Role of protein kinase C in glial cell proliferation

Cell-specific expression of neutral glycosphingolipids in vertebrate brain: immunochemical localization of 3-O-acetyl-sphingosine-series glycolipid(s) in myelin and oligodendrocytes

The tissue- and cell-specific expression of three neutral glycosphingolipids, gangliotetraosylceramide (GA1), gangliopentaosylceramide (GalNAc-GA1), and the novel 3-O-acetyl-sphingosine-series glycolipid (FMC-5), were examined with monospecific polyclonal antibodies. Immunohistochemical studies of rodent brain cross-sections indicated that both GA1 and FMC-5 antibodies stained myelin. In contrast, GalNAc-GA1 antibody distinctly stained neurons in cerebral cortex, but only partially delineated Purkinje cells and other neurons in cerebellum. Preliminary studies of mixed glial cultures suggested the following: 1) both FMC-5 and GA1 antibodies stained oligodendrocytes and oligo progenitors, and 2) GalNAc-GA1 antibody did not stain any cells in the culture. Because the GalNAc-GA1 was associated with neurons, we examined the immunoreactivity of GalNAc-GA1 antibody in primary neuronal cultures. Further studies using primary cultures of rat brain oligodendrocytes, and dissociated cerebellar neuronal cultures indicated that both GA1 and FMC-5 are specifically expressed by oligodendrocytes, whereas GalNAc-GA1 is primarily localized in interneurons and to some extent in Purkinje neurons.

PMA decreases the proliferation of retinal cells in vitro: the involvement of acetylcholine and BDNF

Protein kinase C (PKC) is involved in several cell events including proliferation, survival and differentiation. The aim of this work was to investigate the role of PKC activation on retinal cells proliferation. We demonstrated that PKC activation by phorbol 12-myristate 13-acetate (PMA), a tumor promoter phorbol ester, is able to decrease retinal cells proliferation. This effect was mediated by M1 receptors and dependent on intracellular Ca(2+) increase, tyrosine kinase activity, phosphatidylinositol 3-kinase activity, polypeptide secretion and activation of TrkB receptors. The effect of PMA was not via activation of mitogen-activated protein (MAP) kinase. Carbamylcholine and brain derived neurotrophic factor were both able to decrease retinal cells proliferation to the same level as PMA did. Our results suggest that PKC activation leads to a decrease in retinal cells proliferation through the release of acetylcholine and brain derived neurotrophic factor in the culture, and activation of M1 and TrkB receptors, respectively.

Increase in glutamate-induced neurotoxicity by activated astrocytes involves stimulation of protein kinase C

Activation of astrocytes is a common feature of neurological disorders, but the importance of this phenomenon for neuronal outcome is not fully understood. Treatment of mixed hippocampal cultures of neurones and astrocytes from day 2-4 in vitro (DIV 2-4) with 1 micro m cytosine arabinofuranoside (AraC) caused an activation of astrocytes as detected by a stellate morphology and a 10-fold increase in glial fibrillary acidic protein (GFAP) level compared with vehicle-treated cultures. After DIV 12, we determined 43% and 97% damaged neurones 18 h after the exposure to glutamate (1 mm, 1 h) in cultures treated with vehicle and AraC, respectively. Dose-response curves were different with a higher sensitivity to glutamate in cultures treated with AraC (EC50 = 0.01 mm) than with vehicle (EC50 = 0.12 mm). The susceptibility of neurones to 1 mm glutamate did not correlate with the percentage of astrocytes and was insensitive to an inhibition of glutamate uptake. In cultures treated with vehicle and AraC, glutamate-induced neurotoxicity was mediated through stimulation of the NR1-NR2B subtype of NMDA receptors, because it was blocked by the NMDA receptor antagonist MK-801 and the NR1-NR2B selective receptor antagonist ifenprodil. Protein levels of the NR2A and NR2B subunits of NMDA receptor were similar in cultures treated with vehicle or AraC. AraC-induced changes in glutamate-induced neurotoxicity were mimicked by activation of protein kinase C (PKC), whereas neuronal susceptibility to glutamate was reduced in cultures depleted of PKC and treated with AraC suggesting that the increase in glutamate toxicity by activated astrocytes involves activation of PKC.

Piracetam and vinpocetine exert cytoprotective activity and prevent apoptosis of astrocytes in vitro in hypoxia and reoxygenation

The aim of the present study was to establish whether piracetam (2-pyrrolidon-N-acetamide; PIR) and vinpocetine (a vasoactive vinca alkaloid; VINP) are capable of protecting astrocytes against hypoxic injury. Using the model of astrocyte cell culture we observed the cells treated with PIR and VINP during and after in vitro simulated hypoxia. Cell viability was determined by Live/Dead Viability/Cytotoxicity Assay Kit, LDH release assay and MTT conversion test. Apoptotic cell death was distinguished by a method of Hoechst 33342 staining underfluorescence microscope and caspase-3 colorimetric assay. In addition the intracellular levels of ATP and phosphocreatine (PCr) were evaluated by bioluminescence method. Moreover, the effect of the drugs on the DNA synthesis was evaluated by measuring the incorporation of [3H]thymidine into DNA of astrocytes. PIR (0.01 and 1 mM) and VINP (0.1 and 10 microM) were added to the medium both during 24 h normoxia, 24 h hypoxia or 24 h reoxygenation. Administration of 1 mM PIR or 0.1 microM VINP to the cultures during hypoxia significantly decreases the number of dead and apoptotic cells. The antiapoptic effects of drugs in the above mentioned concentrations was also confirmed by their stimulation of mitochondrial function, the increase of intracellular ATP, and the inhibition of the caspase-3 activity. The prevention of apoptosis was accompanied by the increase in ATP and PCr levels and increase in the proliferation of astrocytes exposed to reoxygenation. The higher concentration of VINP (10 microM) was detrimental in hypoxic conditions. Our experiment proved the significant cytoprotective effect of 1 mM PIR and 0.1 microM VINP on astrocytes in vitro.

