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C/EBP regulates human immunodeficiency virus 1 gene expression through its association with cdk9

¹ Department of Neuroscience, Center for Neurovirology, Temple University School of Medicine, 1900 N 12th Street 015-96, Philadelphia, PA 19122, USA
² Section of Microbiology, Department of Biomedical Sciences, Center of Excellence for Biotechnology Development and Biodiversity Research, Sassari, Italy
³ Department of Biology, College of Science and Technology, Temple University School of Medicine, 1900 N 12th Street 015-96, Philadelphia, PA 19122, USA

Correspondence
Bassel E. Sawaya
sawaya{at}temple.edu

Transcriptional regulation of the human immunodeficiency virus type 1 (HIV-1) is a complex event that requires the cooperative action of both viral (e.g. Tat) and cellular (e.g. C/EBP, NF-B) factors. The HIV-1 Tat protein recruits the human positive transcription elongation factor P-TEFb, consisting of cdk9 and cyclin T1, to the HIV-1 transactivation response (TAR) region. In the absence of TAR, Tat activates the HIV-1 long terminal repeat (LTR) through its association with several cellular factors including C/EBP. C/EBP is a member of the CCAAT/enhancer-binding protein family of transcription factors and has been shown to be a critical transcriptional regulator of HIV-1 LTR. We examined whether Tat–C/EBP association requires the presence of the P-TEFb complex. Using immunoprecipitation followed by Western blot, we demonstrated that C/EBP–cyclin T1 association requires the presence of cdk9. Further, due to its instability, cdk9 was unable to physically interact with C/EBP in the absence of cyclin T1 or Tat. Using kinase assays, we demonstrated that cdk9, but not a cdk9 dominant-negative mutant (cdk9-dn), phosphorylates C/EBP. Our functional data show that co-transfection of C/EBP and cdk9 leads to an increase in HIV-1 gene expression when compared to C/EBP alone. Addition of C/EBP homologous protein (CHOP) inhibits C/EBP transcriptional activity in the presence and absence of cdk9 and causes a delay in HIV-1 replication in T-cells. Together, our data suggest that Tat–C/EBP association is mediated through cdk9, and that phosphorylated C/EBP may influence AIDS progression by increasing expression of HIV-1 genes.