The protein kinase C inhibitor Go6976 [12-(2-cyanoethyl)-6,7,12,13-tetrahydro-13-methyl-5-oxo-5H-indolo(2,3-a)pyrrolo(3,4-c)-carbazole] potentiates agonist-induced mitogen-activated protein kinase activation through tyrosine phosphorylation of the epidermal growth factor receptor

PKC Inhibits Sec61 Translocon-Mediated Sarcoplasmic Reticulum Ca2+ Leak in Smooth Muscle Cells

PKC inhibitors stimulate Ca2+ release from internal stores in diverse cell types. Our data indicate that this action cannot be explained by an increased agonist-induced IP3 production or an overloaded SR Ca2+ pool in smooth muscle cells from guinea pig urinary bladder. The incubation of these cells with three different PKC inhibitors, such as Go6976, Go6983, and BIM 1, resulted in a higher SR Ca2+ leak revealed by inhibition of the SERCA pump with thapsigargin. This SR Ca2+ leakage was sensitive to protein translocation inhibitors such as emetine and anisomycin. Since this increased SR Ca2+ leak did not result in a depleted SR Ca2+ store, we have inferred there was a compensatory increase in SERCA pump activity, resulting in a higher steady-state. This new steady-state increased the frequency of Spontaneous Transient Outward Currents (STOCs), which reflect the activation of high conductance, Ca2+-sensitive potassium channels in response to RyR-mediated Ca2+ sparks. This increased STOC frequency triggered by PKC inhibition was restored to normal by inhibiting translocon-mediated Ca2+ leak with emetine. These results suggest a critical role of PKC-mediated translocon phosphorylation in regulating SR Ca2+ steady-state, which, in turn, alters SR Ca2+ releasing activity.

12(S)-hydroxyheptadeca-5Z,8E,10E-trienoic acid (12-HHT) induces cell growth and improves barrier function through BLT2 interaction in intestinal epithelial Caco-2 cell cultures

12(S)-hydroxyheptadeca-5Z,8E,10E-trienoic acid (12-HHT) is an unusual product of the cyclooxygenase pathway that is an endogenous ligand of the low-affinity receptor for leukotriene 4 (LTB₄), BLT2. Recent findings suggested that BLT2 possibly plays an important role in the healing of intestinal lesions and the regulation of barrier function. Here, we studied the role of 12-HHT on intestinal epithelial cell growth and the paracellular permeability of intestinal epithelium using Caco-2 cell cultures as experimental model. Our results demonstrated that 12-HHT stimulates intestinal epithelial Caco-2 cell growth through 12-HHT-BLT2-p38-PKC axis and improves paracellular permeability in differentiated Caco-2 cell cultures through the regulation of tight junction elements such as myosin light chain phosphorylation through 12-HHT-BLT2-p38-PKC-MYPT1 axis. Thus, 12-HHT-BLT2 interaction can be involved in intestinal epithelial cell growth and consequently in the epithelium regeneration/repair processes, together with an interesting improvement on the paracellular permeability. These effects appoint that 12-HHT/BLT2 axis may be a suitable strategy for treating wound healing epithelium and barrier-disrupted intestinal processes.

CRF1 Receptor Signaling via the ERK1/2-MAP and Akt Kinase Cascades: Roles of Src, EGF Receptor, and PI3-Kinase Mechanisms