Histamine H1 receptor activation stimulates mitogenesis in human astrocytoma U373 MG cells

In human astrocytoma U373 MG cells that express histamine H1 receptors (180 +/- 6 fmol/mg protein) but not H2 or H3 receptors, histamine stimulated mitogenesis as assessed by [3H]-thymidine incorporation (173 +/- 2% of basal; EC50, 2.5 +/- 0.4 microM). The effect of 100 microM histamine was fully blocked by the selective H1 antagonist mepyramine (1 microM) and was markedly reduced (93 +/- 4% inhibition) by the phospholipase C inhibitor U73122 (10 microM). The activator of protein kinase C (PKC) phorbol 12-tetradecanoyl-13-acetate (TPA, 100nM) stimulated [3H]-thymidine incorporation (270 +/- 8% of basal), and this response was not additive with that to 100 microM histamine. The incorporation of [3H]-thymidine induced by 100 microM histamine was partially reduced by the PKC inhibitor Ro 31-8220 (57 +/- 7% inhibition at 300 nM) and by the compound PD 098,059 (30 microM, 62 +/- 14% inhibition), an inhibitor of the mitogen-activated kinase (MAPK) kinases MEK1/MEK2. These results show that histamine H1 receptor activation stimulates the proliferation of human astrocytoma U373 MG cells. The action of histamine appears to be partially mediated by PKC stimulation and MAPK activation.

Remyelination in vitro following protein kinase C activator-induced demyelination

In previous work we found that mezerein, a C kinase activator, as well as basic fibroblast growth factor (FGF-2) induce demyelination and partial oligodendrocyte dedifferentiation in highly differentiated aggregating brain cell cultures. Here we show that following protein kinase C activator-induced demyelination, effective remyelination occurs. We found that mezerein or FGF-2 caused a transient increase in DNA synthesis following a pronounced decrease of the myelin markers myelin basic protein and 2',3'-cyclic nucleotide 3'-phosphohydrolase. Both oligodendrocytes and astrocytes were involved in this mitogenic response. Within 17 days after demyelination, myelin was restored to the level of the untreated controls. Transient mitotic activity was indispensable for remyelination. The present results suggest that myelinating oligodendrocytes retain the capacity to reenter the cell cycle, and that this plasticity is important for the regeneration of the oligodendrocyte lineage and remyelination. Although it cannot be excluded that a quiescent population of oligodendrocyte precursor cells was present in the aggregates and able to proliferate, differentiate and remyelinate, we could not find evidence supporting this view.

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.

Differences in signal transduction pathways by which platelet-derived and fibroblast growth factors activate extracellular signal-regulated kinase in differentiating oligodendrocytes

Treatment of cultured rat oligodendroglial progenitors with either platelet-derived growth factor (PDGF) or fibroblast growth factor-2 (FGF-2) activated extracellular signal regulated kinase 2 (ERK2). Activation was transient in response to PDGF, whereas it was greater and more prolonged in response to FGF-2. ERK2 activation by PDGF was preceded by a very rapid, robust and transient tyrosine phosphorylation of the PDGF receptor. Although there was consistently more activation of ERK2 in response to FGF-2 than to PDGF, immunostaining of FGF receptors 1 (FGFR1) and 2 (FGFR2) and their tyrosine phosphorylation in progenitors was very weak, and both receptors were up-regulated during differentiation to oligodendrocytes. Tyrosine phosphorylation of the FGF receptors was maximal from 15 to 60 min of treatment and was sustained for many hours. Binding of radioiodinated FGF-2 to FGFR1 was predominant in progenitors, whereas binding to FGFR2 was predominant in oligodendrocytes. ERK2 activation by PDGF was more sensitive to inhibition of tyrosine kinases, whereas ERK2 activation by FGF-2 was relatively more sensitive to inhibitors of protein kinase C. These differences in signal transduction pathways probably contribute to the different cellular responses of oligodendroglial lineage cells to PDGF and FGF-2, respectively.

Both cell-surface carbohydrates and protein tyrosine phosphatase are involved in the differentiation of astrocytes in vitro

Astrocytes are important in the development and maintenance of functions of the CNS, acting in cooperation with neurons and other glial cells. The glycans on astrocyte membrane are believed to play important roles in cell-cell communication. Plant lectins are useful probes, because the lectins can bind to certain cell surface receptors and elicit cellular responses that are normally activated by endogenous ligands for those receptors. In the present study, we investigated the effect of Datura stramonium agglutinin (DSA) on astrocytes and characterized several molecular events. The addition of DSA to a culture of flat, polygonal, immature astrocytes derived from the neonatal rat cerebellum caused the cells to become stellate in shape, similar to astrocytes observed in vivo, concomitant with an increase in expression of astrocyte-specific intermediate filament (glial fibrillary acidic protein [GFAP]) and inhibition of proliferation. These results indicate that DSA binds to astrocytes and triggers differentiation. We also found a decrease in the extent of tyrosine-phosphorylation of a 38-kDa protein. To elucidate the molecular events during astrocyte differentiation, we examined the effects of various signal transduction inhibitors on the transformation from the polygonal to stellate shape (stellation). Interestingly, only tyrosine phosphatase inhibitors, orthovanadate and phenylarsine oxide, showed an inhibitory effect. Our results suggest that DSA induced astrocyte differentiation acts via tyrosine dephosphorylation.

