Functional inactivation of p53 by HPV-E6 transformation is associated with a reduced expression of radiation-induced potentially lethal damage

The Effect of p53 Status on Radio-Sensitivity of Quiescent Tumor Cell Population Irradiated With γ-Rays at Various Dose Rates

Background

The aim of the study was to clarify the effect of p53 status of tumor cells on radio-sensitivity of solid tumors following γ-ray irradiation at various dose rates, referring to the response of intratumor quiescent (Q) cells.

Methods

Human head and neck squamous cell carcinoma cells transfected with mutant TP53 (SAS/mp53) or with neo vector (SAS/neo) were injected subcutaneously into hind legs of nude mice. Tumor bearing mice received 5-bromo-2’-deoxyuridine (BrdU) continuously to label all intratumor proliferating (P) cells. They received γ-rays at a high, middle or low dose rate. Immediately or 9 h after the high dose-rate irradiation (HDR, 2.5 Gy/min), or immediately after the middle (MDR, 0.039 Gy/min) or low (LDR, 0.00098 Gy/min) dose-rate irradiation, the tumor cells were isolated and incubated with a cytokinesis blocker, and the micronucleus (MN) frequency in cells without BrdU labeling (Q cells) was determined using immunofluorescence staining for BrdU.

Results

Following γ-ray irradiation, SAS/neo tumor cells, especially intratumor Q cells, showed a marked reduction in sensitivity due to the recovery from radiation-induced damage, compared with the total or Q cells within SAS/mp53 tumors that showed little repair capacity. The recovery capacities following γ-ray irradiation were greater in Q than total cell population and increased in the following order of 9 h after HDR < MDR < LDR. Thus, the difference in radio-sensitivity between the total (P + Q) and Q cells after γ-ray irradiation increased in the same order.

Conclusion

To secure controlling solid tumors as a whole, difference in sensitivity between total and Q tumor cells especially in solid tumors irrespective of p53 status has to be suppressed as irradiation dose rate decreases, for instance, through employing combined method for enhancing the response of Q tumor cells.

The Effect of p53 Status of Tumor Cells on Radiosensitivity of Irradiated Tumors With Carbon-Ion Beams Compared With γ-Rays or Reactor Neutron Beams

Background

The aim of the study was to clarify the effect of p53 status of tumor cells on radiosensitivity of solid tumors following accelerated carbon-ion beam irradiation compared with γ-rays or reactor neutron beams, referring to the response of intratumor quiescent (Q) cells.

Methods

Human head and neck squamous cell carcinoma cells transfected with mutant TP53 (SAS/mp53) or with neo vector (SAS/neo) were injected subcutaneously into hind legs of nude mice. Tumor-bearing mice received 5-bromo-2’-deoxyuridine (BrdU) continuously to label all intratumor proliferating (P) cells. They received γ-rays or accelerated carbon-ion beams at a high or reduced dose-rate. Other tumor-bearing mice received reactor thermal or epithermal neutrons at a reduced dose-rate. Immediately or 9 hours after the high dose-rate irradiation (HDRI), or immediately after the reduced dose-rate irradiation (RDRI), the tumor cells were isolated and incubated with a cytokinesis blocker, and the micronucleus (MN) frequency in cells without BrdU labeling (Q cells) was determined using immunofluorescence staining for BrdU.

Results

The difference in radiosensitivity between the total (P + Q) and Q cells after γ-ray irradiation was markedly reduced with reactor neutron beams or carbon-ion beams, especially with a higher linear energy transfer (LET) value. Following γ-ray irradiation, SAS/neo tumor cells, especially intratumor Q cells, showed a marked reduction in sensitivity due to the recovery from radiation-induced damage, compared with the total or Q cells within SAS/mp53 tumors that showed little repair capacity. In both total and Q cells within both SAS/neo and SAS/mp53 tumors, carbon-ion beam irradiation, especially with a higher LET, showed little recovery capacity through leaving an interval between HDRI and the assay or decreasing the dose-rate. The recovery from radiation-induced damage after γ-ray irradiation was a p53-dependent event, but little recovery was found after carbon-ion beam irradiation. With RDRI, the radiosensitivity to reactor thermal and epithermal neutron beams was slightly higher than that to carbon-ion beams.

