Activation of S6 kinase in astroglial cells by FGFa and FGFb

MAP kinase cascades are activated in astrocytes and preadipocytes by 15-deoxy-Delta(12-14)-prostaglandin J(2) and the thiazolidinedione ciglitazone through peroxisome proliferator activator receptor gamma-independent mechanisms involving reactive oxygenated species

15-Deoxy-Delta(12-14)-prostaglandin J(2) (dPGJ2) and thiazolidinediones are known as ligands for the peroxisome proliferator activator receptor gamma (PPAR gamma) a member of the nuclear receptor superfamily. Herein, we show that dPGJ2 activates, in cultured primary astrocytes, Erk, Jnk, p38 MAP kinase, and ASK1, a MAP kinase kinase kinase, which can be involved in the activation of Jnk and p38 MAP kinase. The activation kinetic is similar for the three MAP kinase. The activation of the MAP kinases is detectable around 0.5 h. The activation increases with dPGJ2 in a dose dependent manner (0-15 microm). A scavenger of reactive oxygenated species (ROS), N-acetylcysteine (NAC) at 20 mm, completely suppresses the activation of MAP kinases and ASK1, suggesting a role for oxidative stress in the activation mechanism. Other prostaglandin cyclopentenones than dPGJ2, A(2), and to a lesser degree, A(1) also stimulate the MAP kinases, although they do not bind to PPAR gamma. Ciglitazone (20 microm), a thiazolidinedione that mimics several effects of dPGJ2 in different cell types, also activates the three MAP kinase families and ASK1 in cultured astrocytes. However the activation is more rapid (it is detectable at 0.25 h) and more sustained (it is still strong after 4 h). NAC prevents the activation of the three MAP kinase families by ciglitazone. Another thiazolidinedione that binds to PPAR gamma, rosiglitazone, does not activate MAP kinases, indicating that the effect of ciglitazone on MAP kinases is independent of PPAR gamma. Ciglitazone and less strongly dPGJ2 activate Erk in undifferentiated cells of the adipocyte cell line 1B8. Ciglitazone also activates Jnk and p38 MAP kinase in these preadipocytes. Our findings suggest that a part of the biological effects of dPGJ2 and ciglitazone involve the activation of the three MAP kinase families probably through PPAR gamma-independent mechanisms involving ROS.

Role of redox status on the activation of mitogen-activated protein kinase cascades by NSAIDs

High concentrations of non steroidal antiinflammatory drugs (NSAIDs) exert preventive effects against carcinogenesis. Their molecular mechanism of action remains to be elucidated. Based on previous reports with salicylate, we have made the hypothesis that various NSAIDs can activate the mitogen-activated protein kinases (MAPK). Moreover, we tested the idea that NSAIDs act by increasing the effects of oxidative stress. We report that in human colorectal carcinoma cells NSAIDs stimulated the three families of MAPK, extracellular regulated kinases, c-Jun N-terminal kinases, p38 MAPK and that this stimulation is prevented by N-acetyl cysteine. In cultured astrocytes, a biological system less sensitive to oxidative stress, we show that a short treatment by NSAIDs strongly activated the three MAP kinases in the presence of H(2)O(2). A 25 microM H(2)O(2), unable to stimulate by itself the MAP kinases, promote an almost complete activation of MAP kinases in the presence of NSAIDs. The activation of MAP kinases by H(2)O(2) and NSAIDs was suppressed by quinone reductase inhibitors, suggesting that "redox cycling" was involved in the activation mechanisms of MAP kinases by H(2)O(2) and NSAIDs. The mobility on SDS-PAGE of the apoptosis signal-regulating kinase, which activates C-Jun N-terminal kinases and p38 MAPK cascades, was reduced by H(2)O(2) and NSAIDs, suggesting, that H(2)O(2) and NSAIDs activated apoptosis signal-regulating kinase by increasing its state of phosphorylation. In conclusion, we demonstrate that various NSAIDs can activate the three families of MAP kinases and that this activation depends on the presence of reactive oxygenated species. These results give a new insight into the mechanism of the action of NSAIDs.

