Protein kinase C-promoted inhibition of Galpha(11)-stimulated phospholipase C-beta activity

Pharmacological characterization of the involvement of protein kinase C in oscillatory and non-oscillatory calcium increases in astrocytes

Evidence increasingly shows that astrocytes play a pivotal role in brain physiology and pathology via calcium dependent processes, thus the characterization of the calcium dynamics in astrocytes is of growing importance. We have previously reported that the epidermal growth factor and basic fibroblast growth factor up-regulate the oscillation of the calcium releases that are induced by stimuli, including glutamate in cultured astrocytes. This calcium oscillation is assumed to involve protein kinase C (PKC), which is activated together with the calcium releases as a consequence of inositol phospholipid hydrolysis. In the present study, this issue has been investigated pharmacologically by using astrocytes cultured with and without the growth factors. The pharmacological activation of PKC largely reduced the glutamate-induced oscillatory and non-oscillatory calcium increases. Meanwhile, PKC inhibitors increased the total amounts of both calcium increases without affecting the peak amplitudes and converted the calcium oscillations to non-oscillatory sustained calcium increases by abolishing the falling phases of the repetitive calcium increases. Furthermore, the pharmacological effects were consistent between both glutamate- and histamine-induced calcium oscillations. These results suggest that PKC up-regulates the removal of cytosolic calcium in astrocytes, and this up-regulation is essential for calcium oscillation in astrocytes cultured with growth factors.

Phospholipase C signaling tonically represses basal atrial natriuretic factor secretion from the atria of the heart

The cardiac hormone atrial natriuretic factor (ANF or ANP) plays significant, well-established roles in a large number of physiological and pathophysiological processes, including water and electrolyte balance, blood pressure regulation, and cardiovascular growth. Understanding the regulation of its production and secretion by atrial cardiomyocytes is incomplete. We have previously established a significant role of G(i/o) protein signaling in modulating ANF secretion as promoted by stretch of the atrial myocardium. In the present study, we investigated the role of G(q) protein signaling and its relationship to G(i/o) protein signaling using pharmacological manipulation of proximal effectors of G(αq) in an ex vivo model of spontaneously beating rat atria. Phospholipase C (PLC) and protein kinase C (PKC) inhibitors dramatically increased basal secretion of ANF. Furthermore, although atrial wall stretch is a potent stimulus for secretion, stretch unexpectedly reduced ANF secretion to basal levels under PLC and PKC inhibitory conditions. Inhibition of the inositol triphosphate receptor did not appear to affect basal secretion but dose-dependently blocked stretch-secretion coupling. The results obtained demonstrate that the PLC and PKC signaling cascades play important albeit unexpected roles in the regulation of basal and stimulated ANF secretion and suggest interplay between the G(q) and G(i/o) protein signaling pathways.

Spatiotemporal characteristics of calcium dynamics in astrocytes

Although Ca(i)(2+) waves in networks of astrocytes in vivo are well documented, propagation in vivo is much more complex than in culture, and there is no consensus concerning the dominant roles of intercellular and extracellular messengers [inositol 1,4,5-trisphosphate (IP(3)) and adenosine-5'-triphosphate (ATP)] that mediate Ca(i)(2+) waves. Moreover, to date only simplified models that take very little account of the geometrical struture of the networks have been studied. Our aim in this paper is to develop a mathematical model based on realistic cellular morphology and network connectivity, and a computational framework for simulating the model, in order to address these issues. In the model, Ca(i) (2+) wave propagation through a network of astrocytes is driven by IP(3) diffusion between cells and ATP transport in the extracellular space. Numerical simulations of the model show that different kinetic and geometric assumptions give rise to differences in Ca(i)(2+) wave propagation patterns, as characterized by the velocity, propagation distance, time delay in propagation from one cell to another, and the evolution of Ca(2+) response patterns. The temporal Ca(i)(2+) response patterns in cells are different from one cell to another, and the Ca(i)(2+) response patterns evolve from one type to another as a Ca(i)(2+) wave propagates. In addition, the spatial patterns of Ca(i)(2+) wave propagation depend on whether IP(3), ATP, or both are mediating messengers. Finally, two different geometries that reflect the in vivo and in vitro configuration of astrocytic networks also yield distinct intracellular and extracellular kinetic patterns. The simulation results as well as the linear stability analysis of the model lead to the conclusion that Ca(i)(2+) waves in astrocyte networks are probably mediated by both intercellular IP(3) transport and nonregenerative (only the glutamate-stimulated cell releases ATP) or partially regenerative extracellular ATP signaling.