Exposure to 60-Hz magnetic fields and proliferation of human astrocytoma cells in vitro

Epidemiological studies have suggested that exposure to electric and magnetic fields (EMF) may be associated with an increased incidence of brain tumors, most notably astrocytomas. However, potential cellular or molecular mechanisms involved in these effects of EMF are not known. In this study we investigated whether exposure to 60-Hz sinusoidal magnetic fields (0.3-1.2 G for 3-72 h) would cause proliferation of human astrocytoma cells. Sixty-Hertz magnetic fields (MF) caused a time- and dose-dependent increase in proliferation of astrocytoma cells, measured by (3)H-thymidine incorporation and by flow cytometry, and strongly potentiated the effect of two agonists (the muscarinic agonist carbachol and the phorbol ester PMA). However, MF had no effect on DNA synthesis of rat cortical astrocytes, i.e., of similar, nontransformed cells. To determine the amount of heating induced by MF, temperatures were also recorded in the medium. Both 1.2 G MF and a sham exposure caused a 0.7 degrees C temperature increase in the medium; however, (3)H-thymidine incorporation induced by sham exposure was significantly less than that caused by MF. GF 109203X, a rather specific protein kinase C (PKC) inhibitor, and down-regulation of PKC inhibited the effect of MF on basal and on agonist-stimulated (3)H-thymidine incorporation. These data indicate that MF can increase the proliferation of human astrocytoma cells and strongly potentiate the effects of two agonists. These findings may provide a biological basis for the observed epidemiological associations between MF exposure and brain tumors.

HMBA inhibits growth and induces differentiation of astrocytes from neonatal rat brain

HMBA, a potent differentiation inducer belonging to the class of hybrid polar compounds, inhibited the growth of astrocytes from neonatal rat brain. At a concentration of 5.0 mM, which is necessary for growth inhibition and differentiation of a number of leukemic and solid tumour cell lines, astrocyte proliferation was reduced by 74.5+/-5.5%. On HMBA treatment astrocyte morphology changed from flat polygonal with phase translucent cytoplasm to compact retracted appearance with phase dark cytoplasm. HMBA also increased expression of GFAP, a marker for mature astrocytes. HMBA thus induces both morphological and biochemical differentiation of astrocytes. Like dbcAMP and staurosporine, two other known differentiation inducers of astrocytes, HMBA was also found to induce expression of the immediate early gene c-fos. However, unlike dbcAMP and staurosporine, which also induce fra-1, HMBA repressed cycloheximide-induced fra-1 gene expression.

Tamoxifen modulation of carboplatin cytotoxicity in a human U-138 glioma cell line

Glioma cells express high protein kinase C (PKC) activity, which may represent an important therapeutic target. Tamoxifen (TAM) has moderate PKC-inhibiting activity, blocking DNA synthesis and cellular proliferation in human glioma cells at concentrations that can be achieved therapeutically. Carboplatin (CBDCA), a second-generation platinum derivative, induces intra- and interstrand DNA-protein crosslinks producing inhibition of tumor-cell growth. In the present study, the effect of TAM, CBDCA, and the combination of both was evaluated against the human established U-138 glioma cell line during the exponential growth phase (48-72 h) by means of both the Biorad protein assay (BPA) method and Trypan blue exclusion study (TBES). Both TAM and CBDCA reduced the cellular growth rate, with a median 50%-inhibiting concentration (IC50) of 12.5 microM for TAM and 350 microM for CBDCA. The U-138 glioma cell line showed a moderate response to 100 microM of CBDCA, with < or = 10% reduction of the growth rate. The association of both chemotherapeutic agents induced a 98% reduction of the IC50 dose of TAM (0.1 microM), and a 71% reduction of the IC50 dose of CBDCA (100 microM). During the combinational TAM CBDCA exposure we observed a cytotoxic effect of TAM at concentrations lower than 0.1 microM, not recognized using it as a single drug. The differences observed among the IC50 doses (TAM, CBDCA, TAM-CBDCA) and among treated and untreated matched control cells were statistically significant (P < 0.01). Our results confirm previous observations about the efficacy in vitro of TAM against human glioma cell lines and show a marked enhancement of this activity by CBDCA.

Differential role of specific PKC isoforms in the proliferation of glial cells and the expression of the astrocytic markers GFAP and glutamine synthetase

In this study, we explored the role of specific protein kinase C (PKC) isoforms in glial cell proliferation and on the expression of the astrocytic markers GFAP and glutamine synthetase using C6 cells as a model. Analysis of the expression of the various PKC isoforms in control and differentiated C6 cells revealed differences in the expression of specific PKC isoforms. Undifferentiated C6 cells, which express low levels of GFAP and glutamine synthetase (GS), have high levels of PKCalpha and delta, whereas differentiated C6 cells, which express higher levels of both GFAP and GS have lower levels of PKCalpha and delta and higher levels of PKCgamma, theta and eta. Using C6 cells overexpressing specific PKC isoforms, we examined the role of these isoforms on the proliferation and differentiation of C6 cells. Cells overexpressing PKCalpha displayed a reduced level of GFAP, whereas GS expression was not affected. On the other hand, cells overexpressing PKCdelta showed reduced GS expression but little effect on GFAP. Finally, cells expressing PKCgamma displayed a marked increase in the levels of both GFAP and GS. The proliferation of C6 cells was increased in cells overexpressing PKCalpha and epsilon and decreased in cells overexpressing PKCgamma, delta and eta. The results of this study suggest that glial cell proliferation and astrocytic differentiation can be regulated by specific PKC isoforms that selectively affect cell proliferation and the expression of the two astrocytic markers GFAP and GS.

Tamoxifen and carboplatin combinational treatment of high-grade gliomas. Results of a clinical trial on newly diagnosed patients

