Abrogation of p53 function by HPV16 E6 gene delays apoptosis and enhances mutagenesis but does not alter radiosensitivity in TK6 human lymphoblast cells

Abstract

In order to gain a better understanding of the role of p53 in radiation-induced mitotic failure, apoptosis and mutagenesis, we introduced the HPV16 E6 gene via a retroviral vector into the TK6 human lymphoblast cell line which expresses wild type p53. Abrogation of p53 function by E6 resulted in a delayed and reduced apoptotic response and a moderate increase in the frequency of mutations at the thymidine kinase (tk) locus following g-irradiation, but failed to alter radiosensitivity. The apoptotic response of the E6-transduced line was intermediate between that of wild type TK6 and the WTK1 cell line. WTK1 is derived from the same parental cell line as TK6 but expresses mutant p53. The spontaneous and g-ray-induced mutation frequencies in E6-transduced TK6 cells, although higher than that of the parental TK6 cell line, were still much lower than that of the WTK1 line. No e ect on apoptosis, radiosensitivity or mutability was observed when the HPV16 E6 gene was introduced into the WTK1 cells. These results indicate that p53 does not regulate the radiosensitivity of TK6 cells through the apoptotic pathway. Furthermore, the previously observed enhanced radioresistance and mutability in WTK1 cells must be attributed to a more complex mechanism than p53 status alone.

The bovine papillomavirus type 1 E6 oncoprotein sensitizes cells to tumor necrosis factor alpha-induced apoptosis

“…There are also multiple reports that E6 has p53 independent transformation properties, implying that HPV E6 genes may uncouple cellular growth and apoptotic responses. However, both proand anti-apoptotic activities of E6 have been reported, and in some experimental systems, the apoptosis modulatory e ects were thought to represent a p53-independent function of E6 (Fan et al, 1995;Hawkins et al, 1996;Griep 1994, 1995;Puthenveetil et al, 1996;Steller et al, 1996;Thomas et al, 1996;Tsang et al, 1995;Wahl et al, 1996;Xu et al, 1995;Yu et al, 1997). Since BPV-1 E6 does not bind p53 in vitro or promote its degradation in vivo, it provides an excellent model to explore the p53-independent functions of E6.…”

“…Since E6 proteins from high-risk HPVs bind and promote the degradation of p53, it is intuitive to propose that they would suppress apoptosis. Multiple diverse studies have addressed the role of high-risk HPV E6 in apoptosis, and both proand anti-apoptosis activities of E6 have been reported (Fan et al, 1995;Hawkins et al, 1996;Griep, 1994, 1995;Puthenveetil et al, 1996;Steller et al, 1996;Thomas et al, 1996;Tsang et al, 1995;Wahl et al, 1996;Xu et al, 1995;Yu et al, 1997). The apparent inconsistency in determining the e ect of E6 on apoptosis could be due to experimental conditions.…”

Response of malignant B lymphocytes to ionizing radiation: Gene expression and genotype

“…12 Although differences in TP53 functionality have been suggested to be responsible for differences in IR-induced apoptosis and cell cycle delays among lymphoma lines, 7-9 the exact role of TP53 for the radiation response is not clear. 5,13 Tumor cells often have defects in the signal transduction for cell cycle control, DNA repair and induction of cell death that can be caused by mutations and/or amplifications/deletions of genes other than TP53. 14 It is therefore expected that the radiation response may differ considerably between tumors at the transcription level, leading to differences in the phenotypic changes.…”

Different Susceptibilities of Postmitotic Checkpoint‐proficient and ‐deficient Balb/3T3 Cells to ICRF‐193, a Catalytic Inhibitor of DNA Topoisomerase II

“…15) Furthermore, there have been conflicting reports about the relationship between defects in the p53-associated G1 checkpoint and susceptibility to the lethal effects of ionizing radiation or antitumor drugs. [16][17][18][19][20] On the one hand, defects in p53, through mutational means or otherwise, confer increased sensitivity to radiation and taxol. 18,19) In contrast, human lymphoma cells with mutated p53 demonstrate increased resistance to DNA damaging agents such as γ-irradiation, 17,20) while in other cases no apparent effect on sensitivity to radiation has been observed upon inactivation of the p53 pathway.…”

“…[16][17][18][19][20] On the one hand, defects in p53, through mutational means or otherwise, confer increased sensitivity to radiation and taxol. 18,19) In contrast, human lymphoma cells with mutated p53 demonstrate increased resistance to DNA damaging agents such as γ-irradiation, 17,20) while in other cases no apparent effect on sensitivity to radiation has been observed upon inactivation of the p53 pathway. 16) ICRF (Imperial Cancer Research Fund)-193, a catalytic inhibitor of topo II, [21][22][23][24] allows cell cycle progression without chromosome segregation, leading to the accumulation of multiploid cells.…”