HIV-1 Tat induces the expression of the interleukin-6 (IL6) gene by binding to the IL6 leader RNA and by interacting with CAAT enhancer-binding protein beta (NF-IL6) transcription factors.

Human immunodeficiency virus type 1 (HIV-1) infection is associated with severe psoriasis, B cell lymphoma, and Kaposi's sarcoma. A deregulated production of interleukin-6 (IL6) has been implicated in the pathogenesis of these diseases. The molecular mechanisms underlying the abnormal IL6 secretion of HIV-1-infected cells may include transactivation of the IL6 gene by HIV-1. Here we report the molecular mechanisms of Tat activity on the expression of the IL6 gene. By using 5' deletion mutants of pIL6Pr-CAT and using IL6:HIV-1-LTR hybrid constructs where discrete regions of the IL6 promoter replaced the TAR sequence in HIV-1 LTR, we identified a short sequence of the 5'-untranslated region of the IL6 mRNA that is required for Tat to trans-activate the IL6 promoter. This sequence acquires a stem-loop structure and includes a UCU sequence that binds to Tat and is necessary for full trans-activation. In addition, we provide the evidence that Tat can function by enhancing the CAAT enhancer-binding protein (C/EBP) DNA binding activity and is able to complex with in vitro translated C/EBPbeta, which is a major mediator of IL6 promoter function. By using the yeast two-hybrid system and immunoprecipitation, we observed that the interaction of Tat with C/EBP proteins also occurred in vivo. The data are consistent with the possibility that Tat may function on heterologous genes by interacting with RNA structures possibly present in a large number of cellular and viral genes. In addition, Tat may function by protein-protein interactions, leading to the generation of heterodimers with specific transcription factors.

Identification of a novel retinoic acid response element in the promoter region of the retinol-binding protein gene.

We have previously demonstrated that the retinol-binding protein (RBP) gene is induced by retinoids in hepatoma cells. In this report, we define in greater detail the region that mediates the retinoic acid response of the gene. It consists of two degenerate retinoic acid response elements, separated by 30 nucleotides that encompass a GC-rich Sp1 consensus-like sequence. We demonstrate that the entire region, as well as each element taken singly, can bind the retinoic acid receptors as homo- and heterodimers with low affinity. However, only the entire region is able to confer retinoic acid inducibility to a heterologous promoter. We also show that the correct phasing of the DNA segment is necessary to achieve full responsiveness. Site-directed mutants in each element retained partial induction after transfection, while the double mutant was no longer responsive, suggesting that the two elements act synergistically. Mutational analysis of the Sp1 binding site and cotransfection experiments revealed that Sp1 or a related protein plays an important role in the transcription of the gene. Thus, the retinoic acid induction of the RBP gene is mediated by a novel and complex responsive unit formed by two distinct elements located in a specific sequence context and the interplay of the retinoid receptors with Sp1 is required for induction.