Muscari Isabella, Adorisio Sabrina, Liberati Anna Marina, Thuy Trinh Thi, Van Sung Tran, Cannarile Lorenza, Ayroldi Emira, Riccardi Carlo, Delfino Domenico V
Department of Medicine, University of Perugia, Santa Maria Hospital, Terni, Italy.
Foligno Nursing School, Department of Medicine, University of Perugia, Foligno (PG), Italy.
J Transl Autoimmun. 2020 Jan 28;3:100035. doi: 10.1016/j.jtauto.2020.100035. eCollection 2020.
Glucocorticoids promote thymocyte apoptosis and modulate transcription of numerous regulators of thymic apoptosis. Among these, glucocorticoid-induced leucine zipper (GILZ) is strongly upregulated in the thymus. We have previously demonstrated that GILZ decreases Bcl-xL expression, activates caspase-8 and caspase-3, and augments apoptosis in mice thymocytes. To better understand the causal links between glucocorticoids, GILZ, Bcl-xL, caspase-8, and caspase-3, we analyzed the thymocytes of Bcl-xL-overexpressing transgenic mice with or without glucocorticoid stimulation . Overexpression of Bcl-xL inhibited the glucocorticoid-induced up-regulation of GILZ in murine thymocytes as well as the glucocorticoid-dependent activation of caspase-8 and caspase-3. By contrast, no appreciable change in caspase-9 activation was observed upon Bcl-xL overexpression. Thus, these experiments highlighted a novel thymocyte apoptotic pathway in which Bcl-xL overexpression inhibited the glucocorticoid-induced activation of caspase-8 and caspase-3, but not caspase-9, as well as the accumulation of GILZ protein. These findings, together with our previous results showing that caspase-8 protects GILZ from proteasomal degradation, suggest the presence of a glucocorticoid-induced apoptosis self-amplification loop in which GILZ decreases Bcl-xL expression with a subsequent activation of caspase-8 and caspase-3; caspase-8 activation then enhances the stability and accumulation of GILZ and ensures the unimpeded and irreversible progression of apoptosis. By contrast, inappropriate increases in Bcl-xL levels could have catastrophic effects on thymic apoptosis as it would shut down caspase-8/3 activation, diminish the expression of GILZ, and impair the fine control necessary for thymic generation of a healthy immune repertoire.
糖皮质激素可促进胸腺细胞凋亡，并调节众多胸腺细胞凋亡调节因子的转录。其中，糖皮质激素诱导亮氨酸拉链蛋白（GILZ）在胸腺中强烈上调。我们之前已经证明，GILZ可降低Bcl-xL的表达，激活半胱天冬酶-8和半胱天冬酶-3，并增强小鼠胸腺细胞的凋亡。为了更好地理解糖皮质激素、GILZ、Bcl-xL、半胱天冬酶-8和半胱天冬酶-3之间的因果关系，我们分析了有或无糖皮质激素刺激的Bcl-xL过表达转基因小鼠的胸腺细胞。Bcl-xL的过表达抑制了糖皮质激素诱导的小鼠胸腺细胞中GILZ的上调以及糖皮质激素依赖性半胱天冬酶-8和半胱天冬酶-3的激活。相比之下，Bcl-xL过表达后未观察到半胱天冬酶-9激活有明显变化。因此，这些实验突出了一条新的胸腺细胞凋亡途径，其中Bcl-xL过表达抑制了糖皮质激素诱导的半胱天冬酶-8和半胱天冬酶-3的激活，但不抑制半胱天冬酶-9的激活，以及GILZ蛋白的积累。这些发现，连同我们之前的结果表明半胱天冬酶-8可保护GILZ免受蛋白酶体降解，提示存在一种糖皮质激素诱导的凋亡自我放大环，其中GILZ降低Bcl-xL的表达，随后激活半胱天冬酶-8和半胱天冬酶-3；半胱天冬酶-8的激活进而增强GILZ的稳定性和积累，并确保凋亡不受阻碍且不可逆转地进行。相比之下，Bcl-xL水平的不适当升高可能对胸腺细胞凋亡产生灾难性影响，因为它会关闭半胱天冬酶-8/3的激活，减少GILZ的表达，并损害胸腺产生健康免疫库所需的精细调控。
