Signal transduction of interleukin 2 in human natural killer cells: Involvement of the p56 lck tyrosine kinase

Abstract

Despite numerous reports, the role of the protein tyrosine kinase p56 lck in IL-2 signal transduction has remained controversial. We show here, using IL-2-dependent human natural killer cell lines, that p56 lck is regulated by IL-2 in two different ways: (1) IL-2 induces a rapid increase of p56 lck kinase activity as assessed in vitro; and (2) following IL-2 stimulation, p56 lck undergoes phosphorylation on serine residues that is reflected by a modification of its electrophoretic mobility in SDS-PAGE. Furthermore, dose response experiments, and blocking studies performed with anti-IL-2Rα antibodies, indicated that binding of IL-2 to the IL-2R β chain was sufficient to produce these modifications of p56 lck. In contrast, activation of the CD2 pathway stimulated the kinase activity of p56 lck, but did not induce a significant shift in NK cells, as opposed to T lymphocytes. Western blot analyses, and immunoprecipitations of cell lysates from 32P-preloaded NK cells demonstrated that seven major proteins are tyrosine phosphorylated in response to IL-2. These phosphoproteins, with apparent molecular weights of 190, 150, 120, 110, 85, 65 and 56, which may not all be p56 lck substrates, undergo phosphorylation and dephosphorylation with different kinetics. Furthermore, pp120 was identified as rasGAP, by Western blot and immunoprecipitation experiments. rasGAP and some of its co-precipitating molecules become phosphorylated in response to IL-2, presumably by p56 lck, which would thus provide a link between IL-2R and downstream events critical for NK cell proliferation and function.