Conclusion

For tumor control, including intratumor Q-cell control, accelerated carbon-ion beams, especially with a higher LET, and reactor thermal and epithermal neutron beams were very useful for suppressing the recovery from radiation-induced damage irrespective of p53 status of tumor cells.

Decay of γ-H2AX foci correlates with potentially lethal damage repair and P53 status in human colorectal carcinoma cells

The influence of p53 status on potentially lethal damage repair (PLDR) and DNA double-strand break (DSB) repair was studied in two isogenic human colorectal carcinoma cell lines: RKO (p53 wild-type) and RC10.1 (p53 null). They were treated with different doses of ionizing radiation, and survival and the induction of DNA-DSB were studied. PLDR was determined by using clonogenic assays and then comparing the survival of cells plated immediately with the survival of cells plated 24 h after irradiation. Doses varied from 0 to 8 Gy. Survival curves were analyzed using the linear-quadratic formula: S(D)/S(0) = exp-(αD+βD²). The γ-H2AX foci assay was used to study DNA DSB kinetics. Cells were irradiated with single doses of 0, 0.5, 1 and 2 Gy. Foci levels were studied in non-irradiated control cells and 30 min and 24 h after irradiation. Irradiation was performed with gamma rays from a ¹³⁷Cs source, with a dose rate of 0.5 Gy/min. The RKO cells show higher survival rates after delayed plating than after immediate plating, while no such difference was found for the RC10.1 cells. Functional p53 seems to be a relevant characteristic regarding PLDR for cell survival. Decay of γ-H2AX foci after exposure to ionizing radiation is associated with DSB repair. More residual foci are observed in RC10.1 than in RKO, indicating that decay of γ-H2AX foci correlates with p53 functionality and PLDR in RKO cells.

Decay of γ-H2AX foci correlates with potentially lethal damage repair in prostate cancer cells

To determine the relationship between ionizing radiation-induced levels of γ-H2AX foci and cell survival in cultured prostate cancer cell lines, three prostate cancer cell lines: LNCaP (wt TP53), DU145 (mut TP53) and PC3 (TP53 null), were studied. For γ-H2AX foci induction, cells were irradiated with a single dose of 2 Gy and foci levels were studied at 30 min and 24 h after irradiation. Cell survival was determined by clonogenic assay, directly and 24 h after irradiation with doses ranging from 0 to 8 Gy. Irradiation was performed with a Siemens Stabilipan 250 KeV X-ray machine at a dose rate of approximately 3 Gy/min. Survival curves were analyzed using the linear-quadratic model S(D)/S(0)=exp-(αD+βD2). LNCaP cells clearly demonstrated potentially lethal damage repair (PLDR) which was assessed as increased survival levels after delayed plating as compared to cells plated immediately after irradiation. DU145 cells demonstrated only a slight PLDR and PC3 cells did not show PLDR at all. Levels of γ-H2AX foci were significantly decreased in all cell lines at 24 h after irradiation, compared to levels after 30 min. The LNCaP cells which demonstrated a clear PLDR also showed the largest decay in the number of γ-H2AX foci. In addition, the PC cells which did not show PLDR had the lowest decay of γ-H2AX foci. A clear correlation was demonstrated between the degree of decay of γ-H2AX foci and PLDR.

