Signaling 1 & 2 PDF

Signaling 1 & 2 Dr. Brad A. Haubrich Dr. Csaba Fulop Mark’s Basic Medical Biochemistry, Chapter 11 1 Learning Objectives (1 of 3) After completing this lecture, you should be able to: 1. [APPLY] Identify and give examples of contact, paracrine, autocrine and endocrine signaling. 2. [UNDERSTAND...

Signaling 1 & 2 Dr. Brad A. Haubrich Dr. Csaba Fulop Mark’s Basic Medical Biochemistry, Chapter 11 1 Learning Objectives (1 of 3) After completing this lecture, you should be able to: 1. [APPLY] Identify and give examples of contact, paracrine, autocrine and endocrine signaling. 2. [UNDERSTAND] Explain the general steps of signaling and their defects through the example of the neuromuscular junction signaling. 3. [REMEMBER] List the 5 major groups of chemical signals, prototypical examples, and their roles in the human body. 4. [ANALYZE] Predict the type of receptors that bind ligands based on the ligands' chemical nature. 5. [UNDERSTAND] Associate physiologically important chemical signals with their receptors. 6. [ANALYZE] Differentiate among the signaling mechanisms of type I/III, type II and nitric oxide intracellular receptors. 7. [ANALYZE] Differentiate among the signaling mechanisms of the 3 main types of cell surface receptors. 8. [UNDERSTAND] Explain the main concepts of second messengers and kinases in signal transduction pathways. 2 Learning Objectives (2 of 3) After completing this lecture, you should be able to: 9. [ANALYZE] Differentiate among the 3 basic pathways of trimeric G-protein signaling (adenylyl cyclase, protein kinase C, and ion channel modulation). 10. [ANALYZE] Differentiate between the mechanisms of abnormal G-protein signaling caused by cholera and pertussis toxins. 11. [ANALYZE] Distinguish among the 3 receptor/receptor associated kinase signaling pathways (JAK/STAT, serine-threonine kinase, tyrosine kinase). 12. [ANALYZE] Categorize insulin-induced signal transduction pathways with characteristic steps in glucose metabolism. 13. [UNDERSTAND] Explain the 5 major mechanisms of signaling regulation. 14. [UNDERSTAND] Explain how signaling regulation fails and leads to chemical signal overproduction in hyperthyroidism. 3 Learning Objectives (3 of 3) After completing this lecture, you should be able to: 15. [UNDERSTAND] Associate characteristic examples of drugs/drug groups that affect the following signaling pathways - neuromuscular junction signaling - nuclear receptor signaling - nitric oxide signaling - adrenergic receptor signaling 4 What is Signaling? Signaling is the process through which an organism senses, responds to, and communicates with its environment A single cell can detect: ❑ chemical signals (vast majority) ❑ light signals (such as rod and cone cells of the eye) ❑ mechanical signals (endothelial cells are able to sense changes in blood flow through mechanoreceptors) 5 Why is Cell Signaling Important in Medicine? ❑ Signaling controls every aspect of homeostasis in the human body. ❑ Defective or uncontrolled signaling pathways can lead to developmental abnormalities or serious diseases. ❑ Signaling targets. pathways are major pharmacological 6 What are the Major Cell Responses to Signals? (1 of 2) ❑ Cell proliferation - Clonal expansion of specific B and T cells after encountering a foreign antigen. ❑ Cell movement - Chemokines induce immune cell migration to inflammatory sites. ❑ Cell differentiation - Embryonic development, organogenesis, hematopoiesis. 7 What are the Major Cell Responses to Signals? (2 of 2) ❑ Altered metabolic and/or secretory activity of the cell (most general) - Tissue repair after injury. - Histamine release by mast cells during an allergic response. - Insulin stimulates glucose uptake, glycogen and lipid synthesis. ❑ Cell death (programmed cell death, apoptosis) - Shrinking of mammary glands after lactation period. - Removal of the web between fingers and toes during embryonic development. 