Definition
Protein Kinase G (PKG) is serine/threonine-specific protein kinases which is dependent on cyclic GMP and catalyzes the phosphorylation of serine or threonine residues of proteins.
Discovery
Ruth P in 1999 studied in vivo functions of PKG by gene targeting. PKG belongs to the AGC family of serine/threonine kinases, is one of the major intercellular receptors for cGMP. Two different genes encode soluble type I (PKG I) and membrane bound type II (PKG II) in mammalian cells. Two major forms of PKG have been identified in mammalian cells, PKG I and PKG II. In addition, there are two splice variants of PKG I, which are designated Iα and Iβ1. Haas B et al., in 2009 found that PKGI controlled insulin signaling in Brown adipose tissue (BAT) by inhibiting the activity of RhoA and Rho-associated kinase (ROCK), thereby relieving the inhibitory effects of ROCK on insulin receptor substrate–1 and activating the downstream phosphoinositide 3-kinase–Akt cascade. PKGI links NO and cGMP signaling with the RhoA-ROCK and the insulin pathways, thereby controlling induction of adipogenic and thermogenic programs during brown fat cell differentiation 2. Sinnaeve P et al., in 2002 tested the effect of PKG overexpression on the neointimal response to vascular injury 3.
Structural Characteristics
The regulatory and catalytic domains of PKG are contained on a single polypeptide chain. PKG dimerization is mediated by a leucine zipper. The N terminal dimerization domain of PKG is known to provide a stable docking surface for interacting proteins that target the kinases to specific cellular locations. Smith-Nguyen et al., solved a crystal structure of the N-terminal domain of PKG Iβ at 1.9 angstrom and 2.1 angstrom. The structure reveals two parallel helices warping around each other into a left-handed helix and forming an extended leucine/isoleucine zipper with 10 pairs of leucine/isoleucines packing in a “knobs-into-holes” manner. Whereas a cluster of four leucines caps the N-terminus, a pair of tyrosine residues caps the C-terminus of the zipper .
Mode of Action
In endothelial cells, activation of PKG I by cGMP is associated with a reduction in thrombin-induced calcium transients and paracellular permeability. Direct evidence for functional roles of PKG I in relaxation of smooth muscle, inhibition of Ca2+ transients, and inhibition of platelet adhesion and aggregation was obtained from studies of PKG I knockout mice. Vasodilator-stimulated phosphoprotein (VASP) is known to be one of the substrates of PKG I, and its phosphorylation plays a role in inhibiting platelet aggregation and focal adhesion. PKG II is expressed mainly in the brush border of the intestinal mucosa and in specific regions of the brain and plays a role in transepithelial Cl- and Na+ transport in the intestine. There is also supporting evidence that activation of PKG by cGMP in cardiomyocytes, pancreatic B cells, and cultured smooth muscle cells can cause growth inhibition and apoptosis. In addition, PKG activation negatively regulates interleukin-2 signaling in T cell lines. Furthermore, recent studies indicate that sulindac sulfone (Aptosyn), a metabolite of the nonsteroidal anti-inflammatory drug sulindac, and two potent derivatives of Aptosyn, OSI-248 and OSI-461, specifically inhibit the cGMP-specific PDEs 2 and 5. Evidence has also been obtained that the resulting increase in cellular levels of cGMP in human colon cancer cells leads to activation of PKG and thereby the induction of apoptosis. These novel effects of Aptosyn and related drugs may explain why these compounds exert anticancer effects in a variety of biological systems even though, in contrast to conventional nonsteroidal anti-inflammatory drugs, they do not inhibit cyclooxygenase activity 4,5.
Functions
PKG in the presence of 8-pCPT-cGMP (cGMP) phosphorylated the two main cardiac-titin isoforms, N2BA and N2B, in human and canine left ventricle. Autoradiography and anti-phosphoserine blotting of recombinant human I-band-titin domains established that PKG phosphorylates titin's N2-B and N2-A domains 6.
PKG I is expressed in platelets, vascular smooth muscle cells, fibroblasts, certain endothelial cells, the lung, the cerebellum, and the heart 7.
PKG activation leads to apoptosis and involve both a decrease in cellular levels of β-catenin and the activation of c-Jun NH2-terminal kinase 1 8.
PKG is essential for brown fat cell differentiation, Induction of adipogenic markers and fat storage was impaired in the absence of PKGI. Furthermore, PKGI mediated the ability of nitric oxide (NO) and guanosine 3',5'-monophosphate (cGMP) to induce mitochondrial biogenesis and increase the abundance of UCP-1 2.
References
1. Ruth P (1999). Cyclic GMP-dependent protein kinases: understanding in vivo functions by gene targeting. Pharmacol Ther., 82:355–372.
2. Haas B, Mayer P, Jennissen K, Scholz D, Diaz MB, Bloch W, Herzig S, Fässler R, Pfeifer A (2009). Protein kinase G controls brown fat cell differentiation and mitochondrial biogenesis. Sci. Signal., 2(99):78.
3. Sinnaeve P, Chiche JD, Gillijns H, Van Pelt N, Wirthlin D, Van De Werf F, Collen D, Bloch KD, Janssens S (2002). Overexpression of a constitutively active protein kinase G mutant reduces neointima formation and in-stent restenosis. Circulation, 105(24): 2911-2916.
4. Uhler MD(1993). Cloning and expression of a novel cyclic GMP-dependent protein kinase from mouse brain. J Biol Chem., 268:13586–13591.
5. Deguchi A, Soh JW, Li H, Pamukcu R, Thompson WJ, Weinstein IB (2002). Vasodilator-stimulated phosphoprotein (VASP) phosphorylation provides a biomarker for the action of exisulind and related agents that activate protein kinase G. Mol Cancer Ther., 1:803–809.
6. Krüger M, Kötter S, Grützner A, Lang P, Andresen C, Redfield MM, Butt E, dos Remedios CG, Linke WA (2009). Protein kinase G modulates human myocardial passive stiffness by phosphorylation of the titin springs. BMC Pharmacology., Circ Res., 104(1):87-94.
7. Lohmann SM, Vaandrager AB, Smolenski A, Walter U, De Jonge HR (1997). Distinct and specific functions of cGMP-dependent protein kinases. Trends Biochem Sci., 22(8):307–312.
8. Soh JW, Mao Y, Liu L, Thompson WJ, Pamukcu R, Weinstein IB (2001). Protein kinase G activates the JNK1 pathway via phosphorylation of MEKK1. J Biol Chem., 276:16406–16410.