Biochemistry

Example

Semester Details: Written Report for BIOL 374/BCHM 370 Summer I semester 2022

Name: Nisar Ahmad

Topic: Atherosclerosis

Topic, Name and semester details- 5 points

Introduction-15 points

Atherosclerosis, also known as hardening of the arteries, is a chronic arterial disease that develops over many decades and is a major cause of deaths worldwide. A raised patch or plaque develops in the arterial wall that is rich in fat, cholesterol and calcium, and over time this hardens and narrows the artery depriving the region supplied by the blood vessel of oxygen (ischemia). Rupture of the plaque causes blood cell fragments called platelets to stick to the surface of the injury, leading to thrombosis (formation of a blood clot) which can result in a total blockage of the affected artery. If a coronary artery is affected, a myocardial infarction (heart attack) may result or if a cerebral artery supplying the brain is affected ischemic stroke may result. (Figures or tables are not presented in the introduction section)

The biochemical process of atherosclerosis. (140 points. Majority of the points are for Biochemical process of disease)

Cholesterol and fatty acids are two common types of lipids, defined as water-insoluble molecules in cells, that are soluble in organic solvents (Figure 1). {Figures to be included in the text at a place where they are mentioned in text} Both molecules have important biological functions. Cholesterol is an important component of cell membranes where it modulates fluidity, and a precursor of vitamin D and steroid hormones produced by the adrenal gland, testes and ovaries (1). {Reference to be embedded in the text} It is also used as a starting point for the synthesis of bile acids in the liver, which are secreted into the intestine where they solubilize fats and aid in the absorption of fat-soluble vitamins (A, D, E and K). Fatty acids are precursors of membrane phospholipids and glycolipids and are fuel molecules that are stored as triglycerides (esters of glycerol and three fatty acids) (Figure 1).

Figures and tables - 30 points- Points will be given based on their size, clarity and legend descriptions

Since lipids are insoluble in water, they are transported in the plasma as protein–lipid complexes (lipoproteins), which are divided into different types (chylomicrons, very low-density lipoproteins (VLDL), low-density lipoproteins (LDL), high-density lipoproteins (HDL)) based on their size, lipid composition and the type of protein they contain. The proteins embedded in the lipoproteins have a stabilizing function and are recognized by specific receptors in the liver and peripheral tissues. In the exogenous pathway, dietary fat in the small intestine is dispersed into small droplets by bile acids and broken down into fatty acids and glycerol. Once in the enterocyte (cell lining the small intestine), the fatty acids are synthesized into triglycerides again, and packaged into lipoproteins called chylomicrons together with a small amount of absorbed cholesterol, which has been converted into its ester form. Each chylomicron contains several different apoproteins (apoB-48, apoA-I, apoA-II) and acquires apoC-II and apoE (2). The chylomicrons pass via the lymphatic system and blood capillaries to muscle and adipose tissue. Here the enzyme lipoprotein lipase, on the surface of endothelial cells, breaks down most of the triglycerides into glycerol and fatty acids. These molecules are taken up by the peripheral tissues and either used as an energy source or stored. The remnant chylomicrons which are depleted in triglycerides but still contain the bulk of their cholesterol ester pass to the liver and, following binding of apoE to the LDL receptor on hepatocytes, the entire particle undergoes endocytosis, resulting in cholesterol being taken up by the liver. From here the cholesterol may be stored, converted into bile acids, secreted directly in bile or may enter the endogenous pathway.

In the endogenous pathway, the liver produces VLDL particles with newly synthesized triglyceride and a small amount of cholesterol ester. These particles deliver glycerol and fatty acids to peripheral tissues, as described above for chylomicrons. Removal of the triglyceride fraction from the particles, while retaining the cholesterol component, results in their conversion into intermediate density particles and ultimately LDL particles, laden with cholesterol ester. These LDL particles are the main carrier of cholesterol to cells for incorporation into membranes and steroid synthesis, but also play a key role in development of atherosclerosis by depositing lipid in the wall of blood vessels. The surface of the LDL particle contains apoB-100 which is a ligand (i.e. binds) for the LDL receptor located on pits on the surface of the hepatocyte. Apo-B-100 binding to the LDL receptor results in internalization of the particle and its removal from plasma. The cholesterol content of the liver cells in turn regulates the levels of LDL receptors and other key genes involved in cholesterol and fatty acid metabolism in order to maintain a balance. The genes that are regulated include the enzyme HMG CoA reductase which is the rate-limiting enzyme in the multistep cholesterol synthesis pathway (Figure 1). The levels of LDL receptor are also regulated by the secreted proprotein convertase subtilisin/kexin type 9 (PCSK9) which binds to the receptor and enhances its degradation in lysosomes (3). Cholesterol can return to plasma from tissues in HDL particles. HDL particles take up cholesterol, converting it into its ester form in the process, and from here it is transported away from the periphery to the liver. This may occur indirectly via transfer to VLDL particles or directly by a process involving the scavenger receptor B1 in hepatocytes which selectively takes up HDL cholesterol.

Atherosclerosis involves damage to, or dysfunction of, the endothelial cells that form the inner lining of blood vessels, resulting in entry of LDL particles into the vessel wall (Figure 2). Lipids and proteins of the LDL particle undergo oxidation by reactive oxygen species (e.g. superoxide, O2−), generated via oxidative stress, to form oxidised LDL (oxLDL). OxLDL molecules participate in atherosclerotic plaque formation in several ways. They activate endothelial cells, promoting movement of monocytes and T cells into the vessel wall. Also the oxLDL is taken up by macrophages via ‘scavenger’ receptors resulting in conversion of the macrophages into lipid-rich foam cells. Accumulation of these cells give rise to the appearance of ‘fatty streaks’ within the endothelium. Various pro-inflammatory mediators are produced during this process which stimulate smooth muscle cell proliferation, and migration of these cells into the subendothelial layer. Matrix proteins such as collagen are deposited in large quantities by the smooth muscle cells leading to formation of a dense fibrous cap overlying the lipid-rich core. The plaque may partially block the lumen of the blood vessel or eventually rupture leading to formation of a thrombus as blood platelets adhere to the exposed subendothelial collagen.

