Resistance to Apo2 Ligand (Apo2L)/Tumor Necrosis Factor-Related Apoptosis-Inducing Ligand (TRAIL)-Mediated Apoptosis and Constitutive Expression of Apo2L/TRAIL in Human T-Cell Leukemia Virus Type 1-Infected T-Cell Lines

Adult T-cell leukemia (ATL), a CD4+-T-cell malignancy caused by human T-cell leukemia virus type 1 (HTLV-1), is difficult to cure, and novel treatments are urgently needed. Apo2 ligand (Apo2L; also tumor necrosis factor-related apoptosis-inducing ligand [TRAIL]) has been implicated in antitumor therapy. We found that HTLV-1-infected T-cell lines and primary ATL cells were more resistant to Apo2L-induced apoptosis than uninfected cells. Interestingly, HTLV-1-infected T-cell lines and primary ATL cells constitutively expressed Apo2L mRNA. Inducible expression of the viral oncoprotein Tax in a T-cell line up-regulated Apo2L mRNA. Analysis of the Apo2L promoter revealed that this gene is activated by Tax via the activation of NF-κB. The sensitivity to Apo2L was not correlated with expression levels of Apo2L receptors, intracellular regulators of apoptosis (FLICE-inhibitory protein and active Akt). NF-κB plays a crucial role in the pathogenesis and survival of ATL cells. The resistance to Apo2L-induced apoptosis was reversed byN-acetyl-l-leucinyl-l-leucinyl-l-norleucinal (LLnL), an NF-κB inhibitor. LLnL significantly induced the Apo2L receptors DR4 and DR5. Our results suggest that the constitutive activation of NF-κB is essential forApo2Lgene induction and protection against Apo2L-induced apoptosis and that suppression of NF-κB may be a useful adjunct in clinical use of Apo2L against ATL.