Hormone Signaling Pathways Lecture (Dr. Zaidi) Flashcards
Hormones
- MESSENGER Molecules
- SYNTHESIZED and SECRETED by specialized Cells called “ENDOCRINE CELLS”
- RELEASED into Extracellular Fluid or BLOOD STREAM
- Exert their influence by BINDING TO SPECIFIC RECEPTORS on the Cells of remote Target tissues
- Result in ACTIVATION OF SIGNAL TRANSDUCTION mechanisms that ultimately lead to CELL TYPE SPECIFIC RESPONSE
- Small amount of Hormone required to Alter cell Metabolism
- Effect greatly magnified via AMPLIFICATION of Signal
Hormone Signaling: A Multi Step Process
- Biosynthesis
- Storage
- Secretions
- Transport to Target Tissue/ Cells
- Recognition and Binding to Receptors
- Activation of Signal Transduction Pathway
- Relay and Amplification of Signal
- Cellular Response
- Degradation
Steps in Cell Signaling
- In RESPONSE to a STIMULUS, a Signaling cells SYNTHESIZES and SECRETES A SIGNALING MOLECULE (Lipid Soluble or Water Soluble)
- The Signaling Molecule TRANSPORTED to Target Cell, where it BINDS to a Specific RECEPTOR PROTEIN
- The Signaling Molecule- Receptor Complex ACTIVATES OR INHIBITS CELLULAR PATHWAYS that elicit a Particular Cellular Response (Metabolism, Gene Expression)
- Signal TERMINATED by REMOVAL of the Signaling Molecule and/or Receptor or INACTIVATION of the Signaling events triggered by the Signaling Molecule- Receptor Complex
Types of Signaling
1) ENDOCRINE SIGNALING:
- Signaling Molecule released by a Cell DISTANT from the Target Cell. Ex: EPINEPHRINE
2) PARACRINE SIGNALING:
- A Signaling Molecule released by One Cell Type and Diffuses to a Neighboring Target Cell of a Different Cell Type. Ex: TESTOSTERONE
3) AUTOCRINE SIGNALING:
- Singlaing Molecule acts on the Same Cell Type as the Secreting Cells themselves. Ex: INTERLEUKIN - 1
4) JUXTACRINE SIGNALING:
- Signaling Molecule stays attached to the Secreting Cell and Binds to a Receptor on an Adjacent Target Cell. Ex: HEPARIN-BIDNING EPIDERMAL GROWTH FACTOR
Hydrophilic Hormone Signaling
- Hydrophilic Hormones CANNOT PENETRATE the PLASMA MEMBRANE
- Interact with Specific RECEPTORS at the CELL SURFACE
- Ex: Epinephrine, Insulin, Glucagon, etc
- Signaling Molecule-Receptor Complex initiates production of SECOND MESSENGER Molecules Inside Cell
- This triggers the CELLULAR RESPONSE
Receptors involved in Hydrophilic Hormone Signaling:
1) G PROTEIN COUPLED RECEPTORS (GPCRs)
2) RECEPTOR TYROSINE KINASES (RTKs)
Lipophilic Hormone Signaling
- LIPOPHILIC HORMONE PASSES through PLASMA MEMBRANE of TARGET CELL
- Ex: Steroid Hormones, Thyroid Hormones and Retinoids
- Hormone (Ligand) binds to Specific RECEPTOR PROTEINS inside the Cell.
- Signaling MOLECULE- RECEPTOR COMPLEX acts as a TRANSCRIPTION FACTOR
Family of DNA Binding Transcription Factors:
1) CYTOPLASMIC RECEPTORS:
- Exist in an Inactive Complex with HSP 90. Upon Bidning to Signal HSP DISSOCIATES. The Hormone- Receptor Complex Translocates to Nucleus where it binds to a Specific DNA Sequence called the HORMONE RESPONSE ELEMENT (HRE) in the Promoter Region of Specific Genes.
