1. ¹Division of Clinical Nephrology and Rheumatology, Niigata University Graduate School of Medical and Dental Sciences, Niigata, Japan
2. ²Department of Applied Molecular Medicine, Niigata University Graduate School of Medical and Dental Sciences, Niigata, Japan
3. ³Department of Structural Pathology, Institute of Nephrology, Niigata University Graduate School of Medical and Dental Sciences, Niigata, Japan
4. ⁴Division of Molecular and Cellular Biology, Niigata University Graduate School of Medical and Dental Sciences, Niigata, Japan
5. ⁵Division of Intensive Care Medicine, Niigata University Hospital, Niigata, Japan
6. ⁶Department of Biology, Faculty of Science, Niigata University, Niigata, Japan
7. ⁷Department of Homeostasis Medicine and Nephrology, Graduate School, Tokyo Medical and Dental University, Tokyo, Japan
8. ⁸Department of Otolaryngology, Hamamatsu University School of Medicine, Hamamatsu, Japan

Correspondence: Dr A Saito, MD, PhD, Department of Applied Molecular Medicine, Niigata University Graduate School of Medical and Dental Sciences, 1-757 Asahimachi-dori, Niigata 951-8510, Japan. E-mail: akisaito@med.niigata-u.ac.jp

Received 30 June 2004; Revised 2 December 2004; Accepted 4 December 2004; Published online 31 January 2005.

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.