Eicosanoids

Hormones are chemical messengers released directly into the blood and carried to respective organs to exert their function. Eicosanoids are unique compared to other hormones. They fall under a category known as local hormones because, unlike other hormones, they act on nearby cells. Therefore, we can state the meaning of eicosanoids as locally acting bioactive hormones that exert their effect near the site of hormone synthesis. Contrary to other hormones, they are not transported in the bloodstream. Many eicosanoids exist, but the main eicosanoid examples include prostaglandin, thromboxane, leukotriene, and lipoxin. Eicosanoids’ functions vary as they act on an assortment of cells. 

An in-depth look into eicosanoids’ meaning.

Eicosanoids arise from arachidonic acid found as a key component in almost all cell membranes. All mammalian cells release eicosanoids except red blood cells. Arachidonic acid is a polyunsaturated fatty acid-containing 20 carbon molecules. Foods such as poultry, meat, fish, and eggs are important fatty acid sources. 

The biosynthesis of these hormones involves two different pathways. These are the cyclic and linear pathways. The cyclic pathway is used in the synthesis of prostaglandins and thromboxanes. On the other hand, leukotrienes use the linear pathway in their synthesis.

Eicosanoids are characterised by a short life span and break down shortly after production. Furthermore, they implement the paracrine mode of action in their activity, meaning that they diffuse from the production site to act on nearby target cells. 

Eicosanoids examples:

There are four major classes of eicosanoids:

• Prostaglandin (PG).

It was originally thought that the prostate gland solely secreted prostaglandins. However, further studies uncovered that prostaglandins are synthesised by almost all nucleated cells. The structure of naturally occurring prostaglandins is denoted by a cyclopentane ring, a ‘trans’ double-bond between C-13 and C-14, and a hydroxyl group at C-15.

The synthesis of prostaglandin begins with the interaction of hormones such as bradykinin, angiotensin II, and thrombin with their cell surface receptors. However, aspirin and ibuprofen inhibit prostaglandin synthesis.

• Thromboxane (TX).

Thromboxane is secreted by activated platelets and plays a major role in blood clotting. Its structure is made up of a 6-membered ring and has an ether. Aspirin inhibits the biosynthesis of thromboxane by inhibiting the synthesis of its precursor molecules by the COX enzyme in platelets.

There are two main types of thromboxanes. Thromboxane A2 (TXA2) is biologically active. However, it is highly unstable and hydrolyses after 30 seconds to form inactive thromboxane B2 (TXB2).

• Leukotriene (LT).

The main sites of synthesis of leukotrienes are the leukocytes. Leukotrienes have a distinct quality as they are strong biological signals. They contain three conjugated double bonds in their structure. 

They have an important function in inflammation. Increased synthesis of leukotrienes above optimum level has undesirable effects and has been linked to the advent of hay fever and asthma symptoms. 

• Lipoxin (LX).

Lipoxins are synthesised from arachidonic acid through transcellular interactions involving three pathways. They have a linear structure that resembles that of leukotrienes. However, lipoxins have a distinctive feature: many hydroxyl groups within the linear chain.  Lipoxins display anti-inflammatory activity. Aspirin increases the production of lipoxins.

Eicosanoid functions with the various sub-types.

Eicosanoids interact with a wide range of cells and have various biological functions. 

• Prostaglandins.

Prostaglandin is a powerful vasodilator. Vasodilation is the relaxation of the muscles present in blood vessels, causing dilation. Dilation resulted in reduced resistance to blood flow and decreased blood pressure. Furthermore, the vasodilatory action of prostaglandin is exemplified in the kidneys. Here, widespread vasodilation of blood vessels leads to increased blood flow to the kidneys and increased sodium excretion from the blood into the urine.

Moreover, prostaglandin has a crucial role in inflammation. During inflammation, several changes occur, such as redness, heat, pain, and swelling. Local blood vessels dilate, causing more blood flow to the affected area. The dilated vessels are more permeable, permitting the movement of leukocytes to the inflamed tissues. 

In women, prostaglandins play a significant role in ovulation and labour for pregnant women at term. They induce smooth muscle contraction in the uterus. The action of prostaglandins also extends to the gut. Here, they can either stimulate or inhibit the contraction of smooth muscles lining the digestive tract. They also inhibit the production of gastric acid in the stomach. 

• Thromboxane

Thromboxane is an essential factor in blood clotting. After an injury, thromboxane causes platelets to adhere to one another and the walls of the blood vessels to form clots. Thromboxane also increases platelet aggregation. 

• Leukotriene

Leukotrienes have a chemotactic effect meaning that it attracts other immune cells towards the secreting cell, often a neutrophil. Leukotrienes produced in the lungs cause bronchoconstriction, the airway narrowing characterised by shortness of breath.

• Lipoxin

Lipoxin has a vital role in reducing inflammation. It is an endogenous, anti-inflammatory molecule that minimises tissue damage and chronic inflammation. 

Conclusion

Hormones are an essential part of our immune system. Though they differ in certain properties, eicosanoids are equally as important as the major hormones. They are much more than simple lipid molecules within our bodies as they contribute to our overall health. A better understanding of these molecules has led to significant advancements in medicine. There is improved knowledge on how certain bodily systems operate, what activates and inactivates these systems, and how best to treat certain diseases. The function of eicosanoids in pharmacology is unmistakable. Eicosanoids influence the choice of drug and the individual’s therapeutic process. All in all, eicosanoids are powerful hormones meant to be understood.