In the present study, we determined the cellular regulators of ERK1/2 and Akt signaling pathways in response to human CRF₁ receptor (CRF₁R) activation in transfected COS-7 cells. We found that Pertussis Toxin (PTX) treatment or sequestering Gβγ reduced CRF₁R-mediated activation of ERK1/2, suggesting the involvement of a G_i-linked cascade. Neither G_s/PKA nor G_q/PKC were associated with ERK1/2 activation. Besides, CRF induced EGF receptor (EGFR) phosphorylation at Tyr¹⁰⁶⁸, and selective inhibition of EGFR kinase activity by AG1478 strongly inhibited the CRF₁R-mediated phosphorylation of ERK1/2, indicating the participation of EGFR transactivation. Furthermore, CRF-induced ERK1/2 phosphorylation was not altered by pretreatment with batimastat, GM6001, or an HB-EGF antibody indicating that metalloproteinase processing of HB-EGF ligands is not required for the CRF-mediated EGFR transactivation. We also observed that CRF induced Src and PYK2 phosphorylation in a Gβγ-dependent manner. Additionally, using the specific Src kinase inhibitor PP2 and the dominant-negative-SrcYF-KM, it was revealed that CRF-stimulated ERK1/2 phosphorylation depends on Src activation. PP2 also blocked the effect of CRF on Src and EGFR (Tyr⁸⁴⁵) phosphorylation, further demonstrating the centrality of Src. We identified the formation of a protein complex consisting of CRF₁R, Src, and EGFR facilitates EGFR transactivation and CRF₁R-mediated signaling. CRF stimulated Akt phosphorylation, which was dependent on G_i/βγ subunits, and Src activation, however, was only slightly dependent on EGFR transactivation. Moreover, PI3K inhibitors were able to inhibit not only the CRF-induced phosphorylation of Akt, as expected, but also ERK1/2 activation by CRF suggesting a PI3K dependency in the CRF₁R ERK signaling. Finally, CRF-stimulated ERK1/2 activation was similar in the wild-type CRF₁R and the phosphorylation-deficient CRF₁R-Δ386 mutant, which has impaired agonist-dependent β-arrestin-2 recruitment; however, this situation may have resulted from the low β-arrestin expression in the COS-7 cells. When β-arrestin-2 was overexpressed in COS-7 cells, CRF-stimulated ERK1/2 phosphorylation was markedly upregulated. These findings indicate that on the base of a constitutive CRF₁R/EGFR interaction, the G_i/βγ subunits upstream activation of Src, PYK2, PI3K, and transactivation of the EGFR are required for CRF₁R signaling via the ERK1/2-MAP kinase pathway. In contrast, Akt activation via CRF₁R is mediated by the Src/PI3K pathway with little contribution of EGFR transactivation.

In vitro identification of mitochondrial oxidative stress production by time-resolved fluorescence imaging of glioma cells

Oxidative phosphorylation and glycolysis are important features, by which cells could bypass oxidative stress. The level of oxidative stress, and the ability of cells to promote oxidative phosphorylation or glycolysis, significantly determined proliferation or cell demise. In the present work, we have employed selective mitochondrial probe MitoTracker™ Orange CMTM/Ros (MTO) to estimate the level of oxidative stress in cancer cells at different stressed conditions. MTO is partially sensitive to decrease of mitochondrial membrane potential and to reactive oxygen species (ROS) generated in mitochondria. We have demonstrated, that fluorescence lifetime of MTO is much more sensitive to oxidative stress than intensity-based approaches. This method was validated in different cancer cell lines. Our approach revealed, at relatively low ROS levels, that Gö 6976, a protein kinase C (PKC) α inhibitor, and rottlerin, an indirect PKCδ inhibitor, increased mitochondrial ROS level in glioma cell. Their involvement in oxidative phosphorylation and apoptosis was investigated with oxygen consumption rate estimation, western blot and flow-cytometric analysis. Our study brings new insight to identify feeble differences in ROS production in living cells.

Human anti-peptidoglycan-IgG-mediated opsonophagocytosis is controlled by calcium mobilization in phorbol myristate acetate-treated U937 cells

Recently, we demonstrated that human serum amyloid P component (SAP) specifically recognizes exposed bacterial peptidoglycan (PGN) of wall teichoic acid (WTA)-deficient Staphylococcus aureus ΔtagO mutant cells and then induces complement-independent phagocytosis. In our preliminary experiments, we found the existence of human serum immunoglobulins that recognize S. aureus PGN (anti-PGNIgGs), which may be involved in complement-dependent opsonophagocytosis against infected S. aureus cells. We assumed that purified serum anti-PGN-IgGs and S. aureus ΔtagO mutant cells are good tools to study the molecular mechanism of anti-PGN-IgG-mediated phagocytosis. Therefore, we tried to identify the intracellular molecule(s) that is involved in the anti-PGN-IgG-mediated phagocytosis using purified human serum anti-PGN-IgGs and different S. aureus mutant cells. Here, we show that anti-PGN-IgG-mediated phagocytosis in phorbol myristate acetate-treated U937 cells is mediated by Ca2(+) release from intracellular Ca2(+) stores and anti-PGN-IgG dependent Ca2(+) mobilization is controlled via a phospholipase Cγ-2-mediated pathway.

