Dissociation of tyrosine phosphorylation and activation of phosphoinositide phospholipase C induced by the protein kinase C inhibitor Ro-31-8220 in Swiss 3T3 cells treated with platelet-derived growth factor

Protein Kinase C-Independent Inhibition of Organic Cation Transporter 1 Activity by the Bisindolylmaleimide Ro 31-8220

Ro 31–8220 is a potent protein kinase C (PKC) inhibitor belonging to the chemical class of bisindolylmaleimides (BIMs). Various PKC-independent effects of Ro 31–8220 have however been demonstrated, including inhibition of the ATP-binding cassette drug transporter breast cancer resistance protein. In the present study, we reported that the BIM also blocks activity of the solute carrier organic cation transporter (OCT) 1, involved in uptake of marketed drugs in the liver, in a PKC-independent manner. Ro 31–8220, in contrast to other pan-PKC inhibitors such as staurosporine and chelerythrine, was thus shown to cis-inhibit uptake of the reference OCT1 substrate tetraethylammonium in OCT1-transfected HEK293 cells in a concentration-dependent manner (IC₅₀ = 0.18 μM) and without altering membrane expression of OCT1. This blockage of OCT1 was also observed in human hepatic HepaRG cells that constitutionally express OCT1. It likely occurred through a mixed mechanism of inhibition. Ro 31–8220 additionally trans-inhibited TEA uptake in OCT1-transfected HEK293 cells, which likely discards a transport of Ro 31–8220 by OCT1. Besides Ro 31–8220, 7 additional BIMs, including the PKC inhibitor LY 333531, inhibited OCT1 activity, whereas 4 other BIMs were without effect. In silico analysis of structure-activity relationships next revealed that various molecular descriptors, especially 3D-WHIM descriptors related to total size, correspond to key physico-chemical parameters for inhibition of OCT1 activity by BIMs. In addition to activity of OCT1, Ro 31–8220 inhibited those of other organic cation transporters such as multidrug and toxin extrusion protein (MATE) 1 and MATE2-K, whereas, by contrast, it stimulated that of OCT2. Taken together, these data extend the nature of cellular off-targets of the BIM Ro 31–8220 to OCT1 and other organic cation transporters, which has likely to be kept in mind when using Ro 31–8220 and other BIMs as PKC inhibitors in experimental or clinical studies.

Differential inhibitor sensitivity between human kinases VRK1 and VRK2

Human vaccinia-related kinases (VRK1 and VRK2) are atypical active Ser-Thr kinases implicated in control of cell cycle entry, apoptosis and autophagy, and affect signalling by mitogen activated protein kinases (MAPK). The specific structural differences in VRK catalytic sites make them suitable candidates for development of specific inhibitors. In this work we have determined the sensitivity of VRK1 and VRK2 to kinase inhibitors, currently used in biological assays or in preclinical studies, in order to discriminate between the two proteins as well as with respect to the vaccinia virus B1R kinase. Both VRK proteins and vaccinia B1R are poorly inhibited by inhibitors of different types targeting Src, MEK1, B-Raf, JNK, p38, CK1, ATM, CHK1/2 and DNA-PK, and most of them have no effect even at 100 µM. Despite their low sensitivity, some of these inhibitors in the low micromolar range are able to discriminate between VRK1, VRK2 and B1R. VRK1 is more sensitive to staurosporine, RO-31-8220 and TDZD8. VRK2 is more sensitive to roscovitine, RO 31–8220, Cdk1 inhibitor, AZD7762, and IC261. Vaccinia virus B1R is more sensitive to staurosporine, KU55933, and RO 31–8220, but not to IC261. Thus, the three kinases present a different pattern of sensitivity to kinase inhibitors. This differential response to known inhibitors can provide a structural framework for VRK1 or VRK2 specific inhibitors with low or no cross-inhibition. The development of highly specific VRK1 inhibitors might be of potential clinical use in those cancers where these kinases identify a clinical subtype with a poorer prognosis, as is the case of VRK1 in breast cancer.

