The biochemistry and molecular biology of glucocorticoid-induced apoptosis in the immune system

Apoptosis is a form of programmed cell death that occurs under numerous developmental and physiological conditions that require the selective elimination of cells from tissues and organs without the production of an inflammatory response. The initiation of apoptosis is controlled by a regulation of the balance between death and life signals perceived by the cell. A typical response of cells to an apoptotic stimulus includes a reduction in cell volume, compaction of intracellular organelles, chromatin condensation, and the generation of apoptotic bodies which contain degraded cellular components. Apoptotic bodies are often engulfed by neighboring cells or macrophages, preventing the occurrence of an inflammatory response in the region of the dying cells. Although the molecular basis for this cellular suicide is poorly understood, evidence indicates that apoptosis is an active process, requiring energy for its effective completion. We have sought to define the catabolic "effector" molecules that carry out the apoptotic process using glucocorticoid-induced apoptosis in rodent and human lymphocytes as model systems. These cells respond to dexamethasone with an arrest of cell growth, chromatin condensation, cell shrinkage, and the selective degradation of DNA, RNA, and protein. These effects are dependent on the presence of functional glucocorticoid receptors and require gene expression. The fragmentation of DNA and its associated cell shrinkage has been a focus of our efforts, because these effects reflect an irreversible commitment to death. Accordingly, we have developed assays to study apoptosis at the single cell level and to identify, purify, and clone the nuclease(s) that cause DNA damage in apoptotic cells. Using these approaches, we have identified and characterized a novel low molecular weight nuclease (NUC18) whose activity correlates with the DNA degradation occurring during apoptosis. NUC18 requires calcium for optimal activity in vitro and is inhibited by zinc and aurintricarboxylic acid, two known inhibitors of apoptosis. The amino acid sequence of pure NUC18 reveals a surprising homology to the cyclophilin family of proteins. Furthermore, recombinant cyclophilins have biochemical and pharmacological properties identical to those of NUC18. We have also studied the molecular basis for the catabolism of RNA and proteins that occurs during lymphocyte apoptosis. Recent experiments have identified selective cleavage of 28S ribosomal RNA and a novel nonlysosomal protease, both of which contribute to the demise of the cell. In summary, we present an evolving model that unifies the activation of apoptosis in lymphocytes by glucocorticoids with the counter-balancing effect of inhibitors such as Bcl-2.