title: Phosphorylation of the activation loop tyrosines is required for sustained Syk signaling and growth factor-independent B-cell proliferation.
abstract: The Syk kinase is regarded as a promising target for the treatment of antigen-driven B-cell malignancies, considering its essential role in propagating antigenic stimuli through the B-cell receptor (BCR). In certain common B-cell malignancies Syk is activated even in the absence of BCR engagement, suggesting a wider role for this kinase in lymphomagenesis. In this paper, we have profiled molecular differences between BCR-induced and constitutive Syk activation in terms of phosphorylation of regulatory tyrosine residues, downstream signaling properties and capacity to sustain B-cell proliferation. Analysis of primary chronic lymphocytic leukemia B-cells and diffuse large B-cell lymphoma cell lines revealed that constitutive and BCR-induced Syk activation differ with respect to the phosphorylation status of the regulatory tyrosines at positions 352 and 525/526, with only the first site being phosphorylated in the case of constitutive and both sites in the case of BCR-induced Syk activation. Syk phosphorylated only on Y352 is capable of downstream signaling, as evidenced by experiments with a phosphomimetic mutant in which the activation loop tyrosines (YY525/526) were replaced with phenylalanines. However, phosphorylation at YY525/526 was shown to significantly increase the enzymatic activity of Syk and to be required for sustained PLCgamma2, Akt and ERK signaling as well as B-cell transformation. These data demonstrate that constitutively active Syk and Syk activated by BCR crosslinking represent separate stages of Syk activation with distinct signaling properties and transforming capacities.
authors.affiliation: ICGEB Molecular Hematology Group, Campus A. Buzzati-Traverso, Via E. Ramarini 32, Monterotondo Scalo, Rome, Italy.
authors.forename: Laura
authors.initials: L
authors.lastname: Carsetti
authors.forename: Luca
authors.initials: L
authors.lastname: Laurenti
authors.forename: Stefania
authors.initials: S
authors.lastname: Gobessi
authors.forename: Pablo G
authors.initials: PG
authors.lastname: Longo
authors.forename: Giuseppe
authors.initials: G
authors.lastname: Leone
authors.forename: Dimitar G
authors.initials: DG
authors.lastname: Efremov
mesh_terms: D000818:Animals; D001402:B-Lymphocytes / Q000166:cytology* / Q000187:drug effects / Q000201:enzymology*; D049109:Cell Proliferation / Q000187:drug effects; D004789:Enzyme Activation / Q000187:drug effects; D006801:Humans; D036341:Intercellular Signaling Peptides and Proteins / Q000494:pharmacology; D047908:Intracellular Signaling Peptides and Proteins / Q000737:chemistry* / Q000378:metabolism*; D051379:Mice; D010766:Phosphorylation / Q000187:drug effects; D019000:Phosphotyrosine / Q000378:metabolism*; D055503:Protein Multimerization / Q000187:drug effects; D017433:Protein Structure, Secondary; D011505:Protein-Tyrosine Kinases / Q000737:chemistry* / Q000378:metabolism*; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D015398:Signal Transduction / Q000187:drug effects; D000072377:Syk Kinase; D014162:Transfection
chemical_list: D036341:Intercellular Signaling Peptides and Proteins; D047908:Intracellular Signaling Peptides and Proteins; D011993:Recombinant Fusion Proteins; D019000:Phosphotyrosine; D011505:Protein-Tyrosine Kinases; C000606896:SYK protein, human; D000072377:Syk Kinase; C000606897:Syk protein, mouse