Cadmium is a toxic substance of increasing significance due to occupational contaminant, industrialization, smoking, and lack of efficient treatment for Cd poisoning (Afifi and Embaby, 2016). Cd has been documented to have a range of biological toxicities, including reproductive toxicity, neurotoxicity, genotoxicity, and teratogenic effects (Abdel Moneim et al., 2014; Phuagkhaopong et al., 2017). Although this heavy metal toxicity is now documented in almost every organ, the central nervous system remains highly sensitive to low levels of Cd exposure (Alnahdi and Sharaf, 2019). Cd accumulation in brain tissue by crossing the blood-brain barrier is considered an important mechanism of brain damage caused by this metal (Ben et al., 2016). Accumulating evidence suggests that Cd attacks thiol group-containing proteins, including GSH, and causes disruptions in energy metabolism, membrane integrity, and mitochondrial function. It has been reported that oxidative damage and excessive ROS production are triggered as a result of the resulting deterioration. In addition, Cd can replace some cofactors such as zinc and iron, which are required for SOD and CAT activities, thus reducing the antioxidant capacity of these enzymes (Al Olayan et al., 2020; Martelli et al., 2006). Recent experimental studies have reported that Cd causes oxidative stress by decreasing the levels of non-enzymatic and enzymatic antioxidants biomarkers in the rat brain tissue (Adefegha et al., 2016; Al Olayan et al., 2020). As shown in Fig. 1A-E, Cd significantly increased lipid peroxidation and decreased antioxidant enzyme activities (SOD, CAT and GPx) and GSH level in brain tissue. However, CRV treatment reduced Cd-induced brain damage by increasing antioxidant enzyme activities that had been reduced by Cd.
Exposure to Cd is associated with neuroinflammation due to increased secretion of proinflammatory cytokines (Al Olayan et al., 2020). It has been documented that Cd can induce neuroinflammatory pathology by mechanisms including activation of microglia, blood-brain barrier leakage, and infiltration of immune cells into the brain (Ashok et al., 2015; Rogers et al., 2016). NF-κB and MAPK signaling pathways play an important role in regulating the expression of proinflammatory genes and are considered promising target molecules for the treatment of neuroinflammatory diseases (Cho et al., 2016; Çelik et al., 2020a). Notably, NF-κB is known as a ROS-sensitive nuclear transcription factor that regulates the expression of proinflammatory mediators such as TNF-α, IL-1β, IL-6, COX-2 and iNOS (Yardım et al., 2021). NF-κB can be up-regulated by p38 MAPK because p38 MAPK affects NF-κB levels by promoting phosphorylation of IκB, resulting in dissociation and degradation of IκB and NF-κB complexes (Kandemir et al., 2020). NF-κB leaving the IκB complex enters the nucleus and regulates the transcription of the above-mentioned target genes (Caglayan et al., 2019a; Liu et al., 2018). In a previous study, it was determined that cadmium increased the levels of NF-κB, p38 MAPK, IL-6 and IL-8 in neuron and astrocyte cell culture (Phuagkhaopong et al., 2017). In the present study, Cd increased the levels of inflammation-related parameters such as NF-κB, p38α MAPK, Bcl-3, COX-2, MPO, PGE2, nNOS and iNOS, as well as transcript levels of TNF-α and IL-1β in brain tissue, while CRV showed anti-inflammatory effect by decreasing levels of these biomarkers.
Matrix metalloproteinases (MMPs) are endopeptidases that arrange the cell-matrix composition and are considered major proteases involved in extracellular matrix degradation (Mehana et al., 2019). In pathophysiological conditions, aberrant expression of MMPs induces chronic inflammation and leads to the progression of neurodegenerative diseases (Brkic et al., 2015). In the case of neuroinflammation, MMPs increase the permeability of central nervous system barriers by destabilizing tight junction proteins or disrupting the extracellular matrix, which in turn leads to infiltration of immune cells into the brain and cell death (Rosenberg, 2009). In this study, our RT-PCR results showed that Cd increased mRNA transcript levels of MMP9 and MMP13 in rat brain tissue. It has been found that therapeutically administered CRV reduces MMP9 and MMP13 transcription levels.
Accumulating evidence suggests that Cd severely affects the function of the nervous system by inducing neuronal apoptosis (López et al., 2003). Apoptosis involves the regulation of activation of proteolytic caspase enzymes as a result of interactions between several protein families. Although multiple pathways can induce apoptosis, it has been reported that the mitochondrial pathway is mostly involved in central nervous system apoptosis (Unsal et al., 2013). Caspase-3, Bcl-2 and Bax have an important function in the regulation of cellular apoptosis as they contribute to the apoptotic process (Eldutar et al., 2017; Kandemir et al., 2021). Consistent with a previous study (Al Olayan et al., 2020), our results revealed that Cd initiates the apoptotic event in brain tissue by upregulating proapoptotic proteins such as caspase-3 and Bax. In spite of that, CRV administration remarkably regulated these apoptotic biomarker levels.
8-OHdG, one of the most studied oxidized metabolites, is recognized as a biomarker for oxidative damage of DNA (Caglayan et al., 2018; Graille et al., 2020). A linear relationship between ROS production and 8-OHdG formation has been documented indicating that ROS triggers 8-OHdG formation (Caglayan et al., 2019b; Kucukler et al., 2020). It has been reported that 8-OHdG is formed as a result of the interaction of the hydroxyl radical, which is the most important oxygen-free radical, with the nucleobases of the DNA chain such as guanine (Graille et al., 2020). In a previous study, it was reported that Cd increased 8-OHdG levels by causing oxidative DNA damage in brain cortex tissue (Al omairi et al., 2018). In the present study, we found a significant increase in the level of 8-OHdG in the Cd-intoxicated group, and treatment with CRV significantly modulated the level of this marker.