Hormone Transport
Receptors & Signal Transduction
Master how hormones travel, bind to receptors, and trigger intracellular cascades that regulate virtually every physiological process.
MCAT Biological & Biochemical Foundations
What Are Hormones?
Hormones are chemical messengers produced by endocrine glands and secreted into the bloodstream. They travel to distant target cells, bind specific receptors, and alter cellular activity. Unlike neurotransmitters (which act locally across synapses), hormones coordinate systemic, long-duration responses.
Three Major Hormone Classes
Peptide / Protein
Chains of amino acids (e.g., insulin, growth hormone). Water-soluble: travel freely in blood, bind surface receptors.
Steroid
Derived from cholesterol (e.g., cortisol, estrogen). Lipid-soluble: need carrier proteins, cross membrane to bind intracellular receptors.
Amino Acid-Derived
Modified single amino acids (e.g., thyroid hormones, epinephrine). Behavior varies: some water-soluble, some lipid-soluble.
How Hormones Travel in Blood
Receptor Location: Surface vs. Intracellular
The Signal Transduction Cascade
Signal transduction converts an extracellular signal into an intracellular response through a chain of molecular events. Each step amplifies the signal: one hormone molecule can activate thousands of downstream effector molecules.
1
Signal: Hormone (first messenger) arrives at target cell.
2
Reception: Hormone binds a specific receptor (surface or intracellular).
3
Transduction: Relay molecules (G-proteins, kinases) propagate and amplify the signal.
4
Response: Cell changes behavior: gene expression, enzyme activation, ion channel opening, etc.
G-Protein Coupled Receptors (GPCRs)
Key Second Messengers
\(cAMP\)
Produced by adenylyl cyclase from \(ATP\). Activates Protein Kinase A (\(PKA\)). Degraded by phosphodiesterase (\(PDE\)).
\(IP_3\) & DAG
\(IP_3\) releases \(Ca^{2+}\) from ER. DAG activates Protein Kinase C (\(PKC\)). Both from \(PIP_2\) cleavage by phospholipase C.
\(Ca^{2+}\)
Released from ER or enters via channels. Binds calmodulin to activate kinases. Key in muscle contraction and secretion.
Receptor Tyrosine Kinases (RTKs)
GPCRs vs. RTKs: Key Differences
| GPCRs | RTKs | |
|---|---|---|
| Structure | 7-transmembrane passes | Single-pass, dimerize on activation |
| Kinase activity | None (uses separate G-protein) | Intrinsic tyrosine kinase |
| Second messenger | \(cAMP\), \(IP_3\)/DAG, \(Ca^{2+}\) | \(Ras/MAPK\) cascade (no classical 2nd messenger) |
| Example ligands | Epinephrine, glucagon, TSH | Insulin, EGF, PDGF |
| Speed of response | Seconds to minutes | Minutes to hours |
| Clinical relevance | ~40% of all drugs target GPCRs | Oncogenes (e.g., \(HER2\) in cancer) |
Intracellular Receptors: Steroid Pathway
1
Steroid hormone dissociates from carrier protein in blood and crosses the plasma membrane (lipid-soluble).
2
Binds an intracellular receptor in the cytoplasm or nucleus. The receptor may be complexed with heat shock proteins (\(HSPs\)) that dissociate upon binding.
3
The hormone-receptor complex acts as a transcription factor: it enters the nucleus and binds to hormone response elements (\(HREs\)) on DNA.
4
Gene transcription is activated or repressed, leading to new protein synthesis. Effects are slow but long-lasting (hours to days).
Signal Termination Mechanisms
Cells must turn off signaling to prevent overstimulation. Multiple mechanisms ensure signals are transient and controllable.
Phosphodiesterase
Degrades \(cAMP\) to \(AMP\), stopping \(PKA\) activation. Target of caffeine (inhibits \(PDE\), so \(cAMP\) stays elevated).
GTPase Activity
\(G_\alpha\) subunit hydrolyzes \(GTP\) to \(GDP\), self-inactivating the G-protein. Cholera toxin blocks this step.
Phosphatases
Remove phosphate groups added by kinases, reversing phosphorylation-based activation of downstream targets.
How to Analyze a Signaling Problem
1
Identify the hormone class: Is it peptide (water-soluble), steroid (lipid-soluble), or amino acid-derived?
2
Determine receptor location: Water-soluble hormones use surface receptors. Lipid-soluble hormones use intracellular receptors.
3
Trace the signal pathway: GPCR \(\rightarrow\) G-protein \(\rightarrow\) second messenger, or RTK \(\rightarrow\) \(Ras/MAPK\), or steroid \(\rightarrow\) nuclear receptor \(\rightarrow\) transcription.
4
Consider amplification: How many molecules are generated at each step? This explains why tiny hormone concentrations produce large effects.
