Phospholipase C. activation

The NHR contains also the conserved Calcineurin docking site, PxlxIT, required for the physical interaction of NEAT and Calcineurin. Dephosphorylation of at least 13 serines residues in the NHR induces a conformational change that exposes the nuclear localization sequences (NLS), allowing the nuclear translocation of NEAT. Rephosphorylation of these residues unmasks the nuclear export sequences that direct transport back to the cytoplasm. Engagement of receptors such as the antigen receptors in T and B cells is coupled to phospholipase C activation and subsequent production of inositol triphosphate. Increased levels of inositol triphosphate lead to the initial release of intracellular stores of calcium. This early increase of calcium induces opening of the plasma membrane calcium-released-activated-calcium (CRAC) channels,... 

The a subunits and the Py complex have actions independent of those on adenylyl cyclase (see Figure 43-4 and Table 43-3). Some forms of tt stimulate channels and inhibit Ca channels, and some ttj molecules have the opposite effects. Members of the G, family activate the phospholipase C group of enzymes. The py complexes have been associated with channel stimulation and phospholipase C activation. G proteins are involved in many important biologic processes in addition to hormone action. Notable examples include olfaction (oColf) <1 vision (aj. Some examples are listed in Table 43-3. GPCRs are implicated in a number of diseases and are major targets for pharmaceutical agents. 

Gai3 41,000 Ca2+ channel (inhibition) PI-Phospholipase C (activation) Phospholipase A2... 

These effects of ATP are blocked by pertussis toxin, and so the putative ATP receptor is G-protein linked. ATP addition results in phospholipase C activation, which may be detected as increased inositol phosphate metabolism and subsequent elevations in cytosolic free Ca2+. Purinergic receptors on many types of cells are classified as type Pi or P2. Neutrophils possess P2-type receptors, which are activated by ATP and ADP, and also Pi-type receptors, which are activated by adenosine. Occupancy of P2-type receptors enhances fMet-Leu-Phe-mediated effects, whilst occupancy of Pi-type receptors has the opposing effect. Some pharmacological evidence suggests that the P2-type receptor on neutrophils is distinct from the P2X and P2y subtypes that have been described in other cell types. 

Stutchfield, J., Cockcroft, S. (1990). Undifferentiated HL60 cells respond to extracellular ATP and UTP by stimulating phospholipase C activation and exocytosis. FEBS Lett. 262,256-8. 

The G-protein that has been termed Gp, and that is linked to phospholipase C activation, may in fact be Gaj 2 or Gc. 3. Ga is designated as the G-protein responsible for activation of phospholipase A2, which results in arachidonic acid release. Some experimental evidence indicates that, at least in HL-60 cells, different agonists can preferentially activate different phospholipases, and some of these are responsible for the activation of secretion. In neutrophils, the two pertussis-toxin-sensitive Ga-proteins (Gaj-2 and G j 3) have been identified by peptide mapping of proteolytic digests of the proteins, by peptide sequencing and by immunoblotting. Complementary-DNA clones for the mRNA of these two molecules have also been isolated from an HL-60 cDNA library. Gai-2 is five to ten times more abundant than Gai.3, the former component comprising 3% of the total plasma membrane proteins. It is possible that these two different Ga-subunits are coupled to different phospholipases (e.g. phospholipases C and D). Pertussis toxin inhibits the secretion of O2 after stimulation of neutrophils by fMet-Leu-Phe, but pertussis-toxin-insensitive G-proteins are also present in neutrophils. These may be members of the Gq family and may be involved in the activation of phospholipase Cp (see 6.3.1). 

Phospholipid turnover also takes place in an asymmetric manner. The enzymes responsible for phospholipid turnover in response to receptor-mediated phospholipase c activation are active from the cytoplasmic surface of the membrane. Likewise, diacylglycerol kinases converting the product of phospholipase c back into the key intermediate of phospholipid biosynthesis, phosphatidic acid, are also located on the cytoplasmic smface of the membrane (Sanjuan et al., 2001). 

Prostacyclin, a potent inhibitor of platelet aggregation, is derived from metabo-lisation of arachidonic acid by endothelial cells, and shear stress increases its production rate [12]. It is postulated that this effect is due to perturbations of the permeability of the plasma membrane changing the cytosolic Ca + content and leading to an increase in phospholipase C activity (through the by-passing of the receptor requirement), which contributes to a higher production of arachidonic metabolites. 

Berg KA, Stout BD, Maayani S, Clarke WP. Differences in rapid desensitization of 5-hydroxytryptamine2A and 5-hydroxytryptamine2C receptor-mediated phospholipase C activation. J Pharmacol Exp Ther 2001 299 593-602. 

Disturbances in lipid metabolism occur in the skin in psoriasis. Increased phospholipase A2 activity is seen in lesional and nonlesional skin, while phospholipase C activity is elevated in lesional skin.183-184 Increased elongase activity is also observed in psoriatic epidermis. A local increase in AA occurs, and this appears to be preferentially metabolized by the lipoxygenases, resulting in a marked increase in 12-HETE and LTB4, while there appears to be a relative or absolute reduction in metabolism by the COX pathway.185 Leukotriene B4 is a very potent chemoattractant, and topical application causes epidermal hyperproliferation in addition to neutrophil microabscesses.186 A defective transmembranous cell-signaling system is also suggested by elevation of both IP3 and DAG in the psoriatic plaque.187... 

Two laboratories reported activation of a cytosolic phospholipase C activity from platelets by guanine nucleotides [126,127] raising the possibility of existence of not only membrane bound Gps but also of a cytosolic Gp. 

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