Indirubin-3’-monoxime, a Derivative of a Chinese Antileukemia Medicine, Inhibits P-TEFb Function and HIV-1 Replication: Potential Role of CDK9 Inhibition as a New Therapeutic Target
Alonso Heredia*, C Davis, D Bamba, N Le, M Gwarzo, M Sadowska, R Gallo, and R Redfield
Univ of Maryland, Baltimore, US
Background: Despite the potent antiviral activity of
HAART, durability of the induction of viral suppression has been limited in
some patients as a consequence of drug side effects, treatment adherence, and
development of drug resistance. Targeting of additional virus proteins may potentiate the durability and antiviral effects of HAART.
In this regard, an inhibitor of HIV-1 Tat protein activity would be
advantageous because transcriptional activation of the HIV-1 LTR promoter
element by Tat is an essential step in the virus life cycle. In the present
study, we have evaluated the effects of the CDK inhibitor indirubin-3′-monoxime
(IM) on Tat-mediated transactivation function, a step
of the HIV-1 cycle that is not currently targeted in ART.
Methods: Kinase assays were
used to determine the activities of IM on CDK2, CDK7, and CDK9. The effect of
IM on Tat transactivation function was evaluated in transfection experiments using HLtat
cells. The activity of IM on Tat-mediated elongation of viral transcripts was
assessed by ribonuclease protection assay (RPA). The
antiviral activity of IM was evaluated in cell lines and primary peripheral
blood mononuclear cells (PBMC) infected with wild type and drug-resistant
strains of HIV-1.
Results: IM selectively inhibits the kinase activity of CDK9 (IC50 of 0.05 µM), the
catalytic subunit of P-TEFb. Inhibition of CDK9
activity by IM results in abrogation of Tat-induced expression of HIV-1 RNA in
cell lines. In addition, IM inhibits the replication of HIV-1 in both PBMC (IC50
of 1 µM) and macrophages (IC50 of 0.5 µM). IM is effective against
primary and drug-resistant strains of HIV-1. Importantly, the antiviral effects
of the drug were seen at concentrations that did not affect cell proliferation.
Conclusions: Non-toxic concentrations of IM inhibit HIV-1
by blocking viral gene expression mediated by the cellular factor P-TEFb. The drug is effective against wild type and
drug-resistant strains of HIV-1. IM may help control replication of HIV-1 in
patients, particularly in those carrying multi-drug resistant variants, by
disrupting a step of the HIV-1 cycle that is not being targeted in current