Recombinant HTLV-1 Gag polyprotein Pr76 (Caribbea HS-35)

Genomic instability is a hallmark of many cancers; however, the molecular etiology of chromosomal dysregulation is not well understood. The human T-cell leukemia virus type-1 (HTLV-1) oncoprotein Tax activates NF-kappa B-signaling and induces DNA-damage and aberrant chromosomal segregation through diverse mechanisms which contribute to viral carcinogenesis. Intriguingly, Stathmin/oncoprotein-18 (Op-18) depolymerizes tubulin and interacts with the p65(RelA) subunit and functions as a cofactor for NF-kappa B-dependent transactivation. We thus hypothesized that the dissociation of p65(RelA)-Stathmin/Op-18 complexes by Tax could lead to the catastrophic destabilization of microtubule (MT) spindle fibers during mitosis and provide a novel mechanistic link between NF-kappa B-signaling and genomic instability. Here we report that the inhibition of Stathmin expression by the retroviral latency protein, p30(II), or knockdown with siRNA-stathmin, dampens Tax-mediated NF-kappa B transactivation and counters Tax-induced genomic instability and cytotoxicity. The Tax-G148V mutant, defective for NF-kappa B activation, exhibited reduced p65(RelA)-Stathmin binding and diminished genomic instability and cytotoxicity. Dominant-negative inhibitors of NF-kappa B also prevented Tax-induced multinucleation and apoptosis. Moreover, cell clones containing the infectious HTLV-1 ACH. p30(II) mutant provirus, impaired for p30(II) production, exhibited increased multinucleation and the accumulation of cytoplasmic tubulin aggregates following nocodozole-treatment. These findings allude to a mechanism whereby NF-kappa B-signaling regulates tubulin dynamics and mitotic instability through the modulation of p65(RelA)-Stathmin/Op-18 interactions, and support the notion that p3(II) enhances the survival of Tax-expressing HTLV-1-transformed cells.

The genetic and molecular alterations responsible for leukemogenesis and progression of HTLV-infected adult T-cell leukemia (ATL) have not been fully clarified. Previously, we reported that various genes are not only overexpressed but also abnormally spliced in ATL cells. Here, we identified various CASP8 transcript variants in PBMCs from a smoldering-type ATL patient, which encode aberrant truncated caspase 8 (Casp8) isoforms. Among those, we focus on the three transcript variants, CASP8L (including the first 136 bp of the intron 8 between exon 8 and exon 9), CASP8-Delta E4 (without the exon 4), and CASP8-DE7 (without the exon 7), because they encode isoforms, Casp8L, Casp8-Delta E4, and Casp8-Delta E7, respectively, without the C-terminal catalytic domains. In this study, we conducted in vitro characterization and functional analysis of those mutant Casp8 isoforms to clarify their changed functions compared with the wild-type (WT)-Casp8. We demonstrated that these abnormal Casp8 isoforms showed lower ability to induce apoptosis than WT-Casp8 due to their dominant-negative interactions with WT-Casp8, which impair WT-Casp8 homodimerization that is essential for induction of apoptosis. Moreover, Casp8L and Casp8-Delta E7, which have only two death-effector domains, significantly activated NF kappa B by forming filament-like structures, which probably function as scaffolds for the IKK complex formation. In view of increasing levels of these abnormal CASP8 transcripts in primary PBMCs from HTLV-1 carriers and patients with ATL, we propose a possibility that overexpression of those Casp8 mutants, with lower proapoptotic activities and higher NF kappa B-activating functions than WT-Casp8, may be one of the molecular abnormalities causing malignant transformation and growth of ATL cells.