Cyclin T1 and CDK9 T-Loop Phosphorylation Are Downregulated during Establishment of HIV-1 Latency in Primary Resting Memory CD4⁺T Cells

Abstract

c P-TEFb, a cellular kinase composed of Cyclin T1 and CDK9, is essential for processive HIV-1 transcription. P-TEFb activity is dependent on phosphorylation of Thr186 in the CDK9 T loop. In resting CD4؉ T cells which are nonpermissive for HIV-1 replication, the levels of Cyclin T1 and T-loop-phosphorylated CDK9 are very low but increase significantly upon cellular activation. Little is known about how P-TEFb activity and expression are regulated in resting central memory CD4؉ T cells, one of the main reservoirs of latent HIV-1. We used an in vitro primary cell model of HIV-1 latency to show that P-TEFb availability in resting memory CD4؉ T cells is governed by the differential expression and phosphorylation of its subunits. This is in contrast to previous observations in dividing cells, where P-TEFb can be regulated by its sequestration in the 7SK RNP complex. We find that resting CD4؉ T cells, whether naïve or memory and independent of their infection status, have low levels of Cyclin T1 and Tloop-phosphorylated CDK9, which increase upon activation. We also show that the decrease in Cyclin T1 protein upon the acquisition of a memory phenotype is in part due to proteasome-mediated proteolysis and likely also to posttranscriptional downregulation by miR-150. We also found that HEXIM1 levels are very low in ex vivo-and in vitro-generated resting memory CD4 ؉ T cells, thus limiting the sequestration of P-TEFb in the 7SK RNP complex, indicating that this mechanism is unlikely to be a driver of viral latency in this cell type.

Histone Deacetylase Inhibitors (HDACis) That Release the Positive Transcription Elongation Factor b (P-TEFb) from Its Inhibitory Complex Also Activate HIV Transcription

“…In these cells, levels of P-TEFb are vanishingly low, due to actions of specific miRNAs and NF-90 (11)(12)(13)55), which block the translation of CycT1 mRNA. This lack of P-TEFb is thought to be critical for the establishment and maintenance of HIV latency (for review see Refs.…”

“…In resting and terminally differentiated cells, levels of P-TEFb are vanishingly low (11)(12)(13). In growing cells, P-TEFb exists in at least two different functional complexes.…”

Elevated Plasma Viral Loads in Romidepsin-Treated Simian Immunodeficiency Virus-Infected Rhesus Macaques on Suppressive Combination Antiretroviral Therapy

“…In addition to low levels of critical host transcription factors, such as pTEFb (25,26), the recruitment of histone deacetylases (HDACs) to the viral long terminal repeat (LTR) region and the associated deacetylation of histone tails and inaccessibility of the resulting tightly wrapped chromatin structures appear to represent a mechanism whereby viral genome expression is inhibited and a state of latency is maintained (27,28). Indeed, histone deacetylase inhibitor (HDACi) compounds have been shown to induce viral transcription and the production of virus in cell line models of HIV-1 latency and in CD4 ϩ T cells treated ex vivo from cART-suppressed patients (29)(30)(31)(32)(33)(34)(35).…”

Cyclin-dependent kinase 9 may as a novel target in downregulating the atherosclerosis inflammation (Review)

“…As opposed to other CDKs, CDK9 does not control the cell cycle. Instead it promotes RNA synthesis in genetic programmes for cell growth, differentiation and viral pathogenesis (18). CDK9 inhibition appears to contribute to the anticancer activity of the majority of CDK inhibitors under clinical investigation.…”