CDDO-Me protects against space radiation-induced transformation of human colon epithelial cells

Radiation-induced carcinogenesis is a major concern both for astronauts on long-term space missions and for cancer patients being treated with therapeutic radiation. Exposure to radiation induces oxidative stress and chronic inflammation, which are critical initiators and promoters of carcinogenesis. Many studies have demonstrated that non-steroidal anti-inflammatory drugs and antioxidants can reduce the risk of radiation-induced cancer. In this study, we found that a synthetic triterpenoid, CDDO-Me (bardoxolone methyl), was able to protect human colon epithelial cells (HCECs) against radiation-induced transformation. HCECs that were immortalized by ectopic expression of hTERT and cdk4 and exhibit trisomy for chromosome 7 (a non-random chromosome change that occurs in 37% of premalignant colon adenomas) can be transformed experimentally with one combined exposure to 2 Gy of protons at 1 GeV/nucleon followed 24 h later by 50 cGy of (56)Fe ions at 1 GeV/nucleon. Transformed cells showed an increase in proliferation rate and in both anchorage-dependent and independent colony formation ability. A spectrum of chromosome aberrations was observed in transformed cells, with 40% showing loss of 17p (e.g. loss of one copy of p53). Pretreatment of cells with pharmacological doses of CDDO-Me, which has been shown to induce antioxidative as well as anti-inflammatory responses, prevented the heavy-ion-induced increase in proliferation rate and anchorage-dependent and independent colony formation efficiencies. Taken together, these results demonstrate that experimentally immortalized human colon epithelial cells with a non-random chromosome 7 trisomy are valuable premalignant cellular reagents that can be used to study radiation-induced colorectal carcinogenesis. The utility of premalignant HCECs to test novel compounds such as CDDO-Me that can be used to protect against radiation-induced neoplastic transformation is also demonstrated.

BCCIP as a prognostic marker for radiotherapy of laryngeal cancer

BACKGROUND

Recent studies have shown that BCCIP (BRCA2 and CDKN1A interacting protein) is essential for maintaining the transactivation activity of wild type p53. We analyzed the expression of BCCIP and p53 in a cohort of laryngeal cancer treated with radiotherapy and assessed whether BCCIP and p53, alone or in combination, would correlate with local control and overall survival.

METHODS

One hundred twenty-three patients treated between 1975 and 2000 for early stage (stages I and II) squamous cell carcinoma of the larynx were included in the study. Treatment consisted of radiation therapy (RT) with standard fields and fractionation to a median dose of 66Gy. Tissue was collected from pre-RT biopsies and constructed in a tissue microarray, and BCCIP expression and p53 expression were determined using immunohistochemistry.

RESULTS

Loss of expression of BCCIP in combination with normal p53 (negative p53 staining) was associated with local recurrence (RR 2.04; 95% CI 0.99-4.56, p=0.05) and poor overall survival (RR 2.09; 95% CI 1.21-4.00, p=0.008) compared to patients who did express BCCIP. Expression of BCCIP or p53 alone was not found to be independently associated with benefits in local control or overall survival.

CONCLUSIONS

This study provides clinical evidence that BCCIP contributes to outcomes in patients with laryngeal cancer treated with RT. This benefit may be a result of increased radiosensitivity in patients who have functional BCCIP and p53. These data may be used to identify sub-groups of laryngeal cancer patients who are more likely to be cured with radiotherapy.

Inhibition of potential lethal damage repair and related gene expression after carbon-ion beam irradiation to human lung cancer grown in nude mice

