Specific down-regulation of interleukin-12 signaling through induction of phospho-STAT4 protein degradation.
Interleukin-12 (IL-12) plays a critical role in modulating the function of T lymphocytes and natural killer cells. IL-12 has potent antitumor effects in animal models, mediated primarily by its ability to enhance cytolytic activity and secretion of interferon-gamma (IFN-gamma). Unfortunately, the antitumor effect of IL-12 has not been demonstrated in clinical trials. Repeated administration of IL-12 in humans results in decreasing levels of IFN-gamma secretion. To understand the mechanism underlying this loss of responsiveness, the effect of IL-12 on its own signaling in activated human T cells was examined. These experiments demonstrate that the level of the signal transducer and activator of transcription 4 (STAT4) protein, a critical IL-12 signaling component, is dramatically decreased 24 hours after IL-12 stimulation, whereas levels of STAT4 messenger RNA are not affected. The decrease of STAT4 protein appears to be due to specific degradation of phospho-STAT4, possibly through the proteasome degradation pathway. Decreased levels of STAT4 protein lead to decreased STAT4 DNA-binding activity and reduced proliferation and secretion of IFN-gamma. This down-regulation of STAT4 is specific for IL-12 signaling, presumably owing to the prolonged activation of STAT4 induced by IL-12. IFN-alpha stimulation, which leads to transient phosphorylation of STAT4, does not reduce the level of STAT4 protein. These findings provide new insights into the regulation of IL-12 signaling in human T cells, where IL-12 promotes T(H)1 responses, but persistent IL-12 stimulation may also limit this response. The cellular depletion of STAT4 following prolonged IL-12 stimulation may also explain the loss of responsiveness following the repeated administration of IL-12 in clinical trials. (Blood. 2001;97:3860-3866)
Wang
K S
KS
Center for Hematologic Oncology, Department of Adult Oncology, Dana-Farber Cancer Institute, 44 Binney St., Boston, MA 02115, USA.
Zorn
E
E
Ritz
J
J
D002478:Cells, Cultured; D003546:Cysteine Endopeptidases / Q000378:metabolism; D004268:DNA-Binding Proteins / Q000187:drug effects / Q000235:genetics / Q000378:metabolism*; D015536:Down-Regulation; D005246:Feedback; D006801:Humans; D018664:Interleukin-12 / Q000494:pharmacology* / Q000502:physiology; D008213:Lymphocyte Activation; D009097:Multienzyme Complexes / Q000378:metabolism; D010766:Phosphorylation; D046988:Proteasome Endopeptidase Complex; D012333:RNA, Messenger / Q000187:drug effects / Q000378:metabolism; D018123:Receptors, Interleukin / Q000187:drug effects / Q000378:metabolism; D053707:Receptors, Interleukin-12; D050797:STAT4 Transcription Factor; D015398:Signal Transduction; D013601:T-Lymphocytes / Q000378:metabolism; D015534:Trans-Activators / Q000187:drug effects / Q000235:genetics / Q000378:metabolism*
D004268:DNA-Binding Proteins; D009097:Multienzyme Complexes; D012333:RNA, Messenger; D018123:Receptors, Interleukin; D053707:Receptors, Interleukin-12; D050797:STAT4 Transcription Factor; C494090:STAT4 protein, human; D015534:Trans-Activators; D018664:Interleukin-12; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex