cAMP-induced morphological changes are counteracted by the activated RhoA small GTPase and the Rho kinase ROKalpha

Identification of glucagon-like peptide-2 (GLP-2)-activated signaling pathways in baby hamster kidney fibroblasts expressing the rat GLP-2 receptor

Glucagon-like peptide-2 (GLP-2) promotes the expansion of the intestinal epithelium through stimulation of the GLP-2 receptor, a recently identified member of the glucagon-secretin G protein-coupled receptor superfamily. Although activation of G protein-coupled receptors may lead to stimulation of cell growth, the mechanisms transducing the GLP-2 signal to mitogenic proliferation remain unknown. We now report studies of GLP-2R signaling in baby hamster kidney (BHK) cells expressing a transfected rat GLP-2 receptor (BHK-GLP-2R cells). GLP-2, but not glucagon or GLP-1, increased the levels of cAMP and activated both cAMP-response element- and AP-1-dependent transcriptional activity in a dose-dependent manner. The activation of AP-1-luciferase activity was protein kinase A (PKA) -dependent and markedly diminished in the presence of a dominant negative inhibitor of PKA. Although GLP-2 stimulated the expression of c-fos, c-jun, junB, and zif268, and transiently increased p70 S6 kinase in quiescent BHK-GLP-2R cells, GLP-2 also inhibited extracellular signal-regulated kinase 1/2 and reduced serum-stimulated Elk-1 activity. Furthermore, no rise in intracellular calcium was observed following GLP-2 exposure in BHK-GLP-2R cells. Although GLP-2 stimulated both cAMP accumulation and cell proliferation, 8-bromo-cyclic AMP alone did not promote cell proliferation. These findings suggest that the GLP-2R may be coupled to activation of mitogenic signaling in heterologous cell types independent of PKA via as yet unidentified downstream mediators of GLP-2 action in vivo.

Increased myosin light chain phosphorylation is not required for growth factor stimulation of collagen matrix contraction

Previous research suggested the possibility that contraction of floating collagen matrices by human fibroblasts required increased myosin light chain (MLC) phosphorylation. In the current studies, we show that increased MLC phosphorylation was neither necessary for platelet-derived growth factor (PDGF)-dependent matrix contraction nor sufficient for lysophosphatidic acid (LPA)-dependent contraction. In contrast, increased MLC phosphorylation did appear to be coupled to the formation of stress fibers by cells spreading in monolayer culture. Signal transduction pathways required for PDGF- and LPA-dependent matrix contraction involved phosphatidylinositol 3-kinase and the G(i) class of heterotrimeric G proteins, respectively. Our results indicate that PDGF- and LPA-dependent contraction of floating collagen matrices can be uncoupled from an increase in MLC phosphorylation.

Inactivation of Rho signaling pathway promotes CNS axon regeneration

Regeneration in the CNS is blocked by many different growth inhibitory proteins. To foster regeneration, we have investigated a strategy to block the neuronal response to growth inhibitory signals. Here, we report that injured axons regrow directly on complex inhibitory substrates when Rho GTPase is inactivated. Treatment of PC12 cells with C3 enzyme to inactivate Rho and transfection with dominant negative Rho allowed neurite growth on inhibitory substrates. Primary retinal neurons treated with C3 extended neurites on myelin-associated glycoprotein and myelin substrates. To explore regeneration in vivo, we crushed optic nerves of adult rat. After C3 treatment, numerous cut axons traversed the lesion to regrow in the distal white matter of the optic nerve. These results indicate that targeting signaling mechanisms converging to Rho stimulates axon regeneration on inhibitory CNS substrates.

Different roles for RhoA during neurite initiation, elongation, and regeneration in PC12 cells

The goal of the present study was to characterize the effects of RhoA at different stages of nerve growth factor (NGF)-induced neuronal differentiation in the PC12 model. This comparative analysis was prompted by previous studies that reported apparently opposite effects for Rho in different models of neuronal differentiation and regeneration. PC12 cells were transfected with activated V14RhoA or dominant negative N19RhoA under the control of either a constitutive or a steroid-regulated promoter. Upon exposure to NGF, V14RhoA cells continued to proliferate and did not extend neurites; however, they remained responsive to NGF, as indicated by the activation of extracellular signal-regulated kinases. This inability to differentiate was reversed by C3 toxin and activation of cyclic AMP signaling, which inactivate RhoA. N19RhoA expression led to an increase in neurite initiation and branching. In contrast, when the RhoA mutants were expressed after NGF priming, only the rate of neurite extension was altered; V14RhoA clones had neurites approximately twice as long, whereas neurites of N19RhoA cells were approximately 50% shorter than those of appropriate controls. The effects of Rho in neurite regeneration mimicked those observed during the initial stages of morphogenesis; activation inhibited, whereas inactivation promoted, neurite outgrowth. Our results indicate that RhoA function changes at different stages of NGF-induced neuronal differentiation and neurite regeneration.

