PLC isozymes

All PLC isozymes have conserved catalytic domains designated X and Y, and a C2 domain similar to that in cPLA2 (Fig. 2). In addition, the (3, y and 8 isozymes have pleckstrin homology (PH) domains and EF-hand domains located in theN-teiminal region. The y isozymes differ in that they have Src homology domains (SH2 and SH3) and an additional PH domain split by the SH domains. The (3 and y isozymes are of140-155 kDa mass, whereas the 8 isozymes are smaller (85 kDa) and the o isozyme is larger (240 kDa). 

The 3 isozymes are activated by G protein-coupled receptors through two different mechanisms [2]. The first involves activated a-subunits of the Gq family of heterotrimeric G proteins (Gq, Gn, Gi4, G15/16). These subunits activate the (31, (33 and (34 PLC isozymes through direct interaction with a sequence in the C terminus. The domain on the Gqa-subunit that interacts with the (3 isozymes is located on a surface a-helix that is adjacent to the Switch III region, which undergoes a marked conformational change during activation. The second mechanism of G protein activation of PLC 3 isozymes involves (3y-subunits released from Gi/0 G proteins by their pertussis toxin-sensitive activation by certain receptors. The 3y-subunits activate the 32 and 33 PLC isozymes by interacting with a sequence between the conserved X and Y domains. 

The phospholipases (PLC) isozymes cleave the phosphodiester bond in phos-phatidyl-inositol-4,5-bisphosphate (PIP2) releasing two second messenger molecules inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG) as shown before. The /1-isozyme are controlled by the Ga or G y subunits of the heterotrimeric G-proteins coupled to surface receptors. The y-isozymes are substrates for tyrosine kinases, such as growth factors. 

Walsh Do you have any information about the Ca2+ sensitivity of different PLC isozymes ... 

A third mode of entry of Ca2+ into the cells is mediated by the coupling of the receptor to G proteins. It is believed that this is the pathway used by chemokines. The On reaction begins when the agonist induces a conformational change in the receptor which is transmitted to the G protein. The heterodimeric G protein dissociates into Ga and G 3y subunits, both of which can activate different phospholipase C (PLC) isozymes. This enzyme is able to catalyze the hydrolysis of the lipid precursor phosphatidylinositol 4,5-bisphosphate to give both 1,2-diacylglycerol (DAG) and inositol 1,4,5-trisphosphate (IP3). The latter then binds to the IP3 receptor which mobilizes both stored Ca2+ and promotes an influx of external Ca2+. 

While profound decreases in SL PLC yi and 8i activities and protein levels occur in CHF, treatment of animals with the ACE inhibitor imidapril partially corrected these changes in PLC isozymes forms as well as cardiac function (Tappia et ah, 1999). This would suggest that RAS may be involved in mediating alterations in PLC isozymes and that PLC isozymes could serve as novel targets for the treatment of CHF. In addition, PLC isozymes could be another target for the mechanisms of action of ACE inhibitors. It is pointed out that an upregulation of the Gqa/PLC-p pathway in the viable, border, and scar tissues in the post-MI hearts is also seen, which may play an important role in cardiac fibrosis and scar remodeling (Ju et ah, 1998). 

PI-PLC is the key enzyme of IP3 production. Some information already exists on its domains that interact with G-proteins and participate in the phospho-diesteratic cleavage of PIP2. A further understanding of the structure-function relationship of PI-PLC is awaiting the determination of its three-dimensional structure. An important question to be addressed is the molecular mechanism of the G-protein-mediated activation of PI-PLC in its cellular environment, because the PI-PLC isozymes are water soluble, whereas the G-proteins are mainly lipid soluble. 

Phosphoinositides are cleaved by a family of phos-phoinositide-specific phospholipase C isozymes. PLC... 

