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Cholesterol Metabolism

Cholesterol is an important lipid molecule that is used for many biologic functions. Cholesterol can either be synthesized from endogenous acetyl-CoA Acetyl-CoA Acetyl CoA participates in the biosynthesis of fatty acids and sterols, in the oxidation of fatty acids and in the metabolism of many amino acids. It also acts as a biological acetylating agent. Citric Acid Cycle or absorbed from food in the GI tract. Because cholesterol is lipophilic, it must be transported through the bloodstream via lipoproteins Lipoproteins Lipid-protein complexes involved in the transportation and metabolism of lipids in the body. They are spherical particles consisting of a hydrophobic core of triglycerides and cholesterol esters surrounded by a layer of hydrophilic free cholesterol; phospholipids; and apolipoproteins. Lipoproteins are classified by their varying buoyant density and sizes. Lipid Metabolism, where it can be picked up by hepatocytes Hepatocytes The main structural component of the liver. They are specialized epithelial cells that are organized into interconnected plates called lobules. Liver: Anatomy or peripheral tissues. There, cholesterol can be stored, used in cellular membranes, or used as a precursor for steroid hormones Steroid hormones Steroid hormones produced by the gonads. They stimulate reproductive organs, germ cell maturation, and the secondary sex characteristics in the males and the females. The major sex steroid hormones include estradiol; progesterone; and testosterone. Hormones: Overview and Types. The human body cannot degrade cholesterol’s ring structure, so the only mechanism for potential excretion is through the production of bile Bile An emulsifying agent produced in the liver and secreted into the duodenum. Its composition includes bile acids and salts; cholesterol; and electrolytes. It aids digestion of fats in the duodenum. Gallbladder and Biliary Tract: Anatomy acids Acids Chemical compounds which yield hydrogen ions or protons when dissolved in water, whose hydrogen can be replaced by metals or basic radicals, or which react with bases to form salts and water (neutralization). An extension of the term includes substances dissolved in media other than water. Acid-Base Balance.

Last updated: Sep 1, 2022

Editorial responsibility: Stanley Oiseth, Lindsay Jones, Evelin Maza

Synthesis

Mevalonate pathway

Cholesterol’s synthesis Synthesis Polymerase Chain Reaction (PCR) pathway overlaps with the synthesis Synthesis Polymerase Chain Reaction (PCR) of ketone bodies Ketone bodies The metabolic substances acetone; 3-hydroxybutyric acid; and acetoacetic acid (acetoacetates). They are produced in the liver and kidney during fatty acids oxidation and used as a source of energy by the heart, muscle and brain. Ketone Body Metabolism. The steps of this process include: 

  • Thiolase Thiolase Fatty Acid Metabolism catalyzes the joining of 2 acetyl-coenzyme A (CoA; precursor) molecules → acetoacetyl-CoA
  • 3-hydroxy-3-methyl-glutaryl-CoA (HMG-CoA) synthase adds another acetyl-CoA Acetyl-CoA Acetyl CoA participates in the biosynthesis of fatty acids and sterols, in the oxidation of fatty acids and in the metabolism of many amino acids. It also acts as a biological acetylating agent. Citric Acid Cycle → β-hydroxy-β-methylglutaryl-CoA (HMG-CoA)
  • HMG-CoA reductase → mevalonate:
    • Critical role in cholesterol synthesis Synthesis Polymerase Chain Reaction (PCR)
    • Only regulated enzyme step in the process (feedback inhibition by cholesterol)
    • Target for cholesterol-lowering medications ( statins Statins Statins are competitive inhibitors of HMG-CoA reductase in the liver. HMG-CoA reductase is the rate-limiting step in cholesterol synthesis. Inhibition results in lowered intrahepatocytic cholesterol formation, resulting in up-regulation of LDL receptors and, ultimately, lowering levels of serum LDL and triglycerides. Statins)
  • Mevalonate → isoprenoid precursors (by decarboxylating mevalonate): 
    • Isopentenyl pyrophosphate (IPP) 
    • Dimethylallyl pyrophosphate (DMAPP)
  • IPP and DMAPP are combined → geranyl pyrophosphate (geranyl-PP)
  • Another IPP is added → farnesyl pyrophosphate (farnesyl-PP)
  • 2 farnesyl-PP molecules are joined → squalene
  • Squalene is modified → lanosterol (Note: Acetyl-CoA Acetyl-CoA Acetyl CoA participates in the biosynthesis of fatty acids and sterols, in the oxidation of fatty acids and in the metabolism of many amino acids. It also acts as a biological acetylating agent. Citric Acid Cycle is the exclusive building block up to this point.)
  • Lanosterol → cholesterol
    • 19 steps
    • Requires a great deal of energy

Importance

Cholesterol is a:

