Richard Pine

Human alpha- and beta-interferons (IFNs) stimulate rapid but transient increases in transcription from a set of previously quiescent genes. Protein synthesis is not required for initial stimulation, but duration of the response is limited to a few hours by a process requiring synthesis of new proteins. An IFN-stimulated response element (ISRE) was identified 5' to an inducible gene by deletion analysis and point mutagenesis, and sequence comparisons with other promoters defined the consensus element YAGTTTC(A/T)YTTTYCC. Two classes of IFN-inducible nuclear factors were found that bind to the ISRE. The most rapidly induced factor appeared without new protein synthesis, whereas a second factor required active protein synthesis for its appearance and maintenance. The kinetics of appearance and loss of these binding activities correlate with the activation and repression of IFN-stimulated genes. These different IFN-activated or induced factors may bind sequentially to the same essential promoter element to first increase and then repress transcription.

Activation of early response genes by interferons (IFNs) requires tyrosine phosphorylation of STAT (signal transducers and activators of transcription) proteins. It was found that the serine-threonine kinase mitogen-activated protein kinase (MAPK) [specifically, the 42-kilodalton MAPK or extracellular signal-regulated kinase 2 (ERK2)] interacted with the alpha subunit of IFN-alpha/beta receptor in vitro and in vivo. Treatment of cells with IFN-beta induced tyrosine phosphorylation and activation of MAPK and caused MAPK and Stat1 alpha to coimmunoprecipitate. Furthermore, expression of dominant negative MAPK inhibited IFN-beta-induced transcription. Therefore, MAPK appears to regulate IFN-alpha and IFN-beta activation of early response genes by modifying the Jak-STAT signaling cascade.

The signal transduction pathway through which interferon-alpha (IFN alpha) stimulates transcription of a defined set of genes involves activation of DNA-binding factors specific for the IFN alpha-stimulated response element (ISRE). IFN-stimulated gene factor-3 (ISGF3), the positive regulator of transcription, was derived in response to IFN alpha treatment from preexisting protein components that were activated first in the cell cytoplasm prior to appearance in the nucleus. Nuclear translocation of ISGF3 required several minutes and could be inhibited by NaF. Formation of active ISGF3 was mimicked in vitro by mixing cytoplasmic extracts from IFN alpha-stimulated cells with extracts of cells treated to contain high amounts of the unactivated factor. Active ISGF3 was found to be formed from association of two latent polypeptide precursors that were distinguished biochemically by differential sensitivity to N-ethyl maleimide. One precursor was modified in response to IFN alpha occupation of its cell-surface receptor, thus enabling association with the second subunit. The resulting complex then was competent for nuclear translocation and binding to ISRE. Cytoplasmically localized transcription factor precursors thus serve as second messengers to translate directly an extracellular signal into specific transcriptional activity in the nucleus.

Interferon-stimulated gene factor 2 (ISGF2) was purified from HeLa cells treated with alpha interferon. The factor, a single polypeptide of 56 kilodaltons (kDa), bound both to the central 9 base pairs of the 15-base-pair interferon-stimulated response element (ISRE) that is required for transcriptional activation of interferon-stimulated genes and to the PRD-I regulatory element of the beta interferon gene. ISGF2 was a phosphoprotein, and dephosphorylation in vitro reduced its DNA-binding activity. However, conditions that changed the amount of ISGF2 did not change the phosphorylated isoforms in vivo. ISGF2 in unstimulated cells existed in trace amounts and was induced by both alpha interferon and gamma interferon as well as by virus infection. Plasmid-bearing Escherichia coli clones encoding ISGF2 were selected with antibody against purified ISGF2. Sequence analysis revealed that the ISGF2 protein was the same as that encoded by the cDNA clone IRF-1, which has been claimed to activate transcription of interferon genes. We show that transcription of the ISGF2 gene was induced by alpha interferon, gamma interferon, and double-stranded RNA. However, ISGF2 was neither necessary nor sufficient for induced transcription of the beta interferon gene, while the factor NF kappa B was clearly involved.