Src kinase and PI 3-kinase as a transduction pathway in ceramide-induced contraction of colonic smooth muscle

Ceramide modulates electrophysiological characteristics and oxidative stress of pulmonary vein cardiomyocytes

BACKGROUND

Ceramide is involved in regulating metabolism and energy expenditure, and its abnormal myocardial accumulation may contribute to heart injury or lipotoxic cardiomyopathy. Whether ceramide can modulate the electrophysiology of pulmonary veins (PVs) remains unknown.

MATERIALS AND METHODS

We used conventional microelectrodes to measure the electrical activity of isolated rabbit PV tissue preparations before and after treatment with various concentrations of ceramide with or without H₂ O₂ (2 mM), MitoQ, wortmannin or 740 YP. A whole-cell patch clamp and fluorescence imaging were used to record the ionic currents, calcium (Ca²⁺ ) transients, and intracellular reactive oxygen species (ROS) and sodium (Na⁺ ) in isolated single PV cardiomyocytes before and after ceramide (1 μM) treatment.

RESULTS

Ceramide (0.1, 0.3, 1 and 3 μM) reduced the beating rate of PV tissues. Furthermore, ceramide (1 μM) suppressed the 2 mM H₂ O₂ -induced faster PV beating rate, triggered activities and burst firings, which were further reduced by MitoQ. In the presence of wortmannin, ceramide did not change the PV beating rate. The H₂ O₂ -induced faster PV beating rate could be counteracted by MitoQ or wortmannin with no additive effect from the ceramide. Ceramide inhibited pPI3K. Ceramide reduced Ca²⁺ transients, sarcoplasmic reticulum Ca²⁺ contents, L-type Ca²⁺ currents, Na⁺ currents, late Na⁺ currents, Na⁺ -hydrogen exchange currents, and intracellular ROS and Na⁺ in PV cardiomyocytes, but did not change Na⁺ -Ca²⁺ exchange currents.

CONCLUSION

C2 ceramide may exert the distinctive electrophysiological effect of modulating PV activities, which may be affected by PI3K pathway-mediated oxidative stress, and might play a role in the pathogenesis of PV arrhythmogenesis.

IL-1β suppresses cLTP-induced surface expression of GluA1 and actin polymerization via ceramide-mediated Src activation

BACKGROUND

Brain inflammation including increases in inflammatory cytokines such as IL-1β is widely believed to contribute to the pathophysiology of Alzheimer's disease. Although IL-1β-induced impairments in long-term potentiation (LTP) in acute hippocampal slices and memory functions in vivo have been well documented, the neuron-specific molecular mechanisms of IL-1β-mediated impairments of LTP and memory remain unclear.

METHODS

This study uses an in vitro approach in primary hippocampal neurons to evaluate the effect of IL-1β on chemical LTP (cLTP)-induced structural plasticity and signaling.

RESULTS

We found that IL-1β reduces both the surface expression of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor subunit GluA1 and the spine growth following cLTP. These effects of IL-1β were mediated by impairing actin polymerization during cLTP, as IL-1β decreased the cLTP-induced formation of F-actin, and the effect of IL-1β on cLTP-induced surface expression of GluA1 can be mimicked by latrunculin, a toxin that disrupts dynamics of actin filaments, and can be prevented by jasplakinolide, a cell-permeable peptide that stabilizes F-actin. Moreover, live-cell imaging demonstrated that IL-1β decreased the stability of the actin cytoskeleton in spines, which is required for LTP consolidation. We further examined the role of sphingolipid signaling in the IL-1β-mediated impairment of spine plasticity and found that both the neutral sphingomyelinase inhibitor GW4869 and the inhibitor of Src kinase PP2 attenuated the IL-1β-mediated suppression of cLTP-induced surface expression of GluA1 and actin polymerization.

CONCLUSIONS

These findings support a mechanism by which IL-1β, via the sphingomyelinase/ceramide/Src pathway, impairs structural spine remodeling essential for LTP consolidation and memory.

