Hormones and Cell Signaling
Cellular Communication
• Everything an animal does involves communication
among cells
• e.g., moving, digesting food
• Cell signaling – communication between cells
• Signaling cell: sends a signal (usually chemical)
• Target cell: receives the signal
Types of Cell Signaling
Direct: via gap junctions
Indirect: signaling cell releases a chemical messenger that binds
to a receptor on the target cell and activates a signal
transduction pathway
•
Short distances
• Paracrine: diffusion to a nearby cell
• Autocrine: diffusion back to the signaling cell
•
Long distances
• Endocrine: hormone is transported by the circulatory
system
• Neural: electrical signal travels along a neuron and
releases a neurotransmitter
Types of Cell Signaling
Chemical Messengers
Two main types
1.Hydrophilic
2.Hydrophobic
Direct Signaling
• Gap junctions – specialized
protein complexes that
create an aqueous pore
between two adjacent cells
• Hydrophilic chemical
messengers can travel
through the lipid membrane
• Typically involves the
movement of ions
Figure 4.2
Indirect Signaling
Three steps
1. Release of chemical messenger from the
signaling cell
2. Transport of the messenger through the
extracellular environment to the target cell
3. Communication of the signal to the target cell
Release of the Chemical Messenger
• Hydrophobic messengers can cross the cell membrane
by diffusion
• Hydrophilic messengers are packed into vesicles
where they are stored until they are released by
exocytosis
• Exocytosis – vesicle fuses with the plasma membrane
and releases its contents
Transport to the Target Cell
• Hydrophilic messengers
dissolve in aqueous
solutions like extracellular
fluid and blood
• Hydrophobic messengers
bind to carrier proteins in
the blood
• Carrier protein – help
hydrophobic messengers
dissolve in aqueous
solutions
• M + C  M-C
Transport of Hydrophobic Messengers
Communication to the Target Cell
• Receptors
• Hydrophilic: transmembrane protein
• Hydrophobic: intracellular proteins
• Ligand – chemical messenger that can
bind to a specific receptor
• The receptor changes shape when it binds
to the receptor
Ligand-Receptor Interactions
• Only the correctly shaped
ligand (natural ligand) can
bind to the receptor
• Ligand mimics (e.g., drugs
and poisons)
• Agonists – activate
receptors
• Antagonists – block
receptors
Ligand-Receptor Dynamics
• L + R  L-R  response
• More free ligand (L) or receptors (R) will increase the
response
• Receptors can become saturated
Ligand-Receptor Dynamics, Cont.
• More receptors   response
• Higher affinity constant (Ka) 
 response
• Down-regulation – constant
exposure to ligands decrease
the number of receptors, e.g.,
habitual coffee drinkers
(mmmmmhhh)
• Up-regulation – opposite, e.g.,
alcohol withdrawal
Signal Transduction Pathways
Convert the change in shape of a receptorligand into a complex intracellular
response
Transducers
• Convert signals from one
form to another
• Four components
• Receiver: ligand binding
receptor
• Transducer:
conformational change
of the receptor
• Amplifier: the signal
transduction pathway
increases the number of
molecules affected
• Responder: something
that responds to the
signal
Figure 4.9
Types of Receptors
Intracellular Receptors
Regulate the transcription of target genes by binding to
specific DNA sequences, and increasing or decreasing
mRNA production
Ligand-Gated Ion Channels
•
•
•
•
Ligand binds to receptor
Receptor changes shape opening a channel
Ions move across the membrane
Concentration and electrical gradients dictate the direction
of ion movement
• Movement of ions change ion concentrations which alters
the membrane potential
Receptor Enzymes
• When activated by a ligand the catalytic domain starts a
phosphorylation cascade
• Named based on the reaction catalyzed
G-Protein-Coupled Receptors
• Transmembrane protein that interacts with intracellular
G-proteins
• G-proteins – named for their ability to bind guanosine
nucleotides
• Activate second messengers
Second Messengers
Systems for Cell Signaling
Autocrine and Paracrine
• Many chemicals act as paracrine messengers
• Eicosanoids – act only in autocrine and paracrine cell
signaling
• Prostaglandins – involved in pain reception; blocked by
many painkillers
Nervous System
• Specialized collection of cells that can carry signals
across long distances
• Neurons allow electrical signals to be propagated across
long distances within a single cell
• Synapse – region in between two neurons or a neuron
and other target cells
• Gap junctions
• Chemical: neurotransmitters
Endocrine System
• Sends chemicals (hormones) through the blood
• Produced by endocrine glands
• Other chemicals can act as hormones, e.g.,
neurohormones
• Non-endocrine organs can produce hormones, e.g.,
heart
• Three types of hormones
• Peptides
• Steroids
• Amines
Three Types of Hormones
Table 4.4
Peptide Hormones
•
•
•
•
Synthesized on the rough ER
Stored in vesicles
Leave signaling cell via exocytosis
Soluble in aqueous solutions and travel to the target cell dissolved in
the extracellular fluid
• Hydrophilic: cannot cross the target cell membrane
• Bind to transmembrane receptors
• Rapid effects on the target cell
Synthesis of Peptide Hormones
Figure 4.23
Steroid Hormones
• Derived from cholesterol
• Hydrophobic: can pass through the plasma membrane
• Enzymes for synthesis are in the smooth ER or
mitochondria
• Cannot be stored within the cell
Steroid Hormones, Cont.
