Role of protein kinase activity in apoptosis

Mechanisms of resveratrol in the prevention and treatment of gastrointestinal cancer

Gastrointestinal (GI) cancer is one of the leading causes of cancer-related deaths worldwide. According to the Global Cancer Statistics, colorectal cancer is the second leading cause of cancer-related mortality, closely followed by gastric cancer (GC). Environmental, dietary, and lifestyle factors including cigarette smoking, alcohol intake, and genetics are the most important risk factors for GI cancer. Furthermore, infections caused by Helicobacter pylori are a major cause of GC initiation. Despite improvements in conventional therapies, including surgery, chemotherapy, and radiotherapy, the length or quality of life of patients with advanced GI cancer is still poor because of delayed diagnosis, recurrence and side effect. Resveratrol (3, 4, 5-trihydroxy-trans-stilbene; Res), a natural polyphenolic compound, reportedly has various pharmacologic functions including anti-oxidant, anti-inflammatory, anti-cancer, and cardioprotective functions. Many studies have demonstrated that Res also exerts a chemopreventive effect on GI cancer. Research investigating the anti-cancer mechanism of Res for the prevention and treatment of GI cancer has implicated multiple pathways including oxidative stress, cell proliferation, and apoptosis. Therefore, this paper provides a review of the function and molecular mechanisms of Res in the prevention and treatment of GI cancer.

ER Stress-Mediated Signaling: Action Potential and Ca(2+) as Key Players

The proper functioning of the endoplasmic reticulum (ER) is crucial for multiple cellular activities and survival. Disturbances in the normal ER functions lead to the accumulation and aggregation of unfolded proteins, which initiates an adaptive response, the unfolded protein response (UPR), in order to regain normal ER functions. Failure to activate the adaptive response initiates the process of programmed cell death or apoptosis. Apoptosis plays an important role in cell elimination, which is essential for embryogenesis, development, and tissue homeostasis. Impaired apoptosis can lead to the development of various pathological conditions, such as neurodegenerative and autoimmune diseases, cancer, or acquired immune deficiency syndrome (AIDS). Calcium (Ca(2+)) is one of the key regulators of cell survival and it can induce ER stress-mediated apoptosis in response to various conditions. Ca(2+) regulates cell death both at the early and late stages of apoptosis. Severe Ca(2+) dysregulation can promote cell death through apoptosis. Action potential, an electrical signal transmitted along the neurons and muscle fibers, is important for conveying information to, from, and within the brain. Upon the initiation of the action potential, increased levels of cytosolic Ca(2+) (depolarization) lead to the activation of the ER stress response involved in the initiation of apoptosis. In this review, we discuss the involvement of Ca(2+) and action potential in ER stress-mediated apoptosis.

Calcium and apoptosis: ER-mitochondria Ca2+ transfer in the control of apoptosis

There is a growing consensus that the various forms of cell death (necrosis, apoptosis and autophagy) are not separated by strict boundaries, but rather share molecular effectors and signaling routes. Among the latter, a clear role is played by calcium (Ca(2+)), the ubiquitous second messenger involved in the control of a broad variety of physiological events. Fine tuning of intracellular Ca(2+) homeostasis by anti- and proapoptotic proteins shapes the Ca(2+) signal to which mitochondria and other cellular effectors are exposed, and hence the efficiency of various cell death inducers. Here, we will review: (i) the evidence linking calcium homeostasis to the regulation of apoptotic, and more recently autophagic cell death, (ii) the discussion of mitochondria as a critical, although not unique checkpoint and (iii) the molecular and functional elucidation of ER/mitochondria contacts, corresponding to the mitochondria-associated membrane (MAM) subfraction and proposed to be a specialized signaling microdomain.

Cell-cell contacts prevent anoikis in primary human colonic epithelial cells

BACKGROUND & AIMS

Colonic epithelial cells (CECs) receive important survival signals from the extracellular matrix and undergo detachment-induced apoptosis (anoikis) as soon as they lose their cell-matrix anchorage. In contrast to the established role of cell-matrix contact, the role of cell-cell contacts as a physiologic survival factor for CECs is less clear.

METHODS

Intact CEC crypts gently centrifuged to form a cell aggregate in which cell-cell contacts were maintained. Induction of apoptosis was assessed by Western Blot analysis, colorimetric assays, DNA electrophoresis, 4',6-diamidino-2-phenylindole staining, and flow cytometry. Activation of survival pathways was analyzed by Western blot. The role of mitogen-activated protein kinase/extracellular signal-regulated kinase (MEK)/extracellular signal-regulated kinase (Erk)1/2, epidermal growth factor receptor, phosphatidylinositol 3-kinase (PI3-K), and Src signaling was investigated using specific inhibitors.

RESULTS

Despite a complete loss of cell-matrix adhesion after CEC isolation, activation of caspases was blocked and anoikis was prevented when cell-cell contacts were preserved. CECs with preserved cell-cell contacts exhibited a rapid dephosphorylation of focal adhesion kinase. Aggregated CECs had stable levels of active beta-catenin and phosphorylated Akt, Erk1/2, and epidermal growth factor receptor, but CECs undergoing anoikis rapidly degraded beta-catenin and dephosphorylated Akt. Inhibition of Src- and PI3-K-dependent signaling reversed the antiapoptotic effect of cell-cell contact preservation, while inhibition of the MEK pathway had no effect.

CONCLUSIONS

Integrity of cell-cell contacts compensates for the loss of cell-matrix contact-mediated survival signals in CECs and prevents apoptosis. Cell-cell contact-triggered CEC survival involves antiapoptotic signaling through beta-catenin-, Src-, and PI3-K/Akt- but not through MEK- and focal adhesion kinase-dependent pathways.

Protein Kinase-zeta inhibits collagen I-dependent and anchorage-independent growth and enhances apoptosis of human Caco-2 cells

Colonic carcinogenesis is accompanied by abnormalities in multiple signal transduction components, including alterations in protein kinase C (PKC). The expression level of PKC-zeta, an atypical PKC isoform, increases from the crypt base to the luminal surface and parallels crypt cell differentiation in normal colon. In prior studies in the azoxymethane model of colon cancer, we showed that PKC-zeta was down-regulated in rat colonic tumors. In this study, we showed that PKC-zeta is expressed predominantly in colonic epithelial and not stromal cells, and loss of PKC-zeta occurs as early as the adenoma stage in human colonic carcinogenesis. To assess the regulation of growth and differentiation by PKC-zeta, we altered this isoform in human Caco-2 colon cancer cells using stable constitutive or inducible expression vectors, specific peptide inhibitors or small interfering RNA. In ecdysone-regulated transfectants grown on collagen I, ponasterone A significantly induced PKC-zeta expression to 135% of empty vector cells, but did not alter nontargeted PKC isoforms. This up-regulation was accompanied by a 2-fold increase in basal and 4-fold increase in insulin-stimulated PKC-zeta biochemical activity. Furthermore, PKC-zeta up-regulation caused >50% inhibition of cell proliferation on collagen I (P < 0.05). Increased PKC-zeta also significantly enhanced Caco-2 cell differentiation, nearly doubling alkaline phosphatase activity, while inducing a 3-fold increase in the rate of apoptosis (P < 0.05). In contrast, knockdown of this isoform by small interfering RNA or kinase inhibition by myristoylated pseudosubstrate significantly and dose-dependently increased Caco-2 cell growth on collagen I. In transformation assays, constitutively up-regulated wild-type PKC-zeta significantly inhibited Caco-2 cell growth in soft agar, whereas a kinase-dead mutant caused a 3-fold increase in soft agar growth (P < 0.05). Taken together, these studies indicate that PKC-zeta inhibits colon cancer cell growth and enhances differentiation and apoptosis, while inhibiting the transformed phenotype of these cells. The observed down-regulation of this growth-suppressing PKC isoform in colonic carcinogenesis would be predicted to contribute to tumorigenesis.

