Lysophosphatidic acid-induced activation of protein Ser/Thr kinases in cultured rat 3Y1 fibroblasts. Possible involvement in rho p21-mediated signalling

Lysophosphatidic acid protects mesenchymal stem cells against hypoxia and serum deprivation-induced apoptosis

Bone marrow-derived mesenchymal stem cells (MSCs) have shown great promise for cardiac repair. However, poor viability of transplanted MSCs within the ischemic heart has limited their therapeutic potential. Our previous studies have documented that hypoxia and serum deprivation (hypoxia/SD), induced MSCs apoptosis through the mitochondrial apoptotic pathway. Since serum lysophosphatidic acid (LPA) levels are known to be significantly elevated after acute myocardial infarction and that LPA enhanced survival of other cell systems, we embarked on determining whether LPA protects MSCs against hypoxia/SD-induced apoptosis. We have also investigated the potential mechanism(s) that may mediate such actions of LPA. All experiments were carried out on rat bone marrow MSCs. Apoptosis was induced by exposure of cells to hypoxia/SD in a sealed GENbox hypoxic chamber. Effects of LPA were investigated in the absence and presence of inhibitors that target either G(i)proteins, the mitogen activated protein kinases ERK1/2, or phosphoinositide 3-kinase (PI3K). The data obtained showed that hypoxia/SD-induced apoptosis was significantly attenuated by LPA through Gi-coupled LPA(1) receptors linked to the downstream ERK1/2 and PI3K/Akt signaling pathways that function in parallel. Additional studies have demonstrated that hypoxia/SD-induced activation of mitochondrial dysfunction was virtually abolished by LPA treatment and that inhibition of the LPA(1) receptor, Gi proteins, the PI3K/Akt pathway, or ERKs effectively reversed this protective action of LPA. Taken together, our findings indicate that LPA is a novel, potent survival factor for MSCs and this may prove to be of considerable therapeutic significance in terms of exploiting MSC-based therapy in the infracted myocardium.

Lysophosphatidic acid signals through mitogen-activated protein kinase-extracellular signal regulated kinase in ovarian theca cells expressing the LPA1/edg2-receptor: involvement of a nonclassical pathway?

We investigated the mechanism of lysophosphatidic acid (LPA) signaling in ovarian theca cells and observed that stimulation with this bioactive lipid markedly enhanced Thr/Tyr phosphorylation of the MAPK ERK1/2. Activation of ERK was transient, showing a peak at 5 min that declined thereafter, and was not associated with a concomitant nuclear translocation of the enzyme, suggesting that a cytosolic tyrosine phosphatase may be responsible for switching off the signal. Epidermal growth factor (EGF)-induced activation of the enzyme in the same cell system was more rapid (peaking at 1 min), sustainable for at least 60 min, and could be suppressed by prior treatment with either pertussis toxin or a noncompetitive inhibitor of Ras acceptor protein, manumycin A. This functional inhibition of either Gi or Ras failed, however, to affect the LPA-induced ERK-phosphorylation. Surprisingly, functional inhibition of Rho-GTPase, in C3-exotoxin-lipofected cells, markedly reduced LPA-stimulated phosphorylation of ERK, without affecting the EGF-induced stimulation of MAPK. Theca cells labeled with anti-LPA1/edg2-type antibody showed a distinct cell surface labeling, which is reflected in the expression of (LPA1)-type LPA receptors at both mRNA and protein levels. The findings indicate that LPA transiently stimulates MAPK ERK in LPA1/edg2-expressing theca cells and suggest an alternative mechanism regulating the activation of ERK that differs from the canonical EGF-Ras-MAPK kinase pathway.

Activation of Rho signaling contributes to lysophosphatidic acid-induced contraction of intact ileal smooth muscle of guinea-pig

To elucidate the possible role of Rho A/Rho-kinase on lysophosphatidic acid (LPA)-induced contraction in intact guinea-pig ileal smooth muscle, we examined effects of pretreatment with a specific inhibitor of Rho-kinase (Y-27632) on the LPA-induced contraction and MLC20 phosphorylation. In addition, we investigated whether LPA actually elicits an activation of Rho A by studying subcellular distribution of Rho A in unstimulated and stimulated smooth muscles by LPA. LPA induced a less intense, but sustained, contraction compared with ACh, and was accompanied by significant increases in MLC20 phosphorylation. The effects of LPA on tension and MLC20 phosphorylation were inhibited by Y-27632. The ACh-induced contraction, but not increases in MLC20 phosphorylation, was partially inhibited by Y-27632. High K+-induced contraction was unaffected by the inhibitor. LPA stimulated translocation of Rho A from the cytosol to the membrane fraction of the muscle. Translocation of Rho A was also induced by ACh and high K+. These results suggest that LPA-induced contraction of intact ileal smooth muscle is dominated through activation of Rho A and Rho-kinase and subsequent increases in MLC20 phosphorylation.Key words: lysophosphatidic acid, Rho, Rho-kinase, ileal smooth muscle.

