Cell Signaling Flashcards
Steps in cell signaling
- Synthesis of signaling molecule by signaling cell
- Release of the aforementioned signaling molecule
- Transport of the signal to target cell
- Detection of signal by specific receptor of target cell
- Change in function of proteins in target cell, triggered by the receptor-signal complex
- Termination of the signal, which usually terminates the cellular response
Types of signaling mechanisms
Broad classification
- Intercellular signal transduction
- Intracellular signal transduction
Intercellular: endocrine, paracrine, autocrine, and juxtacrine
Intracellular signal transduction
Cell signaling
Comprises the biochemical decoding of the signal upon receipt by a target cell, a process that involves stepwise regulation of intracellular signaling proteins
Types of intercellular signal transduction
- Endocrine
- Paracrine
- Autocrine
- Juxtacrine
Endocrine signaling
Specialized endocrine cells produce hormones which circulate in the bloodstream and can act on diverse and distant targets
Paracrine signaling
Local secretion of mediators by cells. These mediators are rapidly taken up, destroyed or sequestered such that they act only on nearby cells. Synaptic or neuronal signaling+
Autocrine
Cells may secrete a molecule to which they themselves respond
Juxtacrine
Aka contact-depedent signaling - specific cell surface proteins (plasma membrane-attached/anchored) are recognized by receptors expressed on an adjacent cell.
Polypeptide Hormones
Insulin and glucagon
List the classes of hormones
- Polypeptide hormones
- Catecholamines
- Steroid hormones
- Thyroid hormone
Neurotransmitters
Synthesized and stored in nerve terminals. When released the interact with receptor-gated ion channels to mediate rpid responses. Mostly small, simple molecules such as amino acids, substituted amines, and nucleotides
Neuropeptides (neurohormones)
Affect the response of neurons to neurotransmitters. These molecules also have hormone-like affects
Ex: Endorphins and enkephalins that regulate pain sensation
Growth factors
Proteins that bind to receptors on the target cell surface and primarily activate cellular proliferation and/or differentiation. Many GFs are quite versatile, stimulating cellular division in numerous different cell types, while others are specific for a particular target cell type.
Cytokines
Are a unique family of growth factors released by immune cells and are particularly important in both innate and adaptive responses as well as the activation of phagocytic cells.
- Lymphokines - secreted by lymphocytes
- Monokines - secreted by monocytes or macrophages
- Interleukins - IL1-IL35
- Chemokines - cytokines that mediated chemoattraction between cells. Responsible for the homing of leukocytes to site of infection/inflammation.
Local mediators
Involved in paracrine/autocrine signaling
- Eicosanoids
- Platelet activating factor (PAF)
- Lysophopholipids
- Nitric oxide (NO)
Eicosanoids
Derived from arachidonic acid (prostaglandins, thromboxanes, and leukotrienes)
Local mediator
Platelet activating factor (PAF)
A potent phospholipid activator released directly from cell membranes that mediates a wide range of potent and specific biological effects including platelet aggregation, inflammation, and anaphylaxis
Lysophospholipids
Novel class of mediators that include lysophosphatidic acids (LPA) and sphingosine 1-phosphate (S1P) that evoke a wide range of biological responses
Nitric oxide
A gas that diffuses freely across cell membranes. NO is biosynthesized from arginine and oxygen by various nitric oxide synthase (NOS) enzymes and by reduction of inorganic nitrate. NO interacts with many molecules and is quickly consumed close to where it is synthesized (autocrine/paracrine)
Group I signaling molecules
Lipophilic and easily diffuse through cell membranes
Bind to intracellular receptors that act directly on gene transcription (i.e. estrogen and progesterone receptors
Group II signaling molecules
Hydrophilic - cannot diffuse through plasma membranes
Bind to transmembrane proteins that act as cell surface receptors (initiate signaling cascades which produces the cellular response via second messenger systems
G-protein coupled receptors (GCPR)
Seven transmembrane receptors (largest class of receptors)
Act through heterotrimeric GTP-binding proteins (G-proteins) to regulate activity of effector enzymes to produce secondary messengers
Or they act as ligand activated ion channels (as a rule - most small molecules and peptides (8-12 a.a.) hormones activated GPCRs)
Secondary messengers
Regulate the activity of various effector proteins - such as kinases and phosphatases which then regulate the activity of other proteins
- Calcium
- Cyclic AMP (cAMP)
- Cyclic GMP (cGMP)
- Phophatidylinositides
- Diacylglyceride
Ligand-gated ion channels
Aka transmitter-gated ion channels - ionotropic receptors and neurotransmitter receptors
Ion channels that open or close in response to binding of a chemical messenger. Such receptors located at synapses convert the chemical signal of presynaptically released neurotranmitter directly and very quickly into a postsynaptic electrical signal
Tyrosine kinase-linked receptors
Lack enzymatic activity, but when bound by a ligand they associate with and activate and intracellular tyrosine kinase which acts on intracellular proteins to alter their activities.
