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Abstract

The tumor suppressor p53 primarily functions as a transcription factor responding to a myriad of cellular stresses. It is a pivotal and pleiotropic regulator in the stress-induced cellular response networks. Diverse activities of p53 are important not only in DNA repair, induction of cell cycle arrest and apoptosis, but also in senescence, autophagy and metabolism. In cells infected with human papillomaviruses (HPVs), the viral oncoproteins E6 and E7 target the tumor suppressors p53 and pRb, respectively, for degradation and inactivation. HPV E6 and E7 synergistically act to promote uncontrolled cell divisions and inhibit apoptosis. Persistent infections with high-risk HPVs are closely linked to cervical cancer as well as other malignancies in the anogenital and oropharyngeal region. Our previous lab results found that HPV E6/E7 oncogenes are repressed under hypoxia, a condition that is frequently detected in solid tumors. Unlike the reactivation of pRb, p53 protein levels did not increase in E6/E7-repressed hypoxic HPV16-positive cancer cells, but even decreased further. The present study aimed to delineate the dynamics of p53 under hypoxic conditions as well as the mechanisms underlying this regulation and elucidate the role of p53 regulation for downstream responses and cellular outcomes/fates in hypoxic HPV16-positive cancer cells. It was revealed that despite a continuous repression of E6/E7 oncogenes, p53 did not immediately recover, but instead showed a biphasic regulation (rapid and strong depletion, then marked recovery). The initial hypoxic reduction of p53 was predominantly mediated via a lysosome-dependent mechanism. The biphasic regulation of p53 appears to serve as a survival and protective strategy of hypoxic HPV16-positive cancer cells under stress conditions. The modulation on p53 downstream target genes that coincides with p53 protein dynamics may contribute to enhance cellular adaptation to hypoxia. p53 target genes associated with terminal fates such as cell death (apoptosis) and permanent cell cycle arrest (senescence) are inactivated through p53 depletion by hypoxia, protecting cells from committing to an irreversible fate. After prolonged hypoxia, the restored p53 might be required by HPV16-positive cancer cells to maintain cellular homeostasis and select cells resistant to cell death by induction of apoptotic genes. Hypoxia-associated initial reduction of p53 facilitates the induction of autophagy, which is critical for the evasion of senescence by hypoxic HPV16-positive cancer cells. Collectively, these findings reveal a new regulation pattern of p53 by hypoxia and provide new insights into the role of p53 regulation in downstream responses and cellular adaptation to hypoxia in HPV16-positive cancer cells. This study further has implications for the development of new treatment strategies.