Mechanisms of Hormone Action

Transcription

1 Mechanisms of Hormone Action

2 General principles: 1. Signals act over different ranges. 2. Signals have different chemical natures. 3. The same signal can induce a different response in different cells. 4. Cells respond to sets of signals. 5. Receptors relay signals via intracellular signaling cascades. 2

3 External signals are converted to Internal Responses Cells sense and respond to the environment Prokaryotes: chemicals Humans: light - rods & cones of the eye sound hair cells of inner ear chemicals in food nose & tongue Cells communicate with each other Direct contact Chemical signals 3

4 General Principles of Signal Transduction Signal transduction refers to the overall process of converting extracellular signals into intracellular responses. Key players in signal transduction are signaling molecules, receptors, signal transduction proteins and second messengers, and effector proteins.

5 Cells respond to signals by changing the activity of existing enzymes (fast) and/or the levels of expression of enzymes and cell components (slower) by gene regulation (Steps 7a & 7b). Receptors and signal transduction systems have evolved to detect and respond to hormones, growth factors, drugs & neurotransmitters.

6 Primary Messenger Secondary Messengers Target Enzymes EXTRACELLULAR FLUID Plasma membrane CYTOPLASM 1 Reception 2 Transduction 3 Response Receptor Activation of cellular response Relay molecules in a signal transduction pathway Signal molecule Cascade Effect 6

7 Each protein in a signaling pathway Amplifies the signal by activating multiple copies of the next component in the pathway primary signal - activates an enzyme activity, processes 100 substrates /second Primary enzyme activates 100 target enzymes Each of the 100 enzymes activates an additional 100 downstream target enzymes Each of the 10,000 downstream targets activates 100 control factors so rapidly have 1,000,000 active control factors 7

8 A Signal Cascade amplification Reception Transduction Binding of epinephrine to G-protein-linked receptor (1 molecule) Inactive G protein Active G protein (10 2 molecules) Inactive adenylyl cyclase Active adenylyl cyclase (10 2 ) ATP Cyclic AMP (10 4 ) Inactive protein kinase A Active protein kinase A (10 4 ) Inactive phosphorylase kinase Active phosphorylase kinase (10 5 ) Inactive glycogen phosphorylase Active glycogen phosphorylase (10 6 ) Response Glycogen Glucose-1-phosphate (10 8 molecules)

9 Receptors relay signals via intracellular SIGNALING CASCADES 9

10 Main Types of Receptors ION CHANNEL RECEPTORS G-PROTEIN-COUPLED RECEPTORS KINASE-LINKED RECEPTORS INTRACELLULAR RECEPTORS

11 Cell-surface receptors -large &/or hydrophilic ligands ion-channel-linked Trimeric G-protein-linked enzyme-linked (tyrosine kinase) 11

12 Activates or deactivates enzyme catalysed reactions within cell Signal transduction Control of ion channels Receptor protein is part of an ion channel protein complex Receptor binds a messenger leading to an induced fit Ion channel is opened or closed Ion channels are specific for specific ions (Na +, Ca 2+, Cl -, K + ) Ions flow across cell membrane down concentration gradient Polarises or depolarises nerve membranes

13 Ion channel receptors Signal molecule (ligand) Gate closed Ions Examples: Ligand-gated ion channel receptor Plasma Membrane Muscle Contraction Gate open Nerve Cell communication Cellular response Gate close 13

14 Review: Remember the Na + /K + ATPase (Na + /K + pump)? [Na + ] inside ~10mM; outside ~150mM [K + ] inside ~100mM; outside ~5mM cell has membrane potential ~ -60mV Na + Cl mV K + A

15 Intercellular Communication When a ligand binds to a receptor protein, the cell has a response. signal transduction: the events within the cell that occur in response to a signal Different cell types can respond differently to the same signal. 15

16 Intercellular Communication A cell s response to a signal often involves activating or inactivating proteins. Phosphorylation is a common way to change the activity of a protein. protein kinase an enzyme that adds a phosphate to a protein phosphatase an enzyme that removes a phosphate from a protein 16

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18 Signal Transduction Components: Kinases/Phosphatases Proteins that participate in intracellular signal transduction fall into two main classes-- protein kinases/phosphatases and GTPase switch proteins. Kinases use ATP to phosphorylate amino acid side-chains in target proteins. Kinases typically are specific for tyrosine or serine/threonine sites. Phosphatases hydrolyze phosphates off of these residues. Kinases and phosphatases act together to switch the function of a target protein on or off.

