Polyphosphoinositide hydrolysis

Just as neutrophils respond to fMLP [91], macrophages and mast cells exposed to IgE and other stimuli show activation of PhL C, formation of IP3 and mobilization of Ca2+ [102]. This is followed by degranulation with release of lysosomal enzymes and histamine. These ligand-induced responses are blocked by PTX. Further, introduction of GTPVS into the cells causes degranulation that is blocked by neomycin, a substance that inhibits polyphosphoinositide hydrolysis by PhL C and interferes with the action of IP3 to mobilize Ca2+ [100]. The data point to the existence of a PTX-sensitive Gp (PTX-sensitive Gp). 

Receptor-mediated stimulation of polyphosphoinositide hydrolysis is discussed in detail by Guy and Kirk in Chapter 2 of this volume (see also Refs. 15-17). These... 

NAKASHMA S, TOHMATSU T, HATTORI H, OKANO Y, NOZAWA Y. Inhibitory action of cyclic GMP on secretion, polyphosphoinositide hydrolysis and calcium mobilization in thrombin-stimulated human platelets. Biochem Biophys Res Commun 135 1099-1104,1986. 

Activation of the a,-ARs causes polyphosphoinositide hydrolysis catalyzed by phospholipase C via pertussis toxin-insensitive G proteins in almost all tissues where this effect has been examined. Recent studies have shown that other signalling pathways can be activated upon a,-AR stimulation such as phosphatidylcholine hydrolysis and phospholipase A2 (for review, see Graham et al., 1996). However, the comparison among different a,-AR-media-ted responses in various tissues has not allowed to assess any conclusive signalling differences among distinct a,-AR subtypes. 

Casabona, G., Knopfel, T., Kuhn, R., Gasparini, E, Baumann, R, Sortino, M.A., Copani, A., and Nicoletti, E (1997) Expression and coupling to polyphosphoinositide hydrolysis of group I metabotropic glutamate receptors in early postnatal and adult rat brain, Eur. J. Neurosci., 9(1), 12-17. 

Five distinct muscarinic receptors have been identified (Mi-Mj) by molecular cloning, and there is evidence to suggest that at least four of them (M,-M4) are of functional importance within the airways (Barnes 1993). Mj, M3, and M5 receptors stimulate polyphosphoinositide hydrolysis, leading to an increase in inositol monophosphate. This occurs predominantly through the stimulation of pertussis... 

Increases of intracellular free Ca concentratirm that are initiated by changes in plasma membrane potential or receptcH -stimulated polyphosphoinositide (Pis) hydrolysis are astonishingly complex. The key to understanc g these complex Ca signals lies in undmtanding the interactions betweoi different cellular pools fi om whidi Ca rapidly enters the cytosol and their transporting systems (74-76). These cellular events are summarized in Figure 3. 

Figure 7. Schematic of a receptor with intrinsic enzymatic activity. Ligand mediated activation results in phosphorylation of phospholipase C (y isoform) leading to hydrolysis of polyphosphoinositides (PIP2). Dia-cyl-glycerol (DAG) and inositol-triphosphate (IP3) are formed. 1P3 increases the release of stored calcium. Calcium together with DAG can activate PKC signalling pathway. Association of this type of receptor with adapter proteins results in activation of Ras/Raf/ERK signalling or in activation of PI3K/Akt signalling.

The detection of labelled polyphosphoinositides were the experimental prerequisite for the search of an auxin effect on the membrane-bound inositol phospholipids [9]. Actually, the kinetics of the lipid phosphorylation in the absence or presence of 1 pM lAA were analyzed. In comparison with the control a very rapid reduction of the p2p]-label was observed in the phospholipid fraction in the presence of auxin (fig.1). This decrease of [ 2pj. a ei vvas not originated from an auxin-mediated inhibition of the lipid phosphorylation reaction but it was really caused by an auxin-mediated hydrolysis of [22p]-iabelled lipids, since p2p]-iabelled inositol polyphosphates (IPx) could be detected in the aqueous extracts of the same assays after anion exchange chromatography (fig.1). The kinetics of the l/ A-mediated release of the p2p]-labelled inositol compounds IP3 and IP2 correspond well with the kinetics of the loss of the [22p]-iabel in the lipid fraction. The auxin-dependent reactions exhibit also a dose response relationship either by a continuous loss of [32p]-iabel of the phospholipids or by a stimulated release of p2p]-iabelled IPx with increasing lAA concentrations as demonstrated previously [9,11]. 

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