Bile Acid & Bile Acid Synthesis

Bile synthesis in the liver:

• One of the 3 forms of cholesterol exported from the liver

• Main cholesterol catabolic pathway

• Two main pathways: classical and ACIDIC pathway

  • Acidic pathway: minor contribution

  • Rodents - 25% of bile synthesis

  • Human – 6%

• Mainly produces two primary bile acids

• Cholic acid and Chenodeoxycholic acid

• Rate limiting enzyme of bile synthesis:

  • CYP7A1 → Cholesterol 7alpha-hydroxylase

    • only in the liver

Bile Acid Conjugation

• Cholic and chenodeoxycholic acids are used to produce amino acid → conjugated bile acids

  • use glycine and taurine

• They are conjugated before they secreted into bile canaliculi

• Conjugated bile acids are:

  • amphipathic

  • H2O soluble – more H2O soluble grps

  • less toxic

  • more easily secreted

• Facilitate emulsification

• Facilitate lipid absorption

Bile Acid Synthesis

• Derived from cholesterol, converting it into water- soluble forms.

• They are amphipathic molecules.

Bile Acid Functions

• Aid in digestion and absorption of fats, cholesterol, and fat-soluble vitamins.

• Regulate cholesterol homeostasis; via excretion & signaling, predominant biliary cholesterol excretion.

• Signal through receptors to control bile acid, lipid, and energy balance.

• Influence the gut microbiome composition.

• Influence AAs metabolism via diversifying gut microbiome, certain bile acids influence growth of certain bacteria that may or may not be beneficial for host organ.

In-born errors of bile acid synthesis:

• Rare genetic disorders

• Neonatal Cholestasis → bile and bilirubin spills into blood and accumulate

• Fat soluble vit. deficiencies

• Failed to produce physiological levels of bile acids

• Accumulation of bile acid intermediates in the blood

• Individual shows normal or low serum bile acid Concentrations

• Failure to diagnosed early results in liver failure or progressive chronic liver diseases (liver lipid deposition as cholesterol excretion is inhibited.

• Early diagnosis is important and can be corrected by oral bile therapy

Betaine & Phospholipid Synthesis

• Amino acid glycine-derived-nutrient

• Widely found in food

• Synthesized from choline

• Methyl Donor

• Use in methionine-homocysteine cycle, produces SAM (S- Adenosine Methionine)

• Important methyl donor for Phosphatidylcholine (PC), a phospholipid (PL) synthesis

• PL are major components of cell membranes

Methionine cycle and PC synthesis and Betaine involvement - Liver:

• Methionine donates Methyl group to SAM

  • SAM: S-Adenosine methionine ;Universal methyl donor

• SAM donates methyl groups to many metabolic reactions:

  • PC (PL) Synthesis, DNA methylation, Creatine synthesis, etc.

• Once SAM donates methyl group to these metabolic pathways, SAM becomes SAH and then to homocysteine

  • Homocysteine: Link exist between cardiovascular diseases and plasma homocysteine

• Homocysteine must be converted back to methionine

• Betain provide methyl groups to Homocysteine to become methionine

• Betaine maintains continuation of the cycle.

• PL synthesize, incorporated VLDL & HDL at liver

• The lipid core of VLDL consists of triglycerides (TGs, 50–70% of particle mass), cholesterol ester (10–25%), and fatty acids (<10%).

• VLDL functions as a cargo carrier, transporting cholesterol, TGs, and proteins to peripheral cells for essential bioactivities.

Rate limiting enzymes for cholesterol biosynthesis:

HMG-CoA reductase:

• HMGCR

• 3-hydroxy-3-methylglutaryl-CoA reductase

  • converts 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) into mevalonic acid

  • found in the ER

Rate limiting step of major bile acid synthesis pathway:

Cholesterol 7-alpha hydroxylase (Gene:CYP7A1):

• converts cholesterol into 7α-hydroxycholesterol in the endoplasmic reticulum of hepatocytes

Dietary Betaine:

• upregulates both rate-limiting enzymes (HMGCR + cholesterol 7-alpha hydroxylase) when betaine was incorporated to high fat diet.

• reduces the liver fat deposition due to high fat diet (yellowish liver)

Five major classes of steroid hormones:

1. Androgens (e.g., testosterone)

2. Estrogens (e.g., estradiol)

3. Progestins (e.g., progesterone)

4. Glucocorticoids (e.g., cortisol)

5. Mineralocorticoids (e.g., aldosterone)

Steroid Hormone Synthesis

Common precursor: Pregnenolone

• Formed from cholesterol in mitochondria

• Rate-limiting step in steroidogenesis

Key enzymes and processes:

• P450scc (CYP11A1): Converts cholesterol to pregnenolone

• This enzyme located on inner mitochondrial membrane

• Requires cholesterol transport from outer to inner mitochondrial membrane

• Adrenal insufficiency – in-born error of first and rate limiting enzyme for synthesis of building block for cortisol, aldosterol, sex hormones

Glucocorticoids (Cortisol)

• Formed in the adrenal cortex from pregnenolone via CYP21A2 and CYP11B1.

