Activation of p53-Regulated Pro-survival Signals and Hypoxia-Independent Mitochondrial Targeting of TIGAR by E6 Oncoproteins of High-Risk HPVs
The high-risk human papillomaviruses are small, non-enveloped double-stranded DNA viruses that infect the basal epithelial keratinocytes. The majority of HPV infections are self-limiting and asymptomatic; however, in 2-4% of all infected individuals, persistent infections with high-risk HPVs lead to malignancies, including head-and-neck cancers and anogenital carcinomas, particularly cervical cancers. The HPV life cycle is strongly tied to the normal differentiation process of the epithelial tissue. Upon cell entry, the viral genome is maintained as episomal DNA within the nuclei of HPV-infected basal keratinocytes and amplified to a low copy number as the cells divide. As a part of the self-renewing nature of epithelium, while some of these cells remain in the basal layer to maintain the proliferating cell reservoir, some infected daughter cells move through the upper epithelial layers and begin the keratinocyte differentiation process. This signals virus to initiate the vegetative phase of viral DNA replication, increasing the copy number to thousands per nucleus. Since differentiation and cell proliferation are mutually exclusive events, this vegetative phase poses a paradox: HPV DNA amplification takes place in cells committed to terminal differentiation. The E6 and E7 oncoproteins of high-risk HPVs subvert many cellular pathways involved in cell cycle progression and cell proliferation to establish a compatible environment for viral replication and persistence. The viral E6 and E7 oncoproteins deregulate cell-cycle checkpoints in differentiating keratinocytes by interfering with the functions of p53 and retinoblastoma (Rb), respectively. E7 induces proteolytic degradation of Rb to force differentiating keratinocytes to re-enter S-phase. E6 targets p53 for proteolytic degradation and inhibits K120 acetylation of p53 by TIP60 histone acetyltransferase to prevent p53-induced apoptosis upon unscheduled cell proliferation. Moreover, E6 and E7 oncoproteins induce the proliferation of differentiating cells by activating c-Myc. While E6 increases the transcriptional activity of c-Myc on the hTERT promoter, E7 increases the expression of c-Myc oncoprotein by enhancing cap-independent translation of c-myc mRNA transcripts. Although these events take place as a natural course of the HPV life cycle in the infected epithelia, the long-term abrogation of these checkpoints upon persistent HPV infections leads to carcinogenesis. In contrast to most human malignancies, the p53 gene is rarely mutated in high-risk HPV+ cervical carcinomas. Moreover, several studies have shown that many HPV+ cervical cancer cell lines have a detectable level of transcriptionally active p53 despite its continuous proteolytic degradation by E6. This leads to a puzzling question regarding how HR-HPVs overcome oxidative stress and genotoxicity associated with dysregulated c-Myc in the presence of wild-type p53, a downstream target of c-Myc. Here, we demonstrate that the E6 oncoproteins of high-risk HPVs functionally cooperate with oncogenic c-Myc by increasing the p53-dependent antioxidant signaling mediated by TIGAR (Tp53-inducable glycolysis and apoptosis regulator). Our results showed that E6 induces the expression and hypoxia-independent mitochondrial targeting of TIGAR to counter ROS accumulation and DNA damage induced by oncogenic c-Myc. Our data suggested that the latter finding requires serine-phosphorylation of TIGAR through a mechanism that involves PI3K/PI5P4K/AKT signaling. Moreover, we demonstrated that TIGAR expression is necessary for c-Myc and E6 induced in vitro cellular transformation and in vivo tumorigenesis of engrafted HPV18-transformed HeLa cervical cancer cells. TIGAR was found upregulated in HPV18+ HeLa xenograft tumors and HPV16+ clinical cervical cancer isolates, which correlates with dysregulated c-Myc and detectable levels of p53. Lastly, the siRNA inhibition of TIGAR expression sensitized HPV18+ HeLa cells to the subinhibitory concentrations of chemotherapy drugs (cisplatin, doxorubicin, etoposide, and cyclophosphamide) and increased the number of apoptotic cells by having a minimal toxic effect on HPV-negative control cells. These findings suggest that TIGAR might be a novel therapeutic target for treating cervical cancer patients, and inhibition of it in combinatorial treatment strategies can significantly improve clinical outcomes.
Cell Biology, Molecular Biology
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Yapindi, Lacin, "Activation of p53-Regulated Pro-survival Signals and Hypoxia-Independent Mitochondrial Targeting of TIGAR by E6 Oncoproteins of High-Risk HPVs" (2022). Biological Sciences Theses and Dissertations. 15.
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