New insight into dexamethasone activity in lymphoblast transformation of mouse thymocytes

Inhibition of lymphoblast transformation (LT) by glucosteroids has been extensively studied using the incorporation of ~H-thymidine (3H-Tdr) into new DNA induced by mitogen in vitro. Assays involving prolonged exposure to steroids (48-72 hr) have been reported [1-4], whereas expression of their activities in the in vitro inflammation test usually required only a few hours pretreatment [5]. This lag time (30 min-2 hr) is attributed to induction of lipocortin [6], a PLA2 inhibitory protein which prevents arachidonate availability and subsequent metabolism into proinflammatory eicosanoids. Conflicting results [ 1, 2] have been published concerning modification of the inhibitory potency of glucocorticosteroids depending on the relative delay between addition of steroid and mitogen to cells. The purpose of this study was to examine the time course of 3H-Tdr incorporation following concanavalin A (con A) treatment as well as the time course of the modification of incorporation induced by dexamethasone (dex). Variation of dex potency according to time of addition relative to con A was also studied. Minimal duration of exposure to steroid necessary for expression of inhibitory activity was also investigated.