Similar Rates of Phosphatidylinositol Hydrolysis Following Activation of Wild-Type and Truncated Rat Neurokinin-1 Receptors

Abstract: The substance P (neurokinin-1) receptor belongs to the family of seven putative transmembrane domain receptors that are coupled via G proteins to phospholipase C activation. Homologous desensitization of substance P-stimulated responses has been described in various systems. The rat neurokinin-1 receptor and a truncated mutant lacking the carboxyl-terminal region were expressed in Chinese hamster ovary cells to examine the mechanisms of substance P-induced desensitization. Wild-type and truncated receptor-bearing cells were indistinguishable in agonist binding affinity and EC₅₀ of substance P-induced accumulation of ³H-inositol phosphates. Substance P-induced responses continued for 30–45 min in cells expressing wild-type and truncated receptors as well as in rat LRM-55 and human U373 cells, which express endogenous neurokinin-1 receptors. In transfected cells expressing the wild-type receptor, CP-96,345 added 15 min after substance P blocked further responses, demonstrating the continuing presence of responsive receptors. The rates of accumulation of ³H-inositol phosphates were four times greater in the initial 15 s of stimulation than for the next 20 min for both wild-type and truncated receptor types. This decrease in rate of substance P-stimulated phosphatidylinositol hydrolysis is therefore not dependent on the carboxyl-terminal region of the rat neurokinin-1 receptor, which contains 26 serine and threonine residues. These results are discussed in relation to current ideas regarding neurokinin-1 receptor desensitization.     

Protein kinase C-dependent and -independent mechanisms regulating the parotid substance P receptor as revealed by differential effects of protein kinase C inhibitors.

Substance P-induced inositol trisphosphate (InsP3) formation was inhibited by 1 microM-4 beta-phorbol 12,13-dibutyrate (PDBu) in rat parotid acinar cells. The inhibitory effect of PDBu was reversed by the protein kinase C inhibitors H-7 or K252a. Substance P also elicits a persistent desensitization of subsequent substance P-stimulated InsP3 formation. However, this desensitization was not inhibited by H-7. In addition, H-7 had no effect on the time course of substance P-induced InsP3 formation. These results suggest that, although activation of protein kinase C by phorbol esters can inhibit the substance P receptor-linked phospholipase C pathway, this mechanism apparently plays little, if any, role in regulating this system after activation by substance P.     

Sphingosine 1-phosphate induces EGFR expression via Akt/NF-kappaB and ERK/AP-1 pathways in rat vascular smooth muscle cells

Sphingosine 1-phosphate (S1P) has been shown to regulate expression of several genes in vascular smooth muscle cells (VSMCs) and contributes to arteriosclerosis. However, the mechanisms regulating epidermal growth factor receptor (EGFR) expression by S1P in aortic VSMCs remain unclear. Western blotting and RT-PCR analyses showed that S1P induced EGFR mRNA and protein expression in a time- and concentration-dependent manner, which was attenuated by inhibitors of MEK1/2 (U0126) and phosphatidylinositide 3-kinase (PI3K; wortmannin), and transfection with dominant negative mutants of ERK and Akt, respectively. These results suggested that S1P-induced EGFR expression was mediated through p42/p44 MAPK and PI3K/Akt pathways in VSMCs. In accordance with these findings, S1P stimulated phosphorylation of p42/p44 MAPK and Akt which was attenuated by U0126 and wortmannin, respectively. Furthermore, S1P-induced EGFR upregulation was blocked by a selective NF-kappaB inhibitor helenalin. Immunofluorescent staining and reporter gene assay revealed that S1P-induced activation of NF-kappaB was blocked by wortmannin, but not by U0126, suggesting that activation of NF-kappaB was mediated through PI3K/Akt. Moreover, S1P-induced EGFR expression was inhibited by an AP-1 inhibitor curcumin and tanshinone IIA. S1P-stimulated AP-1 subunits (c-Jun and c-Fos mRNA) expression was attenuated by U0126 and wortmannin, suggesting that MEK and PI3K/ERK cascade linking to AP-1 was involved in EGFR expression. Upregulation of EGFR by S1P may exert a phenotype modulation of VSMCs. This hypothesis was supported by pretreatment with AG1478 or transfection with shRNA of EGFR that attenuated EGF-stimulated proliferation of VSMCs pretreated with S1P, determined by XTT assay. These results demonstrated that in VSMCs, activation of Akt/NF-kappaB and ERK/AP-1 pathways independently regulated S1P-induced EGFR expression in VSMCs. Understanding the mechanisms involved in S1P-induced EGFR expression on VSMCs may provide potential therapeutic targets in the treatment of arteriosclerosis.     

