Prostratin antagonizes HIV latency by activating NF-kappaB

Effects on Ligand Interaction and Membrane Translocation of the Positively Charged Arginine Residues Situated along the C1 Domain Binding Cleft in the Atypical Protein Kinase C Isoforms

The C1 domain zinc finger structure is highly conserved among the protein kinase C (PKC) superfamily members. As the interaction site for the second messenger sn-1,2-diacylglycerol (DAG) and for the phorbol esters, the C1 domain has been an important target for developing selective ligands for different PKC isoforms. However, the C1 domains of the atypical PKC members are DAG/phorbol ester-insensitive. Compared with the DAG/phorbol ester-sensitive C1 domains, the rim of the binding cleft of the atypical PKC C1 domains possesses four additional positively charged arginine residues (at positions 7, 10, 11, and 20). In this study, we showed that mutation to arginines of the four corresponding sites in the C1b domain of PKCdelta abolished its high potency for phorbol 12,13-dibutyrate in vitro, with only marginal remaining activity for phorbol 12-myristate 13-acetate in vivo. We also demonstrated both in vitro and in vivo that the loss of potency to ligands was cumulative with the introduction of the arginine residues along the rim of the binding cavity rather than the consequence of loss of a single, specific residue. Computer modeling reveals that these arginine residues reduce access of ligands to the binding cleft and change the electrostatic profile of the C1 domain surface, whereas the basic structure of the binding cleft is still maintained. Finally, mutation of the four arginine residues of the atypical PKC C1 domains to the corresponding residues in the deltaC1b domain conferred response to phorbol ester. We speculate that the arginine residues of the C1 domain of atypical PKCs may provide an opportunity for the design of ligands selective for the atypical PKCs.

Effects of prostratin on Cyclin T1/P-TEFb function and the gene expression profile in primary resting CD4+ T cells

Background

The latent reservoir of human immunodeficiency virus type 1 (HIV-1) in resting CD4⁺ T cells is a major obstacle to the clearance of infection by highly active antiretroviral therapy (HAART). Recent studies have focused on searches for adjuvant therapies to activate this reservoir under conditions of HAART. Prostratin, a non tumor-promoting phorbol ester, is a candidate for such a strategy. Prostratin has been shown to reactivate latent HIV-1 and Tat-mediated transactivation may play an important role in this process. We examined resting CD4⁺ T cells from healthy donors to determine if prostratin induces Cyclin T1/P-TEFb, a cellular kinase composed of Cyclin T1 and Cyclin-dependent kinase-9 (CDK9) that mediates Tat function. We also examined effects of prostratin on Cyclin T2a, an alternative regulatory subunit for CDK9, and 7SK snRNA and the HEXIM1 protein, two factors that associate with P-TEFb and repress its kinase activity.

Results

Prostratin up-regulated Cyclin T1 protein expression, modestly induced CDK9 protein expression, and did not affect Cyclin T2a protein expression. Although the kinase activity of CDK9 in vitro was up-regulated by prostratin, we observed a large increase in the association of 7SK snRNA and the HEXIM1 protein with CDK9. Using HIV-1 reporter viruses with and without a functional Tat protein, we found that prostratin stimulation of HIV-1 gene expression appears to require a functional Tat protein. Microarray analyses were performed and several genes related to HIV biology, including APOBEC3B, DEFA1, and S100 calcium-binding protein genes, were found to be regulated by prostratin.

Conclusion

Prostratin induces Cyclin T1 expression and P-TEFb function and this is likely to be involved in prostratin reactivation of latent HIV-1 proviruses. The large increase in association of 7SK and HEXIM1 with P-TEFb following prostratin treatment may reflect a requirement in CD4⁺ T cells for a precise balance between active and catalytically inactive P-TEFb. Additionally, genes regulated by prostratin were identified that have the potential to regulate HIV-1 replication both positively and negatively.

HIV type 1 inhibition by protein kinase C modulatory compounds

The dichotomous effects of the protein kinase C (PKC) modulatory compounds 12-myristate 13-acetate (PMA), prostratin, and ingenol 3-angelate (I3A) on HIV-1 infection were investigated. PKC modulatory compounds were shown to be potent activators of cells latently infected with HIV-1 (I3A > prostratin). Conversely, PKC modulatory compounds inhibited infection of indicator cells (MAGI) with CXCR4-tropic HIV-1 (PMA > I3A > prostratin), and I3A also inhibited infection with CCR5-tropic virus (AD8-1). Pretreatment with the PKC inhibitors prior to treatment with either I3A or PMA resulted in increased infection, indicating inhibition is PKC mediated. Cell infections suggested that I3A rapidly inhibited the virus from infecting cells at an early point in infection. This observation was supported by the demonstration of inhibition at or before the synthesis of early reverse transcription products, and the inability of these compounds to block vesicular stomatitis virus (VSV) pseudotyped HIV-1 particles. As has already been shown with prostratin, treatment with I3A resulted in down-regulation of the CD4 receptor and CXCR4 coreceptor suggesting that this was a contributor to the infection inhibition. Intriguingly, 48 h pretreatment of unstimulated peripheral blood mononuclear cells (PBMC) prior to infection resulted in abrogation of virus production at concentrations where receptor/ coreceptor levels were not significantly reduced. This result hints at the possibility of inhibition by a PKC modulatory compound of an early pathway of viral entry in PBMC.