Most cytokines signal via tyrosine kinase-linked receptors coupled to the JAK/STAT pathway
Receptor guanylate cyclase
i. Receptors with an intracellular guanylate cyclase domain
ii. Activation of the cyclase domain increases cGMP, as a second messenger
iii. Only two:Atrial natriuretic factor (ANF) receptors and guanylin receptors
Receptor tyrosine phosphatases
i. Receptors with and intracellular tyrosine phosphatase domain
ii. Activation of the domain removes phosphate groups from tyrosine side-chains in proteins to alter their functions
Receptor serine/threonine kinases
i. Receptors with an intracellular serine/threonine kinase domain
ii. Ligand binding activates the ser/thr kinase domain
iii. Receptors for transforming growth factor beta (TGF-beta) and bone morphogenic protein (BMP) families
iv. Regulate activity of SMAD transcription factors
Receptor tyrosine kinases (RTKs)
i. Receptors with an intracellular
tyrosine kinase domain. Ligand binding activates the kinase domain.
ii. Signaling involves adaptor proteins to connect to diverse signaling pathways such as the RAS/MAP kinase pathway
iii. Well known examples = insulin receptor and epidermal growth factor (EGF) receptor family
iv. In general, polypeptide (50-400 a.a.) growth factors activiate RTKs
Detection of the signaling molecule by a target cell receptor is coupled to processes that cause either:
Target cell responses to signaling molecules
- An immediate change in cellular metabolism by activating or inhibiting enzymes or stimulating the uptake of metabolites
- The opening or closing of ion channels resulting in a change in the electrical charge across the plasma membrane
- A change in gene expression
Mechanisms that terminate or attenuate response to various signaling molecules
- Reducing the signaling molecules availability in the extracellular space
- Internalizing and degrading the receptors
- Rapidly modifying the receptor (e.g. phosphorylation) so that it becomes inactive or desensitized
- Activating the expression of a gene coding for an inhibitor of the signaling pathway
General mechanism of steroid hormone signaling
Pathway diagram
Hormone response element
Aka HREs - The receptor-hormone complex alters transcription of susceptible genes by binding (as homo/heterodimers) to regulatory gene sequences collectively known as HREs
Glucocorticoid response element (GRE)
Estrogen response element (ERE)
Vitamin D response element (VDRE)
Nuclear receptor response roles
- Reproduction - estrogen receptors, progesterone receptors, androgen receptors
- Glucose metabolism and stress - glucocorticoid receptor (GR)
- Mineral balance - mineralcorticoid receptor (MR)
- Thyroid function - thyroid hormone receptor (TR)
- Lipid metabolism - liver x receptor (LXR) and peroxisome proliferator-activated receptors (PPARs)
Common modular structure of all NR’s
- N-terminal transactivation domain
- A central DNA binding domain, containing two zinc finger DNA binding motifs
- A C-terminal ligand/hormone-binding domain
What nuclear receptor subclass form homodimers that bind to inverted repeat elements?
Steroid receptor subclass (GR, MR, PR, AR, and ER)
NR activation/repression of gene transcription
Activation - due to the recruitment of co-activator proteins by the hormone-receptor complex (co-activators often have HAC activity)
Repression - is due to the recruitment of co-repressors by some unliganded nuclear receptor (co-repressors often have HDAC activity)
Mifepristone (RU486)
Antagonist of progesterone receptors
Thiazolidinedion (TZD) derivatives
Rosiglitazone (Avandia), pioglitazone (Actos)- are usued as antidiabetic drugs. TZDs bind to peroxisome proliferator-activated receptor (PPAR)-ƴ, which forms heterodimers with retinoic acid receptor (RXR) and binds to PPREs in the promoters of genes inolved with control of glucose and lipid metabolism in muscle, adipose, and liver.
Selective estrogen receptor modulators (SERMs)
Tamoxifen and raloxifene - Fx as agonist or antagonists of estrogen receptors depending upon cell type. Used to treat estrogen-dependent breast cancers.
