PKC epsilon -mediated ERK1/2 activation involved in radiation-induced cell death in NIH3T3 cells

Cell Signaling Pathways That Promote Radioresistance of Cancer Cells

Radiation therapy (RT) is a standard treatment for solid tumors and about 50% of patients with cancer, including pediatric cancer, receive RT. While RT has significantly improved the overall survival and quality of life of cancer patients, its efficacy has still been markedly limited by radioresistance in a significant number of cancer patients (intrinsic or acquired), resulting in failure of the RT control of the disease. Radiation eradicates cancer cells mainly by causing DNA damage. However, radiation also concomitantly activates multiple prosurvival signaling pathways, which include those mediated by ATM, ATR, AKT, ERK, and NF-κB that promote DNA damage checkpoint activation/DNA repair, autophagy induction, and/or inhibition of apoptosis. Furthermore, emerging data support the role of YAP signaling in promoting the intrinsic radioresistance of cancer cells, which occurs through its activation of the transcription of many essential genes that support cell survival, DNA repair, proliferation, and the stemness of cancer stem cells. Together, these signaling pathways protect cancer cells by reducing the magnitude of radiation-induced cytotoxicity and promoting radioresistance. Thus, targeting these prosurvival signaling pathways could potentially improve the radiosensitivity of cancer cells. In this review, we summarize the contribution of these pathways to the radioresistance of cancer cells.

Dual effects of active ERK in cancer: A potential target for enhancing radiosensitivity

Ionizing radiation (IR) is an important cancer treatment approach. However, radioresistance eventually occurs, resulting in poor outcomes in patients with cancer. Radioresistance is associated with multiple signaling pathways, particularly pro-survival signaling pathways. The extracellular signal-regulated kinase 1/2 (ERK1/2) cascade is an important signaling pathway that initiates several cellular processes and is regulated by various stimuli, including IR. Although numerous studies have demonstrated the pro-survival effects of active ERK, activation of ERK has also been associated with cell death, indicating that radiosensitization may occur by ERK stimulation. In this context, the present review describes the associations between ERK signaling, cancer and IR, and discusses the association between ERK and its pro-survival function in cancer cells, including stimuli, molecular mechanisms, clinical use of inhibitors and underlying limitations. Additionally, the present review introduces the view that active ERK may induce cell death, and describes the potential factors associated with this process. This review describes the various outcomes induced by active ERK to prompt future studies to aim to enhance radiosensitivity in the treatment of cancer.

The molecular mechanism of anticancer action of novel octahydropyrazino[2,1-a:5,4-a']diisoquinoline derivatives in human gastric cancer cells

Objective The aim of the current study was to examine the anticancer activity and the detailed mechanism of novel diisoquinoline derivatives in human gastric cancer cells (AGS). Methods The viability of AGS cells was measured by MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay. Cell cycle analysis and apoptosis assay were performed by standard flow cytometric method. Confocal microscopy bioimaging was used to demonstrate the expression of pivotal proteins engaged in apoptosis (caspase-8, caspase-3, p53) and cell signaling (AKT, ERK1/2). Results All compounds decreased the number of viable cells in a dose-dependent manner after 24 and 48 h of incubation, although compound 2 was a more cytotoxic agent, with IC₅₀ values of 21 ± 2 and 6 ± 2 μM, compared to 80 ± 2 and 45 ± 2 μM for etoposide. The cytotoxic and antiproliferative effects of novel compounds were associated with the induction of apoptosis. The highest percentage of early and late apoptotic cells was observed after 48 h of incubation with compound 2 (89.9%). The value was higher compared to compound 1 (20.4%) and etoposide (24.1%). The novel diisoquinoline derivatives decreased the expression of AKT and ERK1/2. Their mechanism was associated with p53-mediated apoptosis, accumulation of cells in the G2/M phase of cell cycle and inhibition of topoisomerase II. Conclusion These data strongly support compound 2 as a promising molecule for treatment of gastric cancer.