Regulation of the cardiac L-type calcium channel in L6 cells by arginine-vasopressin

L-type Ca2+ channel activity was measured in L6 cells as nifedipine-sensitive barium (Ba2+; 5 mM) influx in a depolarizing salt solution containing 140 mM KCl. Addition of AVP (arginine-vasopressin) during Ba2+ uptake reduced the rate of Ba2+ influx by 60-100%; this was followed by a gradual restoration of the initial rate of Ba2+ uptake. Blockade of PKC (protein kinase C) by pretreatment with 10 muM bisindolylmaleimide did not affect the initial inhibition of Ba2+ influx, but completely abolished the recovery phase. The effect of AVP was half-maximal at 10 nM AVP and was blocked by the V1a receptor antagonist d-(CH2)(5)-Tyr(Me)-AVP. Activation of G(alphas) by isoprenaline or cholera toxin antagonized the actions of AVP on Ba2+ uptake. This protection persisted in the presence of the PKA (protein kinase A) inhibitor KT5720, and was not mimicked by agents that increase cAMP. Inhibition of Ba2+ influx was also elicited by ATP and ET (endothelin 1) with an order of effectiveness ET<ATP<AVP. Each of these agents has been reported to act through G(q)-coupled receptors. We conclude that activation of G(q)-coupled receptors produces a rapid inhibition of the cardiac L-type Ca2+ channel, which is subsequently overcome by activation of PKC.

Modulation of melatonin receptors and G-protein function by microtubules

Chronic melatonin exposure produces microtubule rearrangements in Chinese hamster ovary (CHO) cells expressing the human MT1 melatonin receptor while at the same time desensitizing MT1 receptors. Because microtubule rearrangements parallel MT1 receptor desensitization, we tested whether microtubules modulate receptor responsiveness. We determined whether depolymerization of microtubules by Colcemid, which prevents melatonin-induced outgrowths in MT1-expressing CHO cells, also prevents MT1 receptor desensitization by affecting G(alpha)-GTP exchange on G-proteins. In this study, we found that depolymerization of microtubules in MT1 receptor expressing cells, prevented melatonin-induced receptor desensitization reflected by an increase in the number of high potency sites when compared with melatonin-treated cells. Further examination of the mechanism(s) underlying this desensitization suggested that these effects occurred at the level of G-proteins. Depolymerization of microtubules during melatonin-induced desensitization, attenuated forskolin-induced cAMP accumulation, the opposite of which usually occurs following melatonin exposure alone. Concomitant to this attenuation in the forskolin response was a reduction in the amount of G(i alpha) protein coupled to MT1 receptors and an increase in [32P] azidoanilido GTP incorporation into G(i) proteins. These data are consistent with the findings that microtubule depolymerization did not affect MT1/G(q) coupling nor did it affect melatonin-induced phosphoinositide hydrolysis following melatonin exposure. However, interestingly, microtubule depolymerization enhanced melatonin-induced protein kinase C activation that was blocked in the presence of pertussis toxin. These data demonstrate that microtubule dynamics can modulate melatonin receptor function through their actions on G(i) proteins and impact on downstream signaling cascades.

Desensitization of angiotensin-stimulated inositol phosphate accumulation in human vascular smooth muscle cells

The effect of angiotensin II treatment on desensitization of phospholipase C (PLC)-mediated inositol phosphate accumulation has not been quantitated in human aortic vascular smooth muscle (HVSM) cells. We determined the angiotensin II pretreatment dose dependency and time course for desensitization of PLC activation in HVSM cells and the effect of protein kinase C (PKC) activators on angiotensin II-mediated inositol phosphate accumulation. Our results with PKC activators and direct G protein stimulators suggest that PKC activation may play a negative feedback role in desensitization of angiotensin II-activated signaling in HVSM cells by modifying the Gq transducer, PLC-beta effector, or related proteins in the signaling pathway. However, neither angiotensin II nor PKC activator affected basal phosphorylation levels of PLC-beta1 or PLC-beta3 in HVSM cells; PLC-beta isoenzymes were shown to be phosphorylated in unstimulated cells independent of PKC inhibition. We suggest that desensitization of G protein-stimulated inositol phosphate accumulation in HVSM differs from other cell types in which phosphorylation of PLC-beta isoenzymes accompanies desensitization.

