Complex Cell Cycle Abnormalities Caused by Human T-Lymphotropic Virus Type 1 Tax

Human T-lymphotropic virus type 1 (HTLV-1) is the causative agent of adult T-cell leukemia/lymphoma (ATL), a malignancy of CD4+ T cells whose etiology is thought to be associated with the viraltrans -activator Tax. We have shown recently that Tax can drastically upregulate the expression of p27Kip1 and p21CIP1/WAF1 through protein stabilization and mRNAtrans -activation and stabilization, respectively. The Tax-induced surge in p21CIP1/WAF1 and p27Kip1 begins in S phase and results in cellular senescence. Importantly, HeLa and SupT1 T cells infected by HTLV-1 also arrest in senescence, thus challenging the notion that HTLV-1 infection causes cell proliferation. Here we use time-lapse microscopy to investigate the effect of Tax on cell cycle progression in two reporter cell lines, HeLa/18x21-EGFP and HeLa-FUCCI, that express enhanced green fluorescent protein (EGFP) under the control of 18 copies of the Tax-responsive 21-bp repeat element andf luorescentu biquitinc ellc yclei ndicators, respectively. Tax-expressing HeLa cells exhibit elongated or stalled cell cycle phases. Many of them bypass mitosis and become single senescent cells as evidenced by the expression of senescence-associated β-galactosidase. Such cells have twice the normal equivalent of cellular contents and hence are enlarged, with exaggerated nuclei. Interestingly, nocodazole treatment revealed a small variant population of HeLa/18x21-EGFP cells that could progress into mitosis normally with high levels of Tax expression, suggesting that genetic or epigenetic changes that prevent Tax-induced senescence can occur spontaneously at a detectable frequency.