Phosphatidylinositol 3-kinase mediates the inhibitory effect of epidermal growth factor on calcium-dependent chloride secretion

Pharmacological modulation of ion transport across wild-type and DeltaF508 CFTR-expressing human bronchial epithelia

Forskolin, UTP, 1-ethyl-2-benzimidazolinone (1-EBIO), NS004, 8-methoxypsoralen (Methoxsalen; 8-MOP), and genistein were evaluated for their effects on ion transport across primary cultures of human bronchial epithelium (HBE) expressing wild-type (wt HBE) and DeltaF508 (DeltaF-HBE) cystic fibrosis transmembrane conductance regulator. In wt HBE, the baseline short-circuit current (I(sc)) averaged 27.0 +/- 0.6 microA/cm(2) (n = 350). Amiloride reduced this I(sc) by 13.5 +/- 0.5 microA/cm(2) (n = 317). In DeltaF-HBE, baseline I(sc) was 33.8 +/- 1.2 microA/cm(2) (n = 200), and amiloride reduced this by 29.6 +/- 1.5 microA/cm(2) (n = 116), demonstrating the characteristic hyperabsorption of Na(+) associated with cystic fibrosis (CF). In wt HBE, subsequent to amiloride, forskolin induced a sustained, bumetanide-sensitive I(sc) (DeltaI(sc) = 8.4 +/- 0.8 microA/cm(2); n = 119). Addition of acetazolamide, 5-(N-ethyl-N-isopropyl)-amiloride, and serosal 4, 4'-dinitrostilben-2,2'-disulfonic acid further reduced I(sc), suggesting forskolin also stimulates HCO(3)(-) secretion. This was confirmed by ion substitution studies. The forskolin-induced I(sc) was inhibited by 293B, Ba(2+), clofilium, and quinine, whereas charybdotoxin was without effect. In DeltaF-HBE the forskolin I(sc) response was reduced to 1.2 +/- 0.3 microA/cm(2) (n = 30). In wt HBE, mucosal UTP induced a transient increase in I(sc) (Delta I(sc) = 15. 5 +/- 1.1 microA/cm(2); n = 44) followed by a sustained plateau, whereas in DeltaF-HBE the increase in I(sc) was reduced to 5.8 +/- 0. 7 microA/cm(2) (n = 13). In wt HBE, 1-EBIO, NS004, 8-MOP, and genistein increased I(sc) by 11.6 +/- 0.9 (n = 20), 10.8 +/- 1.7 (n = 18), 10.0 +/- 1.6 (n = 5), and 7.9 +/- 0.8 microA/cm(2) (n = 17), respectively. In DeltaF-HBE, 1-EBIO, NS004, and 8-MOP failed to stimulate Cl(-) secretion. However, addition of NS004 subsequent to forskolin induced a sustained Cl(-) secretory response (2.1 +/- 0.3 microA/cm(2), n = 21). In DeltaF-HBE, genistein alone stimulated Cl(-) secretion (2.5 +/- 0.5 microA/cm(2), n = 11). After incubation of DeltaF-HBE at 26 degrees C for 24 h, the responses to 1-EBIO, NS004, and genistein were all potentiated. 1-EBIO and genistein increased Na(+) absorption across DeltaF-HBE, whereas NS004 and 8-MOP had no effect. Finally, Ca(2+)-, but not cAMP-mediated agonists, stimulated K(+) secretion across both wt HBE and DeltaF-HBE in a glibenclamide-dependent fashion. Our results demonstrate that pharmacological agents directed at both basolateral K(+) and apical Cl(-) conductances directly modulate Cl(-) secretion across HBE, indicating they may be useful in ameliorating the ion transport defect associated with CF.

CYP2C40, a unique arachidonic acid 16-hydroxylase, is the major CYP2C in murine intestinal tract

We recently identified five different murine CYP2C cDNAs from a murine cDNA library. When expressed in a bacterial cDNA expression system, all five recombinant proteins metabolized arachidonic acid but produced distinctly different profiles. In addition, some CYP2C mRNAs were found in extrahepatic tissues, as well as in liver. Immunoblots with an antibody raised against recombinant CYP2C38, which recognizes all five murine CYP2Cs, demonstrated that among extrahepatic tissues, colon and cecum contained the highest amount of CYP2Cs. The highest concentration of CYP2Cs occurred in cecum and colon (cecum >/= proximal colon >> distal colon), with lower levels in duodenum, jejunum, and ileum. Immunohistochemical studies revealed that CYP2Cs were localized principally in epithelial cells and autonomic ganglia in gut and colon. Polymerase chain reaction amplification of reverse-transcribed mRNA using murine CYP2C-specific primers followed by cloning and sequencing identified CYP2C40 as the major CYP2C isoform expressed in murine intestinal tract. Recombinant CYP2C40 metabolized arachidonic acid in a regio- and stereospecific manner to 16(R)-HETE (hydroxyeicosatetraenoic acid) as the major product. To our knowledge, CYP2C40 is the first enzyme known to produce primarily 16-HETE. We conclude that CYP2C40 is one of the major cytochrome P450 proteins in the mouse intestinal tract. In the light of vasoactive and anti-neutrophilic effects of 16-HETE, we hypothesize that CYP2C40 may play an important role in endogenous biological functions in intestine.