Chronic morphine-induced loss of the facilitative interaction between vasoactive intestinal polypeptide and delta-opioid: involvement of protein kinase C and phospholipase Cbetas

This laboratory recently demonstrated a multiplicative interaction between the pelvic visceral afferent transmitter vasoactive intestinal polypeptide (VIP) and the delta-opioid receptor (DOR)-selective agonist [D-Pen2,5] enkephalin (DPDPE) to regulate cAMP levels in spinal cord [Brain Res. 959 (2003) 103]. Although DOR activation is required for the manifestation of the VIP-DPDPE facilitative interaction, its relevance to opioid antinociception remains unclear. The current study investigates whether or not the VIP-DPDPE facilitation of cAMP formation is subject to tolerance formation, a hallmark characteristic of opioid antinociception. Chronic morphine exposure abolishes the VIP-DPDPE facilitative interaction, consistent with its relevance to DOR antinociception. However, acute in vitro inhibition of protein kinase C (PKC) reinstates the VIP-DPDPE multiplicative interaction characteristic of opioid naïve spinal tissue. This suggests that its chronic morphine-induced loss requires a PKC phosphorylation. PKC phosphorylation negatively modulates phospholipase C (PLC)beta, enzymes intimately associated with phosphoinositide turnover and calcium trafficking. These are essential determinants of acute and chronic opioid effects. Accordingly, the effect of chronic morphine on their state of phosphorylation was also investigated. Central nervous system opioid tolerance is associated with the reciprocal phosphorylation (regulation) of two PLCbeta isoforms, PLCbeta1 and PLCbeta3. However, although chelerythrine reinstates the chronic morphine-induced loss of the multiplicative VIP-DPDPE interaction, it does not alter the associated changes in PLCbeta phosphorylation, possibly indicating different time courses of restitution of function and/or involvement of different kinases for different components of tolerance. These results could provide a mechanistic rubric for understanding positive modulation of opioid antinociception by afferent transmission.

Threonine 308 within a putative casein kinase 2 site of the cytoplasmic tail of leukotriene B(4) receptor (BLT1) is crucial for ligand-induced, G-protein-coupled receptor-specific kinase 6-mediated desensitization

Desensitization of G-protein-coupled receptors may involve phosphorylation of serine and threonine residues. The leukotriene B(4) (LTB(4)) receptor (BLT1) contains 14 intracellular serines and threonines, 8 of which are part of consensus target sequences for protein kinase C (PKC) or casein kinase 2. In this study, we investigated the importance of PKC and GPCR-specific kinase (GRK) phosphorylation in BLT1 desensitization. Pretreatment of BLT1-transfected COS-7 cells with PKC activators caused a decrease of LTB(4)-induced inositol phosphate (IP) accumulation. This reduction was prevented with the PKC inhibitor, staurosporine, and not observed in cells expressing a BLT1 deletion mutant (G291stop) lacking the cytoplasmic tail. Moreover LTB(4)-induced IP accumulation was significantly inhibited by overexpression of GRK2, GRK5, and especially GRK6, in cells expressing wild type BLT1 but not in those expressing G291stop. GRK6-mediated desensitization correlated with increased phosphorylation of BLT1. The G319stop truncated BLT1 mutant displayed functional characteristics comparable with wild type BLT1 in terms of desensitization by GRK6, but not by PKC. Substitution of Thr(308) within a putative casein kinase 2 site to proline or alanine in the full-length BLT1 receptor prevented most of GRK6-mediated inhibition of LTB(4)-induced IP production but only partially affected LTB(4)-induced BLT1 phosphorylation. Our findings thus suggest that Thr(308) is a major residue involved in GRK6-mediated desensitization of BLT1 signaling.

KN-93 inhibition of G protein signaling is independent of the ability of Ca2+/calmodulin-dependent protein kinase II to phosphorylate phospholipase Cbeta3 on 537-Ser

Stimulation of the phospholipase Cbeta (PLC) signaling pathway results in intracellular Ca2+ release and subsequent activation of calmodulin (CaM) and CaM kinase II (CaMK II). KN-93, an inhibitor of CaMK II, reduced the stimulation of phosphatidylinositide (PI) turnover by Galphai-coupled (formyl-Met-Leu-Phe, fMLP) or Galphaq-coupled [M1 muscarinic and oxytocin (OT)] receptors. The inhibitory effect of KN-93 was also observed when PLCbeta3 was stimulated directly by Galphaq or Gbetagamma in overexpression assays. CaMK II phosphorylated PLCbeta3 but not PLCbeta1 in vitro. Phosphorylation occurred exclusively on 537Ser in the X-Y linker region of PLCbeta3. 537Ser was also phosphorylated in the basal state in cells and phosphorylation was enhanced by ionomycin treatment. However, mutation of 537Ser to Glu had no effect on inhibition of Galphaq or Gbetagamma-stimulated PLCbeta3 activity by KN-93. KN-93 also inhibited Galphaq -stimulated PLCbeta1 activity, even though this enzyme is not a substrate for CaMK II. These data indicate that phosphorylation of PLCbeta3 by CaMK II is not directly involved in the inhibitory effect of KN-93 on phosphatidylinositide turnover.