Two modes of regulation of the phospholipase C-linked substance-P receptor in rat parotid acinar cells.

In rat parotid acinar cells prelabelled with [3H]inositol, substance P (100 nM) induced the formation of [3H]inositol 1,4,5-trisphosphate [Ins(1,4,5)P3]. Ins(1,4,5)P3 reached a maximum 7 s after substance P stimulation, and thereafter decreased and reached a stable value at 60 s. When the cells were exposed to substance P for 10, 30, 60, or 300 s, washed, and re-exposed to this peptide, the formation of [3H]inositol trisphosphate (InsP3) was attenuated in a time-dependent manner. In the cells pretreated as described above, the number of [3H]substance-P-binding sites (Bmax) was also decreased. Possible role(s) of Ca2+ and protein kinase (protein kinase C) control mechanisms in regulating substance P responses were investigated. Desensitization of substance P-induced InsP3 was not affected by the Ca2+ ionophore ionomycin, nor was it dependent on Ca2+ mobilization. On the other hand, in the presence of 4 beta-phorbol 12,13-dibutyrate (PDBu) and 12-O-tetradecanoyl-4 beta-phorbol 13-acetate, known activators of protein kinase C, substance P-induced InsP3 formation was inhibited. However, PDBu had no effect on [3H]substance P binding, whether present during the assay or when cells were pretreated. The persistent desensitization of InsP3 formation induced by substance P was not affected by PDBu. These results suggest that the persistent desensitization of InsP3 formation induced by substance P is a homologous process involving down-regulation of the substance P receptor; the mechanism does not appear to involve, or to be affected by, the Ca2+ or protein kinase C signalling systems. Protein kinase C activation can, however, inhibit substance P-induced InsP3 formation, which may indicate the presence of a negative-feedback control on the substance P pathway.     

Sphingosine-1-phosphate induces COX-2 expression via PI3K/Akt and p42/p44 MAPK pathways in rat vascular smooth muscle cells

Sphingosine 1-phosphate (S1P) has been shown to regulate smooth muscle cell proliferation, migration, and vascular maturation. S1P increases the expression of several proteins including COX-2 in vascular smooth muscle cells (VSMCs) and contributes to arteriosclerosis. However, the mechanisms regulating COX-2 expression by S1P in VSMCs remain unclear. Western blotting and RT-PCR analyses showed that S1P induced the expression of COX-2 mRNA and protein in a time- and concentration-dependent manner, which was attenuated by inhibitors of MEK1/2 (U0126) and PI3K (wortmannin), and transfection with dominant negative mutants of p42/p44 mitogen-activated protein kinases (ERK2) or Akt. These results suggested that both p42/p44 MAPK and PI3K/Akt pathways participated in COX-2 expression induced by S1P in VSMCs. In accordance with these findings, S1P stimulated phosphorylation of p42/p44 MAPK and Akt, which was attenuated by U0126, LY294002, or wortmannin, respectively. Furthermore, this up-regulation of COX-2 mRNA and protein was blocked by a selective NF-kappaB inhibitor helenalin. Consistently, S1P-stimulated translocation of NF-kappaB into the nucleus was revealed by immnofluorescence staining. Moreover, S1P-stimulated activation of NF-kappaB promoter activity was blocked by phosphatidylinositol 3-kinase (PI3K) inhibitor LY294002 and helenalin, but not by U0126, suggesting that involvement of PI3K/Akt in the activation of NF-kappaB. COX-2 promoter assay showed that S1P induced COX-2 promoter activity mediated through p42/p44 MAPK, PI3K/Akt, and NF-kappaB. These results suggested that in VSMCs, activation of p42/p44 MAPK, Akt and NF-kappaB pathways was essential for S1P-induced COX-2 gene expression. Understanding the mechanisms involved in S1P-induced COX-2 expression on VSMCs may provide potential therapeutic targets in the treatment of arteriosclerosis.     

