Glial fibrillary acidic protein-solubility characteristics, relation to cell growth phases and cellular localization in rat C-6 glioma cells: an immunoradiometric and immunohistologic study

Class III beta-tubulin isotype (beta III) in the adrenal medulla: III. Differential expression of neuronal and glial antigens identifies two distinct populations of neuronal and glial-like (sustentacular) cells in the PC12 rat pheochromocytoma cell line maintained in a Gelfoam matrix system

BACKGROUND

The rat PC12 pheochromocytoma cell line provides an established system for the study of neuronal differentiation. To our knowledge, glial differentiation has not been reported in this cell line.

METHODS

We have studied, by immunohistochemistry and immunoblotting, the presence of neuronal cytoskeletal antigens [class III beta-tubulin isotype (beta III), microtubule associated proteins MAP2, MAP1B and tau, and different neurofilament (NF) protein components], and synaptophysin in comparison with the glial fibrillary acidic protein (GFAP) and S-100 protein in the PC12 cell line. In three different experiments, PC12 cells were maintained in a three-dimensional gelatin foam (Gelfoam) matrix system for up to 34 days with and without treatment with 1 mM dibutyryl cyclic (dc)AMP. Immunohistochemistry was performed on explants ranging from 2 to 32 days-in vitro, which were fixed in either Bouin's solution, 70% ethanol, or 10% neutral-buffered formalin and embedded in paraffin. Immunoblotting was performed on Gelfoam explants with a panel of antibodies against all aforementioned neuronal and glial markers. Additional immunoblot experiments using anti-GFAP and anti-beta III monoclonal antibodies in cell suspensions and homogenates from PC12 monolayer cultures were carried out to compare growth conditions in relation to the expression of these proteins.

RESULTS

Beta III and MAP2 were demonstrated by immunohistochemistry and immunoblotting of PC12 explants maintained for up to 32 days in Gelfoam matrices with and without treatment with dcAMP. Intense filamentous and granular beta III staining of PC12 cells was observed in dcAMP-treated cultures concomitant with neuronal morphologic alterations (neuritogenesis and ganglionic phenotype). In untreated cultures, beta III staining was present in less differentiated cells, as well in cells undergoing neuritic development. The neuronal phenotype of PC12 cells was confirmed by staining for MAP2, tau, and NF proteins, as well as for synaptophysin. The presence of beta III, MAP2, MAP1B, tau, and NF proteins was confirmed by immunoblotting. Clusters of GFAP-positive and S-100 protein-positive spindle cells, phenotypically distinct from the chromaffin-like or neuronal cells, were demonstrated in Gelfoam explants at 5-30 days in vitro. In 30-day-old cultures treated with dcAMP, there was strong filamentous GFAP and diffuse S-100 protein staining in an increased number of sustentacular-like PC12 cells. GFAP staining was corroborated by immunoblotting of explants maintained under identical conditions in vitro. In contrast, immunoblots performed on homogenates from PC12 suspension and monolayer cultures were GFAP-negative.

CONCLUSIONS

Neuronal and glial-like, presumed sustentacular, phenotypes were demonstrated in PC12 cells grown in Gelfoam matrices with and without treatment with dcAMP for up to 34 days. To our knowledge, the occurrence of glial differentiation in the PC12 line is a hitherto unreported finding. Adult rat medullary sustentacular cells are known to express S-100 and GFA proteins (Suzuki and Kachi, Kaibogaku Zasshi-Anat 70(2): 130-139, 1995), and the organ culture system employed in our study may well have favored this direction of differentiation.