  • Precursor to:
    • Bile Bile An emulsifying agent produced in the liver and secreted into the duodenum. Its composition includes bile acids and salts; cholesterol; and electrolytes. It aids digestion of fats in the duodenum. Gallbladder and Biliary Tract: Anatomy acids Acids Chemical compounds which yield hydrogen ions or protons when dissolved in water, whose hydrogen can be replaced by metals or basic radicals, or which react with bases to form salts and water (neutralization). An extension of the term includes substances dissolved in media other than water. Acid-Base Balance
    • Steroid hormones Steroid hormones Steroid hormones produced by the gonads. They stimulate reproductive organs, germ cell maturation, and the secondary sex characteristics in the males and the females. The major sex steroid hormones include estradiol; progesterone; and testosterone. Hormones: Overview and Types
    • Vitamin D Vitamin D A vitamin that includes both cholecalciferols and ergocalciferols, which have the common effect of preventing or curing rickets in animals. It can also be viewed as a hormone since it can be formed in skin by action of ultraviolet rays upon the precursors, 7-dehydrocholesterol and ergosterol, and acts on vitamin D receptors to regulate calcium in opposition to parathyroid hormone. Fat-soluble Vitamins and their Deficiencies
  • Component of cell membranes

Transport and Uptake

Basics of lipoproteins Lipoproteins Lipid-protein complexes involved in the transportation and metabolism of lipids in the body. They are spherical particles consisting of a hydrophobic core of triglycerides and cholesterol esters surrounded by a layer of hydrophilic free cholesterol; phospholipids; and apolipoproteins. Lipoproteins are classified by their varying buoyant density and sizes. Lipid Metabolism

  • Function: transport of different lipids Lipids Lipids are a diverse group of hydrophobic organic molecules, which include fats, oils, sterols, and waxes. Fatty Acids and Lipids between target cells
    • Cholesterol
    • Fatty acids Acids Chemical compounds which yield hydrogen ions or protons when dissolved in water, whose hydrogen can be replaced by metals or basic radicals, or which react with bases to form salts and water (neutralization). An extension of the term includes substances dissolved in media other than water. Acid-Base Balance
    • Triacylglycerol (triglyceride)
  • Composition:
    • Triglyceride
    • Cholesterol
    • Cholesterol esters Cholesterol esters Fatty acid esters of cholesterol which constitute about two-thirds of the cholesterol in the plasma. The accumulation of cholesterol esters in the arterial intima is a characteristic feature of atherosclerosis. Digestion and Absorption
    • Phospholipids Phospholipids Lipids containing one or more phosphate groups, particularly those derived from either glycerol (phosphoglycerides) or sphingosine (sphingolipids). They are polar lipids that are of great importance for the structure and function of cell membranes and are the most abundant of membrane lipids, although not stored in large amounts in the system. Lipid Metabolism
    • Proteins Proteins Linear polypeptides that are synthesized on ribosomes and may be further modified, crosslinked, cleaved, or assembled into complex proteins with several subunits. The specific sequence of amino acids determines the shape the polypeptide will take, during protein folding, and the function of the protein. Energy Homeostasis (apoproteins (apo) A, B, C, and E)
  • Types of lipoproteins Lipoproteins Lipid-protein complexes involved in the transportation and metabolism of lipids in the body. They are spherical particles consisting of a hydrophobic core of triglycerides and cholesterol esters surrounded by a layer of hydrophilic free cholesterol; phospholipids; and apolipoproteins. Lipoproteins are classified by their varying buoyant density and sizes. Lipid Metabolism (in order of density):
    • Chylomicrons
    • VLDL
    • IDL
    • LDL
    • HDL
  • Lipoprotein density:
    • Chylomicrons are the least dense.
      • ↑ Triglyceride content
      • ↓ Protein content
    • HDL is the densest.
      • ↓ Triglyceride content
      • ↑ Protein content

Composition of lipoproteins Lipoproteins Lipid-protein complexes involved in the transportation and metabolism of lipids in the body. They are spherical particles consisting of a hydrophobic core of triglycerides and cholesterol esters surrounded by a layer of hydrophilic free cholesterol; phospholipids; and apolipoproteins. Lipoproteins are classified by their varying buoyant density and sizes. Lipid Metabolism

Table: Composition of lipoproteins Lipoproteins Lipid-protein complexes involved in the transportation and metabolism of lipids in the body. They are spherical particles consisting of a hydrophobic core of triglycerides and cholesterol esters surrounded by a layer of hydrophilic free cholesterol; phospholipids; and apolipoproteins. Lipoproteins are classified by their varying buoyant density and sizes. Lipid Metabolism
Lipoprotein Cholesterol/
cholesterol ester Cholesterol Ester Fatty acid esters of cholesterol which constitute about two-thirds of the cholesterol in the plasma. The accumulation of cholesterol esters in the arterial intima is a characteristic feature of atherosclerosis. Lipid Metabolism
Triglyceride Protein Phospholipid
Chylomicrons 1%/3% 85% 2% 8%
VLDL 7%/10% 55% 9% 20%
IDL 8%/30% 26% 11% 22%
LDL 10%/35% 10% 20% 20%
HDL 4%/12% 5% 45% 25%

