Reinehr Roland, Häussinger Dieter
Clinic for Gastroenterology, Hepatology, and Infectiology, Heinrich-Heine-University Düsseldorf, Germany.
Methods Enzymol. 2007;428:145-60. doi: 10.1016/S0076-6879(07)28008-5.
Cell shrinkage, nuclear condensation, DNA fragmentation, and apoptotic body formation are hallmarks of programmed apoptotic cell death. Herein, apoptotic volume decrease (AVD) is an early and ubiquitous event. Conversely, in hepatocytes, hyperosmotic cell shrinkage leads to an activation of the CD95 death receptor system, which involves CD95 tyrosine phosphorylation, CD95 oligomerization, and subsequent trafficking of the CD95 to the plasma membrane, and sensitizes hepatocytes toward CD95 ligand (CD95L)-induced apoptosis. Early signaling events leading to CD95 activation by hyperosmolarity have been identified. In hepatocytes, hyperosmotic exposure induces an almost instantaneous acidification of an acidic sphingomyelinase (ASM) containing endosomal compartment, which is followed by an increase in the intracellular ceramide concentration. Inhibition of anion channels or the vacuolar-type H(+)-ATPase abolishes not only endosomal acidification and subsequent ceramide generation, but also the otherwise observed hyperosmotically induced generation of reactive oxygen species (ROS) by NADPH oxidase isoforms. Hyperosmolarity-induced ROS formation then leads to a Src-family kinase Yes-mediated activation of the epidermal growth factor receptor (EGFR) and to an activation of the c-Jun-N-terminal kinase (JNK). JNK then provides a signal for CD95/EGFR association and subsequent CD95 tyrosine phosphorylation, which is mediated by the EGFR tyrosine kinase activity. CD95 tyrosine phosphorylation then allows for CD95 receptor oligomerization, translocation of the CD95/EGFR protein complex to the plasma membrane, and formation of the death inducing signaling complex (DISC). Mild hyperosmotic exposure, that is, 405 mosmol/liter, does not lead to a reduction of cell viability, even if DISC formation and subsequent caspase 8 and 3 activation occur, but sensitizes hepatocytes to CD95L-induced apoptosis. However, activation of the CD95 system by a more severe hyperosmotic challenge (>505 mosmol/liter) is followed by execution of the apoptotic cell death. Other covalent modifications of CD95, such as CD95 tyrosine nitration or CD95 serine/threonine phosphorylation, were shown to inhibit the CD95 activation process.
细胞皱缩、核固缩、DNA片段化以及凋亡小体形成是程序性凋亡细胞死亡的标志。在此，凋亡性体积减小（AVD）是一个早期且普遍存在的事件。相反，在肝细胞中，高渗性细胞皱缩会导致CD95死亡受体系统的激活，这涉及CD95酪氨酸磷酸化、CD95寡聚化以及随后CD95向质膜的转运，并使肝细胞对CD95配体（CD95L）诱导的凋亡敏感。导致高渗激活CD95的早期信号事件已被确定。在肝细胞中，高渗暴露几乎瞬间诱导含有酸性鞘磷脂酶（ASM）的内体区室酸化，随后细胞内神经酰胺浓度增加。抑制阴离子通道或液泡型H(+) - ATP酶不仅会消除内体酸化和随后的神经酰胺生成，还会消除原本观察到的由NADPH氧化酶同工型介导的高渗诱导的活性氧（ROS）生成。高渗诱导的ROS形成随后导致Src家族激酶Yes介导的表皮生长因子受体（EGFR）激活以及c - Jun - N末端激酶（JNK）激活。JNK随后为CD95/EGFR结合以及随后由EGFR酪氨酸激酶活性介导的CD95酪氨酸磷酸化提供信号。CD95酪氨酸磷酸化随后允许CD95受体寡聚化、CD95/EGFR蛋白复合物向质膜转运以及死亡诱导信号复合物（DISC）形成。轻度高渗暴露，即405毫渗摩尔/升，即使发生DISC形成以及随后的半胱天冬酶8和3激活，也不会导致细胞活力降低，但会使肝细胞对CD95L诱导的凋亡敏感。然而，更严重的高渗挑战（>505毫渗摩尔/升）激活CD95系统后会导致凋亡细胞死亡的执行。CD95的其他共价修饰，如CD95酪氨酸硝化或CD95丝氨酸/苏氨酸磷酸化，已被证明会抑制CD95激活过程。