Effect of mitogen-activated protein kinases on chemokine synthesis induced by substance P in mouse pancreatic acinar cells

Substance P, acting via its neurokinin 1 receptor (NK1 R), plays an important role in mediating a variety of inflammatory processes. Its interaction with chemokines is known to play a crucial role in the pathogenesis of acute pancreatitis. In pancreatic acinar cells, substance P stimulates the release of NFκB-driven chemokines. However, the signal transduction pathways by which substance P-NK1 R interaction induces chemokine production are still unclear. To that end, we went on to examine the participation of mitogen-activated protein kinases (MAPKs) in substance P-induced synthesis of pro-inflammatory chemokines, monocyte chemoanractant protein-1 (MCP-I), macrophage inflammatory protein-lα (MIP-lα) and macrophage inflammatory protein-2 (MIP-2), in pancreatic acini. In this study, we observed a time-dependent activation of ERK1/2, c-Jun N-terminal kinase (JNK), NFκB and activator protein-1 (AP-1) when pancreatic acini were stimulated with substance P. Moreover, substance P-induced ERK 1/2, JNK, NFκB and AP-1 activation as well as chemokine synthesis were blocked by pre-treatment with either extracellular signal-regulated protein kinase kinase 1 (MEK1) inhibitor or JNK inhibitor. In addition, substance P-induced activation of ERK 112, JNK, NFκB and AP-1-driven chemokine production were attenuated by CP96345, a selective NK1 R antagonist, in pancreatic acinar cells. Taken together, these results suggest that substance P-NK1 R induced chemokine production depends on the activation of MAPKs-mediated NFκB and AP-1 signalling pathways in mouse pancreatic acini.     

Zinc L-carnosine protects colonic mucosal injury through induction of heat shock protein 72 and suppression of NF-kappaB activation

In this study, we investigated the effects of zinc L-carnosine, an anti-ulcer drug, on acetic acid-induced colonic mucosal injury and the correlation of these effects with expression of 72-kDa heat shock proteins (HSP72) and nuclear factor kappa B (NF-kappaB) activation in rat colonic mucosa in vivo. After intrarectal administration of zinc L-carnosine, the rats received intrarectal infusion of 5% acetic acid (1 ml). The colonic mucosal damage was evaluated by macroscopic assessments 24 h after the intrarectal infusion of acetic acid. Expression of HSP72 in rat colonic mucosa was evaluated by Western blot analysis before and after zinc L-carnosine administration. NF-kappaB activation was evaluated by electrophoretic mobility shift assays (EMSA). Zinc L-carnosine inhibited visible damage in rat colonic mucosa by acetic acid. Expression of HSP72 was significantly increased at 6 h after zinc L-carnosine administration. Furthermore, NF-kappaB activation in colonic mucosa was suppressed 6 h after zinc L-carnosine treatment. These results suggested that zinc L-carnosine protects the colonic mucosa against acetic acid by induction of HSP72 and suppression of NF-kappaB activation and zinc L-carnosine may be a novel therapeutic agent for the therapy of inflammatory bowel disease.     

The mast cell response to substance P: effects of neuraminidase, limulin, and some novel synthetic peptide antagonists

We have investigated certain aspects of the mechanism whereby substance P triggers secretion of 5-hydroxytryptamine (5-HT) from rat peritoneal mast cells in vitro. Substance P-induced release of 5-HT was inhibited following pretreatment of rat peritoneal cells with 0.01-1.0 units/ml neuraminidase; secretion induced by anti-IgE antibody was inhibited by pretreatment with 1.0 units/ml but not by lower concentrations of enzyme. Addition of the sialic acid-rich substances N-acetyl-neuraminlactose (up to 1.0 mM) and mucin (up to 1.0 mg/ml) to substance P in free solution failed to block the activity of the neuropeptide. Limulin, a sialic acid-specific lectin, failed to block substance P-induced secretion of 5-HT, but was found to possess intrinsic non-lytic secretory activity (at 5-20 micrograms/ml). Release of 5-HT induced by limulin was independent of that induced by substance P. A range of octapeptides incorporating the C-terminal sequence Gly-Ser-Phe-Phe, but differing in degree of cationicity and positioning of cationic residues in the four N-terminal positions, were tested for their capacity to antagonise the mast cell-triggering activity of substance P. A peptide incorporating two lysine residues at the N-terminus was found to have partial substance P antagonist activity; no effects on IgE-mediated secretion were observed.     