The role of Rho in G protein-coupled receptor signal transduction

Low molecular weight G proteins of the Rho subfamily are regulators of actin cytoskeletal organization. In contrast to the heterotrimeric G proteins, the small GTPases are not directly activated through ligand binding to G protein-coupled receptors (GPCRs). However, a subset of GPCRs, including those for lysophosphatidic acid and thrombin, induce stress fibers, focal adhesions, and cell rounding through Rho-dependent pathways. C3 exoenzyme has been a useful tool for demonstrating Rho involvement in these and other responses, including Ca2+ sensitization of smooth muscle contraction, cell migration, transformation, and serum response element-mediated gene expression. Most of the GPCRs that induce Rho-dependent responses can activate Gq, but this is not a sufficient signal. Recent data demonstrate that G alpha 12/13 can induce Rho-dependent responses. Furthermore, G alpha 12/13 can bind and activate Rho-specific guanine nucleotide exchange factors, providing a mechanism by which GPCRs that couple to G alpha 12/13 could activate Rho and its downstream responses.

Osmotic swelling-induced activation of the extracellular-signal-regulated protein kinases Erk-1 and Erk-2 in intestine 407 cells involves the Ras/Raf-signalling pathway

Human Intestine 407 cells respond to hypo-osmotic stress with a rapid stimulation of compensatory ionic conductances accompanied by a transient increase in the activity of the extracellular-signal-regulated protein kinases Erk-1 and Erk-2. In this study, we examined the upstream regulators of hypotonicity-induced Erk-1/Erk-2 activation and their possible role in cell-volume regulation. The hypotonicity-provoked Erk-1/Erk-2 activation was greatly reduced in cells pretreated with the specific mitogen-activated/Erk-activating kinase inhibitor PD098059 and was preceded by a transient stimulation of Raf-1. Pretreatment of the cells with PMA, GF109203X, wortmannin or Clostridium botulinum C3 exoenzyme did not appreciably affect the hypotonicity-provoked Erk-1/Erk-2 stimulation, suggesting the osmosensitive signalling pathway to be largely independent of protein kinase C and p21(rho). In contrast, expression of dominant negative RasN17 completely abolished the hypotonicity-induced Erk-1/Erk-2 activation. Stimulation of the swelling-induced ion efflux was independent of activation of these mitogen-activated protein kinases, as revealed by hypotonicity-provoked isotope efflux from 125I-- and 86Rb+-loaded cells after pretreatment with PD098059 and after expression of RasN17. In addition, the epidermal-growth-factor-induced potentiation of the hypotonicity-provoked anionic response did not depend on the increase in Erk-1/Erk-2 activity but, instead, was found to depend on Ca2+ influx. Taken together, these results indicate that hypotonic stress induces Erk-1/Erk-2 activation through the Ras/Raf-signalling pathway, and argue against a direct role for this pathway in cell-volume control.

Intercellular signaling by lysophosphatidate

Lysophosphatidate (LPA) is an intercellular phospholipid messenger with a wide range of biologic effects. The first discovered source of LPA in the human body were activated platelets, but several other sites of LPA generation are now known. The number of cellular interactions is also growing steadily and responses to the compound range wide, from induction of mitogenesis to neurite retraction. LPA acts via a specific G protein-coupled receptor, of which one or more subtypes may exist. Intracellularly, this receptor activates several heterotrimeric G proteins. LPA induces cell proliferation via the small GTP-binding proteins ras, and triggers actin-based cytoskeletal events through rho. This review describes the most relevant recent developments in our understanding of LPA signaling.