The PPARβ/δ agonist GW0742 modulates signaling pathways associated with cardiac myocyte growth via a non-genomic redox mechanism

Peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors that belong to the nuclear hormone receptor superfamily and appear to have beneficial effects in the cardiovascular system. PPARβ/δ has been shown previously to exert an inhibitory effect on cardiac myocyte hypertrophy in vivo and in vitro although the exact mechanism is not fully clear yet. The principal signaling pathways that have been involved in triggering cardiac hypertrophic response are mitogen-activated protein kinases (MAPKs) and PI3K/Akt cascades. In this study, we sought to evaluate the potential effects evoked by PPARβ/δ activation on signaling pathways that are implicated in cardiac myocyte growth responses. The selective PPARβ/δ agonist GW0742 attenuated ERK1/2 and Akt phosphorylation that was stimulated by growth promoting agonists (phenylephrine, insulin or IGF-1). This effect was not reversed by the specific PPARβ/δ antagonist, GSK0660, but was inhibited by vanadate, a potent protein tyrosine phosphatase inhibitor. In addition, GW0742 prevented the oxidation and inactivation of PTEN supporting further the notion that its inhibitory action on the agonist-induced kinase phosphorylation is mediated by the modulation of phosphatase activity. Furthermore, GW0742 abolished the agonist-induced intracellular generation of reactive oxygen species, independently of PPARβ/δ activation. Our data reveals a new non-genomic mechanism of GW0742, which ameliorates the generation of reactive oxygen species and attenuates ERK1/2 and PI3K/Akt signaling, with implications in the regulation of cardiac hypertrophic response.

EGF-induced MAPK signaling inhibits hemidesmosome formation through phosphorylation of the integrin {beta}4

Migration of keratinocytes requires a regulated and dynamic turnover of hemidesmosomes (HDs). We and others have previously identified three serine residues on the integrin β4 cytoplasmic domain that play a critical role in the regulation of HD disassembly. In this study we show that only two of these residues (Ser-1356 and Ser-1364) are phosphorylated in keratinocytes after stimulation with either PMA or EGF. Furthermore, in direct contrast to previous studies performed in vitro, we found that the PMA- and EGF-stimulated phosphorylation of β4 is not mediated by PKC, but by ERK1/2 and its downstream effector kinase p90RSK1/2. EGF-stimulated phosphorylation of β4 increased keratinocyte migration, and reduced the number of stable HDs. Furthermore, mutation of the two serines in β4 to phospho-mimicking aspartic acid decreased its interaction with the cytoskeletal linker protein plectin, as well as the strength of α6β4-mediated adhesion to laminin-332. During mitotic cell rounding, when the overall cell-substrate area is decreased and the number of HDs is reduced, β4 was only phosphorylated on Ser-1356 by a distinct, yet unidentified, kinase. Collectively, these data demonstrate an important role of β4 phosphorylation on residues Ser-1356 and Ser-1364 in the formation and/or stability of HDs.

Mechanisms of angiotensin II-induced ERK1/2 activation in fetal cardiomyocytes

Fetal cardiomyocytes have been utilized in studies on myocardial repair in the damaged hearts of rodents and other species. Changes in angiotensin II (Ang II) receptor expression, especially decline of its type II receptor (AT2), are known to occur during the growth of cardiomyocytes from fetus to adult. However, the extent to which changes in the signaling pathways of Ang II type I (AT1) and AT2 receptors via p42/44 mitogen-activated protein kinase (ERK1/2) activation affect the physiological and pathophysiological functions in cardiomyocytes has not been defined. The roles of these receptors were analyzed by confocal fluorescence microscopy, immunoblot analysis, reverse transcription PCR, measurement of intracellular 3',5'-cyclic AMP levels and siRNA technology in cultured rat fetal cardiomyocytes. These studies revealed that Gq is required for Ang II-induced ERK1/2 activation via the synergy of AT1 and AT2 receptors. It has also been shown that phospholipase Cβ1, protein kinase Cα and protein kinase A mediate the feedback inhibition of ERK1/2 activation via c-Raf and/or other intermediate signaling molecules. The observed mechanism of Ang II-induced ERK1/2 activation in fetal cardiomyocytes could be relevant to the understanding of cardiomyocyte development and turnover, as well as clinical approaches using protein- and cell-based therapy for diseases such as heart failure.