Between April, 1992 and December, 1995, forty consecutive patients with a cerebral malignant glioma (WHO Grade III and IV) were enrolled in a trial consisting in surgery and post-operative administration of radiotherapy (4500-6000 cGy), carboplatin (CBDCA; dose of 450-600 mg/m2), and oral tamoxifen (TAM; at doses of 40, 80 or 120 mg/day). Two patients of the TAM group died in the postoperative period from a pulmonary embolism and myocardial infarction, respectively. The patients (all dosages combined) had a median survival time of 13 months from the time of diagnosis. The 12-month and 24-month survival rates were 52% and 32%, respectively. The median relapse-free survival time was 7 months. Patients treated with higher doses of TAM (80-120 mg/day) demonstrated a longer median survival rate (13 months both) and a longer 12-month survival result (58% and 76%, respectively). Patients who assumed TAM for a period longer than 3 months (group +3) have a higher median survival rate (16 months) and better 12-month and 24-month results (62% and 40%, respectively). Moreover, the median relapse-free survival time was 10 months (versus 6 months in group -3; p = 0.0038). However, it is not possible to exclude that patients of group +3 had a slower growing or a stable tumor and were well enough to assume TAM for a longer period. The results observed in the TAM-group have been compared with those of 40 matched controls treated with surgery, radiotherapy and CBDCA. These patients had a median survival time of 9 months (p = 0.04) and the 12-month and 24-month survival rates were 30% and 0%, respectively. The median relapse-free survival time was 4 months (p = 0.0014). These data suggest a potential role for combinational TAM-CBDCA therapy in the post-operative treatment of cerebral malignant gliomas; further clinical phase III trials, especially those with higher dosages of TAM are warranted.

Selective induction of tumor necrosis factor receptor type II gene expression by tumor necrosis factor-alpha in C6 glioma cells

Using reverse transcription-polymerase chain reaction (RT-PCR) technique, the levels of tumor necrosis factor receptors gene expression in C6 glioma cells upon induction with tumor necrosis factor-alpha (TNF-alpha) were analysed. In control cells, the level of mRNA for tumor necrosis factor receptor type II (TNF-R2; 75/80 kDa) was much lower than that of tumor necrosis factor receptor type I (TNF-R1; 55/60 kDa). Upon exposure to TNF-alpha, the TNF-R2 mRNA level was greatly increased, while the TNF-R1 mRNA level remained unchanged even after 48 h. The induction of TNF-R2 gene expression by TNF-alpha was dose-dependent and seemed to be unique to TNF-alpha, as IL-6 had no effect. Since TNF-R2 was reported to mediate mitogenic effect in many other cell types, it is likely that the reported proliferative effect of TNF-alpha on astrocytes and C6 glioma cells was mediated by this TNF receptor subtype.

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.

Signal transduction pathways and their relevance in human astrocytomas

Aberrations in a number of signal transduction pathways have been identified as playing a key role in the molecular pathogenesis of astrocytomas and their progression to high grade glioblastoma multiforme (GBM). GBMs are characterized by overexpression of the Platelet Derived Growth Factor Receptors (PDGFR) and their ligands (PDGF), as well as the Epidermal Growth Factor Receptor (EGF-R). These receptors activate the Ras pathway, a key cellular signal transduction pathway, culminating in the activation of a wide range of Ras-dependent cellular events. GBMs have also been found to either overexpression or lose expression of various Protein Kinase C (PKC) isoforms. Major strides are being made in developing pharmacological agents which specifically inhibit these growth factor receptors and intracellular signal transduction pathways. Elucidating the role of these pathways in GBMs is thus of major clinical importance, as these novel molecularly-targeted agents may prove of use in the clinical management of GBMs in the future.

Identification and cellular localization of protein kinase C isoforms in cultures of rat type-1 astrocytes

We have examined which isoforms of protein kinase C were present in rat brain astrocytes and found that: (1) the total of calcium-independent isoforms was greater than the total of calcium-dependent isoforms; (2) there were differences in the intracellular distribution of different isoforms; and (3) the abundance of total protein kinase C was greater in astrocytes from cortex than astrocytes from diencephalon.

Protein kinase C and growth regulation of malignant gliomas

This article reviews the role of the signal transduction enzyme protein kinase C in the regulation of growth of malignant gliomas, and describes how targetting this enzyme clinically can provide a novel approach to glioma therapy.

Protein kinase C isoform alpha overexpression in C6 glioma cells and its role in cell proliferation

Previous studies from this laboratory have demonstrated that protein kinase C (PKC) enzyme activity is highly correlated with the proliferation rate of glioma cells, and that glioma cells of both human and rat origin have very high PKC enzyme activity when compared to non-malignant glia including astrocytes, the antecedents of most gliomas. In the present study, by contrasting the rat C6 glioma cells with non-malignant rat astrocytes, we have sought to determine whether the high PKC enzyme activity of glioma cells was due to the overexpression of a specific isoform of PKC. By Western blot analyses, both C6 glioma cells and astrocytes were found to express PKC alpha, beta, delta, epsilon and zeta, but not gamma. Enzyme activity measurements revealed that the elevated PKC activity of glioma cells compared to glia was calcium-dependent, thereby implicating abnormal activity of the alpha or beta isoforms. On Western blots, when compared to astrocytes, glioma cells were determined to overexpress PKC alpha but not beta. An antisense oligonucleotide to PKC alpha, directed at the site of initiation of translation, inhibited the proliferation rate of glioma cells when compared to cells treated with control oligonucleotides; PKC enzyme activity and PKC alpha protein expression were significantly reduced by the antisense treatment. These results suggest that the high PKC enzyme activity of glioma cells, and its correspondence with proliferation rate, is the result of overexpression of isozyme alpha. Targetting PKC alpha in glioma cells may provide a refinement of therapy of glioma patients, some of which are already showing clinical stabilization when treated with drugs with PKC-inhibitory effects.

