TAR AND SP1-INDEPENDENT TRANSACTIVATION OF HIV LONG TERMINAL REPEAT BY THE TAT PROTEIN IN THE PRESENCE OF HUMAN CYTOMEGALOVIRUS IE1 IE2/

Objective: The HIV Tat protein is a transcriptional transactivator of the HIV-1 long terminal repeat (LTR) promoter element. Its activity de pends on its direct interaction with the Irans-activation response (TA R) element, although TAR-independent activation by Tar has been demons trated in different cells. Herpesviruses in general and human cytomega lovirus (HCMV) in particular are often isolated from HIV-1-infected pa tients and could play a role in the activation of latent HIV and in a subsequent increase in HIV replication. HCMV immediate early gene prod ucts (IE1 and IE2) are nuclear phosphoproteins that play a pivotal rol e in HCMV replication and have been shown to transregulate both viral and cellular gene expression. It has repeatedly been shown that HCMV I E1/IE2 can independently transactivate HIV-1 LTR. The aim of this stud y was to investigate IE1/IE2 transactivation of HIV-1 LTR in a CD4+ T- cell line in the absence and presence of HIV-1 Tat to establish whethe r IE1/IE2 can synergize with Tar. Methods: HIV-1 LTR transactivation b y HCMV IE1/IE2 in the presence and absence of HIV-1 Tat was determined by transient transfection experiments of J-Jhan lymphoblastoid cells with a series of different expression vectors. Results: We found a str ong synergistic transactivaton between HIV Tat and the IE1-IE2 complex on HIV LTR activity using vectors driven either by wild-type LTR or b y the nuclear factor NF-KB response element-mutated HIV LTR. IE1/IE2 s ynergism with HIV Tat was also observed in Spl binding site-mutated or TAR-deleted LTR, which cannot be activated by Tat alone. This coopera tion is abolished when the region in IE2 that binds the TATA box bindi ng protein is deleted. Conclusions: The results obtained indicate that Spl-binding and TAR sequences are not strictly required for Tat respo nsiveness when Tat is directed to the HIV promoter by HCMV IE1-IE2. Th is synergistic effect is mediated by the IE2 and TATA-binding region, and could play a major role in HIV activation when cells are infected by both viruses, a feature often observed in AIDS patients.