Using cultured and nude mouse tumor cells (IA) derived from a human lung cancer, we previously demonstrated their radiosensitivity by focusing attention on the dynamics of tumor clonogens and the early and rapid survival recovery (potential lethal damage repair: PLD repair) occurring after X-ray irradiation. To the authors' knowledge, this is the first study demonstrating gene expression in association with PLD repair after carbon-ion beam or X-ray irradiation to cancer cells. In this study we tried to detect the mechanism of DNA damage and repair of the clonogens after X-ray or carbon-ion beam irradiation. At first, colony assay method was performed after irradiation of 12 Gy of X-ray or 5 Gy of carbon-ion beam to compare the time dependent cell survival of the IA cells after each irradiation pass. Second, to search the genes causing PLD repair after irradiation of X-ray or carbon-ion beam, we evaluated gene expressions by using semi-quantitative RT-PCR with the selected 34 genes reportedly related to DNA repair. The intervals from the irradiation were 0, 6, 12 and 24 hr for colony assay method, and 0, 3, 18 hr for RT-PCR method. From the result of survival assays, significant PLD repair was not observed in carbon-ion beam as compared to X-ray irradiation. The results of RT-PCR were as follows. The gene showing significantly higher expressions after X-ray irradiation than after carbon-ion beam irradiation was PCNA. The genes showing significantly lower expressions after X-ray irradiation rather than after carbon-ion beam irradiation were RAD50, BRCA1, MRE11A, XRCC3, CHEK1, MLH1, CCNB1, CCNB2 and LIG4. We conclude that PCNA could be a likely candidate gene for PLD repair.

Differential Response to Radiation of TP53-Inactivated Cells by Overexpression of Dominant-Negative Mutant TP53 or HPVE6

The inactivation of TP53 by transfection of a dominant- negative mutated TP53 (MP53.13 cells) was compared with inactivation of TP53 by transfection with the HPV E6 gene (RC10.1 cells) with respect to PLD repair, G(1)-phase arrest, and induction of color junctions. Functional G(1) arrest was demonstrated in parental (RKO) cells with wild-type TP53, while in RC10.1 cells the G(1) arrest was eliminated. In MP53.13 cells an intermediate G(1) arrest was found. Functionality of endogenous TP53 was confirmed in RKO and MP53.13 cells by accumulation of TP53 protein and its downstream target CDKN1A (p21). Radiation survival of MP53.13 cells was higher than that of RKO cells, and PLD repair was found in RKO cells and MP53.13 cells but not in RC10.1 cells. Both with and without irradiation, the number of color junctions was 50 to 80% higher in MP53.13 cells than in RKO and RC10.1 cells. In the MP53.13 cells, the genetic instability appears to lead to more aberrations and to radioresistance. In spite of the presence of an excess of mutated TP53, wild- type TP53 functions appear to be affected only partly or not at all.

Effect of p53 activation on cell growth, thymidine kinase-1 activity, and 3′-deoxy-3′fluorothymidine uptake

The use of thymidine (TdR) and thymidine analogs such as 3'-deoxy-3'-fluorothymidine (FLT) as positron emission tomography (PET)-based tracers of tumor proliferation rate is based on the hypothesis that measurement of uptake of these nucleosides, a function primarily of thymidine kinase-1 (TK(1)) activity, provides an accurate measure of cell proliferation in tumors. Tumor growth is influenced by many factors including the oxygen concentration within tumors and whether tumor cells have been exposed to cytotoxic therapies. The p53 gene plays an important role in regulating growth under both of these conditions. The goal of this study was to investigate the influence of p53 activation on cell growth, TK(1) activity, and FLT uptake. To accomplish this, TK(1) activity, S phase fraction, and the uptake of FLT were determined in plateau-phase and exponentially growing cultures of an isogenic pair of human tumor cell lines in which p53 expression was normal or inactivated by human papilloma virus type 16 E6 expression. Ionizing radiation exposure was used to stimulate p53 activity and to induce alterations in cell cycle progression. We found that exposure of cells to ionizing radiation induced dose-dependent changes in cell cycle progression in both cell lines. The relationship between S phase percentage, TK(1) activity, and FLT uptake were essentially unchanged in the p53-normal cell line. In contrast, TK(1) activity and FLT uptake remained high in the p53-deficient variant even when S phase percentage was low due to a p53-dependent G2 arrest. We conclude that a functional p53 response is required to maintain the normal relationship between TK1 activity and S phase percentage following radiation exposure.