Expression, induction and regulation of the cytochrome P450 monooxygenase system in the rat glioma C6 cell line

The cytochrome P450 monooxygenase system consists of NADPH-cytochrome P450 reductase (P450 reductase) and cytochromes P450, which can catalyze the oxidation of a wide variety of endogenous and exogenous compounds. P450 reductase transfers reducing equivalents from NADPH to P450, which in turn catalyzes metabolic reactions. In previous studies, we have used the rat glioma C6 cell line as an in vitro model system and identified the presence of P450 reductase and of cytochrome P450 1A1, 1A2, 2A1, 2B1/2, 2C7, 2D1-5 and 2E1 by reverse transcription followed by polymerase chain reaction (RT-PCR). In C6 cells, the induction of P450 1A1 and 2B1/2 at mRNA level after BA (benzo(a)anthracene) or PB (phenobarbital) treatments was detected. In this study, analysis of microsomal preparations of glioma C6 cells was utilized to demonstrate the presence of P450 2B and P450 reductase at the protein level. ELISAs showed that PB induced P450 2B proteins 12-fold. These experiments further establish that the rat glioma C6 cell line contains an active cytochrome P450 monooxygenase system that can be induced by P450 inducers. We also found that the mRNAs of P450 1A1 and 2B1/2 from glioma C6 cells do not bind to the oligo(dT)-based separation techniques efficiently, suggesting that they may have very short poly(A) tails. The half-lives of P450 1A1 and 2B1/2 mRNA in glioma C6 cells are 1/10 and 1/3 of that in liver, respectively. This may partly contribute to the low expression level of P450s in glial cells. The induction of P450s by BA or PB did not change their mRNA half-lives, indicating the induction may be due to transcriptional regulation. In summary of this study, we believe the presence of the cytochrome P450 monooxygenase system in glial cells of the brain may be important in chemotherapy and carcinogenesis of brain tumors.

Occurrence of the opiate alkaloid-selective mu3 receptor in mammalian microglia, astrocytes and Kupffer cells

Evidence is presented for occurrence of opiate alkaloid-selective, opioid-peptide-insensitive receptor binding sites, labeled with [3H]morphine, in primary cultures of cat microglia and cat astrocytes, as well as on highly purified preparations of rat Kupffer cells. These receptors have been designated mu3 on the basis of their close similarity to receptors first found to be present on human peripheral blood monocytes. Exposure of the microglia to morphine and etorphine caused marked quantifiable changes in cellular morphology, including assumption of a more rounded shape and retraction of cytoplasmic processes; in contrast, several opioid peptides were without effect on morphology. The effects of morphine on microglial morphology were blocked by the opiate antagonist naloxone. These effects of drugs on morphology were as predicted for action via the mu3 receptor. Opiate alkaloid binding sites previously detected on the rat C6 glioma cell line were also characterized here as of the mu3 receptor subtype. It is proposed that mu3 receptors have broad distribution in different macrophage cell types of bone marrow lineage, including microglia and Kupffer cells. Furthermore, these receptors are not restricted to cells of bone marrow lineage, since they are also present on astrocytes.

Effects of peroxisomal beta-oxidation antagonist on 2',3'-cyclic-nucleotide 3'-phosphohydrolase, membrane lipid compositions, and membrane fluidity in C-6 glial cells

In order to understand the relationship between peroxisomal dysfunction and clinical manifestations of peroxisomal disorders, the effect of thioridazine, a peroxisomal beta-oxidation antagonist, on the differentiation, membrane lipid composition and membrane fluidity of C-6 glial cells was examined. In our study, induction of 2',3'-cyclic-nucleotide 3'-phosphohydrolase (CNP), which was considered to be a membrane-associated enzyme closely associated with myelination, was inhibited with supplementation of thioridazine, followed by an increase in the relative concentration of longer chain fatty acids in cell membrane lipids, indicating that thioridazine causes impaired differentiation in the glial stem cell system. Membrane fluidity of C-6 glial cells was examined using a fluorescent probe 1,6-diphenyl-1,3,5-hexatriene (DPH). The DPH anisotropy value was decreased in the glial cells treated with thioridazine. These results indicate that the alteration of the membrane lipid composition caused by thioridazine affects the differentiation of glial cells via the changes in membrane properties.

