Differential sensitivity of breast cancer cells to tumor necrosis factor-alpha: involvement of protein kinase C

Paclitaxel Promotes Cell Apoptosis in Uterine Leiomyomas

Uterine leiomyomas are common clinical gynecological tumors, which are a major health concern for many women. In the current study, we aimed to investigate the effect of paclitaxel (PTX) on uterine leiomyomas. A mouse model of uterine leiomyomas was established by estradiol benzoate, followed by treatment with increasing doses of PTX. PTX showed no dose-limiting toxicity that affected the survival of mice, and was able to restore the apoptosis level of uterus tissues of the model mice to normal levels. In this study, we discovered that PTX played a critical role in promoting apoptosis in the mouse model of uterine leiomyomas, which provides a new insight into the therapy of uterine leiomyomas.

Apoptosis, Chemoresistance, and Breast Cancer: Insights From the MCF-7 Cell Model System

The MCF-7 cell line was derived from a patient with metastatic breast cancer in 1970. Since then it has become a prominent model system for the study of estrogen receptor-positive breast cancer. With this model as a focus, this review summarizes important studies addressing tumor necrosis factor-α as a prototypical apoptosis-inducing cytokine in MCF-7 cells. Both survival and death receptor signaling pathways are discussed in terms of their role in chemotherapy-induced apoptosis as well as in chemoresistance. Novel therapeutic approaches to the treatment of breast cancer are proposed utilizing knowledge of these signaling pathways as targets. Specifically, ceramide metabolism is proposed as a novel target for chemosensitivity, perhaps combined with selective inhibitors of Bcl-2 or PI3K/Akt/nuclear factor-κB. Suggested areas of future research include translational studies manipulating candidate survival and death signaling pathways.

Breast Cancer Genetic and Molecular Subtype Impacts Response to Omega-3 Fatty Acid Ethyl Esters

Epidemiological studies have correlated frequent omega-3 (n-3) fatty acid consumption with a lower risk for breast cancer; however, recent prospective studies have been less conclusive. Efforts in the preventive setting have focused on the use of n-3 fatty acids, and the pharmaceutical ethyl esters (EE) of these natural compounds, for high-risk patient populations. Limited understanding of specific mechanisms by which these agents function has hampered identification of the cancer subtype(s) that would gain the greatest therapeutic benefit. In this study, we investigated the in vitro effects of n-3 EEs in four distinct breast cancer subtypes and explored how they affect not only breast cancer cell survival but also modulate the nuclear factor kappa-light-chain enhancer of activated B cells (NF-κB) and peroxisome proliferator-activated receptor gamma signaling pathways. Similar to the high variance in response observed in human studies, we found that the effectiveness of n-3 EEs depends on the molecular characteristics of the MCF-7, CAMA-1, MDA-MB-231, and SKBR3 breast cancer cell lines and is closely associated with the suppression of NF-κB. These data strongly suggest that the use of n-3 fatty acids and their pharmaceutical ether esters in the prevention and therapeutic setting should be guided by specific tumor characteristics.

Ethambutol induces impaired autophagic flux and apoptosis in the rat retina

Ethambutol (EMB), an effective first-line antituberculosis agent, can cause serious visual impairment or irreversible vision loss in a significant number of patients. However, the mechanism underlying this ocular cytotoxicity remains to be elucidated. In this study, we found that there were statistically significant dose- and time-dependent increases in the number of cytoplasmic vacuoles and the level of cell death in EMB-treated RGC-5 cells (retinal ganglion cells). The protein kinase C (PKC)δ inhibitor rottlerin markedly reduced the EMB-induced activation of caspase-3 and the subsequent apoptosis of RGC-5 cells. Western blot analysis revealed that the expression levels of class III PI3K, Beclin-1, p62 and LC3-II were upregulated, and LC3 immunostaining results showed activation of the early phase and inhibition of the late stage of autophagy in retinas of the EMB-intraperitoneal (IP)-injected rat model. We further demonstrated that exposure to EMB induces autophagosome accumulation, which results from the impaired autophagic flux that is mediated by a PKCδ-dependent pathway, inhibits the PI3K/Akt/mTOR signaling pathway and leads to apoptotic death in retina neuronal cells. These results indicate that autophagy dysregulation in retinal neuronal cells might play a substantial role in EMB-induced optic neuroretinopathy.

Summary: This study provides the first evidence that EMB induces autophagosome accumulation, which results from the impaired autophagic flux that is mediated by a PKCδ-dependent pathway, and leads to apoptotic death in retina neuronal cells.

Expression of immunomodulatory neutrophil-activating protein of Helicobacter pylori enhances the antitumor activity of oncolytic measles virus

Helicobacter pylori neutrophil-activating protein (NAP) is a major virulence factor and powerful inducer of inflammatory reaction and Th1-polarized immune response. Here, we evaluated the therapeutic efficacy of measles virus (MV) strains engineered to express secretory NAP forms against metastatic breast cancer. Recombinant viruses encoding secretory NAP forms (MV-lambda-NAP and MV-s-NAP) efficiently infect and destroy breast cancer cells by cell-to-cell viral spread and large syncytia formation independently of hormone receptor status. Intrapleural administration of MV-s-NAP doubled the median survival in a pleural effusion xenograft model: 65 days as compared to 29 days in the control group (P < 0.0001). This therapeutic effect correlated with a brisk Th1 type cytokine response in vivo. Secretory NAP was expressed at high levels by infected tumor cells and increased tumor necrosis factor-α (TNF-α), interleukin-6 (IL-6), and IL-12/23 cytokine concentrations were detected in the pleural effusion. In an aggressive model of lung metastatic breast cancer, MV-lambda-NAP and MV-s-NAP also significantly improved survival of the treated animals (P < 0.05) as compared to the control MV strain. These data suggest that potent immunomodulators of bacterial origin, such as H. pylori NAP, can enhance the antitumor effect of oncolytic viruses and support the feasibility and potential of a combined viroimmunotherapy approach.

