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.     

Human T-cell leukemia virus type 1 HBZ protein bypasses the targeting function of ubiquitination

Human T-cell leukemia virus type 1 (HTLV-1) encodes an antisense viral gene product termed HTLV-1 basic leucine-zipper factor (HBZ). HBZ forms heterodimers with c-Jun, a member of the AP-1 family, and promotes its proteasomal degradation. Although most proteasomal substrates are targeted for degradation via conjugation of polyubiquitin chains, we show that ubiquitination is not required for HBZ-mediated proteasomal degradation of c-Jun. We demonstrate that HBZ directly interacts with both the 26 S proteasome and c-Jun and facilitates the delivery of c-Jun to the proteasome without ubiquitination. HBZ acts as a tethering factor between the 26 S proteasome and its substrate, thereby bypassing the targeting function of ubiquitination. These findings disclose a novel viral strategy to utilize the cellular proteolytic system for viral propagation.     

Transcomplementation of simian immunodeficiency virus Rev with human T-cell leukemia virus type I Rex.

A molecular clone of the simian immunodeficiency virus SIVSMM isolate PBj14, lacking the ATG initiation codon for Rev protein (PBj-1.5), did not produce virus or large unspliced or singly spliced viral RNA upon transfection of HeLa cells. Low but significant levels of virus and large viral RNA production were observed upon transfection of PBj-1.5 into HeLa Rev cells expressing the rev gene of human immunodeficiency virus type 1. Furthermore, abundant virus and large viral RNA production occurred upon transfection of PBj-1.5 into HeLa Rex cells expressing the rex gene of human T-cell leukemia virus type I. Virus produced from HeLa Rex and HeLa Rev transfections was infectious, produced large amounts of virus, and was cytopathic for Rex-producing MT-4 cells. In contrast, no or only low levels of virus production were observed upon infection of H9 cells. These studies show that a defective SIV rev gene can be transcomplemented with human immunodeficiency virus type 1 Rev and with high efficiency by human T-cell leukemia virus type I Rex, and they suggest that rev-defective viruses could serve as a source for production of a live attenuated SIV vaccine.     

Pseudotyping with human T-cell leukemia virus type I broadens the human immunodeficiency virus host range.

Several epidemiologic and clinical studies suggest that patients coinfected with human immunodeficiency virus (HIV), the primary etiologic agent in AIDS, and other viruses, such as cytomegalovirus or human T-cell leukemia virus (HTLV), have a more severe clinical course than those infected with HIV alone. Cells infected with two viruses can, in some cases, give rise to phenotypically mixed virions with altered or broadened cell tropism and could therefore account for some of these findings. Such pseudotypes could alter the course of disease by infecting more tissues than are normally infected by HIV. We show here that HIV type 1 (HIV-1) efficiently incorporates the HTLV type I (HTLV-I) envelope glycoprotein and that both HIV-1 and HTLV-II accept other widely divergent envelope glycoproteins to form infectious pseudotype viruses whose cellular tropisms and relative abilities to be transmitted by cell-free virions or by cell contact are determined by the heterologous envelope. We also show that the mechanism by which virions incorporate heterologous envelope glycoproteins is independent of the presence of the homologous glycoprotein or heterologous gag proteins. These results may have important implications for the mechanism of HIV pathogenesis.     

Intrauterine transmission of human T-cell leukemia virus type I in rats.

To analyze intrauterine transmission, MT-2 cells, a human T-cell line producing human T-cell leukemia virus type I (HTLV-I), were injected into eight pregnant F344 rats, and cesarean section was performed at day 23 of pregnancy. HTLV-I provirus was detected by PCR in the liver and spleen taken from one of the eight fetuses. Moreover, 71 offspring were delivered by cesarean section from the remaining seven dams and fostered by seven normal rats. HTLV-I provirus was detected in peripheral blood mononuclear cells in 2 of the 71 offspring 4 weeks after cesarean section. These results indicate for the first time the intrauterine transmission of HTLV-I. To confirm the postnatal transmission, MT-2 cells were injected into a dam within 24 h after delivery, and six offspring were fostered by this dam. HTLV-I provirus was detected in peripheral blood mononuclear cells of all six offspring. This animal model may be useful for analysis and prevention of mother-to-child transmission of HTLV-I.     

Requirement of N- and C-terminal regions for enzymatic activity of human T-cell leukemia virus type I protease.

The requirement of N- and C-terminal regions for the enzymatic activity of human T-cell leukemia virus type I (HTLV-I) protease was investigated using a series of deletion mutants. The activity was analyzed by autoprocessing of the protease itself or by processing of the gag p53 precursor. The deletional analyses indicated that Asp38-Gly152 with an additional Met-Pro sequence at the N-terminus was probably sufficient for the enzymatic activity, although the mature HTLV-I protease consists of Pro33-Leu157. A molecular model of HTLV-I protease, which was constructed by comparison with the structure of Rous sarcoma virus protease, predicted that Pro33-Leu37 and Gly143-Leu147 would form a beta-sheet. Our experimental results and the model structure suggest that (a) five amino acids in the N-terminal region (Pro33-Leu37), which are thought to be involved in the beta-sheet, are not crucial for the enzymatic activity; (b) Pro153-Leu157 is not necessary but Pro148-Gly152 is important for the enzymatic activity, in addition to Gly143-Leu147 involved in the beta-sheet.     

Identification of a human T-cell leukemia virus type I tax peptide in contact with DNA

The human T-cell leukemia virus Tax protein directs binding of a host factor, cAMP response element binding protein, to an extended recognition sequence in the proviral promoter. Prior cross-linking experiments have revealed that Tax makes restricted contact with this DNA at two symmetric positions, 14 nucleotides apart on opposite strands of the DNA. Tax lacks a conventional DNA binding domain, and the sequences in Tax that are in contact with DNA have not been previously identified. Analysis of cross-linked peptides now shows that the contact occurs between Tax residues 89 and 110, corresponding to a protease-sensitive linker joining two protein structural domains. The linker assumes a protease-resistant conformation in the cross-linked complex. Point mutations within the linker prevent cross-linking and interfere with Tax function. These data suggest that entry of Tax into the ternary complex may be coupled to folding of an unstructured protein domain, which then makes base-specific contacts with DNA.