Endocrine Principles and Signaling Mechanisms

Question 1

Hormone

Answer 1

• A chemical substance secreted by specialized cell types and carried by the blood stream to act on distant target cells.
• A key role is to implement homeostasis.

Question 2

Homeostasis

Answer 2

• Defined as the maintenance of steady states in the body by coordinated physiological mechanisms.
• Regulate exchanges of matter and energy between the external enviroment and the internal enviroment of the body.
• Regulate exchanges between the intracellular environment with extracellular fluid and its components.

Question 3

Hormonal

Direct Regulation

Answer 3

• Oxygen and carbon dioxide tensions
• Concentrations of glucose and other metabolites
• Osmotic pressure
• Ionic concentrations
• Temperature

Question 4

Hormonal

Indirect Regulation

Answer 4

• Cellular proliferation
• Cellular differentiation
• Growth and maturation
• Reproduction
• Senescence
• Behavior

Question 5

Sources of hormones

Answer 5

Produced by both endocrine glands and many cells with endocrine functions.

Question 6

Hormones

Modes of Communication

Answer 6

• Local communication
  
  1. Autocrine
  2. Paracrine
• Distant communication
  
  1. Neuroendocrine
  2. Endocrine

Question 7

Autocrine

Answer 7

Cell releases a chemical messenger that remains in the surrounding extracellular fluid and binds to a receptor on the surface of the same cell.

Ex. Eicosanoids including prostaglandins.

These hormones regulate local processes such as uterine smooth muscle contraction during pregnancy.

Question 8

Paracrine

Answer 8

• The chemical messenger is released from a cell and act on a nearby cell after diffusing for a short distance.
• Hormones acting in this fashion can only affect the immediate environment.
• Extracellular enzymes rapidly destroy paracrine hormones to avoid their diffusion to distant sites.
• Ex. Nitric oxide
  
  • Regulates the tone of smooth muscle cells among other effects.

Question 9

Neuroendocrine

Answer 9

• Neurohormones produced by neurons and released through the axon into the blood stream to circulate and act on distant target cells.
• Ex. ADH and oxytocin by hypothalamus neurons.

Question 10

Endocrine

Answer 10

• Endocrine glands and cells secrete hormones directly into the blood stream.
  
  • Often through specialized portion of the plasma membrane located on one side of the cell body (cell polarization)

Question 11

Hydrophilic hormones

Answer 11

• Dissolves easily in the blood because they are often polar and charged.
• Typically stored in secretory vesicles until secretion occurs.
• Includes amine hormones and peptide hormones.
• Unable to cross the plasma membrane so in order to transmit the signal they carry inside the cells two conditions are necessary:
  
  • Receptor for the hormone on the cell surface
  • Secondary messengers which transmit signal from the cell membrane to intracellular targets

Question 12

Amine Hormones

Answer 12

• Catecholamine hormones
  
  • Derived from amino acid tyrosine
• Includes:
  
  • Dopamine and norepinephrine
    
    • Neurotransmitters in CNS
  • Epinephrine
    
    • Exclusively produced outside of the CNS

Question 13

Peptide Hormones

Answer 13

• Pituitary gland
  
  • Produces peptide hormones which act as releasing factors (RF) acting on peripheral endocrine glands such as thyroid, gonads, and adrenal gland
• Endocrine Pancreas
  
  • Produces insulin, glucagon, and several others

Question 14

Insulin Processing

Answer 14

Typical example of peptide hormone processing.

• Pre-pro-insulin precursor located in the ER.
  
  • There the C-peptide induces a particular 3D conformation which allows the cross-binding of the pro-insulin.
• Pro-insulin transferred to the golgi.
  
  • There the C-peptide is cleaved off and included in secretory vesicles along with insulin.
    
    • No physiological functions but can be measured in the plasma to provide indication of the amount of insulin secreted by the pancreas.
  • A and B chains bound together forming insulin.

Question 15

Intracellular Signaling Machinery

Answer 15

• Receptors: located on the plasma membrane and bind hydrophillic hormones.
  
  • Ex. G-protein coupled receptors (GPCRs) and Tyrosine Kinase Receptors (TKRs).
• Tranducers: molecular mediators which are tightly associated with the plasma membrane receptors.
  
  • Ex. G-protein
• Effectors: Enzymes responsible for receiving the input from transducers and produce the secondary messengers.
  
  • Ex. Adenylyl cyclase and phospholipase C
• Secondary messengers: carry the signal from the hormones (primary messenger) to intracelluar targets.
  
