Direct association of RhoA with specific domains of PKC-α

Molecular characterization, expression and function analysis of Epinephelus coioides PKC-ɑ response to Singapore grouper iridovirus (SGIV) infection

Protein kinase C (PKC) constitutes the main signal transduction pathway, and participates in the signal pathway of cell proliferation and movement in mammals. In this study, PKC-ɑ was obtained from Epinephelus coioides, an important marine fish cultivated in the coastal areas of southern China and Southeast Asia. The full length cDNA of PKC-ɑ was 3362 bp in length containing a 23 bp 5'UTR, a 1719 bp 3'UTR, and a 1620 bp open reading frame encoding 539 amino acids. It contains three conservative domains including protein kinase C conserved region 2 (C2), Serine/Threonine protein kinases, catalytic domain (S_TKc) and ser/thr-type protein kinases (S_TK_X). Its mRNA can be detected in all 11 tissues examined of E. coioides, and the expression was significantly upregulated response to Singapore grouper iridovirus (SGIV) infection, one of the important pathogens of marine fish. Upregulated E. coioides PKC-ɑ significantly inhibited the activation of nuclear factor kappa-B (NF-κB) and activator protein-1 (AP-1), and SGIV-induced cell apoptosis. The results indicated that the PKC-ɑ may play an important role in pathogenic stimulation.

Enhancement of Phosphorylation and Transport Activity of the Neuronal Glutamate Transporter Excitatory Amino Acid Transporter 3 by C3bot and a 26mer C3bot Peptide

In primary murine hippocampal neurons we investigated the regulation of EAAT3-mediated glutamate transport by the Clostridium botulinum C3 transferase C3bot and a 26mer peptide derived from full length protein. Incubation with either enzyme-competent C3bot or enzyme-deficient C3bot156–181 peptide resulted in the upregulation of glutamate uptake by up to 22% compared to untreated cells. A similar enhancement of glutamate transport was also achieved by the classical phorbol-ester-mediated activation of protein kinase C subtypes. Yet comparable, effects elicited by C3 preparations seemed not to rely on PKCα, γ, ε, or ζ activation. Blocking of tyrosine phosphorylation by tyrosine kinase inhibitors prevented the observed effect mediated by C3bot and C3bot 26mer. By using biochemical and molecular biological assays we could rule out that the observed C3bot and C3bot 26mer-mediated effects solely resulted from enhanced transporter expression or translocation to the neuronal surface but was rather mediated by transporter phosphorylation at tyrosine residues that was found to be significantly enhanced following incubation with either full length protein or the 26mer C3 peptide.

AMPA receptors modulate the reorganization of F-actin in Bergmann glia cells through the activation of RhoA

Alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid glutamate receptors have been shown to modulate the morphology of the lamelar processes of Bergmann glia cells in the molecular layer of the cerebellum. Here we suggest that reorganization of F-actin may underlay the changes in the morphology of the lamelar processes. Using the fluorescent staining of F-actin with Phalloidin and the quantification of RhoA activation through immunoprecipitation or pull-down assays, we show that RhoA is activated after stimulation of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptors and leads to the reorganization of the actin cytoskeleton of Bergmann fibers. This reorganization of the actin cytoskeleton is reflected in the form of an increase in the intensity of the F-actin staining as well as in the loss of the number of Bergmann fibers stained with Phalloidin. Moreover, using a pharmacological approach, we show that activation of RhoA and the change in the intensity of the F-actin staining depends on the activation of PI3-K, focal adhesion kinase, and protein kinase C, whereas changes in the number of Bergmann fibers depend on external calcium in a RhoA independent manner. Our findings show that glutamate may induce a form of structural plasticity in Bergmann glia cells through the reorganization of the actin cytoskeleton. This may have implications in the way the synaptic transmission is processed in the cerebellum.

