Activation of the pp90rsk and mitogen-activated serine/threonine protein kinases by ionizing radiation

Irradiation of Epithelial Carcinoma Cells Upregulates Calcium-Binding Proteins That Promote Survival under Hypoxic Conditions

Hypoxia is thought to promote tumor radio-resistance via effects on gene expression in cancer cells that modulate their metabolism, proliferation, and DNA repair pathways to enhance survival. Here we demonstrate for the first time that under hypoxic condition A431 epithelial carcinoma cells exhibit increased viability when exposed to low-dose γ-irradiation, indicating that radiotherapy can promote tumor cell survival when oxygen supply is limited. When assessed using iTRAQ quantitative proteomics and Western blotting, irradiated tumor cells were observed to significantly up-regulate the expression of calcium-binding proteins CALM1, CALU, and RCN1, suggesting important roles for these mediators in promoting tumor cell survival during hypoxia. Accordingly, shRNA-knockdown of CALM1, CALU, and RCN1 expression reduced hypoxic tumor cell resistance to low-dose radiation and increased apoptosis. These data indicate that γ-irradiation of hypoxic tumor cells induces up-regulation of calcium-binding proteins that promote cancer cell survival and may limit the efficacy of radiotherapy in the clinic.

MEK inhibitor GSK1120212-mediated radiosensitization of pancreatic cancer cells involves inhibition of DNA double-strand break repair pathways

PURPOSE

Over 90% of pancreatic adenocarcinoma PC express oncogenic mutant KRAS that constitutively activates the Raf-MEK-MAPK pathway conferring resistance to both radiation and chemotherapy. MEK inhibitors have shown promising anti-tumor responses in recent preclinical and clinical studies, and are currently being tested in combination with radiation in clinical trials. Here, we have evaluated the radiosensitizing potential of a novel MEK1/2 inhibitor GSK1120212 (GSK212,or trametinib) and evaluated whether MEK1/2 inhibition alters DNA repair mechanisms in multiple PC cell lines.

METHODS

Radiosensitization and DNA double-strand break (DSB) repair were evaluated by clonogenic assays, comet assay, nuclear foci formation (γH2AX, DNA-PK, 53BP1, BRCA1, and RAD51), and by functional GFP-reporter assays for homologous recombination (HR) and non-homologous end-joining (NHEJ). Expression and activation of DNA repair proteins were measured by immunoblotting.

RESULTS

GSK212 blocked ERK1/2 activity and radiosensitized multiple KRAS mutant PC cell lines. Prolonged pre-treatment with GSK212 for 24-48 hours was required to observe significant radiosensitization. GSK212 treatment resulted in delayed resolution of DNA damage by comet assays and persistent γH2AX nuclear foci. GSK212 treatment also resulted in altered BRCA1, RAD51, DNA-PK, and 53BP1 nuclear foci appearance and resolution after radiation. Using functional reporters, GSK212 caused repression of both HR and NHEJ repair activity. Moreover, GSK212 suppressed the expression and activation of a number of DSB repair pathway intermediates including BRCA1, DNA-PK, RAD51, RRM2, and Chk-1.

CONCLUSION

GSK212 confers radiosensitization to KRAS-driven PC cells by suppressing major DNA-DSB repair pathways. These data provide support for the combination of MEK1/2 inhibition and radiation in the treatment of PC.

Altering the response to radiation: sensitizers and protectors

A number of agents are used clinically to enhance the efficacy of radiotherapy today, many of which are cytotoxic chemotherapies. Agents that enhance radiation induced tumor cell killing or protect normal tissues from the deleterious effects of ionizing radiation are collectively termed radiation modifiers. A significant effort in radiobiological research is geared towards describing and testing radiation modifiers with the intent of enhancing the therapeutic effects of radiation while minimizing normal tissue toxicity. In this review, we discuss the characteristics of these agents, the testing required to translate these agents into clinical trials, and highlight some challenges in these efforts.

Mitogen-activated protein kinases and their role in radiation response

Ionizing radiation, like a variety of other cellular stress factors, can activate or down-regulate multiple signaling pathways, leading to either increased cell death or increased cell proliferation. Modulation of the signaling process, however, depends on the cell type, radiation dose, and culture conditions. The mitogen-activated protein kinase (MAPK) pathway transduces signals from the cell membrane to the nucleus in response to a variety of different stimuli and participates in various intracellular signaling pathways that control a wide spectrum of cellular processes, including growth, differentiation, and stress responses, and is known to have a key role in cancer progression. Multiple signal transduction pathways stimulated by ionizing radiation are mediated by the MAPK superfamily including the extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase (JNK), and p38 MAPK. The ERK pathway, activated by mitogenic stimuli such as growth factors, cytokines, and phorbol esters, plays a major role in regulating cell growth, survival, and differentiation. In contrast, JNK and p38 MAPK are weakly activated by growth factors but respond strongly to stress signals including tumor necrosis factor (TNF), interleukin-1, ionizing and ultraviolet radiation, hyperosmotic stress, and chemotherapeutic drugs. Activation of JNK and p38 MAPK by stress stimuli is strongly associated with apoptotic cell death. MAPK signaling is also known to potentially influence tumor cell radiosensitivity because of their activity associated with radiation-induced DNA damage response. This review will discuss the MAPK signaling pathways and their roles in cellular radiation responses.

Candidate protein biodosimeters of human exposure to ionizing radiation

PURPOSE

To conduct a literature review of candidate protein biomarkers for individual radiation biodosimetry of exposure to ionizing radiation.

MATERIALS AND METHODS

Reviewed approximately 300 publications (1973 - April 2006) that reported protein effects in mammalian systems after either in vivo or in vitro radiation exposure.

RESULTS

We found 261 radiation-responsive proteins including 173 human proteins. Most of the studies used high doses of ionizing radiation (>4 Gy) and had no information on dose- or time-responses. The majority of the proteins showed increased amounts or changes in phosphorylation states within 24 h after exposure (range: 1.5- to 10-fold). Of the 47 proteins that are responsive at doses of 1 Gy and below, 6 showed phosphorylation changes at doses below 10 cGy. Proteins were assigned to 9 groups based on consistency of response across species, dose- and time-response information and known role in the radiation damage response.

