Animals
Male Fischer 344 rats, aged 5 weeks and with body weight (BW) of 70–90 g, were purchased from Japan SLC, Inc. (Shizuoka, Japan). The animals were maintained in a filtered air laminar flow at the Institute of Laboratory Animal Sciences, Frontier Science Research Center, Kagoshima University. Room temperature was maintained at 25 ± 2°C and relative humidity at 55 ± 10%, with a 12 h light/dark cycle. They were provided with commercial rat chow (CE-2, CLEA Inc., Tokyo, Japan) and tap water, and were used after acclimation for 7 days. After the start of the experiment, sedentary rat groups were fed an amount equivalent to the mean food intake of the exercise groups. The use of animals in this research complied with all relevant guidelines set by Kagoshima University.
Preparation of Fe-NTA solution
Fe(NO3)3 and Na2NTA were purchased from Wako Pure Chemical (Osaka, Japan). Fe-NTA solution was prepared immediately before use by the method of Toyokuni et al. [13]; Fe(NO3)3 and Na2NTA were each dissolved in Milli-Q water and were mixed in molar ratio of 1:4. pH was adjusted to 7.4 with NaHCO3.
Study design
Rats were divided into six groups (Fig. 1): group I, saline treatment and nonexercise (n = 8); group II, Fe-NTA treatment and nonexercise (n = 8); group III, saline treatment and short-term exercise (n = 8); group IV, Fe-NTA treatment and short-term exercise (n = 8); group V, saline treatment and long-term exercise (n = 7); and group VI, Fe-NTA treatment and long-term exercise (n = 8). The short-term exercise regimen was 12 weeks treadmill running with Rat Runner RR-1200 (Agawa Machine Inc., Shimane, Japan). The long-term exercise regimen was 40 weeks treadmill running. Fe-NTA or saline was administered once a day by i.p. injection (5 mg Fe/kg BW) for the initial 7 days, and was then administered 3 times per week (10 mg Fe/kg BW) for 11 weeks. Treadmill training (8 m/min at 0% incline) was carried out for 15 min before injection up to 12 weeks (groups III, IV, V, and VI). Group V and VI rats were then trained up to 24 weeks, 8 m/min at 0% incline for 30 min, 5 days/week. Due to a decline in exercise ability, the exercise intensity was decreased afterwards, i.e., up to 39 weeks, 8 m/min at 0% incline for 15 min, 5 days/week; and up to 40 weeks, 8 m/min at 0% incline for 15 min, 3 days/week.
Necropsy and gross examination
At the end of the experiment, all animals were anesthetized and killed by exsanguination through the abdominal aorta. At necropsy, the surface of the bilateral kidneys was grossly examined for nodules, the kidneys were cut to thickness of 2–3 mm, and transections were then grossly examined for nodules. The longest and shortest diameters of all nodules were measured, and their lengths multiplied together to calculate the area (mm2). The area of each nodule is expressed as mm2/nodule. Each animal, and the bilateral kidneys and adrenal glands, were weighed.
We initially planned six groups, but RCC was not found in the saline treatment groups. Therefore, we investigated the effects of HE on development of RCC in the Fe-NTA treatment groups (II, IV, and VI).
Histopathological examination
The kidneys were fixed in 10% phosphate-buffered formalin, dehydrated, and then embedded in paraffin. The maximum transection surface of the kidney (center of the kidney) and gross nodules were sectioned to 5 μm, stained routinely with hematoxylin and eosin (H&E) stain, and then examined histopathologically. The numbers of microcarcinomas (Mcs), KCs, degenerative tubules (DTs), proximal tubules (PTs) with brown droplet (BD), and BDs in the maximum transection surface were counted. All Mcs in the maximum transection surface were <1 mm in diameter and could not be grossly observed, but features of carcinoma, such as atypia and duct, and relatively clear border were microscopically observed in Mcs (Fig. 2c). The areas (mm2) of Mcs were also measured by Image Processor for Analytical Pathology (IPAP; Sumika Technoservice Corporation, Hyogo, Japan).
Kidney sections were stained histochemically by Berlin blue (BB), Turnbull blue (TB), periodic acid Schiff (PAS), and Schmorl stain for BDs in PTs.
Immunohistochemistry for PCNA
Deparaffinized kidney sections were examined immunohistochemically. After endogenous peroxidase activity had been blocked with 2% hydrogen peroxide, the sections were pretreated with 10 mM citrate buffer (pH 6.0) by microwaves. After rinsing with phosphate-buffered saline (PBS), the sections were stained with Zymed’s proliferating cell nuclear antigen (PCNA) staining kit (Zymed Laboratories Inc., San Francisco, CA) according to the manufacturer’s instructions. The sections were then visualized with VECTASTAIN Elite ABC kit (Vector Laboratories, USA) according to the manufacturer’s instructions. The sections were then washed, counterstained with hematoxylin, dehydrated, cleared in xylene, and mounted. RCC diagnosed in this study was used as a positive control for PCNA, and cells with stained nuclei in PTs were interpreted as positive. PCNA-stained cell nuclei were counted in PTs of five microscopic fields (1 field = approximately 0.148 mm2) selected randomly. Mean numbers of positive nuclei in the five microscopic fields were calculated. Each PT contained approximately 8–24 epithelial cells.
Immunohistochemistry for apoptotic cells by TUNEL
Deparaffinized kidney sections from four animals selected randomly from each group were treated with proteinase K (20 μg/ml) for 15 min at room temperature, and then endogenous peroxidase was blocked with 2% hydrogen peroxidase. The sections were stained with Apop Tag Plus peroxidase in situ apoptosis detection kit (Intergen, Purchase, NY) according to the manufacturer’s instructions. The positive section in the kit was used as a positive control for terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL), and cells with stained nuclei were interpreted as positive in PTs. Immunostained cell nuclei were counted in PTs in five microscopic fields (1 field = approximately 0.148 mm2) selected randomly.
Statistics
Mean differences were evaluated by least significant difference (LSD). Data are shown as mean ± standard deviation (SD). Incidences (percentages) of rats with gross nodule, Mc, and KC were tested using a fourfold contingency table (chi-square test); p < 0.05 was considered statistically significant.