A non-canonical Grb2-PLC-gamma1-Sos cascade triggered by lipovitellin 1, an apolipoprotein B homologue

The injection of the Grb2 adapter in Xenopus oocytes promotes G2/M transition without stimulation from a receptor only the first day after the oocytes removal from the ovaries. This cell cycle reinitiation is Ras-dependent and requires the SH2 and SH3 domains of Grb2. The SH2 domain of Grb2 binds the tyrosine phosphorylated lipovitellin1, a homologue of the human apolipoprotein B. The N-SH3 domain of Grb2 is linked to a proline-rich sequence of the C2 domain of PLC-gamma1, PLC-gamma1 itself is linked, through its SH3 domain, to the C-terminal proline-rich region of Sos. When Grb2-PLC-gamma1-Sos is associated, PLC-gamma1 is not phosphorylated on Y783 but shows a phospholipase activity. Inhibition of lipovitellin 1 or PLC-gamma1 avoids Grb2-induced cell cycle reinitiation. Therefore, the Grb2-lipovitellin 1 association is the starting point of a novel signaling pathway, where PLC-gamma1 binds Grb2 and recruits Sos.

Protein kinase C phosphorylates protein kinase D activation loop Ser744 and Ser748 and releases autoinhibition by the pleckstrin homology domain

Persistent activation of protein kinase D (PKD) via protein kinase C (PKC)-mediated signal transduction is accompanied by phosphorylation at Ser(744) and Ser(748) located in the catalytic domain activation loop, but whether PKC isoforms directly phosphorylate these residues, induce PKD autophosphorylation, or recruit intermediate upstream kinase(s) is unclear. Here, we explore the mechanism whereby PKC activates PKD in response to cellular stimuli. We first assessed in vitro PKC-PKD transphosphorylation and PKD activation. A PKD738-753 activation loop peptide was well phosphorylated by immunoprecipitated PKC isoforms, consistent with similarities between the loop and their known substrate specificities. A similar peptide with glutamic acid replacing Ser(748) was preferentially phosphorylated by PKCepsilon, suggesting that PKD containing phosphate at Ser(748) is rapidly targeted by this isoform at Ser(744). When incubated in the presence of phosphatidylserine, phorbol 12,13-dibutyrate and ATP, intact PKD slowly autophosphorylated in the activation loop but only at Ser(748). In contrast, addition of purified PKCepsilon to the incubation mixture induced rapid Ser(744) and Ser(748) phosphorylation, concomitant with persistent 2-3-fold increases in PKD activity, measured using reimmunoprecipitated PKD to phosphorylate an exogenous peptide, syntide-2. We also further examined pleckstrin homology domain-mediated PKD regulation to determine its relationship with activation loop phosphorylation. The high constitutive activity of the pleckstrin homology (PH) domain deletion mutant PKD-deltaPH was not abrogated by mutation of Ser(744) and Ser(748) to alanines, suggesting that one function of activation loop phosphorylation in the PKD activation mechanism is to relieve autoinhibition by the PH domain. These studies provide evidence of a direct PKCepsilon-PKD phosphorylation cascade and provide additional insight into the activation mechanism.

Agonist-specific differential changes of cellular signal transduction pathways in senescent human diploid fibroblasts

Changes in the signal transduction efficiency of senescent cells led us to compare the signaling events induced by two mitogenic agonists, platelet-derived growth factor (PDGF) and lysophosphatidic acid (LPA) in presenescent and senescent or near-senescent human diploid fibroblasts. When the changes in intracellular [Ca(2+)](i) were analyzed, both PDGF and LPA generated a rhythmic increase in [Ca(2+)](i) in presenescent cells. The frequency of calcium response was reduced and desensitized in PDGF-stimulated senescent cells, while response to a LPA-induced calcium signal was also reduced in frequency, though its magnitude was unaltered. PDGF treatment increased the fibrous actin (F-actin) level in presenescent cells but not in senescent cells in contrast to a reduced but visible increase in F-actin in LPA-treated senescent cells. The effect of PDGF on phospholipase D (PLD) activation was also reduced significantly, as a ca. 60-80% reduction of PLD activity was observed in PDGF-stimulated cells but only a little reduction in LPA-induced cells. Agonist-specific differential changes of cellular signaling events caused a differential effect on DNA synthesis after growth factor stimulation. We observed a dramatic (80-90%) reduction of [3H]thymidine incorporation into DNA in the PDGF-stimulated near-senescent cells. LPA resulted in a 2-3-fold increase in thymidine incorporation even in the near-senescent cells. These differences in the responses of senescent or near-senescent cells to PDGF- and LPA-stimulation raised questions about the differential changes of the respective signaling apparatuses induced by aging. Since PDGF signaling event was affected greatly by aging, we further examined the protein contents involved in PDGF signal transduction pathway. PDGF receptor (PDGFR), protein kinase C-alpha (PKC-alpha), phospholipase C-gamma1 (PLC-gamma1), and PLD1 were examined by Western blot analysis. The protein levels of PKC-alpha and PLC-gamma1 were unchanged, but those of PLD1 and PDGFR were reduced with age. The reduced content of PDGFR protein may be one of the important contributors to the failure of PDGF-stimulated signal transduction in human senescent fibroblasts. Our results strongly suggest that age-dependent agonist-specific changes in signaling events might be in charge of the functional deterioration of senescent cells through imbalance of signal responses.

