PKC (Protein Kinase C)

Definition
Protein kinase C (PKC) is a family of enzymes that are involved in controlling the function of other proteins through the phosphorylation of hydroxyl groups of serine and threonine amino acid residues on these proteins.

Related Peptides
To date, 11 protein kinase C isozymes have been identified and classified into three groups cPKC, nPKC, and aPKC, based on their structure and cofactor regulation. The best characterized and first discovered are the conventional protein kinase Cs: a, two alternatively spliced variants, ßI and ßII, and ?. This class distinguishes itself from the others in that function is regulated by Ca2+; its C2 domain contains a putative Ca2+-binding site. The next well characterized are the novel protein kinase Cs: d, ?, ? (L), ?, and µ. These isozymes are structurally similar to the conventional protein kinase Cs, except that the C2 domain, while maintaining structural residues, does not have the functional groups that appear to mediate Ca2+ binding. The least understood isozymes are the atypical protein kinase Cs: ? and ?. These differ significantly in structure from the other two classes; first, the C1 domain contains only one Cys-rich motif (not two), and second, key residues that maintain the C2 fold do not appear to be present 1.

Structural Characterisitcs
Members of the PKC family are a single polypeptide, comprised of an N-terminal regulatory region (approximately 20-40 kDa) and a C-terminal catalytic region (approximately 45 kDa). Cloning of the first isozymes in the mid-1980s revealed four conserved domains: C1-C. Each is a functional module, and many unrelated proteins have one or the other. The function of each of these domains has been established by extensive biochemical and mutational analysis; the C1 domain contains a Cys-rich motif, duplicated in most isozymes, that forms the diacylglycerol/phorbol ester binding site; this domain is immediately preceded by an autoinhibitory pseudosubstrate sequence; the C2 domain contains the recognition site for acidic lipids and, in some isozymes, the Ca2+-binding site. The C3 and C4 domains form the ATP- and substrate-binding lobes of the kinase core. The regulatory and catalytic halves are separated by a hinge region that becomes proteolytically labile when the enzyme is membrane-bound; the proteolytically generated kinase domain (protein kinase M), freed of inhibition by the pseudosubstrate, is constitutively active 2.

Mode of Action
Diacylglycerol (DAG) Activates PKC: The source of DAG that activates PKC can be derived from the hydrolysis of phosphatidylinositides or from the metabolism of phosphatidylcholine by phospholipase C or D. The principal function of DAG is to activate a family of protein kinases collectively termed PKC. In the absence of hormone stimulation, PKC is present as a soluble cytosolic protein that is catalytically inactive. A rise in the cytosolic Ca2+ level causes protein kinase C to bind to the cytosolic leaflet of the plasma membrane, where the membrane-associated DAG can activate it. Thus activation of PKC depends on an increase of both Ca2+ ions and DAG 3. The activation of PKC in different cells results in a varied array of cellular responses. PKC also phosphorylates various transcription factors; depending on the cell type; these induce synthesis of mRNAs that trigger cell proliferation.

Functions
Cellular and functional alterations in vascular cells induced by DAG-PKC activation: Multiple cellular and functional abnormalities in the diabetic vascular tissues have been attributed to the activation of DAG-PKC pathways. Abnormalities in vascular blood flow and contractility have been found in many organs of diabetic animals or patients, including the kidney, retina, peripheral arteries, and microvessels of peripheral nerves. Multiple lines of evidence have supported that the decreases in retinal blood flow are due to PKC activation. Decreases in retinal blood flow in diabetic rats have been reported to be normalized by PKC inhibitors 4. In addition to the retina, decreases in blood flow have also been reported in the peripheral nerves of diabetic animals that were normalized by PKC inhibition.

Cardiomyopathy
It has been reported that the diabetic state will induce the activation of PKC- isoform in the hearts of rats. To determine whether activation of PKC- isoform can cause cardiac abnormalities, a study was conducted using transgenic mice overexpressing PKC- 2 isoform specifically in the myocardium by the use of tissue-specific promoter myosin heavy chain (MHC). These mice developed cardiac hypertrophy, cardiomyocyte injuries, and fibrosis at 8–12 weeks of life. At the 20th week, cardiac atrophy and severe fibrosis were observed. Treatment with PKC - isoform inhibitor (LY333531) prevented most of the functional and pathological changes in the hearts of the transgenic mice, clearly demonstrating that excessive PKC activation can cause cardiomyopathy6.

References
1.Harry M and Peter J (1998). The extended protein kinase C superfamily. Biochem. J., 332:281-292.
2.Newton AC (1995). Protein Kinase C: Structure, Function, and Regulation. J. Biol.Chem., 270(48):28495-28498.
3.Nishizuka Y (1995). Protein kinase C and lipid signaling for sustained cellular responses. Faseb., 9(7):484-496.
4.Shiba T, Inoguchi T, Sportsman JR, Heath WF, Bursell S, King GL (1993). Correlation of diacylglycerol and protein kinase C activity in rat retina to retinal circulation. Am J Physiol., 265:783-793.
5.Wakasaki H, Koya D, Schoen FJ, Jirousek MR, Ways DK, Hoit BD, Walsh RA, King GL (1997). Targeted overexpression of protein kinase C isoform in myocardium causes cardiomyopathy. PNAS., 94(17):9320-9325.