Cell Signaling Lecture Notes PDF

16 Cell Signaling 16 Cell Signaling Signaling Molecules and Their Receptors G Proteins and Cyclic AMP Signaling Tyrosine Kinases and Signaling by MAP Kinase, PI 3-Kinase, and Phospholipase C/Calcium Pathways Receptors Coupled to Transcription Factor...

16 Cell Signaling 16 Cell Signaling Signaling Molecules and Their Receptors G Proteins and Cyclic AMP Signaling Tyrosine Kinases and Signaling by MAP Kinase, PI 3-Kinase, and Phospholipase C/Calcium Pathways Receptors Coupled to Transcription Factors Signaling Dynamics and Networks Introduction All cells receive and respond to signals from their environment. Bacteria and unicellular eukaryotes respond to environmental signals and to signaling molecules secreted by other cells for mating and other communication. Introduction In multicellular organisms, cell–cell communication is highly sophisticated. Each cell must be carefully regulated to meet the needs of the whole organism. A variety of signaling molecules are secreted or expressed on the surface of one cell, and bind to receptors expressed by other cells. Introduction Binding of signal molecules to receptors initiates a series of reactions that regulate all aspects of cell behavior. Many cancers arise from problems in signaling pathways that control normal cell proliferation. Much of our understanding of cell signaling has come from the study of cancer cells. Signaling Molecules and Their Receptors Signaling molecules range in complexity from simple gases to proteins. Some carry signals over long distances; others act locally. They also differ in modes of action: Some cross the plasma membrane and bind to intracellular receptors; others bind to receptors on the cell surface. Signaling Molecules and Their Receptors Modes of cell signaling include: Direct cell–cell signaling—direct interaction of a cell with its neighbor, (e.g., via integrins and cadherins). Signaling by secreted molecules— three categories are based on the distance over which signals are transmitted. Figure 16.1 Modes of cell–cell signaling (Part 1) Signaling Molecules and Their Receptors Endocrine signaling Signaling molecules (hormones) are secreted by specialized endocrine cells and carried through the circulation to target cells at distant body sites. Example: estrogen Figure 16.1 Modes of cell–cell signaling (Part 2) Signaling Molecules and Their Receptors Paracrine signaling Molecules released by one cell act on neighboring target cells. Example: neurotransmitters Figure 16.1 Modes of cell–cell signaling (Part 3) Signaling Molecules and Their Receptors Autocrine signaling Cells respond to signaling molecules that they themselves produce. Example: T lymphocytes respond to antigens by making a growth factor that drives their own proliferation, thereby amplifying the immune response. Figure 16.1 Modes of cell–cell signaling (Part 4) Signaling Molecules and Their Receptors Abnormal autocrine signaling often contributes to cancer. A cancer cell produces a growth factor to which it also responds, thereby continuously driving its own unregulated proliferation. Signaling Molecules and Their Receptors Receptors may be located on the cell surface or inside the cell. Intracellular receptors respond to small hydrophobic molecules that can diffuse across the plasma membrane. Examples: Steroid hormones, thyroid hormone, vitamin D3, and retinoic acid. Signaling Molecules and Their Receptors Steroid hormones are synthesized from cholesterol: Testosterone, estrogen, and progesterone are the sex steroids, produced by the gonads. Figure 16.2 Structure of steroid hormones, thyroid hormone, vitamin D 3, and retinoic acid Signaling Molecules and Their Receptors Corticosteroids from the adrenal gland: Glucocorticoids—stimulate production of glucose. Mineralocorticoids—act on the kidneys to regulate salt and water balance. Signaling Molecules and Their Receptors Ecdysone is an insect hormone that triggers metamorphosis of larvae to adults. Brassinosteroids are plant steroid hormones that control several processes, including cell growth and differentiation. Signaling Molecules and