BIO 5406 Notes, 1/09/08
MECHANISMS OF HORMONE ACTION
I. Glands.
[Hadley, pp. 7, 18]
A. Gland = Group of epithelial cells specialized for the function of secretion.
B. Exocrine gland -- secretes substances into a duct.
1. Example:
C. Endocrine gland -- secretes substances into the blood.
1. Example:
D. Pancreas is both exocrine and endocrine.
1. Exocrine function:
2. Endocrine function:
E. Histology.
1. All glands result from ingrowth of an epithelial layera into underlying
connective tissue.
2. Because endocrine glands are ductless, each cell must lie close to
a capillary.
a. Clumps (ex.
islets
of Langerhansb).
b. Sheets or cords (ex. adrenal
cortexc).
c. Follicles (ex. thyroid
folliclesc).
a
Fox,
S.I. Human Physiology, 7th ed., 2002. Links are password-protected.
b
Leeson,
T.S., and Leeson, C.R. A Brief Atlas of Histology, 1979.
c
Slides
from the Loyola University Lumen
Histology Slide Series.
II. Chemical Classification of Hormones. [pp. 22-24]
A. Peptide hormones (ex. insulin).
1. Chain of amino acids.
2. Hydrophilic.
3. Easily digested.
B. Catecholamines (ex. epinephrine).
1. Derivative of the amino acid, tyrosine.
(fig. 14.1).
2. Hydrophilic.
3. Short duration of action.
C. Steroid hormones (ex. estradiol).
1. Synthesized from cholesterol (figs. 2.1, 2.2).
2. Lipophilic.
3. Cross cell membranes easily.
D. Thyroid hormones (ex. thyroxine).
1. Iodine-containing molecules
derived from tyrosine (fig. 13.1).
2. Lipophilic.
III. Biosynthesis of Hormones. [pp. 27-29]
A. Peptide hormones (figured).
1. Synthesized on rough endoplasmic reticulum.
2. Initially synthesized as larger protein molecules, called
preprohormones, based on the nucleotide sequence of a specific
gene (fig. 2.7).
a. Synthesis takes place through the processes of transcription and
translation.
b. Initial sequence (signal sequence) binds to site on rough
endoplasmic reticulum.
1. Allows the peptide to enter lumen of ER (figure).
2. Signal peptide is removed by enzymatic action.
3. Remaining protein is called a prohormone.
3. Prohormone = Protein precursor to a peptide hormone or hormones
(ex. proinsulin).
a. May contain a.a. sequences for one hormone, for multiple copies of
a hormone, or for more than one hormone.
4. Prohormone travels from ER to Golgi apparatus where it may be
modified (ex. addition of carbohydrates ───> glycoproteins).
5. Packaged into secretory vesicles.
6. Prohormone is enzymatically cleaved within secretory vesicles,
yielding one or more hormones and other peptide fragments.
7. Amino acid sequences and genes coding for most peptide hormones
have been identified.
B. Catecholamines (fig. 14.3).
1. Synthesized by adrenal chromaffin cells and some neurons.
2. Begin with tyrosine.
3. Undergoes four enzymatic modifications.
4. Final product, epinephrine, is stored in secretory vesicles.
C. Steroid hormones (fig. 15.3).
1. Synthesized in smooth endoplasmic reticulum.
2. All are derived from
cholesterold.
a. Steroid-producing cells contain large amounts of cholesterol in
lipid droplets.
3. Modified by multiple enzymatic steps.
4. Packaged into lipid droplets by Golgi apparatus.
D. Thyroid hormones (fig. 13.5).
1. Synthesized in thyroid follicular cells.
2. Precursor is a large globular protein called thyroglobulin.
3. Tyrosine residues on thyroglobulin are iodinated.
4. Iodinated tyrosine residues are coupled to form iodothyronines.
5. Thyroglobulin broken down to release iodothyronines as thyroid
hormones.
d Widmaier, E.P., Raff, H., and Strang, K.T. Human Physiology: The Mechanisms of Body
Function, 9th ed., 2004. Links are password-protected.