Despite the similar functions of each isozyme, only two regions of amino acid homology exist (X and Y), one of 150 and a second of 120 amino acid residues, which are 54% and 42% identical among the isozymes but are differentially localized within each enzyme (Fig. 20-3). The X and Y domains form the catalytic core of the enzyme. A characteristic of the (3 and 8 isoforms is that relatively few amino acids (40-110) separate the X and Y entities, whereas a much larger separation is observed for the PLCy isoform (approx. 400). In addition, in PLCy, the region between X and Y contains amino acid sequences that are found in nonreceptor tyrosine kinases (SH2 and SH3 domains). All four isoforms possess pleckstrin homology (PH) domains. The latter are considered to enable the enzyme to become tethered to the plasmalemma via an interaction with PI(4,5)P2. In addition, all PLC isoforms possess an E-F hand domain, which is located between PH and X domains, and a C2 domain, which is located close to the Y domain. 

The changes in the proportions of various isozymes of PLC with their different specificity to phosphoinositides indicate that the rate of hydrolysis of inositol lipids may be altered in patients with AD. In the normal human brain matter, PLC-/1 was the main type present. In AD brains, the amount of PLC-/I was... 

PAF receptor is coupled to the activation of both protein kinase C and tyrosine kinase (Shukla, 1992, Izumi and Shimizu, 1995). PAF receptor antagonists block both pathways. The activation of PKC is through the Imown PLC generated diglyceride -t- Ca pathway (leyasu et. al. 1982). Activation of PKC in platelets (eg. by phrobol myristate acetates) inhibited PAF stimulation of platelets. On the other hand PKC inhibitors (eg. staurosporine) enhanced PAF stimulated IP, production. In rabbit platelets PKC activation by PAF was suggested to be independent of enzyme translocation (Pelech et. al. 1990). Human platelets have 6 PKC isozymes (alpha, p, 8, zeta, eta and theta). PAF stimulated 200% and 175% increase in the levels of membrane - bound PKC eta and theta... 

Other studies have indicated that PG and PLD are a major source of PA in cells stimulated with various agonists [197-200] and several studies have confirmed biphasic formation of DAG, with the first phase derived from PIP2 and the second from PC via PA [201-205]. Analyses of the specific fatty acid composition of PA and PtdBut derived from cells treated with butanol and stimulated by bombesin or LPA also showed a predominance of saturated and mono-unsaturated fatty acids in the lipids derived from PLD action [206]. The fatty acid composition was very different from that found in DAG generated by PLC action on PIP2, in which polyunsaturated fatty acids predominate. The key issue is whether the DAG species derived from PC can activate PKC. Although studies indicate that this species is active on PKC in vitro, this has not been demonstrated in vivo. As described above [196], PC-derived DAG can activate Ca +-independent PKC isozymes but, because of the lack of a Ca rise, it cannot activate ( a -dcpcridcril PKC isozymes. 

ANGII, ai-adrenergic agonists and endothelin-1 are relevant stimulants of PLC p isozymes via the a subunits of the heterotrimeric Gq subfamily PLC p has also been shown to be activated by Gpy dimer (Lee et al., 1994). A nontyrosine kinase activation of PLC y isozymes has been reported (Rhee and Bae,... 

Although the PLC 8-Gh pathway may be an important player in the signaling pathway that regulates calcium homeostasis and modulates physiological processes. PLC s isozymes are activated by Ras, Rho and Rap 2B as well as by Gcxi2 (Song et al., 2001). The activation of PLC and p is far less characterized. 

The development of hypertrophy in cultured rat neonatal cardiomyocytes induced by endothelin-1 has been reported to be due to activation of PLC (3 isozymes (Lamers et al., 1995). In addition, recent studies in neonatal rat cardiomyocytes stimulated with different hypertrophic stimuli, have shown an increased mRNA expression and protein level of PLC p isozymes (Schnabel... 

It should be noted that agonists of PLD simulate the effects of IP, whereas the inhibition of PLD blocks the beneficial effects of IP (Ozer et al., 2005 Hashizume et al., 1997). In summary, the two extremes of stress imposed on the heart under I-R change the redox potential of the cardiomyocyte and affect redox-sensitive molecules involved in phospholipid-mediated signal transduction mechanisms. The early activation of specific PLC and PLD isozymes may represent initiation of redox signaling and cell survival pathways, whereas inactivation of specific PLC and PLD isozymes may contribute to impaired cardiac contractile function (Fig. 11.6). 

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