Their Receptors Thyroid hormone: synthesized from tyrosine in the thyroid gland; important in development and metabolism. Vitamin D3 regulates Ca2+ metabolism and bone growth. Retinoic acid and retinoids: synthesized from vitamin A; important in vertebrate development. Signaling Molecules and Their Receptors Receptors for these molecules are members of the nuclear receptor superfamily. They are transcription factors that have domains for ligand binding, DNA binding, and transcriptional activation. The steroid hormones and related molecules directly regulate gene expression. Signaling Molecules and Their Receptors Ligand binding has different effects on different receptors. Some nuclear receptors are inactive in the absence of hormone: Glucocorticoid receptor is bound to Hsp90 chaperones in the absence of hormone. Glucocorticoid binding displaces Hsp90 and leads to binding of regulatory DNA sequences. Figure 16.3 Glucocorticoid action Signaling Molecules and Their Receptors Hormone binding can alter the activity of a receptor: In the absence of hormone, thyroid hormone receptor is associated with a corepressor complex and represses transcription of target genes. Hormone binding results in activation of transcription. Figure 16.4 Gene regulation by the thyroid hormone receptor Signaling Molecules and Their Receptors Lecture Recap: Cell Signaling and its Importance Modes of Signaling Types of Signaling Molecules – Hydrophilic and Hydrophobic Hydrophobic Molecules– Steroid hormones and related molecules directly regulate gene expression via signaling pathway Signaling Molecules and Their Receptors Lecture Outline: Hydrophobic signaling molecules and mechanism of action of Nitric oxide (NO) and Carbon monoxide Hydrophilic signaling molecules Neurotransmitters Peptide hormones Neuropeptides Polypeptide growth factors G protein-coupled receptors and cAMP signaling Signaling Molecules and Their Receptors Nitric oxide (NO) is a gas, It can cross the plasma membrane and alter the activity of enzymes. Nitric oxide (NO) is a paracrine signaling molecule in the nervous, immune, and circulatory systems. Its action is local, because it is extremely unstable, with a half-life of only a few seconds. Figure 16.5 Synthesis of nitric oxide Figure 16.5 Synthesis of nitric oxide Signaling Molecules and Their Receptors The main target of NO is guanylyl cyclase. NO binding stimulates synthesis of cyclic GMP (a second messenger). A second messenger is a molecule that relays a signal from a receptor to a target inside the cell. Signaling Molecules and Their Receptors NO can signal dilation of blood vessels. NO diffuses to smooth muscle cells and stimulates cGMP production. cGMP induces muscle cell relaxation and blood vessel dilation. Signaling Molecules and Their Receptors Carbon monoxide (CO), also functions as a signaling molecule in the nervous system. It is related to NO and acts similarly as a neurotransmitter and mediator of blood vessel dilation. Figure 16.6 Structure of representative neurotransmitters Signaling Molecules and Their Receptors Neurotransmitters carry signals between neurons or from neurons to other cells. They are released when an action potential arrives at the end of a neuron. The neurotransmitters then diffuse across the synaptic cleft and bind to receptors on the target cell surface. Signaling Molecules and Their Receptors Because neurotransmitters are hydrophilic; they can’t cross plasma membranes and must bind to cell surface receptors. Many neurotransmitter receptors are ligand-gated ion channels. Neurotransmitter binding opens the channels. Signaling Molecules and Their Receptors Neurotransmitter receptors are: Ligand –gated ion channels--binding opens the channels. Enzyme-coupled receptors – binding of neurotransmitters to extracellular domain activates cytosolic domains Coupled to G proteins—a major group of signaling molecules that link cell surface receptors to intracellular responses. Signaling Molecules and Their Receptors Peptide signaling molecules are peptide hormones, neuropeptides, and growth factors. These molecules can’t cross the plasma membranes