8 Types of Signaling Abnormalities (1 of 2) Absent or too little signaling 1. Lack of chemical signals Examples: - Certain female infertilities (lack of gonadotropins) - Type I diabetes mellitus (lack of insulin) - Hypothyroidism (reduced thyroid hormone levels) 2. Insensitivity to signaling molecules (cells do not respond as they should) ❑ Deficient signal detection (receptor) ❑ Deficient intracellular signaling pathway ❑ Interference from other signaling pathways Example: -Type II diabetes mellitus 9 Types of Signaling Abnormalities (2 of 2) Too much signaling 3. Hyper-reactivity to signals ❑ Overproduction of signaling molecules ❑ Lack of signaling regulation Examples: - Hyperthyroidism - cAMP overproduction in cholera or whooping cough 10 Cell Signaling and Tumors/ Cancer General cell responses Proliferation Migration Differentiation Metabolic changes Death Tumor/Cancer formation frequently involves abnormal signaling molecules that cause: - Uncontrolled cell proliferation - Decreased cell death 11 Main Types of Signaling Contact signaling – signaling through cell-cell contact Paracrine signaling – short distance signaling between different cells Autocrine signaling – short distance signaling involving the same (or same type of) cell(s) Endocrine signaling – long distance signaling between endocrine glands and target cells 12 Contact Signaling Example: communication between muscle cells the channels are called gap junctions 13 Paracrine Signaling Example: cytokines Chemical signal - low concentration - short half-life Neurotransmitter - high concentration - very short half-life Receptor - high affinity Receptor - relatively low affinity 14 Autocrine Signaling Example: prostaglandins Chemical signal - low concentration - short half-life Receptor - high affinity 15 Endocrine Signaling Endocrine signaling molecules ❑ Produced in endocrine glands ❑ Delivered to the target tissue by the circulation ❑ Highly diluted ❑ Long half-life (mins, hours, days) ❑ Frequently bound to carriers ❑ Recognized by specific, high affinity receptors ❑ Cause relatively slow cell response 16 The 4 Fundamental Questions of Signaling A. What are the chemical signals? B. How do cells detect chemical signals? C. What are the major mechanisms by which cells convert chemical signals into cellular responses? D. How is signaling regulated? 17 Example of the Essential Signaling Steps (Neuromuscular Junction) neuron Chemical signal ❑ acetylcholine (ACh) Signal detection (receptors) ❑ nicotinic ACh receptor (skeletal muscle) Na+/K+ channel ❑ muscarinic ACh receptor (heart muscle) G protein-linked receptor Conversion of signal ❑ nicotinic ACh receptor lets Na+ in and K+ out ❑ muscarinic ACh receptor regulates a K+ channel Regulation ❑ acetylcholine esterase degrades excess ACh acetylcholine esterase 18 Impairment of Acetylcholine Signaling I Too little signaling ❑ Myasthenia gravis – autoimmune neuromuscular disease (muscle weakness, muscle fatigue) ❑ Autoantibodies against the nicotinic ACh receptors -inhibit ACh binding to the receptor -enhance the internalization and destruction of the receptor Low levels of functional ACh receptors on skeletal muscle (inefficient signaling) Management – increase signaling acetylcholine esterase inhibitors (e.g. pyridostigmine, neostigmine and physostigmine, all reversible) elevate ACh levels more efficient signaling through fewer receptors 19 Impairment of Acetylcholine Signaling II Too much signaling Certain insecticides Nerve gases (Sarin, VX) organophosphates Irreversibly inhibit acetylcholine esterase Excess acetylcholine is not destroyed Contraction-relaxation cycle of muscle is impaired (especially dangerous in heart) Death Management – decrease signaling Block ACh receptors: inhibit ACh signaling Example: Atropine, a muscarinic ACh receptor antagonist 20 The 4 Fundamental Questions of Signaling A. What are the chemical signals? B. How do cells detect chemical signals? C. What are the major mechanisms by which cells convert chemical signals into cellular responses? D. How is signaling regulated? 