Risk factors -15 points

Population studies have identified a major role for the type and amount of dietary fat in determining serum cholesterol, and established a strong correlation between total plasma cholesterol, in particular high LDL cholesterol, and coronary heart disease. While high LDL cholesterol, which makes up approximately 70% of total cholesterol, is associated with disease, HDL cholesterol levels are inversely correlated with disease. One of the earliest population studies, started more than 50 years ago, revealed that plasma cholesterol and deaths from coronary heart disease were substantially lower in southern Europe and Japan, while rates in North America and northern Europe were higher (3). The differences were strongly associated with levels of saturated fat consumption and have led to recognition of the healthy Mediterranean diet.

Treatment- 15 points

There are several drugs that are used clinically to lower lipid levels and reduce the risk of cardiovascular disease. Two classes of drugs of note are the ‘statins’ and recently introduced PCSK9 inhibitors. Statins, such as simvastatin and lovastatin, inhibit the rate-limiting enzyme in the multistep cholesterol synthesis pathway which converts HMG-CoA into mevalonate leading to decreased hepatic cholesterol synthesis (Figure 1C). Consequently, there is an increase in hepatic LDL receptor expression and increased clearance of LDL cholesterol from plasma into liver cells, thereby lowering plasma LDL cholesterol levels. PCSK9 inhibitors used clinically are monoclonal antibodies that lower LDL cholesterol levels by inactivating the hepatic protease (PCSK9) that attaches to and internalizes LDL receptors promoting their destruction. These drugs lower plasma LDL cholesterol levels by preventing LDL receptor destruction and are useful for patients who are intolerant to statins or have severely high cholesterol levels (4). Although oxLDL plays a well-established role in the process of atherosclerosis, clinical trials of antioxidant molecules, such as vitamin E, for prevention of atherosclerosis and cardiovascular disease have not demonstrated any benefit.

Conclusion- 5 points

Atherosclerosis starts with fatty streaks formation and progresses with atheroma and atherosclerotic plaque formation. Hypercholesterolemia, LDL increase, HDL decrease, lipid oxidation, hypertension, mal production and dysfunction of NO, and inflammation are the most facilitating factors for atherosclerosis (5). Lipid oxidation, in the form of Ox-LDL, demonstrates the first step of atherosclerosis. MDA shows lipid peroxidation level and is a marker of increased oxidative stress. CRP is an indicative marker of body's response to inflammatory processes. It is one of the most important pathogenesis factors along with fibrinogen in atherogenic processes. Nitric oxide is known as a vasodilator and endothelial survival factor which enhances the endothelial cell proliferation and migration (6). In special pathologic conditions such as severe hypercholesterolemia, per oxynitrate concentration increases, which leads to severe atherosclerotic damage. Considering the role of oxidative stress and lipid oxidation in formation and progress of atherosclerosis and endothelial damage, using antioxidants, especially herbal types can be beneficial.

References- 15 points

References should be cited by number only and in order of appearance.

The example formats provided below should be followed.

Examples:

1. Mukherjee, A. K., Sharma, S., Bagri, S., Kutum, R., Kumar, P., Hussain, A., Singh, P., Saha, D., Kar, A., Dash, D., and Chowdhury, S. (2019) Telomere repeat-binding factor 2 binds extensively to extra-telomeric G-quadruplexes and regulates the epigenetic status of several gene promoters. J. Biol. Chem. 294, 17709-17722

2. Sambrook, J., Fritsch, E. F., and Maniatis, T. (1989) Molecular Cloning: A Laboratory Manual, 2nd Ed., Cold Spring Harbor Laboratory, Cold Spring Harbor, NY

3. Ramachandran, A., Summerville, L., Learn, B. A., DeBell, L., and Bailey, S. (December 30, 2019) Processing and integration of functionally oriented prespacers in the E. coli CRISPR system depends on bacterial host exonucleases. J. Biol. Chem. 10.1074/jbc.RA119.012196

4. Farrell, C. (1992) The Role of SecB During Protein Export in Escherichia coli. Ph.D. thesis, The Johns Hopkins University

5. [dataset] Archer, C. R., Enslow, B. T., Taylor, A. B., De la Rosa, V., Bhattacharya, A., Shapiro, M. S. (2019) Crystal structure of the Ca2+/CaM complex with independent peptides of Kv7.4 (KCNQ4) A & B domains. Protein Data Bank. 6N5W.

6. [preprint] Chen, J. J., Nathaniel, D. L., Raghavan, P., Nelson, M., Tian, R., Tse, E., Hong, J. Y., See, S. K., Mok, S. A., Southworth, D. R., Grinberg, L. T., Gestwicki, J. E., Leonetti, M. D., Kampmann, M. (2019) Compromised function of an ESCRT complex promotes endolysosomal escape of tau seeds and propagation of tau aggregation. bioRxiv. 10.1101/637785

DOIs should be added for any articles that do not have designated volume and page numbers (such as JBC's Papers in Press; see example 3).

Previously deposited/published datasets should be provided as a reference along with the article describing the dataset. The data citation should be formatted using the general format (see example 5): [dataset] Creator(s)/Author(s). (Publication Year) Title. Repository. Version (if applicable), Global Persistent Identifier.

Preprints may be cited in the reference list of the article (see example 6).

All references should be included in this section.