Phosphatidylinositol-4,5-bisphosphate 3-kinase pathway

PIKfyve is a mammalian class III phosphatidylinositol phosphate kinase which synthesizes phosphatidylinositol 3,5-bisphosphate. The physiological role of this pathway is not yet clear. The PI3K inhibitor PI-103 (85) also inhibits PIKfyve. A structural analog, YM201636 (86), was shown to be... 

Progress made in recent years has revealed the central role the phosphoinositide 3-kinase pathway plays in the establishment and the maintenance of cell polarity (Figure 7) [246]. The substrate for the Class I phosphoinositide 3-kinase [223] involved in chemotaxis is the membrane lipid phosphatidylinositol-(4, 5) bisphosphate, or PI(4,5)P2, which becomes phosphorylated at its 3 position to phosphatidylinositol-(3, 4, 5) trisphosphate, or PI(3, 4, 5)P3. A second 3-phosphoinositide, PI(3, 4)P2, can be formed by the dephosphorylation of PI(3, 4, 5)P3 at... 

We have already seen how the two products of phosphatidylinositol bisphosphate hydrolysis can interact through their cellular effects. A good example is liver, where the inositol 1,4,5-triphosphate/Ca pathway controls the activity of phosphorylase kinase whereas the diacylglycerol/ protein kinase C pathway switches off glycogen synthetase. 

FIGURE 14-6 Main signaling pathways for histamine receptors. Histamine can couple to a variety of G-protein-linked signal transduction pathways via its four different receptors. The Hj receptor activates the phosphatidylinositol turnover via Gq/11 proteins. The other receptors either positively (H2 receptor) or negatively (H3 and H4 receptor) regulate adenylyl cyclase activity via Gs and GUo protein activation respectively. Several additional signaling pathways have been described, which are not shown. Abbreviations PfP2, phosphatidylinositol 4,5-bisphosphate PIC, phospholipase C AC, adenylyl cyclase ATP, adenosine triphosphate cAMP, cyclic AMP PKC, protein kinase C PICA, protein kinase A. 

Figure 6.9. Pathways of inositol phosphate metabolism. Ins 1,4,5-P3, generated via the hydrolysis of phosphatidyl 4,5-bisphosphate by phospholipase C, can be metabolised by a kinase (to generate Ins 1,3,4,5-P4) or via a phosphatase (to yield Ins 1,4-P2). These products can be metabolised further to produce inositol, which itself may be sequentially phosphory-lated to regenerate phosphatidylinositol 4,5-bisphosphate.

After activation of the TCR, there is induction of Src family tyrosine kinase (p56lek), which phosphorylates phospholipase Oyl. This is followed by the hydrolysis of phosphatidylinositol 4,5-bisphosphate, resulting in the production of diacyl-glycerol (DAG) and inositol trisphosphate (IP3). Protein kinase C is activated by DAG, which phosphorylates Ras. Ras is a GTPase and its phosphorylation induces Raf and initiation of MAP kinase signaling pathway. IP3 is involved in calcium-dependent activation of IL-2 gene expression via nuclear factor of activated T cells (NFAT). 

The Ca2+-phosphoinositide signaling pathway. Key proteins include hormone receptors (R), a G protein (G), a phosphoinositide-specific phospholipase C (PLC), protein kinase C substrates of the kinase (S), calmodulin (CaM), and calmodulin-binding enzymes (E), including kinases, phosphodiesterases, etc. (PIP2, phosphatidylinositol-4,5-bisphosphate DAG, diacylglycerol. Asterisk denotes activated state. Open arrows denote regulatory effects.)... 

Fig. 1. The major inositol lipids. Phosphatidylinositol (Ptdlns), the major membrane inositol phospholipid, is phosphorylated to phosphatidylinositol 4-phosphate (Ptdlns 4-P) by a phosphatidylinositol kinase (a). Ptdlns 4-P is further phosphorylated to phosphatidylinositol 4,5-bisphosphate (Ptdlns 4,5-P2) by a phosphatidylinositol 4-phosphate kinase (b). Ptdlns 4,5-P, is converted back to Ptdlns 4-P by phosphatidylinositol 4,5-bisphosphate phosphomonoesterase (c) and then to Ptdlns by phosphatidylinositol 4-phosphate monoesterase (d). The pathway of phosphorylation and dephosphorylation constitutes a futile cycle and is only interrupted by agonist-induced hydrolysis of Ptdlns 4,5-P,.

Fig. 2. A schematic representation of some of the mechanisms by which Car fluxes across the plasma membrane are regulated. In the plasma membrane (the striped area) there are both influx (=>) and energy-dependent ( ) efflux pathways. Two mechanisms by which Ca2+ influx can be increased are via the actions of the intracellular messengers inositol 1,3,4,5-tetrakisphosphate, and cAMP generated via activation of specific classes of surface receptors (R, and R2) linked to specific N proteins which activate either phosphatidylinositol 4,5-bisphosphate (PIP,) hydrolysis or adenylate cyclase (AC). Additionally, influx can be increased either by a direct receptor-coupled event or by a membrane depolarization (not shown). A rise in the Ca2+ concentration in the domain just beneath the plasma membrane, [Ca2+Isin, can lead to an activation of the Ca2+ pump either via a direct calmodulin (CaM)-dependent mechanism, or indirectly via the activation of protein kinase C (CK). Additionally, in some cells, an increase in cGMP concentration also increases Ca2+ efflux (not shown), and in still others cAMP may stimulate Ca2 efflux.

Gq protein-coupled phosphatidylinositol-Ca " " pathway. The binding of a hormone at a specific receptor site results in the activation of G-protein which, in turn, activates phospholipase C via Gga-OTP-protein. The action of phospholipase C on phosphatidylinositol 4,5-bisphosphate (PIP2) yields inositol trisphosphate (IP3) and diacylglycerol (DAG) which, along with phosphatidylserine (PS), activates protein kinase C. IP3 binds to receptors on SER, releasing Ca " which, in turn, activates another set of protein kinases. -I-, Activation. 

Figure 7, Phospholipase C-mediated hydrolysis of phosphatidyl inositol 4,5-bisphos-phate. Phosphoinositide-specific phospholipase C is activated during cellular stimulation and mediates the hydrolysis of phosphatidylinositol 4,5-bisphosphate. The two products of this reaction, DAG and IP3, are both intracellular second messengers. Thus, a single hydrolytic reaction initiates a bifurcating pathway of signal transduction mediated by protein kinase C activation and calcium mobilization, respectively.

Gene expression and cellular proliferation are induced by the concomitant activation of protein kinase C and mobilization of Ca ". The activation of both of these pathways by phosphatidylinositol 4,5-bisphosphate hydrolysis, is necessary for DNA synthesis in macrophage-depleted human peripheral lymphocytes. However, the mechanisms regulating rapid cell proliferation are complex as demonstrated by their dependence on the combined presence of both activated protein kinase C and growth factors. 

As discussed in Chapter 13, hormonal stimulation of some G protein-coupled receptors leads to activation of the (3 isoform of phospholipase C (PLCp). This membrane-associated enzyme then cleaves phosphatidylinositol 4,5-bisphosphate (PIP2) to generate two important second messengers, 1,2-diacylglycerol (DAG) and inositol 1,4,5-trisphosphate (IP3). Signaling via the IP3/DAG pathway leads to an increase in cytosolic Ca " and to activation of protein kinase C (see Figure 13-29). 

Phosphatidylinositol kinase is involved in the PI turnover pathway and may be important for the regulation of phosphatidylinositol 4,5-bisphosphate levels. Therefore, we... 

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