The Pathway of Leukemic Cell Death Caused by Glucocorticoid Receptor Fragment 465

The truncated glucocorticoid receptor mutant gene 465* codes for a protein that is interrupted by a frame-shift mutation in the second zinc finger of the natural DNA binding domain. Thus, 465* represents the natural amino acid sequence 1–465 followed by 21 novel amino acids starting at position 466. The entire ligand binding domain is missing. Prior studies have shown that transient transfection of the glucocorticoid-resistant leukemic T-cell clone ICR-27 with a plasmid expressing 465* rapidly reduces the number of viable cells. This response does not require activation by a steroid, and a hybrid protein consisting of green fluorescent protein fused to 465* is found primarily in the cytoplasm. In the present study, we present evidence that the decrease in cell number is due to a form of cell death that bears many of the classic characteristics of apoptosis. Expression of the 465* protein can be detected a few hours after electroporation and is followed by activation of caspase-3 as well as reduction of the mitochondrial inner transmembrane potential. The caspase-3 inhibitor ZVAD-fmk blocks 465*-dependent cell death when added acutely after electroporation, but fails to do so later. We conclude that the novel 465* gene causes cell death by apoptosis.