and his
brother-in-law Ernest H. Starling, both of London University College, in 1904, who
showed that a chemical substance (secretin) from the intestine could stimulate the action of a
pancreatic secretion. These substances were then called as “chemical messengers”. Went and
Thiemann (1937) defined a hormone as “a substance which, produced in any one part of an
organism, is transferred to another part and there influences a specific physiological
process.” The tissues or organs where they are produced are called as effectors and those
where they exert their influence as targets.

Based on their site on action, the hormones are of two types: local and general.

The local hormones, obviously, have specific local effects, whence their
nomenclature. These may be exemplified by acetylcholine, secretin, cholecystokinin
etc.
The general hormones, on the other hand, are secreted by specific endocrine glands
and are transported in the blood to cause physiologic actions at points remote from
their place of origin. A few of the general hormones affect almost all cells of the body,
e.g., growth hormones (GH) and thyroid hormones ; whereas other general hormones,
however, affect specific tissues far more than other tissues, e.g., adrenocorticotropic (a
hormone secreted from adenohyprophysis and stimulating the adrenal cortex) and
ovarian hormones (affecting the uterine endometrium).

The hormones conduct a wide variety of functions ranging from growth, vegetative and
sexual development, cellular oxidation to thermal production and the metabolism of

2
carbohydrates, proteins and fats. The various functions performed by hormones may, in
general, be discussed under following heads:
1. Regulatory or homeostatic function. The hormones have regulatory effects on the
composition of the body fluids, the rate of gaseous exchange and the activity of the
vascular system and the central nervous system (CNS). There always exists a high degree
of precision and constancy in the composition of the body fluids in a normal individual
for the conduction of various activities.
2. Permissive function. Not only does each endocrine gland affect a number of processes,
but these glands also affect the functioning of one another. Thus certain hormones
require the presence (or “permission‘) of another hormone for the expression of their
activity. This helps in maintaining a perfect hormonal balance. Derangements of this
balance, clinical or experimental, lead to a variety of metabolic aberrations.
3. Integrative function. The integrative function of the hormones is reflected in the fact
that they support the role of nervous system. However, the integrative properties of the
endocrine system are slow and steady whereas those of the nervous system are rapid.
This close tie between the two systems has led to the emergence of a new discipline of
science called neuroendocrinology.
4. Morphogenetic function. The hormones govern the ontogenetic development of an
individual from the embryonic to the adult state.

Classes of Hormones
Hormones are classified by various criteria:
1. By Proximity of their site of synthesis to their site of action,
2. By their chemical structure,
3. By their degree of solubility in aqueous medium
How are hormones classified by proximity of site of synthesis to site of action?
3 classes of hormones based on proximity of site of Synthesis to Site of Action: (Fig. 1)
I. Autocrine Hormones: those that act on the same cells that synthesize them;
II. Paracrine Hormones: those that are synthesized very close to their site of action;
III. Endocrine Hormones: those that are synthesized by endocrine glands and
transported in the blood to target cells that contain the appropriate receptors;

3
Four classes of hormones based on chemical structure:
1. Peptides or Protein hormones:

They are synthesized as peptides or large polypeptides precursors that undergo processing
before secretion;

Most peptide hormones circulate unbound to other proteins, but exceptions exist; for
example, insulin-like growth factor-1 binds to one of several binding proteins. In general, the
half-life of circulating peptide hormones is only a few minutes. Examples:

Thyrotropin Releasing Hormone (TRH), made up of three amino acid residues;

Insulin, made up of 51 amino acid residues;
Pituitary Gonadotropins, made up of large Glycoproteins with subunits;
2. Amino acid derivatives: Examples:

• Adrenaline, Catecholamines, Thyroid Hormones;

3. Fatty acid derivatives: Examples:

Eicosanoids are a large group of molecules derived from polyunsaturated fatty acids. The
principal groups of hormones of this class are prostaglandins, prostacyclins, leukotrienes
and thromboxane. Arachidonic acid is the most abundant precursor for these hormones.
Stores of Arachidonic acid are present in membrane lipids and released through the action
of various lipases.

A great variety of cells produce prostaglandins , including those of the liver, kidneys,
heart, lungs, thymus gland, pancreas, brain, and reproductive organs. In contrast to
hormones, prostaglandins usually act locally, affecting only adjacent cells or the very cell
that secreted it. Prostaglandins are potent and are presented in very small quantities. They
are not stored in cells but rather are synthesized just before release. These hormones are
rapidly inactivated by being metabolized, and are typically active for only a few seconds

4. Steroid hormones:

• These are derivatives of Cholesterol; Example: Estradiol, Testosterone, Cortisol, and

Aldosterone;

How are hormones classified according to solubility in aqueous medium in cells?

