Diacylglycerol cycle

Diacylglycerol has long been known to be a weak competitive inhibitor of PLC/fc, whereas phosphorylcholine shows very little inhibition [40, 49, 116]. Recent kinetic assays of PLCB(. activity in the presence of DAG indicate that it is a competitive inhibitor with a Kl of the order of 10 mM, whereas phosphorylcholine was found to be an extremely weak (K = 30-50 mM), mixed inhibitor of PLC/J( [34]. Because diacylglycerol is a competitive inhibitor of the enzyme, the nature of the catalytic cycle dictates that it must be the last product to leave the enzyme active site. 

Fig. 17. Catalytic cycle for PLQ,. After substrate binding, hydrolysis occurs in the rate-determining step, followed by the sequential release of phosphorylcholine and diacylglycerol. Amino acids known to be involved in substrate binding are shown, and zinc ions appear as filled circles...

Figure 21.6 One mechanism of activation of the cell cycle by a growth factor. Binding of growth factor to its receptor activates membrane-bound phospholipase-C. This hydrolyses phosphati-dylinositol bisphosphate in the membrane to produce the messengers, inositol trisphosphate (IP3) and diacylglycerol (DAG). IP3 results in release of Ca from an intracellular store. The increased Ca + ion concentration activates protein kinases including protein kinase-C (PK-C). DAG remains membrane-bound and also activates protein kinase-C (PK-C) which remains in the activated form as it travels through the cell where it phosphory-lates and activates transcription factors. This results in activation of genes that express enzymes involved in nucleotide synthesis, DNA polymerases and cyclins, which are all reguired for the cell cycle (See Chapter 20 for provision of nucleotides and cyclins for the cell cycle).

An additional action of Li" is interruption of the phosphatidylinositide cycle through an inhibitory action on inositol phosphate metabolism. By this mechanism, depletion of membrane inositol and the phosphoinosi-tide-derived second-messenger products diacylglycerol and inositol triphosphate ultimately reduces signaling through receptor systems dependent on the formation of these products. It is presently unclear to what extent inhibition of inositol phosphate metabolism contributes to the therapeutic properties of Li+ in bipolar patients. 

FIPhosphatidylinositol cycle. A = activator CMPPA = cytidine monophosphate phosphatidic acid DAG = diacylglycerol G = G protein Glu-6-P, glucose 6-phosphate IPj = inositol monophosphate IP2 = inositol biphosphate ... 

The inositol phosphates are linked into a metabolic cycle (Fig. 6.5) in which they can be degraded and regenerated. Via these pathways, the cell has the ability to replenish stores of inositol phosphate derivatives, according to demand. Ptdins may be regenerated from diacylglycerol via the intermediate levels of phosphatidic acid and CDP-glycerol. 

In prokaryotes, phosphatidylserine is made from CDP-diacylglycerol (see fig. 19.3). The enzyme for this reaction is absent in animal cells, which rely on a base exchange reaction in which serine and ethanolamine are interchanged (fig. 19.8). Although the reaction is reversible, it usually proceeds in the direction of phosphatidylserine synthesis. Phosphatidylserine can be converted back to phos-phatidylethanolamine by a decarboxylation reaction in the mitochondria. This may be the preferred route for phosphatidylethanolamine biosynthesis in some animal cells. Furthermore these two reactions (see fig. 19.8) establish a cycle that has the net effect of converting serine into ethanolamine. This is the main route for ethanolamine synthesis... 

IP3 is rapidly degraded to inactive IP2 and then on to inositol. Meanwhile, diacylglycerol is phosphorylated and then converted to CDP-diacylglycerol, which combines with inositol to form phosphatidylinositol. The latter is subsequently phosphorylated in two steps to PIP2. The degradation and resynthesis of PIP2 completes the so-called phosphatidylinositol cycle. 