21 What Are the Chemical Signals? ❑ Neurotransmitters – produced by the nervous system - amino acids or derivatives (glycine, glutamate, dopamine, acetylcholine, etc) - neuropeptides (endorphin, enkephalin, etc) ❑ Hormones – produced by the endocrine system (mostly) - amino acid derivatives (thyroid hormone, epinephrine) - peptides (prolactin, vasopressin) - proteins (luteinizing hormone, follicle stimulating hormone, etc) - steroids (testosterone, progesterone, cortisol, etc) - Vitamin D3, retinoic acid – vitamin-derived (not endocrine) ❑ Cytokines – produced by the immune system, regulate immune function - proteins (interferons, interleukins etc) ❑ Eicosanoids – produced in response to injury or inflammation - arachidonic acid derivatives (prostaglandins, leukotrienes, thromboxanes) ❑ Growth factors – regulate cell differentiation and proliferation - proteins (epithelial growth factor, platelet-derived growth factor, etc) 22 Characterization of Chemical Signals Chemical nature (wide variety) - Amino acids and derivatives - Small peptides - Proteins - Steroids - Fatty Acid derivatives Solubility - Water soluble: most of them - Water insoluble: steroid hormones, thyroid hormone, vitamin D3 require transport proteins (albumin or specific transporters) 23 The 4 Fundamental Questions of Signaling A. What are the chemical signals? B. How do cells detect chemical signals? C. What are the major mechanisms by which cells convert chemical signals into cellular responses? D. How is signaling regulated? 24 Signal Detection: Receptors Hydrophobic molecules can pass through the hydrophobic cell membrane Hydrophilic molecules cannot pass through the hydrophobic cell membrane 25 Intracellular Receptors Type I and III Nuclear Receptor Signaling (Steroid Hormones) (1 of 2) Type I Cortisol Aldosterone Progesterone Testosterone NR: Nuclear Receptor HSP: Heat Shock Protein HRE: Hormone Responsive Element Type III Estradiol LBD: Ligand Binding Domain DBD: DNA Binding Domain 26 Intracellular Receptors: Type I and III Nuclear Receptor Signaling (Steroid Hormones) (2 of 2) ❑ Type I and III receptors are localized in the cytosol in complex with heat shock proteins (hsp). ❑ When the steroid hormone binds to the receptor, the hsp is shed and the receptors are dimerized. ❑ The dimerized receptor-hormone complexes translocate from the cytosol into the nucleus. ❑ In the nucleus the receptor-hormone complexes bind to the DNA (to a hormone responsive element) and with the help of coactivators induce gene transcription (the receptors are transcription factors). Pharmacological note: Dexamethasone, an anti-inflammatory steroid drug acts through this signaling pathway. It is ~ 30 times more efficient than the naturally occurring steroid hormone, cortisol. 27 Intracellular Receptors: Type II Nuclear Receptor Signaling (Retinoic Acid, Vitamin D3, Thyroid Hormone, Fatty Acids) (1 of 2) Thyroid hormone Vitamin D3 Retinoic acid Fatty acids LBD: Ligand Binding Domain DBD: DNA Binding Domain HRE: Hormone Responsive Element RXR: Retinoid X receptor TR: Thyroid Receptor 28 Intracellular Receptors: Type II Nuclear Receptor Signaling (Retinoic Acid, Vitamin D3, Thyroid Hormone, Fatty Acids) (2 of 2) ❑ Type II intracellular receptors are localized in the nucleus in a dimer form with different receptors (heterodimers). ❑ The receptor dimer is bound to the DNA, but is unable to induce gene transcription due to the presence of a corepressor. ❑ When the hormone(s) bind(s) the receptor, a coactivator replaces the corepressor. ❑ The activated receptor-hormone complex now can induce gene transcription (the receptors are transcription factors) Pharmacological note: Thiazolidinediones (TZDs), activate an intranuclear fatty acid receptor (PPARγ) and increase insulin sensitivity. They are used to manage Type II diabetes. 