COUP-TF interacting protein 2 represses the initial phase of HIV-1 gene transcription in human microglial cells.

Human immunodeficiency virus type 1 (HIV-1) gene transcription is characterized by two temporally distinct phases. While the initial phase relies solely on cellular transcription factors, the subsequent phase is activated by the viral Tat transactivator. We have previously reported that the subsequent phase of viral gene transcription can be repressed by the chicken ovalbumin upstream promoter transcription factor (COUP-TF)-interacting protein 2 (CTIP2) in human microglial cells [O. Rohr, D. Lecestre, S. Chasserot-Golaz, C. Marban, D. Avram, D. Aunis, M. Leid and E. Schaeffer (2003), J. Virol., 77, 5415-5427]. Here, we demonstrate that CTIP proteins also repress the initial phase of HIV-1 gene transcription, mainly supported by the cellular transcription factors Sp1 and COUP-TF in microglial cells. We report that CTIP2 represses Sp1- and COUP-TF-mediated activation of HIV-1 gene transcription and viral replication as a result of physical interactions with COUP-TF and Sp1 in microglial nuclei. Using laser confocal microscopy CTIP2 was found to colocalize with Sp1, COUP-TF and the heterochromatin-associated protein Hp1alpha, which is mainly detected in transcriptionally repressed heterochromatic region. Moreover, we describe that CTIP2 can be recruited to the HIV-1 promoter via its association with Sp1 bound to the GC-box sequences of the long terminal repeat (LTR). Since our findings demonstrate that CTIP2 interacts with the HIV-1 proximal promoter, it is likely that CTIP2 promotes HIV-1 gene silencing by forcing transcriptionally repressed heterochromatic environment to the viral LTR region.

Recruitment of chromatin-modifying enzymes by CTIP2 promotes HIV-1 transcriptional silencing.

Following entry and reverse transcription, the HIV-1 genome is integrated into the host genome. In contrast to productively infected cells, latently infected cells frequently harbor HIV-1 genomes integrated in heterochromatic structures, allowing persistence of transcriptionally silent proviruses. Microglial cells are the main HIV-1 target cells in the central nervous system and constitute an important reservoir for viral pathogenesis. In the present work, we show that, in microglial cells, the co-repressor COUP-TF interacting protein 2 (CTIP2) recruits a multienzymatic chromatin-modifying complex and establishes a heterochromatic environment at the HIV-1 promoter. We report that CTIP2 recruits histone deacetylase (HDAC)1 and HDAC2 to promote local histone H3 deacetylation at the HIV-1 promoter region. In addition, DNA-bound CTIP2 also associates with the histone methyltransferase SUV39H1, which increases local histone H3 lysine 9 methylation. This allows concomitant recruitment of HP1 proteins to the viral promoter and formation of local heterochromatin, leading to HIV-1 silencing. Altogether, our findings uncover new therapeutic opportunities for purging latent HIV-1 viruses from their cellular reservoirs.