Neuron-associated class III beta-tubulin, tau, and MAP2 in the D-283 Med cell line and in primary explants of human medulloblastoma

The D283 Med human medulloblastoma cell line and primary explants of five surgically excised medulloblastomas were cultured using a three-dimensional Gelfoam matrix system. The cultures were evaluated immunohistochemically for a series of antigenic determinants associated with neuronal or glial differentiation. Focal immunolocalization of class III beta-tubulin, microtubule-associated protein 2 (MAP2), and to a lesser degree tau, was demonstrated in all cultures. Class III beta-tubulin isotype, MAP2, and tau protein were also detected by immunoblot in Gelfoam matrix cultures, monolayer cultures, and suspension cultures of D283 Med cells. Staining for neurofilament protein epitopes was highly variable, even among different cultures derived from the same original tumour, but time-dependent changes in neurofilament protein, which may have reflected neuronal differentiation, were not consistently shown. Widespread gamma-enolase and focal synaptophysin reactivities were visualized in all cultures, but no S-antigen staining was detected. Leu 7 labelling was variably present in half of the cultures of D283 Med cells, but was more abundant in explants derived from four of the five original tumours. Vimentin was consistently found in D283 Med cultures at all time points. No immunoreactivity for glial fibrillary acidic protein was detected in the D283 Med cell line. Conversely, staining for this protein was demonstrated in scattered astrocytic cells in the surgical specimens of all five medulloblastomas. Concomitant with increased time in culture, three of the primary tumours displayed increased numbers of glial fibrillary acidic protein-positive cells when cultured in the Gelfoam system, but the other two tumours had a minimal astrocytic component.(ABSTRACT TRUNCATED AT 250 WORDS)

Reverse transformation and genome exposure in the C6 glial tumor cell line

Reexpression of growth control and differentiation in response to physiological inducers can be demonstrated in some malignant cell lines, showing that they are not irreversibly transformed. This switch in phenotype is likely to reflect a changing pattern of gene expression, but it has not been known whether such cellular transitions involve major or only minor modulation of chromatin structure. We have studied growth control and accessibility of chromatin to DNase I in C6 glioma cells subjected to different growth regimens using an in situ nick translation assay to label the most exposed regions of nuclear chromatin. In fibroblasts and primary glia, exposed chromatin was localized mainly at the nuclear lamina. This readily labeled DNA structure was largely lacking in the malignant C6 glioma. When C6 cells were treated with dibutyryl cyclic AMP, exposed chromatin was reestablished around the nuclear periphery. This restoration of a normal genome exposure pattern required cytoskeletal integrity. Thus large-scale nuclear reorganization events proceed in parallel with phenotypic normalization. The changes in cell morphology, growth control, cytoskeletal organization, and chromatin exposure and localization are similar to the reverse transformation reaction in CHO-K1 cells, which is also regulated by the cyclic nucleotide system. Hydrocortisone and dexamethasone also restored genome exposure in C6 but less markedly than cAMP derivatives. Diverse transformed cells can thus respond to growth control stimuli with similar nuclear restructuring events, which presumably underlie changes in gene expression. Reverse transformation and redifferentiation appear to be alternative terms describing essentially the same biological phenomenon.

Morphological appearance, growth kinetics and glial fibrillary acidic protein (GFAP) expression in primary in vitro explant culture of astrocytic neoplasms

Astrocytomas of different grades of malignancy were cultured as primary explant and their sequential growth pattern, glial fibrillary acidic protein (GFAP) expression and labelling index (LI) using bromodeoxyuridine (BrdU) were assessed and correlated with the grade of malignancy of the original tumor tissue. Low-grade astrocytomas showed patterns of growth that diverged from anaplastic astrocytomas and glioblastoma multiforme. The GFAP expression decreased with increasing time in culture in all astrocytomas irrespective of the grading. Maximum GFAP was, however, expressed in the morphologically well-differentiated stellate cells. Contrary to expectations, lower BrdU LI was observed in glioblastoma multiforme in comparison to low-grade astrocytomas, which suggests some unidentified mechanism of differentiation in vitro for astrocytomas of higher grade of malignancy. Hence, in contrast to reported literature on the prognostic value of studies on primary cultures, the present study cautions the extrapolation of the in vitro findings for astrocytomas.

