IFI 16 gene encodes a nuclear protein whose expression is induced by interferons in human myeloid leukaemia cell lines

Abstract

We have characterized the induction of mRNA and protein products of the human IFI 16 gene in response to IFN-gamma, IFN-alpha, and IFN-beta 2 (IL-6). We demonstrate that the IFI 16 gene product is a novel nucleoprotein expressed in association with the differentiation of myeloid precursor cell lines. In Northern blots, IFI 16 mRNA was increased approximately 25-fold above barely detectable levels in unstimulated promyelocytic HL-60 cells, in response to IFN-gamma. Other myeloid cell lines, U937 and K562, also demonstrated a marked IFN-gamma-inducibility of IFI 16 mRNA. However, all three cell lines were far less responsive to IFN-alpha, and there was no response to IL-6. By comparison, a panel of T and B cell lines demonstrated high constitutive expression of IFI 16 mRNA that was not regulated by these cytokines. Culture of HL-60 cells in medium containing dimethylsulfoxide, retinoic acid, and 1,25 dihydroxyvitamin D3, agents that stimulate the differentiation of HL-60 along myeloid pathways, also caused the induction of IFI 16 mRNA. To characterize the protein product of IFI 16, a monoclonal antibody was raised against a recombinant bacterial protein comprising the amino terminal 159 amino acids of IFI 16 fused to glutathione S-transferase. The antibody, designated 1G7, was used in Western blotting to demonstrate the strong induction of a cluster of proteins of 85-95 kDa in the nuclear extracts of IFN-gamma-treated HL-60. The nuclear localization of IFI 16 antigen was confirmed by immunohistochemical staining of HL-60 cells treated with IFN-gamma, dimethylsulfoxide, and retinoic acid. IFI 16 was also detected in the nuclei of monocytes, neutrophils, and lymphocytes in normal peripheral blood. Database comparisons of the IFI 16 amino acid sequence revealed 51% identity with the recently cloned myeloid cell nuclear differentiation antigen (MNDA), and extensive similarity to protein products of the Gene 200 cluster of IFN-inducible genes, Ifi 202 and Ifi 204. The amino terminal domain of IFI 16 encodes a putative nuclear localization signal, 124PGAQKRKK, which is strongly conserved in MNDA and 204. Nuclear IFI 16 was able to bind double-stranded DNA in vitro and exhibited a similar elution profile from DNA-cellulose as previously observed for MNDA and 204. Therefore, IFI 16 and MNDA are members of a novel family of human DNA-binding proteins whose expression is associated with myeloid cell differentiation induced by cytokines and chemical agents.