**7. Conclusions**

Reactive oxygen species (ROS) represented a matter of debate/concern in the last years due to the dual role played by these compounds: beneficial effects at low concentrations (signal molecules, mediators of cellular homeostasis, activators of different enzymes) and deleterious effects at high concentrations (DNA and protein damage, lipid peroxidation and oxidative stress).

The oxidative stress was described as an underlying mechanism in different pathologies, including: neurodegenerative diseases, multiple sclerosis, diabetes, atherosclerosis, ageing, chronic inflammatory diseases and cancer. In cancer development, a possible theory regards the activity of ROS as regulators of major signaling pathways: extracellular signal-regulated kinases (ERKs), mitogen-activated protein kinases (MAPKs), phosphoinositide 3-kinases (PI3Ks) and transcription factors such as hypoxia-inducible factors (HIFs). In addition, it was demonstrated that the toxicity associated to heavy metals (lead, mercury, arsen, cadmium, thallium, bismuth, manganese, iron) is mediated via ROS: ROS generation, mitochondrial injury or inhibition of the antioxidant cellular systems.
