Biological effects of bovine glia maturation factor on glial cells in culture

Regulation of aquaporin-4 expression in astrocytes

Aquaporin-4 (AQP4), a mercury-insensitive water channel protein, is abundant in the central nervous system and is localized in astrocytes and ependymal cells. AQP4 is speculated to maintain the homeostasis of intracellular and extracellular water in the brain, but little is known about the mechanism of induction of its expression. To investigate the expressional regulation of AQP4, we analyzed changes in its expression during chemically induced differentiation of embryonal carcinoma cells (P19) to neuronal and astrocytic cells, and during the cell cycle of glioma cells. After exposure to retinoic acid for 4 days AQP4 mRNA expression started at the initiation of astrocytic differentiation of P19 cells at 6 days, and increased markedly by 21 days. AQP4 expression was parallel to that of GFAP, a marker intermediate filament of astrocytes. In glioma cell lines, AQP4 mRNA was not detected in the growing phase, but was induced when the cell cycle was arrested at G0/G1 by transient expression of p21. Although quiescent astrocytes in the G0/G1-phase cultured under the serum-free condition exhibited a high expression of AQP4, serum supplement moved them to the S-phase and markedly decreased the AQP expression. These results suggest that AQP4 expression may be induced not only at the initiation of astrocytic differentiation of neural stem cells, but also at the G0/G1-phase during the cell cycle of astrocytes.

Human neuroblastoma growth inhibitory factor (h-NGIF), derived from human astrocytoma conditioned medium, has neurotrophic properties

Investigations on the general characteristics of human astrocytoma cell line NAC-1 revealed neuroblastoma growth inhibitory activity in conditioned medium. Neuroblastoma growth inhibitory factor (NGIF) was partially purified by Econo Q, Econo CM, and Superose 12 column chromatography. The protein is weakly basic with an estimated M(r) of 120,000, possibly having an M(r) 60,000 dimeric structure. NGIF inhibits the growth of human neuroblastoma cell lines but has no effect on morphology nor does it produce any change in the growth of human glioblastoma cell lines. Interestingly, NGIF appears to promote survival and neurite outgrowth of embryonal rat cortical neurons. These neurotrophic properties suggest a role for NGIF in the development of the nervous system.

Production of tumor growth-inhibiting protein by the neonatal mouse brain: dependence on intracellular glutamine metabolism

We previously reported that the mouse brain at the neonatal stage but not at the adult stage secreted a carcinostatic factor of 62,000 Da, termed NBCF, which inhibited clonal growth and DNA synthesis of malignant cells preferentially over those of normal cells. In the present study, NBCF production by the neonatal mouse brain in culture was investigated. Addition of L-glutamine to the culture medium markedly promoted NBCF production in a concentration-dependent manner. The production seemed to be specific to glutamine, since no promotive effect was exerted by L-glutamic acid, its analogue DL-alpha-aminoadipic acid, L-aspargine, or L-aspartic acid or by other amino acids or vitamins. NBCF production was markedly reduced in culture medium either devoid of L-glutamine or containing 6-diazo-5-oxo-L-norleucine, a glutamine antagonist, or L-methionine sulfoximine, an inhibitor of glutamine synthetase. Thus NBCF production was promoted by extracellular supply, intracellular synthesis, and utilization of L-glutamine but was not affected by its deamidated form or homologue amino acids. On the other hand, NBCF production was completely repressed by addition of cycloheximide to the culture medium. The repressive effect was also exerted by actinomycin D although not completely, whereas cytosine beta-D-arabinofuranoside did not repress NBCF production. These results indicated that NBCF production by cultivation was independent of DNA replication but dependent mostly on a transcription stage and its following stages and partly on a translation stage from the preexisting mRNA to the protein.

Mitogenic and morphogenic effects of a bovine salivary gland extract on astrocytes and fibroblasts

The presence of mitogenic and morphogenic activity in extracts of bovine salivary (parotid) glands is reported. The crude and partially purified extracts stimulated cultured rat cerebellar cells (astrocytes) and skin fibroblasts to undergo morphogenesis. The incorporation of [3H]thymidine was also stimulated in astrocytes, skin fibroblasts, and established fibroblastic cell lines. Growth-promoting activity was also demonstrated. The expression of maximum mitogenic activity in skin fibroblast cultures, but not kidney fibroblast cultures, required the presence of serum. The biological activity had an apparent native molecular weight greater than 230,000, was heat-sensitive, trypsin-resistant, and slightly sensitive to the action of papain.

Autocrine regulation of glial proliferation and differentiation: the induction of cytodifferentiation of postmitotic normal glioblast by growth-promoting factor from astrocytoma cell

A growth-promoting factor (GPF) from astrocytoma cells (GA-1) cultured in serum-free medium (N2) exerted on normal glioblasts proliferative and differentiation-promoting effects, which have been observed in glia maturation factor (GMF) stimulation. The serum-free conditioned media of GA-1 provoked DNA synthesis of glioblasts, and subsequently elicited a morphological differentiation characterized by the extrusion of processes as well as biochemical changes including an increased cellular level of glia fibrillary acidic protein (GFA protein), S-100 protein, and alpha-enolase. The transforming growth factor activity was also found in the media. Partially purified GPF had a molecular weight range of 7100-10,000 Mr and acidic isoelectric point (pH 4.6), and showed a susceptibility to heat treatment and denaturation at low and high pHs. The present results and the findings accumulated from our previous studies on gliotrophic growth factors provide a general concept of the growth and differentiation regulations of normal or neoplastic glial cells by growth factors through autocrine systems.

