Glucocorticoid Resistance and the AP-1 Transcription Factor in Leukaemia

Abstract

Glucocorticoids have been used in the treatment of acute lymphoblastic leukaemia (ALL) for many years, initially as the only agent and then as part of multiagent chemotherapy. In ALL 20% of patients are resistant to glucocorticoids at presentation but this rises to greater than 70% on relapse. It has recently been reported that the glucocorticoid receptor inhibits activity of the AP-1 transcription factor by the ligand-dependant binding of glucocorticoid receptor (GR) to the fos and jun components of AP-1. Since AP-1 is necessary for cell proliferation, the upregulation or over-expression of AP-1 may be a mechanism of resistance to glucocorticoids. The aim of the study was to investigate whether AP-1 levels correlate with in vitro glucocorticoid resistance. In vitro sensitivity to glucocorticoids was measured using the MTT assay. AP-1 levels were quantified using gel shift analysis: a consensus sequence for the AP-1 binding site was synthesised, labelled with ³²P and incubated with nuclear extracts of leukaemic blasts from 14 ALL and 26 CLL patients. Leukaemic blasts were treated with prednisolone or with vehicle alone before preparation of nuclear extracts. The gels were dried and bands quantified using a phosphorimager, using an appropriate internal standard and correcting for protein loading and cytoplasmic contamination of nuclear extracts. The patient samples fell into two distinct groups with respect to their sensitivity to glucocorticoids: AP-1 levels were significantly higher (p<0.02) in sensitive blasts than resistant ones. There was no significant change in AP-1 levels after treating blasts for 4 hours with 0.2 mM prednisolone. No change was seen in CLL samples. These data show that glucocorticoid resistance is not associated with increased AP-1. Conversely, glucocorticoid resistance in these samples was apparently associated with decreased AP-1 levels in ALL samples. Whether this has any causal relationship to glucocorticoid resistance is unknown. Clearly, further studies on the role of AP-1 and related transcription factors is essential for understanding the control of proliferation and apoptosis in ALL.