Arachidonate Mobilization

Taking the two receptor model as a le, it may be noted that those activities ch have been shown to require continuous receptor occupancy, such as arachidonate mobilization and the fonnaticm of dx>sphatidic add, are mediated through the high affinity pathway and represent a receptor-hgand mediated mechanism while those receptor activities that require only transient receptor occupancy, such as the synthesis and breakdown of phosphoinositides, correspond to the moderate affinity proteolytic PARI pathway. The feet feat immunoinhibitoty effects at either receptor can be overcome at elevated thrombin levels merely indicates fee relatively high affinities of thrombin binding at both sites ( 0.5nM and lOnM, respectively) as compared with fee relatively low affinities of fee... 

Pestonjamasp, V.K. and Burstein, S.H. (1998) Anandamide synthesis is induced by arachidonate mobilizing agonists in cells of the immune system, Biochimica et Biophysica Acta 1394 249-260. 

In some cases, association of group II PLA2 with cells appears to be unrelated to their non-covalent binding with membrane proteins or polysaccharides. This cell-associated form of group II PIA2 may also contribute to receptor-stimulated arachidonate mobilization... 

In general, calcium-independent PLA2S coexist in cells with other, calcium-dependent PLA2 activities, raising the question of their respective roles in cell lipid metabolism and signaling. One possibility is that, while calcium-dependent PLA2S may be crucial for arachidonate mobilization, their calcium-independent counterparts may provide the lysophospholipid substrate necessary for the reacylation of arachidonic acid into phospholipids. 

As one would expect, for all its elegance, this model does not exhaust the potential functions of calcium-independent PLA2S. Evidence tallies also with a direct participation of this enzyme in arachidonate mobilization in pancreatic p cells, for example, an ATP-activated, heart-type isoform of calcium-independent PLA2 may mediate mobilization of arachidonate and subsequent formation of 12-lipoxygenase metabolites, which are thought to act as intracellular mediators in glucose-induced insulin secretion. 

Activation of the various PLA2 isoforms described in the preceding sections provides a single-step pathway of arachidonate mobilization. In addition to this direct mechanism, arachidonate mobilization may be initiated by the stimulation of PLCs, which... 

Fig. 2.8. Arachidonate mobilization through the phospholipase C and phospholipase D pathways.

Under what physiological conditions may PLC and PLD participate in stimulus-dependent 1,2 diacylglycerol formation, and hence, create the conditions for arachidonate mobilization From what I have just said, a complete and satisfactory answer to this question is still lacking, but the current general consensus may be... 

We have seen that a series of first messenger signaling molecules produce arachidonate mobilization by activating membrane receptors. Too many such effects have been described over the past 20 years to allow me to compile a review of this subject area that is both exhaustive and useful. I will rather illustrate with some examples what I consider to be the three major modalities of receptor-mediated control of arachidonate mobilization stimulation, inhibition and facilitation. Where possible, I will also indicate the specific transmembrane signaling pathway and enzyme route affected. 

Animal cells have evolved two sorts of mechanisms to increase arachidonate mobilization in response to external stimuli shortterm mechanisms, which evolve over a time-scale of seconds to minutes and do not depend on the synthesis of new proteins, and long-term mechanisms, which evolve over several hours and do depend on protein synthesis. 

Let us turn now to our second example of short-term arachidonate mobilization the receptor-dependent activation of group II PLA2 in P388Di cells. Stimulation of these cells with li-popolysaccharide or PAF mobilizes arachidonate in two phases a transient phase (maximal at -1.5 minutes and rapidly declining... 

A THIRD GROUP OF RECEPTORS FACILITATE ARACHIDONATE MOBILIZATION, BUT DO NOT STIMULATE IT DIRECTLY... 

In many cases, this facilitatory effect on arachidonate mobilization requires, like adenylate cyclase inhibition, the participation... 

The arachidonic acid cascade takes part in a large number of functional interactions with other intracellular second messenger systems. As one would expect from their universal regulatory functions in the physiology of mammalian cells, protein phosphorylation and dephosphorylation play a major part in modulating arachidonate mobilization. Two multifunctional (and ubiquitous) protein kinases, PKC and PKA, have been studied extensively in this regard. 

Unlike PKC, the effects of PKA activity on arachidonate mobilization can be either stimulatory or inhibitory, and they largely depend on the cell type and on the nature of the stimulus. Moreover, the available data make it difficult to draw any conclusion as to the biochemical mechanism of these effects, except perhaps that they are not likely to entail the direct phosphorylation of a PLA2-... 

Fig. 2.13. (Opposite page) Dopamine receptor synergism on arachidonate mobilization in transfected Chinese hamster ovary (CHO) fibroblasts. (Modified with permission from Piomelli D et al. Nature 1991 353 164-167.)...

Wijkander J, Sundler R. An 100-kDa arachidonate-mobilizing phospholipase A2 in mouse spleen and the macrophage cell line J774. Purification, substrate interaction and phosphorylation by protein kinase C. Eur J Biochem 1991 202 873-880. 

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