Inhibitors of N-linked oligosaccharide processing glucosidases interfere with oligodendrocyte differentiation in culture

Previous studies have demonstrated that inhibitors of glycoprotein processing glucosidases interfere with the development of oligodendrocyte properties in primary cultures of embryonic rat brain cells (Bhat, J Neurosci Res 20:158-164, 1988). The present study examines the effect of castanospermine, an inhibitor of the processing glucosidases, on the development and differentiation of isolated oligodendrocyte progenitor cells. Treatment of oligodendrocyte progenitors with castanospermine did not affect the developmental progression of the precursors to become committed oligodendrocytes as revealed by comparable increases in the percentages of cells positive for galactocerebroside (a surface marker for terminally differentiated oligodendrocytes) in control and drug-treated cultures. On the other hand, there was an impairment of the expression of differentiated properties of oligodendrocytes [i.e., sulfolipid synthesis, myelin basic protein (MBP)] and 2'3'-cyclic nucleotide 3'-phosphohydrolase in the drug-treated cultures. Immunocytochemical analysis with anti-MBP antibodies revealed a reduced number of MBP-positive cells in inhibitor-treated cultures. Furthermore, a majority of MBP-positive cells in such cultures displayed immunoreactive MBP in their cell body and not the processes, unlike in control cultures where both cell body and the processes of oligodendrocytes stained intensely for MBP. The strong inhibitory effect of castanospermine on the expression of oligodendrocyte-specific activities was contrasted with a relatively smaller effect of swainsonine, a mannosidase inhibitor on oligodendrocyte differentiation. Both castanospermine and swainsonine, however, effectively blocked the formation of complex-type oligosaccharides, suggesting thereby a lack of correlation between the inhibition of the formation of complex-type oligosaccharides and oligodendrocyte differentiation. It is suggested, therefore, that early trimming reactions involving the removal of glucose residues from the high mannose oligosaccharides in the endoplasmic reticulum may be essential for the cell surface localization and function of glycoproteins critically involved in surface interactions of oligodendrocytes with each other and/or with the substratum.

Involvement of protein kinase C in growth regulation of human meningioma cells

In order to investigate the possible role of protein kinase C (PKC)-mediated signal pathways in growth regulation of meningiomas, we examined the effect of two PKC-activating phorbol esters, 12-O-tetradecanoyl-13-phorbol acetate (TPA) and phorbol 12, 13-dibutyrate (PDBu), and PKC inhibitor, staurosporine, on cell proliferation using low-passage human meningioma cells in culture. TPA (0.1 to 100 ng/ml) caused a dose-dependent stimulation of cell proliferation in six of eight meningioma cultures. At optimal concentrations of TPA, the cell growth ranged from 113% to 251% versus control. In contrast, PDBu (0.1 to 100 ng/ml) caused a significant inhibition of cell proliferation in three of five meningioma cultures. At optimal concentrations of PDBu, the cell growth ranged from 52% to 79% of control. Staurosporine exhibited a stimulation of cell proliferation (135% to 178%) in three of four meningioma cultures studied at a concentration of 10(-10) to 10(-9)M, although a tendency of growth inhibition was observed at a lower concentration. A time course of DNA synthesis in response to TPA, assessed by [3H] thymidine incorporation studies, revealed a time- and dose-dependent stimulation and/or inhibition which further depended on the serum concentration of the growth medium used. The overall results indicate that PKC-mediated signal pathways are closely involved in growth regulation of human meningioma cells. The results further suggest that the signalling processes consist of complex mechanisms which await to be elucidated.

Immunohistochemical localization of protein kinase C delta during postnatal development of the cerebellum

In cerebella of neonatal rats, immunoreactivity of protein kinase C delta (PKC delta) was moderate to strong in radial glia, Bergmann fibers and astrocytes but was absent in neurons. Beginning on the 7th day, a few small clusters of Purkinje cells expressed weak PKC delta reactions in caudal lobules of the vermis. From 10 to 20 days, clusters of labeled Purkinje cells increased in number, size and intensity while astrocytes and palisades of aligned Bergmann glia gradually diminished in intensity. At 21 days, glial cells had weak reactions while Purkinje and stellate-basket cells had intensity and distribution patterns found in adults. A transient occurrence of PKC delta in glia and later appearance in selective groups of neurons strongly support a significant role for this enzyme in signal transduction.

Rat brain glial cells in primary culture and subculture contain the delta, epsilon and zeta subspecies of protein kinase C as well as the conventional subspecies

We raised polyclonal antibodies against the C-terminal peptides of protein kinase C (PkC) subspecies alpha, beta 1, beta 2, gamma, delta, epsilon, and zeta and checked their specificity against brain extracts using Western immunoblot analysis. With equal amounts of protein applied to gels PkC subspecies beta 1, delta, epsilon and zeta were detected in primary cultures of mixed glial cells: bands for the alpha and beta 2 subspecies were less prominent. PkC gamma was not detected in primary glial cultures. The epsilon and zeta subspecies of PkC were detected in subcultures of type 1 astrocytes with weaker bands for the alpha, beta 1 and beta 2 subspecies. Blots of O-2A-lineage glia contained PkCs delta and zeta as prominent bands: the alpha, beta 1 and epsilon subspecies were also present. All PkC subspecies including PkC gamma were detected in C6 glioma cells.

Monogalactosyl diglyceride, a marker for myelination, activates oligodendroglial protein kinase C

Protein kinase C (PKC) is activated by 1,2-sn-diacylglycerol (DAG), the source of which can either be phosphatidylinositol bisphosphate or phosphatidylcholine. Here, we show that monogalactosyl diglyceride (MGDG), a minor galactolipid present in oligodendrocytes (OLs) and myelin, which is designated as a marker for myelination, can enhance OL PKC activity. Based on different calcium and substrate requirements we conclude that MGDG and DAG activate different isoforms of PKC group A: MGDG primarily stimulates PKC-alpha, and DAG primarily activates PKC-gamma. The presence of these PKC isoforms in OLs was confirmed by western blotting, whereas PKC-beta was only weakly stained, if at all. Addition of MGDG to the culture medium provided a higher density of regenerating OL fibers, which was not observed when membrane-permeable DAG was used. These findings indicate that MGDG can modulate the OL PKC activity and that PKC-alpha is the major PKC isoform involved in OL process formation.

Involvement of protein kinase C in cAMP regulation of myelin basic protein gene expression

Since synthesis of myelin components has been seen to be stimulated by cAMP in both oligodendrocytes and Schwann cells we have begun investigating the specific sequence(s) in the 5' flanking region of the myelin basic protein (MBP) gene that are responsible for the induction of MBP transcription by cAMP. Using stably transfected cell lines containing various deletions of the MBP promoter directing the bacterial chloramphenicol acetyltransferase (CAT) gene we have identified a region of the MBP gene that is inhibitory to stimulation by increased cAMP levels. This inhibition can be overcome by pretreating the cells with 12-O-tetradecanoylphorbol 13-acetate (TPA) for 48 hr. The effects on MBP gene expression modulated by TPA and cAMP involve altered DNA-protein interactions in the 5' end of the MBP promoter. The effect of TPA also appears to be mediated by down-regulation of protein kinase C.

