Differential translocation of rho family GTPases by lysophosphatidic acid, endothelin-1, and platelet-derived growth factor

Dynamin and rab5 regulate GRK2-dependent internalization of dopamine D2 receptors

Dopamine D2 receptors (D2Rs; short form, which is one of the alternative splicing variants) expressed in COS-7 cells are internalized in an agonist-dependent manner only when G protein-coupled receptor kinase 2 (GRK2) is coexpressed [Ito, K., Haga, T., Lameh, J. & Sadée, W., (1999) Eur. J. Biochem. 260, 112-119]. We have examined the effects of coexpression of dynamin, a small molecular mass GTP-binding protein, rab5A, and their mutants on the internalization of D2Rs in the presence of both dopamine (10 or 100 microM) and GRK2. The rate and extent of D2R internalization was increased or decreased by coexpression of dynamin I or a dominant-negative form of dynamin I (dynamin I K44E), respectively. The effects of coexpressing these two dynamins were more prominent at 10 microM dopamine than at 100 microM. In the presence of 10 microM dopamine, internalization of D2R was completely suppressed when dynamin I K44E was coexpressed, and the half-life (t 1/2) of D2R internalization decreased relative to cells not expressing dynamin from 82 to 29 min when dynamin I was coexpressed. Internalization of D2Rs was facilitated or suppressed by coexpression of a constitutively active form of rab5A (rab5A Q79L) or a dominant-negative form of rab5A (rab5A S34N), respectively. The t 1/2 of D2R internalization at 10 microM dopamine decreased from 82 to 16 min in cells coexpressing rab5A Q79L. The effect of coexpression of rab5A S34N was more apparent at 100 microM dopamine than at 10 microM; the t 1/2 of D2R internalization at 100 microM dopamine increased from 20 to 56 min and the proportion of internalized D2Rs after 120 min decreased from 53 to 28%. These results indicate that the internalization of D2Rs is dependent on the action of dynamin as well as GRK2, and is regulated by the action of rab5A.

Rho and Rho kinase mediate thrombin-stimulated vascular smooth muscle cell DNA synthesis and migration

Aberrant regulation of smooth muscle cell proliferation and migration is associated with the pathophysiology of vascular disorders such as hypertension, atherosclerosis, restenosis, and graft rejection. To elucidate molecular mechanisms that regulate proliferation and migration of vascular smooth muscle cells, we determined whether signaling through the small G protein Rho is involved in thrombin- and phenylephrine-stimulated proliferation and migration of rat aortic smooth muscle cells (RASMCs). Thrombin and the thrombin peptide SFLLRNP stimulated DNA synthesis of RASMCs as measured by [3H]thymidine incorporation. Both ligands also increased cell migration as measured by the Boyden chamber method. L-Phenylephrine failed to induce either of these responses but increased inositol phosphate accumulation and mitogen-activated protein kinase activation in these cells, which indicated that the cells were responsive to alpha1-adrenergic stimulation. The C3 exoenzyme, which ADP-ribosylates and inactivates Rho, fully inhibited both thrombin-stimulated proliferation and migration but had no effect on inositol phosphate accumulation. In addition, Y-27632, an inhibitor of the Rho effector p160ROCK/Rho kinase, decreased thrombin-stimulated DNA synthesis and migration. To directly examine Rho activation, Rho-[35S]GTPgammaS binding was measured. The addition of the thrombin peptide SFLLRNP, but not phenylephrine, to RASMC lysates resulted in a significant increase in Rho-[35S]GTPgammaS binding. Thrombin and SFLLRNP, but not phenylephrine, also increased membrane-associated Rho in intact RASMCs, consistent with selective activation of Rho by thrombin. These results indicate that thrombin activates Rho in RASMCs and establish Rho as a critical mediator of thrombin receptor effects on DNA synthesis and cell migration in these cells.