29 Intracellular Receptors: Soluble Guanylate Cyclase in Nitric Oxide Signaling ❑ Nitric oxide (NO) can freely pass through the plasma membrane. ❑ NO activates its intracellular receptor, guanylate cyclase. ❑ Guanylate cyclase is an enzyme that generates cyclic GMP (cGMP), a second messenger. ❑ Intracellular cGMP increase will cause changes in cell behavior. ❑ Important in vascular smooth muscle cells, causing vasodilation. ❑ The mechanism is utilized by drugs that decompose to nitric oxide, such as nitroglycerin, nitroprusside, and hydroxyurea. Images adapted from Marks’ and Denninger, JW, Marletta MA. Biochim. Biophys Acta Bioenerg 1411.2-3 (1999): 334-350. 30 Types of Cell Surface Receptors ❑ Ion channel-linked receptors ❑ G-protein-linked receptors (G-protein-coupled receptors, GPCRs) ❑ Enzyme or enzyme-linked receptors 31 Cell Surface Receptors: Ion ChannelLinked Receptors ❑ Binding of the chemical signal to the receptor/channel opens (or closes) the channel. ❑ These receptors convert chemical signals into electrical signal. ❑ These receptors mediate communications in both the central nervous system and the peripheral nervous system ❑ Example: nicotinic ACh receptor 32 Cell Surface Receptors: G-Protein-Linked Receptors (1 of 2) ❑ Seven-transmembrane domain receptors that associate with a heterotrimeric G-protein (G stands for GDP-binding) ❑ Binding of the chemical signal induces activation of the G-protein and an intracellular signaling cascade that leads to change in the cell’s behavior. 33 Cell Surface Receptors: G-Protein-Linked Receptors (2 of 2) ❑ Adrenergic (epinephrine, norepinephrine) receptors - A family of receptors (α1, α2, β1, β2, β3) - Regulate heart-rate, smooth muscle constriction, metabolism - Major pharmaceutical targets (e.g. β1 blockers (antagonists) are used for treating cardiac arrhythmias) ❑ Glucagon receptor - Mediates the metabolic effect of glucagon during fasting. ❑ Muscarinic acetylcholine receptor - Regulates heart rate ❑ Rhodopsin - Senses light in the rod and cone cells of the eye. ❑ Dopamine (a neurotransmitter) receptors - Major pharmaceutical targets in treatments of schizophrenia, Parkinson’s disease, attention deficit disorders 34 Cell Surface Receptors: Enzyme or EnzymeLinked Receptors -insulin receptor -most of the growth factor receptors -interleukin 1 receptor -integrins (extracellular matrix receptors) ❑ Binding of the chemical signal to the receptor activates enzymes (kinases that transfer phosphate groups onto proteins) ❑ The activation of enzymes starts an intracellular signaling cascade and changes the behavior of the cell 35 The 4 Fundamental Questions of Signaling A. What are the chemical signals? B. How do cells detect chemical signals? C. What are the major mechanisms by which cells convert chemical signals into cellular responses? D. How is signaling regulated? 36 What Determines the Cell Response to a Specific Signal? ❑ The type of the receptor A signal can induce different cell responses through different receptors - nicotinic acetylcholine receptor (ligand-gated ion channel) moves Na+ inside and K+ outside the cell stimulates skeletal muscle contraction - muscarinic acetylcholine receptor (G-protein-linked) moves K+ outside the cell decreases the contraction of heart muscle ❑ The intracellular (signal transduction) machinery Even if the receptors are the same, a chemical signal can induce different responses depending on what types of proteins are present inside the cells. 