Transport

  • Emulsification Emulsification Gastrointestinal Secretions of cholesterol with bile salts Bile salts Steroid acids and salts. The primary bile acids are derived from cholesterol in the liver and usually conjugated with glycine or taurine. The secondary bile acids are further modified by bacteria in the intestine. They play an important role in the digestion and absorption of fat. They have also been used pharmacologically, especially in the treatment of gallstones. Cholelithiasis in the intestine → emulsion travels through the intestinal wall 
  • Cholesterol and triglycerides Triglycerides Fatty Acids and Lipids are packaged as chylomicrons (synthesized in intestinal epithelial cells):
    • Travel to the lymphatic circulation Circulation The movement of the blood as it is pumped through the cardiovascular system. ABCDE Assessment → bloodstream → capillaries Capillaries Capillaries are the primary structures in the circulatory system that allow the exchange of gas, nutrients, and other materials between the blood and the extracellular fluid (ECF). Capillaries are the smallest of the blood vessels. Because a capillary diameter is so small, only 1 RBC may pass through at a time. Capillaries: Histology
    • Chylomicrons lose some of their contents → chylomicron Chylomicron A class of lipoproteins that carry dietary cholesterol and triglycerides from the small intestine to the tissues. Their density (0. 93-1. 006 g/ml) is the same as that of very-low-density lipoproteins. Lipid Metabolism remnants
      • Apo C activates lipoprotein lipase Lipoprotein lipase An enzyme of the hydrolase class that catalyzes the reaction of triacylglycerol and water to yield diacylglycerol and a fatty acid anion. The enzyme hydrolyzes triacylglycerols in chylomicrons, very-low-density lipoproteins, low-density lipoproteins, and diacylglycerols. It occurs on capillary endothelial surfaces, especially in mammary, muscle, and adipose tissue. Lipid Metabolism (LPL) → triglycerides Triglycerides Fatty Acids and Lipids are digested to fatty acids Acids Chemical compounds which yield hydrogen ions or protons when dissolved in water, whose hydrogen can be replaced by metals or basic radicals, or which react with bases to form salts and water (neutralization). An extension of the term includes substances dissolved in media other than water. Acid-Base Balance
      • Most commonly occurs in muscle and adipose tissue Adipose tissue Adipose tissue is a specialized type of connective tissue that has both structural and highly complex metabolic functions, including energy storage, glucose homeostasis, and a multitude of endocrine capabilities. There are three types of adipose tissue, white adipose tissue, brown adipose tissue, and beige or “brite” adipose tissue, which is a transitional form. Adipose Tissue: Histology
  • Chylomicron Chylomicron A class of lipoproteins that carry dietary cholesterol and triglycerides from the small intestine to the tissues. Their density (0. 93-1. 006 g/ml) is the same as that of very-low-density lipoproteins. Lipid Metabolism remnants travel to the liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver: Anatomy → contents are stored 
  • Liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver: Anatomy can package and release Release Release of a virus from the host cell following virus assembly and maturation. Egress can occur by host cell lysis, exocytosis, or budding through the plasma membrane. Virology these contents as VLDL into the bloodstream: 
    • Travels to peripheral tissue
    • VLDL loses some of its contents (again, via LPL) → converted to IDL
  • IDL can either: 
    • Travel to the liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver: Anatomy → degraded
    • Convert to LDL in peripheral circulation Circulation The movement of the blood as it is pumped through the cardiovascular system. ABCDE Assessment
  • LDL taken up by various cells → digested → cholesterol released
  • HDL produced by the liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver: Anatomy
    • Scavenges lipid components, especially cholesterol, from:
      • Cell membranes
      • Other lipoproteins Lipoproteins Lipid-protein complexes involved in the transportation and metabolism of lipids in the body. They are spherical particles consisting of a hydrophobic core of triglycerides and cholesterol esters surrounded by a layer of hydrophilic free cholesterol; phospholipids; and apolipoproteins. Lipoproteins are classified by their varying buoyant density and sizes. Lipid Metabolism
    • Lecithin Lecithin A complex mixture of phospholipids; glycolipids; and triglycerides; with substantial amounts of phosphatidylcholines; phosphatidylethanolamines; and phosphatidylinositols, which are sometimes loosely termed as 1, 2-diacyl-3-phosphocholines. Lecithin is a component of the cell membrane and commercially extracted from soybeans and egg yolk. The emulsifying and surfactant properties are useful in food additives and for forming organogels (gels). Fatty Acids and Lipids:cholesterol acetyltransferase (LCAT) converts cholesterol → cholesterol esters Cholesterol esters Fatty acid esters of cholesterol which constitute about two-thirds of the cholesterol in the plasma. The accumulation of cholesterol esters in the arterial intima is a characteristic feature of atherosclerosis. Digestion and Absorption
    • Cholesterol esters Cholesterol esters Fatty acid esters of cholesterol which constitute about two-thirds of the cholesterol in the plasma. The accumulation of cholesterol esters in the arterial intima is a characteristic feature of atherosclerosis. Digestion and Absorption are transferred to other lipoproteins Lipoproteins Lipid-protein complexes involved in the transportation and metabolism of lipids in the body. They are spherical particles consisting of a hydrophobic core of triglycerides and cholesterol esters surrounded by a layer of hydrophilic free cholesterol; phospholipids; and apolipoproteins. Lipoproteins are classified by their varying buoyant density and sizes. Lipid Metabolism → transported to the liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver: Anatomy to either be:
      • Packaged with VLDL
      • Converted to bile Bile An emulsifying agent produced in the liver and secreted into the duodenum. Its composition includes bile acids and salts; cholesterol; and electrolytes. It aids digestion of fats in the duodenum. Gallbladder and Biliary Tract: Anatomy acids Acids Chemical compounds which yield hydrogen ions or protons when dissolved in water, whose hydrogen can be replaced by metals or basic radicals, or which react with bases to form salts and water (neutralization). An extension of the term includes substances dissolved in media other than water. Acid-Base Balance → excreted
Movement of cholesterol throughout the body