Interleukin-1β activates an Src family kinase to stimulate the plasma membrane Ca2+ pump in hippocampal neurons

The plasma membrane Ca(2+) ATPase (PMCA) plays a major role in clearing Ca(2+) from the neuronal cytoplasm. The cytoplasmic Ca(2+) clearance rate affects neuronal excitability, synaptic plasticity, and neurotransmission. Here, we examined the modulation of PMCA activity by PTKs in hippocampal neurons. PMCA-mediated Ca(2+) clearance slowed in the presence of pyrazolopyrimidine 2, an inhibitor of Src family kinases (SFKs), and accelerated in the presence of C2-ceramide, an activator of PTKs. Ca(2+) clearance kinetics were attenuated in cells expressing a dominant-negative Src mutant, suggesting that the pump is tonically stimulated by a PTK. Tonic stimulation was reduced in hippocampal neurons expressing short hairpin (sh)RNA directed to mRNA for Yes. shRNA-mediated knockdown of PMCA isoform 1 (PMCA1) removed tonic stimulation of Ca(2+) clearance, indicating that the kinase stimulates PMCA1. IL-1β accelerated Ca(2+) clearance in a manner blocked by an IL-1β receptor antagonist or by an inhibitor of neutral sphingomyelinase, the enzyme that produces ceramide. Thus IL-1β activates an SFK to stimulate the plasma membrane Ca(2+) pump, decreasing the duration of Ca(2+) transients in hippocampal neurons.

Phosphorylation of serine palmitoyltransferase long chain-1 (SPTLC1) on tyrosine 164 inhibits its activity and promotes cell survival

In BCR-ABL-expressing cells, sphingolipid metabolism is altered. Because the first step of sphingolipid biosynthesis occurs in the endoplasmic reticulum (ER), our objective was to identify ABL targets in the ER. A phosphoproteomic analysis of canine pancreatic ER microsomes identified 49 high scoring phosphotyrosine-containing peptides. These were then categorized in silico and validated in vitro. We demonstrated that the ER-resident human protein serine palmitoyltransferase long chain-1 (SPTLC1), which is the first enzyme of sphingolipid biosynthesis, is phosphorylated at Tyr(164) by the tyrosine kinase ABL. Inhibition of BCR-ABL using either imatinib or shRNA-mediated silencing led to the activation of SPTLC1 and to increased apoptosis in both K562 and LAMA-84 cells. Finally, we demonstrated that mutation of Tyr(164) to Phe in SPTLC1 increased serine palmitoyltransferase activity. The Y164F mutation also promoted the remodeling of cellular sphingolipid content, thereby sensitizing K562 cells to apoptosis. Our observations provide a mechanistic explanation for imatinib-mediated cell death and a novel avenue for therapeutic strategies.

The NADPH oxidase inhibitor diphenyleneiodonium is also a potent inhibitor of cholinesterases and the internal Ca(2+) pump

BACKGROUND AND PURPOSE

Diphenyleneiodonium (DPI) is often used as an NADPH oxidase inhibitor, but is increasingly being found to have unrelated side effects. We investigated its effects on smooth muscle contractions and the related mechanisms.

EXPERIMENTAL APPROACH

We studied isometric contractions in smooth muscle strips from bovine trachea. Cholinesterase activity was measured using a spectrophotometric assay; internal Ca(2+) pump activity was assessed by Ca(2+) uptake into smooth muscle microsomes.

KEY RESULTS

Contractions to acetylcholine were markedly enhanced by DPI (10(-4) M), whereas those to carbachol (CCh) were not, suggesting a possible inhibition of cholinesterase. DPI markedly suppressed contractions evoked by CCh, KCl and 5-HT, and also unmasked phasic activity in otherwise sustained responses. Direct biochemical assays confirmed that DPI was a potent inhibitor of acetylcholinesterase and butyrylcholinesterase (IC(50) approximately 8 x 10(-6) M and 6 x 10(-7) M, respectively), following a readily reversible, mixed non-competitive type of inhibition. The inhibitory effects of DPI on CCh contractions were not mimicked by another NADPH oxidase inhibitor (apocynin), nor the Src inhibitors PP1 or PP2, ruling out an action through the NADPH oxidase signalling pathway. Several features of the DPI-mediated suppression of agonist-evoked responses (i.e. suppression of peak magnitudes and unmasking of phasic activity) are similar to those of cyclopiazonic acid, an inhibitor of the internal Ca(2+) pump. Direct measurement of microsomal Ca(2+) uptake revealed that DPI modestly inhibits the internal Ca(2+) pump.