Inhibition of glia maturation factor-induced mitogenesis in glioblasts by calmodulin antagonists

The growth inhibitory activity of calmodulin antagonist, N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7) and trifluoperazine (TFP), was analyzed by the use of rat fetal glioblasts stimulated by glia maturation factor (GMF) or rat astrocytoma cells (C6). The inhibitory effect of W-7 on GMF-induced DNA synthesis of glioblasts was apparent when the drug was added within 10 h after the stimulation by GMF (late G1 phase), but was not shown when W-7 was added at 12 h or later (S phase). The intracellular calmodulin content was built up concurrently with the increase in the DNA synthesis in S phase. The half-maximal inhibition (ID50) of GMF-induced DNA synthesis in glioblasts was observed at 16.5 microM of W-7 or 9.0 microM of TFP. ID50 of DNA synthesis in exponentially growing C6 cells was approximately 3 times higher than that in glioblasts: 24 microM of TFP and as high as 40 microM of W-7. ID50 of growth rate of C6 cells was 15 microM of TFP which was comparable to the ID50 dose for the inhibition of DNA synthesis. Both calmodulin antagonists and W-5, a dechlorinated analog of W-7, however, elicited a curious activation of DNA synthesis of glioblasts at low concentrations (lower than 10 microM of W-7 and W-5, or lower than 5 microM of TFP), indicating non-specific effects of calmodulin antagonists on DNA synthesis. These results suggest that calmodulin antagonists have two conflicting effects on DNA synthesis: the stimulation of DNA synthesis at lower concentrations, and inhibition at higher concentrations.

The induction of glial proliferation by an astrocytoma-derived growth factor resembling glia maturation factor

Glia maturation factor (GMF)-like activity which induces DNA synthesis and morphological differentiation of density-inhibited glioblasts was detected in various glial tumor cells. A polypeptide from C6 cells (rat astrocytoma) which has a molecular weight range of 40,000-50,000 showed the highest activity. This factor also induced DNA synthesis in glioma cells (354A and LRM55) and fibroblast (Swiss 3T3). The activity was susceptible to heat treatment at 70 degrees C for 5 min, or to proteases such as trypsin, chymotrypsin, papain, and subtilisin, but it was devoid of esteropeptidase activity. The isoelectric point was found to be 5.3. Subcellular fractionation localized the activity in cytosomal and microsomal fractions. These properties closely resemble those of GMF from pig and bovine brain.

Interaction of glia maturation factor with the glial cell membrane

Glia maturation factor (GMF) immobilized on agarose beads retained the same mitogenic and morphological transforming activities as free GMF when tested on glioblasts. The exposure of glioblasts to immobilized GMF for 5 min provoked the initiation of DNA synthesis and maximal stimulation was obtained within 30 min. Thiol-reducing agents, such as dithiothreitol and cysteine, increased the biological activity of GMF. These data suggest the presence of a surface receptor to GMF on the glioblasts, while the reduction of the thiol group(s) in GMF promotes the binding to its receptor. Repeated use of immobilized GMF decreased both the mitogenic and the morphological transforming activities. Immobilized GMF used for the third time lost its biological activity. This implies the existence of a certain kind of degradation system such as a proteolytic enzyme located close to the GMF receptor on the glial cell surface.

Inhibition by calmodulin antagonists of glioblast DNA synthesis and morphological changes induced by glia maturation factor

Calmodulin antagonists, N-(6-aminohexyl)-5-chloro-1-naphthalene sulfonamide hydrochlorides (W-7) and trifluoperazine (TFP), markedly inhibited both morphological transforming and mitogenic activities of glia maturation factor (GMF) on rat fetal glioblasts in culture. In the presence of W/-7 (16.5 microM) or TFP (9 microM) the formation of glial processes of glioblasts caused by GMF was decreased, and DNA synthesis (measured by the incorporation of [methyl-3H]thymidine into DNA was inhibited by 50% without changing cell viability. Kinetic analysis of dose-response experiments revealed a noncompetitive fashion of inhibition by W-7 and a mixed fashion by TFP. W-7 appeared to inhibit DNA synthesis at the G1 phase immediately before the beginning of the S phase. The results strongly suggest that calmodulin exerts a significant role on cell multiplication induced by the growth factor.

Glial cell growth-promoting factor in astrocytoma (C6) cell extracts

The presence of glia maturation factor (GMF)-like activity was demonstrated in rat astrocytoma cells (C6 cells). The extracts of C6 cells enhanced the DNA synthesis of cultured glioblasts 3-fold at the maximum, inducing such morphological changes as extrusion of processes. C6 cells also showed a proliferative response to the extracts, though the responsiveness in terms of effective concentration of C6 extracts was about a half of the glioblast responsiveness. The extracts lowered growth rate of neuroblastoma cells and especially decreased their DNA synthesis without a morphological differentiation.