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 Tat to heterochromatin protein HP1 via interaction with CTIP2 inhibits human immunodeficiency virus type 1 replication in microglial cells.

The Tat protein of human immunodeficiency virus type 1 (HIV-1) plays a key role as inducer of viral gene expression. We report that Tat function can be potently inhibited in human microglial cells by the recently described nuclear receptor cofactor chicken ovalbumin upstream promoter transcription factor-interacting protein 2 (CTIP2). Overexpression of CTIP2 leads to repression of HIV-1 replication, as a result of inhibition of Tat-mediated transactivation. In contrast, the related CTIP1 was unable to affect Tat function and viral replication. Using confocal microscopy to visualize Tat subcellular distribution in the presence of the CTIPs, we found that overexpression of CTIP2, and not of CTIP1, leads to disruption of Tat nuclear localization and recruitment of Tat within CTIP2-induced nuclear ball-like structures. In addition, our studies demonstrate that CTIP2 colocalizes and associates with the heterochromatin-associated protein HP1alpha. The CTIP2 protein harbors two Tat and HP1 interaction interfaces, the 145-434 and the 717-813 domains. CTIP2 and HP1alpha associate with Tat to form a three-protein complex in which the 145-434 CTIP2 domain interacts with the N-terminal region of Tat, while the 717-813 domain binds to HP1. The importance of this Tat binding interface and of Tat subnuclear relocation was confirmed by analysis of CTIP2 deletion mutants. Our findings suggest that inhibition of HIV-1 expression by CTIP2 correlates with recruitment of Tat within CTIP2-induced structures and relocalization within inactive regions of the chromatin via formation of the Tat-CTIP2-HP1alpha complex. These data highlight a new mechanism of Tat inactivation through subnuclear relocalization that may ultimately lead to inhibition of viral pathogenesis.