Functional characterization of the ectopically expressed olfactory receptor 2AT4 in human myelogenous leukemia

The olfactory receptor (OR) family was found to be expressed mainly in the nasal epithelium. In the last two decades members of the OR family were detected to be functional expressed in different parts of the human body such as in liver, prostate or intestine cancer cells. Here, we detected the expression of several ORs in the human chronic myelogenous leukemia (CML) cell line K562 and in white blood cells of clinically diagnosed acute myeloid leukemia (AML) patients by RT-PCR and next-generation sequencing. With calcium-imaging, we characterized in greater detail the cell biological role of one OR (OR2AT4) in leukemia. In both cell systems, the OR2AT4 agonist Sandalore-evoked strong Ca²⁺ influx via the adenylate cyclase-cAMP-mediated pathway. The OR2AT4 antagonist Phenirat prevented the Sandalore-induced intracellular Ca²⁺ increase. Western blot and flow cytometric experiments revealed that stimulation of OR2AT4 reduced the proliferation by decreasing p38-MAPK phosphorylation and induced apoptosis via phosphorylation of p44/42-MAPK. Furthermore, Sandalore increased the number of hemoglobin-containing cells in culture. We described for the first time an OR-mediated pathway in CML and AML that can regulate proliferation, apoptosis and differentiation after activation. This mechanism offers novel therapeutic options for the treatment of AML.

Radiation-induced signaling pathways that promote cancer cell survival (review)

Radiation therapy is a staple cancer treatment approach that has significantly improved local disease control and the overall survival of cancer patients. However, its efficacy is still limited by the development of radiation resistance and the presence of residual disease after therapy that leads to cancer recurrence. Radiation impedes cancer cell growth by inducing cytotoxicity, mainly caused by DNA damage. However, radiation can also simultaneously induce multiple pro-survival signaling pathways, such as those mediated by AKT, ERK and ATM/ATR, which can lead to suppression of apoptosis, induction of cell cycle arrest and/or initiation of DNA repair. These signaling pathways act conjointly to reduce the magnitude of radiation-induced cytotoxicity and promote the development of radioresistance in cancer cells. Thus, targeting these pro-survival pathways has great potential for the radiosensitization of cancer cells. In the present review, we summarize the current literature on how these radiation‑activated signaling pathways promote cancer cell survival.

A new goniothalamin N-acylated aza-derivative strongly downregulates mediators of signaling transduction associated with pancreatic cancer aggressiveness

In this study, a novel concise series of molecules based on the structure of goniothalamin (1) was synthesized and evaluated against a highly metastatic human pancreatic cancer cell line (Panc-1). Among them, derivative 8 displayed a low IC50 value (2.7 μM) and its concentration for decreasing colony formation was 20-fold lower than goniothalamin (1). Both compounds reduced the levels of the receptor tyrosine kinase (AXL) and cyclin D1 which are known to be overexpressed in pancreatic cancer cells. Importantly, despite the fact that goniothalamin (1) and derivative 8 caused pancreatic cancer cell cycle arrest and cell death, only derivative 8 was able to downregulate pro-survival and proliferation pathways mediated by mitogen activated protein kinase ERK1/2. Another interesting finding was that Panc-1 cells treated with derivative 8 displayed a strong decrease in the transcription factor (c-Myc), hypoxia-inducible factor-1α (HIF-1α) and vascular endothelial growth factor (VEGF) protein levels. Notably, the molecular effects caused by derivative 8 might not be related to ROS generation, since no significant production of ROS was observed in low concentrations of this compound (from 1.5 up to 3 μM). Therefore, the downregulation of important mediators of pancreatic cancer aggressiveness by derivative 8 reveals its great potential for the development of new chemotherapeutic agents for pancreatic cancer treatment.