  • Ex. cAMP, cGMP, IP3, DAG, Ca++, and arachidonic acid

Question 16

G protein-coupled receptors

(GPRCs)

Answer 16

• Includes more than 1,000 types which use trimeric G proteins as transucers
• Share a very similar structure with:
  
  • 7 transmembrane-spanning regions
  • ligand-binding domain located on extracellular amino terminal
  • G-protein binding site located on the intracellular carboxy-terminal
• The same receptor/hormone can activate different types of G-proteins in different cells/tissues and generate distinct cellular responses.

1. Hormone binding induces a conformational change in the receptor which results in binding of intracellular portion with G-protein.
2. Bound G-protein exchanges GDP for GTP and dissociates in alpha and beta-gamma subunits
   
   • Each subunit will activate different effectors.
3. Hydrolysis of GTP to GDP by the alpha subunit will then induce reassociation of the three subunits and displacement of the G-protein from the receptor.

Question 17

Tyrosine Kinase Receptors

(TKRs)

Answer 17

• Less homogeneous than GPCRs
• Typically organized with a:
  
  • Extracellular domain to bind the hormone
  • Short transmembrane portion
  • Cytoplasmic portion that contains a kinase domain

1. TKRs exist as monomers and upon binding hormone commonly dimerize.
2. Dimerization brings cytoplasmic tails of each receptor in close proximity and induces a conformational change causing activation of the kinase domain.
3. Resulting trans-phosphorylation of cytoplasmic portions of each receptor generates docking sites for effectors molecules that bind through a specific sequence called a SH2-domain.
4. Phosphorylation of the effectors activates them.

Question 18

Hydrophobic Hormones

Properties

Answer 18

• Do not dissolve well in the blood because they are often nonpolar and uncharged.
• Requires carrier proteins to allow their transport in plasma.
• Cannot be contained in secretory vesicles and are almost always secreted from the producing cell immediately after synthesis.

Question 19

Hydrophobic Hormone

Classes

Answer 19

Each class of hydrophobic hormones are derived from a common precursor.

• Steroids derived from cholesterol.
  
  • Glucocorticoids and mineralocorticoids.
• Thyroid hormones derived from amino acid tyrosine
  
  • These are stored within the producing cells despite hydrophobic nature.
• Eicosanoids derived from arachidonic acids.
  
  • Despite being hydrophobic, will bind and activate receptors on surface of target cells.

Question 20

Type I

Intracellular Receptors

Answer 20

• Located in the cytosol
• Binds steroid hormones (glucocorticoids and mineralocorticoids)

1. Hormones easily crosses the plasma membrane and binds to the cytosolic receptor.
2. Hormone-receptor complex moves into the nucleus and binds to promoters → specific DNA sequences responsible for regulating gene expression.
   
   • Bind via the Hormone Response Element (HRE) sequence in the promotor.
3. Interaction with the promotor can inhibit gene expression but typically promotes expression of specific genes and production of mRNA.
4. mRNA → proteins
   
   • The synthesized proteins produce the response of the cell to the hormone.

Question 21

Type II

Intracellular Receptors

Answer 21

• Receptors are located in the nucleus and are already bound to DNA.
  
  • Inactive when unoccupied by the hormone and act as repressors of gene expression in this state.
• Binds Vit A & D, retinoids, and thyroid hormones.
  
  • Hormones must enter the plasma membrane, travel through the cytoplasm, cross the nuclear envelope, and then bind to their receptors.
• Receptor activated after binding hormone are displaced from the DNA allowing gene expression.

Question 22

Storage and Secretion of Hormones

Answer 22

• Hydrophobic hormones are not normally stored and simply diffuse out of the cell and into the plasma after production.
  
  • Except thyroid hormone
• Hydrophillic hormones can be stored in cytoplasmic vesicles and accumulate until secretion triggered.
  
  • Process of secretion
    
    1. Ligand binds cellular receptor.
    2. G-protein activated → turns on phospholipase C.
    3. PLC produces the secondary messenger inositol triphosphate (IP₃).
    4. IP₃ binds to receptors located on ER triggering release of Ca²⁺ into the cytosol.
    5. Increased intracellular [Ca²⁺] activates the fusion of hormone-containing vesicles with the plasma membrane and triggers secretion.
    6. The simultaneous production of additional secondary messengers (AMP and GMP) also capable of modulating the process of hormone secretion.

Question 23

Additional regulation of hormonal secretion

Answer 23

• G-protein/IP₃ regulation
• Pulsatile vs steady-state release
• Oscillating factors
  
  • Development (growth vs maintenance)
  • Menstrual cycle
  • Seasonal
  • Circadian or daily variance
• Feedback control
  
  • Positive vs negative

Question 24

Hormone Transport

Answer 24

• Majority of hydrophilic hormones (peptide and catecholamines) circulate in the plasma in their free form.
• Hydrophobic hormones (steroids and thyroid hormones) circulate bound to specific globulins.
  