Movement of cholesterol throughout the body:
Movement in the blood is highlighted in green.
FFA FFA Macular Degeneration: free fatty acid

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Cellular uptake

Mechanism:

  • LDL binds to cell surface receptor Receptor Receptors are proteins located either on the surface of or within a cell that can bind to signaling molecules known as ligands (e.g., hormones) and cause some type of response within the cell. Receptors
  • Clathrin-mediated endocytosis Endocytosis Cellular uptake of extracellular materials within membrane-limited vacuoles or microvesicles. Endosomes play a central role in endocytosis. The Cell: Cell Membrane → brings cholesterol into cells
  • Low pH pH The quantitative measurement of the acidity or basicity of a solution. Acid-Base Balance in endosome-lysosomes → LDL released from receptor Receptor Receptors are proteins located either on the surface of or within a cell that can bind to signaling molecules known as ligands (e.g., hormones) and cause some type of response within the cell. Receptors
  • LDL receptor Receptor Receptors are proteins located either on the surface of or within a cell that can bind to signaling molecules known as ligands (e.g., hormones) and cause some type of response within the cell. Receptors is recycled → returns to the cell surface
  • LDL is broken down by proteases Proteases Proteins and Peptides and lipases Lipases An enzyme of the hydrolase class that catalyzes the reaction of triacylglycerol and water to yield diacylglycerol and a fatty acid anion. It is produced by glands on the tongue and by the pancreas and initiates the digestion of dietary fats. Lipid Metabolism.
  • Cholesterol and cholesterol esterases Esterases Macrolides and Ketolides can be:
    • Used for:
      • Bile Bile An emulsifying agent produced in the liver and secreted into the duodenum. Its composition includes bile acids and salts; cholesterol; and electrolytes. It aids digestion of fats in the duodenum. Gallbladder and Biliary Tract: Anatomy acids Acids Chemical compounds which yield hydrogen ions or protons when dissolved in water, whose hydrogen can be replaced by metals or basic radicals, or which react with bases to form salts and water (neutralization). An extension of the term includes substances dissolved in media other than water. Acid-Base Balance
      • Steroids Steroids A group of polycyclic compounds closely related biochemically to terpenes. They include cholesterol, numerous hormones, precursors of certain vitamins, bile acids, alcohols (sterols), and certain natural drugs and poisons. Steroids have a common nucleus, a fused, reduced 17-carbon atom ring system, cyclopentanoperhydrophenanthrene. Most steroids also have two methyl groups and an aliphatic side-chain attached to the nucleus. Benign Liver Tumors
    • Stored

Regulation:

  • Within the cell, cholesterol inhibits:
    • LDL receptor Receptor Receptors are proteins located either on the surface of or within a cell that can bind to signaling molecules known as ligands (e.g., hormones) and cause some type of response within the cell. Receptors expression → further ↓ LDL uptake by the cell
    • HMG-CoA reductase → ↓ cholesterol synthesis Synthesis Polymerase Chain Reaction (PCR)
  • Proprotein convertase subtilisin/kexin type 9 (PCSK9) enzyme:
    • Secreted by the liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver: Anatomy
    • Binds to LDL receptors Receptors Receptors are proteins located either on the surface of or within a cell that can bind to signaling molecules known as ligands (e.g., hormones) and cause some type of response within the cell. Receptors → promotes LDL receptor Receptor Receptors are proteins located either on the surface of or within a cell that can bind to signaling molecules known as ligands (e.g., hormones) and cause some type of response within the cell. Receptors endocytosis Endocytosis Cellular uptake of extracellular materials within membrane-limited vacuoles or microvesicles. Endosomes play a central role in endocytosis. The Cell: Cell Membrane and degradation