CONCLUSIONS

ATM (Ataxia telengiectasia mutated), H2AX (histone 2AX), CDKN1A (Cyclin-dependent kinase inhibitor 1A), and TP53 (tumor protein 53) are top candidate radiation protein biomarkers. Furthermore, we recommend a panel of protein biomarkers, each with different dose and time optima, to improve individual radiation biodosimetry for discriminating between low-, moderate-, and high-dose exposures. Our findings have applications for early triage and follow-up medical assessments.

In vitro and in vivo radiosensitization with AZD6244 (ARRY-142886), an inhibitor of mitogen-activated protein kinase/extracellular signal-regulated kinase 1/2 kinase

PURPOSE

The mitogen-activated protein (MAP) kinase pathway is important for cell proliferation, survival, and differentiation, and is frequently up-regulated in cancers. The MAP kinase pathway is also activated after exposure to ionizing radiation. We investigated the effects of AZD6244 (ARRY-142886), an inhibitor of MAP kinase/extracellular signal-regulated kinase 1/2, on radiation response.

EXPERIMENTAL DESIGN

The effects of AZD6244 on the in vitro radiosensitivity of human cancer cell lines (A549, MiaPaCa2, and DU145) were evaluated using clonogenic assays. DNA damage repair was evaluated using gammaH2AX, and mitotic catastrophe was measured using nuclear fragmentation. Cell cycle effects were measured with flow cytometry. Growth delay was used to evaluate the effects of AZD6244 on in vivo tumor radiosensitivity.

RESULTS

Exposure of each cell line to AZD6244 before irradiation resulted in an increase in radiosensitivity with dose enhancement factors at a surviving fraction of 0.1, ranging from 1.16 to 2.0. No effects of AZD6244 on radiation-induced apoptosis or persistence of gammaH2AX foci after irradiation were detected. Cells treated with AZD6244 had an increased mitotic index and decreased Chk1 phosphorylation at 1 and 2 hours after irradiation. Mitotic catastrophe was increased in cells receiving AZD6244 and irradiation compared with the single treatments. In vivo studies revealed that AZD6244 administration to mice bearing A549 tumor xenografts resulted in a greater than additive increase in radiation-induced tumor growth delay (dose enhancement factor of 3.38).

CONCLUSIONS

These results indicate that AZD6244 can enhance tumor cell radiosensitivity in vitro and in vivo and suggest that this effect involves an increase in mitotic catastrophe.

Gene expression profiles in the liver of mice irradiated with (60)Co gamma rays and treated with soybean isoflavone

PURPOSE

To better understand the molecular mechanisms underlying the radio-protective effect of soybean isoflavone that we observed in our recent animal experiments.

MATERIALS AND METHODS

We utilized a cDNA microarray to investigate the expression profiles of 4,096 known genes in the livers of irradiated-mice with or without soybean isoflavone treatment. Dye swap approach was employed to control for gene-specific dye bias and quantitative real-time RT-PCR was performed on several genes to validate the cDNA microarray data.

RESULTS

Compared with the control group, 68 genes were up-regulated and 28 genes were down-regulated in mice treated with irradiation alone, whereas only 6 genes were down-regulated and 35 genes were up-regulated in mice treated with soybean isoflavone. Interestingly, some of the down-regulated genes in the irradiated group, such as DNA repair and stress response genes and cytoskeleton-associated genes, which are markers of cellular damage after irradiation, were maintained at close to normal expression levels after soybean isoflavone treatment.

CONCLUSIONS

Comparison of gene expression profiles in the livers of irradiated-mice treated with or without soybean isoflavone suggested that soybean isoflavone may be an efficient tool to reverse irradiation damage of the liver through multiple-pathways and also provides important clues to further pursue the molecular mechanisms underlying the radio-protective activity of soybean isoflavone.

Gene expression profiles of normal human fibroblasts after exposure to ionizing radiation: a comparative study of low and high doses

Several types of cellular responses to ionizing radiation, such as the adaptive response or the bystander effect, suggest that low-dose radiation may possess characteristics that distinguish it from its high-dose counterpart. Accumulated evidence also implies that the biological effects of low-dose and high-dose ionizing radiation are not linearly distributed. We have investigated, for the first time, global gene expression changes induced by ionizing radiation at doses as low as 2 cGy and have compared this to expression changes at 4 Gy. We applied cDNA microarray analyses to G1-arrested normal human skin fibroblasts subjected to X irradiation. Our data suggest that both qualitative and quantitative differences exist between gene expression profiles induced by 2 cGy and 4 Gy. The predominant functional groups responding to low-dose radiation are those involved in cell-cell signaling, signal transduction, development and DNA damage responses. At high dose, the responding genes are involved in apoptosis and cell proliferation. Interestingly, several genes, such as cytoskeleton components ANLN and KRT15 and cell-cell signaling genes GRAP2 and GPR51, were found to respond to low-dose radiation but not to high-dose radiation. Pathways that are specifically activated by low-dose radiation were also evident. These quantitative and qualitative differences in gene expression changes may help explain the non-linear correlation of biological effects of ionizing radiation from low dose to high dose.

ATM and the catalytic subunit of DNA-dependent protein kinase activate NF-kappaB through a common MEK/extracellular signal-regulated kinase/p90(rsk) signaling pathway in response to distinct forms of DNA damage

We have identified a novel pathway of ataxia telangiectasia mutated (ATM) and DNA-dependent protein kinase (DNA-PK) signaling that results in nuclear factor kappaB (NF-kappaB) activation and chemoresistance in response to DNA damage. We show that the anthracycline doxorubicin (DOX) and its congener N-benzyladriamycin (AD 288) selectively activate ATM and DNA-PK, respectively. Both ATM and DNA-PK promote sequential activation of the mitogen-activated protein kinase (MAPK)/p90(rsk) signaling cascade in a p53-independent fashion. In turn, p90(rsk) interacts with the IkappaB kinase 2 (IKK-2) catalytic subunit of IKK, thereby inducing NF-kappaB activity and cell survival. Collectively, our findings suggest that distinct members of the phosphatidylinositol kinase family activate a common prosurvival MAPK/IKK/NF-kappaB pathway that opposes the apoptotic response following DNA damage.

