Protein kinase C enzymes

TTie family of protein kinase C enzymes includes Ser/Thr-specific protein kinases that require the following cofactors for activation (review articles Dekker and Parker, 1994 Newton, 1997, Oancea and Meyer, 1998) ... 

A property of the protein kinase C enzyme family that is highly valuable for their identification and characterization is their activation by tumor promoters such as phorbol esters (Fig. 7.7). Protein kinase C binds to the tumor promoter, tetradecanoyl phorbol acetate (TPA), with high affinity and is activated by this binding. The specific activation of protein kinase C by phorbol esters is an important tool to demonstrate their involvement in signal transduction pathways. By external addition of TPA, it is possible to use cellular model systems to test which biological responses of a signal transduction pathway involve, and are controlled by, protein kinase C. 

PKC protein kinase c. Enzyme involved in cell signaling. 

PtdIns(3,4,5)P3 formed by PI3-kinase regulates the activity of a series of protein kinases, including the Ser/Thr-specific Akt kinases, protein kinase C enzymes (see Section 7.4), and the tyrosine-specific Tec kinases. Only the regulation of Akt kinase will be discussed in the following. 

Furthermore, protein kinase C enzymes have been shown to be high-affinity receptors for phorbol esters (see below). Regulation by Ca2+ and diacylglycerol identify protein kinase C enzymes as components of signal transduction pathways, in the course of which, phospholipase C is activated and the messenger substances Ins(3,4,5)P3/Ca2+ and diacylglycerol are produced. Therefore, activation of protein kinase C may take place via two central pathways ... 

The sensitivity to Ca2+ and diacylglycerol and specific binding of phorbol esters has been considered for a long time to be the main characteristics of protein kinase C enzymes. Molecular cloning techniques and biochemical work, however, revealed that protein kinase C is a large kinase family that includes a variety of isoenzymes with very different regulatory properties (see below). 

In addition to protein kinase C enzymes, cells contain other receptors for phorbol esters and diacylglycerol. In mammals, proteins named a- and /1-chimaerins have been identified that lack protein kinase activity and bind phorbol esters and diacylglycerol with high affinity. Therefore, not all biological responses elicited by phorbol ester treatment can be attributed to protein kinase C stimulation (review Ron and Kaza-nietz, 1999). 

Shearman MS, Sekiguchi K, Nishizuka Y (1989) Modulation of ion channel activity a key function of the protein kinase C enzyme family. Pharmacol. Rev. 41, 211-237. 

One of the invertebrates collected during the 1994 SIO-RU-SKB collaborative marine invertebrate collection from the Aliwal Shoal off southern Kwazulu-Natal, South Africa was the ascidian Lissoclinum sp.. Bioassay guided fractionation of a methanol extract of this ascidian yielded a novel interleukin-8 inhibitor, lissoclin disulfoxide (88) [84], Interleukin-8 (IL-8), a promoter of neutrophil aggregation and activation, is implicated in a wide range of inflammatory disorders e.g. psoriasis and rheumatoid arthritis [85]. In addition to its potent inhibition of both IL-8 Roc and IL-8 R(3 (IC50 = 0.6 and 0.82 pM respectively) 88 also exhibited activity against the protein kinase C enzyme (IC50 = 1.54 pM) [84]. 

Fig. 5. Phosphorylation of poly(ADP-ribose) synthetase by purified rat brain protein kinase C. Enzyme reaction was performed principally as described by Kikkawa et al. (20) with l-oleoyl-2-acetyl-rac-glycerol (OAG) and phosphatidyl serine as activator. A The reaction was carried out with (O ) or without ( ) C-kinase activator (Ca /OAG/phosphatidyl-serine) the indicated concentration of poly(ADP-ribose) s mthetase was added to the reaction nuxture in place of phosphate-accepting protein. B Time course of the reaction.

FIGURE 2.7 Production of second messengers inositol 1,4,5-triphosphate (IP3) and diacylglycerol (DAG) through activation of the enzyme phospholipase C. This enzyme is activated by the a- subunit of Gq-protein and also by Py subunits of Gj-protein. IP3 stimulates the release of Ca2+ from intracellular stores while DAG is a potent activator of protein kinase C. 

