Structural Biochemistry/Lipid Droplets and Cellular Lipid Metabolism

Definition
Lipid droplets are the lipid storage organelles of all organisms. Their important roles include cellular and organismic energy storage Lipid droplets could be found nearly in every cell. They are cytoplasmic organelles that store lipids; some examples are triglycerides and cholesterol. Under nutrient deprivation, droplet triglycerides are hydrolyzed to create free fatty acids, which are then oxidized and used to provide energy.

Lipid droplets in a cellular organism are typically composed of nonpolar, hydrophobic lipids, also known as neutral lipids. The droplets contain a hydrophobic center that is encircled by a phospholipid monolayer. There are two major types of lipid droplets; sterol esters and triacylglycerols. As the lipid droplets form there can be a variety of lipid droplet proteins along the monolayer. The amount of lipid droplets in each cell can vary and currently scientists are unable create an algorithm to predict the lipid droplet concentration.

Lipid droplets are formed on or in close proximity of endoplasmic reticulum. Through the use of the electron microscope, it was possible to see that lipid droplets and cisternae have a close relationship but the mechanism of formation has yet to be discovered. 2

There are over 800 genes that affect lipid droplet accumulation. When testing to see how these multitudes of genes affect lipid droplet formation, knockdown of genes led to differing lipid droplet formation changes. Depending on the gene that was knocked-down, some of the lipid droplets decreased in concentration, increased in size, or increased in size and were more dispersed. Proteins were also found to affect lipid droplet formation. For example, with the over-expression of fat-inducing transcript-2 (FIT2) proteins, there are more lipid droplets. Consequently, knockdown of FIT2 leads to fewer lipid droplets. 2

Lipid droplets are capable of interacting with other cellular organelles. They interact with the endoplasmic reticulum, endosomes, mitochondria, and peroxisomes. These interactions can sometimes mediate some cellular mechanisms such as lipid trafficking, neutral lipid metabolism, and synthesis/catabolism of steroid hormones. A majority of these interactions occur at the endoplasmic reticulum.2



http://gladstoneinstitutes.org/u/rfarese/research_pics/lipid_droplet.jpg

Properties of Lipid Droplets
1. Almost every cell has lipid droplets or has the capacity to produce them

2. Some bacteria use lipid droplets to store lipids themselves

3. Lipid droplets' abundance varies greatly in cells.

4. Lipid droplets' size also varies greatly depending on cell types. Many cells are known to contain small LDs (100-200 nm). On the other hand, in white adipocytes, for instance, have diameters up to 100 micrometers, occupying most of the cytoplasm.

Functions of Lipid Droplets
1. Generally they are intracellular lipid reservoirs that become useful as they provide building blocks for cell membranes or substrates for energy metabolism

2. Lipid droplets may be used as places for synthesizing some lipids. For instance, TGs are produced in the Er and lipid droplets.

3. Lipids droplets may be used to store some proteins.

4. Lipid droplets are involved in hepatitis C virus assembly

Lipid Droplets and Tissues
Adipose Tissue In mammals and birds, adipose tissues is located in specific areas in the body and are regulated by hormones. They are more prominently used as insulation for endotherms. There are two types of adipocytes: white adipocytes and brown adipocytes. Adipocytes are the cells that are utilized by the body to store lipids in lipid droplets. White adipocytes store lipids in a single large lipid droplet in the cytoplasm. They are also known to store cholesterol esters and fat-soluble vitamins. The white adipocytes utilize leptin to regulate the lipid storage. Brown adipocytes catabolize lipids in order to generate heat. They store lipids in smaller and more numerous lipid droplets in the cytoplasm.

Liver The liver has the second greatest lipid in LD form storage capacity. They store the lipid droplets in hepatocytes. In humans, a high amount of lipid droplets can lead to an illness called fatty liver.

Small Intestine The small intestine is where a majority of nutrient absorption occurs. By utilizing its microvilli, the small intestine is able to absorb 95% of a meal's fat content. The lipid droplets are stored in intestinal enterocytes. The enterocytes have a large surface area and are able to store and synthesize triacylglycerols(TGs).

Yolk Sac The yolk sac stores lipids in a similar manner to intestinal enterocytes and liver hepatocytes. They are able to store lipid droplets and export via apoB-containing lipoproteins.

Skeletal Muscle Muscle is not known to be a large lipid storage unit. When humans store lipids in their skeletal muscles it is usually a side effect of obesity because of insulin resistance. There is an exception with this in highly-trained athletes. Athletes use a high volume of ATP when they are training so the body needed a way to keep a lipid reserve for when the muscles need more energy. This compensation occurs with the lipid droplets being stored in skeletal muscle cells near the mitochondria. This is considered the athelete's paradox because lipid droplet storage in skeletal muscles is usually a bad sign.

Adrenal Cortex The adrenal cortex is able to store large amounts of sterol esters(SEs). The SEs are most likely stored in the form of cholesterols for steroid hormone synthesis. Also the yellow coloring of the tissue can be credited to the lipids.2

Reference
1. Nature.com. Nature Publishing Group, n.d. Web. 07 Dec. 2012.

2. http://www.ncbi.nlm.nih.gov/pubmed/22524315