Rottlerin and cancer: novel evidence and mechanisms

Because cancers are caused by deregulation of hundreds of genes, an ideal anticancer agent should target multiple gene products or signaling pathways simultaneously. Recently, extensive research has addressed the chemotherapeutic potential of plant-derived compounds. Among the ever-increasing list of naturally occurring anticancer agents, Rottlerin appears to have great potentiality for being used in chemotherapy because it affects several cell machineries involved in survival, apoptosis, autophagy, and invasion. The underlying mechanisms that have been described are diverse, and the final, cell-specific, Rottlerin outcome appears to result from a combination of signaling pathways at multiple levels. This paper seeks to summarize the multifocal signal modulatory properties of Rottlerin, which merit to be further exploited for successful prevention and treatment of cancer.

The estrogen receptor alpha-derived peptide ERα17p (P(295)-T(311)) exerts pro-apoptotic actions in breast cancer cells in vitro and in vivo, independently from their ERα status

In recent years, our knowledge on estrogen receptors (ER) has been modified profoundly with the identification and the deciphering of the role of its protein effectors, as well as with the deeper insight of its molecular structure/function dynamics, characteristics associated with its nucleo-cytoplasmic-membrane shuttling properties. Also, significant progress has been made concerning its turn-over and associated final proteasomal degradation processes. These advances could lead in the near future to the design and the synthesis of novel receptor-interacting drugs. Recently, a number of receptor-related peptides acting as specific ER ligands have been identified and extensively studied with respect to their estrogenic/antiestrogenic activities. Among them, ERα17p, a synthetic analog of the P(295)-T(311) sequence of ERα, has been shown to exert pseudo-estrogenic effects by interacting in the close vicinity of its hinge region (BF3 domain). Remarkably, this sequence appears as the epicenter of a number of post-transcriptional modifications as well as of the recruitment of co-regulators, suggesting that it would play a key role in ERα functions. Here, we provide evidence that ERα17p induces apoptosis in ERα-positive (MCF-7, T47D) and -negative (MDA-MB-231, SK-BR-3) breast cancer cells by an ERα-independent membrane mechanism, triggering major pro-apoptotic signaling cascades. Finally, ERα17p induces the regression of breast ERα-negative cancer tumor xenografts, without apparent toxicity, suggesting that it could represent a new attractive tool for the development of future promising therapeutic approaches, and providing a novel insight to ER regulation of cell fate.

Regulation of translocator protein 18 kDa (TSPO) expression in health and disease states

Translocator protein (TSPO) is an 18 kDa high affinity cholesterol- and drug-binding protein found primarily in the outer mitochondrial membrane. Although TSPO is found in many tissue types, it is expressed at the highest levels under normal conditions in tissues that synthesize steroids. TSPO has been associated with cholesterol import into mitochondria, a key function in steroidogenesis, and directly or indirectly with multiple other cellular functions including apoptosis, cell proliferation, differentiation, anion transport, porphyrin transport, heme synthesis, and regulation of mitochondrial function. Aberrant expression of TSPO has been linked to multiple diseases, including cancer, brain injury, neurodegeneration, and ischemia-reperfusion injury. There has been an effort during the last decade to understand the mechanisms regulating tissue- and disease-specific TSPO expression and to identify pharmacological means to control its expression. This review focuses on the current knowledge regarding the chemicals, hormones, and molecular mechanisms regulating Tspo gene expression under physiological conditions in a tissue- and disease-specific manner. The results described here provide evidence that the PKCepsilon-ERK1/2-AP-1/STAT3 signal transduction pathway is the primary regulator of Tspo gene expression in normal and pathological tissues expressing high levels of TSPO.

Emerging biomarkers in breast cancer care

Currently, decision-making for breast cancer treatment in the clinical setting is mainly based on clinical data, histomorphological features of the tumor tissue and a few cancer biomarkers such as steroid hormone receptor status (estrogen and progesterone receptors) and oncoprotein HER2 status. Although various therapeutic options were introduced into the clinic in recent decades, with the objective of improving surgery, radiotherapy, biochemotherapy and chemotherapy, varying response of individual patients to certain types of therapy and therapy resistance is still a challenge in breast cancer care. Therefore, since breast cancer treatment should be based on individual features of the patient and her tumor, tailored therapy should be an option by integrating cancer biomarkers to define patients at risk and to reliably predict their course of the disease and/or response to cancer therapy. Recently, candidate-marker approaches and genome-wide transcriptomic and epigenetic screening of different breast cancer tissues and bodily fluids resulted in new promising biomarker panels, allowing breast cancer prognosis, prediction of therapy response and monitoring of therapy efficacy. These biomarkers are now subject of validation in prospective clinical trials.

PKC inhibition increases gap junction intercellular communication and cell adhesion in human neuroblastoma

Gap junction intercellular communication and cell-cell adhesion are essential for maintaining a normal cellular phenotype, including the control of growth and proliferation. Loss of either cell-cell adhesion or communication is common in cancers, while restoration of function is associated with tumor suppression. Protein kinase C (PKC) isozymes regulate a broad spectrum of cellular functions including growth and proliferation, and their overexpression has been correlated with carcinogenesis. Consequently, PKC inhibitors are currently undergoing clinical trials as an anti-cancer agents although the precise cellular alterations induced by PKC inhibitors remain to be elucidated. In the current study, the effects of PKC inhibitors on cell interactions were investigated using human neuroblastoma (IMR32, SKNMC, and SHSY-5Y) cell lines. An analysis of intercellular communication revealed an increase in gap junctional coupling with PKC inhibition. The observed increase in coupling was not associated with a change in Connexin 43 distribution or an alteration of phosphorylation status of the protein. There was also an increase in cell-cell adhesion with PKC inhibitor treatment as indicated by a cell aggregation assay. Therefore, the growth suppressive abilities of PKC inhibition on tumors may be due to the cancer suppressive effects of increased gap junction intercellular communication and cell-cell adhesion.

