Induction of interleukin-2 receptor α (IL-2Rα) expression by interleukin-2:: important role of the interleukin-2 receptor β chain region between the two Stat5 docking sites

The interleukin-2 receptor alpha (IL-2Ralpha) forms, together with IL-2Rbeta and gammac chains, a high affinity IL-2 receptor that is important for IL-2 responsiveness and normal T cell function. Expression of the IL-2Ralpha gene by T cells is regulated mainly at the transcription level which is transiently activated by antigen and upregulated and then prolonged by stimulation with IL-2. The effect of IL-2 on the IL-2Ralpha gene depends on the activation of the transcription factor Stat5, which acts on an IL-2- responsive enhancer that consists of two Stat5 and an Elf1 binding site. To identify the functional domains of the IL-2 receptor required for the stimulation of IL-2Ralpha gene expression, we introduced, into the CTL44 T cell line, receptor chimeras between the extracellular domain of the IL-9 receptor and the cytoplasmic region of IL-2Rbeta. Analyzing the effect of mutations in the intracellular IL-2Rbeta segment, we found that a minimal receptor containing the Jak boxes and one intact Stat5 docking site (i.e. tyrosine 392 or 510) can, as expected, mediate Stat5 activation, but is unable to stimulate IL-2Ralpha expression. However, when this minimal receptor includes the region between the two tyrosines, its capacity to mediate IL-2Ralpha cell surface expression is restored. These data suggest that the segment between the two Stat5 docking sites of the IL-2Rbeta chain mediates signaling events that, together with Stat5 activation, are essential for the stimulation of IL-2Ralpha gene transcription.