Protein kinase C


Protein kinase C, commonly abbreviated to PKC, is a family of protein kinase 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, or a member of this family. PKC enzymes in turn are activated by signals such as increases in the concentration of diacylglycerol or calcium ions. Hence PKC enzymes play important roles in several signal transduction cascades.
The PKC family consists of fifteen isozymes in humans. They are divided into three subfamilies, based on their second messenger requirements: conventional, novel, and atypical. Conventional PKCs contain the isoforms α, βI, βII, and γ. These require Ca2+, DAG, and a phospholipid such as phosphatidylserine for activation. Novel PKCs include the δ, ε, η, and θ isoforms, and require DAG, but do not require Ca2+ for activation. Thus, conventional and novel PKCs are activated through the same signal transduction pathway as phospholipase C. On the other hand, atypical PKCs require neither Ca2+ nor diacylglycerol for activation. The term "protein kinase C" usually refers to the entire family of isoforms.

Isozymes

The structure of all PKCs consists of a regulatory domain and a catalytic domain tethered together by a hinge region. The catalytic region is highly conserved among the different isoforms, as well as, to a lesser degree, among the catalytic region of other serine/threonine kinases. The second messenger requirement differences in the isoforms are a result of the regulatory region, which are similar within the classes, but differ among them. Most of the crystal structure of the catalytic region of PKC has not been determined, except for PKC theta and iota. Due to its similarity to other kinases whose crystal structure have been determined, the structure can be strongly predicted.

Regulatory

The regulatory domain or the amino-terminus of the PKCs contains several shared subregions. The C1 domain, present in all of the isoforms of PKC has a binding site for DAG as well as non-hydrolysable, non-physiological analogues called phorbol esters. This domain is functional and capable of binding DAG in both conventional and novel isoforms, however, the C1 domain in atypical PKCs is incapable of binding to DAG or phorbol esters. The C2 domain acts as a Ca2+ sensor and is present in both conventional and novel isoforms, but functional as a Ca2+ sensor only in the conventional. The pseudosubstrate region, which is present in all three classes of PKC, is a small sequence of amino acids that mimic a substrate and bind the substrate-binding cavity in the catalytic domain, lack critical serine, threonine phosphoacceptor residues, keeping the enzyme inactive. When Ca2+ and DAG are present in sufficient concentrations, they bind to the C2 and C1 domain, respectively, and recruit PKC to the membrane. This interaction with the membrane results in release of the pseudosubstrate from the catalytic site and activation of the enzyme. In order for these allosteric interactions to occur, however, PKC must first be properly folded and in the correct conformation permissive for catalytic action. This is contingent upon phosphorylation of the catalytic region, discussed below.

Catalytic

The catalytic region or kinase core of the PKC allows for different functions to be processed; PKB and PKC kinases contains approximately 40% amino acid sequence similarity. This similarity increases to ~ 70% across PKCs and even higher when comparing within classes. For example, the two atypical PKC isoforms, ζ and ι/λ, are 84% identical. Of the over-30 protein kinase structures whose crystal structure has been revealed, all have the same basic organization. They are a bilobal structure with a β sheet comprising the N-terminal lobe and an α helix constituting the C-terminal lobe. Both the ATP- and substrate-binding sites are located in the cleft formed by these two lobes. This is also where the pseudosubstrate domain of the regulatory region binds.
Another feature of the PKC catalytic region that is essential to the viability of the kinase is its phosphorylation. The conventional and novel PKCs have three phosphorylation sites, termed: the activation loop, the turn motif, and the hydrophobic motif. The atypical PKCs are phosphorylated only on the activation loop and the turn motif. Phosphorylation of the hydrophobic motif is rendered unnecessary by the presence of a glutamic acid in place of a serine, which, as a negative charge, acts similar in manner to a phosphorylated residue. These phosphorylation events are essential for the activity of the enzyme, and 3-phosphoinositide-dependent protein kinase-1 is the upstream kinase responsible for initiating the process by transphosphorylation of the activation loop.
The consensus sequence of protein kinase C enzymes is similar to that of protein kinase A, since it contains basic amino acids close to the Ser/Thr to be phosphorylated. Their substrates are, e.g., MARCKS proteins, MAP kinase, transcription factor inhibitor IκB, the vitamin D3 receptor VDR, Raf kinase, calpain, and the epidermal growth factor receptor.

Activation

Upon activation, protein kinase C enzymes are translocated to the plasma membrane by RACK proteins. The protein kinase C enzymes are known for their long-term activation: They remain activated after the original activation signal or the Ca2+-wave is gone. It is presumed that this is achieved by the production of diacylglycerol from phosphatidylinositol by a phospholipase; fatty acids may also play a role in long-term activation.