Functional lysophosphatidic acid receptor in bovine luteal cells

The aim of this study was to determine whether luteal cells possess functional receptors for lysophosphatidic acid (LPA). We present evidence that [3H]LPA binds to a 38-40 kDa protein in a membrane fraction prepared from luteal cells and that this phospholipid is able to induce tyrosine phosphorylation of several proteins (65-125 kDa). Furthermore, LPA upregulates forskolin- and LH/GTP-stimulated adenylyl cyclase activity by changing its Vmax. Although a pertussis toxin-sensitive G-protein has been reported to transmit the inhibitory signals between the LPA receptor and adenylyl cyclase, the observed upregulation of the enzyme activity in luteal cells is not abolished after pre-treating the cells with the toxin, suggesting that a different mechanism is operative in these cells. According to the pharmacological regulatory pattern it is suggested that the modulated adenylyl cyclase isoform is the enzyme subtype V expressed in luteal cells.

Glucose activates the carboxyl methylation of gamma subunits of trimeric GTP-binding proteins in pancreatic beta cells. Modulation in vivo by calcium, GTP, and pertussis toxin

The gamma subunits of trimeric G-proteins (gamma1, gamma2, gamma5, and gamma7 isoforms) were found to be methylated at their carboxyl termini in normal rat islets, human islets and pure beta [HIT-T15] cells. Of these, GTPgammaS significantly stimulated the carboxyl methylation selectively of gamma2 and gamma5 isoforms. Exposure of intact HIT cells to either of two receptor-independent agonists--a stimulatory concentration of glucose or a depolarizing concentration of K+--resulted in a rapid (within 30 s) and sustained (at least up to 60 min) stimulation of gamma subunit carboxyl methylation. Mastoparan, which directly activates G-proteins (and insulin secretion from beta cells), also stimulated the carboxyl methylation of gamma subunits in intact HIT cells. Stimulatory effects of glucose or K+ were not demonstrable after removal of extracellular Ca2+ or depletion of intracellular GTP, implying regulatory roles for calcium fluxes and GTP; however, the methyl transferase itself was not directly activated by either. The stimulatory effects of mastoparan were resistant to removal of extracellular Ca2+, implying a mechanism of action that is different from glucose or K+ but also suggesting that dissociation of the alphabetagamma trimer is conducive to gamma subunit carboxyl methylation. Indeed, pertussis toxin also markedly attenuated the stimulatory effects of glucose, K+ or mastoparan without altering the rise in intracellular calcium induced by glucose or K+. Glucose-induced carboxyl methylation of gamma2 and gamma5 isoforms was vitiated by coprovision of any of three structurally different cyclooxygenase inhibitors. Conversely, exogenous PGE2, which activates Gi and Go in HIT cells and which thereby would dissociate alpha from beta(gamma), stimulated the carboxyl methylation of gamma2 and gamma5 isoforms and reversed the inhibition of glucose-stimulated carboxyl methylation of gamma subunits elicited by cyclooxygenase inhibitors. These data indicate that gamma subunits of trimeric G-proteins undergo a glucose- and calcium-regulated methylation-demethylation cycle in insulin-secreting cells, findings that may imply an important role in beta cell function. Furthermore, this is the first example of the regulation of the posttranslational modification of G-protein gamma subunits via nonreceptor-mediated activation mechanisms, which are apparently dependent on calcium influx and the consequent activation of phospholipases releasing arachidonic acid.

Rho effectors and reorganization of actin cytoskeleton

The small GTPase Rho regulates several actomyosin-based cellular processes such as cell adhesion, cytokinesis and contraction. The biochemical mechanisms of these actions remain unknown. Recently, several GTP-Rho binding proteins were isolated. Among them, p140mDia and p160ROCK appear to work as Rho effectors mediating its action on the cytoskeleton. p140mDia induces actin polymerization by recruiting an actin binding protein, profilin, to the site of Rho action. p160ROCK induces focal adhesions and stress fibers by activating integrin and clustering them by the use of myosin-based contractility.

Involvement of the small GTPase rho in integrin-mediated activation of mitogen-activated protein kinase

Engagement and clustering of integrins triggers a number of intracellular signaling events, including activation of the mitogen-activated protein (MAP) kinases Erk1 and Erk2. To investigate the mechanism by which integrins mediate the activation of MAP kinases upon binding of NIH 3T3 cells to fibronectin, we assessed the effects of both inhibiting and activating the small GTPase Rho. We observed that inhibition of Rho by the Rho-specific inhibitor C3 exoenzyme or by a dominant negative Rho A (RhoN19) inhibited MAP kinase activation. Conversely, activation of Rho by expression of an activated Rho A mutant (RhoQ63L), or the Rho-specific guanine nucleotide exchange factor lbc, enhanced and partially mimicked activation of Erk2 by plating on fibronectin. These results therefore show that Rho is involved in the integrin-dependent activation of MAP kinase.

