Biochemistry, LDL Cholesterol

Fundamentals

The LDL particle is made of a monolayer of phospholipid, unesterified cholesterol forms the surface membrane, and fatty acid esters of cholesterol make up the hydrophobic core. One copy of the hydrophobic apo-B protein is embedded in the membrane, mediating the binding of LDL particles to specific cell-surface receptors. [2]

Issues of Concern

LDL receptor function is needed for the uptake of LDL from the blood into hepatocytes.

Defects in LDL receptor function can cause hypercholesterolemia, known as familial hypercholesterolemia, an autosomal dominant disorder. Because LDL receptors on the surface of hepatocytes are necessary for the binding and subsequent uptake of LDL molecules in the blood, a genetic decrease in LDL receptor number would cause a decreased ability of hepatocytes to absorb LDL and increase LDL in the blood. If this mutation is heterozygous, some LDL receptors will be present on the hepatocytes; thus, LDL is usually around 300 mg/dL. However, a homozygous mutation will result in the complete absence of LDL receptors on hepatocytes, increasing the LDL cholesterol levels to 1000 mg/dL. [3]

Cellular Level

The liver produces very low-density lipoprotein (VLDL), metabolized to IDL by lipoprotein lipase (LPL). IDL is converted to LDL by hepatic triglyceride lipase (HTGL). LDL and a portion of IDL particles are cleared from the circulation via the LDL receptors (LDL-Rc) expressed in the liver and other cells. [4]

The LDL receptor consists of a single-chain glycoprotein and is 839 amino acids long. It comprises a 320 residue N-terminal exoplasmic domain that contains the LDL-binding site and consists of disulfide-bonded cysteine residues, a C-terminal cytosolic domain that traps the LDL receptor in clathrin-coated pits, and a sequence of 22 hydrophobic amino acids within the plasma membrane in the form of an alpha helix. [5]

Molecular Level

LDL particles bind to an LDL receptor on the plasma membrane, forming a receptor-ligand complex that is internalized in a clathrin-coated pit that pinches off to become a coated vesicle. After endocytosis, the LDL particle and its receptors are internalized by receptor-mediated endocytosis and degraded in the lysozyme. The clathrin coat depolymerizes, forming an early endosome which fuses with a late endosome where the low pH causes the LDL particles to dissociate from the LDL receptors. In the lysozyme, the apo-B protein of the LDL is degraded to amino acids, and cholesterol esters are hydrolyzed to fatty acids and cholesterol. [2]

Function

Apolipoproteins serve a structural role in phospholipid membranes, act as ligands for lipoprotein receptors, guide the formation of lipoproteins, and serve as activators and inhibitors of enzymes involved in the metabolism of lipoproteins. Lipoproteins are critical for absorbing and transporting dietary lipids by the small intestine and moving lipids from the liver to peripheral tissues and back from peripheral tissues to the liver and intestine. They are also crucial for transporting toxic foreign hydrophobic and amphipathic compounds, including bacterial endotoxin from areas of invasion and infection. [6]

Mechanism

The LDL receptor is found in the liver and most other tissues. It recognizes Apo B 100 and Apo E, mediating the uptake of LDL, chylomicron remnants, and IDL through endocytosis. After internalization, the lipoprotein particle is degraded in lysosomes, and cholesterol is released. When cholesterol enters the cell, HMG CoA reductase activity increases then synthesizes cholesterol and modulates the expression of LDL receptors. LDL receptors on the liver determine plasma LDL levels. When there is a low number of receptors, less LDL can be taken up from the blood by the liver, leading to high plasma LDL levels. Conversely, when there are more LDL receptors, more LDL is taken up from the blood by the liver, leading to low plasma LDL levels.

Cholesterol levels regulate the number of LDL receptors in the cell. If the cell senses a decrease in cholesterol levels, the transcription factor SREBP is transported from the endoplasmic reticulum to Golgi, where proteases cleave and activate SREBP, which moves to the nucleus and increases the expression of LDL receptors. When cholesterol levels are low in the cell, high SREBP levels remain in the endoplasmic reticulum in an inactive form, and the expression of LDL receptors is decreased. [7]

Testing

A fasting lipid panel should be ordered in testing for hypercholesterolemia, along with labs to rule out secondary causes.[8] In the fasting lipid panel, total cholesterol greater than 200 mg/dL and LDL cholesterol greater than 130 mg/dL are considered abnormal.[9] Secondary causes of hyperlipidemia should be ruled out through testing fasting blood glucose, hemoglobin A1c, thyroid-stimulating hormone, alkaline phosphatase, and urinalysis to assess for proteinuria.[10]

Pathophysiology

Hypercholesterolemia occurs from excess cholesterol from diet, bile, or intestines. The liver releases triglycerides into the plasma in the form of VLDL.[11] The intestines release triglycerides into the plasma in the form of chylomicrons. Once in the plasma, the VLDL is converted into LDL. LDL in the plasma then interacts with the LDL receptor on cells in various tissues.[12]

Clinical Significance

Increased LDL is associated with an increased risk of cardiovascular disease.[13] It is commonly associated with diabetes, hypertension, hypertriglyceridemia, and atherosclerosis.[14] Through typically asymptomatic, hypercholesterolemia could present with metabolic syndrome, in which the patient would present with hypertension. In more severe hypercholesterolemia, patients may present with xanthomas with yellow nodules or plaques on the Achilles tendon; for example, xanthelasma on the eyelids or corneal arcus white rings lining the cornea.[15]

For this reason, LDL is clinically significant as it is crucial to monitor LDL levels in patients with hypertension and diabetes. Lifestyle modifications are crucial in overweight patients to lose weight through both exercise and diet control. Diets with lower saturated fats and aerobic exercise can help reduce LDL for patients. Pharmacologic modalities can also be utilized to decrease LDL levels, primarily HMG-Coa Reductase inhibitors, such as pravastatin and lovastatin, help significantly decrease serum LDL levels by inhibiting the conversion of HMG Coa to Mevalonate, which is a precursor to cholesterol. [16] PCSK 9 inhibitors, such as Evolocumab and Alirocumab, also significantly decrease serum LDL levels by inactivation the degradation of LDL receptors on target tissues. With this, more LDL receptors remain on target tissue as they are not degraded, increasing LDL removal from the bloodstream. [17]