Suppression of WEE1 and Stimulation of CDC25A Correlates with Endothelin-dependent Proliferation of Rat Aortic Smooth Muscle Cells

Proliferation of vascular smooth muscle cells plays a key role in the pathogenesis of several disorders of the vascular wall. Endothelin (ET), a vasoactive peptide that signals through a G protein-coupled receptor, has been linked to mitogenesis in vascular smooth muscle cells, but the mechanistic details underlying this activity remain incompletely understood. In the present study, we demonstrate that ET-dependent mitogenesis in rat neonatal and adult aortic smooth muscle (RASM) cells is accompanied by an increase (up to 10-fold) in CDK2 activity, but not CDK2 protein levels. This effect is blocked almost entirely by PD98059 and UO126, implying involvement of the MEK/ERK signal transduction cascade in the activation. Extracts of ET-treated cells phosphorylate the N terminus of WEE1, an inhibitory kinase, which negatively regulates CDK2 activity through phosphorylation at Tyr(15), leading to a decrease in WEE1 activity and a reduction in levels of phospho-Tyr(15) in the CDK2 protein. ET also increases expression and activity of CDC25A, the regulatory phosphatase responsible for dephosphorylating Tyr(15). All of these effects are reversible following treatment with the MEK inhibitor PD98059. ET also increases levels of CDC2 activity in these cells in association with a decrease in levels of phospho-Tyr(15) on the CDC2 molecule. Phosphorylation of WEE1 is linked to ERK while phosphorylation of MYT1 (CDC2-selective inhibitory kinase) is tied to the ribosomal S6 kinase (RSK). In summary, ET controls progression through the cell cycle, in part, by increasing CDK2 and CDC2 activity through the MEK/ERK/RSK signal transduction pathway(s). This results from the phosphorylation and subsequent inactivation of two inhibitory kinases (WEE1 and MYT1) that tonically suppress CDK2 and CDC2 activity and activation of a phosphatase (CDC25A) that increases CDK2 activity.

Gene Therapy and Endovascular Treatment of Intracranial Aneurysms

Background— Endovascular treatment of intracranial aneurysms is safe and effective but too often is followed by recurrences. Gene therapy may improve healing after embolization, and endovascular approaches may offer future in situ delivery systems designed to prevent aneurysm rupture.

Summary of Review— Advances in coil technology have focused on coating strategies designed to modify the biological reaction to the embolic agent. Gene therapy in cardiovascular applications is limited by low efficiency and transient gene expression. Current advances include the potential use of circulating progenitor cells for ex vivo genetic manipulations followed by in vivo delivery. Direct gene transfer may also be enhanced in situ by coils carrying antibody-tethered adenovirus or through the use of cell-specific or radiation-inducible promoters. Candidate genes that may be of value in promoting healing after endovascular treatment include growth factors and metalloproteinase inhibitors. A better understanding of the biology of aneurysm is necessary to conceive strategies designed to control the development of these lesions before their rupture.

Conclusions— Many technical difficulties remain to be solved, but the combination of gene therapy and endovascular techniques offers multiple therapeutic possibilities in the future control of intracranial aneurysms.

Differential activation of kinases in ex vivo and in vivo irradiated mice lymphocytes

Various kinases, such as tyrosine, protein kinase C (PKC) and MAP kinase, play important role in the cellular response to radiation, but little is known about the specific response in the whole animal. Most studies, except a few, are based on single cells. There is a paucity of data where signaling following whole body irradiation is concerned. In this study a comparison has been made between the activities of these kinases following ex vivo and in vivo irradiation. Tyrosine kinase activity showed no difference in the lymphocytes irradiated ex vivo or in vivo. A significant differential dose-dependent response could be observed in PKC activity. PKC was seen to be activated at the higher dose, i.e., 1 Gy in, in vivo irradiated lymphocytes, whereas in ex vivo irradiated lymphocytes, PKC was seen to be activated at the lower dose, i.e., 0.1 Gy. MAP kinase activity was seen to decrease with an increasing dose in ex vivo irradiated lymphocytes. In vivo MAP kinase activity was seen to increase as the dose increased, with maximum activation at 3 Gy. These kinases are being used to manipulate the tumor response to radiotherapy. Thus it is essential to study the behavior of the above kinases in the whole animal because the difference in response of a single cell to the whole animal may be different.

RAF antisense oligonucleotide as a tumor radiosensitizer

The RAF-1 serine-threonine kinase plays a central role in signal transduction pathways involved in cell survival and proliferation. The concept of RAF-1-targeted disruption of cell signaling for therapeutic purposes was first advanced in 1989 with the demonstration of tumor growth inhibition in athymic mice and radiosensitization of human squamous carcinoma cells transfected with a vector expressing antisense cDNA. However, the clinical application of antisense strategies has awaited the development of improved antisense oligonucleotide technologies and drug delivery methods. Nuclease-resistant phosphorothioated antisense oligonucleotides have been the focus of pharmaceutical industry attention. In vivo delivery of nuclease-sensitive, natural backbone/phosphodiester oligonucleotides has remained a formidable challenge. Liposomal encapsulation of antisense oligonucleotides protects them from degradation and enhances drug delivery. Here, we review the importance of targeting RAF-1 signaling in cancer therapy and the preclinical and clinical experiences with a liposomal formulation of a nuclease-sensitive, ends-modified antisense RAF oligonucleotide.

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.

