27.4 Prostaglandins and Other Eicosanoids

Prostaglandins and Other Eicosanoids

Prostaglandins and eicosanoids are lipid signaling molecules that act as local hormones, meaning they exert their effects close to where they're produced rather than traveling through the bloodstream like typical hormones. They regulate processes ranging from inflammation and blood clotting to smooth muscle contraction and stomach lining protection.

These compounds are all derived from arachidonic acid, a 20-carbon polyunsaturated fatty acid (the prefix eicosa- means twenty). Two main enzymatic pathways produce them: the cyclooxygenase (COX) pathway and the lipoxygenase (LOX) pathway. Understanding these pathways matters because many common drugs, including aspirin and ibuprofen, work by targeting them directly.

Structure and Effects of Prostaglandins

Prostaglandins share a common structural feature: a cyclopentane ring with two side chains extending from it. They're classified by letters A through I based on the functional groups on that ring, and by a subscript number indicating how many double bonds are in the side chains. For example, $\text{PGE}_2$ has a keto group on the ring and two double bonds, while $\text{PGF}_{2\alpha}$ has hydroxyl groups on the ring.

Because different prostaglandins have different ring substituents, they can have very different (even opposing) physiological effects:

• Inflammatory response
  • $\text{PGE}_2$ and $\text{PGF}_{2\alpha}$ promote inflammation, contributing to redness, swelling, and pain
  • $\text{PGE}_1$ has anti-inflammatory effects
• Smooth muscle contraction and relaxation
  • $\text{PGE}_2$ and $\text{PGI}_2$ (prostacyclin) cause vasodilation and bronchodilation, widening blood vessels and airways
  • $\text{PGF}_{2\alpha}$ does the opposite: vasoconstriction and bronchoconstriction
• Platelet aggregation
  • $\text{TXA}_2$ (thromboxane $\text{A}_2$) promotes platelet clumping, which drives blood clot formation
  • $\text{PGI}_2$ inhibits platelet aggregation, keeping blood flowing smoothly
  • The balance between $\text{TXA}_2$ and $\text{PGI}_2$ is critical for normal hemostasis
• Gastrointestinal protection
  • $\text{PGE}_2$ and $\text{PGI}_2$ stimulate mucus and bicarbonate secretion, protecting the stomach lining from its own acid
• Renal function
  • $\text{PGE}_2$ and $\text{PGI}_2$ help regulate blood flow to the kidneys and electrolyte balance
• Reproductive processes
  • $\text{PGE}_2$ and $\text{PGF}_{2\alpha}$ play roles in ovulation, menstruation, and labor induction (synthetic prostaglandins are actually used clinically to induce labor)

Biosynthesis of Eicosanoids

The starting material for all eicosanoids is arachidonic acid, which is stored in cell membrane phospholipids. It's released when the enzyme phospholipase $\text{A}_2$ cleaves it from the $sn$-2 position of the phospholipid. Arachidonic acid itself comes from omega-6 fatty acids (especially linoleic acid) in the diet.

Once free, arachidonic acid enters one of two pathways:

Cyclooxygenase (COX) Pathway

1. COX enzymes convert arachidonic acid into $\text{PGG}_2$, then reduce it to prostaglandin $\text{H}_2$ ($\text{PGH}_2$). This is the committed step for this branch.
2. $\text{PGH}_2$ is a common intermediate that specific downstream synthases convert into different products:
   • PGE synthase → $\text{PGE}_2$
   • PGF synthase → $\text{PGF}_{2\alpha}$
   • Prostacyclin synthase → $\text{PGI}_2$ (prostacyclin)
   • Thromboxane synthase → $\text{TXA}_2$

Which product a cell makes depends on which synthase enzymes it expresses. For instance, platelets are rich in thromboxane synthase, so they primarily produce $\text{TXA}_2$.

Lipoxygenase (LOX) Pathway

1. LOX enzymes (5-LOX, 12-LOX, or 15-LOX) oxidize arachidonic acid to form hydroperoxyeicosatetraenoic acids (HPETEs). The number indicates which carbon gets oxidized.
2. HPETEs are then converted into two important product families:
   • Leukotrienes: 5-HPETE → $\text{LTA}_4$ → $\text{LTB}_4$, $\text{LTC}_4$, $\text{LTD}_4$, $\text{LTE}_4$. These are potent mediators of allergic and inflammatory responses. $\text{LTC}_4$, $\text{LTD}_4$, and $\text{LTE}_4$ were originally called "slow-reacting substance of anaphylaxis" because they cause prolonged bronchoconstriction in asthma.
   • Lipoxins: Formed from 15-HPETE ($\text{LXA}_4$ and $\text{LXB}_4$). Unlike most other eicosanoids, lipoxins are anti-inflammatory and help resolve inflammation.

COX-1 vs. COX-2 in Prostaglandin Production

There are two main isoforms of cyclooxygenase, and the distinction between them is clinically significant:

• COX-1 is constitutively expressed, meaning it's always present in most tissues. It produces prostaglandins needed for everyday "housekeeping" functions: protecting the GI lining, supporting platelet aggregation, and maintaining renal blood flow.
• COX-2 is an inducible enzyme, meaning its expression ramps up in response to inflammatory signals like cytokines and bacterial endotoxins. It's primarily responsible for producing the prostaglandins that drive pain, fever, and swelling during inflammation.

This distinction explains both the benefits and side effects of common pain medications:

• Non-selective NSAIDs (aspirin, ibuprofen, naproxen) inhibit both COX-1 and COX-2. They effectively reduce inflammation and pain by blocking COX-2, but because they also block COX-1, they can reduce the protective prostaglandins in the stomach, leading to GI irritation and ulcers with chronic use.
• Selective COX-2 inhibitors (e.g., celecoxib) target only COX-2, reducing inflammation with fewer GI side effects. However, by leaving $\text{TXA}_2$ production (via COX-1 in platelets) intact while reducing $\text{PGI}_2$ production (partly COX-2 dependent in blood vessel walls), they can shift the $\text{TXA}_2$/$\text{PGI}_2$ balance toward clotting, which may increase cardiovascular risk.

Eicosanoids as Lipid Mediators in Cell Signaling

Unlike steroid hormones that enter cells and bind nuclear receptors, eicosanoids signal through G protein-coupled receptors (GPCRs) on the cell surface. Each eicosanoid type has its own receptor (or set of receptors), which triggers specific intracellular signaling cascades involving second messengers like cAMP, $\text{Ca}^{2+}$, or inositol phosphates.

A few key points about eicosanoid signaling:

• Eicosanoids are autocrine and paracrine mediators. They act on the cell that produced them or on nearby cells, not on distant targets.
• They're not stored in vesicles. Instead, they're synthesized on demand from membrane phospholipids when a cell is stimulated.
• Their effects are short-lived because they're rapidly metabolized, which keeps their signaling tightly localized.
• Dysregulation of eicosanoid pathways contributes to chronic inflammatory diseases (rheumatoid arthritis, asthma), cardiovascular disease, and certain cancers, making these pathways important drug targets.