• Regulate metabolism and immune responses.

Mineralocorticoids (Aldosterone)

• Produced through the CYP11B2 enzyme.

• Regulate sodium and potassium balance in kidneys.

Androgens (Testosterone)

• Synthesized in the testes or adrenal cortex.

• Essential for male secondary sexual characteristics and muscle growth.

Estrogens (Estradiol)

• Produced in ovaries from testosterone by aromatase (CYP19A1).

• Involved in the regulation of reproductive functions in females

Progestogens (Progesterone)

• Derived directly from pregnenolone by 3β-hydroxysteroid dehydrogenase (3β-HSD).

• Crucial for pregnancy maintenance and menstrual cycle regulation.

SLOS – Smith-Lemli-Optiz Syndrome

• Developmental disorder (mental retardation) caused by mutations to the gene encoding the enzyme DHCR7 that impair final step of the cholesterol biosynthesis.

• No cholesterol biosynthesis

• Accumulate 7-DHC (7 dehydrocholesterol) – immediate intermediate of cholesterol

• Inadequate synthesis of Bile

• Adrenal insufficiency - steroid hormone production – low plasma sterol hormones in patients, appears to be the reason for abnormal development

SLOS – Smith-Lemli-Optiz Syndrome & Vitamin D Synthesis:

Because Cholesterol cannot be synthesized in these patients, 7-DHC undergoes Vit D synthesis:

• because more substrates are available

• too much Vitamin D → increases Vitamin D derivatives (by-products)

  • impaired DHCR7 activity suggests an increase in vitamin D biosynthesis and elevated vitamin D metabolites, and this has been observed clinically in SLOS patients

What are the key lipid changes in Smith–Lemli–Opitz syndrome (SLOS)?

Decreased plasma cholesterol and accumulation of 7-dehydrocholesterol (7-DHC), the most reactive lipid molecule, with elevated brain 7-DHC and its oxidative products

• Diagnosis:

  • Plasma levels of 7-DHC

  • Prenatal diagnosis via 7-DHC in amniotic fluid

  • Maternal urine 7-DHC levels (non-invasive technique)

• Prognosis: varies → some live longer than others

7-DHC Oxidative Products

Oxysterols are highly reactive:

• 1000s different oxysterols

• play role in disease pathology

  • cytotoxic, reduces cell proliferation, induce cell differentiation, gene transcriptional changes

• mutation in DHCR7 enzyme causes oxidative damage to lipids

• inhibits HMC-CoA reductase (rate-limiting step of cholesterol synthesis)

SLOS Treatment Options

1. Standard treatment: provide more dietary cholesterol: substrate to synthesize adequate steroid hormones, cholesterol for membranes, for bile synthesis

• However, no changes to behaviour improvement in the patients, no surprise as dietary cholesterol won’t cross BBB.

Because 7-DHC and its byproducts may contribute to pathology,

2. Antioxidants:

• Amelioration of pathology

• Inhibit lipid peroxidation:

  • reduces highly reactive oxidative species, no oxidative damage to cellular compartments

3. Gene therapy rodent models: introduction of functional 7-DHCR: targeting increasing cholesterol levels and decreasing 7-DHC levels and its by-products

Alzheimer’s disease (AD)/Dementia

• One pathology for this neurological disease: deposition of extracellular amyloid-beta peptide (Aβ)

• Cholesterol induces changes in the physiochemical properties of membranes, such as changes in fluidity and density packing of lipids in neuronal membranes, which may affect the binding of Aβ to the cell membrane

• Increase in cholesterol levels exerted a protective effect against the toxicity induced by Aβ oligomers in neuroblastoma cells

• Some studies: depletion of cholesterol reduced the aggregation of Aβ

• The underlying pathology of how amyloid beta contributes to Alzheimer’s disease and dementia remains unclear

Membrane Cholesterol

Maintains fluidity without compromising membrane function greatly during extreme high and low temperatures

• Pathophysiology for later-life neurological impairments

  • Alzheimer’s, Parkinsons, etc

Lipid rafts in biological membranes/intracellular:

• Enriched in cholesterol, other PLs, and proteins

• Organizing centers for cell signalling, provide stability, pathogen entry, endotoxicity and exocytosis, and immune response

• Raft-associated cholesterol involvement in migration, invasion, and metastatic processes of cancer cells

  • Target for certain cancer treatment