Human T-cell leukemia virus type 1 tax protein activates transcription through AP-1 site by inducing DNA binding activity in T cells

Human T-cell leukemia virus type I (HTLV-I) Tax protein induces the express ion of various family members of the transcription factor AP-I, such as c-J un, JunD, c-Fos, and Fra-1, at the level of RNA expression in T cells. We e xamined the activity of Tax in transcription through AP-1-binding sites (AP -1 site) in T cells. Transient transfection studies showed that Tax activat ed the expression of a luciferase gene regulated by two copies of an AP-1 s ite in the human Jurkat T-cell line. Tax activates the expression of viral and cellular genes through two different enhancers: a cAMP-responsive (CRE) -like element and a KB element. Two Tax mutants differentially activated ex pression of these two elements. Tax703 preferentially activated the KB elem ent but not the CRE-like one, whereas TaxM22 showed the reverse. In additio n, Tax703 and Tax, but not TaxM22, converted cell growth of a mouse T-cell line from being interleukin (IL)-2-dependent to being Ii-a-independent. Unl ike the wild-type Tax, Tax703 and TaxM22 only weakly activated the AP-1 sit e in the T-cell line. Thus, Tax seems to activate the AP-1 site via mechani sms distinct from those of KB Or CRE-like elements, and the activation of t he AP-1 site is dispensable for IL-2-independent growth of CTLL-2. Electrop horetic mobility shift assays showed that Tax induced strong binding activi ty to an AP-1 site in CTLL-2, whereas Tax703 did not, indicating that the i nduction of binding activity to the AP-1 site is essential for the transcri ptional activation by Tax. The binding complex induced by Tax in CTLL-2 con tained JunD and Fra-2. Other AP-1 proteins were undetectable. Activation of transcription through the AP-1 site in Jurkat cells by JunD and/or Fra-2 w as weak. c-Jun, JunB, and c-Fos activation was greater, although the level was still less than that with Tax. Thus, the induction of AP-1 mRNA by Tax may not be sufficient for a complete activation of AP-1 site by Tax. Our re sults suggest that Tax activates the transcription of cellular genes with A P-1 sites by inducing the DNA-binding activity of AP-1 proteins in T cells. a mechanism distinct from those of CRE-like and KB elements. (C) 2001 Acad emic Press.