Regulation of radiation-induced protein kinase Cdelta activation in radiation-induced apoptosis differs between radiosensitive and radioresistant mouse thymic lymphoma cell lines

Protein kinase Cdelta (PKCdelta) has an important role in radiation-induced apoptosis. The expression and function of PKCdelta in radiation-induced apoptosis were assessed in a radiation-sensitive mouse thymic lymphoma cell line, 3SBH5, and its radioresistant variant, XR223. Rottlerin, a PKCdelta-specific inhibitor, completely abolished radiation-induced apoptosis in 3SBH5. Radiation-induced PKCdelta activation correlated with the degradation of PKCdelta, indicating that PKCdelta activation through degradation is involved in radiation-induced apoptosis in radiosensitive 3SBH5. In radioresistant XR223, radiation-induced PKCdelta activation was lower than that in radiosensitive 3SBH5. Cytosol PKCdelta levels in 3SBH5 decreased markedly after irradiation, while those in XR223 did not. There was no apparent change after irradiation in the membrane fractions of either cell type. In addition, basal cytosol PKCdelta levels in XR223 were higher than those in 3SBH5. These results suggest that the radioresistance in XR223 to radiation-induced apoptosis is due to a difference in the regulation of radiation-induced PKCdelta activation compared to that of 3SBH5. On the other hand, Atm(-/-) mouse thymic lymphoma cells were more radioresistant to radiation-induced apoptosis than wild-type mouse thymic lymphoma cells. Irradiated wild-type cells, but not Atm(-/-) cells, had decreased PKCdelta levels, indicating that the Atm protein is involved in radiation-induced apoptosis through the induction of PKCdelta degradation. The decreased Atm protein levels induced by treatment with Atm small interfering RNA had no effect on radiation-induced apoptosis in 3SBH5 cells. These results suggest that the regulation of radiation-induced PKCdelta activation, which is distinct from the Atm-mediated cascade, determines radiation sensitivity in radiosensitive 3SBH5 cells.

Programmed cell death in trypanosomatids: a way to maximize their biological fitness?

Programmed cell death (PCD) is a biochemical process that plays an essential role in the development of multicellular organisms. However, accumulating evidence indicates that PCD is also present in single-celled eukaryotes. Thus, trypanosomatids might be endowed with a PCD mechanism that is derived from ancestral death machinery. PCD in trypanosomatids could be a process without a defined function, inherited through eukaryotic cell evolution, which might be triggered in response to diverse stimuli and stress conditions. However, recent observations suggest that PCD might be used by trypanosomatids to maximize their biological fitness. Therefore, PCD could represent a potential pharmacological target for protozoan control.

Involvement of Protein Kinase C-Related Anti-apoptosis Signaling in Radiation-Induced Apoptosis in Murine Thymic Lymphoma(3SBH5) Cells

Protein kinase C (PKC; also known as PRKC) is known to be an important participant in radiation-induced apoptosis. However, its role is not fully clarified. Using 3SBH5 cells, which are radiation-sensitive thymic lymphoma cells, the involvement and functions of PKC were assessed in radiation- induced apoptosis. PMA (phorbol 12-myristate 13-acetate), a PKC activator, inhibited the radiation-induced apoptosis in 3SBH5 cells. On the other hand, chelerythrine, a PKC inhibitor, potentiated apoptosis. In addition, Gö6976, a classical PKC (cPKC) inhibitor, which specifically inhibits PKC (alpha and betaI), also promoted apoptosis. Interestingly, post-treatment (20 min after irradiation) with Gö6976 had no effect on the radiation-induced apoptosis. These results suggest that cPKC is activated early after irradiation for anti-apoptosis signaling and contributes to the balance between cell survival and death. Indeed, an increase of cPKC activity involving PKC (alpha, betaI and betaII) was observed in the cytosolic fraction 3 min after irradiation with 0.5 Gy. However, no translocation of cPKC was observed in the cells after irradiation. Our findings indicate that activation of cPKC (alpha or beta) soon after irradiation is critical to the understanding of the regulation of radiation-induced apoptosis in radiation-sensitive cells.

Programmed cell death in trypanosomatids and other unicellular organisms

In multicellular organisms, cellular growth and development can be controlled by programmed cell death (PCD), which is defined by a sequence of regulated events. However, PCD is thought to have evolved not only to regulate growth and development in multicellular organisms but also to have a functional role in the biology of unicellular organisms. In protozoan parasites and in other unicellular organisms, features of PCD similar to those in multicellular organisms have been reported, suggesting some commonality in the PCD pathway between unicellular and multicellular organisms. However, more extensive studies are needed to fully characterise the PCD pathway and to define the factors that control PCD in the unicellular organisms. The understanding of the PCD pathway in unicellular organisms could delineate the evolutionary origin of this pathway. Further characterisation of the PCD pathway in the unicellular parasites could provide information regarding their pathogenesis, which could be exploited to target new drugs to limit their growth and treat the disease they cause.

Protein kinase C zeta nuclear translocation mediates the occurrence of radioresistance in friend erythroleukemia cells

Friend erythroleukemia cells require high doses (15 Gy) of ionizing radiation to display a reduced rate of proliferation and an increased number of dead cells. Since ionizing radiation can activate several signaling pathways at the plasma membrane which can lead to the nuclear translocation of a number of proteins, we looked at the intranuclear signaling system activated by Protein Kinases C, being this family of enzymes involved in the regulation of cell growth and death. Our results show an early and dose-dependent increased activity of zeta and epsilon isoforms, although PKC zeta is the only isoform significantly active and translocated into the nuclear compartment upon low (1.5 Gy) and high (15 Gy) radiation doses. These observations are concomitant and consistent with an increase in the anti-apoptotic protein Bcl-2 level upon both radiation doses. Our results point at the involvement of the PKC pathway in the survival response to ionizing radiation of this peculiar cell line, offering PKC zeta for consideration as a possible target of pharmacological treatments aimed at amplifying the effect of such a genotoxic agent.

4-Methylcatechol stimulates phosphorylation of Trk family neurotrophin receptors and MAP kinases in cultured rat cortical neurons

Effects of 4-methycatechol (4MC), a potent stimulator of nerve growth factor and brain-derived neurotrophic factor (BDNF) synthesis, on phosphorylation of cellular molecules in cultured rat cortical neurons were examined. 4MC stimulated tyrosine phosphorylation of various proteins of molecular weight from 10-300 kDa including Trks, which are high-affinity neurotrophin receptors. Moreover, 4MC enhanced the phosphorylation of serine 133 of mitogen-activated protein kinase (MAPK/ERK) in a dose-dependent manner. Pretreatment of cultures with PD98059, a selective inhibitor of MAPK kinase (MEK-1), inhibited 4MC-induced phosphorylation of ERKs, demonstrating MEK-1-mediated activation. Therefore, it seems that 4MC triggered the phosphorylation of Trks, resulting in the activation of the subsequent MAPK/ERK signal cascade, or perhaps the involvement of BDNF action as 4MC can stimulate neuronal BDNF synthesis. The phosphorylation of MAPK/ERK was unaffected, however, in the presence of cycloheximide, a protein synthesis inhibitor, and K252a, a selective inhibitor of Trks, suggesting that the effect of newly synthesized BDNF was negligible on this event, and that primary sites of 4MC actions are not limited only to Trks. These results suggest that 4MC primarily activates multiple signal transduction molecules such as tyrosine kinases, including Trks. A significant increase in the survival rate of cortical neurons in the presence of 10 or 100 nM 4MC supported this idea, because the concentrations were much lower than those for stimulation of BDNF synthesis. Our results strongly suggest that the neurotrophic actions of 4MC found so far are mediated predominantly by direct activation of some intracellular signals including MAPK/ERK rather than by neurotrophin synthesis.

Phospholipase C-gamma1 is required for cell survival in oxidative stress by protein kinase C

Phospholipase C-gamma1 (PLC-gamma1) activation has been reported to enhance cell survival during the cellular response to oxidative stress. We studied the role of protein kinase C (PKC) pathways in mediating PLC-gamma1 survival signalling in oxidative stress by using mouse embryonic fibroblasts genetically deficient in PLC-gamma1 (Plcg1(-/-)) and its wild type (Plcg1(+/+)). PLC-gamma1 was activated by H(2)O(2) treatment in a dose- and time-dependent manner. Activation of PKC was also markedly increased in both cell lines treated with H(2)O(2) (1-5 mM), but with low doses (50-200 microM), PKC activation was considerably decreased in Plcg1(-/-) cells. After treatment with H(2)O(2), PKC-dependent phosphorylation of Bcl-2 and cell viability of Plcg1(-/-) cells decreased dramatically and caspase-3-like activity increased significantly compared with that of the wild-type cells. Furthermore, pretreatment of Plcg1(+/+) cells with PKC-specific inhibitor decreased levels of PKC-dependent Bcl-2 phosphorylation, enhanced caspase-3 activity and their sensitivity to H(2)O(2). On the contrary, treatment of Plcg1(-/-) cells with PKC-specific activator increased the Bcl-2 phosphorylation, decreased caspase-3 activity and improved their survival. These results suggest that PLC-gamma1 mediates survival signalling in oxidative-stress response by PKC-dependent phosphorylation of Bcl-2 and inhibition of caspase-3.