Phospholipase C-gamma mediates the hydrolysis of phosphatidylinositol, but not of phosphatidylinositol 4,5-bisphoshate, in carbamylcholine-stimulated islets of langerhans

In pancreatic islets the activation of phospholipase C (PLC) by the muscarinic receptor agonist carbamyolcholine (carbachol) results in the hydrolysis of both phosphatidylinositol 4,5-bisphosphate (PtdInsP(2)) and phosphatidylinositol (PtdIns). Here we tested the hypothesis that PtdIns hydrolysis is mediated by PLCgamma1, which is known to be regulated by activation of tyrosine kinases and PtdIns 3-kinase. PtdIns breakdown was more sensitive than that of PtdInsP(2) to the tyrosine kinase inhibitor, genistein. Conversely, the tyrosine phosphatase inhibitor, vanadate, alone promoted PtdIns hydrolysis and acted non-additively with carbachol. Vanadate did not stimulate PtdInsP(2) breakdown. Carbachol also stimulated a rapid (maximal at 1-2 min) tyrosine phosphorylation of several islet proteins, although not of PLCgamma1 itself. Two structurally unrelated inhibitors of PtdIns 3-kinase, wortmannin and LY294002, more effectively attenuated the hyrolysis of PtdIns compared with PtdInsP(2). Adenovirally mediated overexpression of PLCgamma1 significantly increased carbachol-stimulated PtdIns hydrolysis without affecting that of PtdInsP(2). Conversely overexpression of PLCbeta1 up-regulated the PtdInsP(2), but not PtdIns, response. These results indicate that the hydrolysis of PtdIns and PtdInsP(2) are independently regulated in pancreatic islets and that PLCgamma1 selectively mediates the breakdown of PtdIns. The activation mechanism of PLCgamma involves tyrosine phosphorylation (but not of PLCgamma directly) and PtdIns 3-kinase. Our findings point to a novel bifurcation of signaling pathways downstream of muscarinic receptors and suggest that hydrolysis of PtdIns and PtdInsP(2) might serve different physiological ends.

The invasion protein InIB from Listeria monocytogenes activates PLC-gamma1 downstream from PI 3-kinase

Entry of the bacterial pathogen Listeria monocytogenes into non-phagocytic mammalian cells is mainly mediated by the InlB protein. Here we show that in the human epithelial cell line HEp-2, the invasion protein InlB activates sequentially a p85beta-p110 class I(A) PI 3-kinase and the phospholipase C-gamma1 (PLC-gamma1) without detectable tyrosine phosphorylation of PLC-gamma1. Purified InlB stimulates association of PLC-gamma1 with one or more tyrosine-phosphorylated proteins, followed by a transient increase in intracellular inositol 1,4,5-trisphosphate (IP3) levels and a release of intracellular Ca2+ in a PI 3-kinase-dependent manner. Infection of HEp-2 cells with wild-type L. monocytogenes bacteria also induces association of PLC-gamma1 with phosphotyrosyl proteins. This interaction is undetectable upon infection with a deltainlB mutant revealing an InlB specific signal. Interestingly, pharmacological or genetic inactivation of PLC-gamma1 does not significantly affect InlB-mediated bacterial uptake, suggesting that InlB-mediated PLC-gamma1 activation and calcium mobilization are involved in post-internalization steps.