Quantitative kinetic study of the actin-bundling protein L-plastin and of its impact on actin turn-over

Background

Initially detected in leukocytes and cancer cells derived from solid tissues, L-plastin/fimbrin belongs to a large family of actin crosslinkers and is considered as a marker for many cancers. Phosphorylation of L-plastin on residue Ser5 increases its F-actin binding activity and is required for L-plastin-mediated cell invasion.

Methodology/Principal Findings

To study the kinetics of L-plastin and the impact of L-plastin Ser5 phosphorylation on L-plastin dynamics and actin turn-over in live cells, simian Vero cells were transfected with GFP-coupled WT-L-plastin, Ser5 substitution variants (S5/A, S5/E) or actin and analyzed by fluorescence recovery after photobleaching (FRAP). FRAP data were explored by mathematical modeling to estimate steady-state reaction parameters. We demonstrate that in Vero cell focal adhesions L-plastin undergoes rapid cycles of association/dissociation following a two-binding-state model. Phosphorylation of L-plastin increased its association rates by two-fold, whereas dissociation rates were unaffected. Importantly, L-plastin affected actin turn-over by decreasing the actin dissociation rate by four-fold, increasing thereby the amount of F-actin in the focal adhesions, all these effects being promoted by Ser5 phosphorylation. In MCF-7 breast carcinoma cells, phorbol 12-myristate 13-acetate (PMA) treatment induced L-plastin translocation to de novo actin polymerization sites in ruffling membranes and spike-like structures and highly increased its Ser5 phosphorylation. Both inhibition studies and siRNA knock-down of PKC isozymes pointed to the involvement of the novel PKC-δ isozyme in the PMA-elicited signaling pathway leading to L-plastin Ser5 phosphorylation. Furthermore, the L-plastin contribution to actin dynamics regulation was substantiated by its association with a protein complex comprising cortactin, which is known to be involved in this process.

Conclusions/Significance

Altogether these findings quantitatively demonstrate for the first time that L-plastin contributes to the fine-tuning of actin turn-over, an activity which is regulated by Ser5 phosphorylation promoting its high affinity binding to the cytoskeleton. In carcinoma cells, PKC-δ signaling pathways appear to link L-plastin phosphorylation to actin polymerization and invasion.

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.

The actin filament cross-linker L-plastin confers resistance to TNF-alpha in MCF-7 breast cancer cells in a phosphorylation-dependent manner

We used a tumour necrosis factor (TNF)-α resistant breast adenocarcinoma MCF-7 cell line to investigate the involvement of the actin cytoskeleton in the mechanism of cell resistance to this cytokine. We found that TNF resistance correlates with the loss of cell epithelial properties and the gain of a mesenchymal phenotype, reminiscent of an epithelial-to-mesenchymal transition (EMT). Morphological changes were associated with a profound reorganization of the actin cytoskeleton and with a change in the repertoire of expressed actin cytoskeleton genes and EMT markers, as revealed by DNA microarray-based expression profiling. L-plastin, an F-actin cross-linking and stabilizing protein, was identified as one of the most significantly up-regulated genes in TNF-resistant cells. Knockdown of L-plastin in these cells revealed its crucial role in conferring TNF resistance. Importantly, overexpression of wild-type L-plastin in TNF-sensitive MCF-7 cells was sufficient to protect them against TNF-mediated cell death. Furthermore, we found that this effect is dependent on serine-5 phosphorylation of L-plastin and that non-conventional protein kinase C isoforms and the ceramide pathway may regulate its phosphorylation state. The protective role of L-plastin was not restricted to TNF-α resistant MCF-7 cells because a correlation between the expression of L-plastin and the resistance to TNF-α was observed in other breast cancer cell lines. Together, our study discloses a novel unexpected role of the actin bundling protein L-plastin as a cell protective protein against TNF-cytotoxicity.

Hyaluronan-CD44 interaction with protein kinase C(epsilon) promotes oncogenic signaling by the stem cell marker Nanog and the Production of microRNA-21, leading to down-regulation of the tumor suppressor protein PDCD4, anti-apoptosis, and chemotherapy resistance in breast tumor cells

Multidrug resistance and disease relapse is a challenging clinical problem in the treatment of breast cancer. In this study, we investigated the hyaluronan (HA)-induced interaction between CD44 (a primary HA receptor) and protein kinase Cepsilon (PKCepsilon), which regulates a number of human breast tumor cell functions. Our results indicate that HA binding to CD44 promotes PKCepsilon activation, which, in turn, increases the phosphorylation of the stem cell marker, Nanog, in the breast tumor cell line MCF-7. Phosphorylated Nanog is then translocated from the cytosol to the nucleus and becomes associated with RNase III DROSHA and the RNA helicase p68. This process leads to microRNA-21 (miR-21) production and a tumor suppressor protein (e.g. PDCD4 (program cell death 4)) reduction. All of these events contribute to up-regulation of inhibitors of apoptosis proteins (IAPs) and MDR1 (multidrug-resistant protein), resulting in anti-apoptosis and chemotherapy resistance. Transfection of MCF-7 cells with PKCepsilon or Nanog-specific small interfering RNAs effectively blocks HA-mediated PKCepsilon-Nanog signaling events, abrogates miR-21 production, and increases PDCD4 expression/eIF4A binding. Subsequently, this PKCepsilon-Nanog signaling inhibition causes IAP/MDR1 down-regulation, apoptosis, and chemosensitivity. To further evaluate the role of miR-21 in oncogenesis and chemoresistance, MCF-7 cells were also transfected with a specific anti-miR-21 inhibitor in order to silence miR-21 expression and inhibit its target functions. Our results indicate that anti-miR-21 inhibitor not only enhances PDCD4 expression/eIF4A binding but also blocks HA-CD44-mediated tumor cell behaviors. Thus, this newly discovered HA-CD44 signaling pathway should provide important drug targets for sensitizing tumor cell apoptosis and overcoming chemotherapy resistance in breast cancer cells.