Ca2+/calmodulin-dependent protein kinase II regulates Tiam1 by reversible protein phosphorylation

A number of guanine nucleotide exchange factors have been identified that activate Rho family GTPases, by promoting the binding of GTP to these proteins. We have recently demonstrated that lysophosphatidic acid and several other agonists stimulate phosphorylation of the Rac1-specific exchange factor Tiam1 in Swiss 3T3 fibroblasts, and that protein kinase C is involved in Tiam1 phosphorylation (Fleming, I. N., Elliott, C. M., Collard, J. G., and Exton, J. H. (1997) J. Biol. Chem. 272, 33105-33110). We now show, through manipulation of intracellular [Ca2+] and the use of protein kinase inhibitors, that both protein kinase Calpha and Ca2+/calmodulin-dependent protein kinase II are involved in the phosphorylation of Tiam1 in vivo. Furthermore, we show that Ca2+/calmodulin-dependent protein kinase II phosphorylates Tiam1 in vitro, producing an electrophoretic retardation on SDS-polyacrylamide gel electrophoresis. Significantly, phosphorylation of Tiam1 by Ca2+/calmodulin-dependent protein kinase II, but not by protein kinase C, enhanced its nucleotide exchange activity toward Rac1, by approximately 2-fold. Furthermore, Tiam1 was preferentially dephosphorylated by protein phosphatase 1 in vitro, and treatment with this phosphatase abolished the Ca2+/calmodulin-dependent protein kinase II activation of Tiam1. These data demonstrate that protein kinase Calpha and Ca2+/calmodulin-dependent protein kinase II phosphorylate Tiam1 in vivo, and that the latter kinase plays a key role in regulating the activity of this exchange factor in vitro.

Distinct roles for the small GTPases Cdc42 and Rho in endothelial responses to shear stress

Shear stress, the tangential component of hemodynamic forces, plays an important role in endothelial remodeling. In this study, we investigated the role of Rho family GTPases Cdc42 and Rho in shear stress-induced signal transduction and cytoskeleton reorganization. Our results showed that shear stress induced the translocation of Cdc42 and Rho from cytosol to membrane. Although both Cdc42 and Rho were involved in the shear stress-induced transcription factor AP-1 acting on the 12-O-tetradecanoyl-13-phorbol-acetate-responsive element (TRE), only Cdc42 was sufficient to activate AP-1/TRE. Dominant-negative mutants of Cdc42 and Rho, as well as recombinant C3 exoenzyme, attenuated the shear stress activation of c-Jun NH2-terminal kinases (JNKs), suggesting that Cdc42 and Rho regulate the shear stress induction of AP-1/TRE activity through JNKs. Shear stress-induced cell alignment and stress fiber formation were inhibited by the dominant-negative mutants of Rho and p160ROCK, but not by the dominant-negative mutant of Cdc42, indicating that the Rho-p160ROCK pathway regulates the cytoskeletal reorganization in response to shear stress.

Involvement of RhoA and its interaction with protein kinase C and Src in CCK-stimulated pancreatic acini

We evaluated intracellular pathways responsible for the activation of the small GTP-binding protein Rho p21 in rat pancreatic acini. Intact acini were incubated with or without CCK and carbachol, and Triton X-100-soluble and crude microsomes were used for Western immunoblotting. When a RhoA-specific antibody was used, a single band at the location of 21 kDa was detected. CCK (10 pM-10 nM) and carbachol (0.1-100 microM) dose dependently increased the amount of immunodetectable RhoA with a peak increase occurring at 3 min. High-affinity CCK-A-receptor agonists JMV-180 and CCK-OPE (1-1,000 nM) did not increase the intensities of the RhoA band, suggesting that stimulation of RhoA is mediated by the low-affinity CCK-A receptor. Although an increase in RhoA did not require the presence of extracellular Ca2+, the intracellular Ca2+ chelator 1, 2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-AM abolished the appearance of the RhoA band in response to CCK and carbachol. The Gq protein inhibitor G protein antagonist-2A (10 microM) and the phospholipase C (PLC) inhibitor U-73122 (10 microM) markedly reduced RhoA bands in response to CCK. The protein kinase C (PKC) activator phorbol ester (10-1,000 nM) dose dependently increased the intensities of the RhoA band, which were inhibited by the PKC inhibitor K-252a (1 microM). The pp60(c-src) inhibitor herbimycin A (6 microM) inhibited the RhoA band in response to CCK, whereas the calmodulin inhibitor W-7 (100 microM) and the phosphoinositide 3-kinase inhibitor wortmannin (6 microM) had no effect. RhoA was immunoprecipitated with Src, suggesting association of RhoA with Src. Increases in mass of this complex were observed with CCK stimulation. In permeabilized acini, the Rho inhibitor Clostridium botulinum C3 exoenzyme dose dependently inhibited amylase secretion evoked by a Ca2+ concentration with an IC50 of C3 exoenzyme at 1 ng/ml. We concluded that the small GTP-binding protein RhoA p21 exists in pancreatic acini and appears to be involved in the mediation of pancreatic enzyme secretion evoked by CCK and carbachol. RhoA pathways are involved in the activation of PKC and Src cascades via Gq protein and PLC.

