Interaction between TGFβ Signaling Proteins and C/EBP Controls Basal and Tat-Mediated Transcription of HIV-1 LTR in Astrocytes

Abstract

Signal transduction pathways induced by cytokines can modulate the level of HIV-1 gene transcription and replication in a variety of cells including those from the central nervous system. Here, we investigated the effect of TGFbeta-1 signaling the factors, including Smads, on transcription of the viral LTR in human astrocytic cells. Ectopic expression of Smad-3 increased activity of the viral promoter, while its partner protein, Smad-4, caused a slight decrease in viral gene transcription. Further, Smad-4 was able to suppress transcriptional activation of the LTR by Smad-3 as well as by C/EBPbeta, another activator of LTR transcription in these cells. Results from promoter deletion experiments identified the C/EBP-binding site, which is positioned between nucleotides -114 and -102 as one of the targets for Smad-mediated regulation of the LTR. Band-shift studies showed inhibition of C/EBP binding to its target DNA in protein extract from cells overexpressing Smad-3 and Smad-4. Results from GST pull-down assay and combined immunoprecipitation/Western blot of protein extracts from human astrocytes verified the association of Smad-3 and Smad-4 with C/EBPbeta, suggesting that interaction of C/EBPbeta with Smad-3 and Smad-4 may have a negative impact upon C/EBPbeta-mediated activation of the LTR. Interestingly, Smad-4 showed no inhibitory effect on viral gene transcription in cells expressing Tat protein. However, in the presence of Smad-3, expression of Smad-4 exerted a negative effect on Tat-mediated activation of the LTR promoter. These observations pointed to the functional interplay between viral and cellular proteins in modulating LTR transcription.

p27SJ, a novel protein in St John's Wort, that suppresses expression of HIV-1 genome

“…We have previously demonstrated the existence of a functional and physical interaction between C/EBPb and HIV-1 Tat proteins. 9 Expression of p27 SJ caused a substantial decrease in Tat-mediated activation of the LTR and had a negative effect on the cooperativity between C/EBPb and Tat in enhancing viral gene transcription (Figure 4a). Evaluation of the effects of p27 SJ mutants, p20 and p10, on Tat-mediated activation of the HIV-1 promoter revealed the importance of the C-terminus of p27 SJ in the suppression of Tat activity (Figure 4b).…”

Transforming Growth Factor-β1 Inhibition of Vascular Smooth Muscle Cell Activation Is Mediated via Smad3

“…For example, Zauberman et al (49) show that Smad3 blocked C/EBP-␤ transactivation of the haptoglobin promoter in HepG2 cells, an effect enhanced in the presence of Smad4. Likewise, Coyle-Rink et al (50) demonstrate that Smad3 prevented C/EBP-␤ induction of the HIV-LTR promoter in astrocytes, an effect also enhanced with Smad4. We similarly observed enhanced inhibition with Smad3 and Smad4 on the iNOS promoter in RASMCs (Fig.…”

CCAAT enhancer binding protein β has a crucial role in regulating breast cancer cell growth via activating the TGF-β-Smad3 signaling pathway

“…C/EBP functionally and physically interacts with TGF-β1 signaling factors in astrocytes (13). TGF-β1 has a key role in tumor pathogenesis, contributes to cell growth, invasion and metastasis, and inhibits host antitumor immune responses (14).…”