Inflammation-related genetic variants predict toxicity following definitive radiotherapy for lung cancer

Definitive radiotherapy improves locoregional control and survival in inoperable non-small cell lung cancer (NSCLC) patients. However, radiation-induced toxicities (pneumonitis/esophagitis) are common dose-limiting inflammatory conditions. We therefore conducted a pathway-based analysis to identify inflammation-related SNPs associated with radiation-induced pneumonitis or esophagitis. 11,930 SNPs were genotyped in 201 stage I-III NSCLC patients treated with definitive radiotherapy. Validation was performed in an additional 220 NSCLC cases. After validation, 19 SNPs remained significant. A polygenic risk score (PRS) was generated to summarize the effect from validated SNPs. Significant improvements in discriminative ability were observed by adding the PRS into the clinical/epidemiological variable-based model. We then used 277 lymphoblastoid cell-lines to assess radiation sensitivity and eQTL relationships of the identified SNPs. Three genes (PRKCE,DDX58 and TNFSF7) were associated with radiation sensitivity. We concluded that inflammation-related genetic variants could contribute to the development of radiation-induced toxicities. These loci could assist in predicting those unfavorable events.

Mitogen-activated protein kinase phosphatase-1 inhibition and sustained extracellular signal-regulated kinase 1/2 activation in camptothecin-induced human colon cancer cell death

Camptothecins are commonly used chemotherapeutics; in some models, they enhance signaling via the mitogen-activated protein kinase (MAPK) pathway through effects on upstream kinases. To evaluate the impact of camptothecin (CPT) on MAPKs in human colon cancer, we studied HCT116 and CaCo2 colon cancer cells. We found that HCT116 cells highly express mitogen-activated protein kinase phosphatase-1 (MKP1), which selectively inactivates extracellular signal-regulated kinase (ERK), whereas MKP1 levels were undetectable in CaCo2 cells. CPT did not affect ERK activity in CaCo2 cells, but did induce a striking increase in ERK activity in HCT116 cells in association with a corresponding decrease in MKP1. The reduction in MKP1 expression occurred at a posttranscriptional level and was blocked by the proteasome inhibitor MG132, whereas that CPT-induced downregulation of MKP1 was not due to proteasome-mediated degradation. Treatment of HCT116 cells with CPT induced a sustained activation of nuclear ERK, which was required for CPT-induced apoptosis. P38 and JNK activity were unaffected by CPT, suggesting that the effects of CPT are mediated specifically by ERK. These results suggest that targeting dual-specificity MAPK phosphatases in colon cancer cells may be a viable strategy for optimizing camptothecin-based therapeutic protocols.

Alcohol withdrawal and brain injuries: beyond classical mechanisms

Unmanaged sudden withdrawal from the excessive consumption of alcohol (ethanol) adversely alters neuronal integrity in vulnerable brain regions such as the cerebellum, hippocampus, or cortex. In addition to well known hyperexcitatory neurotransmissions, ethanol withdrawal (EW) provokes the intense generation of reactive oxygen species (ROS) and the activation of stress-responding protein kinases, which are the focus of this review article. EW also inflicts mitochondrial membranes/membrane potential, perturbs redox balance, and suppresses mitochondrial enzymes, all of which impair a fundamental function of mitochondria. Moreover, EW acts as an age-provoking stressor. The vulnerable age to EW stress is not necessarily the oldest age and varies depending upon the target molecule of EW. A major female sex steroid, 17β-estradiol (E2), interferes with the EW-induced alteration of oxidative signaling pathways and thereby protects neurons, mitochondria, and behaviors. The current review attempts to provide integrated information at the levels of oxidative signaling mechanisms by which EW provokes brain injuries and E2 protects against it.

ERK and cell death: mechanisms of ERK-induced cell death--apoptosis, autophagy and senescence

The Ras/Raf/extracellular signal-regulated kinase (ERK) signaling pathway plays a crucial role in almost all cell functions and therefore requires exquisite control of its spatiotemporal activity. Depending on the cell type and stimulus, ERK activity will mediate different antiproliferative events, such as apoptosis, autophagy and senescence in vitro and in vivo. ERK activity can promote either intrinsic or extrinsic apoptotic pathways by induction of mitochondrial cytochrome c release or caspase-8 activation, permanent cell cycle arrest or autophagic vacuolization. These unusual effects require sustained ERK activity in specific subcellular compartments and could depend on the presence of reactive oxygen species. We will summarize the mechanisms involved in Ras/Raf/ERK antiproliferative functions.

