C2006/F2402 '05 OUTLINE OF LECTURE #12

(c) 2005 Dr. Deborah Mowshowitz, Columbia University, New York, NY. Last update 02/28/2005 10:44 AM .

Handouts: Need 11B from last time; 12A & B(How epinephrine controls glycogen breakdown)

I. Major types of secreted Signals. See Handout 11B and notes from last time.

II. Properties of Intracellular Receptors (& their lipid soluble ligands). See Purves 15.8 (15.9)

    A. All these receptors are similar -- All members of same gene/protein family

    B. All these receptors are Transcription Factors

        1. Effect on transcription. Some activate and some repress transcription.

        2. HRE. All bind to cis acting regulatory elements upstream of start of transcription. Binding site usually called a hormone response element or HRE. ( HRE's can be considered part of promotor or as separate proximal upstream sites.)

    C. What sorts of ligands use these receptors?

        1. Lipid soluble ligands -- Steroids, thyroxine, retinoids (vitamin A), and vitamin D.

        2. Lipid soluble ligands cannot be stored -- must be made from soluble precursors as needed.

    D. These receptors have (at least) three domains

1.  DNA binding domain --  binds to HRE (different HRE for each dif. hormone)

2. Ligand binding domain -- binds particular steroid (or thyroxine, etc.)

3. Transcription activating (or inhibiting) domain -- also called transactivating domain. Binds to other proteins and activates or inhibits transcription.

4. Other domains -- Receptors also need NLS and region that allows dimerization -- these may be separate or included in domains listed above.

    E. What (usually) happens when receptors bind their ligands (= receptors are activated?)

1. Receptors disassociate from inhibitory proteins.    

2. Receptors dimerize -- form pairs.

3. If receptor is in cytoplasm, moves to nucleus.

4. Receptor (+ ligand) binds DNA if not already bound.

5. Activated receptor binds to other proteins associated with the DNA (other TF's and/or co-activators), and stimulates or inhibits transcription.

    F. Example -- Estrogen

1. Basic Mechanism. E ----> binds to estrogen receptors ---> complex binds to estrogen response elements in regulatory regions of target genes --> transcription of some genes activated; transcription of others repressed.

2. Example of some proteins controlled by E -- controls production of receptors for other hormones. For example, during pregnancy controls production of receptors for oxytocin (in uterus) and prolactin (in breast). :

a. In uterus: estrogen binding --> transcription of gene for oxytocin receptors --> production of new receptors for oxytocin (= up regulation of oxytocin receptors). Receptors needed to allow response to signal (oxytocin) for contraction --> birth.

b. In breast: estrogen binding --> inhibits transcription of gene for prolactin receptors --> down regulation of prolactin receptors; at birth, estrogen level falls and inhibition stops --> transcription of gene B --> synthesis of prolactin receptors --> response to lactation signal (prolactin).

3. Why different results in different tissues?

a. May be different hormone receptors (more than one kind/signal) in different tissues.

b. Combination of TF's in each cell different; more than one TF required to get proper transcription of each gene. In general, what any hormone does depends on combination of proteins (enzymes, TF's, etc.) already in target cell.

c. Reminder: All cells (except immune system) have same HRE's, enhancers, etc. It's the trans acting factors & hormone receptors that vary, not the cis acting regulatory sites.

Examples of how other hormones give different results in different tissues will be discussed next time.

Try problem 6-19.

III. How do G protein Linked Receptors Work?

    A. What is role of G proteins in signaling? A typical case (see handout 12A): Receptor binds signal (first messenger; is extracellular) --> activates G protein --> activates* target enzyme --> produces 2nd messenger (intracellular) --> activates other enzymes, opens channels --> activates/inhibits other enzymes, etc.

* Some G proteins inhibit; others activate.

    B. What are G proteins? Properties of G proteins (See Becker fig. 10-4)

1. Catalyze GTP/GDP Exchange (followed by hydrolysis of GTP --> GDP)

a. Activation (exchange):

Protein-GDP (inactive) + GTP --->  Protein-GTP (active) + GDP

b. Inactivation (hydrolysis):

Protein-GTP (active) ---> Protein-GDP (inactive) + phosphate.

c. Overall: GTP displaces GDP, activating the G protein; GTP is then hydrolyzed (usually rapidly), returning the G protein to its inactive state. 

d. Terminology. Since the overall result is that GTP is hydrolyzed to GDP, G proteins are sometimes called "GTPases."