19 Kinases - Phosphorylation Phosphatase - Dephosphorylation Tyrosine-OH Serine-OH Threonine-OH Tyr-Kinases Ser/Thr-Kinases dual specificity Kinases

20 There are about 600 kinases and 100 phosphatases encoded in the human genome. Activation of many cell-surface receptors leads directly or indirectly to changes in kinase or phosphatase activity. Note that some receptors are themselves kinases (e.g., the insulin receptor).

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22 Growth hormone receptor Tetrameric complex constructed in presence of growth hormone GH GH binding & dimerisation Binding of kinases Activation and phosphorylation GH receptors (no kinase activity) kinases HO OH OH OH ATP ADP PO OP OP OP HO OH OH OH Kinase active site opened by induced fit Growth hormone binding site Kinase active site

23 Intracellular Receptors steroid hormones -have a nonpolar, lipid-soluble structure -can cross the plasma membrane to a steroid receptor -usually affect regulation of gene expression An inhibitor blocks the receptor from binding to DNA until the hormone is present. 23

24 Intracellular receptors Chemical messengers must cross cell membrane CO 2 H Chemical messengers must be hydrophobic Example-steroids and steroid receptors Steroid binding region Zinc DNA binding region ( zinc fingers ) H 2 N Zinc fingers contain Cys residues (SH) Allow S-Zn interactions

25 Intracellular Receptors A steroid receptor has 3 functional domains: 1. hormone-binding domain 2. DNA binding domain 3. domain that interacts with coactivators to affect gene expression 25

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27 Structure of GPCRs G protein-coupled receptors (GPCRs) are the most numerous class of receptors in most eukaryotes. Receptor activation by ligand binding activates an associated trimeric G protein, which in turn interacts with downstream signal transduction proteins. All GPCRs are integral membrane proteins that have a common 7 transmembrane segment structure. The hormone/ligand binding domain is formed by amino acids located on the external side of the membrane and/or membrane interior. GPCRs interact with G proteins via amino acids in the C3 and C4 cytoplasmic regions.

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29 Biological functions mediated by 7TM receptors Smell Taste Vision Neurotransmission Hormone secretion Exocytosis Control of blood pressure Embryogenesis Cell growth and differentiation Development Viral infection Carcinogenesis

30 G Protein Activation of Effectors The trimeric G protein cycle of activity in hormone-stimulated GPCR regulation of effector proteins is summarized in (next slide).

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32 Trimeric G Proteins & Their Effectors There are 21 different G a proteins encoded in the human genome. The G proteins containing these subunits are activated by different GPCRs and regulate a variety of different effector proteins. The most common effectors synthesize second messengers such as camp, IP 3, DAG, and cgmp. In the case of camp, a stimulatory G as subunit activates adenylyl cyclase and camp production, whereas an inhibitory G ai subunit inhibits adenylyl cyclase and camp production.

33 GPCRs that Regulate Adenylyl Cyclase Adenylyl cyclase is an effector enzyme that synthesizes camp. G a -GTP subunits bind to the catalytic domains of the cyclase, regulating their activity. G as -GTP activates the catalytic domains, whereas G ai -GTP inhibits them. A given cell type can express multiple types of GPCRs that all couple to adenylyl cyclase. The net activity of adenylyl cyclase thus depends on the combined level of G protein signaling via the multiple GPCRs. In liver, GPCRs for epinephrine and glucagon both activate the cyclase. In adipose tissue, epinephrine, glucagon, and ACTH activate the cyclase via G as -GTP, while PGE 1 and adenosine inactivate the cyclase via G ai -GTP.

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35 Activation of Gene Transcription by GPCR Signaling GPCRs regulate gene transcription by camp and PKA signaling. camp-released PKA catalytic domains enter the nucleus and phosphorylate the CREB (CRE-binding) protein, which binds to CRE (camp-response element) sequences upstream of camp-regulated genes. Only phosphorylated p- CREB has DNA binding activity. p-creb interacts with other TFs to help assemble the RNA Pol II transcription machinery at these promoters. In liver, glucagon signaling via this pathway activates transcription of genes needed for gluconeogenesis.

