Functional Differences between the Long Terminal Repeat Transcriptional Promoters of Human Immunodeficiency Virus Type 1 Subtypes A through G

Abstract

The current human immunodeficiency virus type 1 (HIV-1) shows an increasing number of distinct viral subtypes, as well as viruses that are recombinants of at least two subtypes. Although no biological differences have been described so far for viruses that belong to different subtypes, there is considerable sequence variation between the different HIV-1 subtypes. The HIV-1 long terminal repeat (LTR) encodes the transcriptional promoter, and the LTR of subtypes A through G was cloned and analyzed to test if there are subtype-specific differences in gene expression. Sequence analysis demonstrated a unique LTR enhancer-promoter configuration for each subtype. Transcription assays with luciferase reporter constructs showed that all subtype LTRs are functional promoters with a low basal transcriptional activity and a high activity in the presence of the viral Tat transcriptional activator protein. All subtype LTRs responded equally well to the Tat trans activator protein of subtype B. This result suggests that there are no major differences in the mechanism of Tat-mediated trans activation among the subtypes. Nevertheless, subtype-specific differences in the activity of the basal LTR promoter were measured in different cell types. Furthermore, we measured a differential response to tumor necrosis factor alpha treatment, and the induction level correlated with the number of NF-B sites in the respective LTRs, which varies from one (subtype E) to three (subtype C). In general, subtype E was found to encode the most potent LTR, and we therefore inserted the core promoter elements of subtype E in the infectious molecular clone of the LAI isolate (subtype B). This recombinant LAI-E virus exhibited a profound replication advantage compared with the original LAI virus in the SupT1 T-cell line, indicating that subtle differences in LTR promoter activity can have a significant impact on viral replication kinetics. These results suggest that there may be considerable biological differences among the HIV-1 subtypes.

Pertussis Toxin B-Oligomer Suppresses IL-6 Induced HIV-1 and Chemokine Expression in Chronically Infected U1 Cells via Inhibition of Activator Protein 1

“…Alternatively, PTX-B could downregulate AP-1 mediated activation of the intragenic gag enhancer (48,60), previously suggested to be induced by stimuli, such as IL-6, in combination with TNF-␣ or glucocorticoids (38,49). In this regard, our findings on the levels of luciferase expression driven by LTRs after IL-6 stimulation add to previous reports on biological differences among the different HIV clades, which are determined in part by differences in the composition of the LTR promoter (52,(77)(78)(79)(80). Thus, differences in the composition of transcription factor binding sites may provide each clade with a promoter responding in a unique matter to cellular activation pathways.…”

“…Although PTX-B suppression of IL-6-induced HIV expression in both U1 and U1-CR1 cells was almost absolute, inhibition of LTRdriven transcription was restricted to those promoters including AP-1 binding sites, such as those derived from clades A, C, E, and F, whereas it was not observed in cells transfected with clade B and D LTR, devoid of these sites (52). This observation suggests that PTX-B inhibitory effects on HIV expression in U1 and U1-CR1 cells, originally infected with the X4 LAI/IIIB subtype B HIV-1 (41), is likely to occur at a posttranscriptional step.…”

“…Having established that AP-1 was transcriptionally active in U1-CR1 cells, we next explored the capacity of PTX-B to interfere with HIV LTR-driven transcription using constructs obtained from different HIV subtypes previously characterized for the presence of AP-1 binding sites (52). Transcription from the LTR of HIV subtypes B and D (lacking functional AP1 binding sites) was upregulated upon IL-6 stimulation, but was not inhibited by PTX-B (Fig.…”

Sequence Insertions in the HIV Type 1 Subtype C Viral Promoter Predominantly Generate an Additional NF-κB Binding Site

“…In contrast, a typical subtype C LTR is endowed with three canonical NF-jB binding sites. 18 Stronger transcription from the subtype C LTR, as compared to the subtype B LTR, has been ascribed to the presence of the higher number of NF-jB binding sites, 12,19 and this quality in part is believed to underlie the global prevalence of the subtype C strains. Given that the C-LTR is already endowed with three canonical jB sites, it is surprising that a few variant viral strains acquired an additional jB site.…”

The Ixodes scapularis salivary protein, salp15, prevents the association of HIV-1 gp120 and CD4

“…HL2/3 cells were mixed at a 1:1 ratio with HeLa-CD4 (not shown) or 3T3.CD4.CCR5 cells [23] that had been transfected with the HIV-1 3′ LTR-luciferase plasmid [24,25] in the absence or presence of 50 μg/ml of Salp15. Luciferase activity, indicative of cell fusion, was determined after 48 h. The presence of Salp15 decreased the LTR-induced transcription of the luc gene ( Figure 1C), indicative that the tick antigen had prevented cell-cell fusion.…”