Protein kinase activity required for an early step in interferon-alpha signaling

Interferon-alpha (IFN alpha) induces an immediate transcriptional response of a restricted set of genes in target cells. Specific transcription is mediated by the cytoplasmic activation of a transcription factor complex termed ISGF3. ISGF3 is a multimeric protein complex composed of a regulatory component (ISGF3 alpha), which is activated following IFN alpha treatment, and a DNA-binding component (ISGF3 gamma), which recognizes the IFN alpha-stimulated response element (ISRE). Following activation, ISGF3 alpha translocates to the nucleus where ISGF3 assembles as a high affinity complex on the ISRE. The biochemical basis for receptor-mediated activation of ISGF3 is unknown. We report that two potent protein kinase inhibitors, staurosporine and K-252a, ablated the transcriptional response to IFN alpha treatment. These inhibitors prevented the activation of the ISGF3 alpha component without affecting the ISGF3 gamma component, resulting in no accumulation of mature ISGF3 in nuclei of treated cells. Although these agents are potent inhibitors of protein kinase C (PKC), PKC does not mediate ISGF3 alpha activation. Down-regulation of PKC by chronic exposure of cells to 12-O-tetradecanoylphorbol-13-acetate, which led to complete loss of PKC-immunoreactive material, failed to ablate the transcriptional response to IFN alpha or the activation of ISGF3 alpha. The PKC-specific inhibitor calphostin C did not perturb activation or nuclear accumulation of ISGF3. We conclude that a novel, staurosporine/K-252a-sensitive kinase is required for ISGF3 activity and may participate in receptor-mediated signal transduction.