Expression of PKA inhibitor (PKI) gene abolishes cAMP-mediated protection to endothelial barrier dysfunction

We investigated the hypothesis that cAMP-dependent protein kinase (PKA) protects against endothelial barrier dysfunction in response to proinflammatory mediators. An E1-, E3-, replication-deficient adenovirus (Ad) vector was constructed containing the complete sequence of PKA inhibitor (PKI) gene (AdPKI). Infection of human microvascular endothelial cells (HMEC) with AdPKI resulted in overexpression of PKI. Treatment with 0.5 microM thrombin increased transendothelial albumin clearance rate (0.012 +/- 0.003 and 0.035 +/- 0.005 microl/min for control and thrombin, respectively); the increase was prevented with forskolin + 3-isobutyl-1-methylxanthine (F + I) treatment. Overexpression of PKI resulted in abrogation of the F + I-induced inhibition of the permeability increase. However, with HMEC infected with ultraviolet-inactivated AdPKI, the F + I-induced inhibition was present. Also, F + I treatment of HMEC transfected with reporter plasmid containing the cAMP response element-directed transcription of the luciferase gene resulted in an almost threefold increase in luciferase activity. Overexpression of PKI inhibited this induction of luciferase activity. The results show that Ad-mediated overexpression of PKI in endothelial cells abrogated the cAMP-mediated protection against increased endothelial permeability, providing direct evidence that cAMP-dependent protein kinase promotes endothelial barrier function.

Inhibition by vasoactive intestinal polypeptide (VIP) of angiogenesis induced by murine Colon 26-L5 carcinoma cells metastasized in liver

We investigated the effect of VIP on the liver metastases and angiogenesis by Colon 26-L5 carcinoma cells in mice. Daily systemic administration of VIP, beginning 3 days after tumor inoculation into a portal vein of mice, inhibited significantly the development of their liver metastases. Immunohistochemical staining for factor VIII-related antigen in the sections of liver metastases showed that the systemic administration of VIP caused significant prevention of angiogenesis within tumor masses. VIP (10-(10) to 10(-6) M) inhibited the invasion of reconstituted basement membrane (Matrigel) by hepatic sinusoidal endothelial (HSE) cells in a concentration-dependent manner in a Transwell chamber assay in vitro and achieved approximately 50% reduction of control at 10(-6) M. VIP (10(-6) M) also significantly suppressed the haptotactic migration of HSE cells to fibronectin, laminin or type I collagen substrates with a similar inhibition rate to the invasion assay. Exposure of VIP to HSE cells induced accumulation of intracellular cAMP in a concentration-dependent manner. The inhibitory effect of VIP (10(-6) M) on HSE cell migration was significantly abrogated in the presence of 3 x 10(-6) M H-89, a cAMP-dependent protein kinase inhibitor. VIP (10(-6) M) inhibited the morphogenesis of HSE cells into capillary-like structures on Matrigel-coated wells. VIP did not affect the proliferation of HSE cells and the production of gelatinases in HSE cells in vitro at the concentrations used in the invasion assay. These observations suggest that the anti-metastatic effect of VIP on liver metastases by Colon 26-L5 carcinoma cells in mice is partly due to the prevention of tumor angiogenesis probably through suppression of the motility of endothelial cells.

Evidence for a calpeptin-sensitive protein-tyrosine phosphatase upstream of the small GTPase Rho. A novel role for the calpain inhibitor calpeptin in the inhibition of protein-tyrosine phosphatases

