Human PRKCE gene (Ensembl ID: ENSG00000171132) encodes the protein PKCε (Uniprot ID: Q02156), which is 737 amino acids in length with a molecular weight of 83.7 kDa. The PKC family of serine-threoninekinases contains thirteen PKC isoforms, and each isoform can be distinguished by differences in primary structure, gene expression, subcellular localization, and modes of activation.[7] The epsilon isoform of PKC is abundantly expressed in adult cardiomyocytes,[8][9][10][11] being the most highly expressed of all novel isoforms, PKC-δ, -ζ, and –η.[12] PKCε and other PKC isoforms require phosphorylation at sites Threonine-566, Threonine-710, and Serine-729 for kinase maturation.[13] The epsilon isoform of PKC differs from other isoforms by the position of the C2, pseudosubstrate, and C1 domains; various second messengers in different combinations can act on the C1 domain to direct subcellular translocation of PKCε.[9][14]
Receptors for activated C-kinase (RACK) have been found to anchor active PKC in close proximity to substrates.[15] PKCε appears to have preferred affinity to the (RACK/RACK2) isoform; specifically, the C2 domain of PKCε at amino acids 14–21 (also known as εV1-2) binds (RACK/RACK2), and peptide inhibitors targeting εV1-2 inhibit PKCε translocation and function in cardiomyocytes,[16] while peptide agonists augment translocation.[17] It has been demonstrated that altering the dynamics of the (RACK/RACK2) and (RACK1) interaction with PKCε can influence cardiac muscle phenotypes.[18]
Protein kinase C (PKC) is a family of serine- and threonine-specific protein kinases that can be activated by calcium and the second messenger diacylglycerol. PKC family members phosphorylate a wide variety of protein targets and are known to be involved in diverse cellular signaling pathways. PKC family members also serve as major receptors for phorbol esters, a class of tumor promoters. Each member of the PKC family has a specific expression profile and is believed to play a distinct role in cells. The protein encoded by this gene is one of the PKC family members. This kinase has been shown to be involved in many different cellular functions, such as apoptosis, cardioprotection from ischemia, heat shock response, as well as insulin exocytosis.
PKCε has also been shown to play a role in modulating mitochondrial permeability transition (MPT); the addition of PKCε to cardiomyocytes inhibits MPT,[40] though the mechanism is unclear. Initially, PKCε was thought to protect mitochondria from MPT through its association with VDAC1, ANT, and hexokinase II;[40] however, genetic studies have since ruled this out[44][45] and subsequent studies have identified the F0/F1 ATP synthase as a core inner mitochondrial membrane component[46][47][48][49] and Bax and Bak as potential outer membrane components[50] These findings have opened up new avenues of investigation for the role of PKCε at mitochondria. Several likely targets of PKCε action affecting MPT have been discovered. PKCε interacts with ERK, JNKs and p38, and PKCε directly or indirectly phosphorylatesERK and subsequently Bad.[51] PKCε also interacts with Bax in cancer cells, and PKCε modulates its dimerization and function.[52][53] Activation of PKCε with the specific activator, εRACK, prior to ischemic injury has shown to be associated with phosphorylation of the F0/F1 ATP synthase.[54] Moreover, the modulatory component, ANT is regulated by PKCε.[40] These data suggest that PKCε may act at multiple modulatory targets of MPT function; further studies are required to unveil the specific mechanism.