Role of tyrosine phosphorylation in thrombin-induced endothelial cell contraction and barrier function

Thrombin-induced endothelial cell (EC) barrier dysfunction is highly dependent upon phosphorylation of serine and threonine residues present on myosin light chains (MLC) catalyzed by a novel EC myosin light chain kinase (MLCK) isoform. In this study, we examined the participation of tyrosine protein phosphorylation in EC contraction, gap formation and barrier dysfunction. We first determined that thrombin significantly increases protein tyrosine kinase activity and protein tyrosine phosphorylation in bovine pulmonary artery EC. Tyrosine kinase inhibitors, genistein and 2,5 DHC, reduced EC tyrosine kinase activities, however, only genistein significantly attenuated thrombin-mediated increases in albumin clearance and reductions in transendothelial electrical resistance. Similarly, genistein but not 2,5 DHC, decreased basal and thrombin-induced Ca2+ increases and MLC phosphorylation in the absence of alterations in Type 1 or 2A serine/threonine phosphatase activities. Immunoprecipitation of the EC MLCK isoform revealed a 214 kD immunoreactive phosphotyrosine protein and genistein pretreatment significantly reduced MLCK activity in MLCK immunoprecipitates. Although thrombin induced the translocation of p60src from the cytosol to the EC cytoskeleton, a detectable increase in the level of MLCK tyrosine phosphorylation was not noted after thrombin challenge. Taken together, our data suggest that genistein-sensitive tyrosine kinase activities are involved in thrombin-mediated EC MLCK activation, MLC phosphorylation, and barrier dysfunction.

Association of phospholipase D activity with the detergent-insoluble cytoskeleton of U937 promonocytic leukocytes

Phospholipase D (PLD) regulates cytoskeletal-dependent antimicrobial responses of myeloid leukocytes, including phagocytosis and oxidant generation. However, the mechanisms responsible for this association between PLD activity and the actin cytoskeleton are unknown. We utilized a cell-free system from U937 promonocytes to test the hypothesis that stimulation of PLD results in stable association of the activated lipase with the detergent-insoluble membrane skeleton. Plasma membrane and cytosol were incubated +/- guanosine 5'-3-O-(thio)triphosphate (GTPgammaS), followed by re-isolation and extraction of the washed membranes with octyl glucoside. The detergent-insoluble fraction derived from membranes incubated with GTPgammaS (DIFGTPgammaS) exhibited 22-fold greater PLD activity than that derived from control membranes (DIF0), when both were assayed in the presence of GTPgammaS. The DIF contained PLD1, RhoA, and ARF, and the level of each was increased by GTPgammaS in a dose-dependent manner. The DIF also contained F-actin, vinculin, talin, paxillin, and alpha-actinin, consistent with its identification as the membrane skeleton. The physiologic relevance of these findings was demonstrated by a similar increase in DIF-associated PLD activity after stimulation of intact U937 cells with opsonized zymosan. These results indicate that stimulation of PLD1 is accompanied by stable association of the activated lipase, RhoA, and ADP-ribosylation factor with the actin-based membrane skeleton.

Rho A regulates sustained smooth muscle contraction through cytoskeletal reorganization of HSP27

The ras-related protein Rho p21 regulates various actin-dependent functions, including smooth muscle contraction. However, the precise mechanism of action of Rho p21 is still not clear. We report here that Rho A is a key regulator of agonist-induced contractile effects in rabbit colonic smooth muscle. Endothelin-1 and C2 ceramide were used. Both seem to activate phosphoinositide 3-kinase (PI 3-kinase) through G protein and pp60(src), respectively. Immunoprecipitation and immunoblotting revealed one form of 21-kDa Rho A that translocated from the cytosol to the membrane in response to stimulation by either endothelin (10(-7) M) or ceramide (10(-7) M) ( approximately 30% increase at 30 s that was sustained at 4 min). The translocation of Rho A to the membrane was confirmed by immunostaining. The translocation of Rho A was inhibited by Clostridium botulinum C3 exoenzyme, which ADP ribosylated Rho A, but was not inhibited by the pp60(src) inhibitor herbimycin A or by the protein kinase C (PKC) inhibitor calphostin C, suggesting that Rho A may be upstream of pp60(src) and PKC or may belong to a different pathway than these proteins. Both ceramide- and endothelin-induced PI 3-kinase activation was inhibited by C3 exoenzyme pretreatment. However, the C3 exoenzyme inhibited endothelin- but not ceramide-induced mitogen-activated protein kinase phosphorylation, indicating that Rho regulates ceramide- and endothelin-induced contraction through different pathways. Furthermore, the dominant negative form of Rho (N19Rho) inhibited the actin binding protein, 27-kDa heat shock protein (HSP27), reorganization in response to ceramide and endothelin observed under confocal microscopy.

