Chemical carcinogenesis - reactive oxygen species - Phascolarctos cinereus (koala)

It is well established that cancer is a multi-step process which involves initiation, promotion and progression. Chemical carcinogens can alter any of these processes to induce their carcinogenic effects. In the majority of instances, chemical carcinogens, directly or after xenobiotic metabolism, induce DNA damage and act in a 'genotoxic' manner. There is, however, a group of carcinogens that induce cancer via nongenotoxic mechanisms. The biochemical modes of action for nongenotoxic carcinogens are diverse, one example of which is induction of oxidative stress. Trace metals and organic xenobiotics are typical classes of environmental pollutants with prooxidant effects. Trace metals generate reactive oxygen species (ROS) depending on their ability to lose electrons and catalyze Haber Weiss and Fenton reactions. ROS are also generated due to induction of various cytochrome P450 isoenzymes during detoxification of chemical carcinogens. Increased ROS generation often has been linked to DNA damage that can lead to mutations and may, therefore, play an important role in the initiation and progression of multistage carcinogenesis. Besides causing DNA damage, ROS further induce multiple intracellular signaling pathways, notably NF-kappa B, JNK/SAPK/p38, as well as Erk/MAPK. These signaling routes can lead to transcriptional induction of target genes that could promote proliferation or confer apoptosis resistance to exposed cells.