2) NUCLEAR RESPONSE:
- Already present in Nucleus BOUND TO DNA. The Hormone Signaling Activated the Complex and allows for INTERACTIONS with Additional Proteins
BOTH REGULATE THE TRANSCRIPTION OF SPECIFIC GENES**
Lipophilic (“Lipid Loving”)
SIGNALING MOLECULES:
- Steroid Hormones: Progesterone, Estradiol, Testosterone, Cortisol, Aldosterone, Vitamin D
- Thyroid Hormone: Thyroxine
- Retinoids: Retinol, Retinoid Acid
RECEPTOR LOCATION AND TYPE:
- Found in the CYTOPLASM and NUCLEUS
- Family of DNA-BINDING TRANSCRIPTION FACTORS
Hydrophilic (“Water Loving”)
SIGNALING MOLECULES:
- Amino Acid Derived: Histamine, Serotonin, Melatonin, Dopamine, Norepinephrine, Epinephrine
- From Lipid Metabolism: Acetylcholine
- Polypeptides: Insulin, Glucagon, Cytokines, Thyroid Stimulating Hormone
RECEPTOR LOCATION and TYPE:
- Found on the Surface of Plasma Membranes
- Includes Transmembrane proteins such as G Protein- Coupled Receptor and Receptor Tyrosine Kinases
Lipophilic and Hydrophilic Medications
1) HYDROPHILIC MEDICATIONS:
- Have a SHORT HALF LIFe (Seconds to Minutes)
- For Ex: EPINEPHRINE contained in Autoinjectors used to Treat Severe ACUTE ALLERGIC Reaction that may lead to ANAPHYLACTIC SHOCK. Given at TIME OF NEED!!!
2) LIPOPHILIC MEDICATIONS:
- Have LONG HALF LIFE (Hours to Days)
- For Ex: ORAL CONTRACEPTIVES contain Ethinyl Estradiol, a derivative of Estradiol. NEED TO TAKE DIALY!
GPCR Signaling
- TRIMERIC G PROTEINS contain THREE SUBUNITS (Alpha, Beta, and Gamma)
- An INACTIVE G PROTEIN has Guanosine Diphosphate (GDP) bound to its Alpha Subunit, which is attached to the Beta and Gamma Subunits
- To become ACTIVE, the G Protein must Exchange its GDP FOR GUANOSINE TRIPHOSPHATE (GTP)
- This occurs via the action of a GUANINE NUCLEOTIDE EXCHANGE FACTOR (GEF)
- The active, GTP Bound Alpha Subunit separates from Beta and Gamma Subunits
- To return to its INACTIVE STATE, the INTRINSIC GTPASE ACTIVITY of the G Protein Hydrolyzes its bound GTP into GDP and Phosphate (Pi)
- This action is Accelerated by a GTPASE- ACTIVATING PROTEIN (GAP)
GPCR Signaling Variations
1) Gs: Stimulates ADENYLATE CYCLASE
2) Gi: INHIBITS ADENYLATE CYCLASE
3) Gt: STIMULATES HYDROLYSIS of cGMP!!!!
G Protein Coupled Receptor (GPCR) Signaling
- Binding of a Singlaing Molecule to a GPCR causes Activation of Gs or Gi, which in turn STIMULATES or INHIBITS ADENYLATE CYCLASE
- Activated AC Converts ATP to cAMP, a SEOND MESSENGER that regulates the activity if Protein Kinase A (PKA). Enzymatically INACTIVE PKA exists as a TETRAMERIC COMPLEX Containing Two Regulatory Subunits (R) and Two Catalytic Subunits. Binding of cAMP to the Regulatory Subunits causes the Complex to DISSOCIATE.