Protein kinase Cdelta supports survival of MDA-MB-231 breast cancer cells by suppressing the ERK1/2 pathway

Mechanisms that mediate apoptosis resistance are attractive therapeutic targets for cancer. Protein kinase Cdelta (PKCdelta) is considered a pro-apoptotic factor in many cell types. In breast cancer, however, it has shown both pro-survival and pro-apoptotic effects. Here, we report for the first time that down-regulation of PKCdelta per se leads to apoptosis of MDA-MB-231 cells. Inhibition of MEK1/2 by either PD98059 or U0126 suppressed the induction of apoptosis of PKCdelta-depleted MDA-MB-231 cells but did not support survival of MCF-7 or MDA-MB-468 cells. Basal ERK1/2 phosphorylation was substantially higher in MDA-MB-231 cells than in the other cell lines. PKCdelta depletion led to even higher ERK1/2 phosphorylation levels and also to lower expression levels of the ERK1/2 phosphatase MKP3. Depletion of MKP3 led to apoptosis and higher levels of ERK1/2 phosphorylation, suggesting that this may be a mechanism mediating the effect of PKCdelta down-regulation. However, PKCdelta silencing also induced increased MEK1/2 phosphorylation, indicating that PKCdelta regulates ERK1/2 phosphorylation both upstream and downstream. Moreover, PKCdelta silencing led to increased levels of the E3 ubiquitin ligase Nedd4, which is a potential regulator of MKP3, because down-regulation led to increased MKP3 levels. Our results highlight PKCdelta as a potential target for therapy of breast cancers with high activity of the ERK1/2 pathway.

Protein kinase C delta regulates airway mucin secretion via phosphorylation of MARCKS protein

Mucin hypersecretion is a major pathological feature of many respiratory diseases, yet cellular mechanisms regulating secretion of mucin have not been fully elucidated. Previously, we reported that mucin hypersecretion induced by human neutrophil elastase involves activation of protein kinase C (PKC), specifically the delta-isoform (PKC delta). Here, we further investigated the role of PKC delta in mucin hypersecretion using both primary human bronchial epithelial cells and the human bronchial epithelial 1 cell line as in vitro model systems. Phorbol-12-myristate-13-acetate (PMA)-induced mucin hypersecretion was significantly attenuated by rottlerin, a PKC delta-selective inhibitor. Rottlerin also reduced PMA- or human neutrophil elastase-induced phosphorylation of myristoylated alanine-rich C kinase substrate (MARCKS) protein in these cells. Both secretion and MARCKS phosphorylation were significantly enhanced by the PKC delta activator bryostatin 1. A dominant-negative PKC delta construct (pEGFP-N1/PKC delta K376R) transfected into human bronchial epithelial 1 cells significantly attenuated both PMA-induced mucin secretion and phosphorylation of MARCKS, whereas transfection of a wild-type construct increased PKC delta and enhanced mucin secretion and MARCKS phosphorylation. Similar transfections of a dominant-negative or wild-type PKC epsilon construct did not affect either mucin secretion or MARCKS phosphorylation. The results suggest that PKC delta plays an important role in mucin secretion by airway epithelium via regulation of MARCKS phosphorylation.

α- and ϵ-Protein Kinase C Activity during Smooth Muscle Cell Apoptosis in Response to γ-Radiation