t10c12 conjugated linoleic acid suppresses HER2 protein and enhances apoptosis in SKBr3 breast cancer cells: possible role of COX2

Background

HER2-targeted therapy with the monoclonal antibody trastuzumab (Herceptin®) has improved disease-free survival for women diagnosed with HER2-positive breast cancers; however, treatment resistance and disease progression are not uncommon. Current data suggest that resistance to treatment in HER2 cancers may be a consequence of NF-κB overexpression and increased COX2-derived prostaglandin E2 (PGE₂). Conjugated linoleic acid (CLA) has been shown to have anti-tumor properties and to inhibit NF-κB activity and COX2.

Methods

In this study, HER2-overexpressing SKBr3 breast cancer cells were treated with t10c12 CLA. Protein expression of the HER2 receptor, nuclear NF-κB p65, and total and phosphorylated IκB were examined by western blot and immunofluorescence. PGE₂ levels were determined by ELISA. Proliferation was measured by metabolism of 3-(4, 5-Dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide (MTT), and apoptosis was measured by FITC-conjugated Annexin V staining and flow cytometry.

Results/Conclusions

We observed a significant decrease in HER2 protein expression on western blot following treatment with 40 and 80 µM t10c12 CLA (p<0.01 and 0.001, respectively) and loss of HER2 protein in cells using immunoflourescence that was most pronounced at 80 µM. Protein levels of nuclear NF-κB p65 were also significantly reduced at the 80 µM dose. This was accompanied by a significant decrease in PGE₂ levels (p = 0.05). Pretreatment with t10c12 CLA significantly enhanced TNFα-induced apoptosis and the anti-proliferative action of trastuzumab (p = 0.05 and 0.001, respectively). These data add to previous reports of an anti-tumor effect of t10c12 CLA and suggest an effect on the HER2 oncogene that may be through CLA mediated downregulation of COX2-derived PGE₂.

Phorbol-12-myristate 13-acetate acting through protein kinase Cepsilon induces translocator protein (18-kDa) TSPO gene expression

Translocator protein (TSPO) is an 18-kDa cholesterol-binding protein that is expressed at high levels in steroid synthesizing and several cancer cells where it is involved in steroidogenesis and cell proliferation, respectively. The factors regulating Tspo expression are unknown. We analyzed Tspo transcriptional responses to the tumor promoter, phorbol-12-myristate 13-acetate (PMA), in cells with varying TSPO levels. PMA induced Tspo promoter activity and Tspo mRNA levels in TSPO-poor nonsteroidogenic cells (NIH-3T3 fibroblasts and COS-7 kidney) but not in TSPO-rich steroidogenic cells (MA-10 Leydig) with high basal Tspo transcriptional activity. The stimulatory effect of PMA was mediated by an 805-515-bp region upstream of the transcription start site. Electrophoretic mobility shift assay (EMSA) revealed that PMA induced binding of c-jun and GA-binding protein transcription factor (GABP-alpha) to their respective activator protein 1 (AP1) and v-ets erythroblastosis virus E26 oncogene homologue (Ets) sites in this region. Protein kinase C (PKC)-specific inhibitors blocked PMA induction of Tspo promoter activity with an inhibition profile suggestive of involvement of PKCepsilon. PKCepsilon expression correlated with TSPO content in the three cell lines. In NIH-3T3 cells, PKCepsilon overexpression induced Tspo promoter activity and mRNA levels and enhanced PMA-induced up regulation of c-jun and TSPO. In MA-10 cells, a PKCepsilon-specific translocation inhibitor peptide reduced basal Tspo promoter activity. PKCepsilon siRNA pool reduced PKCepsilon and TSPO levels in MA-10 cells indicating a role for PKCepsilon in regulating TSPO expression. Taken together, these data suggest that elevated TSPO expression in steroidogenic cells may be due to high constitutive expression of PKCepsilon that renders them unresponsive to further induction while PMA activation of PKCepsilon drives inducible TSPO expression in nonsteroidogenic cells, likely through AP1 and Ets.

Down-regulation of caspase-2 by rottlerin via protein kinase C-delta-independent pathway

Protein kinase C-delta (PKC delta) plays an important role in DNA damage-induced apoptosis. We have previously shown that the PKC delta inhibitor rottlerin protects against cisplatin-induced apoptosis acting upstream of caspase-9. In the present study, we have investigated if rottlerin regulates caspase-2 activation. Knockdown of caspase-2 by siRNA inhibited processing of apical caspase-9 and caspase-8, whereas depletion of caspase-9 had little effect on caspase-2 processing. Rottlerin inhibited activation and processing of caspase-9 and caspase-8 and cleavage of poly(ADP)ribose polymerase. We made a novel observation that rottlerin induced down-regulation of caspase-2 but not of caspase-3, caspase-7, caspase-8, or caspase-9. Pharmacologic inhibitors of PKC, such as Gö 6983 and bisindolylmaleimide, or depletion of PKC delta by siRNA had no effect on the down-regulation of caspase-2 by rottlerin. The proteasome inhibitor MG132 reversed caspase-2 down-regulation by rottlerin, whereas calpain inhibitor had no effect. These results suggest that rottlerin induces down-regulation of caspase-2 via PKC delta-independent but ubiquitin proteasome-mediated pathway. Furthermore, down-regulation of caspase-2 by rottlerin can explain its antiapoptotic function during DNA damage-induced apoptosis.