Resveratrol-induced inactivation of human gastric adenocarcinoma cells through a protein kinase C-mediated mechanism

Resveratrol, a polyphenolic phytochemical present in berries, grapes, and wine, has emerged as a promising chemopreventive candidate. Because there is scant information regarding natural agents that prevent, suppress, or reverse gastric carcinogenesis, the aim of the present study was to determine the chemopreventive potential of resveratrol against gastric cancer by investigating cellular and molecular events associated with resveratrol treatment of human gastric adenocarcinoma cells. We determined the action of resveratrol on cellular function and cellular integrity by measuring DNA synthesis, cellular proliferation, cell cycle distribution, cytolysis, apoptosis, and phosphotransferase activities of two key signaling enzymes, protein kinase C (PKC) and mitogen-activated protein kinases (ERK1/ERK2), in human gastric adenocarcinoma KATO-III and RF-1 cells. Resveratrol inhibited [3H]thymidine incorporation into cellular DNA of normally proliferating KATO-III cells and of RF-1 cells whose proliferation was stimulated with carcinogenic nitrosamines. Treatment with resveratrol arrested KATO-III cells in the G(0)/G(1) phase of the cell cycle and eventually induced apoptotic cell death, but had a minimal effect on cell lysis. Resveratrol treatment had no effect on ERK1/ERK2 activity but significantly inhibited PKC activity of KATO-III cells and of human recombinant PKCalpha. Results indicate that resveratrol has potential as a chemopreventive agent against gastric cancer because it exerts an overall deactivating effect on human gastric adenocarcinoma cells. Resveratrol-induced inhibition of PKC activity and of PKCalpha, without any change in ERK1/ERK2 activity, suggests that resveratrol utilizes a PKC-mediated mechanism to deactivate gastric adenocarcinoma cells.

Characterization of a putative membrane receptor for progesterone in rat granulosa cells

Progesterone (P(4)) inhibits granulosa cell apoptosis in a steroid-specific, dose-dependent manner, but these cells do not express the classic nuclear P(4) receptor. It has been proposed that P(4) mediates its action through a 60-kDa protein that functions as a membrane receptor. The present studies were designed to determine the P(4) binding characteristics of this protein. Western blot analysis using an antibody that recognizes the P(4) binding site of the nuclear P(4) receptor (C-262) confirmed that the 60-kDa protein was localized to the plasma membrane of both granulosa cells and spontaneously immortalized granulosa cells (SIGCs). To determine whether this protein binds P(4), proteins were immunoprecipitated with the C-262 antibody, electrophoresed, transferred to nitrocellulose, and probed with a horseradish peroxidase-labeled P(4) in the presence or absence of nonlabeled P(4). This study demonstrated that the 60-kDa protein specifically binds P(4). Scatchard plot analysis revealed that (3)H-P(4) binds to a single site (i.e., single protein), which is relatively abundant (200 pmol/mg) with a K(d) of 360 nM. (3)H-P(4) binding was not reduced by dexamethasone, mifepristone (RU 486), or onapristone (ZK98299). Further studies with SIGCs showed that P(4) inhibited apoptosis and mitogen-activated protein kinase kinase (MEK) activity, and maintained calcium homeostasis. These studies taken together support the concept that the 60-kDa P(4) binding protein functions as a low-affinity, high-capacity membrane receptor for P(4).

How protein kinase C activation protects nerve cells from oxidative stress-induced cell death

Oxidative stress is implicated in the nerve cell death that occurs in a variety of neurological disorders, and the loss of protein kinase C (PKC) activity has been coupled to the severity of the damage. The functional relationship between stress, PKC, and cell death is, however, unknown. Using an immortalized hippocampal cell line that is particularly sensitive to oxidative stress, I show that activation of PKC by the phorbol ester tetradecanoylphorbol acetate (TPA) inhibits cell death via the stimulation of a complex protein phosphorylation pathway. TPA treatment leads to the rapid activation of extracellular signal-regulated kinase (ERK) and c-Jun NH2-terminal kinase (JNK), the inactivation of p38 mitogen-activated protein kinase (MAPK), and the downregulation of PKCdelta. Inhibition of either ERK or JNK activation blocks TPA-mediated protection, whereas p38 MAPK and PKCdelta inhibitors block stress-induced nerve cell death. Both p38 MAPK inactivation and JNK activation appear to be downstream of ERK because an agent that blocks ERK activation also blocks the modulation of these other MAP kinase family members by TPA treatment. Thus, the protection from oxidative stress afforded nerve cells by PKC activity requires the combined modulation of multiple enzyme pathways and suggests why the loss of PKC activity contributes to nerve cell death.

PKC-delta inhibits anchorage-dependent and -independent growth, enhances differentiation, and increases apoptosis in CaCo-2 cells

BACKGROUND & AIMS

Previous studies showed decreased protein kinase C (PKC)-delta expression in azoxymethane-induced rat and sporadic human colonic tumors. To elucidate the role of PKC-delta on the neoplastic phenotype of human colon cancer cells, we established stable transfectants of this isoenzyme in CaCo-2 cells.

METHODS

Human PKC-delta complementary DNA was subcloned into 2 distinct metallothionein-regulated expression vectors. Polyclonal populations of PKC-delta transfectants were characterized by Western blotting. PKC-delta activity was measured in situ using a PKC-delta-specific substrate. Proliferation was determined by Coulter counter, and cell cycle distribution was analyzed by flow cytometry. In vitro transformation was assessed by growth in soft agar and differentiation by changes in alkaline phosphatase and sucrase isomaltase. Apoptosis was evaluated by 4',6-diamidino-2-phenylindole dihydrochloride and terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick-end labeling staining.

RESULTS

In the presence of Zn(2+), PKC-delta transfectants expressed a 4-fold increase in the protein and a 2-fold increase in activity of PKC-delta. PKC-delta transfectants exhibited a 30% decrease (P < 0.05) in cell growth and an enhanced differentiation phenotype. Increased PKC-delta expression induced a significant G0/G1 arrest, inhibited anchorage-independent growth (50%, P < 0.05), and caused a 2-fold increase in apoptosis (P < 0.05).

CONCLUSIONS

Our studies show that increased expression of PKC-delta inhibits anchorage-dependent and -independent growth, while inducing cellular differentiation and limiting survival of this human colon cancer cell line.

Current concepts in cell toxicity

This overview provides a basic definition of cellular death and the mechanisms that are associated with cell death: apoptosis and necrosis. It includes a description of structural changes and macromolecular degradation and the roles of signaling, mitochondria, and genetic changes in the two forms of cell death.

Anticancer drug resistance in primary human brain tumors

The difficult clinical situation still associated with most types of primary human brain tumors has fostered significant interest in defining novel therapeutic modalities for this heterogeneous group of neoplasms. Beginning in the 1980s chemotherapy has been incorporated into the treatment protocol of a number of intractable brain tumors. However, it has predominantly failed to improve patient outcome. The unsatisfactory results with chemotherapeutic intervention have chiefly been attributed to tumor cell resistance. In recent years, there has been a literal explosion in our understanding about the mechanisms by which cancer cells become chemoresistant. During the course of their evolution (intrinsic resistance) or in response to chemotherapy (acquired resistance) these cells may follow a number of pathways of genetic alterations to possess a common (multidrug) or drug-specific (individual drug) resistant phenotype. Genomic aberrations, deregulation of membrane transporting proteins and cellular enzymes, and an altered susceptibility to commit to apoptosis are among the steps on the way that contribute to the genesis of chemotherapeutic treatment failure. Although, through the years we have come to yield information and inferences as to the roles that different molecular events may have in the resistance phenotype of cancer cells, the actual involvement of single genetic alterations in conferring drug resistance in primary brain tumors remains debatable. This uncertainty and, besides, the lack of proper drug resistance diagnostics, in a vicious circle, hinder the development of effective resistance-modulation strategies. Clinical non-responsiveness to chemotherapy remains a formidable obstacle to the successful treatment of brain tumors and one of the most serious problems to be solved in the therapy of these lesions. Future advances in the chemotherapeutic management of these neoplasms will come with an improved understanding of the significance and interrelationship of the multiple biological systems operative in promoting resistance to this treatment modality. The focus of this review is to summarize current knowledge concerning major drug resistance-related markers, to describe their functional interaction en route to chemoresistance, and to discuss their implication in rendering human brain tumor cells resistant to chemotherapy.