CONCLUSIONS AND IMPLICATIONS

DPI inhibits cholinesterase activity and the internal Ca(2+) pump in tracheal smooth muscle.

Direct association of calponin with specific domains of PKC-alpha

Calponin contributes to the regulation of smooth muscle contraction through its interaction with F-actin and inhibition of the actin-activated Mg-ATPase activity of phosphorylated myosin. Previous studies have shown that the contractile agonist acetylcholine induced a direct association of translocated calponin and PKC-alpha in the membrane. In the present study, we have determined the domain of PKC-alpha involved in direct association with calponin. In vitro binding assay was carried out by incubating glutathione S-transferase-calponin aa 92-229 with His-tagged proteins of individual domains and different combinations of domains of PKC-alpha. Calponin was found to bind directly to the full-length PKC-alpha. Calponin bound to C2 and C4 domains but not to C1 and C3 domains of PKC-alpha. When incubated with proteins of different combination of domains, calponin bound to C2-C3, C3-C4, and C2-C3-C4 but not to C1-C2 or C1-C2-C3. To determine whether these in vitro bindings mimic the in vivo associations, and in vivo binding assay was performed by transfecting colonic smooth muscle cells with His-tagged proteins of individual domains and different combinations of domains of PKC-alpha. Coimmunoprecipitation of calponin with His-tagged truncated forms of PKC-alpha showed that C1-C2, C1-C2-C3, C2-C3, and C3-C4 did not associate with calponin. Calponin associated only with full-length PKC-alpha and with C2-C3-C4 in cells in the resting state, and this association increased upon stimulation with acetylcholine. These data suggest that calponin bound to fragments that may mimic the active form of PKC-alpha and that the functional association of PKC-alpha with calponin requires both C2 and C4 domains during contraction of colonic smooth muscle cells.

Signal transduction underlying the control of urinary bladder smooth muscle tone by muscarinic receptors and beta-adrenoceptors

The normal physiological contraction of the urinary bladder, which is required for voiding, is predominantly mediated by muscarinic receptors, primarily the M₃ subtype, with the M₂ subtype providing a secondary backup role. Bladder relaxation, which is required for urine storage, is mediated by β-adrenoceptors, in most species involving a strong β₃-component. An excessive stimulation of contraction or a reduced relaxation of the detrusor smooth muscle during the storage phase of the micturition cycle may contribute to bladder dysfunction known as the overactive bladder. Therefore, interference with the signal transduction of these receptors may be a viable approach to develop drugs for the treatment of overactive bladder. The prototypical signaling pathway of M₃ receptors is activation of phospholipase C (PLC), and this pathway is also activated in the bladder. Nevertheless, PLC apparently contributes only in a very minor way to bladder contraction. Rather, muscarinic-receptor-mediated bladder contraction involves voltage-operated Ca²⁺ channels and Rho kinase. The prototypical signaling pathway of β-adrenoceptors is an activation of adenylyl cyclase with the subsequent formation of cAMP. Nevertheless, cAMP apparently contributes in a minor way only to β-adrenoceptor-mediated bladder relaxation. BK_Ca channels may play a greater role in β-adrenoceptor-mediated bladder relaxation. We conclude that apart from muscarinic receptor antagonists and β-adrenoceptor agonists, inhibitors of Rho kinase and activators of BK_Ca channels may have potential to treat an overactive bladder.