Activation of RSK by UV-light: phosphorylation dynamics and involvement of the MAPK pathway

Ribosomal S6 kinases (RSKs) are serine/threonine kinases activated by mitogenic signals through the Mitogen-Activated Protein Kinases/Extracellular Signal-Regulated Kinases (MAPK/ERK). RSKs contain two heterologous complete protein kinase domains. Phosphorylation by ERK of the C-terminal kinase domain allows activation of the N-terminal kinase domain, which mediates substrate phosphorylation. In human, there are three isoforms of RSK (RSK1, RSK2, RSK3), whose functional specificity remains undefined. Importantly, we have shown that mutations in the RSK2 gene lead to the Coffin-Lowry syndrome (CLS). In this study, we characterize two monoclonal antibodies raised against phosphorylated forms of the N- and C-terminal domain of RSK2 (P-S227 and P-T577, respectively). Using these two antibodies, we show that stress signals, such as UV light, induce phosphorylation and activation of the three RSKs to an extent which is comparable to Epidermal Growth Factor (EGF)-mediated activation. The use of specific kinase inhibitors indicates that UV-induced phosphorylation and activation of RSK2 is mediated by the MAPK/ERK pathway, but that the Stress-Activated Protein Kinase 2 (SAPK2)/p38 pathway is also involved. These results modify the view of RSKs as kinases restricted to the mitogenic response and reveal a previously unappreciated role of MAPKs in stress induced signaling. Oncogene (2000) 19, 4221 - 4229

Surfactant sealing of membranes permeabilized by ionizing radiation

Acute tissue injury and subsequent inflammation, including tissue edema and erythema, can be caused by sufficiently high levels of exposure to gamma radiation. The mechanism of this tissue injury is related to the generation of reactive oxygen intermediates (ROI) which chemically alter biological molecules and cell physiology. Cell membrane lipids are vulnerable to ROI-mediated lipid peroxidation that then leads to many of the acute tissue effects. We hypothesize that increased cell membrane permeability leading to osmotic swelling and vascular transudation is one of these effects. Thus we used adult postmitotic rhabdomyocytes in culture and microscopic fluorescence techniques to quantify radiation-induced changes in cell membrane permeability. Based on time-resolved dye flux measurements, a characteristic lag time of 34 +/- 3 min was determined between exposure to 160 Gy of gamma radiation and the decrease in membrane permeability. Administration of 0.1 mM nonionic surfactant Poloxamer 188 added to the cell medium after irradiation completely inhibited the dye loss over the time course of 2 h. Thus a reproducible model was developed for studying the mechanism of acute radiation injury and the efficacy of membrane-sealing agents. As only supportive measures now exist for treating the acute, nonlethal injuries from high-dose radiation exposure, agents that can restore cell membrane function after radiation damage may offer an important tool for therapy.

Granulocyte-macrophage colony-stimulating factor stimulation results in phosphorylation of cAMP response element-binding protein through activation of pp90RSK

Granulocyte-macrophage colony-stimulating factor (GM-CSF) activates several kinases and transcription factors through interaction with a heterodimeric receptor complex. We previously demonstrated that phosphorylation of the cyclic adenosine monophosphate (cAMP) response element-binding protein, CREB, occurs through a protein kinase A-independent pathway and is required for GM-CSF–induced transcriptional activation of the immediate early gene, early growth response-1 (egr-1). Recent reports indicate that receptor tyrosine kinases can induce CREB phosphorylation through activation of pp90RSK. We performed immune complex kinase assays in the human myeloid leukemic cell line, TF-1, which revealed that GM-CSF induced pp90RSK activation and phosphorylation of CREB within 5 minutes of stimulation. Transfection with the kinase-defective pp90RSK expression plasmid demonstrated a statistically significant decrease in transcriptional activation of a −116 CAT/egr-1 promoter construct in response to GM-CSF. Furthermore, activation of pp90RSK, CREB and egr-1in GM-CSF–treated cells was inhibited by the presence of the inhibitor, PD98059. In this study, we report that GM-CSF induces CREB phosphorylation and egr-1 transcription by activating pp90RSK through an MEK-dependent signaling pathway.

Granulocyte-macrophage colony-stimulating factor stimulation results in phosphorylation of cAMP response element-binding protein through activation of pp90RSK

Granulocyte-macrophage colony-stimulating factor (GM-CSF) activates several kinases and transcription factors through interaction with a heterodimeric receptor complex. We previously demonstrated that phosphorylation of the cyclic adenosine monophosphate (cAMP) response element-binding protein, CREB, occurs through a protein kinase A-independent pathway and is required for GM-CSF–induced transcriptional activation of the immediate early gene, early growth response-1 (egr-1). Recent reports indicate that receptor tyrosine kinases can induce CREB phosphorylation through activation of pp90RSK. We performed immune complex kinase assays in the human myeloid leukemic cell line, TF-1, which revealed that GM-CSF induced pp90RSK activation and phosphorylation of CREB within 5 minutes of stimulation. Transfection with the kinase-defective pp90RSK expression plasmid demonstrated a statistically significant decrease in transcriptional activation of a −116 CAT/egr-1 promoter construct in response to GM-CSF. Furthermore, activation of pp90RSK, CREB and egr-1in GM-CSF–treated cells was inhibited by the presence of the inhibitor, PD98059. In this study, we report that GM-CSF induces CREB phosphorylation and egr-1 transcription by activating pp90RSK through an MEK-dependent signaling pathway.

