The human interleukin-2 receptor is being studied to understand critical components of the T cell immune response in normal and neoplastic cells. Following T-cell activation, IL-2 and IL-2 receptors are induced; the magnitude and duration of the T-cell immune response is controlled by the amount of IL-2 produced, the levels of receptors expressed, and the time course of these events. Three chains of the IL-2 receptor exist, IL-2Ra, IL-2Rbeta, and gc, with IL-2Ralpha and IL-2Rbeta being significantly regulated at the level of transcription. The laboratory has focused primarily on the types of signals induced by IL-2, particularly the activation of STAT proteins, and the mechanism of regulating IL-2Ralpha in response to mitogen and IL-2. The group previously identified two enhancer-like regions regulating the IL-2Ralpha gene, and shown that proteins binding to these regions could physically interact. Considerable progress has been made in analyzing the STAT proteins (signal transducers and activators of transcription) induced by IL-2. IL-2 can activate both Stat5a and Stat5b (two closely related proteins with >90% amino acid identity) in fresh peripheral blood lymphocytes (PBL) and additionally activates Stat3 in PBL preactivated with phytohemagglutinin. It was reported that an IL-2 response element exists in the 5' regulatory region of the IL-2Ralpha gene. This element is a comprised of canonical and noncanonical GAS motifs that bind Stat5 as well as an Ets family protein, Elf-1. Consistent with an important role for Stat5 in IL-2Ralpha induction, it was also reported that IL-2-induced IL-2Ralpha gene induction cannot occur following mutation of the tyrosines in the IL-2Rbeta cytoplasmic domain that mediate Stat5 activation. Nevertheless, Stat5 activation alone is not sufficient for IL-2Ralpha induction as IL-3 could not induce IL-2Ralpha even though it activates Stat5, suggesting that other pathways as well must be involved. These studies substantially extend our understanding of the molecular basis for IL-2 upregulation of the IL-2Ralpha gene. Purified Stat5a and Stat5b proteins were purified using a baculovirus expression system so that the requirements for binding and activation of these proteins can be studied with purified reagents, and a binding site-selection analysis is in progress. The yeast two hybrid method was used to identify factors that can interact with Stat5, and two interacting proteins have been identified that appear to clarify the basis for Stat5 function. Stat5a-deficient mice have also been analyzed, revealed a defect in the induction of IL-2Ralpha gene. Finally, the IL-2Rbeta gene was shown to be regulated by the early growth response gene, Egr1, consistent with its rapid induction following appropriate stimuli. Together, these studies substantially enhance our understanding of the basis for IL-2R expression as well as IL-2-dependent gene regulation.
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