Molecular mechanisms of radiation-induced accelerated repopulation

A model of cytokine shedding induced by low doses of gamma radiation

A model for sustained shedding of epidermal growth factor (EGF) in response to low doses of gamma radiation was developed based on a time delay in the feedback from mitogen-activated protein kinase (MAPK) activation to metalloprotease activity in an autocrine signaling process. We determined the kinetic parameters of our model using the data available for MAPK activation by gamma irradiation in the 1-2-Gy dose range and then showed that predictions of the model were consistent with experimental results for the kinetics of EGF shedding into the growth medium after exposure of human mammary epithelial cells to 1-5 cGy of gamma radiation in the presence of antibodies that block ligand binding to EGF receptors. The model allowed us to estimate the rate of radiation-induced cytokine release per cell from measurements of EGF concentration in the growth medium and to assess the effectiveness of EGF shedding and subsequent diffusion through the medium as a mechanism for signal transmission between hit cells and bystanders.

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.

Radiation-induced activation of a common variant of EGFR confers enhanced radioresistance

BACKGROUND AND PURPOSE

The type-III EGFR variant (EGFRvIII) is known to promote enhanced tumorigenicity. We have previously defined the importance of EGFRvIII in cellular radiation responses using Chinese hamster ovary cells (CHO). In the current study, we have extended our investigations of EGFRvIII to human tumor cells in vitro and in vivo and further verified the important role of EGFRvIII in modulating radiosensitivity.

MATERIAL AND METHODS

The cell lines MDA-MB-231, U-87 MG, A-431 and U-373 MG were used. Adenoviral (Ad) vectors were produced to overexpress EGFRvIII in vitro or in xenograft tumors in vivo. The EGFR, EGFRvIII expression and tyrosine phosphorylation (Tyr-P) levels were quantified by Western blotting. The relative radiosensitivities were assessed in vitro by standard colony formation and in vivo by tumor growth delay assays.

RESULTS

The presence of EGFRvIII was verified in all xenograft tumors tested with no detectable expression in the corresponding cells under in vitro culture conditions. MDA-MB-231 xenograft tumors demonstrated EGFRvIII expression levels, which were 1.9-fold higher relative to EGFRwt compared to a 14.5-fold higher Tyr-P. Ionizing radiation of these tumors at 4 Gy induced an average 3.2-fold increase in EGFRvIII Tyr-P. EGFRvIII expression in U-373 MG cells significantly enhanced survival after 4Gy, which was completely abolished by dominant-negative EGFR-CD533. Finally, the ability of EGFRvIII to accelerate tumor growth during irradiation was confirmed in vivo.

CONCLUSION

EGFRvIII is frequently expressed in a variety of different tumor types and can confer significant radioresistance, thus further providing evidence for EGFRvIII as an additional important target in our approaches to radiosensitize malignant solid tumors.

H-RAS V12–induced radioresistance in HCT116 colon carcinoma cells is heregulin dependent

The abilities of mutated active K-RAS and H-RAS proteins, in an isogenic human carcinoma cell system, to modulate the activity of signaling pathways following exposure to ionizing radiation is unknown. Loss of K-RAS D13 expression in HCT116 colorectal carcinoma cells blunted basal extracellular signal-regulated kinase 1/2 (ERK1/2), AKT, and c-Jun NH2-terminal kinase 1/2 activity. Deletion of the allele to express K-RAS D13 also enhanced expression of ERBB1, ERBB3, and heregulin but nearly abolished radiation-induced activation of all signaling pathways. Expression of H-RAS V12 in HCT116 cells lacking an activated RAS molecule (H-RAS V12 cells) restored basal ERK1/2 and AKT activity to that observed in parental cells but did not restore or alter basal c-jun NH2-terminal kinase 1/2 activity. In parental cells, radiation caused stronger ERK1/2 pathway activation compared with that of the phosphatidylinositol 3-kinase (PI3K)/AKT pathway, which correlated with constitutive translocation of Raf-1 into the plasma membrane of parental cells. Inhibition of mitogen-activated protein kinase/ERK1/2, but not PI3K, radiosensitized parental cells. In H-RAS V12 cells, radiation caused stronger PI3K/AKT pathway activation compared with that of the ERK1/2 pathway, which correlated with H-RAS V12–dependent translocation of PI3K into the plasma membrane. Inhibition of PI3K, but not mitogen-activated protein kinase/ERK1/2, radiosensitized H-RAS V12 cells. Radiation-induced activation of the PI3K/AKT pathway in H-RAS V12 cells 2 to 24 hours after exposure was dependent on heregulin-stimulated ERBB3 association with membrane-localized PI3K. Neutralization of heregulin function abolished radiation-induced AKT activation and reverted the radiosensitivity of H-RAS V12 cells to those levels found in cells lacking expression of any active RAS protein. These findings show that H-RAS V12 and K-RAS D13 differentially regulate radiation-induced signaling pathway function. In HCT116 cells expressing H-RAS V12, PI3K-dependent radioresistance is mediated by both H-RAS-dependent translocation of PI3K into the plasma membrane and heregulin-induced activation of membrane-localized PI3K via ERBB3.

