Cholesterol Function & Vitamin D

Cholesterol

• A common steroid

• Precursor to biochemically active steroids

• Member of sterol: sterols contain alcohol as functional group; therefore, these are steroids

Cholesterol in cells:

• In all animal membranes, 25% of lipids in membranes are cholesterol, maintain fluidity

• Free cholesterol does not exist outside membrane

• But only as esterified to fatty acids for storage

Cholesterol Absorption

• Cholesterol comes from the diet and from bile or endogenous synthesis:

  • Cholesterol not absorbed is excreted – Avg 150 mg/day

• Once absorbed;

  • Cholesterol may incorporate into enterocyte membranes

  • Majority is esterified in preparation for transport out of the cell as a component of chylomicrons through lymphatic system

• Cholesterol and phytosterols transported into the intestinal cell by:

  • Niemann-Pick C1 Like 1 (NPC1L1) at apical membrane

• Foods and supplements enriched with phytosterols effective at reducing blood cholesterol concentration by 10% or more

Factors influencing absorption efficiency:

• proportion of dietary fats in the diet

• dietary fiber

• presence of plant sterols

• transit time

The diet:

Represents a major route of cholesterol entry to body pool

• Endogenous synthesis

The brain:

• Exclusively depends on its own synthesis

  • BBB impermeable for peripheral or dietary cholesterol

• Brain excretes excess/waste cholesterol as 24S Hydroxycholesterol (24S-HC)

  • 24S-HC crosses the BBB and enter the circulation

  • BBB maintain waste disposal and homeostasis in the brain

Intestinal Sterol/Bile absorption

• Lymphatic vessel-chylomicron

• Remove TAG

• Deliver chylomicron remnant to subclavian vein

• Bring remnants to liver

• At hepatocytes , remove cholesterol from Bile

• Use again for Bile, steroid hormone synthesis or directing to Vit D synthesis

Cholesterol absorption in the intestines:

• Via important transmembrane proteins (e.g., Niemann-Pick C1-like 1,NPC1L1)

• Proteins internalize free cholesterol from dietary cholesterol and biliary free cholesterol

NPC1L1

• Primarily expressed at the apical side (brush-border membrane) of Intestinal enterocytes

  • Controls intestinal cholesterol absorption

  • These proteins are in lysosomes

• Expresses in Canalicular membrane of hepatocytes – may involve in bile excretion

Niemann-Pick Type-C (NPC) Disease:

• Inherited defect in cholesterol storage

• Cholesterol is not transported out of lysosome & accumulates in the lysosome

NPC1 and NPC2:

Moves cholesterol out of lysosome to cytosol to further metabolize

• NPC1: Transmembrane lysosomal protein

• NPC2:soluble protein

At the liver:

NPC1L1 remove cholesterol from bile to use to re-synthesize sterols and bile

Function of cholesterol in-vivo:

Intracellular cholesterol concentration acts to maintain cellular cholesterol homeostasis

Excess cholesterol in biological cells:

Cholesterol cannot used for energy production:

• Must excrete when in excess:

  • Cholesterol generate Oxysterols

  • Oxysterols are signaling molecules : activate LXR in the liver

  • LXR regulate cholesterol homeostasis

• Intracellular cholesterol concentration acts to maintain cellular cholesterol homeostasis

Liver X Receptors (LXR):

Nuclear X receptors (LXRα and LXRβ) that act as transcription factors:

• Regulate cholesterol homeostasis

• Transcription factors modify enzyme expression in multiple ways (repress or activate)

• Lipid metabolizing enzymes

Cholesterol functions in vivo via oxysterols:

Liver X Receptors (LXR):

• Nuclear receptors (LXRα and LXRβ) that act as transcription factors → Regulate cholesterol homeostasis.

• Key Functions in Cholesterol Regulation

Promotion of Cholesterol Efflux:

• Upregulates ABCA1 and ABCG1 transporters, enhancing cholesterol efflux from cells to high-density lipoproteins (HDL) for reverse cholesterol transport.