Excretion

  • The body cannot degrade cholesterol’s ring structure → must be excreted as bile Bile An emulsifying agent produced in the liver and secreted into the duodenum. Its composition includes bile acids and salts; cholesterol; and electrolytes. It aids digestion of fats in the duodenum. Gallbladder and Biliary Tract: Anatomy acids Acids Chemical compounds which yield hydrogen ions or protons when dissolved in water, whose hydrogen can be replaced by metals or basic radicals, or which react with bases to form salts and water (neutralization). An extension of the term includes substances dissolved in media other than water. Acid-Base Balance in the feces
  • Primary bile Bile An emulsifying agent produced in the liver and secreted into the duodenum. Its composition includes bile acids and salts; cholesterol; and electrolytes. It aids digestion of fats in the duodenum. Gallbladder and Biliary Tract: Anatomy acids Acids Chemical compounds which yield hydrogen ions or protons when dissolved in water, whose hydrogen can be replaced by metals or basic radicals, or which react with bases to form salts and water (neutralization). An extension of the term includes substances dissolved in media other than water. Acid-Base Balance (cholic acid, deoxycholic acid) are synthesized from cholesterol in the liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver: Anatomy
    • 7-alpha-hydroxylase converts cholesterol → 7-alpha-hydroxylcholesterol
    • Multiple steps are required to produce:
      • Cholic acid
      • Chenodeoxycholic acid
    • Can then be conjugated with either:
      • Glycine Glycine A non-essential amino acid. It is found primarily in gelatin and silk fibroin and used therapeutically as a nutrient. It is also a fast inhibitory neurotransmitter. Synthesis of Nonessential Amino Acids → glycocholic acid or glycochenocholic acid
      • Taurine → taurocholic acid or taurochenocholic acid
  • Stored in the gallbladder Gallbladder The gallbladder is a pear-shaped sac, located directly beneath the liver, that sits on top of the superior part of the duodenum. The primary functions of the gallbladder include concentrating and storing up to 50 mL of bile. Gallbladder and Biliary Tract: Anatomy
  • Secreted into the intestinal lumen → aid in digestion Digestion Digestion refers to the process of the mechanical and chemical breakdown of food into smaller particles, which can then be absorbed and utilized by the body. Digestion and Absorption and absorption Absorption Absorption involves the uptake of nutrient molecules and their transfer from the lumen of the GI tract across the enterocytes and into the interstitial space, where they can be taken up in the venous or lymphatic circulation. Digestion and Absorption of lipids Lipids Lipids are a diverse group of hydrophobic organic molecules, which include fats, oils, sterols, and waxes. Fatty Acids and Lipids
    • Most are reabsorbed in the ileum Ileum The distal and narrowest portion of the small intestine, between the jejunum and the ileocecal valve of the large intestine. Small Intestine: Anatomy → returned to the liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver: Anatomy through the enterohepatic circulation Enterohepatic Circulation Recycling through liver by excretion in bile, reabsorption from intestines (intestinal reabsorption) into portal circulation, passage back into liver, and re-excretion in bile. Pharmacokinetics and Pharmacodynamics → inhibit further bile Bile An emulsifying agent produced in the liver and secreted into the duodenum. Its composition includes bile acids and salts; cholesterol; and electrolytes. It aids digestion of fats in the duodenum. Gallbladder and Biliary Tract: Anatomy acid synthesis Synthesis Polymerase Chain Reaction (PCR)
    • Some are modified by bacteria Bacteria Bacteria are prokaryotic single-celled microorganisms that are metabolically active and divide by binary fission. Some of these organisms play a significant role in the pathogenesis of diseases. Bacteriology → secondary bile Bile An emulsifying agent produced in the liver and secreted into the duodenum. Its composition includes bile acids and salts; cholesterol; and electrolytes. It aids digestion of fats in the duodenum. Gallbladder and Biliary Tract: Anatomy acids Acids Chemical compounds which yield hydrogen ions or protons when dissolved in water, whose hydrogen can be replaced by metals or basic radicals, or which react with bases to form salts and water (neutralization). An extension of the term includes substances dissolved in media other than water. Acid-Base Balance → can return to enterohepatic circulation Enterohepatic Circulation Recycling through liver by excretion in bile, reabsorption from intestines (intestinal reabsorption) into portal circulation, passage back into liver, and re-excretion in bile. Pharmacokinetics and Pharmacodynamics
    • Approximately 5% are excreted in the feces
Chemical structures of bile acids

Chemical structures of bile Bile An emulsifying agent produced in the liver and secreted into the duodenum. Its composition includes bile acids and salts; cholesterol; and electrolytes. It aids digestion of fats in the duodenum. Gallbladder and Biliary Tract: Anatomy acids Acids Chemical compounds which yield hydrogen ions or protons when dissolved in water, whose hydrogen can be replaced by metals or basic radicals, or which react with bases to form salts and water (neutralization). An extension of the term includes substances dissolved in media other than water. Acid-Base Balance

