Mapping of functional regions of murine retrovirus long terminal repeat enhancers: enhancer domains interact and are not independent in their contributions to enhancer activity.

We have used deletion and recombinant long terminal repeat (LTR) mutants to examine enhancer activity differences between LTRs of the nonpathogenic Akv and the thymus lymphomagenic MCF13 murine retroviruses. Deletion mutant analysis revealed that major control regions for MCF13 and Akv LTR enhancer activity were similar but not identical. For both LTRs, major control regions were distinctly different in a murine T-cell and a fibroblast cell line. Recombinant enhancer analysis showed that LTRs could be divided into three regions capable of altering the level of enhancer activity through cooperative or antagonistic interaction. The contribution of each region to enhancer activity was dependent on its context with respect to the other regions. LTR enhancer function in different cell types appears to be the result of the interaction of enhancer modular elements.     

Signaling through T lymphocyte surface proteins, TCR/CD3 and CD28, activates the HIV-1 long terminal repeat

The state of T cell activation and proliferation controls HIV-1 replication and gene expression. Previously, we demonstrated that the administration of PHA and PMA to the human T cell line Jurkat activates the HIV-1 enhancer, which is composed of two nuclear factor kappa B (NF kappa B) binding sites. Here, we show that PMA alone is sufficient for this effect. In addition, activation of T cells through the surface proteins TCR/CD3 and CD28 increased gene expression directed by the HIV-1 long terminal repeat (LTR) to the same extent as PMA. Analysis of 5' deletions in the LTR revealed that the NF kappa B binding sites and sequences in the upstream U3 region are required for this response. Whereas cyclosporin A did not inhibit the effect of PMA, it reduced the effects of agonists to TCR/CD3 and CD28 on the LTR. H7, an inhibitor of protein kinase C (PKC), blocked the effects of all stimuli. Thus, PMA activates the NF kappa B sites through a PKC-dependent pathway while ligands to TCR/CD3 and CD28 activate the LTR through a cyclosporin A-sensitive, PKC-dependent pathway of T cell activation. We conclude that mechanisms involved in the expression of IL-2 and the alpha-chain of the IL-2R alpha genes also play a role in the regulation of HIV-1. Physiologic stimuli can activate HIV-1 gene expression; agents that block T cell activation also inhibit activation of the LTR. These observations might serve as a model for the regulation of HIV-1 gene expression in peripheral blood T cells.