Fig. 4. Metabolism of inositol lipids and inositol phosphates in stimulated cells. In the lipid cycle. Ptdlns is converted through Ptdlns 4-P to Ptd 4,5-P, which is hydrolysed upon receptor stimulation to yield diacylglycerol (DG) and Ins(1.4,5)P,. DG is phosphorylated via diacylglycerol kinase to give phospha-tidic acid (PA). PA is activated by CTP to form CDP-diacylglycerol which can combine with free inositol (Ins) to re-form Ptdlns. The other product of the Ptdlns 4,5-P, hydrolysis, Ins(l,4,5)P, may be de-phosphorylated to various InsP2 and InsP isomers which ultimately provide free inositol (Ins) for resynthesis of Ptdlns. Ins(l,4,5)Pj may be phosphorylated in steps to give an array of InsP4, InsP, or InsP6 isomers.

Fig. 2. Messengers mediating the initial and sustained phases of the All-induced cellular response. Initial phase All-elicited hydrolysis of PIP2 induces a transient rise in cytosolic calcium (via IP3), a transient activation of calcium-, calmodulin-dependent protein kinases, a transient increase in the phosphorylation of early-phase phosphoproteins (Pra-P), and a transient cellular response. Sustained response All-elicited hydrolysis of phosphoinositides generates a sustained increase in the diacylglycerol (DG) content of the plasma membrane. In conjunction with a sustained increase in plasma membrane calcium cycling, DG induces the sustained activation of protein kinase C (CK), the sustained increase in the phosphorylation of late-phase phosphoproteins (P -P) and the sustained cellular response.

Busa, W., and Gimlich, R., 1989, Lithium-induced teratogenesis in frog embryos prevented by a polyphosphoinositide cycle intermediate or a diacylglycerol analog. Dev. Biol. 132 315-324. 

There are interesting recent reports linking membrane fusion to cubic lipid phases [9] or to reversed (or "inverted") phases [62]. So-called "inter-lamellar attachments", formed between the bilayers of liposomes on fusion, show freeze-fracture electron micrograph textures identical to those of a cubic lipid-water phase (see Fig. 5.9). The inter-lamellar attachments seem to be identical to "lipidic particles" described earlier [63]. It is also interesting to note that diacylglycerols, secondary messengers from the Pl-cycle, produce fusion in... 

Fig. 8. Role of lipophorin in DG delivery to flight muscle. Adipokinetic hormone (AKH) is released from the corpus cardiacum and binds to the fat body, where it cause production of cAMP and entry of Ca . These second messengers activate lipolysis of triacylglycerol (TG) and production of diacylglycerol (DG). The DG leaves the fat body with the assistance of a lipid transfer particle (LTP) and is taken up by HDLp. The capacity of HDLp to carry DG is increased by binding of apoLp-HI to the surface. Ultimately, LDLp is formed and moves to the flight muscle, where a lipoprotein lipase hydrolyzes the DG to produce fatty acid (FA) and regenerate HDLp and apoLp-IIl. The FA enters the flight muscle, where it is oxidized to produce the ATP required to power flight. HDLp and apoLp-HI circulate back to the fat body to complete the cycle.

A further effect of lithium on receptor-activated cells is accumulation of diacylglycerol, which may increase or prolong the activation of protein kinase C (107). It should be noted that diacylglycerol and D-inositol l,4,5-tris(phosphate) are separate branches of two parallel signaling systems that initially are coherent. Metabolic interference by lithium to desynchronize these systems may itself be a signaling system. This is analogous with beat phenomena seen in the desynchronization of biological cycles by rapid movement between time zones. 

The best known receptors for phorbol esters and their derivatives are the isozymes of protein kinase C (PKC), which bind phorbol esters and the physiological second messenger diacylglycerol (DAG) by cysteine-rich domains, the Cl domains. The exact functions of the different PKC isozymes is not known at present however, they have been shown to be involved in synaptic transmissions, the activation of ion fluxes, secretion, cell cycle control, differentiation, proliferation, tumorigenesis, metastasis, and apoptosis. 

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