Identification of human T-cell lymphotropic virus type I 21-base-pair repeat-specific and glial cell-specific...

The human T-cell lymphotropic virus type I (HTLV-I)-encoded protein, Tax, is capable of trans-activating HTLV-I transcription by interacting with specific sequences in the HTLV-I long terminal repeat (LTR) which comprise an inducible enhancer containing three imperfect tandem repeats of a 21-bp sequence. There is no evidence that purified Tax can bind to DNA in the absence of cellular factors, suggesting that Tax most likely regulates transcription via interaction with cellular factors. Since HTLV-I is a documented agent of adult T-cell leukemia and tropical spastic paraparesis, disorders of the immune and nervous systems, respectively, characterization of cellular factors of lymphoid and neuroglial origin which interact with the 21-bp repeat elements is essential to understanding of the mechanisms involved in basal and Tax-mediated transcription in cells of immune and nervous system origin. Utilizing electrophoretic mobility shift (EMS) analyses, we have detected both 21-bp repeat-specific and glial cell-specific DNA-protein complexes. Several 21-bp repeat-specific DNA-protein complexes were detected when nuclear extracts derived from cells of lymphoid (Jurkat, SupT1, and H9), neuronal (IMR-32 and SK-N-MC), and glial (U-373 MG, Hs683, and U-118) origin were used in reactions with each of the three 21-bp repeat elements. In addition, a glial cell-specific DNA-protein complex was detected when nuclear extracts derived from U-373 MG, Hs683, and U-118 glial cell lines reacted with the promoter-distal and central 21-bp repeat elements. Furthermore, EMS analyses performed with nuclear extracts derived from lymphocytic and glial cell origin and a 223-bp fragment of the HTLV-I long terminal repeat encompassing the three 21-bp repeat elements (designated Tax-responsive elements 1 and 2, TRE-1/-2) have also resulted in the detection of glial cell type-specific DNA-protein complexes. Competition EMS analyses with oligonucleotides containing transcription factor binding site sequences indicate the involvement of a cyclic AMP response element binding protein in the formation of DNA-protein complexes which form with all three 21-bp repeat elements and the glial cell-specific DNA-protein complex as well as the involvement of Sp1 or an Sp1-related factor in the formation of the 21-bp repeat III-specific DNA-protein complexes.