Mechanisms of action of peptide hormones

Raymond B. Birge, PhD

Lecture 27- Birge

•Review basic biology of Insulin/Glucagon in homeostasis

•Review receptor classifications (emphasis on GPCRs versus Receptor tyrosine kinases)

•Understand basic biological distinctions between Glucagon and Insulin

•Understand basic signaling mechanisms between Glucagon and Insulin

Learning Objectives

Signaling and Cellular Communication in Metazoans

600-700 million years ago; origin of multicellular organisms

Examples of Hormones

I. Peptides and Proteins (Growth factors)

II. Fatty acid derivatives (Prostaglandins, Eicosanoids)

III. Amino acid derivatives (Thyroxine, Epinephrine)

IV. Steroids (Estrogen, Progesterone)

Examples of second messengers

I. cyclic nucleotides (cAMP, cGMP)

II. Lipid products (DAG, phosphoinositol lipids)

III. Ions, small molecules (Ca2+, NO, Na+, K+)

IV. Modified proteins (phosphorylation, modular (domains)

1. Highly specific transmembranereceptor

Polar Non-polar Polar

The plasma membrane partitions growth factors and signaling cascades

2. Signal is changedand amplified

3. Cell responds withbiological readout anddownmodulates initial signals

Types of growth factor/target cell interactions1. Paracrine GF produced by distinct Insulin/ target cell Glucagon/NGF

2. Autocrine GF produced by receptor- EGF/ expressing cell tumors

3. Juxtacrine GF produced by distinct TGF-target cell but remainsassociated with target cell

4. Holocrine GF has no signal sequence, IL’sreleased from target cell by unknown mechanism

Paracrine/Endocrine Growth Factor Signaling

Examples: Insulin, Glucagon

Autocrine Growth Factors

Examples: Oncogenic cells (ie Breast cancer cells Overproduce EGFR (Her-2) and EGF.

Paracrine Growth factor Signaling

Examples: Neurotrophic fcators (NGF, BDNF, GGF)

Receptor Classification by Mechanism of Action

G Protein-Coupled Receptor (GPCR): 7 transmembrane receptors(Glucagon, ACTH, TSH, LH, Calcitonin, PTH, Thrombin, Dopamine etc)

Receptors with Intrinsic Enzymatic Activity (Type I or Type II):(Insulin, EGF, NGF, FGF, VEGF, NT-3, BDNF, etc)

Receptors with Associated Enzymatic Activity:(Cytokines, EPO, TGF-, BMPs, Interferons)

Receptors that Stimulate Intracellular Proteolysis: (IL-1, TNF-)

How are signals propagated through cells?

4 Distinct Elements:

1. Signal. Generation, processing, and release of growth factor

2. Signal Detection. Surface receptor/cytoplasmic receptor

3. Signal Transduction. (PM -> cytoplasm -> nucleus)

4. Alteration in gene expression.

Biology of Insulin and GlucagonIn maintenance of blood sugar

Insulin: Secreted by beta cells ofthe pancreas in response to high blood sugar.

In response to insulin, cells (muscle, red blood cells, and adipocytes) takeup glucose from the blood.

Glucagon: Secreted by alpha cells ofthe pancreas in response to low bloodsugar.

When blood glucose is high, no glucagonIs secreted

When blood glucose is low, glucagon causes the liver to release stored glucose into the blood.

ATP cAMP

PKA

phosphorylase kinase

glycogen synthase

P P

phosphorylase P

10-10 M

10-10

10-7

10-6

Signal; ie Insulin

Kinetics of induction of immediate and delayed geneexpression following growth factor stimulation

Figure 6.2 The Biology of Cancer (© Garland Science 2007)

Table 6.1 The Biology of Cancer (© Garland Science 2007)

Figure 6.3 The Biology of Cancer (© Garland Science 2007)

Receptor Tyrosine KinasesReceptor protein-tyrosine kinases transmit signals across the plasma membrane, from the cell exterior to the cytoplasm.

In cells transformed by many oncogenic proteins-(ie, ErbB, EGFR, Bcr-Abl)there is dramatic increase in protein tyrosine phosphorylations

PROTEIN KINASES

Each kinase phosphorylates particular sequence motifs

Phosphorylation is not restricted to one site on the polypeptide chain

Proteins can be phosphorylated by more than one kinase (allows convergence of several signaling pathways)

Phosphorylation can effect enzymatic activity* alter electrostatic interactions and alter equilibrium from one conformation to another* instrumental in forming new interactions (ie, H-bonding, SH2 domains)* expose domains or motifs buried in the protein structure (ie NLS and NES)

Consensus amino acids can be used to identify kinase specificity

Polypeptide Growth Factors activate Receptor Tyrosine kinases (RTK) by a common mechanism

GF-induced activation of aReceptor Tyrosine Kinase.

1. Ligand-induced dimerization

2. “Auto-phosphorylation”of the RTK on the cytoplasmicor intracellular domain.

3. The dimeric state of the receptoris said to be “activated”

Figure 6.8b The Biology of Cancer (© Garland Science 2007)

The pocket of the SH2 domain is highly specificfor phospho-tyrosine because of the bulky sizeof the tyrosine.

SH2 specificity is achieved by amino acidsdirectly C-terminal to the pTyr (particularly +1, +2, and +3).

GF-induced activation of aReceptor Tyrosine Kinase-part 2.

4. Phosphorylation on tyrosine actsas a “magnet” to recruit cellular proteins with Src Homology-2 (SH2)domain.

5. One of these proteins is calledGrb2 (Growth Factor Receptor BBindingprotein-clone-2), an intracellular adaptor protein with SH2 and SH3 domains.

6. Simultaneously, Grb2 binds the activated RTKAND SOS, an exchangefactor for the G protein, Ras to providea link in the signal transduction.

GF-induced activation of aReceptor Tyrosine Kinase-part 3.

7. Because Grb2 recruits SOS to theplasma membrane, it meets up with its partner Ras, resulting in the conversion of Ras-GDP into Ras-GTP.

8. Ras-GTP is activated and binds moresignaling proteins, leading to the activation of MAP kinase.

9. Activated MAP kinase is translocated into the nucleus to activate transcriptionfactors.

The Ras Cycle

V12 Ras= Always activei.e, constitutive activation of downstream pathways

N17 Ras=Always inactivei.e, dominant negative foractivation of downstream pathways

V12,N17=Functionally-deadmolecule ie, no effect in downstream pathways

Common Ras mutants

GTP

GDP

GAP

Pi

Ras-GDP (Inactive)

Ras-GTP (Active)

*

The MAP Kinase (Extracellular-Related Kinase ERK) Cascade

Mitogens, Growth Factors

Ras

RAF

MAPK

MAPKK/MEK1

MAPKKK

MAP4K

MAP3K

MAP2K

MAPK

GTPase

Transcriptional Regulation

Cytokines, Cell Stress

Rac, Cdc42

MEKK1

PAK

JNKK/SEK

JNK

} Serine/Threonine Kinases

Cytosol

Nucleus

Raf Localization of MAP K

Many growth factor signals terminate via the activation of transcription factors

Summary and Take-Home Points

1. Identify differences between autocrine, paracrine, andendocrine signaling

2. Distinguish major differences between insulin and glucagon signaling, and differences in how receptorsare activated (ie, RTK versus GPCR)

3. Understand RTK signal transduction mechanisms

4. Relationship between RTK activation and gene expression