Involvement of small GTPases in the regulation of smooth muscle contraction

Neurohumoral stimulation of smooth muscle leads to an increased responsiveness of the myofilaments to Ca2+. This review provides a summary of the data that suggest that the signalling from the membrane-bound serpentine receptors to the contractile apparatus leading to the increase in Ca(2+)-sensitivity requires the activation of the Ras-related low molecular mass GTPase Rho. In smooth muscle permeabilized with alpha-toxin or beta-escin, the increase in force elicited by different agonists at fixed [Ca2+] (Ca(2+)-sensitization) can be inhibited by bacterial toxins (EDIN, and exoenzyme C3) which ADP-ribosylate and inactivate Rho proteins. Moreover, the agonist-induced increase in Ca(2+)-sensitivity can be mimicked by constitutively active recombinant Rho proteins. The physiological relevance of this mechanism is suggested by the fact that toxins that are internalized into intact cells (toxin B from C. difficile and a chimeric toxin (DC3B) consisting of C3 and the (non-catalytic) B fragment of diphteria toxin (inhibit the tonic phase of an agonist-induced contraction. Toxin B inhibits contraction without affecting the intracellular Ca(2+)-transient determined with fura-2. However, it inhibits phosphorylation of the regulatory light chains of myosin (MLC). Rho has been suggested to activate a Rho-associated kinase which in turn phosphorylates the myosin binding subunit of the myosin light chain phosphatase. This would lead to an increase in phosphorylation of MLC and hence of force at constant Ca2+. The Ca(2+)-sensitizing effect of agonists is also inhibited by tyrosine kinase inhibitors. This suggests the possibility that in smooth muscle, like in non-muscle cells, there is a cross-talk between Rho and tyrosine kinases.

Role of rho proteins in agonist regulation of phospholipase D in HL-60 cells

Rho family GTP-binding proteins have been demonstrated to play a role in the regulation of phospholipase D (PLD) activity. In the present study, we examined the role of Rho proteins in PLD activation in differentiated HL-60 cells using C3 exoenzyme from Clostridium botulinum, which ADP-ribosylates and inactivates Rho proteins. Introduction of C3 exoenzyme into differentiated HL-60 cells by electroporation resulted in complete inhibition of PLD activity stimulated by formyl methionine-leucine-phenylalanine (fMLP) and ATP, two receptor agonists. Phorbol myristate acetate-induced PLD activation was also inhibited in C3 exoenzyme-treated cells, but the inhibition was only partial. GTPgammaS-dependent activation of PLD, measured in the absence or presence of ATP in permeabilized cells, was also partially affected by C3 exoenzyme treatment. Thus, these results indicate that Rho proteins play a key role in receptor-mediated PLD regulation in differentiated HL-60 cells, but play a partial role in the in vivo action of PMA and in vitro action of GTPgammaS on PLD. ATP produced a significant enhancement of the in vitro effect of GTPgammaS on PLD activity, but the effect of ATP was not altered by inhibitors of serine/threonine and tyrosine kinases. However, it was markedly reduced by neomycin and accompanied by an increase in phosphatidylinositol 4,5-bisphosphate (PtdInsP2) synthesis. These data indicate that in permeabilized HL-60 cells, the stimulatory effect of ATP on PLD does not involve protein phosphorylation but is due to an increase in PtdInsP2.

