Cell Signaling Flashcards
Mechanism of Ras activation
Growth factor ligand binding to RTK triggers dimerization and autophosphorylation of the RTK; SH2 domain in Grb2 adaptor protein binds to phosphotyrosine residues on the RTK; Grb2 SH3 domains bind proline-containing peptide Sos, a Ras GEF; Sos activates Ras through proximity
Mechanism of TKI vs. Antibodies
Antibodies (i.e. Cetuximab) block the extracellular ligand binding site on EGFR, preventing dimerization
Targeted kinase inhibitors (TKIs) i.e. Gefinitib bind in the phosphorylation site of EGFR, blocking ATP from binding and preventing phosphorylation & downstream signaling
Tumor cell characteristics predictive of response to EGFR TKI
Patients whose tumor cells show gain-of-function mutations in EGFR are good candidates for TKIs, i.e. Gefitinib; EGFR amplification or overexpression as determined by FISH
3 Mechanisms for resistance to TKIs
- Acquired resistance via second mutations that arise under selective pressure from TKI therapy
- Pathological activation of other receptors, i.e. ErbB2
- Primary resistance - i.e. a tumor with a Ras mutation downstream of the receptor will be unaffected by TKIs
4 types of signaling receptors
Ligand-gated ion channels
GPCRs
Enzyme-linked receptors (including RTKs)
Nuclear receptors
5 examples of second messengers
Ca2+ cAMP IP3 DAG NO
5 mechanisms of signaling termination
Re-uptake, degradation, or diffusion of an extracellular signaling molecule
Phosphatases - de-phosphorylate kinase cascades
Phosphodiesterases - hydrolyze cAMP and cGMP; activity increased by allosteric binding of substrate and phosphorylation by c-NMP dependent kinases
Intrinsic GTPase activity
Constitutively active terminators, i.e. Ca2+ pumps
Signal pathway nodes
Nodes are points in a signaling network that receive multiple inputs and/or contribute to multiple outputs
i.e. Ca2+
4 types of cellular signaling
Paracrine - from signaling cell to target cell over a short distance
Contact-dependent - requires physical contact between a membrane-bound mediator on the signaling cell and a receptor on the target cell
Endocrine - from signaling cell to target cell over a long distance
Synaptic - mediated by release of neurotransmitter at a synaptic cleft
Characteristics of lipophilic signaling molecules
Lipophilic signaling molecules can penetrate the cell membrane and bind an intracellular receptor; they cannot be stored intracellularly and therefore they are regulated by synthesis only
Ex: Steroid hormones
Characteristics of hydrophilic signaling molecules
Hydrophilic signaling molecules cannot penetrate the cell membrane and therefore must bind receptors on the cell surface; they can be stored intracellularly within the signaling cell and therefore their release is regulated by vesicular release
Ex: Peptides
G-protein coupled receptor structure
7 transmembrane domain protein with N-terminus oriented toward the cytosol; 7 TM domains fold to form a barrel structure which contains an extracellular ligand-binding domain; intracellular loops and C-tail mediate conformational changes that activate G-protein
G protein signaling
Inactivated GCPRs are bound intracellularly to a heterotrimeric G-protein, composed of an alpha subunit(-GDP) and a beta/gamma subunit; agonist binding triggers a conformational change that favors the disassociation of GDP from the alpha subunit; GTP quickly binds to and activates G-alpha; G-alpha dissociates from beta-gamma and both subunits diffuse through the membrane to reach their effector proteins
Inactivation of G-protein signaling
G-alpha is a GTPase, which hydrolyzes GTP to GDP; hydrolysis triggers G-alpha-GDP to re-bind the beta-gamma subunit as well as the receptor;
GAPs accelerate the process of GTP hydrolysis, shortening the lifespan of the signaling pathway
Switch II
The switch II region of G-alpha is responsible for binding the beta-gamma subunit in its GDP-bound, inactive state; GTP-binding frees up the switch II region to dissociate from beta-gamma, activating the two subunits
Pertussin
PTX enzymatically puts an ADP-ribose onto G-alpha near the C-terminus of the alpha subunit to lock the G protein into it’s inactive state
Cholera
CTX ADP-ribosylates Gs-alpha near the ATP binding site, inhibiting ATPase activity, leading to constitituve activation of the G-protein