ROCK-I and ROCK-II, two isoforms of Rho-associated coiled-coil forming protein serine/threonine kinase in mice

We recently identified a novel human protein kinase, p160 ROCK, as a putative downstream target of the small GTPase Rho. Using the human ROCK cDNA as a probe, we isolated cDNA of two distinct, highly related sequences from mouse libraries. One encoded a mouse counterpart of human ROCK (ROCK-I), and the other encoded a novel ROCK-related kinase (ROCK-II). Like ROCK/ROCK-I, ROCK-II also bound to GTP-Rho selectively. ROCK-I mRNA was ubiquitously expressed except in the brain and muscle, whereas ROCK-II mRNA was expressed abundantly in the brain, muscle, heart, lung and placenta. These results suggest that at least two ROCK isoforms are present in a single species and play distinct roles in Rho-mediated signalling pathways.

Rho: a connection between membrane receptor signalling and the cytoskeleton

The Rho family of GTP-binding proteins has yielded fresh insights into cell signalling in relation to motility, shape and the control of the actin cytoskeleton. Rho itself is probably near the top of several diverse signalling cascades and has been implicated in cell adhesion, actin filament organization, control of mitogen-activated protein kinase pathways and phospholipid synthesis and turnover. As a member of the Ras superfamily, Rho is regulated by GDP-GTP exchange factors (GEFs) that have homology to the dbl oncogene, and by GTPase-activating proteins (GAPs). These proteins regulate the nucleotide (GDP or GTP) bound to Rho, thus determining the activity of Rho and the interactions of Rho with many of its downstream targets. In the past year, many new targets of Rho have been identified, which hopefully will uncover molecular connections among the diverse downstream effects of Rho activation.

Evidence for Rho-mediated agonist stimulation of phospholipase D in rat1 fibroblasts. Effects of Clostridium botulinum C3 exoenzyme

Small GTP-binding proteins of the Rho family are implicated in the in vitro regulation of phosphatidylcholine hydrolysis by phospholipase D (PLD). However, their role in agonist-stimulated PLD activity in whole cells is not clear. The ribosyltransferase C3 from Clostridium botulinum modifies Rho proteins and inhibits their function. When introduced into rat1 fibroblasts by scrape-loading, C3 inhibited PLD activity stimulated by lysophosphatidic acid (LPA), endothelin-1, or phorbol ester. Neither the time course nor agonist dose response for LPA-stimulated PLD activity was altered in C3-treated cells. In contrast to the effects of C3 on PLD activity, agonist-stimulated phosphatidylinositol-phospholipase C activity was not altered in C3-treated cells. Surprisingly, C3 treatment led to a decrease in the amount of RhoA protein, indicating that the loss of PLD activity in response to agonist was partly due to the loss of Rho proteins. As described previously, C3 treatment led to the inhibition of LPA-stimulated actin filament formation. However, disruption of actin filaments with cytochalasin D caused only a minor inhibition of LPA-stimulated PLD activity. Interestingly, stimulation of cells with LPA caused a rapid enrichment of RhoA in the particulate fraction of cell lysates. These data support an in vivo role for RhoA in agonist-stimulated PLD activity that is separate from its role in actin fiber formation.

Protein kinase N (PKN) and PKN-related protein rhophilin as targets of small GTPase Rho

The Rho guanosine 5'-triphosphatase (GTPase) cycles between the active guanosine triphosphate (GTP)-bound form and the inactive guanosine diphosphate-bound form and regulates cell adhesion and cytokinesis, but how it exerts these actions is unknown. The yeast two-hybrid system was used to clone a complementary DNA for a protein (designated Rhophilin) that specifically bound to GTP-Rho. The Rho-binding domain of this protein has 40 percent identity with a putative regulatory domain of a protein kinase, PKN. PKN itself bound to GTP-Rho and was activated by this binding both in vitro and in vivo. This study indicates that a serine-threonine protein kinase is a Rho effector and presents an amino acid sequence motif for binding to GTP-Rho that may be shared by a family of Rho target proteins.