Inhibition of ERK attenuates autophagy and potentiates tumour necrosis factor-alpha-induced cell death in MCF-7 cells

The role of autophagy in cell death is under considerable debate. The process of autophagy has been shown to lead to either cell survival or cell death depending on cell type and stimulus. In the present study, we determined the contribution of ERK1/2 signalling to autophagy and cell death induced by tumour necrosis factor-α (TNF) in MCF-7 breast cancer cells. Treatment of MCF-7 cells with TNF caused a time-dependent increase in ERK1/2 activity. There was an induction of autophagy and cleavage of caspase-7, -8, -9 and PARP. Pharmacological inhibition of ERK1/2 phosphorylation with U0126 or PD98059 resulted in a decrease in TNF-induced autophagy that was accompanied by an increase in cleavage of caspase-7, -8, -9 and PARP Furthermore, inhibition of ERK1/2 signalling resulted in decreased clonogenic capacity of MCF-7 cells. These data suggest that TNF-induces autophagy through ERK1/2 and that inhibition of autophagy increases cellular sensitivity to TNF.

Phorbol ester-induced apoptosis and senescence in cancer cell models

Protein kinase C (PKC) isozymes catalyze the phosphorylation of substrates that play key roles in the control in proliferation, differentiation, and survival. Treatment of cells with phorbol esters, activators of classical and novel PKC isozymes, leads to a plethora of responses in a strict cell-type-dependent specific manner. Interestingly, a few cell models undergo apoptosis in response to phorbol ester stimulation, including androgen-dependent prostate cancer cells. This effect involves the autocrine secretion of death factors and activation of the extrinsic apoptotic cascade. We have recently found that in other models, such as lung cancer cells, phorbol esters lead to irreversible growth arrest and senescence. This chapter describes the methods we use to assess these phorbol ester responses in cancer cell models, focusing on apoptosis and senescence.

Adenine nucleotide (ADP/ATP) translocase 3 participates in the tumor necrosis factor induced apoptosis of MCF-7 cells

Mitochondrial adenine nucleotide translocase (ANT) is believed to be a component or a regulatory component of the mitochondrial permeability transition pore (mtPTP), which controls mitochondrial permeability transition during apoptosis. However, the role of ANT in apoptosis is still uncertain, because hepatocytes isolated from ANT knockout and wild-type mice are equally sensitive to TNF- and Fas-induced apoptosis. In a screen for genes required for tumor necrosis factor alpha (TNF-alpha)-induced apoptosis in MCF-7 human breast cancer cells using retrovirus insertion-mediated random mutagenesis, we discovered that the ANT3 gene is involved in TNF-alpha-induced cell death in MCF-7 cells. We further found that ANT3 is selectively required for TNF- and oxidative stress-induced cell death in MCF-7 cells, but it is dispensable for cell death induced by several other inducers. This data supplements previous data obtained from ANT knockout studies, indicating that ANT is involved in some apoptotic processes. We found that the resistance to TNF-alpha-induced apoptosis observed in ANT3 mutant (ANT3(mut)) cells is associated with a deficiency in the regulation of the mitochondrial membrane potential and cytochrome c release. It is not related to intracellular ATP levels or survival pathways, supporting a previous model in which ANT regulates mtPTP. Our study provides genetic evidence supporting a role of ANT in apoptosis and suggests that the involvement of ANT in cell death is cell type- and stimulus-dependent.

Protein kinase Cepsilon makes the life and death decision

Cancer is caused by dysregulation in cellular signaling systems that control cell proliferation, differentiation and cell death. Protein kinase C (PKC), a family of serine/threonine kinases, plays an important role in the growth factor signal transduction pathway. PKCepsilon, however, is the only PKCepsilon isozyme that has been considered as an oncogene. It can contribute to malignancy by enhancing cell proliferation or by inhibiting cell death. This review focuses on how PKCepsilon collaborates with other signaling pathways, such as Ras/Raf/ERK and Akt, to regulate cell survival and cell death. We have also discussed how PKCepsilon mediates its antiapoptotic signal by altering the level or function of pro- and antiapoptotic Bcl-2 family members.

Protein kinase C delta is required for survival of cells expressing activated p21RAS

Inhibition of protein kinase C (PKC) activity in transformed cells and tumor cells containing activated p21(RAS) results in apoptosis. To investigate the pro-apoptotic pathway induced by the p21(RAS) oncoprotein, we first identified the specific PKC isozyme necessary to prevent apoptosis in the presence of activated p21(RAS). Dominant-negative mutants of PKC, short interfering RNA vectors, and PKC isozyme-specific chemical inhibitors directed against the PKCdelta isozyme demonstrated that PKCdelta plays a critical role in p21(RAS)-mediated apoptosis. An activating p21(RAS) mutation, or activation of the phosphatidylinositol 3-kinase (PI3K) Ras effector pathway, increased the levels of PKCdelta protein and activity in cells, whereas inhibition of p21(RAS) activity decreased the expression of the PKCdelta protein. Activation of the Akt survival pathway by oncogenic Ras required PKCdelta activity. Akt activity was dramatically decreased after PKCdelta suppression in cells containing activated p21(RAS). Conversely, constitutively activated Akt rescued cells from apoptosis induced by PKCdelta inhibition. Collectively, these findings demonstrate that p21(RAS), through its downstream effector PI3K, induces PKCdelta expression and that this increase in PKCdelta activity, acting through Akt, is required for cell survival. The p21(RAS) effector molecule responsible for the initiation of the apoptotic signal after suppression of PKCdelta activity was also determined to be PI3K. PI3K (p110(C)(AAX), where AA is aliphatic amino acid) was sufficient for induction of apoptosis after PKCdelta inhibition. Thus, the same p21(RAS) effector, PI3K, is responsible for delivering both a pro-apoptotic signal and a survival signal, the latter being mediated by PKCdelta and Akt. Selective suppression of PKCdelta activity and consequent induction of apoptosis is a potential strategy for targeting of tumor cells containing an activated p21(RAS).