Clinical significance
Cardiac hypertrophy and heart failure
Findings of PKCε phosphorylation in animal models have been verified in humans; PKCε phosphorylatescTnI, cTnT, and MyBPC and depresses the sensitivity of myofilaments to calcium.[55] PKCε induction occurs in the development of cardiac hypertrophy, following stimuli such as myotrophin,[56] mechanical stretch and hypertension.[57] The precise role of PKCε in hypertrophic induction has been debated. The inhibition of PKCε during transition from hypertrophy to heart failure enhances longevity;[58] however, inhibition of PKCε translocation via a peptide inhibitor increases cardiomyocyte size and expression of hypertrophic gene panel.[59] A role for focal adhesion kinase at costameres in strain-sensing and modulation of sarcomere length has been linked to hypertrophy. The activation of FAK by PKCε occurs following a hypertrophic stimulus, which modulates sarcomere assembly.[60][61] PKCε also regulates CapZ dynamics following cyclic strain.[62]
Transgenic studies involving PKCε have also shed light on its function in vivo. Cardiac-specific overexpression of constitutively-active PKCε (9-fold increase in PKCε protein, 4-fold increase in activity) induced cardiac hypertrophy characterizes by enhanced anterior and posterior left ventricular wall thickness.[63] A later study unveiled that the aging of PKCε transgenic mice brought on dilated cardiomyopathy and heart failure by 12 months of age,[64]] characterized by a preserved Frank-Starling mechanism and exhausted contractile reserve.[65] Crossing PKCε transgenic mice with mutant cTnI mice lacking PKCε phosphorylation sites (Serine-43/Serine-45 mutated to Alanine) attenuated the contractile dysfunction and hypertrophic marker expression, offering critical mechanistic insights.[66]
Cardioprotection against Ischemic injury
JM Downey was the first to introduce the role of PKC in cardioprotection against ischemia-reperfusion injury in 1994,;[67] this seminal idea stimulated a series of studies which examined the different isoforms of PKC. PKCε has been demonstrated to be a central player in preconditioning in multiple independent studies, with its best known actions at cardiacmitochondria. It was first demonstrated by Ping et al. that in five distinct preconditioning regimens in conscious rabbits, the epsilon isoform of PKC specifically translocated from the cytosolic to particulate fraction.[12][68] This finding was validated by multiple independent studies occurring shortly thereafter,[69][70] and has since been observed in multiple animal models[71][72][73] and human tissue,[74] as well as in studies employing transgenesis and PKCε activators/inhibitors.[75]
Knockout and molecular studies in mice suggest that this kinase is important for regulating behavioural response to morphine[86] and alcohol.[87][88] It also plays a role lipopolysaccharide (LPS)-mediated signaling in activated macrophages and in controlling anxiety-like behavior.[89]
Substrates and interactions
PKC-epsilon has a wide variety of substrates, including ion channels, other signalling molecules and cytoskeletal proteins.[90]
^ abPing P, Zhang J, Qiu Y, Tang XL, Manchikalapudi S, Cao X, Bolli R (Sep 1997). "Ischemic preconditioning induces selective translocation of protein kinase C isoforms epsilon and eta in the heart of conscious rabbits without subcellular redistribution of total protein kinase C activity". Circulation Research. 81 (3): 404–14. doi:10.1161/01.res.81.3.404. PMID9285643.
^VanWinkle WB, Snuggs MB, De Hostos EL, Buja LM, Woods A, Couchman JR (Sep 2002). "Localization of the transmembrane proteoglycan syndecan-4 and its regulatory kinases in costameres of rat cardiomyocytes: a deconvolution microscopic study". The Anatomical Record. 268 (1): 38–46. doi:10.1002/ar.10130. PMID12209563. S2CID25711666.
^Noland TA, Raynor RL, Jideama NM, Guo X, Kazanietz MG, Blumberg PM, Solaro RJ, Kuo JF (Nov 1996). "Differential regulation of cardiac actomyosin S-1 MgATPase by protein kinase C isozyme-specific phosphorylation of specific sites in cardiac troponin I and its phosphorylation site mutants". Biochemistry. 35 (47): 14923–31. doi:10.1021/bi9616357. PMID8942657.
^Inagaki K, Iwanaga Y, Sarai N, Onozawa Y, Takenaka H, Mochly-Rosen D, Kihara Y (Oct 2002). "Tissue angiotensin II during progression or ventricular hypertrophy to heart failure in hypertensive rats; differential effects on PKC epsilon and PKC beta". Journal of Molecular and Cellular Cardiology. 34 (10): 1377–85. doi:10.1016/s0022-2828(02)92089-4. PMID12392998.
^Heidkamp MC, Bayer AL, Scully BT, Eble DM, Samarel AM (Oct 2003). "Activation of focal adhesion kinase by protein kinase C epsilon in neonatal rat ventricular myocytes". American Journal of Physiology. Heart and Circulatory Physiology. 285 (4): H1684–96. doi:10.1152/ajpheart.00016.2003. PMID12829427. S2CID2800040.
^Montgomery DE, Rundell VL, Goldspink PH, Urboniene D, Geenen DL, de Tombe PP, Buttrick PM (Nov 2005). "Protein kinase C epsilon induces systolic cardiac failure marked by exhausted inotropic reserve and intact Frank-Starling mechanism". American Journal of Physiology. Heart and Circulatory Physiology. 289 (5): H1881–8. doi:10.1152/ajpheart.00454.2005. PMID15951344. S2CID37813298.
^Scruggs SB, Walker LA, Lyu T, Geenen DL, Solaro RJ, Buttrick PM, Goldspink PH (Apr 2006). "Partial replacement of cardiac troponin I with a non-phosphorylatable mutant at serines 43/45 attenuates the contractile dysfunction associated with PKCepsilon phosphorylation". Journal of Molecular and Cellular Cardiology. 40 (4): 465–73. doi:10.1016/j.yjmcc.2005.12.009. PMID16445938.