The use of gamma-radiation in treatment of pelvic cancer is associated with injury of healthy surrounding tissues and disorders of intestinal motility; however, the cellular mechanisms involved are unclear. We tested the hypothesis that exposure of visceral smooth muscle cells (SMCs) to gamma-radiation induces apoptosis via activation of specific protein kinase C (PKC) isoforms. Cultured SMCs and slices from guinea pig ileum smooth muscle longitudinal layer (GPISMLL) were exposed to 10 to 50 Gy. Flow cytometry in gamma-radiated SMCs showed increased percentage of cells in the sub-G(0)/G(1) phase, a hallmark of apoptosis. gamma-Radiation-induced reduction in cell survival was partially but significantly alleviated with the PKC inhibitors. Sections of gamma-irradiated GPISMLL showed DNA fragmentation and apoptotic bodies analyzed by the terminal deoxynucleotidyl transferase dUTP nick-end labeling method, whereas the plasma and nuclear membranes were preserved. Confocal microscopy in gamma-radiated SMCs labeled with annexin V-fluorescein showed an increase in apoptotic cells and phosphatidylserine externalization. Contraction of GPISMLL strips in response to KCl and acetylcholine was reduced in tissues exposed to 30 and 50 Gy. gamma-Radiation of GPISMLL caused an increase in PKC activity in the particulate fraction, a decrease in the cytosolic fraction, and increased particulate/cytosolic PKC activity ratio. Western blot analysis revealed significant amounts of alpha- and epsilon-PKC in the cytosolic fraction of control GPISMLL. gamma-Radiation caused an increase in the amount of alpha- and epsilon-PKC in the particulate fraction and a decrease in the cytosolic fraction. Data suggest that gamma-radiation induces apoptosis, growth arrest, and contractile dysfunction in visceral SMCs of GPISMLL via activation and translocation of alpha- and epsilon-PKC isoforms.

Radiation-induced cell signaling: inside-out and outside-in

Exposure of tumor cells to clinically relevant doses of ionizing radiation causes DNA damage as well as mitochondria-dependent generation of reactive oxygen species. DNA damage causes activation of ataxia telangiectasia mutated and ataxia telangiectasia mutated and Rad3-related protein, which induce cell cycle checkpoints and also modulate the activation of prosurvival and proapoptotic signaling pathways, such as extracellular signal-regulated kinase 1/2 (ERK1/2) and c-Jun NH(2)-terminal kinase 1/2, respectively. Radiation causes a rapid reactive oxygen species-dependent activation of ERBB family and other tyrosine kinases, leading to activation of RAS proteins and multiple protective downstream signaling pathways (e.g., AKT and ERK1/2), which alter transcription factor function and the apoptotic threshold of cells. The initial radiation-induced activation of ERK1/2 can promote the cleavage and release of paracrine ligands, which cause a temporally delayed reactivation of receptors and intracellular signaling pathways in irradiated and unirradiated bystander cells. Hence, signals from within the cell can promote activation of membrane-associated receptors, which signal back into the cytosol: signaling from inside the cell outward to receptors and then inward again via kinase pathways. However, cytosolic signaling can also cause release of membrane-associated paracrine factors, and thus, paracrine signals from outside of the cell can promote activation of growth factor receptors: signaling from the outside inward. The ultimate consequence of these signaling events after multiple exposures may be to reprogram the irradiated and affected bystander cells in terms of their expression levels of growth-regulatory and cell survival proteins, resulting in altered mitogenic rates and thresholds at which genotoxic stresses cause cell death. Inhibition of signaling in one and/or multiple survival pathways enhances radiosensitivity. Prolonged inhibition of any one of these pathways, however, gives rise to lineages of cells, which have become resistant to the inhibitor drug, by evolutionary selection for the clonal outgrowth of cells with point mutations in the specific targeted protein that make the target protein drug resistant or by the reprogramming of multiple signaling processes within all cells, to maintain viability. Thus, tumor cells are dynamic with respect to their reliance on specific cell signaling pathways to exist and rapidly adapt to repeated toxic challenges in an attempt to maintain tumor cell survival.

MAP kinases: Differential activation following in vivo and ex vivo irradiation

Mitogen activated protein kinases (MAPK) play a critical role in controlling cell survival and repopulation following exposure to ionising radiation. Most investigations on these pathways have been done using cultured cells or by ex vivo treatments. The present study was carried out to determine whether the response of MAPKs in mouse lymphocytes differs following in vivo and ex vivo irradiation with 60Co gamma-rays. We observed that ex vivo treatment resulted in a very significant decrease in the activated p44/42 and p38 MAPK as compared to in vivo. However, stress activated protein kinase (SAPK) response showed no significant difference between in vivo and ex vivo treatments. These observations point towards the differences in response elicited when the treatment is given in vivo as compared to in vitro. Therefore the findings reported from in vitro or ex vivo treatments should be treated with caution especially if it has to be clinically applied.

