Protein load impairs factor H binding promoting complement-dependent dysfunction of proximal tubular cells.

Area of Science:

• Nephrology
• Immunology
• Cell Biology

Background:

• Intrarenal complement activation is a key driver in chronic kidney disease (CKD) progression.
• Proximal tubules are susceptible to injury from filtered plasma proteins and complement factors.
• Understanding complement regulation in proximal tubules under protein overload is crucial for CKD research.

Purpose of the Study:

• To investigate if protein overload in human proximal tubular cells (HK-2) enhances complement activation.
• To determine if protein loading impairs the cells' complement regulatory mechanisms.

Main Methods:

• HK-2 cells were cultured and exposed to albumin or transferrin with diluted human serum (complement source).
• Complement deposition (C3) and activation products (C3, C5b-9) were measured.
• Inhibitors of complement pathways (soluble complement receptor-1, MgCl2-EGTA) were used.
• mRNA expression of inflammatory markers (fractalkine, TGF-β) was assessed.
• Heparan sulfate density and Factor H binding were quantified.

Main Results:

• Albumin and transferrin increased apical C3 deposition, indicating complement activation.
• Soluble complement receptor-1 inhibited deposition, while MgCl2-EGTA did not, suggesting alternative pathway involvement.
• Albumin, but not transferrin, with complement led to increased inflammatory gene expression and C3/C5b-9 generation.
• Protein loading reduced heparan sulfate density and Factor H binding to HK-2 cells.
• These changes enhanced albumin- and serum-dependent complement activation.

Conclusions:

• Protein overload impairs proximal tubule cell complement regulation, specifically by reducing Factor H binding.
• This impaired regulation promotes complement activation and inflammation, contributing to kidney injury in CKD.
• Targeting complement regulation in proximal tubules may offer therapeutic strategies for proteinuric kidney diseases.