Wortmannin inhibition of forskolin-stimulated chloride secretion by T84 cells

The time- and dose-dependent effects of wortmannin on transepithelial electrical resistance (Rte) and forskolin-stimulated chloride secretion in T84 monolayer cultures were studied. In both instances, maximal effects developed over 2 h and were stable thereafter. Inhibition of forskolin-stimulated chloride secretion, as measured by the short-circuit current (Isc) technique, had an IC50 of 200-500 nM, which is 100-fold higher than for inhibition of phosphatidylinositol 3-kinase (PI3K), but similar to the IC50 for inhibition of myosin light chain kinase (MLCK) and mitogen-activated protein kinases (MAPK). Previous work demonstrated that 500 nM wortmannin did not inhibit the cAMP activation of apical membrane chloride channels. We show here that 500 nM wortmannin has no affect on basolateral Na/K/2Cl-cotransporter activity, but inhibits basolateral membrane Na/K-ATPase activity significantly. The MLCK inhibitors ML-7 and KT5926 were without affect on forskolin-stimulated Isc. Similarly, the p38- and MEK-specific MAPK inhibitors SB203580 and PD98059 did not reduce forskolin-stimulated Isc. In contrast, the non-specific MAPK inhibitor apigenin reduced forskolin-stimulated Isc and basolateral membrane Na/K-ATPase activity similar to wortmannin. In isolated membranes from T84 cells, wortmannin did not inhibit Na/K-ATPase enzymatic activity directly. We conclude that one or more MAPK may regulate the functional expression of basolateral membrane Na/K-ATPase by controlling the abundance of enzyme molecules in the plasma membrane.

A role for protein kinase cepsilon in the inhibitory effect of epidermal growth factor on calcium-stimulated chloride secretion in human colonic epithelial cells

Epidermal growth factor (EGF) inhibits carbachol-induced chloride secretion in T(84) colonic epithelial cells and has been shown to activate phosphatidylinositol (PI) 3-kinase, leading to inhibition of a basolateral potassium conductance. We asked whether the inhibitory effect of EGF on secretion is due to activation of specific isoforms of protein kinase C (PKC) by PI 3-kinase. Western analysis revealed that PKCalpha, gamma, epsilon, eta, mu, lambda/iota, and zeta were expressed in T(84) cells. Ro318220 (an inhibitor active against PKCepsilon, 10 micrometer) but not Gö6983 (an inhibitor active against PKCzeta, 10 micrometer) reversed the inhibitory effect of EGF (100 ng/ml) on carbachol-stimulated chloride secretion. EGF induced the rapid translocation of PKCepsilon from the cytoplasm to the membrane. Wortmannin (50 micrometer) and LY294002 (20 nm), which are PI 3-kinase inhibitors that by themselves had no effect on PKCepsilon activity, significantly suppressed PKCepsilon translocation activated by EGF. LY294002 also reversed the inhibitory action of EGF on chloride secretion. PI (3,4)P(2) increased membrane-associated PKCepsilon and reduced carbachol-induced (86)Rb(+) efflux. Antisense oligonucleotides against PKCepsilon decreased PKCepsilon mass and prevented the inhibitory effect of EGF on carbachol-induced (86)Rb(+) efflux. Thus, the inhibitory effect of EGF on carbachol-induced chloride secretion involves the activation of PKCepsilon mediated by PI 3-kinase. Our findings contribute to the understanding of the cellular mechanisms that control chloride secretion.

Inhibition of phosphatidylinositol 3-kinase does not alter forskolin-stimulated Cl(-) secretion by T84 cells

Wortmannin is a potent inhibitor of phosphatidylinositol 3-kinase (PI3K) and membrane trafficking in many cells. To test the hypothesis that cystic fibrosis transmembrane conductance regulator (CFTR) traffics into and out of the plasma membrane during cAMP-stimulated epithelial Cl(-) secretion, we have studied the effects of wortmannin on forskolin-stimulated Cl(-) secretion by the human colonic cell line T84. At the PI3K inhibitory concentration of 100 nM, wortmannin did not affect significantly forskolin-stimulated Cl(-) secretion measured as short-circuit current (I(SC)). However, 500 nM wortmannin significantly inhibited forskolin-stimulated I(SC). cAMP activation of apical membrane CFTR Cl(-) channels in alpha-toxin-permeabilized monolayers was not reduced by 500 nM wortmannin, suggesting that inhibition of other transporters accounts for the observed reduction in T84 Cl(-) secretion. Forskolin inhibits apical endocytosis of horseradish peroxidase (HRP), but wortmannin did not alter forskolin inhibition of apical HRP endocytosis. In the absence of forskolin, wortmannin stimulated HRP endocytosis significantly. We conclude that, in T84 cells, apical fluid phase endocytosis is not dependent on PI3K activity and that CFTR does not recycle through a PI3K-dependent and wortmannin-sensitive membrane compartment.