Repair of Potentially Lethal Damage does not Depend on Functional TP53 in Human Glioblastoma Cells

The functionality of G(1)-phase arrest was investigated in relation to repair of potentially lethal damage (PLD) in human glioblastoma Gli-06 cells. Confluent cultures were irradiated and plated for clonogenic survival either immediately or 24 h after gamma irradiation. Bivariate flow cytometry was performed to assess the distribution over the cell cycle. Levels of TP53 and CDKN1A protein were assessed with Western blotting and levels of CDKN1A mRNA with RT-PCR. Confluence significantly reduced the number of proliferating cells. Marked PLD repair was found in the absence of an intact G(1) arrest. No accumulation of TP53 was observed, and the protein was smaller than the wild-type TP53 of RKO cells. No increased expression of CDKN1A at the mRNA or protein levels was found in Gli-06 cells. The TP53 of Gli-06 cells was unable to transactivate the CDKN1A gene. From this study, it is evident that PLD repair may be present without a functional TP53 or G(1) arrest.

Loss of wild-type Trp53 protein in mouse fibroblasts leads to increased radioresistance with consequent decrease in repair of potentially lethal damage

It has been reported that the loss of function of Trp53 protein is associated with a reduction in the expression of radiation-induced potentially lethal damage (PLD). These studies, however, were carried out using either transformed or transfected cell lines, and other factors may have existed that could interfere with PLD repair. In this study, we used isogenic fibroblasts derived from Trp53 knockout mice to study radiation sensitivity, PLD repair, and repair of DNA double-strand breaks (DSBs). Experiments were carried out using wild-type (Trp53(+/+)), heterozygous (Trp53(+/-)) and homozygous mutant (Trp53(-/-)) cells. This is an ideal system because the only difference in the three cell strains is the status of the Trp53 protein. DSB repair was measured by pulsed-field-gel electrophoresis (PFGE), while radiosensitivity and PLD repair were studied using the clonogenic survival assay. Cells were irradiated in plateau phase and then trypsinized and plated either immediately or 24 h later to allow for PLD repair. The results of Western blot analyses showed that Trp53(-/-) cells expressed a putative mutant form of Trp53 that was unable to transcriptionally activate Cdkn1a (p21) protein in response to irradiation. The Trp53(-/-) cells were significantly more radioresistant than the Trp53(+/+) cells, and this was associated with a moderate reduction in PLD repair. DNA repair experiments showed no difference in DSB rejoining capability between the two cell lines. In conclusion, our results show that loss of wild-type Trp53 leads to increased radioresistance with consequent reduction in PLD repair but with no effect on DNA DSB repair.

Importance of TP53 and RB in the repair of potentially lethal damage and induction of color junctions after exposure to ionizing radiation

Repair of potentially lethal damage (PLD) was investigated in cells with functional G1-phase arrest with wild-type TP53 and wild-type RB and in cells in which G1-phase arrest was abrogated by inactivation of TP53 or RB. Confluent cultures of cells were plated for clonogenic survival assay either immediately or 24 h after irradiation. Induction of color junctions, an exchange between a painted and unpainted chromosome, was studied in chromosomes 18 and 19 after irradiation with 4 Gy gamma rays. Significant repair of PLD was found in cells carrying both wild-type TP53 and wild-type RB. In cells in which TP53 or RB was inactivated, the survival curves from immediately plated and delayed-plated cells were not significantly different. The numbers of radiation-induced color junctions in chromosomes 18 and 19 were similar in all cell lines. From this study we conclude that a functional G1-phase arrest is important for repair of PLD and that TP53 and RB do not affect the frequencies of induction of color junctions in chromosome 18 or 19.