RhoA- and PKC-alpha-mediated phosphorylation of MYPT and its association with HSP27 in colonic smooth muscle cells

Agonist-induced activation of the RhoA/Rho kinase (ROCK) pathway results in inhibition of myosin phosphatase and maintenance of myosin light chain (MLC20) phosphorylation. We have shown that RhoA/ROCKII translocates and associates with heat shock protein (HSP)27 in the particulate fraction. We hypothesize that inhibition of the 130-kDa regulatory myosin-binding subunit (MYPT) requires its association with HSP27 in the particulate fraction. Furthermore, it is not certain whether regulation of MYPT by CPI-17 or by ROCKII is due to cross talk between RhoA and PKC-alpha. Presently, we examined the cross talk between RhoA and PKC-alpha in the regulation of MYPT phosphorylation in rabbit colon smooth muscle cells. Acetylcholine induced 1) sustained phosphorylation of PKC-alpha, CPI-17, and MYPT; 2) an increase in the association of phospho-MYPT with HSP27 in the particulate fraction; 3) a decrease in myosin phosphatase activity (66.21+/-3.52 and 42.19+/-3.85% nM/ml lysate at 30 s and 4 min); and 4) an increase in PKC activity (298.12+/-46.60% and 290.59+/-22.07% at 30 s and 4 min). Inhibition of RhoA/ROCKII by Y-27632 inhibited phosphorylation of MYPT and its association with HSP27. Both Y27632 and a negative dominant construct of RhoA inhibited phosphorylation of MYPT and CPI-17. Inhibition of PKCs or calphostin C or selective inhibition of PKC-alpha by negative dominant constructs inhibited phosphorylation of MYPT and CPI-17. The results suggest that 1) acetylcholine induces activation of both RhoA and/or PKC-alpha pathways, suggesting cross talk between RhoA and PKC-alpha resulting in phosphorylation of MYPT, inhibition of myosin phosphatase activity, and maintenance of MLC phosphorylation; and 2) phosphorylated MYPT is associated with HSP27 and translocated to the particulate fraction, suggesting a scaffolding role for HSP27 in mediating the association of the complex MYPT/RhoA-ROCKII. Thus both pathways (PKC and RhoA) converge on the regulation of myosin phosphatase activities and modulate sustained phosphorylation of MLC20.

Acute exposure of ceramide enhances cardiac contractile function in isolated ventricular myocytes

1. The sphingolipid ceramide, a primary building block for all other sphingolipids, is associated with growth arrest, apoptosis, and lipotoxic dysfunction. Interestingly, ceramide may attenuate high glucose-induced myocyte dysfunction, produce Ca2+ influx, and augment smooth muscle contraction. To determine the role of ceramide on cardiac excitation-contraction (E-C) coupling, electrically paced adult rat ventricular myocytes were acutely exposed to a cell-permeable ceramide analog (10 pm-100 microM) and the following indices were determined: peak shortening (PS), time-to-PS, time-to-90% relengthening, and the maximal velocity of shortening and relengthening (+/-dLdt). Intracellular Ca2+ properties were assessed using fura-2AM fluorescent microscopy. 2. Our results revealed a concentration- and time-dependent increase of PS in ventricular myocytes in response to ceramide associated with an increase in +/-dLdt. The maximal increase in PS was approximately 35% from control value and was maintained throughout the first 20 min of ceramide exposure. However, the ceramide-induced increase in PS was not maintained once the exposure time was beyond 20 min. Acute exposure of ceramide significantly enhanced intracellular Ca2+ release, although at a much lower concentration range. The ceramide-induced augmentation of PS was not significantly affected by inhibition of phosphatidylinositol (PI)-3-kinase, protein kinase C (PKC), ceramide-activated protein phosphatase (CAPP), and nitric oxide (NO) synthase. 3. Our data suggest that ceramide acutely augments the contractile function of cardiac myocytes through an alternative mechanism(s) rather than PI-3-kinase, PKC, CAPP, or NO.