Nuclear retention of multiply spliced HIV-1 RNA in resting CD4+ T cells

HIV-1 latency in resting CD4+ T cells represents a major barrier to virus eradication in patients on highly active antiretroviral therapy (HAART). We describe here a novel post-transcriptional block in HIV-1 gene expression in resting CD4+ T cells from patients on HAART. This block involves the aberrant localization of multiply spliced (MS) HIV-1 RNAs encoding the critical positive regulators Tat and Rev. Although these RNAs had no previously described export defect, we show that they exhibit strict nuclear localization in resting CD4+ T cells from patients on HAART. Overexpression of the transcriptional activator Tat from non-HIV vectors allowed virus production in these cells. Thus, the nuclear retention of MS HIV-1 RNA interrupts a positive feedback loop and contributes to the non-productive nature of infection of resting CD4+ T cells. To define the mechanism of nuclear retention, proteomic analysis was used to identify proteins that bind MS HIV-1 RNA. Polypyrimidine tract binding protein (PTB) was identified as an HIV-1 RNA-binding protein differentially expressed in resting and activated CD4+ T cells. Overexpression of PTB in resting CD4+ T cells from patients on HAART allowed cytoplasmic accumulation of HIV-1 RNAs. PTB overexpression also induced virus production by resting CD4+ T cells. Virus culture experiments showed that overexpression of PTB in resting CD4+ T cells from patients on HAART allowed release of replication-competent virus, while preserving a resting cellular phenotype. Whether through effects on RNA export or another mechanism, the ability of PTB to reverse latency without inducing cellular activation is a result with therapeutic implications.

NF-kappaB p50 promotes HIV latency through HDAC recruitment and repression of transcriptional initiation

Cells latently infected with HIV represent a currently insurmountable barrier to viral eradication in infected patients. Using the J-Lat human T-cell model of HIV latency, we have investigated the role of host factor binding to the kappaB enhancer elements of the HIV long terminal repeat (LTR) in the maintenance of viral latency. We show that NF-kappaB p50-HDAC1 complexes constitutively bind the latent HIV LTR and induce histone deacetylation and repressive changes in chromatin structure of the HIV LTR, changes that impair recruitment of RNA polymerase II and transcriptional initiation. Knockdown of p50 expression with specific small hairpin RNAs reduces HDAC1 binding to the latent HIV LTR and induces RNA polymerase II recruitment. Similarly, inhibition of histone deacetylase (HDAC) activity with trichostatin A promotes binding of RNA polymerase II to the latent HIV LTR. This bound polymerase complex, however, remains non-processive, generating only short viral transcripts. Synthesis of full-length viral transcripts can be rescued under these conditions by expression of Tat. The combination of HDAC inhibitors and Tat merits consideration as a new strategy for purging latent HIV proviruses from their cellular reservoirs.

HIV-1 gp120 induces NFAT nuclear translocation in resting CD4+ T-cells

The replication of human immunodeficiency virus (HIV) in CD4+ T-cells is strongly dependent upon the state of activation of infected cells. Infection of sub-optimally activated cells is believed to play a critical role in both the transmission of virus and the persistence of CD4+ T-cell reservoirs. There is accumulating evidence that HIV can modulate signal-transduction pathways in a manner that may facilitate replication in such cells. We previously demonstrated that HIV gp120 induces virus replication in resting CD4+ T cells isolated from HIV-infected individuals. Here, we show that in resting CD4+ T-cells, gp120 activates NFATs and induces their translocation into the nucleus. The HIV LTR encodes NFAT recognition sites, and NFATs may play a critical role in promoting viral replication in sub-optimally activated cells. These observations provide insight into a potential mechanism by which HIV is able to establish infection in resting cells, which may have implications for both transmission of HIV and the persistence of viral reservoirs.

Human immunodeficiency virus reactivation by phorbol esters or T-cell receptor ligation requires both PKCalpha and PKCtheta

Latently human immunodeficiency virus (HIV)-infected memory CD4(+) T cells represent the major obstacle to eradicating HIV from infected patients. Antigens, T-cell receptor (TCR) ligation, and phorbol esters can reactivate HIV from latency in a protein kinase C (PKC)-dependent manner; however, it is unknown which specific PKC isoforms are required for this effect. We demonstrate that constitutively active (CA) forms of both PKCtheta, PKCthetaA148E, and PKCalpha, PKCalphaA25E, induce HIV long terminal repeat (LTR)-dependent transcription in Jurkat and primary human CD4(+) T cells and that both PKCthetaA148E and PKCalphaA25E cause HIV reactivation in J1.1 T cells. Suppression of both PKCalpha and PKCtheta with short hairpinned (sh) RNA inhibited CD3/CD28-induced HIV LTR-dependent transcription and HIV reactivation in J1.1 T cells. Both prostratin and phorbol myristate 13-acetate induced HIV LTR-dependent transcription and HIV reactivation in J1.1 T cells that was blocked by shRNA against either PKCalpha or PKCtheta. Since suppression of PKCalpha and PKCtheta together has no greater inhibitory effect on HIV reactivation than inhibition of PKCalpha alone, our data confirm that PKCalpha and PKCtheta act in sequence. The requirement for PKCalpha and PKCtheta for prostratin-induced HIV reactivation and the ability of selective PKCalpha or PKCtheta agonists to induce HIV transcription indicate that these PKC isoforms are important targets for therapeutic drug design.