In-Depth Notes on Lipoproteins and Cholesterol Metabolism

  • Metabolism of Chylomicrons and Lipoproteins

    • Chylomicrons:

    • Carry dietary lipids from the gut to cells

    • Contain APOB 48

    • Rich in triglycerides

    • VLDL (Very Low-Density Lipoprotein):

    • Produced by the liver, also carries triglycerides

    • Contains APOB 100

    • Precursor for LDL (Low-Density Lipoprotein)

  • Transition from VLDL to LDL

    • Originates in the liver as nascent VLDL
    • Interaction with HDL:
    • HDL donates APOC-II and APOE to nascent VLDL
    • Fully mature VLDL contains these proteins and can donate triglycerides
    • VLDL interacts with non-hepatic tissues:
    • Activated by APOC-II via lipoprotein lipase (LPL)
    • Triglycerides are broken down into glycerol and fatty acids
    • Fatty acids can be stored in adipose tissue or used as energy

  • Glycerol and Triglyceride Formation

    • Glycerol cannot be utilized directly by adipose tissue due to low glycerol kinase expression
    • Adipose tissue utilizes glucose to form glycerol
    • Insulin stimulates lipogenesis in adipose tissue

  • IDL Transition

    • After unloading triglycerides, VLDL becomes IDL (Intermediate-Density Lipoprotein)
    • IDL interacts with HDL and returns some proteins (APOC-II, APOE)
    • IDL further converts to LDL

  • LDL Characteristics

    • Rich in cholesterol, contains APOB 100 only
    • Can be taken up by liver or peripheral tissues
    • "Bad cholesterol" label due to its potential for being deposited in arteries

  • Oxidized LDL and Cardiovascular Disease

    • LDL can be oxidized, prompting macrophages to engulf it and become foam cells
    • Foam cells contribute to inflammation and vascular lesions
    • LDL's cholesterol rich nature is linked to cardiovascular risk

  • De Novo Lipid Synthesis

    • Liver synthesizes fats (e.g., palmitate) from excess sugar (carbohydrates)
    • Explains why high sugar intake leads to increased fat production (fatty liver disease)

  • Reverse Cholesterol Transport by HDL

    • HDL collects excess cholesterol from tissues and returns it to the liver
    • HDL is produced in the liver and intestine
    • Nascent HDL (discoidal shape) transforms into spherical HDL as it collects cholesterol
    • Enzyme LCAT helps in the formation of cholesterol esters, facilitating material transfer

  • HDL Functionality

    • Receives cholesterol from tissues, reducing free cholesterol levels
    • Final destination is the liver for processing and recycling
    • HDL is often regarded as "good cholesterol" for its protective roles in cardiovascular health

  • Increasing HDL Levels

    • Exercise and intake of saturated fats have shown to increase HDL levels
    • No pharmaceutical drugs currently proven to effectively increase HDL

  • Cholesterol Ester Transfer Protein (CETP)

    • Mediates exchange of cholesterol esters between HDL and other lipoproteins (like VLDL)
    • Attempts to inhibit CETP to maintain HDL's cholesterol contents are ongoing

  • Role of PCSK9

    • A secreted protein by the liver that binds LDL receptors and promotes their degradation
    • Inhibition of PCSK9 enhances LDL receptor availability, decreasing LDL levels in circulation
    • Current pharmacological strategies targeting PCSK9 are under investigation to manage cholesterol levels