Cigarette smoke extract induces COX-2 expression via a PKCalpha/c-Src/EGFR, PDGFR/PI3K/Akt/NF-kappaB pathway and p300 in tracheal smooth muscle cells

Exposure to cigarette smoke extract (CSE) leads to airway or lung inflammation, which may be mediated through cyclooxygenase-2 (COX-2) expression and its product prostaglandin E2 (PGE2) synthesis. The aim of this study was to investigate the molecular mechanisms underlying CSE-induced COX-2 expression in human tracheal smooth muscle cells (HTSMCs). Here, we describe that COX-2 induction is dependent on PKCalpha/c-Src/EGFR, PDGFR/PI3K/Akt/NF-kappaB signaling in HTSMCs. CSE stimulated the phosphorylation of c-Src, EGFR, PDGFR, and Akt, which were inhibited by pretreatment with the inhibitor of PKCalpha (Gö6976 or Gö6983), c-Src (PP1), EGFR (AG1478), PDGFR (AG1296), or PI3K (LY294002). Moreover, CSE induced a significant increase in COX-2 expression, which was reduced by pretreatment with these inhibitors or transfection with siRNA of PKCalpha, Src, or Akt. Furthermore, CSE-stimulated NF-kappaB p65 phosphorylation and translocation were also attenuated by pretreatment with Gö6976, PP1, AG1478, AG1296, or LY294002. CSE-induced COX-2 expression was also mediated through the recruitment of p300 associated with NF-kappaB in HTSMCs, revealed by coimmunoprecipitation and Western blot analysis. In addition, pretreatment with the inhibitors of NF-kappaB (helenalin) and p300 (garcinol) or transfection with p65 siRNA and p300 siRNA markedly inhibited CSE-regulated COX-2 expression. However, CSE-induced PGE2 generation was reduced by pretreatment with the inhibitor of COX-2 (NS-398). These results demonstrated that in HTSMCs, CSE-induced COX-2-dependent PGE2 generation was mediated through PKCalpha/c-Src/EGFR, PDGFR/PI3K/Akt leading to the recruitment of p300 with NF-kappaB complex.

Cholinergic receptor pathways involved in apoptosis, cell proliferation and neuronal differentiation

Acetylcholine (ACh) has been shown to modulate neuronal differentiation during early development. Both muscarinic and nicotinic acetylcholine receptors (AChRs) regulate a wide variety of physiological responses, including apoptosis, cellular proliferation and neuronal differentiation. However, the intracellular mechanisms underlying these effects of AChR signaling are not fully understood. It is known that activation of AChRs increase cellular proliferation and neurogenesis and that regulation of intracellular calcium through AChRs may underlie the many functions of ACh. Intriguingly, activation of diverse signaling molecules such as Ras-mitogen-activated protein kinase, phosphatidylinositol 3-kinase-Akt, protein kinase C and c-Src is modulated by AChRs. Here we discuss the roles of ACh in neuronal differentiation, cell proliferation and apoptosis. We also discuss the pathways involved in these processes, as well as the effects of novel endogenous AChRs agonists and strategies to enhance neuronal-differentiation of stem and neural progenitor cells. Further understanding of the intracellular mechanisms underlying AChR signaling may provide insights for novel therapeutic strategies, as abnormal AChR activity is present in many diseases.

Lysophosphatidic acid induces rapid and sustained decreases in epidermal growth factor receptor binding via different signaling pathways in BEAS-2B airway epithelial cells