of target cells, so they act by binding to cell surface receptors. Abnormalities in growth factor signaling are the basis for many diseases, including many cancers. Signaling Molecules and Their Receptors Peptide signaling molecules include peptide hormones, neuropeptides, and polypeptide growth factors. Peptide hormones include insulin, glucagon, and pituitary gland hormones (e.g., growth hormone, follicle- stimulating hormone, prolactin). Signaling Molecules and Their Receptors Neuropeptides are secreted by some neurons. Enkephalins and endorphins act as neurotransmitters and as neurohormones—natural analgesics that decrease pain responses; they bind to the same receptors on brain cells as morphine does. Signaling Molecules and Their Receptors Nerve growth factor (NGF) is a member of the neurotrophin family that regulates development and survival of neurons. Epidermal growth factor (EGF) stimulates cell proliferation. It is the prototype for the study of growth factors. Figure 16.7 Structure of epidermal growth factor (EGF) Signaling Molecules and Their Receptors Platelet-derived growth factor (PDGF) is stored in blood platelets and released during blood clotting at the site of a wound. It stimulates proliferation of fibroblasts, contributing to regrowth of the damaged tissue. Signaling Molecules and Their Receptors Cytokines regulate development and differentiation of blood cells and activities of lymphocytes during the immune response. Membrane-anchored growth factors remain with the plasma membrane and function as signaling molecules in direct cell–cell interactions. Table 16.1 Representative Peptide Hormones, Neuropeptides, and Polypeptide Growth Factors Signaling Molecules and Their Receptors Plant hormones: Gibberellins—stem elongation Auxins—cell elongation Ethylene—fruit ripening Cytokinins—cell division Abscisic acid—onset of dormancy Figure 16.9 Plant hormones G Proteins and Cyclic AMP Signaling Most ligands responsible for cell–cell signaling bind to surface receptors on target cells. Intracellular signal transduction: The surface receptors regulate intracellular enzymes, which then transmit signals from the receptor to a series of additional intracellular targets. G Proteins and Cyclic AMP Signaling The targets of signaling pathways frequently include transcription factors. Ligand binding to a receptor initiates a chain of intracellular reactions, ultimately reaching the nucleus and altering gene expression. G Proteins and Cyclic AMP Signaling G protein-coupled receptors are the largest family of cell surface receptors. Signals are transmitted via guanine nucleotide-binding proteins (G proteins). G Proteins and Cyclic AMP Signaling G proteins have three subunits designated α, β, and γ. They are called heterotrimeric G proteins to distinguish them from other guanine nucleotide-binding proteins, such as the Ras proteins. The receptors have seven membrane- spanning α helices. Figure 16.11 Structure of a G protein-coupled receptor Figure 16.12 Hormonal activation of adenylyl cyclase G Proteins and Cyclic AMP Signaling Binding of a ligand induces a conformational change that allows the cytosolic domain to activate a G protein on the inner face of the plasma membrane. The activated G protein then dissociates from the receptor and carries the signal to an intracellular target. G Proteins and Cyclic AMP Signaling G proteins were discovered during studies of cyclic AMP (cAMP), a second messenger that mediates responses to many hormones. A G protein is an intermediary in adenylyl cyclase activation, which synthesizes cAMP. G Proteins and Cyclic AMP Signaling The α subunit binds guanine, which regulates G protein activity. In the inactive state, α is bound to GDP in a complex with β and γ. Homone binding to the receptor causes exchange of GTP for GDP. The α and βγ complex then dissociate from the receptor and interact with their targets. Figure 16.13 Regulation of G proteins Signaling Molecules and Their Receptors Lecture Recap: Hydrophobic signaling molecules and mechanism of action of Nitric oxide (NO) and Carbon monoxide Hydrophilic signaling molecules