Key Event Title
|Level of Biological Organization|
Key Event Components
|cholesterol biosynthetic process||cholesterol||decreased|
|cholesterol transport||cholesteryl ester||decreased|
Key Event Overview
AOPs Including This Key Event
|AOP Name||Role of event in AOP|
|HMGCR inhibition to male fertility||KeyEvent|
|PPARa Agonism Impairs Fish Reproduction||KeyEvent|
|All life stages||Moderate|
Key Event Description
Most cholesterol synthesis in vertebrates occurs within the endoplasmic reticulum of hepatic cells. First, acetyl-CoA is converted to HMG-CoA via HMG-CoA synthase. Next, HMG-CoA is converted to mevalonate via HMG-CoA reductase. Several other steps follow, but this is the rate-limiting step of cholesterol synthesis. Statin drugs inhibit HMG-CoA reductase to reduce cholesterol. Cholesterol synthesis may also occur to a limited extent in steroidogenic cells where it’s used to produce steroid hormones (Azhar et al., 2007)
Once cholesterol is produced in the liver, it’s transported in the plasma. Hydrophobic lipids like cholesterol, cholesteryl ester (a cholesterol molecule bound to a fatty acid), and triglycerides are transported via lipoprotein complexes. There are different groups of lipoproteins which use different proteins and ratios of lipids including high-density lipoprotein (HDL), low-density (LDL), and very low-density (VLDL).
How It Is Measured or Detected
Commerical assay kits are available for measuring cholesterol using either colorimetric or fluorometric detection. Total cholesterol assay kits often include cholesteryl esters in the measurement (Cell Bio Labs, ThermoFisher). Additional kits are availalbe for measuring the cholesterol in the different lipoprotein complexes (Cell Bio Labs).
Oil Red O staining can be used for organisms such as zebrafish larvae that are clear, however it stains triglycerides and lipids not just cholesterol (Zhou et al., 2015).
Domain of Applicability
Taxonomic Applicability: Cholesterol is synthesized in plants but acts as a precursor for different products than in animals (Sonawane et al. 2016). Within the animal kingdom most deuterostomes (including vertebrata, cyclostomata, cephalochordate, and echinodermata, but not chordata) possess the genes necessary for cholesterol biosynthesis. However, most protostomes (including arthropoda and nematomorpha) have lost these genes (Zhang et al., 2019). Thus far vertebrates are the primary consideration for this KE.
Lifestage Applicability: Cholesterol can be measured in organisms at all life stages. However, the size of young organisms may limit the ability to collect plasma for cholesterol analysis. Whole-body measurements or pooled samples may be more feasible.
Sex Applicability: Cholesterol measurements are applicable for all sexes
Evidence for Perturbation by Stressor
Juvenile female rainbow trout have decreased cholesterol (including total, HDL, LDL, & VLDL) after exposure to gemfibrozil (Prindiville et al. 2011)
Male and female zebrafish fed gemfibrozil alone or in combination with atorvastatin have decreased cholesterol (Al-Habsi et al. 2016)
Adult male zebrafish fed bezafibrate have decreased cholesterol (Velasco-Santamaría et al. 2011)
Feeding grass carp either a high-fat or high-carbohydrate diet causes increases in total cholesterol, HDL, and LDL. Clofibrate reduces the high cholesterol levels caused by these diets to levels similar to controls (Guo et al. 2015)
Feeding fenofibrate to grass carp on a high fat diet causes a decrease in cholesterol, LDL, body weight, and whole-body lipid content (Du et al. 2008)
Male and female zebrafish fed atorvastatin alone or in combination with gemfibrozil have decreased whole-body cholesterol (Al-Habsi et al., 2016)
Atorvastatin is a statin drug that lowers cholesterol by inhibiting HMG-CoA reductase. Other chemical that work by the same mechanism can be found at: https://comptox.epa.gov/dashboard/chemical_lists/STATINS
Larval Zebrafish fed a high fat and high cholesterol diet show reduced liver cholesterol when given simvastatin (Dai et al., 2015)
Al-Habsi, A.A., A. Massarsky, T.W. Moon (2016) “Exposure to gemfibrozil and atorvastatin affects cholesterol metabolism and steroid production in zebrafish (Danio rerio)”, Comparative Biochemistry and Physiology, Part B, Vol. 199, Elsevier, pp. 87-96. http://dx.doi.org/10.1016/j.cbpb.2015.11.009
Azhar, S., E. Reaven (2007) “Regulation of Leydig cell cholesterol metabolism”, in A.H. Payne, M.P. Hardy (eds.) The Leydig Cell in Health and Disease, Humana Press. https://doi.org/10.1007/978-1-59745-453-7
Dai, W. et al. (2015) "High fat plus high cholesterol diet lead to hepatic steatosis in zebrafish larvae: a novel model for screening anti-hepatic steatosis drugs", Nutrition and Metabolism, Vol. 12(42), Springer Nature. DOI 10.1186/s12986-015-0036-z
Du, Z.Y. et al. (2008) “Hypolipidaemic effect of fenofibrate and fasting in the herbivorous grass carp (Ctenopharyngodon idella) fed a high-fat diet”, British Journal of Nutrition, Vol. 100, Cambridge University Press, pp. 1200-1212. doi:10.1017/S0007114508986840
Guo, X. et al. (2015) “Effects of lipid-lowering pharmaceutical clofibrate on lipid and lipoprotein metabolism of grass carp (Ctenopharyngodon idellal Val.) fed with the high non-protein energy diets”, Fish Physiology and Biochemistry, Vol. 41, Springer, pp. 331-343. doi: 10.1007/s10695-014-9986-8
Prindiville, J.S. et al. (2011) “The fibrate drug gemfibrozil disrupts lipoprotein metabolism in rainbow trout”, Toxicology and Applied Pharmacology, Vol. 251, Elsevier, pp. 201-238. doi:10.1016/j.taap.2010.12.013
Sonawane, P.D. et al. (2016) “Plant cholesterol biosynthetic pathway overlaps with phytosterol metabolism”, Nature Plants, Vol. 3, Nature Publishing Group, https://doi.org/10.1038/nplants.2016.205
Velasco-Santamaría, Y.M. et al. (2011) “Bezafibrate, a lipid-lowering pharmaceutical, as a potential endocrine disruptor in male zebrafish (Danio rerio)”, Aquatic Toxicology, Vol. 105, Elsevier, pp. 107-118. doi:10.1016/j.aquatox.2011.05.018
Zhang, T. et al. (2019) “Evolution of the cholesterol biosynthesis pathway in animals”, Molecular Biology and Evolution, Vol. 36(11), Oxford University Press, pp. 2548-2556. doi:10.1093/molbev/msz167
Zhou, J. et al. (2015) "Rapid analysis of hypolipidemic drugs in a live zebrafish assay", Journal of Pharmacological and Toxicological Methods, Vol. 72, Elsevier, pp. 47-52. http://dx.doi.org/10.1016/j.vascn.2014.12.002