Evidence Indicating that Renal Tubular Metabolism of Leptin Is Mediated by Megalin But Not by the Leptin Receptors

Abstract

Leptin is secreted by adipocytes and is a circulating factor that regulates food intake and energy expenditure. Its serum level is elevated in patients with renal failure and has been suggested to be associated with malnutritional factors in these patients. Leptin has been suggested to be primarily metabolized by the kidneys, although the precise molecular mechanisms are not known. The purpose of this study was to determine the nephron segments and potential receptors involved in renal leptin metabolism. To determine the segment involved in leptin uptake, we performed histoautoradiography of kidney sections obtained from rats that had been injected iv with (125)I-leptin. The ability of megalin, a multiligand endocytic receptor in the proximal tubules, to bind and endocytose leptin was examined by ligand blotting analysis, quartz-crystal microbalance, and degradation assays using megalin-expressing rat yolk sac L2 cells. Immunohistochemistry was performed to localize leptin receptors (LEP-R) in the rat kidney using two antibodies that recognize different epitopes on the LEP-R proteins. Circulating (125)I-leptin was filtered by glomeruli and internalized by proximal convoluted tubules. Megalin bound leptin in the presence of Ca(2+) and mediated its cellular internalization and degradation. On immunohistochemistry, LEP-R were localized in the proximal straight tubules, loops of Henle, distal tubules, and collecting ducts. In conclusion, circulating leptin was filtered by glomeruli and taken up by proximal convoluted tubules, where megalin likely mediates its binding and uptake. The localization of LEP-R suggests that they are not primarily involved in leptin metabolism in the proximal tubules.

Clusterin and LRP2 are critical components of the hypothalamic feeding regulatory pathway

“…LRP2 binds leptin to mediate leptin reuptake in renal tubules and to promote leptin transport across the choroid plexus [21][22][23] , suggesting a molecular link between LRP2 and leptin. LRP2 was also identified as an endocytic receptor for clusterin 24,25 and our preliminary data confirmed the mRNA expression of LRP2 in rodent hypothalamus.…”

Evidence for megalin-mediated proximal tubular uptake of L-FABP, a carrier of potentially nephrotoxic molecules

“…31,33 Ligands that are endocytosed via megalin include multiple low-molecular weight proteins such as parathyroid hormone, 34 insulin, b 2 -microglobulin, epidermal growth factor, prolactin, lysozyme, cytochrome c, 35 a 1 -microglobulin, odorant-binding protein, 36 transthyretin 37 and leptin. 26 The majority of the ligands that are internalized by megalin are transported via endocytotic vesicles to lysosomes, resulting in their dissociation and degradation. 38 Megalin also regulates the endocytosis of complexes of vitamins and carrier proteins such as transcobalamin, 39 vitamin D-binding protein (DBP), 40 and retinol-binding protein, 41 indicating the importance of megalin in both maintaining vitamin homeostasis and metabolizing certain vitamins, notably the renal hydroxylation of vitamin D. 42 The megalin-mediated uptake of 25-(OH) vitamin D 3 by PTC is followed by lysosomal degradation of DBP and subsequent conversion of 25-(OH) vitamin D 3 to 1,25-(OH) 2 vitamin D 3 , which is then returned to the circulation.…”

“…26 Briefly, the QCM sensor chips were activated by a 1:1 mixture of 100 mg/ml 1-(3-dimethylaminopropyl)-3 ethylcarbodiimide hydrochloride (EDC) (Sigma-Aldrich, St Louis, MO, USA) and 100 mg/ml Nhydroxysuccinimide (NHS) (Wako Pure Chemical Industries, Ltd) in water. Rat megalin, which had been prepared from renal microvillar membranes by immunoaffinity chromatography as previously described, 27 at a concentration of 10 mg/ml in buffer B (10 mM HEPES, pH 7.4, 150 mM NaCl, 2 mM CaCl 2 ), or fatty acid (FA)-free BSA (Sigma-Aldrich) in the negative control experiment, was immobilized on the sensor chips.…”

“…The binding affinity was determined from the frequency changes upon cumulative injection of a small volume (8 ml) of 1 mg/ml L-FABP as previously described. 26 …”

LRP2 A Myopic Hotspot between Environment and Genetics

“…LRP2 ligands such as transthyretin, albumin, insulin, leptin or angiotensin II are implicated in HM, diabetes, obesity, hypertension and other pathological conditions which may affect both renal function and retinal health. With the exception of the renal effects of LRP2 dysfunction which have been extensively studied, the pathological mechanisms that relate LRP2 to other systems are not exactly defined [25][26][27][28].…”