37 General Concepts of Intracellular Signal Transduction (1 of 3) ❑ Intracellular signaling activates kinases (special enzymes that add phosphate groups to proteins – post-translational modification). ❑ Phosphorylation can activate or deactivate proteins. ❑ The action of kinases can be negated by phosphatases (special enzymes that remove phosphate groups from proteins). ❑ The balance between kinase and phosphatase activities determine signaling Example: adding or removal of a phosphate group can change the activity of an enzyme. Note that the addition of a phosphate group requires ATP. 38 General Concepts of Intracellular Signal Transduction (2 of 3) (glucagon receptor) (β2 adrenergic receptor) Glucagon acts through a G-proteinlinked receptor and activates adenylyl cyclase and protein kinase A. Protein kinase A phosphorylates phosphorylase kinase (active) and glycogen synthase (inactive). Phosphorylase kinase phosphorylates glycogen phosphorylase (active). Glycogen is degraded 39 General Concepts of Intracellular Signal Transduction (3 of 3) G-protein-linked receptors mediate their action through second messengers Main second messengers (mediators of intracellular signaling) ❑ Cyclic AMP (cAMP) ❑ Cyclic GMP (cGMP) ❑ Ca2+ ❑ Diacylglycerol (DAG) ❑ Inositol triphosphate (IP3) 40 General Pathways of Signal Transduction Inside the Cell G-protein-linked receptor Enzyme or Enzyme-linked receptors G-protein linked receptors induce messengers (signal amplification). second Second messengers activate protein kinases. Protein kinases phosphorylate (activate or deactivate) proteins such as metabolic enzymes, transcription factors, or other kinases (signal amplification). Steven R. Goodman, Medical Cell Biology, 1998 Note: Enzyme or enzyme-linked receptors have kinase activity and generally don’t use secondary messengers 41 Activation of G-Protein-Linked Receptors Most of the time, the Gα subunit mediates signaling, but the βγ subunits also can participate. Gα subunit Enzyme regulation 2nd messenger affected Gs,α stimulates adenylyl cyclase increases cAMP Gi/o,α inhibits adenylyl cyclase decreases cAMP Gq,α activates phospholipase Cβ increases DAG, IP3 and Ca2+ Gt,α stimulates phosphodiesterase decreases cGMP 42 Adenylyl Cyclase Regulation by G-Proteins (the α and βγ subunits separate) The α subunit modifies the activity of adenylyl cyclase Induces cell responses Note: The α subunit can have either stimulatory (Gs,α) or inhibitory (Gi,α) effect on adenylyl cyclase. 43 G-Protein-Linked Receptors Can Induce or Inhibit cAMP Production PGE1: Prostaglandin E1 ACTH: Adrenocorticotrophic hormone Lodish et al., Molecular Cell Biology, Figure 13.32 44 Action of a G-Protein with a Gsα Subunit ❑ Signal molecule binds to related G-protein-linked receptor. ❑ Gsα dissociates from βγ. Gsα dissociates from GDP. Gsα binds to GTP and is active. ❑ Gsα binds to adenylyl cyclase and activates it. ❑ Adenylyl cyclase makes cAMP from ATP. ❑ Concentrations of cAMP increase in the cell. ❑ High cAMP activates protein kinase A (PKA). Protein kinase A phosphorylates other proteins that result in cellular responses. ❑ Over time, Gsα will hydrolyze its GTP to GDP, inactivating itself (and therefore no longer activating adenylyl cyclase). Gsα with GDP will re-associate with βγ until the signal is received again by the G-proteinlinked receptor. 45 Action of a G-Protein with a Giα Subunit ❑ Signal molecule binds to related G-protein-linked receptor. ❑ Giα dissociates from βγ. Giα dissociates from GDP. Giα binds to GTP and is active. ❑ Giα binds to adenylyl cyclase and inhibits it. ❑ Adenylyl cyclase is inhibited. ❑ Concentrations of cAMP decrease in the cell. ❑ Low cAMP inactivates protein kinase A (PKA). Protein kinase A phosphorylates other proteins that result in cellular responses. ❑ Over time, Giα will hydrolyze its GTP to GDP, inactivating itself (and therefore no longer activating adenylyl cyclase). Giα with GDP will re-associate with βγ until the signal is received again by the G-proteinlinked receptor. 