Lysophosphatidic acid (LPA) and epidermal growth factor (EGF) are important mediators of lung cell function and lung diseases. We showed previously that LPA decreases epidermal growth factor receptor (EGFR) binding rapidly in BEAS-2B airway epithelial cells, and this decrease is sustained to at least 18 h. The current studies investigate which LPA signaling pathways mediate the rapid versus sustained decreases in EGFR binding in BEAS-2B cells. The G(i/o) inhibitor pertussis toxin and the Rho kinase inhibitor Y-27632 [(R)-(+)-trans-N-(4-pyridyl)-4-(1-aminoethyl)-cyclohexanecarboxamide] had no effect on the rapid or sustained decreases. However, the mitogen-activated protein kinase kinase (MEK) inhibitor U0126 [1,4-diamino-2,3-dicyano-1,4-bis(o-aminophenylmercapto)-butadiene ethanolate] decreased extracellular signal-regulated kinase (ERK) 1/2 phosphorylation, completely inhibited the rapid decrease in binding, and partially inhibited the sustained decrease. The direct Ca2+- and phospholipid-dependent protein kinase (PKC) activator phorbol-12-myristate-13-acetate stimulated ERK1/2 phosphorylation and decreased EGFR binding at both 15 min and 18 h. Furthermore, inhibitors of PKC partially inhibited ERK1/2 phosphorylation and the 15-min decrease but completely inhibited the 18-h decrease. Inhibitor time course studies showed that PKC induction of the 18-h decrease occurred during the first 3 h of treatment. We showed previously that LPA-stimulated EGFR transactivation contributes to the rapid decrease. Two transactivation inhibitors partially inhibited ERK1/2 phosphorylation, and U0126 partially inhibited EGFR transactivation, indicating that MEK may be involved both upstream and downstream of EGFR activation. Together, the data presented here indicate that LPA mediates the rapid decrease in EGFR binding via EGFR transactivation, MEK/ERK, and PKC, whereas the sustained decrease is regulated primarily by PKC.

Lysophosphatidic acid decreases epidermal growth factor receptor binding in airway epithelial cells

We showed previously that treatment of human airway smooth muscle cells and lung fibroblasts with lysophosphatidic acid (LPA) increases the binding of epidermal growth factor (EGF) to EGF receptors (EGFRs). The purpose of this study was to determine whether LPA also regulates EGFR binding in airway epithelial cells. Airway epithelial cells were incubated in the absence or presence of 10 microM LPA for increasing times, and binding of 125I-EGF to intact cells on ice was measured. Exposure to LPA for only 15 min caused a 30 to 70% decrease in EGFR binding in a dose-dependent manner, depending on the cell line. This decrease in binding was sustained to at least 18 h in BEAS-2B and primary human bronchial epithelial cells. In contrast, the LPA-induced decrease in binding reversed rapidly in two lung cancer epithelial cell lines, H292 and A549, returning to control levels within 3 h. LPA increased phosphorylation of the EGFR in BEAS-2B cells, and this phosphorylation was inhibited by both 4-(3'-chloroanilino)-6,7-dimethoxy-quinazoline (AG1478; EGFR tyrosine kinase inhibitor) and N-[(2R)-2-(hydroxamidocarbonylmethyl)-4-methylpentanoyl]-l-tryptophan methylamide (GM6001; matrix metalloproteinase inhibitor) but not by CRM197 (heparin-binding EGF inhibitor). AG-1478 and GM6001 also inhibited the LPA-induced decrease in EGFR binding but only by 50%, suggesting only partial involvement of EGFR transactivation in the decrease in EGFR binding. In summary, LPA stimulates a decrease in EGFR binding in airway epithelial cells that is sustained in normal cells but that rapidly reverses in cancer cells. LPA-induced transactivation of EGFRs occurs and contributes to the decrease in EGFR binding, but additional pathway(s) may also be involved.

Angiotensin II activates p44/42 MAP kinase partly through PKCepsilon in H295R cells

Using pharmaceutical and overexpression approaches we have previously reported that in H295R cells, (a) angiotensin II (AII) activates PKCepsilon, PKCalpha and p44/42 MAPK pathway, (b) PKCepsilon, PKCalpha and p44/42 MAPK overexpression inhibits AII-induced CYP11B2 gene transcription and (c) overexpression of PKCepsilon inhibits CYP11B2 gene transcription through p44/42 MAPK activation [LeHoux, J.G., Dupuis, G., Lefebvre, A., 2001. Control of CYP11B2 gene expression through differential regulation of its promoter by atypical and conventional protein kinase C isoforms. J. Biol. Chem. 276 (11), 8021-8028; LeHoux, J.G., Lefebvre, A., 2006. Novel protein kinase C-epsilon inhibits human CYP11B2 gene expression through ERK1/2 signalling pathway and JunB. J. Mol. Endocrinol. 36 (1), 51-64]. The aim of the present work was to evaluate the physiological role of endogenous PKCepsilon and PKCalpha isoforms in the activation of p44/42 MAPK by AII. A 50% reduction of PKCepsilon protein by siRNA-PKCepsilon resulted in 35% inhibition of AII-induced p44/42 MAPK activation. Knockdown of PKCepsilon stimulated AII-induced CYP11B2 transcription indicating that the PKCepsilon is not involved in the activation of CYP11B2 gene expression by AII. Furthermore, knockdown of PKCalpha enhanced AII-stimulated CYP11B2 transcription without altering p44/42 MAPK indicating that inhibition of AII-stimulated CYP11B2 gene by PKCalpha does not involve the p44/42 MAPK signalling pathway. These results thus establish that physiologically, PKCepsilon and PKCalpha act through different signalling pathways to inhibit AII-stimulated CYP11B2 gene expression.

