Anoikis: To Die or Not to Die?

Epithelial, endothelial, and many connective tissue cells are normally attached to the extracellular matrix (ECM). These cells rely on the ECM for structural support, signaling, and regulation of their behavior. When these cells lose this attachment or are in an inappropriate location, these cells soon die by a mechanism called anoikis (homelessness). Anoikis is a programmed cell death of an apoptotic nature; however, it can, in certain cases, be overcome, and detached cells can survive in the absence of the correct signals from the ECM. This is the case of malignant cells, where anoikis resistance is a prerequisite for invasion and metastasis. Without anoikis resistance (anchorage-independency), tumors would be unable to abandon their normal sites and would invade neighboring tissues and metastasize at distant locations. Anoikis is the natural barrier against cancer progression. Therefore, overcoming anoikis is a major step in cellular transformation. Cancer cells have developed many successful strategies to bypass anoikis. The main mechanism, albeit not the only one, involves hyper-activating survival pathways and over-expressing anti-apoptotic molecules. There is a strong and intertwining association between epithelial–mesenchymal transition and anoikis resistance that is discussed in depth. A better understanding of these anoikis resistance mechanisms has led to the research and development of pharmaceuticals that can counteract them.