E-cadherin-mediated cell contact prevents apoptosis of spontaneously immortalized granulosa cells by regulating Akt kinase activity

The present studies were designed to determine the role that homophilic E-cadherin binding plays in preventing apoptosis of spontaneously immortalized granulosa cells (SIGCs). Although the levels of E-cadherin were similar to serum control levels, the amount of E-cadherin at the plasma membrane was dramatically reduced by 5 h after serum withdrawal. To determine whether disrupting homophilic E-cadherin binding leads to apoptosis, SIGCs were cultured in serum in the presence of either EGTA or an E-cadherin antibody. Treatment with either EGTA, which disrupts all calcium-dependent contacts, or E-cadherin antibody, induced apoptosis. Exposure to EGTA reduced MEK and Akt kinase activity, whereas E-cadherin antibody only attenuated Akt kinase activity. Because Akt kinase controls caspase-3 activity, an important activator of apoptosis, caspase-3 activity was monitored. Caspase-3 activity increased after serum depletion, or EGTA or E-cadherin antibody treatment. Time-series analysis of caspase-3 activity within single cells revealed that during apoptosis cell contact was disrupted then caspase-3 activity was detected. Finally, the caspase inhibitor, Z-VAD-FMK, blocked apoptosis. These data taken together are consistent with the concept that E-cadherin-mediated cell contact, either directly or indirectly, promotes Akt kinase activity, which in turn, inhibits caspase-3 activation and thereby maintains SIGC viability.

Modulation of protein kinase C in antitumor treatment

PKC isoenzymes were found to be involved in proliferation, antitumor drug resistance and apoptosis. Therefore, it has been tried to exploit PKC as a target for antitumor treatment. PKC alpha activity was found to be elevated, for example, in breast cancers and malignant gliomas, whereas it seems to be underexpressed in many colon cancers. So it can be expected that inhibition of PKC activity will not show similar antitumor activity in all tumors. In some tumors it seems to be essential to inhibit PKC to reduce growth. However, for inhibition of tumor proliferation it may be an advantage to induce apoptosis. In this case an activation of PKC delta should be achieved. The situation is complicated by the facts that bryostatin leads to the activation of PKC and later to a downmodulation and that the PKC inhibitors available to date are not specific for one PKC isoenzyme. For these reasons, PKC modulation led to many contradicting results. Despite these problems, PKC modulators such as miltefosine, bryostatin, safingol, CGP41251 and UCN-01 are used in the clinic or are in clinical evaluation. The question is whether PKC is the major or the only target of these compounds, because they also interfere with other targets. PKC may also be involved in apoptosis. Oncogenes and growth factors can induce cell proliferation and cell survival, however, they can also induce apoptosis, depending on the cell type or conditions in which the cells or grown. PKC participates in these signalling pathways and cross-talks. Induction of apoptosis is also dependent on many additional factors, such as p53, bcl-2, mdm2, etc. Therefore, there are also many contradicting results on PKC modulation of apoptosis. Similar controversial data have been reported about MDR1-mediated multidrug resistance. At present it seems that PKC inhibition alone without direct interaction with PGP will not lead to successful reversal of PGP-mediated drug efflux. One possibility to improve chemotherapy would be to combine established antitumor drugs with modulators of PKC. However, here also very contrasting results were obtained. Many indicate that inhibition, others, that activation of PKC enhances the antiproliferative activity of anticancer drugs. The problem is that the exact functions of the different PKC isoenzymes are not clear at present. So further investigations into the role of PKC isoenzymes in the complex and interacting signalling pathways are essential. It is a major challenge in the future to reveal whether modulation of PKC can be used for the improvement of cancer therapy.

Can melatonin regulate the expression of prohormone convertase 1 and 2 genes via monomeric and dimeric forms of RZR/ROR nuclear receptor, and can melatonin influence the processes of embryogenesis or carcinogenesis by disturbing the proportion of cAMP and cGMP concentrations? Theoretic model of controlled apoptosis

The presented model of controlled apoptosis has been based on the assumption that correct information exchange between an organism as a whole, and each of its cells is conditioned by mutual proportions of cAMP and cGMP concentrations (CcAMP, CcGMP), according to the formula CcAMP x CcGMP = 'a' (constant). The regulation of balance of these 'second messengers' in a cell and an extracellular space would depend on the mutual proportions of concentrations of Melatonin and monomers of Melanin. These indoloderived compounds could be the activators of the transcription factors i.e. RZR and NFkappa-B, regulating the expression of Prohormone Convertase (PC) gen and Nitric Oxide Synthase (NOS) gen, respectively. Additionally, maternal Melatonin and Nitric Oxide (NO), being able to pass through trophoblast or placenta freely, would play decisive role in the synchronization of embryogenesis and intrauterine development of the fetus. In case of an embryo or a fetus, the result of CcAMP and CcGMP multiplication, different from the proper constant 'a'-value, would mean occurrence of disorders in the structure and functioning of the cellular tensegrity system and, in consequence, disturbances in the intercellular information exchange. It would lead to deviation in cellular metabolism, oriented cell movement, uncontrolled apoptosis, and as a consequence, would lead to the development of fetal defects. In case of a child or an adult, a sudden occurrence and prolongation of such disturbances in CcAMP-CcGMP proportions would induce a process of apoptosis of normal cells and an initiation of a cancerogenesis. On the other hand, the recovery of equilibrium in the information exchange system would initiate apoptosis of neoplastic cells, and simultaneously, proliferation of connective tissue cells. According to the presented hypothesis, a decrease in CcAMP and destabilization of the CcAMP-CcGMP balance in an embryo or a fetus would result from relatively excessive amounts of maternal Melatonin (monomers) in fetal circulation, while a decrease of CcAMP and destabilization of the CcAMP-CcGMP balance in a child or an adult would be a consequence of relatively insufficient amounts of Melatonin (dimers) in an organism. It seems possible, that determination of both CcAMP and CcGMP would enable an early detection of high risk of developmental defects occurrence in an embryo or a fetus and neoplastic processes in a child or an adult. This method might also be considerably useful in monitoring a safe substitutional hormonotherapy.

Protein kinase C and meiotic regulation in isolated mouse oocytes

In this study, the possible role of protein kinase C (PKC) in mediating both positive and negative actions on meiotic maturation in isolated mouse oocytes has been examined. When cumulus cell-enclosed oocytes (CEO) were cultured for 17-18 hr in a medium containing 4 mM hypoxanthine (HX) to maintain meiotic arrest, each of the five different activators and five different antagonists of PKC stimulated germinal vesicle breakdown (GVB) in a dose-dependent fashion. One of the activators, phorbol-12-myristate 13-acetate (PMA), also triggered GVB in CEO arrested with isobutylmethylxanthine or guanosine, but not in those arrested with dibutyryl cyclic AMP. When denuded oocytes (DO) were cultured for 3hr in inhibitor-free medium, all PKC activators suppressed maturation (<10% GVB compared to 94% in controls), while the effect of PKC antagonists was negligible. Four of the five antagonists reversed the meiosis-arresting action of HX in DO. PMA transiently arrested the spontaneous maturation of both CEO and DO, with greater potency in DO. The stimulatory action of PMA in HX-arrested oocytes was dependent on cumulus cells, because meiotic induction occurred in CEO but not DO. PKC activators also preferentially stimulated cumulus expansion when compared to antagonists. A cell-cell coupling assay determined that the action of PMA on oocyte maturation was not due to a loss of metabolic coupling between the oocyte and cumulus oophorus. Finally, Western analysis demonstrated the presence of PKCs alpha, beta1, delta, and eta in both cumulus cells and oocytes, but only PKC epsilon was detected in the cumulus cells. It is concluded that direct activation of PKC in the oocyte suppresses maturation, while stimulation within cumulus cells generates a positive trigger that leads to meiotic resumption.

Increased Protein Tyrosine Phosphorylation in Apoptotic Neural Cell Death Due to Microtubule Perturbations

The microtubule-perturbing drugs colchicine and taxol have been found to induce apoptosis in a CNS neuronal cell line. Apoptosis in drug-treated rat B103 neuroblastoma cells was evident in characteristic morphological changes, internucleosomal DNA fragmentation, and loss of nuclear content. Since colchicine and taxol have opposite actions on microtubule integrity, disruption of the active turnover of the microtubule network appears to be a crucial step for apoptosis to occur. It has been suggested that the basis for apoptosis by these drugs derives from their known block of the cell cycle at G₂/M, but this does not appear the sole reason as both colchicine and taxol were able to evoke high levels of apoptosis in cells differentiated by Bt₂cAMP or serum withdrawal. Further tests of cellular consequences of microtubule perturbation revealed a specific impact on signal transduction involving protein tyrosine phosphorylation. Immunoprecipitation with antibodies against tyrosine phosphorylated proteins showed a striking increase in the phosphorylation of a Triton-insoluble ~90 kDa protein, roughly concurrent with the onset of internucleosomal DNA fragmentation. Cycloheximide and genistein significantly reduced cell death and blocked appearance of the ~90 kDa tyrosine phosphorylated protein. Data suggest the hypothesis that signal transduction leading to apoptosis can be triggered by anomalous microtubule turnover and that the mechanism involves tyrosine phosphorylation of a ~90 kDa Triton-resistant protein.