Ethanol-induced in vitro invasion of breast cancer cells: the contribution of MMP-2 by fibroblasts

Ethanol is a tumor promoter and may promote metastasis of breast cancer. However, the underlying cellular/molecular mechanisms remain unknown. Overexpression and high activity of matrix metalloproteinase-2 (MMP-2) are frequently associated with metastatic breast cancers and serve as a prognostic indicator of clinical outcome. MMP-2 is predominantly expressed in stromal fibroblasts and plays a pivotal role in regulating the invasive behavior of breast tumor cells. We hypothesized that ethanol may enhance the invasion of breast tumor cells by modulating the activity of fibroblastic MMP-2. With in vitro models (HS68 and CCD1056SK human fibroblasts), we showed that ethanol at physiologically relevant concentrations (50-200 mg/dl) activated MMP-2; conversely, at a higher concentration (400 mg/dl), it inhibited the MMP-2 activity. Consistently, conditioned medium collected from ethanol (50-200 mg/dl)-exposed fibroblasts markedly enhanced the invasive potential of breast cancer cells and mammary epithelial cells overexpressing ErbB2/HER2 (BT474, SKBR-3 and HB2(ErbB2) cells) but had little effect on cells with low ErbB2 levels (BT20 and HB2 cells). In contrast, conditioned medium obtained from ethanol (400 mg/dl)-treated fibroblasts inhibited cell invasion. Selective inhibitors of MMP-2 (SB-3CT and OA-Hy) eliminated ethanol-stimulated invasion, indicating that the effect of ethanol was mediated by MMP-2. Ethanol activated conventional PKCs and JNKs in fibroblasts; inhibitors of PKC (Go6850 and Go6976) and JNKs (SP600125) significantly inhibited ethanol-mediated MMP-2 activation as well as cell invasion, indicating that PKCs and JNKs play a role in ethanol-induced MMP-2 activation and cell invasion in vitro. Thus, ethanol-promoted breast cancer cell invasion may be mediated by the modulation of fibroblastic MMP-2.

Differential activation of the phosphatidylinositol 3'-kinase/Akt survival pathway by ionizing radiation in tumor and primary endothelial cells

Ionizing radiation induces an intracellular stress response via activation of the phosphatidylinositol 3'-kinase (PI3K)/Akt survival pathway. In tumor cells, the PI3K/Akt pathway is induced through activation of members of ErbB receptor tyrosine kinases. Here, we investigated the receptor dependence of radiation-induced PI3K/Akt activation in tumor cells and in endothelial cells. The integrity of both the ErbB and the vascular endothelial growth factor (VEGF) ligand-activated PI3K/Akt pathway in endothelial cells was demonstrated using specific ErbB and VEGF receptor tyrosine kinase inhibitors. Irradiation of endothelial cells resulted in protein kinase B (PKB)/Akt activation in a similar time course as observed in response to VEGF. More importantly, radiation-induced PKB/Akt phosphorylation in endothelial cells was strongly down-regulated by the VEGF receptor tyrosine kinase inhibitor, whereas the ErbB receptor tyrosine kinase inhibitor did not affect PKB/Akt stimulation in response to irradiation. An opposite receptor dependence for radiation-induced PKB/Akt phosphorylation was observed in ErbB receptor-overexpressing A431 tumor cells. Furthermore, direct VEGF receptor phosphorylation was detected after irradiation in endothelial cells in absence of VEGF, which was almost completely inhibited after irradiation in presence of the VEGF receptor tyrosine kinase inhibitor. These data demonstrate that ionizing radiation induces VEGF ligand-independent but VEGF receptor-dependent PKB/Akt activation in endothelial cells and that PI3K/Akt pathway activation by radiation occurs in a differential cell type and receptor-dependent pattern.