Differential responsiveness of late passage C-6 glial cells and advanced passages of astrocytes derived from aged mouse cerebral hemispheres to cytokines and growth factors: glutamine synthetase activity

In this study, we were interested to compare the responsiveness to growth factors, NGF, b-FGF and EGF and cytokines, IL1 beta, and TNF-alpha, in late passages (74-79) C6 glial cells committed astrocytes and astrocytes of advanced passages (26-28) in cultures derived from aged mouse cerebral hemispheres (MACH). Cultures were grown in either DMEM or chemically defined medium (CDM/TIPS) in order to test the effects of growth factors or cytokines. The activity of glutamine synthetase (GS), a marker for astrocytes, was used as a test parameter. We found that treatment with growth factors increased GS activity in both glial cell culture systems with the exception of EGF in C-6 glial cells. Treatment with cytokines markedly decreased GS activity in the late passage C6 glial cells whereas only TNF-alpha had a similar effect on MACH astrocytes. In view of the generally opposite effects of growth factors and cytokines on GS activity, we speculate that these molecules which are also endogenously present in glial cells may play a role in the maintenance of cellular homeostasis.

Effects of growth factors on phosphatidylinositol-3 kinase in astroglial cells

Growth factors differently regulate astroglial cell differentiation and proliferation. In an effort to understand the early intracellular events promoted by growth factors in astroglial cells, we have determined the effects of insulin-like growth factor I (IGF1), insulin, platelet-derived growth factor (PDGF), epidermal growth factor (EGF) and fibroblast growth factors (FGFs) on phosphatidylinositol-3 kinase (PI(3)-kinase). In astroglial cells cultured in serum-free medium, IGF1, PDGF, and EGF, which stimulate cell proliferation, increased PI(3)-kinase activity immunoprecipitated with anti-phosphotyrosine antibodies as shown by thin layer chromatography and high performance liquid chromatography. FGFa and FGFb, which strongly stimulate proliferation, glutamine synthetase, and deiodinase activities and modify cell morphology, have no effect on PI(3)-kinase activity. Addition of 1 nM PDGF, 10 nM IGF1, or 100 nM EGF to the culture medium rapidly stimulated PI(3)-kinase activity which declined slowly after 2 min. The stimulation of PI(3)-kinase increased with growth factor concentration. The maximum increase in PI(3)-kinase activity occurred with 50 nM IGF1, 1 nM PDGF, or 100 nM EGF. Since insulin was active only at high concentration (1 microM), its effect was probably mediated through IGF1 receptors and not through insulin receptors. IGF1 and PDGF, to a lesser degree, also increased the PI(3)-kinase activity associated with pp60c-src protein. Immunoblots performed with an antibody directed against the p85-subunit of the PI(3)-kinase confirmed that IGF1 increased the number of PI(3)-kinase molecules associated with phosphotyrosine-containing proteins or with c-src protein. Each growth factor affects in a different manner the association of PI(3)-kinase with phosphotyrosine-containing proteins and with pp60c-src and thus probably modulates intracellular signals downstream of PI(3)-kinase in astroglial cells.

12-O-tetradecanoylphorbol 13-acetate and fibroblast growth factor increase the 30-kDa substrate binding subunit of type II deiodinase in astrocytes