Tec/Bmx non-receptor tyrosine kinases are involved in regulation of Rho and serum response factor by Galpha12/13

A transient transfection system was used to identify regulators and effectors for Tec and Bmx, members of the Tec non-receptor tyrosine kinase family. We found that Tec and Bmx activate serum response factor (SRF), in synergy with constitutively active alpha subunits of the G12 family of GTP-binding proteins, in transiently transfected NIH 3T3 cells. The SRF activation is sensitive to C3, suggesting the involvement of Rho. The kinase and Tec homology (TH) domains of the kinases are required for SRF activation. In addition, kinase-deficient mutants of Bmx are able to inhibit Galpha13- and Galpha12-induced SRF activation, and to suppress thrombin-induced SRF activation in cells lacking Galphaq/11, where thrombin's effect is mediated by G12/13 proteins. Moreover, expression of Galpha12 and Galpha13 stimulates autophosphorylation and transphosphorylation activities of Tec. Thus, the evidence indicates that Tec kinases are involved in Galpha12/13-induced, Rho-mediated activation of SRF. Furthermore, Src, which was previously shown to activate kinase activities of Tec kinases, activates SRF predominantly in Rho-independent pathways in 3T3 cells, as shown by the fact that C3 did not block Src-mediated SRF activation. However, the Rho-dependent pathway becomes significant when Tec is overexpressed.

Localization of RhoA GTPase to endothelial caveolae-enriched membrane domains

Caveolae are small microdomains of the plasma membrane that are thought to play important roles in signal transduction processes. In this work, we have investigated the association of Rho proteins with caveolae-enriched membrane domains isolated from cultured endothelial cells. Fractionation of ECV304 cells by sucrose gradient density centrifugation in the absence of detergent resulted in the co-sedimentation of a significant proportion of RhoA and Cdc42 with known caveolae marker proteins, including caveolin, but not with other non-caveolae membrane proteins such as the angiotensin-converting enzyme. Immunoprecipitation experiments carried on crude endothelial cell lysates as well as with solubilized caveolae-enriched membrane domains showed the coimmunoprecipitation of caveolin with RhoA but not with Cdc42. Incubation of endothelial cell lysates with a glutathione-S-transferase (GST)-RhoA fusion protein resulted in the specific precipitation of caveolin, while addition of GST-caveolin-1 to the lysates promoted the precipitation of RhoA. Moreover, incubation of bacterially expressed RhoA with GST-caveolin-1 resulted in the precipitation of RhoA, indicating that RhoA directly interacts with caveolin-1. This interaction was found to be nucleotide-independent and was not affected by prior modification of RhoA with the C3 exoenzyme from C. botulinium or with the cytotoxic necrotinizing factor from E. coli. Taken together, these results suggest the association of RhoA with endothelial caveolae-enriched membrane domains, likely through physical interaction with caveolin-1. These findings may provide new insights into the functions played by Rho proteins and caveolae in signal transduction events.

Gbeta gamma-independent coupling of alpha2-adrenergic receptor to p21(rhoA) in preadipocytes

In preadipocytes, alpha2-adrenergic receptor (alpha2-AR) stimulation leads to a Gi/Go-dependent rearrangement of actin cytoskeleton. This is characterized by a rapid cell spreading, the formation of actin stress fibers, and the increase in tyrosyl phosphorylation of the focal adhesion kinase (pp125(FAK)). These cellular events being tightly controlled by the small GTPase p21(rhoA), the existence of a Gi/Go-dependent coupling of alpha2-AR to p21(rhoA) in preadipocytes was proposed. In alpha2AF2 preadipocytes (a cell clone derived from the 3T3F442A preadipose cell line and which stably expresses the human alpha2C10-adrenergic receptor) alpha2-adrenergic-dependent induction of cell spreading, formation of actin stress fibers, and increase in tyrosyl phosphorylation of pp125(FAK) were abolished by pretreatment of the preadipocytes with the C3 exoenzyme, a toxin which impairs p21(rhoA) activity by ADP-ribosylation. Conversely, C3 exoenzyme had no effect on the alpha2-adrenergic-dependent increase in tyrosyl phosphorylation and shift of ERK2 mitogen-activated protein kinase. alpha2-Adrenergic stimulation also led to an increase in GDP/GTP exchange on p21(rhoA), as well as to an increase in the amount of p21(rhoA) in the particulate fraction of alpha2AF2 preadipocytes. Stable transfection of alpha2AF2 preadipocytes with the COOH-terminal domain of betaARK1 (betaARK-CT) (a blocker of Gbeta gamma-action), strongly inhibited the alpha2-adrenergic-dependent increase in tyrosyl phos- phorylation and shift of ERK2, without modification of the tyrosyl phosphorylation of pp125(FAK) and spreading of preadipocytes. These results show that alpha2-adrenergic-dependent reorganization of actin cytoskeleton requires the activation of p21(rhoA) in preadipocytes. Conversely to the activation of the p21(ras)/mitogen-activated protein kinase pathway, the alpha2-adrenergic activation of p21(rhoA)-dependent pathways are independent of the beta gamma-subunits of heterotrimeric G proteins.

