The kinase domain of Jak2 mediates induction of Bcl-2 and delays cell death in hematopoietic cells

Granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin (IL)-3, and IL-5 stimulate DNA synthesis and proliferation and inhibit apoptosis in hematopoietic cells. Multiple signal pathways are activated by binding of these ligands to their receptors, which share a common β subunit. Janus protein kinase 2 (Jak2) binds to the membrane proximal domain of the β chain and is phosphorylated on receptor ligation. To explore the role of Jak2 in the regulation of specific signal transduction pathways, we constructed fusion proteins with a CD16 external domain, a CD7 transmembrane region, and a Jak2 cytoplasmic domain. This cytoplasmic domain consisted either of wild type Jak2 (CD16/Jak2-W) or Jak2 mutations with deletions of (a) the amino terminus (CD16/Jak2-N), (b) kinase-like domain (CD16/Jak2-B), (c) kinase domain (CD16/Jak2-C), or (d) amino-terminal and kinase-like domains, leaving the kinase domain (CD16/Jak-K) intact. In contrast to the CD16/Jak2-W fusion protein, which requires cross-linking for activation, CD16/Jak2-N, CD16/Jak2- B, and CDI6/Jak2-K were constitutively phosphorylated, and they stimulated Shc phosphorylation and increased binding of STAT to DNA in Ba/F3 cells. Cell lines derived from IL-3-dependent Ba/F3 cells stably transfected with CD16/Jak2-W, CD16/Jak2-N, or CD16/Jak2-B mammalian expression vectors died at a rate similar to that of the parental cells on IL-3 deprivation. In contrast, CD16/Jak2-K cell lines exhibited increased expression of bcl-2 and pim-1 mRNA and maintained their viability when compared with control cell lines. Thus, activation of tyrosine phosphorylation by creating a CD16/Jak2- K fusion is sufficient to activate pathways that prevent cell death.