Activation of I-κB kinases (IKKs) and NF-κB by the human T lymphotropic virus type 1 (HTLV-1) trans-activator/oncoprotein, Tax, is thought to promote cell proliferation and transformation. Paradoxically, expression of Tax in most cells leads to drastic up-regulation of cyclin-dependent kinase inhibitors, p21 CIP1/WAF1 and p27 KIP1, which cause p53-/pRb-independent cellular senescence. Here we demonstrate that p21 CIP1/WAF1-/p27 KIP1-mediated senescence constitutes a checkpoint against IKK/NF-κB hyper-activation. Senescence induced by Tax in HeLa cells is attenuated by mutations in Tax that reduce IKK/NF-κB activation and prevented by blocking NF-κB using a degradation-resistant mutant of I-κBα despite constitutive IKK activation. Small hairpin RNA-mediated knockdown indicates that RelA induces this senescence program by acting upstream of the anaphase promoting complex and RelB to stabilize p27 KIP1 protein and p21 CIP1/WAF1 mRNA respectively. Finally, we show that down-regulation of NF-κB by the HTLV-1 anti-sense protein, HBZ, delay or prevent the onset of Tax-induced senescence. We propose that the balance between Tax and HBZ expression determines the outcome of HTLV-1 infection. Robust HTLV-1 replication and elevated Tax expression drive IKK/NF-κB hyper-activation and trigger senescence. HBZ, however, modulates Tax-mediated viral replication and NF-κB activation, thus allowing HTLV-1-infected cells to proliferate, persist, and evolve. Finally, inactivation of the senescence checkpoint can facilitate persistent NF-κB activation and leukemogenesis.
Transcription factors of the NF-κB/Rel family are critical for the proliferation of lymphocytes and the expression of genes that mediate inflammatory and immune responses. They are often aberrantly activated in human cancers, especially leukemia, where they confer survival and proliferation advantages. Through the study of the trans-activator/oncoprotein, Tax, of the human T-lymphotropic virus type 1 (HTLV-1), we have found that persistent and potentially oncogenic activation of NF-κB triggers a defense mechanism that commits cells into senescence, an irreversible state of cell cycle arrest. This checkpoint is turned on by hyper-activated p65/RelA and is mediated by two cyclin-dependent kinase inhibitors, p21 and p27, in a p53- and pRb-independent manner. It is often impaired in cancer cells with constitutively active NF-κB. Our results anticipate that the anti-sense protein of HTLV-1, HBZ, which down-regulates NF-κB and HTLV-1 trans-activation by Tax, would mitigate or prevent Tax-induced senescence. This prediction has been borne out experimentally. Thus, Tax promotes robust HTLV-1 replication, potent NF-κB activation and senescence, while HBZ attenuates Tax-driven viral replication and NF-κB activation to allow proliferation of infected cells and persistent infection. Finally, our data support the notion that inactivation of the senescence checkpoint facilitates chronic NF-κB hyper-activation, a critical step in leukemia development.