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Steroid Hormones

  • Steroid hormones Steroid hormones Steroid hormones produced by the gonads. They stimulate reproductive organs, germ cell maturation, and the secondary sex characteristics in the males and the females. The major sex steroid hormones include estradiol; progesterone; and testosterone. Hormones: Overview and Types are lipophilic compounds synthesized from cholesterol and include:
    • Progestins Progestins Compounds that interact with progesterone receptors in target tissues to bring about the effects similar to those of progesterone. Primary actions of progestins, including natural and synthetic steroids, are on the uterus and the mammary gland in preparation for and in maintenance of pregnancy. Hormonal Contraceptives
    • Estrogens
    • Androgens Androgens Androgens are naturally occurring steroid hormones responsible for development and maintenance of the male sex characteristics, including penile, scrotal, and clitoral growth, development of sexual hair, deepening of the voice, and musculoskeletal growth. Androgens and Antiandrogens
    • Glucocorticoids Glucocorticoids Glucocorticoids are a class within the corticosteroid family. Glucocorticoids are chemically and functionally similar to endogenous cortisol. There are a wide array of indications, which primarily benefit from the antiinflammatory and immunosuppressive effects of this class of drugs. Glucocorticoids
    • Mineralocorticoids Mineralocorticoids Mineralocorticoids are a drug class within the corticosteroid family and fludrocortisone is the primary medication within this class. Fludrocortisone is a fluorinated analog of cortisone. The fluorine moiety protects the drug from isoenzyme inactivation in the kidney, allowing it to exert its mineralocorticoid effect. Mineralocorticoids
  • Synthesized by endocrine glands Endocrine glands Ductless glands that secrete hormones directly into the blood circulation. These hormones influence the metabolism and other functions of cells in the body. Glandular Epithelium: Histology:
    • Gonads Gonads The gamete-producing glands, ovary or testis. Hormones: Overview and Types:
      • Testes Testes Gonadal Hormones
      • Ovaries Ovaries Ovaries are the paired gonads of the female reproductive system that contain haploid gametes known as oocytes. The ovaries are located intraperitoneally in the pelvis, just posterior to the broad ligament, and are connected to the pelvic sidewall and to the uterus by ligaments. These organs function to secrete hormones (estrogen and progesterone) and to produce the female germ cells (oocytes). Ovaries: Anatomy
    • Adrenal glands Adrenal Glands The adrenal glands are a pair of retroperitoneal endocrine glands located above the kidneys. The outer parenchyma is called the adrenal cortex and has 3 distinct zones, each with its own secretory products. Beneath the cortex lies the adrenal medulla, which secretes catecholamines involved in the fight-or-flight response. Adrenal Glands: Anatomy
  • Hormone synthesis Synthesis Polymerase Chain Reaction (PCR):
    • 1st step is conversion of cholesterol → pregnenolone Pregnenolone A 21-carbon steroid, derived from cholesterol and found in steroid hormone-producing tissues. Pregnenolone is the precursor to gonadal steroid hormones and the adrenal corticosteroids. Gonadal Hormones
    • Pregnenolone Pregnenolone A 21-carbon steroid, derived from cholesterol and found in steroid hormone-producing tissues. Pregnenolone is the precursor to gonadal steroid hormones and the adrenal corticosteroids. Gonadal Hormones is then converted → progesterone Progesterone The major progestational steroid that is secreted primarily by the corpus luteum and the placenta. Progesterone acts on the uterus, the mammary glands and the brain. It is required in embryo implantation; pregnancy maintenance, and the development of mammary tissue for milk production. Progesterone, converted from pregnenolone, also serves as an intermediate in the biosynthesis of gonadal steroid hormones and adrenal corticosteroids. Gonadal Hormones
    • Progesterone Progesterone The major progestational steroid that is secreted primarily by the corpus luteum and the placenta. Progesterone acts on the uterus, the mammary glands and the brain. It is required in embryo implantation; pregnancy maintenance, and the development of mammary tissue for milk production. Progesterone, converted from pregnenolone, also serves as an intermediate in the biosynthesis of gonadal steroid hormones and adrenal corticosteroids. Gonadal Hormones is the precursor to other hormones Hormones Hormones are messenger molecules that are synthesized in one part of the body and move through the bloodstream to exert specific regulatory effects on another part of the body. Hormones play critical roles in coordinating cellular activities throughout the body in response to the constant changes in both the internal and external environments. Hormones: Overview and Types.
  • Functions:
    • Sexual maturation and characteristics
    • Metabolism
    • Inflammation Inflammation Inflammation is a complex set of responses to infection and injury involving leukocytes as the principal cellular mediators in the body’s defense against pathogenic organisms. Inflammation is also seen as a response to tissue injury in the process of wound healing. The 5 cardinal signs of inflammation are pain, heat, redness, swelling, and loss of function. Inflammation and immune function
    • Salt/water balance
    • Blood pressure regulation
Table: Steroid hormones Steroid hormones Steroid hormones produced by the gonads. They stimulate reproductive organs, germ cell maturation, and the secondary sex characteristics in the males and the females. The major sex steroid hormones include estradiol; progesterone; and testosterone. Hormones: Overview and Types and their functions
Synthesis Synthesis Polymerase Chain Reaction (PCR)
