CONDITIONAL ACTIVATION OF JANUS KINASE (JAK) CONFERS FACTOR INDEPENDENCE UPON INTERLEUKIN-3-DEPENDENT CELLS - ESSENTIAL ROLE OF RAS IN JAK-TRIGGERED MITOGENESIS

Cytokines play crucial roles in the growth and differentiation of hema topoietic cells. They bind to specific cell membrane receptors that us ually do not possess intrinsic protein-tyrosine kinase activity. Janus kinases (JAKs) are cytoplasmic protein-tyrosine kinases that physical ly interact with intracellular domains of the cytokine receptors and h ave been implicated in playing important roles in signal transduction triggered by the cytokine-cytokine receptor interaction. However, it i s still uncertain whether JAK activation alone suffices to induce cell proliferation. In this work, we modified Tyk2, a member of the JAK fa mily, by adding a membrane localization sequence and a chemical dimeri zer (coumermycin)-dependent dimerization sequence. The modified Tyk2 w as activated in a coumermycin-dependent manner, and the activated Tyk2 conferred cytokine independence upon interleukin-3-dependent pro-B ly mphoid cells, This cytokine-independent proliferation was completely i nhibited by dominant-negative Ras. These results indicate that activat ion of JAK through membrane-proximal dimerization is sufficient to ind uce cell cycle progression and that Ras is essentially involved in JAK -triggered mitogenesis.