Protein kinase Calpha mediates feedback inhibition of EGF receptor transactivation induced by Gq-coupled receptor agonists

While a great deal of attention has been focused on G-protein-coupled receptor (GPCR)-induced epidermal growth factor receptor (EGFR) transactivation, it has been known for many years that the tyrosine kinase activity of the EGFR is inhibited in cells treated with tumor-promoting phorbol esters, a process termed EGFR transmodulation. Because many GPCR agonists that elicit EGFR transactivation also stimulate the Gq/phospholipase C (PLC)/protein kinase C (PKC) pathway, we hypothesized that PKC-mediated inhibition of EGFR transactivation operates physiologically as a feedback loop that regulates the intensity and/or duration of GPCR-elicited EGFR transactivation. In support of this hypothesis, we found that treatment of intestinal epithelial IEC-18 cells with the PKC inhibitors GF 109203X or Ro 31-8220 or chronic exposure of these cells to phorbol-12,13-dibutyrate (PDB) to downregulate PKCs, markedly enhanced the increase in EGFR tyrosine phosphorylation induced by angiotensin II or vasopressin in these cells. Similarly, PKC inhibition enhanced EGFR transactivation in human colonic epithelial T84 cells stimulated with carbachol, as well as in bombesin-stimulated Rat-1 fibroblasts stably transfected with the bombesin receptor. Furthermore, cell treatment with inhibitors with greater specificity towards PKCalpha, including Gö6976, Ro 31-7549 or Ro 32-0432, also increased GPCR-induced EGFR transactivation in IEC-18, T84 and Rat-1 cells. Transfection of siRNAs targeting PKCalpha also enhanced bombesin-induced EGFR tyrosine phosphorylation in Rat-1 cells. Thus, multiple lines of evidence support the hypothesis that conventional PKC isoforms, especially PKCalpha, mediate feedback inhibition of GPCR-induced EGFR transactivation.

Interleukin-8 stimulates cell proliferation in non-small cell lung cancer through epidermal growth factor receptor transactivation

Interleukin-8 (IL-8; CXCL8) is a cytokine of the CXC chemokine family that is involved in neutrophil recruitment and activation. In addition, IL-8 has been implicated in a wide variety of other processes, including angiogenesis and metastasis in lung cancer. Lung adenocarcinoma and muco-epidermoid carcinoma cells produce substantial amounts of IL-8, and express both CXCR1 and CXCR2 IL-8 receptors. We hypothesized that IL-8 stimulates proliferation of non-small cell lung cancer cells, involving transactivation of the epidermal growth factor receptor (EGFR). The EGFR plays a central role in regulating cell proliferation and it has been therefore implicated in lung cancer. Both EGFR ligands and transactivation of the receptor may lead to downstream signalling events, including mitogen-activated protein kinase (MAPK) activation. Transactivation of the EGFR has been shown to occur in response to ligands of various G-protein coupled receptors (GPCRs) and involves metalloproteinase-mediated release of membrane bound EGFR ligands. The aim of the present study was to investigate the effect of IL-8 on proliferation of lung adenocarcinoma and muco-epidermoid carcinoma cells, and to explore the mechanisms leading to this proliferation in two different non-small cell lung cancer cell lines (A549 and NCI-H292). In both NSCLC cell lines, we observed that IL-8 stimulates epithelial cell proliferation in a dose-dependent manner. The ability of IL-8 to increase cell proliferation was blocked both by an inhibitor of EGFR tyrosine kinase, by a specific anti-EGFR blocking antibody and by a panmetalloproteinase inhibitor. Similar results were obtained using the GPCR inhibitor pertussis toxin. Inhibition of the MAPK p42/44 (ERK1/2) also blocked the mitogenic effect of IL-8, while a p38 MAPK inhibitor did not affect IL-8-induced cell proliferation. These results suggest that IL-8 increases cell proliferation in NSCLC cell lines via transactivation of the EGFR and that this mechanism involves metalloproteinase activity.

