Stimulation of inositol incorporation into lipids of PC12 cells by nerve growth factor and bradykinin

Involvement of nerve growth factor in visual cortex plasticity

The physiological role of nerve growth factor (NGF), the prototype member of the neurotrophin family, has been widely studied. NGF has been shown to promote survival, sprouting and differentiation of sympathetic ganglion cells and sensory neurons in the peripheral nervous system; it has also been shown to support survival and regeneration of cholinergic neurons in the central nervous system. Recent evidence indicates that NGF is also involved in the neuronal plasticity of the visual cortex. Exogenous supplies of NGF have been shown to interfere with normal processes underlying activity- and age-dependent synaptic modifications in both developing and adult visual cortex. In parallel to these physiological effects, numerous neuronal markers in the visual cortex have been found to be influenced by NGF. Several proposals have been introduced to explain the physiological role of NGF in visual cortex plasticity. Although the mechanisms underlying NGF effects in the visual cortex are still under active investigation, current evidence implies that NGF, and perhaps other neurotrophins as well, may be useful for preventing or correcting inappropriate or anomalous connections in the visual cortex, and thus for treating visual dysfunctions such as amblyopia and strabismus.

Decreased myo-inositol uptake is associated with reduced bradykinin-stimulated phosphatidylinositol synthesis and diacylglycerol content in cultured neuroblastoma cells exposed to L-fucose

L-Fucose is a potent, competitive inhibitor of myo-inositol transport by cultured mammalian cells. Chronic exposure of neuroblastoma cells to L-fucose causes a concentration-dependent decrease in myo-inositol content, accumulation, and incorporation into phosphoinositides. In these studies, L-fucose supplementation of culture medium was used to assess the effect of decreased myo-inositol metabolism and content on bradykinin-stimulated phosphatidylinositol synthesis and diacylglycerol production. Chronic exposure of cells to 30 mM L-fucose caused a sustained decrease in bradykinin-stimulated, but not basal, 3H-inositol phosphate release and 32P incorporation into phosphatidylinositol in cells incubated in serum-free, unsupplemented medium. In addition, 32P incorporation into phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate was not altered in L-fucose-conditioned cells. Acute exposure of cells to serum-free medium containing 30 mM L-fucose did not affect either basal or bradykinin-stimulated 32P incorporation into phosphatidylinositol. Basal diacylglycerol content was decreased by 20% in cells chronically exposed to 30 mM L-fucose, although analysis of the molecular species profile revealed no compositional change. Bradykinin stimulated diacylglycerol production in neuroblastoma cells by increasing the hydrolysis of both phosphoinositides and phosphatidylcholine. Bradykinin-stimulated production of total diacylglycerol was similar for control and L-fucose-conditioned cells. However, there was a decrease in the bradykinin-induced generation of the 1-stearoyl-2-arachidonoyl diacylglycerol molecular species in the cells chronically exposed to 30 mM L-fucose. This molecular species accounts for about 70% of the composition of phosphoinositides, but only 10% of phosphatidylcholine. The results suggest that a decrease in myo-inositol uptake results in diminished agonist-induced phosphatidylinositol synthesis and phosphoinositide hydrolysis in cultured neuroblastoma cells grown in L-fucose-containing medium.

Nerve growth factor potentiates bradykinin-induced calcium influx and release in PC12 cells

To investigate how the response to agonists changes during neuronal differentiation, we examined the effect of nerve growth factor (NGF) on bradykinin-induced calcium increases in PC12 cells. Short-term (1 h) treatment with NGF increased the potency of bradykinin to raise intracellular calcium by about 10-fold, whereas long-term (1 week) treatment, which was associated with the expression of the differentiated phenotype, increased the potency about 100-fold. Neither treatment affected the maximal response to bradykinin. NGF alone had no acute effect on calcium levels. Short-term potentiation appeared to be mainly a result of greater release of calcium from intracellular stores, whereas the effect of long-term treatment apparently was due to increases in both release from intracellular stores and calcium influx. [3H]Bradykinin binding to intact PC12 cells was unaltered by short-term NGF treatment, whereas differentiated cells displayed a 50% increase in receptor number and about a twofold increase in affinity as compared with cells not treated with NGF. The production of inositol phosphates in response to bradykinin correlated poorly with the calcium transients, in that large calcium responses were associated with small increases in inositol phosphates. Neither NGF treatment had a significant effect on the appearance of inositol phosphates in response to bradykinin. Experiments with permeabilized cells revealed that differentiated cells did not display a heightened response to exogenously added inositol 1,4,5-trisphosphate. Our results demonstrate that NGF modulates the bradykinin signaling pathway without acutely activating this pathway itself.

