It is now widely recognized that the activation of phospholipase C by specific agonists leads to the formation of two second messengers: (1) inositol trisphosphate, which releases Ca2+ from the endoplasmic reticulum to the cytosol and (2) 1,2- diacylglycerol, which stimulates protein kinase C. In the past few years, GTP-binding proteins have been associated with the regulation of phospholipase C. However, the identity of the GTP-binding protein involved and the type of association with phospholipase C is not yet known. It is now recognized that there are two types of phospholipase C enzymes: (a) a soluble enzyme that has been characterized in several tissues and does not preferentially hydrolyze polyphospholinositides and (b) membrane-bound enzymes that are coupled to the receptors, specifically hydrolyzing polyphosphoinositides and activated by membrane guanine nucleotide-binding proteins. Recent reports have tried to assess the involvement of GTP-binding proteins in the agonist-induced stimulation of phospholipase C, and various related aspects have been reported. These are concerned with: (a) detection of various GTP-binding proteins in platelets, (b) the effects of known inhibitors of GTP-binding proteins such as GDPgS or pertussis toxin on the agonist-induced stimulation of phospholipase C, (c) the direct effects of stimulators of GTP-binding proteins such as GTP, GTP-analogs and fluoride on phospholipase C activity, (d) the possible association of GTP-binding proteins to cytosolic phospholipase C that would then lead to degradation of the membrane-bound inositides and (e) cytosolic phospholipase C response to the activation of cell surface receptors. The emerging information has had contradictory conclusions. (1) Pretreatment of saponin-permeabilized platelets with pertussis toxin has been shown to enhance and to inhibit the thrombin-induced activation of phospholipase C. Therefore, it is not clear if a G protein that is affected by pertussis toxin in a manner similar to Gi or Go plays a central role in activation of phospholipase C. (2) Studies on the effect of GDPβ;S are also conflicting indicating that there may be GTP-independent and/or -dependent pathways for the activation of phosphoinositide hydrolysis. (3) A cytosolic phospholipase C is activated by GTP, and it has been advanced that this activity might trigger the hydrolysis of membrane-bound inositides. A cytosolic GTP-binding protein might be involved in this action, and it is speculated that an α-subunit might be released to the cytoplasm by a receptor-coupled mechanism to activate phospholipase C. However, no direct evidence exists to support this conclusion. Moreover, the exact contribution of phospholipase C from the membranes or the cytosol to inositide hydrolysis in response to cellular agonists and the relationship of those activites to membrane-bound or soluble GTP-binding proteins are unknown. Our results indicate that the stimulation of phospholipase C in platelets by GDPβS and thrombin are affected differently by GDPβS. GDPgSinhibits the formation of inositol phosphates produced by GTPγS but not that induced by thrombin. Thrombin, therefore, can directly stimulate phospholipase C without the involvement of a “stimulatory” GTP-binding protein, such as Gs, for the agonist stimulation of adenylate cyclase. However, an “inhibitory” GTP-binding protein might have some influence on thrombin-stimulated phospholipase C, since in the presence of GDPγS thrombin produces a more profound stimulation of phospholipase C.This “inhibitory” GTP-binding protein might be ADP-ribosylated by pertussis toxin because pertussis toxin can also enhance thrombin action on phospholipase C activity. Therefore, phospholipase C that responds to thrombin could be different from the one that responds to GTPγS. Cytosolic phospholipase C can be activated by GTP or GTP analogs, and the one that responds to thrombin should be coupled to the receptors present in the plasma membrane. The initial action of thrombin is to directly activate the plasma membrane-bound phospholipase C and the mechanism of this activation is probably related to the proteolytic action of thrombin or the activation of platelet proteases by thrombin. In agreement with this, trypsin can also directly activate platelet phospholipase C and, subsequently, GTPyS produces further activation of phospholipase C. If these two mechanisms are operative in platelets, the inhibition of cytosolic phospholipase C by GDPβS would allow a larger fraction of inositides for degradation of the thrombin-stimulated phospholipase C, as our results show.
Current Topics in Heterotrimeric GTP-binding Proteins. Structure and function of heterotrimeric G-proteins.
