Transmembrane signaling by the interleukin-2 receptor: Progress and conundrums

Activation of the multicomponent interleukin-2 receptor (IL-2R) complex leads to a rapid increase in tyrosine phosphorylation of a number of cellular proteins including the IL-2Rβ and IL-2Rγ chains of the IL-2R and the RAF-1 serine threonine kinase. In addition, phosphatidylinositol 3-kinase (PI-3K) protein and activity can be immunoprecipitated with anti-phosphotyrosine and anti-IL-2Rβ antibodies from IL-2-activated but not resting T lymphocytes. We have demonstrated that the SH2 (SRC homology 2) domains of the 85 kDa subunit of PI-3K are sufficient to mediate binding of the PI-3K complex to tyrosine phosphorylated, but not non-phosphorylated IL-2Rβ, suggesting that tyrosine phosphorylation is an integral component of the activation of PI-3K by the IL-2R. Since none of the members of the IL-2R complex contains an intrinsic tyrosine kinase domain, IL-2-induced tyrosine phosphorylation must be the consequence of activation of intracellular tyrosine kinases. SRC family members including lck, lyn and fyn have been demonstrated to associate with IL-2Rβ through binding of the kinase domain to the acidic domain of IL-2Rβ. However, we have demonstrated that the serine rich (SD) region of the cytosolic domain of IL-2Rβ is also required for association of a tyrosine kinase with the IL-2R complex and that IL-2 can induce proliferation and tyrosine phosphorylation in cell lines which lack the known SRC family kinases expressed by T lymphocytes. Th us members of other kinase families besides SRC may also be involved in mediating IL-2 signal transduction. Biochemical studies and studies of cells expressing mutant IL-2 receptors indicate that IL-2-induced tyrosine kinase activation initiates a complex signaling cascade. The cascade includes SRC family kinase members such as lck, fyn, and lyn, activation of Raf-1 and PI-3K, and ras, and increased expression of the fos, fra-1, and jun protooncogenes. In addition, ligation of the IL-2R leads to rapid increases in myc expression and more delayed increases in the expression of the cdc2 and cdk2 kinases and the cyclins through a tyrosine phosphorylation independent pathway. Whether other biochemical processes initiated by IL-2R ligation, including activation of the MAP2, p70S6 and p90RSK serine threonine kinases, activation of NF-NB, and increased expression ofRaf-1, Pim-1, bcl-2, IL-2Rα and IL-2Rβ, are consequences of the IL-2-induced tyrosine kinase cascade remains to be determined. Whereas tyrosine kinase inhibitors prevent IL-2-induced proliferation in murine and human T cells, mutant IL-2Rβ chains which do not couple to the tyrosine kinase pathway retain the ability to induce proliferation in BAF pro-B cells albeit at slightly decreased levels. Thus, the role of the signaling cascade initiated by tyrosine phosphorylation in IL-2-induced proliferation remains controversial. In order to identify additional kinases which may be involved in IL-2 signal transduction, we utilized expression cloning and polymerase chain reaction to clone kinases from IL-2 responsive cells. We have identified two unique human kinases, TTK and EMT, which are induced by IL-2 in at least some T lymphocytes. It is not yet clear what role these kinases play in IL-2-induced cellular activation and proliferation.