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What is the primary action of lipophilic hormones once they enter a target cell?
Thyroid hormones act quickly and do not affect gene expression.
False
What is the major circulating form of thyroid hormones?
T4 (thyroxine)
The nuclear hormone receptor (NHR) contains a _______ domain that binds to DNA of the target gene.
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Match the following hormones with their primary characteristics:
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What is the primary mode of action for lipophilic hormones?
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Water-soluble hormones circulate bound to carrier proteins in the blood.
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What role do carrier proteins play in controlling lipophilic hormones?
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Lipophilic hormones are _____ in water and typically circulate associated with carrier proteins.
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Match the following hormone types with their characteristics:
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Which type of hormone is regulated mainly through feedback loops?
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A small portion of lipophilic hormones remains free to diffuse into tissues and is the active portion of the hormone.
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What is the primary requirement for hormone action on target cells?
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Which type of hormone requires second messengers to elicit a biological response?
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Water-soluble hormones can pass through phospholipid membranes without any assistance.
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What is the primary function of a cell surface receptor?
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Adenylate cyclase hydrolyzes ATP into __________.
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Match the following types of cell surface receptors with their corresponding messenger:
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Which of the following is an exception to water-soluble hormones circulating free?
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What initiates the activation of intracellular second messengers in G-proteins coupled receptors?
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G-proteins coupled receptors are linked to only one type of second messenger.
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What is the role of phospholipase C (PLC) in the signaling pathway?
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Tyrosine kinase receptors require second messengers to activate target proteins.
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What is the effect of the binding of a hormone to a receptor tyrosine kinase?
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Cytokine receptors do not have intrinsic __________ activity.
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Which process follows the dimerization of cytokine receptors?
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Match the following receptors with their characteristics:
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Receptor serine kinases are primarily involved in the activation of G-proteins.
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The G-protein subunit that stimulates PLC is __________.
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Study Notes
Hormone Action Mechanisms
- Hormones act at a distance from the release site and need to travel in the blood, be soluble, survive long enough, and still be active at the target site.
- Hormones trigger specific actions in target cells through specific hormone-receptor recognition.
- The mode of action depends on the hormone's biochemical structure and its receptors.
- Receptors are essential for hormone action.
- Lipophilic hormones diffuse out of producing cells, circulate bound to carriers in the blood, then diffuse into target cells to interact with intracellular receptors.
- Water-soluble hormones are secreted by exocytosis, circulate free in the blood, and bind to surface receptors on target cells.
### Lipophilic Hormones (Steroids & Thyroid Hormones)
- Lipophilic hormones are insoluble in water and circulate bound to carrier proteins.
- Carrier proteins include specific globulins (CBG, DBG, SHBG, TBG) and non-specific proteins (albumin and prealbumin).
- Carrier proteins keep hormones in the bloodstream, prevent degradation, and serve as a hormone reservoir, buffer, and protection.
- A small portion of the hormone remains free and diffuses into tissues, representing the active form.
- Free hormones are susceptible to degradation and are involved in feedback loops.
- In general, carrier proteins help control the amount of free circulating hormones.
Action of Lipophilic Hormones
- Free hormones diffuse through the plasma membrane of target cells and bind to nuclear hormone receptors (NHRs), specific intracellular receptors.
- The hormone-receptor complex translocates to the nucleus and binds to DNA response elements to stimulate gene expression and de novo protein synthesis.
- NHRs act as transcription factors.
- Thyroid hormones, however, have NHRs already present in the nucleus.
- Lipophilic hormones are considered slow-acting because their action affects protein synthesis through gene regulation.
### Nuclear Hormone Receptors
- NHRs contain a ligand-binding domain, a DNA-binding domain, and an activation domain.
- Orphan receptors are cloned NHRs for which no ligand has yet been identified.
Water-Soluble Hormones (Proteins & Catecholamines)
- Water-soluble hormones cannot pass through the phospholipid membrane barrier.
- They are secreted in vesicles by exocytosis and circulate free in the blood.
