Glucocorticoid activation of a calcium-dependent endonuclease in thymocyte nuclei leads to cell death.

TLDR

Glucocorticoid‑induced thymocyte death is a distinct, endogenous “death from within” that differs from toxic or accidental cell death and is absent in mature T cells lacking the inducible activation mechanism. The authors show that a constitutively present nuclear endonuclease is activated by calcium and magnesium, and that glucocorticoid‑induced synthesis of a protein involved in nuclear calcium transport is required for this activation. Dexamethasone and corticosterone kill thymocytes by inducing RNA‑ and protein‑dependent activation of a calcium‑dependent endonuclease that fragments nuclear DNA, and this death is blocked by zinc.

Abstract

Dexamethasone and corticosterone kill mouse thymocytes, as measured by eosin uptake, after several hours of in vitro incubation. This killing requires RNA and protein synthesis, because it is inhibited by cycloheximide, emetine, or actinomycin D. An earlier event than cell death is the extensive fragmentation of nuclear DNA into oligonucleosomal subunits; this fragmentation also requires RNA and protein synthesis. The DNA cleavage results from the action of an endonuclease that preferentially cleaves DNA in the linker regions between nucleosomes. This endonuclease is found constitutively in the nuclei of thymocytes and some other cells, and requires calcium and magnesium ions for its activation; if isolated fresh thymocyte nuclei are incubated with these ions, as much as 77% of their DNA is cleaved within 90 min. Thus, the protein for which synthesis is necessary for glucocorticoid-induced thymocyte death is not the endonuclease itself, but is in some way involved in its activation; we suggest that it may be part of a system for transporting calcium into the nucleus. The endonuclease is inhibited by zinc, which also prevents thymocyte death. It appears that glucocorticoids cause thymocyte death by activating an enzyme that rapidly and extensively degrades DNA. This "death from within" is biochemically and morphologically different from toxic or accidental cell death, such as that induced by azide, heat, or antibody and complement treatment. Although mature T cells also contain the endogenous endonuclease, they lack the glucocorticoid-inducible mechanism for activating it, and are thus glucocorticoid-resistant.