Selective time-dependent effects of insulin on brain phosphoinositide metabolism

Signaling events mediating activation of brain ethanolamine plasmalogen hydrolysis by ceramide

Ceramide is a lipid second messenger that acts on multiple-target enzymes, some of which are involved in other signal-transduction systems. We have previously demonstrated that endogenous ceramide modifies the metabolism of brain ethanolamine plasmalogens. The mechanism involved was studied. On the basis of measurements of breakdown products, specific inhibitor effects, and previous findings, we suggest that a plasmalogen-selective phospholipase A2 is the ceramide target. Arachidonate-rich pools of the diacylphosphatidylethanolamine subclass were also affected by ceramide, but the most affected were plasmalogens. Concomitantly with production of free arachidonate, increased 1-O-arachidonoyl ceramide formation was observed. Quinacrine (phospholipase A2 inhibitor) and 1-O-octadecyl-2-O-methyl-rac-glycerol-3-phosphocholine (CoA-independent transacylase inhibitor) prevented all of these ceramide-elicited effects. Therefore, phospholipase and transacylase activities are tightly coupled. Okadaic acid (phosphatase 2A inhibitor) and PD 98059 (mitogen-activated protein kinase inhibitor) modified basal levels of ceramide and sphingomyelinase-induced accumulation of ceramide, respectively. Therefore, they provided no evidence to determine whether there is a sensitive enzyme downstream of ceramide. The evidence shows that there are serine-dependent and thiol-dependent enzymes downstream of ceramide generation. Furthermore, experiments with Ac-DEVD-CMK (caspase-3 specific inhibitor) have led us to conclude that caspase-3 is downstream of ceramide in activating the brain plasmalogen-selective phospholipase A2.

Platelet-activating factor stimulation of p125(FAK) and p130(Cas) tyrosine phosphorylation in brain

The effect of platelet-activating factor (PAF) on protein tyrosine phosphorylation was studied in rat brain slices. PAF induced a time- and concentration-dependent increase in tyrosine phosphorylation of a doublet of approximately 125 kDa. These proteins were identified by immunoprecipitation as p125(FAK) and p130(Cas), using monoclonal antibodies. This effect was mediated by PAF receptors, as shown by its inhibition by the action of a PAF antagonist. The tyrosine phosphorylation evoked by PAF was dependent, at least in part, on external calcium. The involvement of protein kinase C was demonstrated by the synergistic effect of TPA on PAF-stimulated tyrosine phosphorylation. The finding that PAF stimulates tyrosine phosphorylation of both focal adhesion protein p125(FAK) and p130(Cas) suggests that PAF might modulate the integrin mediated signal transduction in the brain.

Platelet-activating factor modulates brain sphingomyelin metabolism

In the present study the modulatory action of platelet-activating factor (PAF) on sphingolipid metabolism in cerebral cortical slices was studied. PAF did not alter the basal levels of either sphingomyelin (SM) or ceramide. However, the SMase-elicited reciprocal alterations in SM and ceramide levels were partially prevented by the PAF treatment. The PAF effect was dose-dependent, with 10-8 m being the lowest effective concentration, and receptor-mediated as it was abolished by WEB 2086, a PAF receptor antagonist. Neither N-oleoylethanolamine (OE, ceramidase inhibitor) or d,l-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (PDMP, an inhibitor of glucosylceramide synthase and the formation of 1-O-acyl ceramides) prevented the action of PAF. Therefore, the effect of PAF was unlikely to be dependent upon transformation of ceramides into glycosphingolipids, 1-O-acyl ceramides or sphingosine. Experiments with different labeled compounds ([14C]serine, [14C]arachidonate and phosphatidyl [N-methyl-3H]choline) were also performed to test whether PAF could affect the resynthesis of SM. Data obtained agree with the idea that selective pools of both choline and ethanolamine phospholipids were used as precursors for the resynthesis of SM elicited by SMase treatment. PAF itself did not evoke any variation in the lipids analyzed but always prevented the SMase-evoked alterations. Together the data suggest the interesting possibility that PAF increases the overall turnover of SM. In summary, the present data demonstrate that PAF is able to regulate the cellular ceramide levels in brain by accelerating the SM cycle.