Carbachol-stimulated transactivation of epidermal growth factor receptor and mitogen-activated protein kinase in T(84) cells is mediated by intracellular Ca2+, PYK-2, and p60(src)

Ca(2+)-dependent agonists, such as carbachol (CCh), stimulate epidermal growth factor receptor (EGFR) transactivation and mitogen-activated protein kinase activation in T(84) intestinal epithelial cells. This pathway constitutes an antisecretory mechanism by which CCh-stimulated chloride secretion is limited. Here, we investigated mechanisms underlying CCh-stimulated epidermal growth factor receptor (EGFR) transactivation. Thapsigargin (TG, 2 microM) stimulated EGFR and extracellular signal-regulated kinase (ERK) phosphorylation in T(84) cells. Inhibition of either EGFR or ERK activation, with tyrphostin AG1478 (1 microM) and PD 98059 (20 microM), respectively, potentiated chloride secretory responses to TG, as measured by changes in short-circuit current (I(sc)) across T(84) cells. CCh (100 microM) stimulated tyrosine phosphorylation and association of the Ca(2+)-dependent tyrosine kinase, PYK-2, with the EGFR, which was inhibited by the Ca(2+) chelator, BAPTA (20 microM). The calmodulin inhibitor, fluphenazine (50 microM) inhibited CCh-stimulated PYK-2 association with the EGFR and phosphorylation of EGFR and ERK. CCh also induced tyrosine phosphorylation of p60(src) and association of p60(src) with both PYK-2 and the EGFR. The Src family kinase inhibitor, PP2 (20 nM-20 microM) attenuated CCh-stimulated EGFR and ERK phosphorylation and potentiated chloride secretory responses to CCh. We conclude that CCh-stimulated transactivation of the EGFR is mediated by a pathway involving elevations in intracellular Ca(2+), calmodulin, PYK-2, and p60(src). This pathway represents a mechanism that limits CCh-stimulated chloride secretion across intestinal epithelia.

Cross-talk between enteric pathogens and the intestine

Enteric pathogens finely regulate the expression of virulence genes in reply to stimuli generated by the intestinal environment. This minireview focuses on recently discovered strategies developed by enteric bacteria to cause intestinal secretion through the elaboration of factors that share structure and function with specific host counterparts. Such bacterial antigens appear to interfere largely with the epithelial cell signalling that physiologically regulates the numerous and, as yet not fully elucidated, mechanisms controlling both the transcellular and the paracellular secretion pathways. Heat-stable enterotoxins (STs) elaborated by enterotoxigenic Escherichia coli and the enteroaggregative E. coli enterotoxin (EAST1) are both typical examples of enteric toxins that activate the transcellular secretion pathway by mimicking guanylin, the endogenous modulator of cGMP signalling. Alternative strategies have been developed by Salmonella to induce intestinal secretion through the elaboration of a factor (SopB) that resembles at least two of the host cell 4-phosphatases, enzymes that activate the Ca-dependent transcellular secretion pathway. Finally, Vibrio cholerae has developed innovative tactics to activate the paracellular secretion pathway through the elaboration of Zonula occludens toxin (Zot), a factor that mimics a recently described physiological modulator of intercellular tight junctions.

Distinct contributions of glycoprotein VI and alpha(2)beta(1) integrin to the induction of platelet protein tyrosine phosphorylation and aggregation

Platelet activation by collagen depends principally on two receptors, alpha(2)beta(1) integrin (GPIa-IIa) and GPVI. During this activation, the nonreceptor protein tyrosine kinase pp72(syk) is rapidly phosphorylated, but the precise contribution of alpha(2)beta(1) integrin and GPVI to signaling for this phosphorylation is not clear. We have recently found that proteolysis of platelet alpha(2)beta(1) integrin by the snake venom metalloproteinase, jararhagin, results in inhibition of collagen-induced platelet aggregation and pp72(syk) phosphorylation. In order to verify whether the treatment of platelets with jararhagin had any effect on GPVI signaling, in this study we stimulated platelets treated with either jararhagin or anti-alpha(2)beta(1) antibody with two GPVI agonists, an antibody to GPVI and convulxin. Platelet shape change and phosphorylation of pp72(syk) by both GPVI agonists was preserved, as was the structure and function of GPVI shown by (125)I-labeled convulxin binding to immunoprecipitated GPVI from jararhagin-treated platelets. In contrast, defective platelet aggregation in response to GPVI agonists occurred in both jararhagin-treated and alpha(2)beta(1)-blocked platelets. This apparent cosignaling role of alpha(2)beta(1) integrin for platelet aggregation suggests the possibility of a topographical association of this integrin with GPVI. We found that both platelet alpha(2)beta(1) integrin and GPVI coimmunoprecipitated with alpha(IIb)beta(3) integrin. Since platelet aggregation requires activation of alpha(IIb)beta(3) integrin, defective aggregation in the absence of alpha(2)beta(1) suggests that this receptor may provide a signaling link between GPVI and alpha(IIb)beta(3). Our study therefore demonstrates that platelet signaling leading to pp72(syk) phosphorylation initiated with GPVI engagement by either convulxin or GPVI antibody does not depend on alpha(2)beta(1) integrin. However, alpha(IIb)beta(3) integrin may, in this model, require functional alpha(2)beta(1) integrin for its activation.

