Cellular
signal transduction involves the conversion of one signal or stimulus (mechanical or chemical) to another. The transduction process is
usually performed by
enzymes in association with
second messengers.
▼ :
7TM receptors :
classes of receptors :
controlled activities :
coupling :
DAG :
diacylglycerol :
DGKzeta :
extracellular signals :
GEFs :
G-protein coupled receptors :
guanine nucleotide-binding protein-coupled receptors :
GPCR families :
GPCRs :
heptahelical receptors :
hormone receptors :
HREs :
intracellular signals :
intracellular receptors :
intercellular signals :
IP3 :
kinase inhibitors :
ligands :
phospholipases :
phospholipids :
PI3K :
PKCs :
protein kinases :
Ras :
RasGRP :
receptor classes :
serpentine receptors :
targets for control :
7TM receptors : ▼
Extracellular signals impinge upon specialized membranous receptors.
Sensory transduction involves the conversion of mechanical or chemical stimuli to cellular signals or neurophysiological signals.
Intracellular signals enable communication within cells, while
intercellular signals enable communication between cells.
Tables
Cell Adhesion Molecules
Cell signaling
Immune Cytokines
Receptor Tyrosine Kinases (RTKs)
Second Messengers
Chemical signals (
ligands) include :
1.
neurotransmitters :
acetylcholine,
dopamine,
epinephrine,
GABA,
glycine,
norepinephrine,
serotonin (5HT), etc.
2.
hormones3.
phospholipids4. growth factors
5. nutrients
Classes of
receptors:
1.
Membrane-penetrating receptors possessing/connected to
intrinsic enzymatic activity:
…….a)
receptor tyrosine kinases (
RTKs) capable of autophosphorylation as well as
phosphorylation of other substrates (incl.
EGF, FGF, insulin, PDGF receptors),
…….b) tyrosine phosphatases (CD45),
…….c)
guanylate cyclases (natriuretic peptide receptors),
…….d)
serine/threonine kinases (activins, inhibins, bone morphogenetic proteins (BMPs), TGF-beta receptors).
…….e) receptors coupled to intracellular tyrosine kinases by direct protein-protein interactions: 'Multiprotein signaling networks create focal points of enzyme activity that disseminate the intracellular action of many hormones and neurotransmitters. Accordingly, the spatio-temporal activation of protein kinases and phosphatases is an important factor in controlling where and when phosphorylation events occur. Anchoring proteins provide a molecular framework that orients these enzymes towards selected substrates. A-kinase anchoring proteins (
AKAPs) are signal-organizing molecules that compartmentalize the
cAMP dependent protein kinase,
phosphodiesterases, and a variety of enzymes that are regulated by
second-messengers.'[
s].
Phospholipases and phospholipids participate in transmission of ligand-receptor induced signals from the plasma membrane to intracellular proteins, primarily
PKC, which is maximally active in the presence of calcium ion and
diacylglycerol. PKC activity is mediated by receptors that are coupled to activation of
phospholipase C-gamma (PLC-gamma), which contains SH2 domains that enable it to interact with tyrosine phosphorylated
RTKs. PI-3K is tyrosine phosphorylated and activated by various RTKs and receptor-associated
PTKs. PI-3K is activated by the PDGF, EGF, insulin, IGF-1, HGF and NGF receptors. The p85 subunit of PI-3K is activated by tyrosine phosphorylation, but only the 110 kDa subunit is enzymatically active.
Phospholipases D and A2 (PLD, PLA2) sustain the activation of PKC through their hydrolysis of membrane phosphatidylcholine (PC). Activation of PLC-gamma results in hydrolysis of membrane phosphatidylinositol bisphosphate (PIP2), which leads to an elevation of intracellular DAG and inositol trisphosphate (
IP3), which interacts with intracellular membrane receptors to effect release of stored calcium ions (
PKC is maximally active in the presence of
second messengers,
calcium ions and
diacylglycerol).
Diacylglycerol (DAG) is an intracellular messenger, which accumulates transiently in cells exposed to growth factors or other stimuli. Cellular responses such as growth and differentiation are impacted by the binding of DAG to
PKC, thus activating PKC.
Diacylglycerol kinases (DGKs) are responsible for eliminating the function of diacylglycerol (DAG) and for producing phosphatidic acid (PA) (both molecules are connected to cancer).
DGKzeta regulates factors that promote activity of the
oncogene product,
Ras, the activity of which must be precisely regulated lest abnormal cellular proliferation result. An estimated 30% of human tumors have an activating mutation of the Ras gene. Guanine nucleotide exchange factors (GEFs) activate Ras by facilitating
GTP binding. Abnormally high levels of the nucleotide exchange factor, RasGRP can lead to malignant transformation. RasGRP has a diacylglycerol (DAG)-binding domain and its exchange factor activity depends on local availability of the signaling molecule DAG.
Diacylglycerol kinases (DGKs) remove DAG from the cell by converting DAG to PA. DGKzeta, but not other DGKs, can completely eliminate Ras activation induced by RasGRP, and diacylglycerol kinase activity is required for this mechanism.
2.
Serpentine receptors,
guanine nucleotide-binding protein-coupled
receptors, or
GPCRs, in which a characteristic trans-membrane structure spans the
cell membrane seven times. Intracellular signalling is carried out by association of the neurotransmitter with
G-proteins (small GTP-binding and hydrolyzing proteins), which leads to generation of
second messengers. GTP-hydrolytic activity of
G-proteins is regulated by
GTPase activating proteins, GAPs. Ras, is a
proto-oncogenic G-protein involved in
carcinogenesis. Other cancer-active G-proteins include the gene products of the neurofibromatosis type-1 (NF1) susceptibility locus and the BCR locus (break point cluster region gene).
