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.

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.

Advanced glycation end products (AGEs) are formed by the nonenzymatic Maillard reaction between sugars and proteins. Low-molecular weight AGEs are filtered by renal glomeruli and then reabsorbed and metabolized by proximal tubule cells (PTCs). High-molecular weight AGEs are also delivered to PTCs in proteinuric states. In patients with diabetes, AGE generation is increased, and the actions of AGEs on PTCs are likely involved in the pathogenesis of diabetic nephropathy. In patients with chronic renal failure (CRF), reduced renal metabolism of AGEs likely accounts for the accumulation of AGEs in serum, leading to uremic complications including dialysis-related amyloidosis. AGE precursors such as reactive carbonyl compounds also accumulate in the sera of patients with CRF. It is likely that PTCs take up AGEs and AGE precursors via specific endocytotic receptors or transporters. Megalin is a multiligand endocytotic receptor that is abundantly expressed on PTCs. There is evidence that megalin is involved in the cellular uptake and degradation of AGEs. We previously reported a cell therapy model involving implantation of megalin-expressing cells into experimental mice with renal failure for elimination of uremic toxin proteins. Further studies are needed to clarify the molecular mechanisms of the metabolism of AGEs and their precursors to develop a strategy for the treatment of diabetic nephropathy and uremic complications of CRF.

A reduction treatment using carbon powder for reducing TiO 2 to fabricate non-stoichiometric titanium dioxide, TiO 2Àx , was proposed and performed. The carrier density and non-stoichiometric number were calculated by using thermogravimetry (TG) while heating and re-oxidizing TiO 2Àx in air. The thermoelectric properties of TiO 2Àx were measured and evaluated in air. The results show that TiO 2Àx can be simply and safely obtained by reducing insulating TiO 2 through the proposed reduction treatment. The carrier density increases and non-stoichiometric number decrease with the temperature of the reduction treatment and have a good correspondence with the decrease in the electrical resistivity of TiO 2Àx . Significantly improved thermoelectric properties were observed as a consequence of the decreasing electrical resistivity of TiO 2Àx .

Leptin can regulate several immune functions. However, the role of leptin on lymphocyte function has not been recognized in vivo. Accordingly, we have investigated the effect of leptin on starvation-induced immune dysfunction using diet-induced obese mice. To induce obesity, C57BL/6J mice were fed a high-fat diet for 14 weeks and control mice were fed a standard diet for the same period. The obese and control groups of mice were then starved for 48 h, and received intraperitoneal injections of recombinant leptin or phosphate-buffered saline four times during starvation. Other control mice in both diet groups were free fed without being starved. Although starvation of the control mice dramatically reduced the weights of the immune organs, cytokine production and increased proliferation of cultured splenocytes, these levels returned to those of the free-feeding groups with exogenous leptin administration. However, these effects of leptin were not observed in obese mice. These findings provide some evidence that leptin can regulate the immune function in vivo. It is also suggested that the action of leptin might not appear in obesity.