Adjuvant inhibition of the epidermal growth factor receptor after fractionated irradiation of FaDu human squamous cell carcinoma

BACKGROUND AND PURPOSE

Experiments performed by others have shown that inhibition of EGFR before and after single dose irradiation prolonged growth delay and improved local tumour control. This suggests that adjuvant EGFR inhibition can inactivate clonogens that survived irradiation. To test this hypothesis local tumour control was investigated after fractionated radiotherapy and adjuvant EGFR-TK inhibition.

MATERIALS AND METHODS

FaDu hSCC xenografts were irradiated with 30 fractions in 6 weeks with total doses of 30-100Gy. After the end of fractionated irradiation, BIBX1382BS was administered daily orally over a time period of 75 days. Tumour volumes were determined two times per week, the volume doubling time during adjuvant treatment was calculated for progressing and recurrent tumours. Local tumour control was investigated 120 days after end of irradiation.

RESULTS

Adjuvant BIBX1382BS significantly reduced the tumour growth rate but did not improve local tumour control. The TCD(50) values were 66.1Gy (95% C.I.: 59; 73Gy) after adjuvant BIBX1382BS treatment and 67.9Gy (61; 75Gy) for control tumours (P=0.9).

CONCLUSIONS

These data indicate that, although growth of recurrent tumour cells after irradiation is dependent on the EGFR pathway, tumour cells retain their clonogenic potential despite of EGFR inhibition. The results imply also that a decreased tumour growth rate does not necessarily allow conclusions on enhanced inactivation of clonogenic cells when antiproliferative drugs are combined with radiation.

p53 activation in chronic radiation-treated breast cancer cells: regulation of MDM2/p14ARF

Mammalian cells chronically exposed to ionizing radiation (IR) induce stress response with a tolerance to the subsequent cytotoxicity of IR. Although p53 is well documented in IR response, the signaling network causing p53 activation in chronic IR remains to be identified. Using breast carcinoma MCF+FIR cells that showed a transient radioresistance after exposure chronically to fractionated IR (FIR), the present study shows that the basal DNA binding and transcriptional activity of p53 was elevated by FIR. p53-controlled luciferase activity was strikingly induced ( approximately 7.9-fold) with little enhancement of p53/DNA binding activity ( approximately 1.3-fold). The phosphorylated p53 (Thr 55) was increased in the cytoplasm and nucleus of MCF+FIR but not in the sham-FIR control cells. On the contrary, the sham-FIR control MCF-7 cells showed a low p53 luciferase transcription ( approximately 3-fold) but a striking enhancement of p53/DNA binding (12-fold) after 5 Gy of IR. To determine the signaling elements regulating p53 activity, DNA microarray of MCF+FIR using sham-FIR MCF-7 cells as a reference demonstrated that the mRNA of p21, MDM2, and p14ARF was up-regulated. Time course Western blot analysis, however, showed no difference in p21 induction. In contrast, MDM2 that was absent in control cells and was predominantly induced by IR was not induced in MCF+FIR cells. In agreement with MDM2 inhibition, MDM2-inhibitory protein p14ARF was increased in MCF+FIR cells. In summary, these results demonstrate that up-regulation of p14ARF paralleled with MDM2 inhibition contributes to p53 accumulation in the nucleus and causes a high responsiveness of p53 in chronic IR-treated breast cancer cells.

ERBB receptor tyrosine kinases and cellular radiation responses

Ionizing radiation induces in autocrine growth-regulated carcinoma and malignant glioma cells powerful cytoprotective responses that confer relative resistance to consecutive radiation exposures. Understanding the mechanisms of these responses should provide new molecular targets for tumor radiosensitization. ERBB and other receptor Tyr kinases have been identified as immediate early response gene products that are activated by radiation within minutes, as by their physiological growth factor ligands, and induce secondary stimulation of cytoplasmic protein kinase cascades. The simultaneous activation of all receptor Tyr kinases and nonreceptor Tyr kinases leads to complex cytoprotective responses including increased cell proliferation, reduced apoptosis and enhanced DNA repair. Since these responses contribute to cellular radioresistance, ERBB1, the most extensively studied ERBB receptor, is examined as a target for tumor cell radiosensitization. The three methods of ERBB1 inhibition include blockade of growth factor binding by monoclonal antibody against the ligand-binding domain, inhibition of the receptor Tyr kinase-mediating receptor activation, and overexpression of a dominant-negative epidermal growth factor receptor-CD533 that lacks the COOH-terminal 533 amino acids and forms nonfunctional heterodimeric complexes with wild-type receptors. All the three approaches enhance radiation toxicity in vitro and in vivo. The different mechanisms of inhibition have contributed to the understanding of cellular responses to radiation, vary in relative effectiveness and pose different challenges for translation.