Protein kinase C phosphorylates RGS2 and modulates its capacity for negative regulation of Galpha 11 signaling

RGS proteins (regulators of G protein signaling) attenuate heterotrimeric G protein signaling by functioning as both GTPase-activating proteins (GAPs) and inhibitors of G protein/effector interaction. RGS2 has been shown to regulate Galpha(q)-mediated inositol lipid signaling. Although purified RGS2 blocks PLC-beta activation by the nonhydrolyzable GTP analog guanosine 5'-O-thiophosphate (GTPgammaS), its capacity to regulate inositol lipid signaling under conditions where GTPase-promoted hydrolysis of GTP is operative has not been fully explored. Utilizing the turkey erythrocyte membrane model of inositol lipid signaling, we investigated regulation by RGS2 of both GTP and GTPgammaS-stimulated Galpha(11) signaling. Different inhibitory potencies of RGS2 were observed under conditions assessing its activity as a GAP versus as an effector antagonist; i.e. RGS2 was a 10-20-fold more potent inhibitor of aluminum fluoride and GTP-stimulated PLC-betat activity than of GTPgammaS-promoted PLC-betat activity. We also examined whether RGS2 was regulated by downstream components of the inositol lipid signaling pathway. RGS2 was phosphorylated by PKC in vitro to a stoichiometry of approximately unity by both a mixture of PKC isozymes and individual calcium and phospholipid-dependent PKC isoforms. Moreover, RGS2 was phosphorylated in intact COS7 cells in response to PKC activation by 4beta-phorbol 12beta-myristate 13alpha-acetate and, to a lesser extent, by the P2Y(2) receptor agonist UTP. In vitro phosphorylation of RGS2 by PKC decreased its capacity to attenuate both GTP and GTPgammaS-stimulated PLC-betat activation, with the extent of attenuation correlating with the level of RGS2 phosphorylation. A phosphorylation-dependent inhibition of RGS2 GAP activity was also observed in proteoliposomes reconstituted with purified P2Y(1) receptor and Galpha(q)betagamma. These results identify for the first time a phosphorylation-induced change in the activity of an RGS protein and suggest a mechanism for potentiation of inositol lipid signaling by PKC.

Molecular mechanism of the inhibition of phospholipase C beta 3 by protein kinase C

Activation of protein kinase C (PKC) can result from stimulation of the receptor-G protein-phospholipase C (PLCbeta) pathway. In turn, phosphorylation of PLCbeta by PKC may play a role in the regulation of receptor-mediated phosphatidylinositide (PI) turnover and intracellular Ca(2+) release. Activation of endogenous PKC by phorbol 12-myristate 13-acetate inhibited both Galpha(q)-coupled (oxytocin and M1 muscarinic) and Galpha(i)-coupled (formyl-Met-Leu-Phe) receptor-stimulated PI turnover by 50-100% in PHM1, HeLa, COSM6, and RBL-2H3 cells expressing PLCbeta(3). Activation of conventional PKCs with thymeleatoxin similarly inhibited oxytocin or formyl-Met-Leu-Phe receptor-stimulated PI turnover. The PKC inhibitory effect was also observed when PLCbeta(3) was stimulated directly by Galpha(q) or Gbetagamma in overexpression assays. PKC phosphorylated PLCbeta(3) at the same predominant site in vivo and in vitro. Peptide sequencing of in vitro phosphorylated recombinant PLCbeta(3) and site-directed mutagenesis identified Ser(1105) as the predominant phosphorylation site. Ser(1105) is also phosphorylated by protein kinase A (PKA; Yue, C., Dodge, K. L., Weber, G., and Sanborn, B. M. (1998) J. Biol. Chem. 273, 18023-18027). Similar to PKA, the inhibition by PKC of Galpha(q)-stimulated PLCbeta(3) activity was completely abolished by mutation of Ser(1105) to Ala. In contrast, mutation of Ser(1105) or Ser(26), another putative phosphorylation target, to Ala had no effect on inhibition of Gbetagamma-stimulated PLCbeta(3) activity by PKC or PKA. These data indicate that PKC and PKA act similarly in that they inhibit Galpha(q)-stimulated PLCbeta(3) as a result of phosphorylation of Ser(1105). Moreover, PKC and PKA both inhibit Gbetagamma-stimulated activity by mechanisms that do not involve Ser(1105).