Neuronal specific and non-specific responses to cadmium possibly involved in neurodegeneration: A toxicogenomics...

Epidemiological data have linked cadmium exposure to neurotoxicity and to neurodegenerative diseases (e.g., Alzheimer's and Parkinson's disease), and to increased risk of developing ALS. Even though the brain is not a primary target organ, this metal can bypass the blood brain barrier, thus exerting its toxic effects. The coordination chemistry of cadmium is of strong biological relevance, as it resembles to zinc(II) and calcium(II), two ions crucial for neuronal signaling. A toxicogenomics approach applied to a neuronal human model (SH-SY5Y cells) exposed to cadmium (10 and 20 μM) allowed the identification of early deregulated genes and altered processes, and the discrimination between neuronal-specific and unspecific responses as possible triggers of neurodegeneration. Cadmium confirmed its recognized carcinogenicity even on neuronal cells by activating the p53 signaling pathway and genes involved in tumor initiation and cancer cell proliferation, and by down-regulating genes coding for tumor suppressors and for DNA repair enzymes. Two cadmium-induced stress responses were observed: the activation of different members of the heat shock family, as a mechanism to restore protein folding in response to proteotoxicity, and the activation of metallothioneins (MTs), involved in zinc and copper homeostasis, protection against metal toxicity and oxidative damage. Perturbed function of essential metals is suggested by the mineral absorption pathway, with MTs, HMOX1, ZnT-1, and Ferritin genes highly up-regulated. Cadmium interferes also with Ca²⁺ regulation as S100A2 is one of the top up-regulated genes, coding for a highly specialized family of regulatory Ca²⁺-binding proteins. Other neuronal-related functions altered in SH-SY5Y cells by cadmium are microtubules dynamics, microtubules motor-based proteins and neuroprotection by down-regulation of NEK3, KIF15, and GREM2 genes, respectively.