Nuclear factor-kappaB activation leads to down-regulation of fatty acid oxidation during cardiac hypertrophy

Little is known about the mechanisms responsible for the fall in fatty acid oxidation during the development of cardiac hypertrophy. We focused on the effects of nuclear factor (NF)-kappaB activation during cardiac hypertrophy on the activity of peroxisome proliferator-activated receptor (PPAR) beta/delta, which is the predominant PPAR subtype in cardiac cells and plays a prominent role in the regulation of cardiac lipid metabolism. Phenylephrine-induced cardiac hypertrophy in neonatal rat cardiomyocytes caused a reduction in the expression of pyruvate dehydrogenase kinase 4 (Pdk4), a target gene of PPARbeta/delta involved in fatty acid utilization, and a fall in palmitate oxidation that was reversed by NF-kappaB inhibitors. Lipopolysaccharide stimulation of NF-kappaB in embryonic rat heart-derived H9c2 myotubes, which only express PPARbeta/delta, caused both a reduction in Pdk4 expression and DNA binding activity of PPARbeta/delta to its response element, effects that were reversed by NF-kappaB inhibitors. Coimmunoprecipitation studies demonstrated that lipopolysaccharide strongly stimulated the physical interaction between the p65 subunit of NF-kappaB and PPARbeta/delta, providing an explanation for the reduced activity of PPARbeta/delta. Finally, we assessed whether this mechanism was present in vivo in pressure overload-induced cardiac hypertrophy. In hypertrophied hearts of banded rats the reduction in the expression of Pdk4 was accompanied by activation of NF-kappaB and enhanced interaction between p65 and PPARbeta/delta. These results indicate that NF-kappaB activation during cardiac hypertrophy down-regulates PPARbeta/delta activity, leading to a fall in fatty acid oxidation, through a mechanism that involves enhanced protein-protein interaction between the p65 subunit of NF-kappaB and PPARbeta/delta.     

Relationship between NF-kappaB and trypsinogen activation in rat pancreas after supramaximal caerulein stimulation

Intra-acinar cell nuclear factor-kappaB (NF-kappaB) and trypsinogen activation are early events in secretagogue-induced acute pancreatitis. We have studied the relationship between NF-kappaB and trypsinogen activation in rat pancreas. CCK analogue caerulein induces early (within 15 min) parallel activation of both NF-kappaB and trypsinogen in pancreas in vivo as well as in pancreatic acini in vitro. However, NF-kappaB activation can be induced without trypsinogen activation by lipopolysaccharide in pancreas in vivo and by phorbol ester in pancreatic acini in vitro. Stimulation of acini with caerulein after 6 h of culture results in NF-kappaB but not trypsinogen activation. Protease inhibitors (AEBSF, TLCK, and E64d) inhibit both intracellular trypsin activity and NF-kappaB activation in caerulein stimulated acini. A chymotrypsin inhibitor (TPCK) inhibits NF-kappaB activation but not trypsin activity. The proteasome inhibitor MG-132 prevents caerulein-induced NF-kappaB activation but does not prevent trypsinogen activation. These findings indicate that although caerulein-induced NF-kappaB and trypsinogen activation are temporally closely related, they are independent events in pancreatic acinar cells. NF-kappaB activation per se is not required for the development of early acinar cell injury by supramaximal secretagogue stimulation.     

Involvement of phospholipase D in sphingosine 1-phosphate-induced activation of phosphatidylinositol 3-kinase and Akt in Chinese hamster ovary cells overexpressing EDG3