Two classes of hormones based on solubility in aqueous medium;

4
 1. Hydrophilic Hormones (Lipophobic Hormones);
2. Lipophilic Hormones (Hydrophobic Hormones);

Location of receptors for each class of hormone is different;

Hydrophilic Hormones (Lipophobic Hormones): Hormones that are soluble in aqueous

medium; They cannot cross the cell membrane, Thus, they bind to receptor molecules
on the outer surface of target cells, initiating reactions within the cell that ultimately
modifies the functions of the cells; Examples: Insulin, Glucagon, Epinephrine,
Lipophilic Hormones (Hydrophobic Hormones): Hormones that are not soluble in
aqueous medium, but soluble in lipid; They can easily cross the cell membrane, Thus,
they can enter target cells and bind to intracellular receptors to carry out their action;
Examples: Thyroid hormones, Steroid hormones;

Figure 1. Classification of hormones by chemical nature

5
Hormones’ Receptors and Target Cells

Receptors and Target Cells

A given hormone usually affects only a limited number of cells, which are called target cells.
A target cell responds to a hormone because it bears receptors for the hormone.

Target tissue must have two characters in response to the hormone:-

1. The tissue should recognize the hormone by a receptor.
2. The tissue should have intracellular capacity of translating the massage of the
hormone to biochemical event or reaction.

Receptors: - Are specific molecules within the cell membrane, cytoplasm or nucleus of the
target cell that are necessary for recognition and binding of extra cellular messenger
(hormone)

Interactions with receptors

Most hormones initiate a cellular response by

initially combining with either a specific
intracellular or cell membrane associated
receptor protein. A cell may have several
different receptors that recognize the same
hormone and activate different signal
transduction pathways, or a cell may have
several different receptors that recognize different hormones and activate the same
biochemical pathway.

Hormone-receptor complex concentrations are effectively determined by three factors:

1. The number of hormone molecules available for complex formation.

2. The number of receptor molecules available for complex formation.
6
3. The binding affinity between hormone and receptor.
The number of hormone molecules available for complex formation is usually the key factor
in determining the level at which signal transduction pathways are activated, the number of
hormone molecules available being determined by the concentration of circulating hormone,
which is in turn influenced by the level and rate at which they are secreted by biosynthetic
cells. The number of receptors at the cell surface of the receiving cell can also be varied, as
can the affinity between the hormone and its receptor.

Two important terms are used to refer to molecules that bind to the hormone-binding
sites of receptors:

Agonists: are molecules that bind the receptor and induce all the post-receptor events
that lead to a biologic effect. In other words, they act like the "normal" hormone,
although perhaps more or less potently.
Antagonists: are molecules that bind the receptor and block binding of the agonist, but
fail to trigger intracellular signaling events. Hormone antagonists are widely used as
drugs.
Characteristics of Receptors
1- Hormone specificity
Receptors can bind one hormone only. Agonists and antagonists can also bind the receptors.
Many neurohormonal ligands and hormones can bind several different receptors usually
within the same molecular family of receptors (i.e. epinephrine binds α1, α2, β1, and β2
adrenergic receptors).
2- Affinity: refers to the concentration of hormones required to activate sufficient
receptors to trigger a biological effect.
3- Receptors number: On average, a diploid cell may have receptors in the order of 103
to 105. Lower numbers of receptors can sometimes be compensated by higher affinity
of the receptor for the hormone.
4- Tissue Specificity: The presence of the appropriate type of receptors in the respective
tissue is essential for proper hormone function.

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Pathways of Hormone Action
The message a hormone sends is received by a hormone receptor, a protein located either
inside the cell or within the cell membrane. The receptor will process the message by
initiating other signaling events or cellular mechanisms that result in the target cell’s
response. Hormone receptors recognize molecules with specific shapes and side groups, and
respond only to those hormones that are recognized. The same type of receptor may be
located on cells in different body tissues, and trigger somewhat different responses. Thus, the
response triggered by a hormone depends not only on the hormone, but also on the target
cell. Once the target cell receives the hormone signal, it can respond in a variety of ways.