The molecular basis of radiosensitivity and chemosensitivity in the treatment of breast cancer

The molecular basis of sensitivity to therapeutic radiation and chemotherapy is a complex product of cellular and tissue responses. Certain genetic factors can be highlighted as being of special importance in the response of breast cancers to treatment. The breast cancer susceptibility genes, BRCA1 and BRCA2, determine the phenotype of the tumor, with BRCA1- or BRCA2-deficient tumors showing marked sensitivity to ionizing radiation and drugs that produce double-strand breaks. However, the extent to which loss of BRCA1 or BRCA2 function occurs in sporadic cancer has not yet been determined. The ATM protein plays a significant role in determining the response to therapy, but how frequently the function of ATM is disrupted in breast cancer is debated. Although the p53 protein is a major determinant of the response to ionizing radiation and cytotoxic drugs, there is no consistency in how p53 affects the survival of cells, because an impairment of DNA repair is offset by reduced apoptosis. Growth factors that sustain the proliferation of breast cancer cells may impact the response to therapy by inhibiting apoptosis. Loss of cell-cycle checkpoint responses may result in increased sensitivity, particularly if the checkpoint controls the G2 transition. Overexpression of cyclin D, which shortens the duration of the G1 transition, is associated with mild radiation resistance, perhaps by inhibiting apoptosis. Overall, there is much more to be understood in the complex response of breast cancers to therapy, and many other proteins play important roles in the response to treatment. The focus of our investigation is on those genetic alterations in tumors that affect the response to therapy, which will ultimately allow strategies to achieve therapeutic gain.

Immortalization of normal adult human middle ear epithelial cells using a retrovirus containing the E6/E7 genes of human papillomavirus type 16

A human middle ear epithelial cell line (HMEEC-1) was established using human papillomavirus E6/E7 genes. HMEEC-1 has remained morphologically and phenotypically stable, even after 50 passages. The cells are anchorage-dependent and nontumorigenic when injected into nude mice. This cell line thus provides a new tool for the study of normal cell biology and the pathological processes associated with the epithelial cells of the middle ear in otitis media. HMEEC-1 will also be useful in the search for new drugs and biological agents for the treatment of otitis media.

Evidence of an association between human papillomavirus and impaired chemotherapy-induced apoptosis in cervical cancer cells

OBJECTIVES

The aim of this study was to determine cervical cancer cell sensitivity to chemotherapy-induced apoptosis based on human papillomavirus (HPV) status.

METHODS

CaSki (HPV-positive) and C33A (HPV-negative) cells were treated with camptothecin or cisplatin. Cellular viability was determined by trypan blue exclusion. Apoptotic indexes were determined by flow cytometric analysis of annexin V labeling and morphological changes. Mitochondrial release of cytochrome c was determined by immunofluorescence using confocal microscopy. Caspase 3 activation and bax expression were assessed by immunoblotting.

RESULTS

CaSki cells displayed chemoresistance to both camptothecin and cisplatin. Low response to apoptogenic stimuli was evidenced by a marginal increase in the apoptotic cell fraction after camptothecin treatment (22.9 +/- 2.56%) compared with control (17.8 +/- 1.95%). Cisplatin (14.8 +/- 1.01%) caused a slight decrease in apoptosis. Mitochondrial release of cytochrome c and cleavage of caspase 3 could not be demonstrated in CaSki cells after treatment. Despite p53 mutation, C33A cells were sensitive to the antiproliferative effects of camptothecin and cisplatin. Mean apoptotic events were 17.5 +/- 0.33 for control, 42 +/- 1.76 for cisplatin, and 38.1 +/- 0.75 for camptothecin. An intact cytochrome c pathway was demonstrated in C33A cells leading to cleavage of caspase 3 after camptothecin treatment. The constitutive bax expression demonstrated in both cell lines displayed no change after camptothecin treatment.

CONCLUSION

HPV-positive cervical cancer cells have an inherent resistance to chemotherapy-induced apoptosis. HPV-dependant inactivation of apoptotic regulators such as p53 and blockage of downstream events such as cytochrome c release and caspase 3 activation might be elemental to this cellular survival advantage provided by high-risk oncogenic papillomavirus.