Phospholipase D (PLD), phosphatidylinositol 3-kinase (PI3K), and Akt are known to be involved in cellular signaling related to proliferation and cell survival. In this report, we provide evidence that PLD links sphingosine 1-phosphate (S1P)-induced activation of the G protein-coupled EDG3 receptor to stimulation of PI3K and its downstream effector Akt in Chinese hamster ovary (CHO) cells. S1P stimulation of EDG3-overexpressing CHO cells but not vector-transfected cells induced activation of PLD, PI3K, and Akt in a time- and dose-dependent manner. Akt phosphorylation was prevented by the PI3K inhibitors wortmannin and LY294002 (2-(4-monrpholinyl)-8-phenyl-4H-1-benzopyran-4-one), indicating that Akt activation was dependent on PI3K. S1P-induced activation of PI3K and Akt was abrogated by 1-butanol, which inhibited S1P-induced accumulation of phosphatidic acid by serving as a phosphatidyl group acceptor in the transphosphatidylation reaction catalyzed by PLD, whereas both PI3K and Akt activation were not inhibited by 2-butanol without such reaction. Co-expression of wild-type PLD2 with myc-Akt resulted in increased Akt activation in response to S1P. In contrast, co-expression of a catalytically inactive mutant of PLD2 eliminated the S1P-induced Akt activation. The treatment of EDG3-expressing CHO cells with exogenous Streptomyces chromofuscus PLD, which caused an accumulation of phosphatidic acid, resulted in increases in PI3K activity and the phosphorylation of Akt, the latter of which was completely abolished by LY294002. Furthermore, S1P-induced membrane ruffling, which was dependent on PI3K and Rac, was inhibited by 1-butanol, but not by 2-butanol. These results demonstrate that PLD participates in the activation of PI3K and Akt stimulation of EDG3 receptor.     

Sphingosine 1-phosphate induces cyclooxygenase-2 via Ca2+-dependent, but MAPK-independent mechanism in rat vascular smooth muscle cells

The effects of sphingosine 1-phosphate (S1P) on prostaglandin I(2) (PGI(2)) production and cyclooxygenase (COX) expression in cultured rat vascular smooth muscle cells (VSMCs) were investigated. S1P stimulated PGI(2) production in a concentration-dependent manner, which was completely suppressed by NS-398, a selective COX-2 inhibitor, as determined by radioimmunoassay. S1P stimulated COX-2 protein and mRNA expressions in a concentration- and time-dependent manner, while it had no effect on COX-1 expression. S1P(2) and S1P(3) receptors mRNA were abundantly expressed in rat VSMCs. Suramin, an antagonist of S1P(3) receptor, almost completely inhibited S1P-induced COX-2 expression. Pretreatment of VSMCs with pertussis toxin (PTX) partially, but significantly inhibited S1P-induced PGI(2) production and COX-2 expression. S1P also activated extracellular signal-regulated kinase (ERK) and p38 mitogen-activated protein kinase (MAPK). However, neither PD 98059, a selective inhibitor of ERK activation, nor SB 203580, a selective inhibitor of p38 MAPK, had a significant inhibitory effect on S1P-induced COX-2 expression, suggesting that the MAPK activation does not play main roles in S1P-induced COX-2 induction. S1P-induced COX-2 expression was inhibited by PP2, an inhibitor of Src-family tyrosine kinase, Ca(2+) depletion, and GF 109203X, an inhibitor of protein kinase C (PKC). These results suggest that S1P stimulates COX-2 induction in rat VSMCs through mechanisms involving Ca(2+)-dependent PKC and Src-family tyrosine kinase activation via S1P(3) receptor coupled to PTX-sensitive and -insensitive G proteins.     

Differential effect of interleukin-10 on hepatocyte apoptosis

Current data suggests that hepatocyte apoptosis is an essential feature contributing to several chronic liver diseases. It has been shown that IL-10 has diverse and potentially pleiotropic actions that suggest that it may have a direct effect on apoptosis. It has been established that NF-kappaB activation is essential to protect hepatocytes from apoptosis. The purpose of the present work is to evaluate the effect of the anti-inflammatory cytokine, IL-10 on the activation of NF-kappaB in primary cultured rat hepatocytes and hepatoblastoma (HepG2) cell line and explore its consequences on apoptosis. Apoptosis was induced by TNF-alpha and cicloheximide in HepG2 hepatoblastoma cells and by ethanol and a glutathione depletor in primary cultured rat hepatocytes. NF-kappaB activation was determined by EMSA. IL-10 increased ethanol induced apoptosis in primary culture rat hepatocytes (28%). These effects were enhanced when the cells were pre-treated with IL-10 under conditions of oxidative stress (glutathione depletion). The effects of IL-10 on primary cultured hepatocytes were independent of NF-kappaB activation. When apoptosis was induced by cicloheximide and TNF-alpha in hepatoblastoma cells, pretreatment with IL-10 was accompanied by a decrease of 38% in apoptosis. IL-10 did not have any effect on the signaling cascade of apoptosis but caused a significant increase in NF-kappaB activation. When NF-kappaB activation was inhibited by sulfazalazine the decrease in apoptosis was reversed. The present study demonstrates the importance of differential cell marking when trying to characterize the effects of cytokines in their contribution to liver cell apoptosis. The study provides insight into the mechanisms by which IL-10 affects apoptosis through a differential effect on NF-kappaB activation.     

