title: Evidence for megalin-mediated proximal tubular uptake of L-FABP, a carrier of potentially nephrotoxic molecules.
abstract: Liver-type fatty acid binding protein (L-FABP) binds with high affinity to hydrophobic molecules including free fatty acid, bile acid and bilirubin, which are potentially nephrotoxic, and is involved in their metabolism mainly in hepatocytes. L-FABP is released into the circulation, and patients with liver damage have an elevated plasma L-FABP level. L-FABP is also present in renal tubules; however, the precise localization of L-FABP and its potential role in the renal tubules are not known. In this study, we examined the cellular and subcellular localization of L-FABP in the rat kidney and tried to determine from where the L-FABP in kidney tissues had originated. Immunohistochemical studies of kidney sections localized L-FABP in the lysosomes of proximal tubule cells (PTC). In rats with carbon tetrachloride (CCl4)-induced acute liver injury, we detected high levels of L-FABP in the circulation and in the kidney compared with those in the control rat by immunoblotting, while reverse transcription-polymerase chain reaction showed that the level of L-FABP mRNA expression in the kidney of CCl4-treated rats was low and did not differ from that in the control rat. When 35S-L-FABP was intravenously administered to rats, the kidneys took up 35S-L-FABP more preferentially than the liver and heart, and histoautoradiography of kidney sections revealed that 35S-L-FABP was internalized via the apical domains of PTC. Quartz-crystal microbalance analysis revealed that L-FABP bound to megalin, a multiligand endocytotic receptor on PTC, in a Ca2+-dependent manner. Degradation assays using megalin-expressing rat yolk sac tumor-derived L2 cells demonstrated that megalin mediated the cellular uptake and catabolism of 125I-L-FABP. In conclusion, circulatory L-FABP was found to be filtered by glomeruli and internalized by PTC probably via megalin-mediated endocytosis. These results suggest a novel renal uptake pathway for L-FABP, a carrier of hydrophobic molecules, some of which may exert nephrotoxic effects.
authors.affiliation: Division of Clinical Nephrology and Rheumatology, Niigata University Graduate School of Medical and Dental Sciences, Niigata, Japan.
authors.forename: Yuko
authors.initials: Y
authors.lastname: Oyama
authors.forename: Tetsuro
authors.initials: T
authors.lastname: Takeda
authors.forename: Hitomi
authors.initials: H
authors.lastname: Hama
authors.forename: Atsuhito
authors.initials: A
authors.lastname: Tanuma
authors.forename: Noriaki
authors.initials: N
authors.lastname: Iino
authors.forename: Kiyoko
authors.initials: K
authors.lastname: Sato
authors.forename: Ryohei
authors.initials: R
authors.lastname: Kaseda
authors.forename: Meilei
authors.initials: M
authors.lastname: Ma
authors.forename: Tadashi
authors.initials: T
authors.lastname: Yamamoto
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Fujii
authors.forename: Junichiro J
authors.initials: JJ
authors.lastname: Kazama
authors.forename: Shoji
authors.initials: S
authors.lastname: Odani
authors.forename: Yoshio
authors.initials: Y
authors.lastname: Terada
authors.forename: Kunihiro
authors.initials: K
authors.lastname: Mizuta
authors.forename: Fumitake
authors.initials: F
authors.lastname: Gejyo
authors.forename: Akihiko
authors.initials: A
authors.lastname: Saito
mesh_terms: D000818:Animals; D001483:Base Sequence; D002352:Carrier Proteins / Q000096:biosynthesis / Q000378:metabolism*; D017931:DNA Primers; D050556:Fatty Acid-Binding Proteins; D007150:Immunohistochemistry; D007687:Kidney Tubules, Proximal / Q000187:drug effects / Q000378:metabolism*; D026561:Low Density Lipoprotein Receptor-Related Protein-2 / Q000502:physiology*; D008297:Male; D051381:Rats; D017207:Rats, Sprague-Dawley; D020133:Reverse Transcriptase Polymerase Chain Reaction
chemical_list: D002352:Carrier Proteins; D017931:DNA Primers; C493321:Fabp1 protein, rat; D050556:Fatty Acid-Binding Proteins; D026561:Low Density Lipoprotein Receptor-Related Protein-2