Type II 5'-deiodinase (D-II) catalyzes the intracellular conversion of thyroxine (T4) to 3,5,3'-triiodothyronine (T3) in the brain. The D-II activity in astroglial cell cultures is induced by several pathways including cyclic AMP (cAMP), 12-O-tetradecanoylphorbol 13-acetate (TPA), and fibroblast growth factors (FGFs). We have examined the effect of TPA and FGFs on the 30-kDa substrate binding subunit of D-II, by affinity labeling with N-bromoacetyl-[125I]T4 in astroglial cells. TPA (0.1 microM), 20 ng/ml acidic FGF (aFGF), and 1 mM 8-bromo cyclic AMP all caused an increase in the 30-kDa protein. cAMP induced the greatest increase (fivefold) followed by TPA (3.2-fold) and FGF (2.8-fold). Glucocorticoids acted synergistically with cAMP and aFGF and promoted the effect of TPA. Affinity labeling was competitively inhibited by bromoacetyl-T4 > bromoacetyl-T3 > T4 > reverse T3 > iopanoic acid > T3 > 3,5,3'-triiodothyroacetic acid. The effect of TPA (0.1 microM) was maximum at 8 h and then gradually decreased. aFGF (20 ng/ml) plus heparin (17 micrograms/ml) induced a maximal 30-kDa increase at 8 h, which stayed stable for up to 24 h. The effect of aFGF was concentration dependent. Of the other growth factors studied, only basic FGF and platelet-derived growth factor induced small increases in the 30-kDa protein. Epidermal growth factor had little effect. In vitro labeling of cAMP, TPA, and aFGF-stimulated cell sonicates resulted in an increase in the 30-kDa protein that paralleled the increase in D-II activity.(ABSTRACT TRUNCATED AT 250 WORDS)

MAP kinase cascade in astrocytes

We have studied in cultured rat astroglial cells MAP kinases, known for their role in intracellular signal transduction. The MAP kinase activity was stimulated by growth factors (FGFb, FGFa, EGF, PDGF, and IGF1), by a phorbol ester (TPA) activating-protein kinase C (PKC), by a neuropeptide (endothelin-1), and by a neuromediator (carbachol). Astrocytes pretreated for 18 h with TPA were still stimulated by growth factors and endothelin, suggesting that down-regulated isoforms of PKC are not involved in MAP kinase activation. In contrast, the small effect of carbachol was suppressed by TPA pretreatment. Astrocytes contained two proteins (p41 and p44) recognized by MAP kinase antibody. These proteins were phosphorylated on tyrosine residues in the cytosols of stimulated astrocytes. The kinetics of MAP kinase activation by FGFb and IGF1 were very different. FGFb promoted a rapid activation of MAP kinase (about 10 min) plus a prolonged phase that lasted at least 12 h. IGF1 produced only a rapid transient peak of activation at about 20 min. Hence, extracellular signals might generate different effects in astrocytes by differentially modulating the MAP kinase cascade. On a Mono Q column the growth factor-stimulated MAP kinase activity was separated into two peaks containing p41 and p44. Stimulation of astrocytes altered the elution pattern of p44 as a result of its phosphorylation. An ATP-dependent MAP kinase activator (MW = 40-45 kDa) was found in fractions of FGFb-stimulated cells which were not retained on Mono Q column, indicating the existence of a MAP kinase kinase (MEK) in astrocytes. C-Raf, identified in other cells as a MAP kinase kinase kinase, was also present in astrocytes.(ABSTRACT TRUNCATED AT 250 WORDS)

Phosphorylation of the MARCKS protein (P87), a major protein kinase C substrate, is not an obligatory step in the mitogenic signaling pathway of basic fibroblast growth factor in rat oligodendrocytes