A delayed gonadotropin-dependent and growth factor-mediated activation of the extracellular signal-regulated kinase 1/2 cascade negatively regulates aromatase expression in granulosa cells

Human chorionic gonadotropin and human FSH (hFSH) elicit a transient increase in ERK1/2 phosphorylation lasting less than 60 min in immature granulosa cells expressing a low density of gonadotropin receptors. In cells expressing a high density of receptors, human chorionic gonadotropin and human FSH elicit this fast transient increase in ERK1/2 phosphorylation and also a delayed and more sustained increase that is detectable after 6-9 h. Both the early and delayed increases in ERK1/2 phosphorylation can be blocked with inhibitors of protein kinase A, the epidermal growth factor receptor kinase, metalloproteases, and MAPK kinase. The delayed effect, but not the early effect, can also be blocked with an inhibitor of protein kinase C. Because the delayed increase in ERK1/2 phosphorylation correlates with low aromatase expression in response to gonadotropins, we tested the effects of these inhibitors on aromatase expression. These inhibitors had little or no effect on gonadotropin-induced aromatase expression in cells expressing a low density of receptors, but they enhanced gonadotropin-induced aromatase expression in cells expressing a high density of receptors. Phorbol esters also induced a prolonged increase in ERK1/2 phosphorylation and, when added together with hFSH, blocked the induction of aromatase expression by hFSH in cells expressing a low density of hFSH receptor. A MAPK kinase inhibitor reversed the inhibitory effect of the phorbol ester on aromatase induction. We conclude that the effects of gonadotropins on ERK1/2 phosphorylation are mediated by epidermal growth factor-like growth factors and that the delayed effect is partially mediated by protein kinase C and acts as a negative regulator of aromatase expression.

Regulation of lysophosphatidic acid-induced epidermal growth factor receptor transactivation and interleukin-8 secretion in human bronchial epithelial cells by protein kinase Cdelta, Lyn kinase, and matrix metalloproteinases

We have demonstrated earlier that lysophosphatidic acid (LPA)-induced interleukin-8 (IL-8) secretion is regulated by protein kinase Cdelta (PKCdelta)-dependent NF-kappaB activation in human bronchial epithelial cells (HBEpCs). Here we provide evidence for signaling pathways that regulate LPA-mediated transactivation of epidermal growth factor receptor (EGFR) and the role of cross-talk between G-protein-coupled receptors and receptor-tyrosine kinases in IL-8 secretion in HBEpCs. Treatment of HBEpCs with LPA stimulated tyrosine phosphorylation of EGFR, which was attenuated by matrix metalloproteinase (MMP) inhibitor (GM6001), heparin binding (HB)-EGF inhibitor (CRM 197), and HB-EGF neutralizing antibody. Overexpression of dominant negative PKCdelta or pretreatment with a PKCdelta inhibitor (rottlerin) or Src kinase family inhibitor (PP2) partially blocked LPA-induced MMP activation, proHB-EGF shedding, and EGFR tyrosine phosphorylation. Down-regulation of Lyn kinase, but not Src kinase, by specific small interfering RNA mitigated LPA-induced MMP activation, proHB-EGF shedding, and EGFR phosphorylation. In addition, overexpression of dominant negative PKCdelta blocked LPA-induced phosphorylation and translocation of Lyn kinase to the plasma membrane. Furthermore, down-regulation of EGFR by EGFR small interfering RNA or pretreatment of cells with EGFR inhibitors AG1478 and PD158780 almost completely blocked LPA-dependent EGFR phosphorylation and partially attenuated IL-8 secretion, respectively. These results demonstrate that LPA-induced IL-8 secretion is partly dependent on EGFR transactivation regulated by PKCdelta-dependent activation of Lyn kinase and MMPs and proHB-EGF shedding, suggesting a novel mechanism of cross-talk and interaction between G-protein-coupled receptors and receptor-tyrosine kinases in HBEpCs.

