KAPLAN_USMLE_STEP_1_LECTURE_NOTES_2018_BIOCHEMISTRY_and_GENETICS

Hormones 9

Learning Objectives

Understand concepts concerning hormones and signal transduction

Interpret scenarios about mechanism of water-soluble hormones

Answer questions about G-proteins and second messengers in signal transduction

HORMONES AND SIGNAL TRANSDUCTION

Broadly speaking, a hormone is any compound produced by a cell, which by binding to its cognate receptor alters the metabolism of the cell bearing the hormone–receptor complex. Although a few hormones bind to receptors on the cell that produces them (autoregulation or autocrine function), hormones are more commonly thought of as acting on some other cell, either close by (paracrine) or at a distant site (telecrine).

•Paracrine hormones are secreted into the interstitial space and generally have a very short half-life. These include the prostaglandins and the neurotransmitters.

•Telecrine hormones are secreted into the bloodstream, generally have a longer half-life, and include the endocrine and gastrointestinal (GI) hormones. (The endocrine hormones are the classic ones, and it is sometimes implied that reference is being made to endocrine hormones when the word hormones is used in a general sense.)

Hormones are divided into 2 categories: those that are water soluble (hydrophilic) and those that are lipid soluble (lipophilic, or hydrophobic).

Note

The GI and endocrine hormones are discussed in detail in the GI and endocrinology chapters in the Physiology Lecture Notes. Although there is some overlap, this chapter presents basic mechanistic concepts applicable to all hormones, whereas coverage in the Physiology Notes emphasizes the physiologic consequences of hormonal action.

135

Part I ● Biochemistry

Table I-9-1. Two Classes of Hormones

	Water-Soluble				Lipid-Soluble
Receptor in cell membrane					Receptor inside cell
Second messengers often involved					Hormone–receptor complex binds
Protein kinases activated					hormone response elements (HRE)
					of enhancer regions in DNA
Protein phosphorylation to modify					—
activity of enzymes (requires minutes)
Control of gene expression through					Control of gene expression
proteins such as cAMP response element					(requires hours)
binding (CREB) protein (requires hours)
Examples:					Examples:
•		Insulin			•	Steroids
•		Glucagon			•	Calcitriol
•		Catecholamines			•	Thyroxines
					•	Retinoic acid

MECHANISM OF WATER-SOLUBLE HORMONES

Water-soluble hormones must transmit signals to affect metabolism and gene expression without themselves entering the cytoplasm. They often do so via second messenger systems that activate protein kinases.

Protein Kinases

High-Yield

A protein kinase is an enzyme that phosphorylates otherMEDIUMproteins,YIELDchanging their activity (e.g., phosphorylation of acetyl CoA carboxylase inhibits it). Examples of protein kinases are listed in Table I-9-2 along with the second messengers that activate them.

Table I-9-2. Signal Transduction by Water-Soluble Hormones

Pathway

G Protein

Enzyme

Second

Protein Kinase

Examples

Messenger(s)

cAMP

Gs (Gi)

Adenyl cyclase

cAMP

Protein kinase A

Glucagon

Epinephrine (β, α-2)

Vasopressin (V2, ADH)

kidney

PIP

G

q

Phospholipase C

DAG, IP

3

, Ca2+

Protein kinase C

Vasopressin (V1, V3)

2

vascular smooth muscle

Epinephrine (α1)

cGMP

None

Guanyl cyclase

cGMP

Protein kinase G

Atrial natriuretic factor (ANF)

Nitric oxide (NO)

Insulin,

Monomeric

—

—

Tyrosine kinase

Insulin

growth

p21ras

activity of receptor

Insulin-like growth factor (IGF)

factors

Platelet-derived growth factor (PDGF)

Epidermal growth factor (EGF)

136

Chapter 9 ● Hormones

Some water-soluble hormones bind to receptors with intrinsic protein kinase activity (often tyrosine kinases). In this case, no second messenger is required for protein kinase activation. The insulin receptor is an example of a tyrosine kinase receptor.