Endogenous opioid systems and the growth of oligodendrocyte progenitors: paradoxical increases in oligodendrogenesis as an indirect mechanism of opioid action

Endogenous opioids inhibit nervous system development by inhibiting the proliferation of certain neuronal and glial progenitors. To determine whether opioids affect the growth of preoligodendrocytes, the effects of the endogenous opioid [Met5]-enkephalin were examined in preoligodendrocytes in primary mixed-glial and preoligodendrocyte-enriched (> 98% pure) cultures. Proliferating preoligodendrocytes in mixed-glial or preoligodendrocyte-enriched cultures were continuously treated for a total of 40 h with either basal growth media (controls), 1 microM [Met5]-enkephalin, 1 microM [Met5]-enkephalin plus the opioid antagonist naloxone (3 microM), or naloxone alone (3 microM), and incubated in [3H]-thymidine (0.2 microCi/ml/4-6 h) after 34-36 h of opioid exposure. Opioid-dependent changes in DNA synthesis were assessed autoradiographically in O4-immunoreactive oligodendrocyte progenitors. Naloxone alone significantly decreased the rate of DNA synthesis and number of O4-immunoreactive preoligodendrocytes in mixed-glial cultures. However, naloxone and/or [Met5]-enkephalin did not affect DNA synthesis or the number of O4-immunoreactive preoligodendrocytes in cultures enriched in preoligodendrocytes. The results suggest that astrocytes, or perhaps another cell type, play a permissive role in opioid-dependent alterations in preoligodendrocyte proliferation. Endogenous opioids affect the genesis of neural cells by both direct and indirect mechanisms.

Regulation of astrocyte proliferation by prostaglandin E2 and the alpha subtype of protein kinase C

We found that astrocytes expressed the alpha subtype of protein kinase C. Treatment with 12-O-tetradecanoylphorbol 13-acetate (TPA) caused cultured astrocytes to proliferate. This effect of TPA was blocked by staurosporine, a potent protein kinase C inhibitor, suggesting the involvement of protein kinase C in astrocyte proliferation. Indomethacin, an inhibitor of prostaglandin formation, enhanced both the normal and TPA-induced proliferation of astrocytes. Authentic prostaglandin E2 blocked this effect of indomethacin and also partially blocked the effect of TPA, suggesting that the intracellular mechanisms involved in prostaglandin E2-regulated astrocyte growth might differ from those acting in protein kinase-dependent growth. The effect of prostaglandin E2 was blocked by a specific anti-prostaglandin E2 polyclonal antibody. Cultured astrocytes and microglia produced and released prostaglandin E2 in response to stimulants such as lipopolysaccharide, TPA, and lymphokines. Since the sensitivity of astrocytes and microglia to these stimuli was different, prostaglandin E2 may differentially regulate astrocyte proliferation under different physiological conditions, acting in an autocrine fashion for astrocytes and in a paracrine fashion for microglia.

Protein Kinase C and Phagocytic Activity in Amitriptyline-exposed Primary Cultures of Glial Cells

Factors which can have either an aggravating or a protective effect on glial cell activation, as found in the early stages of multiple sclerosis and other neurological disorders, are not well known. Enzyme analyses and time-lapse video film were used to study the mechanisms underlying glial cell activation as induced by exposure to amitriptyline (AT). When the effects on the two enzymes protein kinase C (PKC) and 2'3’-cyclic nucleotide 3’-phosphodiesterase (CNP) were compared, PKC activity was increased by 49% and CNP activity was not affected. The addition of the essential fatty acids arachidonic acid (Ara) and alpha-linolenic acid (Lin), revealed that Lin alone activated PKC by 59%, and when Lin was co-exposed with AT, by 67%. The activation of astroglial and microglial cells and phagocytosis of oligodendroglial cells in an AT-exposed culture was recorded by video film. Further studies on AT-induced events in primary cultures of glial cells and the modulating effects of fatty acids, are in progress.

Transient reversion of O4+ GalC- oligodendrocyte progenitor development in response to the phorbol ester TPA

The physiological importance of protein kinase C during oligodendrocyte progenitor maturation was investigated by analyzing the effects of the protein kinase C activator phorbol 12-myristate 13-acetate (TPA) on the morphology, proliferation, and differentiation of oligodendrocytes at sequential stages of development. Monoclonal antibodies A2B5 and O4 were used to identify the A2B5+O4- and the A2B5+O4+ galactocerebroside- progenitor stages. Anti-galactocerebroside and anti-myelin basic protein were used to identify mature, post-mitotic oligodendrocytes. Proliferation was measured by bromodeoxyuridine incorporation. Within 24 hr after addition, TPA induced a down-regulation of the O4 antigen in OL progenitors, and an increase of expression of the intermediate filament protein vimentin, leading to a phenotypic reversion from the vimentin-A2B5+O4+ phenotype to the less mature vimentin+A2B5+O4- stage. Concomitantly, TPA induced an increase in the number of bromodeoxyuridine-labeled oligodendrocyte progenitors and extensive process elongation. The response of O4+ progenitors was transient. Even with continued exposure to TPA, by 4 days after TPA addition the reverted cells ceased proliferation, reacquired O4 immunoreactivity, became vimentin-negative, and began to express galactocerebroside and myelin basic protein, and to display the complex, highly branched morphology characteristic of terminally differentiated oligodendrocytes. These results indicate that modulation of protein kinase C activity by TPA induces a transient reversion of O4+ progenitors to less mature O4- cells, causing a transient inhibition of terminal differentiation. The relationship of these data to similar responses of the OL lineage to specific growth factors and implications for remyelination after pathologic injury are discussed.

