L9 Insulin Signalling PDF

L9 Insulin Signalling Learning outcomes: To understand the concept of cellular signalling pathways. To be able to describe the core components of the insulin signalling pathway and the cellular processes it regulates. To be able describe the roles of insulin signalling in muscle and liver....

L9 Insulin Signalling Learning outcomes: To understand the concept of cellular signalling pathways. To be able to describe the core components of the insulin signalling pathway and the cellular processes it regulates. To be able describe the roles of insulin signalling in muscle and liver. To understand how insulin signalling contributes to growth control and ageing. To understand the concept of cellular signalling pathways. Cell signalling Gene expression can be controlled by extracellular signals. Signals tell cells to respond in specific ways e.g. survive, divide, differentiate, die. Signal transduction drives cell proliferation. Ligands (first messengers) bind to cell surface receptors which link the extracellular to the intracellular environment. Examples of ligands: hormones, cell surface proteins, neurotransmitters, short range secreted factors (EGF, TNF, NGF etc). Examples of receptors: receptor tyrosine kinases (RTKs), G-protein coupled receptors (GPCRs), ionotropic receptors (ligand gated ion channels), cytokine receptors. Ligand-bound receptors undergo a conformational change which results in the activation of second messengers. Examples of second messengers are phosphatidilinositol-3-phosphate (PIP3), Ca2+, diacylglycerol (DAG), cyclic AMP (cAMP), arachidonic acid Production of second messengers causes a series of post-translational events resulting in changes in gene expression (through regulation of target transcription factors) or posttranslational changes (usually through phosphorylation). Receptor tyrosine kinase pathways RTKs are high affinity cell surface receptors for polypeptide growth factors, cytokines and hormones.  They have roles in development and cancer TRKs are generally activated by dimerization and substrate presentation. Activated receptor tyrosine kinases phosphorylate themselves (autophosphorylation). The binding of the signal protein to the ligand-binding domain on the extracellular side of the receptor activates the tyrosine kinase domain on the cytosolic side. This leads to phosphorylation of tyrosine side chains on the cytosolic part of the receptor, creating phosphotyrosine docking sites for various intracellular signalling proteins that relay the signal. Ligand binding causes the receptors to dimerise, bringing two cytoplasmic kinase domains together and promoting their activation. In many cases, such as the insulin receptor, dimerization simply brings the kinase domains close to each other in an orientation that allows them to phosphorylate each other on specific tyrosines in the kinase active sites, thereby promoting conformational changes that fully activate both kinase domains. Once the kinase domains of an RTK MBoC6 dimer are activated, they phosphorylate multiple additional sites in the cytosolic parts of the receptors, typically in disordered regions outside the kinase domain. This phosphorylation creates high-affinity docking sites for intracellular signaling proteins. Each signaling protein binds to a particular phosphorylated site on the activated receptors because it contains a specific phosphotyrosine-binding domain that recognizes surrounding features of the polypeptide chain in addition to the phosphotyrosine. To be able to describe the core components of the insulin signalling pathway and the cellular processes it regulates. Insulin Pathway The insulin receptor is a TKR. It consists of two intracellular and 2 extracellular domains that interact with disulphide bridges. When activated, it is a dimer of dimers. Binding of insulin or other agonists e.g. IGF-1 triggers autophosphorylation of tyrosine residues, activating tyrosine kinases. Tyrosine kinase phosphorylates insulin receptor substrate (IRS), which phosphorylates phosphoinositide 3 kinase (PI3K)  PI3K catalyses the conversion of pip2 to pip3. PIP3 acts as a second messenger on the internal plasma membrane and it is not present unless the pathway is activated. PIP3 recruits AKT/PKB to the membrane and binds to the pH domain where it is activated. Threonine 309 phosphorylated by PDK1. PKB triggers the translocation of glucose transporter (GLUT4) containing vesicles to the cell membrane, via the activation of SNARE proteins, to facilitate the diffusion of glucose into the cell. PKB also phosphorylates and inhibits glycogen synthase kinase, which is an enzyme that inhibits glycogen synthase. Therefore, PKB acts to start the process of glycogenesis, which ultimately reduces blood- glucose concentration. To be able describe the roles of insulin signalling in muscle and liver. Glucose homeostasis Insulin signalling in glucose homeostasis Insulin is secreted by the pancreatic b cells (express Glut2) Response to insulin in different tissues: Skeletal muscle and adipose: Increase glucose uptake (Glut4), increase glycogen synthesis (muscle), increase in lipid synthesis (adipose) o Activation of PKB inactivates repressors o FOXO1 dephosphorylated and enters nucleus to downregulate genes involved in gluconeogenesis. o GSK3 inactivated so glycogen synthase is activated. Liver: Inhibits production and release of glucose by blocking gluconeogenesis and glycogenolysis (from glycogen) o Upregulation of genes involved in glycogen synthesis Brain and other organs: Glucose uptake is constitutive and not regulated by insulin (Glut1 transporter) To understand how insulin signalling contributes to growth control and ageing. Growth control Insulin signalling regulates organ size. Insulin signalling regulates cell size and proliferation. AKT has many downstream effects including phosphorylating TSC2 and activating the mTOR complex – which upregulates protein synthesis. Ageing Insulin signalling promotes ageing. Summary: Cell surface receptors allow hormonal control of gene expression Extracellular signals are relayed to the nucleus through signal transduction pathways Insulin signalling has outputs through transcriptional and post-translational mechanisms Insulin signalling controls many key processes including glucose homeostasis, growth control and ageing