Human T-cell leukemia virus type 1 Tax protein transforms rat fibroblasts via two distinct pathways

The human T-cell leukemia virus type 1 (HTLV-1) Tax protein activates the transcription of several cellular genes. This function is thought to play a critical role in the Tax-dependent transformation step in HTLV-1 leukemogenesis. Tax activates transcription via three enhancers: the cyclic AMP response element (CRE)-like sequence, the kappaB element, and the CArG box. Their involvement in the transformation of rat fibroblasts by Tax was examined by colony formation of Rat-1 cells in soft agar and Ras cooperative focus formation of rat embryo fibroblasts (REF). Among Tax mutants, those retaining activity for the CArG box transformed REF like wild-type Tax, while those inactive for the CArG box did not. Thus, the activation of the CArG box pathway is essential for the transformation of REF by Tax. In contrast, activation of the kappaB element correlated with the transformation of Rat-1 by Tax. These results show that Tax transforms rat fibroblasts via two distinct pathways.