^Ytrehus K, Liu Y, Downey JM (Mar 1994). "Preconditioning protects ischemic rabbit heart by protein kinase C activation". The American Journal of Physiology. 266 (3 Pt 2): H1145–52. doi:10.1152/ajpheart.1994.266.3.H1145. PMID8160817. S2CID1642830.
^Liu GS, Cohen MV, Mochly-Rosen D, Downey JM (Oct 1999). "Protein kinase C-epsilon is responsible for the protection of preconditioning in rabbit cardiomyocytes". Journal of Molecular and Cellular Cardiology. 31 (10): 1937–48. doi:10.1006/jmcc.1999.1026. PMID10525430.
^Wilson S, Song W, Karoly K, Ravingerova T, Vegh A, Papp J, Tomisawa S, Parratt JR, Pyne NJ (1996). "Delayed cardioprotection is associated with the sub-cellular relocalisation of ventricular protein kinase C epsilon, but not p42/44MAPK". Molecular and Cellular Biochemistry. 160–161: 225–30. doi:10.1007/bf00240053. PMID8901477. S2CID2878489.
^Kawamura S, Yoshida K, Miura T, Mizukami Y, Matsuzaki M (Dec 1998). "Ischemic preconditioning translocates PKC-delta and -epsilon, which mediate functional protection in isolated rat heart". The American Journal of Physiology. 275 (6 Pt 2): H2266–71. doi:10.1152/ajpheart.1998.275.6.H2266. PMID9843828.
^Hassouna A, Matata BM, Galiñanes M (Nov 2004). "PKC-epsilon is upstream and PKC-alpha is downstream of mitoKATP channels in the signal transduction pathway of ischemic preconditioning of human myocardium". American Journal of Physiology. Cell Physiology. 287 (5): C1418–25. doi:10.1152/ajpcell.00144.2004. PMID15294852. S2CID37574971.
^Gregory KN, Hahn H, Haghighi K, Marreez Y, Odley A, Dorn GW, Kranias EG (Feb 2004). "Increased particulate partitioning of PKC epsilon reverses susceptibility of phospholamban knockout hearts to ischemic injury". Journal of Molecular and Cellular Cardiology. 36 (2): 313–8. doi:10.1016/j.yjmcc.2003.12.001. PMID14871559.
^Waza AA, Andrabi K, Hussain MU (Sep 2014). "Protein kinase C (PKC) mediated interaction between conexin43 (Cx43) and K(+)(ATP) channel subunit (Kir6.1) in cardiomyocyte mitochondria: Implications in cytoprotection against hypoxia induced cell apoptosis". Cellular Signalling. 26 (9): 1909–17. doi:10.1016/j.cellsig.2014.05.002. PMID24815185.
^Pyle WG, Smith TD, Hofmann PA (Oct 2000). "Cardioprotection with kappa-opioid receptor stimulation is associated with a slowing of cross-bridge cycling". American Journal of Physiology. Heart and Circulatory Physiology. 279 (4): H1941–8. doi:10.1152/ajpheart.2000.279.4.H1941. PMID11009483. S2CID7497152.
^Pyle WG, Chen Y, Hofmann PA (Sep 2003). "Cardioprotection through a PKC-dependent decrease in myofilament ATPase". American Journal of Physiology. Heart and Circulatory Physiology. 285 (3): H1220–8. doi:10.1152/ajpheart.00076.2003. PMID12763745. S2CID15914929.
^Yang FH, Pyle WG (Mar 2012). "Reduced cardiac CapZ protein protects hearts against acute ischemia-reperfusion injury and enhances preconditioning". Journal of Molecular and Cellular Cardiology. 52 (3): 761–72. doi:10.1016/j.yjmcc.2011.11.013. PMID22155006.
Slater SJ, Ho C, Stubbs CD (Jun 2002). "The use of fluorescent phorbol esters in studies of protein kinase C-membrane interactions". Chemistry and Physics of Lipids. 116 (1–2): 75–91. doi:10.1016/S0009-3084(02)00021-X. PMID12093536.
Aksoy E, Goldman M, Willems F (Feb 2004). "Protein kinase C epsilon: a new target to control inflammation and immune-mediated disorders". The International Journal of Biochemistry & Cell Biology. 36 (2): 183–8. doi:10.1016/S1357-2725(03)00210-3. PMID14643884.
Tolstrup M, Ostergaard L, Laursen AL, Pedersen SF, Duch M (Apr 2004). "HIV/SIV escape from immune surveillance: focus on Nef". Current HIV Research. 2 (2): 141–51. doi:10.2174/1570162043484924. PMID15078178.
External links
Overview of all the structural information available in the PDB for UniProt: Q02156 (Protein kinase C epsilon type) at the PDBe-KB.