Mechanisms of tumor cell necrosis

Until recently, necrosis, unlike apoptosis, was considered as passive and unregulated form of cell death. However, during the last decade a number of experimental data demonstrated that, except under extreme conditions, necrosis may be a well-regulated process activated by rather specific physiological and pathological stimuli. In this review, we consider mechanisms and the role of necrosis in tumor cells. It became recently clear that the major player in necrotic cascade is a protein kinase RIP1, which can be activated by number of stumuli including TNF, TRAIL, and LPS, oxidative stress, or DNA damage (via poly-ADP-ribose polymerase). RIP1 kinase directly (or indirectly via another kinase JNK) transduces signal to mitochondria and causes specific damage (mitochondrial permeability transition). Mitochondrial collapse activates various proteases (e.g., calpains, cathepsin) and phospholipases, and eventually leads to plasma membrane destruction, a hallmark of necrotic cell death. Necrosis, in contrast to apoptosis, usually evokes powerful inflammatory response, which may participate in tumor regression during anticancer therapy. On the other hand, excessive spontaneous necrosis during tumor development may lead to more aggressive tumors due to stimulatory role of necrosis-induced inflammation on their growth.