- An exception is IGF-1, which circulates bound to a carrier protein.
- Water-soluble hormones bind to specific surface receptors on target cells, and do not enter the cell to act.
- They require second messengers to exert their effect.
- The receptor mediates interactions between the hormone and second messengers.
Cell Surface Receptor
- These receptors span the cell membrane, with a hydrophobic transmembrane domain and hydrophilic regions located outside (extracellular domain) and inside (intracellular domain) the cell.
- When a hormone binds to the receptor, it activates intracellular signaling pathways through second messengers.
Major Types of Cell Surface Receptors
- G-protein coupled receptors use adenylate cyclase and cAMP, phospholipase C and Ca2+, or other second messengers.
- Tyrosine kinase receptors trigger auto-phosphorylation.
- Interleukin/cytokine receptors rely on JAK-STAT signaling pathways.
- Serine kinase receptors (TGFβ family) utilize SMAD proteins.
G-Protein Coupled Receptors
- The hormone binds to a receptor coupled to G-proteins (αβγ subunits).
- The binding causes a conformational change in the receptor, leading to the exchange of GDP for GTP on the Gα subunit.
- The activated Gα subunit dissociates from the βγ subunits and activates a membrane protein.
- The activated membrane protein stimulates a cascade of second messengers.
- The second messengers elicit a biological response in the cell.
Adenylate Cyclase-cAMP-PKA Pathway
- Receptors linked to G-proteins αs or αi (e.g., β-adrenergic, LH) trigger this pathway.
- Adenylate cyclase (AC), a membrane-associated enzyme, is activated by αs and inhibited by αi.
- AC hydrolyzes ATP to produce cyclic AMP (cAMP).
- cAMP activates protein kinase A (PKA).
- Activated PKA phosphorylates intracellular proteins and stimulates gene transcription, resulting in a biological action.
- Phosphodiesterase degrades cAMP, and a phosphoprotein phosphatase deactivates PKA.
Phospholipase C Pathway (PLC)
- Receptors coupled to G-protein αq activate this pathway.
- PLC, a membrane-associated protein, is stimulated by αq.
- PLC hydrolyzes phosphoinositol diphosphate (PIP2) into inositol triphosphate (IP3) and diacylglycerol (DAG).
- IP3 stimulates the release of Ca2+, while DAG activates protein kinase C (PKC).
- Ca2+/calmodulin and PKC initiate phosphorylation cascades, leading to a biological action.
Tyrosine Kinase Receptor
- Hormone binding activates the receptor, initiating auto-phosphorylation.
- The receptor acts as a kinase and phosphorylates tyrosines on target proteins.
- This pathway does not require second messengers like G-coupled proteins.
- The receptor consists of transmembrane, extracellular, and cytoplasmic domains.
- The cytoplasmic domain contains the auto-phosphorylation site, regulatory signals, and ATP binding sites.
Cytokine Receptors
- These receptors, which bind cytokines (e.g., GH, prolactin, erythropoietin, interferons, interleukins), lack intrinsic kinase activity.
- Hormone binding causes receptor dimerization and recruitment of JAK tyrosine kinase, which phosphorylates the receptor.
- Phosphotyrosines on the receptor act as docking sites for STATs (signal transducers and activators of transcription).
- STATs activate different genes.
Receptor Serine Kinase
- The TGFβ family (activin, inhibin, MIS) primarily controls cell proliferation and differentiation.
- Hormone binding leads to heterodimer formation between receptor I and receptor II.
- Receptor II, specific to the hormone, recruits receptor I.
- The same receptor I can be recruited by different receptor II complexes.
- The activated receptor phosphorylates SMAD proteins, which then dimerize, translocate to the nucleus, and modulate gene transcription.
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Description
Explore the intricate mechanisms by which hormones act on target cells. This quiz delves into the differences between lipophilic and water-soluble hormones, their modes of action, and the role of receptors in hormone activity. Test your understanding of how these biochemicals function in the body.
