Megalin Antagonizes Activation of the Parathyroid Hormone Receptor

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.