The Receptor-Mediated Hydrolysis of a Rare Phospholipid of the Plasma Membrane Produces Two Different Second Messengers with Different Actions

Anoiher second-messenger system differs from both Ca²' and cAMP in that r∣∣^,o distinct second-messenger molecules are produced as a result of an CfiZy∏]il∏C IlChVfltioil by a single receptor/ligand complex.

FIGURE 1-19 Hydrolysis of a membrane lipid to produce two second messengers. After appropriate receptor and G-protein activation, the rare membrane phospholipid shown to the left, phosphatidylinositol 4,5-bisphosphate (PIP₂), is hydrolyzed into two separate second messengers by phospholipase C.The phosphate "head" of the PIP₂ molecule is cleaved to produce the soluble messenger inositol 1,4,5-trisphosphate (IP₃), which mobilizes intracellular Ca²*, as well as the Iipidic messenger diacylglycerol (DAG), which remains in the membrane and activates protein kinase C.

membrane phospholipid that can accept additional phosphate groups by reaction with the —OH groups on the inositol (Figure 1-19). Phosphatidylinositol 4,5-bisphosphate (PIP₂) is the membrane phospholipid that is broken down to produce two important second messengers. PIP₂ is hydrolyzed to diacylglycerol (DAG) and inositol 1,4,5-trisphosphate (IPJ by a receptor-mediated enzyme called phospholipase C (PLC) or phosphoinositidase. Although many distinct processes are controlled through the PIP₂ path, it plays a particularly important role in control of growth and of receptor-mediated secretion. The effect of acetylcholine acting through mus­carinic receptors (not the nicotinic receptor/ion channel of the nerve-muscle synapse) is often transmitted and transduced through activation of the PIP₂ pathway.

The events involved in the receptor-mediated production of IP₃ and DAG from PIP₂ are similar to those in the pro­duction of cAM P.

The activation of the hydrolytic enzyme increases the concentration of IP₃, which is water soluble and thus diffuses through the cytoplasm. IP_κ binds to and opens ligand­gated Ca² ‘ channels in the endoplasmic reticulum. This releases Ca²' from that high [Ca²J compartment into the cytoplasm. Ca² thus becomes the “third messenger” in this system (although this term is not in widespread use) and is another example of a role of Ca²' in GPCR signaling. The ensuing increase in cytoplasmic ∣Ca²*∣ affects cellular function by the same mechanisms outlined earlier for Ca²* as a second messenger (e.g., binding to calmodulin), with the Ca²'/calmodulin complex in turn activating various enzyme activities. In receptor-mediated secretion, for example, the binding of acetylcholine to muscarinic receptors in the pan­creas (the organ that secretes digestive enzymes) causes an increase in PIP₂ breakdown and an increase in cytoplasmic IP_v The IP; opens ligand-gated Ca^2, channels in the endo­plasmic reticulum, and intracellular [Ca²⁺∣ increases. The process then becomes similar to smooth muscle contraction. Calmodulin binds Ca²∖ and the complex activates a protein kinase.

DAG is also produced on activation of PLC, but it is not at all soluble. DAG diffuses in the plasma membrane, binding to and activating membrane-associated protein kinase, protein kinase C (PKC). PKC is not an intrinsic membrane protein and can bind reversibly to the cytoplasmic face of the plasma membrane. PKC phosphorylates other proteins and changes their activity. Because of the membrane-bound character of the enzyme, most evidence indicates that PKC phosphorylates membrane proteins such as receptors and ion channels, regulating their function. In the case of the secretory response to some hormone/neurotransmitter stimulus, PKC gener­ally acts separately but additively with IP₅ to produce the response. As with cAMP and protein kinase A, however, much interest focuses on longer-term effects of DAG activation of PKC, particularly its role in growth control and cancer. A class of chemicals long known to promote the onset of cancer, phorbol esters, is a potent substitute for DAG at activating PKC. It is now known that PKC indirectly activates an important transcription factor involved in cell proliferation, nuclear factor kappa B (NF- κB). Thus, as a second messenger and as with cAMP» DAG has both short-term effects and longer-term transcriptional effects.