HIV-1 replication is induced by an excess of iron and iron chelation by desferrioxamine (DFO) inhibits viral replication by reducing proliferation of infected cells. Treatment of cells with DFO and 2-hydroxy-1-naphthylaldehyde isonicotinoyl hydrazone (311) inhibit expression of proteins that regulate cell-cycle progression, including cycle-dependent kinase 2 (CDK2). Our recent studies showed that CDK2 participates in HIV-1 transcription and viral replication suggesting that inhibition of CDK2 by iron chelators might also affect HIV-1 transcription. Here we evaluated the effect of a clinically approved orally effective iron chelator, 4-[3,5-bis-(hydroxyphenyl)-1,2,4-triazol-1-yl]-benzoic acid (ICL670) and 311 on HIV-1 transcription. Both ICL670 and 311 inhibited Tat-induced HIV-1 transcription in CEM-T cells, 293T and HeLa cells. Neither ICL670 nor 311 induced cytotoxicity at concentrations that inhibited HIV-1 transcription. The chelators decreased cellular activity of CDK2 and reduced HIV-1 Tat phosphorylation by CDK2. Neither ICL670A or 311 decreased CDK9 protein level but significantly reduced association of CDK9 with cyclin T1 and reduced phosphorylation of Ser-2 residues of RNA polymerase II C-terminal domain. In conclusion, our findings add to the evidence that iron chelators can inhibit HIV-1 transcription by deregulating CDK2 and CDK9. Further consideration should be given to the development of iron chelators for future anti-retroviral therapeutics.

Transcription of human immunodeficiency virus (HIV)-1 genesis activated by HIV-1 Tat protein, which induces phosphorylation of the C-terminal domain of RNA polymerase-II by CDK9/cyclin T1. We previously showed that Tat-induced HIV-1 transcription is regulated by protein phosphatase-1 (PP1). In the present study we demonstrate that Tat interacts with PP1 and that disruption of this interaction prevents induction of HIV-1 transcription. We show that PP1 interacts with Tat in part through the binding of Val 36 and Phe 38 of Tat to PP1 and that Tat is involved in the nuclear and subnuclear targeting of PP1. The PP1 binding mutant Tat-V36A/ F38A displayed a decreased affinity for PP1 and was a poor activator of HIV-1 transcription. Surprisingly, Tat-Q35R mutant that had a higher affinity for PP1 was also a poor activator of HIV-1 transcription, because strong PP1 binding competed out binding of Tat to CDK9/cyclin T1. Our results suggest that Tat might function as a nuclear regulator of PP1 and that interaction of Tat with PP1 is critical for activation of HIV-1 transcription by Tat. Human immunodeficiency virus type 1 (HIV-1)3 is a retrovirus that encodes transcriptional activator (Tat) protein. The activation domain of Tat (amino acids 1-48) interacts with host cell factors, whereas the positively charged RNA-binding domain (amino acids 49 -57) interacts with HIV-1 transactivation response (TAR) RNA (1). In cell-free transcription assays Tat induces elongation of transcription (2, 3). In vivo, Tat also induces initiation of transcription from the integrated HIV-1 promoter (4 -6). In a recent study Tat was shown to stimulate formation of a transcription complex containing TATA box-binding protein but not TATA box-binding protein-associated factors, thus indicating that Tat may enhance initiation of transcription (4). This finding apparently agrees with the early observation that Tat binds directly to the TATA box-binding protein-containing basal transcription factor TFIID (7). Tat activates HIV-1 transcription by recruiting transcriptional coactivators that include positive transcription elongation factor b, containing CDK9/cyclin T1, an RNA polymerase II CTD kinase (3,8,9) and histone acetyltransferases (10 -12). Whereas positive transcription elongation factor b induces HIV-1 transcription from non-integrated HIV-1 template (3,8,9), histone acetyltransferases allow induction of integrated HIV-1 provirus (10 -12). In contrast to the well defined role of protein kinases, the role of protein phosphatases in Tat-mediated HIV-1 transcription is less well understood. FCP1, a CTD phosphatase that dephosphorylates Ser-2 during elongation of transcription (13), is inhibited by Tat and this inhibition may alleviate FCP1-mediated pausing of transcription (14,15). In addition to FCP1, PP2A and PP1 were also shown to be involved in the regulation of HIV-1 transcription. Disregulation of cellular enzymatic activity of PP2A inhibited Tat-induced HIV-1 transcription (16). Expression of the catalytic subunit of PP2A enhanced activa...

We recently reported that protein phosphatase 1 (PP1) dephosphorylates RNA polymerase II C-terminal repeats and regulates HIV-1 transcription in vitro. Here we provide evidence that PP1 is also required for Tat

BACKGROUND AND PURPOSEHIV-1 transcription is activated by the Tat protein which recruits the cyclin-dependent kinase CDK9/cyclin T1 to TAR RNA. Tat binds to protein phosphatase-1 (PP1) through the Q 35 VCF 38 sequence and translocates PP1 to the nucleus. PP1 dephosphorylates CDK9 and activates HIV-1 transcription. We have synthesized a low MW compound 1H4, that targets PP1 and prevents HIV-1 Tat interaction with PP1 and inhibits HIV-1 gene transcription. Here, we report our further work with the 1H4-derived compounds and analysis of their mechanism of action. EXPERIMENTAL APPROACHUsing the 1H4-PP1 complex as a model, we iteratively designed and synthesized follow-up libraries that were analysed for the inhibition of HIV-1 transcription and toxicity. We also confirmed the mechanism of action of the PP1-targeting molecules by determining the affinity of binding of these molecules to PP1, by analysing their effects on PP1 activity, disruption of PP1 binding to Tat and shuttling of PP1 to the nucleus. KEY RESULTSWe identified a tetrahydroquinoline derivative, compound 7, which disrupted the interaction of Tat with PP1. We further optimized compound 7 and obtained compound 7c, renamed 1E7-03, which inhibited HIV-1 with low IC50 (fivefold lower than the previously reported compound, 1H4), showed no cytotoxicity and displayed a plasma half-life greater than 8 h in mice. 1E7-03 bound to PP1 in vitro and prevented shuttling of PP1 into the nucleus. CONCLUSIONS AND IMPLICATIONSOur study shows that low MW compounds that functionally mimic the PP1-binding RVxF peptide can inhibit HIV-1 transcription by deregulating PP1. AbbreviationsCTD, C-terminal domain of RNA polymerase II; HEXIM1, hexamethylene bis-acetamide-inducible protein 1; LTR, long terminal repeats; PP1, protein phosphatase 1; P-TEFb, positive transcription elongation factor b BJP British Journal of Pharmacology

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