Role of guanine nucleotide exchange factor-H1 in complement-mediated RhoA activation in glomerular epithelial cells

Visceral glomerular epithelial cells (GEC), also known as podocytes, are vital for the structural and functional integrity of the glomerulus. The actin cytoskeleton plays a central role in maintaining GEC morphology. In a rat model of experimental membranous nephropathy (passive Heymann nephritis (PHN)), complement C5b-9-induced proteinuria was associated with the activation of the actin regulator small GTPase, RhoA. The mechanisms of RhoA activation, however, remained unknown. In this study, we explored the role of the epithelial guanine nucleotide exchange factor, GEF-H1, in complement-induced RhoA activation. Using affinity precipitation to monitor GEF activity, we found that GEF-H1 was activated in glomeruli isolated from rats with PHN. Complement C5b-9 also induced parallel activation of GEF-H1 and RhoA in cultured GEC. In GEC in which GEF-H1 was knocked down, both basal and complement-induced RhoA activity was reduced. On the other hand, GEF-H1 knockdown augmented complement-mediated cytolysis, suggesting a role for GEF-H1 and RhoA in protecting GEC from cell death. The MEK1/2 inhibitor, U0126, and mutation of the ERK-dependent phosphorylation site (T678A) prevented complement-induced GEF-H1 activation, indicating a role for the ERK pathway. Further, complement induced GEF-H1 and microtubule accumulation in the perinuclear region. However, both the perinuclear accumulation and the activation of GEF-H1 were independent of microtubules and myosin-mediated contractility, as shown using drugs that interfere with microtubule dynamics and myosin II activity. In summary, we have identified complement-induced ERK-dependent GEF-H1 activation as the upstream mechanism of RhoA stimulation, and this pathway has a protective role against cell death.

Protein kinase Cα and integrin-linked kinase mediate the negative axon guidance effects of Sonic hedgehog

In addition to its role as a morphogen, Sonic hedgehog (Shh) has also been shown to function as a guidance factor that directly acts on the growth cones of various types of axons. However, the noncanonical signaling pathways that mediate the guidance effects of Shh protein remain poorly understood. We demonstrate that a novel signaling pathway consisting of protein kinase Cα (PKCα) and integrin-linked kinase (ILK) mediates the negative guidance effects of high concentration of Shh on retinal ganglion cell (RGC) axons. Shh rapidly increased Ca(2+) level and activated PKCα and ILK in the growth cones of RGC axons. By in vitro kinase assay, PKCα was found to directly phosphorylate ILK on threonine-173 and -181. Inhibition of PKCα or expression of a mutant ILK with the PKCα phosphorylation sites mutated (ILK-DM), abolished the Shh-induced macropinocytosis, growth cone collapse and repulsive axon turning. In vivo, expression of a dominant negative PKCα or ILK-DM disrupted RGC axon pathfinding at the optic chiasm but not the projection toward the optic disk, supporting that this signaling pathway plays a specific role in Shh-mediated negative guidance effects.

Protein kinase C-α signals P115RhoGEF phosphorylation and RhoA activation in TNF-α-induced mouse brain microvascular endothelial cell barrier dysfunction

Background

Tumor necrosis factor-α (TNF-α), a proinflammatory cytokine, is capable of activating the small GTPase RhoA, which in turn contributes to endothelial barrier dysfunction. However, the underlying signaling mechanisms remained undefined. Therefore, we aimed to determine the role of protein kinase C (PKC) isozymes in the mechanism of RhoA activation and in signaling TNF-α-induced mouse brain microvascular endothelial cell (BMEC) barrier dysfunction.

Methods

Bend.3 cells, an immortalized mouse brain endothelial cell line, were exposed to TNF-α (10 ng/mL). RhoA activity was assessed by pull down assay. PKC-α activity was measured using enzyme assasy. BMEC barrier function was measured by transendothelial electrical resistance (TER). p115RhoGEF phosphorylation was detected by autoradiography followed by western blotting. F-actin organization was observed by rhodamine-phalloidin staining. Both pharmacological inhibitors and knockdown approaches were employed to investigate the role of PKC and p115RhoGEF in TNF-α-induced RhoA activation and BMEC permeability.

Results

We observed that TNF-α induces a rapid phosphorylation of p115RhoGEF, activation of PKC and RhoA in BMECs. Inhibition of conventional PKC by Gö6976 mitigated the TNF-α-induced p115RhoGEF phosphorylation and RhoA activation. Subsequently, we found that these events are regulated by PKC-α rather than PKC-β by using shRNA. In addition, P115-shRNA and n19RhoA (dominant negative mutant of RhoA) transfections had no effect on mediating TNF-α-induced PKC-α activation. These data suggest that PKC-α but not PKC-β acts as an upstream regulator of p115RhoGEF phosphorylation and RhoA activation in response to TNF-α. Moreover, depletion of PKC-α, of p115RhoGEF, and inhibition of RhoA activation also prevented TNF-α-induced stress fiber formation and a decrease in TER.

