Midterm #3: Signal Transduction

Question 1

Intercellular vs. Intracellular Signal Transduction, Incoming Signals, Initial Receptors

Answer 1

• Intercellular Signal Transduction: a signal passes from one cell to another
• Intracellular Signal Transduction: the receiving cell biochemically decodes that signal
• Signals may include: gas (NO), amino acid (glutamate), steroid (estrogen), peptide hormone (insulin), protein (growth factors)
• The initial receptors: may exist on the surface of the cell (cell surface receptors) or within it (intracellular/nuclear receptors)

Question 2

Intracellular Communication Mechanisms: endocrine, paracrine, autocrine, juxtacrine, matricine

Answer 2

• Endocrine: specialized sender cells (glands) synthesized and secrete molecules (hormones) into the blood. All cells are exposed, but only those containing the appropriate receptors are affected
• Paracrine: Sender cells secretes molecules into the local environment only (local mediators)
• Autocrine: Sender cell secretes molecules into the local environment and receives them itself
• Juxtacrine: Cell-to-cell contact
• Matricine: cell to extracellular matrix

Question 3

Nuclear Receptors

Answer 3

• Hydrophobic Small Molecules
• Diffuse through plasma membrane and bind to intracellular receptors
  
  • Transcription Factors

Question 4

Type 1 Nuclear Receptor

Answer 4

• bind to ligand in the cytoplasm
• homodimerize
• translocated into the nucleus (via nuclear pore complex)
• bind to transcription element
• initiate transcription
• Examples:
  
  • sex hormone receptors
  • glucocorticoid receptor
  • mineralcorticoid receptor

Question 5

Type II Nuclear Receptor

Answer 5

• constituitively exist in the nucleus in DNA bound heterodimers
• Often bound to a corepressor protein
• Ligand binding triggers dissociation of the corepressor and association of a coactivator
  
  • allows the recruitment of RNAP and initiation of transcription
• Examples:
  
  • thyroid hormone receptor (TR)
  • retinoic acid receptor (RAR)
  • aryl hydrocarbon receptor (AhR)*
  • pregnane X receptor (PXR)*
  • constituitive androstane receptor (CAR)*
  • *binds to xenotic response elements, important for CYP induction

Question 6

Cell Surface Receptors

Answer 6

• binds to ligands at the extracellular surface
• wide variety of ligands, small molecules, peptides and large proteins
• Receptor Tyrosine Kinase, G-Couple Protein Receptors

Question 7

Receptor Tyrosine Kinase (RTK) Family

Answer 7

• Bind growth factors and certain hormones
  
  • cytokines
• ​Trigger intracellular phosphorylation cascades to cause effects

Question 8

G-Coupled Protein Receptors

Answer 8

• bind a wide variety of ligands
• intracellular second messanger to ellict their effects

Question 9

Receptor Tyrosine Kinase Mechanism

Answer 9

• Hormone binding extracellular domain activates cytosolic tyrosine kinase domain
  
  • autophosphorylates specific Tyr residues and activates the receptor
• Activated receptor binds to and phosphorylates Tyr residues on specific target proteins, which are often kinase enzymes themselves
• Examples:
  
  • insulin receptor
  • EGFR (epidermal growth factor receptor)
  • VEGFR (vascular endothelial growth factor receptor)

Question 10

EGFR as Therapeutic Target

Answer 10

• EGF binding to EGFR stimulates cell proliferation
• EGFR-targeting drugs include
  
  • monoclonal antibodies such as cetuximab (Erbitux)
    
    • inhibit EGF binding
  • small molecules like **erlotinib **(Tarceva)
    
    • inhibit autophosphorylation and downstream signaling

Question 11

VEFGR as Therapeutic Target

Answer 11

• VEGF binds VEGFR and stimulates blood vessel growth (angiogenesis/vasculogenesis)
• Important in tumor growth and age-related macular degeneration
• Bevacizumab (Avastin) is approved for certain cancers and used off label to treat AMD
• **Ranibizumab **(Lucentis) is a fragment of the same mAb approved to specifically treat AMD

Question 12

What is this structure?

Answer 12

Erlotinib (Tarceva)

Question 13

G-Protein Coupled Receptors (aka 7TM Receptors) Mechanism

Answer 13

• 7 TM alpha helices bundled together
• Cause downstream effects through GTP-ases called G-proteins
• Resting state:
  
  • GPCR associate with **heterotrimeric G-proteins **(alpha, beta, and gamma) with the Ga bound to GDP
• When ligands bind:
  
  • Undergoes confirmational changes that causes Ga to release GDP and bind GTP
  • Ga dissociates from Gbg with activates both complexes
  • Complexes interact with downstream effectors (like adenylate cyclase and phospholipase C) to change second messanger levels
• Activation of G protein is short lived
  
  • Ga slowly hydrolyzes GTP and Ga-GDP rebinds Gbg to reassemble the inactive heterotrimer

Question 14

cAMP second messanger signaling by GPCR

Answer 14

• Adenylate cyclase is a membrane bound protein that catalyzes the synthesis of cAMP
• Activated Ga subunits regulate the activity of adenylate cyclase: Gas stimulates and Gai inhibits
• cAMP regulates the activity of protein kinase A
• PKA is an inactive heterotetramer consisting of 2 regulatory and 2 catalytic subunits
• cAMP binds the regulatory subunits which release the active catalytic subunits
• Activated PKA subunits then phosphorylate Ser and Thr residues on target proteins

Question 15

cAMP Second Messanger Signaling in Muscle Cell

Answer 15

• epinephrine binds to alpha-adrenergic receptor and activates associated Gas
• This activates adenylate cyclase, which generates cAMP, which activates PKA
• PKA phosphorylates:
  
  • glycogen synthase (inactivation)
  • phosphorylase kinase (activation)
• Glycogen is broken down to glucose to power muscle
• PKA also phosphorylates ser and thr residues on the insulin receptor and decreases it’s catalytic activity

Question 16

The Phosphoinositide Cascade

Answer 16

• Mediated by phospholipase C
• Binding of hormone (ex: serotonin) to membrane receptor activates trimeric G-protein that then activates PLC (an integral membrane protein)
• PLC hydrolyzes **phosphatidylinositol **(PIP₂) to diacyl glycerol (DAG) and **inositol triphosphate **(IP₃)
• IP₃ diffuses into the cytosol and binds to ligand-gated calcium channels in ER
• calcium is yet another second messanger.
  
  • DAG and calcium activate **protein kinase C, **which is a ser/thr protein kinase involved in cell growth and differentiation

Question 17

Therapeutic Approaches to GPCR Signaling

Answer 17

• Antihistamines like **loratadine **(blocks the histamine H₁ receptor)
• Heartburn meds like rantidine (histamine H₂ receptor antagonist)
• Antipsychotics like perphenazine (blocks dopamine D₂ receptor)
• Second messangers can also be targeted:
  
  • phosphodiesterases degrade cAMP and return adenylate cyclase cascade to resting state
  • Methylxanthines like caffeine and theobromine** **inhibit phosphodiesterases and prolong the effects of cAMP, potentiating the adenylate cyclase system

Question 18

Name this structure

Answer 18

Caffiene

Question 19

Name this structure

Answer 19

Theobromine