Abstract 2821: Mitigation of radiation-induced colon injuries in mice by oral sepiapterin (PTC923)

The rectum is the primary dose limiting structure for radiation therapy (RT) in treating prostate cancer. Two clinical forms of rectal injury have been described: early and late. Early injury typically develops during RT and occurs to some degree in most patients. The condition is usually self-limiting, but can predispose patients to the development of late injury. Late rectal injury occurs in up to 25% of patients months to years after pelvic RT and is more concerning, since it can be irreversible and can have a major impact on a patient’s lifestyle. Development of new pharmacologic approaches to eliminate the risk of RT-dependent rectal injury remains an important goal. Although detailed mechanisms underlying normal tissue toxicities vary from tissue to tissue, the central underlying mechanism is abnormal wound healing involving endothelial dysfunction (ED) and subsequently fibrosis. Regardless of its cause, ED is characterized by an “uncoupled” endothelial NOS (eNOS) activity that generates superoxide (O2 -) and peroxynitrite (ONOO-) rather than NO. One mechanism for uncoupling in inflammatory conditions is oxidation of the reduced cofactor tetrahydrobiopterin (BH4). The product of uncoupling, ONOO-, oxidizes BH4, initiating a feed forward mechanism to sustain uncoupling. Oral treatment with sepiapterin (PTC923), a tetrahydrobiopterin precursor, decreased infiltrating inflammatory cells and cytokine levels in mice with colitis. We therefore tested whether a synthetic PTC923 might mitigate radiation-induced colon injuries. C57L/J wild-type 6-8-week-old males mice received 13.5 Gy total-body irradiation (TBI). Starting from 24 h post-irradiation, mice were treated once daily with 1 mg/kg PTC923 for six days by oral gavage. Colon injuries were accessed on 6th day after IR by colonoscopy and scored by using distal-proximal endoscopic colitis scoring system (D-PECS). The system has a core inflammatory component of four non-co-linear parameters and decimal units to notable lesions or complications. Relative expression of cytokines IL-1β, IL-6, IL-17α, and TGF-β1 in colon endothelial cells were measured by RT-PCR 8 days after IR. According with D-PECS estimation, post-irradiation colon injury was significantly reduced by PTC923 treatment: on the 6th day after TBI mice demonstrated D-PECS score 5.8±1.48 vs 1.6±0.55 D-PECS score for animals received PTC923 treatment after TBI (p-value=0.00). Increase of cytokines IL-6 and TGF-β1 in the colon endothelium after TBI was effectively blocked by PTC923 treatment (IL-6: IR+Vehicle = 4.31±1.31 fold increase vs IR+PTC923 = 1.29±0.37 fold increase (p-value=0.0016); TGF-β1: IR+Vehicle = 5.18±0.66 fold increase vs IR+PTC923 = 2.01±0.75 fold increase (p-value=0.00029)). In conclusion, these findings support the proposal that oral treatment with sepiapterin is a potential mitigator of colon injury caused by RT.

Citation Format: Vasily A. Yakovlev, Christopher S. Rabender, Ross B. Mikkelsen. Mitigation of radiation-induced colon injuries in mice by oral sepiapterin (PTC923) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 2821.

Radiation induces ESCRT pathway dependent CD44v3+ extracellular vesicle production stimulating pro-tumor fibroblast activity in breast cancer

Despite recent advances in radiotherapeutic strategies, acquired resistance remains a major obstacle, leading to tumor recurrence for many patients. Once thought to be a strictly cancer cell intrinsic property, it is becoming increasingly clear that treatment-resistance is driven in part by complex interactions between cancer cells and non-transformed cells of the tumor microenvironment. Herein, we report that radiotherapy induces the production of extracellular vesicles by breast cancer cells capable of stimulating tumor-supporting fibroblast activity, facilitating tumor survival and promoting cancer stem-like cell expansion. This pro-tumor activity was associated with fibroblast production of the paracrine signaling factor IL-6 and was dependent on the expression of the heparan sulfate proteoglycan CD44v3 on the vesicle surface. Enzymatic removal or pharmaceutical inhibition of its heparan sulfate side chains disrupted this tumor-fibroblast crosstalk. Additionally, we show that the radiation-induced production of CD44v3⁺ vesicles is effectively silenced by blocking the ESCRT pathway using a soluble pharmacological inhibitor of MDA-9/Syntenin/SDCBP PDZ1 domain activity, PDZ1i. This population of vesicles was also detected in the sera of human patients undergoing radiotherapy, therefore representing a potential biomarker for radiation therapy and providing an opportunity for clinical intervention to improve treatment outcomes.

Mitigation of Radiation-Induced Lung and Heart Injuries in Mice by Oral Sepiapterin after Irradiation

After radiation exposure, endothelium-dependent vasorelaxation is impaired due to impaired nitric oxide production. Endothelial dysfunction is characterized by uncoupled endothelial nitric oxide synthase activity, oxidation of the reduced cofactor tetrahydrobiopterin to dihydrobiopterin as one well recognized mechanism. Oral treatment with sepiapterin, a tetrahydrobiopterin precursor, decreased infiltrating inflammatory cells and cytokine levels in mice with colitis. We therefore tested whether a synthetic sepiapterin, PTC923, might mitigate radiation-induced cardiac and pulmonary injuries. C57L/J wild-type 6-8-week-old mice of both sexes received 5 Gy total-body irradiation (TBI), followed by a top-up dose of 6.5 Gy to the thorax (total thoracic dose of 11.5 Gy). Starting from 24 h postirradiation, mice were treated once daily with 1 mg/kg PTC923 for six days by oral gavage. Assessment of lung injury by breathing rate was measured every other week and echocardiography to assess heart function was performed at different time points (8, 30, 60, 90 and 180 days). Plasma proteins (fibrinogen, neutrophil elastase, C-reactive protein, and IL-6) were assessed as well. TBI induced a reduction in cardiac contractile reserve and an impairment in diastolic function restored by daily oral PTC923. Postirradiation lung injury was significantly delayed by PTC923. TBI mice treated with PTC923 experienced a longer survival compared to nonirradiated mice (71% vs. 40% of mice alive after 180 days). PTC923-treated mice showed a reduction in inflammatory mediators, especially IL-6 and IL-1b. In conclusion, these findings support the proposal that PTC923 is a potential mitigator of cardiac and lung injury caused by TBI.

Abstract A64: Exosomal miRNA as a noninvasive prediction marker of normal tissue toxicity after radiotherapy for prostate cancer

Introduction: A potential limiting feature for radiotherapy (RT) in the treatment of cancer is the toxicity that arises due to damage to normal tissues as a result of the radiation. This risk of toxicity is a particular concern in men with localized prostate cancer. Radiation-induced proctitis (RIP) is a common adverse effect of RT in treatment for prostate cancer. RIP limits radiation dose and treatment volumes, interrupts treatment, and lowers patients’ quality of life. Exosomes are 30- to 150-nm-wide nanovesicles originating from the endosomal network and are found in most body fluids. Exosomes are a fundamental driver of intercellular communication by transferring proteins, lipids, and miRNA. Exosomes miRNA signatures may serve as noninvasive prediction biomarkers of late toxicities such as RIP.

Methods: Twelve prostate cancer patients (pts) treated with curative intent with different grades of post-irradiation toxicity were enrolled on an IRB-approved study. Pts were evaluated prospectively weekly during RT and at prescribed intervals following completion of RT for the development of toxicity using a standardized instrument of physician-reported toxicity (CTCAE v4). Blood samples were collected one day before and one year after RT. The 101Bio kit was used to isolate exosomes from plasma of pts. For the miRNAs expression profiling we used ExiLENT SYBR® Green Maser Mix (Exiqon). The expression of 179 miRNA commonly found in human serum/plasma were analyzed by Serum/plasma PCR Panel (QIAGEN) for each sample.

Results: Exosomes miRNAs were analyzed from the plasma of prostate pts divided into two groups: (i) low toxicity grade 0-1 (6 pts) and (ii) high toxicity grades 2-4 (6 pts). Out of the 179 miRNAs analyzed, three miRNAs exhibited differential expression at post-RT compared to pre-RT for low toxicity pts (LTP): miR-132-5p (upregulated, p=0.001), miR-23a-3p (downregulated, p=0.020) and miR-1-3p (upregulated, p=0.047). Four miRNAs exhibited differential expression at post-RT compared to pre-RT for high toxicity pts (HTP): miR-132-5p (downregulated, p=0.003), miR-197-3p (upregulated, p=0.017), miR-151a-5p (upregulated, p=0.031), miR-18b-5p (upregulated, p=0.020). The most interesting results were observed with miR-132-5p. Before RT it showed significantly higher expression in HTP compared to the LTP. However, after RT miR-132-5p expression showed 10.76-fold increase in LTP and 7.87-fold decrease in the HTP. As a result of these dynamic changes, expression of miR-132-5p after RT became significantly higher in LTP compared to the HTP.

Conclusions: Our investigation has revealed miR-132-5p as a potential marker for risk of significant rectal injury following RT for prostate cancer, and changes in the expression of miR-1-3p, miR-18b-5p, miR-23a-3p, miR-151a-5p, and miR-197-3p may reflect the ongoing inflammatory process responsible for radiation-induced rectal injury.

Citation Format: Pratip Rana, Preetam Ghosh, Mitchell S. Anscher, Ross B. Mikkelsen, Vasily A. Yakovlev. Exosomal miRNA as a noninvasive prediction marker of normal tissue toxicity after radiotherapy for prostate cancer [abstract]. In: Proceedings of the AACR Special Conference on Advances in Liquid Biopsies; Jan 13-16, 2020; Miami, FL. Philadelphia (PA): AACR; Clin Cancer Res 2020;26(11_Suppl):Abstract nr A64.