Activation of a protein kinase causes:

•Phosphorylation of enzymes to rapidly increase or decrease their activity.

•Phosphorylation of gene regulatory proteins such as CREB to control gene expression, usually over several hours. The typical result is to add more enzyme to the cell. CREB induces the phosphoenolpyruvate carboxykinase (PEPCK) gene. Kinetically, an increase in the number of enzymes means an increase in Vmax for that reaction.

	ATP	Protein	ADP
		kinase

Proteins:		Proteins:	P
• Gene regulatory proteins		• Gene regulatory proteins
• Enzymes		• Enzymes
Dephosphorylated	Different	Phosphorylated
	activity

Pi	Protein	H2O
	phosphatase

Figure I-9-1. Protein Kinases and Phosphatases

Both represent strategies to control metabolism. The action of protein kinases is reversed by protein phosphatases.

Sequence of Events from Receptor to Protein Kinase

High-Yield

G protein

Receptors in these pathways are coupled through trimetric G proteins in the membrane. The 3 subunits in this type of G protein are α, β, and γ. In its inactive form, the α subunit binds GDP and is in complex with the β and γ subunits. When a hormone binds to its receptor, the receptor becomes activated and, in turn, engages the corresponding G protein (step 1 below). The GDP is replaced with GTP, enabling the α subunit to dissociate from the β and γ subunits (step 2 below).

137

Part I ● Biochemistry

The activated α subunit alters the activity of adenylate cyclase. If the α subunit is αs, then the enzyme is activated; if the α subunit is αi, then the enzyme is inhibited. The GTP in the activated α subunit will be dephosphorylated to GDP (step 3 below) and will rebind to the β and γ subunits (step 4 below), rendering the G protein inactive.

α	β	α	β
GDP		GTP
γ		γ
Inactive G protein		Active G protein
		α
	β	GTP
	γ	Enzyme
		(adenylate cyclase)

α

GDP Pi

Figure I-9-2. Trimeric G Protein Cycle

Cyclic AMP (cAMP) and phosphatidylinositol bisphosphate (PIP2)

The receptors all have characteristic 7-helix membrane-spanning domains.

The sequence of events, leading from receptor to activation of the protein kinase via the cAMP and PIP2 second messenger systems, is as follows:

•Hormone binds receptor

•Trimeric G protein in membrane is engaged

•Enzyme (adenylate cyclase or phospholipase) activated

•Second messenger generated

•Protein kinase activated

•Protein phosphorylation (minutes) and gene expression (hours)

An example of inhibition of adenylate cyclase via Gi is epinephrine inhibition (through its binding to α2 adrenergic receptor) of insulin release from β cells of the pancreas.

138

Chapter 9 ● Hormones

cAMP System

PIP2 System

+

+

Receptor for:

NH3

NH3

Receptor for:

• Glucagon

• Vasopressin

• Epinephrine β (Gs)

• Epinephrine α1

• Epinephrine α2 (Gi)

Membrane

PIP2

DAG

Cytoplasm

α

β

Adenyl

α

β

Phospho-

Protein

COO–

γ

cyclase

ATP

COO–

γ

lipase C

+ kinase C

IP3

G

or G

i

Gq

CREB

s

+

cAMP

+

Ca2+

+

ER

Ca2+

Gene

CREB

P

Protein kinase A

expression

Nucleus

P

+

+

CREB

Protein kinase

P

CRE

Gene

Enzymes

Enzymes

DNA

+

dephosphorylated

phosphorylated

Gene expression in nucleus	(phosphatase)

Figure I-9-3. Cyclic AMP and Phosphatidylinositol Bisphosphate (PIP2)

cGMP

Atrial natriuretic factor (ANF), produced by cells in the atrium of the heart in response to distension, binds the ANF receptor in vascular smooth muscle and in the kidney. The ANF receptor spans the membrane and has guanylate cyclase activity associated with the cytoplasmic domain. It causes relaxation of vascular smooth muscle, resulting in vasodilation, and in the kidney it promotes sodium and water excretion.

