Lecture 26: Phosphodiesterases

cAMP & cGMP Metabolism

Broken down to 5’AMP and 5’GMP by phosphodiesterase enzymes (PDEs)

Equilibrium between synthesis and metabolism

Phosphodiesterases (PDE)

21 genes encoding PDEs in human genome

11 groups based on sequence similarity

PDE Structure

N-terminus contains regulatory and dimerization domains, enzymes act as dimers

R-regulatory features include Calm-binding domains, anchoring domains, phosphorylation sites, cyclic nucleotide binding sites

Conserved catalytic domain

PDE Substrate Specificity

Expressed in tissue-regulated and developmentally-regulated fashions

• PDE 4, 7, 8 are cAMP selective

• PDE 5, 6, 9 are cGMP selective

• PDE 1, 2, 3, 10, 11 bind both cAMP and cGMP

PDE4

• Anchoring domain

• Regulatory activating phosphorylating site (PKA)

• cAMP binding domain

• Upstream conserved region

• Catalytic domain

• Regulatory inhibitory phosphorylation site (ERK2)

PDE5

• Regulatory phosphorylation site (PKG)

• 2 cGMP binding domains

• Catalytic domain

PDE Deletions & Mutations

PDE4D polymorphisms linked to stroke

PDE6B mutations linked to retinal degeneration

• PDE4D: Neonatal lethality

• PDE4B: Altered immune response

• PDE3B: Altered insulin secretion

• PDE3A: Altered fertility

• PDE1B: Increased locomotion

PDE Inhibitors

Common ingestion of non-selective methylxanthines

Xanthine, caffeine (coffee), theophylline (tea), theobromine (chocolate)

Effects of Methylxanthines

• Increased cortical arousal, alertness → deferral of fatigue

• Increased myocardial, cerebral blood vessel contraction

• Relaxation of smooth muscle

• Increased gastric secretion, digestive enzymes

• Weak diuretic effects

• Bronchodilation

Use of Theophylline in Respiratory Disease

Effective bronchodilator

PDE inhibition functions alongside B₂ agonism to increase cAMP

Selective PDE Inhibitors

Non-selective inhibitors have many side effects

Development pursued for treatment of respiratory disease (asthma), COPD, hypertension, MDD, thrombosis, memory/cognition (Alzheimer’s, Parkinson’s)

PDE5 inhibitors successful: Vardenafil, sildenafil, udenafil, tadalafil, IBMX

Rolipram (early 1970s)

Selective PDE4 inhibitor developed for treatment of depression

Effective but side effects of nausea, GI disturbances, emesis

Roflumilast

2nd generation PDE4 inhibitor used in treatment of COPD

GI and other side effects

Mechanism of Penile Erection

Stimulus → sexual arousal → stimulation of non-adrenergic, non-cholinergic neurons

NO production → guanylyl cyclase stimulation → relaxation of corpus cavernosum → increased blood flow → erection

• PDE5 activity reverses effect by hydrolysis of GTP

Erectile Dysfunction

Insufficient blood supply to corpus cavernosum (restrictions to blood vesses)

50% of M >40, associated with cigarette smoking, hypertension, diabetes, depression

PDE5 Inhibition

Maintains concentration of cGMP in muscles surrounding vessels supplying blood to penis → increased blood flow to corpus cavernosum → erection

Does not change sexual desire, only ability to maintain erection

Sildenafil (Viagra)

First introduced in 1998, effective in 65% of M with ED of various causes

Onset of action 25-60 min, lasts 4-5 hours

35% incidence of side effects: Headaches, flushing, dyspepsia, blue vision

Tadalafil (Cialis)

Lasts 17.5 hours

Problems with PDE Inhibitors

Hypotensive shock if combined with nitroglycerin

• Increased cGMP stimulation (NO), PDE inhibition blocks cGMP breakdown

Inhibition of PDE6 in retinal rods causes changes in blue vision