Function

A multiplicity of functions have been ascribed to PKC. Recurring themes are that PKC is involved in receptor desensitization, in modulating membrane structure events, in regulating transcription, in mediating immune responses, in regulating cell growth, and in learning and memory. These functions are achieved by PKC-mediated phosphorylation of other proteins. However, the substrate proteins present for phosphorylation vary, since protein expression is different between different kinds of cells. Thus, effects of PKC are cell-type-specific:
Cell typeOrgan/systemActivators
ligands --> Gq-GPCRs		Effects
smooth muscle cell digestive system
  • prostaglandin F2α -->
  • thromboxanes
contraction
smooth muscle cells in:
  • iris dilator muscle
  • urethral sphincter
  • uterus
  • arrector pili muscles
  • ureter
  • urinary bladder
    		• Various
  • adrenergic agonists --> α1 receptor
    		• contraction
    smooth muscle cells in:
  • iris constrictor muscle
  • ciliary muscle
    		• sensory systemacetylcholine --> M3 receptorcontraction
    smooth muscle cell circulatory system
  • 5-HT --> 5-HT2A receptor
  • adrenergic agonists --> α1 receptor
  • vasoconstriction
    • smooth muscle cell reproductive system
  • adrenergic agonists --> α1 receptor
    		• ejaculation
    smooth muscle cell digestive system
  • 5-HT --> 5-HT2A or 5-HT2B receptor
  • acetylcholine --> M3 receptor
  • contraction
    • smooth muscle cell respiratory system
  • 5-HT --> 5-HT2A receptor
  • adrenergic agonists --> β₂ receptor
  • acetylcholine --> M3 and M1 receptor
    		• bronchoconstriction
    proximal convoluted tubule cellkidney
  • angiotensin II --> AT1 receptor
  • adrenergic agonists --> α1 receptor
  • stimulate NHE3 --> H+ secretion & Na+ reabsorption
  • stimulate basolateral Na-K ATPase --> Na+ reabsorption
    • neurons in autonomic ganglianervous systemacetylcholine --> M1 receptorEPSP
    neurons in CNSnervous system
  • 5-HT --> 5-HT2A receptor
  • glutamate --> NMDA receptor
  • neuronal excitation
  • memory
    • plateletscirculatory system5-HT --> 5-HT2A receptoraggregation
    ependymal cells ventricular system5-HT --> 5-HT2C receptor↑cerebrospinal fluid secretion
    heart musclecirculatory system
  • adrenergic agonists --> β1 receptor
    		• positive inotropic effect
    serous cells digestive system
  • acetylcholine --> M1 and M3 receptors
  • adrenergic agonists --> β1 receptor
  • ↑secretion
  • increase salivary potassium levels.
    • serous cells digestive system
  • acetylcholine --> M3 receptor
  • ↑secretion
    • adipocytedigestive system/endocrine system
  • adrenergic agonists --> β3 receptor
  • glycogenolysis and gluconeogenesis
    • hepatocytedigestive system
  • adrenergic agonists --> α1 receptor
  • glycogenolysis and gluconeogenesis
    • sweat gland cellsintegumentary system
  • adrenergic agonists --> β2 receptor
  • ↑secretion
    • parietal cellsdigestive systemacetylcholine --> M3 receptors↑ gastric acid secretion

    Pathology

    Protein kinase C, activated by tumor promoter phorbol ester, may phosphorylate potent activators of transcription, and thus lead to increased expression of oncogenes, promoting cancer progression, or interfere with other phenomena. Prolonged exposure to phobol ester, however, promotes the down-regulation of Protein kinase C. Loss-of-function mutations and low PKC protein levels are prevalent in cancer, supporting a general tumor-suppressive role for Protein kinase C.
    Protein kinase C enzymes are important mediators of vascular permeability and have been implicated in various vascular diseases including disorders associated with hyperglycemia in diabetes mellitus, as well as endothelial injury and tissue damage related to cigarette smoke. Low-level PKC activation is sufficient to reverse cell chirality through phosphatidylinositol 3-kinase/AKT signaling and alters junctional protein organization between cells with opposite chirality, leading to an unexpected substantial change in endothelial permeability, which often leads to inflammation and disease.

    Inhibitors

    Protein kinase C inhibitors, such as ruboxistaurin, may potentially be beneficial in peripheral diabetic nephropathy.
    Chelerythrine is a natural selective PKC inhibitor. Other naturally occurring PKCIs are miyabenol C, myricitrin, gossypol.
    Other PKCIs : Verbascoside, BIM-1.
    Bryostatin 1 can act as a PKC inhibitor; It was investigated for cancer.
    Tamoxifen is a PKC inhibitor.

    Activators

    The Protein kinase C activator ingenol mebutate, derived from the plant Euphorbia peplus, is FDA-approved for the treatment of actinic keratosis.
    Bryostatin 1 can act as a PKCe activator and as of 2016 is being investigated for Alzheimer's disease.