Main Factors Contributing to Atherosclerosis


There are two main factors that contribute to atherosclerosis
  1. Free radicals
  2. Oxidative stress from oxidation of lipids in the artery.

Free radicals are the number one cause of atherosclerosis; they encourage the transformation of LDL into its oxidized form, which is the most damaging state for LDL. Free radicals are atoms with an unpaired electron; making them extremely reactive and engaging in chain reactions that can destabilize other molecules, forming vascular damaging compounds. In terms of atherosclerosis, the primary free radicals are the superoxide anion, and hydroxyl radical. These two radicals are both active in lipid peroxidation, which is the main mechanism behind oxidizing LDL.

Oxidative stress is described as changes to the vascular system in return for the free radicals that help to modify oxidants and oxidize LDL. Oxidative stress is a primary factor of atherosclerosis and stems from free radicals and the way they can modify oxidants into damaging products. For the process of atherosclerosis, the primary oxidant involved is hydrogen peroxide. The tables below list various oxidants and free radicals present in the vascular system, which when reacted together, can lead to oxidative stress and in turn, the formation of atherosclerosis.

Examples of free radicals in biological systems (Stocker & Keaney, 2004)

Name
Formula
Comments
Carbon-centered radical
carbon.gif
These radicals with the unpaired electron residing on carbon, usually react rapidly with O2 to make peroxyl radicals.
Superoxide anion and hydroperoxyl radical
oxy.gif
The “primary” oxygen-centered radicals in its anionic and protonated form.
Peroxyl and alkoxyl radical
RO2·, RO·
Oxygen-centered radicals that can be formed from reaction of carbon-centered radicals with O2 (RO2·), or from the breakdown of organic peroxides, such as LOOH (RO2·, RO·).
Hydroxyl radical
·OH
Highly reactive, oxygen-centered radical that reacts with all biomolecules.
Nitric oxide (nitrogen monoxide) and nitrogen dioxide
·NO, ·NO2
Nitric oxide is formed from L-arginine and nitrogen dioxide from reaction of ·NO with O2.
Thiol and perthiol radical
RS·, RSS·
A group of radicals with the unpaired electrons residing on sulfur.
Transition-metal ions
Fe, Cu, etc.
Ability to change oxidation numbers by one, allowing them to accept/donate single electrons; hence, they can be catalysts of free radical reactions.

Examples of nonradical oxidants of potential relevance to oxidative stress in the vasculature (Stocker & Keaney, 2004)

Name
Formula
Comments
Hydrogen peroxide
H2O2
A diffusible oxidant that is aweak oxidizing agent and is generally poorly reactive. It may participate in cellular signaling and in the presence of available transition metals, give rise to ·OH.
Hypochlorite, hypochlorous acid
OCl, HOCl
Weak acid (pKa ∼7.5) but strong oxidant. Reacts steadily with Fe·S clusters; metal ions held in proteins by thiolate ligands, heme, amino acid residues (methionine, cysteine) of proteins, and GSH. Can give rise to secondary reactive species including chloramines and amino acid-derived aldehydes.
Ozone
O3
Strong oxidant that attacks protein and lipids including cholesterol. Singlet oxygen may be formed as a by-product.
Singlet oxygen
1ΔgO2
Reacts with other molecules chemically or by transfer of its excitation energy. Reaction with carbon-carbon double bonds is best known. The relevance of 1ΔgO2 in the vasculature is unknown.
Oxoperoxonitrate (1−) or peroxynitrite, peroxynitrous acid
ONOO, ONOOH
Can be formed via reaction of O2−· with ·NO. The protonated form is highly reactive. A major reaction of ONOO is with CO2 that gives rise to nitrating, nitrosating, and oxidizing species.
Alkylperoxynitrites, dinitrogen trioxide, nitryl chloride, and nitronium (nitryl) ion
ROONO, N2O3, NO2Cl, and NO2+
Additional reactive nitrogen species. N2O3 is a major nitrosating species.
Nitrosothiols
RSNO
Formed via reaction of RS· with ·NO, or thiols with higher oxides of nitrogen. Nitrosothiols are weak oxidants.