Diazoxide acts more as a PKC-epsilon activator, and indirectly activates the mitochondrial K(ATP) channel conferring cardioprotection against hypoxic injury

BACKGROUND AND PURPOSE

Diazoxide, a well-known opener of the mitochondrial ATP-sensitive potassium (mitoK(ATP)) channel, has been demonstrated to exert cardioprotective effect against ischemic injury through the mitoK(ATP) channel and protein kinase C (PKC). We aimed to clarify the role of PKC isoforms and the relationship between the PKC isoforms and the mitoK(ATP) channel in diazoxide-induced cardioprotection.

EXPERIMENTAL APPROACH

In H9c2 cells and neonatal rat cardiomyocytes, PKC-epsilon activation was examined by Western blotting and kinase assay. Flavoprotein fluorescence, mitochondrial Ca(2+) and mitochondrial membrane potential were measured by confocal microscopy. Cell death was determined by TUNEL assay.

KEY RESULTS

Diazoxide (100 microM) induced translocation of PKC-epsilon from the cytosolic to the mitochondrial fraction. Specific blockade of PKC-epsilon by either epsilonV1-2 or dominant negative mutant PKC-epsilon (PKC-epsilon KR) abolished the anti-apoptotic effect of diazoxide. Diazoxide-induced flavoprotein oxidation was inhibited by either epsilonV1-2 or PKC-epsilon KR transfection. Treatment with 5-hydroxydecanoate (5-HD) did not affect translocation and activation of PKC-epsilon induced by diazoxide. Transfection with wild type PKC-epsilon mimicked the flavoprotein-oxidizing effect of diazoxide, and this effect was completely blocked by epsilonV1-2 or 5-HD. Diazoxide prevented the increase in mitochondrial Ca(2+), mitochondrial depolarization and cytochrome c release induced by hypoxia and all these effects of diazoxide were blocked by epsilonV1-2 or 5-HD.

CONCLUSIONS AND IMPLICATIONS

Diazoxide induced isoform-specific translocation of PKC-epsilon as an upstream signaling molecule for the mitoK(ATP) channel, rendering cardiomyocytes resistant to hypoxic injury through inhibition of the mitochondrial death pathway.

Minocycline exerts inhibitory effects on multiple mitogen-activated protein kinases and IkappaBalpha degradation in a stimulus-specific manner in microglia

CNS inflammation mediated by microglial activation can result in neuronal and glial cell death in a variety of neurodegenerative and demyelinating diseases. Minocycline, a second-generation tetracycline, has profound anti-inflammatory properties in the CNS mediated, in part, by inhibition of microglia. MAPK and nuclear factor-kappaB (NF-kappaB) activation are hallmarks of activated microglia and they are critical for the expression of many inflammatory mediators. In the present study, we investigated minocycline effects on activation of p38, c-Jun-N-terminal activated protein kinase (JNK) 1/2 and extracellular signal regulated kinase (ERK) 1/2 MAPKs and inhibitor alpha of NF-kappaB (IkappaBalpha) degradation in BV-2 and primary microglial cells. Our results demonstrate that minocycline has the ability to inhibit all MAPKs but these effects strongly depend on the stimulus used for MAPK activation. Minocycline significantly decreased activation of all lipopolysaccharide-stimulated MAPKs but it was without effect on MAPKs activated by H2O2. Minocycline inhibited JNK1/2 and ERK1/2 but not p38 when stimulated by 2',3'-O-(4-benzoylbenzoyl)-adenosine 5'-triphosphate, indicating that minocycline affects only certain upstream signaling target(s) that are stimulus-specific. Our data also suggest that protein kinase C (PKC) inhibition may be partially involved in the minocycline mechanism of MAPK inhibition. In addition, minocycline attenuated lipopolysaccharide-stimulated degradation of IkappaBalpha implying a possible inhibitory role on NF-kappaB transcriptional activity.