Subclones of C6 rat glioma cells differing in intermediate filament protein expression

The C6 rat glioma cell line is shown to consist of a mixed population of cells which either contain vimentin (80% of the cells) or completely lack any cytoplasmic intermediate filament (IF) proteins. Subclones could be established with both phenotypes, indicating that these IF protein expression patterns represent stable phenotypic markers. Absence of IF proteins in C6 subclones could consistently be correlated with an altered cell morphology and a pronounced increase in the number of actin stress fibers. In vitro translation and hybridization assays suggest the absence of vimentin to result from a block at the transcriptional level. The data indicate that subcloning of the C6 cell line on the basis of IF protein expression seems to be a reasonable approach for obtaining homogeneous C6 cell populations which may represent suitable experimental models for studies on vimentin expression and glioma cell differentiation.

Differential effects of tumor necrosis factor on the growth and differentiation of neuroblastoma and glioma cells

We have studied the effect of tumor necrosis factor (TNF-alpha) on transformed neural and glial-derived cell lines. TNF-alpha at physiological doses was able to arrest the growth and inhibit DNA synthesis of N103 neuroblastoma cells. This phenomenon was accompanied by a morphological cell differentiation characterized by the outgrowth of neurites. By contrast, TNF-alpha induced an increase in the growth rate of C6 glioma cells and upon cytokine addition a higher number of C6 cells were found in the S + G2 phase of the cell cycle. C6 cells did not show morphological changes under this treatment. Analogous results were obtained with IFN-gamma. These neurotrophic and mitogenic effects of TNF-alpha suggest a putative role of this cytokine in the regeneration of brain tissue upon brain injury.

Characterization of adult human astrocytes derived from explant culture

Four different human astrocytic cell lines established from either epilepsy surgical specimens or cerebral white matter obtained during thalamotomy for tremor in a patient with multiple sclerosis were characterized using morphologic analysis, ultrastructural attributes, growth characteristics, and immunocytochemical analysis. Immunocytochemical characterization of cultures indicated a mean of 84% of cells contained cytoplasmic glial fibrillary acidic protein (GFAP): to confirm that GFAP(+) cells also proliferated, bromo-deoxyuridine (BrdU) uptake was measured in cell line. Our method of simplified explant culture allows establishment of astrocytic cell lines from a variety of pathologic substrates using limited amounts of human material.

Kinetics and glial fibrillary acidic (GFA) protein production in a transplantable human giant cell glioblastoma (D-212 MG) of near haploid karyotype maintained in an organ culture system. An immunohistochemistry study

A transplantable subcutaneous tumour (designated D-212 MG), sequentially passaged in athymic nude mice and originally derived from a human giant cell glioblastoma, was maintained in an organ culture (matrix) system and studied immunohistochemically after in vitro pulse-labelling with bromodeoxyuridine (BrdU) and for the presence of glial fibrillary acidic (GFA) protein, after 1, 2 and 3 weeks in culture. The histological characteristics of the tumour, showing two cell populations of giant multinucleated cells and small cells, were preserved in the explants. An increased percentage of multinucleated giant cells was found after 3 weeks in vitro. A small but constant fraction (4-6%) of these cells continued to synthesize DNA. The labelling index of the small cells was somewhat higher, but decreased slightly although significantly over the 3-week period in vitro (from approximately 10.5 to 8%). The percentage of small cells that were positive for GFA protein was in the region of 75% and that of the giant multinucleated cells was in the region of 45%; it did not change significantly during the 3 weeks in vitro. The in vitro results confirm the astrocytic nature of both the small cells and the giant multinucleated cells in this tumour, the capacity of both cell populations to synthesize DNA in culture and to demonstrate invasiveness, and suggest the possibility that some of the giant multinucleated cells may have originated from the conversion of a number of small tumour cells.