Abstract 1802: Exosomal miRNA as a non-invasive prediction marker of normal tissue toxicity after radiotherapy for prostate cancer

Introduction: A potential limiting feature for radiotherapy (RT) in the treatment of cancer is the toxicity that arises due to damage to normal tissues as a result of the radiation. This risk of toxicity is a particular concern in men with localized prostate cancer. Radiation-induced proctitis (RIP) is a common adverse effect of RT in treatment for prostate cancer. RIP limits radiation dose and treatment volumes, interrupts treatment, and lowers patients' quality of life.

Exosomes are 30-150 nm wide nanovesicles originating from the endosomal network and are found in most body fluids. Exosomes are a fundamental driver of intercellular communication by transferring proteins, lipids and miRNA. Exosomes miRNA signatures may serve as non-invasive prediction biomarkers of late toxicities such as RIP.

Methods: Twelve prostate cancer patients (pts) treated with curative intent with different grades of post-irradiation toxicity were enrolled on an IRB approved study. Pts were evaluated prospectively weekly during RT and at prescribed intervals following completion of RT for the development of toxicity using a standardized instrument of physician reported toxicity (CTCAE v4). Blood samples were collected one day before and one year after RT. The 101Bio kit was used to isolate exosomes from plasma of pts. For the miRNAs expression profiling we used ExiLENT SYBR® Green Maser Mix (Exiqon). The expression of 179 miRNA commonly found in human serum/plasma were analyzed by Serum/plasma PCR Panel (QIAGEN) for each sample.

Results: Exosomes miRNAs were analyzed from the plasma of prostate pts divided into two groups: (i) Low toxicity grade 0-1 (6 pts), and (ii) High toxicity grades 2-4 (6 pts). Out of the 179 miRNAs analyzed, three miRNAs exhibited differential expression at post-RT compared to pre-RT for low toxicity pts (LTP): miR-132-5p (up-regulated, p=0.001), miR-23a-3p (down-regulated, p=0.020) and miR-1-3p (up-regulated, p=0.047). Four miRNAs exhibited differential expression at post-RT compared to pre-RT for high toxicity pts (HTP): miR-132-5p (down-regulated, p=0.003), miR-197-3p (up-regulated, p=0.017), miR-151a-5p (up-regulated, p=0.031), miR-18b-5p (up-regulated, p=0.020). The most interesting results were observed with miR-132-5p. Before RT it showed significantly higher expression in HTP compared to the LTP. However, after RT miR-132-5p expression showed 10.76-fold increase in LTP and 7.87-fold decrease in the HTP. As a result of these dynamic changes, expression of miR-132-5p after RT became significantly higher in LTP compared to the HTP.

Conclusions: Our investigation has revealed miR-132-5p as a potential marker for risk of significant rectal injury following RT for prostate cancer, and changes in the expression of miR-1-3p, miR-18b-5p, miR-23a-3p, miR-151a-5p, and miR-197-3p may reflect the ongoing inflammatory process responsible for radiation induced rectal injury.

Citation Format: Pratip Rana, Preetam Ghosh, Mitchell S. Anscher, Ross B. Mikkelsen, Vasily A. Yakovlev. Exosomal miRNA as a non-invasive prediction marker of normal tissue toxicity after radiotherapy for prostate cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 1802.

Sepiapterin Enhances Tumor Radio- and Chemosensitivities by Promoting Vascular Normalization

Previously, we demonstrated that nitric oxide (NO) synthase (NOS) is uncoupled in a wide range of solid tumors and that restoring NOS coupling with the tetrahydrobiopterin precursor sepiapterin (SP) inhibits tumor progression. Endothelial dysfunction characterizes the poorly functional vasculature of solid tumors, and since NO is critical for regulation of endothelial function we asked whether SP, by recoupling NOS, improves tumor vasculature structure and function-enhancing chemotherapeutic delivery and response to radiotherapy. MMTV-neu mice with spontaneous breast tumors were treated with SP by oral gavage and evaluated by multispectral optoacoustic tomographic analysis of tumor HbO2 and by tissue staining for markers of hypoxia, blood perfusion, and markers of endothelial and smooth muscle proteins. Recoupling tumor NOS activity results in vascular normalization observed as reduced tumor hypoxia, improved tumor percentage of HbO2 and perfusion, as well as increased pericyte coverage of tumor blood vessels. The normalized vasculature and improved tumor oxygenation led to a greater than 2-fold increase in radiation-induced apoptosis compared with radiation or SP alone. High-performance liquid chromatography analysis of tumor doxorubicin levels showed a greater than 50% increase in doxorubicin uptake and a synergistic effect on tumor cell apoptosis. This study highlights for the first time the importance of NOS uncoupling and endothelial dysfunction in the development of tumor vasculature and presents a new approach for improving the tumoricidal efficacies of chemotherapy and radiotherapy.

Abstract 5196: Recoupling nitric oxide synthetase to normalize tumor vasculature and enhance tumor radiosensitivity and anti-tumor drug uptake

The aberrant nature of tumor vasculature results in uneven, heterogeneous blood flow and leaky, hemorrhagic blood vessels. Due to the unusual nature of tumor vessels, areas of hypoxia develop that contribute to radioresistance and inefficiency of therapeutic drug delivery. Our current studies examine the role of nitric oxide synthase (NOS) in tumor vasculature. NOS has been demonstrated to be “uncoupled” in tumors and isolated tumor cells due to reduced levels of tetrahydrobiopterin (BH4), a necessary cofactor, resulting in superoxide and peroxynitrite formation in lieu of NO (Rabender et al 2015 Molecular Cancer Research). NO signaling is critical for vascular function and thus uncoupling of eNOS in tumor endothelial cells may partly explain the poor vasculature found within solid tumors. Having previously demonstrated that NOS can be “recoupled” and NO production restored through treatment of tumor cells with Sepiapterin (SP), a BH4 precursor, we examined whether SP could normalize tumor vasculature, promoting radiosensitivity and improving drug uptake. Multispectral optoacoustic tomography analysis of both flank tumor xenografts and a spontaneous breast tumor model (MMTV) demonstrate that SP given by oral gavage once a day for 7 days significantly enhances tumor BH4 levels and the percent of oxyhemoglobin in the tumor. Immunohistochemical analysis of tumors from mice treated with SP showed a significant reduction in CD31 staining and a significant increase in smooth muscle actin (SMA), both hallmarks of vascular normalization. A significant decrease in pimonidazole staining and concomitant increase in Hoescht staining in MMTV mice treated with SP confirms the reduction in tumor hypoxia as well as increased tumor perfusion. Increased tumor perfusion was also demonstrated through enhanced doxorubicin uptake in tumors of mice treated with SP. Lastly, the enhanced tumor oxygenation correlated with more than a 2-fold increase in radiation induced cell killing measured by ex vivo clonogenic assay. These preliminary data demonstrate the need for continued study of NOS uncoupling in solid tumors, especially when we take into consideration that SP has been demonstrated to be cytotoxic to both breast and colon tumors. On-going studies are examining the consequences of enhanced drug uptake on both normal and tumor tissue as well as the mechanism behind the vascular normalization.

Note: This abstract was not presented at the meeting.

Citation Format: Christopher S. Rabender, Sundaresan Gobalakrishnan, Li Wang, Jamal Zweit, Ross B. Mikkelsen. Recoupling nitric oxide synthetase to normalize tumor vasculature and enhance tumor radiosensitivity and anti-tumor drug uptake [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5196. doi:10.1158/1538-7445.AM2017-5196

Abstract 3800: Analysis of circulating tumor exosomes: their ability to serve as a biomarker for recurrence in lung cancer

Objectives: Exosomes (Exos) are 30-150 nm wide nanovesicles originating from the endosomal network and are found in most body fluids. Production of Exos increases in cancer, making Exos potential biomarkers. Exos are a fundamental driver of intercellular communication by transferring proteins, lipids and miRNA. Analysis of Exos and their interaction with the microenvironment may uncover cellular pathways involved with disease progression. We hypothesized that Exos may serve as a liquid biopsy for tumor recurrence.

Materials and Methods: Six lung cancer patients (LCPs) were prospectively enrolled on an IRB approved study. Exos were precipitated and purified with a Norgen kit from 0.7 ml of blood samples collected prior to, during and 6 months after radiation therapy (RT). Exos numbers were quantified by Exocet Exosome Quantitation Kit (System Biosciences). Total exosomal RNA was isolated by the Norgen RNA/DNA/Protein Purification kit. For the miRNAs expression profiling we used the miRCURY LNA™ Universal RT microRNA PCR assay (Exiqon) and ExiLENT SYBR® Green Maser Mix (Exiqon). Expressions of 10 different miRNAs (let-7a-5p, miR-20a-5p, miR-21-5p, miR-30b-5p, miR-106a-5p, miR-146a-5p, miR-155-5p, miR-200b-5p, miR-203a, miR-208a) for all samples were determined. Human lung fibroblasts (MRC-5s) in serum-free medium for 24 hrs were treated with Exos, TGF-β (positive control) or PBS (negative control) to determine their relative effects on MRC-5 proliferation.

Results: Of the 6 LCPs, 2 had limited stage small cell and 4 had non-small cell at diagnosis. Three patients were diagnosed with recurrence at the time of their post-RT blood draw. Six miRNAs demonstrated significantly different expressions between recurrent and non-recurrent groups of LCPs: let-7a-5p, miR-20a-5p, miR-21-5p, miR-106a-5p, miR-155-5p, and miR-203a. MRC-5 proliferation increases 2-fold when treated with Exos from blood collected prior to RT compared to PBS (p = 0.01). There is <2-fold increase compared to TGF-β (p = 0.03). Plasma Exos levels decrease during RT as do their relative effects on MRC-5 proliferation when compared at equivalent numbers. Exos collected at 6 months from patients with recurrence significantly stimulated MRC-5 proliferation (p = 0.03), whereas Exos from patients without recurrence do not. Western blot analyses of MRC-5 cells stimulated with Exos revealed decreased PTEN but enhanced CDC25A expression and increased AKT and SMAD phosphorylation, regardless of when the Exos were collected.

