Phosphatidylinositol bisphosphate, hydrolysis

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. 

Activation of Neutrophils Is Similar to Activation of Platelets Involves Hydrolysis of Phosphatidylinositol Bisphosphate... 

Berstein, G., Blank, J. L., Smrcka, A. V. etal. Reconstitution of agonist stimulated phosphatidylinositol 4-5 bisphosphate hydrolysis using purified mi muscarinic receptor, Gq/11, and phospholipase C-Pl. /. Biol. Chem. 267 8081-8088, 1992. 

When the receptor interacts with its associated G protein, the conformation of the guanine-nucleotide-binding site is altered. The subunits then dissociate, and a phosphatidylinositol-specific phospholipase C (PI-PLC) is activated [5]. The subsequent hydrolysis of phosphatidylinositol bisphosphate then produces inositol triphosphate (IP3) and diacylglycerol (DAG), which are known to be secondary messengers. For example, the water soluble IP3 is released into the cell where its ultimate targets are the calcium storage organelles from which Ca2+ is released [3]. The presence of DAG in cells is known to activate the cellular enzyme protein kinase C (PKC) [6, 7], which phosphorylates a number of cellular... 

Figure 6.7. Phosphatidylinositol 4,5-bisphosphate hydrolysis by phospholipase C. Occupancy of receptors (R) results in exchange of bound GDP for GTP on the a-subunit of a het-erotrimeric G-protein. The a-subunit then dissociates from the fi- and y-subunits and activates phospholipase (PLC). This enzyme is calcium dependent and, upon activation, can hydrolyse phosphatidylinositol 4,5-bisphosphate (PIP2). The products of this hydrolysis are inositol 1,4,5-trisphosphate (Ins 1,4,5-P3), which is released into the cytoplasm, and diacylglycerol (DAG), which remains in the membrane. The DAG is an activator of protein kinase C, which moves from the cytoplasm to the membrane, where it forms a quaternary complex with DAG and Ca2+. 

The phosphorylated phospholipid, phosphatidylinositol bisphosphate, is present in cell membranes. On hydrolysis by a phospholipase, it produces two products, inositol trisphosphate and diacylglycerol (Figure 11.25), as follows ... 

Figure 11.25 Structure of phosphatidylinositol bisphosphate and position of hydrolysis by phospholipase. Phosphatidylinositol 4,5-bisphosphate (PIP2) is a component of the inner leaflet of a cell membrane from which inositol 1,4,5-trisphosphate (IP3) is released by hydrolysis catalysed by phospholipase where indicated. The diacylglycerol is retained in the membrane.

Sternweiss PC, Smrcka AV, Gutowski S (1992) Hormone signalling via G-protein regulation of phosphatidylinositol 4,5-bisphosphate hydrolysis by Gq. Philos Trans R Soc Lond B Biol Sci 336(1276) 35 U... 

Products of phosphatidylinositol 4,5-bisphosphate hydrolysis and their roles as second messengers in the cell... 

Vallar L, Muca C, Magni M, Albert P, Bunzow J, Meldolesi J, Civelli O (1990) Differential coupling of dopaminergic D2 receptors expressed in different cell types. Stimulation of phosphatidylinositol 4,5-bisphosphate hydrolysis in LtK-fibroblasts, hyperpolarization, and cytosolic-free Ca2+ concentration decrease in GH4C1 cells. J Biol Chem 265 10320-10326. 

Perera, N.M., Michell, R.H. and Dove, S.K., 2004, Hypo-osmotic stress activates Plclp-dependent phosphatidylinositol 4,5-bisphosphate hydrolysis and inositol hexakisphosphate accumulation in yeast. J. Biol. Chem. 279 5216-5226. 

A signal transduction pathway in which hormone binding to a cell surface receptor induces phospholipase C to catalyze the hydrolysis of phosphatidylinositol bisphosphate to yield the second messengers inositol... 

Structure of phosphatidylinositol 4,5-bisphosphate. Hydrolysis at the dotted line is catalyzed by phospholipase C and releases diacylglycerol and inositol 1,4,5-trisphosphate, which are the intracellular second messengers. 

