Hyperosmotic activation of the CD95 death receptor system
Apoptosis is characterized by cell shrinkage, nuclear condensation, DNA fragmentation and apoptotic body formation. These features distinguish apoptosis from other types of cell death, such as necrosis. Whereas some signs of apoptosis, such as externalization of phosphatidylserine, altered mitochondrial function or activation of caspases are cell type- and death signal-dependent, apoptotic cell volume decrease (AVD) is an early and ubiquitous event and little is known about the signalling events, which are localized upstream of the plasma membrane transport steps leading to AVD and the proapoptotic events, which are induced by osmolyte loss and cell shrinkage. In hepatocytes hyperosmotic shrinkage sensitizes the cells towards CD95 ligand-induced apoptosis by activating the CD95 system. This complex process with a NADPH oxidase-derived reactive oxygen species signal as an important upstream event, allows via Yes, JNK and epidermal growth factor-receptor activation for CD95 tyrosine phosphorylation as a prerequisite for CD95 targeting to the plasma membrane and formation of the death inducing signalling complex. Other covalent modifications such as CD95-tyrosine-nitration or CD95-serine/threonine-phosphorylation can interfere with the CD95 activation process. The findings not only provide a mechanistic explanation for the high susceptibility of dehydrated cells for apoptosis, but also give insight into the role of AVD.