The major vault protein is responsive to and interferes with interferon-γ-mediated STAT1 signals

Abstract

The major vault protein (MVP) is the main component of vaults, large ribonucleoprotein particles implicated in the regulation of cellular signaling cascades and multidrug resistance. Here, we identify MVP as an interferon γ (IFN-γ)-inducible protein. Treatment with IFN-γ resulted in a significant upregulation of MVP promoter activity as well as mRNA and protein levels. Activation of MVP expression by IFN-γ involved transcriptional upregulation through the JAK/STAT pathway based on an interaction of STAT1 with an interferon-γ-activated site (GAS) within the proximal MVP promoter. Mutation of this site distinctly reduced basal as well as IFN-γ-stimulated MVP transcription. IFN-γ also significantly enhanced the translation rate of MVP. Ectopic MVP overexpression in the MVP-negative lung cancer cell model H65 led to a downregulation of three known IFN-γ-regulated genes, namely ICAM-1, CD13 and CD36. Additionally, presence of MVP in H65 cells blocked both basal and IFN-γ-induced ICAM-1 expression whereas downmodulation of endogenous MVP levels by shRNA enhanced IFN-γ-induced ICAM-1 expression in U373 glioblastoma cells. MVP-mediated IFN-γ insensitivity was accompanied by significantly reduced STAT1 phosphorylation at Y701 and diminished translocation of STAT1 into the nucleus. Summarizing, we identify MVP as an IFN-γ-responsive gene interfering with IFN-γ-activated JAK/STAT signals. These data further substantiate that the vault particle functions as a general interaction platform for cellular signaling cascades.