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Cell Signaling Cell Signaling IIIISignal Transduction pathways
Cell BiologyCell BiologyLecture 13Lecture 13
Readings and ObjectivesReadings and Objectives• ReadingReading
– Russell Chapter 8 (not sufficient)
– Cooper: Chapter 15• TopicsTopicsLecture 12• Signaling Molecules and Their Receptors • Functions of Cell Surface ReceptorsLecture 13• Pathways of Intracellular Signal Transduction• Signal Transduction and the Cytoskeleton• Signaling Networks
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Intracellular Signal Transduction PathwaysIntracellular Signal Transduction Pathways• Intracellular signal transduction- chain of reactions,
transmits signals/cell surfaceintracellular targets• First studied for epinephrine• Signals glycogen breakdown to glucose• Earl Sutherland (1958): action of epinephrine was
mediated by an increase in cyclic AMP (cAMP)• Concept: cAMP is a second messenger• Noble prize 1971
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1915-19741915-1974
cAMP Signal Transduction PathwayscAMP Signal Transduction Pathways
• Epinephrine receptor coupled to adenylyl cyclase via a G protein increasing the concentration of cAMP
cAMP signaling & cell responsescAMP signaling & cell responses1.1. Metabolic regulationMetabolic regulation• Cytosolic Protein Kinase A activation (PKA)• tetramer of regulatory and catalytic subunits,
ie R2C2 (inactive)• cAMP binding of “R” dissociation of
catalytic subunits (active)• A serine/threonine kinaseActivation or
inactivation of substrate proteins
4PKA activation
cAMP Signal Transduction PathwayscAMP Signal Transduction PathwaysPhosphorylation of two downstream enzymes:• Glycogen synthase inactivated glycogen synthesis↓
• Phosphorylase kinase activated phosphorylates Glycogen phosphorylase (active) Glu-1P↑
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cAMP Signal Transduction PathwayscAMP Signal Transduction Pathways2. Gene regulation2. Gene regulation• Increased cAMP activate transcription• Free PKA C-subunit translocated to the
nucleus• binds Genes containing a regulatory
sequence—the cAMP response element, or CRE
• phosphorylates the transcription factor CREB (CRE-binding protein).
• Recruits RNA polymerase• expression of cAMP-inducible genes• Proliferation, differentiation, memory,
cognition
• Review article: Transcriptional regulation by cAMP 6
cAMP Signal Transduction PathwayscAMP Signal Transduction Pathways
• Protein phosphorylation is reversed by protein phosphatases
• terminates responses initiated by receptor activation of protein kinase
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Secondary messenger DAG and IP3 signalingSecondary messenger DAG and IP3 signaling
• PLC-γ binds receptor protein tyrosine kinases via SH2 domain phosphorylated (active)
• PLC- γ stimulates hydrolysis of PIP2 to DAG and IP3 (how?)
• DAG and IP3 are secondary messengers
• IP3 regulates Ca2+
• DAG activates PKC family
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PLC=Phospholipase CPIP2: Phosphatidylinositol 4,5-bisphosphate IP3: Inositol 1,4,5-trisphosphateDAG: Diaceyl glycerol
Secondary messenger DAG and IP3 signalingSecondary messenger DAG and IP3 signaling
• PLC-γ binds receptor protein tyrosine kinases via SH2 domain phosphorylated (active)
• PLC- γ stimulates hydrolysis of PIP2 to DAG and IP3 (how?)
• DAG and IP3 are secondary messengers
• IP3 regulates Ca2+
• DAG activates PKC family
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PLC=Phospholipase CPIP2: Phosphatidylinositol 4,5-bisphosphate IP3: Inositol 1,4,5-trisphosphateDAG: Diaceyl glycerol
Secondary messenger DAG and IP3 signalingSecondary messenger DAG and IP3 signaling
• DAG remains associated with the plasma membrane and activates protein-serine/threonine kinases of the protein kinase C family.
• IP3 , a small polar molecule, released to the cytosol
• Stimulates release of Ca2+ from the ER by binding to receptors that are ligand-gated Ca2+ channels 10
Secondary messenger DAG and IP3 signalingSecondary messenger DAG and IP3 signaling
• Calmodulin is activated when Ca2+ concentration increases
• CaM kinase family are activated by Ca2+/calmodulin
• they phosphorylate and activate other proteins such as,
• protein kinases, phosphatases, metabolic enzymes, ion channels, and transcription factors (eg CREB)
• Also regulates synthesis and release of neurotransmitters
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Secondary messenger DAG and IP3 signalingSecondary messenger DAG and IP3 signaling
• nonmuscle cells and smooth muscles, contraction is regulated by phosphorylation of myosin light chain
• catalyzed by myosin light chain kinase, which is regulated by the Ca2+ binding protein calmodulin
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Secondary messenger DAG and IP3 signalingSecondary messenger DAG and IP3 signaling
• Increased [Ca2+ ] signals further release of Ca2+ from the ER by opening Ca2+ channels (ryanodine receptors) in the ER membrane.
