Activation of STAT4 by IL-12 and IFN-alpha: evidence for the involvement of ligand-induced tyrosine and serine phosphorylation.
The immunoregulatory cytokine IL-12 plays a central role in cell-mediated immune responses through its effects on NK cells and T lymphocytes. While IL-12 is known to share some functions with other cytokines, such as IFN-alpha, it also maintains distinct roles, such as its ability to induce Th1 differentiation. The molecular basis for these unique and overlapping functions is not well understood. IL-12 has previously been shown to induce tyrosine phosphorylation and DNA-binding of STAT3 and STAT4, members of the signal transducers and activators of transcription (STAT) family. Because STAT4 has only been shown to be activated in response to IL-12, this specificity has been suggested to be a basis for the unique actions of IL-12. In this study, we demonstrated that STAT4 activation by IL-12 is not unique; IL-12 and IFN-alpha, but not IFN-gamma, induced tyrosine phosphorylation and DNA binding of STAT4. Since tyrosine and serine phosphorylation of STAT1 have previously been shown to be important in IFN-alpha-mediated signaling, we also investigated IL-12- and IFN-alpha-induced serine phosphorylation of STAT4. We demonstrated that both cytokines induced serine phosphorylation. This modification was not required for DNA binding, but may be important in STAT-mediated transcription. Thus, STAT4 activation was not IL-12 specific, and IL-12 and IFN-alpha activated STAT4 through both tyrosine and serine phosphorylation. These findings have significant implications for understanding the role of STAT4 in mediating biologic functions; specifically, the data argue that the unique effects of IL-12 cannot be solely explained by STAT4 activation.
Cho
S S
SS
Arthritis and Rheumatism Branch, National Institute of Arthritis and Musculoskeletal and Skin Diseases, National Institutes of Health, Bethesda, MD 20892, USA.
Bacon
C M
CM
Sudarshan
C
C
Rees
R C
RC
Finbloom
D
D
Pine
R
R
O'Shea
J J
JJ
D000818:Animals; D001483:Base Sequence; D019556:COS Cells; D002478:Cells, Cultured; D004247:DNA / Q000378:metabolism; D004268:DNA-Binding Proteins / Q000737:chemistry / Q000378:metabolism*; D006801:Humans; D007370:Interferon Type I / Q000494:pharmacology*; D018664:Interleukin-12 / Q000494:pharmacology*; D007694:Killer Cells, Natural / Q000276:immunology / Q000378:metabolism; D008024:Ligands; D009838:Oligodeoxyribonucleotides / Q000235:genetics / Q000378:metabolism; D010766:Phosphorylation; D011485:Protein Binding; D011994:Recombinant Proteins / Q000494:pharmacology; D050797:STAT4 Transcription Factor; D012694:Serine / Q000378:metabolism; D013601:T-Lymphocytes / Q000276:immunology / Q000378:metabolism; D015534:Trans-Activators / Q000737:chemistry / Q000378:metabolism*; D014443:Tyrosine / Q000378:metabolism
D004268:DNA-Binding Proteins; D007370:Interferon Type I; D008024:Ligands; D009838:Oligodeoxyribonucleotides; D011994:Recombinant Proteins; D050797:STAT4 Transcription Factor; C494090:STAT4 protein, human; D015534:Trans-Activators; D018664:Interleukin-12; D014443:Tyrosine; D012694:Serine; D004247:DNA