Application of path analysis to urinary findings of cadmium-induced renal dysfunction

Histomorphological and ultrastructural cadmium-induced kidney injuries and precancerous lesions in rats and screening for biomarkers

Long-term exposure to cadmium (Cd) can severely damage the kidney, where orally absorbed Cd accumulates. However, the molecular mechanisms of Cd-induced kidney damage, especially the early biomarkers of Cd-induced renal carcinogenesis, are unclear. In the present study, we established a rat kidney injury model by intragastric administration of Cd to evaluate the morphological and biochemical aspects of kidney injury. We randomly divided Sprague-Dawley rats into control, low Cd (3 mg/kg), and high Cd (6 mg/kg) groups and measured biochemical indices associated with renal toxicity after 2, 4, and 8 weeks of treatment. The Cd-exposed mice had significantly higher Cd concentrations in blood and renal tissues as well as blood urea nitrogen (BUN), β2-microglobulin (β2-MG), urinary protein excretion, and tumor necrosis factor-α (TNF-α) levels. Furthermore, histopathological and transmission electron microscopy (TEM) observations revealed structural disruption of renal tubules and glomeruli after 8 weeks of exposure to the high Cd regimen. Besides, microarray technology experiments showed that Cd increased the expression of genes related to the chemical carcinogenesis pathway in kidney tissue. Finally, combining the protein–protein interaction (PPI) network of the Cd carcinogenesis pathway genes with the microarray and Comparative Toxicogenomics Database (CTD) results revealed two overlapping genes, CYP1B1 and UGT2B. Therefore, the combined molecular and bioinformatics experiments’ results suggest that CYP1B1 and UGT2B are biomarkers of Cd-induced kidney injury with precancerous lesions.

MiR-122-5p and miR-326-3p: Potential novel biomarkers for early detection of cadmium exposure

Cadmium is a common environmental and occupational pollutant and can produce toxic effects in a range of organs, especially in kidneys, after long-term exposure. MicroRNAs are ideal candidate biomarkers for various types of disorders, including renal diseases. In this study, we profiled the global miRNA expressions in rat kidneys using miRNA microarrays and found a collection of differentially expressed miRNAs induced by cadmium exposure. Among all of the candidate miRNAs, we identified miR-122-5p and miR-326-3p as early biomarkers for cadmium-induced nephrotoxicity. The two-miRNA signature was validated by quantitative real-time PCR in HK-2 and NRK-52E cells, rat kidney, serum and urine samples, and serum of an occupational population. Our results indicate that miR-122-5p and miR-326-3p may be potential biomarkers for cadmium exposure.

Evaluation of factors associated with cadmium exposure and kidney function in the general population

Cadmium (Cd) is a nonessential toxic metal which is widely distributed in the environment. The general population is exposed to low levels of Cd and the kidney is the organ most sensitive to Cd toxicity. This study was performed to simultaneously evaluate Cd exposure, kidney function, and oxidative stress biomarkers in the general population. A total of 643 adults were interviewed to document demographic characteristics, lifestyles, past-medical history, and diet during the last 24 h. We estimated daily Cd intake based on the diet of study subjects who had not been exposed to Cd occupationally. Whole blood and urine samples were collected and analyzed to determine Cd concentrations and kidney function indices (β₂ -microglobulin [β₂-MG], N-acetyl-β-D-glucosaminidase [NAG], metallothionein [MT]). The oxidative stress index (malondialdehyde [MDA]) was determined from the urine. The daily Cd intake from diet was established as 7.07 μg/day. The mean concentration of Cd measured in the blood was 1.22 μg/L and urine was 0.95 μg/g creatinine. The concentrations of Cd in blood and urine were higher in females than in males. The blood levels of Cd were affected by sex, age, and smoking, and urine Cd was influenced by sex, age, and blood Cd. The urine Cd was positively correlated with MT, NAG activity, and MDA in females, but with NAG only in males. The blood Cd was associated with MT in males. Increased NAG activity was observed when Cd in urine reached 1.0 μg Cd/g creatinine and was also affected by age, hypertension, and diabetes mellitus. Urinary MT only responded to Cd in urine or blood. In summary, exposure to Cd in the general population was influenced by various factors including sex, age, and smoking habits. Such exposure might eventually cause tubular damage in the kidneys through the oxidative stress mechanism, and females might be more susceptible than males to Cd exposure under the environment.