Nitric oxide (NO) is synthesized by vascular endothelium in response to vasodilators. It diffuses into the surrounding vascular smooth muscle, where it directly binds the heme group of soluble guanylate cyclase, activating the enzyme.

Both the ANF receptor and the soluble guanylate cyclase are associated with the same vascular smooth muscle cells.

Note

Once generated, the second messengers cAMP and cGMP are slowly degraded by a class of enzymes called phosphodiesterases (PDEs).

139

Part I ● Biochemistry

Bridge to Microbiology

E. coli heat stable toxin (STa)

A similar guanylate cyclase receptor in enterocytes is the target of E. coli heatstable toxin (STa). The toxin binds to, and stimulates, the guanylate cyclase increasing cGMP. This causes increased activity of CFTR and diarrhea.

			Drugs:
	+		• Nitroprusside
			• Nitroglycerine
	NH3	Arginine
			• Isosorbide
Receptors for		Nitric oxide	dinitrate
atrial natriuretic
		synthase
factor (ANF)			Nitric oxide (NO)
Membrane
Cytoplasm		COO–
			NO
GTP	Receptor		+
	guanylate	Soluble guanylate cyclase
	cyclase	cGMP
Vascular Smooth		+	GTP
		Protein kinase G
Muscle

Relaxation of smooth muscle (vasodilation)

Figure I-9-4. Cyclic GMP

The sequence from receptor to protein kinase is quite similar to the one above for cAMP, with 2 important variations:

•The ANF receptor has intrinsic guanylate cyclase activity. Because no G protein is required in the membrane, the receptor lacks the 7-helix membrane-spanning domain.

•Nitric oxide diffuses into the cell and directly activates a soluble, cytoplasmic guanylate cyclase, so no receptor or G protein is required.

The Insulin Receptor: A Tyrosine Kinase	High-Yield

Insulin binding activates the tyrosine kinase activity associated with the cytoplasmic domain of its receptor. There is no trimeric G protein, enzyme, or second messenger required to activate this protein tyrosine kinase activity:

•Hormone binds receptor

•Receptor tyrosine kinase (protein kinase) is activated

•Protein phosphorylation (autophosphorylation and activation of other proteins)

Once autophosphorylation begins, a complex of other events ensues. An insulin receptor substrate (IRS-1) binds the receptor and is phosphorylated on tyrosine

140

Chapter 9 ● Hormones

residues, allowing proteins with SH2 (src homology) domains to bind to the phosphotyrosine residues on IRS-1 and become active. In this way, the receptor activates several enzyme cascades, which involve:

•Activation of phosphatidylinositol-3 kinase (PI-3 kinase), one of whose effects in adipose and muscle tissues is to increase GLUT-4 in the membrane

•Activation of protein phosphatases. Paradoxically, insulin stimulation via its tyrosine kinase receptor ultimately may lead to dephosphorylating enzymes

•Stimulation of the monomeric G protein (p21ras) encoded by the normal ras gene

All these mechanisms can be involved in controlling gene expression, although the pathways by which this occurs have not yet been completely characterized.

			Insulin binding
	s s	s s	activates tyrosine	s s	s s
			kinase activity

	P		P		P
ADP			ADP
ATP	Tyrosine kinase		ATP		IRS-1
					P	P
	Autophosphorylation
	of receptor				SH2	SH2
					Protein
	Protein kinase			P		Protein
Enzymes			Enzymes
dephosphorylated		phosphorylated				+
						p21ras
						G protein

P

P

SH2

PI-3 kinase

Insulin receptor substrate (IRS) binds receptor and is phosphorylated

on tyrosine residues

SH2-domain proteins bind phosphotyrosine residues on IRS

Translocation of GLUT-4 to membrane in:

• Adipose

• Muscle

Protein +	Gene expression
phosphatase
	in nucleus

Figure I-9-5. Insulin Receptor

Tyrosine kinase receptors are also involved in signaling by several growth factors, including platelet-derived growth factor (PDGF) and epidermal growth factor (EGF).