• Must be synthesized on demand
• Transported to target cell by carrier proteins
(e.g., albumin)
• Sloooooow effects on the target cell (regulate
transcription) – exception: stress hormone
cortisol has rapid non-genomic effects
Three Classes of Steroid Hormones
• Mineralocorticoids
• Electrolyte balance
• e.g., aldosterone
• Glucocorticoids
• Stress hormones
• e.g., cortisol
• Reproductive hormones
• Regulate sex-specific characteristics and
reproduction
• e.g., estrogen, progesterone, testosterone
Amine Hormones
• Chemicals that possess amine (-NH2)
• e.g., acetylcholine, catecholamines (dopamine,
norepinephrine, epinephrine), serotonin, melatonin,
histamine, thyroid hormones
• Some are true hormones, some are neurotransmitters,
some are both
• Diverse effects
Exocrine Cell Signaling
• Secretions via ducts to the outside of the body including the
skin, respiratory surfaces, and the gut
• e.g., salivary glands, prostate, sweat glands
• Cell-to-cell: pheromones, allelochemicals
Regulation of Cell Signaling
•Feedback
• Positive – output acts as a further stimulus
• Negative – output reduces the stimulus
Regulation of Cell Signaling, Cont.
Direct Endocrine Pathways
• Simplest system
• Endocrine organ
acts as the
receptor, the
integrating center,
and the endocrine
organ
• e.g., parathyroid
hormone
Second Order Endocrine Pathways
•
•
•
•
•
•
Epinephrine – “fight-or-flight” response
Sense organ perceives and alarming stimulus
Sensory nerves send signals to the brain
Brain integrates the signals and sends signal out via the motor nerves
Adrenal medulla responds to this signal by releasing epinephrine
Epinephrine interacts with the heart and muscles
Simultaneous Response Pathways
Insulin is part of a direct
stimulus-response
pathway and a
second-order
pathway
Pituitary Hormones
• The pituitary gland secretes many hormones
• Two distinct sections
• Anterior pituitary
• Posterior pituitary
Posterior Pituitary
• Extension of the
hypothalamus
• Neurons that originate
in the hypothalamus
terminate in the
posterior pituitary
• Cell bodies synthesize
neurohormones that
travel in vesicles down
the axons
• e.g., Oxytocin and
vasopressin
• First-order endocrine
pathway
Anterior Pituitary
Hypothalamus
synthesizes and
secretes
neurohormones

Portal system

Anterior pituitary
• Tropic hormones –
cause the release of
another hormone
• Third-order endocrine
pathway
Regulation of Blood Glucose
• Very tightly controlled
• Too low  brain cannot
function
• Too high  osmotic
balance of the blood is
disturbed
• Hormones: insulin and
glucagon
• Antagonistic pairing –
hormones have opposite
effects
Evolution of Cell Signaling
• Mechanisms for cell signaling share many
similarities in all animal groups
• Must have originated in a common
ancestor
Unicellular Organisms
• Can sense and respond to their environment
• Use mechanisms similar to cell signaling in animals
• Examples
• Motile bacteria: sense and move towards chemoattractants using
transmembrane receptor proteins
• Yeast: secrete mating factor pheromones that bind only to
receptors on cells of the opposite mating type
• Slime molds: free-living amoeboid organisms that form
multicellular colonies; secrete cAMP to attract other cells that have
specific receptors for cAMP
Plants vs. Animals
• Pathways have similar outlines, but different details
• Similarities
• Use Ca2+ as a secondary messenger
• Have many pathways that involve protein kinases
• Differences
• Plants do not have receptor tyrosine kinases or Ras
proteins
• Plants have unique transmembrane serine/threonine
kinases
Vertebrates vs. Invertebrates
•
•
•
All have nervous systems; except sponges
Circulatory systems arose independently in several
groups, e.g., arthropods and vertebrates
Endocrine systems could only arise in groups with
circulatory systems and therefore also arose
independently
• All vertebrates use a series of steroid hormones, e.g.,
estrogens, androgens, and corticosteroids
• Only estrogen has been found in invertebrates
• Insects and crustaceans use a different series of steroids
(e.g., ecdysone) to regulate molting and metamorphosis
Vertebrate Hormones
• Alterations in the way tissues respond to a hormone,
rather than a change in the hormones
• Similarities
• Many hormones are affective across many groups,
e.g., human growth hormone increase growth rate in
fish, estrogen from pregnant mares are used in
menopausal women
• Differences
• Prolactin stimulates milk production in mammals,
inhibits metamorphosis and promotes growth in
amphibians, and regulates water balance in fish
Vertebrate Endocrine Organ Structure
• Adrenal glands
• The location of interrenal and chromaffin tissue differ
across groups