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37 G proteins are important signal transducing molecules in cells. "Malfunction of GPCR [G Protein-Coupled Receptor] signaling pathways are involved in many diseases, such as diabetes, blindness, allergies, depression, cardiovascular defects, and certain forms of cancer. It is estimated that about 30% of the modern drugs' cellular targets are GPCRs." The human genome encodes roughly 800 G protein-coupled receptors, which detect photons of light, hormones, growth factors, drugs, and other endogenous ligands. Approximately 150 of the GPCRs found in the human genome have still-unknown functions. Whereas G proteins are activated by G protein-coupled receptors, they are inactivated by RGS proteins (for "Regulator of G protein signalling"). Receptors stimulate GTP binding (turning the G protein on). RGS proteins stimulate GTP hydrolysis (creating GDP, thus

38 Down-regulation of GPCR/cAMP/PKA Signaling A number of events contribute to the termination of signaling by a GPCR. These include: 1. dissociation of the hormone from the receptor, 2. hydrolysis of GTP by G a 3. hydrolysis of camp via camp phosphodiesterase, 4. phosphorylation and desensitization of receptors by kinases such as PKA and ß- adrenergic receptor kinase (BARK). 5. In addition, GPCRs can be removed from the membrane by vesicular uptake.

39 G-protein activation molecular switch (b) Ligand binds G-protein associates inactive (c) GDP-GTP exchange a-subunit dissociates Active G-Protein-GTP -> allosteric modulator of target effector enzyme active 39

40 All G-proteins similar structure/activation There are TWO broad subclasses of trimeric G-protein-activated signal transduction pathways: depends on their target effector enzymes A. adenylyl cyclase B. phospholipase C 40

41 An activated G a -protein-gtp Can trigger the formation of camp, which then acts as a second messenger in cellular pathways First messenger (signal molecule such as epinephrine) G protein Adenylyl cyclase G-protein-linked receptor GTP ATP camp Protein kinase A Cellular responses 41

42 G-protein-GTP activation of Effector Enzyme adenylyl cyclase produces the 2 nd messenger camp Activated G-protein 42

43 Adenylyl Cyclase & Protein Kinase A Adenylyl cyclase is an integral membrane protein that contains 12 transmembrane segments. It also has 2 cytoplasmic domains that together form the catalytic site for synthesis of camp from ATP. One of the primary targets of camp is a regulatory kinase called protein kinase A (PKA), or camp-dependent protein kinase.

44 PKA exists in two different states inside cells. In the absence of camp, the enzyme forms a inactive tetrameric complex in which 2 PKA catalytic subunits are non-covalently associated with 2 regulatory subunits. When camp concentration rises, camp binds to the regulatory subunits which undergo a conformational change, releasing the active catalytic subunits.

45 Protein Kinase A Phosphorylates downstream target enzymes Phosphorylase kinase inactive + P active Breaks down Glycogen Into Glucose 45

46 What are targets for Protein Kinase A?? camp regulated pathways Function target tissue signal Glycogen breakdown muscle epinephrine Glycogen breakdown liver glucagon Heart rate cardiovascular epinephrine Water reabsorption kidney antidiuretic hormone 46

47 How to shut it off? No ligand G-protein a-subunit is on a timer Auto Shut-off Inherent GTPase activity 47

48 How to shut it off? campphosphodiesterase rapidly cleaves camp (so short lived) 48

49 What if you can t turn off cascade? Vibrio cholera - causes cholera Normal gut: H 2 0, NaCl, NaHCO 3 secretion controlled by hormones via Gs/cAMP signal pathways V. cholera secretes enterotoxin, chemically modifies ags no GTPase activity - stays ON Severe watery diarrhea dehydration, death 49

50 target effector enzyme is Phospholipase C PLC cleaves a membrane phospholipid (Phoshatidyl inositol) to two 2nd Messengers: Inositol-1,4,5-Trisphosphate (InsP3) & Diacylglycerol (DAG) 50

51 PIP 2 DAG Lipid Soluble InsP 3 Water Soluble 51

52 GPCRs That Activate Phospholipase C Another common GPCR signaling pathway involves the activation of phospholipase C (PLC). This enzyme cleaves the membrane lipid, phosphatidylinositol 4,5-bisphosphate (PIP 2 ) to the second messengers, inositol 1,4,5- trisphosphate (IP 3 ) and diacylglycerol (DAG) (Fig.). In this case, the G ao(other) and G aq G a proteins conduct the signal from the GPCR to PLC. This is the pathway used in a 1 -adrenergic GPCR signaling in the liver. *

