Lymphocyte activation and capping of hormone receptors

AbstractIn this study both a Ligand‐dependent treatment [concanavalin A (Con A)] and a ligand‐independent treatment [high‐voltage pulsed galvanic stimulation (HVPGS)] have been used to initiate lymphocyte activation via a transmembrane signaling process. Our results show that both treatments cause the exposure of two different hormone [insulin and interIeukin‐2 (IL‐2)] receptors within the first 5 min of stimulation. When either insulin or IL‐2 is present in the culture medium, the stimulated lymphocytes undergo the following responses: (1) increased free intra‐cellular Ca2+activity; (2) aggregation of insulin or IL‐2 receptors into patch/cap structures; (3) tyrosine‐kinase‐specific phosphorylation of a 32‐kd membrane protein; and finally (4) induction of DNA synthesis.Further analysis indicates that hormone receptor capping is inhibited by (1) cytochalasin D, suggesting the involvement of microfilaments; (2) sodium azide, indicating a requirement for ATP production; and (3) W‐5, W‐7, and W‐12 drugs, implying a need for Ca2 +/calmodulin activity. Treatment with these metabolic or cytoskeletal inhibitors also prevents both the tyrosine‐kinase‐specific protein phosphorylation and DNA synthesis which normally follow hormone receptor capping. Double immunofluorescence staining shows that actomyosin, Ca2+/calmodulin, and myosin light‐chain kinase are all closely associated with the insulin and IL‐2 receptor cap structures.These findings strongly suggest that an actomyosin‐mediated contractile system (regulated by Ca2+, calmodulin, and myosin light‐chain kinase in an energy‐dependent manner) is required not only for the collection of insulin and IL‐2 receptors into patch and cap structures but also for the subsequent activation of tyrosine kinase and the initiation of DNA synthesis. We, therefore, propose that the exposure and subsequent patching/capping of at least one hormone receptor are required for the activation of mouse splenic T‐Iymphocytes.