Are BET Inhibitors yet Promising Latency-Reversing Agents for HIV-1 Reactivation in AIDS Therapy?

Abstract

AIDS first emerged decades ago; however, its cure, i.e., eliminating all virus sources, is still unachievable. A critical burden of AIDS therapy is the evasive nature of HIV-1 in face of host immune responses, the so-called "latency." Recently, a promising approach, the "Shock and Kill" strategy, was proposed to eliminate latently HIV-1-infected cell reservoirs. The "Shock and Kill" concept involves two crucial steps: HIV-1 reactivation from its latency stage using a latency-reversing agent (LRA) followed by host immune responses to destroy HIV-1-infected cells in combination with reinforced antiretroviral therapy to kill the progeny virus. Hence, a key challenge is to search for optimal LRAs. Looking at epigenetics of HIV-1 infection, researchers proved that some bromodomains and extra-terminal motif protein inhibitors (BETis) are able to reactivate HIV-1 from latency. However, to date, only a few BETis have shown HIV-1-reactivating functions, and none of them have yet been approved for clinical trial. In this review, we aim to demonstrate the epigenetic roles of BETis in HIV-1 infection and HIV-1-related immune responses. Possible future applications of BETis and their HIV-1-reactivating properties are summarized and discussed.

Keywords: BET protein; BETi; BRD2; BRD4; HIV-1; LRA; epigenetics; immune response; latency-reversing agent; latently HIV-1-infected cell.

Figure 1

Concepts of “Shock and Kill” and “Block and Lock” strategies in AIDS therapy. Latency-reversing agents induce HIV-1 transcription via epigenetic activations in the “Shock” step. Then, these viral particles will be processed and released, leading to the elimination of the infected cells by immune clearance and to the elimination of the virus by combination antiretroviral therapy in the “Kill” step. Immune clearance involves CD8⁺ cytotoxic T cells (CLTs) that recognize the MHC I: HIV-1 peptide complex on the surface of HIV-1-infected cells and induce apoptosis by secreting granzyme B and perforin. An optimization of immunotherapy by using therapeutic vaccines to enhance CTL responses, broadly neutralizing antibodies and/or immune modulators are needed since viral reservoirs do not die from viral cytopathic effects or via the cytotoxic CTL responses. In the “Block and Lock” strategy, latency-promoting agents are applied for blocking the HIV-1 transcription in the “Block” step, and epigenetic silencing occurs in the “Lock” step. The figure was created with BioRender.com.

Figure 2

Roles of BET proteins and BETis in HIV-1 latency through the Tat-dependent manner. The 7SK snRNP forming complex which contains HEXIM1 and CTIP2 inactivates P-TEFb (CDK9 and CylinT1). During the cell activation, P-TEFb is released from the complex and becomes an active form that is the target for the viral Tat protein and BRD4. In HIV-1 latency, BRD4 competes with the viral Tat protein for recruiting active P-TEFb, leading to the low productive HIV-1 transcription. Thus, BRD4 inhibition by BETis increases the viral transcription via the phosphorylation of negative elongation factors (DSIF and NELF) and the serine 2 in CTD of RNA Pol II by the viral Tat/P-TEFb complex. The figure was created with BioRender.com.

Figure 3

Roles of BET proteins and BETis in HIV-1 latency through the Tat-independent manner. (a) NF-κB and E2F1 initially recruit BRD2 to the viral LTR site (which consists of a regulatory element, an enhancer, and a promoter), and BRD2 recruits further the repressor complex by recognizing the acetylated lysine residue on such complex. The formation of the repressor complex represses HIV-1 transcription. When BETis inhibit BRD2, the activator complex binds to the viral LTR site, leading to the activation of viral transcription. (b) CTCF recruits BRD2 or BRD3 (BED2/3) to interact with the CTCF-cohesin complex and forms a transcriptional boundary that interrupts the viral transcription. BRD2/3 induces a strong transcriptional boundary that may modify the chromatin structure and interfere with the accessibility of HIV-1 transcription components. Hence, BRD2/3 inhibited by BETis weakens the transcriptional boundary and enhance the HIV-1 transcription by causing relaxed chromatin structure and facilitating the accessibility of transcription components to the viral genome site. The figure was created with BioRender.com.

Figure 4

BETis and immune responses. (a) BRD4 recognizes an acetyl group on STATs and NF-κB, which are critical transcription factors in JAK-STAT and NF-κB signaling pathways, respectively. Once immune cells are exposed to BETis, BRD4 is then blocked, resulting in the decreased expression of cytokine gene. (b) BETis directly inhibit the functions of BRD2 and BRD4 mediating Th17 cell differentiation and proliferation in transcription regulation. (c) When BETis block BRD4, the repair complex is not formed, leading to the undeveloped CSR in B cells. Besides, after inhibiting BRD4, immunoglobulin genes are significantly suppressed, causing a low antibody level (AID is an activation-induced deaminase). The figure was created with BioRender.com.