Preclinical evaluation of novel urinary biomarkers of cadmium nephrotoxicity

As a result of the widespread use of Cd in industry and its extensive dissemination in the environment, there has been considerable interest in the identification of early biomarkers of Cd-induced kidney injury. Kim-1 is a transmembrane glycoprotein that is not detectable in normal kidney, but is up-regulated and shed into the urine following ischemic or nephrotoxic injury. Recent studies utilizing a sub-chronic model of Cd exposure in the rat have shown that Kim-1 is an early urinary marker of Cd-induced kidney injury. Kim-1 was detected in the urine 4-5 weeks before the onset of proteinuria and 1-3 weeks before the appearance of urinary metallothionein and Clara cell protein 16, which are standard markers of Cd nephrotoxicity. In the present study, we have compared the time course for the appearance of Kim-1 in the urine with the time course for the appearance of alpha glutathione-S-transferase (alpha-GST), N-acetyl-beta-D-glucose amidase (NAG) and Cd, each of which have been used or proposed as urinary markers of Cd nephrotoxicity. Adult male Sprague-Dawley rats were given daily subcutaneous injections of 0.6 mg (5.36 micromoles)/kg Cd, 5 days per week for up to 12 weeks. One day each week, 24 h urine samples were collected and analyzed for protein, creatinine and the various markers. The results showed that significant levels of Kim-1 appeared in the urine as early as 6 weeks into the treatment protocol and then continued to rise for the remainder of the 12 week treatment period. By contrast, significant levels of alpha-GST and NAG did not appear in the urine until 8 and 12 weeks, respectively, while proteinuria was not evident until 10 weeks. The urinary excretion of Cd was below the level of detection until week 4 and then showed a slow, linear increase over the next 6 weeks before increasing markedly between weeks 10 and 12. These results provide additional evidence that Kim-1 is a sensitive biomarker of the early stages of Cd-induced proximal tubule injury.

Kidney injury molecule-1 is an early biomarker of cadmium nephrotoxicity

Cadmium (Cd) exposure results in injury to the proximal tubule characterized by polyuria and proteinuria. Kidney injury molecule-1 (Kim-1) is a transmembrane glycoprotein not normally detected in the mature kidney, but is upregulated and shed into the urine following nephrotoxic injury. In this study, we determine if Kim-1 might be a useful early biomarker of Cd nephrotoxicity. Male Sprague-Dawley rats were given daily injections of Cd for up to 12 weeks. Weekly urine samples were analyzed for Kim-1, protein, creatinine, metallothionein, and Clara cell protein CC-16. Significant levels of Kim-1 were detected in the urine by 6 weeks and continued to increase throughout the treatment period. This appearance of Kim-1 occurred 4-5 weeks before the onset of proteinuria, and 1-3 weeks before the appearance of metallothionein and CC-16. Higher doses of Cd gave rise to higher Kim-1 excretion. Reverse transcriptase-polymerase chain reaction (RT-PCR) expression analysis showed that Kim-1 transcript levels were increased after 6 weeks at the low dose of Cd. Immunohistochemical analysis showed that Kim-1 was present in proximal tubule cells of the Cd-treated rats. Our results suggest that Kim-1 may be a useful biomarker of early stages of Cd-induced proximal tubule injury.

Cell adhesion molecules in chemically-induced renal injury

Cell adhesion molecules are integral cell-membrane proteins that maintain cell-cell and cell-substrate adhesion and in some cases act as regulators of intracellular signaling cascades. In the kidney, cell adhesion molecules, such as the cadherins, the catenins, the zonula occludens protein-1 (ZO-1), occludin and the claudins are essential for maintaining the epithelial polarity and barrier integrity that are necessary for the normal absorption/excretion of fluid and solutes. A growing volume of evidence indicates that these cell adhesion molecules are important early targets for a variety of nephrotoxic substances including metals, drugs, and venom components. In addition, it is now widely appreciated that molecules, such as intracellular adhesion molecule-1 (ICAM-1), integrins, and selectins play important roles in the recruitment of leukocytes and inflammatory responses that are associated with nephrotoxic injury. This review summarizes the results of recent in vitro and in vivo studies indicating that these cell adhesion molecules may be primary molecular targets in many types of chemically-induced renal injury. Some of the specific agents that are discussed include cadmium (Cd), mercury (Hg), bismuth (Bi), cisplatin, aminoglycoside antibiotics, S-(1,2-dichlorovinyl)-l-cysteine (DCVC), and various venom toxins. This review also includes a discussion of the various mechanisms, by which these substances can affect cell adhesion molecules in the kidney.