ARK5 regulates the growth of HTLV-1-infected T cells during glucose starvation

599 A human retrovirus, HTLV-1 (human T-cell leukemia virus type 1) is the causative agent of ATL (adult T-cell leukemia), which is an aggressive and fatal T cell malignancy characterized by dysregulated proliferation of CD4-positive T cells. HTLV-1-infected T cells are resistant to apoptotic cell death induced by various stimuli. AMP-activated protein kinase-related kinase 5 (ARK5) suppresses cell death in AKT-dependent manner. It has been shown that ARK5 was transcriptionally regulated by large-Maf family proteins including c-Maf in multiple myeloma cells. However, the role of ARK5 on survival of HTLV-1-infected T cells is not elucidated. In this study, we found that ARK5 expression was induced by HTLV-1 transforming protein Tax and played an important role on survival of HTLV-1-infected T cells during glucose starvation. ARK5 and c-Maf mRNA was overexpressed in HTLV-1-infected T-cell lines but not in HTLV-1-uninfected T-cell lines. In vitro infection of T-cell line, TY8-3, with HTLV-1 induced the expression of ARK5 and c-Maf mRNA. Inducible expression of Tax in a T-cell line also up-regulated the expression of ARK5 and c-Maf mRNA. Analysis of the ARK5 promoter revealed that transcription of this gene was activated by Tax but not by c-Maf. Knockdown of c-Maf mRNA expression by using small interfering RNA (siRNA) did not affect ARK5 mRNA expression in HTLV-1-infected T cells. Tax mutant which defects of ability of activation of NF-κB signaling pathway could not increase transcriptional activity of ARK5 gene. Coexpression of IκBα, IkBβ, IKKβ, and IKKγ dominant-negative constructs with Tax inhibited activation of ARK5 gene transcription induced by Tax. Bay11-7082, an IκBα phosphorylation inhibitor, reduced the expression of ARK5 mRNA in HTLV-1-infected T cells. Introduction of NF-κB (p65) expression plasmid increased transcriptional activity of ARK5 gene. Previously, it has been shown that ARK5 suppresses the cell death induced by glucose starvation in human hepatoma cells. Therefore, the role of ARK5 on survival of HTLV-1-infected T-cell lines during glucose starvation was assessed. We found that the viability of non-ARK5-expressing HTLV-1-uninfected T cells was markedly reduced in glucose reduced media. In contrast, more than 70% of ARK5-expressing HTLV-1-infected T-cell lines were viable during glucose starvation, except AKT-inactive MT-4 cells. Knockdown of ARK5 expression by siRNA did not affect the viability of HTLV-1-infected T-cell lines in glucose-containing media, but reduced the viability of these cells in glucose-reduced media. Our results demonstrated that Tax induced the expression ARK5 gene by activating NF-κB signaling pathway and ARK5 contributed to survival of HTLV-1-infected T cells during glucose starvation in AKT-dependent manner. Our findings implicated that ARK5 plays important roles in the survival of ATL cells and is a good therapeutic target of ATL.