Basic fibroblast growth factor (bFGF) is a well-characterized peptide hormone that has mitogenic activity for various cell types and elicits a characteristic set of responses on the cell types investigated. In this report we confirmed that bFGF is a potent mitogen for rat brain-derived oligodendrocyte (OL) precursor cells as well as for differentiated OL in secondary culture. bFGF was shown to induce expression of the protooncogene c-fos in OL. The role of protein kinase C (PKC) in mediating bFGF-stimulated proliferation as well as c-fos expression in OL was investigated. The PKC activator phorbol 12-myristate 13-acetate (PMA) stimulated c-fos expression but did not trigger cell proliferation. When PKC was down-regulated by pretreatment of OL with PMA for 20 h, the bFGF-mediated stimulations of OL proliferation and c-fos mRNA expression were still observed, whereas the induction of c-fos mRNA by PMA was totally inhibited. These data demonstrate that the bFGF mitogenic signaling pathway in OLs does not require PKC. On the other hand, bFGF was found to stimulate specifically the phosphorylation of a limited number of PKC substrates in oligodendroglial cells, including the MARCKS protein. The bFGF-dependent phosphorylation of MARCKS protein was totally inhibited when PKC was first down-regulated, indicating that the phosphorylation of this protein is PKC dependent. Tryptic digestion of the phosphorylated MARCKS protein revealed that bFGF stimulated specifically the phosphorylation of the MARCKS protein on a single phosphopeptide. We provide evidence that bFGF also stimulated fatty acylation of the MARCKS protein, which might explain the observed specific bFGF-dependent phosphorylation of this protein in OL. We propose that bFGF-dependent fatty acylation and phosphorylation of the MARCKS protein are not essential for the transduction of the bFGF mitogenic signal but are probably linked to differentiation processes elicited by bFGF on OL.

Early effect of BCNU on rat astrocytes. Inhibition of S6 kinase activation by growth factors

In primary cultures of astrocytes, methylmethane, 2-N-methyl 9-hydroxy-ellepticinium acetate, ditercalinium, 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea and 1,3 bis (2-chloroethyl)-1-nitrosourea (BCNU) blocked to various extents the activation of S6 kinase by acidic fibroblast growth factor and insulin [or insulin-like growth factor 1 (IGF1)]. The effects of the most active agent, BCNU, were time and concentration dependent. Pretreatment of cells with 50 microM BCNU for 1 hr completely prevented S6 kinase activation by growth factors for at least 2 days. The S6 kinase activity of unstimulated cells was slightly affected. S6 kinase activation by 12-O-tetradecanoylphorbol 13 acetate was also strongly impaired by treating cells with BCNU whereas activation by 8-bromo-cyclic AMP was slightly reduced. Cyclic AMP-dependent protein kinase and phospholipid and Ca(2+)-dependent protein kinase were unaffected. BCNU had no direct effect on IGF1 binding to cell surface receptors or on the S6 kinase activity of cell cytosols.

Induction of 5-deiodinase activity in astroglial cells by 12-O-tetradecanoylphorbol 13-acetate and fibroblast growth factors

In the brain, 5'-deiodinase (5'-D) is responsible for the metabolic activation of thyroxine (T4) into 3,5,3'-triiodothyronine (T3) and 5-deiodinase (5-D) deiodinates T4 and T3 into inactive metabolites. This study examines the effects of factors known to induce astroglial 5'-D activity on the 5-D activity in cultured rat astroglial cells. The potencies of these factors were compared after 8 h of incubation, when stimulations by these factors near their maximal effects. 12-O-Tetradecanoylphorbol 13-acetate (TPA) at 10(-7) M was a potent inducer of 5-D activity, producing a 30- to 80-fold increase after 8 h. The maximal effect of TPA was observed after about 14 h. The TPA stimulation of 5-D activity was not dependent on glucocorticoids, unlike 5'-D activity. In comparison with TPA, 8-bromo-cyclic AMP (10(-3) M) was a poor inducer of 5-D activity whereas it is an excellent inducer of 5'-D activity. It produced a 2- to 20-fold increase in 5-D activity after 8 h. Natural acidic fibroblast growth factor (20 ng/ml) produced a degree of stimulation similar to that of TPA after 8 h. The maximal effect of acidic fibroblast growth factor was observed after about 16 h (until a 120-fold increase). Recombinant acidic fibroblast growth factor also induced 5-D activity. Basic fibroblast growth factor was less potent than acidic fibroblast growth factor for increasing 5-D activity (maximal increase by 40- to 50-fold after 8 h).(ABSTRACT TRUNCATED AT 250 WORDS)

