Small molecules that bind the HIV-1 transactivation response (TAR) RNA inhibit proviral transcription, but the two lead compounds act in opposite reservoir lineages: 110FA suppresses viral transcription and protein expression in infected T cells, while 102FA does so in monocyte-derived macrophages without displacing Tat. The mechanistic basis of this cell-type specificity is unknown, as are the structural binding modes predicted computationally, the regulatory significance of drug-induced HEXIM-1/P-TEFb occupancy on TAR, and whether treatment-associated stalled RNA polymerase II at the three LTR nucleosomes represents true latency or a transient pause. Delineating these linked questions determines whether this compound class can be developed into cell-type-tailored transcription-blocking (block-and-lock) interventions toward an HIV functional cure.

Background

HIV-1 integrates into the host genome, and combination antiretroviral therapy (cART) suppresses but does not eliminate the integrated provirus, because reservoirs of infected cells maintain low-level transcription. A key control point is viral transcription elongation: the viral Tat protein binds the transactivation response (TAR) element — a stem-loop RNA at the 5' end of every viral transcript containing a three-nucleotide UCU bulge and an apical loop — and recruits the positive transcription elongation factor b (P-TEFb, a Cyclin T1/CDK9 kinase complex) to phosphorylate RNA polymerase II (RNAP II) and enable productive elongation. P-TEFb is normally sequestered in an inactive form by HEXIM-1 bound to the 7SK small nuclear RNA, and HIV can manipulate this balance: Tat recruits P-TEFb from the large inactive form, and HEXIM-1 itself has been shown to bind TAR. Transcription from the 5' long terminal repeat (LTR) is further shaped by chromatin: the proviral promoter is organized into positioned nucleosomes (Nuc-0, Nuc-1, Nuc-2), and SWI/SNF chromatin-remodeling complexes (BAF and PBAF subtypes), the coactivator p300, and histone deacetylases such as HDAC1 determine whether the promoter is active or repressive. Because Tat–TAR is virus-specific, it is an attractive drug target; no FDA-approved agent currently targets HIV-1 transcription. One proposed cure strategy, 'block-and-lock', aims to enforce durable, drug-independent proviral silencing rather than latency reversal; its leading candidate, didehydro-cortistatin A, binds Tat directly.

Khatkar et al. (Pharmaceuticals 17:33, 2024) screened five small molecules that bind TAR through a noncanonical mode identified earlier by Abulwerdi et al. (J. Med. Chem. 59:11148, 2016), in chronically infected T cells (J1.1), promonocytic cells (U1), and monocyte-derived macrophages (MDMs), and in peripheral blood mononuclear cells from three donors. Two compounds stood out: 110FA inhibited Tat-mediated TAR and env RNA production and reduced gp120, p24, and Nef expression in infected T cells, whereas 102FA produced comparable suppression in myeloid cells but, by a biotinylated-TAR pulldown assay, did not act by displacing Tat. Molecular docking and molecular dynamics (MD) simulations predicted distinct mechanisms: 110FA occupies the Tat-binding pocket of TAR but adopts a 'pulled-out' position toward Cyclin T1 that sterically excludes the Tat/P-TEFb complex, while 102FA intercalates between the CDK9 and Cyclin T1 subunits of P-TEFb. The pulldown assay additionally showed that HEXIM-1 remains bound to TAR in the presence of 110FA while phosphorylated CDK9 signal is nearly lost, suggesting occupancy of TAR by the inactive HEXIM-1/P-TEFb complex and a 'fine interplay' with the active Tat/P-TEFb complex. Chromatin immunoprecipitation showed the compounds create a repressive LTR environment (reduced recruitment of BRG-1, BAF200/PBAF, Src-1 — steroid receptor coactivator 1, a nuclear coactivator that promotes transcription by recruiting p300 and SWI/SNF — and p300; increased BAF250 and HDAC1) with stalled RNAP II at all three nucleosome positions. Prior work had already documented divergent pharmacology between infected T cells and myeloid cells — histone deacetylase (HDAC) inhibitors reactivate latent provirus in CD4+ T cells but inhibit viral production in macrophages through autophagy, and mycophenolate changes env RNA copy number in opposite directions in T-cell versus promyelocytic lines — and a three-state mathematical model quantified different transcriptional dynamics in T cells versus macrophages. What no study has yet provided is a mechanistic account of why the same TAR-binding molecules act in one reservoir lineage but not the other, a structure of either compound bound to TAR, a functional test of the HEXIM-1/P-TEFb interplay, or a determination of whether the stalled RNAP II state is deep latency or a reversible pause. Khatkar et al. noted that further confirmation of these predicted binding modes is needed by either NMR spectroscopy or X-ray crystallography in future work — for 110FA in the compound–TAR (or compound–TAR–P-TEFb) complex and for 102FA in the compound–P-TEFb (optionally with TAR/Tat) complex.

Problem Statement

For the TAR RNA-binding small molecules 110FA and 102FA, which inhibit HIV-1 transcription in opposite reservoir lineages (110FA in infected T cells, 102FA in myeloid cells), determine their mechanism of action across the two principal HIV-1 reservoir lineages, jointly addressing four linked questions: (1) delineate the mechanistic basis for the cell-type-specific responses — why 110FA suppresses Tat-mediated transcription and viral protein expression in infected T cells while 102FA acts in monocyte-derived macrophages without displacing Tat from TAR, identifying the responsible host and viral factors and testing them causally in both lineages; (2) determine whether the stalled RNA polymerase II observed at nucleosomes Nuc-0, Nuc-1, and Nuc-2 of the HIV-1 LTR after treatment represents a true latent phenotype (durable, drug-independent proviral silencing that resists reactivation after compound withdrawal) or a transient transcriptional pause; (3) validate the computationally predicted binding modes of 110FA (occupying the Tat-binding pocket and adopting a 'pulled-out' position toward Cyclin T1) and 102FA (intercalating between the CDK9 and Cyclin T1 subunits of P-TEFb); and (4) establish the regulatory significance of the observed HEXIM-1/P-TEFb occupancy on TAR in the presence of 110FA — whether the interplay between inactive HEXIM-1/P-TEFb and active Tat/P-TEFb complexes on TAR is a driver of, or bystander to, transcriptional shutdown. Together these results must support a coherent account of whether and how this compound class can be developed into cell-type-tailored transcription-blocking (block-and-lock) interventions against the T-cell and myeloid reservoirs.

References

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