La comunicación en el desarrollo de las habilidades sociales

This transduction process was first recognised by Hokin and Hokin [175,176]. However, the elucidation of the process came about in 1980s [177], when phosphatidylinositol (PI) hydrolysis, calcium mobilization and cellular activation were linked mechanistically. In the last ten years this area o f research has developed rapidly.

Receptor-mediated phosphoinositide breakdown is catalyzed by agonist activation of phospholipase C (PLC). There are at least four or five types of PLC in mammalian tissues (a, p, y, Ô and £) activated by the Gp-linked receptors, such as muscarinic M2 receptors, and gastrin/cholecystokinin (CCK) receptors [178-180]. Three types of Gp proteins are involved in this process and have been described according to their sensitivity to pertussis and cholera toxins [181-185].

39

Each PLC has similar catalytic properties and hydrolyses three, common phosphoinositides: PI, phosphatidylinositol 4-phosphate (PIP) and phosphatidylinositol 4, 5-bisphosphate (PIP2). PLC transforms PIP2 into two messengers, inositol 1, 4, 5- trisphosphate (IP3) and diacylglycerol (DAG). The latter is also the product of phosphatidylcholine hydrolysis [186,187]. When IP3 is released into the cytoplasm, it mobilizes Ca^^ stores from the endoplasmic reticulum. The transitory rise in free calcium and/or its binding to calmodulin induces the activation o f specific protein kinases [157].

Recent studies have also indicated that the Ca^^-CaM pathway may have a critical role in the secretion of pepsinogen induced by carbachol and CCK in rabbit mucosal tissue maintained in organ culture [188]. Evidence is now accumulating to indicate that other membrane phospholipids are also degraded following receptor activation. This is the case for phosphatidylcholine which is probably hydrolysed by both a PLC and a receptor-coupled phospholipase D (PLD), producing phosphatidic acid or phosphatidylbutanol and DAG [189]. In various tissues, PLD might be activated by G protein-coupled receptors, phosphorylation by a receptor tyrosine kinase, the DAG/PKC cascade, or by the IP3/Ca^^ cascade.

The other product of PIP2 breakdown, DAG, has widely been accepted as an activator o f PKC [157,177], a calcium and phospholipid-dependent enzyme which has been detected in a variety of tissues and also in the cytoplasm o f mast cells [190,191]. DAG activates PKC which catalyses phosphorylation o f serine and threonine residues in proteins, in the presence of phospholipids, by reversibly increasing its affinity for calcium ions such that it can be activated at submicromolar concentrations of this cation [192,193]. DAG, in this way, could thus serve as a second messenger to activate protein phosphorylation.

Lipid soluble DAG allows PKC, localized in the cytoplasm in its inactive form, to translocate towards the plasma membrane where it is activated. The family of PKC enzymes consists of at least six isotypes a , (31, (3II, y, 5, e, which exhibit distinct tissue-specific patterns of expression. The y isotype appears to be expressed

differentially expressed in a wide variety of cell types. The presence of elevated levels of Ca^^ and phosphatidylserine are required for the stabilization of the binding o f PKC to the membrane. Phorbol esters and certain mutagens directly stimulate the C kinases in the presence of calcium in a fashion similar to DAG. The PKC contains two phorbol ester binding domains which may have interactive functions [194]. The hydrolysis of DAG produces AA, which is metabolized by the lipoxygenase pathway into LTs and by the cyclo-oxygenase pathway into a series of PGD2, PGE2 and PGF2 and TXA2. Various observations have suggested a potential role for PKC in mast cell stimulus-secretion coupling. Firstly, IgE-dependent stimulation of rat mast cells [195], mouse cultured mast cells [196] and human cultured basophils leukocytes [197] caused a rapid accumulation of DAG which preceded or was concurrent with histamine release. Secondly, White et al have shown that antigen-activation of mouse cultured mast cells stimulated PKC activity associated with the cell particulate fraction. Kinase activation was maximal 30 s after challenge and preceded histamine secretion, and both enzyme activation and histamine release were similarly dependent on antigen concentration [198].

Further evidence for the role of PKC was provided by the use of phorbol esters such as phorbol myristate acetate (PMA), agents which mimicked the effects of DAG on PKC. At low concentrations (<10 ng/ml), these compounds were found to be poor histamine releasers, but potentiated histamine secretion initiated by suboptimal concentrations of IgE-directed ligands and calcium ionophores in human basophils [197,199], RPMCs [200] and RBL cells [201]. These results thus suggest that PKC has a positive modulatory role in histamine secretion. Therefore, the two products of

PIP2 breakdown (IP3 and DAG) may represent two routes o f a second messenger pathway which positively interact with each other [204,211].

However, that PKC can also negatively modulate secretion was suggested by observations with RBL cells. Sagi-Eisenberg et at [199] have shown that at low concentrations (<10 ng/ml), TPA potentiated antigen-induced serotonin release but, in contrast, completely blocked the intracellular calcium increase normally associated

41

with this secretagogue. The concept of PKC acting as a negative modulator of signal transduction is further supported by observations that, in the mast cell [2 0 2] and in a variety of other cell types [203], phorbol esters can inhibit phosphoinositide breakdown and calcium mobilization. These observations may be interpreted either in terms of PKC having two roles: one to interact positively with the calcium signal and one to suppress it or that phorbol esters may exert an effect other than that through PKC activation.

It is now generally accepted that the PKC enzymatic complex plays a crucial role in a number of cellular responses related to differentiation, proliferation and membrane protein functions [193] [Fig 1.6].