| Budget Amount *help |
¥12,800,000 (Direct Cost: ¥12,800,000)
Fiscal Year 2003: ¥6,000,000 (Direct Cost: ¥6,000,000)
Fiscal Year 2002: ¥6,800,000 (Direct Cost: ¥6,800,000)
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| Research Abstract |
Human T-cell leukemia virus type 1 has a function to immortalize normal lymphocytes upon infection. To clarify the role of Tax that is encoded by HTLV-1 and is responsible for this immortalization, I focus to analyze regulatory mechanisms of Tax-dependent transcription as well as mechanism of a cell cycle inhibitory protein, p21, which is known to be up-regulated by Tax.
Tax has the ability to transactivate p21(Waf1 /Cip 1 /Sdi 1), resulting in high expression levels in HTLV-1-immortalized cells. To understand the role of p21 for the proliferation of Tax-transformed Rat-1 cells, I examined the effect of ectopic expression of p21 in these cells. I observed that p21 expression enhanced the transformation of this cell line via at least two mechanisms: (i) the enhancement of NF-kappaB activation and/or CREB signaling and (ii) the excitation of antiapoptotic machinery. To analyze the role of p21 that is overexpressed in HTLV-1-immortalized lymphocytes, p21 expression was suppressed by using
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an antisense oligonucleotide specific for p21 mRNA; these cells then became sensitive to apoptotic induction. These results suggest that p21 plays an important role in the proliferation of Tax-expressing cells through the regulation of at least two independent mechanisms.
Expression of HTLV-1 genes is transcriptionally activated by the cognate oncoprotein Tax which enhances the binding of the cyclin AMP-responsive element binding protein (CREB) to the Tax responsive element (TxRE) located in its long terminal repeat (LTR). TxRE is highly homologous to the cyclic AMP-responsive element (CRE) except for the GC-rich sequence flanking the CRE. We cloned the cDNA for a cellular factor, TAXREB803, of which the DNA-binding domain bound to TxRE and the binding was dependent on the 3' GC-rich sequence in TxRE. TAXREB803 is an SR-related protein composed of 2,752 amino acids including numerous arginine/serine (RS) motifs. TAXREB803 enhanced both the Tax dependent transcription and the CREB binding to TxRE in cooperation with Tax. The interaction of TAXREB803 and Tax was detected by coimmunoprecipitation assays as well as by indirect immunofluorescence assays. Significantly, Tax transactivation for the HTLV-1 LTR decreased dramatically when the expression of the endogenous TAXREB803 was suppressed by the small interfering RNA. These results suggest that. TAXREB803 functions as a transcriptional coactivator for Tax and plays a critical role in the expression of HTLV-1 genes. Less
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