Tamoxifen binds to ER but does not induce the same conformational change as estrogen that is necessary for interaction with co-activator proteins
General principles of hormone signaling disorders
- Excessive hormone production
- Insufficient hormone production
- Receptor defects and/or downstream defects (sometimes referred to as “hormone resistance”)
What is the biologically active thyroid hormone
T3 - tri-iodothyronine
Functions - increase oxidative metabolism and increase basal metabolic rate (BMR)
Analogy: thyroid gland functions as the body’s thermostat (turned up - burn more fuel and prouduce more heat)
Thyroid hormone synthesis
Synthesized by follicular cells in the thyroid gland by iodination and coupling of tyrosines in thyroglobulin by tyrosine peroxidase (TPO)
Thyroid gland secrete mostly T4 -> peripheral tissues deiodinate T4 to produce circulating T3
Hypothalmic-pituitary-thyroid axis
Diagram
Hypothyroidism
sxs: Weakness, fatigue, and lethary due to diminished utilization of oxidative fuels
Cold intolerance due to reduced BMR and reduced heat production
Weight gain due to stored fuels not being utilized
Dry skin and coarse hair due to reduced metabolic activity of sebaceous glands
Elevated LDL (LDL_C) due to redueced LDLR expression in liver (LDLR gene is upregulated by thyroid hormone)
What removes LDL from circulation?
LDLR on hepatocytes bind and internalize LDL_C removing it from the blood
Hypothyroidism Tx
Levothyroxine (synthetic T4)
Iodine supplement for iodine deficiency
Surgery for tumors
Immune modulation for autoimmune conditions
Management of other hormone deficiencies for secondary or tertiary hypothyroidism cases, if present
Types of hypothyroidism
Primary hypothyroidism (↓T3/T4→ ↑TSH): Hashimoto’s thyroiditis, iodine deficiency
Secondary hypothyroidism ( ↓TSH → ↓T3/T4): pituitary defect
Tertiary hypothyroidism( ↓TRH→↓TSH →↓T3/T4): hypothalamus defect
Iodine deficiency
Dx: Low T4, elevated TSH, and low urinary iodine
Tx: iodine supplementation
Primary hypothyroidism
Regulation of cortisol secretion
Diagram
Hypercortisolism
Cushing’s syndrome
Sxs:
Muscle weakness, skin thinning, stretch marks due to protein breakdown
Hyperglycemia due to ↑ gluconeogenes
Hypertension due to ↑ sodium retention and consequent water reabsorption coupled with enhanced sensitivity of blood vessels to catecholamines
Hirutism due to stimluation of androgen production
Nitric Oxide
Synthesized by the deamination of arginine by NO synthase
Activates a cytosolic guanylate cyclase resulting in the production of the second messenger cyclic guanosine monophosphate (cGMP)
Increase in cGMP levels activated cGMP-dependent protein kinases, culminating in a reduction in intracellular Ca2+ conc. and decrease to the sensitivity of the contractile system to Ca2+
NO signaling during SMC relaxation
Diagram
GPCRs
7 transmembrane helices
Activate heterotrimeric guanine-nucleotide binding proteins (G-proteins) to control specific effector proteins
G-proteins
Have three subunits (α, β and γ)
When activated - G-protein interacts with an effector protein which produces a 2nd messenger moecule (effector proteins are usually enzymes or ion channels)
α subunits can bind GDP or GTP
-αGDP is inactive
-αGTP is active
Agonist binding to a GPCR results in a conformational change that promotes interaction with an inactive G-protein (αGDPβγ). - induces conformational change that promotes interaction with inactive G-protein αGTP dissociates from βγ and interacts with an effector protein altering its activity
α contains an intrinsic GTPase activity which hydrolyzes GTP to GDP. The α-GDP then reassociates with βγ to form an inactive αGDPβγ complex. The GTPase activity functions as a self-timer or “auto-off” switch for the α subunit.