• Ca2+ is a versatile second messenger that controls a wide range of cellular processes
• These pathways function coordinately to regulate many cellular responses
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• PIP2 is also the start of another signaling pathway
• PIP2 is phosphorylated by phosphatidylinositide (PI) 3-kinase
• This yields a second messenger, phosphatidylinositol 3,4,5-trisphosphate (PIP3)
PI 3/Akt signaling pathway
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• PIP3 targets a protein-serine/threonine kinase called Akt and also binds protein kinase PDK1
• Activation of Akt also requires protein kinase mTOR (in a complex called mTORC2) which is also stimulated by growth factor
PI3/Akt signaling pathway
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mTOR: mammalian target of rapamycinPDK1: phosphoinositide dependent protein kinase-1GSK3: glycogen synthase kinase 3Bad: Bcl2 associated death promoter (promotes apoptosis)
• Akt phosphorylates several target proteins, transcription factors, and other protein kinases
• Transcription factors include members of the Forkhead or FOXO family
• If growth factors are not present, Akt is not active
• FOXO travels to the nucleus, stimulates transcription of genes that inhibit cell proliferation, or induce cell death
PI3/Akt signaling pathway
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• When growth factors attached to receptor/tyrosine kinases
• Akt is phosphorylated (active)• Akt phosphorylation of FOXO
sequesters it in inactive form• Akt inhibits GSK-3, the general
inhibitor of translation• Inhibition of GSK-3 relieves
translation• Cells are prepared to
proliferate
PI3/Akt signaling pathway
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• MAP kinases (mitogen-activated protein kinases) are protein-serine/threonine kinases
• Conserved across eukaryotic cells; three groups of MAP kinases
MAP Kinase Signaling Pathways
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• ERK (extracellular signal-regulated kinase) family, first to be identified in MAPKs
• regulation of meiosis, mitosis, cell proliferation and differentiation• Ligands:Ligands: growth factors, cytokines and viral infection, carcinogenic
chemicals
ERK Signaling Pathway
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• ERK activationERK activation mediated by Ras, Raf, MEK kinase cascade• Activation of Rasactivation of Raf protein serine/threonine kinase• Raf phosphorylates and activates a second protein kinase called MEK
(MAPK/ERK Kinase)• MEK activates ERK transcriptional activation
ERK Signaling Pathway
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• RasRas:: guanine nucleotide-binding protein that function like α subunits of G proteins
• Ras is activated by guanine nucleotide exchange factors (GEF)
• Sos=specific GEF for Ras• GTPase-activating
proteines GTP hydrolysis
• Ras-GDP becomes inactive
ERK Signaling Pathway
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• Grb2: Grb2: SH2 domain containing protein associated with Sos• RPTK activation by ligand recruits Grb2/Sos to membrane• Grb2/Sos contacts Ras-GDP GTP replaces GDP in Ras• Ras-GTP activated and phosphorylates Raf • Raf initiates a protein kinase cascade ERK activation
ERK Signaling Pathway
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• ERK goes to the nucleus, phosphorylates Elk-1
• transcriptional induction of immediate-early genes (~ 100 genes)
• serum response element (SRE), recognized by transcription factors serum response factor (SRF) and Elk-1
• immediate-early genes encode transcription factors
• Activate downstream genes called secondary response genes
• Cell proliferation and growth
ERK Signaling Pathway
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• Specificity of MAP kinase signaling is maintained in part by their physical association on scaffold proteins
• For example, the KSR scaffold protein organizes ERK and its upstream activators Raf and MEK into a signaling cassette
ERK Signaling Pathway
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• Direct signaling from receptor to nucleus
• Ligand: cytokines• Receptors: Janus Kinases (JAK),
nonreceptor protein-tyrosine kinase• STAT: Signal Tansducer & Activators
of Transcription• Transcription factors, contain SH2
domains that mediate binding to phosphotyrosine sequences
• STATs activated, dimerized, translocate to nucleus
• Activate transcription
JAK/STAT PathwayJAK/STAT Pathway
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• direct cell-cell interactions during development
• Notch a receptor for signaling by transmembrane proteins (e.g., Delta) on adjacent cells
• Ligand binding proteolytic cleavage of cytosolic domain of Notch
• translocated into the nucleus• converts a transcription factor (CSL in
mammals) from a repressor to an activator
• Downstream genes code for other transcriptional factors
• Cell developmental differentiation
Notch PathwayNotch Pathway
26Minireview: Notch signaling
• binding of integrins to the extracellular
• activation of FAK ( focal adhesion kinase), a nonreceptor protein-tyrosine kinase
• provides binding sites for Grb2-Sos complex, leading to activation of Ras/ERK, PI 3-kinase
Integrins and Signal TransductionIntegrins and Signal Transduction
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