Rapid degradation of newly synthesized tubulin in lithium-treated sensory neurons

When cultured chick sensory neurons were labeled with [35S]methionine for 1 h or longer in the presence of 5-25 mM LiCl, we found a dose-dependent reduction in the level of radiolabeled tubulin, to one third of control levels, with no noticeable effect on other proteins. The magnitude of this response was identical after a 1-h or 72-h preincubation in 25 mM LiCl and returned to control values within 1 h after removal of LiCl. Short (5-min) pulse-chase experiments revealed that tubulin synthesis was not affected by Li+, but that newly synthesized tubulin was rapidly degraded, such that 50% of the labeled beta-tubulin was lost within 5 min. There was no enhanced degradation of tubulin present before exposure to Li+. Addition of LiCl at various times before and after a 10-min pulse suggested that tubulin becomes completely refractory to Li(+)-induced degradation within 10 min after translation. Although Li+ treatment resulted in a decrease in the fraction of extant tubulin present in the unassembled form, the Li(+)-induced degradation of nascent tubulin is not a consequence of shifts in assembly state, because colcemid or taxol treatment did not lead to rapid degradation of newly synthesized tubulin, and neither drug altered the response to Li+. We suggest that Li+ interferes with the correct folding of tubulin polypeptides, exposing sites, normally hidden, to the action of a protease(s).

Regulation of bradykinin receptor level by cholera toxin, pertussis toxin and forskolin in cultured human fibroblasts

1. The effect of bacterial toxins on bradykinin-triggered release of arachidonic acid was studied in serum-deprived human foreskin (HSWP) fibroblasts prelabelled with [3H]-arachidonic acid. An 18-h exposure of HSWP cells to cholera toxin, pertussis toxin, or forskolin enhanced the bradykinin-stimulated release of arachidonic acid and metabolites. 2. Prolonged treatment of HSWP cells with these agents also caused a 3 to 4 fold rise in cell surface [3H]-bradykinin binding. The rise was inhibited by concurrent incubation with cycloheximide or actinomycin D. In addition, cholera toxin and foreskolin increased [3H]-bradykinin binding in wildtype PC12 cells, but not in mutant PC12 cells with reduced cyclic AMP-dependent protein kinase type II activity. 3. In conclusion, cholera toxin, pertussis toxin and forskolin enhanced arachidonic acid release in response to bradykinin, and increased the number of bradykinin receptors in HSWP fibroblasts. A cyclic AMP-dependent mechanism appears to mediate the actions of the toxins and forskolin.

Lithium augments fos protoonocogene expression in PC12 pheochromocytoma cells: implications for therapeutic action of lithium

It has been proposed that lithium's antimanic action is due to an effect on phosphoinositide metabolism. Second messengers generated by this pathway regulate calcium mobilization and the activity of the serine and threonine kinase, protein kinase C (PKC). Included among the targets of PKC is activation of fos protooncogene expression, a well-established component of the AP-1 transcription factor. Because of these interactions, we investigated the effect of lithium on fos gene expression in PC12 pheochromocytoma cells. We find that lithium increases the level of fos mRNA that occurs in response to receptor and postreceptor activation of PKC. Treatment with lithium also leads to an augmentation of muscarinic cholinergic-mediated fos gene expression in cells that are down-regulated as a result of excessive cholinergic stimulation. The ability of lithium to enhance the response of a down-regulated cholinergic system suggests a model for its therapeutic efficacy in affective disorders.

Production of 1,2-diacylglycerol in PC12 cells by nerve growth factor and basic fibroblast growth factor

