Nephrotoxicity of uranium: pathophysiological, diagnostic and therapeutic perspectives

Uranium dynamics and developmental sensitivity in rat kidney

Renal toxicity is the principal health concern after uranium exposure. Children are particularly vulnerable to uranium exposure; with contact with depleted uranium in war zones or groundwater contamination the most likely exposure scenarios. To investigate renal sensitivity to uranium exposure during development, we examined uranium distribution and uranium-induced apoptosis in the kidneys of neonate (7-day-old), prepubertal (25-day-old) and adult (70-day-old) male Wistar rats. Mean renal uranium concentrations increased with both age-at-exposure and exposure level after subcutaneous administration of uranium acetate (UA) (0.1-2 mg kg(-1) body weight). Although less of the injected uranium was deposited in the kidneys of the two younger rat groups, the proportion of the peak uranium content remaining in the kidneys after 2 weeks declined with age-at-exposure, suggesting reduced clearance in younger animals. In situ high-energy synchrotron radiation X-ray fluorescence analysis revealed site-specific accumulation of uranium in the S3 segment of the proximal tubules, distributed in the inner cortex and outer stripe of the outer medulla. Apoptosis and cell loss in the proximal tubules increased with age-at-exposure to 0.5 mg kg(-1) UA. Surprisingly, prepubertal rats were uniquely sensitive to uranium-induced lethality from the higher exposure levels. Observations of increased apoptosis in generating/re-generating tubules particularly in prepubertal rats could help to explain their high mortality rate. Together, our findings suggest that age-at-exposure and exposure level are important parameters for uranium toxicity; uranium tends to persist in developing kidneys after low-level exposures, although renal toxicity is more pronounced in adults.

Human dietary exposure to uranium in catalonia, Spain

The purpose of this study was to determine the daily dietary intake of uranium (U) by the general population of Catalonia, Spain. Uranium concentrations were measured in foods widely consumed by the population living in that autonomous community. Food samples were randomly acquired in 12 representative cities of Catalonia. The dietary intake of U was estimated for various age-gender groups: children, adolescents, adults, and seniors. Fish and seafood was the food group showing the highest U concentrations (0.090 μg/g of fresh weight (fw)), followed by dairy products (0.044 μg/g fw). In contrast, the lowest U levels were found in oils and fats (0.003 μg/g fw), while in tubers and milk, U was not detected in any sample. The estimated dietary intake of U for a standard male adult of 70 kg body weight living in Catalonia was 15.48 μg/day. According to the age/gender of the population, the highest dietary intake of U corresponded to children (20.32 μg/day), while senior females was the subgroup with the lowest U intake (10.04 μg/day). Based on the tolerable daily intake established for U, the current dietary intake of this metal by the general population of Catalonia should not mean health risks for any of the different age/gender groups of consumers.

Increased urinary excretion of albumin, hemopexin, transferrin and VDBP correlates with chronic sensitization to gentamicin nephrotoxicity in rats

Drug nephrotoxicity is a serious health and economic problem worldwide. Rats can be acutely sensitized to acute kidney injury (AKI) by subnephrotoxic treatments with potentially nephrotoxic drugs. Acquired sensitization to AKI poses a silent risk impossible to diagnose pre-emptively with the technology available at the clinical level. Herein, we hypothesized whether a chronic, subnephrotoxic insult to the kidneys might result in chronically acquired sensitization to AKI, and whether chronic sensitization might be detected through specific urinary markers. To this end, rats were treated with a subtoxic dosage of the experimental nephrotoxin uranyl nitrate (UN) in the drinking water for 21 weeks, or plain water (as control), and then with low-dose gentamicin for 7 days. Renal function and renal tissue damage were evaluated through the experiment. The mild renal damage caused by gentamicin was markedly magnified in rats having received UN chronically, which was evident both at the functional and histological level. Four proteins, namely albumin, hemopexin, transferrin and vitamin D binding protein were increased in the urine in temporal association with the appearance of chronic predisposition. Although further studies are necessary, our results suggest that these proteins might be potentially used as markers of hidden, chronic predisposition to gentamicin nephrotoxicity, in order to appropriately and pre-emptively stratify and handle individuals according to their specific risk in the long term, and to conveniently optimize their life conditions or additional clinical procedures or treatments that might trigger the disease. This might reduce AKI incidence and severity and the associated costs.

