Glucocorticoid Receptors in Lymphoid Tumors1

There ¡sa range of levels of glucocorticoid receptor numbers seen in the various subclasses of acute lymphatic leukemia (ALL). This variability cannot be explained by the known cor relation between active cell proliferation and an increase in the number of receptors, since the tumors with the highest growth fraction (i.e., Burkitt's lymphoma and T-cell leukemia) tend to have lower average receptor numbers than do tumors with lower growth fractions such as common ALL. All clinical spec imens from patients with lymphatic leukemia have some mea surable level of glucocorticoid receptors; therefore, the resist ance seen in vivo cannot be explained by the lack of receptors. However, there has been a positive correlation, in our hands, with receptor level and prognosis. On the basis of in vitro models, it is proposed that perhaps the high receptor cell lines (i.e., common ALL of childhood) have relative stability of their genetic material making glucocorticoid-resistant mutations less likely to occur in patients with these cells than in low-receptor cell lines (i.e., T-cell leukemia). This greater genetic variability in the low-receptor lines could account for the earlier emer gence of clinical glucocorticoid resistance in these patients. In considering the significance of cellular glucocorticoid re ceptors, it is important to enumerate at least some of the host or environmental factors which may alter receptor level before considering the cell itself. First, one must consider the species in which the cell arises. Steroids exert markedly different effects in various species. Claman (3) has classified the mouse, rat, and rabbit as steroid-sensitive species on the basis of the relative susceptibility of their lymphocytes to lysis by steroids, while lymphocytes from other species (guinea pigs and hu mans) are resistant. Thus interspecies comparisons must be made with caution. The hormonal state of the individual may also influence receptor level. Duval and Homo (5) have dem onstrated, for example, that adrenalectomy in mice can pro duce not only an increase in thymus size and cellularity but also an augmentation of the number of dexamethasone-binding sites per cell. Others have shown a fall in total receptor number after adrenocorticotrophic hormone treatment or adrenal abla tion. The existence of such a mechanism of receptor regulation emphasizes the idea that receptor modulation will occur under conditions when fluctuations of steroid plasma levels are ob served such as circadian rhythms, stress, and steroid therapy. In addition, in humans, Fauci and Dale (6) have demonstrated that steroid treatment induces changes in lymphocyte recircu lation and homing, so that the apparent lymphopenia after glucocorticoid administration to normal volunteers is due to movement of lymphocytes out of the circulation rather than lympholysis. The age of the individual may affect the level of steroid receptor; Roth and Livingston (13) have reported a ' Presented at the Conference on Cell Markers in Acute Leukemia, March 4 and 5. 1980, Bethesda, Md. Supported in part by Grant CA 28476. decrease in receptor content in leukocytes with senescence. The extent of steroid binding to the receptors is also de pendent on other modulating factors such as metabolic condi tions. For example, Munck ef al. (11) have shown that the binding of cortisol to its cytosolic receptors was virtually abol ished in the absence of glucose or under anaerobic conditions. Once one has considered the milieu in which the cell exists, one must also consider the cell itself. In steroid-sensitive spe cies, the most immature T-cells are also the most steroid sensitive. With maturation or with exposure to specific antigen, relative steroid resistance is acquired (8). Although receptors have not been measured in these circumstances, it is probable that there are changes. In a given cell, also, the level of binding sites fluctuates according to the stage of cell proliferation. Cidlowski and Michaels (2) demonstrated in synchronized HeLa cells that the number of glucocorticoid receptors in creases during S phase and falls after mitosis. Blast transfor mation in response to phytohemagglutinin (12) and concanavalin A (14) results in increased levels of glucocorticoid recep tors. Smith ef al. (14) believe that this is related to the stage of the cell in cycle. They have demonstrated in their concanavalin A-stimulated cells that, with increasing time after concanavalin A stimulation, there is an increase in number of receptors and an increase in the amount of tritiated thymidine incorporated into the cells; however, the percentage of inhibition of thymidine incorporation in the presence of dexamethasone remains sta ble. In studying normal human cells, Lippman and Barr (9) used resetting techniques to study purified populations of T-, non-T, and mononuclear cells from human peripheral blood and dis covered that there is no difference between Tand non-T lymphocytes in receptor content on a per cell basis or in affinity for glucocorticoid, with each lymphocyte population containing about 3000 sites/cell. Monocytes under these same conditions contain about 7000 sites/cell. Yarbro et al. (15) attempted to correlate leukemic cell type in humans with glucocorticoid receptor level. These studies were all done with a whole-cell method in which the leukemic cells were incubated with varying concentrations of labeled dexamethasone which was then chased with cold dexametha sone to assure specificity of binding. In this study, T-lymphoblasts (defined by the erythrocyte-rosetting technique) had significantly lower levels of glucocorticoid receptors than did null lymphoblasts. The binding curves for each cell type showed that, although the total number of sites per cell differed markedly, the shape of the curves was the same, with both approximately 70% saturated at a concentration of 0.8 x 10~8 M dexamethasone. The dissociation constants were also about the same, with Kd about 3.9 x 10s M for both cell types. This difference in receptor number cannot be explained on the basis of cell cycle events since the cells with the higher thymidine labeling index (T-cells) had the lower receptor level. The null group of cells can be further subdivided into those cells which did stimulate allogeneic donors in mixed-leukocyte