Adherence of platelets under flow conditions results in specific phosphorylation of proteins at tyrosine residues

Collagen is a powerful platelet activating agent that promotes adhesion and aggregation of platelets. To differentiate the signals generated in these processes we have analyzed the tyrosine phosphorylation occurring in platelets after activation with collagen in suspension or under flow conditions. For the suspension studies, washed platelets were activated with different concentrations of purified type I collagen (ColI). Studies under flow conditions were performed using two different adhesive substrata: ColI and endothelial cells extracellular matrix (ECM). Coverslips coated with ColI or ECM were perfused through a parallel-plate perfusion chamber at 800 s(-1) for 5 min. After activation of platelets either in suspension or by adhesion, samples were solubilized and proteins were resolved by electrophoresis. Tyrosine-phosphorylated proteins were detected in immunoblots by specific antibodies. Activation of platelet suspensions with collagen induced tyrosine phosphorylation before aggregation could be detected. Profiles showing tyrosine-phosphorylated proteins from platelets adhered on ColI or on ECM were almost identical and lacked proteins p95, p80, p66, and p64, which were present in profiles from platelets activated in suspension. The intensity of phosphorylation was quantitatively weaker in those profiles from platelets adhered on ECM. Results from the present work indicate that activation of platelets in suspension or by adhesion induces differential tyrosine phosphorylation patterns. Phosphorylation of proteins p90 and p76 may be related to early activation events occurring during initial contact and spreading of platelets. Considering that adhesion is the first step of platelet activation, studies on signal transduction mechanisms under flow conditions may provide new insights to understand the signaling processes taking place at earliest stages of platelet activation.

ErbB2 and ErbB3 receptors mediate inhibition of calcium-dependent chloride secretion in colonic epithelial cells

We have previously demonstrated that epidermal growth factor (EGF) inhibits calcium-dependent chloride secretion via a mechanism involving stimulation of phosphatidylinositol 3-kinase (PI3-K). The muscarinic agonist of chloride secretion, carbachol (CCh), also stimulates an antisecretory pathway that involves transactivation of the EGF receptor (EGFR) but does not involve PI3-K. Here, we have examined if ErbB receptors, other than the EGFR, have a role in regulation of colonic secretion and if differential effects on ErbB receptor activation may explain the ability of the EGFR to propagate diverse signaling pathways in response to EGF versus CCh. Basolateral, but not apical, addition of the ErbB3/ErbB4 ligand alpha-heregulin (HRG; 1-100 ng/ml) inhibited secretory responses to CCh (100 microM) across voltage-clamped T(84) epithelial cells. Immunoprecipitation/Western blot studies revealed that HRG (100 ng/ml) stimulated tyrosine phosphorylation and dimerization of ErbB3 and ErbB2, but had no effect on phosphorylation of the EGFR. HRG also stimulated recruitment of the p85 subunit of PI3-K to ErbB3/ErbB2 receptor dimers, while the PI3-K inhibitor, wortmannin (50 nM), completely reversed the inhibitory effect of HRG on CCh-stimulated secretion. Further studies revealed that, while both EGF (100 ng/ml) and CCh (100 microM) stimulated phosphorylation of the EGFR, only EGF stimulated phosphorylation of ErbB2, and neither stimulated ErbB3 phosphorylation. EGF, but not CCh, stimulated the formation of EGFR/ErbB2 receptor dimers and the recruitment of p85 to ErbB2. We conclude that ErbB2 and ErbB3 are expressed in T(84) cells and are functionally coupled to inhibition of calcium-dependent chloride secretion. Differential dimerization with other ErbB family members may underlie the ability of the EGFR to propagate diverse inhibitory signals in response to activation by EGF or transactivation by CCh.

Phosphoinositide 3-kinase is required for aldosterone-regulated sodium reabsorption

Aldosterone, a steroid hormone, regulates renal Na+ reabsorption and, therefore, plays an important role in the maintenance of salt and water balance. In a model renal epithelial cell line (A6) we have found that phosphoinositide 3-kinase (PI 3-kinase) activity is required for aldosterone-stimulated Na+ reabsorption. Inhibition of PI 3-kinase by the specific inhibitor LY-294002 markedly reduces both basal and aldosterone-stimulated Na+ transport. Further, one of the products of PI 3-kinase, phosphatidylinositol 3,4,5-trisphosphate, is increased in response to aldosterone in intact A6 monolayers. This increase occurs just before the manifestation of the functional effect of the hormone and is also inhibited by LY-294002. With the use of blocker-induced noise analysis, it has been demonstrated that inhibition of phosphoinositide formation causes an inhibition of Na+ entry in both control and aldosterone-pretreated cultures by reducing the number of open functional epithelial Na+ channels (ENaCs) in the apical membrane of the A6 cells. These novel observations indicate that phosphoinositides are required for ENaC expression and suggest a mechanism for aldosterone regulation of channel function.