Conclusions: Recurrent and non-recurrent groups of LCPs demonstrated significant differences in exosomal miR profile before and after RT. Exos from untreated LCPs, and recurrent LCPs after RT, stimulate MRC-5 proliferation and possible conversion to CAFs. Our results suggest that Exos could serve as a “liquid biopsy” to assess the potential for recurrence when imaging modalities are equivocal.

Citation Format: Nicholas Serrano, John Blue, Asim Alam, Christopher S. Rabender, Elisabeth Weiss, Mitchell S. Anscher, Ross B. Mikkelsen, Vasily A. Yakovlev. Analysis of circulating tumor exosomes: their ability to serve as a biomarker for recurrence in lung cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 3800. doi:10.1158/1538-7445.AM2017-3800

Abstract 3383: Tumor vasculature normalization by recoupling nitric oxide synthase with a tetrahydrobiopterin precursor

The abnormal, irregular nature of tumor vasculature has been well documented. The aberrant nature of the vasculature results in uneven, heterogeneous blood flow and leaky, hemorrhagic blood vessels. Due to the unusual nature of tumor vessels, areas of hypoxia tend to develop that contribute to radioresistance and inefficiency of therapeutic drug delivery. Our current studies examine the role of nitric oxide synthase (NOS) in tumor vasculature. NOS has been demonstrated to be “uncoupled” in tumor cells due to reduced levels of tetrahydrobiopterin (BH4), a necessary cofactor, resulting in peroxynitrite (ONOO-) formation in lieu of nitric oxide (NO). NO signaling is critical for vascular function and thus uncoupling of eNOS in the endothelial cells may partly explain the poor vasculature structure found within solid tumors. Having previously demonstrated that NOS can be “recoupled” and NO production restored through treatment of tumor cells with Sepiapterin (SP), a BH4 precursor, we examined whether SP could normalize tumor vasculature, promoting radiosensitivity and improving drug uptake. Multispectral optoacoustic tomography analysis of both flank tumor xenografts and a spontaneous breast tumor model (MMTV) demonstrate that SP given orally significantly enhances the percent of oxyhemoglobin in the tumor. Immunohistochemical analysis of tumors treated with SP showed a significant reduction in CD31 staining and a significant increase in smooth muscle actin (SMA), both hallmarks of vascular normalization. Ex vivo analysis of tumors revealed that the enhanced tumor oxygenation resulted in over a two-fold increase in radiation induced cell killing. These preliminary data demonstrate great potential for SP as an adjuvant in the treatment of cancer, especially when we take into consideration that SP has been demonstrated to be cytotoxic to both breast and colon tumors. Future studies will examine drug uptake in tumors as well as the mechanism behind the vascular normalization.

Citation Format: Christopher Rabender, Ninu Bruno, Ross B. Mikkelsen. Tumor vasculature normalization by recoupling nitric oxide synthase with a tetrahydrobiopterin precursor. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3383.

A Preliminary Study on Racial Differences in HMOX1, NFE2L2, and TGFβ1 Gene Polymorphisms and Radiation-Induced Late Normal Tissue Toxicity

PURPOSE

This study tested whether racial differences in genetic polymorphisms of 4 genes involved in wound repair and response to radiation can be used to predict the occurrence of normal tissue late effects of radiation therapy and indicate potential therapeutic targets.

METHODS AND MATERIALS

This prospective study examined genetic polymorphisms that modulate the expression of 4 genes involved in inflammation and fibrosis and response to radiation (HMOX1, NFE2L2, NOS3, and TGFβ1). DNA from blood samples of 179 patients (∼ 80% breast and head and neck) collected at the time of diagnosis by their radiation oncologist as exhibiting late normal tissue toxicity was used for the analysis. Patient demographics were as follows: 56% white, 43% African American, 1% other. Allelic frequencies of the different polymorphisms of the participants were compared with those of the general American population stratified by race. Twenty-six additional patients treated with radiation, but without toxicity at 3 months or later after therapy, were also analyzed.

RESULTS

Increased frequency of a long GT repeat in the HMOX1 promoter was associated with late effects in both African American and white populations. The single nucleotide polymorphisms (SNP) rs1800469 in the TGFβ1 promoter and the rs6721961 SNP in the NFE2L2 promoter were also found to significantly associate with late effects in African Americans but not whites. A combined analysis of these polymorphisms revealed that >90% of African American patients with late effects had at least 1 of these minor alleles, and 58% had 2 or more. No statistical significance was found relating the studied NOS3 polymorphisms and normal tissue toxicity.

CONCLUSIONS

These results support a strong association between wound repair and late toxicities of radiation. The presence of these genetic risk factors can vary significantly among different ethnic groups, as demonstrated for some of the SNPs. Future studies should account for the possibility of such ethnic heterogeneity in the late toxicities of radiation.

The Role of Nitric Oxide Synthase Uncoupling in Tumor Progression

Here, evidence suggests that nitric oxide synthases (NOS) of tumor cells, in contrast with normal tissues, synthesize predominantly superoxide and peroxynitrite. Based on high-performance liquid chromatography analysis, the underlying mechanism for this uncoupling is a reduced tetrahydrobiopterin:dihydrobiopterin ratio (BH4:BH2) found in breast, colorectal, epidermoid, and head and neck tumors compared with normal tissues. Increasing BH4:BH2 and reconstitution of coupled NOS activity in breast cancer cells with the BH4 salvage pathway precursor, sepiapterin, causes significant shifts in downstream signaling, including increased cGMP-dependent protein kinase (PKG) activity, decreased β-catenin expression, and TCF4 promoter activity, and reduced NF-κB promoter activity. Sepiapterin inhibited breast tumor cell growth in vitro and in vivo as measured by a clonogenic assay, Ki67 staining, and 2[18F]fluoro-2-deoxy-D-glucose-deoxyglucose positron emission tomography (FDG-PET). In summary, using diverse tumor types, it is demonstrated that the BH4:BH2 ratio is lower in tumor tissues and, as a consequence, NOS activity generates more peroxynitrite and superoxide anion than nitric oxide, resulting in important tumor growth-promoting and antiapoptotic signaling properties.

IMPLICATIONS

The synthetic BH4, Kuvan, is used to elevate BH4:BH2 in some phenylketonuria patients and to treat diseases associated with endothelial dysfunction, suggesting a novel, testable approach for correcting an abnormality of tumor metabolism to control tumor growth.

Role of Interleukin-1 in Radiation-Induced Cardiomyopathy

Thoracic X-ray therapy (XRT), used in cancer treatment, is associated with increased risk of heart failure. XRT-mediated injury to the heart induces an inflammatory response leading to cardiomyopathy. The aim of this study was to determine the role of interleukin (IL)-1 in response to XRT injury to the heart and on the cardiomyopathy development in the mouse. Female mice with genetic deletion of the IL-1 receptor type I (IL-1R1 knockout mice [IL-1R1 KO]) and treatment with recombinant human IL-1 receptor antagonist anakinra, 10 mg/kg twice daily for 7 d, were used as independent approaches to determine the role of IL-1. Wild-type (wt) or IL-1R1 KO mice were treated with a single session of XRT (20 or 14 gray [Gy]). Echocardiography (before and after isoproterenol challenge) and left ventricular (LV) catheterization were performed to evaluate changes in LV dimensions and function. Masson's trichrome was used to assess myocardial fibrosis and pericardial thickening. After 20 Gy, the contractile reserve was impaired in wt mice at d 3, and the LV ejection fraction (EF) was reduced after 4 months when compared with sham-XRT. IL-1R1 KO mice had preserved contractile reserve at 3 d and 4 months and LVEF at 4 months after XRT. Anakinra treatment for 1 d before and 7 d after XRT prevented the impairment in contractile reserve. A significant increase in LV end-diastolic pressure, associated with increased myocardial interstitial fibrosis and pericardial thickening, was observed in wt mice, as well as in IL-1R1 KO-or anakinra-treated mice. In conclusion, induction of IL-1 by XRT mediates the development of some, such as the contractile impairment, but not all aspects of the XRT-induced cardiomyopathy, such as myocardial fibrosis or pericardial thickening.

Current status and recommendations for the future of research, teaching, and testing in the biological sciences of radiation oncology: report of the American Society for Radiation Oncology Cancer Biology/Radiation Biology Task Force, executive summary

In early 2011, a dialogue was initiated within the Board of Directors (BOD) of the American Society for Radiation Oncology (ASTRO) regarding the future of the basic sciences of the specialty, primarily focused on the current state and potential future direction of basic research within radiation oncology. After consideration of the complexity of the issues involved and the precise nature of the undertaking, in August 2011, the BOD empanelled a Cancer Biology/Radiation Biology Task Force (TF). The TF was charged with developing an accurate snapshot of the current state of basic (preclinical) research in radiation oncology from the perspective of relevance to the modern clinical practice of radiation oncology as well as the education of our trainees and attending physicians in the biological sciences. The TF was further charged with making suggestions as to critical areas of biological basic research investigation that might be most likely to maintain and build further the scientific foundation and vitality of radiation oncology as an independent and vibrant medical specialty. It was not within the scope of service of the TF to consider the quality of ongoing research efforts within the broader radiation oncology space, to presume to consider their future potential, or to discourage in any way the investigators committed to areas of interest other than those targeted. The TF charge specifically precluded consideration of research issues related to technology, physics, or clinical investigations. This document represents an Executive Summary of the Task Force report.