Expression of glial fibrillary acidic protein in rat C6 glioma relates to vimentin and is independent of cell-cell contact

Glial fibrillary acidic protein (GFAP) was induced in rat C6 glioma cells grown in M199 and HAM F10 media by addition of 1 mM dibutyryl cyclic AMP. The amount of GFAP per cell increased 7- and 33-fold in M199 and HAM F10 media, respectively. GFAP could be induced in each phase of the cell culture except for the lag phase, where GFAP synthesis was delayed until the onset of the logarithmic growth. The induction took place under conditions where the total protein content of the cell decreased. Measurement of the amount of vimentin indicated that GFAP was induced under conditions of low vimentin concentration. Our results do not support the hypothesis that GFAP induction depends on cell-cell contact or cell proliferation. They indicate a shift from vimentin to GFAP synthesis by an as yet unknown mechanism.

Relation of cellular phospholipid composition to oligodendroglial differentiation in C-6 glial cells

The relation of the polar head group composition of cellular phospholipids to a biochemical expression of oligodendroglial differentiation was studied in cultured C-6 glial cells. Induction of the oligodendroglial enzyme, 2',3'-cyclic nucleotide 3'-phosphohydrolase (CNP), was determined after alteration of the polar head group composition of phospholipids by exposure of the cells to choline analogues, especially N,N'-dimethylethanolamine. To accomplish the phospholipid alteration, cells were grown in the presence of the analogue in medium free of exogenous lipid, i.e., first for 24 h in 10% delipidated serum and then for 48 h in serum-free medium. The 48-h exposure to serum-free medium resulted in untreated C-6 cells in a several fold increase in CNP activity, but in cells treated with 2.5 mM N,N'-dimethylethanolamine, total inhibition of this induction was observed. A graded, concentration-dependent inhibitory effect of the analogue on the induction of CNP was defined. The effect of the analogue was relatively specific, e.g., the activity of another plasma membrane enzyme of C-6 cells, (Na+ + K+)-activated ATPase, was not affected. Morever, there was no evidence of a toxic effect of the analogue; thus, total protein synthesis and cell growth were not altered, and the induction of CNP in serum-free medium recurred after removal of the analogue. N,N'-Dimethylethanolamine was shown to be incorporated into cellular phospholipids, primarily at the expense of phosphatidylcholine. The data define an important role for the polar head group composition of membrane phospholipids in oligodendroglial differentiation in this model system.

Cytoskeletal properties and endogenous degradation of glial fibrillary acidic protein and vimentin in cultured human glioma cells

The cytoskeletal properties and endogenous degradation of intermediate filaments in cultured human glioma cells (U-251MG) were studied using monoclonal antibodies in immunohistochemical and immunochemical methods. Both glial fibrillary acidic protein (GFAP)- and vimentin-antibodies gave a fibrillar cytoplasmic staining of the cells, and double immunofluorescence experiments showed the presence of both types of intermediate filaments in the same cells. GFAP and vimentin could also be located to typical coiling perinuclear bundles after vinblastine treatment of the cultures. In the detergent-resistant, adherent cytoskeletons of the glioma cells, both GFAP and vimentin persisted as fibrillar cytoplasmic arrays. Scanning and transmission electron microscopy showed that only intermediate filaments were left in the cytoplasmic domain. Electrophoretic analysis, combined with the immunoblotting method, revealed that the two major detergent-resistant cytoskeletal polypeptides of the cells, with molecular weights of 51 kD and 58 kD, were GFAP and vimentin, respectively. On the other hand, neither GFAP nor vimentin were detected in the detergent extracts of the glioma cells. Detergent-extraction in low ionic strength medium as well as inclusion of Ca2+ into the extraction medium resulted into a rapid degradation of both GFAP and vimentin. These degradation conditions produced different, partially soluble, lower MW immunoreactive polypeptides as detected by the immunoblotting technique. Interestingly, the degradation also produced soluble intact GFAP and vimentin. These results indicate that GFAP and vimentin have closely similar physicochemical properties in the cytoskeletons of human glioma cells including a nearly quantitative localization in filaments, rearrangement upon microtubule disruption, and resistance to extractions by detergents. Proteolytic degradation of both proteins can be induced by a protease activated by both low ionic strength and Ca2+.