FcepsilonRI-mediated antigen endocytosis turns interferon-gamma-treated mouse mast cells from inefficient into potent antigen-presenting cells

Previous studies in our laboratory have shown that bone-marrow-derived mast cells (BMMC) could present immunogenic peptides, from soluble antigens endocytosed through fluid phase, only if they were subjected to a 48-hr treatment with interleukin-4 (IL-4) and granulocyte-macrophage colony-stimulating factor (GM-CSF). In contrast to GM-CSF, interferon-gamma (IFN-gamma) which highly upregulates major histocompatibility complex (MHC) class II expression, completely inhibits the generation of immunogenic peptides. We have used this model to study the role of FcepsilonRI-mediated antigen internalization in the regulation of the antigen-presenting function of IFN-gamma-treated mast cells. Here, we report that FcepsilonRI can reverse the IFN-gamma-treated mast cells from inefficient to highly efficient antigen-presenting cells. Inhibition of the antigen presenting capacity by piceatannol, a protein tyrosine kinase (PTK) syk inhibitor, indicates that this is an active process resulting from immunoglobulin E (IgE)-antigen-FcepsilonRI engagement which involves tyrosines found in the immunoreceptor tyrosine-based activation motif (ITAM) embedded in the cytoplasmic tail of the FcepsilonRI beta and gamma chains. Antigen-presenting function was also shown to require the activation of phosphatidyl inositol 3 (PI3) kinase, downstream of PTK syk phosphorylation, since this activity was completely blocked by wortmannin, a PI3 kinase inhibitor. These data suggest that signalling generated by FcepsilonRI provides mast cells with IgE-mediated enhanced antigen presentation to T cells and emphasize a so far unknown immunoregulatory mast-cell function that might take place in inflammatory sites.

Adhesion-induced intracellular signalling in endothelial cells depends on the nature of the matrix

The adhesion of a human microvascular endothelial cell line to its own matrix was studied in comparison with adhesion of the same cells to fibronectin or thrombospondin-1. These endothelial cells adhered preferentially to their matrix whereas an equal cell number was attached to fibronectin or thrombospondin-1. The adhesion of cells to thrombospondin-1 was mediated by the N-terminal heparin binding domain of thrombospondin-1 as shown by the use of a recombinant fragment, N18. Cells adhering to their matrix displayed a morphology and a cytoskeleton organization very similar to that observed in vivo with an apical immunostaining for actin stress fibers and a fine basal labeling for vinculin. Cells on fibronectin were extensively spread and rapidly assembled stress fibers and focal contacts. Cells adherent to thrombospondin-1 presented large lamellae rich in actin but devoid of vinculin and only few actin fibers were observed. Depending on the substratum used, adhering endothelial cells displayed also different tyrosine phosphorylation patterns on electrophoresis. Our observations indicate that endothelial cells adhering to their matrix present an activation state intermediate between that induced by a "hyperadhesive" protein like fibronectin and that generated by a moderate, indeed anti-adhesive, protein like thrombospondin-1.

Hexamethylenebisacetamide modulation of thyroglobulin and protein levels in thyroid cells is not mediated by phosphatidylinositol-3-kinase: a study with wortmannin

Hexamethylenebisacetamide (HMBA) induces in murine erythroleukemia cells (MELC) the commitment to terminal differentiation leading to globin gene expression. In the thyroid, HMBA acts as a growth factor and also as a differentiating agent. In the present paper, we studied the effect of HMBA on the very specific thyroid marker thyroglobulin (Tg) in two different thyroid cell systems, i.e., porcine cells in primary culture and ovine cells in long term culture. Using wortmannin, a specific inhibitor of phosphatidylinositol-3-kinase, we investigated whether this enzyme is involved in HMBA mode of action. We found that HMBA is a positive modulator of Tg production in porcine cells, but a negative effector in the OVNIS cell line. As all HMBA effects studied in the present paper, i.e., Tg production and total protein levels, are not inhibited by wortmannin, we suggest the non-involvement of phosphatidylinositol-3-kinase in HMBA mode of action.

