Transcription Regulatory Complexes Bind the Human T-Cell Leukemia Virus 5′ and 3′ Long Terminal Repeats To Control Gene Expression

Abstract

The human T-cell leukemia virus type 1 (HTLV-1) is a retrovirus that integrates randomly into the T-cell genome. Two long terminal repeats (LTRs) flank the integrated provirus. The upstream and downstream LTRs carry identical promoter sequences. Studies with other retroviruses suggest that the downstream promoter is silent and that RNA polymerases initiating at the upstream promoter proceed through the 3′ LTR. In this study, we used the chromatin immunoprecipitation assay to compare the binding of transcription regulatory proteins at both the upstream and downstream promoters in HTLV-1-infected cell lines and adult T-cell leukemia-lymphoma cells. Unexpectedly, we detected a nearly equal distribution of activator (Tax, CREB, ATF-1, ATF-2, c-Fos, and c-Jun) and regulatory protein (CBP, p300, TAF_II250, and polymerase II) binding at both the upstream and downstream promoters. Consistent with this observation, we found that the downstream promoter was transcriptionally active, suggesting that the two promoters are functionally equivalent. We also detected asymmetrical binding of histone deacetylases (HDAC-1, -2, and -3) at both promoters. All three HDACs strongly repressed Tax transactivation, and this repression correlated with displacement of Tax from the HTLV-1 promoter. These effects were reciprocal, as Tax expression reversed HDAC repression and displaced HDACs from the HTLV-1 promoter. These data suggest that HTLV-1 transcriptional regulation at both the 5′ and 3′ LTRs is mediated, in part, through the mutually exclusive binding of Tax and HDACs at the proviral promoters.

We are especially grateful to Robert Harrod, Shigeki Takemoto, and Hirokuni Taguchi for the ATLL cells, K.-T. Jeang for the CHOK1-Luc cells, and Ed Seto for the Flag-HDAC plasmids. We also thank Jill Livengood for critical reading of the manuscript.

This study was supported by grants from the National Institutes of Health, National Cancer Institute: CA55035 to J.K.N. and CA87540 to J.K.N. and P.J.L.