Thyroid hormone action: induction of morphological changes and protein secretion in astroglial cell cultures

The effects of triiodothyronine (T3) on cell morphology and protein secretion were examined in astrocytes cultured in a chemically defined medium devoid of other hormones and growth factors. The flat polygonal astrocytic cells treated with T3 (1-50 nM) and maintained in non-renewed medium cultures were progressively transformed into process-bearing cells. These changes were initially observed 3 days after the end of T3 treatment and accounted for more than 50% of the cells 7-8 days thereafter. The proteins secreted by the T3-stimulated cells were analyzed on SDS-PAGE after cell labeling for 4.5 h with [35S]methionine. The effect of T3 on protein secretion was dose-dependent. Half-maximal stimulation was reached with 0.2-0.5 nM hormone and the proteins of 46, 59, 67, 78, 85 and 140 kDa were over-secreted (greater than 300% of control). These results were only obtained when the cell medium was not renewed after T3 treatment.

Induction of 5'-deiodinase activity in rat astroglial cells by acidic fibroblast growth factor

Acidic fibroblast growth factor (aFGF) induced a large increase in the type II 5'-deiodinase (5'D) activity in astroglial cells. This required a time lag of about 4 h. Half-maximal stimulation was obtained with about 7 ng/ml aFGF. This factor at 20 ng/ml induced several times more 5'D activity than did 20 ng/ml basic fibroblast growth factor (bFGF) after 8 h incubation. aFGF (20 ng/ml) produced a 10-50-fold increase in 5'D activity after 24 h, whereas the effect of 20 ng/ml bFGF had disappeared after 24 h. Heparin (17 micrograms/ml) potentiated the 5'D response to natural and recombinant aFGF. Glucocorticoids amplified the aFGF-induction of 5'D activity. This is the first demonstration in astroglial cells that a growth factor can regulate the 5'D activity.

Effects of transforming growth factor beta 1 on astroglial cells in culture

The effects of transforming growth factor beta 1 (TGF beta 1) on DNA synthesis and functional differentiation of astroglial cells cultured in serum-free medium were investigated. TGF beta 1 diminished and delayed the peak of DNA synthesis induced by serum. TGF beta 1-treated cells were larger than control cells. This factor delayed the appearance of process-bearing cells induced by acidic fibroblast growth factor treatment and also affected the astrocyte-specific enzyme glutamine synthetase (GS), whose accumulation is under hydrocortisone (HC) control. TGF beta 1 inhibited the induction of GS activity by HC in a dose- and time-dependent manner. Moreover, pretreatment with TGF beta 1 for 4 h maintained the inhibition of GS activity for approximately 16 h after removal of this factor from culture medium. These results suggest that TGF beta 1 may be an important regulator of astrocyte growth and differentiation.

Basic fibroblast growth factor (bFGF) and rat C6 glioma cells: regulation of expression, absence of release, and response to exogenous bFGF

Basic fibroblast growth factor (bFGF) is a potent mitogen for several types of cells, including glial cells, which also seem to express bFGF. We have used rat C6 glioma cells as a model system to study the expression and release of bFGF by glioma cells, as well as the effects of exogenous bFGF on these cells. We have shown that C6 cells express 18 kD bFGF and several higher molecular weight immunoreactive forms. The expression of bFGF could be induced by a factor present in fetal calf serum. Subsequent to its initial appearance, bFGF is regulated in a cell density-dependent manner. Neither bFGF-like immunoreactive material, nor bFGF-like neurotrophic activity were found to be released by C6 cells. Exogenously applied bFGF changed C6 cell morphology similar to cyclic AMP induced alterations but had no significant influence on C6 cell proliferation and biochemical differentiation. From these results we conclude that bFGF in C6 cells might act as an endogenous (not autocrine) mitogen. Possible roles for bFGF in glial cells are discussed.