Mammalian caspases: structure, activation, substrates, and functions during apoptosis

Apoptosis is a genetically programmed, morphologically distinct form of cell death that can be triggered by a variety of physiological and pathological stimuli. Studies performed over the past 10 years have demonstrated that proteases play critical roles in initiation and execution of this process. The caspases, a family of cysteine-dependent aspartate-directed proteases, are prominent among the death proteases. Caspases are synthesized as relatively inactive zymogens that become activated by scaffold-mediated transactivation or by cleavage via upstream proteases in an intracellular cascade. Regulation of caspase activation and activity occurs at several different levels: (a) Zymogen gene transcription is regulated; (b) antiapoptotic members of the Bcl-2 family and other cellular polypeptides block proximity-induced activation of certain procaspases; and (c) certain cellular inhibitor of apoptosis proteins (cIAPs) can bind to and inhibit active caspases. Once activated, caspases cleave a variety of intracellular polypeptides, including major structural elements of the cytoplasm and nucleus, components of the DNA repair machinery, and a number of protein kinases. Collectively, these scissions disrupt survival pathways and disassemble important architectural components of the cell, contributing to the stereotypic morphological and biochemical changes that characterize apoptotic cell death.

Effects of neoplastic transformation and teniposide (VM26) on protein kinase C isoform expression in rodent fibroblasts

This study examined changes in protein kinase C (PKC) isoforms in rodent fibroblasts (rat F111 and mouse NIH3T3), transformed by the polyoma virus middle T antigen (mT) and undergoing apoptosis in response to teniposide (VM26). The mT-transformed cells up-regulated PKC delta and down-regulated both PKC epsilon and PKC lambda expression, and were more sensitive to the drug than their non-transformed counterparts. The drug treatment further lowered the expression of PKC epsilon, triggered nuclear translocation of PKC delta and its site-specific proteolysis, consistent with the notion that changes in specific PKC isoforms play a role not only in the neoplastic transformation of fibroblasts, but also in their apoptotic response.

Mechanism of thymocyte apoptosis induced by serum of tumor-bearing host: the molecular events involved and their inhibition by thymosin alpha-1

The observations presented in this paper indicate that serum of Dalton's lymphoma (DL) bearing mice contained certain soluble factor(s) that augmented the induction of apoptosis in thymocytes in a time- and dose-dependent manner. DL-ascitic fluid and DL-conditioned medium could also induce apoptosis of thymocytes in vitro, though the magnitude of the same was consistently lower than that induced by serum of DL-bearing mice. It was observed that the interaction of FasL and TNFalpha with their respective receptors could trigger apoptosis in thymocytes. Elucidation of the signal transduction mechanism revealed involvement of protein tyrosine kinase, protein kinase C and ser/thr phosphatases with concomitant increase in the level of protein products of apoptosis associated genes p53, bax, bad, fas and fas ligand and cleavage of N-terminal 23 kDa fragment of Bcl-2 that exhibited Bax-like death effector properties. Further, we report, for the first time, the ability of thymosin alpha-1, an immunopotentiating thymic hormone, to antagonize apoptosis in thymocytes induced by factors present in serum of DL-bearing mice. The underlying mechanism of tumor serum induced apoptosis inhibition by thymosin alpha-1 was also analyzed. The signal transduction cascade evoked by thymosin alpha-1 involves activation of protein kinase C with a decrease in the level of protein products of proapoptotic genes like bax and bad and increase in the protein products of bcl-2 gene.

Histone H2B phosphorylation in mammalian apoptotic cells. An association with DNA fragmentation

Histone phosphorylation was investigated in several mammalian cells undergoing apoptosis (human HL-60 and HeLa, mouse FM3A and N18 cells, and rat thymocytes). Among the four nucleosomal core histones (H2A, H2B, H3, and H4), H2B, which is not usually phosphorylated in quiescent or growing cells, was found to be phosphorylated after treatment with various apoptotic inducers. The H2B was phosphorylated around the time when nucleosomal DNA fragmentation was initiated and, like this fragmentation, was completely blocked with Z-Asp-CH(2)-DCB, an inhibitor of ICE or ICE-like caspase. The involved single phosphopeptide of H2B proved to be phosphorylatable in vitro with a protein kinase C, and the site Ser-32 was tentatively identified. Despite typical apoptotic chromatin condensation, the H3 phosphorylation was at a low level, and the sites where phosphorylation did occur did not include any mitosis-specific phosphopeptides. Phosphorylation of H4 was increased, but the other two histone proteins (H1 and H2A) were not appreciably changed. These observations imply that 1) H2B phosphorylation occurs universally in apoptotic cells and is associated with apoptosis-specific nucleosomal DNA fragmentation, 2) chromatin condensation in apoptosis occurs by a different biochemical mechanism from those operating during mitosis or premature chromosome condensation, and 3) this unique phosphorylation of H2B is a useful biochemical hallmark of apoptotic cells.

Anti-proliferative effect of two novel palmitoyl-carnitine analogs, selective inhibitors of protein kinase C conventional isoenzymes

Previous studies have shown that palmitoyl-carnitine is an anti-proliferative agent and a protein kinase C inhibitor. Two new palmitoyl-carnitine analogs were synthesized by replacing the ester bond with a metabolically more stable ether bond. An LD50 value in the nM range was found in anti-proliferative assays using HL-60 cells and was dependent on the alkyl-chain length. The inhibitory action of these water-soluble compounds on protein kinase C in vitro was greatly increased with respect to palmitoyl-carnitine and was dependent on the length of the alkyl chain. Its effect was mediated by an increase in the enzyme's requirement for phosphatidylserine. Inhibition of the in situ phosphorylation of a physiological platelet protein kinase C substrate and of phorbol ester-induced differentiation of HL-60 cells was also observed. Finally, to test for isoenzyme selectivity, several human recombinant protein kinase C isoforms were used. Only the Ca2+-dependent classic protein kinase Cs (alpha, betaIota, betaIotaIota and gamma) were inhibited by these compounds, yet the activities of casein kinase I, Ca2+/calmodulin-dependent kinase and cAMP-dependent protein kinase were unaffected. Thus, these novel inhibitors appear to be both protein kinase C and isozyme selective. They may be useful in assessing the individual roles of protein kinase C isoforms in cell proliferation and tumor development and may be rational candidates for anti-neoplasic drug design.

Direct evidence of cytoplasmic delivery of PKC-alpha, -epsilon and -zeta pseudosubstrate lipopeptides: study of their implication in the induction of apoptosis

Protein kinases C (PKC) are serine/threonine kinase enzymes involved in the mechanism of cell survival. Their pseudosubstrate sequences are autoinhibitory domains, which maintain the enzyme in an inactive state in the absence of allosteric activators, thus representing an attractive tool for the modulation of different PKC isoforms. Here, we report the use of palmitoylated modified PKC-alpha, -epsilon, and -zeta pseudosubstrate peptides, and determine their intracellular distribution together with their respective PKC isoenzymes. Finally, we propose that the differential distribution of the peptides is correlated with a selective induction of apoptosis and therefore argues for different involvement of PKC isoforms in the anti-apoptotic program.

Drosophila abelson interacting protein (dAbi) is a positive regulator of abelson tyrosine kinase activity

Human and mouse Abelson interacting proteins (Abi) are SH3-domain containing proteins that bind to the proline-rich motifs of the Abelson protein tyrosine kinase. We report a new member of this gene family, a Drosophila Abi (dAbi) that is a substrate for Abl kinase and that co-immunoprecipitates with Abl if the Abi SH3 domain is intact. We have identified a new function for both dAbi and human Abi-2 (hAbi-2). Both proteins activate the kinase activity of Abl as assayed by phosphorylation of the Drosophila Enabled (Ena) protein. Removal of the dAbi SH3 domain eliminates dAbi's activation of Abl kinase activity. dAbi is an unstable protein in cells and is present at low steady state levels but its protein level is increased coincident with phosphorylation by Abl kinase. Expression of the antisense strand of dAbi reduces dAbi protein levels and abolishes activation of Abl kinase activity. Modulation of Abi protein levels may be an important mechanism for regulating the level of Abl kinase activity in the cell.

