Signal Transduction

Question 1

Nitric Oxide (NO)

Answer 1

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

Question 2

Neuronal NOS (nNOS or NOS-I)

Answer 2

NO can act as a neurotransmitter when it is produced in autonomic nerve terminals and diffuses to adjacent target cells.

Question 3

Macrophage (mNOS or NOS-II)

Answer 3

As a reactive O2 radical, NO had both tumericidal and bacteriacidal actions when produced by macrophages.

Question 4

Endothelial (eNOS or NOS-III)

Answer 4

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.

Question 5

Active Membrane Transport in Cancer

Answer 5

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.

Question 6

Coupled and passive membrane transport in cancer

Answer 6

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.

Question 7

Receptor Mediated Signal Transduction

Answer 7

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.

Question 8

Endocrine

Answer 8

Secreted factor enters the blood stream to reach target cell.

Question 9

Paracrine

Answer 9

Secreted factor acts locally on adjacent cells.

Question 10

Contact-Dependant

Answer 10

Cell-cell contact generates a signal.

Question 11

Synaptic

Answer 11

Communication between neurons and at neuromuscular junctions.

Question 12

Four Classes of Receptors

Answer 12

Ion channel
GPCR
Receptor Kinase/enzyme (intrinsic or auxiliary)

Question 13

GPCR

Answer 13

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.

Question 14

Growth Factors GF

Answer 14

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.

Question 15

Heterotrimeric G-Proteins

Answer 15

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.

Question 16

Ras

Answer 16

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.

Question 17

Phosphorylation

Answer 17

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.”

Question 18

MAP kinase pathway

Answer 18

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.

Question 19

Second Messengers

Answer 19

Diffusible, short lived compound produced inside the cell in response to binding external signal. Act via stimulation of protein kinases frequently.

Question 20

Gs/cAMP-dependent signal transduction

Answer 20

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.

Question 21

Gq Calcium/diacylglycerol/inositol triphosphate-dependent signal transduction.

Answer 21

Several hormones elicit their cellular response by inducing the production of these second messengers. A heterotrimeric G protein is activated.

Question 22

Phospholipase C

Answer 22

Enzyme which converts phosphatidylinositol 4,5-bisphosphate (PIP2) to diacylglycerol and inositol triphosphate (IP3).

Question 23

IP3

Answer 23

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.

Question 24

Acute Responses

Answer 24

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.

Question 25

Long Term

Answer 25

Alterations in gene expression. TF. Play a role in cancer.

Question 26

Beta 1 Adrenergic Signaling Pathway

Answer 26

Target of beta blockers. Adrenaline binds to B1-AR and activates G alpha S, production of cAMP, and the activation of PKA, which then phosphorylates L-type Ca++ channels and the ryanodine receptor, both of which regulate Ca++ influx. Phospholamban is alpha phosphorylated causing it to dissociate from the SERCA2 Ca++ pump which produces a greater transmembrane gradient and thus a stronger driving force for Ca++. The therapeutic benefit of the beta blocker could re sensitize the B1-AR to adrenaline.

Question 27

Skeletal Muscle Glycolysis

Answer 27

PKA (from Gs alpha, AC, cAMP pathway) phosphorylates phosphorylase kinase, activating it to phosphorylate glycogen phosphorylase, which converts glycogen into glucose-1-phosphate.

Question 28

MAP Kinase

Answer 28

Dephosphorylated it is monomeric and cytosolic. Phosphorylation allows it to form dimers that are capable of entering into the nucleus where it phosphorylates a TF that regulates expression of genes by interacting with serum response element (SRE).

Question 29

PKA and gene expression

Answer 29

The catalytic subunit of PKA (called cAPK) can also enter the nucleus where it phos the cAMP response element binding protein (CREB) which regulates the expression of genes downstream from the cAMP response element (CRE).

Question 30

Reversibility

Answer 30

Bind and release with non-covalent bonds. Agonist vs mass action balances the forward and back reactions. A refractory state is for when the transient process must shut off more rapidly than the agonist can be removed or dissolved. Spontaneously in a conformational change or by covalent or non-covalent modification of the receptor following activation. Opposing catalytic mechanisms (ie phos dephos).

Question 31

Signal Cascades

Answer 31

Allow repeated signal amplification by the enzymatic components.

Question 32

Cross-Talk

Answer 32

The sharing of signaling pathway “modules” among multiple receptors means that they integrate the input from multiple pathways, providing a unified readout.

Question 33

Feedback

Answer 33

Desensitization (adjust responsiveness acutely) or chronically (adaptation). For example eyesight.

Question 34

Wnt

Answer 34

Cell fate. Embryo patterning, CNS patterning, control of stem cell number during differentiation. Regulates the decision to keep dividing or to terminate. Cysteine rich. Interaction with its receptor. In its absence, axin and APC assemble a destruction complex including kinases CKI and GSK that binds to beta-catenin and phosphorylates it. Phosphorylated beta-catenin is Ub. When Wnt is present, Dvl recruited to the site frizzled forming a polymeric assembly that recruits axin away. Beta catenin enters the nucleus, binds to TCF and activates transcription of key division genes. c-myc.

Question 35

Steroid Hormone Signaling

Answer 35

Synthesized from cholesterol and secreted into the blood stream with carrier proteins. Can diffuse across the plasma membrane. Bind to receptors in cytoplasm and they enter the nucleus and modulate gene expression. Altered gene expression.