Reversible inhibition of the hydrocortisone induction of glycerol phosphate dehydrogenase by cytochalasin B in rat glial C6 cells

Oligodendrocytes as glucocorticoids target cells: functional analysis of the glycerol phosphate dehydrogenase gene

Previous research has established that the development and function of oligodendrocytes are influenced by glucocorticoids. The enzyme glycerol phosphate dehydrogenase (E.C.1.1.1.8) has been used as a model to study glucocorticoid regulation of gene expression in oligodendrocytes and the C6 glial cell line. In the rat brain this enzyme is exclusively localized to oligodendrocytes. The sequence of the 5' flanking region for the rat gene encoding Glycerol Phosphate Dehydrogenase (GPDH; EC 1.1.1.8) was determined. 4 kb of sequence from the 5' flanking region, exon 1, and part of intron 1 of the rat GPDH gene was compared to the corresponding mouse sequence. Dotplot matrix comparison revealed that the rat sequence is more than 80% similar to the mouse sequence, but differs from the mouse sequence in two regions: the rat sequence is devoid of 200 bp of B1 repeat sequence that is present in the mouse, and the rat sequence has an excess 700 bp of B2 repeat sequence inserted between -0.7 kb and -1. 4 kb that is absent in the mouse. To determine the regulatory activity of the rat GPDH 5' flanking region, various portions of the rat GPDH 5' flanking region were placed in luciferase reporter constructs and tested for transcriptional activity. Transient transfection of reporter constructs into the C6 glial cell line revealed that the distal end of the 5' flanking region was glucocorticoid-inducible. A 385 bp Glucocorticoid Response Unit (GRU) was identified whose glucocorticoid induction was enhanced by dibutyryl-cAMP and reduced by phorbol esters. Sequence analysis of the GRU revealed the presence of four consensus GRE sequences and other putative consensus elements. Results here suggest that the 5' flanking region of the GPDH gene mediates the ligand-inducible regulation of GPDH, and that multiple signaling pathways converge at the 5' regulatory sequence to modulate GPDH gene expression in oligodendrocytes.

The acute effect of lead acetate on glucocorticoid receptor binding in C6 glioma cells

Lead exerts significant toxic effects on the nervous system, the hematopoietic system and the kidney. Specific cellular sites of action of this environmental pollutant have not been elucidated in the central nervous system. The present investigations were conducted to test the hypothesis that lead exposure perturbs glucocorticoid-mediated events in central nervous system hormonal target tissues. Utilizing the C6 glioma cell culture model in these studies, glucocorticoid receptor binding to its cytosolic receptor was investigated. Receptor binding studies yielded a Kd= 10.5 +/- 0.5 nM and a Bmax = 486 +/- 27 fmol/mg protein in untreated cells versus a Kd = 23.1 +/- 2.6 nM and Bmax = 472 +/- 35 fmol/mg protein in cells exposed to 10 microM lead acetate for 24 h. Presence of lead in these glial cells may decrease affinity of the glucocorticoid for its receptor without affecting receptor number. Treatment with 10 microM lead acetate for 48 h, resulted in a significant reduction in glucocorticoid-regulated glycerol phosphate dehydrogenase (GPDH) specific activity. These effects were not due to cell cytotoxicity assessed as cell number growth curves, [3H]thymidine incorporation or trypan blue exclusion. In protein kinase C (PKC) activity assays, treatment of cells with sodium or lead acetate and dexamethasone indicated that both lead and dexamethasone affect the distribution of PKC. In lead-treated cells cytosolic PKC activity was reduced 48% when compared to sodium acetate treated controls. Taken together, these results suggest that acute exposure of C6 cells to lead may inhibit processes involved in glucocorticoid-mediated signal transduction events within central nervous system hormonal target cells. Lead may perturb initial glucocorticoid binding events possibly by affecting PKC-mediated phosphorylations in the glucocorticoid signal transduction system.

Trypsin promotes C6 glioma cell proliferation in serum- and growth factor-free medium

C6 glioma cells could be successively subcultured and maintained in serum- and growth factor-free medium (SF/GFF medium). C6 cell proliferation in SF/GFF medium was positively correlated with the initial cell density at plating. This correlation disappeared when the medium had been renewed early after cell adhesion (3 h after plating), suggesting that C6 cell growth depends on some diffusible factor in the medium before renewal, and that this factor is not secreted from C6 cells in the assay culture but is transferred from the cell suspension. The supernatant of trypsinized C6 cell suspension (SCS), trypsin-EDTA solution for routine cell harvesting use, and modified trypsin of protein sequencing grade all promoted C6 cell proliferation at, appropriate dilutions or concentrations under SF/GFF conditions. The growth promoting effects of SCS and trypsin-EDTA solution were completely inhibited by soybean trypsin inhibitor. These results demonstrate that the serine protease trypsin has a proliferative effect on C6 cells continuously subcultured in SF/GFF medium. In addition, it is suggested that trypsin used for cell dispersion is transferred from cell suspension into the culture, where it promotes C6 cell growth after passage in our SF/GFF subculture system.