Neurotransmitters Peptide hormones Neuropeptides Polypeptide growth factors G protein-coupled receptors Figure 16.13 Regulation of G proteins Signaling Molecules and Their Receptors Lecture Recap: G protein-coupled receptors- indirectly linked to enzymes and ion channels Example --G protein coupled receptors indirectly activate adneylyl cyclase G protein coupled receptors indirectly regulate ion channels Example: action of the neurotransmitter acetylcholine on heart muscle. Figure 16.12 Hormonal activation of adenylyl cyclase G Proteins and Cyclic AMP Signaling A large array of G proteins connect receptors to distinct targets. In addition to enzyme regulation, G proteins can also regulate ion channels. Example: action of the neurotransmitter acetylcholine on heart muscle. G Proteins and Cyclic AMP Signaling Heart muscle cells have acetylcholine receptors that are G protein-coupled. The α subunit of this G protein (Gi) inhibits adenylyl cyclase. The Gi βγ subunits open K+ channels in the plasma membrane, which slows heart muscle contraction. Signaling Molecules and Their Receptors Lecture Outline: Formation of cAMP Effects of cAMP 1. Activating protein kinase A 2. Regulation of gene expression 3. Direct regulation of ion channels Signaling by receptor tyrosine kinases and non- receptor tyrosine kinases G Proteins and Cyclic AMP Signaling The role of cAMP as a second messenger was discovered in 1958 by Sutherland in studies of epinephrine, which signals the breakdown of glycogen to glucose in muscle cells. cAMP is formed from ATP by adenylyl cyclase and degraded to AMP by cAMP phosphodiesterase. Figure 16.14 Synthesis and degradation of cAMP G Proteins and Cyclic AMP Signaling Effects of cAMP are mediated by cAMP- dependent protein kinase, or protein kinase A. Inactive form has two regulatory and two catalytic subunits. cAMP binds to the regulatory subunits, which dissociate. The free catalytic subunits can then phosphorylate serine on target proteins. Figure 16.15 Regulation of protein kinase A G Proteins and Cyclic AMP Signaling In glycogen metabolism, protein kinase A phosphorylates two enzymes: Phosphorylase kinase is activated, and in turn activates glycogen phosphorylase, which catalyzes glycogen breakdown. Glycogen synthase is inactivated, so glycogen synthesis is blocked. Figure 16.16 Regulation of glycogen metabolism by epinephrine Example of Signal Transduction and Signal Amplification G Proteins and Cyclic AMP Signaling Signal amplification: Binding of a hormone molecule leads to activation of many intracellular target enzymes. Example: Each molecule of epinephrine activates one receptor. Each receptor may activate up to 100 molecules of Gs. G Proteins and Cyclic AMP Signaling Gs then stimulates adenylyl cyclase, which catalyzes synthesis of many molecules of cAMP. Each molecule of protein kinase A phosphorylates many molecules of phosphorylase kinase, which phosphorylate many molecules of glycogen phosphorylase. G Proteins and Cyclic AMP Signaling In many animal cells, increases in cAMP activate transcription of genes that have a regulatory sequence called cAMP response element (CRE). Figure 16.17 Cyclic AMP-inducible gene expression cAMP response element (CRE). CREB (CRE- binding protein). G Proteins and Cyclic AMP Signaling In many animal cells, increases in cAMP activate transcription of genes that have a regulatory sequence called cAMP response element (CRE). The free catalytic subunit of protein kinase A goes to the nucleus and phosphorylates transcription factor CREB (CRE-binding protein). G Proteins and Cyclic AMP Signaling Phosphorylation of CREB leads to recruitment of coactivators and expression of cAMP-inducible genes. Regulation of gene expression by cAMP plays important roles in many aspects of cell behavior such as proliferation, survival, differentiation, learning and memory. Figure 16.16 Regulation of glycogen metabolism by epinephrine Kinases are inactivated by phosphatases G Proteins and Cyclic AMP Signaling Protein kinases don’t function in isolation. Protein phosphorylation is rapidly reversed by protein phosphatases, which terminate responses