Evidence for megalin-mediated proximal tubular uptake of L-FABP, a carrier of potentially nephrotoxic molecules.
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.
Oyama
Yuko
Y
Division of Clinical Nephrology and Rheumatology, Niigata University Graduate School of Medical and Dental Sciences, Niigata, Japan.
Takeda
Tetsuro
T
Hama
Hitomi
H
Tanuma
Atsuhito
A
Iino
Noriaki
N
Sato
Kiyoko
K
Kaseda
Ryohei
R
Ma
Meilei
M
Yamamoto
Tadashi
T
Fujii
Hiroshi
H
Kazama
Junichiro J
JJ
Odani
Shoji
S
Terada
Yoshio
Y
Mizuta
Kunihiro
K
Gejyo
Fumitake
F
Saito
Akihiko
A
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
D002352:Carrier Proteins; D017931:DNA Primers; C493321:Fabp1 protein, rat; D050556:Fatty Acid-Binding Proteins; D026561:Low Density Lipoprotein Receptor-Related Protein-2