location
Class Primary hormone Functions
Adrenal glands Adrenal Glands The adrenal glands are a pair of retroperitoneal endocrine glands located above the kidneys. The outer parenchyma is called the adrenal cortex and has 3 distinct zones, each with its own secretory products. Beneath the cortex lies the adrenal medulla, which secretes catecholamines involved in the fight-or-flight response. Adrenal Glands: Anatomy Glucocorticoids Glucocorticoids Glucocorticoids are a class within the corticosteroid family. Glucocorticoids are chemically and functionally similar to endogenous cortisol. There are a wide array of indications, which primarily benefit from the antiinflammatory and immunosuppressive effects of this class of drugs. Glucocorticoids Cortisone Cortisone A naturally occurring glucocorticoid that has been used in replacement therapy for adrenal insufficiency and as an anti-inflammatory agent. Cortisone itself is inactive; it is converted in the liver to the active metabolite hydrocortisone. Glucocorticoids
  • Antiinflammatory
  • Immunosuppressive
  • Regulates stress response
Mineralocorticoids Mineralocorticoids Mineralocorticoids are a drug class within the corticosteroid family and fludrocortisone is the primary medication within this class. Fludrocortisone is a fluorinated analog of cortisone. The fluorine moiety protects the drug from isoenzyme inactivation in the kidney, allowing it to exert its mineralocorticoid effect. Mineralocorticoids Aldosterone Aldosterone A hormone secreted by the adrenal cortex that regulates electrolyte and water balance by increasing the renal retention of sodium and the excretion of potassium. Hyperkalemia
  • Sodium Sodium A member of the alkali group of metals. It has the atomic symbol na, atomic number 11, and atomic weight 23. Hyponatremia/water retention
  • Potassium Potassium An element in the alkali group of metals with an atomic symbol k, atomic number 19, and atomic weight 39. 10. It is the chief cation in the intracellular fluid of muscle and other cells. Potassium ion is a strong electrolyte that plays a significant role in the regulation of fluid volume and maintenance of the water-electrolyte balance. Hyperkalemia excretion
Gonads Gonads The gamete-producing glands, ovary or testis. Hormones: Overview and Types Progestins Progestins Compounds that interact with progesterone receptors in target tissues to bring about the effects similar to those of progesterone. Primary actions of progestins, including natural and synthetic steroids, are on the uterus and the mammary gland in preparation for and in maintenance of pregnancy. Hormonal Contraceptives Progesterone Progesterone The major progestational steroid that is secreted primarily by the corpus luteum and the placenta. Progesterone acts on the uterus, the mammary glands and the brain. It is required in embryo implantation; pregnancy maintenance, and the development of mammary tissue for milk production. Progesterone, converted from pregnenolone, also serves as an intermediate in the biosynthesis of gonadal steroid hormones and adrenal corticosteroids. Gonadal Hormones
  • Stimulates the endometrium Endometrium The mucous membrane lining of the uterine cavity that is hormonally responsive during the menstrual cycle and pregnancy. The endometrium undergoes cyclic changes that characterize menstruation. After successful fertilization, it serves to sustain the developing embryo. Embryoblast and Trophoblast Development during the menstrual cycle Menstrual cycle The menstrual cycle is the cyclic pattern of hormonal and tissular activity that prepares a suitable uterine environment for the fertilization and implantation of an ovum. The menstrual cycle involves both an endometrial and ovarian cycle that are dependent on one another for proper functioning. There are 2 phases of the ovarian cycle and 3 phases of the endometrial cycle. Menstrual Cycle
  • Maintains pregnancy Pregnancy The status during which female mammals carry their developing young (embryos or fetuses) in utero before birth, beginning from fertilization to birth. Pregnancy: Diagnosis, Physiology, and Care
  • Inhibits lactation Lactation The processes of milk secretion by the maternal mammary glands after parturition. The proliferation of the mammary glandular tissue, milk synthesis, and milk expulsion or let down are regulated by the interactions of several hormones including estradiol; progesterone; prolactin; and oxytocin. Breastfeeding during pregnancy Pregnancy The status during which female mammals carry their developing young (embryos or fetuses) in utero before birth, beginning from fertilization to birth. Pregnancy: Diagnosis, Physiology, and Care
Estrogens Estradiol Estradiol The 17-beta-isomer of estradiol, an aromatized C18 steroid with hydroxyl group at 3-beta- and 17-beta-position. Estradiol-17-beta is the most potent form of mammalian estrogenic steroids. Noncontraceptive Estrogen and Progestins Sexual development, differentiation, and maturation of primary and secondary sex Sex The totality of characteristics of reproductive structure, functions, phenotype, and genotype, differentiating the male from the female organism. Gender Dysphoria characteristics
Androgens Androgens Androgens are naturally occurring steroid hormones responsible for development and maintenance of the male sex characteristics, including penile, scrotal, and clitoral growth, development of sexual hair, deepening of the voice, and musculoskeletal growth. Androgens and Antiandrogens Testosterone Testosterone A potent androgenic steroid and major product secreted by the leydig cells of the testis. Its production is stimulated by luteinizing hormone from the pituitary gland. In turn, testosterone exerts feedback control of the pituitary LH and FSH secretion. Depending on the tissues, testosterone can be further converted to dihydrotestosterone or estradiol. Androgens and Antiandrogens