IV. Hormone Secretion. [pp. 29-30]
A. Peptides and catecholamines.
1. Membrane-bound secretory vesicles, containing hormone, bud off from
Golgi apparatus and travel to cell membrane.
2. Fuse with plasma membrane, spilling contents into extracellular
space ───> exocytosise.
3. Diffuses to nearby capillary.
4. Mechanism.
a. Stimulus (physical or chemical) opens Ca++ channels on cell
membrane.
b. Ca++ enters cell ───> increased cytoplasmic Ca++.
c. Fusion of vesicle membrane with plasma membrane.
B. Steroid hormones.
1. Present in form of lipid droplets.
2. Lipid-soluble ───> easily pass through cell membrane.
3. Not stored, but secreted as quickly as they are synthesized.
C. Thyroid hormones.
1. Released within cytoplasm by breakdown of thyroglobulin.
2. Lipid-soluble ───> easily pass through cell membrane.
e Sherwood, L. Human Physiology: From Cells to Systems, 6th ed., 2007.
V. Transport of Hormones in Blood. [pp. 31-33]
A. Very low concentrations (10-12 M).
B. Peptides and catecholamines -- polar.
1. Dissolve in blood.
C. Steroid and thyroid hormones -- nonpolar.
1. Would rapidly disappear from blood.
2. High proportion bound to plasma proteins.
a. Ex. testosterone is >95% bound to sex hormone binding globulin
(SHBG).
3. Only free hormone is available for binding to receptors or for metabolism.
D. Hormone turnover rates.
1. Biological half-life = Time required for half of molecules to be
inactivated or removed from blood.
a. Peptides and catecholamines ───> minutes.
b. Steroid hormones ───> hours.
c. Thyroid hormones ───> days.
VI. Hormone Receptors. [pp. 39-42]
A. Target cell = Cell on which a hormone exerts its effects.
B. Receptor = Component of a cell with which a chemical messenger
combines to exert its effect.
C. Binding is reversible and subject to Law of Mass Action.
H + R <────> HR
D. Hormone receptors are proteins.
1. Most membrane receptors are glycoproteins
(ex. ß-adrenergic receptord).
E. Characteristics
of hormone receptors (same as characteristics of protein
binding sites).
1.
2.
3.
4.
VII. Mechanisms of Hormone Action. [pp. 45-53, 57-64]
A. Peptide and catecholamine hormones.
1. Example: Epinephrine (catecholamine) or glucagon (peptide
hormone).
a. Both cause release of glucose from liver.
b. Glycogen phosphorylase.
1. Catalyzes the breakdown of glycogen to glucose-1-phosphate.
2. Is present in two forms, active and inactive.
3. Epi and glucagon both activate glycogen phosphorylase in liver.
2. Experiments of Earl Sutherland and Theodore Rall (1957).
a. Add epi to a homogenate of liver ───> phosphorylase is activated.
b. Add epi to purified enzyme preparation ───>
c. Centrifuge homogenate of liver at low speed so that cell membrane
components are removed, then add epi to supernatant ───>
d. Combine pellet and supernatant, then add epi ───>
e. Same experimental results using glucagon.
f. Conclusion:
3. Hypothesized a cell membrane receptor and a "second messenger"
that took the signal from the cell membrane to the enzyme --
second messenger hypothesis.
a. Discovered that the second messenger was adenosine
3',5'-monophosphate (cyclic AMP)
(fig. 1.2).
b. Discovered the enzyme responsible for formation of cyclic AMP
from
ATP -- adenylate cyclase (fig.
3.6a).
c. Found that adenylate cyclase was associated with the cell membrane.
4. Sutherland received the Nobel Prize in 1971.
5. Current view of mechanism of cAMP activation.
a. Three components (fig. 3.7).