Sepiapterin ameliorates chemically induced murine colitis and azoxymethane-induced colon cancer

The effects of modulating tetrahydrobiopterin (BH4) levels with a metabolic precursor, sepiapterin (SP), on dextran sodium sulfate (DSS)-induced colitis and azoxymethane (AOM)-induced colorectal cancer were studied. SP in the drinking water blocks DSS-induced colitis measured as decreased disease activity index (DAI), morphologic criteria, and recovery of Ca(2+)-induced contractility responses lost as a consequence of DSS treatment. SP reduces inflammatory responses measured as the decreased number of infiltrating inflammatory macrophages and neutrophils and decreased expression of proinflammatory cytokines interleukin 1β (IL-1β), IL-6, and IL-17A. High-performance liquid chromatography analyses of colonic BH4 and its oxidized derivative 7,8-dihydrobiopterin (BH2) are inconclusive although there was a trend for lower BH4:BH2 with DSS treatment that was reversed with SP. Reduction of colonic cGMP levels by DSS was reversed with SP by a mechanism sensitive to 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), a specific inhibitor of the NO-sensitive soluble guanylate cyclase (sGC). ODQ abrogates the protective effects of SP on colitis. This plus the finding that SP reduces DSS-enhanced protein Tyr nitration are consistent with DSS-induced uncoupling of NOS. The results agree with previous studies that demonstrated inactivation of sGC in DSS-treated animals as being important in recruitment of inflammatory cells and in altered cholinergic signaling and colon motility. SP also reduces the number of colon tumors in AOM/DSS-treated mice from 7 to 1 per unit colon length. Thus, pharmacologic modulation of BH4 with currently available drugs may provide a mechanism for alleviating some forms of colitis and potentially minimizing the potential for colorectal cancer in patients with colitis.

Physical activity and sedentary behavior of cancer survivors and non-cancer individuals: results from a national survey

Increasing physical activity and decreasing sedentary behavior are associated with a higher quality of life and lower mortality rates for cancer survivors, a growing population group. Studies detailing the behavior of cancer survivors are limited. Therefore, we investigated physical activity and sedentary behavior of cancer survivors using data from the National Health and Nutrition Examination Survey (NHANES) 2007–2010. Participants were those who provided physical activity and sedentary behavior data. Those who were pregnant, <20 years old, or <3 years from their cancer diagnosis were excluded. A cancer case was a self-reported diagnosis by a physician. We identified 741 cancer survivors and 10,472 non-cancer participants. After adjustment for age, race, gender, education status, body mass index, and smoking status, cancer survivors (n = 10,472) reported significantly longer duration of sedentary behavior (OR = 1.42, 95% CI (1.12, 1.80) for 8 or more hours, p-value for trend = 0.09), compared to non-cancer participants (n = 741). They also reported non-significant increases in maximum intensity, duration, frequency, and energy expenditure, whereas they reported significant increases in moderate intensity (OR = 1.26, 95% CI (1.01, 1.57)), moderate frequency (1–4 times/week) (OR = 1.32, 95% CI (1.00, 1.74)), and moderate energy expenditure (4018.5–7623.5 kcal) (OR = 1.30, 95% CI (1.00, 1.71)) of physical activity, compared to non-cancer participants. These patterns are similar for breast and prostate cancer survivors, with prostate cancer survivors more likely to engage in physical activity for more than one hour per day (OR = 1.98, 95% CI (1.05, 3.71)). Our findings suggest that cancer survivors tend to have more physical activity, but they are also more likely to engage in sedentary behavior.

In situ vaccination with CD204 gene-silenced dendritic cell, not unmodified dendritic cell, enhances radiation therapy of prostate cancer

Given the complexity of prostate cancer progression and metastasis, multimodalities that target different aspects of tumor biology, for example, radiotherapy in conjunction with immunotherapy, may provide the best opportunities for promoting clinical benefits in patients with high-risk localized prostate cancer. Here, we show that intratumoral administration of unmodified dendritic cells (DC) failed to synergize with fractionated radiotherapy. However, ionizing radiation combined with in situ vaccination with DCs, in which the immunosuppressive scavenger receptor A (SRA/CD204) has been downregulated by lentivirus-mediated gene silencing, profoundly suppressed the growth of two mouse prostate cancers (e.g., RM1 and TRAMP-C2) and prolonged the lifespan of tumor-bearing animals. Treatment of subcutaneous tumors with this novel combinatorial radioimmunotherapeutic regimen resulted in a significant reduction in distant experimental metastases. SRA/CD204-silenced DCs were highly efficient in generating antigen or tumor-specific T cells with increased effector functions (e.g., cytokine production and tumoricidal activity). SRA/CD204 silencing-enhanced tumor cell death was associated with elevated IFN-γ levels in tumor tissue and increased tumor-infiltrating CD8(+) cells. IFN-γ neutralization or depletion of CD8(+) cells abrogated the SRA/CD204 downregulation-promoted antitumor efficacy, indicating a critical role of IFN-γ-producing CD8(+) T cells. Therefore, blocking SRA/CD204 activity significantly enhances the therapeutic potency of local radiotherapy combined with in situ DC vaccination by promoting a robust systemic antitumor immunity. Further studies are warranted to test this novel combinatorial approach for translating into improved clinical outcomes in patients with prostate cancer.

Factors influencing protein tyrosine nitration--structure-based predictive models

Models for exploring tyrosine nitration in proteins have been created based on 3D structural features of 20 proteins for which high-resolution X-ray crystallographic or NMR data are available and for which nitration of 35 total tyrosines has been experimentally proven under oxidative stress. Factors suggested in previous work to enhance nitration were examined with quantitative structural descriptors. The role of neighboring acidic and basic residues is complex: for the majority of tyrosines that are nitrated the distance to the heteroatom of the closest charged side chain corresponds to the distance needed for suspected nitrating species to form hydrogen bond bridges between the tyrosine and that charged amino acid. This suggests that such bridges play a very important role in tyrosine nitration. Nitration is generally hindered for tyrosines that are buried and for those tyrosines for which there is insufficient space for the nitro group. For in vitro nitration, closed environments with nearby heteroatoms or unsaturated centers that can stabilize radicals are somewhat favored. Four quantitative structure-based models, depending on the conditions of nitration, have been developed for predicting site-specific tyrosine nitration. The best model, relevant for both in vitro and in vivo cases, predicts 30 of 35 tyrosine nitrations (positive predictive value) and has a sensitivity of 60/71 (11 false positives).

Protein tyrosine nitration in cellular signal transduction pathways

How specificity and reversibility in tyrosine nitration are defined biologically in cellular systems is poorly understood. As more investigations identify proteins involved in cell regulatory pathways in which only a small fraction of that protein pool is modified by nitration to affect cell function, the mechanisms of biological specificity and reversal should come into focus. In this review experimental evidence has been summarized to suggest that tyrosine nitration is a highly selective modification and under certain physiological conditions fulfills the criteria of a physiologically relevant signal. It can be specific, reversible, occurs on a physiological time scale, and, depending on a target, can result in either activation or inhibition.

Nitration of the tumor suppressor protein p53 at tyrosine 327 promotes p53 oligomerization and activation

Previous studies demonstrate that nitric oxide (NO) promotes p53 transcriptional activity by a classical DNA damage responsive mechanism involving activation of ATM/ATR and phosphorylation of p53. These studies intentionally used high doses of NO donors to achieve the maximum DNA damage. However, lower concentrations of NO donors also stimulate rapid and unequivocal nuclear retention of p53 but apparently do not require ATM/ATR-dependent p53 phosphorylation or total p53 protein accumulation. To identify possible mechanisms for p53 activation at low NO levels, the role of Tyr nitration in p53 activation was evaluated. Low concentrations of the NO donor, DETA NONOate (<200 microM), exclusively nitrate Tyr327 within the tetramerization domain promoting p53 oligomerization, nuclear accumulation, and increased DNA-binding activity without p53 Ser15 phosphorylation. Molecular modeling indicates that nitration of one Tyr327 stabilizes the dimer by about 2.67 kcal mol(-1). Significant quantitative and qualitative differences in the patterns of p53-target gene modulation by low (50 microM), non-DNA-damaging and high (500 microM), DNA-damaging NO donor concentrations were shown. These results demonstrate a new posttranslational mechanism for modulating p53 transcriptional activity responsive to low NO concentrations and independent of DNA damage signaling.

Proteomic analysis of radiation-induced changes in rat lung: Modulation by the superoxide dismutase mimetic MnTE-2-PyP(5+)

PURPOSE

To identify temporal changes in protein expression in the irradiated rat lung and generate putative mechanisms underlying the radioprotective effect of the manganese superoxide dismutase mimetic MnTE-2-PyP(5+).

METHODS AND MATERIALS

Female Fischer 344 rats were irradiated to the right hemithorax with a single dose of 28 Gy and killed from day 1 to 20 weeks after irradiation. Proteomic profiling was performed to identify proteins that underwent significant changes in abundance. Some irradiated rats were administered MnTE-2-PyP(5+) and changes in protein expression and phosphorylation determined at 6 weeks after irradiation.

RESULTS

Radiation induced a biphasic stress response in the lung, as shown by the induction of heme oxygenase 1 at 1-3 days and at 6-8 weeks after irradiation. At 6-8 weeks after irradiation, the down-regulation of proteins involved in cytoskeletal architecture (filamin A and talin), antioxidant defense (biliverdin reductase and peroxiredoxin II), and cell signaling (β-catenin, annexin II, and Rho-guanosine diphosphate dissociation inhibitor) was observed. Treatment with MnTE-2-PyP(5+) partially prevented the apparent degradation of filamin and talin, reduced the level of cleaved caspases 3 and 9, and promoted Akt phosphorylation as well as β-catenin expression.