Activation of p38 mitogen-activated protein kinase by PYK2/related adhesion focal tyrosine kinase-dependent mechanism

The stress-activated p38 mitogen-activated protein kinase (p38 MAPK), a member of the subgroup of mammalian kinases, appears to play an important role in regulating inflammatory responses, including cytokine secretion and apoptosis. The upstream mediators that link extracellular signals with the p38 MAPK signaling pathway are currently unknown. Here we demonstrate that pp125 focal adhesion kinase-related tyrosine kinase RAFTK (also known as PYK2, CADTK) is activated specifically by methylmethane sulfonate (MMS) and hyperosmolarity but not by ultraviolet radiation, ionizing radiation, or cis-platinum. Overexpression of RAFTK leads to the activation of p38 MAPK. Furthermore, overexpression of a dominant-negative mutant of RAFTK (RAFTK K-M) inhibits MMS-induced p38 MAPK activation. MKK3 and MKK6 are known potential constituents of p38 MAPK signaling pathway, whereas SEK1 and MEK1 are upstream activators of SAPK/JNK and ERK pathways, respectively. We observe that the dominant-negative mutant of MKK3 but not of MKK6, SEK1, or MEK1 inhibits RAFTK-induced p38 MAPK activity. Furthermore, the results demonstrate that treatment of cells with 1, 2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, tetra(acetoxymethyl)-ester, a membrane-permeable calcium chelator, inhibits MMS-induced activation of RAFTK and p38 MAPK. Taken together, these findings indicate that RAFTK represents a stress-sensitive mediator of the p38 MAPK signaling pathway in response to certain cytotoxic agents.

Mitogen-activated protein kinase kinase 2 activation is essential for progression through the G2/M checkpoint arrest in cells exposed to ionizing radiation

An increasing body of evidence suggests that mitogen-induced activation of the RAF/ERK signaling pathway is functionally separate from the stress-induced activation of the SEK/JNK/p38 signaling pathway. In general, stress stimuli strongly activate the p38s and the JNKs while only weakly activating ERK1 and ERK2. However, a number of independent groups have now shown that the RAF/ERK signaling pathway is strongly activated by ionizing radiation. In this work, we examine this paradox. We show that both mitogen-activated protein (MAP) kinase kinase 1 (MEK1) and MAP kinase kinase 2 (MEK2) are activated by ionizing radiation. Blockage of this activation through the use of dominant negative MEK2 increases sensitivity of the cell to ionizing radiation and decreases the ability of a cell to recover from the G2/M cell cycle checkpoint arrest. Blocking MEK2 activation does not affect double-strand DNA break repair, however. Although MEK1 is activated to a lesser extent by ionizing radiation, expression of a dominant negative MEK1 does not affect radiation sensitivity of the cell, the G2/M checkpoint of the cell, or double-strand break repair. Because ionizing radiation leads to a different cell cycle arrest (G2/M arrest) than that typically seen with other stress stimuli, and because we have shown that MEK2 can affect G2/M checkpoint kinetics, these results provide an explanation for the observation that the MEKs can be strongly activated by ionizing radiation and only weakly activated by other stressful stimuli.

Ionizing radiation-induced MEK and Erk activation does not enhance survival of irradiated human squamous carcinoma cells

PURPOSE

Ionizing radiation (IR) triggers several intracellular signaling cascades that have commonly been regarded as mitogenic, including the Raf-MEK-Erk kinase cascade. In addition to promoting proliferation, activated MEK and Erk may also prevent cell death induced by cytotoxic stimuli. Because Raf, MEK, and Erk are activated by IR in some tumor cell lines, this suggests that IR-induced activation of the kinase cascade may enhance the survival of irradiated cells.

METHODS AND MATERIALS

IR-induced activation of MEK and Erk was assessed in irradiated UM-SCC-6 cells, a human squamous carcinoma cell line. Activation of MEK and Erk was blocked with the pharmacological inhibitor of MEK activation, PD098059. Clonogenic survival was assessed in irradiated UM-SCC-6 cells that were pretreated with nothing or with the MEK inhibitor.

RESULTS

In UM-SCC-6 cells, IR doses as low as 2 Gy rapidly activated MEK and Erk. Pretreatment of the cells with the pharmacological inhibitor of MEK activation, PD098059, effectively blocked IR-induced activation of MEK and Erk. However, inhibition of the kinase cascade did not affect the clonogenic survival of irradiated cells in either early or delayed-plating experiments.

CONCLUSION

Taken together, these results suggest that although MEK and Erk are rapidly activated by IR treatment, these protein kinases do not affect the clonogenic survival of irradiated UM-SCC6 cells.

Ionizing radiation activates c-Jun NH2-terminal kinase (JNK/SAPK) via a PKC-dependent pathway in human thyroid cells

Thyroid gland is known to be higher sensitive to carcinogenic effects of external ionizing radiation (IR) than other tissues. To clarify the cell-specific response following irradiation, activations of c-Jun NH2-terminal kinases (JNKs), which is one of mitogen-activated protein kinases (MAPKs) family members, and extracellular signal-regulated kinase (ERK) were examined in primary cultured human thyroid cells in comparison with human diploid fibroblast cells, WI-38. Although UV exposure strikingly induced JNK activity in both cells, the dose-response increase following IR exposure was observed in thyroid cells with the maximal JNK activity (3.5 fold induction) obtained at 10 Gy exposure, but no increase in WI-38 cells. The JNK activity was reached a maximum of 2.2 fold induction at 30 min after 5 Gy exposure and then sustained for at least 12 hr. On the other hand, ERK activity was not stimulated in thyroid cells following irradiation. The effects of 12-O-tetradecanoylphorbol beta-acetate (TPA) mimicked those of radiation on JNK cascade and 1-(5-isoquinolinesulphonyl)-2,5-dimethylpiperazine 2HCl (H7) and pretreatment with TPA blocked JNK activation following irradiation. Our results demonstrate that IR stimulates JNK activity in cultured human thyroid cells but not in fibroblasts indicating distinct activation and regulation mechanisms of JNK cascade. The JNK activation following IR exposure is mediated at least partially through a PKC-dependent pathway.

