Functional interaction between cyclin T/cdk9 and Purα determines the level of TNFα promoter activation by Tat in glial cells

In addition to its stimulatory effect on transcription of the HIV-1 LTR, the early protein of HIV-1, Tat, exhibits detrimental effects on the CNS by deregulating the expression of several cytokines and immunomodulators including TNFα. Activation of the viral promoter by Tat requires several cellular proteins including cyclin T1 and its partner, cdk9, which upon association with the TAR sequence of the LTR, forms a complex that enhances the activity of RNA polymerase II. Here, we examined the involvement of cyclin T1/cdk9 in Tat-mediated transcriptional activation of the TNFα promoter which has no TAR sequence. Results from transfection of human astrocytic cells revealed that both cyclin T1 and cdk9 stimulate the basal promoter activity of TNFα, although the level of such activation is decreased in the presence of Tat. Ectopic expression of Purα, a brain-derived regulatory protein which binds to Tat, enhanced the basal level of TNFα transcription, yet exerted a negative effect on the level of Tat activation of the TNFα promoter. The antagonistic effect of Purα and Tat upon the TNFα promoter was diminished in the presence of cyclin T1 and cdk9, suggesting cooperativity of Purα with cyclin T1 and cdk9 in Tat activation of the TNFα promoter. Results from protein-protein binding studies showed the interaction of Purα with both cyclin T1 and cdk9 through distinct domains of Purα which are in juxtaposition with each other. Interestingly, the site for cyclin T1 binding within Purα is adjacent to the region which is important for Tat/Purα association. In light of these observations, we propose a model which ascribes a bridging role for Purα in assembling Tat, cyclin T1, and cdk9 around the promoter region of TAR-negative genes such as TNFα, which is responsive to Tat activation. © 2001 Elsevier Science B.V. All rights reserved.