Protein kinase Cepsilon activates protein kinase B/Akt via DNA-PK to protect against tumor necrosis factor-alpha-induced cell death

We have previously shown that protein kinase Cepsilon (PKCepsilon) protects breast cancer cells from tumor necrosis factor-alpha (TNF)-induced cell death. In the present study, we have investigated if the antiapoptotic function of PKCepsilon is mediated via Akt and the mechanism by which PKCepsilon regulates Akt activity. TNF caused a transient increase in Akt phosphorylation at Ser473 in MCF-7 cells. Overexpression of PKCepsilon in MCF-7 cells increased TNF-induced Akt phosphorylation at Ser473 resulting in its activation. Knockdown of PKCepsilon by small interfering RNA (siRNA) decreased TNF-induced Akt phosphorylation/activation and increased cell death. Introduction of constitutively active Akt protected breast cancer MCF-7 cells from TNF-mediated cell death and partially restored cell survival in PKCepsilon-depleted cells. Depletion of Akt in MCF-7 cells abolished the antiapoptotic effect of PKCepsilon on TNF-mediated cell death. Akt was constitutively associated with PKCepsilon and DNA-dependent protein kinase (DNA-PK), and this association was increased by TNF treatment. Overexpression of PKCepsilon enhanced the interaction between Akt and DNA-PK. Knockdown of DNA-PK by siRNA inhibited TNF-induced Akt phosphorylation and the antiapoptotic effect of Akt and PKCepsilon. These results suggest that PKCepsilon activates Akt via DNA-PK to mediate its antiapoptotic function. Furthermore, we report for the first time that DNA-PK can regulate receptor-initiated apoptosis via Akt.

Apoptosis of non-small-cell lung cancer cell lines after paclitaxel treatment involves the BH3-only proapoptotic protein Bim

A significant variation in susceptibility to paclitaxel-mediated killing was observed among a panel of short-term cultured non-small-cell lung cancer (NSCLC) cell lines. Susceptibility to killing by paclitaxel correlated with expression of the BH3-only protein, Bim, but not with other members of Bcl-2 family. NSCLC cell lines with the highest level of Bim expression are most susceptible to apoptosis induction after paclitaxel treatment. Forced expression of Bim increased paclitaxel-mediated killing of cells expressing an undetectable level of Bim. Conversely, knock down of Bim, but not Bcl-2 expression, decreased the susceptibility of tumor cells to paclitaxel-mediated killing. Similar observations were made using a panel of breast and prostate cancer cell lines. Paclitaxel impairs microtubule function, causes G2/M cell cycle blockade, mitochondria damage, and p53-independent apoptosis. These results established Bim as a critical molecular link between the microtubule poison, paclitaxel, and apoptosis.

Enhancement of tumor necrosis factor-alpha-induced growth inhibition by insulin-like growth factor-binding protein-5 (IGFBP-5), but not IGFBP-3 in human breast cancer cells

Expression of IGF-binding protein-3 (IGFBP-3) and IGFBP-5 in human breast cancer cells induces apoptosis and is associated with modulations in Bcl-2 proteins, suggesting that these IGFBPs induce an intrinsic apoptotic pathway. In this study we demonstrate that although both IGFBPs induced the activation of caspase-8 and caspase-9, the expression of IGFBP-5, but not IGFBP-3, sensitized MDA-MB-231 breast cancer cells to the inhibitory effects of TNFalpha. This sensitivity to TNFalpha was associated with a block in nuclear factor-kappaB-mediated cell survival signals. IGFBP-5 expression was also associated with a caspase-8-independent activation of Bid, increased levels of cytosolic second mitochondria-derived activator of caspase (Smac)/direct inhibitor of apoptosis proteins (IAP) binding protein with low pI (DIABLO), and an enhanced phosphorylation of c-Jun N-terminal kinase, both basally and in response to TNFalpha. These results suggest that IGFBP-5 expression may influence extrinsic apoptotic pathways via a differential modulation of downstream cell survival and cell death pathways. Furthermore, although IGFBP-3 and IGFBP-5 share much structural and functional homology, they can modulate distinct apoptotic pathways in human breast cancer cells.

Apoptosis and production of TNF-alpha by tumor-associated inflammatory cells in histological grade III breast cancer

Tumor necrosis factor alpha (TNF-alpha) is a cytokine that acts as an important mediator of the apoptotic process that also demonstrates selective citotoxicity against malignant breast tumor cells. In the present study, the presence of apoptotic tumor cells and the synthesis of TNF-alpha by inflammatory cells were investigated in tissue samples from grade III invasive breast cancer with long-term follow-up. In situ detection of tumor apoptotic cells was investigated by direct immuno-peroxidase of digoxigenin-labeled genomic DNA. The production of TNF-alpha and tumor cell proliferation were investigated by immunohistochemical procedures. Our data demonstrated that patients with a clinical history of cancer recurrence and metastasis presented a lower number of cancerous apoptotic cells, higher tumor proliferation rates, and lower TNF-alpha expression rates by inflammatory cells than what is observed among patients diagnosed with the same histopathological breast cancer type but in the absence of tumor recurrence and metastasis.

