Role of Ca2+ in the regulation of hormone receptor exposure during lymphocyte activation

Ca2+ is known to be required for mitogen-mediated lymphocyte activation. In order to further define the regulatory role of Ca2+, we have examined the activation events which occur following treatment with ionomycin (a Ca2+ ionophore), as compared to those occurring following concanavalin A (Con A) treatment of mouse splenic T-lymphocytes. Our results indicate that ionomycin and Con A induce the exposure of both interleukin-2 (IL-2) and insulin receptors on the surface of the lymphocytes within the first 5 min of treatment. The exposed insulin and IL-2 receptors have the following properties: (1) they consist of both high- and low-affinity receptors; and (2) they appear on the cell surface in small clusters (i.e., patches) or, occasionally, a large aggregate (i.e., cap). c-myc gene expression and DNA synthesis occur in both the ionomycin and Con A-treated lymphocytes when either IL-2 or insulin is present in the culture medium. Furthermore, the exposure of both hormone receptors can be inhibited by either EGTA (a Ca2+ chelator), bepridil (a Ca2+ channel blocker), W-7 (a calmodulin antagonist) or cytochalasin D (a microfilament inhibitor). Treatment with these inhibitors also blocks the expression of c-myc gene and DNA synthesis which occur at later times during IL-2 and insulin-induced activation of ionomycin- and Con A-treated lymphocytes. These findings suggest that a Ca2+ and calmodulin-mediated contractile system is involved in the exposure of certain hormone receptors which appear to be required for complete lymphocyte activation.