CONCLUSION

A significant down-regulation of proteins and an increase in protein markers of apoptosis were observed at the onset of lung injury in the irradiated rat lung. Treatment with MnTE-2-PyP(5+), which has been demonstrated to reduce lung injury from radiation, reduced apparent protein degradation and apoptosis indicators, suggesting that preservation of lung structural integrity and prevention of cell loss may underlie the radioprotective effect of this compound.

Abstract 1321: ERK1/2 show different activity profiles in vitro and in vivo subsequent to NOS inhibition

Nitric oxide (NO) is a crucial mediator of several survival mechanisms in tumor cells. Previous studies demonstrated that ionizing radiation (IR) stimulates NO generation activating pro-survival mechanisms including receptor tyrosine kinase and NF-κB-dependent pathways. In vitro, the IR-induced activation of these pathways survival pathways is inhibited by the NO synthase inhibitor, NG-nitro-L-arginine (L-NNA). Furthermore, the addition of L-NNA to the drinking water of flank xenograft bearing animals slows tumor progression, kills tumor cells and with radiation enhances animal survival. To elucidate the mechanism(s) by which L-NNA acts in such tumors we conducted experiments to study intracelluar signaling pathway activation by L-NNA with and without radiation.

Animals bearing flank xenografts received 10 Gy whole body irradiation either with or without prior L-NNA treatment. Tumors were excised at times post IR up to 30 minutes and protein lysates prepared for analysis by western blot. Cell monolayers were also similarly treated prior to cell lysis and protein analysis.

The activation (phosphorylation) of ERK1 and ERK2 was assayed in cultured monolayers and in flank xenografts using the A431 and FaDu cell lines. In vitro, the addition of L-NNA to the culture medium had little effect upon the activation of either ERK. In vivo however, L-NNA treatment up-regulated the phosphorylation of both ERK isoforms without irradiation and following irradiation elicited a significant enhancement of ERK1 phosphorylation and an inhibition of ERK2.

The current consensus is that ERK1 and ERK2, due to the similarities of their protein sequence, are similarly regulated and in general activate the same substrates. However, recent evidence has emerged to indicate distinct roles for ERK1/2 in addition to their overlapping actions. Indeed, the targeting of ERK2 has been seen to abolish Ras-dependent cell proliferation, an effect not observed with ERK1 targeting. Distinct signaling roles, and the opposite effects we observe upon ERK1 and ERK2 phosphorylation in vivo following L-NNA treatment may explain our previously reported findings of enhanced tumor control. In addition, these findings my shed light upon why ERK inhibitors show only moderate efficacy in clinical trials.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 1321.

Abstract 1400: Mitigation of radiation-induced injury with the nitric oxide synthase co-factor, tetrahydrobiopterin

The purpose of this study was to determine the effect of modulation of the nitric oxide (NO) and the cGMP/PKG signaling pathway on radiation-induced myofibroblast differentiation and radiation-induced injury in mice. Nitric oxide (NO) is a diverse second messenger critical for a variety of cell functions produced by nitric oxide synthases (NOS). NOS activity is regulated in part by the availability of co-factors including tetrahydrobiopterin (BH4). In the absence of BH4, NOS can become ‘uncoupled’ to produce superoxide anion which upon reaction with NO can form peroxynitrite, a highly reactive molecule known to contribute to protein tyrosine nitration. BH4 is approved for human consumption and therefore may represent a safe, viable strategy to reduce tissue injury from radiation. We show here that BH4 administration to mice prior to whole body irradiation with 9.5 Gy significantly increases mouse survival. In contrast, inhibition of NOS activity either before or after irradiation significantly reduced survival. We also show that the phosphodiesterase 5 inhibitor, sildenafil, inhibits the radiation-induced differentiation of human myofibroblasts. The mechanisms underlying the protective effects of BH4 and the role of NOS in radiation-induced tissue injury will be discussed.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 1400.

Transport of ions in erythrocytes infected by plasmodia

Throughout its erythrocytic cycle the plasmodial parasite modifies the plasma membrane of its host cell. Some changes derive from parasite metabolism. Intraerythrocytic forms use glucose at more than 10-fold normal red cell rates. The H+ accompanying the lactate end-product is exported into the host cell cytoplasm by an electrogenic proton pump in the parasite membrane. This maintains a pH greater than 7.0 in the parasite cytoplasm, but lowers erythrocyte cytoplasmic pH from approximately 7.2 to 6.5. Ca2+ transport across parasite membranes is coupled to the proton pump, possibly a Ca2+/H+ antiporter. The Ca2+, Mg2+-ATPase and Na+,K+-ATPase activities of erythrocyte membranes from schizont-infected erythrocytes have been studied. Under optimal assay conditions (pH = 7.0; [ATP] = 1 mM; +/- calmodulin) membranes from infected cells showed a 30% reduction in Ca2+,Mg2+-ATPase activity but no difference from normal in Na+,K+-ATPase activity. The calmodulin levels of infected cells were depressed by about 30%. The [ATP] in the cytoplasm of infected erythrocytes was only 0.2 mM (as against 1.3 mM in normals) and at this ATP concentration the activities of both ATPases are only 30% of normal. Shifting the pH from 7.0 to 6.5 decreases Na+,K+-ATPase activity by an additional 50% but is without effect on the Ca2+,Mg2+-ATPase. The results provide a partial explanation for the increased Ca2+ permeability and altered Na+/K+ content of plasmodia-infected erythrocytes.

Tyrosine nitration of IkappaBalpha: a novel mechanism for NF-kappaB activation

The NF-kappaB family of transcription factors is an important component of stress-activated cytoprotective signal transduction pathways. Previous studies demonstrated that some activation mechanisms require phosphorylation, ubiquitination, and degradation of the inhibitor protein, IkappaBalpha. Herein, it is demonstrated that ionizing radiation in the therapeutic dose range stimulates NF-kappaB activity by a mechanism in which IkappaBalpha tyrosine 181 is nitrated as a consequence of constitutive NO* synthase activation, leading to dissociation of intact IkappaBalpha from NF-kappaB. This mechanism does not appear to require IkappaBalpha kinase-dependent phosphorylation or proteolytic degradation of IkappaBalpha. Tyrosine 181 is involved in several noncovalent interactions with the p50 subunit of NF-kappaB stabilizing the IkappaBalpha-NF-kappaB complex. Evaluation of hydropathic interactions of the IkappaBalpha-p50 complex on the basis of the crystal structure of the complex is consistent with nitration disrupting these interactions and dissociating the IkappaBalpha-NF-kappaB complex. Tyrosine nitration is not commonly studied in the context of signal transduction. However, these results indicate that tyrosine nitration is an important post-translational regulatory modification for NF-kappaB activation and possibly for other signaling molecules modulated by mild and transient oxidative and nitrosative stresses.

Compensatory ErbB3/c-Src signaling enhances carcinoma cell survival to ionizing radiation

EGFR and ErbB2 are two members of the ErbB family of receptor Tyr Kinases identified as therapeutic targets for treating carcinomas. Breast carcinoma cells express different complements and variable proportions of ErbB receptor Tyr kinases, which activate unique and redundant signaling cascades that are essential for cell survival. Previously it was shown that a COOH-terminal truncation mutant of the EGFR (EGFR-CD533) blocks EGFR dependent signals and radiosensitizes breast carcinoma cells. In this study the effects of EGFR-CD533 and an analogous truncation mutant of ErbB2 (ErbB2-CD572) on ErbB receptor family dimerization and signaling are further investigated. Using adenoviral vectors in breast carcinoma cell lines with variable ErbB expression profiles, we demonstrate different effects for each deletion mutant. EGFR-CD533 blocks ligand stimulation of EGFR, ErbB2, and ErbB4, but is associated with a compensatory Tyr kinase activity resulting in phosphorylation of ErbB3. In contrast, ErbB2-CD572 produces a weaker, non-specific pattern of ErbB receptor family inhibition, based upon the ErbB expression pattern of the cell type. Investigation of the compensatory Tyr kinase activity associated with EGFR-CD533 expression identified an ErbB3/c-Src signaling pathway that regulates expression of anti-apoptotic Bcl family proteins. This signaling is active in the T47D cell line, which inherently over-express ErbB3, absent in MDA-MB231 cells, which have low ErbB3 expression levels, and is restored in a MDA-MB231 cell line engineered to over-express ErbB3. Furthermore we demonstrate that ErbB3/c-Src signaling is radio-protective, and that its elimination through pharmacologic inhibition of c-Src enhances radiation-induced apoptosis. In summary, these studies identify a novel ErbB3/c-Src survival signal and point to ErbB3 expression levels as an important variable in therapeutic targeting of ErbB receptors in breast carcinoma cells.

Inhibition of the type III epidermal growth factor receptor variant mutant receptor by dominant-negative EGFR-CD533 enhances malignant glioma cell radiosensitivity

PURPOSE

The commonly expressed variant epidermal growth factor receptor (EGFR), the type III EGFR variant (EGFRvIII), functions as an oncoprotein promoting neoplastic transformation and tumorigenicity. The role of EGFRvIII in cellular responses to genotoxic stress, such as ionizing radiation, is only minimally defined. Thus, we have investigated EGFRvIII as a potential modulator of cellular radiation responses and explored the feasibility of adenovirus (Ad)-mediated expression of dominant-negative EGFR-CD533 as a gene therapeutic approach for inhibiting EGFRvIII function in vitro and in vivo.