Inhibition of Cholesterol Absorption and Synthesis:

• Reduces expression of NPC1L1, a transporter responsible for intestinal cholesterol uptake at apical membrane of enterocytes

• Suppresses genes involved in cholesterol biosynthesis (e.g., HMG-CoA reductase).

Regulation of Inflammation:

• LXR activation has anti-inflammatory effects, modulating gene expression to reduce atherosclerosis risk.

Cholesterol synthesis:

Is expensive → requires ATP energy

• but cholesterol cannot be used to generate energy

Unlike other biomolecules, cholesterol and steroids cannot be degraded to small molecules:

• Instead, they are converted to derivatives

• Some of these derivatives are more soluble (eg Bile) → allow excretion

De novo cholesterol synthesis:

• regulated by endogenous cholesterol availability

  • LXR regulate HMG-CoA reductase

Oxysterol Production

• Metabolic intermediates

• Several of them are bioactive

• Different intermediates show different effects

• At physiological levels they are safe and required as we described

• But too much production → not favourable

Too much oxysterols – derivatives of oxysterols:

• Over production of oxysterols can be harmful to cells as 1000s of oxidative products are accumulated in the body and oxidize other cellular metabolites and membrane lipids

  • oxidation/cellular oxidative damage

• Some of these derivatives are not soluble, but toxic and accumulate in the body (oxidative products of oxysterols) → these are highly reactive species

• Research: Etiology for cancer, atherosclerosis, Alzherimer’s disease, cataracts, age related neurological other diseases, osteoporosis, inflammatory diseases, etc.,

• Reducing agents such as Vit E, Vit C and Glutathione can reduce the oxysterol loads and oxidative products of oxysterols

Cholesterol in Membrane

Cholesterol content:

• Maintain fluidity without compromising membrane function greatly during extreme high and low temp

  • Facilitates smooth phase transition of membrane → With cholesterol, fluidity increases (but linear increase

  • Less drastic changes to cellular bi-layers

• Gives fluidity and rigidity within the hydrophobic middle tail areas of phospholipid bilayer

Cholesterol's amphipathic architecture enables dual regulatory functions:

• Hydrophobic sterol ring embeds within the hydrophobic tails of phospholipids, associating with saturated fatty acid chains to increase membrane density. By filling the gaps between phospholipid chains, cholesterol reduces membrane defects and thereby enhances mechanical strength.

• The flexible portion of cholesterol’s alkyl chain interact with the hydrophobic tails of unsaturated fatty acid chains of PL, diminishing the disorderly packing caused by their kinks and preventing excessive fluidity when required.

Cholesterol functions as a buffer:

Prevents inhibition of fluidity at lower temperatures and preventing increasing fluidity at higher temperatures:

• A pure phospholipid bilayer exists in a gel (rigid) phase below its transition temperature (TM) and in a liquid crystalline (fluid) phase above Tₘ

• Chol interact with PL chains leading to buffer the phase transition – linear changes

  • Stabilize the liquid crystalline phase over broader temp range

• At environment Temp below or above near Tm , chol restrict vigorous movement – especially lateral motion of PL chains – prevent excess membrane fluidity – prevent sudden and quick changes to fluidity

• AT environment Temp very lower or extreme high, sterol ring of choles hinders orderly arrangement of PL chain –prevent formation of rigid gel state

• thereby stabilize the liquid crystalline phase over a broader temperature range by preventing the formation of a rigid gel state

Cholesterol’s ability to modulate membrane fluidity used treat for fungal diseases:

• Fungal cells membrane contain Ergosterol

• Ergosterol is cholesterol-like structure

• Antifungal bind to Ergosterol →

• Reduce the Ergosterol function as it is no more available for membrane function →

• Decrease the fluidity of fungal cell membrane → more rigid, not favor and break easily →

• Leak the inner contact of fungal cell →

• Fungal death

• These antifungal won’t bind to cholesterol

• No harm to our cell membrane

Fates of cholesterol in the body:

• 50% of body cholesterol excreted via bile formation

Vitamin D

• Vitamin D deficiency: osteomalacia

  • softening and weakening of bones

• More common in northern climate

• Play role in immunity:

  • cancer etiology, cardiovascular diseases, diabetes