Ceramides modulate protein kinase C activity and perturb the structure of Phosphatidylcholine/Phosphatidylserine bilayers

We studied the effects of natural ceramide and a series of ceramide analogs with different acyl chain lengths on the activity of rat brain protein kinase C (PKC) and on the structure of bovine liver phosphatidylcholine (BLPC)/dipalmitoylphosphatidylcholine (DPPC)/dipalmitoylphosphatidylserine (DPPS) (3:1:1 molar ratio) bilayers using (2)H-NMR and specific enzymatic assays in the absence or presence of 7.5 mol % diolein (DO). Only a slight activation of PKC was observed upon addition of the short-chain ceramide analogs (C(2)-, C(6)-, or C(8)-ceramide); natural ceramide or C(16)-ceramide had no effect. In the presence of 7.5 mol % DO, natural ceramide and C(16)-ceramide analog slightly attenuated DO-enhanced PKC activity. (2)H-NMR results demonstrated that natural ceramide and C(16)-ceramide induced lateral phase separation of gel-like and liquid crystalline domains in the bilayers; however, this type of membrane perturbation has no direct effect on PKC activity. The addition of both short-chain ceramide analogs and DO had a synergistic effect in activating PKC, with maximum activity observed with 20 mol % C(6)-ceramide and 15 mol % DO. Further increases in C(6)-ceramide and/or DO concentrations led to decreased PKC activity. A detailed (2)H-NMR investigation of the combined effects of C(6)-ceramide and DO on lipid bilayer structure showed a synergistic effect of these two reagents to increase membrane tendency to adopt nonbilayer structures, resulting in the actual presence of such structures in samples exceeding 20 mol % ceramide and 15 mol % DO. Thus, the increased tendency to form nonbilayer lipid phases correlates with increased PKC activity, whereas the actual presence of such phases reduced the activity of the enzyme. Moreover, the results show that short-chain ceramide analogs, widely used to study cellular effects of ceramide, have biological effects that are not exhibited by natural ceramide.

Involvement of protein kinase Cepsilon (PKCepsilon) in thyroid cell death. A truncated chimeric PKCepsilon cloned from a thyroid cancer cell line protects thyroid cells from apoptosis

The protein kinase C (PKC) family has been implicated in the regulation of apoptosis. However, the contribution of individual PKC isozymes to this process is not well understood. We reported amplification of the chromosome 2p21 locus in 28% of thyroid neoplasms, and in the WRO thyroid carcinoma cell line. By positional cloning we identified a rearrangement and amplification of the PKCepsilon gene, that maps to 2p21, in WRO cells. This resulted in the overexpression of a chimeric/truncated PKCepsilon (Tr-PKCepsilon) mRNA, coding for N-terminal amino acids 1-116 of the isozyme fused to an unrelated sequence. Expression of the Tr-PKCepsilon protein in PCCL3 cells inhibited activation-induced translocation of endogenous PKCepsilon, but its kinase activity was unaffected, consistent with a dominant negative effect of the mutant protein on activation-induced translocation of wild-type PKCepsilon and/or displacement of the isozyme to an aberrant subcellular location. Cell lines expressing Tr-PKCepsilon grew to a higher saturation density than controls. Moreover, cells expressing Tr-PKCepsilon were resistant to apoptosis, which was associated with higher Bcl-2 levels, a marked impairment in p53 stabilization, and dampened expression of Bax. These findings point to a role for PKCepsilon in apoptosis-signaling pathways in thyroid cells, and indicate that a naturally occurring PKCepsilon mutant that functions as a dominant negative can block cell death triggered by a variety of stimuli.

The effect of transient global ischemia on the interaction of Src and Fyn with the N-methyl-D-aspartate receptor and postsynaptic densities: possible involvement of Src homology 2 domains

Transient ischemia increases tyrosine phosphorylation of N-methyl-D-aspartate (NMDA) receptor subunits NR2A and NR2B in the rat hippocampus. The authors investigated the effects of this increase on the ability of the receptor subunits to bind to the Src homology 2 (SH2) domains of Src and Fyn expressed as glutathione-S-transferase-SH2 fusion proteins. The NR2A and NR2B bound to each of the SH2 domains and binding was increased approximately twofold after ischemia and reperfusion. Binding was prevented by prior incubation of hippocampal homogenates with a protein tyrosine phosphatase or by a competing peptide for the Src SH2 domain. Ischemia induced a marked increase in the tyrosine phosphorylation of several proteins in the postsynaptic density (PSD), including NR2A and NR2B, but had no effect on the amounts of individual NMDA receptor subunits in the PSD. The level of Src and Fyn in PSDs, but not in other subcellular fractions, was increased after ischemia. The ischemia-induced increase in the interaction of NR2A and NR2B with the SH2 domains of Src and Fyn suggests a possible mechanism for the recruitment of signaling proteins to the PSD and may contribute to altered signal transduction in the postischemic hippocampus.

The Src-Homology Domain 2-Bearing Protein Tyrosine Phosphatase-1 Inhibits Antigen Receptor-Induced Apoptosis of Activated Peripheral T Cells

Restimulation of Ag receptors on peripheral T lymphocytes induces tyrosine phosphorylation-based signaling cascades that evoke Fas ligand expression and induction of Fas-mediated programmed cell death. In view of the role for the Src homology domain 2-bearing protein tyrosine phosphatase-1 (SHP-1) in modulating TCR signaling, we investigated the influence of SHP-1 on TCR-mediated apoptosis by assaying the sensitivity of peripheral T cells from SHP-1-deficient viable motheaten (mev) mice to cell death following TCR restimulation. The results of these studies revealed mev peripheral T cells to be markedly more sensitive than wild-type cells to induction of cell death following TCR stimulation. By contrast, PMA/ionophore and anti-Fas Ab-induced apoptotic responses were no different in mev compared with wild-type activated cells. Enhanced apoptosis of TCR-restimulated mev lymphocytes was associated with marked increases in Fas ligand expression as compared with wild-type cells, but was almost abrogated in both mev and wild-type cells by Fas-Fc treatment. Thus, the increased sensitivity of mev T cells to apoptosis following TCR restimulation appears to reflect a TCR-driven phenomenon mediated through up-regulation of Fas-Fas ligand interaction and induction of the Fas signaling cascade. These findings, together with the hyperproliferative responses of mev peripheral T cells to initial TCR stimulation, indicate that SHP-1 modulation of TCR signaling translates to the inhibition of both T cell proliferation and activation and, as such, is likely to play a pivotal role in regulating the expansion of Ag-stimulated T cells during an immune response.

Human munc13 is a diacylglycerol receptor that induces apoptosis and may contribute to renal cell injury in hyperglycemia

We have previously shown that human munc13 (hmunc13) is up-regulated by hyperglycemia under in vitro conditions in human mesangial cell cultures. The purpose of the present study was to determine the cellular function of hmunc13. To do this, we have investigated the subcellular localization of hmunc13 in a transiently transfected renal cell line, opossum kidney cells. We have found that hmunc13 is a cytoplasmic protein and is translocated to the Golgi apparatus after phorbol ester stimulation. In addition, cells transfected with hmunc13 demonstrate apoptosis after treatment with phorbol ester, but cells transfected with an hmunc13 deletion mutant in which the diacylglycerol (C1) binding domain is absent exhibit no change in intracellular distribution and no induction of apoptosis in the presence of phorbol ester stimulation. We conclude that both the diacylglycerol-induced translocation and the apoptosis represent functional activity of hmunc13. We have also demonstrated that munc13-1 and munc13-2 are localized mainly to cortical epithelial cells in rat kidney and both are overexpressed under conditions of hyperglycemia in a streptozotocin-treated diabetic rat model. Taken together, our data suggest that hmunc13 serves as a diacylglycerol-activated, PKC-independent signaling pathway capable of inducing apoptosis and that this pathway may contribute to the renal cell complications of hyperglycemia.