Steroid Inhibition of Neural Micro vessel Morphogenesis In Vitro: Receptor Mediation and Astroglial Dependence

Steroid hormones alter several aspects of microvascular function within the CNS. Both microvessel formation and blood-brain barrier expression appear to be influenced by interactions between astrocytes and endothelial cells. To determine if steroids alter astrocyte-endothelial interactions, we studied their effects on astroglial-induced microvessel morphogenesis in vitro. C6 astroglial cells induce bovine retinal microvascular endothelial cells to differentiate into capillary-like structures. Dexamethasone, hydrocortisone, and progesterone at 10 nM inhibited C6-induced microvessel morphogenesis by 75, 35, and 30%, respectively. Inhibition by dexamethasone was both time and concentration dependent, reaching 80-100% at 1 microM. Tetrahydrocortisone and 17 alpha-hydroxyprogesterone had only marginal inhibitory effects. Cortexolone, a glucocorticoid receptor antagonist, blocked inhibition by dexamethasone. Progesterone receptors were expressed in C6 but not bovine retinal microvascular endothelial cells, identifying the astroglial cell as the likely effector of progesterone-mediated inhibition. Astroglial cells were further implicated as the effectors of steroid-mediated inhibition because none of the steroids inhibited astroglial-independent capillary-like structure formation in response to a reconstituted extracellular matrix, Matrigel. These findings are evidence that steroids modulate neural microvascular endothelial cell functions indirectly through perivascular astrocytes via a receptor-mediated mechanism.

Differential regulation of myelin gene expression in SV40 T antigen-transfected rat glioma C6 cells

Rat glioma C6 cells were stably transfected with a pSV3-neo plasmid containing SV40 T antigen gene, and geniticin-resistant transfectants (designated C6T cells) were cloned. The C6T cells grew as well-defined foci of cells showing squamous or irregular morphology. The doubling time for transfected cells was reduced by approximately 40% as compared to control C6 cells. The transfection with T-antigen also affected the expression of genes coding for structural myelin proteins and for myelin-associated enzymes. The steady-state level of proteolipid protein (PLP)-specific mRNA in C6T cells was 44% lower than in parental C6 cells. On the other hand, the transfection upregulated the expression of myelin-associated glycoprotein (MAG) by 153%. The activity of 2':3' cyclic AMP phosphodiesterase (CNP) was increased by approximately 80% in the C6T cells as compared to untransfected, control cells. The activity of calcium-activated neutral proteinase (CANP) was also significantly elevated in the transfectants by approximately 50% and 220% for millimolar and micromolar form respectively. The results indicate that T antigen affects the expression of myelin genes, although, individual genes appear to be differently regulated implying the existence of several independent regulatory mechanisms.

Intracellular location of the Ah receptor

The subcellular distribution of the Ah receptor from the mouse hepatoma line, Hepa-1, was investigated following cytochalasin B treatment and cell enucleation. Probing the resultant cytoplast and nucleoplast fractions with radiolabelled tetrachlorodibenzo-p-dioxin (TCDD) revealed the presence of a specifically bound peak of receptor only in the cytoplast fraction. However, the quantity of receptor recovered in these experiments was only 10-12% of the expected value. We therefore undertook an investigation to determine the fate of the Ah receptor in the presence of cytochalasin B. Incubation of Hepa-1 cells with this compound resulted in a rapid loss or inactivation of cytosolic binding activity with a concomitant decrease in the amount of receptor partitioned into the nucleus at all time periods examined. Control experiments indicated that cytochalasin B did not compete with TCDD for binding to the Ah receptor and furthermore, that its mechanism of action could not be attributed to a non-specific effect on all cytosolic proteins. The results obtained are discussed in relation to the proposed models for induction by the estrogen and glucocorticoid binding receptors.

