Involvement of an Inducible Factor in Interferon-γ-Mediated Accumulation of HLA Gene Transcripts

Earlier studies with a cDNA clone (C5-4) complementary to an interferon (IFN)-gamma-inducible mRNA showed that in human fibroblasts (FS-4), IFN-gamma induced the transcription of the cognate gene, but it required new protein synthesis (Caplen and Gupta, J. Biol. Chem. 263, 332-339, 1988). To determine whether such a strategy is used for the regulation of other cellular genes by IFN-gamma, the regulation of the HLA class I and class II genes and another cellular gene for which a cDNA clone was isolated (C13) was studied. The results indicate that: (i) HLA-B (class I) and C13 gene expression was transcriptionally activated by IFN-gamma and IFN-alpha 2, and it did not require new protein synthesis. (ii) In contrast, the transcription of the HLA-DR alpha was activated by IFN-gamma (and not by IFN-alpha 2), but the accumulation of -DR alpha gene transcripts was strongly inhibited by cycloheximide or anisomycin, which indicated that there was a requirement for some newly synthesized protein factor(s) in this process, apparently at a step subsequent to transcriptional activation. We obtained evidence indicating that the putative protein factor(s) required is actually induced by IFN-gamma. (iii) IFN-gamma-induced transcription of the HLA-B gene was not inhibited by anisomycin or cycloheximide, but the accumulation of HLA-B transcripts plateaued sooner. This latter effect was not due to any toxicity of these inhibitors because it was observed if cycloheximide was added together with IFN-gamma, but not if it was added a few hours later. Furthermore, if cycloheximide was added 24 h after IFN-gamma, it actually caused a superinduction of HLA-B transcripts. The results suggest that some newly synthesized protein factor(s) may be required also for maximal accumulation of HLA-B gene transcripts following treatment with IFN-gamma. The results indicate a dual regulation of HLA class I and class II genes by IFN-gamma, and involvement of multiple mechanisms in the regulation of cellular gene expression by IFN-gamma.