Phospholipase C-gamma, protein kinase C and Ca2+/calmodulin-dependent protein kinase II are involved in platelet-derived growth factor-induced phosphorylation of Tiam1

In Swiss 3T3 fibroblasts, the Rac1-specific guanine nucleotide exchange factor Tiam1 is phosphorylated by several different agonists. We show here that PDGF induces threonine phosphorylation of Tiam1 in a time- and dose-dependent manner. Tiam1 phosphorylation was significantly reduced by the selective protein kinase C inhibitor Ro-31-8220 and by KN93, an inhibitor of Ca2+/calmodulin-dependent protein kinase II. The Ca2+ chelator BAPTA/AM totally abrogated Tiam1 phosphorylation, indicating that Ca2+ is essential for this phosphorylation. Moreover, PDGF-stimulated Tiam1 phosphorylation was markedly reduced by 72 +/- 10% in PLC-gamma1 deficient mouse fibroblasts, compared to wild-type cells, indicating that phosphoinositide phospholipase C is involved.

RhoB is stabilized by transforming growth factor beta and antagonizes transcriptional activation

Transforming growth factor beta (TGF-beta) is the prototype for an evolutionarily conserved superfamily of secreted factors implicated in diverse biological phenomena. The pleiotropic responses to TGF-beta are initiated by a heteromeric receptor complex that binds and phosphorylates downstream effectors. Among these, the Smads have been extensively studied. However, less attention has been directed toward alternative downstream effectors and their participation in TGF-beta signal transduction. We show that TGF-beta promotes accumulation of the labile monomeric GTPase RhoB by antagonizing its normal proteolytic destruction, presumably via the 26 S proteasome. RhoB accumulates in its isoprenylated form. Transient overexpression of wild type RhoB but not its dominant negative mutant RhoB-N19 antagonizes TGF-beta-mediated transcriptional activation. These results suggest a novel mechanism of regulation by TGF-beta and implicate RhoB as a negative regulator of TGF-beta signal transduction.

Angiotensin II activates RhoA in cardiac myocytes: a critical role of RhoA in angiotensin II-induced premyofibril formation

The organization of actin into striated fibers (myofibrils) is one of the major features of cardiac hypertrophy. However, its signal transduction mechanism is not well understood. Although Rho-family small G proteins have been implicated in actin organization in many cell types, it is not fully elucidated whether Rho mediates the organization of actin fibers by hypertrophic stimuli in cardiac myocytes. Therefore, we examined (1) whether Rho is activated by the hypertrophic stimulus, angiotensin II (Ang II), and (2) whether Rho mediates the Ang II-induced organization of actin fibers in cultured neonatal rat cardiac myocytes. Treatment of myocytes with Ang II caused a rapid formation of both striated (mature myofibrils) and nonstriated (premyofibrils) actin fibers within 30 minutes, as determined by phalloidin stainings of the polymerized actin and troponin T stainings. Immunoblot analyses and immunostainings have indicated that cardiac myocytes express RhoA, but RhoB is undetectable. In the control state, RhoA was observed predominantly in the cytosolic fraction, but it was translocated in part to the particulate fraction in response to Ang II, consistent with activation of RhoA by Ang II. Incubation of myocytes with exoenzyme C3 for 48 hours completely ADP-ribosylated Rho in vivo. The C3 treatment abolished formation of premyofibrils induced by Ang II, suggesting that Ang II causes premyofibril formation via a Rho-dependent mechanism. The Ang II-induced mature myofibril formation was only partly abolished by C3. Expression of constitutively active RhoA (V14RhoA) caused the formation of premyofibrils but not mature myofibrils. The C3 treatment inhibited Ang II-induced atrial natriuretic factor induction, whereas it had no effect on c-fos induction. These results indicate that RhoA is activated by Ang II and mediates the Ang II-induced formation of premyofibrils and induction of a subset of genes. Distinct signaling mechanisms seem to be responsible for striated mature myofibril formation by Ang II.