Phosphorylated Forms of Activated Caspases Are Present in Cytosol From HL-60 Cells During Etoposide-Induced Apoptosis

Treatment of HL-60 human leukemia cells with etoposide induces apoptotic cell death and activation of at least 18 electrophoretically distinct cysteine-dependent aspartate-directed protease (caspase) isoforms, several of which differ only in their isoelectric points. The purpose of the present study was to determine whether activated caspases are phosphorylated. Phosphatase treatment of cytosolic extracts containing active caspases followed by affinity labeling with N-(N-benzyloxycarbonylglutamyl-N-biotinyllysyl)aspartic acid [(2,6-dimethylbenzoyl)oxy] methyl ketone (Z-EK(bio)D-aomk) showed a mobility shift in several of the labeled species, suggesting that phosphorylated forms of these enzymes are present in the extracts. Metabolic labeling with 32P followed by etoposide treatment and subsequent affinity purification of affinity-labeled caspases confirmed that at least three caspase species were phosphorylated. To detect effects of the phosphorylation on enzymatic activity, caspase-mediated cleavage of aspartylglutamylvalinylaspartyl-7-amino-4-trifluoromethylcoumarin (DEVD-AFC) and poly(ADP-ribose) polymerase (PARP) by phosphorylated and dephosphorylated extracts was measured. No significant changes in Km or vmax were detected using DEVD-AFC. In contrast, a slight, but significant enhancement of PARP cleavage was observed in dephosphorylated extracts, suggesting that phosphorylation of active caspases could have an inhibitory effect on enzyme activity. These observations, which provide the first evidence that caspases are phosphoproteins, suggest that caspases may be targets for some of the growing list of protein kinases that are involved in apoptotic events.

© 1998 by The American Society of Hematology.

Phosphorylated Forms of Activated Caspases Are Present in Cytosol From HL-60 Cells During Etoposide-Induced Apoptosis

Treatment of HL-60 human leukemia cells with etoposide induces apoptotic cell death and activation of at least 18 electrophoretically distinct cysteine-dependent aspartate-directed protease (caspase) isoforms, several of which differ only in their isoelectric points. The purpose of the present study was to determine whether activated caspases are phosphorylated. Phosphatase treatment of cytosolic extracts containing active caspases followed by affinity labeling with N-(N-benzyloxycarbonylglutamyl-N-biotinyllysyl)aspartic acid [(2,6-dimethylbenzoyl)oxy] methyl ketone (Z-EK(bio)D-aomk) showed a mobility shift in several of the labeled species, suggesting that phosphorylated forms of these enzymes are present in the extracts. Metabolic labeling with 32P followed by etoposide treatment and subsequent affinity purification of affinity-labeled caspases confirmed that at least three caspase species were phosphorylated. To detect effects of the phosphorylation on enzymatic activity, caspase-mediated cleavage of aspartylglutamylvalinylaspartyl-7-amino-4-trifluoromethylcoumarin (DEVD-AFC) and poly(ADP-ribose) polymerase (PARP) by phosphorylated and dephosphorylated extracts was measured. No significant changes in Km or vmax were detected using DEVD-AFC. In contrast, a slight, but significant enhancement of PARP cleavage was observed in dephosphorylated extracts, suggesting that phosphorylation of active caspases could have an inhibitory effect on enzyme activity. These observations, which provide the first evidence that caspases are phosphoproteins, suggest that caspases may be targets for some of the growing list of protein kinases that are involved in apoptotic events.

© 1998 by The American Society of Hematology.

Molecular mechanisms of programmed cell death

Programmed cell death and apoptosis have now been recognised as biological phenomena which are of fundamental importance to the integrity of organisms. What may have evolved as an altruistic defence against pathogen invasion in simple organisms is now a major regulatory mechanism in the development and maintenance of multi-cellular organisms. The classically defined morphological characteristics of apoptosis are now accompanied by a plethora of information regarding common biochemical and genetic mediators of programmed cell death. It is apparent that life and death decisions are taken by individual cells based on their interpretation of physiological signals, or their own self-assessment of internal damage. The knowledge that cell death is a genetically regulated process has highlighted an inherent potential for manipulation and offered new avenues for research into several diseases, and also productivity improvements in the biotechnology industry. This relatively "new frontier" in cell science has undoubtedly widened our perspectives and may provide novel strategies to expedite both medical and biotechnological research.

Dysmorphogenic effects of a specific protein kinase C inhibitor during neurulation

Protein kinase C (PKC) plays a key role in signal transduction and is an important mediator of events throughout development. However, no information exists regarding the effect of a specific PKC inhibitor on mammalian embryogenesis during neurulation. This investigation was undertaken to examine the effects of a specific inhibitor of PKC, as well as inhibitors of other important kinases, on cultured mouse embryos. CD-1 mouse embryos (3 to 6 somite stage) were exposed to bisindolylmaleimide I (a specific PKC inhibitor) as well as specific inhibitors of PKA, PKG, and MAP kinase kinase for 24 h. The PKC inhibitor was a potent embryotoxicant and elicited malformations at concentrations as low as 0.01 microM. Inhibitors of other kinases also produced malformations but at much higher concentrations than those required to produce similar defects with the PKC inhibitor. These data suggest that PKC plays an important role in mammalian neurulation. Further research is required to clarify the mechanism by which PKC inhibition at this developmental stage produces malformations and the potential effects of environmental toxicants with PKC inhibitory properties on this signal transduction pathway.

Intercomparison of apoptosis morphology with active DNA cleavage on single cells in vitro and on testis tumours

Apoptosis morphology (DNA condensation) and endonucleolytical DNA cleavage (TdT assay) were measured simultaneously on double fluorescence labelled cells employing confocal laser scanning and conventional immunofluorescence microscopy. In vitro experiments on irradiated HL-60 cells revealed a high correspondence of non-apoptotic (normal) cells without detectable DNA cleavage, versus apoptotic cells and apoptotic bodies showing DNA cleavage. Experiments performed on histological slides of testis tumours reflected a heterogeneous picture: non-apoptotic (normal) cells, apoptotic cells, and apoptotic bodies appeared either with or without detectable DNA cleavage. These data allowed the characterization and quantitation of the grade of disturbance/heterogeneity of the apoptosis programme in vivo. Furthermore, the measured apoptotic index (AI) based on apoptosis morphology was lower than the AI assessed by DNA cleavage, in contrast to published work. Taken together, these methods represent a new approach and might be suitable for improved correlation with clinical parameters. In addition, the data presented confirm frequently published doubts regarding the ability of the TdT assay to detect apoptosis as defined by morphological criteria in tumours.

Transforming growth factor-beta-mediated apoptosis in the Ramos B-lymphoma cell line is accompanied by caspase activation and Bcl-XL downregulation

Upon transforming growth factor-beta (TGF-beta) treatment, Ramos cells, a B-cell lymphoma cell line, undergo apoptosis, as measured by annexin V labeling, DNA fragmentation, and propidium iodide staining. Apoptosis could be observed by 24 h after TGF-beta exposure and occurred before the development of a significant blockage of cell cycle progression. TGF-beta-mediated apoptosis was also accompanied by a strong induction of caspase-3 subfamily activity. Incubation of cells with the caspase inhibitor Z-VAD.FMK at 20 microM, but not at 10 microM, prevented TGF-beta-induced apoptosis from occurring. By comparison, caspase-3 subfamily activity was 87% inhibited at 10 microM Z-VAD.FMK and completely inhibited at 20 microM. Because of TGF-beta's well-established role of regulating gene transcription, the mRNA levels for proteins associated with apoptosis (Fas- and Fas-associated proteins, Bcl-2 family members, IAP proteins, and I kappa B) were also studied. After 24 h of TGF-beta treatment, the most significant mRNA changes occurred with Bcl-XL (two-fold decrease) and Bik (twofold increase). TGF-beta treatment also resulted after 48 h in a fivefold decrease in Bcl-XL protein levels, based on immunoblotting analysis. Therefore, TGF-beta-mediated apoptosis involves the activation of caspases. In addition, TGF-beta transcriptionally regulates Bcl-2 family members, Bcl-XL and Bik, to further influence the apoptosis process.