There are several
families of GPCRs, including:
(a)
GPCRs that modulate
adenylate cyclase activity
(b)
GPCRs that activate
phospholipase C-gamma, leading to hydrolysis of polyphosphoinositides (such as PIP2) and generating the
second messengers,
diacylglycerol (DAG) and inositol trisphosphate (
IP3). This class of receptors includes receptors for angiotensin, bradykinin and vasopressin.
(c) Photoreceptors coupled to a G-protein (transducin) that activates a
phosphodiesterase, depressing the level of
second messenger cGMP. The drop in cGMP causes closing of a
Na+/Ca2+ ion channel, leading to hyperpolarization of the cell.
3.
Intracellular receptors that migrate to the
nucleus after binding to the ligand – here the ligand-receptor complex directly affects gene
transcription.
Hormone receptors are
cytoplasmic proteins that bypass membrane-bound signal transduction pathways – receptors for lipophilic steroid/thyroid hormones, the glucocorticoid, vitamin D, retinoic acid and thyroid hormones. All hormone receptors are capable both of binding hormone and of directly activating gene transcription (bi-directional). After binding the hormonal ligand, the hormone-receptor complex translocates to the nucleus and binds to specific DNA sequences (hormone
response elements,
HREs), resulting in altered
transcription rates of the associated gene.
Coupling of ligand-receptor interactions to intracellular events
1.
phosphorylations by tyrosine kinases and/or serine/threonine kinases –
two-component systemsIntracellular events
controlled by signaling:
1.
gene expression (
transcription)
2.
chemotaxis3. cellular growth, proliferation, and
differentiation (
tyrosine and
serine/threonine phosphorylation)
Protein kinases are targetted by pharmaceuticals because PKs play a variety of roles in disease states.
Kinase inhibitors bind to the kinase in at least four different binding modes:
(1) direct competition with ATP at the ATP binding site;
(2) engagement of an adjacent allosteric binding site in the ATP pocket, which is usually accessible when the activation loop is in the inactive conformation; and
(3) binding at sites remote from the ATP site (but still close to the ATP) that impact kinase activity;
(4) binding outside of the ATP binding pocket (truly allosteric).
Kinases can escape inhibition by mutating key residues in their catalytic domain, thus becoming resistant to the kinase inhibitors. Those kinase that have or gain functional mutations may be more sensitive or resistant to inhibition by kinase inhibitors than is the wt form of the kinase.
▲:
7TM receptors סּ
adhesion סּ
cell membranes סּ
chemotaxis :
classes of receptors :
controlled activities :
coupling :
DAG ~
DAG ~
DAGKs ~
diacylglycerol ~
diacyl glycerol kinase :
diacylglycerol :
DGKzeta :
extracellular signals :
GEFs :
G-protein coupled receptors :
guanine nucleotide-binding protein-coupled receptors :
GPCR families :
GPCRs סּ
GPCRs סּ
GPCR families s :
heptahelical receptors סּ
hormones:
hormone receptors :
HREs :
intracellular signals :
intracellular receptors :
intercellular signals :
IP3 :
kinase inhibitors :
ligands סּ
microtubules סּ
migration ₪
molecular switches סּ
neurotransmission סּ
neuronal interconnections :
phospholipases ~
phospholipase C-gamma :
phospholipids :
PI3K :
PKCs :
protein kinases :
Ras :
RasGRP ¤
Ras :
receptor classes סּ
receptor-mediated endocytosis סּ
receptor proteins :
serpentine receptors ~
signaling items ¤
signaling molecules :
targets for control :
7TM receptors : ▲
Chemotaxis •
GPCRs •
GPCR families •
hormones •
neurotransmission •
Nitric Oxide •
neuronal interconnections •
phosphotransfer-mediated signaling pathways •
Protein Kinase Signaling Networks •
receptor tyrosine kinases •
Receptor Tyrosine Kinases (RTKs) Tables
Cell signaling
Cell Adhesion
Second Messengers
Immune Cytokines
Apoptosis vs Necrosis
Apoptosis
Malignant Transformation
Oncogenes Proto-oncogenes
Regulatory Proteins Sequences •
signaling gradients •
two-component systems • animation
MAPK signal transduction :
more :
Signaling pathways:
Pathway
ABC transporters : Pathway
Phosphotransferase system (PTS) : Pathway
Two-component system : Pathway
MAPK signaling pathway : Pathway
Wnt signaling pathway : Pathway
Notch signaling pathway : Pathway
Hedgehog signaling pathway : Pathway
TGF-beta signaling pathway : Pathway
VEGF signaling pathway : Pathway
Jak-STAT signaling pathway : Pathway
Calcium signaling pathway : Pathway
Phosphatidylinositol signaling system : Pathway
mTOR signaling pathway : Pathway
Neuroactive ligand-receptor interaction : Pathway
Cytokine-cytokine receptor interaction : Pathway
ECM-receptor interaction : Pathway
Cell adhesion molecules (CAMs) : Orthology
Transporters (+diseases) : Orthology
Two-component system : Orthology
Receptors and channels (+diseases) : Orthology
Cytokines : Orthology
Cell adhesion molecules (CAMs) : Orthology
CAM ligands : Orthology
CD molecules : Orthology
GTP-binding proteins :
▲
Top ▲
Labels: AKAP, DAG, extracellular, GPCR, intracellular, pathways, PK, regulatory, RTK, SerThrK, signaling, transduction
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Biologically active molecules (ligands) exert their physiological effect by activating or inducing 3D conformational changes in receptor proteins, which participate in signal transduction, cellular signaling, gene regulation, cellular proliferation and differentiation, or regulation of cellular metabolic processes.