EGFRvIII-mediated radioresistance through a strong cytoprotective response

The constitutively active, truncated epidermal growth factor receptor EGFRvIII lacks the ability of EGF binding due to a deletion of the NH(2)-terminal domain. EGFRvIII confers increased tumorigenicity, is coexpressed with EGFR wild type (wt) in human carcinoma and malignant glioma cells when grown as xenografts, but is not expressed in vitro. The effects of EGFRvIII expression on cellular radiation responses were studied in Chinese hamster ovary (CHO) cells transfected with plasmids expressing EGFRvIII (CHO.EGFRvIII) or EGFRwt (CHO.EGFRwt). CHO cells expressing similar levels of either receptor were employed to define their roles in response to EGF and ionizing radiation. EGF activated EGFRwt with no effect on EGFRvIII. In contrast, a single radiation exposure of 2 Gy resulted in a 2.8- and 4.3-fold increase in Tyr phosphorylation of EGFRwt and EGFRvIII, respectively. Downstream consequences of this radiation-induced activation were examined by inhibiting EGFRwt and EGFRvIII with AG1478 (kinase inhibitor). The radiation-induced 8.5-fold activation of the pro-proliferative mitogen-activated protein kinase and the 3.2-fold stimulation of the antiapoptotic AKT/phosphatidylinositol-3-kinase pathways by EGFRvIII far exceeded that in CHO.EGFR wt cells. Thus, based on colony formation and apoptosis assays, EGFRvIII expression conferred a stronger cytoprotective response to radiation than EGFRwt, resulting in relative radioresistance. Therefore, disabling EGFRvIII in addition to EGFRwt needs to be considered in any therapeutic approach aimed at targeting EGFR for tumor cell radiosensitization.

Overexpression of ErbB2 enhances ethanol-stimulated intracellular signaling and invasion of human mammary epithelial and breast cancer cells in vitro

Both epidemiological and experimental studies indicate that ethanol is a tumor promoter and may promote metastasis of breast cancer. However, the molecular mechanisms underlying ethanol-mediated tumor promotion remain unknown. Overexpression of ErbB proteins in breast cancer patients is generally associated with poor prognosis. The ErbB proteins are a family of receptor kinases that include four closely related members: epidermal growth factor receptor (EGFR/ErbB1), ErbB2/neu, ErbB3, and ErbB4. Particularly, ErbB2 plays a pivotal role in ErbB-mediated activities. Here we demonstrated that amplification of ErbB2 expression sensitized a specific cellular response to ethanol. Human breast cancer cells or mammary epithelial cells with a high expression of ErbB2 exhibited an enhanced response to ethanol-stimulated cell invasion in vitro. Ethanol also stimulated cell proliferation; however, this stimulation was independent of ErbB2 levels. Ethanol triggered divergent intracellular signaling among cells expressing different ErbB2 levels. In the cells overexpressing ErbB2, ethanol was more effective in the activation of c-Jun NH2 terminal protein kinases (JNKs) and p38 mitogen-activated protein kinase (p38 MAPK) as well as the induction of reactive oxygen species (ROS) than the cells with normal ErbB2 expression. Blockage of either JNKs or p38 MAPK activation eliminated ethanol-mediated cell invasion. In contrast, the reduction of hydrogen peroxide concentration by catalase exposure had little effect on ethanol-induced cell invasion. These results indicated that ethanol-induced cell invasion was primarily mediated by JNKs and p38 MAPK, whereas the involvement of ROS formation might be minimal. Our study suggests that overexpression of ErbB2 may augment ethanol-elicited signaling and promote ethanol-stimulated tumor metastasis.

Combined-modality treatment of solid tumors using radiotherapy and molecular targeted agents

PURPOSE

Molecular targeted agents have been combined with radiotherapy (RT) in recent clinical trials in an effort to optimize the therapeutic index of RT. The appeal of this strategy lies in their potential target specificity and clinically acceptable toxicity.