Identification of caspase-independent PKCepsilon-JNK/p38 MAPK signaling module in response to metabolic inhibition in H9c2 cells

To understand the molecular mechanism of ischemia-induced cardiac myocyte cell death, H9c2 cells were studied by chemical hypoxia (CH), using metabolic inhibition buffer. CH suppressed the activities of caspase-3, -8, and -9. c-Jun N-terminal kinase (JNK), and p38 mitogen-activated protein kinase (MAPK) were activated, whereas extracellular regulated kinase (ERK) was inactivated. Only protein kinase Cepsilon (PKCepsilon) among PKC isotypes was translocated to the membrane fraction implying its activation. Moreover, the administration of PKCepsilon inhibitor suppressed the phosphorylations of JNK/p38 MAPK and reduced CH-induced cell death. An administration of JNK/p38 MAPK inhibitors also decreased CH-induced cell deaths, implying JNK/p38 MAPK's causative roles in the deaths. Collectively, this study identified a novel caspase-independent PKCepsilon-JNK/p38 MAPK signaling module induced by CH in cardiac myocytes. Our data show that the PKCepsilon-JNK/p38 MAPK signaling module contributes to CH-induced H9c2 cell death. This contrasts with previous notions, i.e., PKCepsilon's protective effect against ischemic death. Thus our data suggest that PKCepsilon can mediate alternative signals, i.e., beneficiary or deleterious signals, depending on the cell type, intensity, and/or type of injury.

Enhanced retinoid-induced apoptosis of MDA-MB-231 breast cancer cells by PKC inhibitors involves activation of ERK

Retinoids are vitamin A derivatives, which cause growth inhibition, differentiation and/or apoptosis in various cell types, including some breast cancer cells. In general, estrogen receptor (ER)-positive cells are retinoic acid (RA) sensitive, whereas ER-negative cells are resistant. In this report, we show that ER-negative MDA-MB-231 cells are strongly growth inhibited by retinoids in combination with a PKC inhibitor. While neither RA nor GF109203X (GF) has a significant growth inhibitory effect in these cells, RA+GF potently suppress proliferation. We found that RA+GF induce apoptosis, as shown by an increase in fragmented DNA, Annexin-V-positive cells and caspase-3 activation. Apoptosis was also induced by GF in combination with two synthetic retinoids. Expression of phosphorylated as well as total PKC was decreased by GF and this was potentiated by RA. In addition, treatment with GF caused a strong and sustained activation of ERK1/2 and p38-MAPK, as well as a weaker activation of JNK. Importantly, inhibition of ERK but not p38 or JNK suppressed apoptosis induced by RA+GF, indicating that activation of ERK is specifically required. In support of this novel finding, the ability of other PKC inhibitors to cause apoptosis in combination with RA correlates with ability to cause sustained activation of ERK.

Synergistic Growth of Stem Cell Factor and Granulocyte Macrophage Colony-stimulating Factor Involves Kinase-dependent and -independent Contributions from c-Kit

Stem cell factor (SCF) binds and activates the receptor tyrosine kinase c-Kit, and this interaction is critical for normal hematopoiesis. SCF also synergizes with a variety of growth factors, including those binding members of the cytokine receptor superfamily. The mechanisms mediating this synergy remain to be defined. The present study investigates both structural and biochemical cross-talk between c-Kit and the receptor for granulocyte macrophage colony-stimulating factor (GM-CSF). We have found that c-Kit forms a complex with the beta-chain of the GM-CSF receptor, and this interaction involves the first part of the c-Kit kinase domain. Although inhibition of c-Kit kinase activity completely blocked SCF-induced proliferation, there was still greater than additive growth induced by SCF in combination with GM-CSF. In contrast, an inhibitory antibody against the extracellular domain of c-Kit (K-27) completely inhibited growth in response to SCF alone or in combination with GM-CSF. These results support a kinase-independent component of the synergistic growth induced by SCF and GM-CSF that may relate to interaction of these receptors. It is also clear that a significant part of the synergistic growth is dependent of c-Kit kinase activity. Although synergistic increases in phosphorylation of c-Kit and the beta-chain of the GM-CSF receptor were not observed, SCF and GM-CSF in combination prolonged the duration of Erk1/2 phosphorylation in a phosphatidylinositol 3-kinase-dependent manner. Consistent with these findings, phosphatidylinositol 3-kinase is synergistically activated by SCF and GM-CSF together. Hence, c-Kit makes both kinase-independent and -dependent contributions to the proliferative synergy induced by SCF in combination with GM-CSF.