Conclusions

Taken together, our results show that PKC-α phosphorylation of p115RhoGEF mediates TNF-α signaling to RhoA, and that this plays a critical role in signaling F-actin rearrangement and barrier dysfunction in BMECs.

Regulation of smooth muscle contraction by small GTPases

Next to changes in cytosolic [Ca(2+)], members of the Rho subfamily of small GTPases, in particular Rho and its effector Rho kinase, also known as ROK or ROCK, emerged as key regulators of smooth muscle function in health and disease. In this review, we will focus on the regulation of the contractile machinery by Rho/ROK signaling and its interaction with PKC and cyclic nucleotide signaling. We will briefly discuss the emerging evidence that remodeling of cortical actin is necessary for contraction.

Effects of nitroglycerin or pentaerithrityl tetranitrate treatment on the gene expression in rat hearts: evidence for cardiotoxic and cardioprotective effects

Nitroglycerin (NTG) and pentaerithrityl tetranitrate (PETN) are organic nitrates used in the treatment of angina pectoris, myocardial infarction, and congestive heart failure. Recent data show marked differences in the effects of NTG and PETN on the generation of reactive oxygen species. These differences are attributed to different effects of NTG and PETN on the expression of antioxidative proteins like the heme oxygenase-I. To analyze the expressional effects of NTG and PETN in a more comprehensive manner we performed whole genome expression profiling experiments using cardiac total RNA from NTG- or PETN-treated rats and DNA microarrays containing oligonucleotides representing 27,044 rat gene transcripts. The data obtained show that NTG and PETN together significantly modify the expression of >1,600 genes (NTG 532, PETN 1212). However, the expression of only a small group of these genes (68) was modified by both treatments, indicating marked differences in the expressional effects of NTG and PETN. NTG treatment resulted in the enhanced expression of genes that are believed to be markers for cardiotoxic processes. In addition, NTG treatment reduced the expression of genes described to code for cardioprotective proteins. In sharp contrast, PETN treatment enhanced the expression of cardioprotective genes and reduced the expression of genes believed to perform cardiotoxic effects. In conclusion, our data suggest that NTG treatment results in the induction of cardiotoxic gene expression networks leading to an activation of mechanisms that result in pathological changes in cardiomyocytes. In contrast, PETN treatment seems to activate gene expression networks that result in cardioprotective effects.

Direct association of calponin with specific domains of PKC-alpha

Calponin contributes to the regulation of smooth muscle contraction through its interaction with F-actin and inhibition of the actin-activated Mg-ATPase activity of phosphorylated myosin. Previous studies have shown that the contractile agonist acetylcholine induced a direct association of translocated calponin and PKC-alpha in the membrane. In the present study, we have determined the domain of PKC-alpha involved in direct association with calponin. In vitro binding assay was carried out by incubating glutathione S-transferase-calponin aa 92-229 with His-tagged proteins of individual domains and different combinations of domains of PKC-alpha. Calponin was found to bind directly to the full-length PKC-alpha. Calponin bound to C2 and C4 domains but not to C1 and C3 domains of PKC-alpha. When incubated with proteins of different combination of domains, calponin bound to C2-C3, C3-C4, and C2-C3-C4 but not to C1-C2 or C1-C2-C3. To determine whether these in vitro bindings mimic the in vivo associations, and in vivo binding assay was performed by transfecting colonic smooth muscle cells with His-tagged proteins of individual domains and different combinations of domains of PKC-alpha. Coimmunoprecipitation of calponin with His-tagged truncated forms of PKC-alpha showed that C1-C2, C1-C2-C3, C2-C3, and C3-C4 did not associate with calponin. Calponin associated only with full-length PKC-alpha and with C2-C3-C4 in cells in the resting state, and this association increased upon stimulation with acetylcholine. These data suggest that calponin bound to fragments that may mimic the active form of PKC-alpha and that the functional association of PKC-alpha with calponin requires both C2 and C4 domains during contraction of colonic smooth muscle cells.