Endothelin stimulates tyrosine phosphorylation of p125FAK and p130Cas in rat cerebral cortex

Stimulation of rat cerebral cortex with endothelin-1 (ET-1) caused an increase in the tyrosine phosphorylation of several proteins. Two of these phosphoproteins were identified by the immunoprecipitation assays as being the focal adhesion kinase p125FAK and crk-associated substrate p130Cas. This effect was time- and dose-dependent, with an EC50 value of 3.9 x 10(-8) M. In addition, the cerebral cortex ET receptor subtype involved in this action was determined by using BQ-123 and BQ-788, which are ET(A) and ET(B) receptor antagonists respectively. Our results indicate that the ET-1 effect on protein tyrosine phosphorylation occurred through ET(B) receptors. The requirement for extracellular Ca2+ on ET-1 action was also studied. ET-1-stimulated tyrosine phosphorylation of both p125FAK and p130Cas was abolished in the absence of external Ca2+ or in the presence of nimodipine, a Ca2+ channel-blocker. These results suggest that the ET-1-stimulated protein tyrosine phosphorylation was secondary to Ca2+ influx through the dihydropyridine Ca2+-channel. In slices where protein kinase C was inhibited, ET-1-stimulated tyrosine phosphorylation of both proteins was reduced. These results indicate that ET-1 modulates the tyrosine phosphorylation of specific proteins, which may be involved in adhesion processes in the brain.

Involvement of sphingolipids in the endothelin-1 signal transduction mechanism in rat brain

In cerebral cortex, endothelin-1 (ET-1) evoked a decrease of 40% in sphingomyelin (SM) levels together with an increase in both ceramide and glycosphingolipid (GSL) levels (100 and 56% respectively). These facts indicate that ET-1 increases sphingomyelinase activity and, possibly, activates the synthesis of GSL. By contrast, in cerebellum ET-1 seems to activate the hydrolysis of both SM and GSL, since the peptide evoked a decrease (near 30%) of their levels concomitantly with an increased production of ceramides (200%). These ceramides are clearly different from those produced in cerebral cortex which come from the SM hydrolysis only. It is suggested that ETB receptor subtype is involved in these responses.

Endothelin stimulates phosphoinositide hydrolysis and PAF synthesis in brain microvessels

Treatment of brain microvessels with the three endothelin (ET) isoforms resulted in an increase of phosphoinositide turnover by activation of phospholipase C in a dose- and time-dependent manner. Both ET-1 and ET-2 are maximally effective, whereas the effect evoked by ET-3 was smaller. Concomitantly, there was an enhanced production of a platelet-activating factor (PAF)-like material. This was identified by standard and biological probes in platelets, such as induction of aggregation, phosphatidic acid (PA) production, increase of endogenous protein phosphorylation, and reversal of these responses by a PAF antagonist. The effects evoked by endothelins on phosphoinositide metabolism and PAF production were, to a certain extent, dependent on the presence of extracellular Ca2+. In addition, ET induced changes in Ca2+ dynamics, evoking an initial and rapid intracellular mobilization and influx of Ca2+ and, later, a maintained Ca2+ influx. These findings contribute to the understanding of the pathophysiological role of ET in the blood-brain barrier (BBB).

Further studies on the mechanism of action of substance P in rat brain, involving selective phosphatidylinositol hydrolysis

We have suggested that substance P, in cerebral cortex, causes a phosphatidylinositol (PI) breakdown by a dual mechanism suggesting the involvement of either phospholipase A2 or phospholipase C. We have presently characterized further these effects. Substance P (65 pM) provoked an increase in lysoPI concomitant with a decrease in PI level. This finding confirms the involvement of phospholipase A2 activation. To study the involvement of phospholipase C in the action of higher doses (0.65 microM) of the peptide, we used pulse-chase experiments (where phospholipid depletion was monitored) and short-term 32P-labeled slices (where phospholipid synthesis was studied). Substance P evoked an acceleration of both hydrolysis and resynthesis of PI as early as 15 s. A prolonged exposure (30 min) resulted in stimulation of PI hydrolysis without subsequent resynthesis. The peptide did not cause any effect on inositol 1,4-bisphosphate and inositol 1,4,5-trisphosphate. These alterations in PI metabolism take place simultaneously with a generation of diacylglycerol which showed two maxima at both indicated times.

Involvement of calcium in phosphoinositide metabolism in the blood-brain barrier

The Ca2+ effect on phosphoinositide metabolism in the blood-brain barrier was studied by using rat cerebral microvessels prelabelled with either [32P]orthophosphate or myo-[3H]inositol and stimulated with Ca2+ ionophore A23187. In radioactivity steady-state conditions, addition of ionophore caused a rapid and marked loss of labelling in both phosphatidylinositol-4-phosphate (PIP) and phosphatidylinositol-4,5-bisphosphate (PIP2), without significant alterations in phosphatidylinositol (PI) and phosphatidic acid (PA) labelling. These facts were accompanied by a rise in labelling of both inositol 1-monophosphate (IP) and inositol 1,4-bisphosphate (IP2), but not in inositol 1,4,5-trisphosphate (IP3). In addition, a Ca(2+)-dependent inhibition of phosphoinositide kinase activities from isolated membranes was also found. These data suggest that elevated intracellular Ca2+ level evokes a PIP and PIP2 hydrolysis by phosphodiesterasic and phosphomonoesterasic activities respectively, and also partially inhibits the synthesis of these phosphoinositides. Our results constitute evidence that a reciprocal control mechanism between polyphosphoinositide metabolism and mobilization of Ca2+ exists in the blood-brain barrier.