Signal transduction pathways mediated by glycoprotein Ia/IIa in human platelets: comparison with those of glycoprotein VI

Human platelets were activated either by glycoprotein (GP) Ia/IIa agonist (rhodocytin) or by a GPVI agonist (collagen-related peptide, CRP), and the intracellular signal transduction pathways were compared in the presence of various inhibitors. Rhodocytin isolated from Calloselasma rhodostoma venom was verified as a GPIa/IIa agonist, based on the inhibitory effects of three mAbs directed against GPIa. Platelet activation mediated by GPIa/IIa led to overt platelet aggregation, elevation of intracellular Ca2+, and tyrosine phosphorylation of several proteins, similar to that of GPVI. p72(syk) and phospholipase Cgamma2 (PLCgamma2) tyrosine phosphorylation were also observed with GPIa/IIa-mediated platelet aggregation, although they peaked slightly later than that of GPVI. In contrast to GPVI-mediated platelet activation, most of these phenomena induced by GPIa/IIa activation were markedly suppressed by acetylsalicylic acid (ASA) or cytochalasin D. These findings suggest that the requirements for thromboxane A2 (TXA2) production and actin polymerization, which are the characteristics of collagen-induced platelet activation, are derived from the GPIa/IIa-mediated signal transduction, but not from that of GPVI.

Role of autocrine stimulation on the effects of cyclic AMP on protein and lipid phosphorylation in collagen-activated and thrombin-activated platelets

We compared several responses in thrombin-stimulated and collagen (type I)-stimulated platelets with and without forskolin and inhibitors of autocrine stimulation (IAS: an ADP-removing system of creatine phosphate/creatine phosphokinase, Arg-Gly-Asp-Ser peptide to prevent fibrinogen/fibronectin binding to GPIIb/IIIa, SQ 29.548 as a thromboxane A2 receptor antagonist, cyproheptadine as a serotonin receptor antagonist, BN 52021 as a platelet-activating factor receptor antagonist). The pattern of tyrosine-phosphorylated proteins, the phosphorylation of lipids in the polyphosphoinositide cycle and phosphorylation of pleckstrin (P47) were studied as markers for signal-transducing responses, exposure of CD62 (P-selectin) and CD63 (Glycoprotein 53), as well as secretion of ADP + ATP and beta-N-acetyl-glycosaminidase were studied as final activation responses. Clear differences between thrombin-stimulated and collagen-stimulated platelets were observed. First, practically all protein-tyrosine phosphorylation induced by thrombin was inhibited by IAS, while a partial inhibition was observed for collagen; the phosphorylation due to collagen alone was apparently stimulated by elevation of cAMP. Secondly, the other responses to thrombin were inhibited by increased levels of cAMP, independent of autocrine stimulation. In contrast, only the autocrine part of the collagen-induced responses was inhibited by elevation of cAMP. Thus, the inhibition by elevated cAMP seen in collagen-stimulated platelets seems to be due to removal of the G-protein-mediated activation from secreted autocrine stimulators either by IAS or forskolin. The remaining activity is a pure collagen effect which is not affected by elevated levels of cAMP.

Epoxyeicosatrienoic acids and their sulfonimide derivatives stimulate tyrosine phosphorylation and induce mitogenesis in renal epithelial cells

In our present studies utilizing a well characterized proximal tubule cell line, LLCPKcl4, we determined that all four EET regioisomers (5,6-EET, 8,9-EET, 11,12-EET, and 14,15-EET) stimulated [3H]thymidine incorporation, with 14,15-EET being the most potent. In contrast, no mitogenic effects were seen with arachidonic acid, other cP450 arachidonate metabolites (12R-hydroxyeicosatetraenoic acid (12R-HETE), 14,15-dihydroxyeicosatrienoic acid (14,15-DHET), or 20-HETE), or lipoxygenase metabolites (5S-HETE, leukotriene B4, or lipoxin A4). We found that their metabolically more stable sulfonimide (SI) analogs (11,12-EET-SI and 14,15-EET-SI) were also potent mitogens. In addition 14,15-EET-SI also increased cell proliferation as well as expression of both c-fos and egr-1 mRNA. The protein kinase C and A inhibitors, H-7 and H-8, or the cyclooxygenase inhibitor, indomethacin, had no effect upon 14, 15-EET-induced [3H]thymidine incorporation, but the selective tyrosine kinase inhibitor, genistein, significantly inhibited it. Immunoprecipitation and immunoblotting demonstrated increased tyrosine phosphorylation of PI3-kinase and epidermal growth factor receptor (EGFR) within 1 min of EET administration. EETs also stimulated association of PI3-kinase with EGFR. PI3-kinase inhibitors, wortmannin and LY 294002, markedly inhibited 14, 15-EET-SI-stimulated [3H]thymidine incorporation. In addition, 14, 15-EET-SI administration stimulated tyrosine phosphorylation of src homologous and collagen-like protein (SHC) and association of SHC with both growth factor receptor-binding protein (GRB2) and EGFR. Mitogen-activated protein kinase was also activated within 5 min. Pretreatment of the cells with the mitogen-activated protein kinase kinase inhibitor, PD98059, inhibited the 14,15-EET-SI-stimulated [3H]thymidine incorporation. Moreover, immunoblotting indicated that 14,15-EET stimulated tyrosine phosphorylation of the specific pp60(c-src) substrate p120 and c-Src association with EGFR. 14, 15-EET increased src kinase activity within 1 min. Our data indicate that EETs are potent mitogens for renal epithelial cells, and the mitogenic effects of the EETs are mediated, at least in part, by the activation of Src kinase and initiation of a tyrosine kinase phosphorylation cascade.