PKCdelta and MAPK mediate G(1) arrest induced by PMA in SKBR-3 breast cancer cells

The effects of activating endogenous protein kinase C (PKC) on cell proliferation and the cell cycle were investigated by treating the breast cancer cell line SKBR-3 with phorbol 12-myristate 13 acetate (PMA). This inhibited cell growth in a concentration-dependent manner, causing a marked arrest of cells in G(1). Pre-treatment with GF109203X completely blocked the antiproliferative effect of PMA, and pre-treatment with the PKCdelta inhibitor rottlerin partially blocked it. Infecting SKBR-3 cells with an adenovirus vector containing wild-type PKCdelta, WTPKCdeltaAdV, had similar effects on PMA. Infecting the cells with a dominant-negative PKCdeltaAdV construct blocked the growth inhibition induced by PMA. Downstream of PKC, PMA treatment inhibited extracellular signal-regulated kinase mitogen-activated protein kinase phosphorylation, up-regulated c-jun NH(2)-terminal kinase phosphorylation, and inhibited retinoblastoma (Rb) phosphorylation. These results strongly implicated PKC (mainly PKCdelta) in the G(1) arrest induced by PMA and suggested PKC as a target for breast cancer treatment.

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.

Cisplatin resistance is associated with deregulation in protein kinase C-delta

Proteolytic activation of protein kinase C (PKC)-delta has been associated with cell death induced by the DNA damaging agent cisplatin. In the present study, we have examined if PKCdelta is affected when cells acquire resistance to cisplatin. The level of PKCdelta was elevated in cisplatin-resistant HeLa (HeLa/CP) cells compared to parental HeLa cells. Prolonged cellular exposure to the PKC activator phorbol-12,13-dibutyrate (PDBu), caused downregulation of PKCdelta in HeLa cells but not in HeLa/CP cells. Treatment of HeLa cells with PDBu resulted in the translocation of PKCdelta from the cytosol to the membrane but it failed to induce PKCdelta translocation in HeLa/CP cells. PDBu, however, induced translocation and downregulation of PKCalpha in both HeLa and HeLa/CP cells. The ability of PDBu to enhance cisplatin-induced cell death was attenuated in cisplatin-resistant HeLa cells. Thus, a deregulation in PKCdelta was associated with reduced cellular sensitivity to cisplatin.

Proteolytic activation of protein kinase C-epsilon by caspase-mediated processing and transduction of antiapoptotic signals

Several novel protein kinase C (PKC) isozymes have been identified as substrates for caspase-3. We have previously shown that novel PKCepsilon is cleaved during apoptosis in MCF-7 cells that lack any functional caspase-3. In the present study, we show that in vitro-translated PKCepsilon is processed by human recombinant caspase-3, -7, and -9. Tumor necrosis factor-alpha (TNF) triggered processing of PKCepsilon to a 43-kDa carboxyl-terminal fragment, and cell-permeable caspase inhibitors prevented TNF-induced processing of PKCepsilon in MCF-7 cells. PKCepsilon was cleaved primarily at the SSPD downward arrow G site to generate two fragments with an approximate molecular mass of 43 kDa. It was also cleaved at the DDVD downward arrow C site to generate two fragments with molecular masses of 52 and 35 kDa. Treatment of MCF-7 cells with TNF resulted in the activation of PKCepsilon, and mutation at the SSPD downward arrow G (D383A) site inhibited proteolytic activation of PKCepsilon. Overexpression of wild-type but not dominant-negative PKCepsilon in MCF-7 cells delayed TNF-induced apoptosis, and mutation at the D383A site prevented antiapoptotic activity of PKCepsilon. These results suggest that cleavage of PKCepsilon by caspase-7 at the SSPD downward arrow G site results in the activation of PKCepsilon. Furthermore, activation of PKCepsilon was associated with its antiapoptotic function.

Cytokine receptor as a sensitizer for targeted cancer therapy

Introducing a cytokine receptor as a sensitizer into cancer cells offers a unique opportunity for receptor-targeted cancer therapy. It has been shown that transfection of the tumor necrosis factor (TNF) receptor gene in cancer cells or exposing cancer cells to certain reagents which increase expression of TNF receptors results in enhancement of the cytotoxic effect of TNF. In addition, the literature suggests that Fas/CD95-mediated apoptotic tumor cell killing is augmented by gene transfer of Fas into cancer cells or treatment of cells with agents like cisplatin and interferon (IFN)-gamma. In contrast to these approaches, we have discovered a new approach to cancer therapy; wherein introduction of a cytokine receptor chain into cancer cells sensitizes them to receptor-directed cytotoxins. We have demonstrated that when interleukin (IL)-13 receptor (IL-13R) alpha2 chain, one of the two known IL-13 binding proteins, is introduced into cancer cells that do not express this chain the cells acquire extreme sensitivity to a chimeric fusion cytotoxin composed of IL-13 and a mutated form of Pseudomonas exotoxin (IL13-PE). Cells that do not express this chain or express low levels show limited sensitivity to IL13-PE. Acquisition of sensitivity to IL13-PE was observed both in vitro and in vivo when IL-13R alpha2-transfected human tumor cells were implanted in immunodeficient animals followed by systemic or regional IL13-PE therapy. Our third generation experiments suggest that this approach is feasible for clinical situations as intratumor administration of plasmid carrying the IL-13R alpha2 chain gene sensitized these tumors to systemic or regional IL13-PE therapy. This unique approach comprising gene transfer of cytokine receptor chain and receptor-targeted cytotoxin administration represents a novel strategy for cancer therapy.