The response may include; 3. Alteration in the permeability of the

1. the stimulation of protein synthesis cell membrane,
through gene modification. 4. altered rates of mitosis and cell
2. activation or deactivation of growth,
enzymes, 5. Stimulation of the secretion of
products.
Moreover, a single hormone may be capable of inducing different responses in a given cell.

Two main type of hormone receptor

1. Pathways Involving Intracellular Hormone Receptors (Group I)

2. Pathways Involving Cell Membrane Hormone Receptors (Group II)

8
More specifically, when a receptor becomes bound to a hormone, it undergoes
conformational changes that allow it to interact productively with other components of the
cells, leading ultimately to an alteration in the physiologic state of the cell. Despite the
molecular diversity of hormones, all hormone receptors can be categorized into one of two
types, based on their location within the cell;

Intracellular Hormone Receptors

Intracellular hormone receptors are located
inside the cell. Hormones that bind to this type
of receptor must be able to cross the cell
membrane. Steroid hormones are derived
from cholesterol and therefore can readily
diffuse through the lipid bilayer of the cell
membrane to reach the intracellular receptor.
Thyroid hormones, which contain benzene
rings studded with iodine, are also lipid-
soluble and can enter the cell.

The location of steroid and thyroid hormone

binding differs slightly: a steroid hormone may bind to its receptor within the cytosol or
within the nucleus. In either case, this binding generates a hormone-receptor complex that
moves toward the chromatin in the cell nucleus and binds to a particular segment of the cell’s
DNA. In contrast, thyroid hormones bind to receptors already bound to DNA. For both
steroid and thyroid hormones, binding of the hormone-receptor complex with DNA triggers
9
transcription of a target gene to mRNA, which moves to the cytosol and directs protein
synthesis by ribosomes.

Thus, the mechanism of action of these hormones is to modulate gene expression in

target cells. By selectively affecting transcription from a battery of genes, the concentration
of those respective proteins are altered, which clearly can change the phenotype of the cell.

Structure of Intracellular Receptors

Steroid and thyroid hormone receptors are members of a large group of transcription factors.
All of these receptors are composed of a single polypeptide chain that has, three distinct
domains:

1- The amino-terminus: In most cases, this region is involved in activating or stimulating

transcription by interacting with other components of the transcriptional machinery. The
sequence is highly variable among different receptors.
2- DNA binding domain: Amino acids in this region are responsible for binding of the
receptor to specific sequences of DNA.
3- The carboxy-terminus or ligand-binding domain: This is the region that binds
hormone.

Hormone-Receptor Binding and Interactions with DNA

Being lipids, steroid hormones enter the cell by simple diffusion across the plasma
membrane. Thyroid hormones enter the cell by facilitated diffusion. The receptors exist
either in the cytoplasm or nucleus, which is where they meet the hormone.

When hormone binds to receptor, a characteristic series of events occurs:

Receptor activation is the term used to describe conformational changes in the

receptor induced by binding hormone. The major consequence of activation is that the
receptor becomes competent to bind DNA.
Activated receptors bind to hormone response elements, which are short specific
sequences of DNA which are located in promoters of hormone-responsive genes.
Transcription from those genes to which the receptor is bound is affected. Most
commonly, receptor binding stimulates transcription.
The hormone-receptor complex thus functions as a transcription factor. The end
10
 Chemistry and Biochemistry department/ College of medicine
/Mustansiriyah University
Dr. Ali al-bayati Endocrinology Lec. 3

Hormones with Cell Surface Receptors

Protein and peptide hormones, Catecholamines like epinephrine, and eicosanoids such as
prostaglandins find their receptors decorating the plasma membrane of target cells. Binding
of hormone to receptor initiates a series of events which leads to generation of so-called
second messengers within the cell (the hormone is the first messenger). The second
messengers then trigger a series of molecular interactions that alter the physiologic state of
the cell. Another term used to describe this entire process is signal transduction.

Structure of Cell Surface Receptors was described in lecture 1.

Second Messenger Systems

Nonsteroid hormones (water soluble) do not enter the cell but bind to plasma membrane
receptors, generating a chemical signal (second messenger) inside the target cell. Second
messengers activate other intracellular chemicals to produce the target cell response.

Second Messenger produced in response to binding of signal molecules (hormones) to

receptors. They bring about activation of enzyme cascades or changes in plasma membrane
potential.

They may be water soluble or lipid soluble.

Water soluble messengers are cAMP, cGMP, Inositol triphosphate (ITP).

