Cells of adult T-cell leukemia evade HTLV-1 Tax/NF-κB hyperactivation-induced senescence

Abstract

Human T-cell leukemia virus type 1 (HTLV-1) is the etiological agent of adult T-cell leukemia/lymphoma (ATL). The HTLV-1 viral trans-activator/oncoprotein Tax is a major driver of ATL, yet it induces rapid p21^Cip1/Waf1 (p21)- and p27^Kip1-mediated cellular senescence through constitutive activation (hyperactivation) of NF-κB. Although constitutive NF-κB activation is a common feature of T/B-cell leukemia/lymphoma, including ATL, it is not known how ATL cells maintain chronic NF-κB activation without undergoing senescence. Here, we demonstrate that, in contrast to HTLV-1^- T-cell lines, ATL cell lines no longer undergo Tax-induced senescence. Although Tax⁺ and Tax^- ATL cell lines showed signatures of constitutive NF-κB activation, their ability to progress through the cell cycle was unaffected. In some cases, ATL cell lines continued to proliferate despite significant upregulation of p21; additionally, many cell lines displayed altered expression of G1 and G1/S cyclins, particularly overexpression of cyclin D2. We propose that, during the course of ATL development, leukemia cells acquire genetic/epigenetic changes that can mitigate the senescence response triggered by NF-κB hyperactivation. Restoring the NF-κB-induced senescence response would likely help to control the development and progression of ATL and similar lymphoid malignancies.

Graphical abstract

Figure 1.

ATL cells are resistant to Tax-induced senescence. T cells were transduced with the HTLV-1 oncogenic protein Tax and an EGFP Tax-reporter plasmid and allowed to grow undisturbed for 7 to 10 days. Transduced T cells were monitored for proliferation in semisolid media, as described in “Materials and methods.” This experiment was repeated 3 times; representative images acquired using a 10× objective are shown.

Figure 2.

NF-κB activation and cell-cycle dysregulation in ATL and control T cells. Whole cell lysates were prepared as reported and analyzed by standard immunoblotting using the indicated antibodies. (A) Evaluation of NF-κB pathway activation. (B) Evaluation of cyclin-dependent kinase inhibitor, cyclin, and CDK expression. Each immunoblot shown used the same protein lysates; the β-actin control in panel B is applicable to panel A. Each blot was repeated ≥5 times with the same and different lysates.