  • Complexes increase solubility of hydrophobic hormones in plasma.
  • Also increases a hormone’s distribution in different tissues.
  • Only the free form of a hormone is active and capable of activating cellular responses.

Question 25

Hormone Half-Life

Answer 25

Defined as the time needed for one-half of a hormone to disappear from the plasma.

• Half-life of a hydrophobic hormone is predominantly correlated to the percentage of it that is bound to carrier proteins at any given time.
  
  • Because only free hormones are available to be degraded, successively cleared from the plasma, and secreted from the body.
• Hormones that circulate mostly bound to carrier proteins (hydrophobic ones) will have a longer half-life as compared to hormones that circulate in free form (hydrophilic ones).

Question 26

Methods of hormone removal

Answer 26

1. Uptake and degradation operated by target cells.
2. Metabolic degradation (predominately by the liver).
3. Urinary or biliary excretion.

Question 27

Metabolic Clearance Rate

(MCR)

Answer 27

Measurement of the speed and efficiency of the elimination of a hormone from the plasma.

Question 28

Radioimmunoassay

(RIA)

Answer 28

• Monoclonal antibodies bind to a radioactive form of the same hormone that needs to be measured in biological fluids.
• The hormone in the unknown sample will compete with the radioactive hormone in binding to the antibody.
• The higher the concentration of the hormone in the unknown sample, the lower the radioactivity in the antibody complex.
• Indicated by a RIA standard curve in which the non-radioactive hormone to be measured progressively reduces the concentrations of the radioactive hormone bound to the antibody depending on its concentration in the unknown sample.

Question 29

Enzyme-linked Immunosorbent Assay

(ELISA)

Answer 29

• An antibody is bound to a substrate and captures the hormone from the biological sample.
• Another antibody - conjugated with an enzyme - will detect the captured hormone.
• Enzyme catalyzes a reaction in which a substrate is converted into a colored or fluorescent product.
• The intensity of the colorimetric or fluorescent reaction indicates the concentration of the hormone in the sample.

Question 30

Hormonal Biological Assays

Answer 30

• Used to estimate the concentration of a biologically active hormone by comparing an unknown sample to standard concentrations.
• Necessary to obtain correct physiological or diganostic information when the activity of a hormone is not dependent on its concentration but on its physiological state
  
  • Ex. a hormone whose activity is dependent on the level of glycosylation

Question 31

Hormone Receptor Regulation

Answer 31

Receptor down-regulation and up-regulation determines dose-response relations and directly dictates the changes in responsiveness and sensivity to a hormone.

• Down-regulation
  
  • # of receptors for a given hormone expressed on the cell surface can decrease due to physiological and regulatory events as well as pathological conditions
    • Ex. as a compensating response to exaggerated increases in the plasma levels of a hormone
      
      • The binding of the hormone to its receptors induces the endocytosis of the receptor and/or its inactivation.
  • Pathological conditions may be associated with auto-antibodies which target specific hormone receptors.
    
    • Induces a down-regulation of the effects observed on targeted cells.
  • Functional down-regulation may be associated with a reduced affinity of a hormone for its receptor even in the presence of a normal number of receptors on the cell surface.
    
    • Ex. Insulin and Type II Diabetes
• Up-regulation:
  
  • Can be the result of a compensatory measure in normal cells for a reduction in the levels of a hormone in the interstitial fluid.
  • In cancer cells, over-expression of hormone receptors, often acting as oncogenes, can be observed.

Question 32

Dose-Response Relationship

Answer 32

The relationship between the dose of a hormone and the resulting biological response observed in the target cells and the expected changes in body functions.

Dependent on the target cell’s responsiveness and sensivity to a hormone.

Question 33

Responsiveness to a Hormone

Answer 33

• Indicates the maximum functional response that can be obtained by progressively increasing the concentration of a hormone.
• Depends on:
  
  • The number of functional receptors expressed
  • The maximum possible number of transducers, effectors, and secondary messengers that can be recruited by the receptors in a given time
• If there is a reduction in the responsiveness for a given hormone, then an increase in the hormone concentration will not produce any relevant increase in the biological response.
  
  • Excess ligand cannot be matched by a comparable number of receptors or transducers.

PANEL A

Question 34

Sensitivity to a hormone

Answer 34

• Defines the affinity between a hormone and its receptors.
• Sensitivity is measured as the concentration of a hormone capable of inducing 50% of the maximal response.
• A reduction in sensitivity is expressed as a shift to the right of the dose-response curve.
  
  • Need for higher concentrations of a hormone in order to induce the 50% of maximal response.
• A reduction in sensitivity can be temporarily compensated for with an increase in hormone concentration.

PANEL B