Protein kinase C and ERK activation are required for TFF-peptide-stimulated bronchial epithelial cell migration and tumor necrosis factor-alpha-induced interleukin-6 (IL-6) and IL-8 secretion

TFF-peptides (formerly P-domain peptides, trefoil factors) are typical secretory products of many mucous epithelia and are aberrantly secreted during chronic inflammatory diseases. They are known to enhance the migration of intestinal, corneal, and bronchial epithelial cells. Using the human bronchial epithelial cell line BEAS-2B as a model, it is shown here for the first time that TFF-peptides are capable of modulating the inflammatory response in vitro by regulating tumor necrosis factor-alpha-induced secretion of interleukin (IL)-6 and IL-8. In contrast, TFF2 itself does not change IL-6 and IL-8 secretion but triggers sustained activation of the extracellular signal-regulated kinases (ERK1/2) as well as phosphorylation of c-Jun N-terminal kinase (JNK). A complex differential regulation of tumor necrosis factor-alpha-induced IL-6 and IL-8 secretion by TFF2 is observed that involves signaling via protein kinase C and ERK1/2. Furthermore, the motogenic effect of TFF2 on BEAS-2B cells is analyzed using a modified Boyden chamber assay. This migratory effect is shown to be dependent not only on protein kinase C and ERK1/2 but also on the activation of the Src family of tyrosine kinases. Taken together, the data presented indicate an important physiological role of TFF-peptides during inflammatory conditions of mucous epithelia.

Protein kinase D complexes with C-Jun N-terminal kinase via activation loop phosphorylation and phosphorylates the C-Jun N-terminus

Protein kinase D (PKD), a downstream effector of protein kinase C (PKC), is implicated in suppression of the c-Jun N-terminal kinase (JNK) signaling pathway, however, its mechanism of action is unclear. Transphosphorylation of the PKD activation loop at serines 744/748 by a PKC mediated signal transduction pathway enhances its catalytic activity. Here we show that PKD activation loop phosphorylation at serines 744/748 via PKC, or mutation of these serines to glutamic acid (PKD-S744/748E) also results in complex formation with JNK, indicating that suppression of JNK signaling by PKD involves a direct interaction with JNK. Because catalytically active PKD associates with JNK we determined whether it could phosphorylate the c-Jun N-terminus as a potential mechanism by which it suppresses c-Jun Ser 63 phosphorylation when it complexes with JNK. Purified human PKD and either wild-type PKD from phorbol 12, 13-dibutyrate (PDB)-stimulated cells or unstimulated constitutively active PKD (PKD-S744/748E), phosphorylated the c-Jun N-terminus between amino acids 1-89 at sites distinct from those phosphorylated by JNK. These results demonstrate, for the first time, phosphorylation dependent association of PKD with another signaling molecule and reveal a potential mechanism by which PKD could modulate the ability of JNK to phosphorylate c-Jun by phosphorylating alternative sites in the c-Jun N-terminus when it is complexed with JNK.

Targeting protein kinase C: new therapeutic opportunities against high-grade malignant gliomas?

A large body of evidence suggests that the abnormal phenotype of neoplastic astrocytes, including their excessive proliferation rate and high propensity to invade surrounding tissues, results from mutations in critical genes involved in key cellular events. These genetic alterations can affect cell-surface-associated receptors, elements of signaling pathways, or components of the cell cycle clock, conferring a gain or a loss of relevant metabolic functions of the cells. The understanding of such phenomena may allow the development of more efficacious forms of cancer treatment. Examples are therapies specifically directed against overexpressed epidermal growth factor receptor, hyperactive Ras, excessively stimulated Raf-1, overproduced ornithine decarboxylase, or aberrantly activated cyclin-dependent kinases. The applicability of some of these approaches is now being assessed in patients suffering from primary malignant central nervous system tumors that are not amenable to current therapeutic modalities. Another potentially useful therapeutic strategy against such tumors involves the inhibition of hyperactive or overexpressed protein kinase C (PKC). This strategy is justified by the decrease in cell proliferation and invasion following inhibition of the activity of this enzyme observed in preclinical glioma models. Thus, interference with PKC activity may represent a novel form of experimental cancer treatment that may simultaneously restrain the hyperproliferative state and the invasive capacity of high-grade malignant gliomas without inducing the expected toxicity of classical cytotoxic agents. Of note, the experimental use of PKC-inhibiting agents in patients with refractory high-grade malignant gliomas has indeed led to some clinical responses. The present paper reviews the current status of the biochemistry and molecular biology of PKC, as well as the possibilities for developing novel anti-PKC-based therapies for central nervous system malignancies.

Protein kinase C inhibitors as novel anticancer drugs

The role of PKC isoforms in signal transduction pathways involved in regulation of the cell cycle, apoptosis, angiogenesis, differentiation, invasiveness, senescence and drug efflux are reviewed, along with the clinical results on the current crop of PKC inhibitors, including midostaurin (PKC-412, CGP 41251, N -benzoylstaurosporine), UCN-01 (7-hydroxystaurosporine), bryostatin 1, perifosine, ilmofosine, Ro 31-8220, Ro 32-0432, GO 6976, ISIS-3521 (CGP 64128A) and the macrocyclic bis (indolyl) maleimides (LY-333531, LY-379196, LY-317615). An appreciation of the complex, often contradictory roles of PKC isoforms in signal transduction pathways involved in cancer is important for interpreting the clinical results observed with PKC inhibitors of varying selectivity. An antisense oligonucleotide, ISIS-3521 and two orally available small molecule inhibitors, LY 333531 and midostaurin, have now advanced to latter stage development for cancer and/or other indications. These compounds have varying levels of selectivity for the PKC isoforms and for the kinase and initial safety and early clinical efficacy have been encouraging. At this stage, the potential of PKC inhibition for the treatment of cancer has not been fully realised. The concurrent inhibition of multiple PKC isoforms may yet provide an improved clinical outcome in treating cancers in view of the complex interrelated roles of the PKC isoforms.