EXPERIMENTAL DESIGN AND RESULTS

EGFR-CD533 and EGFRvIII were expressed in vitro and in vivo in malignant U-373 MG glioma cells through transduction with an Ad vector, Ad-EGFR-CD533 and Ad-EGFRvIII, respectively. In vivo studies defined the importance of EGFRvIII as a modulator of radiation responses, demonstrating a 2.6-fold activation of EGFRvIII in U-373 malignant glioma tumors. Concomitant expression of EGFR-CD533 inhibited the radiation-induced activation of EGFRvIII in vitro and completely abolished the enhanced clonogenic survival conferred by EGFRvIII. The ability of EGFR-CD533 to inhibit EGFRvIII function was further confirmed in vivo through complete inhibition of EGFRvIII-mediated increased tumorigenicity and radiation-induced activation of EGFRvIII. Growth delay assays with U-373 xenograft tumors demonstrated that the expression of EGFR-CD533 significantly enhanced radiosensitivity of tumor cells under conditions of intrinsic and Ad-mediated EGFRvIII expression.

CONCLUSIONS

We conclude that EGFRvIII confers significant radioresistance to tumor cells through enhanced cytoprotective responses, and we have demonstrated that dominant-negative EGFR-CD533 effectively inhibits EGFRvIII function. These data affirm the broad potential of EGFR-CD533 to radiosensitize human malignant glioma cells.

Requirement of Tyr-992 and Tyr-1173 in phosphorylation of the epidermal growth factor receptor by ionizing radiation and modulation by SHP2

The epidermal growth factor receptor (EGFR) is activated by ionizing radiation (IR) in many human carcinomas, mediating a cytoprotective response and subsequent radioresistance. The underlying molecular mechanisms remain to be understood, and we propose here a specific role for the Tyr-992 residue of EGFR and examine its regulation by the phosphatase, SHP2. The -fold increase in phosphorylation of Tyr-992 in response to IR is twice that seen with ligand (EGF) binding. Mutation of Tyr-992 blocked completely IR-induced EGFR phosphorylation and reduced activation of the downstream signaling molecule, phospholipase Cgamma. IR has previously been demonstrated to inhibit activity of protein-tyrosine phosphatases. Following protein-tyrosine phosphatase inhibition by sodium vanadate both EGFR expressing Chinese hamster ovary (CHO) and A431 exhibited up to an 8-fold increase in the basal level of Tyr-992 phosphorylation, significantly higher than that seen with Tyr-1173, Tyr-1068, and total EGFR Tyr. CHO cells expressing a SHP2 mutant also demonstrated up to an 8-fold increase in the basal level of Tyr-992 phosphorylation. In this study we show the unique association of SHP2 with EGFR in response to IR, with up to a 2.5-fold increase in the direct association of endogenous SHP2 with EGFR-wt in response to 2 gray of IR in both CHO and A431 cells. Mutation of Tyr-992 abolished this response. In conclusion we have identified several differentially activated Tyr residues, one of which is not only more sensitive to activation by IR, translating into differential activation of downstream signaling, but uniquely modulated by the phosphatase SHP2.

Inhibition of protein-tyrosine phosphatases by mild oxidative stresses is dependent on S-nitrosylation

Previous studies have shown that a Ca(2+)-dependent nitric-oxide synthase (NOS) is activated as part of a cellular response to low doses of ionizing radiation. Genetic and pharmacological inhibitor studies linked this NO signaling to the radiation-induced activation of ERK1/2. Herein, a mechanism for the radiation-induced activation of Tyr phosphorylation-dependent pathways (e.g. ERK1/2) involving the inhibition of protein-Tyr phosphatases (PTPs) by S-nitrosylation is tested. The basis for this mechanism resides in the redox-sensitive active site Cys in PTPs. These studies also examined oxidative stress induced by low concentrations of H(2)O(2). S-Nitrosylation of total cellular PTP and immunopurified SHP-1 and SHP-2 was detected as protection of PTP enzymatic activity from alkylation by N-ethylmaleimide and reversal by ascorbate. Both radiation and H(2)O(2) protected PTP activity from alkylation by a mechanism reversible by ascorbate and inhibited by NOS inhibitors or expression of a dominant negative mutant of NOS-1. Radiation and H(2)O(2) stimulated a transient increase in cytoplasmic free [Ca(2+)]. Radiation, H(2)O(2), and the Ca(2+) ionophore, ionomycin, also stimulated NOS activity, and this was associated with an enhanced S-nitrosylation of the active site Cys(453) determined by isolation of S-nitrosylated wild type but not active site Cys(453) --> Ser SHP-1 mutant by the "biotin-switch" method. Thus, one consequence of oxidative stimulation of NO generation is S-nitrosylation and inhibition of PTPs critical in cellular signal transduction pathways. These results support the conclusion that a mild oxidative signal is converted to a nitrosative one due to the better redox signaling properties of NO.

Biological chemistry of reactive oxygen and nitrogen and radiation-induced signal transduction mechanisms

In the past few years, nuclear DNA damage-sensing mechanisms activated by ionizing radiation have been identified, including ATM/ATR and the DNA-dependent protein kinase. Less is known about sensing mechanisms for cytoplasmic ionization events and how these events influence nuclear processes. Several studies have demonstrated the importance of cytoplasmic signaling pathways in cytoprotection and mutagenesis. For cytoplasmic signaling, radiation-stimulated reactive oxygen species (ROS) and reactive nitrogen species (RNS) are essential activators of these pathways. This review summarizes recent studies on the chemistry of radiation-induced ROS/RNS generation and emphasizes interactions between ROS and RNS and the relative roles of cellular ROS/RNS generators as amplifiers of the initial ionization events. Cellular mechanisms for regulating ROS/RNS levels are discussed. The mechanisms by which cells sense ROS/RNS are examined in terms of how ROS/RNS modify protein structure and function, for example, interactions with metal-thiol clusters, protein tyrosine nitration, protein cysteine oxidation, S-thiolation and S-nitrosylation. We propose that radiation-induced ROS are the initiators and that nitric oxide (NO*) or derivatives are the effectors activating these signal transduction pathways. In responding to cellular ionization events, the cell converts an oxidative signal to a nitrosative one because ROS are too reactive and unspecific in their reactions for regulatory purposes and the cell is equipped to precisely modulate NO* levels.

Dominant-negative cAMP-responsive element-binding protein inhibits proliferating cell nuclear antigen and DNA repair, leading to increased cellular radiosensitivity

Selective inhibition of the epidermal growth factor receptor or mitogen-activated protein kinase (MAPK) results in radiosensitization of cancer cells. One potential mechanism involves cAMP-responsive element-binding protein, which is activated by radiation via the epidermal growth factor receptor/MAPK pathway and which regulates synthesis of proliferating cell nuclear antigen (PCNA), a protein involved in repair of ionizing radiation-induced DNA damage. To test for a role of CREB in cellular radiosensitivity, CHO cells were transfected with plasmids expressing dominant-negative CREB mutants (CR133 or KCREB), and various end-points were measured 48 h later. Basal levels of PCNA-CAT reporter construct activity were reduced by 60 and 40% following expression of CR133 and KCREB, respectively; similar decreases were observed in PCNA protein levels. Pulsed-field gel electrophoresis measurements showed that CR133 inhibited the repair of radiation-induced DNA double-strand breaks, and this effect was reversed by over-expression of PCNA; dominant-negative CREB also significantly inhibited split-dose recovery. Clonogenic assays were used to determine surviving fraction; the dose enhancement ratios for dominant-negative CREB-expressing cells compared with control (vector alone) were 1.5 and 1.3 for CR133 and KCREB, respectively. Importantly, co-transfection of mutant CREB and a construct constitutively expressing PCNA protein restored radiosensitivity of CHO cells back to wild-type levels. Moreover, cells expressing either CREB mutant showed no significant cell cycle redistribution. These data demonstrate that genetic disruption of CREB results in radiosensitization, and that this effect can be explained by a mechanism involving decreased PCNA expression and inhibition of DNA repair.

A simple method of producing low oxygen conditions with oxyrase for cultured cells exposed to radiation and tirapazamine

Several methods of establishing low O(2) conditions have been used in studies on the response of cultured cells to radiation and other agents. These methods, eg, gassing culture vessels with O(2)-free nitrogen with or without carbon dioxide or placing high cell-density suspensions in sealed glass ampoules to consume O(2) in the ampules, can be technically demanding and have experimental limitations. We introduce a simple, versatile, and reliable method of producing low O(2) conditions without special equipment or changes in culture conditions unrelated to hypoxia. The method is based on the ability of Oxyrase (Oxyrase, Inc., Mansfield, OH), membrane fragments prepared from Enterococcus coli, to consume O(2) in solution and is confirmed in the present study by 2 analytical methods. The effects of low O(2) conditions induced by Oxyrase on cellular responses to radiation and treatment with the bioreductive agent tirapazamine (TPZ) were examined with Chinese hamster V79 and human glioma U373 cells. Measured by clonogenic and MTT assays, these cells were less sensitive to radiation but more sensitive to TPZ in treatment media containing native Oxyrase than in media containing heat-inactivated Oxyrase. In addition, Oxyrase treatment increased the basal activity of mitogen-activated protein kinase (ERK1/2) but suppressed its activation induced by radiation. The results suggest that this method might also be useful for other in vitro cancer biologic investigations requiring a low O(2) condition.

