21.1 Introduction to Lipoprotein and Apolipoproteins

Lipoproteins are crucial for transporting fats in our blood. They come in different types, each with a unique job. Without them, we'd struggle to move essential fats around our body and use them properly.

Having the right balance of lipoproteins is key to heart health. Too much of certain types can lead to plaque buildup in our arteries, while others help keep our blood vessels clean. Understanding these tiny fat-carriers helps us grasp how our body handles cholesterol.

Lipoproteins and Apolipoproteins

Role of lipoproteins

• Lipoproteins transport hydrophobic lipids (cholesterol and triglycerides) through the aqueous bloodstream
  • Lipids are insoluble in water requiring lipoproteins for transport
• Lipoproteins classified based on density, size, and composition
  • Major classes: chylomicrons, ###Very_low-density_lipoproteins_(VLDL)_0###, low-density lipoproteins (LDL), and high-density lipoproteins (HDL)
• Apolipoproteins are protein components of lipoproteins that stabilize structure and guide metabolism
  • Act as ligands for specific receptors on cell surfaces facilitating uptake and delivery of lipids (ApoB-100 for LDL, ApoA-I for HDL)

Abnormal lipid levels and atherosclerosis

• Elevated LDL cholesterol (LDL-C), triglycerides, and low HDL cholesterol (HDL-C) increase risk of atherosclerosis and coronary artery disease
  • Atherosclerosis: plaque buildup in arterial walls causing narrowing and hardening (aorta, coronary arteries)
  • Coronary artery disease: atherosclerosis affecting arteries supplying the heart (left anterior descending, right coronary artery)
• LDL particles infiltrate arterial wall, undergo oxidation, trigger inflammatory response, and form foam cells
  • Foam cells accumulate contributing to atherosclerotic plaque development (fatty streaks, fibrous plaques)
• High triglyceride levels increase production of small, dense LDL particles which are more atherogenic than larger, buoyant LDL
• Low HDL-C reduces reverse cholesterol transport removing excess cholesterol from peripheral tissues and arterial wall

Major lipoprotein types and functions

• Chylomicrons
  • Produced by intestines to transport dietary triglycerides and cholesterol
  • Deliver triglycerides to adipose tissue and muscle for storage or energy (lipoprotein lipase)
  • Remnant particles taken up by liver (ApoE receptor)
• Very low-density lipoproteins (VLDL)
  • Produced by liver to transport endogenous triglycerides and cholesterol
  • Deliver triglycerides to peripheral tissues (lipoprotein lipase)
  • As triglycerides are removed, VLDL particles become intermediate-density lipoproteins (IDL) and eventually LDL
• Low-density lipoproteins (LDL)
  • Derived from VLDL and IDL particles
  • Primary carriers of cholesterol to peripheral tissues
  • Bind to LDL receptors on cell surfaces for cellular uptake and use in membrane synthesis and steroid hormone production (adrenal glands, gonads)
  • Elevated LDL-C major risk factor for atherosclerosis
• High-density lipoproteins (HDL)
  • Produced by liver and intestines
  • Participate in reverse cholesterol transport removing excess cholesterol from peripheral tissues and arterial wall
  • Transport cholesterol back to liver for excretion or recycling (bile acids, VLDL production)
  • Higher HDL-C levels associated with reduced risk of atherosclerosis and cardiovascular disease (myocardial infarction, stroke)