Lecture 5: Signaling Through PI3 Kinase

Signaling through pi3 kinase Comparison of PIP3 and PIP2 PIP2 Has 2 phosphates at positions 4 and 5 of inositol ring Can be cleaved by phospholipase C (beta or gamma): PIP2 —> DG + IP3 PI3 kinase can convert PIP2 to PIP3: PIP2 + P —> PIP3 PIP3 Has 3 phosphates at 3,4,5 PIP3 serves as a docking site for signaling proteins These proteins bind to it through PH domains Cannot be cleaved by phospholipase C Specific phosphatases dephosphorylate PIP3: PIP3 —> PIP2 + P Phosphoinositides are _____ • A. Receptors • B. Extracellular signaling molecules • C. Transmembrane intracellular signaling molecules RTK can activate PI3 kinase PI3 kinase binds to phosphorylated tyrosines on RTK and gets activated PI3K phosphorylates PIP2 to PIP3 Akt kinase and PDK1 (phosphoinositides-dependent kinase) bind to PIP3 through PH domains PDK1 and mTOR kinases phosphorylate and activates Akt and other proteins Activated Akt dissociates from the membrane Cells respond to specific combinations of signals An example of integration: two signals, A and B, lead to phosphorylation of protein Y at two different locations Protein Y is also called coincidence detector Which of the following can serve as a docking site? • A. PIP2 • B. PIP3 • C. DG • D. IP3 Activation of Akt promotes cell survival and growth Akt phosphorylates and inactivates protein Bad, which promotes cell survival Non-phosphorylated Bad promotes cell death by sequestering apoptosis-inhibitory protein Tyrosine-kinase-associated receptors depend on cytoplasmic tyrosine kinases for their activity The cytoplasmic domain of the receptor associates with a tyrosine kinase rather than being a kinase itself These tyrosine kinases come in many flavors, each working in a different pathway Cytokine receptors activate the Jak-STAT signaling pathway, providing a fast track to the nucleus Cytokines are hormone-like peptide signaling molecules gamma-interferon - activates macrophages (phagocytes immune cell) alpha-interferon - activates neighboring cells to resist viral infection Erythropoietin- stimulates production of red blood cells Prolactin - stimulates milk production Growth hormone - stimulates growth Interleukins - stimulate blood cell development In contrast to hormones, cytokines are produced by a large variety of cell types Janus tyrosine kinases (Jaks) associate with the cytosolic side of these receptors Certain Jaks work with certain receptors Jaks phosphorylate themselves, the receptor, and STATs STATs (signal transducers and activators of transcription) associated with the phosphorylated receptor to become phosphorylated Phosphorylated STATs dissociate from receptor, dimerize, enter the nucleus, and act as transcriptional activators Which of the following usually forms dimers? • A. GPCR • B. RTK • C. Cytokine receptor is • D. Both B and C The TGF-beta family of signaling proteins TGF-beta are secreted dimeric signaling proteins There are 30-40 in humans They act as hormones or local mediators They bind to enzyme-coupled receptors. The cytosolic domain is a serine/threonine kinase Involved in: Body patterning during development Cell proliferation Extracellular matrix production Apoptosis Tissue repair and immune regulation TGF-beta activates signaling pathway resulting in activation Smads Dimeric TGF-beta binding causes the dimeric receptors type I and II form tetramers Serine/threonine kinase domains are brought together and type II receptor phosphorylates type I Type I receptor phosphorylates Smads, which form complexes with each other and activated transcription Which receptor is not a kinase? • A. RTK • B. Cytokine receptor • C. TGFbeta receptor • D. Both B and C Signaling via proteolysis Has a huge role in development Signaling is regulated by a specific protein degradation Degradation of a particular signaling protein may stop signal transduction Cleavage of a protein at a specific site can generate prod outs with completely new properties. Par of a receptor or a signaling protein can translocate to the nucleus and serve as a gene regulatory protein Such pathways are widely used in the development of different tissues and different animals. They are used in many different unrelated circumstances Activation of notch by proteolytic cleavage Upon binding to delta, extracellular domain of notch is cleaved by extracellular protease Delta-notch complex is endocytosed by signaling cell The membrane-bound part of notch is cleaved again by a protease complex gamma-secretase Gamma-secretase is also involved in Alzheimer’s disease In alzheimer’s (mechanism of the disease is unknown), it cleaves an amyloid precursor protein, which releases its extracellular part, amyloid beta Amyloid beta forms fibrils, which eventually form plaques in the brain These plaques might not be causing the disease, and alternative mechanisms are now discussed Signals that diffuse through plasma membrane Signal molecules: Nitric oxide (NO) Carbon monoxide (CO) Steroid hormones Thyroid hormones Retinoids Vitamin D Hydrophobic extracellular signals that can pass through plasma membrane Steroid hormones Retinoids Vitamins D All of them have different chemical structures and functions, but work by similar mechanism. they bind to intracellular receptors called: nuclear receptor superfamily Nuclear receptors Located inside the cell Can bind signaling molecules (ligands), which diffuse inside the cell Nuclear receptors are transcription factors: after binding a ligand, nuclear receptors regulate transcription of the genes In the absence of ligand, nuclear receptors are usually bound to inhibitor proteins and are inactive Ligand binding cause conformational change, which causes dissociation of the inhibitor Binding of a ligand often causes a nuclear receptor to enter the nucleus, while its unbound form remains inactive in the cytoplasm; however, some of them are always in the nucleus Which of the following is true? • A. All hormone. receptors are gene regulatory proteins • B. Nuclear receptors are always in the cytoplasm before they bind to a signal • C. Nuclear receptors can be located in the nucleus even in the absence of a corresponding signaling molecule Responses of different cells to the same signal may differ To respond to a signal, a cell is supposed to have a receptor for it Nuclear receptors usually interact with other regulatory proteins at a specific gene. The set of these other proteins is different in each cell