Lipid soluble messengers are Diacyl glycerol (DAG) and Phosphatidyl inositol
triphosphate (PIP3).

Currently, four second messenger systems are recognized in cells, Note that not only do
multiple hormones utilize the same second messenger system, but a single hormone can
utilize more than one system.

In all cases, the seemingly small signal generated by hormone binding its receptor is
amplified within the cell into a cascade of actions that changes the cell's physiologic state.

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Presented below are two examples of second messenger systems commonly used by
hormones.

Hormones Acting through Cyclic AMP

Cyclic AMP (cAMP) was first discovered by Earl Sutherland in 1961, who was awarded
Nobel Prize in 1971. Signal transduction pathways are like a river flowing in one direction
only; components closer to the receptor are called “upstream” and closer to the response are
called “downstream”.

Many hormones exert their effect on cells by first causing the formation of a substance,
cyclic 3′, 5′-adenosine monophosphate (figure) in the cell.

Once formed, the cyclic AMP causes the hormonal effects inside the cell. Thus, cyclic AMP
acts as an intracellular hormonal mediator. It is also frequently referred to as the second
messenger for hormone mediation; the first messenger being the original hormone itself.

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Cyclic adenosine monophosphate (cAMP) is a nucleotide generated from ATP through the
action of the enzyme adenylate cyclase. The intracellular concentration of cAMP is
increased or decreased by a variety of hormones and such fluctuations affect a variety of
cellular processes.

Hormones stimulate adenyl cyclase: ACTH, ADH, Calcitonin, CRH, FSH, Glucagon,
epinephrine, hCG, LH, LPH, MSH, PTH and TSH.

Hormones inhibit adenyl cyclase: Acetylcholine, angiotensin II and somatostatin.

The cell contains receptor for hormones in the plasma membrane (G-protein coupled
receptors). The stimulating hormone acts at the plasma membrane of the target cell and
combines with a specific receptor for that particular type of hormone. The specificity of the
receptor determines which hormone will affect the target cell. The combination of the
hormone with its receptor leads to the activation of the enzyme, adenyl cyclase, which is also
bound to the plasma membrane. The portion of the adenyl cyclase that is exposed to the
cytoplasm causes immediate conversion of cytoplasmic ATP into cAMP.

The cAMP then acts inside the cell to initiate a number of cellular functions before it itself is
destroyed. The various functions initiated include:

a) activating the enzymes b) altering the cell permeability

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 c) synthesizing the intracellular proteins d) contracting or relaxing the muscles
e) Releasing other hormones (third messengers).

Cyclic AMP is, however, destroyed (or inactivated) by a specific enzyme called
phosphodiesterase, which hydrolyzes it to AMP. Like adenyl cyclase, the phosphodiesterase
is present in practically all tissues.

Cyclic AMP elicits many of its effects by activating protein kinases. Protein kinases are
ubiquitous in nature and are activated by cAMP at extremely low concentrations of 10–8 M.
these kinases molecules mediate the activities of different proteins in different cells by
phosphorylating them. The enzyme protein kinase (figure) consists of two subunits: a
catalytic subunit and a regulatory subunit which can bind cAMP. In the absence of cAMP,
the catalytic and regulatory subunits form a complex that is enzymatically inactive.

In the presence of cAMP, however, the complex disintegrates, freeing the catalytic subunit
which now becomes catalytically active. The regulatory subunit binds cAMP to form a
complex. Thus, the binding of cAMP to the regulatory subunit relieves its inhibition of the
catalytic subunit. The cAMP acts as an allosteric effector.

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Clinical Significances;

 Cholera toxins are produced by the bacteria Vibrio cholerae. The enterotoxin
modified alpha subunit of Gs protein. This results in the inhibition of the inherent
GTPase activity and irreversible activation of G protein. Therefore, adenyl cyclase
remains continuously active and keeps cyclic AMP levels high. This prevents
absorption of salts from intestine leading to watery diarrhea and loss of water from
body.

Second Messenger in Tyrosine Kinase Systems;

Insulin is an example of a hormone whose receptor is a tyrosine kinase.