Carbachol stimulates transactivation of epidermal growth factor receptor and mitogen-activated protein kinase in T84 cells. Implications for carbachol-stimulated chloride secretion

We have examined the role of tyrosine phosphorylation in regulation of calcium-dependent chloride secretion across T84 colonic epithelial cells. The calcium-mediated agonist carbachol (CCh, 100 microM) stimulated a time-dependent increase in tyrosine phosphorylation of a range of proteins (with molecular masses ranging up to 180 kDa) in T84 cells. The tyrosine kinase inhibitor, genistein (5 microM), significantly potentiated chloride secretory responses to CCh, indicating a role for CCh-stimulated tyrosine phosphorylation in negative regulation of CCh-stimulated secretory responses. Further studies revealed that CCh stimulated an increase in both phosphorylation and activity of the extracellular signal-regulated kinase (ERK) isoforms of mitogen-activated protein kinase. Chloride secretory responses to CCh were also potentiated by the mitogen-activated protein kinase inhibitor, PD98059 (20 microM). Phosphorylation of ERK in response to CCh was mimicked by the protein kinase C (PKC) activator, phorbol myristate acetate (100 nM), but was not altered by the PKC inhibitor GF 109203X (1 microM). ERK phosphorylation was also induced by epidermal growth factor (EGF) (100 ng/ml). Immunoprecipitation/Western blot studies revealed that CCh stimulated tyrosine phosphorylation of the EGF receptor (EGFr) and increased co-immunoprecipitation of the adapter proteins, Shc and Grb2, with the EGFr. An inhibitor of EGFr phosphorylation, tyrphostin AG1478 (1 microM), reversed CCh-stimulated phosphorylation of both EGFr and ERK. Tyrphostin AG1478 also potentiated chloride secretory responses to CCh. We conclude that CCh activates ERK in T84 cells via a mechanism involving transactivation of the EGFr, and that this pathway constitutes an inhibitory signaling pathway by which chloride secretory responses to CCh may be negatively regulated.

Effects of orally administered epidermal growth factor on enteropathogenic Escherichia coli infection in rabbits

The increased intestinal absorption induced by epidermal growth factor (EGF) is associated with diffuse lengthening of brush border microvilli. The aim of this study was to examine the in vivo effects of oral administration of EGF during infection with enteropathogenic Escherichia coli. New Zealand White rabbits (4 weeks old) received orogastric EGF daily starting 3 days prior to infection with enteropathogenic E. coli RDEC-1 and were compared with sham-treated infected animals and uninfected controls. Weight gain, food intake, fecal E. coli, and stool consistency were assessed daily. On day 10, segments of jejunum, ileum, proximal, and distal colon were assessed for gram-negative bacterial colonization, disaccharidase activities, and epithelial ultrastructure. Effects of EGF on E. coli RDEC-1 proliferation were studied in vitro. E. coli RDEC-1 caused diarrhea and reduced weight gain. Seven days postinfection, the small and large intestines were colonized with numerous bacteria, brush border microvilli were disrupted, and maltase and sucrase activities were significantly reduced in the jejunum. Daily treatment with EGF prevented the occurrence of diarrhea and reduction of weight gain. These effects were associated with significant inhibition of E. coli colonization in the small and large intestine, improved jejunal maltase and sucrase activities and reduced microvillous injury. EGF did not affect the proliferation of E. coli in vitro. The findings suggest that EGF protects the gastrointestinal tract against colonization by enteropathogenic E. coli.

Involvement of reactive oxygen intermediates in lectin-induced protein-tyrosine phosphorylation of Syk in THP-1 cells

Oxidative stress (H2O2) has been shown to be associated with tyrosine phosphorylation and activation of protein-tyrosine kinase Syk. In the present study, we examined the possibility that reactive oxygen intermediates (ROI) were involved in concanavalin A (Con A)-induced tyrosine phosphorylation in THP-1 cells. Rapid tyrosine phosphorylations of Syk, Fc gamma receptor(s) and phospholipase C gamma 2 (PLC gamma 2) were induced by Con A treatment in THP-1 cells. Pretreatment of cells with antioxidants N-acetylcysteine (NAC) and glutathione (GSH) almost completely blocked tyrosine phosphorylations of Syk, Fc gamma receptor(s) and PLC gamma 2. In addition, THP-1 cells showed significant levels of ROI from the early period of Con A treatment and the levels of ROI were inhibited by antioxidant treatment. These data suggest that ROI have an important role in Con A-induced protein-tyrosine kinase(s) signaling pathways.