Inadequate activation of the GTPase RhoA contributes to the lack of fibronectin matrix assembly in von Hippel-Lindau protein-defective renal cancer cells

The von Hippel-Lindau (VHL) tumor suppressor gene regulates extracellular matrix deposition. In VHL negative renal cancer cells, VHL(-), the lack of fibronectin matrix assembly is thought to promote and maintain tumor angiogenesis allowing vessels to infiltrate tumors. Therefore, and considering the importance of this process in tumor growth, we aimed to study why VHL(-) renal cancer cells fail to form a proper extracellular matrix. Our results showed that VHL(-) cells were not defective in fibronectin production and that the fibronectin produced by these cells was equally functional in promoting cell adhesion and matrix assembly as that produced by VHL+ cells. We have previously reported that VHL(-) cells fail to form beta1 integrin fibrillar adhesions and have a diminished organization of actin stress fibers; therefore, we aimed to study if the small GTPase family is involved in this process. We found that activation of the RhoA GTPase was defective in VHL(-) cells, and this was possibly mediated by an increased activation of its inhibitor, p190RhoGAP. Additionally, the expression of constitutively active RhoA in VHL(-) cells resulted in formation of a fibronectin matrix. These results strongly suggest an important role for RhoA in some of the defects observed in renal cancer cells.

Myosin light chain kinase activation and calcium sensitization in smooth muscle in vivo

Ca(2+)/calmodulin (CaM)-dependent phosphorylation of myosin regulatory light chain (RLC) in smooth muscle by myosin light chain kinase (MLCK) and dephosphorylation by myosin light chain phosphatase (MLCP) are subject to modulatory cascades that influence the sensitivity of RLC phosphorylation and hence contraction to intracellular Ca(2+) concentration ([Ca(2+)](i)). We designed a CaM-sensor MLCK containing smooth muscle MLCK fused to two fluorescent proteins linked by the MLCK CaM-binding sequence to measure kinase activation in vivo and expressed it specifically in mouse smooth muscle. In phasic bladder muscle, there was greater RLC phosphorylation and force relative to MLCK activation and [Ca(2+)](i) with carbachol (CCh) compared with KCl treatment, consistent with agonist-dependent inhibition of MLCP. The dependence of force on MLCK activity was nonlinear such that at higher concentrations of CCh, force increased with no change in the net 20% activation of MLCK. A significant but smaller amount of MLCK activation was found during the sustained contractile phase. MLCP inhibition may occur through RhoA/Rho-kinase and/or PKC with phosphorylation of myosin phosphatase targeting subunit-1 (MYPT1) and PKC-potentiated phosphatase inhibitor (CPI-17), respectively. CCh treatment, but not KCl, resulted in MYPT1 and CPI-17 phosphorylation. Both Y27632 (Rho-kinase inhibitor) and calphostin C (PKC inhibitor) reduced CCh-dependent force, RLC phosphorylation, and phosphorylation of MYPT1 (Thr694) without changing MLCK activation. Calphostin C, but not Y27632, also reduced CCh-induced phosphorylation of CPI-17. CCh concentration responses showed that phosphorylation of CPI-17 was more sensitive than MYPT1. Thus the onset of agonist-induced contraction in phasic smooth muscle results from the rapid and coordinated activation of MLCK with hierarchical inhibition of MLCP by CPI-17 and MYPT1 phosphorylation.

Oxidoreductase, morphogenesis, extracellular matrix, and calcium ion-binding gene expression in streptozotocin-induced diabetic rat heart