Relation of cholesterol to astrocytic differentiation in C-6 glial cells

The relation of cellular cholesterol content to a biochemical expression of astrocytic differentiation was investigated in cultured C-6 glial cells. The astrocytic marker, glutamine synthetase, was studied. Cellular sterol content was perturbed with compactin, a specific inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A reductase and, thereby, cholesterol biosynthesis. Depletion of cellular sterol resulted in 72 h in a more than twofold increase in glutamine synthetase activity. Production of various degrees of sterol depletion with different concentrations of compactin demonstrated a striking inverse relationship between glutamine synthetase activity and the cellular sterol/phospholipid molar ratio. That the effect of compactin, in fact, is mediated by depletion of sterol was shown further by prevention of the compactin-induced increase in synthetase activity by simultaneous addition of exogenous cholesterol. Moreover, addition of cholesterol alone to the culture medium led to both a decrease in glutamine synthetase activity and an increase in the sterol/phospholipid molar ratio. The possibility that the compactin-induced increase in glutamine synthetase activity is caused by an increase in synthesis of the enzyme was suggested by prevention of the increase by cycloheximide. The data suggest that astrocytic differentiation is stimulated by a decrease in cellular sterol content. When considered with our previous observation that oligodendroglial differentiation is inhibited by such a decrease, the findings suggest that cellular sterol content is a critical determinant of the direction of glial differentiation, i.e., whether along astrocytic or oligodendroglial lines.

Cytoskeletal structures and oligodendroglial differentiation in C-6 glial cells

The relationship of the cytoskeleton to a biochemical expression of oligodendroglial differentiation was studied in cultured C-6 glial cells. Specifically, we investigated the effect of the cytoskeletal perturbants, colchicine and cytochalasin D, on the induction of the oligodendroglial marker enzyme, 2',3'-cyclic nucleotide 3'-phosphohydrolase (CNP), caused by removal of serum from the culture medium. Each drug inhibited CNP induction in a concentration-dependent manner, and essentially complete inhibition of induction was observed with 0.25 microM colchicine or 2.0 microM cytochalasin D. Detailed study of the effect of colchicine was carried out. This antimicrotubular agent not only totally prevented induction if added at the onset of serum removal, but also prevented further induction when added at various times after serum removal. That the effect of colchicine related to the drug's effect on microtubules was supported by the demonstration that lumicolchicine, a colchicine isomer which has no effect on microtubules, had no effect on the CNP induction. Moreover, colchicine, but not lumicolchicine, prevented the morphological signs of differentiation provoked by serum removal. The effect of colchicine was reversible and relatively specific. Thus, no concomitant effect of colchicine on the activity of another plasma membrane enzyme of C-6 cells, i.e., (Na+ + K+)-activated ATPase, or on the rate of incorporation of [3H]leucine into total protein of intact cells could be discerned. The possibility that the site of the effect of colchicine is on intracellular events was suggested by the observation that the drug inhibited the induction of CNP by dibutyryl cyclic AMP. The data suggest that the cytoskeleton is involved in oligodendroglial differentiation.

Divergent glial and neuronal differentiation in a cerebellar medulloblastoma in an organ culture system: in vitro occurrence of synaptic ribbons

A cerebellar medulloblastoma from a 2-year-old boy was maintained in vitro in an organ culture system for 6.5 months, and the explants studied by light and electron microscopy at different time intervals. The tumor cells progressively demonstrated divergent differentiation into astrocytes and neuroblasts. Astrocytic differentiation, confirmed by immunohistochemistry for GFA protein, became maximal after about 7 weeks in vitro and was thereafter maintained in different areas of the explants. Concomitantly, neuroblastic differentiation was expressed in other cells, with the progressive development of cell processes filled with many microtubules, of neuroblastic rosettes, of increased numbers of dense-core and clear-centered vesicles, of occasional 9 + 0 cilia, and of synaptic ribbons appearing in vitro. Neuroblastic differentiation was most pronounced in 4- and 6-month-old explants, but synapses were not found. The differentiating features reported are in contrast to those of the original tumor, which was largely undifferentiated. The alternative interpretation of a divergent glial and pineocytic differentiation is also considered. These findings support the concept of the differentiating bipotential of the cerebellar medulloblastoma.