DESIGN

This article integrates the salient, published research findings into the underlying molecular mechanisms, preclinical efficacy, and clinical applicability of combining RT with molecular targeted agents. These agents include inhibitors of intracellular signal transduction molecules, modulators of apoptosis, inhibitors of cell cycle checkpoints control, antiangiogenic agents, and cyclo-oxygenase-2 inhibitors.

RESULTS

Molecular targeted agents can have direct effects on the cytoprotective and cytotoxic pathways implicated in the cellular response to ionizing radiation (IR). These pathways involve cellular proliferation, DNA repair, cell cycle progression, nuclear transcription, tumor angiogenesis, and prostanoid-associated inflammation. These pathways can also converge to alter RT-induced apoptosis, terminal growth arrest, and reproductive cell death. Pharmacologic modulation of these pathways may potentially enhance tumor response to RT though inhibition of tumor repopulation, improvement of tumor oxygenation, redistribution during the cell cycle, and alteration of intrinsic tumor radiosensitivity.

CONCLUSION

Combining RT and molecular targeted agents is a rational approach in the treatment of solid tumors. Translation of this approach from promising preclinical data to clinical trials is actively underway.

Manganese superoxide dismutase-mediated gene expression in radiation-induced adaptive responses

Antioxidant enzymes are critical in oxidative stress responses. Radioresistant variants isolated from MCF-7 human carcinoma cells following fractionated ionizing radiation (MCF+FIR cells) or overexpression of manganese superoxide dismutase (MCF+SOD cells) demonstrated dose-modifying factors at 10% isosurvival of 1.8 and 2.3, respectively. MCF+FIR and MCF-7 cells (exposed to single-dose radiation) demonstrated 5- to 10-fold increases in MnSOD activity, mRNA, and immunoreactive protein. Radioresistance in MCF+FIR and MCF+SOD cells was reduced following expression of antisense MnSOD. DNA microarray analysis and immunoblotting identified p21, Myc, 14-3-3 zeta, cyclin A, cyclin B1, and GADD153 as genes constitutively overexpressed (2- to 10-fold) in both MCF+FIR and MCF+SOD cells. Radiation-induced expression of these six genes was suppressed in fibroblasts from Sod2 knockout mice (-/-) as well as in MCF+FIR and MCF+SOD cells expressing antisense MnSOD. Inhibiting NF-kappa B transcriptional activity in MCF+FIR cells, by using mutant I kappa B alpha, inhibited radioresistance as well as reducing steady-state levels of MnSOD, 14-3-3 zeta, GADD153, cyclin A, and cyclin B1 mRNA. In contrast, mutant I kappa B alpha was unable to inhibit radioresistance or reduce 14-3-3 zeta, GADD153, cyclin A, and cyclin B1 mRNAs in MCF+SOD cells, where MnSOD overexpression was independent of NF-kappa B. These results support the hypothesis that NF-kappa B is capable of regulating the expression of MnSOD, which in turn is capable of increasing the expression of genes that participate in radiation-induced adaptive responses.

Organ preservation trials for laryngeal cancer

Organ-preservation strategies include definitive radiation therapy alone, induction chemotherapy followed by radiotherapy, and concurrent chemoradiotherapy. Over the past decade, induction chemotherapy followed by radiotherapy has been the standard for the nonsurgical management of advanced laryngeal cancer. Over this same period, however, other nonsurgical strategies have been under evaluation. These approaches include radiotherapy with concurrent chemotherapy to take advantage of the radiosensitizing properties of most cytotoxic drugs with activity against squamous cell cancer, altered fractionation radiotherapy, and the incorporation of molecularly targeted therapeutics into multimodality treatment.