Urea activates ribosomal S6 kinase (RSK) in a MEK-dependent fashion in renal mIMCD3 cells

Urea activates a characteristic subset of signaling pathways in a tissue-specific fashion, including transcription of immediate early genes through activation of the mitogen-activated protein kinase (MAPK), ERK (extracellular signal-regulated kinase), and activation of its transcription factor substrate, Elk-1. The ability of urea to activate the ERK effector and pivotal regulatory kinase, ribosomal S6 kinase (RSK), was investigated in mIMCD3 renal inner medullary collecting duct cells. Urea upregulated RSK activity in a time-dependent fashion in serum-deprived mIMCD3 cells; the effect was maximal at 5 min. Activation by hypertonic NaCl, in contrast, was negligible at 5 min and peaked at 15 min. Both stimuli induced the nuclear translocation of cytosolic RSK, as determined via immunofluorescence. Importantly, activation of RSK by both solutes was MAPK/ERK kinase (MEK) dependent, as determined by the ability of the specific MEK inhibitor, PD-98059, to abrogate the response. Taken together, these data indicate that urea activates the ERK effector, RSK, in cells of the renal medulla in an ERK-dependent fashion, further emphasizing the functional significance of urea signaling through ERK activation in renal medullary cells.

Angiotensin II stimulates p90rsk in vascular smooth muscle cells. A potential Na(+)-H+ exchanger kinase

Angiotensin II is a multifunctional agonist for vascular smooth muscle cells (VSMCs), stimulating increases in signal events, cell growth, and ion flux. We previously defined protein kinase C (PKC)-dependent and -independent mechanisms by which angiotensin II stimulated activity of the Na(+)-H+ exchanger isoform-1 (NHE-1) and identified a 90-kD kinase that exhibited increased activity in VSMCs isolated from genetically hypertensive rats. To determine whether this 90-kD kinase was p90rsk (RSK), VSMCs were stimulated with 100 nmol/L angiotensin II, and NHE-1 kinase activity was measured by phosphorylation of recombinant NHE-1 (a glutathione S-transferase fusion protein containing amino acids 516 to 815 of the cytoplasmic carboxyl tail) in vitro. NHE-1 kinase (90 kD) activity was markedly decreased by immunodepletion of RSK. Characterization of RSK activation by angiotensin II revealed many similarities to the 90-kD NHE-1 kinase, including time course and NHE-1 domain phosphorylation, as well as regulation by extracellular signal-regulated kinases (ERK1/2), intracellular Ca2+, and PKC. Specifically, angiotensin II stimulated a rapid and transient (peak, 5 minutes) increase in RSK activity. Analysis of several NHE-1 fusion proteins revealed that only proteins containing amino acids 670 to 714 were phosphorylated by RSK. Inhibiting ERK1/2 (30 mumol/L PD098059 for 30 minutes) or chelating intracellular Ca2+ prevented RSK activation. In contrast, downregulating PKC (1 mumol/L phorbol dibutyrate for 24 hours) had little effect. These findings establish RSK as a putative NHE-1 kinase and potential mediator of increased Na(+)-H+ exchange in hypertension.

Finkel-Biskis-Reilly mouse osteosarcoma virus v-fos inhibits the cellular response to ionizing radiation in a myristoylation-dependent manner

DNA damage is recognized as a central component of carcinogenesis. DNA-damaging agents activate a number of signal transduction pathways that lead to repair of the DNA, apoptosis, or cell cycle arrest. It is reasoned that a cell deficient in DNA repair is more likely to acquire other cancer-promoting mutations. Despite the recent interest in the link between DNA damage and carcinogenesis, retroviral oncogenes have not yet been shown to affect the DNA damage-signaling pathway. In this report, we show that Finkel-Biskis-Reilly mouse osteosarcoma virus (FBR) v-fos, the retroviral homologue of the c-fos proto-oncogene, inhibits the cellular response to ionizing radiation. Cells that express FBR v-Fos show a decreased ability to repair DNA damage caused by ionizing radiation, and these cells show decreased survival in response to ionizing radiation. In addition, FBR v-Fos inhibits DNA-dependent protein kinase, a kinase specifically activated upon exposure to ionizing radiation. These effects were specific to ionizing radiation, as no effect of FBR v-Fos on the UV light signaling pathway was seen. Last, these effects were dependent on a lipid modification required for FBR v-Fos tumorigenesis, that of myristoylation of FBR v-Fos. A non-myristoylated mutant FBR v-Fos caused none of these effects. This study suggests that a retroviral oncogene can lead to an increased genomic instability, which can ultimately increase the carcinogenic potential of a cell.

Regulation and interaction of pp90(rsk) isoforms with mitogen-activated protein kinases

Each of the three known mammalian 90-kDa S6 kinase (pp90(rsk)) isoforms (RSK1, RSK2, and RSK3) was expressed in transfected cells and further characterized. The kinase activity (immunocomplex toward S6 peptide) of each isoform was activated by in vivo growth factor (epidermal growth factor (EGF)) stimulation; RSK1 was more responsive (10-15-fold) versus RSK2 and RSK3 (2-4-fold). Pretreatment with PD98059 (MEK1 inhibitor) partially (80%) blocked EGF-mediated ERK1 activation and had similar effects on EGF stimulation of each ribosomal S6 kinase (RSK). Cotransfection with dominant-negative MEK1 inhibited activation of each RSK; furthermore, the kinase activity of RSK1, RSK2, and RSK3 was markedly increased by cotransfection with constitutively active MEK1. A specific association between mitogen-activated protein kinases (MAPKs) (ERK1 and ERK2) and RSK isoforms was tested by MAPK immunoblotting after immunoprecipitation of RSKs. ERK1 and ERK2 were present in RSK3 (and to a lesser extent, RSK2) immunoprecipitates, but were absent in RSK1 immunoprecipitates. Both dephosphorylated (from quiescent cells) and phosphorylated (from stimulated cells) MAPKs were associated with RSK2 and RSK3. Deletion mutants of RSK3 were characterized: the C terminus (33 residues) was shown to be required for association with MAPKs. The kinase activity of RSK1 or RSK2 was enhanced by in vitro incubation with ERK1. In contrast, RSK3 activity was not affected by exposure to ERK1. Furthermore, MAPKs in RSK3 immunoprecipitates were phosphorylated by purified MEK1; however, RSK3 kinase activity was unaffected. We conclude that 1) the MEK1-MAPK signaling pathway is both necessary and sufficient for in vivo growth factor-mediated activation of all three RSK isoforms; 2) RSK isoforms differ with respect to growth factor responsiveness and their physical association with MAPK; and 3) formation of the MAPK.RSK complex is mediated by the RSK C terminus.