46 Action of a Gsα Subunit That Has Been Overactivated by Cholera Toxin ❑ When Gsα is active (dissociated from βγ and bound to GTP), it is ADP-ribosylated. This prevents GTP from hydrolyzing, and so Gsα never deactivates itself. ❑ Adenylyl cyclase makes excessive cAMP from ATP because it is activated in an uncontrolled manner. ❑ Concentrations of cAMP increase in the cell. ADP-ribose cholera toxin G T P ❑ High cAMP activates protein kinase A (PKA). Protein kinase A phosphorylates other proteins that result in extreme salt and water efflux from gut epithelial cells to the lumen, causing diarrhea. overactive! overactive! 47 Action of a Giα Subunit That Has Been Inactivated by Pertussis Toxin ❑ When Giα is inactive, it is ADP-ribosylated. This prevents Giα from inhibiting adenylyl cyclase. ❑ Adenylyl cyclase makes excessive cAMP from ATP because it is not inactivated, and its activity is not controlled. ❑ Concentrations of cAMP increase in the cell. ADP-ribose overactive! inactive! ❑ High cAMP activates protein kinase A (PKA). Protein kinase A phosphorylates other proteins that result in increased mucus secretion in airway epithelium. pertussis toxin 48 cAMP Regulates Cell Function Through cAMP-Dependent Protein Kinase A ❑ cAMP-dependent protein kinase A has 4 subunits (2 catalytic and 2 regulatory) ❑ When the regulatory subunits are attached to the catalytic subunits, the enzyme is inactive. ❑ Binding cAMP to the regulatory subunits dissociates the subunits of protein kinase A and activates the catalytic subunits. ❑ The catalytic subunits phosphorylate target proteins (using ATP). ❑ This phosphorylation can change enzyme activities or gene transcriptions in the cell. ❑ The effect of phosphorylation can be reversed by phosphatases. 49 G-Protein Linked Receptors Can Activate Protein Phospholipase Cβ This mechanism uses 3 second messengers Note: α1-adrenergic receptors on smooth muscle cells mediate vasoconstriction through this mechanism. Pharmacological note: α1-adrenergic agonists are used in decongestants and eye drops. Image adapted from Molecular Biology of the Cell 4th ed., B Albert, A Johnson, J. Lewis 50 G-Protein-Linked Receptors Can Alter cGMP Levels Phototransduction in rod and cone cells ❑ A photon activated rhodopsin (receptor) activates transducin (G-protein). ❑ The α subunit of transducin activates phosphodiesterase (an enzyme). ❑ Phosphodiesterase hydrolyzes cGMP GMP (degrades 2nd messenger!). to ❑ The fall in cGMP levels closes cGMP-gated ion channels. Image from Arshavsky VY., Burns ME. 2012. J Biol Chem 287(3):1620. 51 The Subunits of Heterotrimeric G-Proteins Can Directly Activate Ion Channels (Muscarinic ACh Receptor) Lodish et al., Molecular Cell Biology This is the exception from the general rule 1. Channel activation is performed by the βγ subunits and not by the α subunit 2. There are no second messengers or kinases involved in this process. 52 G-Protein Involvement in Heart Rate Regulation (Summary) Signaling Molecule Receptor Gα subunit Mechanism Heart Rate Acetylcholine Muscarinic ACh receptor Gi,α βγ opens K+ channel  cAMP  Epinephrine β1 adrenergic receptor Gs,α  cAMP  Adenosine Adenosine receptor Gi,α  cAMP  ❑ Pharmacological note: Caffeine is an antagonist of the adenosine receptor and increases heart rate. 53 Enzyme or Enzyme-Linked Receptors These receptors are either kinases or associated with kinases kinase associated with a kinase kinase General principles ❑ The receptor has to dimerize to induce intracellular signal transduction. ❑ Initiating intracellular signaling requires phosphorylation of the receptor. ❑ The phosphorylated receptor binds signal transducer protein(s). 