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 ac tivity and secretion of interferon-gamma (IFN-gamma), Unfortunately, the an titumor effect of IL-12 has not been demonstrated in clinical trials. Repea ted administration of IL-12 in humans results in decreasing levels of IFN-g amma secretion. To understand the mechanism underlying this loss of respons iveness, the effect of IL-12 on its own signaling in activated human T cell s 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 stim ulation, whereas levels of STAT4 messenger RNA are not affected. The decrea se of STAT4 protein appears to be due to specific degradation of phospho-ST AT4, possibly through the proteasome degradation pathway. Decreased levels of STAT4 protein lead to decreased STAT4 DNA-binding activity and reduced p roliferation and secretion of IFN-gamma, This down-regulation of STAT4 is s pecific for IL-12 signaling, presumably owing to the prolonged activation o f STAT4 induced by IL-12, IFN-a stimulation, which leads to transient phosp horylation of STAT4, does not reduce the level of STAT4 protein. These find ings 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 stimulat ion may also limit this response. The cellular depletion of STAT4 following prolonged IL-12 stimulation may also explain the loss of responsiveness fo llowing the repeated administration of IL-12 in clinical trials. (C) 2001 b y The American Society of Hematology.