Thomas, P. and Meizel, S. (1989), Phosphatidylinositol 4,5-bisphosphate hydrolysis in human sperm stimulated with follicular fluid or progesterone is dependent upon Ca influx. Biochem. J. 264 539-546,... 

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. 

Answer C. Muscarinic receptors present in bronchiolar smooth muscle are of the M3 subtype coupled via Gq proteins to phospholipase C. Activation of this enzyme causes hydrolysis of phosphatidylinositol bisphosphate, with release of IP3 and DAG (the latter activates protein kinase C). Decreased formation of cAMP mediated via a G protein occurs with activation of M2 receptors such as those in the heart. Cation channel opening occurs in response to activation of nicotinic receptors. 

It is now generally accepted that both of the products of phosphatidylinositol 4,5-bisphosphate hydrolysis can function as intracellular second messengers. 1,2-Diacylglycerol can affect a variety of intracellular processes by activation of protein kinase C [ 148, 229, 230]. Inositol 1,4,5-trisphosphate, on the other hand, has been shown to release calcium ions from non-mitochondrial stores in a number of peripheral tissues and may thus be the link between the receptor and the intracellular calcium store in many pharmacological responses [231-233]. Furthermore, it remains a possibility that inositol phospholipid hydrolysis may also have a r61e in calcium gating [221,234]. If inositol phospholipid metabolism is closely coupled to receptor-mediated calcium mobilization, then this response may be a more general consequence of H,-receptor stimulation than other H,-responses. 

Einspahr KJ, Thompson GA Jr. Transmembrane signalling via phosphatidylinositol 4,5-bisphosphate hydrolysis in plants. Plant Physiol 1990 93 361-366. 

It is now known that the initial event following binding of the agonist to its receptor (usually within 30 secs) is a rapid hydrolysis of phosphatidylinositol bisphosphate by a specific phospholipase C. This produces inositol 1,4,5-triphosphate and diacylglycerol, both of which have second messenger functions. 

The hormone-sensitive inositol lipid pool is confined to the plasma membrane where most of the phosphatidylinositol bisphosphate is located. Although the other inositol lipids have been suggested to be used for cell signalling, it is phosphatidylinositol bisphosphate which is by far the most important. Increased hydrolysis of this lipid is caused by a host of stimuli including neurotransmitters, releasing factors, hormones, growth factors, fertilization and light. 

TRPVl, also known as the capsaicin- or vanilloid-receptor, is a nonselective cation channel expressed e.g., in neurons of the dorsal root and trigeminal ganglions, which integrates multiple pain-producing stimuli including heat, protons, capsaicin, and resiniferatoxin. In addition, TRPVl currents can be activated by ananda-mide, protein kinase C (PKC), and by hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2). 

Covalent regulation. Following occupation and activation of the M2 acetyl choline receptors, phospholipase C (PLC), is activated and both inositol (l,4,5)-trisphosphate (IP3), and diacylglycerol (DAG), are formed by hydrolysis of phosphatidylinositol (4,5)-bisphosphate (PIP2). 

The family of heterotrimeric G proteins is involved in transmembrane signaling in the nervous system, with certain exceptions. The exceptions are instances of synaptic transmission mediated via receptors that contain intrinsic enzymatic activity, such as tyrosine kinase or guanylyl cyclase, or via receptors that form ion channels (see Ch. 10). Heterotrimeric G proteins were first identified, named and characterized by Alfred Gilman, Martin Rodbell and others close to 20 years ago. They consist of three distinct subunits, a, (3 and y. These proteins couple the activation of diverse types of plasmalemma receptor to a variety of intracellular processes. In fact, most types of neurotransmitter and peptide hormone receptor, as well as many cytokine and chemokine receptors, fall into a superfamily of structurally related molecules, termed G-protein-coupled receptors. These receptors are named for the role of G proteins in mediating the varied biological effects of the receptors (see Ch. 10). Consequently, numerous effector proteins are influenced by these heterotrimeric G proteins ion channels adenylyl cyclase phosphodiesterase (PDE) phosphoinositide-specific phospholipase C (PI-PLC), which catalyzes the hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2) and phospholipase A2 (PLA2), which catalyzes the hydrolysis of membrane phospholipids to yield arachidonic acid. In addition, these G proteins have been implicated in... 

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.

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