P-TEFb as a target to reactivate latent HIV : Two Brds are now in hand

Abstract

P-TEFb is a protein kinase required for RNA polymerase II transcriptional elongation of most, if not all, mammalian protein-coding genes. P-TEFb is believed to be a therapeutic target for cancer, and inhibitors of P-TEFb are currently being evaluated in clinical trials. Additionally, P-TEFb has potential as a therapeutic target for HIV infection, as transcriptional elongation of the integrated virus is dependent upon the viral Tat protein’s recruitment of P-TEFb to the TAR RNA element at the 5′ end of nascent viral transcripts. Resting CD4+ T lymphocytes that contain integrated but transcriptionally silent HIV are clinically significant, as when patients stop antiviral drugs that effectively suppress viral replication, some viruses in this latent reservoir reactivate and rekindle infection. Substantial effort in the HIV/AIDS field is currently directed toward identifying cell-permeable small molecules that can reactivate latent viruses and thereby reduce or even purge the latent reservoir, perhaps curing infection. This research activity is illustrated by five recent publications, including one from Boehm and colleagues in a recent issue of Cell Cycle, which shows that a molecule termed JQ1 targets P-TEFb and reactivates latent HIV under some conditions. 1 - 5 Although P-TEFb exists in multiple complexes in cells, its core is composed of CDK9 and either Cyclin T1 or Cyclin T2, with Cyclin T1 being the predominant subunit in most human tissues examined. Three P-TEFb complexes have been biochemically characterized: core P-TEFb + the bromodomain protein Brd4, the 7SK snRNP and the super elongation complex. 6 JQ1 was identified as a molecule that recognizes protein bromodomains, or acetyl-lysine recognition motifs, and it has highest specificity for Brd4, although it also has specificity for Brd2 and Brd3. 7 All five of the recent publications found that JQ1 can reactivate HIV in cell line models of latency. P-TEFb is clearly involved in this reactivation, as shRNA depletions of Cyclin T1 largely abolished JQ1’s activity in cell lines. 5 Reactivation of latent virus by JQ1 is, however, not strictly dependent upon the viral Tat protein, as reactivation was observed in a cell line harboring a latent virus that lacked the Tat gene. 5 JQ1 appears to function as an antagonist of Brd4 and Brd2, and shRNA depletions of either protein also reactivated latent HIV in cell lines. 2 , 4 , 5 The mechanisms whereby Brd4 and Brd2 mediate JQ1’s reactivation activity remain to be clarified, but both proteins were found in complexes with P-TEFb in a large-scale co-immunoprecipitation study. 8 Although its HIV reactivation activity in cell lines suggests that JQ1 and related molecules have therapeutic potential, a critical difference between cell lines and primary resting CD4+ T lymphocytes is that P-TEFb is expressed at high levels in cell lines but is repressed in resting lymphocytes. In primary resting CD4+ T cells, very low levels of Cyclin T1 are expressed, and phosphorylation of the CDK9 T-loop is absent, a modification required for P-TEFb catalytic activity. 9 JQ1’s abilities to reactivate latent HIV in CD4+ cells from patients or primary cell models of latency were mixed in the recent studies. JQ1 reactivated the virus in one of three patients’ samples in one study, 1 while its effects were variable in another study, either enhancing or suppressing reactivation of virus in patients’ samples when used in combination with prostratin or SAHA, other molecules that have some ability to reactivate latent HIV. 4 JQ1 activity in primary CD4+ T cell models of latency was also variable—it was able to reactivate latent virus in a model developed by the Siliciano lab but was inactive in an alternative model developed by the Planelles lab. 5 Elucidating JQ1’s differential activity in these two primary cell models may give important clues into mechanisms of HIV latency, as may the study of the roles of Brd2 and Brd4 in latency and reactivation. Similar to effective suppression of HIV replication by a combination of antiviral drugs that target different steps in the viral life cycle, it is thought that reactivation of latent viruses will require a combination of small molecules that reactivate latent virus through different mechanisms. The identification of JQ1and both Brd2 and Brd4 as targets for reactivation are hopefully steps forward in the development of effective strategies to reactivate latent viruses in infected individuals.