RESUMO
In resting peripheral T cells, Csk is constitutively present in lipid rafts through an interaction with the Csk SH2-binding protein, PAG, also known as Cbp. Upon triggering of the T cell antigen receptor (TCR), PAG/Cbp is rapidly dephosphorylated leading to dissociation of Csk from lipid rafts. However, tyrosine phosphorylation of PAG/Cbp resumes after 3--5 min, at which time Csk reassociates with the rafts. Cells overexpressing a mutant Csk that lacks the catalytic domain, but displaces endogenous Csk from lipid rafts, have elevated basal levels of TCR-zeta-chain phosphorylation and spontaneous activation of an NFAT-AP1 reporter from the proximal interleukin-2 promoter as well as stronger and more sustained responses to TCR triggering than controls. We suggest that a transient release from Csk-mediated inhibition by displacement of Csk from lipid rafts is important for normal T cell activation.
Assuntos
Ativação Linfocitária/fisiologia , Microdomínios da Membrana/fisiologia , Proteínas Tirosina Quinases/metabolismo , Linfócitos T/imunologia , Antígenos Transformantes de Poliomavirus/genética , Proteína Tirosina Quinase CSK , Células Cultivadas , Humanos , Células Jurkat , Modelos Biológicos , Muromonab-CD3/farmacologia , Fosforilação , Fosfotirosina/metabolismo , Proteínas Tirosina Quinases/genética , Receptores de Antígenos de Linfócitos T/fisiologia , Proteínas Recombinantes/metabolismo , Linfócitos T/efeitos dos fármacos , Transfecção , Vanadatos/farmacologia , Domínios de Homologia de src , Quinases da Família srcRESUMO
Transcription factors of the STAT family have been implicated in regulation of cell proliferation. EGF activates several STAT proteins in liver. We have studied the relationship between STAT activation and the growth-stimulatory effect of EGF in rat hepatocytes, assessing specific DNA-binding activity of STAT proteins in electrophoretic mobility-shift and supershift assays. In freshly isolated hepatocytes, EGF activated Stat1, Stat3, and, particularly, Stat5b. However, the ability of EGF to produce this activation was rapidly attenuated when the cells were cultured, while the activation by IFN-gamma (Stat1) and IL-6 (Stat3) was sustained. Hepatocytes cultured for 24-48 h are highly sensitive to the stimulatory effect of EGF on S phase entry. In these cells EGF did not detectably activate Stat1, Stat3, or Stat5b but markedly stimulated MAP kinase (Erk1/2). Thus, although EGF has the ability to activate several STAT proteins, this did not seem to be part of the mitogenic mechanisms used by the EGF receptor in hepatocytes.