Direct association and translocation of PKC-alpha with calponin

Calponin has been implicated in the regulation of smooth muscle contraction through its interaction with F-actin and inhibition of the actin-activated MgATPase activity of phosphorylated myosin. Calponin has also been shown to interact with PKC. We have studied the interaction of calponin with PKC-alpha and with the low molecular weight heat-shock protein (HSP)27 in contraction of colonic smooth muscle cells. Particulate fractions from isolated smooth muscle cells were immunoprecipitated with antibodies to calponin and Western blot analyzed with antibodies to HSP27 and to PKC-alpha. Acetylcholine induced a sustained increase in the immunocomplexing of calponin with HSP27 and of calponin with PKC-alpha in the particulate fraction, indicating an association of the translocated proteins in the membrane. To examine whether the observed interaction in vivo is due to a direct interaction of calponin with PKC-alpha, a cDNA of 1.3 kb of human calponin gene was PCR amplified. PCR product encoding 622 nt of calponin cDNA (nt 351-972 corresponding to amino acids 92-229) was expressed as fusion glutathione S-transferase (GST) protein in the vector pGEX-KT. We have studied the direct association of GST-calponin fusion protein with recombinant PKC-alpha in vitro. Western blot analysis of the fractions collected after elution with reduced glutathione buffer (pH 8.0) show a coelution of GST-calponin with PKC-alpha, indicating a direct association of GST-calponin with PKC-alpha. These data suggest that there is a direct association of translocated calponin and PKC-alpha in the membrane and a role for the complex calponin-PKC-alpha-HSP27, in contraction of colonic smooth muscle cells.

Neutral sphingomyelinase inhibitor scyphostatin prevents and ceramide mimics mechanotransduction in vascular endothelium

Recently, we showed that neutral sphingomyelinase (N-SMase) is concentrated at the endothelial cell surface in caveolae and is activated to produce ceramide in an acute and transient manner by increase in flow rate and pressure in rat lung vasculature (Czarny M, Liu J, Oh P, and Schnitzer JE, J Biol Chem 278: 4424-4430, 2003). Here, we report further on our investigations of this new acute mechanotransduction pathway. We employed three experimental models to explore the role of N-SMase and ceramides in mechanosignaling: 1) a cell-free, in vitro model using isolated luminal plasma membranes of rat lung endothelium; 2) a fluid shear stress model using monolayers of intact bovine aorta endothelial cell in culture; and 3) an in situ model using controlled perfusion of the rat lung vasculature. Scyphostatin, which specifically inhibited N-SMase but not acid SMase activity, prevented mechanoactivation of N-SMase as well as downstream tyrosine and mitogen-activated protein kinases. Cell-permeable ceramide analogs (N-acetylsphingosine, C2-ceramide, and N-hexanoylsphingosine, C6-ceramide) but not the inactive dihydroderivatives D2-ceramide and D6-ceramide (N-acetylsphinganine and N-hexanoylsphinganine, respectively) mimic rapid mechano-induced tyrosine phosphorylation of cell surface proteins as well as mechanoactivation of Src-like kinases and the extracellular regulated kinase pathway. The responses common to ceramide and mechanical stress were inhibited by genistein, herbamycin A, and PP2, but not PP3, which suggests an obligate role of Src-like kinases in ceramide-mediated mechanotransduction. Ceramides also induced serine/threonine phosphorylation to activate the Akt/endothelial nitric oxide synthase pathway. Thus N-SMase at the plasma membrane in caveolae may be an upstream initiating mechanosensor, which acutely triggers mechanotransduction by generation of the lipid second messenger ceramide.

Normal cells control the growth of neighboring transformed cells independent of gap junctional communication and SRC activity

The growth of many types of cancer cells can be controlled by surrounding normal cells. However, mechanisms underlying this phenomenon have not been defined. We used a layered culture system to investigate how nontransformed cells suppress the growth of neighboring transformed cells. Direct physical contact between transformed and nontransformed cells was required for growth suppression of transformed cells in this system; communication by diffusible factors was not sufficient. However, significant gap junctional communication was not required, indicating that other intercellular junctions mediated this growth regulatory response. We also report that the Src kinase activity in transformed cells was not directly inhibited by contact with nontransformed cells. Instead, nontransformed cells increased the expression of serum deprivation-response protein and the transcription factor four and a half LIM domain 1 in tumor cells. In addition, these results suggest mechanisms by which normal cells may block Wnt signaling, inhibit insulin-like growth factor activity, and promote host recognition of neighboring tumor cells.