54 JAK-STAT Receptors ❑ Used by most cytokines for signaling. ❑ Kinase associated receptors. ❑ JAKs are tyrosine kinases that associate with the receptor. ❑ STATs are the signal transducer proteins. regulates gene transcription 55 Serine-Threonine Kinase Receptors ❑ Used by the transforming growth factor β family. ❑ The receptor subunits in the dimer are different. ❑ R-Smad is the signal transducer protein that binds to the receptor. regulates gene transcription 56 Tyrosine Kinase Receptors ❑ Used by many growth factors and insulin ❑ Can activate multiple intracellular signaling pathways -Mitogen activated protein (MAP)-kinase pathway. -Phospholipase Cγ (PLC γ) pathway. -Phosphatidylinositol-3’-kinase (PI 3-kinase) pathway. regulates transcription & translation 57 The Complexity of the Insulin Receptor Signaling (Example of Tyrosine Kinase Signaling) When blood glucose is high, insulin stimulates glucose metabolism Insulin receptor Saltiel A.R. FASEB J 8:1034, 1994 PI-3K Glucose transporter (GLUT4) is shuttled to the plasma membrane to enhance glucose uptake by muscle and adipose tissue. MAP-kinase pathway PLC γ pathway PI 3-kinase pathway Activated pathways MAPK regulate transcription & translation of genes necessary for glucose metabolism excess glucose can be stored in glycogen or fatty acids (lipids) PLCγ 58 Signaling Pathways Do Not Act Alone! Image: Nair A, Chauhan P, Saha B, Kubatzky KF. 2019 Int J Mol Sci 20(13) : 3292. 59 Signaling Pathways Do Not Act Alone! Medical Significance Managing diseases by modifying intracellular signaling pathways (through drugs) can be advantageous or disadvantageous. ❑ Disadvantage: Interfering with a pathway can cause side effects in other pathways ❑ Advantage: An alternate pathway can partially restore the function of an impaired pathway Example: A type of treatment for Type II diabetes mellitus. Activating an intracellular peroxisome proliferator activated receptor (PPAR) induces the transcription of genes involved in glucose metabolism. 60 The 4 Fundamental Questions of Signaling A. What are the chemical signals? B. How do cells detect chemical signals? C. What are the major mechanisms by which cells convert chemical signals into cellular responses? D. How is signaling regulated? 61 Regulation of Signaling (Signal Termination) ❑ Destruction of the chemical signal - Acetylcholine esterase destroys ACh. ❑ Decreased synthesis of the chemical signal - Negative feedback regulation of hormone synthesis in the hypothalamus and the pituitary gland. ❑ Reduction of the functional receptors - Desensitization of some receptors by phosphorylation (i.e. binding of the signal to the receptor no longer results in signaling). - Removal of receptors from the cell surface by endocytosis. ❑ Destruction of second messengers - Phosphodiesterase destroys cAMP, cGMP. ❑ Reversing the effects of kinases - Phosphatases remove phosphate groups from proteins. 62 Example of Reducing Chemical Signals by Negative Feedback Regulation - Thyroid hormone downregulates its own production by inhibiting hormone production by the hypothalamus and the pituitary gland. negative feedback regulation - Hypothalamus TSH-releasing hormone Anterior pituitary Thyroid stimulating hormone (TSH) Thyroid gland + Autoantibodies Thyroid hormone Hyperthyroidism (Grave’s disease): ❑ Autoantibodies stimulate TSH receptors in the thyroid gland → increased thyroid hormone production ❑ Thyroid hormone downregulates TSH production but has no effect on autoantibodies ❑ The negative feedback does not work → runaway thyroid gland 63 A Few of the Connections to Other MB Lectures ▪ We will see insulin and glucagon a lot...e.g. in Amino Acids in Proteins (2, Unit 1), Glycolysis (11), Gluconeogenesis (15), Glycogen Metabolism (16), and in Unit 5. Phosphorylation of amino acids in proteins will be particularly important here. ▪ ADP-ribosylation by bacterial toxins was one of our modifications in Amino Acids in Proteins (2, Unit 1). Phosphorylation occurs a lot through this course, including Enzymes (4/5, Unit 1). ▪ Signaling will stick with us throughout the whole semester. From a DO25 student in the spring: “Signaling shows up all the time!” 64