Transactivation of Krox-20 and Krox-24 promoters by the HTLV-1 Tax protein through common regulatory elements.

The HTLV-1 Tax protein has been shown to induce the expression of host cellular genes, some of which play crucial roles in cell proliferation and differentiation. We have examined the effect of Tax on the expression of two immediate-early genes, Krox-20 and Krox-24, which encode transcription factors. Several HTLV-1-infected T-cell lines and a HeLa cell line that constitutively expresses the Tax protein have a high level of expression of the Krox-20 and Krox-24 genes. In addition, Tax transactivates the promoters of both Krox-20 and Knox-24 in a co-transfection assay. Tax-responsive elements in Krox-20 and Krox-24 include the serum response elements (SREs) and the putative cAMP-responsive element (CRE). A correlation exists between the ability of these elements to mediate Tax transactivation and their affinity for their cognate factors, the serum response factor (SRF) or the CRE-binding protein (CREB) respectively. Since Tax is also able to transactivate the human c-fos promoter through the SRE and the CRE-60, our findings support the idea that the HTLV-1 Tax protein uses common mechanisms for transactivation of these three immediate-early genes. Deregulation of their expression may contribute to malignant transformation associated with HTLV-1 infection.