Early transcriptional events in glucocorticoid-induced thymocyte apoptosis.

Abstract

Apoptosis, or programmed cell death, occurs in response to various biological signals, providing a means for the deletion of unnecessary or potentially harmful cells. Following genetic rearrangement of the variable region of the T-cell receptor (TCR), only thymocytes that express a functional TCR which does not recognize self-antigen survive to undergo positive selection, while both self-reactive and non-functional thymocytes undergo apoptosis. Glucocorticoid hormones trigger apoptosis of immature thymocytes. Although the physiological role of this sensitivity to glucocorticoids is not completely understood, it provides a useful model for thymocyte apoptosis. The glucocorticoid receptor (GR) is a ligand-dependent transcription factor, capable of both gene activation and repression. I have addressed the role of GR in steroid-induced apoptosis. Some controversy exists over whether gene activation or repression by GR is important for thymocyte apoptosis. By altering the N-terminal transcriptional activation domain of GR, I have shown that increasing the activation capacity of GR increases its ability to initiate apoptosis, suggesting that the activation function of GR is important for steroid-induced apoptosis. My work does not rule out an additional role for repression; the apoptotic pathway may be initiated by a combination of activation and repression of multiple GR targets. To identify potential GR target genes which might be involved in apoptosis, I used differential display PCR. I identified four transcripts which are up-regulated by glucocorticoids in murine thymocyte cell lines. One, the G/T binding protein, has recently been implicated in DNA repair. The remaining three represent novel transcripts. GIG1 (for glucocorticoid induced gene 1) is a 10.5 kB transcript which is induced within 2 h of hormone treatment, reaching a maximum of 20-fold over basal levels by 8 h. Similarly, a 7.5 kB transcript called GIG10 is also rapidly induced to high levels. GIG18, a 7.5 kB thymus specific transcript, is induced as early as 30 min following treatment with glucocorticoids, reaching 10-fold basal levels by 8 h. Partial cDNAs of all three genes have been obtained. However, comparisons to sequence databases have to date revealed no significant homology to known genes. Determination of the possible involvement of any of these genes in glucocorticoid-induced thymocyte apoptosis will require further characterization.