1. Receptor protein -- outer surface.
2. Adenylate cyclase -- inner surface.
3. Guanylyl regulatory protein (G protein) --
composed of three
subunits (a,b,g).
b. Resting state.
1. No hormone bound to receptor.
2. GDP bound to a-subunit of G protein.
3. Adenylate cyclase is inactive.
c. Consequences of hormone binding to receptor.
1. Conformational change occurs in receptor.
2. Activated receptor binds to G protein.
3. G protein releases GDP and binds to GTP.
4. Activated a-subunit separates from b,g-subunits, binds to
adenylate cyclase and activates the enzyme.
d. Alfred Gilman and Martin Rodbell received Nobel Prize in 1994
for their discoveries related to G-proteins.
e. Family of G proteins.
1. Some increase cAMP, some decrease cyclic AMP.
2. Others regulate ion channels.
6. Cyclic AMP cascade.
a. Cyclic AMP activates a protein kinase (fig. 3.7)
b. Triggers a series of phosphorylations of various enzymes (figure).
1. Activation of some enzymes, inactivation of others.
2. Reaction is amplified at each level (figure).
c. Results in different effects, depending on the hormone and target cell.
7. Examples of cyclic AMP-mediated effects (figure).
a. TSH -- thyroxine synthesis in thyroid cells.
b. Parathyroid hormone -- increased phosphate excretion in renal
tubular cells.
c. Vasopressin -- water reabsorption in renal medullary cells.
d. Glucagon -- glycogenolysis in hepatocytes.
8. Other second messengers.
a. Cyclic guanosine monophosphate (cyclic GMP) (fig.
3.6b).
b. Inositol triphosphate (IP3).
c. Calcium ion (Ca++).
B. Steroid hormones.
1. Cellular responses to steroid hormones are not apparent for hours
or days.
2. Steroid hormones act by inducing the synthesis of specific
mRNA's and their subsequent translation into specific proteins.
3. Evidence (1960's).
a. Inhibitors of RNA and protein synthesis block the effects
of
adrenal steroids on target tissues.
b. Puffs on chromosomes (figure).
1. Administered ecdysone to midge larvae.
2. Observed giant chromosomes from salivary glands.
3. Ecdysone produced visible puffs at specific sites on
chromosomes.
4. Puffs represent areas of increased RNA synthesis.
c. Induction of specific mRNA (1970's).
1. Administered estradiol to chicks.
2. Increased levels of mRNA for a specific protein in chick oviduct.
4. Site of steroid hormone action.
a. Radioactive isotopes of estradiol were found disappear from blood
and accumulate in target tissues.
b. Centrifugation studies.
1. Cytosolic and nuclear fractions can be separated by
centrifugation.
2. Incubate fractions with radioactive hormone ───>
hormone
binds
to cytosolic fraction.
3. Add hormone to whole cells, incubate, then fractionate ───>
hormone is found in nuclear fraction.
c. Two-step hypothesis (fig. 3.15, figure).
1. Steroid hormones bind to cytoplasmic receptors.
2. Receptor-hormone complex is translocated into nucleus.
3. Binds to nuclear chromatin and directs synthesis of specific
proteins.
4. Specific sites on DNA are called hormone response
elements (HRE) (figure).
d. Controversies.
1. Increasing evidence for nuclear location of bound and unbound
receptors.
a. Antibodies specific for steroid hormone receptors bind in
nucleus.
b. Immunocytochemical studies.
2. Membrane receptors.
a. Some responses to steroid hormones occur in minutes.
b. Not blocked by inhibitors of RNA or protein synthesis.
C. Thyroid hormones.
1. Bind to nuclear receptors (fig.
13.10,
figure).
a. Same superfamily as steroid hormone receptors.
2. Induce the synthesis of specific mRNA's and their subsequent
translation into specific proteins.
3. May also have mitochondrial receptors.