2. Subunits -- Ordinary G proteins are trimeric = they have 3 subunits.

a. Inactive G prot = heterotrimer of alpha, beta, gamma

b. On activation --> alpha (with the GTP) separates from other 2.

c. Either part -- alpha, or beta + gamma -- may be the "active" part -- act as activator or inhibitor of target

d. Hydrolysis of GTP to GDP causes alpha to reassociate with other subunits --> inactive heterotrimer

e. Trimeric G proteins (unlike small G proteins) catalyze both GTP/GDP exchange and GTP hydrolysis.

3. Small G proteins have no subunits (example: ras); need other proteins to catalyze the addition/exchange of the GTP. Only reaction they catalyze (by themselves) is hydrolysis of GTP.

4. What do Activated G proteins do? 

a. There are many different G proteins. G proteins are involved in a very large number of cellular processes, not just signaling. (We have ignored their importance until now. See Becker for details.) 

b.  Active G proteins can be inhibitory or stimulatory.

c. Activated G proteins work by binding to and activating (or inhibiting) other target enzymes/proteins

d. Terminology: The different G proteins are usually known as G_p, G_q G_i, G_s etc. (Books differ on details of naming.)

e. Targets: G proteins involved in signaling usually activate enzymes that generate second messengers (see Purves 15.7 (15.8)) or open/close ion channels. More details below and/or next time.

5. Comparison of Protein Kinases, Trimeric (ordinary) G proteins & Small G proteins

Try problem 6-2.

    C. How do activated G proteins produce second messengers? (See handout 12A) or Purves 15.7 (15.8)

1.  The usual second messengers -- see handout for structures and mode of action

2. General Idea: Active G protein (subunit) --> binds to & activates enzyme in membrane ---> generates second messenger in cytoplasm. (See Becker fig. 10-6 or Purves 15.7 (15.8) & 15.10 (15.12) for cAMP; Becker fig. 10-9 or Purves 15.11 (15.13) for IP3 etc.)

IV. An example of a second messenger -- cAMP  & its target (PKA) (see handout 12A)

    A.   How is cAMP level regulated? What does it do?

1. How is cAMP made?

a. G protein activates adenyl cyclase (AC)

b. cAMP made from ATP by adenyl cyclase (also called adenylyl cyclase); for structure of cAMP see handout and Becker fig. 10-5 [23-7] or Purves 15.10 (15.12).

2. What does cAMP do?

a. cAMP binds to and activates protein kinase A = PKA. (Also called cyclic AMP dependent protein kinase = cAPK) 

b. PKA adds phosphates to other proteins

(1). Phosphorylation by PKA can activate or inhibit target protein (target = substrate of PKA)

(2). PKA action can modify other kinases/phosphatases and start a cascade

(3). End result varies. Depends on which kinases and phosphatases in that tissue are targets (substrates) of PKA and/or the other kinases/phosphatases (at end of cascade). See example below.

2. How does signal system turn off when hormone leaves?

a. G protein doesn't stay activated for long: Activated G protein hydrolyzes its own GTP --> GDP (--> inactive G protein).

b. cAMP is short lived -- it's hydrolyzed by phosphodiesterase (PDE)

c. In absence of cAMP, PKA becomes inactivated and phosphatases are active that reverse effects of kinases.

    B. How do hormones work through cAMP?

1.  Same hormone can have different effects on different tissues using cAMP.

a. An example:

(1). In skeletal muscle: epinephrine causes glycogen breakdown.

(2). In smooth muscle of lung: epinephrine causes muscle relaxation.

b. Why does this make sense?

(1). Epinephrine (also called adrenaline) is produced in response to stress.

(2). In response to stress,  need to "mobilize" glucose -- release it from storage so it can be broken down to provide energy. Therefore need to increase glycogen breakdown (and decrease glycogen synthesis) in muscle (& liver).

(3). In response to stress, need to breathe more deeply. Therefore need smooth muscle around tubes that carry air (bronchioles) to relax.

c. How is this possible? In some cases, different receptors or 2nd messengers may be involved; examples of this below and/or next time. However, in this case, same receptors, same 2nd messenger (cAMP) are used. (Solution to be discussed in class.)

