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 (CCl 4 )-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 CCl 4 -treated rats was low and did not differ from that in the control rat. When 35 S-L-FABP was intravenously administered to rats, the kidneys took up 35 S-L-FABP more preferentially than the liver and heart, and histoautoradiography of kidney sections revealed that 35 S-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 Ca 2 þ -dependent manner. Degradation assays using megalin-expressing rat yolk sac tumor-derived L2 cells demonstrated that megalin mediated the cellular uptake and catabolism of 125 I-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.

Sex differences in the excretion levels of traditional and novel urinary biomarkers of nephrotoxicity in rats

“…In the kidney, there are receptors and transporters such as sodium/glucose co-transporter-2 and organic cation transporter 2, which have been reported that testosterone changed their expression levels in rats (Pávó et al, 1995;Urakami et al, 2000;Ljubojevic et al, 2004;Sabolic et al, 2012;Loh et al, 2017). Meanwhile, sex differences of megalin and cubilin, receptors related to reabsorption of LMWPs and albumin which are localized in the renal proximal tubules (Cui et al, 1996;Orlando et al, 1998;Birn et al, 2000;Oyama et al, 2005;Hvidberg et al, 2005;Kaseda et al, 2007;Amsellem et al, 2010), have been ambiguous. It might be possible that one or more receptors and transporters contributing to reabsorption of LMWPs and albumin were affected by testosterone, and their excretion levels had varied.…”

Biomarkers of nephrotoxic acute kidney injury

“…The primary function of FABP is the facilitation of long-chain free fatty-acid transport from the plasma membrane to sites for oxidation (mitochondria, peroxisomes) (Oyama et al, 2005;Pelsers et al, 2005). Two types of FABP have been identified in the human kidney, liver-type FABP (L-FABP) in the proximal tubule and heart-type FABP (H-FABP) in the distal tubule (Maatman et al, 1991;Maatman et al, 1992).…”

Mapping the Extracellular and Membrane Proteome Associated with the Vasculature and the Stroma in the Embryo

“…Fatty acid binding proteins are a family of small, highly conserved carrier proteins that bind long-chain fatty acids and other hydrophobic ligands. This protein can also be found outside the cell and is able to bind bile acids (48). It is thought that the roles of fatty acid binding proteins include fatty acid uptake, transport, and metabolism and are regulated by liver-enriched transcription factors HNF3␤ and C/EBP␣ (49).…”