Differential associations between the cytoplasmic regions of the interleukin-12 receptor subunits beta 1 and beta 2 and JAK kinases

The role of the cytoplasmic regions of interleukin-12 receptors (IL-12R) beta 1 and beta 2 in stimulating proliferation was examined, The transmembrane and cytoplasmic regions of IL-12R beta 1 or IL-12R beta 2 were fused to the extracellular domain of the epidermal growth factor (EGF) receptor, yielding chimeric receptors E12R1 and E12R2, respectively, These chimeras were stably transfected into BaF3 cells, a factor-dependent murine pro-B cell line, Only E12R2 or E12R1+E12R2 transfectants were capable of EGF-dependent proliferation, EGF-dependent phosphorylation of E12R2, JAK2, Tyk2, and STAT3 was observed, JAK2 was phosphorylated in E12R1-, E12R2-, and E12R1 +E12R2-expressing cells, However, direct associations were detectable only between E12R2 and JAK2. Tyk2 phosphorylation was observed only in cells expressing E12R1 or E12R1+E12R2. In parallel with this activation pattern, direct interactions only between Tyk2 and E12R1 were demonstrable. Phosphorylation of STAT3 was observed in cells expressing E12R1, E12R2, and E12R1+E12R2. The expression levels of STAT4 protein in BaF3 cells are undetectable by the methods employed here; therefore, STAT4 phosphorylation was not observed. Taken together, the data indicate that differential interactions take place between the cytoplasmic regions of the two IL-12R subunits and JAK2/Tyk2 and that the cytoplasmic region of IL-12R beta 2 alone is capable of delivering a proliferative signal.