Proliferation of human and mouse astrocytes in vitro: signalling through the protein kinase C pathway

While several mitogens for astrocytes have been described, the signal transduction pathway(s) that mediates their proliferative effect remains unclear; in this report, a major role for the protein kinase C (PKC) system is suggested by several lines of evidence. Firstly, biologically active phorbol esters, 4 beta-phorbol-12,13-dibutyrate and phorbol-12-myristate-13-acetate, increase the proliferation of astrocytes as determined by [3H]thymidine incorporation or bromodeoxyuridine immunofluorescence; this effect is not reproduced by a phorbol ester that binds to PKC but does not activate it (4 alpha-phorbol-12,13-didecanoate). Secondly, 2 relatively selective inhibitors of PKC, H7 and staurosporine, attenuate the basal rate of proliferation of astrocytes in concentrations that were not cytotoxic to cells. Thirdly, mitogen-enhanced proliferation of astrocytes can be blocked by PKC inhibitors; this is observed for all astrocyte mitogens tested. Fourthly, measurements of PKC enzyme activity in astrocytes in response to serum-mitogenic factors, or to staurosporine, revealed a statistically significant correlation with proliferation rate. The mediation by PKC is not dependent on species- or age factors, since neonatal mouse or adult human astrocytes gave comparable results. The results have relevance to normal development and reactive gliosis post-injury, 2 conditions where astrocytes undergo proliferation, and to glioma growth.

Cell proliferation and protooncogene induction in oligodendroglial progenitors

Cell proliferation and the expression of the protooncogenes c-fos and c-jun have been examined in the primary cultures of oligodendroglial (OL) progenitor cells in response to phorbol 12-myristate 13-acetate (PMA), serum, insulin, insulin-like growth factor-I (IGF-I), platelet-derived growth factor (PDGF), and fibroblast growth factor (FGF). Combined [3H]thymidine autoradiography and immunocytochemistry was used to assess the mitogenic response of O4 (an oligodendrocyte-specific marker)-positive OL progenitors. In addition, the rate of DNA synthesis was measured by the incorporation of [3H]thymidine into acid-precipitable material. It was found that all of the agents tested stimulated DNA synthesis in OL progenitors and induced a rapid increase in c-fos and c-jun protooncogene expression. The induction of c-fos gene expression and DNA synthesis in response to PMA was completely blocked by 1-(5-isoquinolinyl-sulfonyl)-2-methylpiperazine (H-7), a potent inhibitor of protein kinase C (PKC), thereby suggesting a role for PKC in the control of c-fos expression and cell proliferation in OL progenitors.

The treatment of intracranial malignant gliomas using orally administered tamoxifen therapy: preliminary results in a series of "failed" patients

In vitro studies have shown that the nonsteroidal antiestrogen tamoxifen can suppress deoxyribonucleic acid synthesis and cell proliferation in cultured human gliomas. This growth suppression is independent of its antiestrogenic properties. Tamoxifen may act through the inhibition of the enzyme protein kinase C, which transduces mitogenic signals from the cell surface to the nucleus. Based on these preclinical studies, we initiated a clinical trial of orally administered tamoxifen, 20 mg twice daily, to patients with recurrent, progressive malignant gliomas who were not candidates for other "failed" protocols, such as brachytherapy. No limits were placed on age, Karnofsky Performance Score (KPS), or expected survival. Thirty-two patients were entered in the study, 29 with a glioblastoma multiforme and 3 with an anaplastic astrocytoma. The mean age of the group was 48 years, and the mean KPS was 65. Median survival of the entire cohort from the onset of tamoxifen therapy was 17 weeks; the median survival of those patients with an initial KPS of 70 or more was 21 weeks. Seven patients survived for more than 6 months with no change in their baseline computed tomographic scans or KPS on tamoxifen, including 2 patients with computed tomographic evidence of regression during the course of therapy. There were no significant patient-reported side effects of the treatment. Three patients had thromboembolic complications during tamoxifen administration. We conclude that tamoxifen can be administered safely to these patients and may show some efficacy against glial neoplasms.

Interleukin-1-beta and tumor necrosis factor-alpha increase peripheral-type benzodiazepine binding sites in cultured polygonal astrocytes

Peripheral-type benzodiazepine binding sites (PTBBS) are markedly increased in the injured CNS. Astrocytes appear to be the primary cell type which express increased PTBBS. Because certain cytokines within the injured CNS are potent mitogens for astrocytes, we examined the effects of two such cytokines, interleukin (IL)-1 beta and tumor necrosis factor (TNF), on PTBBS in cultured astrocytes using [3H]Ro 5-4864 as the specific ligand. Purified cultures of either polygonal or process-bearing astrocytes were prepared from neonatal rat cerebral hemispheres. At a concentration of 1.8 nM, specific binding of the radioactive ligand to polygonal astrocytes reached equilibrium within 60 min and was half-maximal by 5-10 min. By contrast, specific binding to process-bearing astrocytes barely exceeded background levels. IL-1 and TNF increased PTBBS within polygonal astrocytes in both dose- and time-dependent manners. At 10-50 ng/ml, IL-1 beta and TNF-alpha elevated [3H]Ro 5-4864 binding in polygonal astrocyte cultures 65 and 87%, respectively, above the level in control cultures. However, no changes in PTBBS were seen within polygonal astrocytes after IL-2 treatment. Scatchard analysis of saturation binding experiments suggested that the increase in PTBBS promoted by TNF was due to an increased number of binding sites present in polygonal astrocytes and not due to an increase in receptor affinity. Binding data suggested that PTBBS within cultures of process-bearing astrocytes were virtually absent irrespective of the treatment. These in vitro data suggest that certain cytokines found in the injured brain may be involved in up-regulating PTBBS within a particular subtype of astrocyte.

