What is the origin of lipoprotein in the body, particularly in the context of a woman undergoing In Vitro Fertilization (IVF) treatment?

Origin of Lipoproteins in the Body

Lipoproteins originate from two primary sources: the liver, which produces VLDL, IDL, LDL, and HDL, and the small intestine, which produces chylomicrons from dietary lipids. 1

Hepatic Production (Liver-Derived Lipoproteins)

The liver serves as the primary manufacturing site for most lipoproteins in the body:

• VLDL (Very Low-Density Lipoprotein) is assembled in the endoplasmic reticulum of hepatocytes, where triglycerides derived from either plasma fatty acids or newly synthesized fatty acids are combined with apolipoprotein B-100 and phospholipids 1
• HDL (High-Density Lipoprotein) is secreted by both the liver and intestine, containing two or three ApoA-I protein molecules per particle 1
• LDL (Low-Density Lipoprotein) is not directly secreted but rather formed through the metabolic cascade as VLDL is catabolized by lipoprotein lipase into IDL, which is then further processed into LDL 1
• Lipoprotein(a) is biosynthesized exclusively in the liver (and only in humans and old world monkeys), where apolipoprotein(a) is expressed primarily in hepatocytes, undergoes glycosylation in the Golgi apparatus, and is secreted as a mature glycoprotein 1

Intestinal Production (Diet-Derived Lipoproteins)

The small intestine produces lipoproteins from absorbed dietary nutrients:

• Chylomicrons are formed in intestinal cells after dietary fat and cholesterol absorption, incorporating these lipids into nascent particles that contain a single ApoB-48 protein molecule (a truncated form of ApoB-100) 1
• These chylomicrons represent 80-95% triglyceride by composition and are secreted into the lymphatic system before entering the bloodstream at the junction of the internal jugular and subclavian veins 1

Lipoprotein Classification and Abundance

Lipoproteins are classified by density and size, with specific molecular identifiers:

• Chylomicrons are the largest and least dense, containing one ApoB-48 molecule per particle 1
• VLDL, IDL, and LDL each contain a single ApoB-100 molecule per particle 1
• HDL contains two or three ApoA-I molecules per particle, affecting their shape 1
• Lipoproteins are by far the most abundant colloid particles present in blood, plasma, and serum 1

Context for IVF Patients

In women undergoing IVF treatment, lipoprotein metabolism follows the same fundamental pathways:

• Blood lipid levels, including lipoproteins, can influence embryo quality parameters during IVF, with TG, TC, LDL, and Lp(b) showing negative correlations with embryo quality 2
• HDL is the sole lipoprotein present in follicular fluid, where it may play protective roles in oocyte health and early embryo development 3
• Pregnancy itself induces physiological hyperlipidemia with elevated triglycerides, total cholesterol, HDL-C, and LDL-C compared to non-pregnant women 1
• In pregnant women with obesity, altered lipoprotein metabolism has been observed, including lower HDL-cholesterol and decreased LCAT activity 1

Metabolic Regulation

The production and secretion of lipoproteins is tightly regulated:

• Hepatic VLDL assembly and secretion are influenced by substrate availability, hormones, and neural signals, with significant proportions of newly synthesized apoB-100 potentially degraded before secretion when hepatic lipids are scarce 1
• Lipoprotein(a) concentrations are primarily determined by production rate rather than catabolism, with plasma levels strongly correlating with hepatic synthesis rates 1
• The liver is also the major organ of lipoprotein degradation, with approximately 50% of Lp(a) taken up by the liver, followed by kidney, spleen, and muscle 1