TNFα Cooperates with the Protein Kinase A Pathway to Synergistically Increase HIV-1 LTR Transcription via Downstream TRE-like cAMP Response Elements

AbstractActivating protein-1 (AP-1) binding TPA responsive elements (TRE) are located downstream of the transcription initiation site in the U5 region of the HIV-1 long terminal repeat (LTR). These downstream sequence elements, termed DSE, can bind both AP-1 and CREB/ATF transcription factors. Recently, we demonstrated that the DSE are also cAMP-responsive elements (CRE), since they mediated activation signals elicited by cholera toxin (Ctx), a potent activator of the cAMP-dependent protein kinase A (PKA) signal transduction pathway. In the present study, we demonstrate that the HIV-1 DSE can mediate the transcriptional synergy elicited by the combination of Ctx and TNFα. Ctx combined with TNFα or IL-1β to produce a synergistic increase in p24 antigen production in U1 promonocytic cells. Transfection studies of LTR reporter constructs indicated that mutation of the DSE sites abrogated the LTR-mediated synergy induced by Ctx and TNFα, whereas the synergy induced by Ctx and IL-1β was unaffected, suggesting TNFα and IL-1β cooperate differently with the cAMP/PKA activation pathway to induce HIV-1 expression in U1 cells. Because the DSE are also TRE sites, we assessed the effect of the agonist combinations on AP-1-dependent transcription. TNFα as well as IL-1β cooperated with Ctx to produce a synergistic activation of AP-1-mediated transcription. These data indicate that the TRE-like cAMP-responsive DSE sites within the 5′-untranslated leader can mediate the transcriptional cooperativity between TNFα and the cAMP/PKA pathway. Since the DSE and TRE sites cannot bind CREB/ATF homodimers, we propose a mechanism in which the HIV-1 DSE bind heterodimers composed of both AP-1 and CREB/ATF proteins