Functional LCK Is required for optimal CD28-mediated activation of the TEC family tyrosine kinase EMT/ITK.
Activation of CD28 on T lymphocytes initiates a cascade of intracellular events, which in concert with activation of the T cell receptor, culminates in production of cytokines and a functional immune response. One of the earliest biochemical changes observed following stimulation of CD28 is tyrosine phosphorylation. We have demonstrated that both the LCK and the EMT/ITK/TSK (EMT) intracellular tyrosine kinases are activated following cross-linking of CD28. Utilizing somatic cell mutants lacking LCK, we demonstrate that functional LCK is required for CD28-induced activation of EMT as evidenced by increased tyrosine phosphorylation and kinase activity. In support of a role for LCK in EMT activation, reconstitution of a LCK-negative Jurkat T cell line by transfection with normal LCK recreates CD28-mediated EMT activation. Furthermore, co-transfection of LCK and EMT into COS-7 cells showed that EMT becomes phosphorylated in the presence of LCK. In addition, increases in EMT association with CD28 were eliminated in a LCK-negative Jurkat cell line, but were restored following transfection of wild type LCK. The data are most compatible with a model in which LCK, either directly or indirectly, initiates EMT activation and association with CD28 following ligation of CD28.
Gibson
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S
Molecular Oncology, Division of Medicine, University of Texas, M. D. Anderson Cancer Center, Houston, 77030, USA.
August
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Branch
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Dupont
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B
Mills
G M
GM
D000595:Amino Acid Sequence; D000818:Animals; D018106:CD28 Antigens / Q000378:metabolism*; D002460:Cell Line; D004789:Enzyme Activation; D006801:Humans; D019860:Lymphocyte Specific Protein Tyrosine Kinase p56(lck); D008969:Molecular Sequence Data; D010766:Phosphorylation; D011505:Protein-Tyrosine Kinases / Q000378:metabolism*; D014162:Transfection; D019061:src-Family Kinases / Q000378:metabolism*
D018106:CD28 Antigens; D011505:Protein-Tyrosine Kinases; D019860:Lymphocyte Specific Protein Tyrosine Kinase p56(lck); C080813:emt protein-tyrosine kinase; D019061:src-Family Kinases