The hormone binds to domains exposed on the cell's surface, resulting in a conformational
change that activates kinase domains located in the cytoplasmic regions of the receptor. In
many cases, the receptor phosphorylates itself as part of the kinase activation process. The
activated receptor phosphorylates a variety of intracellular targets, many of which are
enzymes that become activated or are inactivated upon phosphorylation. As seen with cAMP
second messenger systems, activation of receptor tyrosine kinases leads to rapid modulation
in a number of target proteins within the cell. Some of the targets of receptor kinases are
protein phosphatases which, upon activation by receptor tyrosine kinase, become competent
to remove phosphates from other proteins and alter their activity. Again, a seemingly small
change due to hormone binding is amplified into a multitude of effects within the cell.

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Calcium-based Signal Transduction;
Calcium is an important intracellular regulator of cell function like contraction of muscles,
secretion of hormones and neurotransmitters, cell division and regulation of gene regulation.
Rapid but transient increase in cytosolic calcium result from either opening of calcium
channels in the plasma membrane or calcium channels in the ER. The released calcium can
be rapidly taken-up by ER to terminate the response.

Hormones can increase the cytosolic calcium level by the following mechanisms:

A. By altering the permeability of the membrane.

B. The actions of Ca-H+-ATPase pump which extrudes calcium in exchange for H+.
C. By releasing the intracellular calcium stores.
D. Calmodulin, the calcium dependent regulatory protein within the cell has four
calcium binding sites. When calcium binds there is a conformational change to the
calmodulin, which has a role in regulating various kinases.
Hormones Acting through PIP2 Cascade
The major player in this type of signal transduction is phospholipase C that hydrolyses
phosphatidyl inositol in membrane lipids to 1, 4, 5-Inositol triphosphate (IP3). The
phospholipase C may be activated either by G-proteins or calcium ions.

The binding of hormones like serotonin to cell surface receptor triggers the activation of the
enzyme phospholipase-C which hydrolyzes the phosphatidyl inositol to diacyglycerol. IP3
can release Ca++ from intracellular stores, such as from endoplasmic reticulum and from
sarcoplasmic reticulum. The elevated intracellular calcium then triggers processes like
smooth muscle contraction, glycogen breakdown and exocytosis.

Diacylglycerol (DAG,) the other second messenger, activates the enzyme protein kinase C
(PKC), which then alter physiological processes. Thus the hydrolysis of phosphatidylinositol
bisphosphate leads to activation of protein kinase – C and promotes an increase of
cytoplasmic calcium ion.

Cyclic guanosine monophosphate (cyclic GMP);

Cyclic GMP is a nucleoside similar to cAMP and is found in most tissues. It can probably
catalyze some intracellular functions in a manner similar to that of cAMP. cGMP involved in
contractile function of smooth muscles, visual signal transduction and maintenance of blood
volume.

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Control of Endocrine Activity;
The physiologic effects of hormones depend largely on their concentration in blood and
extracellular fluid.

The concentration of hormone as seen by target cells is determined by three factors:

1- Rate of production: Synthesis and secretion of hormones are the most highly
regulated aspect of endocrine control. Such control is mediated by positive and
negative feedback circuits.
2- Rate of delivery: An example of this effect is blood flow to a target organ or group of
target cells:
3- Rate of degradation and elimination: Hormones, like all biomolecules, have
characteristic rates of decay, and are metabolized and excreted from the body through
several routes.

TRANSPORT OF HORMONES:
Hormones must be transported at least some distance to their target organs. The primary
transport medium is the plasma, although the lymphatic system and the cerebrospinal fluid
are also important. Since delivery of the hormone to its target tissue is required before a
hormone can exert its effects, the presence or absence of specific transport mechanisms play
a major role in mediating hormonal action.

A. The water-soluble hormones (peptide hormones, catecholamines) are transported in

plasma in solution and require no specific transport mechanism. Because of this, the
water-soluble hormones are generally short-lived. These properties allow for rapid
shifts in circulating hormone concentrations, which is necessary with the pulsatile
tropic hormones or the catecholamines. This is consistent with the rapid onset of
action of the water-soluble hormones.
B. The lipid-soluble hormones (thyroid hormone, steroids) circulate in the plasma bound
to specific carrier proteins. Many of the proteins have a high affinity for specific
hormone, such as thyroxine-binding globulin (TBG), sex hormone-binding globulin
(SHBG), and cortisol-binding globulin (CBG). Non-specific, lowaffinity] binding of
these hormones to albumin also occurs. Carrier proteins act as reservoirs of hormone.
Since it is generally believed that only the free hormone can enter cells, a dynamic

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 equilibrium must exist between the bound and free hormone. Thus, alterations in the
amount of binding protein available, or in the affinity of the hormone for the binding
protein, can markedly alter the total circulating pool of hormone without affecting the
free concentration of hormone.

