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Megalin antagonizes activation of the parathyroid hormone receptor
J Hilpert1, A Nykjaer, C Jacobsen
1Franz-Volhard-Clinic, Humboldt-University, 13125 Berlin, Germany.
Abstract:
Parathyroid hormone (PTH) is predominantly cleared from the circulation by glomerular filtration and degradation in the renal proximal tubules. Here, we demonstrate that megalin, a multifunctional endocytic receptor in the proximal tubular epithelium, mediates the uptake and degradation of PTH. Megalin was purified from kidney membranes as the major PTH-binding protein and shown in BIAcore analysis to specifically bind full-length PTH and amino-terminal PTH fragments (Kd 0.5 microM). Absence of the receptor in megalin knockout mice resulted in 4-fold increased levels of amino-terminal PTH fragments in the urine. In F9 cells expressing both megalin and the PTH/PTH-related peptide receptor (PTH/PTHrP receptor), uptake and lysosomal degradation of the hormone was mediated through megalin. Blocking megalin-mediated clearance of PTH resulted in 3-fold increased stimulation of the PTH/PTHrP receptor. These data provide evidence that megalin is involved in the renal catabolism of PTH and potentially antagonizes PTH/PTHrP receptor activity in the proximal tubular epithelium.
Insights
The kidney receptor megalin clears parathyroid hormone (PTH) from the blood. Megalin deficiency increases PTH fragments in urine and enhances PTH receptor activity.
Area of Science:
- Nephrology
- Endocrinology
- Molecular Biology
Background:
- Parathyroid hormone (PTH) is primarily cleared by the kidneys.
- Renal proximal tubules are key sites for PTH degradation.
- The role of specific receptors in PTH clearance is not fully understood.
Purpose of the Study:
- To investigate the role of megalin in the renal clearance and catabolism of parathyroid hormone (PTH).
- To determine if megalin directly binds and mediates the uptake of PTH in renal proximal tubules.
Main Methods:
- Purification of megalin from kidney membranes and assessment of PTH binding using BIAcore analysis.
- Analysis of urinary PTH fragments in megalin knockout mice.
- Studying PTH uptake and degradation in F9 cells expressing megalin and the PTH/PTHrP receptor.
Main Results:
- Megalin was identified as the major PTH-binding protein in kidney membranes, binding full-length PTH and N-terminal fragments.
- Megalin knockout mice exhibited a 4-fold increase in urinary N-terminal PTH fragments.
- Megalin mediated PTH uptake and lysosomal degradation in F9 cells, and blocking this pathway increased PTH/PTHrP receptor stimulation.
Conclusions:
- Megalin plays a significant role in the renal catabolism of parathyroid hormone (PTH).
- Megalin-mediated clearance of PTH may antagonize PTH/PTHrP receptor activity in proximal tubular cells.
- These findings elucidate a novel mechanism for PTH homeostasis involving megalin.