Identification of Functions of Peroxisome Proliferator- Activated Receptor in Proximal Tubules

Peroxisome proliferator-activated receptor (PPAR ) is a member of the steroid/nuclear receptor superfamily that is intensively expressed in the kidney, but its physiologic function is unknown. In this study, PPAR -null mice were used to help clarify the function. Starved PPAR null mice were found to secrete significantly more quantities of urine albumin than starved wild-type mice. Furthermore, the appearance of giant lysosomes, marked accumulation of albumin, and an impaired ability concerning albumin digestion were found only in proximal tubules of the starved PPAR null mice. These abnormalities were probably derived from ATP insufficiency as a result of the starvation-induced decline of carbohydrate metabolism and a lack of PPAR -dependent fatty acid metabolism. It is interesting that these abnormalities disappeared when glucose was administered. Taken together, these findings demonstrate important functions of PPAR in the proximal tubules, the dynamic regulation of the proteindegradation system through maintenance of ATP homeostasis, and emphasize the importance of the fatty acid metabolism in renal physiology. Recently, considerable attention has been paid to the peroxisome proliferator-activated receptor (PPAR ), which is known as a member of the steroid/nuclear receptor superfamily (1). According to some studies (2,3), a high level of PPAR is found in the kidney and mainly localizes in the proximal tubules, although its physiologic function has not yet been clarified. Proximal tubular epithelial cells are highly differentiated cells that reabsorb many substances that are essential to the body from glomerular filtrate and secrete several physiologically active compounds. It was suggested that ATP produced in the proximal tubular epithelial cells, which contain a greater density of mitochondria, is necessary for supporting its specific functions and maintaining basic cell functions (4). To elucidate the physiologic role of PPAR in the kidney, we examined the reabsorption process in the proximal tubules, using PPAR -null mice. We focused on albumin reabsorption, a typical component of the filtered protein handling, done through efficient receptor-mediated endocytosis in which megalin acts as the main receptor (5–8). The relationship between albumin reabsorption and energy production was also determined, because PPAR is known to play an important role as a potent regulator of mitochondrial energy production in the liver and heart (9,10). In addition, we used starved mice to increase the dependence on fatty acids as an energy fuel source and to reduce the effect of carbohydrate metabolites. Materials and Methods Materials Anti-mouse albumin IgG was purchased from Bethyl Laboratories (Montgomery, TX). Anti-rat cathepsin D IgG, ATP, acetyl-CoA, and tripalmitin were from Wako (Osaka, Japan). Anti-Rab5a IgG, antiRab7 IgG, and anti-mouse cathepsin L IgG were from Santa Cruz Biotechnology (Santa Cruz, CA). Anti–LAMP-1 IgG was from American Research Products (Belmont, MA). Anti-megalin polyclonal antiserum (11) was provided by Dr. Marilyn G. Farquhar (University of California, CA), and anti-rat -glucuronidase IgG (12) was from Dr. K. Sukegawa (Gifu University School of Medicine, Gifu, Japan). Animals and Starvation Treatment PPAR -null mice with an Sv/129 genetic background were produced as described previously (13). Wild-type Sv/129 were used as controls in all experiments. Two groups of mice, wild-type and PPAR -null mice, were starved for 48 h with water provided ad libitum. They were fed again for 48 h and then subjected to a second period of starvation for 48 h. After the second starvation, 150 l of 5% and 50% glucose solution and that of 2.2% and 22% tripalmitin suspension were respectively administered to the ( / ) mice by gavage. The administration was repeated three times every 3 h. Received October 30, 2001. Accepted March 15, 2002. Correspondence to Dr. Kazuhiko Hora, Second Department of Internal Medicine, Shinshu University School of Medicine, 3-1-1 Asahi, Matsumoto, 3908621, Japan. Phone: 81-263-37-2634; Fax: 81-263-32-9412; E-mail: yujibeat@hsp.md. shinshu-u.ac. jp