An Introduction of Adipokines

Introduction

The adipokines, also adipocytokines, are now widely accepted to be cytokines (cell signaling proteins), acute phase reactants, growth factors, other inflammatory mediators, hormones secreted by adipose tissue. Many of them are proteins, protein hormones, or genetic substances existing in the body cells, only to be detected in high-power microscope. The first adipokine, i.e. leptin, was discovered in 1994. After that, hundreds of adipokines have been discovered. They're differentiated in shapes, structures, sources, functions, and affections to human body development.

Adipokines play important roles in affecting obesity, metabolic syndrome, cardiovascular disease, immunoreaction, and blood glucose regulation. In such case, they have great potentials for future pharmacological treatment of a wide range of diseases. They would also serve as markers for the diagnosis of obsessive, metabolic, cardiovascular, inflammatory, or other diseases. 

Leptin, The First Adipokine Found | Source

Classification of Adipokines by Functions and Affections

According to their different functions and affections towards human body, adipokines could mainly classified into five groups.

1. Energy Balance and Metabolism

This group includes but not only leptin, adiponectin, omentin, apelin, adipsin(complement factor D), chemerin, lipocalin, visfatin (pre-B-cell enhancing factor), vaspin, and resistin. All are mainly connected to or affecting the body's energy regulation and metabolism.

Leptin is a protein hormone produced by human body's fat cells. The levels of leptin are positively related to the body fat that fat cells carry. It's currently the most widely studied adipokine due to its regulation in food intake, fat storage and energy expenditure, though through a negative feedback loop from the fat cells to leptin, leptin to the brain via the bloodstream, and the brain back to the fat cells. Leptin resistance, which means the body does not response or shows little response to leptin, could result in control disorder in fat gain.

2. Inflammation

This group mainly includes interleukin 1, interleukin 8, tumor necrosis factor α (TNFα), interleukin 6, interleukin 10, and monocyte chemotactic protein-1 (MCP-1). All are affecting the body in inflammation.

Interleukin 6 (IL-6) is an interleukin produced by adipocytes. IL-6 acts as both a pro-inflammatory cytokine and an anti-inflammatory myokine. It is a major mediator of fever and has acute phase response. Just like leptin, IL-6 could also cross the blood-brain barrier and acts as a messenger to the brain in initiating synthesis of PGE2 in the hypothalamus, thus controlling the regulation of body temperature.

3. Thrombosis and Hypertension

This group mainly includes serum amyloid A (SAA), plasminogen activator inhibitor-1 (PAI-1), C-reactive protein (CRP), and proteins of the renin angiotensin system. This group of adipokines belong to the protein family and are all in connection with thrombosis and hypertension.

Plasminogen activator inhibitor-1 (PAI-1), also known as endothelial plasminogen activator inhibitor or serpin E1, is a protein mainly produced by the endothelium (cells lining blood vessels) or secreted by other tissue types. In humans, PAI-1 is encoded by the SERPINE1 gene. Its main function is to inhibit the serine proteases tPA and uPA/urokinase, thus inhibiting fibrinolysis, the physiological process that degrades blood clots.

4. Growth Factors

This group mainly includes nerve growth factor (NGF) and hepatocyte growth factor (HGF).

NGF is a protein acting as a neurotrophic factor and neuropeptide primarily involved in growth regulation, maintenance, proliferation, and survival of certain target neurons. It plays a critical role not only in the survival of pancreatic beta cells but also in the regulation of both innate and acquired immunity. Since NGF was first isolated by Nobel Laureates Rita Levi-Montalcini and Stanley Cohen in 1956, numerous biological processes involving NGF have been identified.

5. Brown Fat

This group mainly includes fibroblast growth factor 21(FGF21) and insulin-like growth factor 1 (IGF-1).

FGF21, as one of the fibroblast growth factors family, is a cell signaling protein involved in the processes of normal development. IGF-1 is a protein hormone similar in molecular structure to insulin.That's why it's called "insulin-like". IGF-1 acts a great part in childhood growth and continues to have anabolic effects in adults.