The addition of nerve growth factor (NGF) or basic fibroblast growth factor (bFGF) to PC12 cells prelabeled with [3H]inositol and preincubated for 15 min in the presence of 10 mM LiCl stimulated the production of inositol phosphates with maximal increases of 120-180% in inositol monophosphate (IP), 130-200% in inositol bisphosphate (IP2), and 45-50% in inositol trisphosphate (IP3) within 30 min. The majority of the overall increase (approximately 85%) was in IP; the remainder was recovered as IP2 and IP3 (approximately 10% as IP2 and 5% as IP3). Under similar conditions, carbachol (0.5 mM) stimulated about a 10-fold increase in IP, a sixfold increase in IP2, and a fourfold increase in IP3. The mass level of 1,2-diacylglycerol (DG) in PC12 cells was found to be dependent on the incubation conditions; in growth medium [Dulbecco's modified Eagle's medium (DME) plus serum], it was around 6.2 mol %, in DME without serum, 2.5 mol %, and after a 15-min incubation in Dulbecco's phosphate-buffered saline, 0.62 mol %. The addition of NGF and bFGF induced an increase in the mass level of DG of about twofold within 1-2 min, often rising to two- to threefold by 15 min, and then decreasing slightly by 30 min. This increase was dependent on the presence of extracellular Ca2+, and was inhibited by both phenylarsine oxide (25 microM) and 5'-deoxy-5'-methylthioadenosine (3 mM). Under similar conditions, 0.5 mM carbachol stimulated the production of DG to the same extent as 200 ng/ml NGF and 50 ng/ml bFGF. Because carbachol is much more effective in stimulating the production of inositol phosphates, the results suggest that both NGF and bFGF stimulate the production of DG primarily from phospholipids other than the phosphoinositides.

Effects of bradykinin on PC-12 cell differentiation

PC-12 cells are used as a model for neuronal differentiation because they assume a neuronal phenotype, including the extension of neurites, when exposed to nerve growth factor (NGF). The present results show that bradykinin (BK) also causes PC-12 cells to extend neurites. In addition, BK potentiates the neurite-extending effect of nerve growth factor (NGF), an action which is attenuated by a BK antagonist. The potentiation of neurite extension produced by the combination of BK and NGF may be mediated at the receptor level, as indicated by an NGF-induced alteration of BK binding.

Nerve growth factor stimulates the hydrolysis of glycosylphosphatidylinositol in PC-12 cells: a mechanism of protein kinase C regulation

Treatment of PC-12 pheochromocytoma cells with nerve growth factor (NGF) results in the differentiation of these cells into a sympathetic neuron-like phenotype. Although the initial intracellular signals elicited by NGF remain unknown, some of the cellular effects of NGF are similar to those of other growth factors, such as insulin. We have investigated the involvement of a newly identified inositol-containing glycolipid in signal transduction for the actions of NGF. NGF stimulates the rapid generation of a species of diacylglycerol that is labeled with [3H]myristate but not with [3H]arachidonate. NGF stimulates [3H]myristate- or [32P]phosphate-labeled phosphatidic acid production over the same time course. Although NGF alone has no effect on the turnover of inositol phospholipids, it does stimulate the hydrolysis of glycosylphosphatidylinositol. The NGF-dependent cleavage of this lipid is accompanied by an increase in the accumulation of its polar head group, an inositol phosphate glycan, which is generated within 30-60 sec of NGF treatment. In an unresponsive PC-12 mutant cell line, neither the diacylglycerol nor inositol phosphate glycan response is detected. A possible role for the NGF-stimulated diacylglycerol is suggested by the inhibition of NGF-dependent c-fos induction by staurosporin, a potent inhibitor of protein kinase C. These results suggest that, like insulin, some of the cellular effects of NGF may be mediated by the phospholipase C-catalyzed hydrolysis of glycosylphosphatidylinositol.

Lithium stimulation of membrane-bound phospholipase C from PC12 cells exposed to nerve growth factor

LiCl stimulated the formation of inositol monophosphate in PC12 cells that had been exposed to nerve growth factor (NGF) for 4-5 days. Half-maximal accumulation was observed at approximately 8 mM LiCl. Stimulation of formation of inositol bisphosphate plus inositol trisphosphate was half-maximal at approximately 1 mM LiCl. With membranes isolated from PC12 cells differentiated with NGF, the hydrolysis of added phosphatidylinositol 4,5-bisphosphate (PIP2) was stimulated by LiCl in a biphasic manner, with the first stimulation half-maximal at approximately 0.7 mM and the second half-maximal at approximately 15 mM LiCl. The apparent Km for PIP2 was lowered in the presence of 1.1 mM LiCl from approximately 200 to approximately 70 microM. Membranes from cells grown in the absence of NGF did not respond to LiCl. Although observations with intact cells are difficult to interpret without ambiguity, the results obtained with isolated membranes support our interpretation of the stimulatory action of lithium in the intact PC12 cells.