Oxidative stress induces protein kinase D activation in intact cells. Involvement of Src and dependence on protein kinase C

Protein kinase D (PKD) is a protein serine kinase that is directly stimulated in vitro by phorbol esters and diacylglycerol in the presence of phospholipids, and activated by phorbol esters, neuropeptides, and platelet-derived growth factor via protein kinase C (PKC) in intact cells. Recently, oxidative stress was shown to activate transfected PKC isoforms via tyrosine phosphorylation, but PKD activation was not demonstrated. Here, we report that oxidative stress initiated by addition of H(2)O(2) (0.15-10 mm) to quiescent Swiss 3T3 fibroblasts activates PKD in a dose- and time- dependent manner, as measured by autophosphorylation and phosphorylation of an exogenous substrate, syntide-2. Oxidative stress also activated transfected PKD in COS-7 cells but not a kinase-deficient mutant PKD form or a PKD mutant with critical activating serine residues 744 and 748 mutated to alanines. Genistein, or the specific Src inhibitors PP-1 and PP-2 (1-10 micrometer) inhibited H(2)O(2)-mediated PKD activation by 45%, indicating that Src contributes to this signaling pathway. PKD activation by H(2)O(2) was also selectively potentiated by cotransfection of PKD together with an active form of Src (v-Src) in COS-7 cells, as compared with PDB-mediated activation. The specific phospholipase C inhibitor, partly blocked H(2)O(2)-mediated but not PDB-mediated PKD activation. In contrast, PKC inhibitors blocked H(2)O(2) or PDB-mediated PKD activation essentially completely, suggesting that whereas Src mediates part of its effects via phospholipase C activation, PKC acts more proximally as an upstream activator of PKD. Together, these studies reveal that oxidative stress activates PKD by initiating distinct Src-dependent and -independent pathways involving PKC.

Nongenomic action of progesterone: activation of Xenopus oocyte phospholipase C through a plasma membrane-associated tyrosine kinase

Using a plasma membrane-cortex preparation (wherein the nucleus and >90% of the total cell protein are removed), progesterone stimulated tyrosine kinase activity that stimulated phospholipase C. Although it has been known for over 20 yr that progesterone acts at the plasma membrane of Xenopus oocytes to induce oocyte maturation, this is the first report that progesterone stimulates this tyrosine kinase activity that is associated with the oocyte plasma membrane and cortex. A tyrosine kinase inhibitor (tyrphostin B46) inhibited steroid stimulation of tyrosine kinase and phospholipase C (PLC) activities, but did not block lipase C stimulation by G protein activators. A fusion protein that contains tandem N- and C-terminal SH2 domains of PLCgamma also blocked progesterone stimulation of PLC (a fusion protein with the SH2 domain from Shc was ineffective). Lowering the Ca2+ concentration in the medium inhibited progesterone, but not guanosine 5'-O-(3-thiotriphosphate), stimulation of PLC, and the effects of progesterone and a G protein agonist were additive. However, neither progesterone nor insulin increased phosphotyrosine on PLCgamma. To evaluate another tyrosine kinase path involving phosphatidylinositol 3-kinase, we added wortmannin to our membrane preparation, but wortmannin did not inhibit progesterone's ability to activate PLC.

Translocation of the Rac1 guanine nucleotide exchange factor Tiam1 induced by platelet-derived growth factor and lysophosphatidic acid

Several guanine nucleotide exchange factors for the Rho family of GTPases that induce activation by exchanging GDP for GTP have been identified. One of these is the tumor invasion gene product Tiam1, which acts on Rac1. In this study, we demonstrate that platelet-derived growth factor (PDGF) and lysophosphatidic acid induce the translocation of Tiam1 to the membrane fraction of NIH 3T3 fibroblasts in a time-dependent manner. Previously, we have shown that Tiam1 is phosphorylated by protein kinase C (PKC) and calcium/calmodulin kinase II (CaMK II) after stimulation with agonists. Here we show, by pretreatment of cells with kinase inhibitors, that CaMK II, but not PKC, is involved in the membrane translocation of Tiam1. Addition of the calcium ionophore ionomycin alone induced the translocation of Tiam1. However, the cell-permeable diacylglycerol oleoylacetylglycerol was without effect and did not enhance the effect of ionomycin. These data further indicated a role for CaMK II and not PKC. Inhibition of phosphoinositide 3-kinase by wortmannin had little effect on the translocation of Tiam1. The role of phosphorylation was further studied by comparing the phosphorylation pattern of Tiam1 in the membranes versus whole cell Tiam1. PDGF-induced phosphorylation of membrane-associated Tiam1 occurred more rapidly than that of the total Tiam1 pool, and CaMK II, but not PKC, played a significant role in this process. Furthermore, by using the p21-binding domain of PAK-3, we show that PDGF, but not lysophosphatidic acid, activates Rac1 in vivo and that this activation involves CaMK II and PKC, but not 3-phosphoinositides. Our results indicate that Tiam1 is translocated to and phosphorylated at membranes after agonist stimulation and that CaMK II, but not PKC, is involved in this process. Also, these kinases are involved in the activation of Rac in vivo.