Glucocorticoid regulation of phenylethanolamine N-methyltransferase (PNMT) in organ culture of superior cervical ganglia

Glucocorticoid regulation of the adrenergic enzyme, phenylethanolamine N-methyltransferase (PNMT) was studied in organ cultures of the superior cervical ganglion (SCG) from newborn rats. Although PNMT catalytic activity was present in control ganglia, enzyme levels were too low to allow visualization of PNMT immunofluorescent cells. Addition of dexamethasone (DEX) or corticosterone to the medium resulted in a large increase in PNMT activity and bright PNMT immunoreactive (PNMT-IR) staining in cells resembling small, intensely fluorescent (SIF) cells. Addition of non-glucocorticoid steroids was ineffective. Exposure to a brief, 2-hr pulse of DEX (10(-6) M) in vitro elicited the same increase in PNMT as continual exposure to DEX. Studies using metabolic inhibitors demonstrated that the steroid-dependent increase in PNMT activity required both protein and RNA synthesis. Furthermore, the increase was inhibited by cytochalasin B and by the glucocorticoid receptor antagonists, DEX 21-mesylate and cortisol 21-mesylate. These observations suggest that glucocorticoids increase PNMT protein in SIF cells by interacting with specific steroid receptors that undergo translocation to the nucleus.

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.

Interrelationship between differentiation and malignancy-associated properties in glioma

The phenotypic expression of cells derived from human anaplastic astrocytomas, rat glioma, normal human adult and foetal brain tissue have been examined for differentiated and malignancy-associated properties. Glial fibrillary acidic protein (GFAP), high affinity glutamate and gamma-amino butyric acid (GABA) uptake and glutamine synthetase were used as indicators of astroglial differentiation. Plasminogen activator and tumour angiogenesis factor were the malignancy-associated markers. The normal adult brain-derived lines showed some differentiated astroglial features and expressed low levels of the malignancy-associated properties. The foetal cultures contained highly differentiated astroglia while the glioma lines showed considerable phenotypic heterogeneity from highly differentiated to undifferentiated. The least differentiated glioma cells exhibited the highest plasminogen activator activities. The density-dependent control of phenotypic expression was also investigated. High affinity GABA uptake, and GFAP in rat C6 glioma cultures, increased with increasing monolayer cell density, events probably mediated by an increase in the formation of cell-cell contacts at confluence. Plasminogen activator activity decreased with increasing cell density.

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.

Immunocytochemical localization of glycerol-3-phosphate dehydrogenase in rat oligodendrocytes

In this study, two indirect immunoperoxidase staining procedures were used to investigate the cellular localization of rat brain glycerol-3-phospate dehydrogenase (EC 1.1.1.8;GPDH). At the light and electron microscopic level, we found that the use of monospecific rabbit antibodies to GPDH consistently resulted in the specific staining of only one glial cell population. GPDH-positive cells in perineuronal, interfascicular and perivascular positions were identified as oligodendrocytes by classical morphological criteria. The specificity of GPDH antigen-antibody reaction was determined by qualitative and quantitative immunochemical methods and by imunocytochemical controls for immunologic and methodologic sources of nonspecific reaction product. The illustrative data from this study serve to qualitatively define GPDH as a biochemical marker for oligodendrocytes in rat central nervous tissue. In view of the fact that the synthesis of rat brain GPDH is specifically regulated by glucocorticoids, the positive results obtained in this study further warrant the interpretation that rat oligodendrocytes are target cells for glucocorticoids.

Effect of hydrocortisone on cell morphology in C6 cells

Hydrocortisone has been found to induce cell spreading in rat glial C6 cells by 24 hours after its addition. This spreading phenomenon is correlated with an increase in the fraction of the peripheral cytoplasm occupied by microfilaments. Cytochalasin B causes disorganization of microfilaments in the peripheral cytoplasm of the cells. Additionally, it also prevents cell spreading in response to hormonal stimulation. High levels of calcium prevent recovery of normal microfilament organization and cell spreading following removal of cytochalasin B, but have no effect on normal microfilament organization alone. Additionally both the hydrocortisone induced spreading of C6 cells and increases in peripheral microfilaments are shown to be dependent on RNA ans protein synthesis. The levels of protein co-electrophoresing with actin are not effected by hydrocortisone.

Reversible inhibition of the norepinephrine induction of lactate dehydrogenase by cytochalasin B in rat glial C6 cells

The cyclic AMP mediated induction of lactate dehydrogenase (LDH: E.C. 1.1.1.27) activity by norepinephrine in the rat glial cell line C6 is inhibited by cytochalasin B. Doses of 5, 15, and 25 microgram/ml of cytochalasin B inhibited the induction equally. Twenty-five microgram/ml of cytochalasin B inhibited the induction reversibly, and had no effect on basal enzyme level. No effect of cytochalasin B on general protein synthesis was found, nor did it increase the rate of decline of enzyme activity in deinduced cells. It therefore appears to block LDH induction by selectivity inhibiting its synthesis. Cytochalasin B had no effect on the transient (intracellular and extracelllular) rise in cyclic AMP generated in response to norepinephrine treatment. Cytochalasin B was effective when added during the transcription dependent phase (first 3 hours) but not during the translation dependent phase (after 3 hours) of LDH induction. The suggestion is discussed that cytochalasin B inhibits one of the early events of the inductive process.