Platelet-activating factor inhibits (Na+,K+) ATPase activity in rat brain

In the present study, experiments were conducted to determine the effect of platelet-activating factor (PAF) on (Na+,K+)-ATPase in rat cerebral cortex. PAF, but not lysoPAF, inhibited (Na+,K+)ATPase activity, in a dose- and time-dependent manner, 10(-7) to 10(6) M being the most effective dose. These effects were abolished in the presence of PCA-4248, a PAF antagonist, indicating that the PAF effect may be mediated by its specific membrane receptors. Omission of external calcium caused an increase in the basal activity and abolished the PAF effect on (Na+,K+)ATPase. The present study demonstrates that PAF inhibits (Na+,K+)ATPase activity in the cerebral cortex and suggests that PAF released during certain pathological conditions, such as ischemia, may act on ATPase. This could be one possible mechanism of PAF action that needs further attention.

Insulin-like growth factor I modulates voltage-dependent Ca2+ channels in neuronal cells

Insulin and insulin-like growth factors are neuroactive peptides. We investigated the effect of insulin-like growth factor I (IGF-I) on Ca2+ channel currents in 108CC15 neuroblastoma x glioma (N x G) cells and a possible role of protein kinase C (PKC). Whereas the native IGF-I enhanced the Ca2+ channel current density in N x G cells, the boiled IGF-I had no effect. The effect of IGF-I occurred after 1-2 h incubation and reversed within 24 h. Ca2+ channel currents recorded in control cells were mainly of a low-threshold fast inactivating type and showed a mean density of 5.9 +/- 0.3 pA/pF. Current density in cells incubated with IGF-I (0.2 micrograms/ml) for 2 h increased to 9.2 +/- 0.8 pA/pF. Ca2+ channel currents in cells treated with IGF-I showed an enhanced amount of a high-threshold slowly inactivating Ca2+ current type sensitive to the dihydropyridine isradipine and the snail toxin omega-conotoxin. The effect of IGF-I was suppressed by coincubation with the PKC inhibitors 1-(5-isoquinolinylsulfonyl)-2-methyl-piperazine (H-7) and staurosporin which were both without effect on current density in control cells. Whereas the inactive phorbol ester phorbol 12-myristate 13-acetate (PMA) failed to modulate Ca2+ channels in N x G cells, stimulation of PKC by the active phorbol ester PMA mimicked the effect of IGF-I. The effects of IGF-I and phorbol ester were not additive. Our data suggest an intracellular mechanism dependent on PKC and we propose a physiological relevance of the observed Ca2+ channel modulation by IGF-I in the neuroactivity of the peptide.

Inhibitory effect of insulin and cytoplasmic factor(s) on brain (Na(+) + K+) ATPase

(Na+ + K+)ATPase activity in cerebral cortex was modulated by insulin action depending on the Mg2+ concentration. Thus, in homogenates in the presence of 1-3 mM Mg2+, insulin stimulated the enzyme, whereas in the presence of 4-6 mM Mg2+ inhibition was observed. Exposure of synaptosomal membranes to the soluble fraction resulted in inhibition of ATPase activity in a dose-dependent manner. The inhibitory effect of insulin was regulated by a cytoplasmic factor in a dose-dependent manner. Similar variations to those obtained with a crude synaptosomal fraction were obtained by using a partially purified ATPase. These results indicated the importance of soluble factors in the modulation of ATPase by insulin and add more evidence in support for a role of insulin as a neuromodulator.

PAF-induced activation of polyphosphoinositide-hydrolyzing phospholipase C in cerebral cortex

The action of platelet-activating factor (PAF) on phosphoinositide hydrolysis was studied in rat brain slices. PAF produced a significant increase of 32P incorporation into phosphoinositides and phosphatidic acid (PA), in a dose- and time-dependent manner. Concomitantly, an increase of inositol phosphates and diacylglycerol (DAG) production was observed. Both inositol bisphosphate (IP2) and inositol trisphosphate (IP3) were detected as early as 5 s and they returned immediately to basal levels; concomitantly, formation of inositol monophosphate (IP) was detected. These findings demonstrated that PAF causes a rapid hydrolysis of polyphosphoinositides in cerebral cortex by a phospholipase C-dependent mechanism followed by subsequent resynthesis.