Diabetes has far-ranging effects on cardiac structure and function. Previous gene expression studies of the heart in animal models of type 1 diabetes concur that there is altered expression of genes involved in lipid and protein metabolism, but they diverge with regard to expression changes involving many other functional groups of genes of mechanistic importance in diabetes-induced cardiac dysfunction. To obtain additional information about these controversial areas, genome-wide expression was assessed using microarrays in left ventricle from streptozotocin-diabetic and normal rats. There were 261 genes with statistically significant altered expression of at least +/-1.5-fold, of which 124 were increased and 137 reduced by diabetes. Gene ontology assignment testing identified several statistical significantly overrepresented groups among genes with altered expression, which differed for increased compared with reduced expression. Relevant gene groups with increased expression by diabetes included lipid metabolism (P < 0.001, n = 13 genes, fold change 1.5 to 14.6) and oxidoreductase activity (P < 0.001, n = 17, fold change 1.5 to 4.6). Groups with reduced expression by diabetes included morphogenesis (P < 0.00001, n = 28, fold change -1.5 to -5.1), extracellular matrix (P < 0.02, n = 9, fold change -1.5 to -3.9), cell adhesion (P < 0.05, n = 10, fold change -1.5 to -2.7), and calcium ion binding (P < 0.01, n = 13, fold change -1.5 to -3.0). Array findings were verified by quantitative PCR for 36 genes. These data combined with previous findings strengthen the evidence for diabetes-induced cardiac gene expression changes involved in cell growth and development, oxidoreductase activity, and the extracellular matrix and also point out other gene groups not previously identified as being affected, such as those involved in calcium ion homeostasis.

RhoA signaling in phorbol ester-induced apoptosis

Exposure of cells to phorbol ester activates protein kinase C (PKC) to induce apoptosis or differentiation, depending on the cellular context. In erythroblastic cell lines, TF-1 and D2, upregulation of the RhoA signaling promotes phorbol ester-induced apoptosis through activating Rho-associated kinase (ROCK)/phosphorylation of myosin light chain (MLC), thus generating membrane contraction force. As a result, cell adhesion is inhibited and death receptor-mediated death pathway is activated in these cells with a concurrent changes in nucleocytoplasmic signaling for protein trafficking. A microtubule-regulated GEF-H1, which is a specific RhoA activator, was identified to contribute to RhoA activation in these cells. Thus, a cytoskeleton-regulated RhoA signaling cooperates with PKC activation constitutes a cellular context to determine the cell fate in response to phorbol ester stimulation.

RhoA- and PKC-alpha-mediated phosphorylation of MYPT and its association with HSP27 in colonic smooth muscle cells

Agonist-induced activation of the RhoA/Rho kinase (ROCK) pathway results in inhibition of myosin phosphatase and maintenance of myosin light chain (MLC20) phosphorylation. We have shown that RhoA/ROCKII translocates and associates with heat shock protein (HSP)27 in the particulate fraction. We hypothesize that inhibition of the 130-kDa regulatory myosin-binding subunit (MYPT) requires its association with HSP27 in the particulate fraction. Furthermore, it is not certain whether regulation of MYPT by CPI-17 or by ROCKII is due to cross talk between RhoA and PKC-alpha. Presently, we examined the cross talk between RhoA and PKC-alpha in the regulation of MYPT phosphorylation in rabbit colon smooth muscle cells. Acetylcholine induced 1) sustained phosphorylation of PKC-alpha, CPI-17, and MYPT; 2) an increase in the association of phospho-MYPT with HSP27 in the particulate fraction; 3) a decrease in myosin phosphatase activity (66.21+/-3.52 and 42.19+/-3.85% nM/ml lysate at 30 s and 4 min); and 4) an increase in PKC activity (298.12+/-46.60% and 290.59+/-22.07% at 30 s and 4 min). Inhibition of RhoA/ROCKII by Y-27632 inhibited phosphorylation of MYPT and its association with HSP27. Both Y27632 and a negative dominant construct of RhoA inhibited phosphorylation of MYPT and CPI-17. Inhibition of PKCs or calphostin C or selective inhibition of PKC-alpha by negative dominant constructs inhibited phosphorylation of MYPT and CPI-17. The results suggest that 1) acetylcholine induces activation of both RhoA and/or PKC-alpha pathways, suggesting cross talk between RhoA and PKC-alpha resulting in phosphorylation of MYPT, inhibition of myosin phosphatase activity, and maintenance of MLC phosphorylation; and 2) phosphorylated MYPT is associated with HSP27 and translocated to the particulate fraction, suggesting a scaffolding role for HSP27 in mediating the association of the complex MYPT/RhoA-ROCKII. Thus both pathways (PKC and RhoA) converge on the regulation of myosin phosphatase activities and modulate sustained phosphorylation of MLC20.