Ionizing radiation activates Erb-B receptor dependent Akt and p70 S6 kinase signaling in carcinoma cells

In this study we have investigated the effects of low dose ionizing radiation (2 Gy) on p70 S6 kinase and Akt signaling with respect to Erb-B receptors in both the A431 squamous and the MDA-MB-231 mammary carcinoma cell lines. Ionizing radiation caused a 2-3-fold increase in p70 S6 kinase activity that was blocked pharmacologically using an EGFR inhibitor (AG1478) alone, or in combination with an Erb-B2 inhibitor (AG825). These results suggested that both EGFR and Erb-B2 receptors could initiate radiation-induced activation of p70 S6K. EGFR dependent Erb-B3 signaling also contributed to p70 S6 kinase activity through recruitment and activation of PI3K, which has been shown to regulate p70 S6 kinase activity. Furthermore, inhibition of the EGFR blocked IR stimulated increases in protein translation, a biologic consequence of p70 S6 kinase activation. We also report that ionizing radiation stimulated Akt activity that was partially independent of PI3K activity, but dependent on Erb-B2 function. Erb-B2 inhibition also correlated with enhanced apoptosis following IR exposure, suggesting an important role for Erb-B2 in cell survival. Together this work demonstrates that the Erb-B receptor tyrosine kinase network stimulates cytoprotective p70 S6 kinase and Akt activity in response to clinically relevant doses of ionizing radiation.

Activation of constitutive nitric-oxide synthase activity is an early signaling event induced by ionizing radiation

Ionizing radiation at clinical dose levels activates both pro- and anti-proliferative signal transduction pathways, the balance of which determines cell fate. The initiating and amplifying mechanisms involved in the activation are poorly understood. We demonstrate that one mechanism involves stimulation of constitutive nitric-oxide synthase (NOS) activity. NOS activity of Chinese hamster ovary cells was measured by the arginine --> citrulline conversion assay. Irradiation stimulated a transient activation of NOS with maximal activity at 5 min of post-irradiation. Western blot analysis and genetic manipulation by overexpression of wild type or dominant negative NOS mutant identify the radiation-induced isoform as NOS-1. Further evidence that NOS-1 is activated by radiation was the demonstration of radiation-induced cGMP formation in cells transiently transfected with the NO-dependent soluble guanylate cyclase. Protein Tyr nitration, a footprint of peroxynitrite formation, followed radiation exposure and was inhibited by expression of a dominant negative NOS-1 mutant. Radiation-induced ERK1/2 kinase activity, a cytoprotective response to radiation, was also blocked by inhibiting NOS activity. These experiments establish NO-dependent signal transduction pathways as being radio-responsive. Given the lipophilic and relatively stable properties of NO, these results also suggest a possible mechanism by which ionization events in one cell may activate signaling processes in adjacent cells.

Epidermal growth factor receptor as a genetic therapy target for carcinoma cell radiosensitization

BACKGROUND

Exposure of human cancer cells to ionizing radiation activates the epidermal growth factor receptor (EGFR), which, in turn, mediates a cytoprotective response that reduces the cells' sensitivity to ionizing radiation. Overexpression of a dominant-negative EGFR mutant, EGFR-CD533, disrupts the cytoprotective response by preventing radiation-induced activation of the receptor and its downstream effectors. To investigate whether gene therapy with EGFR-CD533 has the potential to increase tumor cell radiosensitivity, we introduced an adenoviral vector containing EGFR-CD533 into xenograft tumors in nude mice and evaluated the tumor response to ionizing radiation.

METHODS

Xenograft tumors established from the human mammary carcinoma cell line MDA-MB-231 were transduced via infusion with the adenoviral vector Ad-EGFR-CD533 or a control vector containing the beta-galactosidase gene, Ad-LacZ. The transduced tumors were then exposed to radiation in the therapeutic dose range, and radiation-induced EGFR activation was assessed by examining the tyrosine phosphorylation of immunoprecipitated EGFR. Radiosensitization was determined in vitro by colony-formation assays. All statistical tests were two-sided.

RESULTS

The transduction efficiency of MDA-MB-231 tumors by Ad-LacZ was 44%. Expression of EGFR-CD533 in tumors reduced radiation-induced EGFR activation by 2.94-fold (95% confidence interval [CI] = 2.23 to 4.14). The radiosensitivity of Ad-EGFR-CD533-transduced tumors was statistically significantly higher (46%; P<.001) than that of Ad-LacZ-transduced tumors, yielding a dose-enhancement ratio of 1.85 (95% CI = 1.54 to 2.51).

CONCLUSIONS

Transduction of MDA-MB-231 xenograft tumors with Ad-EGFR-CD533 conferred a dominant-negative EGFR phenotype and induced tumor radiosensitization. Therefore, disruption of EGFR function through overexpression of EGFR-CD533 may hold promise as a gene therapeutic approach to enhance the sensitivity of tumor cells to ionizing radiation.

Ionizing radiation-induced, mitochondria-dependent generation of reactive oxygen/nitrogen

Transient generation of reactive oxygen or nitrogen (ROS/RNS), detected with dihydrodichlorofluoroscein by fluorescence microscopy, occurs within minutes of exposing cells to ionizing radiation. In the 1-10 Gy dose range, the amount of ROS/RNS produced/cell is constant, but the percentage of producing cells increases with dose (20 to 80%). Reversible depolarization of the mitochondrial membrane potential () and decrease in fluorescence of a mitochondria-entrapped dye, calcein, are observed coincidentally. Radiation-induced ROS/RNS, depolarization, and calcein fluorescence decrease are inhibited by the mitochondrial permeability transition inhibitor, cyclosporin A, but not the structural analogue, cyclosporin H. Radiation-stimulated ROS/RNS is also inhibited by overexpressing the Ca(2+)-binding protein, calbindin 28K, or treating cells with an intracellular Ca(2+) chelator. Radiation-induced ROS/RNS is observed in several cell types with the exception of rho(o) cells deficient in mitochondrial electron transport. rho(o) cells show neither radiation-induced ROS/RNS production nor depolarization. We propose that radiation damage in a few mitochondria is transmitted via a reversible, Ca(2+)-dependent mitochondrial permeability transition to adjacent mitochondria with resulting enhanced ROS/RNS generation. Measurements of radiation-induced mitogen-activated protein kinase activity indicate that this sensing/amplification mechanism is necessary for activation of some cytoplasmic signaling pathways by low doses of radiation.

The relative role of ErbB1-4 receptor tyrosine kinases in radiation signal transduction responses of human carcinoma cells

Activation of the epidermal growth receptor (ErbB1) occurs within minutes of a radiation exposure. Immediate downstream consequences of this activation are currently indistinguishable from those obtained with growth factors (GF), e.g. stimulation of the pro-proliferative mitogen-activated protein kinase (MAPK). To identify potential differences, the effects of GFs and radiation on other members of the ErbB family have been compared in mammary carcinoma cell lines differing in their ErbB expression profiles. Treatment of cells with EGF (ErbB1-specific) or heregulin (ErbB4-specific) resulted in a hierarchic transactivations of ErbB2 and ErbB3 dependent on GF binding specificity. In contrast, radiation indiscriminately activated all ErbB species with the activation profile reflecting that cell's ErbB expression profile. Downstream consequences of these ErbB interactions were examined with MAPK after specifically inhibiting ErbB1 (or 4) with tyrphostin AG1478 or ErbB2 with tyrphostin AG825. MAPK activation by GFs or radiation was completely inhibited by AG1478 indicating total dependance on ErbB1 (or 4) depending on which ErbB is expressed. Inhibiting ErbB2 caused an enhanced MAPK response simulating an amplified ErbB1 (or 4) response. Thus ErbB2 is a modulator of ErbB1 (or 4) function leading to different MAPK response profiles to GF or radiation exposure.

Radiosensitization of malignant glioma cells through overexpression of dominant-negative epidermal growth factor receptor

The epidermal growth factor receptor (EGFR) plays an important role in neoplastic growth control of malignant gliomas. We have demonstrated that radiation activates EGFR Tyr-phosphorylation (EGFR Tyr-P) and the proliferation of surviving human carcinoma cells, a likely mechanism of accelerated cellular repopulation, a major cytoprotective response after radiation. We now investigate the importance of radiation-induced activation of EGFR on the radiosensitivity of the human malignant glioma cells U-87 MG and U-373 MG. The function of EGFR was inhibited through a genetic approach of transducing cells with an Adenovirus (Ad) vector containing dominant-negative (DN) EGFR-CD533 (Ad-EGFR-CD533) at efficiencies of 85-90%. The resulting cells are referred to as U-87-EGFR-CD533 and U-373-EGFR-CD533. After irradiation at 2 Gy, both of the cell lines exhibited a mean 3-fold increase in EGFR Tyr-P. The expression of EGFR-CD533 completely inhibited the radiation-induced activation of EGFR. In clonogenic survival assays after a single radiation exposure, the radiation dose for a survival of 37% (D37) for U-87-EGFR-CD533 cells was 1.4- to 1.5-fold lower, relative to cells transduced with AdLacZ or untransduced U-87 MG cells. This effect was amplified with repeated radiation exposures (3 x 2 Gy) yielding a D37 ratio of 1.8-2.0. In clonogenic survival studies with U-373 MG cells, the radiosensitizing effect of EGFR-CD533 was similar. Furthermore, in vivo studies with U-87 MG xenografts confirmed the effect of EGFR-CD533 on tumor radiosensitization (dose enhancement ratio, 1.8). We conclude that inhibition of EGFR function via Ad-mediated gene transfer of EGFR-CD533 results in significant radiosensitization. As underlying mechanism, we suggest the disruption of a major cytoprotective response involving EGFR and its downstream effectors, such as mitogen-activated protein kinase. The experiments demonstrate for the first time that radiosensitization of malignant glioma cells through disruption of EGFR function may be achieved by genetic therapy approaches.

Signal transduction and cellular radiation responses

Exposure of cells to ionizing radiation results in complex cellular responses resulting in cell death and altered proliferation states. The underlying cytotoxic, cytoprotective and cellular stress responses to radiation are mediated by existing signaling pathways, activation of which may be amplified by intrinsic cellular radical production systems. These signaling responses include the activation of plasma membrane receptors, the stimulation of cytoplasmic protein kinases, transcriptional activation, and altered cell cycle regulation. From the data presented, there is increasing evidence for the functional links between cellular signal transduction responses and DNA damage recognition and repair, cell survival, or cell death through apoptosis or reproductive mechanisms.

