Chloé Gillard

Involvement of the cellular factor UHRF1 and its interactants in the transcriptional regulation of HIV-1

Abstract

Human immunodeficiency virus type 1 (HIV-1) persists due to the existence of latent reservoirs, which constitute the major obstacle to its eradication. HIV-1 latency is controlled by a multitude of molecular mechanisms acting at the transcriptional, post-transcriptional and epigenetic levels. A better understanding of these mechanisms by identifying the factors involved in the establishment and maintenance of HIV-1 latency are essential for obtaining new targets and therapeutic strategies aimed at eradicating the virus. In this context, our laboratory demonstrated for the first time the involvement of the cellular factor UHRF1 in the epigenetic latency of HIV-1 (Verdikt et al., 2022). Subsequently, the host laboratory investigated the involvement of a repressive activity of UHRF1 independent of the methylation of the viral promoter. Indeed, our laboratory has demonstrated that UHRF1 is able to repress the transcription of HIV-1 in the absence of methylation at the level of this promoter. At the same time, Liang and colleagues reported a repressive role of UHRF1 on HIV-1 transcription that would involve the degradation of the viral protein Tat, mediated by the E3-ubiquitin ligase activity of UHRF1 (Liang et al., 2021 ). This thesis aims to investigate different aspects of the transcriptional repression of HIV-1 mediated by UHRF1, independently of its epigenetic function. First of all, we could not confirm the degradation of the Tat protein by the E3-Ubiquitin Ligase activity of UHRF1, leaving in question the mechanisms by which UHRF1 could act. We hypothesized that the UHRF1 protein could act on different partners of Tat, such as the P-TEFb complex or even p300. Together, our results shed light on a repressive effect of UHRF1 on the initiation and elongation of HIV-1 transcription by destabilizing important partners of the viral transactivator Tat, namely (1) the catalytic subunit of the complex P-TEFb, CDK9 and (2) histone acetyl-transferase p300. In addition to the protein partners of the Tat protein, UHRF1 is known to interact with a large panel of proteins. Our laboratory has identified for the first time an in vivo interaction between UHRF1 and the cellular factor Yin Yang 1 (YY1). Interestingly, YY1 is known in the literature as a transcriptional repressor of HIV-1 under latency conditions (Coull et al., 2000). However, Chang and his colleagues identified a positive activity of YY1 on the transcriptional regulation of HIV-1. Consistent with this publication, we also observed an increase in HIV-1 transcription in the presence of YY1 overexpression. However, the addition of UHRF1 significantly reduces this activation of the 5' LTR promoter. In this context, we have shown that although the RING domain of UHRF1 seems to be important for its interaction with, none of the five domains seems essential for the repressive activity of UHRF1, in the presence or absence of YY1. In view of the literature, our results and the dual function of YY1 – repressor/activator – we hypothesized that under latency conditions, YY1 could associate with other proteins, such as UHRF1, to repress HIV-1 transcription. Moreover, under productive conditions, YY1 associates with other positive regulators, such as p300, to promote the activity of the 5' LTR promoter. In this context, the interaction between UHRF1 and YY1 would probably disrupt the "YY1-p300" complex given the deleterious effect of the UHRF1 protein on the protein stability of p300. Together, our results contribute to a better understanding of the complex mechanisms involved in HIV-1 latency, which ultimately would allow the development of new therapeutic strategies aimed at the eradication of HIV-1.