INHIBITION OF LYSOZYME SYNTHESIS BY DEXAMETHASONE IN HUMAN MONONUCLEAR LEUKOCYTES - AN INDEX OF GLUCOCORTICOID SENSITIVITY

Glucocorticoids inhibit translation of the lysozyme gene. This effect may be the basis of an improved method of measuring glucocorticoid res ponsiveness in human tissues. We have compared lysozyme synthesis in v arious types of white blood cells and examined the specificity of inhi bitory responses to various steroid hormones. The dose-related effects of the glucocorticoid receptor antagonist RU486 on dexamethasone resp onses were also assessed. Glucocorticoid receptor binding in mononucle ar leukocytes (HML) was characterized by homologous displacement of [H -3]dexamethasone and compared with the dose-related inhibitory effect of dexamethasone on lysozyme synthesis. Lysozyme activity was measured photometrically as the ability to cause lysis of Micrococcus lysodeik ticus in the medium. The greatest effect of dexamethasone was observed after 72 h of culture. Qualitatively similar effects of dexamethasone were observed on cell lysozyme content and lysozyme activity in the m edium, but for convenience, activity in medium, rather than cell conte nt, was measured in subsequent assays. Lysozyme activities in various cell types prepared from the blood of healthy volunteers were ranked a s follows: polymorphonuclear cells > monocytes > mononuclear cells > l ymphocytes. However, dexamethasone inhibited lysozyme synthesis to a s imilar degree for all types. As mononuclear cells are more convenientl y prepared in greater yield compared with other cells, this HML fracti on formed the basis of a method of assessing glucocorticoid responsive ness and sensitivity. Lysozyme activity from HML was not significantly affected by incubation with 1 mu mol/L estradiol, progesterone, dehyd roepiandrosterone, or aldosterone. Dexamethasone and cortisol at 1 mu mol/L both inhibited release by 45-50%. Although RU486 when added alon e partially inhibited lysozyme activity, the same concentration (1 mu mol/L) antagonized glucocorticoid responses and shifted the IC50 and t hreshold values for the effect of dexamethasone from 1.2 nmol/L to mor e than 1 mu mol/L and from less than 1.0 to 19 nmol/L, respectively. T he equilibrium dissociation constants (K-d) for dexamethasone binding to the glucocorticoid receptor ranged from 2.8-12.5 nmol/L and were po sitively correlated with dexamethasone IC50 values for lysozyme synthe sis (r = 0.57; P = 0.002). In conclusion, the inhibition of lysozyme s ynthesis by dexamethasone in human mononuclear cells is a convenient a nd specific method of measuring responsiveness to glucocorticoids.