Chemical carcinogenesis - receptor activation - Ursus americanus (American black bear)

Carcinogenesis is a multistage process that consists of initiation, promotion, and progression stages. Chemicals or environmental factors may act at any of these stages to induce and/or enhance the carcinogenic process. Based on their mode-of-action, carcinogens can be classified as genotoxic or non-genotoxic. Genotoxic agent begins their action at the deoxyribonucleic acid (DNA) level, causing DNA damage through several mechanisms. Non-genotoxic carcinogens are chemicals that cause the development of tumors through multiple non-genotoxic events and epigenetic alterations without direct interaction with DNA. One non-genotoxic mechanism involves receptor activation. Biological mechanisms involving receptor activation fall into two broad categories: (i) those that involve cell surface receptors and some intracellular receptors that activate signal transduction pathways, resulting in biological responses, including gene transcription, and (ii) those that involve intracellular receptors that translocate into the nucleus and act as transcription factors regulating gene expression. Both classes of receptors can be involved in mechanisms of carcinogenesis.