A 3′-Transcribed Region of the HLA-A2 Gene Mediates Posttranscriptional Stimulation by IFN-γ

Abstract

The expression of several MHC class I genes is up-regulated at the transcriptional level by IFN-γ. Posttranscriptional mechanisms also have been implicated, but not well characterized. To investigate the mechanism of IFN-γ stimulation of the human MHC class I gene HLA-A2, several human tumor cell lines were transfected with reporter gene constructs driven by the HLA-A2 promoter. We have previously shown that the extended 525-bp HLA-A2 promoter alone, which includes a 5′ IFN-stimulated response element consensus sequence, is not sufficient for IFN-γ response in either K562 or Jurkat cells. In the current study, stable transfection of a genomic HLA-A2 gene construct, containing both 5′- and 3′-flanking sequences, resulted in stimulation of the gene by IFN-γ. Nuclear run-on assays revealed that, unlike other class I genes, IFN-γ stimulation of HLA-A mRNA accumulation occurs almost entirely through posttranscriptional mechanisms. RNA stability assays showed that the effect is not mediated by alteration of the half-life of the HLA-A2 mRNA. Formation of the 3′ end was unaffected by IFN-γ treatment. Sequences that mediate the majority of IFN-γ induction of HLA-A2 mRNA reside in a 127-bp 3′-transcribed region of the gene. This region contains the terminal splice site, the usage of which is not affected by IFN-γ treatment. These results demonstrate a novel posttranscriptional mechanism of regulation of MHC class I genes by IFN-γ.