The Reference Dose for Subchronic Exposure of Pigs to Cadmium Leading to Early Renal Damage by Benchmark Dose Method

toxicological sciences 128(2), 524–531 (2012) doi:10.1093/toxsci/kfs173 Advance Access publication May 18, 2012 The Reference Dose for Subchronic Exposure of Pigs to Cadmium Leading to Early Renal Damage by Benchmark Dose Method Xiaosheng Wu,*,† Shuai Wei,* Yimin Wei,*,1 Boli Guo,* Mingqi Yang,† Duoyong Zhao,* Xiaoling Liu,* and Xianfeng Cai* *Institute of Agro-Products Processing Science and Technology, Chinese Academy of Agricultural Sciences/Key Laboratory of Agro-Products Processing, Ministry of Agriculture, P.O. Box 5109, Beijing 100193, China; and †Department of Veterinary Medicine, Northwest A&F University, Yangling; Shaanxi 712100, China 1 To whom correspondence should be addressed. Fax: +86-010-62895141. E-mail: . Received December 3, 2011; accepted April 30, 2012 Cadmium (Cd) is a relatively rare element that occurs naturally in ores together with zinc and lead. Industrial uses of Cd and agricultural uses of phosphate fertilizers have caused widespread dispersion of the metal at trace levels into the environment and human foodstuffs (Satarug et al., 2003; WHO, 1992). Cd is efficiently retained in the kidney and liver in human body through dietary exposure, with a very long biological half-life ranging from 10 to 30 years (Nordberg, 2007; Staessen, 1994; WHO, 1992). Cd can cause several adverse health effects. Together with other heavy metals such as lead and mercury, Cd is considered to be a threat to human health (Jarup, 2003; WHO, 2000). Kidney is the critical target organ for dietary exposure to Cd. Renal damage is characterized by Cd accumulation in convoluted proximal tubules followed by glomerular damage (Åkesson et al., 2005; Järup and Åkesson, 2009; Suwazono Y., 2010; WHO, 1996). On the basis of epidemiological data, the Joint Food and Agriculture Organization/World Health Organization (FAO/WHO) Expert Committee on Food Additives set the provisional tolerable weekly intake (PTWI) of Cd at 7 μg/kg body weight, which corresponds to 1 μg/kg body weight for each day of the week (i.e., 70 μg/day for a person of 70-kg body weight). Recently, the European Food Safety Authority (EFSA) carried out a meta-analysis applying the benchmark dose (BMD) approach (EFSA, 2009). Based on the one-compartment model, translating urine cadmium (U-Cd) into dietary exposure (Amzal et al., 2009), EFSA arrived at a tolerable weekly intake (TWI) of 2.5 μg Cd/ kg body weight, which was much lower than the provisional TWI of 7 μg Cd/kg body weight established by the Joint FAO/ WHO Expert Committee on Food Additives. Furthermore, emissions of Cd from various industries and the combustion of waste and fossil fuels have resulted in a large increase in the concentrations of Cd in soils over the last century (Järup et al., 1998; Thomas et al., 2009). The high transfer rate of Cd from soil to plants, coupled with continuing mobilization of small amounts of the metal from non-bioavailable geologic matrices into biologically accessible media, led to the prediction that human exposure to dietary Cd will gradually increase in the next 10–20 years (Satarug and Moore, 2004). Exposure to Cd in the general population is already close to the critical level, particularly in susceptible population groups and populations living close to polluting industries. However, the concentration © The Author 2012. Published by Oxford University Press on behalf of the Society of Toxicology. All rights reserved. For permissions, please email: Pigs were exposed to cadmium (Cd) (in the form of CdCl2) concentrations ranging from 0 to 32 mg Cd/kg feed for 100 days. Urinary cadmium (U-Cd) and blood cadmium (B-Cd) levels were determined as indicators of Cd exposure. Urinary levels of β2-microglobulin (β2-MG), α1-microglobulin (α1-MG), N-acetyl-β-D-glucosaminidase (NAG), cadmium-metallothionein (Cd-MT), and retinol binding protein (RBP) were determined as biomarkers of tubular dysfunction. U-Cd concentrations were increased linearly with time and dose, whereas B-Cd reached two peaks at 40 days and 100 days in the group exposed to 32 mg Cd/kg. Hyper-metallothionein-urinary (HyperMTuria) and hyper-N-acetyl-β-D-glucosaminidase-urinary (hyperNAGuria) emerged from 80 days onwards in the group exposed to 32 mg Cd/kg feed, followed by hyper-β2-microglobulin-urinary (hyperβ2-MGuria) and hyper-retinol-binding-protein-urinary (hyperRBPuria) from 100 days onwards. The relationships between the Cd exposure dose and biomarkers of exposure (as well as the biomarkers of effect) were examined, and significant correlations were found between them (except for α1-MG). Dose– response relationships between Cd exposure dose and biomarkers of tubular dysfunction were studied. The critical concentration of Cd exposure dose was calculated by the benchmark dose (BMD) method. The BMD10/BMDL10 was estimated to be 1.34/0.67, 1.21/0.88, 2.75/1.00, and 3.73/3.08 mg Cd/kg feed based on urinary RBP, NAG, Cd-MT, and β2-MG, respectively. The calculated tolerable weekly intake of Cd for humans was 1.4 μg/kg body weight based on a safety factor of 100. This value is lower than the currently available values set by several different countries. This indicates a need for further studies on the effects of Cd and a re-evaluation of the human health risk assessment for the metal. Key Words: cadmium; renal dysfunction; biomarkers; benchmark dose; subchronic exposure. SUBCHRONIC EXPOSURE TO CADMIUM AND RENAL DAMAGE MATERIALS AND METHODS Ethical approval of the study protocol. The study protocol was approved by the Ethics Committee of the Institute of Agro-Food Science and Technology (Beijing, China). Experimental setup. Twenty WZSPs (F18, i.e., 2.5–3 months) of inbred strain (inbreeding coefficient, 0.979) were purchased from the Institute of Animal Sciences, Chinese Academy of Agricultural Sciences (Beijing, China). They were housed under conventional conditions at 24–32°C, with natural light and humidity of 45–65%. They were fed twice a day (7:30 am and 4:30 pm). The feed intake was 3% of body weight (Feng et al., 1999). The feed was mainly composed of 67.6% corn, 5% wheat, 20% soybean meal, and 7.4% concentrates. The concentrates contained fish meal, cottonseed meal, soy protein concentrate, copper (Cu), iron (Fe), zinc (Zn), manganese (Mn), vitamin A, vitamin C, vitamin E, and lysine. The concentrations of Cu, Fe, Zn, Mn, Pb are 144.02, 55.26, 161.27, 135.28, and 0.046 mg/kg feed, respectively. Tap water (pH, 6.5–6.8; Cd concentration < 0.001 μg/L) was supplied continuously. Pigs were chosen randomly and subdivided into five groups (four each group). They were exposed to 0, 0.5, 2, 8, and 32 mg Cd/kg in the feed for 100 days, respectively. Cd was added to the feed in the form of cadmium chloride (CdCl2: > 99%; Shanghai Chemical Reagent Company, Shanghai, China). Pigs were weighed before exposure and placed in a metabolic cage for urine collection. Drinking water was available ad libitum in the metabolic cage. Body weight was measured every 20 d (...truncated)