initiated by receptor activation of protein kinases. Figure 16.18 Regulation of protein phosphorylation by protein kinase A and protein phosphatase 1 G Proteins and Cyclic AMP Signaling cAMP can also directly regulate ion channels: It is a second messenger in sensing smells—odorant receptors are G protein- coupled. They stimulate adenylyl cyclase, leading to increased cAMP. cAMP opens Na+ channels in the plasma membrane, leading to initiation of a nerve impulse. Tyrosine Kinases and Signaling by MAP Kinase, PI 3-Kinase, and Phospholipase C/Calcium Pathways Other cell surface receptors are directly linked to intracellular enzymes. The largest family of these are the tyrosine kinases, which phosphorylate their substrates on tyrosine residues. Receptor Tyrosine Kinases Non-Receptor Tyrosine Kinases Figure 16.20 Organization of receptor and non- receptor tyrosine kinases Receptor Tyrosine Kinases Non-Receptor Tyrosine Kinases Tyrosine Kinases and Signaling by MAP Kinase, PI 3-Kinase, and Phospholipase C/Calcium Pathways Receptor tyrosine kinases Includes the receptors for most polypeptide growth factors. The human genome encodes 58 receptor tyrosine kinases, including the receptors for EGF, NGF, PDGF, insulin, and many other growth factors. Tyrosine Kinases and Signaling by MAP Kinase, PI 3-Kinase, and Phospholipase C/Calcium Pathways All receptor tyrosine kinases have: An N-terminal extracellular ligand- binding domain One transmembrane α helix A cytosolic C-terminal domain with protein-tyrosine kinase activity Figure 16.20 Organization of receptor tyrosine kinases Tyrosine Kinases and Signaling by MAP Kinase, PI 3-Kinase, and Phospholipase C/Calcium Pathways Binding of ligands (growth factors) to the extracellular domains activates the cytosolic kinase domains. This results in phosphorylation of both the receptors and intracellular target proteins that propagate the signal. Tyrosine Kinases and Signaling by MAP Kinase, PI 3-Kinase, and Phospholipase C/Calcium Pathways The first step is ligand-induced receptor dimerization. This results in receptor autophosphorylation, as the two polypeptide chains cross-phosphorylate each other. Figure 16.21 Dimerization and autophosphorylation of receptor tyrosine kinases Tyrosine Kinases and Signaling by MAP Kinase, PI 3-Kinase, and Phospholipase C/Calcium Pathways Autophosphorylation has two roles: Phosphorylation of tyrosine in the catalytic domain increases protein kinase activity. Phosphorylation of tyrosine outside the catalytic domain creates binding sites for other proteins that transmit signals downstream from the activated receptors. Tyrosine Kinases and Signaling by MAP Kinase, PI 3-Kinase, and Phospholipase C/Calcium Pathways Downstream signaling molecules have domains, such as SH2 (Src homology 2 ), that bind to specific phosphotyrosine- containing peptides of the activated receptors. SH2 domains were first recognized in tyrosine kinases related to Src, the oncogenic protein of Rous sarcoma virus. Figure 16.22 Association of downstream signaling molecules with receptor tyrosine kinases Tyrosine Kinases and Signaling by MAP Kinase, PI 3-Kinase, and Phospholipase C/Calcium Pathways Nonreceptor tyrosine kinases stimulate intracellular tyrosine kinases with which they are noncovalently associated. The cytokine receptor superfamily includes receptors for most cytokines and some polypeptide hormones. Tyrosine Kinases and Signaling by MAP Kinase, PI 3-Kinase, and Phospholipase C/Calcium Pathways The structure of cytokine receptors is similar to receptor tyrosine kinases, but the cytosolic domains have no catalytic activity. Ligand binding induces dimerization of receptors, and cross-phosphorylation of associated nonreceptor tyrosine kinases. Figure 16.24 Activation of nonreceptor tyrosine kinases (Part 1) Figure 16.24 Activation of nonreceptor tyrosine kinases (Part 2) Tyrosine Kinases and Signaling by MAP Kinase, PI 3-Kinase, and Phospholipase C/Calcium Pathways The activated kinases then phosphorylate the receptor. This provides phosphotyrosine-binding sites for recruitment of downstream signaling molecules with SH2 domains.