Histochemical characterization of low density lipoprotein receptors in internalization-defective familial...

Area of Science:

• Cell biology
• Molecular genetics
• Biochemistry

Background:

• Familial hypercholesterolemia is a genetic disorder characterized by high LDL cholesterol levels.
• LDL internalization via receptors is crucial for cholesterol homeostasis.
• Defects in LDL internalization lead to hypercholesterolemia.

Purpose of the Study:

• To investigate the cellular localization of low-density lipoprotein (LDL) in fibroblasts from a patient with homozygous familial hypercholesterolemia (MN).
• To compare LDL localization in MN cells with normal fibroblasts and another cell line (GM2408A) with defective LDL internalization.
• To elucidate the specific defect in LDL internalization in MN cells.

Main Methods:

• Cultured human fibroblasts from normal subjects, familial hypercholesterolemia patients (MN and parents), and GM2408A cells.
• Incubation with 125I-labeled or ferritin-labeled LDL.
• Observation of LDL binding and localization using autoradiography and electron microscopy.
• Quantitative analysis of ferritin-LDL localization in coated pits at 4°C and 37°C.

Main Results:

• MN's cells demonstrated a defect in LDL internalization at 37°C.
• Ferritin-labeled LDL localized mainly in non-coated regions in MN cells at 4°C and was absent from endocytic vesicles at 37°C.
• Quantitative analysis showed significantly reduced binding of ferritin-LDL in coated pits in MN cells (11%) and GM2408A cells (12%) compared to normal cells (62%).

Conclusions:

• The LDL internalization defect in MN's cells is functionally identical to that observed in GM2408A cells.
• Neither MN cells nor GM2408A cells can correctly localize the LDL-LDL receptor complex within coated pits for internalization.
• This impaired localization in coated pits is the underlying cause of the internalization defect in these familial hypercholesterolemia cells.