Carrier proteins act as buffers to both blunt sudden increases in hormone concentration and
to diminish degradation of the hormone once it is secreted. Thus, the half-life of hormones
that utilize carrier proteins is longer than those that are not protein bound. Indeed, carrier
proteins have a profound effect on the clearance rate of hormones; the greater the capacity
for high affinity binding of the hormone in the plasma, the slower the clearance rate. Also,
the carrier proteins allow slow, tonic delivery of the hormone to its target tissue. This is
consistent with the slower onset of action of the lipid-soluble hormones.

HORMONE METABOLISM:
Clearance of hormones from the circulation plays a critical role in the modulation of
hormone levels in response to varied physiologic and pathologic processes. The time
required to reach a new steady-state concentration in response to changes in hormone release
is dependent upon the half-life of the hormone in the serum. Thus, an increase in hormone
release or administration will have a much more marked effect if the hormone is cleared
rapidly from the circulation as opposed to one that is cleared more slowly.

Most peptide hormones have a plasma half-life measured in minutes, consistent with the
rapid actions and pulsatile nature of the secretion of these hormones. This rapid clearance is
achieved by the lack of protein binding in the plasma, degradation or internalization of the
hormone at its site of action, and ready clearance of the hormone by the kidney. Binding to
serum proteins markedly decreases hormone clearance, as is observed with the steroid
hormones and the iodothyronine.

Metabolism of the steroid hormones occurs primarily in the liver by reductions,

conjugations, oxidations, and hydroxylation, which serve to inactivate the hormone and
increase their water-solubility, facilitating their excretion in the urine and the bile. Metabolic
transformation also may serve to activate an inactive hormone precursor, such as the
deiodination of thyroxine to form T3. Hormone metabolism is not as tightly regulated as is

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hormone synthesis and release. However, alterations in the metabolic pathways may be
clinically important.

Feedback Control of Hormone Production

Feedback circuits are at the root of most control mechanisms in physiology, and are
particularly prominent(obvious) in the endocrine system. Instances of positive feedback
certainly occur, but negative feedback is much more common. Feedback loops are used
extensively to regulate secretion of hormones in the hypothalamic-pituitary axis. An
important example of a negative feedback loop is seen in control of thyroid hormone
secretion. The thyroid hormones thyroxine and triiodothyronine ("T4 and T3") are
synthesized and secreted by thyroid glands and affect metabolism throughout the body. The
basic mechanisms for control in this system are:

 Neurons in the hypothalamus secrete thyroid releasing hormone (TRH), which

stimulates cells in the anterior pituitary to secrete thyroid-stimulating hormone (TSH).
 TSH binds to receptors on epithelial cells in the thyroid gland, stimulating synthesis
and secretion of thyroid hormones, which affect probably all cells in the body.
 When blood concentrations of thyroid hormones increase above a certain threshold,
TRH-secreting neurons in the hypothalamus
are inhibited and stop secreting TRH.

This is an example of "negative feedback".

Inhibition of TRH secretion leads to shut-off of

TSH secretion, which leads to shut-off of

thyroid hormone secretion. As thyroid hormone

levels decay below the threshold, negative

feedback is relieved, TRH secretion starts again,

leading to TSH secretion.

General principles of Endocrine Diagnosis

1- A hormone is a substance that is secreted by an endocrine gland and transported in blood
to regulate the function of another tissue or other glands.
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The physiologic effects of hormones depend largely on their concentration in blood and
extracellular fluids Almost , inevitably, disease results when hormone concentrations are
either too high or too low, and precise control over circulating concentrations of hormones is
therefore crucial.

2- The hormone secretion may vary over a 24-hr: circadian or may be episodic or may
respond to physiological stimuli (stress).

3- If hypo function is suspected, the samples are taken when the levels should be high and
vice versa when investigate hyper function.

4- Results near to limits of normal reference range, the (Dynamic Tests) should be carried
when the response of gland or the feedback mechanism is assessed after stimulation or
suppression by administration of exogenous H.

a) Suppression Tests

Used for differential diagnosis of excessive H. secretion so failure to suppress implies that
the secretion is not under normal feedback control.

b) Stimulation Tests Used mainly for differential diagnosis of deficient H. secretion when
normal response exclude the abnormality of the target gland, while failure to respond will
confirm the diagnosis.

The end

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