Induction or suppression of expression of cytochrome C oxidase subunit II by heregulin beta 1 in human mammary epithelial cells is dependent on the levels of ErbB2 expression

The ErbB family of receptor kinases is composed of four members: epidermal growth factor receptor (EGFR/ErbB1), ErbB2/neu, ErbB3, and ErbB4. Amplification of the ErbB2/neu is found in about 30% of breast cancer patients and is associated with a poor prognosis. Heregulin (HRG) activates the ErbB2 via induction of heterodimerization with ErbB3 and ErbB4 receptors. With suppression subtractive hybridization, we demonstrated that the expression of cytochrome c oxidase subunit II (COXII) is HRG-responsive. Two nontransformed human mammary epithelial cell lines, the HB2 and the HB2(ErbB2) (the HB2 engineered to overexpress ErbB2), displayed an opposite response to HRG-mediated regulation. HRG upregulated mRNA expression of COXII in the HB2 cells, but suppressed COXII expression in the HB2(ErbB2) cells. A human breast cancer cell line (T47D), which expresses ErbB2 at a level similar to that of the HB2 cells, also responded to HRG by increasing COXII mRNA levels. Therefore, HRG regulation of COXII expression depends on the levels of ErbB2 expression. Furthermore, the expression of COXII was inversely correlated to the levels of ErbB2, i.e., the cells overexpressing ErbB2 exhibited lower COXII levels. HRG-evoked signal transduction differed between the cells with normal ErbB expression and the cells overexpressing ErbB2. The activation of both ERK and PI3-K was essential for HRG regulation of COXII, i.e., blockage of either pathway eliminated HRG-mediated alteration. This is the first report demonstrating that the expression of mitochondria-encoded COXII is HRG-responsive. The levels of ErbB2 expression are decisive for the diverse biological activities of HRG.

Epidermal growth factor receptor dependence of radiation-induced transcription factor activation in human breast carcinoma cells

Ionizing radiation (1-5 Gy) activates the epidermal growth factor receptor (EGFR), a major effector of the p42/44 mitogen-activated protein kinase (MAPK) pathway. MAPK and its downstream effector, p90 ribosomal S6 kinase (p90RSK), phosphorylate transcription factors involved in cell proliferation. To establish the role of the EGFR/MAPK pathway in radiation-induced transcription factor activation, MDA-MB-231 human breast carcinoma cells were examined using specific inhibitors of signaling pathways. Gel-shift analysis revealed three different profile groups: 1) transcription factors that responded to both radiation (2 Gy) and epidermal growth factor (EGF) (CREB, Egr, Ets, and Stat3); 2) factors that responded to radiation, but not EGF (C/EBP and Stat1); and 3) those that did not respond significantly to either radiation or EGF (AP-1 and Myc). Within groups 1 and 2, a two- to fivefold maximum stimulation of binding activity was observed at 30-60 min after irradiation. Interestingly, only transcription factors that responded to EGF had radiation responses significantly inhibited by the EGFR tyrosine kinase inhibitor, AG1478; these responses were also abrogated by farnesyltransferase inhibitor (FTI) or PD98059, inhibitors of Ras and MEK1/2, respectively. Moreover, radiation-induced increases in CREB and p90RSK phosphorylation and activation of Stat3 and Egr-1 reporter constructs by radiation were all abolished by AG1478. These data demonstrate a distinct radiation response profile at the transcriptional level that is dependent on enhanced EGFR/Ras/MAPK signaling.

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.

Expression of transforming growth factor-alpha, epidermal growth factor receptor and platelet-derived growth factors A and B in oropharyngeal cancers treated by curative radiation therapy

BACKGROUND AND PURPOSE

Epidermal growth factor receptor (EGFR) has been implicated in cellular responses to ionizing radiation and represents a major target for current radiosensitizing strategies. We wished to ascertain whether a correlation existed between the expression of EGFR, transforming growth factor-alpha (TGFalpha) and platelet-derived growth factors A and B (PDGF-A and PDGF-B) and treatment outcome in a group of patients with oropharyngeal cancer who had undergone curative radiation therapy. We also assessed the relationship existing between each of the aforementioned proteins and intratumoral microvessel densities (IMD) which have been previously reported (Int J Radiat Oncol Biol Phys 2000;48:17-25.

MATERIALS AND METHODS

Pretherapeutic tumor biopsies from 95 patients were immunohistochemically stained and their immunoreactivities evaluated semi-quantitatively. The statistical analyses included Cox regression for calculating risk ratios of survival endpoints and logistic regression for determining odds ratios for the development of distant metastasis.

RESULTS

Local tumor control as well as disease-free and overall survival were independent of protein expression levels, whereas combined TGFalpha and EGFR immunoreactivities were closely related to IMD (P = 0.003). The expression levels of these two proteins were also correlated to each other (P = 0.015). Expression of PDGF-B occurred in 54% of cases and was associated with an increase in the risk of developing distant metastasis (P = 0.011).