Muscarinic receptor stimulation increases regulators of G-protein signaling 2 mRNA levels through a protein kinase C-dependent mechanism

RGS2, a member of the Regulators of G-protein Signaling family, modulates the activity of G-proteins coupled to the phosphoinositide signal transduction system, but little is known about what regulates RGS2. In human neuroblastoma SH-SY5Y cells stimulation of muscarinic receptors by carbachol activates phosphoinositide signaling and also caused a rapid, large, and long lasting increase in RGS2 mRNA levels. Direct activation of protein kinase C also rapidly increased RGS2 mRNA levels. Inhibition of protein kinase C with Ro31-8220, GF109203x, or Go6976 or down-regulation of protein kinase C inhibited increases in RGS2 mRNA levels induced by carbachol or by the activation of protein kinase C. Blockade of calcium signaling did not alter carbachol-induced increases in RGS2 mRNA levels. Neither activation of epidermal growth factor receptors nor stimulation of cyclic AMP production with forskolin increased RGS2 mRNA levels. Pretreatment with actinomycin D blocked increases in RGS2 mRNA levels but caused a surprisingly small, although statistically significant, partial blockade of protein kinase C-mediated feedback inhibition of carbachol-induced phosphoinositide hydrolysis. Thus, RGS2 mRNA levels are increased by activation of muscarinic receptors coupled to the phosphoinositide signal transduction system through a protein kinase C-dependent mechanism. This action may contribute to negative feedback control of this signaling cascade, but because the small contribution to negative feedback contrasts with the large and prolonged elevations in RGS2 mRNA levels, we speculate that its primary role may be in modulating other signaling components.

Bisindolylmaleimide VIII facilitates Fas-mediated apoptosis and inhibits T cell-mediated autoimmune diseases

Fas-mediated apoptosis is essential for the elimination of cells, and impaired apoptosis can have severe detrimental consequences. Bisindolylmaleimide VIII potentiated Fas-mediated apoptosis in human astrocytoma 1321N1 cells and in Molt-4T cells, both of which were devoid of apoptosis induced by anti-Fas antibody in the absence of bisindolylmaleimide VIII, and in Jurkat and CEM-6 T cells, which showed slight and moderate apoptotic responses, respectively, to low levels of Fas stimulation. Potentiation of Fas-mediated apoptosis by bisindolylmaleimide VIII was selective for activated, rather than non-activated, T cells, and was Fas-dependent, as it was not observed in T cells from Fas-deficient lpr/lpr mice. Administration of bisindolylmaleimide VIII to rats during autoantigen stimulation prevented the development of symptoms of T cell-mediated autoimmune diseases in two models, the Lewis rat model of experimental allergic encephalitis and the Lewis adjuvant arthritis model. Thus, the use of agents such as bisindolylmaleimide VIII may be therapeutically useful for supporting more effective elimination of detrimental cells through enhancement of Fas-dependent apoptosis signaling.

Signal transduction via platelet-derived growth factor receptors

Platelet-derived growth factor (PDGF) exerts its stimulatory effects on cell growth and motility by binding to two related protein tyrosine kinase receptors. Ligand binding induces receptor dimerization and autophosphorylation, allowing binding and activation of cytoplasmic SH2-domain containing signal transduction molecules. Thereby, a number of different signaling pathways are initiated leading to cell growth, actin reorganization migration and differentiation. Recent observations suggest that extensive cross-talk occurs between different signaling pathways, and that stimulatory signals are modulated by inhibitory signals arising in parallel.