141

Part I ● Biochemistry

Recall Question

Epinephrine, through inhibition of adenylate cyclase, prevents the secretion of which of the following products?

A.Gastrin

B.Glucagon

C.Insulin

D.Lipoprotein lipase

Answer: C

Functional Relationship of Glucagon

and Insulin

High-Yield

Insulin (associated with well-fed, absorptive metabolism) and glucagon (associated with fasting and postabsorptive metabolism), usually oppose each other with respect to pathways of energy metabolism. Glucagon works through the cAMP system to activate protein kinase A favoring phosphorylation of rate-limiting enzymes, whereas insulin often activates protein phosphatases that dephosphorylate many of the same enzymes.

Glucagon promotes phosphorylation of both rate-limiting enzymes (glycogen phosphorylase for glycogenolysis and glycogen synthase for glycogen synthesis). The result is twofold in that synthesis slows and degradation increases, but both effects contribute to the same physiologic outcome, release of glucose from the liver during hypoglycemia.

Insulin reverses this pattern, promoting glucose storage after a meal. The reciprocal relationship between glucagon and insulin is manifested in other metabolic pathways, such as triglyceride synthesis and degradation.

Glucagon

					+
			ATP		Protein		ADP
					kinase A
			Glycogen phosphorylase			Glycogen phosphorylase			P
			(glycogenolysis)				(glycogenolysis)
Glycogen			LESS ACTIVE				ACTIVE			Glycogenolysis
synthesis			Glycogen synthase			Glycogen synthase
										(release glucose)
(store glucose)			(glycogen synthesis)			(glycogen synthesis)			P
			ACTIVE				LESS ACTIVE
			Pi		Protein		H2O
				phosphatase
					+

Insulin

Figure I-9-6. Opposing Activities of Insulin and Glucagon

142

Chapter 9 ● Hormones

G PROTEINS IN SIGNAL TRANSDUCTION

Table I-9-2 seen earlier summarizes the major components of water-soluble hormone pathways reviewed in this section. There are several G proteins (GTP-binding) involved. Trimeric G proteins include Gs, Gi, Gq, and in the photoreceptor pathway reviewed in Chapter 10, Gt (transducin). Receptors that engage these all have the 7-helix membrane-spanning structure. Receptor stimulation causes the Gα subunit to bind GTP and become active. The Gα subunit subsequently hydrolyzes the GTP to GDP, terminating the signal. The p21ras G protein is monomeric.

G-protein defects can cause disease in several ways.

Table I-9-3. Abnormal G Proteins and Disease

	Defect				Example			Disease
	ADP-ribosylation by:
	•	Cholera toxin			Gsα			Diarrhea of cholera
	•	E. coli toxin			Gsα			Traveler’s diarrhea
	•	Pertussis toxin			Giα			Pertussis (whooping cough)
	Oncogenic mutations				p21ras (ras)			Colon, lung, breast, bladder tumors

ADP-Ribosylation by Bacterial Toxins

High-Yield

Certain bacterial exotoxins are enzymes which attach theMEDIUMadenosineYIELDdiphosphate (ADP)-ribose residue of NAD to Gα subunits, an activity known as ADPribosylation. In humans, some ADP-ribosylation is physiological but it may also be pathological:

•Vibrio cholerae exotoxin ADP-ribosylates Gsα, leading to an increase in cAMP and subsequently chloride secretion from intestinal mucosal cells, and causing the diarrhea of cholera.

•Certain strains of Escherichia coli release toxins (heat labile or LT) similar to cholera toxin, producing traveler’s diarrhea.

•Bordetella pertussis exotoxin ADP-ribosylates Giα, dramatically reducing its responsiveness to the receptor, thus increasing cAMP. It is not known how this relates to the persistent paroxysmal coughing symptomatic of pertussis (whooping cough).

143