[The regulation of anoikis in tumor invasion and metastasis]

As a barrier to metastasis of cancer, cells that lost contact with the neighbouring cells or extracellular matrix(Extracellular matrix, ECM) will be subjected to apoptosis. This cell death process has been termed "anoikis". When normal epithelial cells or solid tumor cells without metastatic potential detach from the primary site, and then enter into the circulatory system, the anoikis mechanism will be activated. The significance of anoikis is to prevent the shedding cells from growing and implanting into other inappropriate sites. Tumor cells, especially several malignant cells that is prone to transfer to distant sites, have properties of anti-anoikis, which facilitates metastasis as well as invasion of tumor cells. The studies found that tumor cells can resist anoikis through multiple mechanisms: the pro-survival pathways are activated by cells autocrine growth factors and paracrine factors derived from neighboring cells; cells change the pattern of integrin expression so that they can receive survival signals from new environment; reactive oxygen species (ROS) activates growth factor receptors in a ligand-independent way to avoid apoptosis; and epithelial-mesenchymal transformation(EMT) is activated etc.. All of these mechanisms lead to activation of survival signals and inhibition of apoptotic pathways, and ultimately cause resistance to anoikis as well as metastasis. This paper summarizes the key mechanisms of the current studies on metastasis, which also suggest important targets for cancer therapy.