PKC-delta and -epsilon regulate NF-kappaB activation induced by cholecystokinin and TNF-alpha in pancreatic acinar cells

Although NF-kappaB plays an important role in pancreatitis, mechanisms underlying its activation remain unclear. We investigated the signaling pathways mediating NF-kappaB activation in pancreatic acinar cells induced by high-dose cholecystokinin-8 (CCK-8), which causes pancreatitis in rodent models, and TNF-alpha, which contributes to inflammatory responses of pancreatitis, especially the role of PKC isoforms. We determined subcellular distribution and kinase activities of PKC isoforms and NF-kappaB activation in dispersed rat pancreatic acini. We applied isoform-specific, cell-permeable peptide inhibitors to assess the role of individual PKC isoforms in NF-kappaB activation. Both CCK-8 and TNF-alpha activated the novel isoforms PKC-delta and -epsilon and the atypical isoform PKC-zeta but not the conventional isoform PKC-alpha. Inhibition of the novel PKC isoforms but not the conventional or the atypical isoform resulted in the prevention of NF-kappaB activation induced by CCK-8 and TNF-alpha. NF-kappaB activation by CCK-8 and TNF-alpha required translocation but not tyrosine phosphorylation of PKC-delta. Activation of PKC-delta, PKC-epsilon, and NF-kappaB with CCK-8 involved both phosphatidylinositol-specific PLC and phosphatidylcholine (PC)-specific PLC, whereas with TNF-alpha they only required PC-specific PLC for activation. Results indicate that CCK-8 and TNF-alpha initiate NF-kappaB activation by different PLC pathways that converge at the novel PKCs (delta and epsilon) to mediate NF-kappaB activation in pancreatic acinar cells. These findings suggest a key role for the novel PKCs in pancreatitis.     

Assembly of adherens junctions is required for sphingosine 1-phosphate-induced matriptase accumulation and activation at mammary epithelial cell-cell contacts

Sphingosine 1-phosphate (S1P), a bioactive phospholipid, simultaneously induces actin cytoskeletal rearrangements and activation of matriptase, a membrane-associated serine protease in human mammary epithelial cells. In this study, we used a monoclonal antibody selective for activated, two-chain matriptase to examine the functional relationship between these two S1P-induced events. Ten minutes after exposure of 184 A1N4 mammary epithelial cells to S1P, matriptase was observed to accumulate at cell-cell contacts. Activated matriptase first began to appear as small spots at cell-cell contacts, and then its deposits elongated along cell-cell contacts. Concomitantly, S1P induced assembly of adherens junctions and subcortical actin belts. Matriptase localization was observed to be coincident with markers of adherens junctions at cell-cell contacts but likely not to be incorporated into the tightly bound adhesion plaque. Disruption of subcortical actin belt formation and prevention of adherens junction assembly led to prevention of accumulation and activation of the protease at cell-cell contacts. These data suggest that S1P-induced accumulation and activation of matriptase depend on the S1P-induced adherens junction assembly. Although MAb M32, directed against one of the low-density lipoprotein receptor class A domains of matriptase, blocked S1P-induced activation of the enzyme, the antibody had no effect on S1P-induced actin cytoskeletal rearrangement. Together, these data indicate that actin cytoskeletal rearrangement is necessary but not sufficient for S1P-induced activation of matriptase at cell-cell contacts. The coupling of matriptase activation to adherens junction assembly and actin cytoskeletal rearrangement may serve to ensure tight control of matriptase activity, restricted to cell-cell junctions of mammary epithelial cells.