Cell lines (VMDk) derived from a spontaneous murine astrocytoma. Morphological and immunocytochemical characterization

Three cell lines (VMDk) derived from a spontaneous, murine astrocytoma, which produce tumours when injected either subcutaneously or intracranially into syngeneic mice, have been examined in vitro. Ultrastructurally, the cells show astrocytic features but each line differs in its degree of differentiation. Treatment with both dexamethasone and dibutyryl adenosine 3',5'-cyclic monophosphate (dbcAMP) increases intracytoplasmic differentiation and causes surface structural changes. The addition of dbcAMP also induces a statistically significant increase in the length and number of cell processes. All three cell lines express the astrocyte-specific markers, glial fibrillary acidic protein (GFAP) and glutamine synthetase (GS), by indirect immunofluorescence, with two lines showing an increased propensity to stain for GFAP following dbcAMP treatment. The cell surface antigen fibronectin is also detected in all lines. Thus these VMDK cell lines exhibit both the morphological and antigenic characteristics of astrocytes and respond to dexamethasone and dbcAMP and may be used to provide a suitable in vivo-in vitro model system for the study of astrocytoma.

Vimentin: a phosphoprotein under hormonal regulation

Past studies of norepinephrine-stimulated protein phosphorylation in intact C-6 glioma cells had identified a 58,000 molecular weight, 5.7 isoelectric point protein (58K-5.7) as a cyclic AMP-dependent phosphoprotein and had shown that 58K-5.7 was one of the most abundant proteins of the nuclear fraction. Initial experiments of present studies showed that the 58K-5.7 protein remained with the nuclear ghost, or matrix structure, after removal of chromatin. Based on the size, acidity, abundance, nonsolubilization by nonionic detergent and salt, and solubilization by urea, the hypothesis was advanced that the 58K-5.7 protein was the vimentin-type intermediate filament protein. The hypothesis was tested by two types of immunochemical experiments. Antisera against hamster vimentin reacted selectively with only the 58K-5.7 protein in polyacrylamide gels of urea-solubilized cellular residues (i.e., nonionic detergent and 0.6 M salt-insoluble material) as determined by immunoautoradiography. Antisera against the pure 58K-5.7 protein of C-6 cells bound selectively to a fibrous array of cellular material typical of vimentin filaments as determined by indirect immunofluorescence. It is concluded that the 58K-5.7 protein is vimentin.

Rat C-6 glioma cells maintained in an organ culture system: a study of kinetic parameters

Rat C-6 glioma cells were grown on a sponge foam matrix in an organ culture system and the cell cycle parameters, including the growth fraction (GF), were assessed after autoradiography. The zones of growth consisted of a compact upper layer (UL) at the gaseous interface, a central necrotic layer and a deeper lower layer (LL) which invaded the matrix. The fraction of continuously labeled mitoses (FCLM) was similar in both the UL and LL cells. The derivatives of the FCLM curves obtained in three experiments gave an average modal TG2 of 5 hr. A mathematical model relating GF, TG2, TC and labeling index as a function of time, LI(t), was devised for cells in a steady state exposed continuously to tritiated thymidine and was applied to data obtained from UL cells. A mean GF of 9% (range: 8-10%) and a mean cell cycle time (TC') of 27 hr (range: 13-47 hr) were obtained. The mean TS was calculated to be 11 hr (range: 8-16 hr) by the method of grain counts per mitotic figure or grain index (GI). Knowledge of TS permitted alternative calculation of the cell cycle time from the equation Ts/TC = LI(0)/GF:this gave a mean cell cycle time (TC") of 29 hr (range: 20-45hr). Except for the GF, the cell kinetics were comparable to those of the same cell line grown in monolayer culture. The GF in the in vitro system described is in the lower range reported in some human malignant gliomas in vivo.

