Regulation of the dephosphorylation of Stat6. Participation of Tyr-713 in the interleukin-4 receptor alpha, the tyrosine phosphatase SHP-1, and the proteasome.

Signal transducer and activator of transcription 6 (Stat6) plays an important role in interleukin (IL)-4-induced responses. To analyze the regulation of Stat6 phosphorylation, cells were cultured in the continuous presence of IL-4 or after a pulse and washout. In the continual presence of IL-4, Stat6 remained phosphorylated for an extended period. After IL-4 removal and inhibition of the Janus family kinase, tyrosine-phosphorylated Stat6 decayed at a rate dependent upon the length of IL-4 stimulation. The decay of tyrosine-phosphorylated Stat6 was similar in the presence or absence of either cycloheximide or actinomycin D. In the absence of functional Src homology-containing phosphatase-1 (SHP-1), the early loss of tyrosine-phosphorylated Stat6 was substantially reduced. Furthermore, the rate of loss of tyrosine-phosphorylated Stat6 in cells expressing a mutation of the human IL-4 receptor alpha in the immunoreceptor tyrosine-based inhibitory motif sequence (Y5F) was dramatically decreased compared with wild-type cells. The early rate of decay was similar in the presence or absence of MG132, an inhibitor of the proteasome, but the later rate of decay was decreased 5-fold. These results suggest that the loss of tyrosine phosphorylation of Stat6 is regulated by the action of SHP-1 and the proteasome but is not dependent on new protein synthesis.