Alpha 2 adrenoceptor-mediated vasoconstriction in porcine palmar lateral vein: role of phosphatidylinositol 3-kinase and EGF receptor transactivation

1 Alpha(2) adrenoceptors cause vasoconstriction in the porcine palmar lateral vein through a mechanism involving the ERK signal transduction cascade, calcium influx, and a Src tyrosine kinase. The aim of the present study was to determine if phosphatidylinositol 3-kinase (PI 3-kinase) and/or epidermal growth factor (EGF) receptor transactivation are also involved. 2 alpha(2) Adrenoceptor-mediated vasoconstriction and ERK2 activation in the porcine palmar lateral vein was inhibited in the presence of either the PI 3-kinase inhibitor LY294002, or the EGF receptor tyrosine kinase inhibitor AG1478 suggesting the involvement of both PI 3-kinase and EGF receptor transactivation. 3 Akt phosphorylation was increased in segments of porcine palmar lateral vein contracted with UK14304 indicating an increase in Akt activation. This is a further indication that PI 3-kinase is involved in alpha(2) adrenoceptor-mediated vasoconstriction. Akt activation was inhibited by the Src tyrosine kinase inhibitor PP2, and removal of extracellular calcium. 4 UK14304 (10 microM) stimulated an increase in intracellular calcium in segments of palmar lateral vein. This was inhibited by removal of extracellular calcium, but not by nifedipine suggesting the rise in calcium is due to influx of calcium through non-L type calcium channels. The increase in calcium was also inhibited by LY294002 indicating that PI 3-kinase is upstream of calcium influx. 5 These data indicate that alpha(2) adrenoceptor-mediated vasoconstriction in the porcine palmar lateral vein is dependent upon stimulation of PI 3-kinase, leading to an influx of calcium. This results in activation of the EGF receptor tyrosine kinase, and finally activation of ERK-MAP kinase.

Cell-permeable ceramides increase basal glucose incorporation into triacylglycerols but decrease the stimulation by insulin in 3T3-L1 adipocytes

OBJECTIVE

To investigate mechanisms for the regulation of glucose incorporation into triacylgycerols in adipocytes by ceramides, which mediate some actions of tumour necrosis factor-alpha (TNFalpha).

DESIGN

The effects of C(2)- and C(6)-ceramides (N-acetyl- and N-hexanoyl-sphingosines, respectively) on glucose uptake and incorporation into triacylglycerols and pathways of signal tansduction were measured in 3T3-L1 adipocytes.

RESULTS

C(6)-ceramide increased basal 2-deooxyglucose uptake but decreased insulin-stimulated uptake without changing the EC(50) for insulin. Incubating 3T3-L1 adipocytes from 2 to 24 h with C(2)-ceramide progressively increased glucose incorporation into the fatty acid and especially the glycerol moieties of triacylglycerol. These effects were accompanied by increased GLUT1 synthesis resulting from ceramide-induced activation phosphatidylinositol 3-kinase, ribosomal S6 kinase and mitogen-activated protein kinase. C(2)-ceramide also increased p21-activated kinase and protein kinase B activities. However, C(2)-ceramide decreased the insulin-stimulated component of these signalling pathways and also glucose incorporation into triacylglycerol after 2 h.

CONCLUSIONS

Cell-permeable ceramides can mimic some effects of TNFalpha in producing insulin resistance. However, ceramides also mediate long-term effects that enable 3T3 L1 adipocytes to take up glucose and store triacylglycerols in the absence of insulin. These observations help to explain part of the nature and consequence of TNFalpha-induced insulin resistance and the control of fat accumulation in adipocytes in insulin resistance and obesity.

Comparison of signalling mechanisms involved in rat mesenteric microvessel contraction by noradrenaline and sphingosylphosphorylcholine