Protein kinase C (PKC) is an enzyme family whose members are activated by agonists that cause receptor-mediated generation of lipid second messengers. The activated enzymes transduce information from such agonists by phosphorylating relevant downstream substrates. 

Recent evidence indicates that the 5-HT transporter is subject to post-translational regulatory changes in much the same way as neurotransmitter receptors (Blakeley et al. 1998). Protein kinase A and protein kinase C (PKC), at least, are known to be involved in this process. Phosphorylation of the transporter by PKC reduces the Fmax for 5-HT uptake and leads to sequestration of the transporter into the cell, suggesting that this enzyme has a key role in its intracellular trafficking. Since this phosphorylation is reduced when substrates that are themselves transported across the membrane bind to the transporter (e.g. 5-HT and fi -amphetamine), it seems that the transport of 5-HT is itself linked with the phosphorylation process. Possibly, this process serves as a homeostatic mechanism which ensures that the supply of functional transporters matches the demand for transmitter uptake. By contrast, ligands that are not transported (e.g. cocaine and the selective serotonin reuptake inhibitors (SSRIs)) prevent the inhibition of phosphorylation by transported ligands. Thus, such inhibitors would reduce 5-HT uptake both by their direct inhibition of the transporter and by disinhibition of its phosphorylation (Ramamoorthy and Blakely 1999). 

Protein kinase C theta plays a critical role in T cell signaling and thus the inhibition of this enzyme has the potential to be useful for treating inflammatory diseases. Cywin et al. [55] describe the parallel optimization of potency against the enzyme,... 

Lead also has been shown to substitute for calcium in the activation of calmodulin, but this requires higher levels of lead than does the activation of protein kinase C. Nevertheless, the affinity of lead for calmodulin is higher than that of calcium. Once activated, calmodulin regulates the activity of certain enzymes and transporters. For example, it activates c-AMP phosphodiesterase to hydrolyze and terminate the action of cAMP, another second messenger (Bressler and Goldstein 1991 Goldstein 1993 Goering 1993). 

Protein kinase An enzyme activated by second messengers like cyclic AMP (protein kinase A) and 1,2-diacylgycerol (protein kinase C). 

Several independent laboratories have now demonstrated that both lithium and valproate (VPA) exert complex, isozyme-specific effects on the PKC (protein kinase C) signaling cascade (reviewed in [3, 5, 11-13]). Not surprisingly, considerable research has recently attempted to identify changes in the activity of transcription factors known to be regulated (at least in part) by the PKC signaling pathway - in particular the activator protein 1 (AP-1) family of transcription factors. In the CNS, the genes that are regulated by AP-1 include those for various neuropeptides, neurotrophins, receptors, transcription factors, enzymes involved in neurotransmitter synthesis, and proteins that bind to cytoskeletal elements [14]. 

Chen et al. [62] have proposed that protein kinase C is involved in the fi receptor desensitization since activators of this enzyme potentate fi receptor uncoupling from the K+ channel. The fi receptor has consensus protein kinase C phosphorylation sites in its intracellular domains [8]. Arden et al. [88] have reported that morphine can induce the phosphorylation of the cloned fi receptor and Zhang et al. [64] have reported that activators of protein kinase C can induce the phosphorylation of the ft receptor. 

A few enzymes, such as the previously mentioned CNP, are believed to be fairly specific for myelin/oligodendro-cytes. There is much more in the CNS than in peripheral nerve, suggesting some function more specialized to the CNS. In addition, a unique pH 7.2 cholesterol ester hydrolase is also enriched in myelin. On the other hand, there are many enzymes that are not myelin-specific but appear to be intrinsic to myelin and not contaminants. These include cAMP-stimulated kinase, calcium/calmodulin-dependent kinase, protein kinase C, a neutral protease activity and phosphoprotein phosphatases. The protein kinase C and phosphatase activities are presumed to be responsible for the rapid turnover of MBP phosphate groups, and the PLP acylation enzyme activity is also intrinsic to myelin. 

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