Attenuation of Peroxisome Proliferator-activated Receptor γ (PPARγ) Mediates Gastrin-stimulated Colorectal Cancer Cell Proliferation

Peroxisome proliferators-activated receptor gamma (PPARgamma) has been shown to suppress cell proliferation and tumorigenesis, whereas the gastrointestinal regulatory peptide gastrin stimulates the growth of neoplastic cells. The present studies were directed to determine whether changes in PPARgamma expression might mediate the effects of gastrin on the proliferation of colorectal cancer (CRC). Initially, using growth assays, we determined that the human CRC cell line DLD-1 expressed both functional PPARgamma and gastrin receptors. Amidated gastrin (G-17) attenuated the growth suppressing effects of PPARgamma by decreasing PPARgamma activity and total protein expression, in part through an increase in the rate of proteasomal degradation. G-17-induced degradation of PPARgamma appeared to be mediated through phosphorylation of PPARgamma at serine 84 by a process involving the biphasic phosphorylation of ERK1/2 and activation of the epidermal growth factor receptor (EGFR). These results were confirmed through the use of EGFR antagonist AG1478 and MEK1 inhibitor PD98059. Furthermore, mutation of PPARgamma at serine 84 reduced the effects of G-17, as evident by inability of G-17 to attenuate PPARgamma promoter activity, degrade PPARgamma, or inhibit the growth suppressing effects of PPARgamma. The results of these studies demonstrate that the trophic properties of gastrin in CRC may be mediated in part by transactivation of the EGFR and phosphorylation of ERK1/2, leading to degradation of PPARgamma protein and a decrease in PPARgamma activation.

CysLT1 receptor-induced human airway smooth muscle cells proliferation requires ROS generation, EGF receptor transactivation and ERK1/2 phosphorylation

Background

Cysteine-containing leukotrienes (cysteinyl-LTs) are pivotal inflammatory mediators that play important roles in the pathophysiology of asthma, allergic rhinitis, and other inflammatory conditions. In particular, cysteinyl-LTs exert a variety of effects with relevance to the aetiology of asthma such as smooth muscle contraction, eosinophil recruitment, increased microvascular permeability, enhanced mucus secretion and decreased mucus transport and, finally, airway smooth muscle cells (ASMC) proliferation. We used human ASMC (HASMC) to identify the signal transduction pathway(s) of the leukotriene D₄ (LTD₄)-induced DNA synthesis.

Methods

Proliferation of primary HASMC was measured by [³H]thymidine incorporation. Phosphorylation of EGF receptor (EGF-R) and ERK1/2 was assessed with a polyclonal anti-EGF-R or anti-phosphoERKl/2 monoclonal antibody. A Ras pull-down assay kit was used to evaluate Ras activation. The production of reactive oxygen species (ROS) was estimated by measuring dichlorodihydrofluorescein (DCF) oxidation.

Results

We demonstrate that in HASMC LTD₄-stimulated thymidine incorporation and potentiation of EGF-induced mitogenic signaling mostly depends upon EGF-R transactivation through the stimulation of CysLT₁-R. Accordingly, we found that LTD₄ stimulation was able to trigger the increase of Ras-GTP and, in turn, to activate ERK1/2. We show here that EGF-R transactivation was sensitive to pertussis toxin (PTX) and phosphoinositide 3-kinase (PI3K) inhibitors and that it occurred independently from Src activity, despite the observation of a strong impairment of LTD₄-induced DNA synthesis following Src inhibition. More interestingly, CysLT₁-R stimulation increased the production of ROS and N-acetylcysteine (NAC) abolished LTD₄-induced EGF-R phosphorylation and thymidine incorporation.

Conclusion

Collectively, our data demonstrate that in HASMC LTD₄ stimulation of a G_i/o coupled CysLT₁-R triggers the transactivation of the EGF-R through the intervention of PI3K and ROS. While PI3K and ROS involvement is an early event, the activation of Src occurs downstream of EGF-R activation and is followed by the classical Ras-ERK1/2 signaling pathway to control G1 progression and cell proliferation.