CONCLUSIONS

Tumoral levels of TGFalpha, EGFR and PDGF-A/B are not predictive of radioresponsiveness in oropharyngeal cancers. The association between IMD and immunoreactivity for TGFalpha and EGFR indicates the involvement of these proteins in the promotion of angiogenesis in these tumors. PDGF-B should be further evaluated as a prognostic marker for squamous cell cancer of the head and neck.

Autocrine loops with positive feedback enable context-dependent cell signaling

We describe a mechanism for context-dependent cell signaling mediated by autocrine loops with positive feedback. We demonstrate that the composition of the extracellular medium can critically influence the intracellular signaling dynamics induced by extracellular stimuli. Specifically, in the epidermal growth factor receptor (EGFR) system, amplitude and duration of mitogen-activated protein kinase (MAPK) activation are modulated by the positive-feedback loop formed by the EGFR, the Ras-MAPK signaling pathway, and a ligand-releasing protease. The signaling response to a transient input is short-lived when most of the released ligand is lost to the cellular microenvironment by diffusion and/or interaction with an extracellular ligand-binding component. In contrast, the response is prolonged or persistent in a cell that is efficient in recapturing the endogenous ligand. To study functional capabilities of autocrine loops, we have developed a mathematical model that accounts for ligand release, transport, binding, and intracellular signaling. We find that context-dependent signaling arises as a result of dynamic interaction between the parts of an autocrine loop. Using the model, we can directly interpret experimental observations on context-dependent responses of autocrine cells to ionizing radiation. In human carcinoma cells, MAPK signaling patterns induced by a short pulse of ionizing radiation can be transient or sustained, depending on cell type and composition of the extracellular medium. On the basis of our model, we propose that autocrine loops in this, and potentially other, growth factor and cytokine systems may serve as modules for context-dependent cell signaling.

Adenovirus-mediated overexpression of dominant negative epidermal growth factor receptor-CD533 as a gene therapeutic approach radiosensitizes human carcinoma and malignant glioma cells

PURPOSE

Epidermal growth factor receptor (EGFR) and other members of the ErbB family of receptor tyrosine kinases (RTK) mediate autocrine growth regulation in a wide spectrum of human tumor cells. We have previously demonstrated that in stably transfected mammary carcinoma cells a dominant negative (DN) mutant of EGFR, EGFR-CD533 is a potent inhibitor of EGFR and its cytoprotective signaling after exposure to ionizing radiation. In the present study, we further investigate the capacity of a genetic approach, using replication-incompetent adenovirus (Ad)-mediated transfer of EGFR-CD533 (Ad-EGFR-CD533), to enhance the radiosensitivity in vitro of four cell lines representative of three major cancer phenotypes.

METHODS AND MATERIALS

The cell lines MDA-MB-231 and T-47D mammary carcinoma, A-431 squamous carcinoma, and U-373 MG malignant glioma cells were used. The ErbB expression profiles and the EGFR tyrosine phosphorylation (Tyr-P) levels following irradiation were quantified by Western blotting. The relative radiosensitivities of tumor cells were assessed by standard colony formation assays after infection with control vector (Ad-LacZ) or Ad-EGFR-CD533.

RESULTS

The expression profiles demonstrated varying levels of EGFR, ErbB2, ErbB3, and ErbB4 expression. The overexpression of EGFR-CD533 after infection with Ad-EGFR-CD533 completely inhibited the radiation-induced stimulation of EGFR Tyr-P relative to the immediate 2.4- to 3.1-fold increases in EGFR Tyr-P in control infected cells (Ad-LacZ). Ad-EGFR-CD533-infected cells demonstrated significant (p < 0.001) radiosensitization over a range of radiation doses (1-8 Gy), yielding dose-enhancement ratios (DER) between 1.4 and 1.7. This radiosensitization was maintained under conditions of repeated radiation exposures, using 3 x 2 Gy, yielding DERs of 1.6 and 1.7 for MDA-MB-231 and U-373 cells, respectively.

CONCLUSIONS

Overexpression of EGFR-CD533 significantly sensitizes human carcinoma and glioma cells to single and repeated radiation exposures irrespective of their ErbB expression levels. Therefore, transduction of human tumor cells with EGFR-CD533 holds promise as a gene therapeutic approach for the radiosensitization of neoplastic cells that are growth-regulated by EGFR or other ErbB receptors.

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.

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.