Divergence and complexities in DAG signaling: looking beyond PKC

For many years protein kinase C (PKC) has been the subject of extensive studies as a molecular target for the treatment of cancer and other diseases. To better define the role of PKC isozymes in the control of cell proliferation, survival and transformation, the examination of PKC-mediated signal transduction pathways by isozyme-specific intervention has become essential. However, issues related to the selectivity of activators and inhibitors of PKC isozymes, in addition to convoluted cross-talks between phorbol ester-regulated pathways, have greatly complicated our understanding of PKC-mediated responses. An additional level of complexity is provided by the fact diacylglycerol (DAG) signals can be transduced by phorbol ester receptors other than PKC. These receptors include chimaerins, RasGRPs, MUNC13s, PKD (PKC mu) and DAG kinases beta and gamma. Thus, it is conceivable that some of the effects that were originally attributed to PKC isozymes in response to phorbol esters might be mediated by PKC-independent pathways. A key issue for the design of novel therapeutic strategies that target PKC isozymes is a comprehensive analysis of isozyme-specific signal transduction pathways in different cell types and the development of pharmacological and molecular tools that can distinguish between the various PKC and 'non-PKC' phorbol ester receptors.

Role for PKC-ε in neuronal death induced by oxidative stress

We investigated which isoforms of PKCs can be modulated and what their roles are during l-buthionine-S,R-sulfoximine (BSO)-induced neuronal death. We observed the isoform specific translocation of PKC-epsilon from the soluble fraction to the particulate in cortical neurons treated with 10 mM BSO. The translocation of PKC-epsilon by BSO was blocked by antioxidant trolox, suggesting the PKC-epsilon as a downstream of reactive oxygen species (ROS) elevated by BSO. Trolox inhibited the ROS elevation and the neuronal death in BSO-treated cortical cells. The BSO-induced neuronal death was remarkably inhibited by both the pharmacological inhibition of PKC-epsilon with epsilonV1-2 and the functional blockade for PKC-epsilon through overexpression of PKC-epsilon V1 region, suggesting the detrimental role of PKC-epsilon. These results suggest that PKC-epsilon is the major PKC isoform involved in the pathways triggered by ROS, leading to neuronal death in BSO-treated cortical neurons.

N-Nitrosocarbofuran Induces Apoptosis in Mouse Brain Microvascular Endothelial Cells (bEnd.3)

In this study, we investigated whether carbofuran, a commonly used carbamate pesticide, and N-nitrosocarbofuran (NOCF), the N-nitroso metabolite of carbofuran, have cytotoxicity in mouse brain microvascular endothelial cells (bEnd.3). Results from the MTT assay in bEnd.3 cells showed that NOCF but not carbofuran caused a remarkable decrease in cell viability. The cell death induced by NOCF appeared to involve apoptosis, based on our results from annexin V staining and electron microscopy. To investigate the mechanism of the NOCF-induced cell death, we examined the effects of selective inhibitors for MAP kinase pathways, PD98059 (for MEK/ERK), SB202190 (for p38 MAP kinase), and SP600125 (for JNK), on the NOCF-induced cell death. The NOCF-induced cell death was significantly reduced by PD98059, but not by SB202190 or SP600125. NOCF increased ERK phosphorylation as early as 15 min after the treatment and this increase was maintained for 2 h. In summary, our results suggest that NOCF can induce apoptotic cell death, at least in part, through the ERK pathway in brain microvascular endothelial cells.