Endothelin receptors on cultured fetal rat diencephalic glia

The recently described family of proteins, the endothelins, are produced in neurons and bind to extravascular sites in the CNS. To characterize these receptors, we carried out studies on cultures of fetal rat diencephalic glia. Scatchard analysis of saturation binding studies was done for astrocytes (greater than 95% glial fibrillary acidic protein positive). For endothelin 3 (ET-3) and ET-1, respectively, a single receptor class of KD 0.41 +/- 0.05 and 0.62 +/- 0.04 nM and a receptor density of 42 +/- 0.8 and 58 +/- 1.1 fmol/mg of glial protein was found. Bound and cross-linked 125I-ET-3 or ET-1 showed a single predominant receptor band at Mr 52,000 on sodium dodecyl sulfate-polyacrylamide gel electrophoresis; a minor band at 50,000 was also seen. At concentrations equal to the receptor KD, the major brain form of ET, ET-3, stimulated a nearly 200% increase in the incorporation of tritiated thymidine into glia. ET-3 and ET-1 significantly impaired the ability of atrial natriuretic peptide (ANP) to generate cyclic GMP, and isoproterenol to generate cyclic AMP. The ability of ET to inhibit ANP-induced cyclic GMP generation was reversed by cycloheximide and actinomycin-D, whereas the inhibition of isoproterenol-induced cyclic AMP generation was partially and significantly blocked by inhibitors of calcium influx, protein kinase C action, or G protein activation, as well. Astrocytes from this part of the brain are a potential target cell for endothelin, assuming these findings are present in vivo. This neuropeptide may serve as a growth stimulator for astrocytes and modulator of the actions of catecholamines or ANP on glia by inhibiting second messenger generation.

Phosphorylation of MARCKS (80-kDa) protein, a major substrate for protein kinase C in oligodendroglial progenitors

We have recently reported a potent mitogenic stimulation of oligodendroglial (OL) progenitors by the protein kinase C (PKC) activating phorbol ester, i.e., phorbol 12-myristate 13-acetate (PMA) (Bhat NR, J Neurosci Res 22:20-27, 1989). The present study deals with PMA-induced protein phosphorylation reactions in cultured OL progenitors. The phorbol ester induced the phosphorylation of several cytosol and membrane-associated proteins, including a major protein with an apparent molecular weight of 80 kDa. In both control and PMA-treated cultures, phosphorylation level of the 80-kDa protein in cytosol was higher than that in the particulate fraction. Okadaic acid, an inhibitor of protein phosphatases, also increased the phosphorylation of several proteins and substantially enhanced protein phosphorylation induced by PMA. In vitro incubation of the cell membranes with phosphatidylserine and diacylglycerol (a physiological activator of PKC) in the presence of [gamma 32p]-ATP resulted in an increased phosphorylation of the 80-kDa protein. The induction of phosphorylation of the 80-kDa protein under both in situ and in vitro conditions was subject to inhibition by 1-[5[isoquinolinyl sulfonyl)-3-methylpiperazine (H-7), a potent inhibitor of PKC. The 80-kDa phosphoprotein was identified as the prominent PKC substrate, i.e., myristoylated alanine-rich C-kinase substrate (MARCKS) protein by immunoprecipitation with anti-MARCKS antibodies.

Distribution of protein kinase C isozymes in rat optic nerves

Light (LM) and electron (EM) microscopic immunocytochemical methods were used to study the distribution of protein kinase C (PKC) isozymes in adult rat optic nerves. In cryostat and vibratome sections examined by LM, type II (beta) isozyme was localized almost exclusively in the axons. In the EM, immunoreaction products were found to associate with microtubules and neurofilaments. The inner surface of axonal membranes were occasionally stained. Analysis of PKC isozyme composition of the optic nerves by using immunoblot techniques revealed that type II (beta) isozyme accounted for approximately 80% of the total immunoreactivity. By contrast, type III (alpha) isozyme, which accounted for the remaining 20% of PKC, was found mainly in the astrocytes. Astrocytic processes next to blood vessels and between myelinated axons were stained. In the EM, immunoreaction products were found in the cytoplasm and along astroglial filaments. Segments of plasma membranes also were stained; but nuclei were unstained. Adult glial cells were not stained by an antibody to type II (beta) isozyme except for the occurrence of a few punctate cytoplasmic densities in occasional astrocytes. Very faint or no immunostaining was observed in sections treated with a monoclonal antibody to type I (gamma) isozyme. Immunoblot analyses also did not reveal this subspecies. The absence of type I (gamma) isozyme in optic nerves is not due to a down-regulation of the enzyme during development. In developing (5 and 11 day) rats, immunoreactivity of protein kinase C was very faint or absent. After 15 days, reaction products of both type III (alpha) and type II (beta) isozymes were found throughout the nerve. These findings suggest that type II (beta) isozyme may be involved in axonal transport whereas type III (alpha) isozyme may play a role in some astrocyte functions in mature optic nerves.

Altered protein kinase C activity in different subfields of hippocampus following cerebral ischemia

Protein kinase C (PKC) activity was determined in different (membrane, nuclear and soluble) subcellular fractions prepared separately from the CA1 and CA3 subfields of Mongolian gerbils hippocampus at various time intervals following a single 5-min occlusion of the common carotid arteries. Soluble and nuclear PKC activities of the CA1 sector were found to be elevated at 24 hours following the ischemic injury, while PKC activities did not increase in the CA3 region until the 3rd day after ischemia. The ratio of soluble/membrane-associated PKC activities followed a similar pattern, predominantly because the activation/elevation and then down regulation of the cytosolic enzyme pool changing correspondingly to the ongoing pathological processes. PKC activity returned to the normal level in each subfraction of the CA3 subfield by the 7th day. However, PKC activity remained elevated in the soluble fraction of the CA1 sector even after the delayed death of pyramidal neurons, presumably because of the reactive response of astrocytes. Conceivably, the transient activation and rapid down regulation of PKC in the CA1 sector may contribute to the initiation of postischemic neuronal death in the CA1 subfield.