These solid slags, including copper concentrate slag, iron ore slag, copper and sulfur ore slag, and cobalt-nickel-manganese waste residue—are commonly imported by customs as recoverable metals and are recognized for their toxic, mutagenic, teratogenic, and carcinogenic effects on humans and various animals due to exposure to heavy metals from slag waste (HMSWs)[18]. Iron, copper, manganese, cobalt, and nickel serve as crucial trace elements in the human body, playing a pivotal role in biological processes at low concentrations. However, heavy metals pose concerns due to the challenges they present in body excretion. When heavy metal concentrations surpass a certain threshold, they tend to accumulate in multiple organs and tissues, particularly the kidneys[19], leading to serious damage. This accumulation can trigger diseases, including cancer and malignant tumors[20].
In toxicology experiments, body weight serves as a crucial indicator reflecting bodily damage[21]. Our experiment revealed a significant decrease in the rate of rat body weight growth within 4 weeks of exposure to metal solid waste leachate, as compared to the control group. This decrease signifies the toxic effect of exposure to HMSWs, resulting in growth retardation. The toxicity of various HMSWs leachate exhibited a distinct, time-dependent relationship, indicating the accumulation of observable toxicity. These results support the findings of Fiati Kenston et al[22].
Multiple parameters such as body weight growth rate, biochemical indices, and oxidative system balance must be considered to assess the toxicity of different HMSWs accumulated in the kidney. The hematopoietic system stands as one of the most sensitive organs indicating organismal toxicity[23]. In our study, we observed significantly decreases in rat erythrocyte counts in various HMSWs exposure groups, while the leukocyte counts significantly increased. Notably, in the Cu&S and Cobalt-Nickel-Manganese ore exposure groups, hemoglobin content decreased significantly, whereas in Cu&S and Fe ore exposure groups, it increased slightly or remained unchanged. This variation might relate to the necessity of Cu and Fe in Hb synthesis. Elevated concentrations of copper and CoNiMn induced microcytic anemia and erythrocyte hemolysis, while lower doses of copper and iron might mitigate these conditions[24].
In this study, Heavy metal solid wastes (HMSWs) include transition elements, namely iron, copper, manganese, cobalt, and nickel. They have the potential to disrupt both non-enzymatic and enzymatic antioxidant components, hinder enzyme activity, accelerate the lipids and proteins degradation, and trigger excessive production of reactive oxygen species (ROS)[25]. The excessive production of ROS, combined with irregular function of the antioxidant system, induces oxidative stress and disrupts signaling transduction pathways. SOD function as the primary defense against the adverse effects of ROS adverse in the antioxidant system, while MDA serves as a crucial indicator of oxidative stress, amplifying damage by accelerating the oxidative cascade through binding to subcellular molecules such as proteins and nucleic acids[26]. The biochemical indexes of kidney homogenate indicated significant reductions in SOD levels and notable increases in MDA content among rats in various groups exposed to solid waste dyeing compared to the control group. These findings indicated that HMSWs worsen induced oxidative damage in rat kidney tissues by increasing MDA content and decreasing SOD activity, which aligns with the study conducted by Jadhav et al[27].
Studies demonstrate a significant increase in blood lactate dehydrogenase (LDH) activity following exposure to heavy metals exposure[28]. In toxicology, the release of LDH serves as an indicator of irreversible cellular damage[29]. Metabolic disturbances induced by cellular damage result in the significant release of LDH into the bloodstream, leading to varying levels of increased serum LDH activity[30]. Furthermore, since the kidney is the organ responsible for LDH excretion, kidney injury is closely associated with changes in serum LDH levels[31], which aligns with our findings showing higher LDH levels in various HMSWs compared to the control group. Principal component analysis of kidney-related variables indicated elevated loadings of malondialdehyde (MDA), superoxide dismutase (SOD), and lactate dehydrogenase (LDH), suggesting a strong correlation among these substances within the rat kidney.
The glomerular filtration rate (GFR) is a common test used to assess renal function. Decreases in GFR occur when toxic substances are reabsorbed from the filtrate, leading to damage in renal tubules and indicating renal injury[32]. Our study revealed a decrease in GFR in rats exposed to solid waste dyeing, indicating the detrimental impact of HMSWs on glomerular filtration. This toxic effect may be associated with tubular necrosis and glomerular failure. Alterations in creatinine (Cr) and blood urea nitrogen (BUN) indices, commonly used for diagnosing renal injuries, reflect kidney damage[33]. Our study revealed higher levels of creatinine (Cr) and blood urea nitrogen (BUN) in both serum and urine of rats exposed to various groups exposed to heavy metal solid waste dyeing compared to the control, signifying kidney damage and confirming renal insufficiency while indicating the extent of glomerular filtration damage. Furthermore, the BUN/Cr ratio is a reliable method for detecting renal insufficiency[34, 35]. Elevated urea nitrogen/creatinine ratios correlate with acute kidney injury, indicating that exposure to solid waste dyeing treatments induced renal insufficiency in rats, thereby increasing the risk of nephropathy development.
Investigation using Pearson correlation and a decision tree model revealed a strong association between changes in serum lactate dehydrogenase (LDH) and early kidney toxicity. LDH showed strong correlation with indicators for superoxide dismutase (SOD), malondialdehyde (MDA), and glomerular filtration rate (GFR). Moreover, LDH levels exceeding 6.82 U/gprot indicated severe kidney injury in the cobalt, nickel, manganese, and copper-sulfur slag dyeing groups, and showed significant correlation coefficients. Collectively, these findings suggested that LDH as a crucial indicator for early evaluation of oxidative stress injury caused by heavy metal solid waste. When combined with creatinine (Cr), blood urea nitrogen (BUN), and other functional markers, it enables comprehensive early screening for nephrotoxicity. LDH, an NAD+-dependent kinase crucial in intracellular oxidative and antioxidant processes, responds to levels of in vivo oxidative stress. These findings validate the application of toxicogenomics in predicting renal mechanisms of oxidative stress injury.
The toxicogenomic analysis revealed that heavy metal-induced kidney injury and repair processes at the cellular level demonstrated oxidative stress. Accumulation of reactive oxygen species (ROS) leads to both apoptosis and necrosis, along with a substantial rise in lipid peroxides. Redox imbalance prompts uncontrolled cell proliferation. Cells manage oxidative stress responses by regulating signaling molecules such as MAPK, Ferroptosis, and PI3K-Akt, which play roles in defense, death, and proliferation. The oncogenicity of heavy metals appears associated with ROS mediation. Understanding how ROS activate signaling pathways in diverse heavy metal stresses, including defense mechanisms, warrants further investigation. Increased leukocyte levels might stem from immune system dysregulation linked to pathogen activation, culminating in inflammation. Studies suggest a link between renal cell injury and leukocytosis. Emerging evidence indicates that heavy metal-induced nephrotoxicity and the inflammation correlate with heightened oxidative stress and proinflammatory factors production, aligning with our toxicogenomic findings.