Subtle Shifts in the Ratio Between Pro- and Antiapoptotic Molecules After Activation of Corticosteroid Receptors Decide Neuronal Fate

Glucocorticoid receptor (GR) activation induces apoptosis of granule cells in the hippocampus. In contrast, neuroprotection is seen after mineralocorticoid receptor (MR) activation. To date there is no in vivo evidence for direct interactions between corticosteroids and any of the key regulatory molecules of programmed cell death. In this report, we show that the opposing actions of MR and GR on neuronal survival result from their ability to differentially influence the expression of members of the bcl-2 gene family; specifically, in the rat hippocampus, activation of GR induces cell death by increasing the ratio of the proapoptotic molecule Bax relative to the antiapoptotic molecules Bcl-2 or Bcl-x(L); the opposite effect is observed after stimulation of MR. The same results were obtained in both young and aged animals; however, older subjects (which were more susceptible to GR-mediated apoptosis) tended to express the antiapoptotic genes more robustly. Using a loss-of-function mouse model, we corroborated the observations made in the rat, demonstrating Bax to be essential in the GR-mediated cell death-signaling cascade. In addition, we show that GR activation increases and MR activation decreases levels of the tumor suppressor protein p53 (a direct transcriptional regulator of bax and bcl-2 genes), thus providing new information on the early genetic events linking corticosteroid receptors with apoptosis in the nervous system.