- The FREE, ACTIVE Catalytic Subunits Phosphorylate Target Proteins. cAMP is Hydrolyzed into AMP by the Action of PHOSPHODIESTERASE (PDE), an Enzyme that is INHIBITED by CAFFEINE
- Light triggers the GPCR- mediated activation of Gt, which, in turn, stimulates the Hydrolysis of CYCLIC GUANOSINE MONOPHOSPHATE (cGMP) by cGMP PDE
G Protein- Coupled Receptor (GPCR) Signaling via Gq, Phospholipase C (PLC) and Protein Kinase C (PKC)
- Binding of a Singlaing Molecule to a GPCR Triggers the ACTIVATION OF Gq, which stimulates the CLEAVAGE OF PHOSPHATIDYL INOSITOL 4,5 BISPHISPHATE (PIP2) by PHOSPHOLIPASE C (PLC) to yield INOSITOL 1,4,5- TRIPHOSPHATE (IP3) and DIACYLGLYCEROL (DAG)
- IP3 causes the RELEASE OF CA2+ from the Endopalsmic/ Sarcoplasmic Reticulum (ER/ SR) into the Cytosol
- An Increase in Cytoplasmic CA2+ causes the Cytosolic Enzyme PROTEIN KINASE C (PKC) TO TRANSLOCATE TO THE PLASMA MEMBRANE, where it is activated by DAG
- Ca2+ also binds to the Cytosolic Protein CALMODULIN, forming a Complex that activates Ca2+-Calmodulin-Dependent Proteins, which include CA2+-CALMODULIN-DEPENDENT PROTEIN KINASE (CaM Kinase) and MYOSIN LIGHT CHAIN (MLC) KINASE
- Activated CaM Kinase Phosphorylates Target Proteins to alter their activities, whereas activated MLC Kinase Phosphorylates MYOSIN LIGHT CHAINS, causing SMOOTH MUSCLES TO CONTRACT
Gs GPCR with EPINEPHRINE
Physiological Response:
- RELAXATION of BRONCHIAL and INTESTINAL Smooth muscle
- CONTRACTION of Heart Muscle
- INCR Breakdown of Triacylglycerides in Adipose Tissue
- INCR Breakdown of Glycogen in Liver and Muscle
- INCR Glycolysis in Muscle
***EPINEPHIRNE NON-SELECTIVE AGONIST OF ALL ADRENERGIC RECEPTORS (MAJOR SUBTYPES Alpha1, Alpha2, Beta1, Beta2, Beta3) UNDERGO MULTIPLE GPCR SIGNALING PATHWAYS!
Gs GPCR with HISTAMINE
Physiological Response:
- BRONCHOCONSTRICTION and SYMPTOMS of ALLERGIC REACTIONS (Itchy, Watery Eyes)
Gi GPCR with Epinephrine/ Norepinephrine
- CONSTRICTION of Smooth Muscle
Receptor Tyrosine Kinase
- ISNULIN Hormoen (Ligant) bidns to its Receptor (RTK) to regulate GLUCOSE METABOLSIM
1) RAS- DEPENDENT Signaling
2) RAS- INDEPENDENT Signaling
Primary Structure of Insulin
- Insulin composed of Two Peptide Chains referred to as the A Chain and B CHAIN
- They are linked together by TWO DISULFIDE BRIDGES, and an Additional DISULFIDE is formed WITHIN the A Chain
- In most Species, the A Chain consists of 21 Amino Acids and the B CHAIN of 30 Amino Acids
Secondary and Tertiary Structure of Insulin
- High Resolution Model of 6 INSULIN Molecules assembled in a HEXAMER
- Has a 3 FOLD Symmetry with ZINC in the Center connected to the Polypeptides via HISTIDINES
*****INACTIVE INSULIN is stored in the BODY as a HEXAMER, while the ACTIVE FORM is the MONOMER
Insulin Synthesis and Secretions
- PROPROINSULIN mRNA Transcription and Translation (Protein Synthesis)
- PROPROINSULIN (110 Amino Acids) has a N Terminal Hydrophobic Signal Peptide which interacts with the Signal Particle that directs it to ER
- Translocated into Lumen of ER
- Cleaved by a Protease to for PROINSULIN
- Proinsulin folds into Correct Conformation and 3 Disulfide Added. ER Chaperone Protein help in this
- Transported into GOLGI Apparatus
- Packaged into Immature CLATHRIN-COATED Granules
- Cleaved by PROTEASES to form INSULIN and C PEPTIDE
(ENDOPEPTIDASES aka Prohormone Converts (PC1 and 1) and a Carboxypeptidase) - The Immature Granules then become MATURE Granules containing HEXAMETRIC Crystaliized Insulin (Three Dimers). Also contain Amylin and other B Cell Secretory Products