The low molecular weight GTPase Rho regulates myofibril formation and organization in neonatal rat ventricular myocytes. Involvement of Rho kinase

The assembly of contractile proteins into organized sarcomeric units is one of the most distinctive features of cardiac myocyte hypertrophy. In a well characterized in vitro model system using cultured neonatal rat ventricular myocytes, a subset of G protein-coupled receptor agonists has been shown to induce actin-myosin filament organization. Pretreatment of myocytes with C3 exoenzyme ADP-ribosylated Rho and inhibited the characteristic alpha1-adrenergic receptor agonist-induced myofibrillar organization, suggesting involvement of the Rho GTPase in cardiac myofibrillogenesis. We used adenoviral mediated gene transfer to examine the effects of activated Rho and inhibitory mutants of one of its effectors, Rho kinase, in myocytes. Rho immunoreactivity was increased in the particulate fraction of myocytes infected with a recombinant adenovirus expressing constitutively activated Rho. Rho-infected cells demonstrated a striking increase in the assembly and organization of sarcomeric units and in the expression of the atrial natriuretic factor protein. These Rho-induced responses were markedly inhibited by co-infection with adenoviruses expressing putative dominant negative forms of Rho kinase. A parallel pathway involving Ras-induced myofibrillar organization and atrial natriuretic factor expression was only minimally affected. alpha1-Adrenergic receptor agonist-induced myofibrillogenesis was inhibited by some but not all of the Rho kinase mutants. Our data demonstrate that activated Rho has profound effects on myofibrillar organization in cardiac myocytes and suggest that Rho kinase mediates Rho-induced hypertrophic responses.

Substrate specificities of neuronal nuclear acetyltransferases involved in the synthesis of platelet-activating factor: differences in the use of 1-alkyl and 1-acyl lysophospholipid acceptors

The selectivity of alkylglycerophosphate (AGP) acetyltransferase and lyso-platelet-activating factor (lyso-PAF) acetyltransferase was studied in neuronal nuclei isolated from cerebral cortices of 15-day-old rabbits. Specifically, 1-alkyl and 1-acyl analogues were compared as acceptors in these acetylation reactions. A number of observations supported one nuclear activity in the acetylation of AGP and lyso-PA. Lyso-PA was a competitive substrate for AGP, Km values for AGP and lyso-PA were similar, as were acetylation rates measured at individual AGP or lyso-PA concentrations, and the acetylation of both substrates was unaffected by preincubations with protein phosphatase 1 (PP-1). In contrast, there were a number of differences seen in the acetylation of lyso-PAF and lyso-PC. The kinetics for lyso-PC acetylation (as a function of lyso-PC concentration) were not hyperbolic, and lyso-PC was not a competitive substrate for the acetylation of lyso-PAF. Unlike acetylation rates with lyso-PAF, lyso-PC acetylation was not reduced by preincubations with PP-1, and was less susceptible to inhibition particularly at high levels of free fatty acid. In addition, rates of acetylation of lyso-PC were selectively increased by the presence of lyso-PA. When neuronal nuclear envelope fractions (NE) were prepared from N1, the specific acetylation activity with lyso-PAF was significantly lower in NE, while the activities for lyso-PC were comparable in NE and the parent N1 fraction. The results with the acetylation of lyso-PC and lyso-PAF suggest that the lyso-PC acetyltransferase may be in a uniquely sequestered state within the neuronal nucleus. This could explain the smaller inhibition of lyso-PC acetylation by free fatty acid, the maintenance of lyso-PC acetylation during PP-1 preincubations, the non-hyperbolic response to lyso-PC concentrations and the selective preservation of lyso-PC acetylation during NE isolation. This protected status could result from a more internal location for this acetyltransferase within the membranes of the nuclear envelope, or possibly an association of the enzyme with the nuclear matrix that is disrupted with the exposure of N1 to lyso-PA.

Lysophosphatidic acid induces threonine phosphorylation of Tiam1 in Swiss 3T3 fibroblasts via activation of protein kinase C

The Rho family of GTPases plays an important role in the control of cell shape, adhesion, movement, and growth. Several guanine nucleotide exchange factors have been identified that activate Rho family GTPases by promoting the binding of GTP to these proteins. However, little is known concerning the regulation of these GDP/GTP exchange factors. In this study, we demonstrate that lysophosphatidic acid (LPA) induces a rapid, sustainable phosphorylation of the Rac1-specific nucleotide exchange factor Tiam1 in Swiss 3T3 fibroblasts. LPA stimulated Tiam1 phosphorylation in a dose-dependent manner, and the protein was phosphorylated on threonine, but not tyrosine or serine. Tiam1 phosphorylation was also induced by platelet-derived growth factor, endothelin-1, bombesin, and bradykinin but not by epidermal growth factor. Significantly, pretreatment of Swiss 3T3 fibroblasts with 1 microM phorbol 12-myristate 13-acetate for 24 h, or with the selective protein kinase C inhibitor Ro-31-8220, reduced LPA-stimulated phosphorylation of Tiam1 by approximately 75%. Moreover, acute stimulation with 100 nM phorbol 12-myristate 13-acetate was sufficient to induce Tiam1 phosphorylation in vivo, and protein kinase C could phosphorylate purified Tiam1 on threonine residues in vitro. These data indicate that agonist-induced phosphorylation of Tiam1 is a general mechanism and suggest that it is likely to be important in its regulation. Protein kinase C appears to play a key role in phosphorylation of Tiam1.

