Inhibition of geranylgeranyl transferase-I decreases cell viability of HTLV-1-transformed cells.

Human T-cell leukemia virus type-1 (HTLV-1) is the etiological agent of adult T-cell leukemia (ATL), an aggressive and highly chemoresistant malignancy. Rho family GTPases regulate multiple signaling pathways in tumorigenesis: cytoskeletal organization, transcription, cell cycle progression, and cell proliferation. Geranylgeranylation of Rho family GTPases is essential for cell membrane localization and activation of these proteins. It is currently unknown whether HTLV-1-transformed cells are preferentially sensitive to geranylgeranylation inhibitors, such as GGTI-298. In this report, we demonstrate that GGTI-298 decreased cell viability and induced G(2)/M phase accumulation of HTLV-1-transformed cells, independent of p53 reactivation. HTLV-1-LTR transcriptional activity was inhibited and Tax protein levels decreased following treatment with GGTI-298. Furthermore, GGTI-298 decreased activation of NF-κB, a downstream target of Rho family GTPases. These studies suggest that protein geranylgeranylation contributes to dysregulation of cell survival pathways in HTLV-1-transformed cells.

Drug targets in human T-lymphotropic virus type 1 (HTLV-1) infection.

Human T-lymphotropic virus type 1 (HTLV-1), the first known human retrovirus, induces various human diseases with a long latency period. The mechanism by which the virus causes diseases is still unknown. Studies indicate that viral replication is important at least for the development of HTLV-1 associated myelopathy, and therefore treatments based on our knowledge of human immunodeficiency virus type-1 (HIV-1) can be utilized to develop potent antiretroviral therapies targeting the replication enzymes reverse transcriptase, protease and integrase as well as the envelope glycoproteins. Furthermore, accessory gene products such as Tax and HBZ may also provide targets for chemotherapy. Treatment targeting these viral proteins may prevent the development of other HTLV-1-related diseases including adult T-cell leukemia, although such treatment may not be useful during the progression of the disease. This review describes the characteristics of HTLV-1 replication enzymes, envelope glycoproteins, and accessory proteins Tax and HBZ, and discusses the status of drug development strategies.

Arsenic/interferon specifically reverses 2 distinct gene networks critical for the survival of HTLV-1-infected leukemic cells.

Adult T-cell leukemia (ATL) is a severe chemotherapy-resistant malignancy associated with prolonged infection by the human T cell-lymphotropic virus 1 (HTLV-1) retrovirus. Although the Tax viral transactivator is clearly an oncogene, the role of its continuous expression in the maintenance of the transformed phenotype is controversial. Because arsenic trioxide (As) and interferon alpha (IFN) synergize to induce cell cycle arrest and apoptosis of ATL cells both ex vivo and in vitro, we investigated the effects of As alone and As/IFN combination on gene networks in HTLV-1-infected leukemic cells. The As/IFN combination reduced Tax expression and, accordingly, reversed the Tax-induced constitutive nuclear factor kappaB (NF-kappaB) activation. Using DNA microarray analyses, we demonstrated that As rapidly and selectively blocks the transcription of NF-kappaB-dependent genes in HTLV-1-infected cells only. Reversal of NF-kappaB activation by As alone resulted from dramatic stabilization of IkappaB-alpha and IkappaB-epsilon, independently of IkappaB kinase (IKK) activity modulation or Tax degradation. In contrast, only the As/IFN combination induced late and massive down-regulation of cell cycle-regulated genes, concomitantly with Tax degradation by the proteasome and cell death induction, indicating the importance of continuous Tax expression for ATL cell survival. These 2 successive events likely account for the potent and specific effects of the As/IFN combination in ATL.

Human T cell leukemia virus-I (HTLV-I) Tax-mediated apoptosis in activated T cells requires an enhanced intracellular prooxidant state.

We have shown that an estradiol-dependent activation of human T cell leukemia virus-I Tax leads to the inhibition of cell proliferation and to the induction of apoptosis. The present study demonstrates that a hormone-dependent activation of Tax promotes an enhanced prooxidant state in stably transfected Jurkat cells as measured by changes in the intracellular levels of glutathione and H2O2; these changes are followed by apoptotic cell death. Additional stimulation of the CD3/TCR pathway enhances the oxidative and apoptotic effects. Both Tax-mediated apoptosis and oxidative stress can be potently suppressed by antioxidants, as is seen with the administration of recombinant thioredoxin (adult T cell leukemia-derived factor) or pyrrolidine dithiocarbamate. Hormone-induced Tax activation induces a long-lasting activation of NF-kappaB, which is a major target of reactive oxygen intermediates. The long-term exposure of Jurkat cells to hormone eventually results in a selection of cell clones that have lost Tax activity. A subsequent transfection of these apparently "nonresponsive" clones allows the recovery of Tax responses in these cells. Our observations indicate that changes in the intracellular redox status may be a determining factor in Tax-mediated DNA damage, apoptosis, and selection against the long-term expression of Tax function.

Inhibition of the association with nuclear matrix of pRB, p70 and p40 proteins along with the specific suppression of c-MYC expression by geldanamycin, an inhibitor of Src tyrosine kinase.

Geldanamycin is an antibiotic that preferentially inhibits G1/S transition and causes G2/M arrest in human leukemia HL-60 cells. With it, we selectively inhibited recombinant Src tyrosine kinase without significantly inhibiting protein kinase A. The perturbation of cell cycling by geldanamycin was accompanied by marked suppression of c-MYC expression. In contrast to this, pRB expression was remarkably enhanced by geldanamycin. In the untreated HL-60 cells, c-MYC was apparently enriched in nuclear matrix preparation, and significant amounts of hyperphosphorylated pRB, p70 and p40 proteins were observed to associated with the nuclear matrix. The amounts of these proteins associated with the nuclear matrix, however, were markedly decreased by treatment with geldanamycin. This finding suggests that the association of c-MYC, hyperphosphorylated pRB, p70 and p40 proteins with the nuclear matrix is essential in cell cycling, especially in G1/S and G2/M progressions, and that this association is a part of signal transduction pathway in Src kinase activation.