Major G-protein families
The 3 major Gα families: αs, αi,αq (aka Gs, Gi, and Gq)
Gs and Gi modulate adenylate cyclase to alter levels of 3’,5’-cAMP:
Gs - stimulates adenylate cyclase to increase cAMP
Gi - inhibits adenylate cyclase to decrease cAMP
The net adenylate cyclase activity in a cell
is the result of simultaneous action of Gs and Gi signals
cAMP-dependent protein kinase A (PKA)
Activated by cAMP
PKA is a tetramer with two regulatory (R) and two catalytic (C) subunits
When cAMP levels rise, cAMP binding sites in the R subunits become occupied and the C subunits dissociate as active ser/thr kinases
PKA regulates the activites of secondary kinases and phosphatases, with change pohsphorylation states of various enzymes turning them on or off. A classic example is regulation of glycogen synthesis and breakdown in the liver
PKA regulation of glycogen synthesis and breakdown in the liver
Figure
Active PKA inactivated glycogen synthase
Posphorylation of cAMP-regulatory element binding protein (CREB)
Performed by PKA - phosphorylated CREB binds to specific DNA sequences, cAMP regulatory elements (CRE), in the promoters of some genes to produce changes in gene expression
Gq (aka GP or GPLC)
Activates phospholipase C (PLC).
PLC cleaves phosphatidyl inositol bisphosphate (PIP2) in the membrane giving rise to 2nd messengers:inositol triphosphate (IP3) and diacylglycerol (DAG)
IP3 diffuses in the cytosol and binds to IP3-gated Ca2+ release channels on Ca2+ storage organelles (sarcoplasmic and endoplasmic reticulum) causing an increase in cytosolic Ca2+ which activates calcium-dependent processes
A) Cushing’s syndrome
B) Hypothyroidism
C) Grave’s disease
D) Addison’s disease
Cushing’s syndrome
A mutation in the Gs protein increases the rate of GTP hydrolysis. How would this mutation most likely affect protein kinase A (PKA) activity?
Decrease PKA activity
In many cells, epinephrine will bind to alpha-1 adrenergic receptors resulting in activation of the Gq signaling pathway. Which of the following happens when epinephrine binds alpha-1 adrenergic receptors?
Phospholipase C will be activated
A 48 year old male is diagnosed with type II diabetes mellitus. After evaluating various treatment modalities, you recommend therapy with pioglitazone, a thiazolidinedione (TZD) with affinity for PPAR gamma, a nuclear hormone receptor. Which of the following best describes the mechanism by which pioglitazone therapy exerts beneficial effects?
Modulation of gene transcription
A) Phosphorylation of STAT factors
B) GTP/GDP exchange on Ras
C) Dimerization of SMAD factors
D) Activation of Janus kinases (JAK)
Dimerization of SMAD factors
A) Prohpsatidylinositol-3-kinase (PI-3K)
B) Protein kinase B (Akt/PKB)
C) Grb2
D) Ras
A) PI-3K
Goes on to activate Akt and PKC
The mechanism by which Ras operates downstream of an activated receptor tyrosine kinase is most similar to which of the following signaling proteins?
Alpha subunits of heterotrimeric G proteins
Proteins that contain a Src homology 2 (SH2) domain_______________________.
Bind to proteins that have a phosphotyrosine residue within a specific context
The hematopoietic system is made up of all adult blood cell types including erythrocytes and cells of the myeloid and lymphoid lineages. All these cells are derived from multipotent hematopoietic stem cells (HSCs) through a succession of precursors with progressively limited potential under the control of specific cytokines, such as interleukins and granulocyte/monocyte-stimulating factors. Maturation of HSCs involves which of the following signal transduction events?
C) activation of ctyosolic kinases that phosphorylate STAT proteins
The insulin receptor has in common with many growth factor receptors, such as the epidermal growth factor receptors (EGFR) and the fibroblast growth factor receptors (FGFR), ________________.
A) the ability to phosphorylate tyrosine residues on proteins
Which of the following occurs when epidermal growth factor binds to EGF receptors (EGFR) and activates the RAS/MAP kinase pathway?
Grb2 binds to phosphorylated tyrosine residues of the receptor
A 45-year-old patient with a history of type 2 diabetes mellitus presents to the clinic with complaints of unexplained weight loss, excessive thirst, and frequent urination. Laboratory tests reveal hyperglycemia and glycosuria. The patient’s medical history includes a sedentary lifestyle and a diet high in processed foods. Given the symptoms and history, the physician suspects insulin resistance and decides to investigate the involvement of the phosphatidylinositol 3-kinase (PI3K) signaling pathway. Which of the following statements concerning the reaction catalyzed by phosphatidylinositol 3-kinase (PI-3K) is true?
B) phosphaatidylinositol 4,5-bisphophate (PIP2) is a substrate
RAS activation in cells is opposed by the activity of _________________?
E) GTP’ase activating proteins (GAPs)
