Hyperlipidemia in Nephrotic Syndrome

1. Increased Hepatic Synthesis

• Low oncotic pressure upregulates hepatic lipoprotein synthesis. The liver interprets the low oncotic pressure as a signal to increase protein output, and because lipoproteins and albumin share a common secretory pathway, lipoprotein synthesis rises in parallel with albumin synthesis attempts. Infusion of albumin has been shown to partially correct nephrotic hyperlipidemia in experimental models.
• HMG-CoA reductase activity is elevated, increasing cholesterol synthesis. Serum cholesterol levels are inversely proportional to serum albumin levels, and they generally normalize on remission.
• Apolipoprotein synthesis increases selectively: Apo A-I mRNA increases transcriptionally (~6-fold increase in secretion), while Apo B and Apo E increase ~2-fold (likely at the post-transcriptional/translational level). Apo C synthesis is not increased.
• Hepatic VLDL and LDL production increase, raising circulating VLDL, IDL, and LDL.
• PCSK9 is upregulated (both liver- and kidney-derived), which degrades LDL receptors and further impairs LDL clearance.
• Lipoprotein(a) synthesis is increased, adding additional atherogenic risk.

• Brenner and Rector's The Kidney, p. 1319: "There is general agreement that hepatic lipid and apolipoprotein synthesis are both increased and that the clearance of chylomicrons and VLDL is reduced in the nephrotic syndrome."
• Goldman-Cecil Medicine: "low oncotic pressures upregulate hepatic synthesis of apolipoproteins."

2. Impaired Clearance and Catabolism

• LPL activity bound to the vascular endothelium is reduced by ~90%.
• This delays catabolism of chylomicrons and VLDL, raising triglycerides and IDL.
• The half-life of VLDL triglycerides is prolonged from ~4 to ~11 hours.
• Urinary loss of LPL activators (and possibly GPIHBP1, the LPL anchor protein) likely contributes, since analbuminemic rats with normal oncotic pressure still show this defect - pointing to proteinuria as the direct cause rather than hypoalbuminemia alone.

• LCAT (lecithin-cholesterol acyltransferase) activity is decreased. LCAT normally esterifies cholesterol and promotes maturation of HDL3 to HDL2, supporting reverse cholesterol transport. Its deficiency (partly due to urinary losses of LCAT and its activators) results in accumulation of HDL3 at the expense of HDL2 and impairs cholesterol removal from tissues.
• CETP (cholesterol ester transfer protein) activity is increased. CETP transfers cholesterol esters from HDL2 to VLDL remnants, converting them to LDL. This raises LDL at the expense of HDL, worsening the atherogenic profile.
• Smaller HDL3 particles are filtered and lost in the urine, further depleting HDL.

Lipid Profile Summary

Diagram: Pathophysiology of Nephrotic Hyperlipidemia

Clinical Consequences

• Nephrotic patients have roughly a 5-fold increased risk of coronary death (except in minimal change disease, where the nephrotic state is usually transient).
• Excess LDL deposited in the extracellular matrix of mesangial cells gets oxidized, releasing cytotoxic agents (prostaglandin E2, TNF) that damage glomerular epithelial and endothelial cells - potentially worsening the kidney disease itself.
• Clinical signs of hyperlipidemia (xanthelasmas) can appear rapidly.
• Lipiduria (oval fat bodies, fatty casts with "Maltese cross" birefringence under polarized light) is a direct consequence.