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The HIV Tat protein is a transcriptional transactivator of the HIV-1 long terminal repeat (LTR) promoter element. Its activity depends on its direct interaction with the trans-activation response (TAR) element, although TAR-independent activation by Tat has been demonstrated in different cells. Herpesviruses in general and human cytomegalovirus (HCMV) in particular are often isolated from HIV-1-infected patients and could play a role in the activation of latent HIV and in a subsequent increase in HIV replication. HCMV immediate early gene products (IE1 and IE2) are nuclear phosphoproteins that play a pivotal role in HCMV replication and have been shown to transregulate both viral and cellular gene expression. It has repeatedly been shown that HCMV IE1/IE2 can independently transactivate HIV-1 LTR. The aim of this study 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 whether IE1/IE2 can synergize with Tat.HIV-1 LTR transactivation by 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.We found a strong synergistic transactivaton between HIV Tat and the IE1–IE2 complex on HIV LTR activity using vectors driven either by wild-type LTR or by the nuclear factor NF-κB response element-mutated HIV LTR. IE1/IE2 synergism with HIV Tat was also observed in Sp1 binding site-mutated or TAR-deleted LTR, which cannot be activated by Tat alone. This cooperation is abolished when the region in IE2 that binds the TATA box binding protein is deleted.The results obtained indicate that Sp1-binding and TAR sequences are not strictly required for Tat responsiveness when Tat is directed to the HIV promoter by HCMV IE1–IE2. This 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.