Association of the insulin receptor with phospholipase C-gamma (PLCgamma) in 3T3-L1 adipocytes suggests a role for PLCgamma in metabolic signaling by insulin

Phospholipase C-gamma (PLCgamma) is the isozyme of PLC phosphorylated by multiple tyrosine kinases including epidermal growth factor, platelet-derived growth factor, nerve growth factor receptors, and nonreceptor tyrosine kinases. In this paper, we present evidence for the association of the insulin receptor (IR) with PLCgamma. Precipitation of the IR with glutathione S-transferase fusion proteins derived from PLCgamma and coimmunoprecipitation of the IR and PLCgamma were observed in 3T3-L1 adipocytes. To determine the functional significance of the interaction of PLCgamma and the IR, we used a specific inhibitor of PLC, U73122, or microinjection of SH2 domain glutathione S-transferase fusion proteins derived from PLCgamma to block insulin-stimulated GLUT4 translocation. We demonstrate inhibition of 2-deoxyglucose uptake in isolated primary rat adipocytes and 3T3-L1 adipocytes pretreated with U73122. Antilipolytic effect of insulin in 3T3-L1 adipocytes is unaffected by U73122. U73122 selectively inhibits mitogen-activated protein kinase, leaving the Akt and p70 S6 kinase pathways unperturbed. We conclude that PLCgamma is an active participant in metabolic and perhaps mitogenic signaling by the insulin receptor in 3T3-L1 adipocytes.

IkappaBalpha degradation and nuclear factor-kappaB DNA binding are insufficient for interleukin-1beta and tumor necrosis factor-alpha-induced kappaB-dependent transcription. Requirement for an additional activation pathway

Two closely related IkappaBalpha kinases as well as the upstream kinase, NIK, which integrates interleukin-1beta (IL-1beta)- and tumor necrosis factor (TNF)-alpha-dependent activation of the transcription factor NF-kappaB have recently been described. However, in this emerging pathway the role of previously identified components of cytokine-induced NF-kappaB activation, namely phosphatidylcholine-specific phospholipase C and protein kinase C, remains unclear. We now show that, in A549 human alveolar epithelial cells, the activation of a stably transfected NF-kappaB-dependent reporter gene by TNF-alpha and IL-1beta is completely blocked by the phosphatidylcholine-specific phospholipase C inhibitor D609 and the protein kinase C inhibitor RO31-8220. However, IL-1beta-induced IkappaBalpha degradation as well as NF-kappaB nuclear translocation and DNA binding, as determined by Western blot and electro-mobility shift assay, respectively, are not affected by these inhibitors. A similar effect, although less pronounced, is observed with the p38 mitogen-activated protein kinase inhibitor SB 203580. On the basis of these data we propose the existence of a second signaling pathway induced by IL-1beta and TNF-alpha that is activated in parallel to the cascade leading to IkappaBalpha degradation and is specifically required for NF-kappaB-dependent transcriptional competency.

Stimulation of G-protein coupled receptors in vascular smooth muscle cells induces tyrosine kinase dependent increases in calcium without tyrosine phosphorylation of phospholipase C gamma-1

It is often believed that increases in intracellular Ca2+ ([Ca2+]i) resulting from stimulation of G-protein coupled receptors in vascular smooth muscle cells (VSMC) require activation of the beta1 isoform of phospholipase C (PLC). However, recent studies showed that rat aortic VSMC do not express PLC beta-1 and that stimulation with angiotensin-II induces tyrosine kinase dependent increases in [Ca2+]i and tyrosine phosphorylation of PLC gamma-1. Whether this pathway is activated by other vasoactive agents that stimulate G-protein coupled receptors is unknown. Here, we show that A10 VSMC express PLC beta-2, PLC beta-3, PLC delta-1, and PLC gamma-1. The cells also expressed Galpha(q/11). However, neither PLC beta-1 nor PLC beta-4 was detected. Stimulation with angiotensin-II, vasopressin, serotonin, or endothelin induced tyrosine kinase dependent increases in [Ca2+]i. However, tyrosine phosphorylation of PLC gamma-1 did not occur. In contrast, stimulation with platelet derived growth factor increased [Ca2+]i and tyrosine phosphorylation of PLC gamma-1. The results show that tyrosine phosphorylation of PLC gamma-1 is not required for tyrosine kinase dependent increases in [Ca2+]i resulting from stimulation of diverse G-protein coupled receptors in VSMC.