Multiple forms and fragments of cytosolic glucocorticoid receptors from human leukemic cells and normal lymphocytes

Therapy with glucocorticoids is generally more effective in acute lymphoblastic leukemia than in other types of human leukemia. Previous studies, however, have not revealed any consistent relationship between clinical responsiveness and the cellular or cytosolic concentration of glucocorticoid-binding sites. The objectives of this study were to determine whether there are intrinsic structural differences among the glucocorticoid receptors in various types of leukemic cells and normal lymphocytes and to investigate the role of endogenous peptidases in receptor degradation. Cytosols were prepared from fresh or rapidly frozen leukocytes from 6 healthy adults and 35 high-risk leukemia patients (median white blood cell count, 150,000 cells/microliter; median age, 13 years). Receptors were labeled with [3H]triamcinolone acetonide and quantitated by charcoal-dextran treatment or Sephadex LH-20 chromatography. Mean and median cytosolic receptor concentrations in 12 acute lymphoblastic leukemia specimens lacking the standard B-cell or T-cell markers ("null cells") were approximately 4-fold higher than in 23 other leukemic cell specimens. No other consistent differences in receptor content were observed. Agarose filtration and ultracentrifugation in hypotonic buffers containing 20 mM Na2MoO4 revealed complexes of similar size and shape in all clinical specimens tested and two established leukemic cell lines. They had Stokes radii (Rs) of 8.1 +/- 0.5 (S.D.) nm (n = 50), sedimentation coefficients of 9.5 +/- 0.3S (n = 40), molecular weights of approximately 330,000, and axial ratios (a/b) of approximately 12. In hypertonic, molybdate-free buffer, these oligomeric complexes were dissociated into subunits with Rs of 5.9 +/- 0.3 nm (n = 12) and a/b of 11 to 12, as observed previously for other receptors. Fragmentation of the oligomer and the subunit was evident in some cytosols. High activities of peptidases of various specificities were detected in leukemic cell cytosols, as in other cytosols, by fluorometric assays with derivatives of 7-amino-4-methylcoumarin. Receptor cleavage by these and other endogenous enzymes may account for previous observations of "abnormal" receptors in cytosols from some leukemic specimens. We conclude that intrinsic structural defects in the receptors are unlikely explanations for the unresponsiveness of some types of leukemia to steroid therapy.