2. How does this work at molecular level -- an example: Regulation of glycogen breakdown and synthesis in response to epinephrine. (See handout  & Purves fig. 15.15 (15.17) or Becker figs. 10-25 (10-24) & 6-18 ). See handout 12B.

a. When hormone present: Hormone activates PKA (protein kinase A or cAMP dependent protein kinase) through pathway explained above:

epinephrine --> receptor --> G prot --> Adenyl cyclase --> cAMP --> PKA.

b. PKA initiates a cascade that activates glycogen phosphorylase and inactivates glycogen synthetase. Therefore, glycogen breaks down --> glucose.

PKA --> activates phosphorylase kinase --> activates phosphorylase --> degrades glycogen

PKA --> inactivates glycogen synthetase

c. When hormone is absent, cAMP is degraded, PKA is not active, and phosphatases reverse effects of PKA. Result is to activate glycogen synthetase and inactivate glycogen phosphorylase. Therefore, glucose polymerized  -->  synthesis of glycogen

Phosphatases  --> activate glycogen synthetase --> synthesis of glycogen  from glucose.

Phosphatases inactivate phosphorylase kinase & phosphorylase

d. Have two controlled processes -- glycogen synthesis and breakdown; system ensures only one works at a time.

Try problems 6-6 to 6-8.

    C. The same hormone can generate different second messengers & therefore different effects (in different cells). (See Becker fig. 10-24 [10-23] & table below). An example -- effects of epinephrine (adrenaline) on smooth muscle.

1. The phenomenon: Epinephrine has different effects on different smooth muscles:

            On some smooth muscles --> contraction (helps divert blood from peripheral circulation to essential internal organs.)
            On other smooth muscles -->  relaxation (example as above -- allows lungs to breathe more deeply)

2. How Ca⁺⁺ fits in:

a. Ca⁺⁺ stimulates muscle contraction.

b. Epinephrine binds to receptors on some smooth muscles --> Ca⁺⁺ released from ER --> intracellular Ca⁺⁺ up --> stimulates contraction. 

c. Epinephrine binds to receptors on some smooth muscles --> Ca⁺⁺ pump activated ---> Ca⁺⁺ removed --> intracellular Ca⁺⁺ down --> relaxation! 

3. How does epinephrine work two ways on same type of tissue?

a. Two basic types of epinephrine  receptors -- called alpha and beta adrenergic receptors (adrenergic = for adrenaline). The two types are distinguished by their relative affinities for epinephrine and norepinephrine. (See note to table below.)

b. Some types of smooth muscle have mostly one type of receptors; some the other.

c. Two types activate different G proteins and generate different second messengers as on handout 12A.

(1). Beta receptors --> G protein type (G_s)--> cAMP response --> PKA --> phosphorylation of Ca⁺⁺ pump --> removal of cell Ca⁺⁺ --> relaxation

(2). Alpha₁  receptors --> different G protein (G_p) --> different second messenger (IP3) --> Ca⁺⁺ release from ER --> contraction

4. How 2 receptor types help to respond appropriately to stress (epinephrine).

a. Beta type receptors. Beta receptors are found in lung tissue in smooth muscle surrounding bronchioles. Stress (pop quiz, lion in street, etc.)--> epinephrine --> muscles relax --> bronchioles dilate -->  deeper breathing --> more oxygen --> energy to cope with stress. 

b. Alpha type receptors. Alpha receptors are found in smooth muscle surrounding blood vessels of peripheral circulation. Stress --> epinephrine --> muscles contract --> constrict peripheral circulation --> direct blood to essential organs for responding to stress (heart, lungs, skeletal muscle).

5. Medical Uses of all this.

Epinephrine can be used during an asthmatic attack to relax bronchi and ease breathing. Overuse of this type of broncho-dilator eases breathing temporarily but masks underlying problem (inflammation of lung tissue) and can have additional serious long term effects (from overstimulation of heart which also has beta receptors). Heart and lungs have slightly different types of beta receptors, so drugs have been developed that stimulate one and not the other (unlike epinephrine). 

6. Summary of epinephrine effects on smooth muscle (in lung vs peripheral circulation)*

* There are more than two types of epinephrine receptors on smooth muscle cells, so epinephrine may affect smooth muscle in other tissues in other ways.  (There are subtypes of alpha and subtypes of beta.)