LDL Oxidation and Atherosclerosis Development Mechanisms


LDL Oxidation Mechanism

  • Initially, a free radical takes a hydrogen atom from a polyunsaturated fatty acid (PUFA) within the low density lipoprotein (LDL) molecule. This initial free radical is most likely a lipid hydroperoxide or hydrogen peroxide, “which decomposes in the presence of metal ions into lipid alkoxyl and peroxl radicals, and then to hydroxyl radicals” (Esterbauer, 1993).
  • The PUFA, now a carbon-centered radical, reacts with oxygen to form a lipid peroxyl radical. This lipid peroxyl radical takes a hydrogen atom from a nearby PUFA, making a lipid hydroperoxide and another PUFA radical, thus continuing the cycle.
  • Antioxidants within LDL also combine with lipid peroxyl radicals to form lipid hydroperoxides. As the PUFA’s become lipid hydroperoxides the LDL loses its antioxidants.
  • These lipid hydroperoxides then decompose into aldehydes, hydrocarbon gases, epoxides, ketones, and alcohols with the help of transition metal ions (such as copper).
  • The aldehydes that are formed play an important role in oxidizing LDL; they modify the apolipoprotein B in LDL to attach to the scavenger receptor cells of macrophages.

mechanism.png
Mechanism of polyunsaturated fatty acids into Oxidized LDL (Esterbaur, Puhl & Wag,1993)

Mechanism to Form Atherosclerosis:

  • Very low density lipoprotein (vLDL) is produced by the liver and is changed into LDL by means of lipoprotein lipase. This process removes triglycerides from vLDL by hydrolysis, releasing fatty acids and leaving greater numbers of cholesterol, thus increasing the density of the molecule.
  • The LDL crosses the endothelium and moves into the extracellular matrix where it is oxidized (by the aforementioned steps above), and forms oxidized LDL (OxLDL).
  • OxLDL is a cause of inflammation and signals monocytes (white blood cells) to enter the arterial wall to fix the inflammation. As monocytes enter the arterial wall, they transform into macrophages.
  • Since the LDL is now oxidized due to aldehydes and lipid hydroperoxides, the modified apolipoprotein B in LDL attaches to macrophage scavenger receptor cells. At this stage, OxLDL has a very high number of cholesterol and cholesterol esters, since it lost antioxidants, triglycerides, and fatty acids in previous steps. Macrophages are supposed to remove cholesterol by use of high density lipoprotein (HDL) particles, but if there is too much excess cholesterol, it causes the macrophages to enlarge and fill with lipids.
  • Eventually the macrophages build up and convert into lipid-laden foam cells (a collection of fatty materials and cholesterol) which die and become part of the plaque that causes atherosclerosis. As this process continues, more and more LDL becomes trapped within the tunica intima (the innermost layer of the arterial wall) creating a pool of cholesterol called a fatty streak. This is held in place by a fibrous cap formed by means of a fibrous matrix: a combination of elastin, collagen, and proliferated smooth muscle cells. The smooth muscle cells move from the tunica media (the thickest layer of the artery) to the tunica intima and become proliferated by way of released cytokines (proteins that help with immunity) within the macrophages.
  • The major atherosclerosis causing plaque has a fibrous cap, which sticks out into the artery, causing vasoconstriction, and blocking blood flow (the plaque always forms in the lumen, which is between the intima and the musculature of the wall).

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Mechanism of LDL into foam cells (Stocker & Keaney, 2004)


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Diagram showing plaque build-up (Rader & Daughtery, 2008)

References:


Biotech100. (2005). Biotechnology Encyclopedia: Atherosclerosis. Retrieved October 2012, from Biotech100: http://www.biotech100.com/biotechnology_encyclopedia/atherosclerosis.htm

Brock, T. (2008). Inflammation in Atherosclerosis: Oxidants and Oxidized Phospholipids. Cayman Chemical, Atherosclerosis catalog.

Esterbauer, H., Wäg, G., & Puhl, H. (1993). Lipid peroxidation and its role in atherosclerosis. British Medical Bulletin, 49(3), 566-576.

QIAGEN . (2000-2010). LDL Oxidation in Atherogenesis. Retrieved October 15, 2012, from SABiosciences: http://www.sabiosciences.com/pathway.php?sn=LDL_Oxidation_in_Atherogenesis

Rader, D. J., & Daughtery, A. (2008). Translating molecular discoveries into new therapies for atherosclerosis. Nature, international weekly journal of science, 904-913.

Steinberg, D. (1997). Low Density Lipoprotein Oxidation and Its Pathobiological Significance*. Journal of Biological Chemistry, 20963-20966.

Stocker, R., & Keaney, J. F. (2004). Role of Oxidative Modifications in Atherosclerosis. Physiological Reviews, 1381-1478.