Expression of the adaptor protein Lnk in leukemia cells

Abstract

Tyrosine kinases are involved in cytokine signaling and are frequently aberrantly activated in hematological malignancies. Lnk, a negative regulator of cytokine signaling, plays critical nonredundant roles in hematopoiesis. By binding to phosphorylated tyrosine kinases, Lnk inhibits major cytokine receptor signaling, including c-KIT; erythropoietin receptor-Janus kinase 2 (JAK2); and MPL-JAK2. In the present study, we investigated Lnk expression and possible function in transformed hematopoietic cells. Coimmunoprecipitations were performed to identify binding between Lnk and mutant tyrosine kinases. Proliferation assays were done to examine the affect of Lnk overexpression on cancer cell growth. Real-time polymerase chain reaction analysis was used to determine Lnk expression in patient samples. We show that, in parallel to binding wild-type JAK2 and c-KIT, Lnk associates with and is phosphorylated by mutant alleles of JAK2 and c-KIT. In contrast, Lnk does not bind to and is not phosphorylated by BCR-ABL fusion protein. Ectopic expression of Lnk strongly attenuates growth of some leukemia cell lines, while others as well as most solid tumor cancer cell lines are either moderately inhibited or completely insensitive to Lnk. Furthermore, Lnk-mediated growth inhibition is associated with differential downregulation of phosphatidylinositol 3 kinase/Akt/mammalian target of rapamycin and mitogen-activated protein kinase/extracellular signal-regulated kinase signaling in leukemia cell lines. Surprisingly, analysis of Lnk in a large panel of myelodysplastic syndrome and acute myeloid leukemia patient samples revealed high levels of Lnk in nearly half of the samples. Although how leukemic cells overcome the antiproliferative effects of Lnk is not yet clear, our data highlight the multifaceted role negative feedback mechanisms play in malignant transformation.