Bioengineered chimeric spider silk-uranium binding proteins

Heavy metals constitute a source of environmental pollution. Here, novel functional hybrid biomaterials for specific interactions with heavy metals are designed by bioengineering consensus sequence repeats from spider silk of Nephila clavipes with repeats of a uranium peptide recognition motif from a mutated 33-residue of calmodulin protein from Paramecium tetraurelia. The self-assembly features of the silk to control nanoscale organic/inorganic material interfaces provides new biomaterials for uranium recovery. With subsequent enzymatic digestion of the silk to concentrate the sequestered metals, options can be envisaged to use these new chimeric protein systems in environmental engineering, including to remediate environments contaminated by uranium.

Association between drinking water uranium content and cancer risk in Bavaria, Germany

OBJECTIVE

To evaluate the possible association between uranium (U) content in public drinking water on the one hand and the risk of cancer of the colorectum, lung, female breast, prostate, kidney, and urinary bladder, total cancer, and leukemia on the other hand in Bavaria, an ecologic study on the level of municipalities was performed.

METHODS

Cancer incidence data for the years 2002-2008 were obtained from the population-based cancer registry Bavaria according to sex. Current U content data of public drinking water on the level of municipalities were obtained from a publicly available source. The possible association between drinking water U content and cancer risk adjusted for average socio-economic status was evaluated using Poisson regression.

RESULTS

Drinking water U content was below 20 μg/L in 458 out of 461 included municipalities. We found a significantly increased risk of leukemia in men in the intermediate (U level, 1.00-4.99 μg/L; relative risk [RR], 1.14) and in the highest U exposure category (U level, ≥5 μg/L; RR, 1.28). Moreover, in women, a significantly elevated risk was identified with respect to kidney cancer in the highest exposure category (RR, 1.16) and with respect to lung cancer in the intermediate exposure category (RR, 1.12).

CONCLUSION

The slightly increased risk of leukemia in men, kidney cancer in women, and lung cancer in women may require further investigation. If an increased cancer risk is confirmed, preventive measures (e.g., introduction of U filters in public water systems) may be considered.

Toxicity of depleted uranium on isolated rat kidney mitochondria

BACKGROUND

Kidney is known as the most sensitive target organ for depleted uranium (DU) toxicity in comparison to other organs. Although the oxidative stress and mitochondrial damage induced by DU has been well investigated, the precise mechanism of DU-induced nephrotoxicity has not been thoroughly recognized yet.

METHODS

Kidney mitochondria were obtained using differential centrifugation from Wistar rats and mitochondrial toxicity endpoints were then determined in both in vivo and in vitro uranyl acetate (UA) exposure cases.

RESULTS

Single injection of UA (0, 0.5, 1 and 2mg/kg, i.p.) caused a significant increase in blood urea nitrogen and creatinine levels. Isolated mitochondria from the UA-treated rat kidney showed a marked elevation in oxidative stress accompanied by mitochondrial membrane potential (MMP) collapse as compared to control group. Incubation of isolated kidney mitochondria with UA (50, 100 and 200μM) manifested that UA can disrupt the electron transfer chain at complex II and III that leads to induction of reactive oxygen species (ROS) formation, lipid peroxidation, and glutathione oxidation. Disturbances in oxidative phosphorylation were also demonstrated through decreased ATP concentration and ATP/ADP ratio in UA-treated mitochondria. In addition, UA induced a significant damage in mitochondrial outer membrane. Moreover, MMP collapse, mitochondrial swelling and cytochrome c release were observed following the UA treatment in isolated mitochondria.

GENERAL SIGNIFICANCE

Both our in vivo and in vitro results showed that UA-induced nephrotoxicity is linked to the impairment of electron transfer chain especially at complex II and III which leads to subsequent oxidative stress.