Signal Transduction Flashcards
(35 cards)
Nitric Oxide (NO)
Gas, can diffuse across the membrane and alter cellular activity by activating guanylyl cyclase activity to generate cGMP. Short half life of 5-10 seconds because it rapidly converted to nitrates and nitrites with oxygen and water. Generated from arginine by NO synthase. L-arginine + NADPH +O2—-> L-citrulline + NADP+ + NO
Neuronal NOS (nNOS or NOS-I)
NO can act as a neurotransmitter when it is produced in autonomic nerve terminals and diffuses to adjacent target cells.
Macrophage (mNOS or NOS-II)
As a reactive O2 radical, NO had both tumericidal and bacteriacidal actions when produced by macrophages.
Endothelial (eNOS or NOS-III)
Regulate blood flow by influencing smooth muscle contractility in blood vessel walls.NO is a potent vasodilator. Produced in endothelial cells and diffuses directly into smooth muscle cells in vessel walls where it activates the enzyme guanylyl cyclase to generate cGMP. cGMP induces the relaxation of smooth muscle cells causing vasodilation.
Active Membrane Transport in Cancer
Ion pumps and ATP-Binding Cassette (ABC) transporters. Use ATP hydrolysis to drive the transport of substrates across the membrane, even against concentration/charge gradients. Ion pumps charge the membrane battery. Others deliver cargo. For example, multidrug resistance protein/p-glycoprotein (MDR/pgp) pumps drugs out of cells. Upregulation of pgp in tumor cells enable chemotherapeutic resistance. Down-regulation of TAP, which transports peptides into the ER where they are loaded onto class I MHC molecules and then exported to cell surface for immune surveillance.
Coupled and passive membrane transport in cancer
K+ and Cl- channels are required to mediate cell shrinkage associated with apoptosis. Other ion channels control the secretion of hormones that facilitate tumor growth. Colorectal cancer hERG1 potassium channel regulates VEGF secretion, which in turn stimulates angiogenesis. Symporters and antiporters couple unfavorable with favorable. “Rocker-switch.” Rapid proliferation is pH sensitive. Acid extrusion pumps are required to maintain intracellular pH in acidic microenvironments accomplished by NHE1 exchanger (Na/H). Wore outcomes in pancreatic ductal adenocarcinoma with upregulated NHE1.
Receptor Mediated Signal Transduction
Ligand binding on one face of the membrane stabilizes a conformational change that creates different structure(s) on opposite face of the membrane. Activate or suppress a signaling transduction in the cell. Cell differentiation, tissue adaptation, quiescence, and division. Account for 10-20% of human genes.
Endocrine
Secreted factor enters the blood stream to reach target cell.
Paracrine
Secreted factor acts locally on adjacent cells.
Contact-Dependant
Cell-cell contact generates a signal.
Synaptic
Communication between neurons and at neuromuscular junctions.
Four Classes of Receptors
Ion channel
GPCR
Receptor Kinase/enzyme (intrinsic or auxiliary)
GPCR
Hormones and neurotransmitters and small molecules bind to GPCR. They bind the Galpha subunit of heterotrimeric G-proteins (alpha beta gamma). The gamma seems to interact with the receptor. The GDP is exchanged for GTP and the conformational changes dissociate from G beta gamma. This activates the enzyme or second messenger system.
Growth Factors GF
Small proteins that stimulate growth or differentiation. Epidermal growth factor, insulin, transforming growth factors alpha and beta, platelet-derived growth factor (PDGF) and macrophage growth factor (CSF). Bind to specific receptors on the cell surface and activate protein kinase activity (tyrosine kinase) on the cytoplasmic side of the receptor protein. Activated kinase undergoes autophosphorylation in trans and the phosphorylated cytoplasmic domains then interact wiht and activate downstream pathways, involving ras and MAP kinase. Also phospholipase C. Assymetric dimer in EGFR family, one being pseudo-cyclin and the other a pseudo-CDK.
Heterotrimeric G-Proteins
Alpha beta gamma. GPCR binds to the alpha subunit which trades GDP for GTP and expulsion of beta gamma. Cholera and pertussis binds the G s alpha which irreversibly activates adenyl cyclase and G i alpha which irreversibly inhibits AC respectively. Go and Gq regulate phospholipase C.
Ras
A small GTP binding protein (monomeric G protein) that switches between active GTP-bound form and the inactive GDP-bound form. The phosphorylated cytoplasmic domains of the activated RTK are recognized by the SH2 domains of adaptor proteins (Grb2) which in turn recruit Guanine-nucleotide exchange factors (GEFs) such as SOS. Sos catalyzes the expulsion of GDP from Ras, which can then bind GTP but does so much less effectively. GAPs (GTPase activiating proteins) have evolved to bind Ras and accelerate its hydrolysis.
Phosphorylation
Activated by signaling proteins or second messengers. Kinase transfers a phosphate from ATP onto hydroxyl of serine, threonine, tyrosine residue. Phosphatase is the opposite. Autophosphorylation of RTK and downstream pathways often contain multiple phosphorylation events in series known as “cascade.”
MAP kinase pathway
Ras protein stimulates a cascade that includes Raf and MEK kinases culminating in the phosphorylation of the Mitogen-Activated kinase. Phosphorylated MAP can dimerize, enter the nucleus, where is can activate TF leading to changes in transcription and possibly translation.
Second Messengers
Diffusible, short lived compound produced inside the cell in response to binding external signal. Act via stimulation of protein kinases frequently.
Gs/cAMP-dependent signal transduction
Gs alpha activates adenyl cyclase (membrane bound) to catalyze the conversion of ATP to cAMP. Destruction by cAMP phosphodiesterase. Activates cAMP dependent protein kinase PKA. This can phosphorylate and activate targets.
Gq Calcium/diacylglycerol/inositol triphosphate-dependent signal transduction.
Several hormones elicit their cellular response by inducing the production of these second messengers. A heterotrimeric G protein is activated.
Phospholipase C
Enzyme which converts phosphatidylinositol 4,5-bisphosphate (PIP2) to diacylglycerol and inositol triphosphate (IP3).
IP3
Water soluble. Diffuses through the cytoplasm to the ER where it induces the release of Ca++ from the lumen of the ER into the cytoplasm. Ca++ and diacylglycerol combine to activate the Ca+ dependent protein kinase C or PKC. at the inside surface of the cell membrane. Some tyrosine kinases also activate it independently. Ca+ can also bind calmodulin to interact with downstream kinases (CaM Kinase). Ca+ also acts independently.
Acute Responses
Tend to act on cytoplasmic function, not gene expression. Example 1: Rhodopsin activates phospholipase C the heterotrimeric G protein. Second messengers cause Ca++ influx which activates the action potential to indicate light. Example 2: Epinephrine binding to receptor surface of skeletal muscle cells activates a heterotrimeric G protein coupled to adenyl cyclase. The cAMP generated activates PKA which phosphorylates and activates enzymes that break down glycogen to glucose.