Membrane-permeant esters of phosphatidylinositol 3,4,5-trisphosphate

Phosphoinositide 3-OH kinases and their products, D-3 phosphorylated phosphoinositides, are increasingly recognized as crucial elements in many signaling cascades. A reliable means to introduce these lipids into intact cells would be of great value for showing the physiological roles of this pathway and for testing the specificity of pharmacological inhibitors of the kinases. We have stereospecifically synthesized di-C8-PIP3/AM and di-C12-PIP3/AM, the heptakis(acetoxymethyl) esters of dioctanoyl- and dilauroylphosphatidylinositol 3,4,5-trisphosphate, in 14 steps from myo-inositol. The ability of these uncharged lipophilic derivatives to deliver phosphatidylinositol 3,4,5-trisphosphate across cell membranes was demonstrated on 3T3-L1 adipocytes and T84 colon carcinoma monolayers. Insulin stimulation of hexose uptake into adipocytes was inhibited by the kinase inhibitor wortmannin and was largely restored by di-C8-PIP3/AM, which had no effect in the absence of insulin. Thus phosphatidylinositol 3,4,5-trisphosphate or a metabolite was necessary but not sufficient for stimulation of hexose transport. In T84 epithelial monolayers, di-C12-PIP3/AM mimicked epidermal growth factor in inhibiting chloride secretion and potassium efflux, suggesting that phosphatidylinositol 3,4, 5-trisphosphate was sufficient to modulate these fluxes and mediate epidermal growth factor's action.

Inhibition of Ca(2+)-dependent Cl- secretion in T84 cells: membrane target(s) of inhibition is agonist specific

Previous studies have indicated that Ca(2+)-dependent Cl- secretion across monolayers of T84 epithelial cells is subject to a variety of negative influences that serve to limit the overall extent of secretion. However, the downstream membrane target(s) of these inhibitory influences had not been elucidated. In this study, nuclide efflux techniques were used to determine whether inhibition of Ca(2+)-dependent Cl- secretion induced by carbachol, inositol 3,4,5,6-tetrakisphosphate, epidermal growth factor, or insulin reflected actions at an apical Cl- conductance, a basolateral K+ conductance, or both. Pretreatment of T84 cell monolayers with carbachol or a cell-permeant analog of inositol 3,4,5,6-tetrakisphosphate reduced the ability of subsequently added thapsigargin to stimulate apical 125I-, but not basolateral 86Rb+, efflux. These data suggested an effect on an apical Cl- channel. Conversely, epidermal growth factor reduced Ca(2+)-stimulated 86Rb+ but not 125I- efflux, suggesting an effect of the growth factor on a K+ channel. Finally, insulin inhibited 125I- and 86Rb+ effluxes. Thus effects of agents that inhibit transepithelial Cl- secretion are also manifest at the level of transmembrane transport pathways. However, the precise nature of the membrane conductances targeted are agonist specific.

D-myo-Inositol 1,4,5,6-tetrakisphosphate produced in human intestinal epithelial cells in response to Salmonella invasion inhibits phosphoinositide 3-kinase signaling pathways

Several inositol-containing compounds play key roles in receptor-mediated cell signaling events. Here, we describe a function for a specific inositol polyphosphate, D-myo-inositol 1,4,5,6-tetrakisphosphate [Ins(1,4,5,6)P4], that is produced acutely in response to a receptor-independent process. Thus, infection of intestinal epithelial cells with the enteric pathogen Salmonella, but not with other invasive bacteria, induced a multifold increase in Ins(1,4,5,6)P4 levels. To define a specific function of Ins(1,4,5,6)P4, a membrane-permeant, hydrolyzable ester was used to deliver it to the intracellular compartment, where it antagonized epidermal growth factor (EGF)-induced inhibition of calcium-mediated chloride (Cl-) secretion (CaMCS) in intestinal epithelia. This EGF function is likely mediated through a phosphoinositide 3-kinase (PtdIns3K)-dependent mechanism because the EGF effects are abolished by wortmannin, and three different membrane-permeant esters of the PtdIns3K product phosphatidylinositol 3,4,5-trisphosphate mimicked the EGF effect on CaMCS. We further demonstrate that Ins(1,4,5,6)P4 antagonized EGF signaling downstream of PtdIns3K because Ins(1,4,5, 6)P4 interfered with the PtdInsP3 effect on CaMCS without affecting PtdIns3K activity. Thus, elevation of Ins(1,4,5,6)P4 in Salmonella-infected epithelia may promote Cl- flux by antagonizing EGF inhibition mediated through PtdIns3K and PtdInsP3.

Bowditch lecture. Integrated regulation of intestinal epithelial transport: intercellular and intracellular pathways

The intestinal epithelium is an important site of active solute transport processes. Such processes include the secretion of electrolytes into the lumen, predominantly chloride and bicarbonate. These secretory mechanisms subserve a variety of functions, both physiological and pathophysiological, including maintenance of the fluidity of intestinal contents and mucosal defense. Both chloride and bicarbonate secretion are the subject of integrated regulatory mechanisms at both the intercellular and intracellular levels. The goal of this article is to discuss data that exemplify these two levels of regulation, which have been the subject of research in my laboratory. It is likely that some of these principles are also broadly applicable to secretory epithelial cells outside of the intestinal tract, such as those in the airway. I also discuss the ways in which we believe these regulatory mechanisms are involved not only in intestinal physiology but also perhaps in the pathogenesis of specific disease states.