- INSULIN + C PEPTIDE released TOGETHER
Regulation of Insulin Synthesis and Secretion
- After GLUCOSE Stimulation INSULIN Granules exhibit 2 Characteristics:
1) Rapidly INITATED but TRANSIENT
2) SUSTAINED
- The Granules are divided into 2 Different Pools
- (1) A LIMITED POOL of Granules (5%) ready for IMMEDIATE Release and is referred to as the “READILY RELEASABLE POOL” (RRP), which account for the FIRST Release Phase
- (2)Most of the Granules (95%) belong to a RESERVE POOL responsible for the SECOND-PHASE of Insulin Secretion. Granules in this pool must undergo MOBILIZATION before they can GAIN Release Competence
RAS Dependent Insulin Signaling
- Insulin binds to RTK which exists as a Performed Dimer
- Causes Autophosphorylation of the Receptors TYROSINE Residues
- Phosphotyrosine residues recognized and bound by Protien called INSULIN RECEPTOR SUBSTRATE 1 (IRS-1)
- IRS-1 Phosphorylated on its TYROSINE by INSULIN Receptor
- Phsophrylated IRS-1 recognized and bound by Adaptor Protein GRB-2 initiating the Transcription of RAS and the MAP Kinase Pathway
- Results in Phosphorylation of Nuclear Proteins that INCREASE Transcription of Glucokinase
- GLUCOKINASE phosphorylated Glucose in the First Step of Glycolysis and Glycogen Synthesis
- IRS-2 similar to IRS-1 but has different Specificity for Adaptor Proteins
RAS- Independent Insulin Signaling
- INSULIN binds to RTK which exists as a PERFORMED DIMER
- Causes AUTOPHOSPHORYLATION of the Receptor’s Tyrosine Residues
- PHOSPHOTYROSINE residues recognized and bound by IRS-1
- IRS-1 Phosphorylated on its Tyrosine by Insulin Receptor
- Phosphorylated IRS-1 recruits PHOSPHOIONOSITIDE 3 KINASE (PI3Kinase)
- PI3 KINASE Phosphorylates Phosphoinositides to form Phosphatidyl Inositol 3,4- Bisphosphate and Phosphoinositol 3, 4, 5 Triphsopahte (PIP3)
- These Membrane bound Phosphoinositides act as Second Messengers. Stimulate the Recruitment of PROTEIN KINASE B (PKB) to the membrane and its Activation via PHOSPHORYLATION
- Active PKB (serine Threonine Kinase) also known as AKT PHOSPHORYLATES and alters the activity of several Intracellular Proteins
- Has stimulatory effects on Glucose uptake and Storage
- For example, PKB plays a role in the Insulin- Induced movement of GLUCOSE TRANSPORTER (GLUT 4) form Cytoplasm to Plasma Membrane of Muscle and Adipose Cells
- PKB also promoted Glycogen Synthesis by PHOSPHORYLATING and INHIBITING GLYCOGEN SYNTHASE KINASE 3 (GSK-3)
Insulin Resistance
- Quantifiable Parameter: Measured as the AMOUNT OF GLUCOSE CLEARED FROM THE BLOOD IN RESPONSE TO A DOSE OF INSULIN
- Failure of Normal Amounts of Insulin to ELICIT the expected response is referred to as INSULIN RESISTANCE
- Mechanism not fully understood
- DEFECTS IN INSULIN SINGLAING (Receptor Activation to Translocation of GLUT4 transporter in ADIPOSE and SKELETAL MSUCLE TISSUE)
- > 75 different Mutations in INSULIN RECEPTOR Identified. Impair Action of the Receptor
- Defects in Insulin binding Domain on the Extracellular Side- SEVER RESISTANCE
- INCREASED PHOSPHORYLATION OF SERINE (Via Ser/ The Kinase) INSTEAD OF TYROSINE in the IR and the IRS- Inhibits ACTIVATION AND SIGNALING
- PHOSPHORYLATION of IRS NEEDED For RECRUITMENT of PI3 KINASE but Ser/ The PHOSPHORYLATION APPEARS to INACTIVATE the IRS 1 and 2, leading to DEGRADATION!!!!!
- Ser/ Thr KINASE ACTIVATED by CYTOKINES, FREE FATTY ACIDS, DAG, CERAMIDE, INFLAMMATORY MOLECULES!!!