Ionizing radiation stimulates existing signal transduction pathways involving the activation of epidermal growth factor receptor and ERBB-3, and changes of intracellular calcium in A431 human squamous carcinoma cells

Previous studies demonstrated that ionizing radiation activates the epidermal growth factor receptor (EGFR), as measured by Tyr autophosphorylation, and induces transient increases in cytosolic free [Ca2+], [Ca2+]f. The mechanistic linkage between these events has been investigated in A431 squamous carcinoma cells with the EGFR Tyr kinase inhibitor, AG1478. EGFR autophosphorylation induced by radiation at doses of 0.5-5 Gy or EGF concentrations of 1-10 ng/ml is inhibited by >75% at 100 nM AG1478. Activation of EGFR enhances IP3 production as a result of phospholipase C (PLC) activation. At the doses used, radiation stimulates Tyr phosphorylation of both, PLCgamma and erbB-3, and also mediates the association between erbB-3 and PLCgamma not previously described. The increased erbB-3 Tyr phosphorylation is to a significant extent due to transactivation by EGFR as >70% of radiation- and EGF-induced erbB-3 Tyr phosphorylation is inhibited by AG 1478. The radiation-induced changes in [Ca2+]f are dependent upon EGFR, erbB-3 and PLCgamma activation since radiation stimulated IP3 formation and Ca2+ oscillations are inhibited by AG1478, the PLCgamma inhibitor U73122 or neutralizing antibody against an extracellular epitope of erbB-3. These results demonstrate that radiation induces qualitatively and quantitatively similar responses to EGF in stimulation of the plasma membrane-associated receptor Tyr kinases and immediate downstream effectors, such as PLCgamma and Ca2+.

Downregulation of delta CaM kinase II in human tumor cells

Over two dozen alternative splice variants of CaMK-II, the type II Ca(2+)/CaM-dependent protein kinase, are encoded from four genes (alpha, beta, gamma and delta) in mammalian cells. Isozymes of alpha and beta CaMK-II are well characterized in brain; however, an understanding of the relative endogenous levels of CaMK-II isozymes in a wide variety of non-neuronal cells has not yet been described. In this study, we have demonstrated that CaMK-II consists primarily of the 54 kDa delta CaMK-II (delta(2) or delta(C)) isozyme in rodent fibroblasts. beta and gamma CaMK-II isozymes are minor and alpha CaMK-II was not expressed. The primary delta CaMK-II in human fibroblasts and the MCF10A mammary epithelial cell line was the 52 kDa delta(4) CaMK-II, an isozyme identical to delta(2) except for a missing 21-amino-acid C-terminal tail. delta CaMK-II levels were diminished in both human and rodent fibroblasts after SV40 transformation and in the mammary adenocarcinoma MCF7 cell line when compared to MCF10A cells. In fact, most tumor cells exhibited CaMK-II specific activities which were two- to tenfold lower than in untransformed fibroblasts. We conducted complementary CaMK-II studies on the NGF-induced differentiation of rat PC-12 cells. Although no new synthesis of CaMK-II occurs, neurite outgrowth in these cells is accompanied by a preferential activation of delta CaMK-II. Endogenous delta CaMK-II has a perinuclear distribution in fibroblasts and extends along neurites in PC-12 cells. These findings point to a role for delta CaMK-II isozymes in cellular differentiation.

Dominant negative EGFR-CD533 and inhibition of MAPK modify JNK1 activation and enhance radiation toxicity of human mammary carcinoma cells

Exposure of MDA-MB-231 human mammary carcinoma cells to an ionizing radiation dose of 2 Gy results in immediate activation and Tyr phosphorylation of the epidermal growth factor receptor (EGFR). Doxycycline induced expression of a dominant negative EGFR-CD533 mutant, lacking the COOH-terminal 533 amino acids, in MDA-TR15-EGFR-CD533 cells was used to characterize intracellular signaling responses following irradiation. Within 10 min, radiation exposure caused an immediate, transient activation of mitogen activated protein kinase (MAPK) which was completely blocked by expression of EGFR-CD533. The same radiation treatment also induced an immediate activation of the c-Jun-NH2-terminal kinase 1 (JNK1) pathway that was followed by an extended rise in kinase activity after 30 min. Expression of EGFR-CD533 did not block the immediate JNK1 response but completely inhibited the later activation. Treatment of MDA-TR15-EGFR-CD533 cells with the MEK1/2 inhibitor, PD98059, resulted in approximately 70% inhibition of radiation-induced MAPK activity, and potentiated the radiation-induced increase of immediate JNK1 activation twofold. Inhibition of Ras farnesylation with a concomitant inhibition of Ras function completely blocked radiation-induced MAPK and JNK1 activation. Modulation of EGFR and MAPK functions also altered overall cellular responses of growth and apoptosis. Induction of EGFR-CD533 or treatment with PD98059 caused a 3-5-fold increase in radiation toxicity in a novel repeated radiation exposure growth assay by interfering with cell proliferation and potentiating apoptosis. In summary, this data demonstrates that both MAPK and JNK1 activation in response to radiation occur through EGFR-dependent and -independent mechanisms, and are mediated by signaling through Ras. Furthermore, we have demonstrated that radiation-induced activation of EGFR results in downstream activation of MAPK which may affect the radiosensitivity of carcinoma cells.

The inducible expression of dominant-negative epidermal growth factor receptor-CD533 results in radiosensitization of human mammary carcinoma cells

Ionizing radiation activates the epidermal growth factor receptor (EGFR) and downstream signaling involving the cytoprotective mitogen-activated protein kinase (MAPK) pathway. In our effort to investigate the role of EGFR in cellular responses to radiation, we generated mammary carcinoma cell clones, MCF-TR5-EGFR-CD533 and MDA-TR15-EGFR-CD533, that inducibly express EGFR-CD533, a truncated EGFR mutant lacking mitogenic and transformation activity. EGFR-CD533 expression inhibits radiation- and EGF-induced EGFR autophosphorylation and MAPK activation and, therefore, functions as a dominant-negative mutant without blocking the expression of EGFR or erbB-2, another member of the erbB receptor Tyr kinase family. Expression of EGFR-CD533 only minimally inhibited cell growth and did not alter radiosensitivity to single radiation exposures. However, repeated 2 Gy radiation exposures of cells, under conditions of EGFR-CD533 expression, essentially abolished their ability for subsequent cell growth. These results identify the inhibition of EGFR function through genetic manipulation as a potential therapeutic maneuver. The concept of such an intervention would be the radiosensitization of cells by counteracting a radiation-induced cytoprotective proliferation response.

Calcium-dependent stimulation of mitogen-activated protein kinase activity in A431 cells by low doses of ionizing radiation

Ionizing radiation at 2 Gy activates the epidermal growth factor receptor (EGFR) kinase activity in A431 squamous carcinoma cells and as a consequence transiently activates a downstream effector, mitogen-activated protein kinase (MAPK). A dose-response analysis shows fourfold activation 3-5 min after irradiation at 0.5 Gy with no additional activation after doses up to 4 Gy. Activation is independent of protein kinase C as defined by marginal effects of protein kinase C down-regulation and the protein kinase C inhibitor, chelerythrine. In contrast, an intracellular Ca2+ chelator (BAPTA/AM), a Ca2+ antagonist (TMB-8) and a phospholipase C inhibitor (U73223), which inhibits radiation-induced Ca2+ oscillations, all block MAPK stimulation. The upstream component, Raf-1, is also activated through a mechanism that is dependent on EGFR and Ca2+. Activation of Raf-1, monitored by tyrosine phosphorylation and co-immunoprecipitation with Ras, was inhibited by BAPTA/AM and TMB-8, indicating that the Ca2+-dependent step occurs at or before the interaction of Ras and Raf-1. Neither the Ras guanosine triphosphate exchange protein, SOS, nor Ca2+-activated tyrosine kinases linked to the MAPK pathway, focal adhesion kinase and PYK2, were stimulated by radiation. In contrast, EGF activated SOS as shown by the enhanced association of SOS with EGFR in co-immunoprecipitation experiments. These results suggest that activation of EGFR-dependent downstream signaling induced by radiation differs from that induced by the natural ligands of EGFR.

Radiation-induced proliferation of the human A431 squamous carcinoma cells is dependent on EGFR tyrosine phosphorylation

Accelerated cellular repopulation has been described as a response of tumors to fractionated irradiation in both normal tissue and tumor systems. To identify the mechanisms by which cells enhance their proliferative rate in response to clinically used doses of ionizing radiation (IR) we have studied human mammary and squamous carcinoma cells which are autocrine growth regulated by the epidermal growth factor receptor (EGFR) and its ligands, transforming growth factor-alpha and EGF. Both EGF and IR induced EGFR autophosphorylation, comparable levels of phospholipase C gamma activation as measured by inositol-1,4,5-triphosphate production, and as a consequence oscillations in cytosolic [Ca2+]. Activities of Raf-1 and mitogen-activated protein kinase (MAPK) were also stimulated by EGF and IR by Ca(2+)-dependent mechanisms. All these responses to EGF and IR were dependent upon activation of EGFR as judged by the use of the specific inhibitor of EGFR autophosphorylation, tyrphostin AG1478. Importantly, IR-induced proliferation of A431 cells was also inhibited by AG1478. This is the first report which demonstrates a link between IR-induced activation of proliferative signal transduction pathways and enhanced proliferation. We propose that accelerated repopulation of tumors whose growth is regulated by EGFR is initiated by an IR-induced EGFR activation mechanism that mimics the effects of growth factors.