Clinical Relevance

  • Statins Statins Statins are competitive inhibitors of HMG-CoA reductase in the liver. HMG-CoA reductase is the rate-limiting step in cholesterol synthesis. Inhibition results in lowered intrahepatocytic cholesterol formation, resulting in up-regulation of LDL receptors and, ultimately, lowering levels of serum LDL and triglycerides. Statins: competitive inhibitors of HMG-CoA reductase in the liver Liver The liver is the largest gland in the human body. The liver is found in the superior right quadrant of the abdomen and weighs approximately 1.5 kilograms. Its main functions are detoxification, metabolism, nutrient storage (e.g., iron and vitamins), synthesis of coagulation factors, formation of bile, filtration, and storage of blood. Liver: Anatomy. Inhibition results in lowered intrahepatocytic cholesterol formation, resulting in up-regulation Up-Regulation A positive regulatory effect on physiological processes at the molecular, cellular, or systemic level. At the molecular level, the major regulatory sites include membrane receptors, genes (gene expression regulation), mRNAs, and proteins. Pharmacokinetics and Pharmacodynamics of LDL receptors Receptors Receptors are proteins located either on the surface of or within a cell that can bind to signaling molecules known as ligands (e.g., hormones) and cause some type of response within the cell. Receptors and, ultimately, lowering levels of serum LDL and triglycerides Triglycerides Fatty Acids and Lipids. Indications for prescribing include prevention of primary or secondary cardiovascular disease in individuals with dyslipidemia.
  • PCSK9 inhibitors PCSK9 inhibitors Lipid Control Drugs: monoclonal antibodies Antibodies Immunoglobulins (Igs), also known as antibodies, are glycoprotein molecules produced by plasma cells that act in immune responses by recognizing and binding particular antigens. The various Ig classes are IgG (the most abundant), IgM, IgE, IgD, and IgA, which differ in their biologic features, structure, target specificity, and distribution. Immunoglobulins: Types and Functions to PCSK9, which block binding of this enzyme to LDL receptors Receptors Receptors are proteins located either on the surface of or within a cell that can bind to signaling molecules known as ligands (e.g., hormones) and cause some type of response within the cell. Receptors. This binding prevents degradation of the LDL receptors Receptors Receptors are proteins located either on the surface of or within a cell that can bind to signaling molecules known as ligands (e.g., hormones) and cause some type of response within the cell. Receptors, allowing for increased LDL cellular uptake and clearance from the bloodstream. For this reason, PCSK9 inhibitors PCSK9 inhibitors Lipid Control Drugs are prescribed for hyperlipidemia. 
  • Dyslipidemia: commonly occurs as high plasma Plasma The residual portion of blood that is left after removal of blood cells by centrifugation without prior blood coagulation. Transfusion Products cholesterol and/or triglycerides Triglycerides Fatty Acids and Lipids. There are many potential etiologies, including genetics Genetics Genetics is the study of genes and their functions and behaviors. Basic Terms of Genetics, dietary intake, medications, and medical conditions (e.g., diabetes Diabetes Diabetes mellitus (DM) is a metabolic disease characterized by hyperglycemia and dysfunction of the regulation of glucose metabolism by insulin. Type 1 DM is diagnosed mostly in children and young adults as the result of autoimmune destruction of β cells in the pancreas and the resulting lack of insulin. Type 2 DM has a significant association with obesity and is characterized by insulin resistance. Diabetes Mellitus, CKD CKD Chronic kidney disease (CKD) is kidney impairment that lasts for ≥ 3 months, implying that it is irreversible. Hypertension and diabetes are the most common causes; however, there are a multitude of other etiologies. In the early to moderate stages, CKD is usually asymptomatic and is primarily diagnosed by laboratory abnormalities. Chronic Kidney Disease, hypothyroidism Hypothyroidism Hypothyroidism is a condition characterized by a deficiency of thyroid hormones. Iodine deficiency is the most common cause worldwide, but Hashimoto’s disease (autoimmune thyroiditis) is the leading cause in non-iodine-deficient regions. Hypothyroidism). Sequelae may include cardiovascular disease, stroke, pancreatitis Pancreatitis Inflammation of the pancreas. Pancreatitis is classified as acute unless there are computed tomographic or endoscopic retrograde cholangiopancreatographic findings of chronic pancreatitis. The two most common forms of acute pancreatitis are alcoholic pancreatitis and gallstone pancreatitis. Acute Pancreatitis, and hepatic steatosis Steatosis Nonalcoholic Fatty Liver Disease
  • Atheroslerosis: common form of arterial disease in which lipid deposition forms a plaque Plaque Primary Skin Lesions in the blood vessel walls. Atherosclerosis Atherosclerosis Atherosclerosis is a common form of arterial disease in which lipid deposition forms a plaque in the blood vessel walls. Atherosclerosis is an incurable disease, for which there are clearly defined risk factors that often can be reduced through a change in lifestyle and behavior of the patient. Atherosclerosis manifests as vessel stenosis Stenosis Hypoplastic Left Heart Syndrome (HLHS) and a source of thromboembolic disease. The clinical manifestations depend on the specific vessels affected and include, most notably, coronary artery Coronary Artery Truncus Arteriosus disease, carotid disease, and peripheral vascular disease.

References

  1. Botham KM, Mayes PA. (2018). Cholesterol synthesis, transport, & excretion. Chapter 26 of Rodwell BW, Bender DA, Botham KM, Kennelly PJ, Weil PA (Eds.), Harper’s Illustrated Biochemistry, 31st ed. McGraw-Hill Education. Retrieved from accessmedicine.mhmedical.com/content.aspx?aid=1160189958
  2. Chumsri S, Howes T, Bao T, Sabnis G, Brodie A. (2011). Aromatase, aromatase inhibitors, and breast cancer. Journal of Steroid Biochemistry and Molecular Biology 125:13–22. https://doi.org/10.1016/j.jsbmb.2011.02.001
  3. Alphonse PA, Jones PJ. (2016). Revisiting human cholesterol synthesis and absorption: the reciprocity paradigm and its key regulators. Lipids 51:519–536. https://doi.org/10.1007/s11745-015-4096-7
  4. Luo J, Yang H, Song BL. (2019). Mechanisms and regulation of cholesterol homeostasis. Nat Rev Mol Cell Biol 21:225–245. https://doi.org/10.1038/s41580-019-0190-7
  5. Infante RE, Radhakrishnan A. (2017). Continuous transport of a small fraction of plasma membrane cholesterol to endoplasmic reticulum regulates total cellular cholesterol. Elife. https://doi.org/10.7554/eLife.25466
  6. Robinson JG. (2020). Disorders of lipid metabolism. In: Goldman L, Schafer AI, eds. Goldman-Cecil Medicine, 26th ed. Elsevier.

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