Activating protein-1 cooperates with phorbol ester activation signals to increase HIV-1 expression

To determine whether Jun and Fos, components of the activating protein-1 (AP-1) transcription factor, transactivate HIV-1 proviral expression. The effects of phorbol myristate acetate (PMA) and Jun or Fos transcription factors on HIV-1 expression were investigated using a provirus clone and long terminal repeat (LTR)-reporter gene constructs. The influence of PMA stimulation on AP-1 binding activity was determined with antibodies in gel mobility shift assays. Activation of HIV-1 provirus and transcription of HIV-1 LTR sequences in response to cotransfection of Jun or Fos expression plasmids into a permissive colon cancer cell line, SW480, were assessed by p24 core antigen capture and reporter gene assays, respectively. The effect of protein kinase C activation was evaluated by comparing cells grown in the presence or absence of PMA (20 ng/ml). Cotransfection of HIV-1 provirus and expression plasmids for c-Jun or JunB into SW480 cells resulted in increased p24 core antigen and this response was markedly increased following PMA stimulation of cells. c-Fos or JunD alone did not increase p24 production but markedly increased p24 production in PMA-stimulated cells. PMA increased c-Fos and JunD binding activity on an AP-1 binding site within the U5 region of the LTR, as shown in gel mobility shift assays. Functional analysis of this site by transient transfections demonstrated it was required to mediate c-Fos and JunD transactivation of the HIV-1 LTR. Specific Jun and Fos transcription factors can transactivate the HIV-1 provirus and this response is markedly increased in cooperation with cellular activation signals elicited by PMA. Taken together, the data indicate that AP-1 binding sites downstream of the transcriptional start site in the HIV-1 LTR are capable of binding c-Fos and JunD and may contribute to transactivation of HIV-1 provirus.