Inhibition of CPP32-like proteases rescues axotomized retinal ganglion cells from secondary cell death in vivo

The majority of retinal ganglion cells (RGCs) degenerate and die after transection of the optic nerve (ON) in the adult rat. This secondary cell death can primarily be ascribed to apoptosis. Recent work strongly suggests a decisive role for a family of cysteine proteases, termed caspases, as mediators of neuronal apoptosis. In this study, we investigated whether activation of caspases contributes to delayed death of RGCs after axotomy. Intraocular application of various caspase inhibitors rescued up to 34% of RGCs that would otherwise have died 14 d after ON transection. Using a modified affinity-labeling technique, we detected a 17 kDa protease subunit upregulated after axotomy. Upregulation was prevented by caspase inhibitor treatment. The 17 kDa protein was identified as a CPP32-like protease by Western blot analysis and affinity labeling with biotinylated acetyl-Asp-Glu-Val-Asp-aldehyde, which specifically inhibits CPP32-like caspases. In vivo application of the irreversible caspase inhibitor benzyloxycarbonyl-Asp-Glu-Val-Asp-chloromethylketone revealed CPP32-like proteases to be major mediators of caspase-induced apoptosis in axotomized RGCs, because this inhibitor showed an even higher neuroprotective potential than the irreversible wide-range inhibitor benzyloxycarbonyl-Val-Ala-DL-Asp-fluoromethylketone. In summary, the data presented here provide further insight into the mechanisms of injury-induced neuronal apoptosis and could give rise to more effective therapeutic intervention strategies in CNS trauma and neurodegenerative diseases.

Simultaneous measurement of cell cycle and apoptotic cell death

Understanding the dynamics of cell death in conjunction with those of cell cycle can be illuminating in the investigation of various cellular behaviors. Robust assays for measuring such parameters are invaluable. Many assays of apoptosis and/or cell cycle use flow cytometry. This chapter describes two different assays to measure apoptosis and cell cycle simultaneously using flow cytometry. The first involves the use of terminal transferase (the "TUNEL" assay) together with propidium iodide for identification of cell cycle. The second uses fluorescently labeled annexin V, together with propidium iodide as an indicator of cell membrane integrity; and additionally Hoechst 33342 for determination of cell cycle. Each assay has positive and negative attributes. The terminal transferase assay is performed using fixed cells and is therefore useful in the analysis of samples collected over time. The annexin V assay is performed using unfixed cells, and thus provides information regarding membrane integrity. Other practical aspects of both assays are discussed.

Effect of curcumin on cell cycle progression and apoptosis in vascular smooth muscle cells

1. The possible mechanisms of the antiproliferative and apoptotic effects of curcumin (diferuloylmethane), a polyphenol in the spice turmeric, on vascular smooth muscle cells were studied in rat aortic smooth muscle cell line (A7r5). 2. The proliferative response was determined from the uptake of [3H]-thymidine. Curcumin (10(-6)-10(-4) M) inhibited serum-stimulated [3H]-thymidine incorporation of both A7r5 cells and rabbit cultured vascular smooth muscle cells in a concentration-dependent manner. Cell viability, as determined by the trypan blue dye exclusion method, was unaffected by curcumin at the concentration range 10(-6) to 10(-5) M in A7r5 cells. However, the number of viable cells after 10(-4) M curcumin treatment was less than the basal value (2 x 10(5) cells). 3. To analyse the various stages of the cell cycle, [3H]-thymidine incorporation into DNA was determined every 3 h. After stimulation with foetal calf serum, quiescent A7r5 cells started DNA synthesis in 9 to 12 h (G1/S phase), then reached a maximum at 15 to 18 h (S phase). Curcumin (10(-6)-10(-4) M) added during either the G1/S phase or S phase significantly inhibited [3H]-thymidine incorporation. 4. Following curcumin (10(-6)-10(-4) M) treatment, cell cycle analysis utilizing flow cytometry of propidium iodide stained cells revealed a G0/G1 arrest and a reduction in the percentage of cells in S phase. Curcumin at 10(-4) M also induced cell apoptosis. It is suggested that curcumin arrested cell proliferation and induced cell apoptosis, and hence reduced the [3H]-thymidine incorporation. 5. The apoptotic effect of 10(-4) M curcumin was also demonstrated by haematoxylin-eosin staining, TdT-mediated dUTP nick end labelling (TUNEL), and DNA laddering. Curcumin (10(-4) M) induced cell shrinkage, chromatin condensation, and DNA fragmentation. 6. The membranous protein tyrosine kinase activity stimulated by serum in A7r5 cells was significantly reduced by curcumin at the concentration range 10(-5) to 10(-4) M. On the other hand, the cytosolic protein kinase C activity stimulated by phorbol ester was reduced by 10(-4) M curcumin, but unaffected by lower concentrations (10(-6)-10(-5) M). 7. The levels of c-myc, p53 and bcl-2 mRNA were analysed using a reverse transcription-polymerase chain reaction (RT-PCR) technique. The level of c-myc mRNA was significantly reduced by curcumin (10(-5)-10(-4) M) treatment. And, the level of bcl-2 mRNA was significantly reduced by 10(-4) M curcumin. However, the alteration of the p53 mRNA level by curcumin (10(-5)-10(-4) M) treatment did not achieve significance. The effects of curcumin on the levels of c-myc and bcl-2 mRNA were then confirmed by Northern blotting. 8. Our results demonstrate that curcumin inhibited cell proliferation, arrested the cell cycle progression and induced cell apoptosis in vascular smooth muscle cells. Curcumin may be useful as a template for the development of drugs to prevent the pathological changes of atherosclerosis and post-angioplasty restenosis. Our results suggest that the antiproliferative effect of curcumin may partly be mediated through inhibition of protein tyrosine kinase activity and c-myc mRNA expression. And, the apoptotic effect may partly be mediated through inhibition of protein tyrosine kinase activity, protein kinase C activity, c-myc mRNA expression and bcl-2 mRNA expression.

Reactive oxygen species and intracellular Ca2+, common signals for apoptosis induced by gallic acid

Gallic acid (3,4,5-trihydroxybenzoic acid), a naturally occurring plant phenol, induces cell death in apparently different manners, depending on cell lines. Flow cytometric analysis and agarose gel electrophoresis indicated that internucleosomal breakdown of chromatin DNA was observed in HL-60RG cells but not in dRLh-84, HeLa, and PLC/PRF/5 cells, and that the action of gallic acid was independent of cell cycle. A detailed study of signal transduction revealed that the gallic acid-induced cell death of all cells tested in this study was prevented by treatment with the intracellular thiol antioxidant N-acetyl-L-cysteine, catalase, and the intracellular calcium chelator bis-(o-aminophenoxy)-N,N,N,N'-tetraacetic acid acetoxymethyl ester (BAPTA-AM). However, the effects of ascorbic acid, superoxide dismutase, EGTA, the endonuclease inhibitor zinc sulfate, the calmodulin inhibitor N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7), and the NADPH oxidase inhibitor diphenyleneiodonium chloride on cell death were different depending on the cell type, suggesting that the death signal induced by gallic acid was diverse among different cell types, although the production of reactive oxygen species, such as H2O2, and the elevation of intracellular calcium concentration were required as common signals.

Endogenous activation of apoptosis in bursal lymphocytes: inhibition by phorbol esters and protein synthesis inhibitors

The bursa of Fabricius represents the primary immune organ where immature B cells undergo maturational changes in avian species. Isolation of bursal lymphocytes for analysis in cell culture results in the rapid endogenous activation of apoptosis. After 2 h of incubation, over 45% of the lymphocytes were shown to be undergoing apoptosis and by 6 h 80% were undergoing apoptosis as demonstrated by a terminal deoxynucleotidyltransferase-mediated dUTP-fluorescein isothiocynate nick end-labeling flow-cytometric analysis. These results were corroborated by a propidium iodide-staining flow-cytometric assay and by an agarose gel electrophoresis DNA fragmentation assay that demonstrated internucleosomal DNA cleavage of genomic DNA in apoptotic bursal lymphocytes. Endogenous activation of apoptosis in bursal lymphocytes could be inhibited in a dose-dependent fashion with the phorbol ester, phorbol 12-myristate 13-acetate, but not the phorbol ester antagonist 4 alpha-phorbol 12,13-didecanoate. In addition, apoptosis could be inhibited in a dose-dependent fashion with inhibitors of protein translation, cycloheximide, and puromycin, as well as the transcriptional inhibitor actinomycin D. These results suggest that endogenous activation of bursal lymphocyte apoptosis may be mediated by the protein kinase C signal transduction pathway and activation of this process appears to be dependent upon de novo protein biosynthesis.

The role of protein kinase C (PKC) in the evolution and proliferation of malignant gliomas, and the application of PKC inhibition as a novel approach to anti-glioma therapy

The present article reviews the role of the second messenger enzyme protein kinase C (PKC) in the growth regulation of high-grade gliomas, and evaluates the efficacy of therapeutic strategies directed against PKC for blocking the proliferation of these malignancies in in vitro and in vivo models. The translation of such strategies to the treatment of patients with malignant gliomas may provide a novel approach for improving the otherwise grim outlook associated with these neoplasms.