Activation of the thiol protease calpain results in proteolysis of focal adhesion-associated proteins and severing of cytoskeletal-integrin links. We employed a commonly used inhibitor of calpain, calpeptin, to examine a role for this protease in the reorganization of the cytoskeleton under a variety of conditions. Calpeptin induced stress fiber formation in both forskolin-treated REF-52 fibroblasts and serum-starved Swiss 3T3 fibroblasts. Surprisingly, calpeptin was the only calpain inhibitor of several tested with the ability to induce these effects, suggesting that calpeptin may act on targets besides calpain. Here we show that calpeptin inhibits tyrosine phosphatases, enhancing tyrosine phosphorylation particularly of paxillin. Calpeptin preferentially inhibits membrane-associated phosphatase activity. Consistent with this observation, in vitro phosphatase assays using purified glutathione S-transferase fusion proteins demonstrated a preference for the transmembrane protein-tyrosine phosphatase-alpha over the cytosolic protein-tyrosine phosphatase-1B. Furthermore, unlike wide spectrum inhibitors of tyrosine phosphatases such as pervanadate, calpeptin appeared to inhibit a subset of phosphatases. Calpeptin-induced assembly of stress fibers was inhibited by botulinum toxin C3, indicating that calpeptin is acting on a phosphatase upstream of the small GTPase Rho, a protein that controls stress fiber and focal adhesion assembly. Not only does this work reveal that calpeptin is an inhibitor of protein-tyrosine phosphatases, but it suggests that calpeptin will be a valuable tool to identify the phosphatase activity upstream of Rho.

Myelin and collapsin-1 induce motor neuron growth cone collapse through different pathways: inhibition of collapse by opposing mutants of rac1

Precise growth cone guidance is the consequence of a continuous reorganization of actin filament structures within filopodia and lamellipodia in response to inhibitory and promoting cues. The small GTPases rac1, cdc42, and rhoA are critical for regulating distinct actin structures in non-neuronal cells and presumably in growth cones. Collapse, a retraction of filopodia and lamellipodia, is a typical growth cone behavior on contact with inhibitory cues and is associated with depolymerization and redistribution of actin filaments. We examined whether small GTPases mediate the inhibitory properties of CNS myelin or collapsin-1, a soluble semaphorin, in chick embryonic motor neuron cultures. As demonstrated for collapsin-1, CNS myelin-evoked growth cone collapse was accompanied by a reduction of rhodamine-phalloidin staining most prominent in the growth cone periphery, suggesting actin filament disassembly. Specific mutants of small GTPases were capable of desensitizing growth cones to CNS myelin or collapsin-1. Adenoviral-mediated expression of constitutively active rac1 or rhoA abolished CNS myelin-induced collapse and allowed remarkable neurite extension on a CNS myelin substrate. In contrast, expression of dominant negative rac1 or cdc42 negated collapsin-1-induced growth cone collapse and promoted neurite outgrowth on a collapsin-1 substrate. These findings suggest that small GTPases can modulate the signaling pathways of inhibitory stimuli and, consequently, allow the manipulation of growth cone behavior. However, the fact that opposite mutants of rac1 were effective against different inhibitory stimuli speaks against a universal signaling pathway underlying growth cone collapse.

Bidirectional signaling between the cytoskeleton and integrins

Clustering of integrins into focal adhesions and focal complexes is regulated by the actin cytoskeleton. In turn, actin dynamics are governed by Rho family GTPases. Integrin-mediated adhesion activates these GTPases, triggering assembly of filopodia, lamellipodia and stress fibers. In the past few years, signaling pathways have begun to be identified that promote focal adhesion disassembly and integrin dispersal. Many of these pathways result in decreased myosin-mediated cell contractility.

Activation of G12/G13 results in shape change and Rho/Rho-kinase-mediated myosin light chain phosphorylation in mouse platelets

Platelets respond to various stimuli with rapid changes in shape followed by aggregation and secretion of their granule contents. Platelets lacking the alpha-subunit of the heterotrimeric G protein Gq do not aggregate and degranulate but still undergo shape change after activation through thromboxane-A2 (TXA2) or thrombin receptors. In contrast to thrombin, the TXA2 mimetic U46619 led to the selective activation of G12 and G13 in Galphaq-deficient platelets indicating that these G proteins mediate TXA2 receptor-induced shape change. TXA2 receptor-mediated activation of G12/G13 resulted in tyrosine phosphorylation of pp72(syk) and stimulation of pp60(c-src) as well as in phosphorylation of myosin light chain (MLC) in Galphaq-deficient platelets. Both MLC phosphorylation and shape change induced through G12/G13 in the absence of Galphaq were inhibited by the C3 exoenzyme from Clostridium botulinum, by the Rho-kinase inhibitor Y-27632 and by cAMP-analogue Sp-5,6-DCl-cBIMPS. These data indicate that G12/G13 couple receptors to tyrosine kinases as well as to the Rho/Rho-kinase-mediated regulation of MLC phosphorylation. We provide evidence that G12/G13-mediated Rho/Rho-kinase-dependent regulation of MLC phosphorylation participates in receptor-induced platelet shape change.