Inhibition of RhoA translocation and calcium sensitization by in vivo ADP-ribosylation with the chimeric toxin DC3B

Pretreatment of intact rabbit portal vein smooth muscle with the chimeric toxin DC3B (10(-6) M, 48 h; ; ) ADP-ribosylated endogenous RhoA, including cytosolic RhoA complexed with rhoGDI, and inhibited the tonic phase of phenylephrine-induced contraction and the Ca2+-sensitization of force by phenylephrine, endothelin and guanosine triphosphate (GTP)gammaS, but did not inhibit Ca2+-sensitization by phorbol dibutyrate. DC3B also inhibited GTPgammaS-induced translocation of cytosolic RhoA () to the membrane fraction. In DC3B-treated muscles the small fraction of membrane-associated RhoA could be immunoprecipitated, even after exposure to GTPgammaS, which prevents immunoprecipitation of non-ADP-ribosylated RhoA. Dissociation of cytosolic RhoA-rhoGDI complexes with SDS restored the immunoprecipitability and ADP ribosylatability of RhoA, indicating that both the ADP-ribosylation site (Asn 41) and RhoA insert loop (Wei et al., 1997) are masked by rhoGDI and that the long axes of the two proteins are in parallel in the heterodimer. We conclude that RhoA plays a significant role in G-protein-, but not protein kinase C-mediated, Ca2+ sensitization and that ADP ribosylation inhibits in vivo the Ca2+-sensitizing effect of RhoA by interfering with its binding to a membrane-associated effector.

Cloning and characterization of phospholipase D from rat brain

The regulation of phospholipase D cloned from rat brain (rPLD) was examined in vivo and in vitro. The enzyme was a shorter splice variant of human phospholipase D 1 (Hammond, S. M., Altshuller, Y. M. , Sung, T.-C., Rudge, S. M., Rose, K., Engebrecht, J. A., Morris, A. J., and Frohman, M. A. (1995) J. Biol. Chem. 270, 29640-29643). Its expression in COS-7 cells led to increased phospholipase D (PLD) activity that was further stimulated by constitutively active V14RhoA. V14RhoA had no effect on the endogenous PLD of the COS-7 cells, but constitutively active L71ARF3 increased its activity. In contrast, L71ARF3 did not activate rPLD expressed in the cells. Addition of phorbol ester markedly increased the endogenous PLD activity of COS-7 cells, and there was a further increase in the cells expressing rPLD. In membranes from COS-7 cells expressing rPLD, addition of myristoylated ADP-ribosylation factor (ARF) and RhoA in vitro stimulated PLD activity. The effect of ARF was greater than that of RhoA, although the concentrations for half-maximal stimulation (0.08-0.2 microM) were similar. Membranes isolated from cells expressing rPLD plus L71ARF3 and/or V14RhoA also showed higher PLD activity but no synergism between the two G proteins. Addition of phorbol ester and protein kinase C alpha (PKCalpha) also stimulated PLD activity in membranes from COS-7 cells expressing rPLD, but it had no effect on the activity in control (vector) membranes and did not enhance the effects of constitutively active ARF or Rho. The stimulation by PKCalpha did not require ATP and was not increased by addition of this nucleotide. No synergism between ARF and Rho and between these and PKCalpha on PLD activity was observed when these were added to membranes from cells expressing rPLD. Oleate inhibited the PLD activity of membranes from both control and rPLD-expressing cells. In summary, these results indicate that in vitro, rPLD is stimulated by ARF, RhoA, and PKCalpha and inhibited by oleate. However, in intact COS-7 cells, ARF activates endogenous PLD but not rPLD, whereas the reverse is true for RhoA. In addition, the effects of phorbol ester are much greater in the intact cells. It is concluded that the regulation of rPLD in intact COS-7 cells differs significantly from that seen in vitro; possible reasons for this are discussed.