Activation of Raf by ionizing radiation

The critical pathways through which ionizing radiation induces malignant transformation and cell death are not well defined. Raf-1, a cytoplasmic serine-threonine protein kinase, mediates the transmission of mitogenic signals initiated at the cell membrane to the nucleus, resulting in the activation of transcription factors that regulate cell growth and proliferation. Moreover, Raf-1 overexpression and activation increases the survival response of mammalian cells to the toxic effects of ionizing radiation by an as-yet unknown mechanism (refs 3, 4 and V. Soldatenkov et al.; manuscript submitted). Somewhat analogous to mitogen-induced signalling, radiation stimulates protein-tyrosine kinase(s) and transcription factors. No direct biochemical link has been established, however, between radiation-stimulated protein tyrosine phosphorylation and downstream signals. Here we report a series of radiation-responsive events in which protein-tyrosine phosphorylation is followed by membrane recruitment, then tyrosine phosphorylation and activation of Raf-1 in vivo. Our results show that radiation-stimulated protein-tyrosine kinase(s) modify Raf-1, and implicate Raf-1 in the ionizing-radiation signal-transduction pathway.

The stress response to ionizing radiation involoves c-Abl-dependent phosphorylation of SHPTP1

c-Abl is a nonreceptor tyrosine kinase that is activated by certain DNA-damaging agents. The present studies demonstrate that nuclear c-Abl binds constitutively to the protein tyrosine phosphatase SHPTP1. Treatment with ionizing radiation is associated with c-Abl-dependent tyrosine phosphorylation of SHPTP1. The results demonstrate that the SH3 domain of c-Abl interacts with a WPDHGVPSEP motif (residues 417-426) in the catalytic domain of SHPTP1 and that c-Abl phosphorylates C terminal Y536 and Y564 sites. The functional significance of the c-Abl-SHPTP1 interaction is supported by the demonstration that, like c-Abl, SHPTP1 regulates the induction of Jun kinase activity following DNA damage. These findings indicate that SHPTP1 is involved in the response to genotoxic stress through a c-Abl-dependent mechanism.

Expression of the ATDC (ataxia telangiectasia group D-complementing) gene in A431 human squamous carcinoma cells

The ATDC gene was originally identified by its ability to complement the radiosensitivity defect of an ataxia telangiectasia (AT) fibroblast cell line. Because hypersensitivity to ionizing radiation is an important feature of the AT phenotype, we reasoned that ATDC may function generally in the suppression of radiosensitivity. Previous work in our laboratory focused on radiosensitization mechanisms in human squamous carcinoma (SC) cells, especially A431 cells. To establish a basis for investigating the role of ATDC in radiation-responsive signaling pathways in human SC cells, we characterized ATDC message and protein expressions in A431 cells. ATDC message expression was also compared among human epidermoid cells (A431 cells, HaCaT spontaneously immortalized human keratinocytes and normal human epidermal keratinocytes) and a normal human fibroblast cell line (LM217). We made the following major observations: (i) the relative abundance of ATDC message is substantially higher in the epidermoid cells than in the fibroblast cell line, which has a message level comparable to those reported for other fibroblast lines; (ii) ATDC is constitutively phosphorylated on serine/threonine in A431 cells; (iii) in A431 cells, ATDC is a substrate for the serine/threonine protein kinase C (PKC) but not the epidermal growth factor (EGF) receptor tyrosine kinase; and (iv) EGF decreases ATDC message and protein expressions in A431 cells after a 24-hr exposure. The phosphorylation studies suggest that the ability of ATDC to modulate cellular radiosensitivity may be mediated in part through a PKC signaling pathway.

C-jun and Egr-1 participate in DNA synthesis and cell survival in response to ionizing radiation exposure

Exposure of mammalian cells to ionizing radiation results in the induction of the immediate early genes, c-jun and Egr-1, which encode transcription factors implicated in cell growth as well as the cellular response to oxidative stress. We studied the role of these immediate early genes in cell cycle kinetics and cell survival following x-irradiation of clones containing inducible dominant negatives to c-jun and Egr-1. The dominant negative constructs to c-jun (delta 9) and Egr-1 (WT/Egr) prevented x-ray induction of transcription through the AP-1 and Egr binding sites, respectively. Twenty percent of confluent, serum-deprived SQ20B human tumor cells, normal fibroblasts, and fibroblasts from patients with ataxia telangiectasia entered S phase within 5 h of irradiation. Clones containing inducible delta 9 and WT/Egr dominant negative constructs demonstrated attenuation of the percentage of cells exiting G1 phase and reduced survival following irradiation. These data indicate that the dominant negatives to the stress-inducible immediate early genes Egr-1 and c-jun prevent the onset of S phase and reduce the survival of human cells exposed to ionizing radiation.

Mitogenic signals and transforming potential of Nyk, a newly identified neural cell adhesion molecule-related receptor tyrosine kinase

Nyk/Mer is a recently identified receptor tyrosine kinase with neural cell adhesion molecule-like structure (two immunoglobulin G-like domains and two fibronectin III-like domains) in its extracellular region and belongs to the Ufo/Axl family of receptors. The ligand for Nyk/Mer is presently unknown, as are the signal transduction pathways mediated by this receptor. We constructed and expressed a chimeric receptor (Fms-Nyk) composed of the extracellular domain of the human colony-stimulating factor 1 receptor (Fms) and the transmembrane and cytoplasmic domains of human Nyk/Mer in NIH 3T3 fibroblasts in order to investigate the mitogenic signaling and biochemical properties of Nyk/Mer. Colony-stimulating factor 1 stimulation of the Fms-Nyk chimeric receptor in transfected NIH 3T3 fibroblasts leads to a transformed phenotype and generates a proliferative response in the absence of other growth factors. We show that phospholipase C gamma, phosphatidylinositol 3-kinase/p70 S6 kinase, Shc, Grb2, Raf-1, and mitogen-activated protein kinase are downstream components of the Nyk/Mer signal transduction pathways. In addition, Nyk/Mer weakly activates p90rsk, while stress-activated protein kinase, Ras GTPase-activating protein (GAP), and GAP-associated p62 and p190 proteins are not activated or tyrosine phosphorylated by Nyk/Mer. An analysis comparing the Nyk/Mer signal cascade with that of the epidermal growth factor receptor indicates substrate preferences by these two receptors. Our results provide a detailed description of the Nyk/Mer signaling pathways. Given the structural similarity between the Ufo/Axl family receptors, some of the information may also be applied to other members of this receptor tyrosine kinase family.

