Lipids are hydrophobic or amphiphilic biomolecules whom include a chemically diverse group of compounds that are non-polar solvents. Their most common and defining feature is their inability to dissolve in water. Functions of lipids are equally as diverse and include, storing energy, signalling and acting as structural components of cell membranes. Fats and oils are typically the primary stored forms of energy in most organisms while phospholipids and steroids including cholesterol compose of up to half the mass of biological membranes.
Lipid transport in the blood is typically carried out by a biochemical assembly known as a lipoprotein. They have a single-layer phospholipid and cholesterol outer shell oriented in such a way that the hydrophilic sections face outward towards the water and the hydrophobic portions are oriented inwards towards the lipid’s molecules within the particles. In fact, these lipoproteins are used to transport cholesterol and other fats throughout the body via the bloodstream. This is due to the polar nature of the phospholipid bilayer of the lipoprotein which contains the cholesterol. Lipoproteins have specific receptors that direct them to certain tissues in the body and based on their density there are numerous types of lipoproteins.
In particular to this study are, high density lipoprotein (HDL) which is the lipoprotein used to transport cholesterol from body tissues to the liver and low-density lipoprotein (LDL) which is utilised to transport cholesterol from the liver to the many cells in the body. These particular molecules are quantifiable and constitute the most important measurable variables of cholesterol and is what is assayed when blood is drawn to check a patient’s lipid profile. LDL is considered the ‘bad’ cholesterol as too much is unhealthy for the body as it transports cholesterol to the arteries where it has the potential to build up on artery walls leading to a condition known as atherosclerosis. Moreover, HDL is referred to as the ‘good’ cholesterol as it is protective in the manner that it transports cholesterol to the liver to be expelled from the body, essentially helping to rid the body of excess cholesterol so there is less chance of it ending up in the arteries.
Elevated levels of non-HDL cholesterol and LDL in the blood can be linked to hereditary diseases but is also commonly the result of an unhealthy diet, obesity or the presence of other conditions such as type 2 diabetes. Elevated non-HDL cholesterol concentrations correlate with high rates of atherosclerosis progression.
The pathway for uptake of LDL into cells is a characteristic example of receptor-facilitated endocytosis. Cells express LDL receptor complexes on the plasma membrane of the cell, the receptor then fuses itself to binding sites in LDL and bound receptors cluster in clathrin-coated pits and are then endocytosed. Essentially through various processes these LDL receptors facilitate the uptake of cholesterol from LDL to the cells via endocytosis on the outer membrane of the cell. Therefore, cells can effectively eliminate LDL from serum if they express a sufficient quantity of LDL receptors on the cell surface. However, genetic mutations in this receptor can reduce its numbers at the cell membrane, leading to excess serum LDL and consequentially, hypercholesterolemia.