5
Check termination: How is the signal turned off? Phosphodiesterase, GTPase, phosphatases, receptor internalization.
Example 1: Epinephrine on Liver Cells
Scenario: During a fight-or-flight response, the adrenal medulla releases epinephrine. Trace the signal from release to glucose entering the bloodstream.
1
Epinephrine is amino acid-derived (from tyrosine) and water-soluble: it travels freely in blood.
2
Binds \(\beta\)-adrenergic receptor (a GPCR) on liver cell surface.
3
\(G_s\alpha\) activates adenylyl cyclase \(\rightarrow\) \(cAMP\) rises \(\rightarrow\) activates \(PKA\).
4
\(PKA\) activates phosphorylase kinase \(\rightarrow\) activates glycogen phosphorylase \(\rightarrow\) glycogen breakdown to glucose.
Example 2: Cortisol and Gene Expression
Scenario: Chronic stress causes sustained cortisol release from the adrenal cortex. How does cortisol suppress the immune system?
1
Cortisol is a steroid (lipid-soluble): transported bound to cortisol-binding globulin (\(CBG\)).
2
Free cortisol crosses the target cell membrane and binds a cytoplasmic glucocorticoid receptor.
3
Receptor-hormone complex enters the nucleus, binds glucocorticoid response elements (\(GREs\)) on DNA.
4
Represses transcription of pro-inflammatory cytokines (\(IL-1\), \(IL-6\), \(TNF-\alpha\)) \(\rightarrow\) immune suppression.
Common Mistakes
✕ Confusing 1st and 2nd Messengers
The hormone is the first messenger. \(cAMP\), \(IP_3\), and \(Ca^{2+}\) are second messengers generated inside the cell.
✕ Steroid = Surface Receptor
Steroids are lipid-soluble and cross the membrane. They bind intracellular receptors, not surface receptors.
✕ Mixing Up G-Protein Types
\(G_s\) stimulates adenylyl cyclase (raises \(cAMP\)). \(G_i\) inhibits it (lowers \(cAMP\)). Same receptor type, opposite effects.
Master Summary: Signaling Pathways
| GPCR | RTK | Intracellular | |
|---|---|---|---|
| Hormone type | Peptide, amines | Growth factors | Steroids, thyroid |
| Receptor | 7-TM surface | 1-TM surface (dimer) | Cytoplasmic / nuclear |
| Mechanism | G-protein \(\rightarrow\) 2nd messenger | Kinase cascade (\(Ras/MAPK\)) | Direct gene regulation |
| Speed | Seconds-minutes | Minutes-hours | Hours-days |
| Amplification | Very high | High | Low (1:1 binding) |
| Example | Epinephrine | Insulin (metabolic) | Cortisol, estrogen |
Question 1 of 5
A researcher adds a phosphodiesterase inhibitor to liver cells treated with glucagon. What is the most likely result?
Phosphodiesterase degrades \(cAMP\). Inhibiting \(PDE\) causes \(cAMP\) to accumulate, prolonging \(PKA\) activation and therefore glycogen phosphorylase activity, increasing glycogen breakdown.
Question 2 of 5
Which of the following hormones would you expect to bind an intracellular receptor and directly alter gene transcription?
Estrogen is a steroid hormone: it is lipid-soluble, crosses the cell membrane, and binds a cytoplasmic/nuclear receptor that acts as a transcription factor. Insulin uses RTKs, epinephrine uses GPCRs, and growth hormone uses a JAK-STAT surface receptor.
Question 3 of 5
Cholera toxin permanently activates the \(G_s\alpha\) subunit by preventing \(GTP\) hydrolysis. Which downstream effect would this cause?
\(G_s\alpha\) normally inactivates by hydrolyzing \(GTP\) to \(GDP\). If \(GTP\) hydrolysis is blocked, \(G_s\alpha\) remains permanently active, continuously stimulating adenylyl cyclase. This leads to massive \(cAMP\) accumulation: in intestinal cells, this causes the severe watery diarrhea of cholera.
Question 4 of 5
A mutation causes a receptor tyrosine kinase to dimerize constitutively (without ligand binding). This would most likely lead to:
RTK dimerization triggers autophosphorylation and \(Ras/MAPK\) signaling. If this occurs without ligand, the growth signal is always on: this is the mechanism behind many oncogenes (e.g., mutant \(HER2\) in breast cancer).
Question 5 of 5
A cell is treated with a drug that blocks \(IP_3\) receptors on the endoplasmic reticulum. Which signaling event is most directly impaired?
\(IP_3\) receptors on the ER are ligand-gated \(Ca^{2+}\) channels. When \(IP_3\) binds, they open and release stored \(Ca^{2+}\) into the cytoplasm. Blocking these receptors directly prevents \(Ca^{2+}\) release, impairing downstream calmodulin-dependent signaling.