Mutational analysis of human T-cell leukemia virus type 2 Tax.

A mutational analysis of human T-cell leukemia virus type 2 (HTLV-2) Tax (Tax-2) was performed to identify regions within Tax-2 important for activation of promoters through the CREB/ATF or NF-kappaB/Rel signaling pathway. Tax-2 mutations within the putative zinc-binding region as well as mutations at the carboxy terminus disrupted CREB/ATF transactivation. A single mutation within the central proline-rich region of Tax-2 disrupted the transactivation of the NF-kappaB/Rel pathway. Surprisingly, this mutation, which is thought to be in a separate activation domain, was suppressed by mutations within or around the putative zinc-binding region, suggesting an interaction between these two regions. These analyses indicate that the functional regions or domains important for transactivation through the CREB/ATF or NF-kappaB/Rel signaling pathway are similar, but not identical, in Tax-1 and Tax-2. Identification of these distinct Tax-2 mutants should facilitate comparative biological studies of HTLV-1 and HTLV-2 and ultimately lead to the determination of the functional importance of Tax trans-acting capacities in T-lymphocyte transformation by HTLV.     

Human T-cell leukemia virus type 1 Tax transactivates the human proliferating cell nuclear antigen promoter.

The human T-cell leukemia virus type 1 (HTLV-1) transforming protein, Tax, is a potent transactivator of both viral and cellular gene expression. The ability of Tax to transform cells is believed to depend on its transactivation of cellular-growth-regulatory genes. Expression of proliferating cell nuclear antigen (PCNA) is intimately linked to cell growth and DNA replication and repair. By testing a series of PCNA promoter deletion constructs, we have demonstrated that the PCNA promoter can be transactivated by Tax. The smallest construct that was activated did not include the ATF/CRE binding site at nucleotide -50, and mutations in the ATF/CRE element in the context of a larger promoter were still activated by Tax. In addition, a Tax mutant that is defective for activation of the CRE pathway retained the ability to activate the -397 promoter construct. When a series of linker scanner mutations that span the region from nucleotide -45 to -7 were assayed, mutations in and around a repeat sequence were found to abolish Tax transactivation. Multimerized copies of either half of the repeat were Tax responsive. A single protein complex was shown to bind specifically to the Tax-responsive region, and the binding of this complex was enhanced in the presence of Tax. These results demonstrate that the PCNA promoter contains a Tax-responsive element located between nucleotides -45 and -7 whose sequence is different from those of other, previously identified Tax-responsive elements. The ability of Tax to activate the PCNA promoter may play an important role in cellular transformation by HTLV-1.     

Mitochondria to nucleus stress signaling: a distinctive mechanism of NFkappaB/Rel activation through calcineurin-mediated inactivation of IkappaBbeta

Mitochondrial genetic and metabolic stress causes activation of calcineurin (Cn), NFAT, ATF2, and NFkappaB/Rel factors, which collectively alter the expression of an array of nuclear genes. We demonstrate here that mitochondrial stress-induced activation of NFkappaB/Rel factors involves inactivation of IkappaBbeta through Cn-mediated dephosphorylation. Phosphorylated IkappaBbeta is a substrate for Cn phosphatase, which was inhibited by FK506 and RII peptide. Chemical cross-linking and coimmunoprecipitation show that NFkappaB/Rel factor-bound IkappaBbeta forms a ternary complex with Cn under in vitro and in vivo conditions that was sensitive to FK506. Results show that phosphorylation at S313 and S315 from the COOH-terminal PEST domain of IkappaBbeta is critical for binding to Cn. Mutations at S313/S315 of IkappaBbeta abolished Cn binding, inhibited Cn-mediated increase of Rel proteins in the nucleus, and had a dominant-negative effect on the mitochondrial stress-induced expression of RyR1 and cathepsin L genes. Our results show the distinctive nature of mitochondrial stress-induced NFkappaB/Rel activation, which is independent of IKKalpha and IKKbeta kinases and affects gene target(s) that are different from cytokine and TNFalpha-induced stress signaling. The results provide new insights into the role of Cn as a critical link between Ca2+ signaling and NFkappaB/Rel activation.     

Antiapoptotic effect of human T-cell leukemia virus type 1 tax protein correlates with its creb transcriptional activity

Defects in the regulation of programmed cell death play a fundamental role in the development of neoplasia and neurological disorders, both of which are linked to the human T-cell leukemia/lymphoma virus type 1 (HTLV-1) infection. We previously showed that the HTLV-1 Tax protein protects from apoptosis induced by serum starvation by preventing cytochrome c release and Bax relocation to mitochondria, two early events in the mitochondrial apoptotic pathway. As a natural extension of these findings, and to better define the action of Tax, in the present study, we investigated the outcome of Tax and two mutants which are inactive in CREB/ATF (M47) or NF-kappaB (M22) pathways, in the control of apoptosis induced by the proapoptotic Bax protein. We found that activation of CREB, rather than NF-kappaB, is a key phenomenon in preventing apoptosis. Furthermore, the importance of CREB activation is strengthened by experiments with CREB mutants, treatment with forskolin, and in situ analysis of P-CREB status in cells transfected with Tax or its nonprotecting M47 mutant. Considered together, these results underscore a primary role of CREB in preventing apoptosis triggered by Bax, and suggest that Tax might act by affecting the phosphorylation state of CREB.