Divergent functional roles for p90rsk kinase domains

A unique and highly conserved structural feature of approximately 90-kDa ribosomal S6 kinase (p90rsk or RSK) is the presence of two non-identical kinase domains. To explore the mechanism of RSK activation, a cloned human RSK cDNA (RSK3) was used to generate and characterize several site-directed RSK mutants; K91A (N-Lys, NH2-terminal ATP-binding mutant), K444A (C-Lys, COOH-terminal ATP-binding mutant), N/C-Lys (double ATP-binding mutant) T570A (C-Thr, mutant of the putative MAPK phosphorylation site in subdomain VIII of the C-domain), S218A (N-Ser, mutant of the corresponding NH2-terminal residue). Epitope-tagged RSKs were expressed in transfected COS cells followed by immunoprecipitation with or without prior in vivo epidermal growth factor stimulation. Kinase activity (S6 peptide) of N/C-Lys and N-Lys was ablated (and partially impaired with N-Ser). In contrast, both C-Lys and C-Thr retained high levels of kinase activity and were capable of responding to stimulation. C-Lys also retained partial kinase activity toward other substrates (c-Fos, S40 ribosomes, protein phosphatase 1 G-subunit, histones, and Leu-Arg-Arg-Ala-Ser-Leu-Gly (Kemptide)) whereas N-Lys did not. The isolated NH2-and COOH-terminal domains were also expressed; the C-domain was inactive, whereas the N-domain retained partial activity. Relative to wild-type, both N-Lys and C-Lys (as well as N-Ser and C-Thr) underwent partial in vitro autophosphorylation that was further stimulated by EGF protein tyrosine phosphatase. We conclude that 1) the NH2-terminal RSK kinase domain mediates substrate phosphorylation; 2) both domains contribute to autophosphorylation; 3) the putative MAPK phosphorylation site is not required for growth factor-stimulated autophosphorylation or kinase activation.

Ionizing radiation stimulates a Grb2-mediated association of the stress-activated protein kinase with phosphatidylinositol 3-kinase

The stress-activated protein (SAP) kinases are induced by tumor necrosis factor, oncoproteins, and UV light. The present studies demonstrate that ionizing radiation (IR) activates p54 SAP kinase. IR-induced activation of SAP kinase is associated with binding to the SH2/SH3-containing adaptor protein Grb2. This interaction is mediated by the SH3 domains of Grb2 and the proline-rich sequence PPPKIP in the carboxy-terminal region of SAP kinase. We also demonstrated that SAP kinase and the p85 alpha-subunit of phosphatidylinositol (PI) 3-kinase form a complex in irradiated cells. The results indicate that this complex involves binding of the p85 alpha subunit of PI 3-kinase to the SH2 domain of Grb2. The functional role of linking SAP kinase to PI 3-kinase is further supported by the finding that wortmannin, an inhibitor of PI 3-kinase, stimulates SAP kinase activity. These results suggest that the cellular response to IR may include regulation of SAP kinase by a PI 3-kinase-dependent signaling pathway.

RSK3 encodes a novel pp90rsk isoform with a unique N-terminal sequence: growth factor-stimulated kinase function and nuclear translocation

A novel pp90rsk Ser/Thr kinase (referred to as RSK3) was cloned from a human cDNA library. The RSK3 cDNA encodes a predicted 733-amino-acid protein with a unique N-terminal region containing a putative nuclear localization signal. RSK3 mRNA was widely expressed (but was predominant in lung and skeletal muscle). By using fluorescence in situ hybridization, the human RSK3 gene was localized to band q27 of chromosome 6. Hemagglutinin epitope-tagged RSK3 was expressed in transiently transfected COS cells. Growth factors, serum, and phorbol ester stimulated autophosphorylation of recombinant RSK3 and its kinase activity toward several protein substrates known to be phosphorylated by RSKs. However, the relative substrate specificity of RSK3 differed from that reported for other isoforms. RSK3 also phosphorylated potential nuclear target proteins including c-Fos and histones. Furthermore, although RSK3 was inactivated by protein phosphatase 2A in vitro, the enzyme was not activated by ERK2/mitogen-activated protein (MAP) kinase. In contrast, the kinase activity of another epitope-tagged RSK isoform (RSK-1) was significantly increased by in vitro incubation with ERK2/MAP kinase. Finally, we used affinity-purified RSK3 antibodies to demonstrate by immunofluorescence that endogenous RSK3 undergoes serum-stimulated nuclear translocation in cultured HeLa cells. These results provide evidence that RSK3 is a third distinct isoform of pp90rsk which translocates to the cell nucleus, phosphorylates potential nuclear targets, and may have a unique upstream activator. RSK3 may therefore subserve a discrete physiologic role(s) that differs from those of the other two known mammalian RSK isoforms.

The MAP kinase cascade: its role in Xenopus oocytes, eggs and embryos

Mitogen-activated protein kinase (MAPK) was originally identified as a serine/threonine kinase that is activated by mitogens. Now MAPK and its activator, MAPK kinase (MAPKK), are thought to function in a wide variety of intracellular signalling pathways from yeast to vertebrate. We describe here a brief summary of the dissection of the MAPK cascade and its possible functions, especially in Xenopus oocytes and embryos.