53 IP 3 /DAG Signaling Elevates Cytosolic Ca 2+ The steps downstream of PLC that make up the IP 3 /DAG signaling pathway are illustrated in the figure. IP 3 diffuses from the cytoplasmic membrane to the ER where it binds to and triggers the opening of IP 3 -gated Ca 2+ channels (Steps 3 & 4). Another kinase, protein kinase C (PKC) binds to DAG in the cytoplasmic membrane and is activated (Step 6). In liver, the rise in cytoplasmic [Ca 2+ ] activates enzymes such as glycogen phosphorylase kinase, which phosphorylates and activates glycogen phosphorylase. Glycogen phosphorylase kinase is activated by Ca 2+ -calmodulin. In addition, PKC phosphorylates and inactivates glycogen synthase.

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55 DAG Activates Protein Kinase C (Starts Cascade) InsP 3 Ligand for ER ligandgated Ca ++ channels Ca ++ levels 55

56 Response: Protein Kinase C phosphorylates target proteins (ser & thr) cell growth regulation of ion channels cytoskeleton increases cell ph Protein secretion Ca++ Binds & activates calmodulin Calmodulin-binding proteins activated (kinases & phosphatases) 56

57 Signal Trans. Components: GTPase Switches GTPase switch protein also play important roles in intracellular signal transduction. GTPases are active when bound to GTP and inactive when bound to GDP. The timeframe of activation depends on the GTPase activity (the timer function) of these proteins. Proteins known as. guanine nucleotide-exchange factors (GEFs) promote exchange of GTP for GDP and activate GTPases. Proteins known as GTPase-activating proteins (GAPs), stimulate the rate of GTP hydrolysis to GDP and inactivate GTPases.

58 We will cover two classes of GTPase switch proteins-- trimeric (large) G proteins, and monomeric (small) G proteins. Trimeric G proteins interact directly with receptors, whereas small G proteins interact with receptors via adaptor proteins and GEFs.

59 Signal Trans. Components: 2nd Messengers While there are a large number of extracellular receptor ligands ("first messengers"), there are relatively few small molecules used in intracellular signal transduction ("second messengers"). In fact, only 6 second messengers occur in animal cells. These are camp, cgmp, 1,2-diacylglycerol (DAG), and inositol 1,4,5- trisphosphate (IP 3 ), and calcium and phosphoinositides. Second messengers are small molecules that diffuse rapidly through the cytoplasm to their protein targets. Another advantage of second messengers is that they facilitate amplification of an extracellular signal.

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61 Nitric Oxide (NO)/cGMP Signaling A related signaling pathway involving phospholipase C operates in vascular endothelial cells and causes adjacent smooth muscle cells to relax in response to circulating acetylcholine (Fig.) In the NO/cGMP signaling pathway, the downstream target of Ca 2+ /calmodulin is nitric oxide synthase, which synthesizes the gas NO from arginine. NO diffuses into smooth muscle cells and causes relaxation by activating guanylyl cyclase and increasing [cgmp]. As a result arteries in tissues such as the heart dilate, increasing blood supply to the tissue. NO also is produced from the drug nitroglycerin which is given to heart attack patients and patients being treated for angina.

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63 Second messenger DAG, IP3 and Ca M 10-7 M

64 Metabolism of Diacylglycerol. Diacylglycerol may be (1) phosphorylated to phosphatidate or (2) hydrolyzed to glycerol and fatty acids.

65 Summary - signaling is endocrine, paracrine, synaptic, or direct cell contact - signal transduction is mediated by receptor proteins - Receptors bind primary signal (ligand) - Some amplification event occurs - Example: ligand gated ion channel opens influx of ions triggers change in activity (vesicle fusion in nerve end, contraction in muscle) - Example: ligand binds to 7-pass membrane receptor catalyzes GTP exchange to G a -subunit of trimeric G-protein active G a -subunit-gtp is allosteric activator of effector enzymes: - ADENYLATE CYCLASE: makes cyclic AMP - PHOSPHOLIPASE C: makes DAG and IP 3 these second messengers activate target enzymes Trigger cascades - Must shut off cascade: removal of ligand, hydrolysis of GTP, phosphodiesterase, protein phosphatases, Ca ++ ion pumps 65