1 We have compared the signalling mechanisms involved in the pertussis toxin-sensitive and -insensitive contraction of rat isolated mesenteric microvessels elicited by sphingosylphosphorylcholine (SPC) and noradrenaline (NA), respectively. 2 The phospholipase D inhibitor butan-1-ol (0.3%), the store-operated Ca(2+) channel inhibitor SK>F 96,365 (10 microM), the tyrosine kinase inhibitor genistein (10 microM), and the src inhibitor PP2 (10 microM) as well as the negative controls (0.3% butan-2-ol and 10 microM diadzein and PP3) had only little effect against either agonist. 3 Inhibitors of phosphatidylinositol-3-kinase (wortmannin and LY 294,002, 10 microM each) or of mitogen-activated protein kinase kinase (PD 98,059 and U 126, 10 microM each) did not consistently attenuate NA- and SPC-induced contraction as compared to their vehicles or negative controls (LY 303,511 or U 124). 4 The phospholipase C inhibitor U 73,122 (10 microM) markedly inhibited the SPC- and NA-induced contraction (70% and 88% inhibition of the response to the highest NA and SPC concentration, respectively), whereas its negative control U 73,343 (10 microM) caused only less than 30% inhibition. 5 The rho-kinase inhibitors Y 27,632 (10 microM) and fasudil (30 microM) caused a rightward-shift of the NA concentration-response curve by 0.7-0.8 log units and reduced the response to 10 microM SPC by 88% and 83%, respectively. 6 These data suggest that SPC and NA, while acting on different receptors coupling to different G-protein classes, elicit contraction of rat mesenteric microvessels by similar signalling pathways including phospholipase C and rho-kinase.

Coupling of M(2) muscarinic receptors to Src activation in cultured canine colonic smooth muscle cells

The purpose of this study was to determine whether Src tyrosine kinases are one of the signaling intermediaries linking M(2) receptor stimulation to extracellular signal-regulated kinase (ERK) mitogen-activated protein kinase (MAPK) in cultures of canine colonic smooth muscle cells (CSMC). RT-PCR studies demonstrate expression of multiple Src tyrosine kinases, including Src, Fyn, and Yes, in CSMC. Muscarinic stimulation of CSMC with 10 microM ACh results in a twofold increase in Src activity within 10 min but does not increase the activity of Fyn. Treatment with the M(2) antagonist AF-DX 116 (10 microM) blocks ACh-stimulated Src activation in primary CSMC cultures that express both M(2) and M(3) receptors and in first-passage CSMC cultures that express predominantly M(2) receptors. Alkylation of M(3) receptors with 100 nM N,N-dimethyl-4-piperidinyl diphenylacetate mustard has no effect on Src activity. Treatment with the pyrazolopyrimidine Src inhibitor PP1 (10 microM) or AF-DX 116 (10 microM) blocks ACh-stimulated ERK phosphorylation. Together these results indicate that M(2) receptors are coupled to Src tyrosine kinase and subsequent activation of ERK in cultured CSMC.

Role of the extracellular signal-regulated kinase (Erk) signal transduction cascade in alpha(2) adrenoceptor-mediated vasoconstriction in porcine palmar lateral vein

The mechanism of alpha(2) adrenoceptor-mediated vasoconstriction is unknown, but may involve activation of voltage-sensitive calcium channels, and/or a protein tyrosine kinase. Recently the extracellular signal-regulated kinase (Erk) cascade, often an event downstream of tyrosine kinase activation, has been shown to mediate vasoconstriction to a variety of agents. The aim of this present study was to determine the involvement of the Erk signal transduction cascade in alpha(2) adrenoceptor-mediated vasoconstriction, and to confirm the involvement of activation of voltage-sensitive calcium channels, and protein tyrosine kinase. Contractions to the alpha(2) adrenoceptor agonist UK14304 in the porcine palmar lateral vein in vitro were reduced 70 - 80% by the MEK inhibitors PD98059 (10 - 50 microM) and U0126 (10 - 50 microM), indicating the involvement of the Erk signal transduction cascade. Immunoblots also demonstrated an increase in the phosphorylated (activated) form of Erk in palmar lateral vein segments after contraction with UK14304, which was inhibited by PD98059 and U0126. The calcium channel blockers nifedipine and verapamil, or removal of extracellular calcium inhibited UK14304-induced contractions and phosphorylation of Erk, demonstrating the importance of an influx of extracellular calcium. UK14304-induced contractions were inhibited by PP2 (1 - 10 microM), a selective inhibitor of Src tyrosine kinases, but not by PP3, an inactive analogue. PP2 also prevented the phosphorylation of Erk by UK14304. These data demonstrate that alpha(2) adrenoceptor-mediated vasoconstriction in the porcine palmar lateral vein is dependent upon activation of the Erk signal transduction cascade, which is downstream of an influx of extracellular calcium, and activation of Src tyrosine kinases.