Expression of the adaptor protein Lnk in leukemia cells.
OBJECTIVE
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.


METHODS
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.


RESULTS
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.


CONCLUSIONS
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.
Division of Hematology/Oncology, Cedars-Sinai Medical Center, UCLA School of Medicine, Los Angeles, Calif. 90048, USA. gerys@cshs.org
Sigal
S
Gery
Saskia
S
Gueller
Verena
V
Nowak
Julia
J
Sohn
Wolf K
WK
Hofmann
H Phillip
HP
Koeffler
D048868:Adaptor Proteins, Signal Transducing; D000483:Alleles; D045744:Cell Line, Tumor; D002471:Cell Transformation, Neoplastic / Q000235:genetics / Q000378:metabolism / Q000473:pathology; D005260:Female; D016044:Fusion Proteins, bcr-abl / Q000235:genetics / Q000378:metabolism; D015973:Gene Expression Regulation, Leukemic; D006801:Humans; D047908:Intracellular Signaling Peptides and Proteins; D053614:Janus Kinase 2 / Q000235:genetics / Q000378:metabolism*; D015470:Leukemia, Myeloid, Acute / Q000235:genetics / Q000378:metabolism* / Q000473:pathology; D008297:Male; D020929:Mitogen-Activated Protein Kinase Kinases; D009154:Mutation; D009190:Myelodysplastic Syndromes / Q000235:genetics / Q000378:metabolism* / Q000473:pathology; D019869:Phosphatidylinositol 3-Kinases / Q000235:genetics / Q000378:metabolism; D011494:Protein Kinases / Q000235:genetics / Q000378:metabolism; D011506:Proteins / Q000235:genetics / Q000378:metabolism*; D051057:Proto-Oncogene Proteins c-akt / Q000235:genetics / Q000378:metabolism; D019009:Proto-Oncogene Proteins c-kit / Q000235:genetics / Q000378:metabolism*; D058570:TOR Serine-Threonine Kinases
D048868:Adaptor Proteins, Signal Transducing; D016044:Fusion Proteins, bcr-abl; D047908:Intracellular Signaling Peptides and Proteins; D053614:Janus Kinase 2; D020929:Mitogen-Activated Protein Kinase Kinases; D019869:Phosphatidylinositol 3-Kinases; D011494:Protein Kinases; D011506:Proteins; D051057:Proto-Oncogene Proteins c-akt; D019009:Proto-Oncogene Proteins c-kit; D058570:TOR Serine-Threonine Kinases; C490778:SH2B3 protein, human; C546842:MTOR protein, human; C507924:JAK2 protein, human