No abstract

Despite being discovered in animals in the early 20th century, the scientific interest in retroviruses was boosted with the discovery of human retroviruses (human T-leukemia/lymphoma virus [HTLV] and HIV), which are responsible for significant morbidity and mortality. HTLV was identified more than 25 years ago as the etiological agent of adult T-cell leukemia/lymphoma. It was then shown to be a complex retrovirus, given that it not only encodes the characteristic retroviral Gag, Pol and Env proteins, but also regulatory and accessory proteins. Since the first studies documenting the role of these proteins in viral expression, the picture has become increasingly more complex. Indeed, owing to the limited size of its genome that contains overlapping open-reading frames, HTLV has evolved unique ways to regulate its expression. Retroviral expression was originally thought to be mainly controlled through the regulation of transcription from the 5 long-terminal repeats, but we now know that the 3 long-terminal repeats also serve as promoters. Regulation of splicing and mRNA export, and post-translational modifications of viral protein also play a major role. This review discusses the latest insights gained into the field of HTLV gene expression.

We recently discovered the antisense protein of human T-cell leukemia virus (HTLV) type 2 (APH-2), whose messenger RNA is encoded by the antisense strand of the HTLV-2 genome. We quantified proviral load, level of tax, and APH-2 in a series of blood samples obtained from a cohort of HTLV-2 carriers. We determined whether APH-2 promotes cell proliferation. APH-2 was detectable in most samples tested and was correlated with proviral load. APH-2 levels were not correlated with lymphocyte count in vivo, consistent with the inability of APH-2 to promote cell proliferation in vitro. APH-2 does not promote cell proliferation and does not cause lymphocytosis.

Since the identification of the antisense protein of HTLV-2 (APH-2) and the demonstration that APH-2 mRNA is expressed in vivo in most HTLV-2 carriers, much effort has been dedicated to the elucidation of similarities and/or differences between APH-2 and HBZ, the antisense protein of HTLV-1. Similar to HBZ, APH-2 negatively regulates HTLV-2 transcription. However, it does not promote cell proliferation. In contrast to HBZ, APH-2 half-life is very short. Here, we show that APH-2 is addressed to PML nuclear bodies in T-cells, as well as in different cell types. Covalent SUMOylation of APH-2 is readily detected, indicating that APH-2 might be addressed to the PML nuclear bodies in a SUMO-dependent manner. We further show that silencing of PML increases expression of APH-2, while expression of HBZ is unaffected. On the other hand, SUMO-1 overexpression leads to a specific loss of APH-2 expression that is restored upon proteasome inhibition. Furthermore, the carboxy-terminal LAGLL motif of APH-2 is responsible for both the targeting of the protein to PML nuclear bodies and its short half-life. Taken together, these observations indicate that natural APH-2 targeting to PML nuclear bodies induces proteasomal degradation of the viral protein in a SUMO-dependent manner. Hence, this study deciphers the molecular and cellular bases of APH-2 short half-life in comparison to HBZ and highlights key differences in the post-translational mechanisms that control the expression of both proteins.

Unlike HTLV-1, HTLV-2 does not induce leukemia and has been tentatively associated with an HTLV-1-associated myelopathy-like disorder. It has been reported that HTLV-2/HIV-1 co-infected patients progress less rapidly to AIDS than HIV-1-infected individuals. Tax2 has been suggested to mediate this protective state by inducing MIP-1α expression and blocking HIV-1 infection. As cells from HTLV-2-infected individuals mainly express Antisense Protein 2 (APH-2), our objective was to determine if this protein might also intervene in controlling HIV-1 replication in dually infected individuals. Using Jurkat cells, we first demonstrated that both APH-2 and HBZ, the HTLV-1 analogue, equally induced MIP-1α in unstimulated and stimulated Jurkat T cells. To assess if APH-2 might directly affect HIV-1 replication, a full length luciferase-expressing proviral DNA was tested in Jurkat cells. Surprisingly, upon co-transfection with an APH-2 expression vector, an increase in luciferase activity was observed, while HBZ expression rather led to reduced reporter gene expression. Western blot analyses and ELISA assay further indicated that HIV-1 p24 levels were more important in APH-2-expressing cells. To determine if APH-2 was directly modulating HIV-1 LTR activity, both NF-B and NFAT were tested in stimulated Jurkat cells. Unexpectedly, HBZ and APH-2 inhibited NF-B and NFAT activation, albeit at different extent. In addition, LTR activation was also inhibited by both antisense proteins although APH-2 had a more modest effect. Our results thus highlight the complex interplay between HTLV antisense transcript-encoded proteins and HIV-1 expression and further studies will be required to determine the potential impact of APH-2 in HTLV-2/HIV-1-infected individuals.

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