Continuous culture of rat C6 glioma in serum-free medium

In this communication we describe serum-free culture conditions for the serial propagation of the C6 glioma cell line. The growth rate, saturation density, and morphology of these cells are equivalent to those of their serum-grown counterparts when cultured in a 3:1 mixture of Dulbecco's modified Eagle's medium and Ham's medium F-12 supplemented with trace elements, insulin, transferrin, fibroblast growth factor, linoleic acid complexed to fatty acid-free bovine serum albumin, and a serum-spreading factor (SSF) partially purified from human plasma. The requirement for SSF in the medium can be satisfied by preincubating the tissue culture dishes with SSF. Tissue culture dishes sequentially pretreated with poly-D-lysine and purified cold insouluble globulin will also substitute for this requirement. The fatty acid-free bovine serum albumin/linoleic acid complex increases the growth rate of these cells but has no appreciable effect on their morphology, saturation density, or ability to grow with repeated subculture. The growth stimulation caused by this complex appears to be dependent on the fatty acid, as the fatty acid-free bovine serum albumin alone has no effect on the growth rate. Linoleic acid is cytotoxic in the absence of bovine serum albumin, and the fatty acid-free bovine serum albumin prevents this toxicity. Other fatty acids including oleic, arachidonic, and palmitic only partially substitute for the growth-promoting effect of linoleic acid.

Glial fibrillary acidic protein in monolayer cultures of C-6 glioma cells: effect of aging and dibutyryl cyclic AMP

Aging induced a dramatic increase in the number of glial fibrillary acidic protein (GFA) positive cells in monolayer cultures of C-6 cells. Dibutyryl cyclic AMP (DBcAMP) had a similar effect on subconfluent cultures. Both aging and DBcAMP treatment also helped C-6 cells to develop mature astroglia like morphology.

Induction of an oligodendroglial enzyme in C-6 glioma cells maintained at high density or in serum-free medium

The relationship between cell density and the activity of 2':3'-cyclic nucleotide 3'-phosphohydrolase (CNP), an enzyme believed to be specific to oligodendroglial cells and myelin in the brain, has been studied in cultured C-6 glioma cells. Over a 12-day period, the specific activity of CNP underwent a 4-fold increase in conjunction with an increase in the cell density (total protein/flask) and a decline in the growth rate of the cultures. In contrast, the specific activity of Na+,K+-ATPase was not influenced by cell density. Experiments with cultures seeded at different initial densities indicated that the increase in CNP activity coincided with the attainment of a specific cell density rather than with the length of time that the cells were maintained in culture. Arrest of cell proliferation in non-confluent C-6 cells by means of thymidine blockade was not sufficient to cause an increase in the activity of CNP; however, removal of serum from the culture medium resulted in a 3-fold induction of the enzyme in the absence of a high degree of cell contact. The induction of CNP in cells maintained in serum-free medium paralleled the development of a series of distinct morphological changes reminiscent of glial differentiation, which occurred within 48 hours after removal of the serum. Inhibition of protein synthesis by cycloheximide prevented the induction of CNP in serum-free cultures. The demonstration that an enhancement of an oligodendroglial characteristic in C-6 glioma cells can be obtained by growing the cells to high density or by removing serum from the medium, provides further support for the suggestion that these cells may be analogous to the glial stem cells present in the developing brain.

GFA expression in aggregation cultures of rat C6 glioma

Few C6 glioma cells synthesize the astroglia-specific GFA protein in monolayer culture. A uniform population of GFA-positive cells was obtained by aggregation C6 cells in suspension culture, as previously reported for C6 glioma maintained on sponge foam matrices. These results strongly suggest that cell-to-cell interactions promote GFA expression.